J. Ma et al. /Journal of the European Ceramic Society 24(2004)825-831 2. Experimental procedure fine-polished cross-section face of the samples using Scanning Electron Microscope (Jeol, JSM5310) The ceramic powders used in the present work were ne-grained high purity alumina powders(AKP 30 Sumitomo Chemicals, Japan) with a average particle 3. Results and discussions size of 0.4 um. The PMMa powders(Acrylic Powder, Buehler) used to generate pores in the porous inter- 3. 1. Sintering behavior layers, on the other hand, have an average particle size of 40 um. Tapes for forming the respective layers in the Fig. I shows the cross-section SEM micrograph of the layered systems were produced by tape casting of a layered system formed with 60% PMMA addition. It aqueous slurry containing 58 wt. of alumina powders, can be seen that the layers were very well integrated I wt. of fish oil, 6.5 wt. of polyethylene glycol 400, without any delamination between the dense and por- 4.5 wt. of benzyl butyl phthalate, 7 wt. of polyvinyl ous layers. The dense layers have been fully densified butyral and 23 wt. of ethanol(99.86%). For the os nation using SEM and image analyzer showed that or- during the sintering process and microstructural exami ous tapes, various volume percent of PMMA powde were added to the slurry with respect to the amount of relative density of more than 97% has been achieved. It the alumina powder in the slurry. The different volume is also observed that the addition of PMMa produced percent added were 40, 50, 60, 70 and 80 vol % The uniformly distributed large spherical pores throughout slurries were then de-gased in a pressure de-gas system the por he ize of these large pores and finally tape casted using a continuous feed tape (70 um) is measured to be more than two orders of casting machine (Unique, USA)onto a polypropylene magnitude larger than the average grain size of alumina carrier tape running at a speed of 20 cm/min. The (0.4 um). These large pores introduced from the burnt thickness of the dense and porous tapes fabricated was out PMMa powders are termed as"macropores"in the both 0.3 mm. After drying, the tapes were cut into a subsequent discussions for clarity. It is also noted from rectangle of size 60 4 mm. The dense and porous layers the microstructural examinations that the actual volume were then stacked and pressed together at room tem- fraction of porosity in the porous layer, which is mainly perature to a final thickness of 3.3 mm. For layered contributed by the large macropores since the matrix is systems, dense and porous layers were stacked alter- almost fully densified, is lower than that of the initially nately. The stacked green samples were then trimmed to added PMMa volume percent. For example, in the 50x4x3.3 mm block before sintering at 1550C for 3 h. sample shown(Fig. 1), although the added amount of The sintered layered systems and the monolithic sam- PMMA was 60 vol % the porosity result obtained from les were finally subjected to microstructural exami- image analysis showed a porosity of 48.7%. Similarly nations and four point bending tests with a loading span the final porosity after sintering for samples with other of 30 mm to evaluate the fracture energies of the volume fraction of PMMA addition was found to be samples. Microstructural studies were performed on the lower than the actual PMMA volume fraction (Table 1) 2kU75 1m141129 Fig. I. SEM cross-section micrograph of a layered system with 60 vol. PMMA2. Experimental procedure The ceramic powders used in the present work were fine-grained high purity alumina powders (AKP 30, Sumitomo Chemicals, Japan) with a average particle size of 0.4 mm. The PMMA powders (Acrylic Powder, Buehler) used to generate pores in the porous inter￾layers, on the other hand, have an average particle size of 40 mm. Tapes for forming the respective layers in the layered systems were produced by tape casting of a aqueous slurry containing 58 wt.% of alumina powders, 1 wt.% of fish oil, 6.5 wt.% of polyethylene glycol 400, 4.5 wt.% of benzyl butyl phthalate, 7 wt.% of polyvinyl butyral and 23 wt.% of ethanol (99.86%). For the por￾ous tapes, various volume percent of PMMA powders were added to the slurry with respect to the amount of the alumina powder in the slurry. The different volume percent added were 40, 50, 60, 70 and 80 vol.%. The slurries were then de-gased in a pressure de-gas system and finally tape casted using a continuous feed tape casting machine (Unique, USA) onto a polypropylene carrier tape running at a speed of 20 cm/min. The thickness of the dense and porous tapes fabricated was both 0.3 mm. After drying, the tapes were cut into a rectangle of size 604 mm. The dense and porous layers were then stacked and pressed together at room tem￾perature to a final thickness of 3.3 mm. For layered systems, dense and porous layers were stacked alter￾nately. The stacked green samples were then trimmed to 5043.3 mm block before sintering at 1550 C for 3 h. The sintered layered systems and the monolithic sam￾ples were finally subjected to microstructural exami￾nations and four point bending tests with a loading span of 30 mm11 to evaluate the fracture energies of the samples. Microstructural studies were performed on the fine-polished cross-section face of the samples using Scanning Electron Microscope (Jeol, JSM5310). 3. Results and discussions 3.1. Sintering behavior Fig. 1 shows the cross-section SEM micrograph of the layered system formed with 60% PMMA addition. It can be seen that the layers were very well integrated without any delamination between the dense and por￾ous layers. The dense layers have been fully densified during the sintering process and microstructural exami￾nation using SEM and image analyzer showed that a relative density of more than 97% has been achieved. It is also observed that the addition of PMMA produced uniformly distributed large spherical pores throughout the porous layer. The average size of these large pores (70 mm) is measured to be more than two orders of magnitude larger than the average grain size of alumina (0.4 mm). These large pores introduced from the burnt￾out PMMA powders are termed as ‘‘macropores’’ in the subsequent discussions for clarity. It is also noted from the microstructural examinations that the actual volume fraction of porosity in the porous layer, which is mainly contributed by the large macropores since the matrix is almost fully densified, is lower than that of the initially added PMMA volume percent. For example, in the sample shown (Fig. 1), although the added amount of PMMA was 60 vol.%, the porosity result obtained from image analysis showed a porosity of 48.7%. Similarly, the final porosity after sintering for samples with other volume fraction of PMMA addition was found to be lower than the actual PMMA volume fraction (Table 1). Fig. 1. SEM cross-section micrograph of a layered system with 60 vol.% PMMA. 826 J. Ma et al. / Journal of the European Ceramic Society 24 (2004) 825–831