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Sodium ion-conductive silicon nitride ceramic with Na2O–Al2O3–SiO2 glass as the grain boundary phase was fabricated by adding Na2CO3, Al2O3, and SiO2 as sintering additives. The electrical conductivity was two and four orders of magnitude higher than that of Si3N4 ceramic with Y2O3 and Al2O3 additives at 100 and 1000 °C, respectively. This result clearly indicates that ionic conductivity can be provided to insulating structural ceramics by modification of the grain boundary phase without dispersion of conductive particles.

By providing electrical conductivity for structural ceramics, not only the replacement of electronic ceramics or metal products but also the improvement of machinability by applying electrical discharge machining can be expected. Silicon nitride is one of most promising structural ceramics because it has high mechanical properties and excellent resistance to thermal shock, oxidation, and corrosion over a wide temperature range. Composite materials between insulative Si3N4 matrix and conductive secondary phases (e.g., TiN,1 SiC,2 and MoSi23) have been intensively investigated to provide electrical conductivity to Si3N4 ceramics.1–3 A conductive path was formed by the conductive secondary phase particles in the Si3N4 matrices. The electrical conductivity of the binary composite varies generally according to the percolation theory.4,5 Because massive dispersion of the secondary phase may alter the essential properties of structural ceramics, the amount of the secondary phase is desirably small. To reduce the volume of the dispersed phase, reducing the particle size is thought to be an effective approach.6,7 However, it is not easy to reduce the amount of the conductive phase particles required to provide sufficient electrical conductivity to ceramic materials because of the grain growth and aggregation during the high-temperature sintering. In the previous reports, the addition of over 40, 20, and 30 vol% of TiN,1 SiC,2 and MoSi2 particles3 was required to sufficiently reduce the electrical resistance. It is well known that the amount and composition of the grain boundary

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Present address: Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.

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J. Mater. Res., Vol. 18, No. 12, Dec 2003

phase derived from sintering additives have a large influence on the mechanical and thermal properties and the oxidation resistance of Si3N4 ceramics. However, the influence on the electrical properties has not been investigated as far as we know. We will report here a new approach to improve the electrical conductivity of Si3N4 ceramics. By forming an electrically conductive glass in the grain boundary, in this approach, the grain boundary phase can function as a conductive path. Therefore, the introduction of secondary phase particles other than sintering additives is not necessary. As a first model, a Si3N4 ceramic with Na2O–Al2O3– SiO2 glass in the grain boundary was fabricated. The Na2O–Al2O3–SiO2 glass ha