WO2023003511A2

WO2023003511A2 - Methods of stereolithography 3d printing of transparent yag ceramics

Info

Publication number: WO2023003511A2
Application number: PCT/SG2022/050509
Authority: WO
Inventors: Li Ying LIU; Zehui DU; Chee Lip GAN; Wang GUO
Original assignee: Nanyang Technological University; Fujian Institute Of Research On The Structure Of Matter Chinese Academy Of Sciences
Priority date: 2021-07-19
Filing date: 2022-07-19
Publication date: 2023-01-26
Also published as: WO2023003511A3

Abstract

Disclosed herein is a formulation for use in vat photopolymerization based additive manufacturing comprising particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element, a photosensitive resin precursor, a dispersant, and a photoinitiator, and a ceramic product comprising a layer formed from one or more of the group consisting of YAG, and a YAG doped by a rare earth element, which has a first surface, a second surface and a body portion therebetween, wherein the second surface has a plurality of projections extending from the second surface and/or a plurality of depressions that extend into the body portion. Also disclosed herein are methods of stereolithography 3D printing of YAG ceramics and the application of the printed YAG ceramics for LED lighting and thermal management and also for imaging.

Description

METHODS OF STEREOLITHOGRAPHY 3D PRINTING OF TRANSPARENT YAG

CERAMICS

Field of Invention

The current invention relates to a formulation for use in additive manufacturing, a ceramic product, and methods of stereolithography 3D printing of yttrium aluminium garnet (YAG) ceramics.

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

In recent years, additive manufacturing of ceramics has gained growing interest and importance in academia and industry sectors as it offers new manufacturing solutions and opens more shaping possibilities. YAG doped with lanthanide ions has attracted much attention recently because they can be used in various optical applications such as white light solid-state light emitting diodes (LED), thermal sensors, lenses, X-ray scintillators, solar cells, electro-optics, solid state lasers and photonic circuits. Compared to single crystals, transparent polycrystalline YAG ceramics can be mass-produced into near-net shapes at low cost without the need for extensive post-processing. However, the production of transparent ceramics with the desired properties remains challenging as the optical transparency of the ceramics is largely dependent on their microstructures and the inclusion of secondary phases from impurities or the presence of pores which usually causes light scattering that can significantly degrade the optical quality of the ceramics.

Traditionally, doped YAG ceramics are fabricated by die pressing, injection molding or slip casting, followed by vacuum sintering or vacuum pre-sintering and hot isostatic press (HIP) post- treatment. However, such conventional approaches only allow YAG ceramics with simple geometries to be obtained and this limits their practicability to be used in advanced applications that require complex structures. Furthermore, there are very few reports on the fabrication of transparent YAG ceramics using stereolithography (SLA)-based 3D printing. For example, Hostasa et al. have fabricated Yb:YAG ceramics using SLA 3D printing, but only square plates with optical transparency of 60% have been achieved (Hostasa, J. et al., Scr. Mater. 2020, 187, 194-196). Nd:YAG (neodymium-doped yttrium aluminium garnet, Nd:Y₃Al₅0i₂) have also been explored through direct ink writing (DIW), but only 3D shaped rods were attempted at low printing resolution (Jones, I. et al., Opt. Mater. 2018, 75, 19-25). SLA 3D printing can be an alternative solution to allow easy customisation of complex geometries with high printing resolution and high transparency to meet the demand for various applications such as light converters for white light emitting diodes (WLED), photocatalytic supports, optical lens, and windows without the need to re-design expensive molds and post machining.

It is well-known that phosphor converted WLED traditionally manufactured by mixing phosphor powders with silicone or organic resins tend to undergo thermal degradation and aging issues due to poor heat dissipation from the blue-light irradiation, thereby drastically deteriorating the light-conversion efficiency of the phosphors and inducing thermal quenching. To overcome the inherent defects induced by thermal quenching, cerium-doped YAG (YAG:Ce) transparent ceramic phosphors have been proposed to replace the organic resin substrates (Zhou, B. et al., Acta Mater. 2017, 130, 289-296).

To date, many approaches have been reported to change the surface topology of the YAG:Ce ceramics to enhance their light extraction efficiency, such as laser patterning, electron beam lithography and nanoimprint. Various micro/nano structures such as nanosphere arrays, nanohole arrays, nanobowl arrays and nanopillars have been fabricated (Huang, S. etal., Int. J. Appl. Ceram. Technol. 2009, 6, 465-469; and Zhu, P. etal., Opt. Express 2019, 27, A1297- A1307). The light extraction efficiency for normal YAG:Ce ceramics with flat surface is usually very low because most of the converted light is trapped inside the ceramic due to total internal reflection (TIR). The critical angle 0_C of TIR is approximately 33° for YAG:Ce (refractive index, n: 1.82, 0_C = arcsin(n_air/n_YAG). With such small critical angle, only ~ 8% of light can be extracted from the top surface of the ceramics (1/2 / ^c sin(0)d Q » 8%) (Sun, H. et al., IEEE Photon. J. 2016, 8, 1-10).

Therefore, there exists a need to discover new methods for improving the light extraction efficiency of flat YAG:Ce ceramics while maintaining their high external quantum efficiency.

Summary of Invention

Aspects and embodiments of the invention will now be discussed by reference to the following numbered clauses. 1. A formulation for use in additive manufacturing comprising: particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; a photosensitive resin precursor; a dispersant; and a photoinitiator, wherein the formulation has a viscosity of from 0.5 to 100 Pa s as determined at a temperature of 25 °C and a shear rate of from 10 to 120 (e.g. 30) s^_1.

2. The formulation for use in additive manufacturing according to Clause 1, wherein the photosensitive resin precursor comprises a multifunctional monomer and/or a multifunctional oligomer.

3. The formulation for use in additive manufacturing according to Clause 2, wherein the multifunctional monomer and/or the multifunctional oligomer is selected from one or more of the group consisting of dipentaerythritol hexaacrylate, dimethylolpropane tetraacrylate, tris(2- hydroxy ethyl) isocyanurate triacrylate, alkoxylated pentaerythritol tetraacrylate and, more particularly, trimethylolpropane ethoxylate triacrylate (TMPETA) (e.g. TMPETA 428, 692, and 912), trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), pentaerythritol tetraacrylate (PETTA), trimethylolpropane propoxylate triacylate (TMPPTA), an amine-modified polyether acrylate, and a urethane acrylate.

4. The formulation for use in additive manufacturing according to Clause 2 or Clause 3, wherein the photosensitive resin precursor further comprises a diluent; and/or the formulation has a viscosity of from 5 to 10 Pa s as determined at a temperature of 25 °C and a shear rate of 30 s^_1.

5. The formulation for use in additive manufacturing according to Clause 4, wherein the diluent is selected from one or more of the group consisting of diethylene glycol dimethylate, polyethylene glycol diacrylate, 2-ethylhexyl methacrylate, isobornyl acrylate, isodecyl acrylate and. more particularly, neopentyl glycol propoxylate (1PO/OH) diacrylate (NPG2PODA), 1,6- hexanediol diacrylate (HDDA), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxyethyl acrylate (HEA), and triethylene glycol dimethacrylate (TEGDMA).

6. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein the rare earth element is selected from one or more of the group consisting of Tm, Er, Ho and, more particularly, Ce, Tb, Sm, Dy, Nd and Yb, optionally wherein the rare earth element is selected from one or more of the group consisting of Ce, Nd and Yb, such as Ce.

7. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein the rare earth element has an atomic percentage concentration in the YAG of from 0.001 at% to 20 at%, such as from 0.0075 to 10 at%, such as from 0.01 to 1.0 at%, such as from 0.015 to 0.5 at%, such as from 0.02 to 0.1 at% (e.g. about 0.01 at%).

8. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein the YAG particles have a size of from 0.1 to 5pm, such as from 0.1 to 2 pm.

9. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein:

(ai) the dispersant is Solsperse 85000; and/or

(aii) the photoinitiator is 2-hydroxy-2-methylpropiophenone or a blend of 2-hydroxy-2- methylpropiophenone with diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide.

10. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein the formulation further comprises one or more photo-absorber additives, optionally wherein the photo-absorber additives are selected from one or more of the group consisting of Sudan Orange G, Sudan III, and Para red.

11. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein the formulation comprises: from 50 to 90 wt% of particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; from 9.3 to 45 wt% of the photosensitive resin precursor; from 0.5 to 5.0 wt% (e.g. from 0.5 to 2.0 wt%) of the dispersant; from 0.1 to 2.5 wt% of the photoinitiator; and from 0 to 0.1 wt% of a photo-absorber additive, optionally wherein the formulation comprises: from 70 to 85 wt% of particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; from 14 to 27 wt% of the photosensitive resin precursor; from 0.77 to 2.0 wt% of the dispersant; from 0.2 to 1.0 wt% of the photoinitiator; and from 0 to 0.03 wt% of a photo-absorber additive.

12. The formulation for use in additive manufacturing according to Clause 11 , wherein the formulation comprises:

(i) from 70 to 85 wt% of particles of YAG; from 10 to 20 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 1 to 12 wt% in total of one or both of NPG2PODA and HDDA; from 0.2 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2-hydroxy-2- methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; from 0.5 to 5.0 wt% (e.g. from 0.5 to 2.0 wt%) of Solsperse 85000; and from 0.02 to 0.03 wt% of Sudan I, such as: from 70 to 85 wt% of particles of YAG; from 11 to 16 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 3 to 11 wt% in total of one or both of NPG2PODA and HDDA; from 0.3 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2- hydroxy-2-methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; from 0.5 to 2.0 wt% of Solsperse 85000; and from 0.02 to 0.03 wt% of Sudan I; or

(ϋ) from 70 to 85 wt% of particles of YAG doped by Ce; from 10 to 20 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 1 to 12 wt% in total of one or both of NPG2PODA and HDDA; from 0.2 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2-hydroxy-2- methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; and from 0.5 to 2.0 wt% of Solsperse 85000, such as from 70 to 85 wt% of particles of YAG doped by Ce; from 11 to 16 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 3 to 11 wt% in total of one or both of NPG2PODA and HDDA; from 0.3 to 0.5 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2- hydroxy-2-methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; and from 0.7 to 2.0 wt% of Solsperse 85000.

