GB2564860A

GB2564860A - Housing envelope for multi-anvil press, and methods for fabricating same

Info

Publication number: GB2564860A
Application number: GB1711851.4A
Authority: GB
Inventors: Najafi Sani Mohammad; Salehi Sedigheh; Nigel Dodge Carlton; Fiona Ambury Rachael; Manuel Marques De Azevedo Sales Victor
Original assignee: Element Six UK Ltd
Current assignee: Element Six UK Ltd
Priority date: 2017-07-24
Filing date: 2017-07-24
Publication date: 2019-01-30
Also published as: GB201711851D0

Abstract

A housing envelope 110 for a capsule assembly 100, the capsule assembly comprising an inner assembly 109, a pair of end plugs 115 and the housing envelope. The housing envelope is configured for use with a multi-anvil high-pressure, high-temperature (HPHT) press, such as a cubic press, and comprises an envelope wall defining a central longitudinal cavity 105 for housing the inner assembly and receiving the end plugs into the cavity from the proximal and distal ends. The envelope wall has at least a pair of opposite lateral external sides and a lateral axis T passing through the respective centres of the lateral external sides. The lateral sides may comprise recesses 130. The capsule assembly and the inner assembly have respective longitudinal stiffness along the longitudinal axis, and respective lateral stiffness along the lateral axis. The envelope wall is configured such that the difference between the longitudinal and lateral stiffness of the capsule assembly is less than the difference between the longitudinal and lateral stiffness of the inner assembly, at an ambient condition. A method of making a housing envelope may comprise compacting a powder precursor against a mould to form a green body which is then subjected to temperature and pressure.

Description

HOUSING ENVELOPE FOR MULTI-ANVIL PRESS, AND METHODS FOR FABRICATING SAME

FIELD OF THE INVENTION

This disclosure relates generally to housing envelopes for use in multi-anvil high-pressure, high-temperature (HPHT) press apparatuses, particularly but not exclusively for cubic presses, and particularly but not exclusively for synthesising or sintering synthetic diamond or cubic boron nitride (cBN) crystals.

BACKGROUND

United States patent number 8 371 212 discloses cell assemblies for use in a high-pressure cubic press used for fabricating polycrystalline diamond compacts, comprising a gasket medium for housing a substantially tubular heating element, a pressure transmitting medium and a can assembly containing a plurality of diamond grains to be sintered.

Bach, Kevin Christian (An Improved Cube Cell Assembly for the Use with High Pressure/High Temperature Cubic Apparatus in Manufacturing Polycrystalline Diamond Compact Inserts (2009), All Theses and Dissertations, Brigham Young University, Utah, USA; paper 4244; pages 7 and 8) discloses a cubic press capsule assembly comprising a can assembly, a heater assembly and a cube assembly. The can assembly comprises components for sintering a polycrystalline diamond (PCD) insert and is placed inside a liner made out of isostatic material such as salt to ensure a uniform pressure distribution and provide electrical insulation.

European patent application, publication number 2 694 453 discloses a gasket for a cubictype apparatus for generating an ultra-high pressure and high temperature in a reaction volume housed within the gasket. The gasket may comprise synthetic refractory material and have the general external shape of a cube.

There is a need for improving the isotropy of the magnitudes of forces applied onto components within a high-pressure, high-temperature (HPHT) capsule assembly, to subject the components to an ultra-high pressure of at least about 2 GPa; particularly but not exclusively for increasing the isotropy of load applied to components within a six-anvil (cubic) press apparatus for synthesising or sintering synthetic diamond or cubic boron nitride (cBN) crystals.

SUMMARY

Viewed from a first aspect, there is provided a housing envelope for a capsule assembly, which comprises an inner assembly (which, when assembled, may be referred to herein simply as a capsule), a pair of end plugs and the housing envelope, and is configured for use with a multi-anvil high-pressure, high-temperature (HPHT) press; an envelope wall of the housing envelope defining a longitudinal cavity that extends from a proximal end to a distal end of the housing envelope, the cavity defining a longitudinal axis and being configured for housing the inner assembly and receiving the end plugs into the cavity from the proximal and distal ends; and the envelope wall having at least a pair of opposite lateral external sides, a lateral axis passing through the respective centres of the lateral external sides; the housing envelope configured for containing the inner assembly when the capsule assembly is installed within the HPHT press and the inner assembly is at an HPHT condition; in which the capsule assembly and the inner assembly have respective longitudinal stiffness along the longitudinal axis, and respective lateral stiffness along the lateral axis; in which the envelope wall is configured such that the relative difference between the longitudinal and lateral stiffness of the capsule assembly is less than the relative difference between the longitudinal and lateral stiffness of the inner assembly, at an ambient condition.

The longitudinal and lateral stiffness will be measured as the stiffness along the longitudinal and the lateral axis, respectively. The relative difference between the lateral and longitudinal stiffness is calculated as the magnitude of the difference between the lateral and longitudinal stiffness divided by the smaller of the magnitudes of the lateral and longitudinal stiffness. The relative difference between the lateral and longitudinal stiffness may be expressed in fraction or percentage terms. Expressed differently, the ratio of the lateral and longitudinal stiffness of the capsule assembly will be closer to unity than the ratio of the lateral and longitudinal stiffness of the inner assembly, at ambient condition.

There can be provided a capsule assembly comprising the inner assembly, the housing envelope and the pair of end plugs, in assembled or disassembled arrangement.

Viewed from a second aspect, there is provided a method of making an example housing envelope, including providing powder of precursor material for the housing envelope (the precursor material being material that can be transformed into the material of the finished housing envelope by means of at least heat treatment); providing a mould having at least one mould surface configured for forming a lateral side of the housing envelope; compacting the powder against the mould surface to form a compacted green body, a lateral side of which has substantially the same shape as the corresponding lateral side of the housing envelope; and heating the green body to a sufficiently high temperature to transform the precursor material and form the housing envelope.

Viewed from a third aspect there is provided a method of making an example housing envelope, the method including providing powder of precursor material for the housing envelope; uni-axially compacting the precursor material to form a compact body, such that the density of the compact body varies continuously by at least 5% between the uni-axial compaction axis and lateral sides; and processing the compact body to form the cavity.

Viewed from a fourth aspect there is provided a method of making an example housing envelope, including providing a blank body comprising the material of the housing envelope; and processing the side of the blank green body to remove material and configure a lateral side of the housing envelope.

A method can be provided for fabricating a super-hard body comprising super-hard material, the method including providing a capsule assembly for a multi-anvil HPHT press, in which the capsule assembly comprises an example housing envelope as disclosed herein, the housing envelope housing a reaction cell containing raw material for the super-hard body, and a heater assembly; placing the (assembled) capsule assembly between the anvils of the press; pressurising the capsule by means of the press to a pressure sufficiently high for the superhard body to form, and heating the raw materials to a sufficiently high temperature for the super-hard body to form.

