CHARACTERIZATION OF COBALT-HARDENED GOLD ELECTRODEPOSITS BY Co57-Fe57 MÖSSBAUER SPECTROSCOPY

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CHARACTERIZATION OF COBALT-HARDENED GOLD ELECTRODEPOSITS BY Co57-Fe57

MÖSSBAUER SPECTROSCOPY

R. Cohen, F. Koch, L. Schoenberg, K. West

To cite this version:

R. Cohen, F. Koch, L. Schoenberg, K. West. CHARACTERIZATION OF COBALT-HARDENED GOLD ELECTRODEPOSITS BY Co57-Fe57 MÖSSBAUER SPECTROSCOPY. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-349-C1-350. �10.1051/jphyscol:19801130�. �jpa-00219614�

JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au n ^O 1 , Tome 41, janvier 1980, page C1-349

CHARACTERIZATION OF COBALT-HARDENED GOLD ELECTRODEPOSITS BY C O ~ ~ - F ~ 57 ~ S B A U E R SPECTROSCOPY

R.L. Cohen, F.B. Koch, L.N. Schoenberg and K.W. West BeZl Laboratories, Murray Hill, New Jersey 07974, U.S.A.

Gold electrodeposits containing small amounts of other metals are widely used as contact materials in separable electronic connectors and in relays for low-energy switching applications. Typically, $0.6 at% Co is codeposited with the gold, from slightly acid solutions of KAu(CN)2. This small addition raises the hardness of the plated layer to ~ 2 0 0 kg/mm2 and produces surfaces which have good wear resistance and low, stable contact resistance. How- ever, ~ 1 0 at% H , 3 at% C, 2 at% N, and other light elements, all of which are completely insoluble in gold, are also co- deposited. Thus, the material is one of extremely complex structure and ~hernistry.~

Elemental analysis of the impurities has been carried out2 by dissolving the host in aqua regia, or, more recently, mercury, but this destroys the identity of combined inclusions in the original deposit. In contrast, the MGssbauer technique provides the possibility of in situ measurements to def85mine the identity of the inclusions.

Au experiments we have previously re- ported3 have shown there are no significant quantities of gold cyanides present in the deposit.

In these experiments,l we have studied deposits from two different "hard gold"

solutions, identified as "CI" (a product of SelRex Company) and "HS", a proprietary Bell S stem electrolyte. After addition of the Coz7 dopant, deposition was carried out from both solutions under conditions iden- tical to those used in production. The Mossbauer spectra were taken at 78 K source temperature, using a standard Doppler drive and a resonance counter with a sensitive layer of K4Fe (CN) (je3H20.

Spectra of the deposits are shown in Fig. 1.

There are two components: The major line is due to Co substitutionally dissolved in gold; the isomer shift of the line is that of dilute Fe in Au, and we refer to this as arising from "dissolved" Co. Identifica- tion of the weaker line, from 7qcomplexed"

Co, is much more difficult. As seen in Fig. 1, high temperature annealing and rapid cooling converts all Co to the sub- stitutional form.

SOURCE VELOCITY Imm / see,

Fig. I. A. Spectrum of as-deposited CI material. The strong line at -0.68 mm/sec comes from the ~ 0 5 7 substitutional in gold, the weak line near O.S7mm/sec from complexed Co

.

Note relatively weaker line from complexed COST.

C. Spectrum of CI sample from which spectrum A was made, after sample had been treated for 40 minutes at 800°C in Hz. This treatment converted all the ~ 0 5 7 to dissolved Co.

Thus, simple inspection of the spectra allows us to say that (for CI) ~ 7 5 % of the Co is dissolved in the gold; this had been a matter of controversy for many years.

Our conclusion about the dissolved Co has been anticipated by another recent

Wossbauer study4 of these materials; but our conclusions about the complexed Co are

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801130

JOURNAL DE PHYSIQUE

quite different. Additional information is provided by spectra of the residues ob- tained by dissolving the deposits in aqua regia and mercury (Fig. 2). The spectrum of the aqua regia extract (which has re-.

cently been identified2 as

~ ( c o I I I (CN) 6) .xH20) is completely ferent from tiat of the cdmplexed CO in the deposit, The spectrum of the mercury extract (which has recently been identi- fied2 as K3Co(CN)6) shows a superficial resemblance to the spectrum of the com- plexed Co, but a careful comparison shows that the K3Co(CN)6 spectrum cannot be in- cluded in that of the deposit.

SOURCE VELOCITY ( m m / ~ l

Fig. 2. A, spectrum of CI deposit, as in Fig. 1 A ; B, filtrate after aqua regia dissolution of deposit; C, spectrum of filtrate after mercury dis- solution.

The recent chemical studies2 suggest that some form of cobalt cyanide is present in the deposit. However, examination of the Mossbauer literature on spectra of ~ 0 5 7 cyanides, and synthesis of a number of pre- viously unreported Co-containing materials, did not show any producing a single line at 0.14 mm/sec, as shown in Fig. 1 and Table I.

TABLE I

Messbauer parameters of ~ o ~ ~ - d o ~ e d deposits at 78OK

Isomer shifts referenced to ~ 0Fe at 78OK ~ ~ :

Error limits are 1 standard dGiation Dissolved Co Complexed Co I.S. FWHM I.S. FWHM Frac- Type mm/sec mm/sec mmsec

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This puzzle can be explained as follows:

When ~ 0 5 7 compounds are studied by MSssbauer spectroscopy, the radiolysis resulting from the decay can break up the complexes immediately surrounding the Fe daughter. Thus, the spectrum of the daughter is not that of Fe in the configu- ration the Co parent was in. For

K3Co(CN)6, 80% of the daughter Fe ions are present as the (F~III(cN )2 species, and only 20% retain the

(F~II~(CN)~)~-

figuration. Thus, most of the spectrum from K3Co(CN)6 (a spectrum which is identi- cal to that of Fig. 2C) arises from a new species generated in the radioactive decay.

The spectrum of Fe in (F~III(cN)~)~- consists of a single line, with a width of

%0.8 mm/sec and an isomer shift of 0.2 mm/sec,5 identical with the line of the complexed Co, as shown in Fig. 1 and Table I.

We believe that the complexed Co in the hard gold deposits is present in the form of Co(CN)6 complexes, which are included in the deposit. In this situation, the radiolysis could be suppressed due to the metallic matrix, and the resultant daughter complex could be essentially entirely the Fe(CN)6 complex, which appears to be the only cyanide species having a line at the appropriate position. We believe this represents the first such observation of suppression of radiolysis due to a metallic matrix. It is interesting that this should occur as a result of study of a techno- logically important material, and shows the close coupling between basic science and leaterials technology.

References

1. Material in this paper is discussed more extensively by the authors in J.

Electrochem. Soc.,

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2. Y. Okinaka, F. B. Koch, C. Wolowodiuk, and D. R. Blessington, J. Electrochem.

SOC.

125

3. R. L. Cohen, K. W. West, and M. Antler, J. Electrochem. Soc., 124 (1977), 342.

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4. H. Leidheiser, Jr., A. Vdrtes, M. L.

Varsgnyi and I. Czak6-Nagy, J. Electro- chem. Soc. %(1979), 391.

5. K. E. Siekierska, J. Fenger and J.

Olsen, J. Chem. Soc. (Dalton), 72

(1972), 2020.

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