In view of the perspective to produce near-net shapes of ultra-refractory ceramics, ZrB 2 -SiC composite were produced by slip casting on plaster mould and pressureless sintered at 21501C with a holding time of 120 min, using silicon nitride as a sintering aid. After sintering, slip-cast samples reached a very high relative density over 98%, while for comparison, the density of cold isostatic pressed samples was about 94%. Traces of the formation of secondary phases such as B 4 C, graphite, and BN were found in all samples, while no grain boundary phases were revealed. RT flexural strength of as-sintered slip cast bars was affected by the presence of an external porous layer of coarse ZrB 2 particles, while at 15001C the flexural strength increased, as a glassy layer sealed the superficial defects. Because of the absence of softening grain boundary phases, subcritical crack growth did not occur during high-temperature strength tests.

Several PbZr 1−x Ti x O 3 (PZT) compositions in the proximity of the morphotropic phase boundary (MPB) were examined by means of Raman spectroscopy in the 15-800 K temperature range. Previous studies performed by other researchers using various techniques evidenced that, in the phase diagram of PZT, areas with rhombohedral/monoclinic and tetragonal/monoclinic phases coexist across the MPB. For these compositions, either long-range or short-range symmetry ordering of the monoclinic phase should be considered, so that no true rhombohedral-monoclinic-tetragonal phase boundary exists. In addition, the onset of antiferrodistortive phase transitions between high-T and low-T perovskite phases has been predicted by ab initio calculations and experimentally reported. In the present work, low-T and high-T Raman scattering spectra were collected on undoped PbZr 1−x Ti x O 3 with compositions x = 0.42, 0.45, 0.465, 0.48 and 0.53 in an attempt to clarify the current open issues on the phase diagram of PZT. Spectra clearly belonging to the respective phases were observed in the rhombohedral, monoclinic and tetragonal areas, thus confirming that monoclinic ordering is long-range only for a narrow range of compositions. Raman measurements at cryogenic temperatures allowed detecting all predicted low-T phases, including the tetragonal one. These results are in good agreement with both ab initio calculations and experimental results obtained by other authors on the same compositions.

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