Nagaiyar Krishnamurthy

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.

Hydrogen absorption kinetics of V–Al alloy

The vanadium–aluminum alloy has been prepared by aluminothermy process. The alloy ingot obtained was refined by electron beam melting and homogenized by vacuum arc melting technique. The refined alloy was crushed into small pieces. These pieces were kept isothermally in a thermobalance attached to the Sieverts apparatus for the hydrogen charging. Reacted fraction α was calculated using isothermal thermo-gravimetry method. The reacted fraction α–t data thus obtained have been linearly fitted over a suitable reaction mechanism function. Rate constants at different temperatures are determined using slope of these linearly fitted curves. Activation energy of hydrogen charging has been calculated using Arrhenius equation.

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.

Gas solid techniques for preparation of pure lanthanum hexaboride

The processes reported for the preparation of lanthanum hexaboride (LaB6) from lanthanum oxide involve the use of carbon either elemental or in the form of boron carbide or elemental boron itself as reducing agents, fused salts at high temperatures or reactions which require the product boride to be leached out. Each of these techniques either increases the process costs and/or increases chances of contamination in final product. Pure LaB6 can best be prepared by a reaction which produces a gaseous byproduct. In the present study, such a reaction was successfully used to yield pure lanthanum hexaboride. The process involved mixing of anhydrous lanthanum chloride with aluminium and boron and heating the charges under dynamic argon flow. Lanthanum chloride is known to be highly hygroscopic; hence the process using improperly dehydrated LaCl3 led to the formation of lanthanum oxychloride which does not convert to LaB6 under conditions wherein LaCl3 converts. Not only the formation of AlCl3 but also its continuous removal from the reaction zone is necessary for the success of the process.

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.

Reactive DC Magnetron Sputtered NbN Films on Mild Steel with Electroless Nickel Interlayer

Authorscontributions This work was carried out in collab oration between all authors. AuthorKS designed the study,carried out the study, wrote the protocoland wrote the first draft of the manuscript. AuthorMRG performed the XRD analysisand authorNK guided the experimental plan, corrected the manuscriptand analyzed the results.All authors readand approved the final manuscript. ABSTRACT Aims: To explore thebenefits of NbN coating on mild steel (MS) with electroless nickel (EN) interlayer. Study Design: To deposit NbN coatings at various N

Interaction of α-silicon carbide with lead-lithium eutectic

Compatibility of silicon carbide with molten lead-lithium eutectic has been studied by differential scanning calorimetry (DSC) and prolonged heating of α - SiC pellet in molten Pb-17Li eutectic alloy at 823K. Multiple peaks were present in the DSC analysis. However, XRD analysis has not shown any new phase formation, which indicated that no chemical reaction occurred. An experiment have also been carried out by dipping α - SiC pellet in molten Pb-17Li eutectic alloy at 823K for 500 h to check the solubility of α - SiC in molten Pb-17Li. Substantial mass loss of α-SiC pellet was observed which could be due to dissolution of α-SiC or its component into the eutectic melt.

Simulating condensation of ammonium fluoride during fluorination of yttria

Abstract The preparation of yttrium fluoride (YF3) by fluorination of yttria with ammonium bifluoride is an established process. During fluorination, large amounts of ammonium fluoride (NH4F) vapours are generated in the reaction zone. In order to prevent back reaction, the vapours must be suitably removed from the reaction zone. In usual practice, gas purging (argon or dry air), at fairly high flowrate, removes the vapours suitably. The high flowrates have a considerable effect on the temperature inside the reactor. The ammonium fluoride vapours, transported with the inert gas, condense on the cooler regions of the reactor. The condensed vapours being highly corrosive attack the reactor wall. The present study attempts to address the above issue. A three-dimensional numerical heat transfer model has been developed for the fluorination reactor using a commercial software – Fluent 6·1. The simulations were attempted with varying argon flow velocities, to identify the heat taken away by argon during the reaction. When surface cooling was applied, condensation of ammonium fluoride vapours occurred on the inner surface of the walls irrespective of the argon flow velocity; the simulated results for the extent of condensation from the exit end of the reactor were in fair agreement with the corresponding experimental results.