Cordierite Ceramics Prepared from Poor Quality Kaolin for Electric Heater Supports: Sintering Process,Phase Transformation,Microstructure Evolution and Properties

To efficiently utilize the kaolin, an economical way of preparing cordierite ceramic with high performance for electric heater supports was put forward. In this study, sintering process, phase transformation, microstructure evolutions were systematically studied by heating microscope, X-ray diffraction, scanning electronic microscope and thermal analysis. Properties(physical properties, electrical properties and coefficient of thermal expansion(CTE)) were tested for comprehensive performance evaluation. The results showed that the utilization of poor quality kaolin broadened the firing range of cordierite ceramic which was from 1 200 to 1 380 ℃. Microstructure becomes loose with increasing of the pore size, which had significant influence on bending strength and electrical properties. High content of K2 O in poor quality kaolin was the reason for liquid phase generation in sintering process, which further leads to microstructural changes. The cordierite ceramic sintered at 1 320 ℃ had the properties as follows: CTE of 1.98×10~(-6) ℃~(-1)(500 ℃), bending strength of 90 MPa, apparent porosity of 15.1%, dielectric constant of 7.5(100 Hz), and volume resistivity of 1.05×109 Ω·cm(100 Hz). The comprehensive properties are very suitable for use as electric heater supports.     

基于FPGA的基带信号发生器的设计

本文采用一种全数字实现基带信号的方法,避免了模拟方式因正交调制信号相位或幅度不平衡产生的边带及本振泄漏,同时可以通过软件更新来适应不同的数据速率、调制方式及滤波器特性等,具有很大的灵活性和适应能力。该方法适用于数字通信、数字电视等领域。