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Composition and structure are the two most essential factors for insulator materials performance. In this paper, the differences in composition, structure and mechanical properties of three typical alumina insulator for ultra high voltage direct current (UHVDC) were systematically studied, and the reasons were analyzed. The results show that compared with NGK, two domestic manufacturers have added more calcined industrial alumina, less potash feldspar and quartz into the formula, and have not properly raised the sintering temperature according to the formula, the porcelains of two domestic manufacturers show the following characteristic: 1) the content of Al2O3 are 47.22% and 45.24%, respectively, 7.31% and 5.33% higher than that of NGK, the content of SiO2 are 46.32% and 48.44%, respectively, 7.09% and 4.97% lower than that of NGK, and the total content of K2O + Na2O are 3.21% and 3.14%, respectively, 0.41% and 0.48% lower than that of NGK; 2) the true porosity are 5.24% and 4.18%, respectively, 3.46% and 4.52% lower than that of NGK. The internal pores are basically less than 5 µm, and the pore size is generally smaller. There are many flat, slender, crack like pores, as well as annular pores and cracks surrounding the particles; 3) the content of internal crystalline phase is 48.10% and 49.04%, respectively, which is nearly 10% higher than NGK, with corundum phase being 10% higher, quartz phase being 4% higher, mullite phase being 4% less. The mullite phase is thinner and shorter, and the degree of crosslinking is lower; 4) the average value of the three-point bending strength of the porcelain head test strip are 155 and 158 MPa, respectively, which are higher than NGK, but the dispersion strength of the two domestic manufacturers is higher than NGK. [ABSTRACT FROM AUTHOR]

Byline: Fei Chen, Ling Li, Faqiang Yan, Qiang Shen, Lianmeng Zhang In this study, a gradient porous Si3N4 ceramic radome wall structure is promoted for broadband (1-18 GHz) application. By optimising the thickness and dielectric constant, the transmission efficiency at 1-18 GHz is calculated based on the microwave equivalent network method. Si3N4 ceramics with gradient porous structure are then prepared. Si3N4 ceramics of controlled relative density ranging 53-99% and dielectric constant ranging 3-8 are prepared by adding different additives such as ZrP2O7, MgO and Al2O3, using pressureless sintering and spark plasma sintering. The results show that Si3N4 ceramics is still α phase and exhibit a good gradient porous structure.

A new low-temperature fired dense (1 – x)BaMoO4–xBa3(VO4)2 (BM–BV) composite ceramics were prepared by a standard solid-state reaction process. The microstructure and phase composition were characterized by scanning electron microscopy and X-ray diffraction analysis. In this system, the consequences explained that BM and BV crystal phases are coexisted perfectly. With the BV addition increase, the εr and the Q × f increase first and then decrease, and temperature coefficient of dielectric constant can be adjusted to nearly zero. In particular, the 0.48BM–0.52BV microwave dielectric ceramics sintered at 950 °C for 4 h possesses optimal dielectric performance: εr = 11.9, Q × f = 43613 GHz and τf = 0.38 ppm/°C. [ABSTRACT FROM AUTHOR]

Abstract: In this study, a novel sintering method, SPS is applied to prepare α-Si3N4 ceramics of different relative densities with MgO and AlPO4 as the sintering additives. The sintering behavior of SPS process is discussed and the factors influencing the relative density of the prepared samples are analyzed. The sintering mechanism is explained by both the reaction between MgO and SiO2 presenting on the Si3N4 powders and solid state sintering of AlPO4. Microstructures of the sintered samples are observed by SEM images and the phase compositions are analyzed by XRD. The mechanical properties of the sintered ceramics are also investigated, as a function of the relative densities. The resultant α-Si3N4 ceramics sintered from 1300°C to 1500°C show fine microstructure with nearly no grain growth and phase transformation and have a relative high bending strength. The α-Si3N4 ceramics with relative density of 58–98% and bending strength of 50–420MPa are obtained. [Copyright &y& Elsevier]