"Electrochemically Obtained Powders and Coatings of Various Tantalum Silicides"

Figure 1 Cyclic voltammogram of melt (NaCl-KCl)equimol.-NaF(10wt.%)-K2SiF6-K2TaF7v=1.0V s-1,Т=1073К, C(K2SiF6)-1.95410-4molcm-3, Ta : Si=1: 1. Quasi-reference electrode - platinum

Table 1 Electrochemical synthesis parameters of tantalum silicide powders and coatings

Meanwhile, the open-circuit chronopotentogram (Figure 2) recorded on the silicon electrode shows five plateaus (a, b, c, d, e) of potential delay, which according to the equilibrium state diagram of tantalum-silicon [12], The plateaus correspond to the formation of the two-phase regions Ta3Si + Ta, Ta3Si + Ta2Si, Ta2Si + Ta5Si3, Ta5Si3 + TaSi2, and TaSi2 + Si in the surface layer, respectively. Potentiostatic electrolysis at plateau potential (Figure 2) results in the formation of cathode deposits with the following composition: At a potential corresponding to plateau "a" - a mixture of metallic tantalum and tantalum silicide Ta3Si, plateau "b" - composition For the silicide of Ta3Si + Ta2Si, platform "c" - two-phase product of silicide Ta2Si + Ta5Si3, at potential "d" - mixture of silicide Ta5Si3 + TaSi2, at potential "e" - tantalum silicide composed of TaSi2 + Si.

Figure 2 OCP (sodium chloride-potassium chloride) equimolarity of the melt. -Sodium fluoride (100,000 tons.%)-K2TaF7, Т=1073К, C(K2SiF6)-1.95410-4molcm-3, Ta:Si=1:1. Quasi-Reference Electrode-Ta

Potentiostatic electrolysis in all cases resulted in the deposition of tantalum silicide powder on the cathode. The tantalum silicide coating on the silver substrate was deposited in the melt (NaCl-KCl) by galvanostatic electrolysis.

The morphology of the powder and coating was investigated by optical microscopy (Figure 3). Individual particles of Ta3Si powder are 800-900 nm in size, while these particles form agglomerates of 10-20 µm in size (Figure 3(a)). Figure 3(b), (c), (d) show the coating structures deposited using galvanostatic current densities at different current densities. The coating obtained at a current density of 30 mA cm-2 (Figure 3(b)) is biphasic and a mixture of tantalum and silicon. The composition has an unfolded surface and is approximately 20-30 µm in length and 1-3 µm in cross-section. The Ta5Si3 (c) and Ta2Si (d) coatings synthesized at cathodic current densities of 80 and 90 mA cm-2, respectively, are solid with a thickness of about 8 µm and repeat the texture of the pristine substrate surface.

Figure 3 Morphology: (a) Ta3Si powder and tantalum silicide-coated silver substrate; (b)-TaSi2+Si (j=30mAcm-2); (c)-Ta5Si3 (j=80mAcm-2); (d)– Ta2Si(j=90mA cm-2)