Liquid Phase Formation and Sintering Mechanism of Nitride-Oxide Ceramics.

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The liquid phase formation in the system AlN-Y_2O_3-Al_2O_3 and the sintering mechanism in Y_2O_3 doped AlN system were investigated using self-made high-temperature DTA and quenching method. We prove the AlN itself reacts with grain boundary phases to form liquid phase at high temperature using the high-temperature DTA.High-temperature DTA applicable upto 2000ﾟC for non-oxygen atmosphere was made using the DTA unit that composed BN cell and Re20%W/Re40%W thermocouple. The DTA cell was constructed for reducing the leak current and carbon vapor. The D.C.power source was used for reducing the noise.Y_4Al_2O_9-AlN,Y_3Al_5O_<12>-AlN,and the eutectic composition between Y_3Al_5O_<12> and Al_2O_3 and AlN systems were used for detecting the liquid phase formation temperature by means of the DTA.Phase chemistry for the samples after DTA measurement revealed no compounds between AlN and Y_2O_3-Al_2O_3 system. The liquid phase formation temperature decreased significantly (80 to 230ﾟC) by the addition of AlN to Y_2O_3-Al_2O_3 composition. The lowest temperature of the liquid phase formation in this experiment was 1690ﾟC.Lattice parameter of Y_4A_<12>O_9 changed slightly in the sample Y_4Al_2O_9-AlN system quenched at above liquid phase formation temperature. EPMA shows, however, no direct observation of nitrogen dissolution to Y_4Al_2O_9. This means the phase diagram of the system Y_2O_3-Al_2O_3-AlN is eutectic type. Partial phase diagram of the system Y_3Al_5O_<12>-AlN was proposed using DTA and quenchying results.The liquid phase sintering of the Y_2O_3 added AlN system proceeds through the large amount of liqauid phase made of Y_2O_3 additive, impurity oxygen and AlN itself. During cooling liquid phase decomposes into Y-Al-O and AlN,oxygen remains in the Y-Al-O phases and decomposed AlN is purified. This produces the high thermal conductive AlN.