Papers by Author: Xin Wen Zhu

Abstract: This paper reviews the most important results by the authors on the processing of textured -Si3N4 and -sialon by slip casting in a strong magnetic filed of 12 T and reaction-sintering. The a, b-axis textured -Si3N4 and -sialon have been obtained using the static magnetic field because of the magnetic susceptibility of ca, b > c c for -Si3N4. However, the c-axis textured -Si3N4 has also been successfully obtained using a rotating magnetic field. The -Si3N4 crystal was found to exhibit substantially stronger orientation ability than the a-Si3N4 crystal regardless of the Si3N4 raw powders. It reveals that the -Si3N4 nuclei play a key role in the texture development in -Si3N4/-sialon.

Abstract: This paper reports the texture development in Si3N4 by strong magnetic field alignment (SMFA), using slip casting of α-Si3N4 raw powder (SN-E10) and pressureless sintering. The texture of β-Si3N4 in the green and sintered bodies was characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The a, b-axis and c-axis aligned β-Si3N4 has been obtained by the static and rotating magnetic field of 12 T, respectively. The β-seed addition and prolonged sintering both enhance the texture, but the former is more efficient. This work suggests an efficient SMFA strategy of producing highly textured β-Si3N4, particularly the unidirectionally c-axis aligned β-Si3N4 by seeding the α-raw powder using the less-agglomerated β-phase particles.

Abstract: Recently, textured microstructure has received particular interest in the processing of advanced Si3N4 ceramics because of significant improvement in the mechanical properties and thermal conductivity. This work will report a highly textured β-Si3N4 ceramic by aqueous slip casting in a magnetic field and subsequent pressureless sintering, using commercial α-Si3N4 raw powder and a mixture of Y2O3 and Al2O3 as sintering aids. To obtain the well-dispersed Si3N4- Y2O3-Al2O3 slurries, polyethylenimine (PEI) was chosen as a dispersant. Effects of the sintering aids, PEI amount, pH and stirring time on the stability of the Si3N4 slurries were studied. It is shown that PEI is an effective dispersant for stabilizing the Si3N4-Y2O3-Al2O3 slurries that does not show a time-dependent behavior at an optimum pH ≈ 10, compared to the case in the absence of PEI. Using the 30 vol% Si3N4-Y2O3-Al2O3 slurries stabilized with 1.5 dwb% PEI at pH 10, the highly textured β-Si3N4 with 97 % relative density could be obtained by slip casting in a magnetic field of 12 T and subsequent sintering at 1800 oC for 1 h. The textured microstructure is featured by the alignment of c-axis of β-Si3N4 crystals perpendicular to the magnetic field, and the Lotgering orientation factor, f, is determined to be 0.8.

Abstract: This work will report the deformation behavior of silicon carbide reticulated porous ceramics (SiC RPCs) under three-point bend test. SiC RPCs were fabricated by replication processing using an open-cell polyurethane sponge with cell size of ~ 13 pores per inch (ppi). It is shown that the deformation behavior strongly depends on the loading uniformity and the macrostructure. Using a compliant layer (0.5 mm paper pad), the uniform loading leads to a significant transition in the load-displacement curve of RPCs from the complex saw-tooth shape to the one similar to dense ceramics, despite the presence of some macrostructural flaws and partial clogged cells. However, this dependence of loading uniformity is alleviated by developing highly uniform macrostructure with fewer flaws and clogged pores. Even, this dependence becomes less important as the struts become thicker and stronger, leading to an increase in relative density, accordingly. The bend result of RPCs with highly uniform macrostructure is in excellent agreement with the GA (Gibson and Ashby) model, but the one with un-uniform macrostructure deviates from the GA model. This work shows that the macrostructure plays an important role in deformation behavior of RPCs under bend.