13. The formulation for use in additive manufacturing according to any one of the preceding clauses, wherein the formulation further comprises particles of a thermally conductive ceramic material, optionally wherein:

(bi) the particles of a thermally conductive ceramic material is selected from one or more of the group consisting of AI2O3_, MgO, and MgAhCU; and/or

(bii) the amount of the particles of a thermally conductive ceramic material is from 5 to 40 wt%, such as from 10 to 35 wt%, relative to the weight of the particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element.

14. A ceramic product comprising: a layer formed from one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element, which has a first surface, a second surface and a body portion therebetween, wherein the second surface has a plurality of projections extending from the second surface and/or a plurality of depressions that extend into the body portion.

15. The ceramic product according to Clause 14, comprising a first layer formed from a yttrium aluminium garnet (YAG); and a second layer formed from YAG doped by a rare earth element, wherein the second layer has a first surface facing and is attached to the first layer, a second surface facing in the opposite direction to the first surface and a body portion therebetween, wherein the second surface has a plurality of projections extending from the second surface and/or a plurality of depressions that extend into the body portion.

16. The ceramic product according to Clause 15, wherein:

(ci) the first layer and the second layer are not susceptible to delaminate from one another; and/or

(cii) the first layer and the second layer independently have a thickness of from 0.5 to 5.0 mm, such as from 0.75 to 3.0 mm, such as from 1.0 to 2.0 mm. 17. The ceramic product according to Clause 15 or Clause 16, wherein the plurality of projections extending from the second surface are selected from one or more of pyramidal projections, conical projections, or convex projections.

18. The ceramic product according to Clause 17, wherein the plurality of projections extending from the second surface are asymmetric pyramidal projections, optionally wherein the asymmetric pyramidal projections have an oblique angle of from 75 to 135°, such as from 100 to 110°, such as about 105°.

19. The ceramic product according to Clause 17, wherein the plurality of projections extending from the second surface are convex projections, optionally wherein each convex projection has a diameter of from 0.1 mm to 5.0 mm, such as from 1.0 mm to 3.0 mm, such as about 1.0 mm or about 2.0 mm and a height of from 0.1 mm to 2.5 mm, such as from 0.5 mm to 2.0 mm, such as about 1.0 mm.

20. The ceramic product according to any one of Clauses 15 to 19, wherein the plurality of depressions that extend into the body portion are concave depressions, optionally wherein each concave depression has a diameter of from 1.0 mm to 3.0 mm, such as 2.0 mm and a depth of from 0.5 mm to 2.0 mm, such as about 0.5 mm or about 1.0 mm.

21. The ceramic product according to any one of Clauses 14 to 20, wherein the rare earth element is selected from one or more of the group consisting of Tm, Er, Ho and, more particularly, Ce, Tb, Sm, Dy, Nd and Yb, optionally wherein the rare earth element is selected from one or more of the group consisting of Ce, Nd and Yb, such as Ce.

22. The ceramic product according to any one of Clauses 14 to 21 , wherein the rare earth element has an atomic percentage concentration in the YAG of from 0.001 at% to 20 at%, such as from 0.0075 to 10 at%, such as from 0.01 to 1.0 at%, such as from 0.015 to 0.5 at%, such as from 0.02 to 0.1 at% (e.g. about 0.01 at%).

23. The ceramic product according to any one of Clause 14 and Clauses 21 to 22 as dependent upon Clause 14, wherein the ceramic product further incorporates a thermally conductive ceramic material, optionally wherein:

(di) the thermally conductive ceramic material is selected from one or more of AI₂O₃, MgO, and MgAhCU; and/or (dii) the amount of the thermally conductive ceramic material is from 5 to 40 wt%, such as from 10 to 35 wt%, relative to the weight of the particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element.

24. The ceramic product according to any one of Clauses 15 and 16 to 22, as dependent upon Clause 15, wherein the first and/or second layer of the ceramic product further incorporates a thermally conductive ceramic material, optionally wherein:

(di) the thermally conductive ceramic material is selected from one or more of the group consisting of AI₂O₃, MgO, and MgAfeO^ and/or

(dii) the amount of the thermally conductive ceramic material is from 5 to 40 wt%, such as from 10 to 35 wt%, relative to the weight of the particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element.

25. The ceramic product according to any one of Clauses 14 to 24, wherein the ceramic product has a first portion and a second portion, where the first portion corresponds to the ceramic product of Clauses 14 to 24 and the second portion provides a plurality of interconnected pores suitable to remove heat, optionally wherein:

(ei) the second portion occupies a periphery of the ceramic product, with the first portion occupying a central region; or

(eii) the first portion occupies a periphery of the ceramic product, with the second portion occupying a central region.

26. The ceramic product according to Clause 25, wherein the second portion has a cellular ceramic structure of one or more of the group consisting of an array of thin sheets or sheet- based triply periodic minimal surface (TPMS) lattices such as Schwarz primitive, Schwarz diamond, Schoen gyroid and Schoen l-WP.

27. A white-light LED device comprising a ceramic product according to any one of Clauses 14 to 26 mounted on a blue-light LED chip, such that the light from the LED passes through the ceramic product to generate a white light emission.

28. A process of forming a green body product, wherein the process comprises the steps of:

(a) providing a formulation according to any one of Clauses 1 to 13 to an additive manufacturing device; and (b) forming a green body product layer by layer using the additive manufacturing device according to a set of instructions readable by the additive manufacturing device to produce the green body product.

29. The process according to Clause 28, wherein the green body product results in a unitary body formed of a single material.

30. The process according to Clause 28, wherein the green body product is formed of: a first layer according to a first formulation comprising particles of yttrium aluminium garnet (YAG); and a second layer according to a second formulation comprising particles of a YAG doped by a rare earth element.

31. The process according to Clause 30, wherein each layer of the first and second formulations is subjected to curing before the next layer is applied, where a percentage difference in a cure depth used in the first layer compared to a cure depth used in the second layer is from 5 to 20%, optionally wherein the cure depth in the first layer and second layer are independently selected from 50 to 120 pm (e.g. from 90 to 110 pm).

32. A process of forming a debound product from a green body product, which process comprises the steps of:

(aa) providing a green body product as described in any one of Clauses 28 to 31 ;

(ab) subjecting the green body product to a temperature in the range of from 100 to 650 °C, such as from 250 to 600 °C, such as from 300 to 500 °C, under an inert atmosphere for a first period of time, followed by subjecting the green body product to a temperature in the range of from 600 to 900 °C, such as from 600 to 800 °C, under an atmosphere comprising oxygen for a second period of time to provide the debound body product, wherein the debound body product is substantially free of organic matter.

33. A process for forming a ceramic product, the process comprising the steps of:

(ba) providing a debound body product as described in Clause 32; and

(bb) subjecting it to sintering under vacuum at a temperature of from 1500 to 2000 °C, such as from 1720 to 1890 °C for a period of time to provide the ceramic product, optionally wherein the vacuum is from 1x1 O³ to 1.0 Pa. 34. An imaging device comprising a ceramic product according to any one of Claims 14 to 26, wherein the ceramic product is in the form of an optical lens, optionally wherein the optical lens is a convex or concave lens.

Drawings

FIG. 1 depicts the multi-step debinding protocol of 3D printed YAG:Ce green bodies in different atmospheres.

FIG. 2 depicts the morphology and particle size of (A) YAG:Ce, and (B) YAG powder.

FIG. 3 depicts the plot of the viscosities of YAG and YAG:Ce paste as a function of shear rate.

FIG.4 depicts the 3D digital models of asymmetric triangle array with oblique angles of: (A)75° , (B)90° and (C) 135°, respectively, (D) photo of the printed YAG:Ce ceramic green bodies with the asymmetric triangle array and some with other structures such as microcones, kelvin cells, and octet truss, and (E) photo of the sintered samples.

FIG. 5 depicts the photo of the bilayer YAG/YAG:Ce ceramic green body with asymmetric triangle surface topology.

FIG. 6 depicts the scanning electron microscopy (SEM) images of (A) YAG:0.1%Ce sintered at 1800 °C for 10 h; and (B) pure YAG layer sintered at 1820 °C for 15 h, after polishing and thermal etching.

FIG. 7 depicts the 3D printed transparent YAG & YAG:Ce ceramics with complex shapes.

FIG. 8 depicts (A) photo of the 3D printed transparent single layer YAG:Ce and bilayer YAG/YAG:Ce ceramics, and (B) optical transmittances of the sintered YAG:Ce ceramics (sample thickness: 1.8 mm). Samples were sintered at 1800 °C for 10 h or 1820 °C for 15 h.

FIG. 9 depicts the photoluminescence excitation and emission spectra of 3D printed YAG:0.1%Ce monolayer ceramics sintered at 1800 °C for 10 h.

FIG. 10 depicts (A-B) list of the size of the asymmetric triangles in the printed samples; (C) far-field emission intensity patterns; and (D) plot of luminous efficacy and divergent angles (taken at 50% of the full luminosity) of the 3D printed YAG:Ce asymmetric triangle arrays with different oblique angles. The ceramics were excited by blue LED chip with an injection current of 50 mA. The far-field emission intensity unit is candela.

FIG. 11 depicts (A-B) 3D digital models of YAG/YAG: 0.1% Ce bilayer convex and concave lens; (C) schematic illustration of the sample cross section for the bilayer samples; and (D-E) photos of the bilayer samples with convex lens after sintering at 1800 °C for 10 h.

FIG. 12 depicts the far-field radiation patterns of the 3D printed YAG/YAG:0.1%Ce bilayer samples with convex and concave lens arrays and the YAG:0.1%Ce monolayer sample with concave lens array on InGaN/GaN blue LED flip chips mounted on aluminium nitride substrates. The sample thickness was ~2.0 mm.

