WO1982000510A1

WO1982000510A1 - Solar energy collector having semiconductive coating formed from metal and dielectric

Info

Publication number: WO1982000510A1
Application number: PCT/US1980/000987
Authority: WO
Inventors: Illinois Inc Owens
Original assignee: Spanoudis L; Emch B
Priority date: 1980-07-28
Filing date: 1980-07-28
Publication date: 1982-02-18
Also published as: AU7174581A; EP0056373A1

Abstract

A solar energy collector and process for preparing it, in which the collector is characterized by a semiconductive coating (12) formed by the co-deposition of a metal (13) and a dielectric material (14) and having a relatively high solar absorptivity and low infrared emissivity. The simultaneous deposition of the metal (13) and dielectric material (14) may be carried out by vapor-vacuum deposition techniques onto a substrate (10) and results in a semiconductive coating (12) of a solid solution of atoms of the metal dispersed in a matrix of the dielectric material. In the preferred practice, the amounts of metal (13) and dielectric material (14) deposited are varied during the co-deposition and desirably in reverse order, so that there are reverse concentration gradients of each component within the coating (12) in which the concentration of metal is the greatest adjacent the substrate.

Description

Solar Energy Collector Having Semiconductive Coating Formed from Metal and Dielectric

Technical Field The present invention relates to a solar energy collector having a semiconductive coating formed by the co-deposition of a metal and a dielectric material and having a relatively high solar absorptivity and low infrared emissivity.

Background Art

Realization that the fossil fuel supply of the world is finite and may be rapidly depleted at the present rate of national energy consumption has led to a search for substitute energy resources . Use of solar radiation is one possibility for providing clean and reliable energy.

Solar energy is an extensive, constant energy source whose economic feasibility depends on efficient collection, retention, and utilization. The efficiency of some solar collecting systems has been low due to excessive heat losses. One area in which improvement has been sought is in solar selective absorber coatings, that is, coatings which absorb energy particularly well in the solar spectrum. For example, such coatings are designed to collect thermal energy from exposure to solar radiation and then transmit the collected energy through other media either to heat or cool homes and buildings through heat exchangers . In general, when radiant energy from the sun impinges on a cooler object, part of the energy is reflected and lost and the balance either absorbed or transmitted away. The absorbed energy may be re-radiated at a longer wavelength. Accordingly, a coating which absorbs in the range of solar radiation becomes heated, provided the surface does not reradiate or emit most or all of the energy collected. Solar radiation reaching the surface of the earth is almost entirely confined to the range of 0.3 to 2.5 microns. It is estimated that about 90% of solar radiation is at wavelengths of about 0.4 micron to about 1.5 microns. The amount of radiation above 2.5 microns is negligible. Solar energy selective coatings, therefore, are designed to differentiate in their absorption, reflection or transmission characteristics between wavelengths above about 2.5 microns and wavelengths below about 2.5 microns. Thus, solar energy can be collected at wavelengths below about 2.5 microns and the collected energy then transferred to useful application at wavelengths above about 2.5 microns .

This also means that for reflective collection and retention, a solar collector should absorb strongly at wavelengths below about 2.5 microns and not radiate at wavelengths greater than 2.5 microns . A coating which has a high absorptivity, usually termed alpha, in the solar spectrum but a low emissivity, epsilon, at the temperature at which the collector operates may be called a solar selective coating. Even though a high alpha to epsilon ratio is desirable, it is essential that the alpha value be near one to collect as much of the available energy as possible. Solar selective coatings are one important way to increase the efficiency of solar energy collectors, primarily by maximizing the absorption of solar energy and minimizing the energy lost by radiation.

There are three fundamental classes of matter in the solid phase:

1. Metals: These are materials in which the highest occupied energy band is only partially filled with electrons. The electrons are highly mobile and the energy gap between the valence band and the conduction band is less than about 0.1 electron volts (eV) . Metals have low emissivities in the infrared.

