Fully dense doped‐ZnO varistors with sub‐ micrometer grain size (see Figure for a TEM image) can be fabricated by a strategic two‐stage, one‐cycle, thermal processing method at temperatures as low as 825 °C. The nonlinear coefficients α of such fully dense varistors are as high as 270, which is more than one order of magnitude higher than those currently given for commercial ZnO varistors.

Homogeneous and nanosized (28 nm crystallite size) dopedZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore-size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquidphase sintering at 850°and 940°C for 1-5 h. Apparently, the low pore-coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°-850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 m. Ceramics so-fabricated showed a nonlinear coefficient, ␣, of >70, and a breakdown voltage, V b (1 mA/cm 2 ), of >1500 V/mm. The high electrical performance of the doped-ZnO dense ceramics was attributed to liquid-phase recession on cooling, which enhanced the ZnO-ZnO direct contacts and the potential barrier effect.

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The study of BaTiO 3 crystallization from X-ray amorphous (Ba,Ti) polymeric organic powders has been carried out by comparing samples previously heat-treated in air at 250 and 400 uC. From thermal analysis, X-ray diffraction, infrared and Raman spectroscopies, and 13 C NMR spectroscopy, it has been found that the BaTiO 3 formation strongly depends upon the initial structure of the used precursor. It is concluded that an intermediate oxycarbonate phase was formed prior to the formation of BaTiO 3 above 550 uC when the precursor used was a (Ba,Ti)-mixed metal organic complex heat-treated at 250 uC, and a nanocrystalline BaCO 3 intermediate when the metal organic complex had been initially heat-treated at 400 uC. Although not well crystallized, thermoanalytical measurements, the unique XRD pattern, and new IR and Raman structural features revealed that such a metastable intermediate oxycarbonate phase has a stoichiometry close to Ba 2 Ti 2 O 5 ?CO 3 , which is characterized by having CO 3 22 groups different to those of pure BaCO 3 located, probably, in an open interlayer BaTiO 3 metastable structure. Irrespective of the used precursor, thermal decomposition of the (Ba,Ti)-mixed metal organic above 550 uC led to the formation of a mixture of tetragonal and hexagonal BaTiO 3 polymorphs rather than cubic. The synthesized BaTiO 3 powders are characterized by a high surface area of 40 m 2 g 21 up to 700 uC, and an equivalent particle size smaller than 25 nm. Raman spectra indicated asymmetry inside the TiO 6 octahedra of the BaTiO 3 structure.

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