Various housing envelope arrangements and methods of making and using same are envisaged by this disclosure, of which the following are non-limiting, non-exhaustive examples.

In some example arrangements, the envelope wall may be configured such that the difference between the longitudinal and lateral stiffness of the (assembled) capsule assembly is less than the difference between the longitudinal and lateral stiffness of the inner assembly, as measured by isostatic loading at a pressure of at least about 50 MPa, at least about 70 MPa, at least about 0.1 GPa, at least about 0.5 GPa, or at least about 1 GPa, or at least about 2 GPa; and / or at most about 2 GPa, or at most about 1 GPa, or at most about 0.1 GPa; and a temperature of at least about 100°C, or at least about 500°C, or at least about 1,000°C, or at least about 1,500°C; and / or a temperature of at most about 2,500°C, or at most about 1,500°C, or at most about 1,000°C.

In some example arrangements, the ratio of the longitudinal stiffness of the capsule to its lateral stiffness may be at least about 0.8, or at least about 0.9, or at least about 0.95; and / or at most about 1.2, or at most about 1.1, or at most about 0.95; and in some examples, this ratio may be approximately 1.

The envelope wall may extend azimuthally all the way around the cavity and define a plurality of lateral external sides of the housing envelope, each connecting the proximal and distal ends. The cavity may be configured for accommodating an inner assembly suitable for fabricating a super-hard article at an HPHT condition. When the capsule assembly is assembled, the pair of end plugs may abut a respective proximal and distal end of the inner assembly.

In various example arrangements, the envelope wall may have one, or a plurality of, lateral internal sides, which will entirely or partly define the cavity (in other words, the lateral internal side, or sides, of the envelope wall may define at least an area of the cavity surface). The one or more lateral internal sides may extend from the proximal to the distal ends of the housing envelope. In some example arrangements, the envelope wall may have a respective lateral internal side opposite each lateral external side; or the envelope wall may have a cylindrical lateral internal side. The envelope wall may be coaxial with the cavity. The envelope wall may include a recess, and / or a boss, on at least each of the lateral external sides, and I or on each of the corresponding lateral internal sides.

In some example arrangements, the multi-anvil HPHT press may comprise six anvils, arranged as pairs of opposing anvils on three orthogonal axes, in cubic symmetry; such press arrangements may be referred to as cubic presses. The housing envelope may have four lateral sides, each connecting the distal and proximal ends; the four lateral sides may be arranged as a square when viewed in a lateral cross-section (in other words, the lateral sides may be arranged as two sets of pairs of oppositely facing sides, the two sets of pairs being at about 90 degrees to each other). The inner assembly may have four lateral sides connecting distal and proximal ends of the inners assembly.

In some example arrangements, the cavity of the housing envelope may be substantially cylindrical, and circular when viewed in transverse cross-section; or the cavity may be polygonal when viewed in transverse cross-section.

In some example arrangements, the housing envelope may be configured such that when load is applied onto the capsule assembly (in the assembled condition) by the anvils of the multi-anvil press to pressurise the reaction cell or cells, material of the housing envelope extrudes between side surfaces of the anvils to form a gasket having a substantially uniform thickness when the HPHT condition is achieved within the capsule assembly. For example, the thickness of the gasket may vary by at most about 10%, or at most about 5% (for any given distance from the nose of the adjacent anvils). By balancing the stiffness of the capsule along the longitudinal and lateral axes, it may be possible to create substantially equal gasket thickness formation and flow in use in an HPHT press, which may result in a more stable capsule at HPHT conditions.

In some example arrangements, the envelope wall may be configured such that the magnitude of the difference between the longitudinal and mean lateral stiffness of the capsule assembly is less than the magnitude of the difference between the longitudinal and mean lateral stiffness of the inner assembly, along the longitudinal and lateral axes, respectively, at an ambient condition.

In some example arrangements, a recess provided in a lateral external, or internal, side of the housing envelope may have substantially the same effect on the Young’s modulus, and I or the lateral stiffness of the housing envelope as at least about 5%, or at least about 10 %, and I or at most about 20% porosity within the housing envelope.

In some example arrangements, the envelope wall may be configured such that the difference between the longitudinal and lateral Young’s modulus of the capsule assembly is less than the difference between the longitudinal and lateral Young’s modulus of the inner assembly, along the longitudinal and lateral axes, respectively, at an ambient condition.

In some example arrangements, each of the pairs of lateral external sides of the housing envelope may include at least one respective recess, and I or at least one respective boss. In some example arrangements, the cavity surface may include one or more recess, or one or more boss, or at least one recess and at least one boss. In some example arrangements, mutually opposite lateral external sides of the housing envelope may have the same configuration as each other. In some examples, each lateral external side of the housing envelope may have a different configuration than at least one of the adjacent sides.

In some example arrangements, each external, or internal, lateral side of the housing envelope may be provided with a recess or boss having a circular, oval, square, or polygonal shape (when viewed from a side). The recess or boss may be arranged coaxially with the corresponding lateral axis; in other words, the lateral axis may pass through the centre of the recess or boss. An example recess may have a depth into the lateral side of at least about 0.5 mm, or at least about 1 mm, or at least about 2 mm, or at least about 3 mm; and I or at most about 6 mm, or at most about 5 mm; and an example boss may project a distance from the lateral side of at least about 0.5 mm, or at least of about 1 mm, or at least about 2 mm; and I or at most about 5 mm. In some examples, the depth of the recess, or the height of the boss may be at least 15%, or at least 50% the thickness of the end plugs; and I or the depth of the recess, or the height of the boss may be at most about equal to the thickness of the end plugs, or at most about 90% the thickness of the end plugs. In some example arrangements, the thickness of each end plug may be about 6 mm to about 7.5 mm; or about 7 mm to about 12 mm. Each of the pair of external, or internal, lateral sides of the housing envelope may have a recess, the depth of which is such that the thickness of side wall between its inner surface and the recessed surface may be about 4 mm to about 5 mm.

Each of the anvils of the multi-anvil HPHT press may have an anvil nose that will contact the housing envelope in use, as the anvils are driven convergently against the housing envelop to apply load onto the capsule assembly. In some examples, the anvil nose may have a square shape characterised by a side dimension, or a circular shape characterised by a diameter, and in some examples, the recess or boss may have a mean diameter (for example an equivalent circle diameter) or mean breadth that is less than the side dimension or diameter of the anvil nose. In some example arrangements, respective recesses formed into lateral sides of the housing envelope may be substantially circular, square or polygonal, and configured such that the advancing anvil nose will contact an area of the lateral side surrounding the recess, and at least initially the anvil nose may not contact the recessed surface area (until a sufficiently high load is achieved and the anvil nose contacts the recessed surface). In some example arrangements, the anvil noses may be substantially square, the lateral side of the housing envelope may be substantially square, and the recess or boss may be substantially square.

