(Zinc, Cadmium) Aluminate-Gallate-Containing Organic-Binder Paint and Film Articles

This invention relates to passive thermal control articles, and, more particularly, to paint and freestanding film articles useful in spacecraft thermal control applications.

Spacecraft are subjected to a wide range of thermal environments during service. One side of the spacecraft may face free space, while the other side faces the sun. Heat is radiated into free space to cool the spacecraft, but the spacecraft can be heated intensively in direct sunlight.

Active and passive temperature control techniques are used to maintain the interior temperature of the spacecraft, which contains persons or sensitive instruments, within acceptable operating limits. Active temperature control usually involves machinery or electrical devices, such as electrical heaters and heat pipes. The present invention deals with passive temperature control.

An established approach to passive temperature control is the use of surface coatings, typically termed "paints", on the external surface of the spacecraft, or solar films (sometimes termed "blankets") overlying the external surface of the spacecraft. A white paint or film, for example, has a low solar absorptance, while a black paint or film has a high solar absorptance. The selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature. Solar films function similarly, except that they are freestanding articles that are bonded to or suspended over a surface. The present invention deals with a white paint and a white solar film that are useful in spacecraft temperature control applications.

In most cases, the paint or film article must dissipate electrostatic charges that develop on the external surface of the spacecraft, as well as provide passive thermal control. The charges would otherwise accumulate to cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft. In order to dissipate electrostatic charge, the paint or film article must be somewhat electrically conductive, with a surface resistivity on the order of about 10.sup.9 ohms per square or less.

A spacecraft paint, whatever its color, should exhibit additional characteristics for spacecraft applications. The paint should be stable during long-term service in a space environment. The paint is desirably moderately tough and flexible so that it does not crack and flake away as it is flexed due to mechanical or thermal strains.

There are white, electrostatic-dissipative, organic-binder paints known for spacecraft use. The best of the known organic-binder paints, based upon the aluminum-doped zinc oxide pigment of the type disclosed in U.S. Pat. No. 5,094,693, whose disclosure is incorporated by reference, typically has a solar absorptance of from about 0.18 to about 0.22. No white, electrostatic-dissipative paints of lower solar absorptance are available. The lower the value of the solar absorptance, the lower the heating of the paint and thence the underlying substrate, in intense direct sunlight.

There is a need for improved white thermal-control paint and film articles that are operable and stable in a space environment, which have a low solar absorptance, and which can manage electrostatic discharge (ESD). The present invention fulfills this need, and further provides related advantages.

The present invention provides white, particulate-containing paint and film articles. The particulate material is mixed with an organic binder and with a paint vehicle to form a liquid paint, which is then applied to a substrate and dried, or is mixed with an organic binder and processed to form a film. The resulting paint and film articles have a lower solar absorptance than any other paint or film articles suitable for spacecraft applications. The pigment, paint, and film may be made in an electrically conductive form that dissipates electrostatic charge on the painted surface. The paint and film of the invention reach the limit of optical performance for passive white thermal control paints with organic binders.

In accordance with the invention, an article comprises a mixture of pigment particles and an organic binder. Each particle has a composition of A›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D), wherein A is selected from the group consisting of zinc and cadmium, D is a cationic dopant having an ionic valence greater than +2, the value of x is from 0 to 1, and the value of .delta. is from 0 to about 0.2. A is most preferably zinc. The dopant D, where present, is preferably indium, supplied as indium oxide.

The solid particle composition A›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D) is a shorthand notation for a range of doped and undoped particle compositions extending between AAl.sub.2 O.sub.4 and AGa.sub.2 O.sub.4, which range includes the end points and the compositions therebetween. These undoped compositions, measured in an organic paint binder, have solar absorptances a of about 0.14-0.17, well below the solar absorptance available in any other organic-binder paint suitable for spacecraft use. Varying the composition of the particles (by varying x) allows the index of refraction of the particle to be changed from a lower value when x is 1 to a higher value when x is 0. The index of refraction in part determines the "hiding power" or opacity of the paint. The ability to vary the index of refraction of the particle allows the selection of the hiding power according to the application.

Additionally, the composition may be doped with small amounts of cationic dopants D, such as indium (In), having a valence of greater than +2, which impart electrical conductivity to the composition. The notation (.delta.D) as used herein indicates an addition of a small concentration .delta. of a dopant element D into the composition, usually as an oxide, and which substitutes for some of the zinc or cadmium in the spinel crystal structure.