2. Semiconductors: These are materials in which the highest energy band is only partially filled at absolute zero. The energy gap for semiconductors is of the order of 0.1 eV to about 5 eV. Semiconductors are characterized by high solar absorptivity and, especially in thin layers high infrared transmissivity. 3. Dielectrics or Insulators: These are materials in which the highest occupied energy band is completely filled. Such materials have energy gaps larger than 4 eV. The prior art describes a number of solar selective coatings in which a semiconductor is deposited over a substrate. Typical semiconductors are silicon, germanium, copper oxide, and lead sulfide. Unfortunately, semiconductors have high indicies of refraction which provide high reflectivities at an air or vacuum interface. Disclosure of Invention

An object of the invention is to provide an improved solar energy collector. Another object is to provide a solar element for a collector comprising an improved semiconductive coating. A related object is to provide a semiconductive coating for a solar energy collector that comprises a solid solution of metal atoms dispersed in a matrix of a dielectric material. These and other objects are realized by a solar energy collector which, in one form, comprises a metallic or metallized substrate having an overlying semiconductive coating that is formed by the co-deposition of a metal and a dielectric material. The simultaneous co-deposition of the metal and dielectric material may be carried out by known vapor evaporating techniques under vacuum and results in a semiconductive coating forming a solid solution of atoms of the metal dispersed in a matrix of the dielectric material. The semiconductive coating may comprise by weight from about 5% to about 95% of the metal and from about 5% to about 95% of the dielectric material.

In one aspect of the invention, the amounts of the metal and dielectric material are varied relative to each other during the co-deposition. In this way, the concentration of each can be varied to meet diverse requirements widthwise of the semiconductive coating. Preferably, the amount of metal gradually decreases during the co-deposition and the amount of dielectric material gradually increases to form concentration gradients of the metal and dielectric material in the coating. This results in the greatest concentration of metal being adjacent the substrate and the greatest concentration of dielectric material being adjacent the outer surface of the semiconductive coating remote from the substrate.

Brief Description of Drawings

The details of the invention will be described in connection with the accompanying drawings, in which:

Figure 1 is a fragmentary, highly magnified, cross-section of a solar energy collector of the present invention

Figure 2 is a diagrammatic representation of the collector of Figure 1 and illustrates the reverse concentration gradients of the metal and dielectric material in the semiconductive coating; Figure 3 is a side elevational view, partly in section, of a tubular solar energy collector having a solar absorptive coating of the present invention; and

Figure 4 is a graph of comparative spectral reflectance curves of three solar absorptive coatings , including two of the present invention and one of the prior art, and shows the decreased reflectance obtained by the present semiconductive coatings in the visible solar spectrum.

Best Mode for Carrying Out the Invention and The Preferred Embodiments The relatively high reflectance of certain, known semiconductive coatings are discussed, followed by a description of the present semiconductive coating and its preparation. Examples of solar collectors embodying the present semiconductive coating are described as well as the decrease in reflectance (increase in absorptivity) possible with the present semiconductor in such collectors as compared to a prior, commercially used semiconductive coating.

Semiconductors have been suggested for use as solar selective coatings. While such coatings are, in fact, solar selective, they suffer from high refractive indices. For example, Table A summarizes the optical properties of some known semiconductive materials. The reflectivity was calculated by the Fresnel Equation based on the refractive index of the semiconductors indicated.

This table indicates that a solar selective coating using one of the indicated semiconductors reflects a minimum of 20% of the solar energy received for copper oxide to as much as 37% for lead disulfide. The corresponding absorptivity would be 0.80 and 0.63, respectively (A = 1 - R) . Thus, absorptivity is a numeric although it is sometimes also expressed as a percentage such as, in these instances, 80% and 63%. The value of 0.63 is quite low, while the absorptivity of 0.80 represents about the best possible with prior commercially available semiconductive coatings. Either absorptivity when coupled with radiation, convection and conduction losses can result in a relatively low overall efficiency of a solar energy collector.

The present invention relates to a solar selective or absorptive semiconductor coating for a solar energy collector in which the coating has an improved absorptivity while avoiding the introduction of any deleterious property. Semiconductor coatings of the present invention may have an absorptivity of at least 0.85 to as high as 0.96. The higher value is intended to be exemplary only and not limiting of the present invention.

In accordance with the present invention, the semiconductive coating is formed by the co deposition of a metal and a dielectric material onto a metal or metallized substrate. The codeposition takes place in proportions rendering the coating semiconductive and forming a solid solution of atoms of the metal dispersed in a matrix of the dielectric material.

In a desired practice of the invention, an outer portion of the semiconductive coating, that is, a portion remote from the substrate, contains more of the dielectric material than the metal; while an inner portion of the semiconductive coating adjacent the substrate contains more of the metal than the dielectric material. Preferably, the semiconductive coating contains the metal and dielectric material in reverse gradients. More particularly, the concentration of the metal in the semiconductive coating gradually increases in a direction from an outer surface thereof toward the substrate; and the concentration of the dielectric material in the semiconductive coating gradually increases in a direction from the substrate toward the outer surface of the semi conductive coating.

