High Positive Temperature Coefficient Research Articles

Ceramic Manufacturing Technology for the High Performance PTC Thermistor

The roles and the effects of transition elements and other additives on the resistivity and the ratio of the resistivity rise of the semiconducting barium titanate ceramics are presented. The variety of the typical experimental data are illustrated to explain that the exact controlling up to ppm order of the small amount components including both impurities and additives as well as the controlling of the characteristics attributing to the powder, is very important technology for the industrial manufacturing of the high performance positive temperature coefficient (PTC) thermistor. The importance of the exact controlling of the whole firing program is also discussed, indicating the existence of particular temperature ranges at high temperature regions, both in the heating and cooling processes.

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Radiolytic stability at room temperature of crystalline alkaline azides, bromates and nitrates has been compared to their respective thermal stability. The observed variation of radiation yields (of N 2 and Me 0 in azides, of Br − and O 2 in bromates, and of No 2- in nitrates) is accounted for by a correspondence between the size of radiolytic fragments and the “free volume” in the crystal lattice. The dependence of radiation yield on LET (linear energy transfer) has been investigated, irradiation sources being 60Co (dose rate 29 × 10 16 eV/g s) and a 4·7 MeV proton beam (average rate 0·5–1·5 × 10 19 eV/g s). The hypothesis concerning the predominant role of thermal spikes in the radiolysis of ionic salts is critically analysed, and experimental data which are not in agreement with the “hot spike” hypothesis are given. Radiolytic yields from alkali azides radiolysis decrease to one-third when LET increases in spite of the high positive temperature coefficient of azide radiolysis. On the contrary the radiolytic yields in alkali perchorates increase when LET decreases despite the low corresponding temperature coefficient. In sodium nitrate radiolysis two temperature regions of LET effect are observed: thermal effects in tracks are predominant in the low temperature region (20–100° C), whereas above 180° C secondary reactions are of increasing importance. Action of additives incorporated into the lattice of silver oxalate and potassium nitrate on the radiolysis of these salts has been examined. The inhibiting action of cadmium ions introduced into silver oxalate is explained by a diminution of the interstitial silver ion concentration. The effect of Tl +, Sr 2+, Pb 2+, Ce 3+, La 3+, Pr 3+, Nd 3+ ions on the radiolysis of potassium nitrate is regarded as resulting from ionic defects produced by the incorporation of these additives into the lattice; the resulting defects increase the radiation yield due to localization of excitation or to local increase of the free volume in the lattice but, in addition, they can improve the annealing of radiation defects. Doping of potassium nitrate with additives which can act as electron donors (Tl +, Ce 3+, Pr 3+) increases the radiation yields. Doping of nitrates in the anionic sublattice (e.g. with SO 4 2−) does not change the radiation yields.