FI67267C - PHOTOTHERMAL ABSORBER - Google Patents

Description

I55FH M m) KUU.LUtusjumelistus 67267 JHa l J 1 'utlAggningsskmft u' (^) PatenL i ;: idc lr. t, ^ ^ (51) /h eng.ct3 G 02 B 1/10 FINLAND — FINLAND <”> 77263¾ (22) HrifihpIhH- * ΐϋίΙηΐΗ» <Μ 06.09.77 (23) GWrtghitwItf 06.09.77 (41) T « N «|» * lMfcX-MMteffMdH 17.03.78

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Patent) · och registerstyraben '' AmMm ock «u» a «pAfani 31.10.8¾ (32) (33) (31) P | rr» «** r · βΝΛηι · —es *« rd prtorttst 16.09.76 USA (US ) 723857 (71) International Business Machines Corporation, Armonk, NY 1050¾.

USA (US) (72) Jerome John Cuomo, New York, N.Y., Thomas Herman DiStefano,

Bronxville, New York, Jerry MacPherson Woodall, Mt. Kisco, New York, USA (US) (7¾) Oy Kolster Ab (5¾) Photothermal absorbs and - Photothermal absorbers

The invention relates to a photothermal absorber having a minimized reflectivity in a selected wavelength band comprising a photon absorbing body having a surface structure, preferably a rock pile or dendritic structure, so that the light hitting it is reflected many times.

When converting photon energy into thermal energy, the efficiency depends on the ratio of the absorbed part of the photon energy to the radiating or reflecting part of the heat. Metals with good thermal properties absorb or are opaque at substantially all wavelengths, while also reflecting much of the energy applied to them. Highly reflective surfaces generally have both low absorbency and low emissivity. Because of the interrelationship of absorbency and emissivity, multilayer structures have hitherto been developed in the art in which one layer has one desirable property and the other layer has another desirable property. An example of such a structure is disclosed in U.S. Patent No. 3,920,413. However, such structures are subject to structural limitations in that the effect of one layer may interfere with the best advantage of the other. In addition, the fabrication of multilayer structures often involves a lot of processing during fabrication.

The object of the invention is to provide a photothermal absorber which has a substantially higher efficiency in the conversion of photon energy into thermal energy than previously known devices.

This is achieved by a photothermal absorber according to the invention characterized by a reflective surface coating of said body consistent with said surface structure, said coating having a first reflectivity and the boundary layer between said body and the coating having a second reflectivity and wherein said coating has has a specific refractive index value and the photon-absorbing body has a refractive index and an extinction coefficient, both of which have a value such that the combination of the three values gives essentially the same values for said first and second reflectivities.

Other essential features of the invention are set out in claims 2-4.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 shows a schematic view of the optical operation of the invention, Figure 2 shows a photomicrograph of a rock pile type tungsten surface, Fig. 3 shows a photomicrograph of dendritic Figure 6 shows a diagram of the thickness and wavelength of the surface area of tungsten oxide or tungsten at maximum absorption.

When converting light energy into heat, the efficiency can be expressed as:

Equation 1

Efficiency - accented energy - repetitive energy energy-in

Figure 1 shows a schematic view of the effect of the invention on light absorption and reflection. It shows the surface area 1 adjusting the reflection 3 67267 as an optically transparent material for the desired wavelength, the surface 2 of which is parallel to the surface 3 of the photon absorbing material and the thickness 4 of which is proportional to the wavelength of the input light. The optical and physical properties of the reflection adjusting surface area are interrelated, as will become apparent.

By reflection is meant energy that strikes and becomes restored without penetrating the substance as opposed to repetition radiation, where the energy enters the substance and due to the change in the temperature of the substance the substance radiates energy.

In Fig. 1, the light impinging on the surface 2 has a reflectivity component 5 and a series of decreasing successive components, three of which are shown as parts 6, 7 and 8. In operation, the light reflected from the surface 3 increases or decreases when it hits the light returning from the surface 2.

In the following description, the oxide of a metallic photon-absorbing substance is used as an example, although in the light of the principles described, it is clear that coatings other than oxides as well as substances with a different composition than the base metal can be used to achieve the desired properties.

In Figure 1, the first reflection coefficient (part 5) can be expressed as follows:

Equation 2.

1/2 ~ "

Initial reflectance (part 5) = (R ^) '^ ——— o where R.j is the reflectance between air and oxide and N is the refractive index of the oxide.