Expressed in another manner, the present semiconductive coating comprises a material which, in the preferred form of the invention, has a graded energy gap extending from a relatively high electron volt value adjacent the outer surface of the coating to a relatively low electron volt value adjacent the substrate on which the semiconductive coating lies. The term "energy gap" refers to the energy difference between the electron bands which concentrically surround the nucleus of an atom. Metals have relatively low or small energy gaps since their highest energy bands are only partially filled. Electrons are loosely held by these bands and are easily transported. Dielectric materials have relatively high energy gaps since their highest energy bands are completely filled. Accordingly, as one approaches a relatively high energy gap, the electrons of the outer bands become less free to move about and therefore materials comprising such atoms become more and more like an insulator.

Preferably, a semiconductive coating has an energy gap from about 0.5 eV to about 1.24 eV. Metals useful in the present invention are those having a relatively low emissivity and include aluminum, silver, copper, gold, chromium, nickel, molybdenum, tungsten, stainless steel, alloys thereof, and the like. Aluminum, copper, silver, and chromium are preferred for the metal. Dielectric materials useful in the invention include those of Table B in which energy gaps in electron volts are given for certain of the dielectrics and reflectivities are state for all of the dieletrics. The reflectivities are calculated as in Table A by the Fresnel Equation. The absorptivities were calculated from the equation A = 1-R, where R represents reflectivity, assuming that all transmitted energy is absorbed.

Preferred dielectric materials include magnesium fluoride, aluminia, calcium fluoride, magnesia, and silica. While specific examples of metals and dielectric materials are given, the number of combinations is limited only by the periodic table, the vapor pressures of the materials, and deposition techniques available.

The metal and dielectric materials are jointly applied to a substrate by standard, known, vacuum evaporation techniques, such as electric resistance heating, election beam, sputtering, and the like, followed by condensation on a substrate to produce a synthesized, semiconductive coating. For example, small reservoirs of each material may be heated in vacuum by electric resistant circuits to evaporate the material and deposit it on an adjacent substrate. The co-deposition of the two materials can vary in any ratio and/or time-space program desired, although it is preferred to deposit them in reverse gradients, as previously described, so that the resulting semiconductive coating has properties approaching those of a metal at its lower surface adjacent the substrate and properties approaching those of an insulator at an upper surface remote from the substrate.

In any event, the resulting semiconductive coating has a relatively high absorptivity in the solar spectrum and a relatively low emissivity at normal operating temperatures of a solar collector. Deposition of the metal and dielectric material can occur either randomly or in a precise constant or varying ratio. In any case, the metal is diluted with the dielectric material so that the resulting coating is a solid solution of metal atoms commingled or dispersed in a matrix of the dielectric material. The resulting semiconductive coating has the optical properties of a semiconductor, that is, relatively high solar absorptance and relatively high infrared transmittance. But in addition, the present semiconductive coating therefore possesses the reflective properties of a dielectric.

The semiconductive coating may comprise from about 5% to about 95% of the metal and from 5% to about 95% of the dielectric material by weight.

In the present invention the substrate can be composed entirely of one of the indicated metals. Or the substrate can comprise a non-metallic substrate that is conventionally covered or metallized by one of such metals. A typical non-metallic substrate that can be used in glass, especially for tubular collectors as hereinafter described, the composition of the glass is not critical and may comprise a soda-lime glass or a borosilicate glass. Other non-metallic substrates that may be used include porcelain, refractory materials, organic polymeric materials, and the like. Figure 1 illustrates a collector system that can contain the present solar selective semiconductive coating. Figure 1 semi-schematically shows a panel that is part of a flat plate. In this embodiment, a non-metallic substrate 10, which can be glass, plastic, ceramic, and the like has a metallized film 11. If the substrate is composed entirely of metal, it effectively replaces strata 10 and 11. A semiconductive coating containing co-deposited metal and a

' dielectric material in accordance with the present invention is generally represented at 12 and overlies the metallized substrate. The relative thicknesses of the layers of coatings have no significance in any of the figures and are for purposes of illustration only.

