The effect of the GdH 2 on the shrinkage behavior, microstructure, thermal conductivity , and flexural strength was investigated. The pre-sintering enables the elimination of native SiO 2 and the simultaneous formation of Gd 2 O 3 . The N/O ratio in the liquid phase was increased because of the increasing Gd 2 O 3 /SiO 2 ratio, resulting in the restricted shrinkage during gas pressure sintering. The high N/O ratio in the liquid phase benefits not only the nucleation and growth of β-Si 3 N 4 but also the removal of lattice oxygen, leading to the increase of thermal conductivity . Although the replacement of Gd 2 O 3 by GdH 2 leads to enhanced flexural strength at low temperature, it tends to reduce the flexural strength at high temperature. This variation related largely to the exaggerated bimodal microstructure. This work signifies that GdH 2 is an effective sintering additive for obtaining Si 3 N 4 ceramics with both high flexural strength and high thermal conductivity.
A two-step sintering process was conducted to produce β-Si 3 N 4 ceramics with high thermal conductivity. During the first step, native SiO 2 was eliminated, and Y 2 O 3 was in situ generated by a metal hydride reduction process, resulting in a high Y 2 O 3 /SiO 2 ratio. The substitution YH 2 for Y 2 O 3 endow Si 3 N 4 ceramics with an increase of 29% in thermal conductivity from 95.3 to 123 W m −1 K −1 after sintered at 1900°C for 12 hours despite an inferior sinterability. This was primarily attributed to the purified enlarged grains, devitrified grain boundary phase, and reduced lattice oxygen content in the YH 2 -MgO-doped material.
Qiang Zhi,Bo Wang,Shan Zhao,Zhao Zhang,Yu-Chen Deng,Bao-Qiang Hou,Nan-Long Zhang,Jian-Feng Yang
Si 3 N 4 ceramic with ultrafine fibrous grains are expected to exhibit remarkable mechanical properties . In this work, highly porous Si 3 N 4 ceramic monoliths composed of ultrafine fibrous grains were developed via a novel vapor-solid carbothermal reduction nitridation (V-S CRN) reaction between SiO vapor and green bodies comprised of carbon nanotubes (CNTs), α-Si 3 N 4 diluents and Y 2 O 3 in a N 2 atmosphere. The unique fibrous grains-interconnected structure was developed through in-situ formation of Si 3 N 4 and following liquid phase sintering . The porous Si 3 N 4 monoliths with porosity of 61–78% was developed by controlling the contents of α-Si 3 N 4 diluents and densities of the CNT green bodies. With increasing of the α-Si 3 N 4 contents, Si 3 N 4 fibrous grains with an aspect ratio of approximate or higher than 20 could be achieved, and the grains were gradually refined. For the samples with 40 wt% α-Si 3 N 4 , the minimum mean grain diameter and pore size of 164 nm and 0.79 μm were achieved, respectively, and the resultant porous Si 3 N 4 monolith exhibited a flexural strength of as high as 73–102 MPa with the porosity of 61–73%, which is much higher than that of the reported in literature. The improvement of mechanical strength could be attributed to the densely interconnected bird's nests structure formed by the ultrafine fibrous grains. The effects of the α-Si 3 N 4 diluents on the resulting porous Si 3 N 4 monolith via this method were analyzed.
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