Conventionally batched varistors in the ZnO–Co3O4–Pr6O11 system that were sintered at or above 1280 °C exhibited a sharp increase in the average breakdown voltage per grain boundary. In these samples, barrier heights and donor concentration values, derived from capacitance‐voltage analysis, exhibited sharp changes at the same soaking temperature. At or above 1280 °C, grain growth in this system proceeds with the assistance of an eutectic liquid. The improvement of the distribution of Pr6O11, by precipitation of Pr(OH)3 during batching, resulted in varistors markedly more insensitive to firing temperature than conventionally batched ones. The presence of a liquid phase, at least during grain growth, results in an efficient distribution of grain boundary materials and dopants in general. Suitable chemisorbed gases along the grain boundaries are believed to be responsible for the formation of interface states. Segregated materials appeared to provide for enhanced transport of gaseous species along the grain boundaries. The need for the addition of oxides with large ionic‐radius cations, i.e., varistor‐forming oxides such as bismuth, barium, praseodymium, or lanthanum oxide, to varistor compositions is explained within this context.

The effect of liquid‐phase sintering on the properties of Pr6O11‐based ZnO varistors

Determination of cadmium at ultra-trace levels by CPE-molecular fluorescence combined methodology.

Effects of PVA content and calcination temperature on the properties of PVA/boehmite composite film

This paper reports the effect of the variation of citrate to nitrate ratio on the thermal decomposition characteristics of alumina precursor and the properties of nanocrystalline alumina synthesized using this auto-ignition process. The technique involves the auto-ignition of a citrate-nitrate gel occurring between Al(NO3)3 and citric acid to yield an ash powder that upon calcination at 1373 K produced α-alumina. The auto-ignition was restricted to a particular range of citrate to nitrate ratio in the gel. The resulting powder exhibited large surface area (40–50 m2 gm−1) and fine crystallite size. It was established from various characterization techniques that the alumina powder prepared with a C/N ratio of 0.3 has got the optimum powder characteristics compared to the rest of the batches, thus establishing the importance of maintaining a stoichiometric or near stoichiometric C/N ratio. The process has a higher degree of reproducibility and a good potential for large-scale production of alumina.

Effect of citrate to nitrate ratio on the decomposition characteristics and phase formation of alumina

Precipitation and characterization of nickel hydroxide powders were investigated A comprehensive precipitation model incorporating the metal ion hydrolysis, complexation and precipitation reactions was developed for the production of the powders with urea precipitation method Model predictions on Ni{sup 2+} precipitation rate were confirmed with precipitation experiments carried out at 90 C Experimental data and model predictions were in remarkable agreement Uncertainty in the solubility product data of nickel hydroxides was found to be the large contributor to the error There were demonstrable compositional variations across the particle cross-sections and the growth mechanism was determined to be the aggregation of primary crystallites This implied that there is a change in the intercalate chemistry of the primary crystallites with digestion time Predicted changes in the concentrations of simple and complex ions in the solution support the proposed mechanism The comprehensive set of hydrolysis reactions used in the model described above allows the investigation of other systems provided that accurate reaction constants are available the fact that transition metal ions like Ni{sup 2+} form strong complexes with ammonia presents a challenge in the full recovery of the Ni{sup 2+} On the other hand, presence of Al{sup 3+} facilitates the complete precipitation of Ni{sup 2+} in about 3 hours of digestion A challenge in their predictive modeling studies had been the fact that simultaneous incorporation of more than one metal ion necessitates a different approach than just using the equilibrium constants of hydrolysis, complexation and precipitation reactions Another limitation of using equilibrium constants is that the nucleation stage of digestion, which is controlled mainly by kinetics, is not fully justified A new program released by IBM Almaden Research Center (Chemical Kinetics Simulator{trademark}, Version 101) lets the user change the order of kinetic components of a reaction which was set to stoichiometric constant with which the species appear in the reaction in KINSIM by default For instance, in the case of LDH precipitation, the new program allows to change the order of species in the reactions associated with Al{sup 3+} and let the Ni{sup 2+} reactions take over This could be carried on iteratively until a good fit between the experimental data and the predictions were observed However for such studies availability of accurate equilibrium constants (especially for the solubility products for the solid phase) is a prerequisite

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1730&context=rtd

Homogeneous precipitation of nickel hydroxide powders

The large scale manufacture of sodium chromate is carried out by heating finely ground chromite ore mixed with sodium carbonate and lime in air. The essential reaction leading to the formation of sodium chromate is

$$2Cr_2 O_3 + 4 Na_2 CO_3 + 3 O_2 \xrightarrow{{\Delta {\rm H}_{R^0 } }}4Na_2 CrO_4 + CO_2 $$

Reaction between chromium(III) oxide and oxygen in the presence of sodium carbonate

Thermal analysis of alumina precursors prepared by two different PFHS (precipitation from homogeneous solution) methods and a conventional method is described. All three precursors exhibit distinct thermal behaviour patterns, marked by multiple phase transformations, to yieldγ-Al2O3 ultimately. Thermal analysis studies, coupled with XRD, IR and elemental analysis data, indicate that the precursors obtained by the PFHS methods are monophasic in nature, and hence yield relatively small, uniform microspheroidal alumina in comparison with the alumina obtained by the conventional method.

Thermal analysis of alumina precursors prepared by ‘PFHS’ methods

Precipitation from a homogeneous solution of aluminium nitrate by neutralisation using urea in presence of succinic acid leads to the formation of a well-defined alumina precursor, basic aluminium succinate, which on calcination yields microspheroidal γ-alumina with excellent free-flowing characteristics.

Basic aluminium succinate: An alumina precursor formed by precipitation from homogeneous solution

The preparation of a novel nickel trihydroxy isocyanate, along with its distinguishing physicochemical characteristics has been reported for the first time.

M. N. Sastri

Papers

Reaction between chromium(III) oxide and oxygen in the presence of sodium carbonate

Thermal analysis of alumina precursors prepared by ‘PFHS’ methods

Basic aluminium succinate: An alumina precursor formed by precipitation from homogeneous solution

A novel nickel trihydroxy isocyanate — preparation and characterisation