FIG. 13 depicts (A) photos of the 3D printed YAG/YAG:0.1% Ce bilayer ceramics with different convex lens sizes (thickness: ~2.5 mm) on InGaN/GaN blue vertical chips mounted on aluminium nitride substrates; and (B) far-field emission patterns of the samples.

FIG. 14 depicts the 3D model of the YAG ceramic disks with primitive, gyroid and diamond structures at the lateral surface for heat dissipation.

FIG. 15 depicts (A) infrared thermal images of the respective 3D printed 10 wt% AI2O3- YAG:0.1%Ce composite ceramics at 1.5 A and 6.1 V; (B) comparison of the surface temperatures of the 3D printed composite ceramic samples with the control round disk at different currents; (C) 3D printed 10 wt% Al₂O₃-YAG:0.1%Ce composite ceramic with primitive, gyroid and diamond structures; and (D) photo of a 3D printed 10 wt% Al₂O₃-YAG:0.1%Ce diamond structure mounted on the blue LED chip. The ceramics were excited by blue LED chip with an injection current of 1 5A and a voltage of 6.1 V.

FIG. 16 depicts the photo of the polished convex lens C3 with the designed lens diameter of 1.0mm and height of 0.5 mm, showing a good imaging ability. Letter “1” and grids can be seen clearly. The ceramic is YAG:0.1% Ce.

Description

It has been surprisingly found that the use of additive manufacturing enables the generation of YAG:Ce ceramics (e.g. flat YAG:Ce ceramics) with improved light extraction efficiency and high external quantum efficiency. As will be appreciated, the YAG ceramics disclosed herein are obtained by additive manufacturing. As such, in a first aspect of the invention, there is provided a formulation for use in additive manufacturing comprising: particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; a photosensitive resin precursor; a dispersant; and a photoinitiator, wherein the formulation has a viscosity of from 0.5 to 100 Pa s as determined at a temperature of 25 °C and a shear rate of from 10 to 120 (e.g. 30) s^_1.

The viscosity of the formulation may be measured as described in the examples.

In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa.

When used herein, the term “additive manufacturing” refers to the construction of a three- dimensional object from a CAD model or a digital 3D model. Any suitable form of additive manufacturing may be used herein. In particular embodiments of the invention that may be mentioned herein, the additive manufacturing may be by vat photopolymerization 3D printing according to ISO ASTM 52900-15 or stereolithography 3D printing.

As noted above, it is possible to mix both of a pure YAG and a YAG doped by a rare earth element in a single formulation. However, this may result in a lower transparency for the resulting composite between the wavelengths of 550-800 nm due to the slight difference in refractive index between the pure YAG and the doped YAG. As such, it may be preferred herein to make use of only one of these materials in each formulation. For example, to maintain high transparency of the composite ceramics, a YAG microlens array may be printed above the YAG:Ce to improve the light extraction efficiency and also to achieve directional control of emission from the YAG:Ce layer by reducing loss of total internal reflection at the interface, as is discussed in more detail below in the examples. When used herein, the term “pure YAG” may refer to a YAG with substantially no impurities - and certainly one that has almost no (or no) impurities in the form of rare earth elements. For example, the total amount of impurity in the pure YAG may be less than or equal to 0.1 at%, while the amount of any rare earth element impurity, if present at all, will be less than 0.0001 at%. When used herein, the term “at%” refers to atomic percentage.

The YAG particles used herein (whether doped or undoped) may have any suitable size. For example, the YAG particles may have a size of from 0.1 to 5pm, such as from 0.1 to 2 pm.

For the avoidance of doubt, it is explicitly contemplated that where a number of numerical ranges related to the same feature are cited herein, the end points for each range are intended to be combined in any order to provide further contemplated (and implicitly disclosed) ranges.

Thus, the YAG particles discussed above may be selected from any of the following ranges: from 0.1 to 2 pm, from 0.1 to 5 pm; and from 2 to 5 pm.

Examples of YAG particle sizes are provided by FIG. 2.

The technology disclosed herein provides a formulation of paste containing large quantity of micro-sized ceramic powders, exhibiting excellent self-holding and/or shear thinning behavior and low viscosity. The paste comprises YAG:Ce or undoped YAG ceramic powder, photosensitive resin precursor, photoinitiator, and dispersant. The YAG powders disclosed herein (even when doped) should have a purity of over 99.9 at%, and may have a particle size ranging from 0.1 to 5 pm (e.g. from 0.1 to 2 pm) to ensure a good sintering activity, with no impurity phase in the final ceramic body. The solid load of the YAG powder in the paste may be at least 50 wt%.

The photosensitive resin precursor when used herein refers to at least one photosensitive monomeric or oligomeric material, though it may include two or more photosensitive monomers and/or oligomers instead, provided that the monomers and/or oligomers selected are capable of providing the desired level of viscosity when it forms part of the formulation.

As will be appreciated, the photosensitive resin precursor material should be a material that is capable of light curing under conditions conducive to additive manufacturing. In particular embodiments of the invention that may be mentioned herein, the photosensitive resin precursor may be formed of free-radical polymerizable acrylate monomers and/or oligomers. In particular, monomers and/or oligomers with high chemical functionality and low viscosities may be selected as components of the paste.

More particularly, the photosensitive resin precursor may comprise a multifunctional monomer and/or a multifunctional oligomer. When used herein, the term “multifunctional” is intended to refer to a monomer or oligomer that has two or more (e.g. 2, 3, 4, 5, 6, 7 or 8) functional groups that are capable of taking part in a polymerisation reaction. The multifunctional monomer and/or the multifunctional oligomer is selected from one or more of the group consisting of dipentaerythritol hexaacrylate, dimethylolpropane tetraacrylate, tris(2-hydroxy ethyl) isocyanurate triacrylate, alkoxylated pentaerythritol tetraacrylate and, more particularly, trimethylolpropane ethoxylate triacrylate (TMPETA) (e.g. TMPETA 428, 692, and 912), trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), pentaerythritol tetraacrylate (PETTA), trimethylolpropane propoxylate triacylate (TMPPTA), an amine- modified polyether acrylate, and a urethane acrylate.

As will be appreciated, if used alone the multifunctional monomer and/or multifunctional oligomer may provide a formulation that is too viscous for use (or potentially one that is not viscous enough). As such, the photosensitive resin precursor may also include a diluent material. When used herein, the term “diluent” may refer to a monomer or oligomer that is less or more viscous than the multifunctional monomer and/or multifunctional oligomer so as to provide a formulation with the desired level of viscosity. Examples of such diluents include, but are not limited to, diethylene glycol dimethylate, polyethylene glycol diacrylate, 2- ethylhexyl methacrylate, isobornyl acrylate, isodecyl acrylate and. more particularly, neopentyl glycol propoxylate (1PO/OH) diacrylate (NPG2PODA), 1,6-hexanediol diacrylate (HDDA), 2- hydroxyethyl methacrylate (HEMA), 2-hydroxyethyl acrylate (HEA), triethylene glycol di methacrylate (TEGDMA), and combinations thereof.

In embodiments that may be mentioned herein, the formulation may be one that utilises NPG2PODA or HDDA, which both have a low viscosity, as a diluent for a multifunctional oligomer such as TMPETA to achieve a formulation that has a viscosity suitable for printing. At the shear rates commonly set during the paste recoating process (e.g. from about 30 to 100 s ¹), the viscosity of the paste can be tuned in the range of from about 0.5 to 100 Pa s, such as from about 5 to 10 Pa s and can be further adjusted by the solid load of the YAG powder and dispersant concentration, as discussed hereinbelow. As will be appreciated, a shear thinning behaviour at a shear rate lower than 100 s^_1 may be required for paste recoating without causing large shear stress. As noted above, the formulations disclosed herein may make use of a pure YAG or a YAG that is doped by a rare earth element. As such, in certain embodiments of the formulation that may be mentioned herein, the rare earth element, when present, may be selected from one or more of the group consisting of Tm, Er, Ho and, more particularly, Ce, Tb, Sm, Dy, Nd and Yb, optionally wherein the rare earth element is selected from one or more of the group consisting of Ce, Nd and Yb, such as Ce. Any suitable amount of the rare earth element may be present in embodiments disclosed herein. For example, the rare earth element may have an atomic percentage concentration in the YAG of from 0.001 at% to 20 at%, such as from 0.0075 to 10 at%, such as from 0.01 to 1.0 at%, such as from 0.015 to 0.5 at%, such as from 0.02 to 0.1 at% (e.g. about 0.01 at%).

Any material suitable for use as a dispersant may be used in the formulations disclosed herein, provided that it is compatible with the photosensitive resin precursor. For example, the dispersant may be a super-dispersant, such as Solsperse 85000. Solsperse 85000 includes a hydrophilic group which can form hydrogen bonding with the hydroxyl functional group on the surface of YAG powders, while also having a hydrophobic tail that can interact with the photosensitive resin precursor components (e.g. with the acrylate groups thereof), which may help to improve the dispersion of the YAG particles in the photosensitive resin precursor thereby providing a homogeneous paste.

The amount of dispersant may vary depending on the powder particle size of the YAG particles in the formulation in an inverse relationship. That is, the larger the particle size of the particles, the less dispersant needed.

The photoinitiator used in the paste must be compatible with the photosensitive resin precursor components in order to form a resin. Any suitable photoinitiator may be used. For example, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide or, more particularly, 2-hydroxy-2-methylpropiophenone, 2-hydroxy-2- methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide blend, can be used as the photoinitiator for curing the paste under a UV laser at a suitable wavelength (e.g. 355 nm).

As will be understood, the amount of photoinitiator added will be proportional to the Ce (or other) dopant’s concentration. A higher Ce dopant concentration will require a higher photoinitiator concentration. For example, pastes doped with 0.1% Ce may require a photoinitiator concentration of about 0.7 wt% as compared to about 0.3 wt% for a paste doped with 0.01% Ce. The addition of Sudan I to pure YAG paste results in a lower cure depth and so 0.5 wt% photoinitiator concentration can be added (see below). Small square stripes of the YAG paste can be cured to determine the cure depth before printing.

