B.P. Sharma

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.

Carbothermal route for preparation of boron carbide powder from boric acid–citric acid gel precursor

Abstract Boron carbide (B4C) powder has been prepared by carbothermal process using boric acid and citric acid as raw materials. Aqueous solution of boric acid in presence of citric acid forms a stable gel under controlled pH condition. The gel on subsequent pyrolysis under vacuum yields a precursor powder consisting of boron oxide and carbon. The precursor is heated under vacuum at 50 °C/h up to 1450 °C and the product obtained has been characterized by X-ray diffraction (XRD), chemical analysis, particle size analysis and scanning electron microscopy. XRD pattern shows that the product consists of B4C and carbon (graphite). Presence of free carbon (11.1 wt%) indicates boron loss in the carbothermal reaction. Boron loss has been calculated to be 11.3 wt%. Boron carbide powder obtained is composed of near equiaxial particles of narrow size distribution having a median size of 2.25 μm.

Preparation of copper powder by glycerol process

A simple, least energy intensive, efficient, low temperature chemical process for the production of copper powder of narrow size distribution is described. It involves reduction of copper salts by glycerol under atmospheric conditions at a temperature below 240°C. The powders produced by this process have been characterized by X-ray diffraction, chemical composition, BET surface area analysis and scanning electron microscopy. The powder is well crystalline and contains oxygen, carbon, and hydrogen as impurity elements. The purity of the powder produced by the process depends on the starting compound of copper. The glycerol process using copper acetate as starting compound yields copper powder having a purity of 99.7%.

Preparation of silver powder through glycerol process

High purity fine silver powder with uniform particle morphology was prepared through glycerol process. The process involves reduction of silver nitrate by glycerol under atmospheric conditions at a temperature below 175°C. Glycerol, din this process, acts as a solvent as well as a reducing agent. The powders prepared through this process were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical analysis. The powders were well crystalline and contained oxygen, carbon and hydrogen as impurities. Overall purity was better than 99.9%. The yield of silver powder was better than 99%.

Development of calcium phosphate based bioceramics

Two bioceramics (Ca-P-O glass and A-W glass ceramic) were produced using conventional methods of ceramic technology. X-ray powder diffraction patterns were used for identifying the phases and 3-point bend test was carried out for the determination of fracture strength of the bioceramics. Biocompatibility of both ceramics was evaluated using animal model experiments. Histological studies showed that A-W glass ceramic implanted in the tibia of rat formed an intimate contact with newly grown bone and provided enough strength to the bone to bear the animal weight. Implants made of Ca-P-O glass was almost fully resorbed and was replaced by new bone. The implants made of both the bioceramics were biocompatible and did not exhibit any kind of adverse effect to the surrounding tissues.

Novel route for preparation of high voltage varistor powder

Abstract A novel technique for preparing ZnO varistor powder is described. It involves preparation of gel by adding citric acid to nitrate solution of varistor constituents. The gel on subsequent calcination forms submicron reactive varistor powder that exhibits the homogeneity and stoichiometric control associated with the chemical methods of preparation. SEM and BET surface area measurements show the resultant powder to comprise crystallite of submicron size. High field varistors with good nonlinear coefficient result from sintering at 1150 °C for 1 hour. The high value of nonlinear coefficient of the varistor sample can be attributed to the more uniform distribution of dopants and formation of more varistor active grain boundaries. The electrical characteristics of the citrate gel derived varistor are compared to those of a varistor sample prepared through the urea route.

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.