B. T. Lee, S. K Sarkar/ Scripta Materialia 61(2009)686-689 and microporosity presents another concern requiring t-ZrO2(TZ-3Y, Tosoh, Japan) with a particle size of further investigation. A through-channel porosity with 80 nm. Carbon powder(<15 um, Aldrich, USA)was defined geometry can be seen as a compromise for this used as a pore-forming agent. Al2O3 25% m-ZrO2 strength and porosity consideration. It is also highly was ball milled in ethanol and dried. t-ZrO2, AlO3- desirable in case of applications associated with fluid flow (m-ZrO2), and carbon powder were separately shear or porosity where high interconnectivity is required. mixed with a polymer binder, ethylene vinyl acetate Channelled microporosity with improved strength can (EVA)(Elvax 250 and Elvax 210, Dupont, USA), in a act as a holder or container of other kinds of mesoporous heated blender( Shina Platec Co., Suwon, South Korea) system and may find a range of applications with stearic acid as a lubricant In the first step of the Recently, a multipass extrusion process has been experiment, a t-ZrO2 shell was made by warm pressing investigated with respect to the fabrication of a porous and the Al2O3 core was fabricated by extrusion to make body [13-15]. Ordered microchanneled porous bodies a 30 mm diameter feed roll. The t-ZrO2/Al,O3 volume with monolithic ceramics were fabricated. However ratio was chosen to be 60/40. The feed roll was then ex their mechanical properties were significantly lower truded in a cylindrical die at 120 C to obtain 3.5 mm diameter filaments. These were the first-pass core/shell rable or higher compared to those yielded by conven- filaments, and the core/ shell arrangement of the filament tional porous composite fabrication processes such as accounts for the core/shell microstructure of the frame emplate [4] foaming [5] and leaching methods [6]. For region in the final porous composites. Sixty-one fila- all porous materials, monolithic ceramics or composites ments were then reloaded in the same die and extruded with random distributions of each constituent phase to obtain second-pass filaments. These second-pass have been investigated thus far. Microstructural modifi- core/shell filaments are 3.5 mm in diameter and contain cation in the frame region of the macroporous body has 61 core/shells in a close-packed arrangement. Polymer not been investigated previously. It is very difficult mixed carbon powder was extruded with the same die tailor the microstructure in the frame using existing fab- at 120C to obtain 3.5 mm diameter filaments rication methods. The present study is thought to be the To make a porous body, the carbon filaments and the first attempt to tailor the microstructure of a pore frame, second-pass core/shell Al2O3(m-ZrO2)t-zrO2 fila in approach that could dramatically improve the ments were arranged in the previously used steel die. mechanical properties of Al2O3-ZrO2 porous composite In the die, two outer layers were comprised of second- systems pass core/shell filaments and the inner layers were filled n this work, a fibrous monolithic process is used for with carbon filaments. The arrangement was compacted the first time to tailor the frame region of a microchan and extruded at 120oC at an extrusion rate of neled macroporous body, yielding a highly ordered and 8 mm min. The obtained 3. 5 mm diameter filament unidirectional fibrous microstructure resembling a bam- contained an inner carbon core and a ceramic shell with boo-like biomimetic microstructure(see Figure 1). The characteristic core/shell arrangements. This is termed frame of the porous body was made with Al2O3(tetrag the first-pass green porous body. Again, 61 onal-ZrO2) fibrous phase homogeneously distributed ir of these filaments were arranged in the steel die and ex continuous t-ZrO2 phase. The bending strength of the truded yielding the final second-pass green porous fila Al2O3 and 2-fold relative to ta a6-rold over that of ments. Figure I shows the schematic representation of having the same degree of porosity [13, 14]. The same ceramIcs he polymer binder was removed from the concept of tailoring the frame region of the porous body green body with a slow heating rate under a flowing can be extended to other types of microstructures such nitrogen atmosphere and then pore former carbon wa as laminated and fibrous laminated types. Using this burnt out at 1000C in an air atmosphere. Sintering method, the fibrous microstructure along with the pore of the samples was carried out in an air atmosphere at size and porosity can be controlled with great flexibility. 1450 and 1500C for 2 h. All the m-ZrO2 was trans- This method is attractive for incorporating various func formed to t-ZrO, after sintering. The density of the sin- tionalities by selection of microstructure and materials tered Al2O3 t-ZrO2)/t-ZrO2 porous bodies was and by innovative design measured using the weight dimension method. Bending The starting powders were AlO3(AKP-50, Sumim- strength was measured by a four-point bending test oto, Japan) with a particle size of 300 nm, monoclinic- The sintered bodies were used without any prior prepa rO2TZ-0Y, Tosoh, Japan), and yttria-stabilized ration, as the surface was smooth and the samples were ylindrical. The microstructure of the composites wa examined by scanning electron microscopy (SEM JALOr(s%em-ZrO)I JSM 6335F, JEOL, Tokyo, Japan)and transmission ●,●杰● electron microscopy (TEM, JEM2010, JEOL, Tokyo, Japan). Figure 2a shows the sintered first-pass porous body which is cylindrical with a circular pore inside. Prior to burn-out, the green body's outer diameter was 3.5 mm, which was reduced to 2.75 mm after sintering are/sDeⅡl Feed roll for 1 Green 1 pss Green 2 ps In the frame region the microstructure was fibrous, run ning unidirectionally through the composites, with a Figure 1. Schematic representation of the microstructure development. uniform distribution of Al,Ox(t-ZrO2) fiber heand microporosity presents another concern requiring further