In some example arrangements, respective bosses formed on each of at least a pair of opposite lateral sides of the housing envelope may be substantially circular, square or polygonal, and configured such that the advancing anvil nose will contact the embossed surface before contacting an area of the lateral side surrounding the boss. In other words, the area of the recess or boss may be smaller than the area of the anvil nose. In some other examples, the area of the recess or boss may be greater than, or equal to the area of the anvil nose.

In some example arrangements, respective recesses may be formed into each of at least a pair of opposite side walls of the housing envelope, such that the thickness of side wall between the inner surface of the wall (in other words, from an inner lateral side surface of the cavity) and the recessed surface of the exterior lateral side is within about 20%, or within about 10%, or within about 5% of the thickness of the end plugs. In some example arrangements, each of the pair of external, or internal, lateral sides of the housing envelope may have a recess having a depth of 15% - 50% the thickness of each end plug. In some examples, the thickness of the side wall of the housing envelope from the recesses surface to the inner surface may be substantially equal to the thickness of the end plugs. A relatively thick side wall of the housing envelope may have the aspect of achieving greater thermal insulation, reducing the rate of heat conduction from the inner assembly to the anvils in use. The side walls of the housing envelope may be sufficiently thick that the temperature of the anvil noses will remain at less than about 400°C, or less than about 300°C in use, when the reaction cell or cells are at the HPHT condition at least about 5 minutes (for example, when the reaction cell or cells are at a temperature of about 1,600°C).

In some example arrangements, the mean density of the envelope wall may vary substantially monotonically with depth (or radially) from a lateral internal side to the lateral external side; the density of the envelope wall may increase, or decrease, along a lateral axis, from its lateral internal side to its lateral external side; the density may increase, or decrease, over its thickness along a lateral axis by at least about 5%, or at least about 10%, or at least about 20%. The material comprised in the envelope wall may vary along the lateral axis, from its lateral internal side to its lateral external side; or the envelope wall may comprise or consist of the same material throughout its thickness along a lateral axis, and the density of the material may lie on a gradient (in other words, the density of the same material may vary with depth along the lateral axis, from the lateral internal side to the corresponding lateral external side).

For example, the envelope wall may comprise a plurality of different materials, arranged such that the mean density of the envelope wall varies along a lateral axis by at least 10%. In some examples, the mean density of a first volume of the envelope wall coterminous with the lateral external side may be at least 10% lower than the mean density of a second volume of the envelope wall coterminous with the lateral internal side.

In some example arrangements, respective recesses may be formed into at least opposite side walls of the housing assembly (into an external or an internal lateral side), and a plug comprising or consisting of material that is substantially softer than the material of the housing envelope, and which may have a relatively lower density at the HPHT condition, may be accommodated within the recesses. For example, respective plugs may be accommodated within each recess into at least a pair of opposite internal sides of the housing envelope.

In some example methods of fabricating an example housing envelope, a housing envelope for a cubic HPHT press may be made using a mould having four mould surfaces, arranged as opposing pairs of inner walls of a mould cavity, each mould surface configured for forming respective lateral sides of the housing envelope. The powder precursor material may be poured into the mould cavity, and compacted within the mould cavity, thus forming the compacted green body, the lateral sides of which have substantially the same configuration of the corresponding lateral sides of the housing envelope. For example, the mould surface may include a boss, configured to form a corresponding recess in the lateral side of the green body; and / or a corresponding boss on the lateral side of the green body.

In some examples, the housing envelope may comprise a plurality of elements, each of which may be formed separately and cooperatively configured for assembly to provide the housing envelope. For example, two halves of the housing envelope may be formed as two respective parts.

Since the density of the compacted green body will likely be substantially less than that of the finished housing envelope, the dimensions of features of the lateral side, for example one or more recesses, or one or more bosses, may differ from the corresponding dimensions on the lateral side of the finished housing envelope. In some examples, the dimensions of the compacted green body, for example a depth of a recess or a height of a boss, may differ by at most about 0.5 mm, or at most about 0.2 mm, or at most about 0.1 mm from the corresponding dimension of the finished housing envelope. In some examples, the volume, and / or other dimensions of the body may shrink in response to the heat and / or pressure treatment, by at least about 5%, or at least about 10%; and I or at most about 20%.

Some example methods may include processing the compacted green body to remove precursor material from its lateral side, such the volume of precursor material removed extends at most 0.5 mm, or at most about 0.2 mm, or at most about 0.1 mm into the lateral side of the green body.

In some example methods, the powder precursor material may comprise natural, synthetic or reconstituted pyrophyllite, and the method may include subjecting a volume of powder precursor material contacting the mould wall to uni-axial or isostatic compaction, such that compacted green body has a hardness of at least about 50 Rockwell B; and I or at most about 100 Rockwell B; and / or a flexural strength of about 69 MPa; and / or a compressive strength of about 172 MPa; and / or a tensile strength of about 17 MPa.

In some example methods of fabricating the housing envelope, uni-axial compaction of precursor material powder to form a compact body may result in a density gradient within the precursor body and I or the finished housing envelope, in which the density of may decrease, or increase, with radial distance from a longitudinal axis along which compaction load is uniaxially applied. In some examples, the variation of material density within the housing envelope (arising from uni-axial compaction) may be sufficient to reduce the difference between the longitudinal and lateral stiffness of the assembled capsule, according to the first aspect; it may not be necessary for form recesses or bosses on the lateral external or internal sides of the housing envelope. For example, a precursor body in the general form of a cube of compacted powder material may be uni-axially compressed along a longitudinal axis passing through the centres of a distal and proximal end of the precursor body, and material may be removed from the precursor body to form a cavity extending longitudinally from the proximal to the distal ends, providing a housing envelope according to the first aspect. In some examples, the density of a uni-axially compacted precursor body may vary radially (from the longitudinal axis) by at least about 5%, or at least about 10%; and I or at most about 10%, or at most about 20%.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting example arrangements of post boxes will be described with reference to the accompanying drawings, of which

Fig. 1 shows a schematic longitudinal cross-section view through an example capsule assembly, and a schematic indication of lateral and longitudinal forces applied to the capsule;

Fig. 2A shows a schematic illustration of the effect of subjecting an inner assembly to an isostatic pressure, and Fig. 2B shows a schematic illustration of the effect of subjecting an example capsule to the isostatic pressure;

Fig. 3A shows a schematic perspective view of an example housing envelope, Fig. 3B shows a schematic side view (along the x- or y-axis) of the example housing envelope, Fig. 3C shows a schematic longitudinal (z-plane) cross-section view through the example housing envelope, and Fig. 2D shows a schematic top or bottom view of the example housing envelope (viewed along the z-axis);

Fig. 4A shows a schematic perspective view of an example housing envelope, Fig. 4B shows a schematic top or bottom view (along the z-axis) of the example housing envelope, Fig. 4C shows a schematic longitudinal (z-plane) cross-section view through the example housing envelope, and Fig. 4D shows a schematic transverse cross-section view (x-y plane) through the example housing envelope;

Fig. 5A shows a schematic perspective view of an example housing envelope, Fig. 5B shows a schematic side view (along the x- or y-axis) of the example housing envelope, Fig. 5C shows a schematic longitudinal (z-plane) cross-section view through the example housing envelope, and Fig. 5D shows a schematic top or bottom view of the example housing envelope (along the z-axis); and

Fig. 6 shows a photographic perspective view of an example capsule assembly.

