Photocatalytic coatings on optical solar reflectors to decompose organic contaminants

PHOTOCATALYTIC COATINGS ON OPTICAL SOLAR REFLECTORS TO DECOMPOSE ORGANIC CONTAMINANTS

SPECIFICATION

5 TECHNICAL FIELD

The present invention relates to techniques for improving the long term stability of

optical solar reflectors. More particularly, this invention relates to photocatalytic coating

compositions for applying to optical solar reflectors to prevent/decompose organic

residues on the reflector surfaces.

i o BACKGROUND OF THE INVENTION

An optical solar reflector (OSR) is a second-surface reflector on a transparent

substrate, which is attached to the outer surfaces of a spacecraft for ( 1 ) reflecting solar

energy incident on the spacecraft (reflection), and (2) radiating heat energy produced in

the spacecraft (emission). The desired function of optical solar reflectors on satellites is

15 to minimize thermal variations in sensitive satellite electronics caused by the electronic

themselves and by solar radiation, which cause detrimental effects on phase array device

operation. Thus, one of the requirements for OSR materials is to have low <* /E ratio (« :

absorption coefficient; E: emissivity).

Currently, indium-tin oxide (ITO) coated glass in conjunction with a silver layer is

20 used as the preferred OSR tile material. Unfortunately, condensation of thin organic films

from non-metallic materials out gassing (e.g., sealants) reduces the optical performance of

the OSR, and consequently the thermal control feature. For example, the absorption

coefficient of the OSR can increase from 0.1 to 0.4 due to the formation of contaminant

films, usually 10-100 nm in thickness, over a period of years. Thus, it is critical to

25 overcome this contamination problem in order to maintain the long term effectiveness of the OSR. The invention described herein involves the application of a thin semiconductor

coating (e.g., Ti0₂) to promote decomposition of organic contaminants based on

photocatalytic principles.

It is known that photo assisted heterogeneous catalysis can destroy hydrocarbon

and chlorocarbon contaminants in water. See, for example, the following references: D.

F. Ollis, "Contaminant degradation in water", Environ. Sci. Technol., 1985, Vol. 19, No.

6, pp 480-484; C. S. Turchi and D. F. Ollis, "Mixed Reactant Photocatalysis:

Intermediates and Mutual Rate Inhibition", Journal of Catalysis, 1989, 199, pp 483-496;

A. E. Hussain and N. Serpone, "Kinetic Studies in Heterogeneous Photocatalysis. 1.

Photocatalytic Degradation of Chlorinated Phenols in Aerated Aqueous Solutions over

TiO₂ Supported on a Glass Matrix", J. Phys. Chem., 1988, 92, pp 5726-5731 ; and M. A.

Fox and C. C. Chen, "Mechanistic Features of the Semiconductor Photocatalyzed Olefm-

to-Carbonyl Oxidative Cleavage", J. Am. Chem. Soc, 1981, 103, pp 6757-6759. This

process is achieved by illuminating a semiconductor catalyst, typically titanium dioxide

(Ti0₂), with greater-than-bandgap ultraviolet or near ultraviolet light in order to create

electron excitation within the solid. Electron-hole pairs generated by the photoexcitation

can then react with water or oxygen to lead to the formation of hydroxyl and other

oxygen-containing free radicals. These radicals may attack and oxidize organics.

For hydrocarbon compounds, the primary decomposition products are often

carbon dioxide and water. One of the advantages of Ti0₂ as a photocatalyst is its chemical

stability and strong oxidizing/reducing power. In fact, Ti0₂ has been extensively used for

organic decomposition in aqueous systems based on photocatalytic principles (see the

four references noted above). Ti0₂ has also been used for heavy metal removal in aqueous systems based on photocatalytic principles. See, for example, H. Yoneyama et

al., "Heterogeneous photocatalytic reduction of dichromate on n-type semiconductor

catalysts", Nature, 1979, Vol. 282, pp 817-818; and W. Lin et al., "Hexavalent Chromium

at Titanium Dioxide in Aqueous Basic Media", J. Electrochem. Soc, Sept. 1993, Vol.

140, No. 9, pp 2477-2482. In addition, Ti0₂ has been used for bacteria killing in aqueous

systems based on photocatalytic principles. See, for example, T. Matsunaga et al.,

"continuous-Sterilization System That Uses Photosemiconductor Powders", Appl.

Environ. Microbiol., 1988, Vol. 54, pp 1330-1333.

To our knowledge, there are no applications which teach or suggest the use of

photoactive semiconductor materials like TiO₂ to decompose organic residues under

vacuum conditions, for example Space, where there is little or no oxygen. The invention

described herein is directed to photocatalytic coatings for preventing formation of and/or

decomposing organic residue formed at the OSR surface under space conditions.