As will be appreciated, the formulation above may also contain one or more photo-absorber additive materials, which may be selected depending on the application for the resulting YAG materials. Suitable photo-absorber additives that may be mentioned herein include, but are not limited to, Sudan Orange G, Sudan III, Para red, and combinations thereof. A photo absorber may be used in a situation where one is applying two (or more) formulations according to the current invention to form an additive-manufactured product, which can be treated further - as discussed in more detail hereinbelow. In such cases, one of the formulations may make use of a pure YAG, while the other makes use of a YAG doped with a rare earth element. In such cases, there may be a need to attenuate the cure depths of the two formulations so that they are approximately the same. In such cases, the pure YAG formulation may be adapted to include a photo-absorber and to have a lower concentration of the photoinitiator in order to complement a YAG:Ce formulation. For example, 0.7 wt% of a photoinitiator may be added to a YAG:Ce formulation as compared to 0.5 wt% of photoinitiator for a YAG formulation, which may also contain 0.029 wt% of Sudan I. The amount of photoinitiator and photo-absorber added will depend largely on the rare earth element (e.g. Ce) concentration in the doped YAG, and will vary accordingly.

Particular formulations that may be mentioned in embodiments of the invention include those listed below. A first formulation may be one that comprises: from 50 to 90 wt% of particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; from 9.3 to 45 wt% of the photosensitive resin precursor; from 0.5 to 5.0 wt% (e.g. from 0.5 to 2.0 wt%) of the dispersant; from 0.1 to 2.5 wt% of the photoinitiator; and from 0 to 0.1 wt% of a photo-absorber additive.

A second formulation may comprise: from 70 to 85 wt% of particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; from 14 to 27 wt% of the photosensitive resin precursor; from 0.77 to 2.0 wt% of the dispersant; from 0.2 to 1.0 wt% of the photoinitiator; and from 0 to 0.03 wt% of a photo-absorber additive. The first and second formulations above may be generic formulations that cover the situation where the YAG is a pure YAG or a YAG doped by a rare earth element.

A third formulation that may be mentioned herein may be one that comprises: from 70 to 85 wt% of particles of YAG; from 10 to 20 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 1 to 12 wt% in total of one or both of NPG2PODA and HDDA; from 0.2 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2-hydroxy-2- methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; from 0.5 to 5.0 wt% (e.g. from 0.5 to 2.0 wt%) of Solsperse 85000; and from 0.02 to 0.03 wt% of Sudan I.

A fourth formulation that may be mentioned herein may be one that comprises: from 70 to 85 wt% of particles of YAG; from 11 to 16 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 3 to 11 wt% in total of one or both of NPG2PODA and HDDA; from 0.3 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2- hydroxy-2-methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; from 0.5 to 2.0 wt% of Solsperse 85000; and from 0.02 to 0.03 wt% of Sudan I.

The third and fourth formulations may be ones with a pure YAG and so may have a moderated amount of photoinitiator and include a photo-absorber in order to complement a YAG doped by a rare earth element.

A fifth formulation that may be mentioned herein may be one that comprises: from 70 to 85 wt% of particles of YAG doped by Ce; from 10 to 20 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 1 to 12 wt% in total of one or both of NPG2PODA and HDDA; from 0.2 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2-hydroxy-2- methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; and from 0.5 to 2.0 wt% of Solsperse 85000. A sixth formulation that may be mentioned herein may be one that comprises: from 70 to 85 wt% of particles of YAG doped by Ce; from 11 to 16 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 3 to 11 wt% in total of one or both of NPG2PODA and HDDA; from 0.3 to 0.5 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2- hydroxy-2-methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; and from 0.7 to 2.0 wt% of Solsperse 85000.

The fifth and sixth formulations may be suitable for use in combination with the third and fourth formulations in the manufacture of a product that makes use of both pure YAG and a YAG doped by a rare earth element.

In certain embodiments that may be mentioned herein, the formulations above may further comprise particles of a thermally conductive ceramic material. Any suitable thermally conductive ceramic material may be used in the formulations disclosed herein. For example, the particles of a thermally conductive ceramic material may be selected from one or more of the group consisting of AI2O3_, MgO, and MgAI₂04. Any suitable amount of the particles of a thermally conductive ceramic material may be used herein. For example, the amount of the particles of a thermally conductive ceramic material may be from 5 to 40 wt%, such as from 5 to 35 wt%, relative to the weight of the particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element.

The formulation disclosed hereinbefore is suitable for the formation of a ceramic product. Thus, in a second aspect of the invention, there is provided a ceramic product comprising: a layer formed from one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element, which has a first surface, a second surface and a body portion therebetween, wherein the second surface has a plurality of projections extending from the second surface and/or a plurality of depressions that extend into the body portion.

In particular embodiments of the invention, the ceramic product may comprise: a first layer formed from a yttrium aluminium garnet (YAG); and a second layer formed from YAG doped by a rare earth element, wherein the second layer has a first surface facing and is attached to the first layer, a second surface facing in the opposite direction to the first surface and a body portion therebetween, wherein the second surface has a plurality of projections extending from the second surface and/or a plurality of depressions that extend into the body portion.

As will be appreciated, the ceramic product may be formed by the combination of at least one formulation described above using pure YAG and at least one formulation described above using a YAG doped with a rare earth element.

When the ceramic product has the first and second layers discussed above, the first layer and the second layer may not be susceptible to delaminate from one another. This is a particular advantage of the process described herein, as the additive manufacturing process using two formulations of a pure YAG and a YAG doped with a rare earth element can be formulated to be compatible with one another and be resistant to delamination. It is believed that pastes which have the same cure depth using the same laser parameters will not suffer from delamination. Therefore, small square sample stripes of pure YAG formulations and formulations of the YAG doped with a rare earth element were subjected to curing to determine if their cure depths matched before use. Delamination may occur during the print or following debinding, in which case it can be observed by examination of the resulting products at each stage.

The first layer and the second layer may have any suitable thickness. For example, the first and second layer may independently have a thickness of from 0.5 to 5.0 mm, such as from 0.75 to 3.0 mm, such as from 1.0 to 2.0 mm.

In embodiments of the invention, the plurality of projections extending from the second surface may be selected from one or more of pyramidal projections, conical projections, or convex projections.

In some embodiments, the plurality of projections extending from the second surface may be asymmetric pyramidal projections. In particular embodiments of the invention that may be mentioned herein, the asymmetric pyramidal projections may have an oblique angle of from 75 to 135°, such as from 100 to 110°, such as about 105°. As described in Example 4 below, the asymmetric pyramidal projections may have a great influence on the luminous intensity distribution, and the asymmetric pyramidal projections can provide strong far-field emission. In some embodiments, the plurality of projections extending from the second surface may be convex projections, optionally wherein each convex projection has a diameter of from 0.1 mm to 5.0 mm, such as from 1.0 mm to 3.0 mm, such as about 1.0 mm or about 2.0 mm and a height of from 0.1 mm to 2.5 mm, such as from 0.5 mm to 2.0 mm, such as about 1.0 mm.

In some embodiments, the plurality of depressions that extend into the body portion may be concave depressions, optionally wherein each concave depression has a diameter of from 1.0 mm to 3.0 mm, such as 2.0 mm and a depth of from 0.5 mm to 2.0 mm, such as about 0.5 mm or about 1.0 mm. As noted in the examples below, embodiments with convex projections or concave depressions can provide good optical transparency (see FIG. 11).

As will be appreciated, the plurality of projections and depressions may be mixed in any suitable manner, depending on the end application for the ceramic product.

As will be appreciated, as the ceramic product may be formed from the formulations disclosed above, the rare earth element may be selected from one or more of the group consisting of Tm, Er, Ho and, more particularly, Ce, Tb, Sm, Dy, Nd and Yb. In yet more particular embodiments, the rare earth element may be selected from one or more of the group consisting of Ce, Nd and Yb, such as Ce. The rare earth element may be present in any suitable amount in the YAG doped with a rare earth element. For example, the rare earth element may have an atomic percentage concentration in the YAG of from 0.001 at% to 20 at%, such as from 0.0075 to 10 at%, such as from 0.01 to 1.0 at%, such as from 0.015 to 0.5 at%, such as from 0.02 to 0.1 at% (e.g. about 0.01 at%).

In certain embodiments, the inclusion of a thermally conductive ceramic material may be useful. Thus, the ceramic product may further incorporate a thermally conductive ceramic material. Any suitable thermally conductive ceramic material may be used herein. For example, the thermally conductive ceramic material may be selected from one or more of AI₂O₃, MgO, and MgAl₂0₄. Any suitable amount of the thermally conductive ceramic material may be used in embodiments in which the thermally conductive ceramic material is present. For example, the thermally conductive ceramic material may be present in an amount of from 5 to 40 wt%, such as from 10 to 35 wt%, relative to the weight of the particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element. For the avoidance of doubt, the presence of a thermally conductive ceramic material is agnostic to the structure of the ceramic product, which may be provided as a single layer or as a material having two (or more) layers (e.g., one layer formed from a pure YAG, with a second layer formed from a YAG doped by a rare earth element). As described in Example 8 below, the presence of a thermally conductive ceramic material in the ceramic product can provide the resulting product with strong heat dissipation capability, suggesting a promising potential application in the field of thermal management in LED lighting.

The ceramic product can have any suitable structure, which may be governed by the desired application of said product. For example, the ceramic product may have a first portion and a second portion, where the first portion corresponds to the ceramic product as described hereinbefore and the second portion provides a plurality of interconnected pores suitable to remove heat. As will be appreciated, the second portion may be formed from any suitable ceramic material and may actually be formed of the same material as the first portion, but one in which it is shaped differently in order to channel heat through the ceramic product. In certain embodiments that may be mentioned herein, the second portion may occupy a periphery of the ceramic product, with the first portion occupying a central region. In alternative embodiments of the invention that may be mentioned herein, the first portion may occupy a periphery of the ceramic product, with the second portion occupying a central region. In such embodiments, the second portion may have any suitable structure that can channel heat. For example, the second portion may have a cellular ceramic structure of one or more of the group consisting of an array of thin sheets or sheet-based triply periodic minimal surface (TPMS) lattices such as Schwarz primitive, Schwarz diamond, Schoen gyroid and Schoen l-WP. The volume fraction of the lattices may be constant or varied along the height or radial direction. The thickness of the thin sheets may be in the range of from 0.1 to 2 mm.