DETAILED DESCRIPTION

With reference to Fig. 1, a capsule assembly 100 for a cubic HPHT press (not shown) may be adapted to increase the number of reaction cells 103 from one to two, by providing a nest of pressure-transmitting material having two cavities positioned coaxially along the longitudinal axis L, and providing a housing envelope 110 configured according to this disclosure. For example, a respective recess 130 may be formed into each of four lateral sides of the housing envelope 110. An inner assembly 109 comprising the reaction cells 103 and an inner envelope 107 may be accommodated within a cylindrical cavity 105 of the housing envelope 110. A pair of end plugs 115, which may be referred to as thumbtacks, are configured to fit into the open proximal and distal ends of the cavity 105. The inner envelope 107 may contain a heater assembly and pressure-transmitting material (these components are not specifically shown).

The inner assembly 109 may be arranged such that the reaction cells 103 are spaced apart by a layer of the pressure-transmitting material between them. In an example adaptation of the inner assembly 109 to comprise an additional reaction cell 103, the volume of the pressuretransmitting material within the inner assembly 100 may be reduced (owing to the presence of the additional reaction cell) and the stiffness of the inner assembly 109 may be substantially increased. The configuration of the example housing envelope 110 described with reference to Fig. 1 will at least partly compensate for the increase in the anisotropy of the stiffness of the inner assembly 109.

The cavity 105 may have a central longitudinal axis L, aligned with a z-axis in an orthogonal x-y-z coordinate system. An HPHT cubic press (not shown) will have six anvils, arranged as three pairs of opposing anvils, each pair arranged along a respective axis, such that each of the six external sides of the cubic capsule 100 will be impinged by a respective anvil in use. Fig. 1 shows a schematic longitudinal cross-section view through a capsule 100, in which the opposing anvils along the x and z axes will apply forces of magnitude F to respective opposite sides of the capsule 100. One pair of the anvils will impinge the thumbtacks 115 from opposite directions, longitudinally along the z-axis.

In general, the stiffness of the inner assembly 109 may be anisotropic as a result of the arrangement of components comprising various materials within the capsule assembly 110, particularly the reaction cells 103. In other words, the stiffness of the (assembled) inner assembly 109 may differ substantially along the x, y, and z-axes, and the inner assembly 109 may deform (compress) by different magnitudes in response to compressive forces applied to it along these axes. While the reaction cells 103 may be substantially enclosed within pressure-transmitting material such as molten sodium chloride in certain example arrangements, the volume of this material relative to the volume of the reaction cells 103 may not be high enough to achieve a desired degree of pressure isotropy.

In some arrangements, the components of a capsule assembly 100 (principally the components comprising the inner assembly) may be arranged such that the stiffness of the capsule 100 is substantially greater along the x- and y-axes than along the z-axis, along which the thumbtacks 115 will tend to be urged towards each other in response to the opposing forces applied by the anvils. In certain example arrangements, the overall capsule 100 stiffness along each of the axes can be substantially equalised in magnitude by forming recesses into each of the side external surfaces of the housing envelope 110, through which the x- and y-axes will pass. Example capsules 100 may be able to house a greater number of reaction cells than capsules 100 that have not been modified by the formation of recesses or bosses on side walls of the housing envelope 110, without the reaction cells being subjected to substantially increased pressure anisotropy in an HPHT manufacturing cycle. In some examples, modification of a housing envelope as disclosed may have the aspect of enabling two reaction cells 103 instead of one reaction cell, without compromising the quality or consistency of the fabricated product (the number and length of the reaction cells within a capsule will generally affect the magnitude and geometry of the stiffness and elastic moduli of the capsule).

In the particular example arrangement schematically illustrated in Fig. 1, the capsule 100 contains a reaction cell assembly of two reaction cells 103, arranged along a longitudinal axis L (parallel to thez-axis). Each reaction cell 103 may contain stiff bodies such as Co-cemented tungsten carbide substrates, having relatively high Young’s modulus, and so the stiffness of the reaction cell assembly along the z-axis may be substantially less than its stiffness along the x- and y-axes. To compensate for the anisotropy in the stiffness of the inner assembly

109, recesses 130 may be provided in the four lateral external sides ofthe housing envelope

110, perpendicular to the x- and y-axes. The recesses 130 may be configured to reduce the magnitude of the difference between the stiffness of the capsule assembly 100 along the lateral x- and y-axes on the one hand, and along the longitudinal axis L on the other. In some examples, this ratio may be approximately 1, or about 0.9 to about 1.1. When load of substantially the same magnitude is applied to the capsule 100 along the x-, y-, and z-axes as in use, the presence of the recesses may result in the reaction cells 103 being subject to pressure (for example, about 5 GPa to about 10 GPa) that is substantially more isotropic, which will be transmitted between the housing envelope 110 and the thumbtacks 115 on the one hand, and the reaction cells 103 on the other, via the inner envelope 107, specifically pressure-transmitting material such as sodium chloride comprised in the inner envelope 107.

Fig. 2A schematically illustrates an inner assembly 109 at ambient conditions (left-hand drawing), and being subjected to an isostatic pressure IP by means of a hot isostatic press (right-hand side). In the drawing on the right-hand side, the isostatic pressure IP is about 60 MPa and the temperature is about 100°C to about 1,600°C, such that the sodium chloride will be molten and contained within the capsule assembly 100. In the drawing on the left-hand side, the housing envelope and end-plugs are indicated by dashed lines merely for ease of comparison with the arrangement illustrated in Fig. 2B.

As an example, the inner assembly 109 may comprise sodium chloride salt surrounding one, two or more reaction cells for fabricating one or more sintered diamond or PCBN bodies (or bodies comprising other super-hard materials) attached to respective cemented tungsten carbide substrates (not shown). The sodium chloride may have the effect of reducing or substantially eliminating the anisotropy of the pressure applied to the reaction cell 103 or cells when the temperature of the inner assembly 109 is sufficiently high for the sodium chloride to be molten and a pressure sufficiently high to contain the molten sodium chloride within the capsule assembly 100. At sea level atmospheric pressure (101 kPa), the sodium chloride will have a melting point of 801 °C; and at higher pressures, the sodium chloride will have higher melting points.

