R. D. Purohit

Ultrafine ceria powders via glycine-nitrate combustion

The ultrafine ceria powders have been synthesized by the combustion technique using glycine as a fuel and nitrate as an oxidizer. The auto-ignition (at ≈200°C) of the viscous liquids containing cerium nitrate and glycine resulted in voluminous ceria powders. An interpretation based on an adiabatic flame temperature, for different fuel-to-oxidant ratios, has been proposed for the nature of combustion and its correlation with the powder characteristics. The combustion synthesized ceria powders have been characterized by XRD, HRTEM, surface area analysis, and sinterability. Specific surface area and primary crystallite size of the ceria powder obtained through fuel-deficient precursor was found to be ≈75 m2/g and 2.5–12 nm, respectively. The powder, when cold pressed and sintered in air at 1250°C for 1 h, attained the sintered density ≈94% of its theoretical density, with submicron grain size.

Synthesis of monophasic Ba2Ti9O20 through gel combustion

Monophasic Ba2Ti9O20 has been prepared using the powder derived through an auto-ignition route. The process involves the formation of a viscous gel by thermal dehydration of the pH adjusted citrate–nitrate solution. The process parameters, for example, the amount of citric acid used for gelation and pH of the starting solution were optimized experimentally to get a stable gel with a desired decomposition behavior and favorable powder characteristics. The auto-ignition (at an external temperature of ≈225 °C) of the gel resulted in a voluminous, nanocrystalline (<50 nm) powder containing intimate blending of BaTi4O9 and TiO2 with traces of carbonaceous material. The monophasic Ba2Ti9O20 could be obtained at a calcination temperature of 1200 °C. The powder obtained after auto-ignition, when calcined at 600 °C and sintered at 1250 °C for 6 hours, produced ≈96% dense, phase pure Ba2Ti9O20.

Nanocrystalline Electroceramics by Combustion Method

Solution‐combustion is a simple and cost‐effective technique, to synthesize phase pure, nano crystalline powders having high surface area and better sinterability. The potential of combustion technique in synthesizing several electro ceramics, having application in Solid Oxide Fuel Cells (SOFC), has been explored. These nanopowders were characterized in terms of X‐ray diffraction, surface area analysis, electron microscopy, sinterability and conductivity data. The main outcome of this work was preparation of highly sinter‐active powders.

Fabrication and testing of unileg oxide thermoelectric device

A prototype of oxide thermoelectric unileg device was fabricated. This device was based on only n-legs made of La doped calcium manganate. The powder was synthesized, characterised and consolidated in rectangular thermoelements. A 3×3 device was fabricated by fitting 9 rectangular bars in alumina housing and connected by silver strips. The device has been tested under large temperature difference (ΔT=480°C) using an indegenous system. An open circuit voltage of 468 mV was obtained for a nine leg ‘unileg’ device. The device exhibits a internal resistance of ∼1Ω. The maximum power output for this nine leg device reached upto 50 mW in these working condition.

Development of Ca-doped LaCr03 feed material and its plasma coating for SOFC applications

In order to realize SOFC as power generating devices, multiple cells are connected in series through an interconnect material to accumulate the voltage output. The interconnect should have very low permeability for the gases used. A novel solution combustion process has been developed for producing the phase pure, well-sinterable powders of Ca-doped LaCrO3 interconnect material. A process has been developed to produce the coarse granules as a feed material using combustion-synthesized powder for plasma spray through (a) preparation of granules through cold iso-static pressing followed by breaking and sieving (b) sintering of the green granules followed by sieving. The flow ability and deposition efficiency studies on +45-75 and 75-125 μm powders suggested that +45-75 powder is more suitable for the plasma spray coating. The plasma process parameters; plasma power, flow rate of carrier gases and distance between substrate and plasma gun have been optimized to achieve required coating characteristics. The as-produced coating using 20 kW power plasma gun on the porous Sr-doped LaMnO3 cathode substrates has been examined by SEM. An adherent coating of about 100 μm has been observed in the micrographs. No large cracks were observed throughout the coating. However, the coating was not found to be impervious in nature. Also the micrographs showed incomplete melting of the plasma-coated material. The similar experiments were performed using a higher power (≈ 60 Kw) plasma gun. The coated coupons were tested for leakage by checking water penetration. It was found that water did not penetrate for quite a long time. Therefore, the coupon was further tested for leakage by keeping it over a port connected to vacuum pump. The vacuum attained was 7×10-3 mbar and it was maintained for four consecutive days. The SEM studies on the coated sample showed a quite dense coating along with a very few small local pores.