Patent Application
20080057204
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
The present invention is directed to various embodiments of a tunable variable emissivity material that can be fabricated to passively reduce the temperature extremes experienced by aerospace systems, such as satellites, high-altitude inflatables, deep-spacecraft, and the like. A material with a high emissivity efficiently radiates heat while a material with a low emissivity radiates less heat. A variable emissivity material is any material that, when heated above a critical temperature Tc, experiences a phase change that alters the material's emissivity. At Tc, the material transforms from a low emissivity metal into a high emissivity semiconductor. As a result of this behavior, the material tends to self-regulate its temperature near Tc, radiating heat at temperatures above Tc and retaining heat below Tc. The material can be applied to any surface where temperature control through radiative emission is required (e.g., satellite radiators).
In accordance with an exemplary embodiment of the present invention, a variable emissivity material can be represented by the following:
M1(1−(x+y))M2xM3yMnO3,
where M1 comprises lanthanum (La), scandium (Sc), yttrium (Y), praseodymium (Pr), neodymium (Nd), or samarium (Sm) and preferably is lanthanum. M2 is an alkali earth metal and M3 is an alkali earth metal that is not M2. In a preferred embodiment of the invention, M2 comprises beryllium (Be) or magnesium (Mg), more preferably Be, and M3 comprises barium (Ba), strontium (Sr), or calcium (Ca), more preferably Ba. The sum (x+y) is less than 1.
The variable emissivity material has a perovskite oxide structure 10, such as that illustrated in
The emissivity change at Tc is accompanied by a magnetic (ferromagnetic-paramagnetic) transition. Accordingly, it can be detected with a Superconducting Quantum Interference Device (SQUID). As the variable emissivity material is heated in the presence of a weak magnetic field, a large drop in the magnetic moment of the material will occur as it undergoes the ferromagnetic-paramagnetic transition.
The critical temperature Tc and the transition width of the variable emissivity material can be selected for or “tuned” to predetermined values by varying the identity and quantity of the alkali earth metal ions M2 and M3. For example,
For example, in accordance with one embodiment of the invention, the variable emissivity material may be La0.5Ba0.25Be0.25MnO3.
The variable emissivity material, in accordance with various embodiments of the present invention, may be formed using any suitable method. For example, the variable emissivity material may be synthesized using ion-beam doping of M1MnO3 films with M2 and M3 atoms, oxidation of vapor-deposited stoichiometric metal films of M1, M2, M3 and Mn, or reaction under mechanically energetic conditions such as ball-milling, jar milling, or hand-grinding of reactant metal oxides M1O, M22O3, M32O3, and MnO3, followed by post-annealed compositions.
(1−(x+y))M1(acac)3+xM2(acac)2+yM3(acac)2+Mn(acac)3→M1(1−(x+y))M2xM3yMnO3,
(step 252), to form a variable emissivity material. The solvent may be any material capable of dissolving the reactants. To form a film, the variable emissivity material may be deposited onto a binder, such as film of poly(vinyl alcohol) (PVA), and dried (step 254). The variable emissivity film then may be heat-treated to further crystallize the product (step 256). Alternatively, if the variable emissivity material is to be in the form of a powder, the material may be mixed with a PVA aqueous solution and dried (step 254). The mixture then is heat-treated to further crystallize the product (step 256). The heat-treated powder then may be mixed with an organic or inorganic binder so that it can be rolled, painted, sprayed, spin-coated, or the like onto a substrate or so that a substrate can be dipped in the mixture (step 258). As illustrated in
The following is an example of the preparation of La0.5Ba0.25Be0.25MnO3 film, in accordance with an exemplary embodiment of the present invention. Lanthanum acetylacetonate (La(acac)3.H20), manganese acetylacetonate (Mn(acac)3), beryllium acetylacetonate (Be(acac)2) and barium acetylacetonate (Ba(acac)2) may be dissolved in a mixed 4:1 solution of propanoic acid and methanol to form a variable emissivity material. To form a film, a PVA aqueous solution (about 15%) may be spin-coated onto a substrate, and the variable emissivity material may be spin-coated on the PVA film, followed by drying at 150° C. for about 10 hours. The resulting film then may be heat-treated in air at 400 to 600° C., preferably 450 to 550° C. To form a powder, the perovskite product may be combined with a PVA aqueous solution until a viscous gel is formed (about 6 wt % PVA) and dried at about 150° C. for about 4 to about 10 hours. The resulting product then may be heat-treated in air at 1000° C. for about 5 hours. The heat-treated powder may be mixed with an inorganic or organic binder so that it can be rolled, painted, sprayed, spin-coated, or the like onto a substrate or so that a substrate can be dipped in the mixture. Suitable binders include potassium silicate, such as Kasil® available from PQ Corporation of Philadelphia, Pa., silicone-modified epoxy, polyurethane, poly(dimethylsiloxane), poly(dimethylsiloxane-co-methylphenylsiloxane), polyamide, Dow Corning® HC 2000, available from Dow Corning Corporation of Midland, Mich., or a combination thereof. For example, 300 g of the heat-treated powder may be mixed with 175 g of Kasil® and 100 g of H2O to form an inorganic-based paint.
Accordingly, a variable emissivity material, a method for fabricating a variable emissivity material, and a method for controlling the temperature of a spacecraft using a variable emissivity material have been presented. The variable emissivity material radiates heat quickly at temperatures above the material's critical temperature Tc and slowly at temperatures below Tc. The Tc and the transition width of the variable emissivity material can be tuned to desired values by the strategic selection of the identity and quantity of the components of the variable emissivity material. While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.
