S. P. Garg

The reduction of niobium and tantalum pentoxides by silicon in vacuum

Abstract The use of silicon as a reducing agent for Nb 2 O 5 and Ta 2 O 5 to obtain the metals under pyrovacuum conditions has been investigated. Reduction occurs due to the removal of oxygen by evaporation of volatile silicon monoxide, SiO (v) . Pourbaix–Ellingham diagrams for Nb–Si–O and Ta–Si–O systems were constructed using the published thermodynamic data. The sequences of reduction, as indicated by these diagrams, were also observed by vacuum thermogravimetry experiments and X-ray diffraction (XRD) analysis of products. The reduction starts at around 1150°C for both niobium and tantalum systems and proceeds smoothly as the temperature is raised to 1600°C under vacuum. The extents of reduction were found to be more than that observed in carbothermic reductions at comparable levels of temperature, pressure and durations. The product was an Nb–Si–O or Ta–Si–O alloy, which is refineable to the pure metal by pyrovacuum treatment at a higher temperature.

Measurement of contact angle in systems involving liquid metals

A technique has been developed for measurement of the contact angle of liquid metals with solid materials. The liquid metal, when contained in a small cup (inner diameter <4 mm) forms a concave or convex mirror surface. For the present technique, an optical method was devised to measure the radius of curvature of this mirror surface, from which the contact angle can be determined. The surface of the liquid metal approaches sphericity as the diameter of its container is decreased. The optical system formed in this method is non-paraxial. An analytical formula, valid for a spherical non-paraxial mirror, for calculation of image size was deduced. The technique has been demonstrated using the mercury - graphite (15.9% total porosity) system at room temperature and the contact angle was determined to be .

Diffusion studies in Hf-Mo, Zr-Mo, Cr-Nb, Cr-Ta and Th-Re systems above 1900 K

Abstract The paper describes an experimental procedure for carrying out high temperature diffusion studies involving liquid phase. The method has been successfully demonstrated for studying diffusion in binary refractory alloys in Hf–Mo, Zr–Mo, Cr–Nb, Cr–Ta and Th–Re systems at temperatures between 1930 K and 2360 K. The present method provides diffusion data for both solid solutions and the intermediate phases present in each of these systems. The paper reports the tracer diffusion coefficients for Hf and Zr in Mo and Cr in Nb and Ta at infinite dilution without using radio tracers. The values of interdiffusion coefficients using both concentration gradient and chemical potential gradient as the driving force have been determined. The advantages in using interdiffusion coefficient with chemical potential gradient as the driving force particularly in the intermediate phases are discussed.

Determination of Standard Free Energy of Formation for Niobium Silicides by EMF Measurements

EMF measurements were carried out at temperatures ranging from 1280 to 1490 K in the following cells: ⊖Mo, Si + NbSi 2 + SiO 2 /SiO 2 -sat. Li 2 O-SiO 2 /NbSi 2 + Nb 5 Si 3 + SiO 2 , Mo○+, ⊖Mo, Si + NbSi 2 + SiO 2 /SiO 2 -sat. Li 2 O-SiO 2 /Nb 5 Si 3 + NbO + SiO 2 , Mo○+, and ⊖Mo, NbSi 2 + Nb 5 Si 3 + SiO 2 /SiO 2 -sat. Li 2 O-SiO 2 /Nb 5 Si 3 + NbO + SiO 2 , Mo○+, using SiO 2 -saturated lithium silicate liquid electrolyte. Each of the cells showed a reliable electromotive force (emf) corresponding to the difference in silicon potential between the electrodes. Based on these emf values measured, the molar standard free energy of formation for NbSi 2 and Nb 5 Si 3 were determined to be ΔG 0 NbSi2/kJ = -165 + 0.008 (T/K) ± 13 and ΔG 0 Nb 5Si3 /kJ = -526 + 0.009 (T/K) ± 63, respectively.