The dopant D, when used, is normally provided in the form of an oxide. The particle composition may be equivalently expressed as (A,.delta.D)›xAl (1-x)Ga!.sub.2 O.sub.4+.gamma., wherein the value of x is from 0 to 1, the value of .delta. is less than about 0.2 (i.e., 20 atomic percent), and the value of .gamma. is less than about 0.2. This notation also emphasizes the fact that the dopant D, preferably indium, substitutes for zinc or cadmium atoms and preferentially resides on the zinc and/or cadmium lattice sites. The beneficial results are not achieved if the dopant D of +3 valence, such as indium or other elements of similar size, resides on the aluminum or gallium lattice sites. However, beneficial results may also be achieved if a dopant resides on the (Zn,Cd), Al, or Ga sites when the dopant has a +4 valence or greater, as, for example, in the case of Ti.sup.+4, which may reside on the Al and/or Ga sites. The quantity .gamma. indicates additional small amounts of oxygen that may be added to the composition with the dopant D, as when indium is added as indium oxide. This small addition of oxygen does not-adversely affect the operability of the invention, and may be beneficial in reducing the tendency for formation of color centers in the material.

The particle compositions A›xAl (1-x)Ga!.sub.2 O.sub.4 have a normal (as distinct from inverse) spinel crystal structure, whether undoped or doped with the small amount of the dopant D. Within the range covered by this formulation, the indicated elements may be varied or minor amounts of other elements may be substituted for the indicated elements in a substitutional solid solution. For example, when the dopant indium is added in the form of indium oxide, the oxygen content increases a small amount above the nominal O=4 value, which is acceptable as long as the structure remains a spinel or spinel solid solution. In another example, a solute element may be substituted for the zinc, aluminum, or gallium, as long as the composition remains a single-phase, normal spinel crystal structure. That is, subject to the indicated constraints, small solid-solution variations away from the indicated nominal composition A›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D) are within the scope of the invention.

The solid particulate material may be mixed with a binder such as an organic silicone, initially in a flowable liquid composition but subsequently in a solid composition after curing and/or drying. The binder binds the particles together into a coherent mass, as for a paint bonded to the surface of a substrate article or a freestanding film. For use as a paint, the particulate and binder are initially mixed with a liquid paint vehicle, which serves to allow the mixing and applying of the paint to the substrate and thereafter dries and evaporates to leave the paint coating mixture of particles and binder adhered to the substrate.

The solid particulate material may instead be mixed with a film-forming binder such as a polyimide, a polyester, polytetrafluoroethylene, or Mylar and formed into a plastic-like freestanding polymer film having the desired solar absorptance properties. This polymer film may be applied to or suspended above the surface of the substrate for thermal control, instead of painting. Significantly, the present approach allows the film itself to be made with a white color and in a thickness that is typically about 0.002-0.003 inches. In prior approaches to the making of white films for spacecraft use, a white paint having a thickness of 0.004-0.005 inches was applied to a film substrate that was 0.002-0.003 inches thick, for a total of 0.006-0.008 inches thick. The present approach reduces the thickness and weight of the white film to about 1/3-1/2 of the thickness and weight attained with the prior approach, while achieving superior thermal properties.

The present invention provides an important advance in the art ofthermal-control paints, films, and other articles. The pigment used in the paint is white and has a very low solar absorptance. It may be made either as an electrical insulator or as an electrical conductor with sufficiently low electrical resistivity to dissipate electrostatic charges. The paint prepared with organic binders is easily applied and is very smooth after drying. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.

FIG. 1 is a side elevational view of a paint layer according to the invention;

FIG. 2 is a side elevational view of the paint layer of FIG. 1, as applied to a substrate;

FIG. 3 is a side elevational view of a freestanding film according to the pigment of the invention;

FIG. 4 is a block diagram of a method for the preparation of a white paint according to the invention and the painting of a substrate;

FIG. 5 is a graph of the solar energy absorption by ZnO powder and by ZnAlGaO.sub.4 powder; and

FIG. 6 is reflection spectra of organic-binder paint specimens using ZnO and ZnAlGaO.sub.4 particles.