The inner assembly 109 may be cylindrical, and its stiffness along the central longitudinal axis L may be substantially less than the stiffness along an orthogonal lateral axis T. The inner assembly 109 will respond to the isostatic pressure IP and temperature by shrinking in volume, its longitudinal height along the central longitudinal axis decreasing from hi to h₂, and its lateral width along the lateral axis through the centres of opposite sides decreasing from ti to t₂. Since the lateral stiffness of the inner assembly 109 is substantially greater than its longitudinal stiffness in this example, the difference between t₂ and ti (i.e. t₂ - ti) will be substantially less than the difference between h₂ and hi (i.e. h₂ - hi).

In Fig. 2B, the inner assembly 109 having the same structure, dimensions and mechanical properties as that described with reference to Fig. 2A is assembled within a capsule assembly comprising a housing envelope 110 and a pair of end-plugs 115 received in opposite ends of a central longitudinal cavity defined by the housing envelope 110, and contacting respective opposite ends of the inner assembly 109. When the capsule assembly is subjected to the same isostatic pressure IP and temperature as in the arrangement described with reference to 2A, it will be compressed. Recesses 130 are provided in each of four lateral sides of the housing envelope 110. The size, shape and depth of the recesses 130 are selected to compensate at least partially for the difference between the longitudinal and lateral stiffness of the inner assembly 109, such that the changes in longitudinal height and lateral width of the inner assembly will differ substantially less than for the arrangement in Fig. 2A. The difference the difference between t₂ and ti (i.e. t₂ - ti) and the difference between h₂ and hi (i.e. h₂ - hi) will be substantially less than in Fig. 2A, in which the housing envelope 110 and end plugs

115 are not present; in some example arrangement, the recesses 130 can be configured and arranged such that h2 - hi is substantially the same as t2 - ti.

With reference to Fig. 3A - 3D, an example housing envelope 110 for a cubic HPHT press (not shown) may have a generally cubic external shape, and include a cylindrical central cavity 105 connecting proximal and distal ends. The diameter D of the cavity 105 may be about 38 mm, and the length L of each of the square lateral sides may be about 58 mm. Each of the four lateral sides may have a respective central, circular boss 120 having a diameter W of about 36 mm (each anvil nose, not shown, may be square having a side width of about 46 mm), and projecting a height of about 1 mm from an area of a respective lateral side surrounding each boss 120.

With reference to Fig. 4A - 4D, an example housing envelope 110 for a cubic HPHT press (not shown) may have a generally cubic external shape, and include a cylindrical central cavity 105 connecting proximal and distal ends. The diameter D of the cavity 105 may be about 38 mm, and the length L of each of the square lateral sides may be about 58 mm. Each of the four lateral sides may have a respective central, circular recess 130 having a diameter W of about 36 mm (each anvil nose, not shown, may be square having a side width of about 46 mm), and extending a depth of about 2 mm from an area of a respective lateral side surrounding each boss 130.

With reference to Fig. 5A - 5D, an example housing envelope 110 for a cubic HPHT press (not shown) may have a generally cubic external shape, and include a cylindrical central cavity 105 connecting proximal and distal ends. The diameter D of the cavity 105 may be about 28 mm, and the length L of each of the square lateral sides may be about 58 mm. Each of the four lateral sides may have a respective central, cruciform boss 122 formed of four arms extending a distance W4 of about 10 mm from a central portion having a width W3 of about 8 mm (each anvil nose, not shown, may be square having a side width of about 46 mm), and projecting a height of about 4 mm from an area of a respective lateral side surrounding each cruciform boss 122.

With reference to Fig. 6, an example housing envelope 110 for a cubic HPHT press (not shown) may have a generally cubic external shape, and include a cylindrical central cavity 105 connecting proximal and distal ends. The diameter D of the cavity may be about 38 mm, and the length L of each of the square lateral sides may be about 58 mm. Each of the four lateral sides may have a respective central, square recess 132 having a side width of about 36 mm (each anvil nose, not shown, may be square having a side width of about 46 mm), and extending a depth of about 3 mm from an area of a respective lateral side surrounding each recess 132.

In response to the capsule assembly being subjected to a substantially isotropic compressive loading force, the longitudinal and lateral dimensions of the capsule assembly will decrease; similarly, the longitudinal and lateral dimensions of the inner assembly (in the assembled condition) will decrease in response to the same substantially isotropically-applied loading force; and the difference in the magnitudes, or percentages, by which the longitudinal and lateral dimensions of the capsule assembly change may be less than the difference in the magnitudes, or percentages, by which the longitudinal and lateral dimensions of the inner assembly change, as a consequence of the configuration of example housing envelopes.

In some examples, the reaction compact may comprise materials suitable for converting a source of carbon into diamond when subjected to a pressure of at least about 5.5 GPa and a temperature of at least about 1,250°C. In some examples, the reaction compact may comprise a pre-sinter compact comprising grains of super-hard material such as diamond or cBN and material suitable for promoting the binding, sintering and or intergrowth of the super-hard grains to produce PCD or PCBN material. The reaction compact may contain metal such as iron, cobalt and or nickel, which are examples of catalyst and I or binder materials (or precursor materials for catalyst or binder materials) for promoting diamond synthesis or sintering, or compounds including lithium, aluminium and or titanium (or any of these elemental form) for promoting cBN synthesis or sintering.

In some example arrangements, the inner assembly may comprise a heating element that is capable of heating the inner assembly to the high temperature of the HPHT condition in use, in response to an electric current flowing through the heating element. For example, the heating element may comprise or consist of graphitic material, or metal material, such as titanium, tantalum, molybdenum, or niobium. The inner assembly may comprise a pressuretransmitting volume, comprising or consisting of material, for example sodium chloride or potassium bromide, that will be capable of transmitting and distributing pressure from the end plugs onto one or more reaction cells contained within the inner assembly.

In use, an example capsule assembly can be placed between a plurality of anvils, which will be driven by a hydraulic mechanism against pairs of opposite sides of the capsule to subject the capsule to opposing forces, and consequently to generate ultra-high pressure within the pressure-transmitting material, the reaction cell or cells and other components. Heat may be generated within the capsule by means of an electric current passing through the heating assembly. The housing envelope may be configured such that in response to the force being applied by the anvils, it will change shape and adopt a configuration for containing the reaction cell or cells, heating assembly and pressure-transmitting medium (and other components, in some examples), at the ultra-high pressure and high temperature. The housing assembly may be configured such that when the anvils are driven against the ends and sides of the housing envelope to generate the HPHT condition in use, portions of the housing envelope may extrude into the gaps between the converging anvils and a form gasket. Consequently, the anvils may apply load onto the gasket formation between them, as well as along the longitudinal and lateral axes, and the compression of the gasket formation may have the effect of containing the material of the inner assembly at the HPHT condition.

