Chika Matsunaga

Sintering of reaction-bonded silicon nitride (SRBSN) is a method of preparing Si3N4 ceramics with high thermal conductivity and good mechanical properties. In the present study, by using a high purity silicon powder as the starting material and 2 mol % of Y2O3 and 5 mol % of MgO as sintering additives, the phenomena including phase transformation, grain growth, densification, and pore size evolution occurred in the SRBSN process were studied. It was found that the α- to β-Si3N4 phase transformation could complete at 1700°C. Obvious grain growth started at around 1700°C. Substantial densification took place at temperatures between 1800 and 1900°C. When sintered at 1900°C for 30 min, not only the total number of pores drastically decreased as a result of the shrinkage of the compact, but also a fraction of open pores turned to be closed pores due to microstructural changes such as grain growth and pore coalescence.

A series of Sr‐ion deficient perovskites La _0.8 Sr _{0.2− x} Ga _0.8 Mg _0.2 O _{2.8− δ} (LSGM8282, x = 0.00, 0.05, 0.10, 0.15, 0.20), was synthesized by a conventional solid‐state reaction method and their electric conductivity and chemical reactivity with Gd‐doped ceria were investigated. Reactivity tests between the LSGMs and Ce _0.9 Gd _0.1 O _{2− δ} (GDC) were carried out by X‐ray diffraction, SEM‐EDS, and electric conductivity measurements. The Sr‐ion deficient LSGMs have a lower reactivity against the formation of high‐resistivity phases than the stoichiometric ( x = 0.00) LSGM. The reaction layer formed at the interface of LSGM and GDC during the sintering process due to the mutual diffusion of the cations was classified into five layers depending on the composition. The introduction of the Sr‐ion deficient LSGM suppressed the formation of the highly resistive Sr‐rich (La _{1+ x} Sr _{1− x} )Ga ₃ O _{7− δ} phase. It was suggested that the Sr‐ion deficient LSGM (La _0.8 Sr _{0.2− x} Ga _0.8 Mg _0.2 O _{2.8− δ} ) of x = 0.15 was the best composition for suppressing the reaction with the GDC interlayer while retaining a relatively good electric conductivity.

A variety of combinations of Y ₂ O ₃ and MgO were used as additives in preparing Si ₃ N ₄ ceramics by the sintering of reaction‐bonded silicon nitride (SRBSN) method. By varying the amount of Y ₂ O ₃ in the range of 0‐5 mol% and that of MgO in the range of 0‐8 mol%, the effects of Y ₂ O ₃ and MgO additives on nitridation and sintering behaviors as well as thermal conductivity were studied. It was found that appropriate amount and combination of Y ₂ O ₃ and MgO additives were essential for attaining full densification and achieving high thermal conductivity. The sample doped with 2.5 mol% of Y ₂ O ₃ and 5 mol% of MgO attained a thermal conductivity of 128 Wm ⁻¹ K ⁻¹ when sintered at 1900°C for 6 hours, and the sample doped with 2 mol% of Y ₂ O ₃ and 4 mol% of MgO achieved a thermal conductivity of 156 Wm ⁻¹ K ⁻¹ when sintered for 24 hours.

The relationship between thermal conductivity and unidirectional cellular mullite microstructures created with the freeze casting route is discussed using actual three‐dimensional (3D) images, in which a homogenization method was conducted with microscopic models created from 3D images, the temperature variation in macroscopic models by the finite element method (FEM), and local heat conduction in microscopic models. The simulation results were consistent with the experimental results. This image‐based modeling technique was thus confirmed to be a strong tool for the prediction of various thermal properties of porous ceramics.

The effects of the nitriding temperature (1300 and 1350°C), holding time (0‐4 hours), and thickness of Si powder compacts on the nitridation behavior of silicon were investigated by examining the nitridation rates, analyzing phase compositions, and observing the microstructures of nitrided compacts. Si powder compacts doped with Y ₂ O ₃ and MgO as sintering additives were prepared with thicknesses of 3, 6, and 9 mm. The phases of nitrided compacts were transformed from Si to α‐Si ₃ N ₄ and β‐Si ₃ N ₄ with an increase in the nitriding temperature and holding time. The degree of nitridation increased with the nitriding temperature and holding time. The β/(α+β) ratio increased with the nitriding temperature and holding time, and with a decrease in the thickness of the Si powder compacts. However, all compacts exhibited the same tendency for a higher β/(α+β) ratio at the compact surface than in the bulk of the compact. The variation in the β/(α+β) ratio for each compact decreased with an increase in the nitriding temperature and holding time.

Nanoporous alumina membrane and continuous zeolite L membrane were fabricated on the inner surface of microporous alumina tubes. In the former case, an electrophoretic deposition (EPD) technique was used for the deposition of bimodal alumina particles for the subsequent low temperature sintering. In the latter case, the EPD was used for the seeding process of zeolite L particles for the subsequent hydrothermal synthesis. A thin layer of polypyrrole was synthesised on the inside wall of the porous tubes by the chemical polymerisation of pyrrole to give the wall electric conduction for the EPD electrode. The thickness of the coating layers was controlled by altering the applied voltage and deposition time. The interfacial connection of the alumina or zeolite coated layer and the substrate was evaluated by SEM observations before and after the thermal treatment. The nanoporous structure of the alumina membrane was also characterised by a pore size analyser.