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Various compositions of Li1-2xCaxSi2N3 (x = 0–0.2) were synthesized by the reaction of Li3N, Si3N4, and Ca3N2 at temperatures of 1873–2073 K. Ca was incorporated into the LiSi2N3 host lattice to form a solid solution of Li1-2xCaxSi2N3. The activation energy for ionic conduction was decreased and ionic conductivity at room temperature was enhanced by Ca doping. At 298 K, the ionic conductivity of densified Li1-2xCaxSi2N3 (x = 0.075) ceramic reached 1.6 × 10−5 S m−1, almost four orders of magnitude higher than that of densified Li1-2xCaxSi2N3 (x = 0) ceramic (3.1 × 10−9 S m−1). The change in the LiSi2N3 framework upon Ca doping decreased the interaction between the ions and increased the number of defects in the structure, making it easier for mobile Li+ ions to migrate. Moreover, the incorporation of aliovalent substitutional Ca2+ ions in the LiSi2N3 lattice is expected to create Li+ vacancies (VLi) for charge compensation (Li1-2xCaxVLiSi2N3), thereby increasing the number of mobile Li+ ions.

We studied tensile creep behavior in two silicon nitride ceramics, i.e., 4.8 mol% Lu2O3 (SN48) and 1.2 mol% Lu2O3 (SN12), at 1400–1500 °C under applied stress of 137–300 MPa. Time to failure of SN48 increased with decreasing applied stress and minimum strain rate. The stress–rupture parameter was 10.7 at 1400 °C and 11.4 at 1500 °C. Pore formation was confirmed in a creep-tested specimen of SN48 by transmission electron microscopy. These results suggest that SN48 was fractured by creep rupture. The minimum strain rate of SN12 was almost below the measurement system limitation at temperatures below 1500 °C. Time to failure tended to increase with decreasing applied stress. The stress–rupture parameter was 41 at 1400 °C and 73 at 1500 °C. These results suggest that SN12 was fractured by subcritical crack growth.

Lutetium nitride (LuN), an end member of a new quaternary system Si3N4–SiO2–Lu2O3–LuN, was synthesized by direct nitridation of a lutetium metal. The nitridation extent of the lutetium ingot (10 × 5 × 2 mm) reached about 97% by heating at 1600 °C for 8 h with an applied N2 pressure of 0.92 MPa; the initial shape of the bulk metal was maintained in the course of nitridation. The resulting nitrided lutetium possessed a moderately low oxygen content (∼0.7 wt%), which enables the preparation of uncharacterized high nitrogen-containing phases in the Lu–Si–O–N system.

High-temperature fracture behavior of a Yb2O3–SiO2–doped hot-pressed silicon nitride (Si3N4) ceramic was investigated in four-point flexure between 1000 and 1500 °C at five crosshead speeds, using the specimens precracked with three indentation loads. Above 1000 °C, a temperature and stressing rate dependence of fracture stress was seen. At 1200 °C the fracture stress of the precracked specimens increased with decreasing stressing rates due to a toughening effect, the absence of slow crack growth (SCG). However, at 1400 and 1500 °C the fracture stress decreased with decreasing stressing rate. In particular, this dependence was stronger at 1500 °C than at 1400 °C. The SCG was observed only in the specimens precracked with indentation loads of 98 and 196 N. This crack extended with increasing test temperature and/or decreasing stressing rate. The dependence of fracture stress on stressing rate was attributed to a SCG behavior at higher temperatures.

Large-sized β–Si3N4 crystals up to 10 mm in length and 0.3 mm in diameter with low impurity concentration are successfully grown from silicon melt in a nitrogen atmosphere. By controlling the concentration of impurities in the silicon melt, a new kind of β–Si3N4 crystal, that is, a transparent coloring one with an absorption edge around a wavelength of 520 nm, is obtained.