Volume 14, Issue 5
October 1991, pages 1171-1278
pp 1171-1182 October 1991
A second ceramic age started in the mid-twentieth century as a new, exciting materials frontier. Electroceramics with phenomenally wide range of electrical resistivity (spread over 30 orders of magnitude) span insulators, semiconductors, metal-like conductors, ionic conductors, and, recently, superconductors. They also include ferroelectrics, piezoelectrics, pyroelectrics and electro-optics beside ferrites. Advances in electroceramics have been fascinating and rapid, leading to unprecedented rates of industrial growth. Age-old limitations of poor mechanical strength and brittleness of ceramics are being overcome by outstanding toughness and strength achieved in zirconiabased ceramics exploiting the martensitic transformation at the tetragonal-monoclinic phase change. The dimensional changes at this transition which prevented the use of zirconia earlier has now been turned into a mechanism for toughening ceramics to significant levels. Ceramics with near-zero overall thermal expansion coefficient offer new opportunities to science and industry.
pp 1183-1195 October 1991
The performance of concrete is dictated mostly by the characteristics of the transition zone. Internal bleeding leads to a higher effective water/cement ratio and hence a more open, porous structure at the cement/aggregate interface. Besides, more of AFt, AFm and CH phases and halides, sulphate, alkalis and other admixture components and less of C-S-H are found in this zone. These cause the deterioration of the reinforcing materials like steel, silicious aggregates, glass and natural fibres. The orientation of the crystalline phases parallel to the interface also weakens the mortar. The porosity and related effects are considerably modified by the use of plasticizers, fine pozzolanic additives and reduced water cement ratio. The mechanism of interfacial structure formation and their resulting effects on the reinforcement are discussed together with the possible precautionary measures.
pp 1197-1203 October 1991
Fracture toughness tests were performed on two aluminium alloy matrices, 2014-0 and 2024-0 reinforced with alumina particulates of different volume fractions and particulate sizes so as to investigate the fracture mechanisms operative in such composites and to determine how microstructural parameters such as volume fraction, particulate size and interparticle spacing affect the fracture toughness. The results indicate that fracture occurred by a locally ductile mechanism. The fracture toughness increased with increasing particle spacing provided that the particle size was less than a limiting value, above which unstable crack growth occurred and the toughness lowered.
pp 1205-1211 October 1991
The effect of pressure on non-ohmic conduction and electrical switching in the charge transfer complex benzidine-DDQ has been studied up to a pressure of 7·66 GPa at a temperature of 300K. Pulsed I-V measurements reveal heating contribution to non-ohmicity and switching. At high electric fields (∼ 3 × 103 V/cm), the sample switches from high resistance OFF state of several kiloohms to low resistance ON state of several ohms. Temperature dependence of conductivity of ON state show semiconducting behaviour with very low activation energy.
pp 1213-1218 October 1991
Phase transitions in LiKSO4 have been visually observed in a diamond-anvil cell up to 16 GPa. The observations confirm a symmetry change at 0·9 GPa and a solidstate crystal to amorphous transition at 12 GPa. The crystal exhibits a memory across the crystalline transitions. Interesting microstructural features were observed in the range of the amorphous phase.
pp 1219-1223 October 1991
Characterization of the gel-grown barium hydrogen phosphate (BHP) crystals was performed by utilizing the techniques of chemical analysis, X-ray diffraction, infrared and thermal behaviour. The results show that BHP crystals had chemical composition BaHPO4 at room temperature. TG and DTA studies revealed the BHP crystals to be anhydrous at room temperature and decomposed at temperatures above 370°C and the decomposition was an endothermic process. Magnetic susceptibility measurements indicate the material to be diamagnetic.
pp 1225-1230 October 1991
Magnesium zinc ferrites with the general formula MgxZn(1 −x)Fe2O4 were prepared by the standard ceramic technology route involving double sintering. X-ray analysis was carried out to confirm the single-phase formation as well as to calculate the lattice parameters. Two sets of samples were prepared by sintering the samples at 1100°C for 15 and 30 h respectively. The high-field loop tracer was used to measure the hysteresis parameters. It is observed that the sintering conditions effectively modify the magnetization characteristics of these ferrites.
pp 1231-1240 October 1991
Ion beam-induced and thermal reactions at Fe:GaAs interface are studied by using conversion electron Mössbauer spectroscopy and small angle X-ray diffraction measurements. A thin film of Fe (enriched to 30% in57Fe Mössbauer isotope) was deposited in UHV environment on 〈100 〉 oriented semi-insulating GaAs substrates. Some of the samples were ion-mixed by 130 keV Ar+ ions at dose values of 3 × 1015 and 1016 ions/cm2. The asdeposited and ion-mixed samples were annealed at different temperatures up to a maximum of 500° C. It was observed that ion mixing led to precipitation of disordered and/or defective binary phase along with ferromagnetic Fe3GaAs ternary phase which upon vacuum annealing at 500°C for 1 h leads to a mixture of structurally well-defined Fe3Ga, FeAs and FeAs2 phases. The combined analysis of Mössbauer and X-ray data is shown to reveal the location of the phases below the sample surface. The mechanism for phase formation and associated reaction kinetics at Fe/GaAs interface is discussed in the light of the experimental results.
pp 1241-1248 October 1991
The influence of stacking fault energy (SFE) on the mechanism of dynamic recrystallization (DRX) during hot deformation of FCC metals is examined in the light of results from the power dissipation maps. The DRX domain for high SFE metals like Al and Ni occurred at homologous temperature below 0·7 and strain rates of 0·001 s−1 while for low SFE metals like Cu and Pb the corresponding values are higher than 0·8 and 100 s−1. The peak efficiencies of power dissipation are 50% and below 40% respectively. A simple model which considers the rate of interface formation (nucleation) involving dislocation generation and simultaneous recovery and the rate of interface migration (growth) occurring with the reduction in interface energy as the driving force, has been proposed to account for the effect of SFE on DRX. The calculations reveal that in high SFE metals, interface migration controls DRX while the interface formation is the controlling factor in low SFE metals. In the latter case, the occurrence of flow softening and oscillations could be accounted for by this model.
pp 1249-1255 October 1991
Non-contact processing technique involving the use of CW and pulsed CO2 laser irradiation has been used for reducing the core loss of cold-rolled grain-oriented silicon steel. Laser scribing perpendicular to the rolling direction resulted in a refinement of domain wall spacing which subsequently reduced the loss of silicon steel. It was found that laser irradiation was more effective in a specimen with higher magnetic induction (Hi-B) and the loss was reduced by more than 10% under optimum conditions of the laser irradiation and the scribing speed. Since laser processing is a non-contact technique, it can be easily applied to the production line of the silicon steel.
pp 1257-1278 October 1991
Synthesis and some interesting properties of amorphous silicon-based superlattices are reported. Both quantum-well type and doping modulated structures are studied. Quantum confinement, phonon folding and persistent photoconductivity are some of the fascinating effects which are described and their current interpretations discussed.
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