Ultrafine yttria additive on nitride ceramics (Si3N4, AIN) sintering

Abstract

Yttria is an important additive for sintering Si3N4, A1N and other nitride ceramics. Silicon nitride with yttria additive has been widely investigated [1-3]. The addition of yttria influences the density and elastic moduli of products [2]. Also, the sinterability of Si3 N4 is increased by the smaller particle size of yttria powders [3-6]. Yttria addition helps a higher density [7] and higher thermal conductivity [8] of A1N to be obtained. Oxygen solute atoms in A1N are phonon-scattering sites, which reduce the thermal diffusivity [9, 10]. Yttria consists of a crystalline YAG phase with grain boundary oxygen to prevent the formation of the oxygen-aluminium nitride solid solution [11]. Generally, the grainsize of commercial metallic oxide additives is of the order of 100nm. However, commercial yttria powders are likely to agglomerate [6]. Their effective particle size is normally greater than that of sintering raw materials, i.e. Si3N 4 and A1N. Therefore, ultrafine metallic oxide additives are favourable if they do not agglomerate. Another interesting account of ultrafine additives is the liquefying temperature, which can be reduced by using a smaller particle size of metallic oxides [6]. There have been many reports on production processes for ultrafine powders by plasma methods [12-15]. In the present study, ultrafine yttria powder (UFP) was produced by a radio-frequency (r.f.) plasma method from metallic yttrium (with impuritied (in p.p.m.):gadolinium 50, terbium 50, dysprosium 50, erbium 50, ion 440 and magnesium 30) and used as additive for normal sintering. Fig. 1 is a schematic diagram of the r.f. plasma reactor for UFP production. The gas flow used was argon 0.18 m 3 h-J with oxygen 1.2 m 3 h -l , which was introduced through the cooling gas inlet shown in the figure. The pressure in the chamber was about 1 Pa, and the feeding rate of raw materials was 0.15 kg h l The ultrafine and commercial powders used were evaluated by analysing specific surface area (BET method), particle size (microtrac-SPA, Nikkisou), X-ray diffraction pattern and transmission electron micrographs (TEM). Powders Y1 and Y2 denote the commercial yttria powders supplied by Shin-Etsu Chemical and Asahi Chemical respectively. Some of the properties of the ultrafine and commercial powders are listed in Table I. The Y2 powder was likely to form large agglomerates, and consequently had a greater specific surface area with a larger particle size than the other powders.