Platelet-like Grains Research Articles

The synthesis and characterization of tapes and the Ag-induced effect on their microstructure

The present study is based on improving the intergranular connectivity of Tl HTSC tapes by passivation of grain boundaries through the addition of silver. HTSC tapes to which silver had been added were fabricated by the diffusion of Tl vapour in the precursor tape to which silver had been added. The tapes were cast by using a doctor-blade tape-casting machine. These tapes having Tl:2223 as the majority phase were found to possess in the range 107 - 113 K. The critical current density of the Tl HTSC tapes was found to scale with the silver content and optimum (20 K) and (77 K) were obtained for HTSC tapes. SEM of pure (without silver) and Ag-admixed Tl:2223 tapes revealed that, as the silver content increased, the grain size and intergranular connectivity increased. With an optimum concentration of Ag (y = 0.40), a well-oriented platelet-like grain structure became stabilized, leading to an improvement in . The most curious microstructural feature, observed for the first time in the present study, was the occurrence of sequential layer growth features resembling spiral characteristics. The influence of these microstructural features on the superconducting properties, in particular the transport critical current density, is described and discussed.

Properties of YBa 2Cu 3O 7−δ-Ag superconductors prepared by the citrate method

YBa 2Cu 3O 7−δ (123) ceramic superconductors and YBa 2Cu 3O 7−δ-Ag composite superconductors have been prepared by the citrate method. Microstructural analyses have been done by X-ray diffractometry and optical ceramography. The superconducting behavior has been studied by 4-probe dc resistivity in the 77 K–140 K temperature range. Silver percolation in the ceramic matrix was studied by electrical resistivity measurements at room temperature; the percolation threshold was found to be approximately 25 vol% (35.5 wt%) Ag. Specimens with silver addition showed improvement in the flexural strength of the 123 compound. The main results show that the critical temperature does not depend on the silver content in the composite specimens, and that approximately 3 wt% (1.8 vol%) Ag doping yields an optimized composite superconductor from the microstructural point of view, with platelet-like grain shapes.

Experiments on the microstructure of evaporated CuInS 2 thin films

The morphology and microstructure of evaporated CuInS 2 films is investigated with the emphasis on the effect of stoichiometry deviations. The compositions of In-rich and Cu-rich films obey the relations In 2S 3-CuInS 2 and CuS-CuInS 2 in the ternary phase diagram, respectively. In-rich films exhibit a platelet-like grain structure. This morphology can be explained by the formation of twin dislocations. Cu-rich films show a textured morphology with grain sizes in the range of 1–4 μm.

Hot rolling of Bi1.6Pb0.4Sr2Ca2Cu3Ox superconducting pellets

Alignment of grains has been studied in sintered pellets of Bi1.6Pb0.4Sr2Ca2Cu3Ox upon rolling in hot condition. The pellets of diameter 12.7 mm and thickness 2.5-4.5 mm were kept in a pre-heated furnace for 15 min. Two furnace temperatures, namely 865 and 880 degrees C, were used in two different sets of experiments. The hot pellets were wrapped in ZrO2 cloth and rolled in a roller at room temperature. During transfer from the furnace to the roller the temperature at the surface of the pellet dropped by 10-15 degrees C. About 50% reduction in thickness was imparted to the pellets in a single pass. Considerable alignment of platelet-like grains was observed in hot-rolled pellets with the help of a scanning electron microscope (SEM), but superconductivity was destroyed due to the formation of a glassy phase upon rapid quenching (during rolling) of the liquid phase, which was formed by partial melting. By annealing the rolled pellets at 840 degrees C for 24 h in air the superconductivity was recovered. The superconducting transition temperature at zero resistance (Tc) and the transport critical current density (Jc) at 77 K of the pellets, rolled after pre-heating at 865 degrees C and subsequently annealed, were 105.5+or-1 K and 124+or-7 A cm-2 respectively, compared with 100.1+or-0.6 K and 41+or-4 A cm-2 for the sintered pellets. However, Tc of the pellets rolled after heating at 880 degrees C reached only 60+or-2 K upon subsequent annealing. Rolling increased the density and hardness of the pellets.

Grain growth enhancement in silver-doped YBa2Cu3O7−x superconductor

The effects of silver addition on the microstructural and superconducting properties of YBa2Cu3O7−x (YBCO) have been investigated. It was found that the microstructure of silver-doped YBCO samples is characterized by highly oriented platelet-like grains (up to 3 mm × 0.5 mm × 15 μm). The superconducting transition temperatureT itc (R= 0) is decreased with increasing wt% of silver. The critical transport density (J cT) is, however, found to be improved (up to two orders of magnitude) in the doped materials (JitcT (T= 77 K andH = 0 T) = 1250 Acm−2). It is believed that the addition of silver lowers the melting temperature of the system and thereby enhances the decomposition of the compound and promotes grain growth during solidification.

The anisotropic powder metallurgy of n-type Bi2Te2.85Se0.15 thermoelectric material

The anisotropic powder metallurgy of n-type Bi2Te2.85Se0.15 doped with bromines was studied by both x-ray diffraction and thermoelectric measurements. The statistical orientation of platelet-like grains in the samples was characterized using the orientation factor estimated by the Lotgering formula from (0,0,1) x-ray diffraction intensities. It is demonstrated that the orientation factor which is strongly influenced by the hot-pressing conditions and the particle size of the starting material has a strong effect on the thermoelectric properties of the samples in this system. This implies that the powder metallurgy has the additional freedom of controlling the thermoelectric properties in addition to the doping level in the grains.