Volume 17, Issue 2
April 1994, pages 95-199
pp 95-103 April 1994
The processing of alumina ceramics using the sol-gel technique was investigated. Alumina sols were prepared by precipitation-peptization of aluminium hydroxide obtained from aluminium nitrate and aluminium oxyhydroxide obtained from aluminium isopropoxide. Sols cast on Teflon substrate were gelled by controlled dehydration at near-ambient temperature. Gels were converted to ceramics through processes of drying, calcination and sintering in the temperature range 773–1473 K. Ceramics derived from both the sols retained a porosity of 35–45% even after sintering at 1273 K. However, at 1473 K densification was drastically enhanced and bodies could be sintered to about 90–93% of theoretical density (TD) and was accompanied by theϑ-to-α transformation. The progress of the reactions occurring at various stages of the process was studied using pycnometry, TG-DSC, XRD, SEM, MIP and TEM techniques.
pp 105-110 April 1994
Calcium pyrophosphate dihydrate crystals, which are responsible for various arthritic diseases, were grown in sodium metasilicate gel. Variations in gel density, pH value and concentration of the reactants were found to have profound effect on the growth of the crystals. The grown crystals were characterized using IR, XRD, TGA and optical microscopy.
pp 111-119 April 1994
XRD and microstructure studies were carried out on Ti4+ and Zr4+-substituted Li-Zn ferrites prepared by standard ceramic technique. All the ferrite compositions exhibit single phase formation. The lattice parametera increases linearly with the content of Zn2+Zr4+ and Zn2+Ti4+, which is attributed to the ionic volumes of the cations involved. With substitution by Zr4+ the average size decreases, while with substitution by Ti4+ the grain size increases. In both the series grain size varies with the composition. Excess substitution of Zr4+ (x>0·4) leads to the formation of secondary images and discontinuous grain growth. Both Zr4+ and Ti4+ compositions obey Kurtz theory.
pp 121-139 April 1994
Four binary Al-Co alloys containing 2·4, 14, 20 and 25 at.% Co and a ternary Al-Co-Si alloy with 20 at.% Co and 5 at.% Si were rapidly solidified from the liquid state by melt spinning. It has been shown that the solid solubility of Co in Al can be increased up to at least 1·7at.% from the negligible value under equilibrium conditions. Two metastable crystalline intermediate phases and a D1a-type ordered phase were observed in localized regions in the rapidly solidified Al-2·4at.%Co alloy. A quasicrystalline decagonal phase has been observed in all the other rapidly solidified alloys. This decagonal phase was shown to contain dislocations, dislocation loops and also twin-like defects. The phenomenon of polytypism was also observed in this decagonal quasicrystalline phase. In extremely thin areas of the Al-14at.% Co alloy ribbons on amorphous phase was detected. Transmission electron microscopy was used to extensively characterize the phases in the as-solidified condition and also those produced during and after transformation to the equilibrium constitution.
pp 141-152 April 1994
Phonon spectra for mixed systems of II–VI compounds are worked out in an MRI model and the Debye Waller factor evaluated from these phonons are compared with the recent experimental values of Baudouret al where some interesting behaviours are reported.
pp 153-162 April 1994
DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andHmax, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJcgm on low-field M-H loop. The estimation of intergranular critical current densityJcjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJc(T) with silver content under low AC fields only, consistent with the transportJc data; beyond thatJc(T) decreases. This improvement inJc(T) and transportJc with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (Hmax>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionHc2j and lower critical field of intragrain regionHc1g also decrease with silver content.
pp 163-169 April 1994
A detailed investigation on luminescence properties of gem-grade zirconia (NFC) as a function of Ti doping is presented. The effect of various parameters such as Ti concentration, environment of heat treatment and temperature was studied in detail and the optimum conditions for producing zirconia with luminescence properties comparable to standard material determined.
pp 171-179 April 1994
In this paper we describe the toxicological tests done on a machinable glassceramic, prepared at the Defence Metallurgical Research Laboratory. Tests show no toxicity and biocompatibility is inferred.
pp 181-194 April 1994
This paper deals with accelerated and long-term durability tests of glassfibre-reinforced cement composites. AR glassfibres REZAL, ESAP and SVUS 16, developed in Czechoslovakia, and Cem-FIL 1 fibres developed in the United Kingdom were applied. REZAL and ESAP fibres are of a low zirconium dioxide content, SVUS 16 and Cem-FIL 1 fibres are of a high zirconium dioxide content. The accelerated and the long-term ageing process were studied. The modulus of rupture was determined after 28, 90, 180, 360, 720 and 1080 days, and 6 years of ageing. The investigated glassfibre types have shown a high alkali resistance, mostly when dry curing was applied. Within the testing period of six years, glassfibres of low as well as high zirconium dioxide content show similar behaviour in the cement matrix.
pp 195-199 April 1994
α-Si3N4 fibres have been synthesized by carbothermal reduction and nitridation of pre-oxidized SiO1·7. The fibres were characterized using X-ray diffraction, infrared spectroscopy and electron microscopic techniques. The likely mechanism of reaction has been outlined.
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