Figure 2 diagramatically illustrates reverse concentration gradients of the metal and dielectric material which are present in a preferred embodiment of the invention. Using the flat plate solar collector of Figure 1 as a model, there are shown in Figure 2 the non-metallic substrate 10, its metallized film 11, and semiconductor coating 12. The content of coating 12 is graphically indicated by the crossing lines 13 and 14 in which line 13 represents the content of the metal and line 14 represents the content of a dielectric material. Arbitrarily taking the left hand margin of the diagrammatic representation of Figure 2 as indicating zero or substantially zero content and taking the right hand margin as indicating a maximum or substantially maximum content represented by X, it will be noted that the metal content, as indicated by line 13 , increases from a minimum content at a point adjacent the outer surface of coating 12 to a maximum value at a point adjacent the metallized coating 11. Conversely, the dielectric content, as indicated by line 14 decreases from a maximum content at a point adjacent the outer surface of coating 12 to a minimum value at a point adjacent the metallized coating 11. Figure 3 illustrates a double-wall, glass tubular collector of the type described in U.S. Patent 4,033,327, to Y. K. Pei . This embodiment includes a collector generally represented at 15 comprising concentric, transparent glass tubes. An outer or cover tube 16 is circumferentially transparent, open at the right hand end, as viewed in Figure 3, and closed at the opposite end when tipping off the tubulation as at 17. The open end of cover tube 16 is sealed to an inner glass absorber tube 18 by a glass-to-glass hermetic seal at 19. The sealed space between the tubes

16 and 18, respectively, is evacuated to a hard vacuum (e.g. 10 ^-4 to 10^-6 Torr) and the tubulation 17 is sealed off in a known manner.

Absorber tube 18 is preferably made of glass and has a less outside diameter and slightly greater length than the inside diameter and length, respectively, of cover tube 16. Tube 18 is closed at end 18a and opened at opposite end 18b. Prior to assembly, the exterior peripheral glass surface of absorber tube 18 is coated with the energy absorbing, solar selective layer of the present invention which is illustrated in Figure 2 by the shaded area 20. In one example of such a tubular collector, a central feeder tube 21 of smaller diameter than tube 18 may be inserted into open end 18b of the absorber tube to extend longitudinally of concentric tubes 16 and 18 to a point near the closed end 18a of absorber tube 18. End 18a nests within a coiled spring 22 which resiliently retains that end of tube 18 in place. The open end 18b of the absorber tube together with the open end of feeder tube 21 seat in a sealed relation within a manifold (not shown) which carries a number of collector tube assemblies like that illustrated by Figure 3. In operation, a fluid circulates from the manifold, through tube 21, the annular area between tube 21 and tube 18 and then back to the manifold to remove heat energy collected in collector 15. Another example of a fluid circuit in an operating tubular collector of this type is shown in U.S. Patent 4,120,285.

The operation of the present semiconductive coating is the same as for other corresponding semiconductive coatings in known solar energy collectors. In general, the semiconductive coating absorbs energy directly from solar radiation. In the present invention, the semconductive coating also has higher absorptivity and lower reflectance than prior semiconductor coatings and therefore helps retain the absorbed energy and reduce radiation losses as well.

The following examples only illustrate the invention and should not be construed as imposing limitations on the claim.

EXAMPLES 1 THROUGH 4 These examples illustrate four different operations for co-depositing a semiconductive coating of the invention in which the metals used were silver, aluminum, chromium, and copper, respectively, and the dielectric material was always magnesium fluoride.

In each example, a glass substrate was used measuring 50.8mm by 50.8mm by 3.17mm and comprising a borosilicate glass. Each substrate has been previously metallized with aluminum in a standard manner to a film thickness of about 1,000 angstroms. Each substrate in its turn was placed in a bell jar along with a tungsten boat in the case of Examples 1, 2 and 4. The boat containing the metal is in powder or pellet form and was adapted for heating by electric resistance circuitry. In the case of Example 3, a coil filament of tungsten metal was used and chromium evaporated from it by resistance heating. A hearth carrying a supply of magnesium fluoride was included within the bell jar for each run.

In operation, the bell jar was evacuated to 2 x 10^-5 Torr and evaporation of the metal then begun. When evidence of some metal deposition on the substrate was registered, as by a quartz crystal monitor, co-deposition with magnesium fluoride was begun by subjecting the magnesium fluoride on the hearth to an electron beam gun (EB) . At this point the deposition of the metal was slowly phased out, and the deposition of the magnesium fluoride was maintained at least at a constant value and optionally could be slowly increased after which its deposition was also terminated. The total elapsed time of the deposition for each example was about three minutes. The total coating thickness of the resulting semiconductive coating including both the metal and the magnesium fluoride was about 2,000 angstroms. Table C summarizes the results of these examples in which Example 2 provided an absorptivity of 0.961.