The inner assembly may comprise one, two, three, four, five, six, or more reaction cells, each reaction cell comprising raw material for forming and I or sintering grains of super-hard material at the HPHT condition. Each reaction cell may comprise a substrate comprising or consisting of cemented tungsten carbide material. In some example arrangements, the inner assembly may comprise a plurality of reaction cells, each having a substantially cylindrical outer shape; and the inner assembly may comprise a nest including a plurality of respective cavities configured to accommodate the reaction cells such that the cylindrical axes of the reaction cells will be parallel to the longitudinal axis of the housing envelope when assembled as in use. Each reaction cell may comprise a substantially cylindrical, or disc-shaped substrate comprising or consisting of cemented carbide material, for example cobaitcemented tungsten carbide material. In some examples, the reaction cell may contain diamond or cBN grains to be sintered, or raw materials for synthesising diamond or cBN crystals, the pressure-transmitting material may comprise or consist of sodium chloride salt, and the housing envelope may comprise or consist of pyrophyllite, or suitable synthetic material.

In some example in which a housing envelope has respective recesses formed into each of four lateral sides, the depth of the recess may be selected such that the gasket that will form when the load is applied to the capsule assembly may be substantially uniform in thickness (for example, the thickness of the gasket may vary by less than about 10% or less than about 5% for any distance along the side of the adjacent anvil from the anvil nose), and such that the inner assembly material is contained while the reaction cell or cells are maintained at the HPHT condition. In some examples, if the depth of the recess is too small, then the lateral stiffness of the capsule assembly may exceed its longitudinal stiffness by too great a magnitude, and the material of the inner assembly may not be sufficiently contained at the HPHT condition. In some examples if the depth of the recesses is too great, then excessive extrusion of material may result, the lateral and longitudinal stiffness may not be sufficiently balanced, and too much heat may be lost through the lateral side walls of the housing envelope.

In some examples, a high pressure, high temperature (HPHT) condition may be a condition at which the pressure may be at least about 2 GPa, or at least about 3 GPa, or at least about 5 GPa; and / or the pressure may be at most about 25 GPa, or at most about 10 GPa, or at most about 8 GPa; and the temperature may be at least about 1,000°C, or at least about 1,200°C; and I or the temperature may be at most 30,000°C, or at most about 10,000°C, or at most about 5,000°C, or at most about 3,000°C. In some examples, an HPHT condition may be a condition at which the pressure is about 1,000°C to about 3,000°C, and the pressure may be about 2 GPa to about 25 GPa.

In some examples, the housing envelope may comprise or consist of compacted and I or sintered grains of refractory ceramic material. The end plugs may comprise or consist essentially of (apart from an electrically conducting element arranged to enable an electric current to pass between an anvil and a heater element contained within the housing envelope) substantially the same kind of material as the housing envelope. The refractory ceramic material may comprise a phase containing a silicon compound, being sufficiently thermodynamically and kinetically stable to contain the material of the inner assembly at the HPHT condition, for at least about 15 minutes. The ceramic may comprise a phase of material derived from natural, synthetic, or reconstituted pyrophyllite. In some examples, the housing envelope may be formed by a method including heat treating a compacted green body comprising or consisting of pyrophyllite material. The housing envelope may comprise ceramic or mineral material selected from the group consisting of quartz, garnet, zirconium, magnesium oxide, alumina (AI2O3), and natural (mined), synthetic or reconstituted pyrophyllite, or material phases or compounds derived from heat treating pyrophyllite. The housing envelope may include iron oxide (Fe2Os), titania (T1O2), potassium oxide (K2O), sodium oxide (Na2<D) and phosphorus pentoxide (POs), which may be present in relatively minor amounts.

In some examples, the housing envelope may comprise mullite and / or a polymorph of mullite, and I or talc, and I or kaolinite, and I or kyanite, and I or polymorphs of kaolinite or kyanite. The housing envelope may comprise a plurality of grains of these or other minerals. The housing envelope may comprise silica or glassy phases, which may be present at up to about 15 weight per cent (wt.%), or up to about 10 wt.% silica, silicate compounds or other glassy phases, or the housing envelope may be substantially free of silicate compounds. The housing envelope may comprise, or be substantially devoid of, magnesium carbonate or precursor material for magnesium carbonate. The housing envelope may comprise cordierite, a material comprising magnesia, alumina and silicate compounds. The first material may comprise mullite.

In some examples, the housing envelope may comprise grains of a first hard material having a first internal friction and grains of a second hard material having a second internal friction. The second hard material may be a different phase of the first hard material. The second material may promote the plasticity, handleability and or machinability of the housing envelope. The second material may promote plasticity, flowability and or pressure transmission at ultra-high pressure and or elevated temperature of at least about 900°C.

In some examples, the overall stiffness, or shear strength of the reaction cell may be substantially anisotropic. For example, the reaction cell stiffness may vary along different orthogonal axes, which may be undesirable, particularly - but not exclusively - in arrangements where the reaction cell is surrounded by material having relatively high bulk modulus, such as tungsten carbide. Certain example disclosed arrangements may have the aspect of balancing the overall stiffness of the capsule along different axes, thus achieving a more isotropic distribution on the magnitude of applied force. This may be achieved by forming recesses and I or bosses on the housing envelope, for example by machining a housing envelope to the desired configuration and dimensions. In particular, recesses may be formed into volumes of the housing envelope that would exhibit the greatest overall stiffness, thus reducing the effective stiffness along axes passing through the volumes. Such example arrangements may have the aspect of allowing one or more additional reaction cell to be housed within the capsule, while achieving sufficient isotropy of capsule stiffness. This may increase the production yield of product manufactured during an HPHT cycle.

If the anisotropy, or asymmetry, in the magnitude of the forces applied by the anvils of an HPHT press is too great, then there may be a risk of material escaping explosively from the capsule (in an event that may be referred to as a “blow-out”), which may damage an anvil, and may result in substantial down-time for the anvil to be replaced, and I or for the HPHT press to be re-set and re-calibrated. In addition, the batch may need to be written off. Even if there is no blow-out, anisotropic applied forces may result in anisotropic pressure within the capsule, which may deleteriously affect the shape or quality of the product being fabricated within a reaction cell contained by the capsule. A substantially distorted product may need to be discarded.

Example disclosed housing envelopes may have the aspect of reducing the risk of fabricated product having substantial distortion or other reduction in its quality, and consequently, of increasing the manufacturing efficiency; and I or the risk of loss of containment of material of the inner assembly may be reduced; and I or the capsule assembly may contain a greater number of reaction cells, and I or it may be possible to pressurise the reaction cells to higher pressures, without substantial increase in the risk of loss of containment or loss of quality of the product.

