The effect of silver ion on electrodeposition of tungsten and tungsten carbide from molten chloride

EksrroclumicaAces Val 35 No 1 pp Ig7- 1 8 9 19A1 Primed m Creai Bmmn

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W13-4686,9O$300iOW 1989 Pergamon Press plc

THE EFFECT OF SILVER ION ON ELECTRODEPOSITION OF TUNGSTEN AND TUNGSTEN CARBIDE FROM MOLTEN CHLORIDE HIDEKI YABE, KEIKO EMA

and

YASUHIKO ITO

Department of Nuclear Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan (Received 30 March 1989)

Abstract-Tungsten and tungsten carbide can be electrodeposlted from molten chloride However, the figure of the electrodeposit is usually needle-like or dendritic By an addition of very small amount of silver ion in a molten chlonde, cubic tungsten could be obtained

INTRODUCTION

added, to give tungstate or carbonate ion concentration ranging from 01-10mol°/ If necessary, silver chloride was added at concentrations ranging from 0001-001 mol% As a quasi-reference electrode, the Ag/Ag' electrode was used For Ag/Ag*electrode, silver wire was dipped into the eutectic salt containing silver chloride in a Pyrex glass tube whose tip was very thin For potential calibration of the quasi-reference electrode, alkali metal electrode Li/Li'm the same electrolyte was used[7] This consisted of a nickel wire, on which a small amount of alkali metal (equivalent to about I C) was electrodeposited, before addition of K,WO, and K 2CO 3 This procedure did not give any noticeable impurity effect of K 2 WO, and K 2C0 3 in the electrolyte The alkali metal electrode thus prepared showed good reproducibility As an anode, glassy carbon was used, and as a cathode, nickel plate (dia 7 mm, thickness 3 mm) was used which was set inside the alumina tube The electrodepositton of tungsten or tungsten carbide (60C) was conducted on the cathode at 823-973 K with current density of 20-500mAcm -2 The potential was kept more negative than the reduction potential of tungsten (1 3±0 1 V vs Lt/Lt ') determined from voltammograms[8] The electrodeposlted sample was taken out from the vessel several hours

It is well known that tungsten or tungsten carbide can be electrodeposited from molten halide [1-6] However, the texture of the electrodeposit obtained from molten chlonde is usually needle-like or dendntic It is desirable to find appropriate electrolytic condition to get the other type ofcrystals Recently, we have found that cubic tungsten crystals can be obtained when a very small amount of silver ion is added in a molten chloride electrolyte The experimental procedure and the results arc given in the following EXPERIMENTAL A commercial LiCl-KCI (Reagent Grade, Wako Chemicals Co Ltd) mixture was prepared and the eutectic mixture was placed in a hight punty alumina crucible (99 5% A120 3 , Nippon Kagaku Togyo Co Ltd SSA-S) and further it was kept at 573 K under vacuum condition in the cell assembly, for a few days Then it was melted and kept at experimental temperature (823-973 K) under argon atmosphere After melting the salt, vacuum dried potassium tungstate, or potassium tungstate and potassium carbonate were

Fig

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Scanning electron micrograph of tungsten {LiCl-KCI, 10mol% K,WO„ current density 100 mA cm ', (a) 823 K, (b) 973 K, (c) 973 K and 0001 mot°/ Agcl} 187



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H YABE et al

after the end of plating, dried after washing by distilled water, and then, kept in a desiccator For analyses of the samples, SEM, EPMA, EDX, ESCA, RBS and Xray diffraction were used

RESULTS AND DISCUSSION Figures la- show SEMs of samples electrodeposited by a current density of 100 mA Cut' 2 in LiCI-KCI system, containing 1 0 mol% of K, WO, In the case of (a), deposition temperature was 823 K From X-ray diffraction, the deposition of tungsten could be confirmed In this case, needlelike or dendritic deposits are observed (b) shows the influence of temperature This sample was obtained at 973 K, keeping the other conditions the same as (a) From (b), it was found that at high temperature, crystallization of tungsten was promoted (c) Shows SEM of a sample which was obtained from the electrolyte containing a small amount (0001 mol%) of silver chloride under the same condition as (b) In this case, cubes of 0 1 mm size can be observed Figure 2 shows the results of EDX analysis (beam diameter -' 600 nm) at the top corner of this cubic deposit White peaks in this figure are tungsten Figure 3 shows the X-ray diffraction pattern of this sample From these figures, deposition of tungsten was certainly realized Moreover, taking into account that tungsten has a body centered cubic structure, this is considered to be an electrodeposit of a highly pure tungsten crystal Figures 4a--c show typical SEMs when tungsten carbide was obtained from the chloride The samples

