Additive Sintering, Postannealing, and Dielectric Properties of SrTaO₂N

Abstract

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.

High pressure densification and dielectric properties of perovskite-type oxynitride SrTaO2N

“…The conductivity may be related to a partial loss of nitrogen and the formation of rock-salt type TaO 0.9 impurity, as reported in our previous paper [6].…”

“…SrTaO 2 N was prepared by the ammonolysis reaction of SrTaO 3.5 , as reported in our previous paper [6]. The SrTaO 2 N precursors were encapsulated in Pt or Au tubes (6 mm diameter, 2-3 mm height) under a dried Ar atmosphere.…”

“…High pressure was applied in the range of 2.5-7.7 GPa at room temperature and the sample was then heated to temperatures in the range of 600-1000 °C and held for 30 or 60 min, as summarized in Table 1. After sintering under pressure, the 6 sample was quenched to room temperature within a few minutes by turning off the heating power under the pressure. After release the pressure, the SrTaO 2 N compacts were removed from the Pt (Au) wrapping.…”

“…To clarify the intrinsic dielectric properties of the oxynitride, highly densified SrTaO 2 N ceramics were prepared by sintering at 1400 °C in 0.2 MPa of N 2 and subsequent annealing at 1000 °C in NH 3 flow [5,6]. SrCO 3 (2.5 wt%) was necessary as a sintering additive to compensate for the partial loss of SrO during high temperature sintering.…”

“…Thermal annealing of SrTaO 2 N in an evacuated silica tube also partially released nitrogen and a secondary phase of rock-salt type TaO 0.9 was formed together with the perovskite phase at 1100 °C [6]. Thin film formation is another way of material shaping different from ceramic sintering.…”