SUMMARY OF THE INVENTION

The objective of this invention is to maintain long term stability of optical solar

reflectors (OSR) by applying photocatalytic coatings to prevent/decompose organic

residues on the OSR surfaces.

In accordance with the present invention, a thin film layer of photoactive

semiconductor material is deposited or otherwise applied onto the OSR surface in order to

photodecompose organic contaminants that tend to form on the OSR surfaces as a result

from out gassing of organic materials (e.g., sealants). In use, the applied photoactive

semiconductor material is excited by the UV or near UV components of solar radiation to

generate free carriers. The resulting conduction-band electrons and valence-band holes can then interact with bound oxygen in the organic residues to form radicals and

eventually break down the organic contaminants.

In accordance with an important aspect of the invention, the selected coating

material should be sufficiently thin so that the optical properties of OSR are not affected.

Methods and apparatus which incorporate the features described above and which

are effective to function as described above constitute specific objects of this invention.

Other and further objects of the present invention will be apparent from the

following description and claims and are illustrated in the accompanying drawings, which

by way of illustration, show preferred embodiments of the present invention and the

principles thereof and what are now considered to be the best modes contemplated for

applying these principles. Other embodiments of the invention embodying the same or

equivalent principles may be used and structural changes may be made as desired by

those skilled in the art without departing from the present invention and the purview of

the appended claims.

BRIEF DESCRIPTION OF THE DRAWING VIEWS

Fig. 1 is cross-section view through a typical optical solar reflector (OSR) tile

which includes a photocatalytic coating in accordance with the present invention.

Fig. 2 is cross-section view through an optical solar reflector (OSR) tile as in Fig.

1, mounted to a portion of the outer surface of a spacecraft.

Fig. 3 is a graph showing frequency change (mass change) as a function of time

under UV radiation illustrating the deposition rates of organic contaminants on a thermal

quartz microbalance sensor both with and without a TiO₂ coating. Fig. 4 is a graph showing frequency change (mass change) as a function of time

for a pre-deposited film with UV radiation.

Fig. 5 is a graph showing CO response as a function of UV radiation time

illustrating the photocatalytical effectiveness of W0₃ in preventing and/or removing

organics under vacuum conditions.

Fig. 6 is a graph showing CO₂ response as a function of UV radiation time

illustrating the photocatalytical effectiveness of ZnO in preventing and/or removing

organics under vacuum conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 is a cross-section through an improved optical solar reflector (OSR) tile 10

in accordance with the present invention.

In the preferred embodiment, the OSR tile 10 is of conventional manufacture and

includes a substrate 12 (e.g., glass, quartz, or silicon nitride) having a reflective coating

14 (e.g., a silver coating) affixed to its space craft mounting side 12m. A layer 16 of

transparent, electrically conductive material (e.g., indium oxide or indium-tin oxide) is

coated onto its space- facing side 12s, and laps onto the edges 18 of the tile and is in

electrical contact with the reflective coating 14.

In accordance with the present invention, a coating layer 20 of oxidation material

is applied as an outermost layer onto the space- facing side 12s and side edges of the tile

and overlies layer 16. Oxidation coating layer 20 is effective to facilitate removal of

organic residues that tend to accumulate on the OSR surfaces over time. In a preferred

embodiment, the oxidation material is photocatalytic. A photocatalyst useful in the

invention is a semiconductor material selected from the group consisting of photoactive transition metal oxides, including, but not limited to TiO₂, ZnO, WO₃, CaTiO₃, SnO₂,

CaTiO₃, MoO , NbO₅, Fe₂O₃, Ta₂O₅, and Ti^Zr,.^)O₂, where X has a value of between 0

and 1. Preferred photocatalysts are Ti02, ZnO, and WO3. Compound semiconductors,

including, but not limited to ZnS, ZnSe, and CdS are also useful photocatalysts. The

coating layer 20 may be applied as a thin film to the space- facing side 12s of the OSR tile

10 using any number of a variety of conventional techniques for forming thin films in the

fabrication of integrated circuits, including, but not limited to sputtering, chemical vapor

deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and physical

vapor deposition (PVD).

In Fig. 2, the OSR tile 10 is shown mounted to the outer surface of a spacecraft

mounting panel 22 by use of adhesive means 24. The adhesive means may comprise a

double sided transfer adhesive of the type disclosed in U.S. Pat. No. 5,400,986 or may

comprise a suitable liquid resin. Coating layer 20 may also be used in conjunction with

other OSR configurations in addition to the embodiment specifically shown and described

in Figs. 1 and 2.

EXAMPLES

The following examples are designed to illustrate certain aspects of the present

invention such as the capability of decomposing organic materials via photocatalytic

processes utilizing bound oxygen from absorbing residues. The examples are not

intended to be comprehensive of all features and all embodiments of the present

invention, and should not be construed as limiting the claims presented herein.

