Alok Awasthi

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 .

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.

Studies on calcium reduction of yttrium fluoride

The reduction of yttrium fluoride (YF3) by calcium in a molybdenum crucible was investigated. A visual observation technique was used to monitor the reduction process. It was observed that the reaction initiated immediately after the melting of calcium. Based on the results, a two-step reduction sequence was attempted. First, the charge was heated to 950°C to minimise the calcium loss by vaporisation and to ensure completion of reaction. The products were then fast heated to 1600°C for slag/metal separation. Molybdenum content of yttrium was found to be in a range of 3.5–4.75 wt-%, which is much lower than the equilibrium content predicted by Mo–Y phase diagram. The yttrium/molybdenum interface was studied using an electron probe microanalyser, which confirmed that the overall equilibrium was not reached, indicating the possibility of lowering the molybdenum contamination further by process optimisation.