The ceramic products disclosed herein may be particularly suitable for use in a white-light LED device. Thus, in a further aspect of the invention, there is provided a white-light LED device comprising a ceramic product as described hereinbefore mounted on a blue-light LED chip, such that the light from the LED passes through the ceramic product to generate a white light emission.

As will be appreciated, the products disclosed herein may be particularly suited to optical applications. Thus, in a further aspect of the invention, there is provided an imaging device comprising a ceramic product as described herein, wherein the ceramic product is in the form of an optical lens, optionally wherein the optical lens is a convex or concave lens

In order to obtain the ceramic products described above, one performs an additive manufacturing process to obtain a green body, which is then subjected to heat treatment and sintering to provide the desired ceramic body. Thus, in a further aspect of the invention, there is provided a process of forming a green body product, wherein the process comprises the steps of:

(a) providing a formulation as described herein to an additive manufacturing device; and

(b) forming a green body product layer by layer using the additive manufacturing device according to a set of instructions readable by the additive manufacturing device to produce the green body product.

As will be appreciated, more than one formulation may be used in the manufacture of the green body product - and the application (e.g. into layers) will be controlled by the CAD model or 3D digital model.

The green body product may be obtained as a unitary body formed of a single material. Alternatively, the green body product may be formed of: a first layer according to a first formulation comprising particles of yttrium aluminium garnet (YAG); and a second layer according to a second formulation comprising particles of a YAG doped by a rare earth element. In embodiments of this latter option, each layer of the first and second formulations may be subjected to curing before the next layer is applied, where a percentage difference in a cure depth used in the first layer compared to a cure depth used in the second layer may be from 5 to 20%. In certain embodiments, the cure depth in the first layer and second layer may be independently selected to be from 50 to 120 pm, such as from 90 to 110 pm.

As noted hereinbefore, the YAG (both the pure YAG and the YAG doped with a rare earth element) should have a very high purity (e.g. > 99.99%), with the particles being of a suitable micro-sized powder and with a suitable dispersant. The formulation should be a homogeneous submicro-sized ceramic powder formulation with a shear thinning and/or self-holding ability for printing complex structures. Such a formulation may be obtained as described hereinbefore and in the examples.

In embodiments having layers from two different YAG formulations (e.g. doped and undoped), the curing depths for both YAG:Ce and undoped YAG ceramic formulations have to be similar to achieve a successful print of YAG/YAG:Ce bilayer ceramics. Parameters for this may be found hereinbefore and in the examples section below. This control may help prevent delamination of the two layers from one another. As discussed in the examples below, a bilayer YAG/YAG:0.1%Ce concave lens arrangement (160.8 Im/W) has a higher light extraction efficacy than a monolayer YAG:0.1%Ce concave lens arrangement (150.0 Im/W). Without wishing to be bound by theory, it is believed that this indicates that bilayer YAG:Ce patterned samples generally result in a stronger emission and that this arrangement may also have a significant influence in the light extraction efficacy of the resulting WLEDs.

The resulting green body product obtained from the additive manufacturing process may then be transformed into a debound product that is essentially free of organic matter following a heat treatment step. Thus, in a further aspect of the invention, there is provided a process of forming a debound product from a green body product, which process comprises the steps of:

(aa) providing a green body product as described hereinbefore;

When used herein, the term “substantially free” refers to the debound product having less than 1 wt%, such as less than 0.5 wt%, such as less than 0.1 wt%, such as less than 0.01 wt%, such as less than 0.001 wt%, such as less than 0.0001 wt%, such as less than 0.0001 wt%, such as less than 0.00001 wt% of organic matter present therein. In other words, no residual carbon is present in the ceramic green body after debinding.

The ramping rate used to provide the debound product may be in a range of about 0.1 to 3 °C/min, such as from 0.1 to 1.0 °C/min. The inert atmosphere can be in nitrogen or argon, with a flow rate of from about 0.1 to 1 Litre/min. The dwelling time at each temperature may be varied in a range of from about 1 to 10 hours, such as from 3 to 6 hours.

The debound product can be turned into the ceramic product following a sintering process. Thus, in a further aspect of the invention, there is provided a process for forming a ceramic product, the process comprising the steps of:

(ba) providing a debound body product as described hereinbefore; and

(bb) subjecting it to sintering under vacuum at a temperature of from 1500 to 2000 °C, such as from 1720 to 1890 °C (e.g. from 1720 to 1790 °C) for a period of time to provide the ceramic product, optionally wherein the vacuum is from 1x10^-3 to 1.0 Pa. It is believed that the process parameters above may assist in providing transparent YAG ceramics.

It is believed that the additive manufacturing approach adopted here allows for the production of YAG ceramics with improved properties that cannot otherwise be achieved.

Further aspects and embodiments of the invention will now be described by reference to the following non-limiting examples.

Examples

Materials

Solsperse 85000 was purchased from Lubrizol. High pure a-AI₂03 (99.99 %, Sumitomo Chemical Co. Ltd, Japan), Y2O3 (99.99 %; Alfa Aesar), and Ce0₂ (99.99 %, Alfa Aesar) were employed as the precursor materials. 0.5 wt% tetraethoxysilane (TEOS, 99.999 %, Alfa Aesar) and 0.5 wt% oleic acid (99 %, Alfa Aesar) were employed as sintering aid and dispersant agency, respectively. Trimethylolpropane ethoxylate triacrylate (TMPETA) 692, difunctional neopentyl glycol propoxylate (1PO/OH) diacrylate (NPG2PODA) and 2-hydroxy-2- methylpropiophenone, Sudan I were purchased from Sigma-Aldrich. 1,6-hexanediol diacrylate, 99% was purchased from Alfa Aesar

The viscosity was measured using Anton Paar Rheometer MCR501 with parallel plate configuration and gap distance of 0.5 mm, at varying shear rates from 1 to 1000 s^-1.

Example 1. Paste formulation and post heat treatment of YAG and YAG:Ce ceramics

3D printing of both YAG:Ce and YAG/YAG:Ce ceramics mainly comprises of four steps. First, photocurable YAG paste is prepared by mixing the raw materials using vacuum planetary mixer (AVR310, Thinky, Japan). Then, the ceramic green body is printed using SLA-based 3D printer (Ceramaker C900 FLEX, 3D Ceram). The ceramic green bodies will be debinded in a tube furnace (GHA 12/450, Carb Lite Gero) with controlled nitrogen and air atmospheres to fully decompose the organic compounds in the ceramic green bodies without causing stress build-up from the oxidised gases. Finally, the fully dense transparent YAG:Ce ceramic is obtained through a vacuum sintering process. During this procedure for fabrication of transparent YAG:Ce and YAG/YAG:Ce ceramics, a few key points must be rigidly controlled as highlighted below.

1) High purity (³99.99%) micro-sized raw powders with suitable dispersant to be used.

2) Excellent homogeneous micro-sized ceramic powder paste with self-holding ability for printing complex structure.

3) No residual carbon in the ceramic green body after debinding.

4) Narrow vacuum sintering window of 1720-1820 °C with sufficient dwelling duration to obtain transparent YAG ceramics.

5) The curing depths for both compositions of YAG:Ce and undoped YAG ceramic pastes have to be similar to achieve a successful print of YAG/YAG:Ce bilayer ceramics.

Computer-designed 3D-shaped YAG green bodies were printed by ceramic 3D printer such as Ceramaker C100/C900/3600 (3D Ceram), CeraBuidler 100pro/160pro/2500/3000/3000P (iLaser), or other commercial SLA ceramic printers with the printing parameters: 3.5 mm/s recoating speed, 30 pm recoating thickness, 3000 mm/s laser speed, 76% laser power, a hatch distance of 0.04 mm, and a curing depth of 100 pm. The computer designed structure can also be printed with the pastes or slurries by Digital Light Processing (DLP) printers, such as Admaflex 130 (from Admatec), Cerafab 7500/8500 (Lithoz), Promaker V10 (Prodways) and Autoceram M(Ten Dim) printers, etc.

Preparation of YAG powder

YAG powder was prepared by ball-milling of high purity yttria and alumina powder in a stoichiometric ratio of 3:5, with 0.5 wt% tetraethoxysilane and 0.5 wt% oleic acid as the sintering aid and dispersant, respectively. The obtained powders were sieved and annealed at 800 °C for 3 h in air to convert tetraethoxysilane to S1O2 before mixing with the photocurable resin precursors. Preparation of the YAG:Ce powder was performed similarly, with doping of 0.1 at% or 0.01 at% ceria prior to ball-milling of the powder precursors.

An example of the paste includes 75 wt% YAG powder doped with 0.1 wt% Ce, 0.9 wt% Solsperse 85000, 9.4 wt% NPG2PODA, 14.0 wt% TMPETA and 0.7 wt% 2-hydroxy-2- methylpropiophenone.

YAG:Ce paste 2 Another example of the paste includes 78 wt% YAG powder doped with 0.01 wt% Ce, 1.9 wt% Solsperse 85000, 3.9 wt% NPG2PODA, 11.9 wt% TMPETA, 3.9 wt% amine modified polyether acrylate and 0.3 wt% 2-hydroxy-2-methylpropiophenone.

The printed samples were debound by using the following heating profile: heating up to 250, 300, 400, 500 and 600 °C, respectively, and dwelling for 5 h at each step in nitrogen atmosphere, followed by heating up to 800 °C and dwelled for 15 h in air atmosphere. The ramping rate for heating up was 0.2 °C/min. The ceramic green bodies were then sintered at 1800 °C for 10 h.