EXAMPLE 1 To demonstrate the effectiveness of photocatalyst coatings in preventing organic

residues formed on the OSR surfaces, experiments were performed under vacuum

condition to simulate space environments, and a thermal quartz crystal microbalance

(TQCM, sensitivity: 1.56 ng.Hz '.cm ²) was employed to monitor organic contaminant

deposition.

In this first example, a thin layer Ti0₂ (photocatalyst, 2-40 run) was coated on a

TQCM surface, and organic contaminants were then deposited on the TQCM sensor with

or without UV radiation. The deposition rate of the organics was monitored by recording

the frequency change (i.e., mass change) of TQCM sensors. Fig. 3 shows typical TQCM

results. As seen in Fig. 3, the rate of organics deposition on Ti0₂-coated surface was

significantly lower compared to that at the plain TQCM surface under UV radiation.

These results suggested that Ti0₂ coatings are effective in preventing organic residues,

which are further supported by the fact that a significant increase in deposition rate was

observed when no UV radiation was applied. In addition, the experiments also

demonstrated that pre-deposited organic films on Ti0₂ surfaces can be partially removed

under UV illumination (Fig. 4). In this case, frequency decrease at the Ti0₂-coated

TQCM sensor with a thin organic film was observed upon UV radiation, which indicated

the removal of such materials. On the other hand, significantly less change in frequency

was seen for a plain TQCM (no Ti0₂ coating) sensor under similar conditions. To

investigate the effect of Ti0₂ coating on the optical properties of the OSR, the absorption

coefficient « was measured as a function of Ti0₂ thickness. The results showed that

essentially no <* change was observed on OSRs with such a thin TiO₂ coating. In

summary, the test data obtained clearly indicates that Ti0₂ coating is effective. EXAMPLE 2

In this example, a thin layer of WO₃ (i.e., less than 200 nm in thickness) was

coated on a quartz substrate, and organic contaminates (dioctyl phthalate) were then pre-

deposited on the WO₃ surface. The test sample was then placed in an ultrahigh vacuum

chamber (UHV, 10^~10 ~10^~" torr) and radiated with UV light. Mass spectrometry was

used to detect potential decomposition products of the organic contaminants. For

photocatalytic processes, one of the decomposition products is carbon dioxide (CO₂). The

graph of Fig. 5 shows CO₂ response as a function of UV radiation time. Here, the circles

denote a WO₃ sample exposed to UV radiation at 9xl0 ¹⁰ torr. The triangles denote

background condition (no sample) before WO₃ UV exposure. The squares denote

background condition (no sample) after WO₃ UV exposure.

As is seen in Fig. 5, a significant increase in CO₂ signal was observed compared to the

background signal when the test sample was exposed to UV radiation. This demonstrates

the occurrence of photocatalytic processes at the surface. The results revealed that WO₃

is photocatalytically effective in preventing and/or removing organics under vacuum

conditions.

EXAMPLE 3

In this example, a thin layer of ZnO (i.e., less than 200 nm in thickness) was

coated on a quartz substrate, and organic contaminates (dioctyl phthalate) were then pre-

deposited on the ZnO surface. The sample was then placed in an ultrahigh vacuum

chamber (UHV, 10^'10 -10^"" torr) and radiated with UV light. Mass spectrometry was

used to detect potential decomposition products of the organic contaminants. For

photocatalytic processes, one of the decomposition products is carbon dioxide (CO₂). Fig. 6 shows CO₂ response as a function of UV radiation time. Here, the circles denote a

ZnO sample exposed to UV radiation at 8xl0^"10 torr. The triangles denote background

condition (no sample) before ZnO UV exposure. The squares denote background

condition (no sample) after ZnO UV exposure. As is seen in Fig. 6, significant increase

in CO₂ signal was observed compared to the background signal when the sample was

exposed to UV radiation. This demonstrates the occurrence of photocatalytic processes at

the surface. The results revealed that ZnO is photocatalytically effective in preventing

and/or removing organics under vacuum conditions.

The above examples clearly indicate that other photoactive semiconductor

materials, including but not limited to CaTiO₃, SnO₂, CaTiO₃, MoO₃, NbO₅, Fe₂O₃, Ta₂O₅,

Ti_A Zr_/.^)O₂, where X has a value of between 0 and 1, and SiC, should prove effective in

removing and/or decomposing organic materials from the OSR surface under vacuum

conditions (e.g., space environment). As noted above, the layer thickness of selected thin

film semiconductor material should be selected so as not to interfere with the optical

properties of the OSR.

While we have illustrated and described the preferred embodiments of our

invention, it is to be understood that these are capable of variation and modification, and

we therefore do not wish to be limited to the precise details set forth, but desire to avail

ourselves of such changes and alterations as fall within the purview of the following

claims.