Jianghao Liu

Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting

Porous mullite ceramics were prepared at 1300–1600°C for 2 h via a foam-gelcasting route using industrial-grade mullite powders as the main raw material, Isobam 104 as the dispersing and gelling agent, triethanolamine lauryl sulphate as the foaming agent and sodium carboxymethyl cellulose as the foam stabilising agent. The effects of firing temperature on the sintering behaviour of green samples as well as microstructures and properties of final porous mullite products were investigated. With increasing the temperature from 1300 to 1600°C, linear shrinkage and bulk density values of fired samples increased, whereas their porosity decreased. Mechanical strength and thermal conductivity values of fired samples decreased with increasing their porosities. Even at a porosity level as high as 79.4%, compressive and flexural strengths of fired samples (with average pore size of 314 μm) remained as high as 9.0 and 3.7 MPa, respectively, and their thermal conductivity (at 200°C) remained as low as 0.21 W (m−1 K−1).

Highly Efficient and Low-Temperature Preparation of Plate-Like ZrB2-SiC Powders by a Molten-Salt and Microwave-Modified Boro/Carbothermal Reduction Method

To address the various shortcomings of a high material cost, energy-intensive temperature conditions and ultra-low efficiency of the conventional boro/carbothermal reduction method for the industrial preparation of ZrB2-SiC powders, a novel molten-salt and microwave-modified boro/carbothermal reduction method (MSM-BCTR) was developed to synthesize ZrB2-SiC powders. As a result, phase pure ZrB2-SiC powders can be obtained by firing low-cost zircon (ZrSiO4), amorphous carbon (C), and boron carbide (B4C) at a reduced temperature of 1200 °C for only 20 min. Such processing conditions are remarkably milder than not only that required for conventional boro/carbothermal reduction method to prepare phase pure ZrB2 or ZrB2-SiC powders (firing temperature of above 1500 °C and dwelling time of at least several hours), but also that even with costly active metals (e.g., Mg and Al). More importantly, the as-obtained ZrB2 particles had a single crystalline nature and well-defined plate-like morphology, which is believed to be favorable for enhancing the mechanical properties, especially toughness of their bulk counterpart. The achievement of a highly-efficient preparation of such high-quality ZrB2-SiC powders at a reduced temperature should be mainly attributed to the specific molten-salt and microwave-modified boro/carbothermal reduction method.

Preparation of ZrB2-SiC Powders via Carbothermal Reduction of Zircon and Prediction of Product Composition by Back-Propagation Artificial Neural Network

Phase pure ZrB2-SiC composite powders were prepared after 1 450 °C/3 h via carbothermal reduction route, by using ZrSiO4, B2O3 and carbon as the raw materials. The influences of firing temperature as well as the type and amount of additive on the phase composition of final products were detailedly investigated. The results indicated that the onset formation temperature of ZrB2-SiC was reduced to 1 400 °C by the present conditions, and oxide additive (including CoSO4·7H2O, Y2O3 and TiO2) was effective in enhancing the decomposition of raw ZrSiO4, therefore accelerating the synthesis of ZrB2-SiC. Moreover, microstructural observation showed that the as-prepared ZrB2 and SiC respectively had well-defined hexagonal columnar and fibrous morphology. Furthermore, the methodology of back-propagation artificial neural networks (BP-ANNs) was adopted to establish a model for predicting the reaction extent (e g, the content of ZrB2-SiC in final product) in terms of various processing conditions. The results predicted by the as-established BP-ANNs model matched well with that of testing experiment (with a mean square error in 10-3 degree), verifying good effectiveness of the proposed strategy.