FIG. 1 illustrates a paint layer 20 prepared according to the invention. The paint layer 20 comprises particles 22 mixed with a binder 24. (The particles are also sometimes referred to as a "pigment".) Prior to drying, a paint vehicle is also present, but the paint vehicle is evaporated during the drying operation. The composition of the particles, the binder, and the paint vehicle, their proportions, and the preparation of the paint layer will be described in greater detail subsequently.

FIG. 2 illustrates the paint layer 20 applied to a substrate 26.

FIG. 3 illustrates a polymer film 28 having particles 22 dispersed therein. The polymer film is prepared as a freestanding article which may be applied to a surface, placed above a surface, or affixed to a surface as with hook-and-loop fasteners.

FIG. 4 depicts a preferred method for preparing the particles 22, for preparing the paint used in the paint layer 20 and the polymer film 28, and for painting the substrate.

To prepare the pigment or particles 22, the components are provided and mixed together, numeral 40. In the most general form, the particles have a composition stated by A›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D). wherein A is zinc or cadmium, D is a cationic dopant having an ionic valence greater than +2, the value of x is from 0 to 1, and the value of .delta. is from 0 to about 0.2 (i.e., 20 atomic percent). The dopant D, where present, is preferably indium, supplied as indium oxide. The composition of the particles is selected from this formulation, and may be any composition within this range.

Preferably, A is zinc, and the application of the invention will be discussed primarily in terms of this preferred embodiment. In this preferred embodiment, if x and .delta. are both 0, the composition is the undoped ZnGa.sub.2 O.sub.4, a material termed a zinc gallate If x is 1 and .delta. is 0, the composition is the undoped ZnAl.sub.2 O.sub.4, a material termed a zinc aluminate If x is between 0 and 1, and .delta. is 0, the composition is the undoped Zn›xAl (1-x)Ga!.sub.2 O.sub.4, a material termed a zinc aluminate-gallate. Indium-doped versions of all of these compositions may be made by making .delta. nonzero, but no greater than the maximum value of about 0.2 indicated above.

The compositions of the form Zn›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D) are of the normal spinel crystal structure and are solid solutions based upon the end point compositions ZnGa.sub.2 O.sub.4 and ZnAl.sub.2 O.sub.4. In the normal spinel structure, generally notated as AB.sub.2 O.sub.4, oxygen anions form a face-centered-cubic close packed structure, with the zinc atoms in the tetrahedral A sites and the aluminum and/or gallium atoms in the octrahedral B sites. This composition range is chosen because of the properties achieved. The compositions are white in color with very low solar absorptances. Zinc is selected as the A-site cation in the particle composition of the invention, rather than other cations such as magnesium, because the resulting structure has a large band gap and because the resulting structure may be doped with small amounts of indium or other dopants to make the composition sufficiently electrically conductive to permit dissipation of electrostatic charges on the surface of the spacecraft. The aluminum and gallium cations are selected as the B-site cations in the particle composition.

The zinc aluminates, zinc gallates, and zinc aluminate-gallates comprise a range of compositions of a very few known pure white normal spinels which can be doped to achieve electrical conductivity. Another, but less preferred, such normal spinel is of the form Cd›xAl(1-x)Ga!.sub.2 O.sub.4 (.delta.D), where x, .delta., and D are as discussed above, and this normal spinel is also within the scope of the invention. A number of other normal spinels have been evaluated and excluded from the present invention. All of the following normal spinels are not white, and instead are colored, rendering them unacceptable as particles in the low-solar absorptance white paint: ZnFe.sub.2 O.sub.4, CdFe.sub.2 O.sub.4, ZnCr.sub.2 O.sub.4, CdCr.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, MnAl.sub.2 O.sub.4, and NiAl.sub.2 O.sub.4. The normal spinel MgAl.sub.2 O.sub.4 is white in color, but it cannot be readily doped to increase its electrical conductivity for electrostatic charge dissipation, because the magnesium cation is highly stable in the +2 ionization state.

The Zn›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D) compositions exhibit a lower solar absorptance over a wider wavelength range than do prior paint particles, as illustrated in FIG. 5. The energy band gaps of the Zn›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D) normal spinels are greater than about 4.0 eV (electron volts). By comparison, the energy band gap of the ZnO wurtzite structure used in prior pigment formulations in U.S. Pat. No. 5,049,693 is about 3.2 eV. Consequently, the normal spinels of the invention reflect light with a very low solar absorptance much deeper into the ultraviolet range than do paints based upon ZnO pigments, such as described in U.S. Pat. No. 5,094,693. The band gap of ZnO may be increased by doping with aluminum, gallium, or indium, but only very slightly. Increasing additions of aluminum and/or gallium to ZnO result in the transformation back to an insulating phase such as ZnAl.sub.2 O.sub.4.