Preparation of YAG/YAG.Ce bilayer ceramic

The YAG/YAG:Ce bilayer ceramic with convex/concave lens topology was printed using the two paste formulations listed in Tables 1 and 2. To fabricate the YAG/YAG:Ce bilayer ceramic, it is mandatory for the curing depths of both ceramic pastes to be similar, with maximum 20% difference in cure depth to eliminate any delamination or warping issues. To achieve ideal printing results, the curing depths for both pastes should be within 100-120 pm. For example, YAG layer has a curing depth of 120 pm and YAG:Ce has a curing depth of 100 pm. In order to reduce the overcuring of the undoped YAG paste at the printing z-axis, about 0.02-0.03 wt% Sudan I was added as a photo-absorber to the pure YAG paste. The 3D printing process was carried out by following the 3D printing protocol above. The phosphor layer, YAG:0.1% Ce, was first printed by casting the paste in a layer-by-layer manner till a thickness of 1.0 mm followed by casting another 1.0 mm of the pure YAG interlayer on top of the phosphor layer before finally printing the lens structures with YAG.

Asymmetric triangles were chosen as the designed model as they were reported to have a larger surface interaction area and better randomization effect than their symmetric counterparts (Chen, C. etai, Optik 20 9 182, 400-407).

Tables 1 and 2 list the typical compositions of YAG:Ce and undoped YAG printing pastes, respectively.

Table 1. Compositions of YAG:Ce printing pastes.

Table 2. Compositions of undoped YAG printing pastes.

Multi-step debinding procedure The printed samples can be debound under a multi-step debinding procedure with dwelling at 250, 300, 400, 500 and 600 °C for 5 h at each step, in nitrogen atmosphere, followed by heating up to 800 °C and dwelled for 15 h in air atmosphere. The ramping rate for heating up was 0.1-1.0 °C/min to minimize the thermal stress induced from the shrinkage of the ceramic green body. FIG. 1 shows an example of the debinding heat-treatment profile. The ceramic green bodies were then sintered at 1720-1890 °C for 8-15 h.

Example 2. Characterization of YAG powder, YAG:Ce powder, YAG:Ce and YAG/YAG:Ce bilayer ceramics The materials prepared in Example 1 were characterized.

Optical transmittance studies

The optical transmittance was measured using UV-Vis-NIR spectrophotometer (Cary 5000, Agilent) in the wavelength range of 200 to 2000 nm. Samples are double-side polished before the measurement.

SEM

SEM imaging was carried out on a FEI Helios NanoLab 600i Dualbeam system. Results and discussion FIG. 2 shows the morphology of the YAG:Ce and YAG powders prepared. FIG. 3 shows the typical rheological behaviour of the YAG and YAG:Ce pastes. The pastes exhibit a shear thinning behaviour at the shear rate lower than 100 s^_1 which is essential for recoating a new paste over the cured parts underneath without causing large shear stress.

FIG. 4A-C shows the 3D digital models used for the printing, FIG. 4D shows the green bodies of asymmetric triangles, microcones, some Kelvin cells and octet truss lattices printed from the YAG:Ce paste 2, and FIG. 4E are the samples sintered at 1790 °C for 8-15 h.

FIG. 5 shows the photo of the YAG/YAG:Ce bilayer ceramic green bodies printed. Sample has the geometry matching with the original design well and no warping or delamination was observed. Samples were debound with a multi-step debinding procedure: heating up to 250, 300, 400, 500 and 600 °C, respectively, and dwelling for 5 h at each step in nitrogen atmosphere, followed by heating up to 800 °C and dwelled for 15 h in air atmosphere. The ramping rate for heating up is 0.2 °C/min. The ceramic green bodies were then sintered at 1800 °C for 10 h or 1820 °C for 15 h.

FIG. 6 shows the microstructures ofYAG:0.1%Ce and pure YAG layer of the bilayer ceramics after polishing and thermal etching. The ceramics have relatively dense, with almost pore-free grain packing and grain size distribution of -10-30 pm.

The 3D printed transparent YAG ceramics with complex structures are shown in FIG. 7. The samples are made by the optimized 3D printing, debinding and vacuum sintering process.

Example 3. Optical and photoluminescence properties of the printed YAG:Ce ceramics

The optical and photoluminescence properties of the printed YAG:Ce ceramics in Example 1 were investigated.

Optical and photoluminescence studies

The photoluminescence and photoluminescence spectra were recorded with a spectrometer (FLS1000, Edinburgh instrument, United Kingdom). The maximum emission spectra were recorded at the wavelength of 550 nm with an excitation light with wavelength of 450 nm, while the excitation spectra were measured at 340 and 463 nm with an emission wavelength of 550 nm. Sample dimensions were 10 mm in diameter and -2 mm in thickness.

Results and discussion FIG. 8A shows the photo of the transparent YAG:0.1 % Ce monolayer and YAG/YAG:0.1 % Ce bilayer ceramics and their optical transmittance spectra, respectively. The samples were double-side polished with diamond paste to a thickness of -1.8 mm before measurement with UV-Vis NIR spectrophotometer. FIG. 8A shows that both the single and bilayer ceramics sintered at the different temperatures are transparent. The samples sintered at 1800 °C for 10 h have better transparency. When the sintering temperature was increased to 1820 °C for 15 h, some of the bilayer YAG:Ce ceramics became slightly translucent, as further evidenced by the optical transmittance shown in FIG. 8B. FIG. 8B shows that the transmittance decreases sharply from -68.0% in the near-infrared region to 53.0% in the visible light region. The bilayer YAG:Ce ceramic sintered at 1800 °C reveals to have the highest transparency with a maximum transmittance of 77.6% at 1550 nm and 72.7% at 800 nm, while the monolayer ceramic has a comparable transmittance of 73.3% and 66.6% at 1500 nm and 800 nm, respectively. The absorption bands at 345 and 455 nm wavelengths presented in the transmission spectra of the samples are generated by the absorption corresponding to Ce³⁺ transitions of ²Fs/2 5d and ²F7/2 5d, respectively (Y. Pan, M. Wu & Q. Su, J. Phys. Chem. Solids 2004, 65, 845-850). The absorption intensity of ²F7/2 5d transition (at 455 nm) is stronger than ²Fs/2 5d (at 345 nm), which is critical to excitation and emission of phosphors when pumped by 460 nm blue laser or LED.

FIG. 9 shows the photoluminescence excitation and emission spectra of 3D printed YAG:0.1% Ce tested at room temperature. The maximum emission spectra were recorded at the wavelength of 550 nm with an excitation light with wavelength of 450 nm, while the excitation spectra were measured at 340 and 463 nm with an emission wavelength of 550 nm. The following phenomenon was observed due to the splitting of the two ground states, ²Fs/2 and ²F7/2 of the trivalent Ce³⁺ to their 5d excited states. The former excitation band can be assigned to electronic transitions from 4f(²Fs_/2)^®5d(²Ai_/g) while the latter transition can be ascribed to 4f(²F_7/2)^®5d(²Ai_/g) transition, respectively. The excitation band observed between 400-500 nm indicates that the YAG:Ce phosphor is able to absorb the blue emission of the InGaN/GaN LED effectively while the intense emission band at 500 to 650 nm is complementary to the blue light emitted by InGaN/GaN chips to produce white light (Y. Pan, M. Wu & Q. Su, J. Phys. Chem. Solids 2004, 65, 845-850). It should be noted that the photoluminescence excitation and emission spectra of the YAG/YAG:0.1%Ce bilayer ceramics should resemble the YAG: 0.1 %Ce monolayer samples as only a thin layer of pure YAG was added to the structure which has no contribution to the photoluminescence properties of the YAG/YAG:0.1%Ce bilayer samples. Example 4. Light emission behaviour of the white light LED with the printed YAG:Ce

The effect of the printed surface patterns on the light extraction efficiency and light uniformity of YAG:Ce ceramics prepared in Example 1 was studied.

Light extraction efficiency and light uniformity studies

The YAG:Ce ceramics were sliced into a small size of 1 mm x 1 mm and then covered on a blue LED with wavelength of 447-454 nm, and excited with an injection current of 50 mA and power of 134 mW. The spatial radiation spectrum was measured using a goniophotometer (LED626, Everfine Co., Hangzhou, China).

Results and discussion

FIG. 10A-B shows the dimensions of the asymmetric triangle arrays on the printed YAG:Ce ceramics. Thefar-field emission intensity patterns of the YAG:Ce ceramics samples are shown in FIG. 10C. The asymmetric triangle patterns have great influence on the luminous intensity distribution. It was observed that the flat YAG:Ce ceramic had a luminous intensity of ~2.8 cd and the intensity was almost constant at the -45° to 45° range. With the asymmetric triangle patterns on the surface, the luminous intensity at normal direction (0°) was the lowest, while the intensity at the directions away from the normal direction were significantly increased with increasing angle. The highest intensity was found at the ±30-45° range. Compared with the flat sample, patterned samples generally result in stronger emission. The sample with 105° oblique angle had the highest luminous intensity of ~3.4 cd, which is ~ 21% higher than that of the flat sample.

FIG. 10D shows the effect of oblique angle (i.e. the angle a listed in the table in FIG. 10B) on the luminous efficacy of the emission from the LED modules with the ceramic phosphor. The luminous efficacy of the flat YAG:Ce covered blue LED was -90.7 Im/W. Comparatively, the patterned YAG:Ce covered blue LEDs had much higher luminous efficacy, with the luminous efficacy increasing with the oblique angles. The lowest luminous efficacy was found at the oblique angle of 75° (only -93.1 Im/W) while the highest luminous efficacy reached 111.9 Im/W for the asymmetric triangle YAG:Ce with oblique angle of 105°. The improvement is - 23.4% compared with flat YAG:Ce ceramics and this can be attributed to the increased light extraction efficiency of patterned YAG:Ce ceramics. FIG. 10D also shows the effect of oblique angle on the divergent angle of the far-field emission patterns of the YAG ceramic covered blue LEDs. The divergent angle is an angle range taken at 50% of full luminosity, indicative of the extent of light divergence. The divergent angle of the flat YAG:Ce covered blue LED was 150.8°. It is generally increased for LEDs covered with patterned YAG:Ce ceramic with different oblique angles. The divergent angle was increased to ~ 153.4° and 159.3° when the oblique angle of the patterned YAG:Ce was 75° and 90°, respectively. With the oblique angle increased to 105- 135°, the divergent angle was varied in -157-159°. The divergence of the emission light is induced by the tilted planes of the asymmetric triangle patterns presented at the sample surfaces. In summary, the asymmetric triangle patterned YAG:Ce ceramics can promote the luminous efficacy of white-light LED lighting and at the same time, cause light divergence in wider angle. The 3D printed YAG:Ce phosphors could therefore be useful for LED lighting in open space where high light intensity and large divergent angle are required.