investigation. A through-channel porosity with defined geometry can be seen as a compromise for this strength and porosity consideration. It is also highly desirable in case of applications associated with fluid flow or porosity where high interconnectivity is required. Channelled microporosity with improved strength can act as a holder or container of other kinds of mesoporous system and may find a range of applications. Recently, a multipass extrusion process has been investigated with respect to the fabrication of a porous body [13–15]. Ordered microchanneled porous bodies with monolithic ceramics were fabricated. However, their mechanical properties were significantly lower compared to those of the dense body, although compa￾rable or higher compared to those yielded by conven￾tional porous composite fabrication processes such as template [4], foaming [5] and leaching methods [6]. For all porous materials, monolithic ceramics or composites with random distributions of each constituent phase have been investigated thus far. Microstructural modifi- cation in the frame region of the macroporous body has not been investigated previously. It is very difficult to tailor the microstructure in the frame using existing fab￾rication methods. The present study is thought to be the first attempt to tailor the microstructure of a pore frame, an approach that could dramatically improve the mechanical properties of Al2O3–ZrO2 porous composite systems. In this work, a fibrous monolithic process is used for the first time to tailor the frame region of a microchan￾neled macroporous body, yielding a highly ordered and unidirectional fibrous microstructure resembling a bam￾boo-like biomimetic microstructure (see Figure 1). The frame of the porous body was made with Al2O3–(tetrag￾onal-ZrO2) fibrous phase homogeneously distributed in continuous t-ZrO2 phase. The bending strength of the porous composites was improved 6-fold over that of Al2O3 and 2-fold relative to that of ZrO2 porous body having the same degree of porosity [13,14]. The same concept of tailoring the frame region of the porous body can be extended to other types of microstructures such as laminated and fibrous laminated types. Using this method, the fibrous microstructure along with the pore size and porosity can be controlled with great flexibility. This method is attractive for incorporating various func￾tionalities by selection of microstructure and materials and by innovative design. The starting powders were Al2O3 (AKP-50, Sumim￾oto, Japan) with a particle size of 300 nm, monoclinic￾ZrO2 (TZ-0Y, Tosoh, Japan), and yttria-stabilized t-ZrO2 (TZ-3Y, Tosoh, Japan) with a particle size of 80 nm. Carbon powder (<15 lm, Aldrich, USA) was used as a pore-forming agent. Al2O3 + 25% m-ZrO2 was ball milled in ethanol and dried. t-ZrO2, Al2O3– (m-ZrO2), and carbon powder were separately shear￾mixed with a polymer binder, ethylene vinyl acetate (EVA) (Elvax 250 and Elvax 210, Dupont, USA), in a heated blender (Shina Platec. Co., Suwon, South Korea) with stearic acid as a lubricant. In the first step of the experiment, a t-ZrO2 shell was made by warm pressing and the Al2O3 core was fabricated by extrusion to make a 30 mm diameter feed roll. The t-ZrO2/Al2O3 volume ratio was chosen to be 60/40. The feed roll was then ex￾truded in a cylindrical die at 120 C to obtain 3.5 mm diameter filaments. These were the first-pass core/shell filaments, and the core/shell arrangement of the filament accounts for the core/shell microstructure of the frame region in the final porous composites. Sixty-one fila￾ments were then reloaded in the same die and extruded to obtain second-pass filaments. These second-pass core/shell filaments are 3.5 mm in diameter and contain 61 core/shells in a close-packed arrangement. Polymer￾mixed carbon powder was extruded with the same die at 120 C to obtain 3.5 mm diameter filaments. To make a porous body, the carbon filaments and the second-pass core/shell Al2O3–(m-ZrO2)/t-ZrO2 fila￾ments were arranged in the previously used steel die. In the die, two outer layers were comprised of second￾pass core/shell filaments and the inner layers were filled with carbon filaments. The arrangement was compacted and extruded at 120 C at an extrusion rate of 8 mm min1 . The obtained 3.5 mm diameter filament contained an inner carbon core and a ceramic shell with characteristic core/shell arrangements. This is termed hereafter the first-pass green porous body. Again, 61 of these filaments were arranged in the steel die and ex￾truded, yielding the final second-pass green porous fila￾ments. Figure 1 shows the schematic representation of the microstructure development. To obtain porous ceramics, the polymer binder was removed from the green body with a slow heating rate under a flowing nitrogen atmosphere and then pore former carbon was burnt out at 1000 C in an air atmosphere. Sintering of the samples was carried out in an air atmosphere at 1450 and 1500 C for 2 h. All the m-ZrO2 was trans￾formed to t-ZrO2 after sintering. The density of the sin￾tered Al2O3–(t-ZrO2)/t-ZrO2 porous bodies was measured using the weight dimension method. Bending strength was measured by a four-point bending test. The sintered bodies were used without any prior prepa￾ration, as the surface was smooth and the samples were cylindrical. The microstructure of the composites was examined by scanning electron microscopy (SEM, JSM 6335F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEM2010, JEOL, Tokyo, Japan). Figure 2a shows the sintered first-pass porous body, which is cylindrical with a circular pore inside. Prior to burn-out, the green body’s outer diameter was 3.5 mm, which was reduced to 2.75 mm after sintering. In the frame region the microstructure was fibrous, run￾ning unidirectionally through the composites, with a Figure 1. Schematic representation of the microstructure development. uniform distribution of Al2O3–(t-ZrO2) fiber in the B. T. Lee, S. K. Sarkar / Scripta Materialia 61 (2009) 686–689 687