Papers by Keyword: Grain Boundary Phase

Abstract: Y₂O₃ is a common sintering additive of AlN ceramics to achieve densification and remove the oxygen impurity, resulting in a typically grain boundary phase (GBP) Y₃Al₅O₁₂ (YAG). Two AlN ceramics with 3wt% and 5wt% Y₂O₃ intended for thermal conductivity study were sintered at 1800 °C for 4h. X-ray diffraction (XRD) indicates that GBP could either be YAG or YAP (YAlO₃) phase, while the selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) in TEM identifies it as YAP instead of YAG. The electron back-scattering diffraction (EBSD) in SEM further confirms the general presence of YAP phase in both samples. In meanwhile, two types of Al-rich GBPs were also detected by TEM, which could account for extra dopant in the microstructure. GBP contents in the both samples were quantified by K-value method (XRD) and from backscattered electron images. Such analyses of GBPs are helpful to understand the sintering mechanism and evaluate their contribution to the thermal conductivity of AlN.

Abstract: In this study, we analyzed the effect of microstructure and oxygen content on the thermal conductivity of dense AlN ceramics by Slack’s plot, which plots the oxygen content estimated from the lattice parameter c versus the thermal resistivity, which is inverse of the thermal conductivity. The analyses were carried out for the AlN ceramics sintered with oxide additives such as Y2O3, Y2O3-Al2O3, or Y2O3-CaO-B. The data of AlN ceramics sintered with Y2O3 or Y2O3-CaO-B located on the plot of the relation between oxygen content in AlN lattice and thermal resistivity of AlN presented by Slack (Slack’s line) or Harris et al (Harris’s line), respectively. In contrast, the data of the sample sintered with Y2O3-Al2O3, which contains excess grain boundary phases, located above the Harris’s line. These results suggest that the thermal conductivities of AlN ceramics are influenced by not only the oxygen contents in the AlN lattices but also the contents of grain boundary phases.

Abstract: AlN ceramics doped with yttrium oxide (Y2O3) as the sintering additive were prepared via the spark plasma sintering (SPS) technique. The sintering behaviors and densification mechanism were mainly investigated. The results showed that Y2O3 addition could promote the AlN densification. Y2O3-doped AlN samples could be densified at low temperatures of 1600-1700oC in 20-25 minutes. The AlN samples were characterized with homogeneous microstructure. The Y-Al-O compounds were created on the grain boundaries due to the reactions between Y2O3 and Al2O3 on AlN particle surface. With increasing the sintering temperature, AlN grains grew up, and the location of grain boundaries as well as the phase compositions changed. The Y/Al ratio in the aluminates increased, from Y3Al5O12 to YAlO3 and to Y4Al2O9. High-density, the growth of AlN grains and the homogenous dispersion of boundary phase were helpful to improve the thermal conductivity of AlN ceramics. The thermal conductivity of 122Wm-1K-1 for the 4.0 mass%Y2O3-doped AlN sample was reached.

Abstract: Contact damage resistances of silicon nitride ceramics with various grain boundary phases are investigated in this study. The grain boundary phases are controlled by the addition of different types of sintering additives, or the crystallization of intergranular phase in a silicon nitride. We control the microstructures of materials to have similar grain sizes and the same phases to each other. Contact testing with spherical indenters is used to characterize the damage response. The implication is that the grain boundary phase can be another controllable factor against contact damage and strength degradation even though it is not critical relative to the effect of grain morphology.

Abstract: We investigated grain boundary crystallization of gas-pressure-sintered silicon nitride with zirconia and magnesia as sintering aids. Cation compositions were mostly uniform throughout the specimen however, ZrO2 was crystallized in the surface region, while ZrN in the inside. When the specimen was heat-treated at 1 atm nitrogen atmosphere, ZrO2 in the surface region transformed to ZrN. The transformation, however, was suppressed when alumina was incorporated as an additional sintering aid. Based on these results, we propose a model describing the reaction among Si3N4, SiO2, ZrO2, ZrN and N2. Observed microstructures and measured mechanical properties were consistent with the model.