Similarly, the reflection coefficient (part 6) can be expressed by Equation 3:

Equation 3: “„. "

1 ij -N + N - i K

(Part 6) reflection coefficient = (R „) '~ TJ.m

λ N + N + iK

m o m where R2 is the reflectivity between the oxide and the metal, N is the refractive index of the metal, K is the extinction coefficient of the oxides and i is "V-1.

m Thus, the ratio of the reflection coefficients of parts 5, 6, 7 and 8, etc. is: 67267 s = γί 6 = r2 Π - τλ2) 7 = τ2τλ (1 - τ2) 8 = r23ri2 (1 - r, 2) Thus, the control surface 1 the reflectivity is expressed in Equation 4: Equation 4 47 / e 2 2 total ~ r1 + r2 e 1 ~ r1 '-r1r2 where d is the thickness 4 and λ is the wavelength, and JJ indicates absolute values.

Equation 5 1 2 2 I * r 'r ^ - r2 1 £ desired reflectance with the smallest value of ^ r1r2 λ This is approximately Equation 6 2 £ desired reflectance with the smallest value of r1 Γ2 λ

If the application involves the conversion of solar energy into heat, the desired reflectance at the lowest wavelength (minimum λ) should be less than 0.05.

The desired goal is that it is as small as possible and the reflectivity of surface 2 is almost the same as the reflectivity of surface 3.

The criteria for the radiation regulating surface area 1 for the desired wavelength can be expressed as:

Equation 7 2 1-Nq Nm ”No ~ ^ Km ^ desired reflectance with the smallest 1 + No Nn + No_i Km with λ: η

According to Equation 7, the criteria for the reflection-adjusting surface area work so that the effect of the reflection components of the surface 3 becomes the same as the initial reflection of the input light from the surface 2.

The thickness d (part 4 in Fig. 1) acts in two ways.

It is part of the calculations of Equation 4 determining the desired reflectance at the lowest wavelength / and, as described in connection with Figure 6, it allows the minimum wavelength to be shifted.

In such a relationship, it is apparent that one desired goal is to absorb all radiation in the desired wavelength band, to reflect all unwanted wavelengths, and to keep the desired reflected wavelength energy to a minimum.

This is done according to the invention by using a wavelength-selective, reflection-adjusting surface area on the surface of the photon-absorbing substance, so that the reflectivity, thickness and contour between the air and the region 1; the refractive index of area 1; and the photon-absorbing material index and the extinction coefficient all work together so that the light reflected from the photon-absorbing material becomes shortened.

The best surface shape to choose is rough or shaped so that light that comes perpendicular to the surface must be reflected substantially more than once before it can escape the surface. This rough or shaped surface and the reflection adjusting surface provide an absorption that is greater and covers a larger wavelength band than a simple, non-reflective coating with smooth metal. For example, the reflectivity of the latter is "which varies with wavelength, while the reflectance of a reflective coating on a roughened surface, where light is reflected 2 twice before return, is ^] ζ0] ^ 0η3ΐ5 r j ° ka is less than g total"

The reflection control area is distinguished from the passivating coatings by the fact that the main purpose of the latter is to chemically protect the inert properties and the substances are selected with this in mind.

Figures 2 and 3 show a light micrograph of tungsten surfaces with correspondingly increasing absorption rates. The surface of Figure 2 is called a rock pile surface, which is well known in the art, and the surface of Figure 3 is called a dendritic surface. Both surfaces are made by chemical steam treatment well known in the art. The stone pile structure is much thinner than the dendritic structure, so it is cheaper. The light micrograph shows the magnification. When the reflection adjusting surface area according to the invention 6 67267 is manufactured together with the surfaces of Figs. 2 and 3 and a flat surface (not shown), a sudden reduction in the total reflectance for a given wavelength is selectable according to the above criteria.

This is shown in the diagram of Figure 4, which plots the total reflection of the incoming light perpendicular to the wavelength in microns. Three curves are shown. Dotted line means flat tungsten, dashed line rock aggregate in Figure 2 and intact curved dendritic material in Figure 3. It should be noted that the reflection adjusting surface area of the invention provides an absorption peak in the vicinity of about 0.62 microns. This wavelength is considered by the industry to be at or near the peak of the sun's emissivity.

It can be seen from the logarithmic scale of Figure 4 that when the dendritic material of Figure 3 is provided with a reflection adjusting surface area according to the invention, it absorbs 99.94% of the incoming light at a wavelength of 0.55 microns.

Figure 5 shows the effect of the invention on varying directions of light entering the dendritic surface. It has the total reflectance plotted relative to the wavelength in nanometers with an incident light angle of 0 °, 20 °, 40 °, 60 ° and 80 °, respectively. In each case, the absorption peak occurs at approximately the same wavelength.

