Low-temperature Sintering Of Ba(W 1/2 Mg 1/2 )O 3 Ceramics And Its Microwave Dielectric Properties

With the rapid development of mobile communication and internet technology or global positioning system （GPS）, small and light microwave devices have been paid much more attention. The low temperature co-firing technology （LTCC） which can reduce the size of the device, realize the integration effectively has become the focus among researchers. Except a suitable dielectric constant （εr）, high quality factor （Q×f） and near zero temperature coefficient of resonant frequency （τf）, LTCC materials are also required to have a low sintering temperature. Ba（Mg1/2W）/2)O3 ceramics have been reported with excellent microwave properties. However, their sintering temperatures all about 1550℃ were too high to be used in the LTCC multilayer devices. Thus, it is worth to carry out a research on it.In this paper, LiF was added to Ba(Mg_1/2W_1/2)O₃ to lower the sintering temperature, Ti2 addition and Ca²⁺ substitution for Mg²⁺to improve the microwave dielectric properties. The influences of composition on the sintering behavior, phase components, microstructure and microwave dielectric properties of Ba(Mg_1/2W_1/2)O₃ ceramics were investigated. The principal experiment results are shown as follows:（1） The Ba(Mg_1/2W_1/2)O₃-x wt% LiF （x=2.0,4.0,6.0,8.0） ceramics were prepared by a conventional solid-state route. With an amount of LiF addition, the sintering temperature of the ceramics was reduced to 900-975 ℃ from 1550 ℃. XRD showed that all the compounds exhibits reflections of a cubic perovskite structure with space group Fm-3m and a small amount of secondary BaWO4 phase which was mainly formed on the surfaces of LiF-doped samples. SEM indicated all samples have a dense microstructure with low porosity. The liquid phase sintering process at low temperature produced a smaller grain size ～ 1.91 μm owning to the low-melting point of LiF （845 ℃）, and with increasing x there was no marked change of grain size. For the specimens with x=2.0, the density of the samples increased to a maximum value of 6.683 g/cm3 （96% theoretical density） at 975℃. For x=4.0, maximum values of εrand Q×f（εr～18.1, Q×f～71,600 GHz） were achieved sintered at 950 ℃ for 6 h. However, the tf values shifted towards more negative values as x increases attributed to the changes of cell volume of Ba(Mg_1/2W_1/2)O₃ as well as impurity phases. The low-temperature sintering ceramics are suitable for the LTCC application.（2） In order to adjust the temperature coefficient of resonant frequency （τf） to near-zero, TiO2 was added. All the main peaks of （1-x） Ba（Mgi/2W1/2）O3-x TiO2-4.0 wt% LiF （x=0,0.02,0.04,0.06） composites could be indexed in terms of cubic perovskite structure with a small amount of secondary phase BaWO4. TiO2 phase was not detected in our XRD results and no peak shift was observed with increasing of composition x. The well-densified microstructure could be observed for all samples. As increasing of TiO2 content x, the average grain size of samples increased from 2.10 μm to 3.19 μm. As x value increases from 0 to 0.06 at 950 ℃, the εr gradually increased from 18.1 to 20.9. Conversely, the Q×f value was drastically decreased from 71,600 GHz to 46,300 GHz. As expected, the τf changed from negative （-29 ppm/℃） to positive （+8.1 ppm/℃） values with increasing of x. Excellent combined microwave dielectric properties with εr=20, Q×f=48,000 GHz and τf= 1.2 ppm/℃ was obtained at x=0.04 composition at 950℃ for 6 h.（3） To improve the microwave dielectric properties of Ba(Mg_1/2W_1/2)O₃-4.0 wt% LiF ceramics, the Ba（Mg（1-x）/2Cax/2W1/2）O3 solid solution can be well sintered at 925℃-1000℃ with Ca²⁺ substitution for Mg²⁺ in the range of 0≤x≤0.2.The cell volume increased with increasing x due to the substitution of larger Ca²⁺ for smaller Mg²⁺. No BaW04 or other impurity phases could be detected within its detecting limit. Small level doping of Ca²⁺（x=0.05） increased the Q×f values to 87,100 GHz at 1000 ℃. As expected, the substitution of Ca+ tuned the temperature coefficient of resonant frequency （τf） from negative to positive value. Near-zero τf value was obtained for x=0.2 composition with well densified structure and excellent microwave dielectric properties of εr=19.2, Qxf=48,700 GHz, and τf= 0 ppm/℃ sintered at 975℃ for 6 h.（4） A novel microwave dielectric ceramic LiAlW₂O₈ was prepared through a conventional solid-state reaction method in a low temperature range from 740℃ to 800℃. The X-ray diffraction shows that LiAlW₂O₈ ceramic has a monoclinic structure coupled with a minor of second unknown phase. The ceramics could be well sintered at 780℃ for 4 h with 95.6% relative density which consist of rod-shaped grains with a diameter of 1～3 μm and a length of 3～6 μm. Al and W element were detected at a ratio of ～1:2, which confirms the LiAlW₂O₈ phase. When sintered at 780 ℃, this new dielectric ceramic could be well densified and exhibited good microwave dielectric properties:εr=11.7, Q×f=23,000 GHz, and τf=-5.3 ppm/℃. This novel temperature stable material is an attractive candidate of low-firing microwave ceramics.