Volume 3, Issue 1
February 1981, pages 1-73
pp 1-8 February 1981
Some new potential dielectric materials have been made through the cross-substitution of Ba2+ by a 1:1 molar combinations of tri-(La3+) and mono-valent (Li+, Na+, K+) ions at the Ba2+ site in BaTiO3. Chemical analysis shows that compositions in the potassium series are about 6·1% deficient in their K2O content and exhibit interesting dielectric relaxations. The cross-substitution has lowered theTc down to room temperature. The disorder in the larger ‘A’ cation sublattice of the three systems has been established by infra-red and x-ray data. This leads to microscopic compositional variations, which in turn can account for the diffuseness of the dielectric anomalies observed in the barium titanate-rich compositions (0⩽x⩽0·3).
pp 9-13 February 1981
Hydrothermal phase stability studies were carried out in the systems (a) Fe2O3-H2O, (b) Fe2O3-H2O-CO2, (c) Fe2O3-H2O-CO2-H2 and (d) Fe2O3-HNO3. In the pure system Fe2O3-H2O, onlya-Fe2O3 and Fe3O4 are the stable solids, with the phase boundary nearly pressure independent around 700°C. The introduction of CO2 into the system catalyses the stability ofα-FeOOH in the system at low temperatures. The presence of H2 in the system produces traces of Fe3O4 at temperatures as low as 230°C and at around 500°C, Fe3O4 is the only stable phase. The stabilisation of Fe3O4 is suppressed even up to 850°C, when NO3− ion is present in the fluid phase.
pp 15-17 February 1981
Structural characteristics of new type of perovskites with formula Bax (Nd0·5Li0·5)1−x TiO3 (0⩽x⩽1) are reported. Nd0·5Li0·5TiO3 crystallises with orthorhombic distorted perovskite structure, while Bax (Nd0·5Li0·5)1−xTiO3 crystallises with tetragonal distorted perovskite structure.
pp 19-23 February 1981
The Thermoelectric emfs of thermocouples formed by amorphous METGLAS 2826 (Fe40Ni40P14B6) and METGLAS 2826B (Fe29Ni49P14B6Si2) with standard thermocouple wires like copper, chromel, alumel, etc., were measured as a function of temperature between −196° C and 30° C to assess their suitability as thermoelectric temperature sensors. Thermoelectric emfs generated by METGLAS 2826/Cu and METGLAS 2826B/Cu thermocouples at −196° C are about an order of magnitude smaller when compared to thermal emfs of a standard copper/constantan thermocouple at the same temperature.
pp 25-28 February 1981
Magnetic susceptibility and ESR studies of As-Se glasses indicate that these glasses are diamagnetic containing only charged defect pairs and that they correspond to a chemically ordered network structure.
pp 29-36 February 1981
Densities and microhardnesses of As-Se glasses have been measured over a wide range of composition. Anomalous variations of densities and microhardnesses have been noted. It has been found possible to explain the variation on the basis of chemical ordering in these glasses. Scanning electron microscopy has been used to investigate the nature of etching in various compositions. The chemical stabilities of these glasses towards etchants also seem to support chemical ordering in them.
pp 37-55 February 1981
Heat conduction equations applicable to the solidification of grey cast iron cast in moulds of the same material were solved for the cases of plate and cylindrical shaped castings made by pouring the metal at different temperatures into moulds of different wall thicknesses preheated to different temperatures, when the heat transfer coefficient at the casting-mould interface was assumed to have different values. An explicit finite difference method was used to solve the equations with the aid of a digital computer. Local solidification times at different nodal points were determined from the solutions and a relationship between the local solidification time and the location in a ‘casting’ was established. The application of local solidification times for predicting the microstructure and estimating the ultimate tensile strength of these “castings” cast in metallic moulds has been demonstrated.
pp 57-65 February 1981
The poor quality of natural scheelite necessitates growing these crystals in the laboratory. The flux growth method has been modified to obtain crystals of better quality. Various habits of the grown crystals are reported. The effect of the growth conditions like temperature, cooling rate, charge composition, etc. to yield different morphology has been investigated.
pp 67-73 February 1981
Basal cleavages of gypsum are etched in nitric acid and potassium hydroxide solution. Terraced pits are commonly observed independent of the etchants used. Varying degrees of widths of terraces are observed on etching matched cleavage pairs as well as thin flakes. The terraced nature of etch pits has been attributed to the inhibitive action of the etchant due to the precipitation of impurities along dislocation lines. On prolonged etching of a thin flake holes are formed at some dislocation sites while at other dislocation sites, holes are not formed. Preferential etching of fission tracks gives rise to winged etch pits. The implications are discussed.
Volume 42 | Issue 6
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