| With the rapid developing of modern communication technique, miniaturization and lightweight of microwave device have been increasingly addressed. Recently, much attention has been paid to the development of microwave telecommunication technologies because of the increased requirements for microwave applications. With the continuing growth of mobile telecommunications, high quality microwave dielectric ceramics are strongly required. These applications demand microwave substrate materials with high quality factor (Q×f) to achieve high selectivity, low dielectric constant (ε r) to reduce the delay time of electronic signal, and nearly zero temperature coefficient of resonant frequency (τ f) for frequency stability. However, many microwave dielectric ceramics prepared by conventional milling method with very high sintering temperature, which have excellent microwave dielectric properties. At present, the microwave dielectric ceramics are developing in the small grain size, high frequency components, and low temperature sintered. Thus, it is worth to lower the sintering temperature and improve the microwave dielectric properties of the microwave dielectric ceramics.High energy ball milling (HEBM), which is also known as mechanochemistry and mechanical alloying, is a simple and efficient method for preparing nanometer powders. Furthermore, compared with other methods, HEBM is suitable for mass production. HEBM of starting powder is effective to enhance the sintering kinetics. Due to the nanometer scale size and very high homogeneity, the mechanochemically derived ceramic powders demonstrate much better sinter ability than those synthesized by the conventional solid state reaction and soft chemical processes. Also, the HEBM can greatly improve the reactivity of precursors by reducing the phase formation temperatures of many materials. Therefore, high energy ball milling can be used as a cost-efficient and energy-saving technology to preparation of nanometer materials.In this work, nanopowders were prepared by high energy ball milling, combined with subsequent calcinations at low temperatures, and the forsterite Mg2SiO4ceramics sintered at1075℃. It was investigated that nanopowders had a far reaching influence upon the grain size of Mg2SiO4nanopowders, phase structure, synthesis temperature, and the microwave dielectric properties of Mg2SiO4ceramics. Moreover, the AWO4(A=Ca, Ba, Sr) pure phases were only obtained by using high energy ball milling method, and the ceramics sintered at relatively low temperature of900～1000℃. The grain size and phase structure of AWO4(A=Ca, Ba, Sr) powders, sintering behavior, microstructure and microwave dielectric properties of ceramics were researched by differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and network analyzer. The principal experiment results were shown as follows:(1) Mg2SiO4nanopowders were prepared from MgO and SiO2mixtures by using a high energy ball milling method for5-30h, combined with subsequent calcination at low temperatures. It is found that the qualitative concentration of Mg2SiO4phase increased with increasing the milling time and calcining temperature. After milling for30h, pure phase Mg2SiO4nanopowders with an average grain size of147.4nm were obtained calcined at850℃,300℃lower than that required by a conventional solid state reaction process. Mg2SiCO4ceramics sintered at a low temperature of1075℃showed almost full density, the relative density reaches to96.56%, and excellent microwave dielectric properties: ε r=7.2, Q×f=193,800GHz, τ f=-58ppm/℃.(2) The AWO4(A=Ca, Ba, Sr) ceramics were synthesized by high energy ball milling method for the first time. All AWO4nanopowders formed single-phase materials by milled for30h directly, and they all had a tetragonal scheelite structure. CaWO4nanopowders were obtained by milling for30h, particle size of the milled powders were about112nm. Moreover, the CaWO4ceramics with single phase were sintered at900℃, and the ceramics have dense microstructures, the relative density reaches to97.53%. The samples of CaWO4scheelite structure had good microwave dielectric properties, ε r=10.7, Q×f=61,500GHz.BaWO4nanocrystals were synthesized with a grain size of143nm by using HEBM for30h. BaWO4ceramics were sintered at1000℃,200℃lower than that required by a conventional solid state reaction technique. The relative density of BaWO4ceramics was96.16%, the average grain size was less than2μm, ε r=11.2, Q×f=54,900GHz.SrWO4nanopowders with an average grain size of138nm were prepared from SaC03and WO3mixtures by using a high energy ball milling method for30h. For the specimens sintered at950℃, the relative density was96.32%, and well microwave dielectric properties: ε r=10.5, Q×f=52,300GHz. Therefore, AWO4(A=Ca, Ba, Sr) ceramics via HEBM technique were investigated as a promising microwave dielectric material for LTCC applications. |
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