Fig 2 EDX of tungsten sample (cf Fig lc)

in these figures were all obtained by electrodepositions at 973 K and 1 0 V (vs Li/Li') from the LiCI-KCI containing 0 1 mol% of K 2 WO4 and 0 1 mol% of K 2 CO 3 In the case of (a), the mixture of squat particles, needle like crystals and dendrites are observed Figure 5 shows X-ray diffraction pattern of the sample of Fig 4a From this figure, the deposit of Fig 4a was found to be a mixture of W 2C, W and WC Figures 4b and c show an effect of a small amount of

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Fig 3 X-ray diffraction pattern (Cu-K„ of Fig Ic)

Fig 4 Scanning electron micrograph of tungsten carbide (LiCI-KCI, 01 mol%` 2WO 4, 0 1 mol%K 2CO 2, deposition potential 1 0 V vs Li/Li', 973 K, (a) 0 mol% Agcl, (b) 0001 mol% AgCI, (c) 001 mot% AgCl)



Electrodeposition of tungsten and tungsten carbide

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W2C 1 3 4 12 13 15,22 W 5 ,11 14 21 WC2,8 N, 7,10 17,19 Morel 6 , 9 16 16,20 (Sartple holder)

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Fig 5 X-ray diffraction pattern (Cu-K„ cf Fig 4a)

W 5,9,11,13,16 W2C 2 3 4 10 WC 1 7 Mone I 6 , a ,12, 14, 15 (Sanple holder)

Fig 6 X-ray diffraction pattern (Cu-K„ of Fig 4b)

silver ion added in the electrolyte (b) Shows the result when 0001 mol% of silver chloride was added under the same condition as (a) The surface of the electrodeposit was changed drastically, and cubic crystals can be observed Figure 6 shows X-ray diffraction pattern of the sample of Fig 4b From this figure, the deposit is a mixture of W, W2C and WC Tungsten and tungsten carbide are body centered cubic and hexagonal system, respectively Hence, it is possible that the squat or mossy deposit is tungsten carbide and cubic crystals are tungsten, though it has not been proved yet Anyway, the fact is, the effect of silver ion on electrodeposition is confirmed (c) Shows the result when 001 mol% of silver chloride was added In this figure, cubic crystals can not be observed From these results, by an addition of silver ion of 0 001 mol% in a molten chlonde, cubic crystals could be obtained However, there is no effect when the silver ion concentration is high, about 001 mol% The mechanism of the effect of silver ion concentration on the morphology of the electrodeposit is not clear yet, which is one of our future research subjects

Acknowledgements-We would like to thank Mr Teruyosht Unezaki of Kyoto University for EPMA, Prof Tsuyosht Nakajima of Kyoto University for ESCA and Mr Seyi Terada of the Kawasaki Heavy Industries, Ltd for SEM, EDX and EPMA This work was carried out under support from a Grant-lnAid from the Japanese Ministry of Education, Science and Culture REFERENCES I S Senderoff and G W Mellors, J electrochem Soc 114, 586 (1967) 2 S Senderoff and G W Mellors, Science 153, 1475 (1966) 3 3 M Grimes and M M Wong, U S Bur Mines Rep Invest, 7247 (1972) 4 K H Stern and I L Singer, Thin Soled Films 108, 9 (1983) 5 K H Stern and M L Deanhardt, J electrochem Soc 132, 1891(1985) 6 H Yabe, K Ema and Y Ito, to be published 7 T Takenaka, Y Ito and J Oishi, Denke Kagaku 53, 476 (1985) 8 H Yabe, S Hikino, K Ema and Y Ito, to be published