Example 5. Printed convex and concave lens monolayer and bilayer samples

The sample surface topology was tuned to convex lens or concave lens array and the layer structure was changed from monolayer to bilayers with YAG/YAG:Ce with the goal to further enhance the light extraction efficiency of the WLED devices by inducing more light scattering centres in the YAG:Ce layer and pure YAG, facilitating light extraction without causing light divergence.

Fabrication of convex and concave lens monolayer and bilayer samples Fusion 360 software was used to design the 3D digital models of the YAG/YAG:0.1%Ce bilayer convex, concave lens arrays and YAG:0.1%Ce monolayer concave lens arrays, as shown in FIG. 11A-B. Each convex or concave lens has a diameter of 2.0 mm and height of 1.0 mm, respectively, with a total thickness of 3.0 mm and area of 10 x 10 mm². The phosphor layer, YAG:0.1% Ce, was first printed by casting the paste in a layer-by-layer manner till a thickness of 1.0 mm followed by casting another 1.0 mm of the pure YAG interlayer on top of the phosphor layer before finally printing the lens structures with YAG. FIG. 11 C schematically illustrates the sample cross section. The printed convex and concave lens monolayer and bilayer samples were then debound and vacuum sintered at 1800 °C for 10 h.

Results and discussion

The samples after sintering had around 30% shrinkage. FIG. 11 D-E show some of the bilayer convex lens samples. All the samples exhibited a good optical transparency.

Example 6. Enhancement of the light extraction efficiency of WLED devices

Fabrication of WLED devices

The 3D printed YAG/YAG:0.1 % Ce bilayer and YAG:0.1 % Ce monolayer samples in Example 5 were placed on InGaN/GaN LED chips mounted on aluminium nitride substrates. The devices were excited by a blue LED with wavelength of 447-454 nm, injection current of 50 mA and voltage of ~2V.

The control sample has a phosphor layer (YAG: 0.1% Ce, a thickness of 1.0 m ) and a pure YAG interlayer (1.0 mm) coated on top of the phosphor layer.

The spatial radiation spectrum was measured using a goniophotometer (LED626, Everfine Co., Hangzhou, China).

Determination of luminous efficacy

The measurement of far-field emission pattern was made over all the forward hemisphere of an emitting LED. The set-up consists of an optical rail with a mounted rotation stage rotating the LED from -90 degree to +90 degree in a horizontal plane. LED light emission were measured every 2 degrees. The divergent angle is an angle range taken at 50% of full luminosity, indicative of the extent of light divergence. Max light intensity is the maximum intensity across the emission spectra. Luminous efficacy is the luminous flux over the input electrical power.

Results and discussion

FIG. 12 shows the far-field emission patterns of the 3D printed YAG/YAG:0.1%Ce bilayer samples with convex and concave lens arrays, YAG:0.1% Ce monolayer sample with concave lens, and the bilayer sample with flat surface as control sample. It can be observed that the light intensity distribution of all the samples is isotropic and resembles that of a Lambertian pattern emission, which is akin to the intensity distribution curve of the blue LED flip chip. This suggests that the emitted radiance is equal when observed from all directions and the Lambertian-like profile is suitable for applications in WLED lighting and displays. Table 3 shows that the maximum intensities of the ceramic samples improved when the divergent angle is closer to that of the blue LED chip. Of all, the bilayer sample with convex lens has the highest maximum intensity of 5.861 cd at normal direction (0°) and the divergent angle is closest to that of blue LED’s (131.1 degrees), taken at 50% of the full luminosity. The luminous efficacy of the bilayer convex sample is also the highest (160.8 Im/W), as compared to the bilayer control sample (158.9 Im/W) and bilayer concave sample (151.1 Im/W). Comparison of the bilayer concave lens with the monolayer concave lens also revealed that the former has a higher light extraction efficacy than the latter (150.0 Im/W). This indicates that the bilayer YAG:Ce patterned samples generally result in a stronger emission and has a significant influence in the light extraction efficacy of the WLEDs. Table 3. List of divergent angle, luminous efficacy and maximum light intensity (Lax) at normal angle of 3D printed transparent YAG:Ce monolayer and YAG/YAG:Ce bilayer ceramics mounted on blue LED flip chips.

Example 7. Optimal lens size for better luminous efficacy of the WLEDs

The printing resolution of a ceramic printer is limited to -200 pm. Further reduction of the resolution to a few microns to print finer structures is not possible due to light scattering in the paste and the laser beam diameter, which is -50 pm. The lens’ size effect of the bilayer convex lens samples was studied to determine the optimal size for better luminous efficacy of the WLEDs. Five types of lens samples (C1, C2, C3, C4 and C5) were prepared and printed by following the protocol in Example 5 except the pure YAG layer before casting the lens structure has thickness of 1.5 mm instead of 1.0 mm. Table 4 lists the lens and overall sample dimensions.

Table 4. List of the dimensions of the as-printed YAG/YAG:0.1%Ce bilayer ceramics with convex lens at surface.

Results and discussion FIG. 13A-B show the photos of 3D printed YAG/YAG:0.1%Ce bilayer ceramics with different lens sizes on InGaN/GaN blue vertical chips mounted on aluminium nitride substrates and their respective emission patterns, excited by a blue LED with wavelength of 447-454 nm, injection current of 50 mA and voltage of -5V. The results show that the size of lens has a significant influence on the light extraction efficacy of the emitting phosphors. C3 sample, which has a diameter of 1.0 mm and height of 0.5 mm, was found to have the highest maximum intensity at the angle normal to the incident blue light. In contrast, C1, which has a diameter and height 5 times the size of C3, was proven to have its luminous efficacy reduced by half when the same voltage and current were applied. Reducing the size of the lens to C4 and C5 did not significantly ameliorate the luminous efficacy of the YAG:Ce and hence, C3 was concluded to be the ideal printable dimension to improve the light extraction efficacy of WLEDs.

Example 8. Thermal management application of the printed YAG:Ce ceramics

Thermal management is another challenge in LED lumination. To enhance the thermal conductivity of the YAG:Ce ceramic, an excess of 10 wt% AI₂O₃ was introduced as the secondary phase in the ceramics due to its high thermal conductivity property (32-35 W-nr¹K ¹).

Preparation of 10 wt% AI203- YAG.0.1%Ce

10 wt% AI₂O₃- YAG:0.1%Ce composite ceramics were prepared by ball-milling the 90 wt% of high purity yttria and alumina powder in stoichiometric ratio of 3:5, with 0.5 wt% tetraethylorthosilicate and 0.05 wt% magnesium oxide (relative to the total mass of the powder precursors) as the sintering aids and additional 10 wt% of alumina powder for 20 h at 140 rpm. The obtained powders were sieved and annealed at 800 °C for 3 h in air before mixing with the photocurable resin.

Heat dissipation experiment

A heat dissipation experiment of the 3D printed 10 wt% AI₂O₃-YAG:0.1%Ce was performed to evaluate the heat transmission capability of the composite samples. The ceramics were designed and printed with three different lattice structures at the lateral surfaces, namely primitive, gyroid and diamond structures with a volume fraction of 0.35 and sheet thickness of 0.3 mm for evaluation, as shown in FIG. 14. All the samples have a diameter of 10 mm and height of 3 mm. InGaN/GaN LED chips were mounted on AIN substrate and the composite ceramics were then packaged on these chips. Thermal equilibration was allowed to achieve for a few min before measurement by a thermal imaging camera system (Flir A300, America).

Results and discussion FIG. 15A shows the infrared thermal images of the 3D printed 10 wt% Al₂C>₃-YAG:0.1%Ce ceramics with the core surface temperature recorded at forward current and voltage of 1.5 A and 6.1 V, respectively.

FIG. 15B illustrates the surface temperatures of the printed AhOs-YAGiCe composite ceramics with their respective designs at different applied currents in comparison to the control round disk. It can be observed that the thermal stability of all the 3D printed complex structures was improved and the operating surface temperatures decreased from 103.1 °C to 71.7 °C at 1.5 A and 6.1 V current/voltage condition. Amongst all the composite samples, the diamond structure has the lowest surface temperature (71.7 °C), which is 30% lower in temperature as compared to the control round disk, followed by the primitive structure (73.1 °C) and the gyroid structure (93.7 °C). This can be attributed to the high surface area to volume ratio of the samples with lattice structures at the lateral surface (526.397 mm²) which allows faster heat dissipation rate as compared to the control disk (251.327 mm²). It is expected that the surface core temperature of the structures increases with an increasing current in the LED devices. The strong heat dissipation capability of the complex-shaped Al₂O₃-YAG:0.1%Ce composite ceramics demonstrated by this experiment suggests a promising potential application in the field of thermal management in LED lighting.

Example 9. Imaging application of the printed YAG:Ce lens

The printed YAG:Ce lens can be used for imaging application. One example is the lens sample C3 with dimensions listed in Table 4 and was prepared and printed by following the protocol in Example 5. The lens array was polished and then tested for imaging ability.

Optical imaging performance test

The optical imaging performance of the printed convex lens array was characterized on an optical microscope equipped with a CCD camera (SZX16, OLYMPUS).

Results and discussion

FIG. 16 shows the image taken through the YAG:Ce lens. The grid and the digital number “1” are clearly seen without distortion. The images of the microgrids viewed from the 3 different lenses have equally clear quality with high contrast and sharpness. These demonstrations indicate the great potential of the 3D printed YAG ceramics in the application of various optical lens, windows, and artificial compound eyes for cutting-edge applications in robotics, medical endoscopes, surveillance devices, and reconnaissance systems.

