Structural order in Ba(Zn1/3Ta2/3)O3, Ba(Zn1/3Nb2/3)O3 and Ba(Mg1/3Ta2/3)O3 microwave dielectric ceramics

The process and nature of structural ordering and the factors that influence them have been investigated in the microwave dielectric perovskites, barium zinc tantalate (BZT), barium zinc niobate (BZN), and barium magnesium tantalate (BMT), sintered at various temperatures. The samples were characterized mainly by X-ray powder diffraction and transmission electron microscopy. The results show that short-range 1 : 1 B-site order features strongly in the early stages of ordering in BZT and BZN, but it is extremely rare in BMT, for which most grains commence with 1 : 2 order. As sintering progresses, 1 : 1 order is replaced by 1 : 2 long-range order in BZT and by disorder in BZN. Orientational variants of the ordered domains within grains occur in similar numbers when order is fine-scale, but their distribution is less homogeneous in well-ordered samples. Local inhomogeneities in the degree of order within grains, which will affect dielectric properties, correlate with both residual non-stoichiometry and the presence of dislocations. Incompletely reacted starting materials which may persist to late stages of sintering can also strongly influence order. Anomalously large ordered domains at grain boundaries are attributed to grain-boundary migration accompanied by enhanced diffusion. The results indicate that with starting materials that are well-mixed and homogeneous at the nanoscale, tailoring of physical properties should be possible by controlling the type and degree of order through chemical composition. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructures and dielectric properties of Ba[(Co0.7Zn0.3)1/3Nb2/3]O3-based ceramics prepared by aqueous gelcasting-assisted solid-state method

The aqueous gelcasting-assisted solid-state method (AGAS) and traditional solid-state reaction method (TSSR) were used to prepare Ba[(Co_0.7Zn_0.3)_1/3Nb_2/3]O₃ (BCZN)-based ceramics. The effects of different powder-preparation methods on the microstructures and dielectric properties of the BCZN-based ceramics were investigated. X-ray diffraction analysis showed completely the same phase compositions regardless of the preparation method adopted. The relative permittivity (ε _r) did not significantly vary between the two methods. However, BCZN with CeO₂-added ceramics prepared by AGAS had higher and more uniform density (6.374 g/cm³) and high quality factor of resonant frequency (Q × f) value (75,843 GHz) than those prepared by TSSR because of the more uniform microstructures, as shown by scanning electron microscopy images. The temperature coefficients of resonant frequency (τ _f  = 7.4 ppm/°C) of the ceramics prepared by AGAS were also closer to zero than those prepared by TSSR.     

Low-firing and microwave dielectric properties of Ba[(Ni0.6Zn0.4)0 .33Nb0.67]O3 ceramics doped with Sb2O5 and B2O3

The purpose of this study was to reduce the sintering temperature of Ba[(Ni0.6Zn0.4)0.33Nb0.67]O3 ceramics by doping with Sb2O5 and B2O3. Phase formation and dielectric properties were analyzed using X-ray diffraction and the post resonator method (10 GHz), respectively. It was observed that an addition of 1 mol % Sb2O5 or 1 mol % B2O3 was very effective in reducing the sintering temperature from 1500 to 1300 °C. However, these samples showed a temperature coefficient of resonant frequency far from 0 ppm/°C. The two additions produced a temperature coefficients with opposite signs. The combination of the two dopants produced a temperature coefficient very close to 0 ppm/°C as well as a better quality factor.     

Effect of sintering temperature on dielectric properties, vibrational modes and crystal structures of Ba[(Ni0.7Zn0.3)1/3Nb2/3]O3 ceramics

The Ba[(Ni_0.7Zn_0.3)_1/3Nb_2/3]O₃ solid solutions were sintered at 1450, 1500, and 1550 °C for 3 h, respectively, by conventional solid-state sintering method, so as to clarify the effect of the sintering temperatures on vibrational modes, crystal structures, and dielectric properties. Ceramics were characterized by X-ray diffraction (XRD), Raman spectroscopy and Fourier transform far-infrared reflection (FTIR) spectroscopy. Space group and crystal symmetry of Pm[`3]m Pm\bar{3}m were determined by XRD. Lattice vibrational spectra, obtained by Raman and FTIR spectroscopy, show the correlation among polar phonon modes, crystal structures, and dielectric properties of Ba[(Ni_0.7Zn_0.3)_1/3Nb_2/3]O₃ ceramics as a function of the sintering temperatures. The results demonstrate that the dielectric constant e_r \varepsilon_{r} reaches a maximum value of 35.713 at 1500 °C and is related to Raman shifts of the A_1g(O) modes and the FWHM values of the E_g(O) modes, the temperature coefficient of the capacitance t_c \tau_{c} , which gradually increases from −6 × 10⁻⁶/°C to 0, is closely related to the Raman shifts of E_g(O) modes, and the dielectric loss values are closely connected with the full width at half-maximum of E_g(O) active modes and the Raman shifts of of the A_1g(O) modes with the increasing temperatures. FTIR shows that the dielectric properties are closely related to the far-infrared phonon modes. Raman and FTIR active modes were indicated according to the group theory.     