While wishing not to be bound by a particular theory, it may generally be desirable for the pressure applied onto one or more reaction cells within the inner assembly to be as isotropic as possible, because the thermodynamics and kinetics of crystallisation and I or sintering processes that may take place during an HPHT cycle likely depend on pressure and temperature. In addition, relatively isotropic pressure distribution within the inner assembly will likely reduce the likelihood and extent of non-uniform deformation of the heating elements, and consequently of non-uniform heating of the reaction cell or cells. It may also be generally desirable to have as many reaction cells as possible within the capsule assembly, in order to achieve as great a yield of product as possible from each HPHT cycle. However, since reaction cells may have a substantially anisotropic stiffness, increasing the number of reaction cells in the inner assembly will likely tend to increase the anisotropy of the stiffness of the capsule assembly, and consequently to increase the anisotropy of pressure and heat generation. Reaction cells comprising a layer of raw material on a cemented carbide substrate will likely have an especially anisotropic stiffness, owing to the highly anisotropic arrangement of the relatively very stiff carbide material. Increasing the number of reaction cells, the combined volume of the reaction cells will likely increase at the expense of the volume of pressure-transmitting material within the capsule assembly. Therefore, the product yield per HPHT cycle is driven by opposing factors: on the one hand, maximising the number of reaction cells; and on the other hand, minimising the combined relative volume of the reaction cells within the capsule assembly. Example arrangements may have the aspect of allowing the number of reaction cells to be increased without substantial increase in the anisotropy of pressure generation, or even enhancing the isotropy of the pressure applied to the reaction cells. In some alternative examples, it may be desirable to engineer the capsule assembly so that more, or less, load will be applied axially.

Certain terms and concepts as used herein are briefly explained below.

As used herein, a multi-anvil press is an apparatus for applying force onto the body by driving at least four anvils into the body from different directions. The anvils may be driven by a hydraulic mechanism with sufficient force to generate an ultra-high pressure within the body. A cubic multi-anvil press will comprise six anvils arranged as three opposing pairs, each pair capable of moving in mutually opposite directions towards the body, along respective orthogonal axes. A tetrahedral multi-anvil press will comprise four anvils, each arranged to be capable of moving towards the body along a respective one of four axes tetrahedral to each other.

Refractory materials are substantially chemically and physically stable at higher temperatures. According to a standard of the American Society for Testing and Materials International (ASTM), namely ASTM C71, refractory material is non-metallicand has chemical and physical properties that make them suitable for structures or components that will be exposed in use to temperatures above 538°C. A refractory material may be characterised in terms of its “refractoriness”, which is the temperature at which it will deform under its own load, such as when the material is in a plastic or fluid state, and which may be indicated by PCE (pyrometric cone equivalent), according to the ASTM C24-01 standard. As used herein, refractory material suitable for a housing envelope has a refractoriness temperature of at least 2,000°C. Refractory materials may be fabricated from raw materials including oxides of aluminium, or silicon, or magnesium; oxide of calcium may also be used in the fabrication of refractory material. Examples of refractory material may include tungsten carbide, boron nitride, hafnium carbide, tantalum hafnium carbide, magnesium oxide, calcium oxide, chrome-magnesite, chromium oxide, dolomite, quartz, silica, mullite, zirconia, alumino-silicate material. Refractory material may be fabricated by processing and potentially combining (mined) mineral material, including a step of heat treating (such as calcining) the mineral material. Unless indicated otherwise herein, the term “refractory material” includes precursor material that may not be refractory, but which can be transformed into refractory material by a process including heat treatment.

Pyrophyllite is generally a mined mineral comprising aluminium silicate hydroxide (Al2Si4Ow(OH)), which will undergo phase changes when heated to sufficiently high temperatures at sufficiently high pressures. Mullite, which may also be referred to as porcelainite, is a silicate mineral, and can form the two stoichiometric forms 3Al203(2SiC>2) or 2Al203.Si02. Mullite occurs in a platelet form and needle form. Kaolinite (Al2Si2O5(OH)4) is a mineral that may be found in rock formations comprising kaolin. Kyanite is a member of the aluminosilicate series, which also includes the polymorph andalusite and the polymorph sillimanite. Kyanite is strongly anisotropic and its hardness varies depending on its crystallographic direction. At temperatures above 1,100°C, kyanite decomposes into mullite and vitreous silica, in a transformation that results in an increase in volume (exo-volumetric). Talc, mullite, kaolinite and kyanite are naturally occurring minerals, or may be derived from naturally occurring minerals, and so may contain a proportion of other components and impurities.

As used herein, the stiffness of a body or an assembly is the extent to which it will resist deformation in response to an applied force. Stiffness may be referred to as rigidity. The stiffness of a construction or assembly of members will depend on the materials from which the members are formed, the configuration of the members and their arrangement in the construction or assembly. In general, the stiffness of a construction or assembly may not be isotropic; in other words, the construction or assembly may deform by different amounts in response to forces applied along different axes. For example, if a capsule assembly is subjected to compressive forces along three orthogonal axes, it may compress by different amounts along each axis, all else being equal.

As used herein, an ambient condition means that the ambient pressure is 101 kPa and the ambient temperature is 20°C to 25°C, for example 20°C.

As used herein, pressure-transmitting material, or a pressure-transmitting medium, will be a compressible fluid or relatively soft solid. Materials, such as certain alkali halide materials, having a melting point of less than about 1,000°C at sea level pressure and become soft and relatively quasi-hydrostatic at elevated pressure, such as pressure over about 1 GPa. Sodium chloride, potassium chloride, potassium bromide and magnesium oxide may be an effective pressure-transmitting material at relatively high temperature, when under relatively high pressure. For example, reported data indicates that polycrystalline sodium chloride should not be able to support differential stress of more than about 0.4 GPa, or 1.3 GPa (depending on the study reporting the data) at pressures up to about 10 GPa, at ambient temperature (about 20°C to about 30°C). At temperatures of about 1,000°C to about 2,000°C (at least) under pressures of about 5 GPa to about 10 GPa (at least), sodium chloride does not fully melt, but substantially softens.

As used herein in relation to structures, tubes, chambers, heater assemblies, presses that are substantially symmetric about a cylindrical (also referred to as a longitudinal) axis, aspects may be described in terms of cylindrical coordinates, including radial and azimuthal coordinates. As used herein, a longitudinal axis is the axis of a capsule assembly along which a pair of anvils apply opposing forces onto the capsule assembly to pressurise it, and references to ‘lateral’ are in relation to the longitudinal axis; a lateral plane is perpendicular to a longitudinal axis. The word ‘radial’ may also be used to refer to ‘lateral’ when cylindrical coordinates are being used. ‘Longitudinal’ is not intended to imply or suggest that there are only the two anvils that define it and there may be more than the pair of anvils; it is also not intended to imply or suggest ‘vertical’, and a longitudinal axis as used herein may be vertical, horizontal, or at some other orientation with respect to gravity. Similarly, ‘lateral’ is not intended to imply or suggest ‘horizontal’ with respect to gravity. For example, a belt-type press system will have only two anvils, with lateral support for the capsule assembly being provided by a die, and a cubic press will have six anvils arranged as opposing pairs in cubic symmetry, and no die. Therefore, there are three potential longitudinal axes for a capsule assembly in a cubic press.

