Y Bao, P.S. Nicholson /Journal of the European Ceramic Society 28(2008)3041-3048 3043 (a)100% 1270° : 2.0% 15.0% Seeds(wt%) 2(° (b)34 300% Fig. 2. Phase evolution of the reaction-bonded mullite containing 7.5 wt%o MREO 200%自 ◆ Bulk density ica is totally consumed >1300C but the alumina trace persists The mullitization temperature was further decreased by addin -Apparent densityt15.0% mullite"seeds"(Fig 3). With 0.5 wt% seeds, the mullitization process was complete at 1270C. However, the sintering shrink- age(6-8%)was still too large(should be <x3% to avoid matrix cracks around the volume stable fibers). Fig 4 tracks the effect of mullite seed content on the shrinkage, density and open porosity, for rbm with 7.5 wt%o mreo, a small amount of mullite seed 20 can prompt the formation of mullite. However, when seed con- Seeds(wt%) centration is high, it serves as refractory inclusion and retards Fig 4 Influence of mullite seed addition on the sintering shrinkage, density and the rBm sintering. Fig. 5 shows the dilatometric measurement open porosity for RBM sintered at 1300C for 2h. curve for rBM with 7.5 wt% MREO and 5.0 wt% seeds. Lengt expansion occurs due to oxidation of Si metal powder and increase of temperature, RBM sintering shrinkage takes place reaches maximum(<3%)at 170C, close to the eutectic point and final length change is <3%.When(seeds)=5wt%, the RBM of MREO-Al2o3-SiO2. Formation of MREO-Al2o3-Sioz iq- shrinkage is <3% and open porosity, < 20%. This composition is uid phase prompts the sintering shrinkage and a steep drop in optimum and is used in the EPID processing length was detected at 1170-1200oC Si metal is completely oxi- dized into silica when soaked at 1200 C for 10h. With further Si particles are negatively charged. These particles tend to hetero-coagulate. A dispersant was added to induce a com- 0.5wt% seeds mon surface charge sign and stabilize the mixed suspension 1300c PEl, protonated with acetic acid, adsorbs on both surfaces ren- 1250° 2e( 1200 ▲ mullite口:Sio2◆:Al2O3 Sinering Time(min. Fig. 5. Dilatometric measurement of rbM with 7.5 wt% MREO and 5. 0wt% Fig. 3. Effect of mullite seed addition on the mullitization temperatureY. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 3043 Fig. 2. Phase evolution of the reaction-bonded mullite containing 7.5 wt% MREO. ica is totally consumed >1300 ◦C but the alumina trace persists. The mullitization temperature was further decreased by adding mullite “seeds” (Fig. 3). With 0.5 wt% seeds, the mullitization process was complete at 1270 ◦C. However, the sintering shrink￾age (6–8%) was still too large (should be <∼3% to avoid matrix cracks around the volume stable fibers). Fig. 4 tracks the effect of mullite seed content on the shrinkage, density and open porosity, for RBM with 7.5 wt% MREO. A small amount of mullite seeds can prompt the formation of mullite. However, when seed con￾centration is high, it serves as refractory inclusion and retards the RBM sintering. Fig. 5 shows the dilatometric measurement curve for RBM with 7.5 wt% MREO and 5.0 wt% seeds. Length expansion occurs due to oxidation of Si metal powder and reaches maximum (<3%) at 1170 ◦C, close to the eutectic point of MREO–Al2O3–SiO2. Formation of MREO–Al2O3–SiO2 liq￾uid phase prompts the sintering shrinkage and a steep drop in length was detected at 1170–1200 ◦C. Si metal is completely oxi￾dized into silica when soaked at 1200 ◦C for 10 h. With further Fig. 3. Effect of mullite seed addition on the mullitization temperature. Fig. 4. Influence of mullite seed addition on the sintering shrinkage, density and open porosity for RBM sintered at 1300 ◦C for 2 h. increase of temperature, RBM sintering shrinkage takes place and final length change is <3%. When [seeds] = 5 wt%, the RBM shrinkage is <3% and open porosity, <20%. This composition is optimum and is used in the EPID processing. Alumina particles in ethanol are positively charged whereas Si particles are negatively charged. These particles tend to hetero-coagulate. A dispersant was added to induce a com￾mon surface charge sign and stabilize the mixed suspension. PEI, protonated with acetic acid, adsorbs on both surfaces ren￾Fig. 5. Dilatometric measurement of RBM with 7.5 wt% MREO and 5.0 wt% seeds