L C Pathak

A review on the synthesis of Y–Ba–Cu-oxide powder

The preparation of Y?Ba?Cu-oxide (YBCO) powder by various techniques is critically reviewed and the synthesis of ultrafine superconducting YBa2Cu3Ox(123) powders is specially emphasized. The fine powder has assumed a large importance, requiring the minimization of processing parameters (time and temperature) during calcination, homogenization and sintering to avoid the problems of evaporation of the constituent oxides. From the voluminous research activities on YBCO it is known that by adopting any preparation technique the superconducting transition temperature (Tc) value of ?92?K could be achieved in the bulk samples, thus in this review the Tc values are not emphasized. Also, the other transport properties such as critical current densities (Jc) and magnetic properties are not discussed, since these properties are mainly controlled by many other parameters related to the weak link across the grain boundaries of the specimens, and a very little role is played by the synthetic procedure. The weak links across the grain boundaries are known to be the cause for the poor Jc values in bulk specimens and those weak links are observed due to the presence of impurities at the grain boundaries, misorientation of grains and oxygenation of the samples. By selecting the proper synthetic procedure, only the impurity problem can be solved, and to remove the other problems, a proper fabrication method needs to be adopted. This review will guide the new researcher to find the appropriate technique to synthesize good quality YBCO powder.

Carbothermal synthesis of zirconium diboride (ZrB2) whiskers

Abstract In the present investigation, carbothermal synthesis technique is used for the production of ZrB2 whiskers. The synthesis is carried out by heating the mixture of ZrB2, H3BO3 and carbon black over the temperature range of 1300 to 1700°C in argon atmosphere. Different elements such as Ni, Co and Fe are tested for their role as catalysts. The synthesised powder contains ZrB2 as the major phase with minor phases of ZrC and B4C. Whisker yield is found to be low at lower temperatures (<1500°C) and then increases with the pyrolysis temperature. The Ni addition seems more effective as a catalyst than Co or Fe. Shorter length whiskers are found in the case of Co catalyst, whereas use of Fe catalyst shows whisker with rod shape and a special birds nest type whiskers. The electron microscope study of whisker reveals the presence of various defects.

Corrosion behavior of titanium boride composite coating fabricated on commercially pure titanium in Ringer's solution for bioimplant applications.

The boriding of commercially pure titanium was performed at 850°C, 910°C, and 1050°C for varied soaking periods (1, 3 and 5h) to enhance the surface properties desirable for bioimplant applications. The coating developed was characterized for the evolution of phases, microstructure and morphology, microhardness, and consequent corrosion behavior in the Ringers solution. Formation of the TiB2 layer at the outermost surface followed by the TiB whiskers across the borided CpTi is unveiled. Total thickness of the composite layer on the substrates borided at 850, 910, and 1050°C for 5h was found to be 19.1, 26.4, and 18.2μm respectively which includes <3μm thick TiB2 layer. The presence of TiB2 phase was attributed to the high hardness ~2968Hv15gf of the composite coating. The anodic polarization studies in the simulated body fluid unveiled a reduction in the pitting corrosion resistance after boriding the CpTi specimens. However, this value is >0.55VSCE (electrochemical potential in in-vivo physiological environment) and hence remains within the safe region. Both the untreated and borided CpTi specimens show two passive zones associated with different passivation current densities. Among the CpTi borided at various times and temperatures, a 3h treated shows better corrosion resistance. The corrosion of borided CpTi occurred through the dissolution of TiB2.

Synthesis of submicron Ba-hexaferrite powder by a self-propagating chemical decomposition process

A simple and versatile method using a self-propagating chemical decomposition technique has been developed. Synthesised powders are submicron in size and require low temperature of calcination and sintering to yield single phase products with good magnetic properties.