Figure 4 compares the reflectance spectra of the semiconductive coatings of Examples 2 and 3 with a prior semiconductive coating comprising black chrome, CrOx, over an aluminized substrate. Figure 4 includes reflectance spectra from a wavelength of about 350 nanometers to about 2500 nanometers. Of this range a wavelength of about 350 nanometers to about 700 nanometers represents the visible.

The superiority of the present semiconductive coatings, Curves 2 and 3 in Figure 4 over the black chrome semiconductive coating, Curve 1, in affording reduced reflectance is clearly evident in the range of about 350 nanometers to about 1,000 nanometers. In particular, it will be noted that the present semiconductive coatings in the wavelength of about 600 to about 700 nanometers decreases the reflectance from approximately 28% to a range of about 2% to about 4%. The improvements in absorptivity are from 0.77 for the black chrome semiconductive coating to 0.93 and 0.96 for the coatings of Examples 3 and 2, respectively. EXAMPLE 5

A substrate of borosilicate glass measuring

50.8mm by 50.8mm by 3.17mm was coated by the vacuum deposition of aluminum, using a filament evaporation technique known in the art, until a metallized film having a thickness of about 1500 angstroms was developed. A charge of 0.3 gram of aluminum was placed in an alumina coated tantalum boat, and a charge of 0.3 gram of magnesium fluoride was placed in a tungsten boat. The metallized substrate and two boats were then placed inside a bell jar with the two boats electrically connected in parallel.

The aluminum and magnesium fluoride were then simultaneously co-evaporated and co-deposited onto the aluminum metallized substrate under the following conditions:

Pressure: 7 x 10⁵ Torr

Maximum current: 320 amperes, alternating current Electrical potential: 20 volts

Distance from boats 45.7cm to substrate:

The electrical power was raised slowly to the maximum indicated in about 90 seconds and held there for an additional 30 seconds. The thickness of the resulting synthesized semiconductive coating was about 2,000 angstroms. The absorptivity of this coating calculated from its reflectance curve using

ASTM method E-424-71 was 0.913.

EXAMPLE 6

A procedure was carried out like the procedure of Example 5 with the same materials within a bell jar with these exceptions: Weight of aluminum: 0.2 gram

Weight of magnesium fluoride : 0.2 gram

Maximum current: 325 amperes, alternat ing current

The resulting semiconductive coating had a thickness of about 1200 angstroms and an absorptivity of 0.839.

Although the foregoing describes several embodiments of the present invention, it is understood that the invention may be practiced in still other forms within the scope of the following claims .

Claims

1. In a solar energy collector having a semiconductive coating, the improvement comprising a semiconductive coating formed by the co- deposition of a metal and a dielectric material.

2. A solar energy collector comprising a metallic substrate, and a solar absorptive coating on the substrate, said coating comprising a co- deposited metal and dielectric material in. proportions rendering said coating semi conductive and forming a solid solution of atoms of said metal dispersed in a matrix of said dielectric material.

3. The solar energy collector of claim 2 in which said substrate is composed substantially entirely of a metal.

4. The solar energy collector of claim 2 in which a surface of said substrate is metallized to form said metallic substrate.

5. The solar energy collector of claim 2 in which the metal of said metallic substrate has a relatively low emissivity.

6. The solar energy collector of claim 2 in which said semiconductive coating has an absorptivity of at least 0.85.

7. The solar energy collector of claim 2 in which said semiconductive coating comprises from about 5% to about 95% of said metal and from about

5% to about 95% of said dielectric material by weight.

8. The solar energy collector of claim 2 in which an outer portion of said semiconductive coating remote from said substrate contains more of said dielectric material than said metal, and an inner portion of said semiconductive coating adjacent said substrate contains more of said metal than said dielectric material.

9. The solar energy collector of claim 2 in which the concentration of said metal in said semi conductive coating gradually increases in a direction from an outer surface thereof toward said substrate, and the concentration of said dielectric material in said semiconductive coating gradually increases in a direction from said substrate toward said outer surface of the semiconductive coating.

10. The solar energy collector of claim 2 in which said metal is selected from the group consisting of aluminum, silver, copper, gold, chromium, nickel, molybdenum, tungsten, stainless steel, and alloys thereof.

11. The solar energy collector of claim 2 in which said dielectric material is selected from the group consisting of magnesium fluoride, magnesium oxide, silicon dioxide, aluminum oxide, calcium fluoride, calcium oxide, strontium oxide, barium oxide, and titanium oxide.