Claims

1. A housing envelope for a capsule assembly, which comprises an inner assembly, a pair of end plugs and the housing envelope, and is configured for use with a multianvil high-pressure, high-temperature (HPHT) press;

an envelope wall of the housing envelope defining a longitudinal cavity that extends from a proximal end to a distal end of the housing envelope, the cavity defining a longitudinal axis and being configured for housing the inner assembly and receiving the end plugs into the cavity from the proximal and distal ends; and the envelope wall having at least a pair of opposite lateral external sides, a lateral axis passing through the respective centres of the lateral external sides;

the housing envelope configured for containing the inner assembly when the capsule assembly is installed within the HPHT press and the inner assembly is at an HPHT condition; in which the capsule assembly and the inner assembly have respective longitudinal stiffness along the longitudinal axis, and respective lateral stiffness along the lateral axis; in which the envelope wall is configured such that the relative difference between the longitudinal and lateral stiffness of the capsule assembly is less than the relative difference between the longitudinal and lateral stiffness of the inner assembly, at an ambient condition.

2. A housing envelope as claimed in claim 1, in which the envelope wall is configured such that the difference between the longitudinal and lateral Young’s modulus of the capsule assembly is less than the difference between the longitudinal and lateral Young’s modulus of the inner assembly, along the longitudinal and lateral axes, respectively, at an ambient condition.

3. A housing envelope as claimed in claim 1 or claim 2, in which the multi-anvil HPHT press is a cubic press, and the housing envelope comprises four lateral sides.

4. A housing envelope as claimed in any of the preceding claims, in which the envelope wall is configured such that the magnitude of the difference between the longitudinal and lateral stiffness of the capsule assembly is less than the magnitude of the difference between the longitudinal and lateral stiffness of the inner assembly, at an ambient condition.

5. A housing envelope as claimed in any of the preceding claims, in which the housing envelope comprises ceramic material selected from the group consisting of quartz, garnet, zirconium, magnesium oxide, alumina (AI2O3), sodium oxide (Na2O) and natural (mined), synthetic or reconstituted pyrophyllite, or material phases or compounds arising from heat treating pyrophyllite.

6. A housing envelope as claimed in any of the preceding claims, including housing envelope includes iron oxide (Fe2O3), titania (TiO2), potassium oxide (K2O), sodium oxide (Na2O) and phosphorus pentoxide (PO5).

7. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external sides includes at least one respective recess.

8. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external sides includes at least one respective boss.

9. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external sides have the same configuration as each other.

10. A housing envelope as claimed in any of the preceding claims, in which each lateral external side of the housing envelope has a different configuration to at least one of the lateral external sides adjacent to it.

11. A housing envelope as claimed in any of the preceding claims, in which a lateral internal side of the envelope wall includes one or more recess, or one or more boss, or at least one recess and at least one boss.

12. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external or internal sides of the housing envelope has a recess or boss having a circular, oval, square, or polygonal shape (when viewed from a side).

13. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external or internal sides of the housing envelope has a recess having a depth of 0.5 mm to 6 mm; or a boss projecting a height of 0.5 mm to 6 mm.

14. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external or internal side of the housing envelope has a recess having a depth of 15% - 50% of the thickness of each end plug.

15. A housing envelope as claimed in any of the preceding claims, in which the multianvil HPHT press is a cubic press, and each of the pair of lateral external or internal sides of the housing envelope has a recess or boss, the recessed or embossed area being smaller than the area of the nose of each anvil.

16. A housing envelope as claimed in any of the preceding claims, in which the multianvil HPHT press is a cubic press, and the nose of each anvil is square, and the lateral external sides of the housing envelope are square, and each of the pair of lateral external or internal sides of the housing envelope has a square recess or boss.

17. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external or internal sides of the housing envelope has a recess, the depth of which is such that the mean thickness of the recessed portion of the envelope wall is equal to the thickness of each end plug plus-or-minus 20%.

18. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral external or internal sides of the housing envelope has a recess, the depth of which is such that the mean thickness of the recessed portion of the envelope wall is 4 mm - 5 mm, and the thickness of each end plug is 7 mm - 12 mm.

19. A housing envelope as claimed in any of the preceding claims, in which the mean density of the envelope wall varies substantially monotonically with depth from a lateral internal side to the corresponding lateral external side.

20. A housing envelope as claimed in any of the preceding claims, in which the envelope wall comprises a plurality of different materials, arranged such that the mean density of the envelope wall varies along a lateral axis by at least 10%.

21. A housing envelope as claimed in any of the preceding claims, in which each of the pair of lateral internal sides of the housing envelope has a recess, and a respective plug comprising material that is substantially softer than the material of the housing envelope is accommodated within each recess.

22. A housing envelope as claimed in any of the preceding claims, configured such that when load is applied onto the capsule assembly by the anvils of the multi-anvil HPHT press to pressurise the reaction cell or cells, material of the housing envelope will extrude between side surfaces of the anvils to form a gasket, the thickness of the gasket varying by at most about 10%, for any given distance along the side surface of an anvil from the anvil nose.

23. A method of making a housing envelope as claimed in any of the preceding claims, including:

providing powder of precursor material for the housing envelope;

providing a mould having at least one mould surface configured for forming a lateral side of the housing envelope;

compacting the powder against the mould surface to form a compacted green body, a lateral side of which has substantially the same shape as the corresponding lateral side of the housing envelope; and heating the green body to a temperature and pressure sufficiently high to form the housing envelope.

24. A method as claimed in claim 23, in which the mould has four mould surfaces, arranged as opposing pairs of inner walls of a mould cavity, each mould surface configured for forming respective lateral external sides of the housing envelope; and the method includes:

pouring the powder into the mould cavity, and compacting the powder within the mould cavity.

25. A method as claimed in claims 23 or 24, in which the mould surface includes a boss, configured to form a corresponding recess in the lateral side of the green body.

26. A method as claimed in any of claims 23 to 25, in which the mould surface includes a recess, configured to form a corresponding boss on the lateral side of the green body.

27. A method as claimed in any of claims 23 to 26, in which the dimensions of the lateral side of the green body are within 0.5 mm of the corresponding dimensions of the lateral side of the housing envelope.

28. A method as claimed in any of claims 23 to 27, including processing the green body to remove precursor material from its lateral side to form a recess of at most 0.5 mm into the lateral side of the green body.

29. A method of making a housing envelope as claimed in any of claims 1 to 22, including: providing powder of precursor material for the housing envelope;

uni-axially compacting the precursor material to form a compact body, such that the density of the compact body varies continuously by at least 5% between the uni-axial compaction axis and lateral sides; and processing the compact body to form the cavity.

30. A method of making a housing envelope as claimed in any of the preceding claims, including:

providing a blank body comprising the material of the housing envelope; and processing the blank green body to form the housing envelope.