NASA Technical Memorandum 106607
5 ?
A Novel Method for Depositing Precious
Metal Films on Difficult Surfaces
L. C. Veitch and W. H. Phillip
Lewis Research Center
Cleveland, Ohio
(NASA-TM- 106607) A NOVEL METHOD
FOR DEPOSITING PRECIOUS METAL FILMS
ON DIFFICULT SURFACES (NASA. Lewis
Research Center) 5 p
G3/27 0012652
N9A-35260
Unc 1 as
June 1994
National Aeronautics and
Space Administration
A NOVEL METHOD FOR DEPOSITING PRECIOUS METAL FILMS ON DIFFICULT SURFACES
L.C. Veitch and W.H. Phillip
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
A guanidine-based vehicle was developed to deposit precious metal coatings on surfaces known
to be difficult to coat. To demonstrate this method, a platinum coating was deposited on alumina fibers
using a guanidine-platinum solution. X-ray diffraction confirmed that the only species present in the
coating was platinum, and, that all of the carbon species had been removed upon heat treatment. SEM
results showed that some porosity was present but that the coatings uniformly covered the fiber surface
and adhered well to the fiber.
INTRODUCTION
The preparation of thin metallic films by the decomposition of metallo-organic solutions is a
technology which has been known for many years; the first publications reporting on this technology ap-
peared more than one hundred years ago. Although the technique has been known for a long time, its
primary application has been in the decorative trade. The main emphasis is producing films of precious
metals on ceramic and glass articles (ref. 1).
Almost all of the work prior to 1980 used metallo-organic compounds derived from resins or
other natural products and are commonly called metal resinates. These resinates are suitable for most
applications in the decorative trade. The variability of the chemistry is a result of the preparation from
natural products, and is a major impediment to the extensive development of the technical applications of
the metal-organic decomposition (MOD) processes. In recent years, MOD films have been produced
from pure, well-characterized compounds and are now used in technical applications (ref. 2). For ex-
ample, silver films are fabricated by thermally decomposing the MOD silver solutions on silicon for use
as collector grids for photovoltaic cells. These collector grids required good adhesion, low contact resis-
tance, low sheet resistance and long-term electrical stability. The fired films are 100 percent solder leach-
resistant and have good definition and excellent long-term adhesion. This method of metallizing solar
cells is inexpensive since less precious precursor material is required as compared to source targets
needed for sputtering silver on silicon (ref. 3).
The conventional metallo-organic solutions have the advantage of producing film densities near
the theoretical limit at low temperatures. However, adhesion to some ceramics is poor. Altering the vis-
cosity of the solutions can alleviate the problems with adhesion. With more organics present in the solu-
tion, however, there is a greater tendency towards blistering.
A guanidine vehicle has been developed (ref. 4) that solves the adhesion and the blistering prob-
lems. This vehicle can be used to deposit precious metal films or metal oxide films on smooth, cylindri-
cal surfaces and on smooth, flat metal surfaces. Guanidine is a strong base equal in strength to sodium
hydroxide; thus, guanidine forms stable soaps with organic fatty acids that have high ionicity. This en-
ables them to wet and to bond to the ceramic substrates. The advantage of these soaps is that they com-
pletely bum off and leave no residue when fired. The disadvantage of sodium-containing soaps is that
they leave a sodium oxide ash upon firing. This ash can reduce the high-temperature strength of some
ceramics.
1
For this study, single-crystal A1 2 0 3 fibers were coated with a Pt-guanidine solution. This particu-
lar coating was chosen because of its ability to act as a sintering barrier for the fibers when placed in an
oxide matrix. Al 0 O 3 fibers were pulled through a melted guanidine-platinum solution in the guanidine 2-
ethylhexanoate vehicle, air dried and subjected to a rapid thermal anneal to eliminate the organics and
produce a porous Pt coating. The coating and firing procedures were repeated several times to increase
the coating thickness on the fibers. Then, the fibers were annealed at 1000 °C to densify the Pt coating.
X-ray diffraction and SEM were used to analyze the coatings.
Debye-Scherrer results indicated that only Pt peaks were present in the coatings. Thermal gravi-
metric analysis (TGA) of the Pt-guanidine solution confirmed that the organics were completely removed
at 600 °C and that only some trace metals were evident. These trace metals were also present in the Pt-
guanidine solution container walls.
Figures 1(a) and (b) show the surface and cross-section of one of the Pt-coated fibers after the
anneal. Even though some porosity is still present in the coatings, the adhesion of the Pt coating is good.
The coating is also fairly uniform over the length of the fiber (fig. 1(a)). Slower heating cycles were tried
after each coating to reduce the porosity. However, as shown in figure 2, the coating was not as dense as
those that were fired rapidly after each coating.