Effect of preparation methods on microstructures and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3 ceramics

Effect of preparation methods on microstructures and microwave dielectric properties of Ba(Mg_1/3Nb_2/3)O₃ ceramics was investigated. Ba(Mg_1/3Nb_2/3)O₃(BMN) ceramics were prepared by the conventional mixed oxides method and the molten salt synthesis method. It was shown that the single-phase of BMN was obtained at 900 °C in the molten salt synthesis method. No single-phase BMN was obtained in the conventional mixed oxides method, although the calcining temperature was increased up to 1400 °C. BMN powders prepared by the molten salt synthesis method had better sinterability than that prepared by the conventional mixed oxides method. Because of the very different nature of the powders, different microstructures were observed. The molten salt synthesis method ceramics have a higher B-site ordering parameter (S) and larger grain size than that of the conventional mixed oxides ceramics at same sintering temperature. The variation of Qf, ε _r and τ _f were also explained based on the difference in microstructures.     

High dielectric non-linear properties of the Pb[(Mg1/3Nb2/3)0.8(Sc1/2Nb1/2)0.2]O3 ceramics

Pure perovskite Pb[(Mg_1/3Nb_2/3)_0.8(Sc_1/2Nb_1/2)_0.2]O₃ (PMNSN) ceramics were prepared via a modified ‘columbite-type’ method. The 1:1 B-site ordered structure was detected by X-ray diffraction and transmission electron microscopy. The dielectric relaxation behavior was described by a Lorentz relation and a small polaron over the Curie temperature range. The dielectric tunability properties were investigated at 300 K and 10 kHz. A high dielectric tunability (～ 75% at 30 kV/cm), low dielectric loss (～ 0.003) and high figure of merit (～ 250) of PMNSN, indicate that PMNSN is a promising dielectric tunable material. The high dielectric non-linear behavior of PMNSN was further discussed in terms of the Landau–Ginsberg–Devonshire thermodynamic theory completed with a Langevin term that describes the contribution of the re-orientation of the polar nanoclusters to the non-linear ?(E) dependences.     

Effect of dopants on the low temperature microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics

Ba(Zn_1/3Ta_2/3)O₃ has been prepared with different dopants that gave best microwave dielectric properties at room temperature. Effects of different dopants on the low temperature microwave dielectric properties of BZT were investigated. With decrease in temperature, loss tangent was found to decrease marginally and then increase at temperatures lower than 100 K. Increase in loss factor at lower temperatures were found to be less for dopants with smaller ionic radii. Dielectric constant was found to be almost independent of temperature. Temperature coefficient of resonant frequency slowly decreased from a positive value to negative value when temperature was lowered. Temperature at which τ_f becomes zero was found to vary for different dopants. There is a temperature stable region for tanδ, ɛ_r and τ_f which varies for different dopants.     

Preparation and dielectric properties of Pb(Zn1/3Nb2/3)O3-based ferroelectric ceramics

The sintering behavior of mechanochemically prepared 0.9Pb(Zn_1/3Nb_2/3)O₃-0.1ABO₃ (ABO₃ = BaBiO₃, BaMnO₃, BaTiO₃) powders is studied. PbO and Bi₂O₃ are shown to volatilize at relatively low temperatures owing to partial reduction of these oxides during the mechanochemical synthesis. Dense (97% of theoretical density) ceramics are obtained under mild sintering conditions, and their dielectric properties are studied at different frequencies. The observed variations of their dielectric permittivity and loss tangent with frequency are typical of relaxor ferroelectrics, but the ceramics have a reduced dielectric permittivity, which is attributable to nanostructuring.     

