Ferricyanide/ferrocyanide couple prevents zinc oxide pigment degradation in thermal control coatings. Chemical couple retards physical optical property changes

Freund, T.

Morrison, S. R.

English

NASA Technical Reports Server

December 1972	 B72-10711 
S NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Improved Zinc Oxide Thermal Control Coatings 
Electron-hole pairs are formed when pigment ma-
terials such as zinc oxide are illuminated; light energy 
is transformed into the energy of the electron-hole 
pair. In thermal control coatings, the hole can oxidize 
chemical species in contact with the surface of the 
pigment particles or even the surface molecules of the 
pigment itself, and the electron can reduce surface 
chemical species or the cation of the pigment; as a 
result, the energy of illumination is transformed into 
chemical changes of the pigment or the vehicle. 
Eventually, the thermal control coating is degraded 
by the accumulation of new chemical species which 
alter its optical and physical properties. 
Pigments used in thermal-control coatings can be 
improved by inclusion of chemical species which can 
alternately capture electrons and holes by a cyclic 
mechanism which regenerates the original species: 
(1)R+e-+ R;	 (2)R+p-9R. 
The chemical speies is termed a recombination center 
because it leads to the simple recombination of the 
electron-hole pair with no net chemical change; the 
energy of illumination is thus converted to heat. Chem-
ical species which can perform as efficient recombina-
tiort centers exhibit the following characteristics: (a) 
They form stable oxidation states differing by one 
electronic charge. (b) They can be applied to the sur-
face of the pigment in adequate quantities and in both 
oxidation states. (c) They have reasonable capture 
cross-section for holes in the reduced form and for 
electrons in the oxidized form.
The results of a study of zinc oxide have demon-
strated that the ferricyanide/ferrocyanide couple is 
very effective for preventing the degradation of the 
pigment. Zinc oxide pigments doped with the redox 
couple exhibited marked resistance to chemical 
changes as determined by measurements of conduct-
ivity and optical absorption. If at least 0.3 monolayer 
of the ferricyanide/ferrocyanide couple is present, 
neglible changes occur when the pigment is illumi-
nated by ultraviolet light. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP 72-10711 
Patent status: 
Title to this invention has been waived under the 
provisions of the National Aeronautics and Space Act 
[42 USC 2457 (f)], to the Stanford Research Institute, 
Menlo Park, California 94025. 
Source: Stanley R. Morrison and
Thomas Freund of
Stanford Research Institute
under contract to
NASA Pasadena Office
(NPO-11139) 
Category 04 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
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Improved zinc oxide thermal control coatings

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