The coating thickness is dependent on the viscosity of the Pt-guanidine solution and on the num-
ber of applications. The coating and firing procedures for the fibers in this study were repeated four
times, yielding approximately a 10 pm coating.
Although no chemical reactions were observed between the Pt and A1 2 0 3 fiber, pitting on the
surface of the fibers was observed by Jaskowiak (ref. 5) when the fibers were subjected to further heat
treatments and when placed in an oxide matrix material. This has also been observed when other similar
sol-gel coatings have been applied to these A1 2 0 3 fibers (ref. 6). The pitting can be correlated back to
impurities trapped at the surface by the coatings. These impurities, identified as Ca, Ti and other light
metals, have also been observed using x-ray photoelectron spectroscopy (XPS) on the surface of the fibers
before and after cleaning and are not necessarily from the Pt-guanidine solution or other chemical pro-
cesses for coating the fibers. Jaskowiak (ref. 5) also observed that the Pt coating no longer adhered to the
surface of the fiber above 1300 °C. However, the coating remained intact below 1300 °C.
It is possible to coat smooth, cylindrical surfaces with noble metals by means of a modified MOD
process using a guanidine soap solution for the organic vehicle. Application of this process can range
from thin films used as electric conductors in the microelectronics industry to additional coatings for
composite materials. The Pt-guanidine solution utilized in this study adhered well to the A1 2 0 3 fibers
below 1300 °C. The coating was fairly uniform over the entire surface. Some porosity is present in the
coatings, however, the Pt coating still provided the necessary barrier between the A1 2 0 3 fiber and the
oxide matrix for high temperature use.
REFERENCES
1. Mopper, R.T.: Ceram. Ind., 1963, 80, pp. 74-76.
2. Langley, R.C.: AFMLTR 65-262, Wright Patterson AFB, OH (1985).
3. Wachtman, J.B., and Haber, R.A: Ceramic Films and Coatings (Noyes Publications, New Jersey,
1993), Chapter 9.
4. DeGuire, M.R., and Phillip, W.H.: J. Mater. Chem., 1993, 3 (6), pp. 571—574.
5. Jaskowiak, M.H., Phillip, W.H., Eldridge, J.I. , Setlock, J.A., Norkitis, M.E., Helman, J.R.: 5th
Annual Hi-Temp Rev. Proc., 1992, pp. 59-1-59-14.
6. Matthieson, M.M., and Jayne, D. T.: to be published in the Journal of the American Ceramic Society.
2
(a) Surface view. (b) Cross section.
Figure 1 .—A platinum-coated single crystal alumina fiber after four coating and firing cycles and final anneal at 1 000 °C.
Figure 2. — The surface of a platinum-coated alumina fiber
after 8 coating and slower firing cycles and a final anneal
at 1 000 °C.
3
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1. AGENCY USE ONLY ( Leave blank)
2. REPORT DATE
June 1994
3. REPORT TYPE AND DATES COVERED
Technical Memorandum
4. TITLE AND SUBTITLE
A Novel Method for Depositing Precious Metal Films on Difficult Surfaces
S. FUNDING NUMBERS
WU-510-01-01
6. AUTHOR(S)
L. C. Veitch and W. H. Phillip
7.
PERFORMING ORGANIZATION NAME(S) AND ADDRESSES)
8. PERFORMING ORGANIZATION
REPORT NUMBER
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESSES)
E-8886
SPONSORING/MONITORING
AGENCY REPORT NUMBER
11 .
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
NASA TM-1 06607
SUPPLEMENTARY NOTES
Responsible person, Lisa C. Veitch, organization code 5160, (216) 433-2456.
12a. DISTRIBUTION/AVAILABILITY STATEMENT
12b. DISTRIBUTION CODE
Unclassified - Unlimited
Subject Category 50
13. ABSTRACT (Maximum 200 words)
A guanidine-based vehicle was developed to deposit precious metal coatings on surfaces known to be difficult to coat.
To demonstrate this method, a platinum coating was deposited on alumina fibers using a guanidine-platinum solution.
X-ray diffraction confirmed that the only species present in the coating was platinum, and, that all of the carbon
species had been removed upon heat treatment. SEM results showed that some porosity was present but that the
coatings uniformly covered the fiber surface and adhered well to the finer.
14. SUBJECT TERMS
Guanidine; Platinum coating
15. NUMBER OF PAGES
5
16. PRICE CODE
A02
17. SECURITY CLASSIFICATION
18. SECURITY CLASSIFICATION
19. SECURITY CLASSIFICATION
20. LIMITATION OF ABSTRACT
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OF ABSTRACT
Unclassified
Unclassified
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