Dielectric properties of Pb(Zn1/3, Nb2/3)O3 ceramics modified by Ba(Zn1/3, Nb2/3)O3 and BaTiO3

Dielectric properties of lead zinc niobate (PZN) ceramics modified by barium zinc niobate (BZN) and BaTiO₃ (BT) were investigated. By adding the modifier of BT and BZN, the stabilization of perovskite phase of PZN increased, but its Curie temperature decreased linearly with the amount of added modifier. Room temperature dielectric constant of PZN increased by addition of stabilizers up to 12 and 15 mol% of BZN and BT, respectively. The maximum room temperature dielectric constant was observed to be 7800 at 12 mol% of BZN, and 9800 at 15 mol% of BT, respectively.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3 ceramics

B₂O₃ added Ba(Mg_1/3Nb_2/3)O₃ (BBMN) ceramics cannot be sintered below 930 °C. However, when CuO was added to them, they were sintered even at 850 °C. The amount of the Ba₂B₂O₅ second phase, which was formed in the BBMN ceramics decreased with the addition of CuO. Therefore, the CuO additive is considered to react with the B₂O₃ inhibiting the reaction between B₂O₃ and BaO. A dense microstructure without pores developed with the addition of a small amount of CuO. The bulk density, dielectric constant (ɛ_r) and Q-value increased with the addition of CuO, but decreased when a large amount of CuO was added. Excellent microwave dielectric properties were obtained for the Ba(Mg_1/3Nb_2/3)O₃ + 2.0 mol% B₂O₃ + 10.0 mol% CuO ceramic sintered at 875 °C for 2 h, with values Q_xf = 21 500 GHz, ɛ_r = 31 and temperature coefficient of resonance frequency (τ_f) = 21.3 ppm/°C.     

Dielectric characteristics of Mg-replaced Pb[(Zn1/3Ta2/3)0.2(Zn1/3Nb2/3)0.6Ti0.2]O3 ceramics

Powders of a Pb[(Zn_1/3Ta_2/3)_0.2(Zn_1/3Nb_2/3)_0.6Ti_0.2]O₃ system with gradual replacement of Zn by Mg were prepared by employing a B-site precursor method and developed structures were studied by X-ray diffractometry. Dielectric properties of the sintered ceramics were characterized under weak-field low-frequency conditions. A high maximum dielectric constant value of 38,000 (@1 kHz) was realized when 3/4 of octahedral Zn was replaced by Mg. Field-dependent polarization responses were also monitored at various temperatures to investigate ferroelectricity. Microstructures of the fractured ceramics were examined using a scanning electron microscope.     

Pb(Zn1/3Nb2/3)O3-Ba(Zn1/3Nb2/3)O3铁电陶瓷结构与性能的研究

采用固相反应的方法系统地研究了BZN稳定PZN基陶瓷的相结构与介电性能.随着BZN含量的增加,PZN-BZN陶瓷中钙钛矿相的稳定性增强,居里温度近似呈线性下降,室温介电常数和介质损耗随也显著降低,最小值分别为380和0.002.为获得100%钙钛矿结构的PZN基陶瓷所需BZN的最小用量为8mol%～10mol%,当BZN的mol%超过15mol%时,PZN基陶瓷中钙钛矿相所占的百分比不再受烧结工艺的影响,基本保持100%.1kHz时Pb0.9Ba0.1Zn1/3Nb2/3O3陶瓷的最大介电常数Kmax=8680,tgδ=0.02,相应的居里温度Tm为24℃.     

Effects of CaTiO3 on crystal structures and dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics via X-ray diffraction and Raman spectroscopy

Ba(Zn_1/3Nb_2/3)O₃–CaTiO₃ (BZN–CT) ceramics were synthesized at 1,395 °C for 4 h using a conventional solid-state sintering technique, with CT contents of 0, 30, 60, 75, and 90 wt%. Crystal structures were analyzed by X-ray diffraction, and vibrational modes were obtained by Raman spectroscopy to evaluate the correlation between crystal structures, dielectric properties, and phonon modes of these ceramics. Increasing CT content can remarkably affect the crystal structures and dielectric properties of BZN–CT ceramics. Crystal symmetries decrease with CT concentration from cubic to hexagonal structure. The lattice constant and the degree of order also change accordingly. The ordered phases transform from 1:1 to 1:2 ordered structure with changing crystal structures. The relationship between the Raman shifts, the FWHM values, and the dielectric properties were obtained by Raman spectral analyses.