The maintaining of the chemical composition and electrical insulativity of SrTaO 2 N ceramics was investigated during sintering and annealing, using powders prepared by the nitridation of Sr 2 Ta 2 O 7 . Due to the low thermal stability of SrTaO 2 N, the partial loss of SrO and nitrogen induced the formation of a TaO 0.9 impurity after heat-treating at above 1100°C. The sintering additive SrCO 3 and postannealing in NH 3 were employed to compensate for the loss of SrO and nitrogen to obtain ceramics with the original chemical composition. The as-sintered SrTaO 2 N ceramics with various relative density (RD) were annealed in NH 3 to observe the recovery of color and electrical insulativity. It was found that the inner part of the well-sintered samples with RD = 95.1% could not be recovered by annealing, and continued to exhibit semiconducting behavior and a black color. On the other hand, for the as-sintered SrTaO 2 N ceramics with RD < 84%, both the nitrogen content and electrically insulating behavior were completely recovered after annealing. The postannealed SrTaO 2 N ceramics (RD = 83.3%) possessed a relatively large dielectric constant of 450 with a low dielectric loss of less than 0.1 at 100 Hz, almost independent of frequency and temperature.

Oxynitride perovskite SrTaO2N has been attracting attention as a possible new dielectric material owing to the cis-type anion ordering in its crystal structure. It is currently difficult to obtain it in bulk form because it easily loses a part of its nitrogen to become semiconducting during densification at high temperature. We found that its surface layer recovered its original orange color and dielectric properties after postannealing in ammonia. Piezoresponse force microscopy measurements clearly revealed a ferroelectric behavior on the entire orange surface layer peeled off from the black sintered body inside. The present sample was free-standing at about 8 μm thick and clearly distinct from the compressively strained SrTaO2N thin films for which ferroelectricity was recently reported in small domains (10–102 nm) in the locally assumed trans-type anion ordering. The present results represent the first experimental observation of ferroelectric response in a bulk oxynitride perovskite with cis-type anion ordering.

Transmission electron microscopy study of pressureless sintered ZrB2–20 vol% SiC composites with WC (5–10 vol%) additions was carried out in this work. Two independent chemical reactions in which the ZrO2 impurities were removed from ZrB2 grain surface were confirmed. Three new formed phases, (W, Zr)ssB, (Zr, W)ssC, and (W, Zr)ssSi2, apart from the plate‐like ZrB2 and SiC grains, were identified in the samples. Microstructure observations strongly suggest that the liquid phases in terms of (W, Zr)ssB and (W, Zr)ssSi2, appear in this system during the sintering process below 2200°C, and the former could well wet the ZrB2 grain boundaries. The mass transport process between ZrB2 and (W, Zr)ssB was also confirmed by scanning‐transmission electron microscope analysis. The above results supports that the densification at 2200°C is assisted by liquid phase and the elongation of ZrB2 platelets resulted from the Ostwald ripening during the dissolution–diffusion–precipitation process.

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