The undoped Zn›xAl (1-x)Ga!.sub.2 O.sub.4 normal spinel compositions normally do not provide sufficient electrical conductivity to dissipate surface electrostatic charges that may be present on the surface of the painted substrate, because they are insulators. To increase the electrical conductivity (or, alternatively stated, reduce the electrical resistivity), the composition may be reduced in a hydrogen-containing atmosphere or doped with a semiconductor material. The reduction in a hydrogen-containing atmosphere is not preferred, because it produces color centers that color the material so that it is no longer white. If the composition is to be doped, it is doped with a cationic dopant having a valence of +2 or greater. Preferably, the doping is with indium to produce a composition Zn›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.In), where .delta. is less than 0.2 (i.e., 20 atomic percent). As used herein, the terminology (.delta.In) means that the composition is doped with a small amount .delta. of indium.

The doping is preferably accomplished by adding indium oxide, which also adds an inconsequential amount of oxygen to the composition. The composition may then equivalently be expressed as (Zn,.delta.In)›xAl (1-x)Ga!.sub.2 O.sub.4+.gamma., emphasizing the fact that the indium substitutes for zinc atoms on the A-sites of the normal spinel structure. In this expression of the formulation, the .gamma. quantity, which is also about 0.2 or less, reflects the fact that for preferred formulations the indium is added as the oxide In.sub.2 O.sub.3, and the oxygen in this compound adds a small increment to the oxygen content of the spinel. This small addition does not adversely affect the properties of the spinel and in fact may be beneficial by reducing the incidence of color centers in the particles. Indium is chosen as the dopant because the In.sup.+3 ion in four-coordination with oxygen has a size of 76 picometers. The size of the Zn.sup.+2 ion in four-coordination in the spinel is 74 picometers, nearly the same. The indium ion therefore preferentially substitutes for the zinc ion in the spinel. The presence of the +3 valence indium ion in place of some +2 valence zinc ions in the spinel structure forces the zinc to accommodate an extra electron in order to preserve charge neutrality, resulting in an electrically conductive, n-type semiconductor spinel. Other dopants D meeting the indicated limitations, such as Ti.sup.+4 (having an ionic diameter of 74.5 picometers in six-coordination) may be used and provided as TiO.sub.2, for substitution on the Al or Ga sites.

There may also be minor substitutions for the zinc, aluminum, and gallium cations in the formulation, as long as these substitutions result in a single phase, solid solution material having the normal spinel crystal structure. For example, cadmium may be substituted for a portion of the zinc, producing (Zn,Cd)›xAl (1-x)Ga!.sub.2 O.sub.4 (.delta.D)..

The components of the particles are provided and mixed together, numeral 40. In the preferred formulation procedure, readily available components ZnO, Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3, and In.sub.2 O.sub.3 are used as starting materials. Thus, to prepare ZnAl.sub.2 O.sub.4, the appropriate amounts of ZnO and Al.sub.2 O.sub.3 are mixed together. To prepare ZnGa.sub.2 O.sub.4, the appropriate amounts of ZnO and Ga.sub.2 O.sub.3 are mixed together. To prepare Zn›xAl (1-x)Ga!.sub.2 O.sub.4, the appropriate amounts of ZnO, Al.sub.2 O.sub.3, and Ga.sub.2 O.sub.3 are mixed together. If any of these compositions is to be doped with indium, the appropriate amount of In.sub.2 O.sub.3 is added to the mixture. A mixing medium, which later is removed, may be added to promote the mixing of the components. Preferably, water is used as the mixing medium.

The components and the mixing medium are milled together to form a mechanical mixture, numeral 42. After milling is complete, the mixing medium is removed by evaporation, numeral 44. The dried mixture is fired to chemically react the components together, numeral 46, at a temperature that is preferably in the range of from about 1000.degree. C. to about 1300.degree. C. A preferred firing treatment is 1160.degree. C. for 6 hours, in air. After cooling, the agglomerated mass resulting from the firing is lightly pulverized, as with a mortar and pestle, numeral 48. The resulting particulate has a size range of from about 0.1 micrometer to about 5 micrometers. The preparation of the particulate pigment is complete.