According to the invention, the production of the reflection-adjusting surface area according to Fig. 1 is carried out by providing the region 1 material shaped according to the surface shape of the photon absorbing material with desirable parameters which are: the reflection coefficient from the surface 2 material surface 2 is approximately the same as between the photon absorbing material and the region 1 the reflectance of the dividing surface 3. These reflection coefficients are proportional to the refractive index of the substance in region 1, the refractive index of the photon absorbing substance, and the extinction coefficient of the latter. These are well-defined parameters in the art and can be found in most industry manuals. However, in order for one skilled in the art to reduce the experiment, Table 1 gives the values determined for Equations 2 to 7 for the substance W0 ^ used as the reflection adjusting surface area 1 with dendritic W, as shown in Fig. 3.

7 67267

Table 1 w wo3 W - wo3 n 3.43 2.26 --- k 2.96 0.0 --- --- --- 0.386 r2 --- --- 0.496 2 UI - Ir21 - I - ° '12 °

The fabrication of the reflection adjusting surface area 1 is particularly well applicable to processes in which chemical compounds are formed from a photon absorbing material. These use a photon absorber as a single component, form the same contour as the surface contour, and are generally easily adjustable over the desired thickness range of the surface adjustment range. Such processes include, for example, anodic treatment or oxidation, nitriding and carburization. One easily adjustable manufacturing method is anodic treatment, where the photon absorbing agent and the formed region allow it according to the said criteria of the invention. In this method, an oxide is often formed which limits the flow of current so that the thickness of the region corresponds exactly to the voltage. Metals useful according to the invention which form preferred oxides include, for example: W, Mo, Hf, V, Ta and Nb.

In order to facilitate the implementation of the invention, the relationship between the thickness gauge 4 according to Fig. 1 and the anode treatment voltage for the substance W03 on the substance W is indicated in Table 2.

Table 2 Voltage Thickness in volts ^ μm_ 20 0.035 25 0.045 30 0.055 35 0.065 40 0.075 8 67267

The following experimental results of a particular embodiment illustrate the great advantages of the invention. The rock-stack tungsten surface according to Figure 2 was treated anodically in a phosphoric acid bath at a voltage of 30 V. In this case, the WO 2 region stops the anodic reaction at a certain thickness, which is adjusted by the applied voltage. For this surface at 150 ° C, the ratio of "absorbency" to output radiation "compared to" "hemispherical emissivity", i.e. (aL / ζ) 3.9, The efficiency calculated in Equation 1 in the following table compares the efficiency of a plain black body at different temperatures.

Table 3

Efficiency,% Repeat radiation, watts T_Tungsten Black body_Tungsten Black body 50 ° C 80% 32% 0.015 0.063 75 ° C 75% 13% 0.020 0.083 100 ° C 68% 0 0.027 0.1125 150 ° C 51% 0 0.044> 0.1 200 ° C 26% 0 0.069> 0.1

The table shows that efficiencies of more than 50% are achieved at temperatures up to 150 ° C.

An important advantage is that the method of the invention makes it possible to provide a new photon-absorbing agent, since the advantages of an anti-reflective coating can now be applied to substrates having photon absorption properties due to surface irregularities.

Most applications for converting solar energy want to use photon absorbers that absorb more than 90% of the solar spectrum. This result cannot be achieved with flat metal, rough metal, or simple, non-reflective coatings on top of these. In contrast, when the reflection-adjusting surface area according to the invention is used in combination with certain types of shaped or rough metal surfaces, such as tungsten, low reflection over a wide spectral range can be achieved. It has been found that with respect to the solar spectrum, the desired absorption is achieved by means of shaped or roughened surfaces in which the incident light 9 67267 is reflected many times over the surface of the reflection-adjusting layer. In contrast, non-reflective coatings on bonded metals have an absorption that covers only a small portion of the solar spectrum.

Although the invention has been presented in connection with a particular embodiment using anodically treated tungsten, it will be apparent to one skilled in the art that many different embodiments are possible in light of the principles presented.

Claims (4)

10 67267 A photothermal absorber having minimized reflectivity in a selected wavelength band, comprising a photon absorbing body having a surface structure, preferably a rock pile or dendritic structure, so that light impinging thereon is reflected multiple thereof, characterized by a coating surface of said body uniform with wherein said coating has a first reflectivity and the boundary layer between said body and the coating has a second reflectivity, and wherein said coating is a substance having a specific refractive index value and the photon absorbing body has a refractive index and an extinction coefficient both of which have a value such that values for said first and second reflectivities. A photothermal absorber according to claim 1, characterized in that the variations of said first and second reflectances are of the order of 0.05. Photothermal absorber according to Claim 1 or 2, characterized in that the photon absorber is tungsten and the reflection-adjusting surface area is tungsten oxide. Photothermal absorber according to Claim 1 or 2, characterized in that the reflection-adjusting surface area is anodized oxidized tungsten oxide.