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Mechanically flexible aluminum nitride–silicone rubber composites (SAN) were prepared by hot pressing technique for different filling fractions. The effects of filler content on the dielectric, thermal and mechanical properties as well as on moisture absorption were investigated. The relative permittivity and dielectric loss of the composite were found to vary linearly with filler content. The variation in relative permittivity of SAN composites with temperature was also investigated at a frequency of 1 MHz. Theoretical modelling of relative permittivity of the composites was performed and the results were correlated with the experimental data. Among the theoretical models effective medium theory is in good agreement with experimental values of relative permittivity. The coefficient of thermal expansion and specific heat capacity of the composite were found to decrease with filler content and thermal conductivity, thermal diffusivity and the moisture absorption increased with filler loading. The SAN composite is found to be a good candidate for a thermally conductive flexible microwave substrate application.

Non Linear ceramic Resistor (NLR) was processed using ZnO-La2O3 rare earth composition via combustion technique involving urea and glycine as organic fuels. The La2O3 NLR ceramic was sintered at 1300 °C/ 3 h that showed dense microstructures with average grain size of 1 μm. The current v/s voltage characteristics of La2O3 NLR were compared among the fuels. In addition to that the combustion chemistry of urea and glycine was monitored via C-H-N-S analysis and the functional property was assessed by UV-Vis analysis. The I-V analysis confirmed that the La2O3 NLR ceramic prepared with urea fuels exhibits only a linear resistor behavior whereas an excellent non-ohmic nature was obtained in glycine fuels. The grain-grain boundary property was further examined by impedance analysis. The study brought out a promising, eco-friendly rare earth composition for obtaining high-energy NLR ceramics.

Nano size ZnO–Bi2O3 varistor precursor powders containing Y2O3 and Pr6O11 rare earth dopants were prepared by low temperature refluxing at 80 °C. Effect of rare earth dopants, densification by two-step sintering, evolution of microstructures and their influence on varistor properties were investigated. Chemically synthesized nano- precursor varistor powders produced controlled grain size in two-step sintering in which the average sintered ZnO grain size was reduced to at least half compared to the conventionally processed ZnO–Bi2O3 varistors. The study revealed that such grain size reduction is highly beneficial to attain enhanced varistor properties.

Nanocrystalline ZnO nanorods were successfully grown by ultrasonication using an acidic ethanolic zinc acetate precursor solution followed by a flow coating process and annealing at 600 °C. The ZnO nanorods obtained were hexagonal in shape and showed a high degree of uniformity in size and distribution. These samples were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Raman spectrophotometry and the results are discussed. This approach appears to be the easiest way to fabricate bulk ZnO nanorods.

This paper describes the process for the hydrolysed aluminium treatment (HA) on alumina surface and its influence on the rheological characteristics of alumina slurries. Three different commercial grade aluminas were provided with the surface treatment with hydrolysed aluminium using aluminium nitrate and hexamethylenetetramine (HMTA) under controlled conditions. The HA treatment increased the concentration of Al-OH surface sites resulting in higher H+ adsorption on the alumina surface. A highly concentrated (>55 vol%) electrostatically stabilized alumina slurry was prepared from HA treated alumina powders. The rheology of such slurry was studied and the results on the viscosity and yield stress are presented. The alumina slurries followed the Casson Model flow behaviour. The ‘wet’ and ‘flow’ behaviour of the alumina with and without HA treatment was also studied and the results are compared. The surface treatment showed the advantage of maintaining low viscosity and yield stress of alumina slurries even at higher solids loading (>55 vol%) that are prepared in the acidic aqueous medium. The results on viscosity and yield stress were compared with that of the polyelectrolyte dispersed system.

Silica gel has been prepared by the hydrolysis of TEOS under three different pH conditions. These gels have been subjected to drying in an air oven, under humidity, and also by exposure to microwaves, followed by further calcination at 500°C under slow heating schedule (3°C/min). Silica samples thus obtained are characterised by B.E.T. specific surface area and pore volume data. The thermal decomposition behaviour of the precursor gels are also reported. The adsorption isotherms of these samples indicate different behaviour related to the method of synthesis. Increase in pH of hydrolysis of the TEOS from 3 to 8 results in increase in specific surface area in tune with the earlier reports. However more significant observation is the variation in pore size distribution as evidenced from adsorption isotherms related to method of drying. Silica gels prepared at pH 3 show Type I behaviour irrespective of the method of drying. However gel prepared at pH 6 shows Type II behaviour when dried under microwave with specific surface area as high as 635 m2/g and pore volume 0.9733 cc/g. The precursor gels prepared at still higher pH exhibit Type IV behaviour when subjected to microwave drying. The pH conditions of synthesis of precursor gels along with drying techniques appear to affect not only the surface areas and porosities but also the resultant adsorption isotherms, in sol-gel silica.

Sintered ZnO ceramics containing additions such as Bi203, Pr6Oll and SrO with minor dopants of Co304, Cr203 and MnO2 have shown non-linear current-voltage properties, and hence have found wide use in electronic circuits to protect solid-state components against transient overvoltages [1]. The electrical characteristics, such as breakdown voltage (VB) and non-linearity coefficient (0l), of these sintered polycrystalline ceramics are determined primarily by the additives and the microstructure. The microstructure of ZnO varistors comprises semiconducting n-type ZnO grains surrounded by insulating barriers at the ZnO grain boundaries. The electrical conduction through the grain boundary barrier is explained by a double Schottky barrier (DSB) defects model [2]. During sintering, the presence of secondary phase particles forms a liquid phase between grain-grain junctions. The processing and sintering of the ZnO compositions sometimes results in inhomogeneous grain growth, which in turn leads to random grain size distribution. A microstructure having large uniformly distributed grains is essential for low voltage varistors [3]. Different approaches have been taken to control the grain growth in zinc oxide to result in uniform size. Titanium oxide has been introduced up to 0.5 mol % in the varistor composition, with marginal success [4]. Ready-grown ZnO grains prepared by calcining ZnO particles above 1300 °C, usually denoted by the term "seed", have been added to varistor composition and such seeds can arrest the non-uniform growth in bulk ZnO. A similar seeding technique was successfully used to control the grain growth in BaTiO3 ceramics, essentially for suppression of duplex microstructures [5]. Seed grains are generally monocrystalline particles which are slightly larger than those of the original powder [6]. An attempt has been made here to study the effect of such calcined zinc oxide grains in a varistor composition such as ZnO-SrO, which is less extensively studied. Details of the preparation of the varistors, their non-ohmic behaviour and microstructural features are reported. Calcined zinc oxide grains (CZO) were prepared by calcination of ZnO powders at near sintering temperature (1100 °C/2 h) in an alumina crucible. The pre-fired powders were gently crushed in an agate mortar and subsequently fractionated by sieving. The resultant grains consist of crystalline fragments with a grain size about 10-15/xm, which