opean Ceramic Society 22(2002)2333-2342 (EDX) unit. The surfaces of selected thermally cycled of particles analysed, respectively. Particle size distribu samples were also observed by SEM tion is a very critical issue in green body formation using EPD or pressure filtration. A mixture of coarse, medium and very fine particles, where sufficient fine 3. Results and discussion particles are present to fill in the interstices, results in a green body compact of high green density As shown in 3.. Materials characterisation Fig. Ib, there are some very fine particles(as fine as 55 nm)within the synthesised powders as well as very coarse The particle size distribution results and a SEM ones(7. 1 um). However, the mean particle size is in the micrograph of the hydrothermally synthesised mullite range of 2-3 um Fig. Ib also shows the presence of very plus 5 wt zirconia powder used in this work are fine(20-30 nm) zirconia precipitates located within the shown in Fig. 1. The cumulative statistics shown in mullite particles and these very fine particles are expec- Fig. 1(a) for as-produced powder is given as D25, D50, ted to enhance the sintering rates of monophasic mullite D75 and Doo values, which indicate the particle sizes powders via controlling grain growth. Overall, the ver below which are 25, 50, 75 and 90% of the total volume fine particles(<100 nm) are considered to be ideal for D2sD5o D5 D90 Mean Median 6 VolumeSize(pm)0951.7424|7.1|265245 statistics ÷ 00l 0.i Particle Diameter (um) Fig. I(a) Particle size analysis and (b) SEM micrograph of the hydrothermally synthesised mullite plus 5 wt zirconia powder showing the presence of very fine(20-30 nm) zirconia precipitates(as arrowed) within the mullite particles(EDX) unit.The surfaces of selected thermally cycled samples were also observed by SEM. 3. Results and discussion 3.1. Materials characterisation The particle size distribution results and a SEM micrograph of the hydrothermally synthesised mullite plus 5 wt.% zirconia powder used in this work are shown in Fig.1.The cumulative statistics shown in Fig.1(a) for as-produced powder is given as D25, D50, D75 and D90 values, which indicate the particle sizes below which are 25, 50, 75 and 90% of the total volume of particles analysed, respectively.Particle size distribu￾tion is a very critical issue in green body formation using EPD or pressure filtration.A mixture of coarse, medium and very fine particles, where sufficient fine particles are present to fill in the interstices, results in a green body compact of high green density.As shown in Fig.1b, there are some very fine particles (as fine as 55 nm) within the synthesised powders as well as very coarse ones (7.1 mm).However, the mean particle size is in the range of 2–3 mm.Fig.1b also shows the presence of very fine (20–30 nm) zirconia precipitates located within the mullite particles and these very fine particles are expec￾ted to enhance the sintering rates of monophasic mullite powders via controlling grain growth.Overall, the very fine particles (<100 nm) are considered to be ideal for Fig.1. (a) Particle size analysis and (b) SEM micrograph of the hydrothermally synthesised mullite plus 5 wt.% zirconia powder showing the presence of very fine (20–30 nm) zirconia precipitates (as arrowed) within the mullite particles. 2336 C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342