The paint is prepared by providing the particulate material, prepared as described above or otherwise. A binder is provided, numeral 50, to adhere the particles together in the final product. The binder is selected to provide good adherence of the particles to each other and of the particles to the underlying substrate, with acceptable physical properties. The binder must withstand the environment to which the paint is exposed, such as a space environment.

The binder is preferably cross-linked and polymerized dimethyl silicone copolymer, which is flexible and resistant to degradation in ultraviolet (UV) light. This binder is disclosed in greater detail in U.S. Pat. No. 5,589,274, whose disclosure is incorporated by reference. The silicone polymer exhibits a good degree of deformability without cracking, both when the pigment is present at moderate levels and when it is not present. This deformability permits the final solid paint to deform during the bending of the substrate when a thin substrate is used, or to permit the film to deform. The deformability of the binder also improves the resistance of the paint or film to cracking as a result of impacts and the like during service. Other flexible polymeric materials may be used for the matrix, such as silicone-modified epoxy, polyurethane, poly(dimethylsiloxane), poly(dimethylsiloxane-co-methylphenylsiloxane), and polyamide. However, experience has shown that the dimethyl silicone copolymer has the highest resistance to UV degradation, and it is therefore preferred.

The binder is present in an operable amount. In a typical case, the binder is present in an amount such that the ratio, by weight, of the pigment to the binder is from about 3:1 to about 5:1. If the ratio is less than about 3:1, the resulting paint tends to be transparent after drying. If the ratio is more than about 5:1, the critical pigment volume concentration (CPVC) may be exceeded, the paint has insufficient mechanical strength, and the paint falls apart when dried.

The mixture of pigment and binder is ordinarily a solid, and a paint vehicle may be added to form a solution or a slurry that may be applied using conventional painting techniques, numeral 52. The preferred paint vehicle is naphtha or xylene. The amount of the paint vehicle is selected to provide a consistency that will permit application of the paint by the desired approach. For example, application by spraying requires the use of more of the paint vehicle than application by brush or roller.

The paint may instead be applied by a technique where no vehicle is used, and in that case the step 52 is omitted.

The particles, binder, and paint vehicle are mixed together and milled together, numeral 54, to form a liquid paint formulation in which the particles do not rapidly separate. There may be some separation over extended periods of time, but the paint is normally stirred or agitated just before or at the time of application.

The preparation of the paint is complete.

Slight modifications to the approach discussed above are used to form a freestanding polymer film containing the particles. The pigment particles are prepared as described in steps 40-48. Instead of a paint binder, a polymeric film binder material such as a polyimide is supplied. (Other film binder materials such as polyester or polytetrafluoroethylene may be used, as well.) The particles are mixed with the polymer film binder and an appropriate film vehicle solvent, the mixture is formed into a film, sheet or other form by conventional procedures, and the vehicle is removed. The resulting structure is as illustrated in FIG. 3. The film may be applied over a surface by cohesion, adhesion with an adhesive, or covering and mechanically securing. It may also be suspended over the surface using fasteners such as hook-and-loop fasteners. The film imparts to the surface the same low absorptance properties as provided by the paint.

Returning to FIG. 4, the paint is used by providing the substrate 26 to be coated, numeral 56, and cleaning the substrate, numeral 58. There is no known limitation on the type of substrate. The surface of the substrate is cleaned by any operable non-aqueous technique, such as scrubbing and then wiping away residue with an organic solvent.

The paint is applied to the surface of the substrate, numeral 60. At the outset of the application, the surface of the substrate may be primed to improve the adhesion of the paint, but priming is not required in most cases where an organic binder is used.

The paint layer is thereafter applied by any operable technique, with spraying being preferred. As indicated earlier, the amount of paint vehicle present in the paint is selected to permit application by the preferred approach. At this point, the paint is a thin film of a liquid material. Tests with application of the paint prepared by the present approach have demonstrated that the paint layer is much smoother than achieved with prior approaches such as that of U.S. Pat. No. 5,094,693. This smoothness is particularly important when the paint is applied to an aircraft where skin drag must be minimized. It is also important for all applications in order to allow the surface to be readily cleaned.

