Microstructure and Properties of Oxide Ceramic-Based Nanocomposites with Transition Metal Nanoparticles
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Mat. Res. Soc. Symp. Proc. Vol. 501 © 1998 Materials Research Society
for A120 3/Co system and at 450'C for ZrO 2/Ni system in air to obtain A120.Co30 4 and ZrO 2/NiO mixtures, respectively. Subsequently, the mixture was ball-milled again. For the ZrO 2/Ni system, conventional mixing of commercial coarse NiO powder (99.9%, particle size of 60 pm) and 3YZrO 2 powder was also carried out by ball milling in ethanol solvent for 24 hr. The sintering was carried out using the two-step heating sequence [2,3]. The obtained mixtures were kept into graphite-die and reduced by hydrogen gas at 900'C and 700'C for A120 3/Co and ZrO 2/Ni system, respectively. Hot-press sintering was successively accomplished at 1400'C to 1800'C for 1 hr in argon atmosphere under an applied pressure of 30 MPa. Microstructure and Mechanical Properties Investigation To confirm the oxidation phenomena of oxide/nitrate mixtures, high temperature X-ray diffraction (XRD) analysis was performed for both systems up to 600'C in air. All sintered samples were cut and polished by diamond pastes to get the mirror surfaces. The phase composition of the obtained specimen was characterized by the XRD. Further, we used transmission electron microscopy (TEM) to observe powder morphology and microstructure of the prepared material. The fracture toughness was estimated by the indentation fracture (IF) method loaded up to 49 N, and partly by the indentation strength in bending (ISB) technique. The fracture strength measurement was carried out by 3-point bending test with 30 mm span. The magnetization of the composite was estimated by the vibrating sample magnetometer (VSM) with an applied magnetic field up to ± 9 kOe at room temperature. RESULTS AND DISCUSSION Characterization of Mixed Powders Both cobalt and nickel nitrates were completely dissolved in alcohol. TEM investigation for the dried A120 3/Co nitrate and ZrO 2/Ni nitrate mixtures revealed that the nitrate was found to coat the A120 3 or ZrO 2 primary particles as a shapeless matter, which had the same morphology as observed for the ZrO 2/Ni system [8]. High temperature XRD analysis was performed for A120 3/Co nitrate and ZrO2/Ni nitrate mixtures up to 600'C in air. Cobalt nitrate disappeared at 100°C, and Co 30 4 started to appear above 200'C, whereas NiO was found to precipitate above 300'C in the case of nickel nitrate mixed with ZrO2 . No by-product such as Co spinel or nickel zirconate was observed in this experiment. From these results, we fixed calcination temperatures to be 400'C and 450'C for A120 3/Co nitrate and ZrO2/Ni nitrate mixtures, respectively. TEM observation for both calcined mixtures revealed that primary particle size of C030 4 and NiO was approximately 30 to 50 nm. Produced Co 30 4 particles mainly existed around A120 3 particles as a spherical. However, most NiO nano-particles produced by calcining agglomerated in the mixed powder. When ZrO2/NiO mixture was obtained by conventional ball milling of corresponding oxides, dispersed NiO was larger, around 300 nm, compared to that prep
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