Claims

1. A formulation for use in additive manufacturing comprising: particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; a photosensitive resin precursor; a dispersant; and a photoinitiator, wherein the formulation has a viscosity of from 0.5 to 100 Pa s as determined at a temperature of 25 °C and a shear rate of from 10 to 120 (e.g. 30) s^_1.

2. The formulation for use in additive manufacturing according to Claim 1, wherein the photosensitive resin precursor comprises a multifunctional monomer and/or a multifunctional oligomer.

3. The formulation for use in additive manufacturing according to Claim 2, wherein the multifunctional monomer and/or the multifunctional oligomer is selected from one or more of the group consisting of dipentaerythritol hexaacrylate, dimethylolpropane tetraacrylate, tris(2- hydroxy ethyl) isocyanurate triacrylate, alkoxylated pentaerythritol tetraacrylate and, more particularly, trimethylolpropane ethoxylate triacrylate (TMPETA) (e.g. TMPETA 428, 692, and 912), trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), pentaerythritol tetraacrylate (PETTA), trimethylolpropane propoxylate triacylate (TMPPTA), an amine-modified polyether acrylate, and a urethane acrylate.

4. The formulation for use in additive manufacturing according to Claim 2 or Claim 3, wherein the photosensitive resin precursor further comprises a diluent; and/or the formulation has a viscosity of from 5 to 10 Pa s as determined at a temperature of 25 °C and a shear rate of 30 s^_1.

5. The formulation for use in additive manufacturing according to Claim 4, wherein the diluent is selected from one or more of the group consisting of diethylene glycol dimethylate, polyethylene glycol diacrylate, 2-ethylhexyl methacrylate, isobornyl acrylate, isodecyl acrylate and, more particularly, neopentyl glycol propoxylate (1PO/OH) diacrylate (NPG2PODA), 1,6- hexanediol diacrylate (HDDA), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxyethyl acrylate (HEA), and triethylene glycol dimethacrylate (TEGDMA).

6. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein the rare earth element is selected from one or more of the group consisting of Tm, Er, Ho and, more particularly, Ce, Tb, Sm, Dy, Nd and Yb, optionally wherein the rare earth element is selected from one or more of the group consisting of Ce, Nd and Yb, such as Ce.

7. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein the rare earth element has an atomic percentage concentration in the YAG of from 0.001 at% to 20 at%, such as from 0.0075 to 10 at%, such as from 0.01 to 1.0 at%, such as from 0.015 to 0.5 at%, such as from 0.02 to 0.1 at% (e.g. about 0.01 at%).

8. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein the YAG particles have a size of from 0.1 to 5pm, such as from 0.1 to 2 pm.

9. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein:

(ai) the dispersant is Solsperse 85000; and/or

10. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein the formulation further comprises one or more photo-absorber additives, optionally wherein the photo-absorber additives are selected from one or more of the group consisting of Sudan Orange G, Sudan III, and Para red.

11. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein the formulation comprises: from 50 to 90 wt% of particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; from 9.3 to 45 wt% of the photosensitive resin precursor; from 0.5 to 5.0 wt% of the dispersant; from 0.1 to 2.5 wt% of the photoinitiator; and from 0 to 0.1 wt% of a photo-absorber additive, optionally wherein the formulation comprises: from 70 to 85 wt% of particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element; from 14 to 27 wt% of the photosensitive resin precursor; from 0.77 to 2.0 wt% of the dispersant; from 0.2 to 1.0 wt% of the photoinitiator; and from 0 to 0.03 wt% of a photo-absorber additive.

12. The formulation for use in additive manufacturing according to Claim 11, wherein the formulation comprises:

(i) from 70 to 85 wt% of particles of YAG; from 10 to 20 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 1 to 12 wt% in total of one or both of NPG2PODA and HDDA; from 0.2 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2-hydroxy-2- methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; from 0.5 to 5.0 wt% of Solsperse 85000; and from 0.02 to 0.03 wt% of Sudan I, such as from 70 to 85 wt% of particles of YAG; from 11 to 16 wt% of a TMPETA (e.g. one or more of TMPETA 428, 692, and 912) with or without an amine-modified polyether acrylate; from 3 to 11 wt% in total of one or both of NPG2PODA and HDDA; from 0.3 to 1.0 wt% of 2-hydroxy-2-methylpropiophenone or a blend of 2- hydroxy-2-methylpropiophenone and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide; from 0.5 to 2.0 wt% of Solsperse 85000; and from 0.02 to 0.03 wt% of Sudan I; or

13. The formulation for use in additive manufacturing according to any one of the preceding claims, wherein the formulation further comprises particles of a thermally conductive ceramic material, optionally wherein:

(bi) the particles of a thermally conductive ceramic material is selected from one or more of the group consisting of AI2O3_, MgO, and MgAI₂C>4; and/or

(bii) the amount of the particles of a thermally conductive ceramic material is from 5 to 40 wt%, such as from 5 to 35 wt%, relative to the weight of the particles of one or more of the group consisting of yttrium aluminium garnet (YAG), and a YAG doped by a rare earth element.

15. The ceramic product according to Claim 14, comprising a first layer formed from a yttrium aluminium garnet (YAG); and a second layer formed from YAG doped by a rare earth element, wherein the second layer has a first surface facing and is attached to the first layer, a second surface facing in the opposite direction to the first surface and a body portion therebetween, wherein the second surface has a plurality of projections extending from the second surface and/or a plurality of depressions that extend into the body portion.

16. The ceramic product according to Claim 15, wherein:

(ci) the first layer and the second layer are not susceptible to delaminate from one another; and/or (cii) the first layer and the second layer independently have a thickness of from 0.5 to 5.0 m , such as from 0.75 to 3.0 mm, such as from 1.0 to 2.0 mm.

17. The ceramic product according to Claim 15 or Claim 16, wherein the plurality of projections extending from the second surface are selected from one or more of pyramidal projections, conical projections, or convex projections.

18. The ceramic product according to Claim 17, wherein the plurality of projections extending from the second surface are asymmetric pyramidal projections, optionally wherein the asymmetric pyramidal projections have an oblique angle of from 75 to 135°, such as from 100 to 110°, such as about 105°.

19. The ceramic product according to Claim 17, wherein the plurality of projections extending from the second surface are convex projections, optionally wherein each convex projection has a diameter of from 0.1 mm to 5.0 mm, such as from 1.0 mm to 3.0 mm, such as about 1.0 mm or about 2.0 mm and a height of from 0.1 mm to 2.5 mm, such as from 0.5 mm to 2.0 mm, such as about 1.0 mm.

20. The ceramic product according to any one of Claims 15 to 19, wherein the plurality of depressions that extend into the body portion are concave depressions, optionally wherein each concave depression has a diameter of from 1.0 mm to 3.0 mm, such as 2.0 mm and a depth of from 0.5 mm to 2.0 mm, such as about 0.5 mm or about 1.0 mm.

21. The ceramic product according to any one of Claims 14 to 20, wherein the rare earth element is selected from one or more of the group consisting of Tm, Er, Ho and, more particularly, Ce, Tb, Sm, Dy, Nd and Yb, optionally wherein the rare earth element is selected from one or more of the group consisting of Ce, Nd and Yb, such as Ce.

22. The ceramic product according to any one of Claims 14 to 21, wherein the rare earth element has an atomic percentage concentration in the YAG of from 0.001 at% to 20 at%, such as from 0.0075 to 10 at%, such as from 0.01 to 1.0 at%, such as from 0.015 to 0.5 at%, such as from 0.02 to 0.1 at% (e.g. about 0.01 at%).

23. The ceramic product according to any one of Claim 14 and Claims 21 to 22 as dependent upon Claim 14, wherein the ceramic product further incorporates a thermally conductive ceramic material, optionally wherein: (di) the thermally conductive ceramic material is selected from one or more of AI₂O₃, MgO, and MgAhCU; and/or

24. The ceramic product according to any one of Claims 15 and 16 to 22, as dependent upon Claim 15, wherein the first and/or second layer of the ceramic product further incorporates a thermally conductive ceramic material, optionally wherein:

25. The ceramic product according to any one of Claims 14 to 24, wherein the ceramic product has a first portion and a second portion, where the first portion corresponds to the ceramic product of Claims 14 to 24 and the second portion provides a plurality of interconnected pores suitable to remove heat, optionally wherein:

26. The ceramic product according to Claim 25, wherein the second portion has a cellular ceramic structure of one or more of the group consisting of an array of thin sheets or sheet- based triply periodic minimal surface (TPMS) lattices such as Schwarz primitive, Schwarz diamond, Schoen gyroid and Schoen l-WP.

27. A white-light LED device comprising a ceramic product according to any one of Claims 14 to 26 mounted on a blue-light LED chip, such that the light from the LED passes through the ceramic product to generate a white light emission.

(a) providing a formulation according to any one of Claims 1 to 13 to an additive manufacturing device; and (b) forming a green body product layer by layer using the additive manufacturing device according to a set of instructions readable by the additive manufacturing device to produce the green body product.

29. The process according to Claim 28, wherein the green body product results in a unitary body formed of a single material.

30. The process according to Claim 28, wherein the green body product is formed of: a first layer according to a first formulation comprising particles of yttrium aluminium garnet (YAG); and a second layer according to a second formulation comprising particles of a YAG doped by a rare earth element.

31. The process according to Claim 30, wherein each layer of the first and second formulations is subjected to curing before the next layer is applied, where a percentage difference in a cure depth used in the first layer compared to a cure depth used in the second layer is from 5 to 20%, optionally wherein the cure depth in the first layer and second layer are independently selected from 50 to 120 pm, such as from 90 to 110 pm.

(aa) providing a green body product as described in any one of Claims 28 to 31 ;

(ba) providing a debound body product as described in Claim 32; and

(bb) subjecting it to sintering under vacuum at a temperature of from 1500 to 2000 °C, such as from 1720 to 1890 °C fora period of time to provide the ceramic product, optionally wherein the vacuum is from 1x1 O³ to 1.0 Pa.

34. An imaging device comprising a ceramic product according to any one of Claims 14 to 26, wherein the ceramic product is in the form of an optical lens, optionally wherein the optical lens is a convex or concave lens.