12. The solar energy collector of claim 2 in which said collector comprises a flat plate solar collector.

13. The solar energy collector of claim 2 in which said collector is tubular, and said solar absorptive coating is on an outer surface of a tubular absorber element of said collector.

14. The solar energy collector of claim 2 in which said solar absorptive coating comprises a material having a graded energy gap extending from a relatively high electron volt value adjacent the outer surface of said coating to a relatively low electron volt value adjacent said substrate.

15. A wavelength selective solar energy collector comprising :

a) a metallic substrate and b) a coating of a semiconductive material characterized by a relatively high solar absorptivity and low infrared emissivity overlying the metallic substrate, said coating comprising co-deposited metal and a dielectric material in proportions rendering said coating semiconductive and forming a solid solution of atoms of said metal dispersed in a matrix of said dielectric material, said coating contain ing said metal and dielectric material in reverse concentration gradients widthwise of said coating adjacent said substrate.

16. In a process for forming a solar energy collector having a semiconductive coating, the improvement comprising forming such semiconductive coating by the co-deposition of a metal and a dielectric material.

17. A process for forming a solar energy collector comprising simultaneously co- depositing on a metallic substrate a metal and a dielectric material, and controlling their relative proportions during such co- deposition to form a solid solution of atoms of said metal dispersed in a matrix of said dielectric material and define a coating having semiconductive properties.

18. The process of claim 17 comprising varying the amounts of said metal and dielectric material that are deposited relative to each other during said co-deposition step.

19. The process of claim 17 comprising gradually decreasing the amount of metal deposited and simultaneously gradually increasing the amount of said dielectric material deposited during such co-deposition to form concentration gradients of said metal and dielectric material widthwise of such semiconductive coating.

20. The process of claim 17 in which said metal and dielectric material are co-deposited to form a semiconductive coating comprising by weight from about 5% to about 95% of said metal and from about 5% to about 95% of said dielectric material.

21. The process of claim 17 in which said metal is selected from the group consisting of aluminum, silver, copper, gold, chromium, nickel, molybdenum, stainless steel, and alloys thereof.

22. The process of claim 17 in which said dielectric material is selected from the group consisting of magnesium fluoride, magnesium oxide, silicon dioxide, aluminum oxide, calcium fluoride, calcium oxide, strontium oxide, barium oxide, and titanium dioxide. AMENDED CLAIMS

(received by the International Bureau on 23 September 1981 (23.09.81))

1. In a solar energy collector having a semiconductive coating, the improvement comprising a semicoiiductive coating formed by the co- deposition of a metal and a dielectric material.

5% to about 95% of said dielectric material by weight.

15. A wavelength selective solar energy collector comprising :

a) a metallic substrate and b) a coating of a semiconductive material characterized by a relatively high solar absorptivity and low infrared emissivity overlying the metallic substrate, said coating comprising co-deposited metal and a dielectric material in proportions rendering said coating semiconductive and forming a solid solution of atoms of said metal dispersed in a matrix of said dielectric material, said coating containing said metal and dielectric material in reverse concentration gradients widthwise of said coating adjacent said substrate.

22. The process of claim 17 in which said dielectric material is selected from the group consisting of magnesium fluoride, magnesium oxide, silicon dioxide, aluminum oxide, calcium fluoride, calcium oxide, strontium oxide, barium oxide, and titanium dioxide. (New) 23. A solar energy collector having a semi-conductive coating, the coating comprising an inner surface portion of substantially all aluminum, a central portion on one side of the inner surface portion comprising a solid solution of aluminum and magnesium fluoride with a greater concentration of aluminum near the surface portion, and an outer surface portion on the other side of the central portion containing substantially all MgF₂ having no distinct interface between the outer surface portion and the central portion and no distinct interface between the inner surface portion and the central portion.

(New) 24. A solar energy collector having a semi-conductive coating, coating comprising (1) an inner surface portion consisting of substantially all aluminum, (2) a central portion comprising a first portion containing MgF₂ in aluminum next to the inner surface portion, said central portion comprising a second portion adjacent the first portion containing a solid solution of aluminum and MgF₂ with a greater concentration of aluminum near the first portion, said central portion further comprising a third portion adjacent the second portion and containing a solid solution of aluminum in magnesium fluoride in which there is less aluminum than in the second portion, and (3) an outer surface portion consisting of substantially all MgF₂; there being no distinct interface between the inner surface portion and the central portion and no distinct interface between the central portion and the outer surface portion.