Abstract: The sintering behavior, microstructure and microwave dielectric properties of A-site modified Pb(Mg1/2W1/2)O3 with Ca2+, Ba2+and La3+ were investigated in this paper. It is discovered that a series of single-phase perovskite type solid solutions with A-site vacancies were formed for Pb1-3/2xLax(Mg1/2W1/2)O3 (0 £ x £ 2/3)). The solid solution took cubic perovskite type structure (Fm3m)with random distribution of A-site vacancies when 0 < x < 0.5, and tetragonal or orthorhombic structure with the ordering of A-site vacancies when 0.5 £ x £ 2/3. However, the solid solubility of Ba(Mg1/2W1/2)O3 and Ca(Mg1/2W1/2)O3 in Pb(Mg1/2W1/2)O3 is limited to 15 and 5 mol%, respectively, in spite of no structural difference between the end member. It was also found that the A-site substitution with Ca2+, Ba2+and La3+ for Pb2+ has no influences on the degree of B-site ordering between Mg2+ and W6+. However, the antiferroelectric Tc decreases with increase in the content of A-site substitution. The dielectric constants and temperature coefficient of resonant frequency of La3+ modified Pb(Mg1/2W1/2)O3 are much lower than that of. Ca2+and Ba2+ modified Pb(Mg1/2W1/2)O3, and decrease with increasing La content. However, its Q×f values is much higher than that of Ca2+and Ba2+ modified Pb(Mg1/2W1/2)O3,
and increase with increase in La content. Relatively good combination microwave dielectric properties were obtained for Pb1-3/2xLax(Mg1/2W1/2)O3 with x=0.56: er=28.7, Q×f=18098, and tf=-5.8ppm/oC.
Abstract: The different amounts of manganese dioxide were doped to Ba4.5Sm9Ti18O54 (BST)ceramics. It was found that the doping sequence of Mn had serious influences on the microwave dielectric properties of the BST ceramics. When manganese dioxide was added before the mixtures were calcined, the dielectric properties of ceramics were poor and decreased with the increase of the
Mn content. When the Mn content was between 2.0wt% and 3.0wt%, the BST ceramics had no microwave characteristic. But adding manganese dioxide in a range of 0.0 to 1.0wt% after the mixtures were calcined enhanced the microwave dielectric properties of the BST ceramics. When the Mn content changed from 0 to 1.0wt%, the microwave properties of the BST ceramics increased, but the microwave properties decreased while the Mn content was over 1.0wt%. The ceramics with 3.0wt% Mn content had also no microwave characteristic. The optimum microwave dielectric properties were as follows: εr=76, Q·f=7280,τf=-3.6ppm/k.
Abstract: The Ba5(Nb1-xTax)4O15 (x=0~0.5) solid-solution ceramics with hexagonal perovskite structure were prepared by solid-state oxide route. The phases and structure of the sintered ceramics were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of composition and sintering temperature on the bulk density, microstructure and microwave dielectric
properties were also investigated. As the value of x increased from 0 to 0.5, the Q×f value increases from 36,049 to 49,493 GHz (at ~9GHz), whereas the dielectric constant (εr) decreases from 39.21 to 33.03 and the temperature coefficient of resonant frequency (τf ) decreases from 78.6 to 19.4 ppm/oC. The results
indicate that the microwave dielectric properties of Ba5(Nb1-xTax)4O15 ceramics can be easily modified in a large range by changing the relative content of Ta to Nb.
Abstract: Effect of Co2O3 on microstructure and dielectric properties of Mg2TiO4-Mg2SiO4 microwave ceramic with low permittivity and high Q value was studied. Co2O3 acts and results in decreasing of the sintering temperature and densification of the ceramic material. As a result, the dielectric loss decreases and quality factor Q increases. Mg2+ is replaced partially with Co2+ in Mg2TiO4 to form (Mg,Co)2TiO4. It is demonstrated that the system with 2.4 wt% Co2O3 possesses better dielectric properties.
Abstract: (Ag1-xNax)(Nb1-yTay)O3 (ANNT) dielectric ceramic material with perovskite structure was prepared through a wet chemical synthetic method. The precursor solution for the material was obtained from the citric acid solutions of different metal oxides or nitrates with exact molar ratio by a sol-process. The precursor solution thus prepared was then heated under an infrared lamp to
evaporate the solvent. This period showed a self-propagating combustion behavior. Ceramic powder was obtained after the residue was calcined at a certain temperature. It is proved that ANNT sintered at 1040°C was finely grained. The obtained ceramic showed excellent high frequency dielectric properties with considerable high dielectric constant and rather low dissipation factor under 1MHz.
Abstract: The microstructure and microwave dielectric properties of dielectric ceramics comprised of ZnNb2(1-x)TixO(6-3x) (x = 0 ~ 1) were investigated systematically using X-ray powder diffraction, SEM and a network analyzer in this study. The results showed that four phases were observed in this system. The dielectric properties at microwave frequencies exhibit a significant dependence on the composition and crystal structure of the ceramics. The dielectric constant ( er) of ZnNb2(1-x)TixO(6-3x)ceramics increases with increasing Ti content and the quality factors (Q ×f) significantly decreased. However, the temperature coefficient of resonant frequency ( t f ) changes from –59.08 ppm/oC at x = 0
to + 25.1 ppm/oC at x = 1, and a zero temperature coefficient of resonant frequency ( t f ) for this ceramics system was obtained at x = 0.84.
Abstract: MnCO3 was added into ZnNb2O6 ceramics to obtain excellent microwave dielectric properties. The samples were prepared by conventional solid-state reaction method. The effects of the amount of MnCO3 on sintering temperatures, ceramic densities and contraction were systematically investigated. The crystalline structure of ceramic body was analyzed by XRD. The ceramic microstructure was observed
by SEM. The dielectric properties of ZnNb2O6 ceramics were measured by a vector network analyzer at microwave frequency, which showed: er = 22.65, Q×f = 36700 GHz (loaded value) and tf = -40 ppm/°C.
Abstract: The effects of additions on the microstructure and microwave dielectric properties of
0.42ZnNb2O6-0.58TiO2 (ZNT) ceramics were investigated systematically. The sintering temperature of ZNT ceramics doped with different amounts of CuO, Bi2O3 and V2O5 can be effectively reduced. The phase of doped samples was studied compared to the undoped ZNT ceramics. For densified ZNT ceramics with the increasing amount of dopant CuO-Bi2O3-V2O5, the dielectric constants (εr) of
densified samples ranges from 38 to 42, the Q×f values decrease evidently and ranges from 13300 GHz to 5000 GHz and the τf values are shifted toward negative.
Abstract: By means of 34.5 GHz millimeter-wave (MMW) sintering, BaO-Nd2O3-TiO2 microwave ceramic dielectric was prepared from nano-particles of the xBaO×(0.35-x)Nd2O3×0.65TiO2 ternary compound, which possess fine grain, low porosity and good dielectric characteristics. The materials which have ultra-low dielectric loss can be fired by MMW heating at very high frequency, which is unable to be done by normal microwave heating at 2.45 GHz. The sintering temperature is 160 ~ 400°C lower than that of normal firing. The ceramic sample consists of the main phase of Nd2Ti2O7 with perovskite-like layer structure (PLS) and the secondary phase of BaNd2Ti4O12 (Ba4.5Nd9Ti18O54)with perovskite-like tungsten bronze structure, being the same as the results from normal sintering. The excellent microwave characteristics were observed.