Key Engineering Materials Vols. 512-515

Abstract: Microstructures and microwave dielectric properties of Ca(Sm_0.5Nb_0.5)O₃ ceramics, prepared by a conventional solid-state reaction method, were systematically investigated by varying calcining temperature, sintering temperature and cooling rate. The XRD result showed that a single Ca(Sm_0.5Nb_0.5)O₃ phase could be synthesized at a calcining temperature of 1200 °C. Optimized combination of microwave dielectric properties of ε_r = 22.36, Q×f = 18030 GHz and τ_f = -31.2 ppm/°C was obtained for furnace-cooled Ca(Sm_0.5Nb_0.5)O₃ ceramics sintered at 1550 °C for 4 h. However, some microcracks were found from the microstructures of the furnace-cooled specimens. Further, the Q×f value could be increased by controlling the cooling rate during the sintering process due to the disappearance of microcracks in the final material. With a cooling rate of 2 °C/min, Ca(Sm_0.5Nb_0.5)O₃ceramics exhibited an enhanced Q×f value of 37130 GHz.

Abstract: Bismuth zinc niobate [(Bi_1.5Zn_0.5)(Nb_1.5Zn_0.5)O₇, abbreviated as BZN] ceramics are receiving increasing attention due to their excellent dielectric properties in the microwave frequency range. This work is aimed at improving the fabrication technology of BZN ceramics. BZN ceramic specimens were prepared using the general electronic ceramic technique including milling, calcining, crushing, pressing, and sintering. Different sintering temperatures in the range of 950-1080°C were used to study how sintering temperature affects the structure and dielectric properties of BZN ceramics. The crystallinity and microstructure of the BZN ceramics, which were measured respectively by X-ray diffraction and scanning electron microscopy, were improved with increasing of the sintering temperature. The frequency dependence of the dielectric constant and loss tangent was measured at room temperature from 1 kHz to 1 MHz. The dielectric properties of the specimen sintered at 1050°C were found to be the best, for which the relative permittivity (ε_r) and the loss tangent (tanδ) are around 146 and 0.005, respectively. It was also foεund that when the sintering temperature was higher than 1000°C, the ε_r and the tanδ of BZN ceramics sintered at different temperatures were similar. As a result, 1000°C may be an appropriate sintering temperature for BZN ceramics.

Abstract: We calculate the phase velocity(V), power flow angle(PFA) and electromechanical coupling factor(k2) on X,Y,Z-cut of Sr3NbGa3Si2O14(SNGS),Sr3TaGa3Si2O14 (STGS), Ca3NbGa3Si2O14(CNGS) and Ca3TaGa3Si2O14(CTGS)crystal,. The calculated results are in excellent agreement with the experimental results. As these crystals are denser than quartz, they have lower phase velocity. The results also indicates that they have high k2 and cuts of zero PFA. Taking various SAW properties into consideration, some typical propagation directions like 167.5° of X-cut SNGS ,0 of Y-cut SNGS, 170° of X-cut STGS, 0 of Y-cut STGS,11 of X-cut CNGS, 0 of Y-cut CNGS, 175.0 of X-cut CNGS and 0 of Y-cut CTGS are selected, which can be used in the application of SAW devices.

Abstract: Aim of the present research was to fabricate and study crystalline structure as well as dielectric properties of BiNbO₄ ceramics. In the present study BiNbO₄ ceramics was fabricated by solid state reaction from the mixture of simple oxides viz. Bi₂O₃, and Nb₂O₅. Stoichiometric mixture of the powders was thermally analyzed so parameters of the thermal treatment were determined. The EDS measurements proved conservation of the chemical composition of the ceramic powder after calcination (T=800°C). Free sintering at T=870 – 1080 °C was used for fabrication of the ceramic samples. Morphological analysis using scanning electron microscopy was performed. The crystalline structure of the sintered samples were examined by X-ray diffraction method at room temperature. It was found that for low temperature of sintering (T0C) bismuth niobate crystallized in orthorhombic symmetry Pnna (52) whereas for high sintering temperature (T>1000 °C) BiNbO₄ exhibited anorthic symmetry P-1 (2). Low frequency (ν=100Hz – 1MHz) impedance spectroscopy was applied to study influence of the sintering temperature on dielectric properties of bismuth niobate at room temperature. The equivalent circuit method was used for analysis of the impedance data.

Abstract: The structure and electrical characteristics of the lead-free Li_x(K_0.5Na_0.5)_1-x(Nb_0.8Ta_0.2)O₃ (x=0~0.05) piezoelectric ceramics for the conventional solid-state reaction method and the B-side pre-calcined method were achieved and compared. For the B-side pre-calcined method, the lead-free ceramic material exhibited the excellent electrical and piezoelectric properties. The relative dielectric constant (ε_r) and loss (tan δ) of the Li_x(K_0.5Na_0.5)_1-x(Nb_0.8Ta_0.2)O₃ for x=0.03 using the B-side pre-calcined method were 1223 and 0.021, respectively. In addition, the electromechanical coupling factors (k_p) and Curie temperature (T_c) was 48.5 % and 315°C. Finally, the electrical properties of the lead-free Li_x(K_0.5Na_0.5)_1-x(Nb_0.8Ta_0.2)O₃(x=0~0.05) homogeneity ceramics improved by the B-side pre-calcined method were also investigated and discussed.

