Volume 22, Issue 1
February 1999, pages 1-69
pp 1-7 February 1999 Rapid Communication
Axially compressed Kevlar 49 fibres have been examined by X-ray diffraction methods. The most prominent effect of axial compression is the anisotropic deformation of the unit cell. Whereas thec-axial length, which corresponds to the chain axis, undergoes contraction, the basal plane dimensions manifest enlargement. The deformations increase with the extent of axial compression. The half-widths and the azimuthal spread of reflections also exhibit changes. The compression induced structural changes provide qualitative support to the experimentally observed reduction in tensile strength and modulus.
pp 9-10 February 1999 Rapid Communication
Studies on the models of hot deformation of materials indicate that there is a linear relationship between the dissipation efficiency parameter of the dynamic materials model, and the intrinsic hot workability parameter of the polar reciprocity model, and both models need a stability criterion for delineating the regimes of flow instabilities in the processing maps.
pp 11-13 February 1999 Crystal Growth
High quality 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal has been successfully grown from saturated methanol solution by a slow cooling method in a Teflon vessel. The full width at half maximum (FWHM) of (001) X-ray diffraction (XRD) rocking curve depends on the growth rate, and the FWHM could be decreased to 15·8 arcsec. Close relation was observed between the mechanical property and crystallinity. The Vickers hardness increased up to 49 on decreasing FWHM.
pp 15-16 February 1999 Mechanical Properties
The crystals of InxBi2−xTe3 (x=0·1 to 0·5) have been grown by zone-melting method. In order to study anisotropy exhibited by the (0001) plane of the crystals, the directional hardness was determined by producing indentations at various azimuthal orientations of the indentor with respect to the surface over a range 0–180°. The crystal was rotated about the indentor axis in steps of 15° while keeping applied load and loading time constant at 50 g and 20 sec, respectively. For annealing study, the sample was kept at a temperature of 375°C. It was observed that softening of crystal takes place and the hardness decreases to a considerable extent.
pp 17-20 February 1999 Mechanical Properties
CP aluminium was subjected to axysymmetric compression testing at 303, 373, 473, 573 and 673 K. Strength coefficient and strain hardening exponent were determined from log-log plot of true stress and true strain at different temperatures. True stress vs temperature at different strains indicated that dynamic strain ageing (DSA) occurred between 473 and 673 K. Warm working is to be done between 423 K and 473 K, so that optimum substructure hardening and strain hardening can be achieved during mechanical processing.
pp 21-23 February 1999 Mechanical Properties
Temperature dependence of the Vickers microhardness of Bi2Te2·8Se0·2 single crystals has been studied. Loading time dependence of microhardness at different temperatures has been used for creep study in the temperature range 303 K-373 K. The activation energy for indentation creep of the crystals has been evaluated.
pp 25-32 February 1999 Mechanical Properties
This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SiCw) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.
pp 33-35 February 1999 Composites
The precipitation hardening of a Al-Zn-Mg-Al2O3(p) composite is explored. It is found that the peak hardness achieved is almost double that of precipitation hardening of Al-Zn-Mg alloy or dispersion strengthening of Al-Zn-Mg with 5% Al2O3(p). Toughness is marginally improved and tensile strength is one and half times that of precipitation hardened Al-Zn-Mg alloys. The ageing time for peak hardness is reduced due to acceleration of formation of precipitate.
pp 37-47 February 1999 Composites
Several zirconia-mullite and alumina-zirconia composites were prepared and their physical, chemical, mechanical and thermal properties determined. The phase assemblage, size of ZrO2 crystals and microstructure of the composites were ascertained. This paper embodies a comparison of the alumina-zirconia and zirconia-mullite composites with respect to their properties and also the constitution-property relationships between each group of composites. Alumina-zirconia composites (AZ) were found to be superior to zirconia-mullite composites (prepared through ZMS or ZMC route) in almost all respects. Thermal-shock resistance and hydration resistance of the clay-based zirconia-mullite (ZMC) composites and alumina-zirconia (AZ) composites were almost at par.
pp 49-57 February 1999 Electrical Properties
Varistors in the new system ZnO-Bi2O3-SiO2 were prepared through conventional ceramic processing route. The effect of sintering temperature and time (0·5 h to 2 h between 1000° and 1250°C) on the microstructure and current/voltage characteristics of the varistors of the new system were investigated and the results were compared with those of ZnO-Bi2O3 system varistors prepared. An increase in nonlinear coefficient (α) value was observed in the SiO2 added varistors. The microstructure and the phase of the varistors were examined by means of SEM and XRD. The Zn2SiO4 spinel phase was found to be present in the intergranular region. The grain growth exponent was determined to be 2·5±0·2 and the activation energy for the ZnO grain growth was estimated to be 251±11 kJ/mol. These values were compared with those estimated for ZnO-Bi2O3 system varistors.
pp 59-64 February 1999 Electrical Properties
Pyroelectric properties of bismuth layer structured calcium substituted sodium bismuth titanate (Na0·5Bi0·5)1−xCaxBi4Ti4O15 withx=0 (NBT) andx=0·1 (0·1 NCBT) were studied by measuring the current under short circuit condition. Spontaneous polarization and figures of merit were evaluated from the measured values of pyroelectric coefficients, dielectric constant and loss tangent. The pyroelectric properties were studied in poled and short circuit condition from 30°C to 700°C. The results obtained were discussed in the light of dielectric and conductivity properties of the samples.
pp 65-69 February 1999 Organic Solids
Powder XRD of dotriacontane and hexadecane hydrocarbon mixtures was made. The patterns were analysed and phase behaviour recorded. Results were interpreted based on the concept of phase separation.
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