E D. Rodeghiero et al/ Materials Science and Engineering 4244(1998)11-21 13 EHT=20. 00 kV Photo No. =28 Detector= Q Fig. 3. Backscattered electron SEM micrograph of the microstructure of a 5/95 vol. Ni/a-AL,O, composite; (Ni= light contrast, a-Al, O,= dark solution of either aluminum isopropoxide, Altwo the gels were vacuum dried and OCH(CH3)213(Aldrich Chemical), or zirconium isopro and to 231 groun 230 E esh powder form with the use of an oxide isopropanol complex, Zr[OCH(CH3)2]4 agate and pestle (CH,),CHOH(Aldrich Chemical), was prepared and Reduction of the gro heated until boiling. Next an aqueous 0. 2 M solution of formed by placing the powders in a quartz tube furnace nickel formate dihydrate, Ni(CHO,)2 2H,O(Johnson and heating under flowing 99.99% hydrogen(20 cm3 Matthey), was prepared and added to the ethanol solu- min-)for I h at a temperature of 1000-1100C. The tion. The addition of the aqueous metal salt solution to role of this heat treatment was 2-fold. First, the metal the alkoxide solution typically caused immediate gela- salt was decomposed to its metallic state(either Ni or tion of the ceramic precursor. Nevertheless, the gel Fe). Second, the condensation reaction in the ceramic formed was stirred vigorously for at least an additional phase was driven to completion, forcing the elimination 10 min at 70oC, in order to ensure complete inter-d of all excess water and hydroxyls persion of the metallic and ceramic precursors. Natu The metal-ceramic powders which resulted after re- rally, the amount of metal salt solution added depended duction were then uniaxially hot-pressed at 1400oC and pon the final metal-ceramic composite compositio 10 MPa for 3 h in 0.5 in inside diameter a-Al,O3 dies desired. The gel was then transferred to crystallization This hot-pressing was carried out under a reducing gas dishes for drying at 100C for 24 h. Finally, the dried mixture of Co and Co2 flowing at a total rate of 10 gels were ground with an agate mortar and pestle to a cm min-. The oxygen partial pressure typically em- powder size of 230 mesh. ployed was 10-12 atm. The role of this CO atmosphere In the case of the Fe/a-Al2O3 composites, a 500 ml, was not to promote further reduction but rather to 0. 1 M solution containing an appropriate ratio of alu- prevent the occurrence of any reoxidation. (A thorough minum nitrate nonahydrate, Al(NO3)3 9H,O(Aldrich review of the thermodynamics of the Ni/Alyo system in Chemical), and ferric nitrate nonahydrate, regard to both the 1000oC hydrogen reduction and the Fe(NO3)3. 9H,O(Aldrich Chemical), was first synthe- 1400C CO/CO2 hot-pressing has been previously re- sized. Then while stirring, aqueous 1 M NaOH was ported [14]. The thermodynamic aspects of the other slowly added until the ph of the nitrate mixture sur- composite systems investigated here are very similar. passed 7. The gel which resulted was next centrifuged at Following hot-pressing, the sintered composite pellets 7000 rpm for I h and twice washed with deionized were removed from the dies with the use of a circular water. Then, after repeating the centrifuge/washing step diamond blade saw. High quality, optically smoothE.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 13 Fig. 3. Backscattered electron SEM micrograph of the microstructure of a 5/95 vol.% Ni/a-Al2O3 composite; (Ni=light contrast, a-Al2O3=dark contrast). solution of either aluminum isopropoxide, Al [OCH(CH3)2]3 (Aldrich Chemical), or zirconium isopro￾poxide isopropanol complex, Zr[OCH(CH3)2]4 · (CH3)2CHOH (Aldrich Chemical), was prepared and heated until boiling. Next an aqueous 0.2 M solution of nickel formate dihydrate, Ni(CHO2)2 · 2H2O (Johnson Matthey), was prepared and added to the ethanol solu￾tion. The addition of the aqueous metal salt solution to the alkoxide solution typically caused immediate gela￾tion of the ceramic precursor. Nevertheless, the gel formed was stirred vigorously for at least an additional 10 min at 70°C, in order to ensure complete inter-dis￾persion of the metallic and ceramic precursors. Natu￾rally, the amount of metal salt solution added depended upon the final metal–ceramic composite composition desired. The gel was then transferred to crystallization dishes for drying at 100°C for 24 h. Finally, the dried gels were ground with an agate mortar and pestle to a powder size of 230 mesh. In the case of the Fe/a–Al2O3 composites, a 500 ml, 0.1 M solution containing an appropriate ratio of alu￾minum nitrate nonahydrate, Al(NO3)3 · 9H2O (Aldrich Chemical), and ferric nitrate nonahydrate, Fe(NO3)3 · 9H2O (Aldrich Chemical), was first synthe￾sized. Then while stirring, aqueous 1 M NaOH was slowly added until the pH of the nitrate mixture sur￾passed 7. The gel which resulted was next centrifuged at 7000 rpm for 1 h and twice washed with deionized water. Then, after repeating the centrifuge/washing step two more times, the gels were vacuum dried and ground to 230 mesh powder form with the use of an agate mortar and pestle. Reduction of the ground precursor gels was per￾formed by placing the powders in a quartz tube furnace and heating under flowing 99.99% hydrogen (20 cm3 min−1 ) for 1 h at a temperature of 1000–1100°C. The role of this heat treatment was 2-fold. First, the metal salt was decomposed to its metallic state (either Ni or Fe). Second, the condensation reaction in the ceramic phase was driven to completion, forcing the elimination of all excess water and hydroxyls. The metal–ceramic powders which resulted after re￾duction were then uniaxially hot-pressed at 1400°C and 10 MPa for 3 h in 0.5 in. inside diameter a-Al2O3 dies. This hot-pressing was carried out under a reducing gas mixture of CO and CO2 flowing at a total rate of 10 cm3 min−1 . The oxygen partial pressure typically em￾ployed was 10−12 atm. The role of this CO atmosphere was not to promote further reduction but rather to prevent the occurrence of any reoxidation. (A thorough review of the thermodynamics of the Ni/Al/O system in regard to both the 1000°C hydrogen reduction and the 1400°C CO/CO2 hot-pressing has been previously re￾ported [14]. The thermodynamic aspects of the other composite systems investigated here are very similar.) Following hot-pressing, the sintered composite pellets were removed from the dies with the use of a circular diamond blade saw. High quality, optically smooth