Abstract: MgTa₂O₆ powders with the tetragonal phase have been synthesized by mixed oxide method at 1100 °C when the molar ratios of MgO and Ta₂O₅ increase from 1:1 to 1.2:1. The microwave dielectric properties of sintered MgTa₂O₆ ceramics without additives at 1400°C for 6h, which consists of tetragonal and orthorhombic phases have been investigated. The results show that the ceramic possesses a dielectric constant of 27.5, a high Q×f value of 156000 GHz and a temperature coefficient of resonant frequency of 21.6 ppm/°C at 6-10 GHz. Different amounts of CuO has been used as sintering additives to lower the sintering temperature of MgTa₂O₆ ceramics. The effect of the CuO doping agent on the microstructure, densification and the microwave dielectric properties of MgTa₂O₆ ceramics has been studied using X-ray diffraction, scanning electron microscope and HP8720ES network analyzer. The results indicate that the 1-5 wt. % CuO dopant can lower the sintering temperatures of this materials dramatically. The second phase of CuTa₂O₆ was found in MgTa₂O₆ ceramics, which caused the deterioration of Q×f value of the ceramics.

Abstract: The structure, microwave dielectric properties and low-temperature sintering of a new Li2O-Nb2O5-TiO2 system ceramic with the Li2O: Nb2O5: TiO2 mole ratio of 1.52: 0.36: 1.34 have been investigated in this study. The 1.52Li2O-0.36Nb2O5-1.34TiO2 (LNT) ceramic is composed of two phases, the “M-Phase” and Li2TiO3 solid solution (Li2TiO3ss) phase. This new microwave dielectric ceramic has low intrinsic sintering temperature ( ~ 1100 oC ) and good microwave dielectric properties of middle permittivity (εr ~38.6), high Q×f value up to 7712 GHz, and near zero τf value (~ 4.64 ppm/oC). In addition, the sintering temperature of the LNT ceramics could be lowered down effectively from 1100 oC to 900 oC by adding 1 wt.% B2O3. Good microwave dielectric properties of εr = 42.5, Q*f =6819 GHz and τf = 2.7 ppm/oC could be obtained at 900 oC, which indicate the ceramics would be promising candidates for low-temperature co-fired ceramics (LTCC) applications.

Abstract: CaCu3Ti4O12 (CCTO) powder was prepared by solid-state reaction. CCTO-ZnTiO3 composite ceramic were prepared by reaction of CCTO powder with molten ZnCl2 in air, 400°C. The products were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electric and dielectric properties of CCTO-ZnTiO3 composite ceramic were investigated. It is shown that the ceramic composites have higher grain-boundary resistivity and nonlinear coefficient. The addition of Zn mainly formed in the form of ZnTiO3 also affected the low frequency capacitance values, reduced the CCTO ceramic dielectric loss, which showed a Schottky-type nature of potential barrier. Moreover, the mechanisms involved in the NBLC model were discussed, where it is working with the IBLC model.

Abstract: Bismuth ferrite (BFO), one of very few multiferroics with simultaneous coexistence of ferroelectric and magnetic ordering, has attracted much attention due to its potential applications for new memory devices. In this work, 30~90 nm BFO powders with single rhombohedral phase were prepared by chemical coprecipitation method and after calcining of 500oC. Photocatalytic properties of the powders and dielectric properties of BFO ceramic were characterized, respectively. Cut-off wavelength of the powders at 590 nm exhibited efficient ultraviolet photocatalytic activity, which has been demonstrated by a photocatalytic result. 0.3g BFO powders can make 10 ppm RhB aqueous solution (100 ml) to decolorize and RhB decomposition rate reached to 95% during 3 hr UV irradiation. BFO ceramic can be obtained by sintering the green disc compacted with BFO powders at 800oC for 1h. Its dielectric constant is t about 250, and its dielectric loss is 0.03 at below 100 oC in the frequency range of 102 ~ 104 Hz and up to 1200 at 200 oC under 1 kHz, respectively. Reduced polarization is found due to higher loss and lower dielectric constant of the ceramic when the measuing frequency is more than 104 Hz. Magnetic hysteresis loop of BFO ceramic also shows weaker magnetic property, which may be attributed to size confinement effect of the nanostructures.

Abstract: A series of Cr doped multiferroic BiFeO₃ (BFO) with the composition of BiFe_1-xCr_xO₃ have been prepared using hydrothermal method, in which x varies as 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5. The synthesis temperature is carried out as low as 230 °C under hydrothermal conditions. With the increase of the content of Cr, the second phase is gradually increased. When the value of Cr is 0.5, the main peak (110) and (104) of BFO tends to disappear, in which the structure of BFO is changed. The permittivity of as-prepared samples decreases with the increase of Cr content and excellent frequency stability is exhibited. Better grain structure can be obtained for samples sintered at 830 °C