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.

High-temperature piezoelectric ceramics based on W 6+doped Bi 4 Ti 3 O 12 (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800°and 1150°C in air. A rapid densification, >99% of the theoretical density ( th ) at 900°C/2 h, took place in the chemically prepared W 6+ -doped Bi 4 Ti 3 O 12 ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of th , at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W 6+ -doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO 2 produced a further decrease of the electrical conductivity.

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The subsolidus phase relations in the entire system Zr02-Y,03 were established using DTA, expansion measurements, and room-and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution-monoclinic zirconia solid solution + cubic zirconia solid solution at 4.5 mol% YZO3 and =490"C, ( b ) cubic zirconia solid solutiow Sphase Y4Zr3012+ hexagonalphase YsZrOlr at 45 mol% Y203 and -1325"+25"C, and (c) yttria C-type solid solutiowcubic zirconia solid solution+ hexagonal phase YcZrOll at =72mol% Yzo3 and 1650"k 50°C. W o ordered phases were also found in the system, one at 40 mol% Yzo3 with ideal formula Y4Zr3012, and another, a new hexagonal phase, at 75 mol% Yz03 with formula Y6ZrOI1. They decompose at 1375" and >175O"C into cubic zirconia solid solution and yttria C-type solid solution, respectively. The extent of the cubic zirconia and yttria C-type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagramis given for the system ZrOZ-Yz03.

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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.