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We prepared sintered reaction‐bonded silicon nitride (Si3N4) ceramics using various amounts of yttria (Y2O3) and a fixed amount of magnesia as sintering additives and evaluated their effects on the microstructure and dielectric breakdown strength (DBS). The DBS tests were conducted at substrate thicknesses of approximately.3,.2,.1, and.05 mm for each sample. All the prepared samples exhibited a high relative density (> 98%) and were composed of grains of almost identical sizes and shapes. The DBS decreased as the amount of added Y2O3 increased, with the degree of decrease being more pronounced for the thinner substrates. In the case of substrates with the largest Y2O3 amount (5 mol%), the DBS decreased to the level observed for the thinnest substrate of.05 mm. The presence of yttrium ions around the grains increased the electrical conductivity of the glassy phase, resulting in a decreased DBS. Therefore, a smaller amount of Y2O3 led to a higher DBS. The observed decrease in the DBS (.05 mm) was attributed to the connection between the top and bottom surfaces of the substrate through a highly conductive path. [ABSTRACT FROM AUTHOR]

We prepared sintered reaction‐bonded silicon nitride ceramics by using yttria and magnesia as sintering additives and evaluated effects of the nitridation temperature on their microstructure, bending strength, fracture toughness, and thermal conductivity. The effects of the nitridation temperature were large, but different depending on the property. The ratio of β‐phase in the nitrided compacts significantly increased with increasing the nitridation temperature, whereas their microstructures had no clear difference. Although the bending strength varied, it maintains a high value of 800 MPa. Fracture toughness was almost constant regardless the temperature. The thermal conductivity improved as the β‐phase in the nitrided compact increases. This resulted in a decrease of the lattice oxygen content and increase of the thermal conductivity. Therefore, elevating the nitridation temperature and consequently the β‐phase ratio should be a promising strategy for achieving compatibly high strength and high thermal conductivity, which are generally known to be in a trade‐off relationship. [ABSTRACT FROM AUTHOR]

Silicon nitride (Si3N4) was prepared from silicon by a sintered reaction‐bonded silicon nitride method using yttria and magnesia as sintering additives. Post‐sintering (PS) of nitrided compacts was carried out at 1850°C under a nitrogen pressure of 1 MPa. Effect of PS time on microstructure and dielectric breakdown strength (DBS) of the prepared Si3N4 ceramics was evaluated. The DBS was measured using specimens with four different thicknesses (0.30, 0.20, 0.10, and 0.05 mm) in order to examine the thickness dependence. The porosity of the sintered Si3N4 decreased by prolonging the PS time, and the full density could be achieved at the PS time of over 6 h. After full densification, rod‐like β‐Si3N4 grains grew up, and their maximum grain size increased from 45.1 to 154.7 μm by prolonging the PS time from 6 to 48 h. The DBS of the thick Si3N4 substrates (0.30 mm) showed little variation from 35.4 to 47.0 kV/mm, regardless of the PS time. On the other hand, that of the thin ones (0.05 mm) dramatically decreased from 99.5 to 9.8 kV/mm with increased the PS time from 6 to 48 h. Because the DBS sharply decreased at the thin substrate sintered for longer time in which some large‐elongated grains might span the substrate thickness‐wise throughout, it was inferred that the interface between β‐Si3N4 grains and grain boundary phase/intergranular glassy films might be a path of the dielectric breakdown. [ABSTRACT FROM AUTHOR]