Xinwen Zhu

Textured silicon nitride: processing and anisotropic properties

Abstract Textured silicon nitride (Si3N4) has been intensively studied over the past 15 years because of its use for achieving its superthermal and mechanical properties. In this review we present the fundamental aspects of the processing and anisotropic properties of textured Si3N4, with emphasis on the anisotropic and abnormal grain growth of β-Si3N4, texture structure and texture analysis, processing methods and anisotropic properties. On the basis of the texturing mechanisms, the processing methods described in this article have been classified into two types: hot-working (HW) and templated grain growth (TGG). The HW method includes the hot-pressing, hot-forging and sinter-forging techniques, and the TGG method includes the cold-pressing, extrusion, tape-casting and strong magnetic field alignment techniques for β-Si3N4 seed crystals. Each processing technique is thoroughly discussed in terms of theoretical models and experimental data, including the texturing mechanisms and the factors affecting texture development. Also, methods of synthesizing the rodlike β-Si3N4 single crystals are presented. Various anisotropic properties of textured Si3 N4 and their origins are thoroughly described and discussed, such as hardness, elastic modulus, bending strength, fracture toughness, fracture energy, creep behavior, tribological and wear behavior, erosion behavior, contact damage behavior and thermal conductivity. Models are analyzed to determine the thermal anisotropy by considering the intrinsic thermal anisotropy, degree of orientation and various microstructure factors. Textured porous Si3N4 with a unique microstructure composed of oriented elongated β-Si3N4 and anisotropic pores is also described for the first time, with emphasis on its unique mechanical and thermal-mechanical properties. Moreover, as an important related material, textured α-Sialon is also reviewed, because the presence of elongated α-Sialon grains allows the production of textured α-Sialon using the same methods as those used for textured β-Si3N4 and β-Sialon.

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Abstract Yb2O3 is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si3N4 ceramics. Here we report the fabrication of dense Si3N4 ceramics with high thermal conductivity by the gas pressure sintering of α-Si3N4 powder compacts, using only Yb2O3 as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb2O3 content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb2O3 content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb2O3 and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb2O3 content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m-1 K−1 upon 1 mol% Yb2O3 addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m−1 K−1) at 7 mol% Yb2O3. The thermal conductivity value was strongly related to the microstructure.