The paint may also be applied by a plasma spray technique or the like wherein the mixture of pigment and binder is supplied to a heated region such as a plasma and directed toward the substrate. The plasma-heated mixture of pigment and binder strikes the substrate and solidifies thereon.

The paint is dried as necessary to leave a thin film of a solid material, numeral 62. Drying is preferably accomplished at ambient temperature with a 35 percent or greater humidity and for a time of 7 days. Drying removes the paint vehicle by evaporation. Additionally, the drying step may accomplish a degree of curing of any curable components, as where a curable organic binder is used. The paint layer is preferably from about 0.003 to about 0.006 inches thick.

The painting is complete.

The following is a specific example of the preparation of ZnAlGaO.sub.4 pigment, preparation of paint using the pigment, and painting according to the procedure discussed above in relation to FIG. 4. A mixture of 89.99 grams of ZnO powder, 56.37 grams of Al.sub.2 O.sub.3 powder, and 103.64 grams of Ga.sub.2 O.sub.3 powder is weighed and mixed. The powders are added to a Roalox.TM. #0 jar mill containing a 50 percent charge of 1/2 inch cylindrical end-radius Burundum.TM. milling media A milling aid of 600 milliliters of de-ionized water is added to the jar mill. The jar mill is sealed and the jar is rotated to mill the mixture for 12-24 hours, resulting in a homogeneous mixture. The slurried mixture is removed from the jar mill and placed into a pneumatically stirred container overnight, resulting in drying of the mixture. The resulting ceramic cake is lightly pulverized with a mortar and pestle. The pulverized material is fired in an oven in air at 1160.degree. C. for 6 hours, producing the undoped ZnAlGaO.sub.4 spinel material. The resulting ceramic cake is lightly pulverized in a mortar and pestle, resulting in the powder pigment material.

The pigment material is used to prepare a paint. A mixture of 4 parts by weight of the pigment material prepared as in the preceding paragraph, 1 part by weight of the dimethyl silicone binder, and about 1 to about 3 parts by weight of naphtha or xylene paint vehicle was prepared. This amount of the organic liquid as the paint vehicle is chosen to permit spray application of the paint, as described in the next paragraph, and may be varied as convenient. The mixture is added to ajar mill with a 50 percent charge of Burundum milling media. The mixture is milled for 5-8 hours to form the liquid paint.

The paint is applied by first furnishing a substrate, in this case a sheet of aluminum alloy or graphite/resin composite material. For application of this paint having an organic binder, the sheet is mechanically cleaned with sandpaper or a scouring pad. After scrubbing, any residual particles are removed by wiping with methylethylketone (MEK). Within five minutes after cleaning, the liquid paint formulation is applied by spraying. After the spraying is complete, the coupon is dried and cured at room temperature and 35 percent or greater humidity, for a time of at least 7 days.

The procedure for preparing pigment and paint, and for painting, described in the preceding three paragraphs is repeated, except that about 0.1 weight percent of indium oxide is added to the mixture of powders. The result is a pigment having a composition ZnAlGaO.sub.4 doped with about 0.1 weight percent In.sub.2 O.sub.3. As discussed previously, the oxygen content of the compound is slightly greater than stoichiometric, but that is acceptable within the scope of the invention. The remainder of the procedures are as described above.

Tests have been performed of the optical performance of the paints made according to the invention and of prior paints. The paints of the present invention were prepared as described above. For a silicone-binder paint using undoped ZnAlGaO.sub.4 on a Kapton substrate, .alpha. was measured as 0.142; in a 0.004 inch coating on an aluminum substrate a was measured as 0.157. For comparison, a silicone-binder, ZnO-based paint using particles made according to the '693 patent had a higher .alpha. of about 0.18-0.22. The reflection spectra for specimens of these paints are presented in FIG. 6. The differences in solar absorptance between the paints of the invention and the prior ZnO-base paints are highly significant. The paints of the invention achieve much lower solar absorptances than the prior art ZnO-based paints, so that structures painted with these paints experience far less heating than structures painted with the prior paints.

Specimens were successfully prepared of powder pigments of both stoichiometric and non-stoichiometric ZnAl.sub.2 O.sub.4, of both stoichiometric and non-stoichiometric ZnGa.sub.2 O.sub.4, and of ZnAlGaO.sub.4 (x=0.5 in the general form representation of the composition presented earlier). X-ray measurements demonstrated that the formulation of these powders was successful in preparing the indicated spinel materials.

Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.