Variable emittance thermochromic material and satellite system

Abstract

The emittance value is a measure of an amount of energy expelled from a given surface area relative to a black-body reference. Depending on the specific coating a change in the emittance value is actively or passively effected. There are known active variable emittance thermal control coatings. However, such coatings are actually panels housing a mixture of both high and low emissivity materials that are electrically manipulated to control the emittance value of the panel. These “coatings” are classified as either electrochromic or electrophorectic. Both electrochromic and electrophorectic coatings require an applied voltage to cause a change in the emittance value of the coating. By contrast, aspects of the present invention do not include active variable emittance thermal control coatings. Aspects of the present invention do include passive variable emittance thermal control coatings and materials. In accordance with aspects of the present invention “passive” means that the variable emittance value changes in response to changes in the environment without active control (e.g. neither a voltage nor a current is applied). More specifically, in accordance with one aspect of the invention a passive variable emittance thermochromic material is provided that has a relatively low emittance value at low temperatures and a relatively high emittance value at high temperatures.

1 . A variable emittance thermochromic material comprising:
 spatially isolated particles, wherein the particles are of a transition metal based complex having temperature dependent properties including a range of non-metallic properties at relatively low temperatures including low emissivity, a range of metallic properties at relatively high temperatures and a range of semi-metallic properties, including high emittance, within a temperature range below relatively high temperatures, and the particles are dimensioned such that the particles retain essentially the semi-metallic properties at relatively high temperatures, but would otherwise transition to a range of metallic properties if not spatially isolated and appropriately dimensioned; and   a separator for maintaining spatial isolation of the particles.   
     
     
         2 . A variable emittance thermochromic material according to  claim 1 , wherein the transition metal based complex includes at least one transition metal chemically bonded such that there is energy splitting of the d-orbitals resulting in:
 (1) relatively empty e g -orbitals at low temperatures, which in turn results in the complex having non-metallic characteristics at low temperatures;   (2) the promotion of electrons from filled d-orbitals into the e g -orbitals at higher temperatures, which in turn results in the complex having semi-metallic characteristics at high temperatures; and   (3) a switching temperature T s  at which the complex transitions from having non-metallic characteristics to semi-metallic characteristics.   
     
     
         3 . A variable emittance thermochromic material according to  claim 2 , wherein the transition metal is one of Tungsten (W), Manganese (Mn), Lanthanum (La) and Vanadium (V). 
     
     
         4 . A variable emittance thermochromic material according to  claim 2 , wherein the transition metal is bonded with an oxide (O n ) group. 
     
     
         5 . A variable emittance thermochromic material according to  claim 1 , wherein the complex is a Vanadium Oxide (VO n ). 
     
     
         6 . A variable emittance thermochromic material according to  claim 1  further comprising a dopant for changing the switching temperature T s . 
     
     
         7 . A variable emittance thermochromic material according to  claim 6 , wherein the dopant is a donor of electrons. 
     
     
         8 . A variable emittance thermochromic material according to  claim 6 , wherein the dopant is an acceptor donor of electrons. 
     
     
         9 . A variable emittance thermochromic material according to  claim 6 , wherein the switching temperature ranges over the approximate range of −20° C. to +68° C. 
     
     
         10 . A variable emittance thermochromic material according to  claim 1  wherein the separator comprises a dielectric matrix separating the particles. 
     
     
         11 . A variable emittance thermochromic material according to  claim 10 , wherein the dielectric matrix comprising Silicon Dioxide (SiO 2 ). 
     
     
         12 . A variable emittance thermochromic material according to  claim 1  wherein the separator comprises some portion of a substrate material onto when the spatially isolated particles are deposited. 
     
     
         13 . A variable emittance thermochromic material according to  claim 12 , wherein the portion of the substrate material is patterned. 
     
     
         14 . A variable emittance thermochromic material according to  claim 12 , wherein the portion of the substrate is a metal oxide layer. 
     
     
         15 . A variable emittance thermochromic material according to  claim 12  further comprising:
 a plurality of layers of spatially isolated particles; and   a plurality of buffer layers interleaved between spatially isolated particle layers;   wherein each of the plurality of layers of spatially isolated particles is separated from another one of the plurality of layers of spatially isolated particles by at least one of the buffer layers.   
     
     
         16 . A variable emittance thermochromic device according to  claim 15 , wherein at least one of the buffer layers is patterned. 
     
     
         17 . A variable emittance thermochromic device according to  claim 15 , wherein at least a portion of the substrate is patterned. 
     
     
         18 . A variable emittance thermochromic device according to  claim 12 , wherein the spatially isolated particles are separated by textured features on the substrate.