Sintering of ultrafine zirconium diboride powder prepared by modified SHS technique

Abstract Pressureless sintering of ultrafine zirconium diboride ZrB2 powder produced by a self-propagating high temperature synthesis (SHS) route was investigated at 1973–2223 K with continuous flow of pure argon gas (20 mL min−1). Ultrafine ZrB2 powder produced by the addition of an inert SHS diluent showed excellent sinterability, and a maximum densification of 97·5% was observed at 2223 K. In contrast, the identically sintered diboride powder produced without the addition of SHS diluent showed only 81·2% densification. The apparent activation energy for sintering was found to vary with particle size from around 37 to 43 kJ. Boron diffusion through ZrB2 is likely the rate controlling factor for the sintering reaction.

Deposition of nanostructured Si–C–N superhard coatings by rf magnetron sputtering

Systematic investigation on the deposition of thin silicon-carbon-nitride films by reactive rf magnetron sputtering from SiC–C composite target in nitrogen-argon atmosphere was studied. The significant effect of deposition pressures on the hardness of the deposited SiCN films was found, which varied between 4.7 and 44GPa. The films were found to be amorphous from x-ray diffraction analysis but localized crystallization was noticed during atomic force microscopy (AFM) studies on these deposited films. The AFM studies also suggested that the increased hardness was due to reduction in particle size and localized formation of β-C3N4 and β-Si3N4 phase in the films. The x-ray photoelectron spectroscopy analyses showed the formation of C–N and Si–N bonds for the harder film. The increased nitrogen concentration in the sputtering gas mixture to 99% resulted in large particle growth and graphitic phase formation, which exhibited a low hardness value of 4.7GPa. The high C content and low Si content in the deposited...

Sintering characteristics of Y-Ba-Cu-oxide-Ag x superconductors

The sintering characteristics of Y-Ba-Cu-oxide (YBCO)-Ag-x (x = 0 to 1.2) using thermomechanical analyzer were systematically investigated to understand the sintering mechanism of the metal superconductor composites. The addition of ag was observed to lower the sintering temperatures, and the apparent densities of the sintered compacts increased with x from 0 to 0.6. A further increase of x above 0.6 decreased the apparent densities of the sintered compacts. The presence of Ag globules in the YBCO-Ag compacts was observed by scanning electron microscopy and energy dispersive x-ray spectroscopy. The apparent activation energies for sintering of the powder compacts were estimated and observed to vary between 900 to 2000 kJ/mol. The formation of AgOx by absorbing oxygen from YBCO and sintering atmosphere possibly controls the sintering and superconducting behavior, incorporation of Ag into the matrix modifies the weak-link characteristics from superconductor-insulator-normal- superconductor (S-I-N-S) to superconductor-normal-superconductor (S-NS) type.

Sintering characteristics of Y-Ba-Cu oxide-Ag-x superconductors under argon atmosphere

Sintering studies on Y-Ba-Cu oxide (YBCO)-Ag-x (x = 0, 0.6, and 1.0 mol) powder were carried out in argon atmosphere to understand the role of silver addition on the densification behavior of these materials. The increase of sintered densities of the compacts with silver addition in argon atmosphere contradicted our earlier observation on sintering of YBCO-Ag-x powder compacts in air, where the densities decreased for x > 0.6. Thermogravimetric (TG) studies under argon atmosphere indicate a continuous decrease of mass on heating suggesting an enhanced rate of oxygen removal from the YBCO matrix that facilitated the sintering in argon atmosphere. Sintering studies of YBCO-Ag-x powder compacts in argon in conjunction with earlier observations in air has substantiated our claim that high-temperature oxygen desorption by the silver from the YBCO matrix to the sintering atmosphere controls the rate of densification for these superconducting composites. However, the apparent activation energies for sintering, suggest that the sintering process is controlled by yttrium ion diffusion along bulk and grain boundaries.

Degradation of BPSCCO superconductors during processing

Evaporation losses of Bi, Sr, Cu and Pb during processing of Bi-Pb-Sr-Ca-Cu-oxide (BPSCCO) superconductors were observed, which affected the superconducting properties. Incongruent loss of Bi, Sr, Cu and Pb was reduced by covering the pellets by excess BPSCCO powder during sintering and formation of the 2223 phase was observed to be enhanced. Formation of low temperature eutectic by silver addition was observed to accelerate the rate of evaporation. Lower temperature processing was required for fabrication of high quality Ag sheathed BPSCCO tapes.