Key Engineering Materials Vols. 368-372

Abstract: CaCu3Ti4O12 (CCTO) barrier layer ceramic capacitor was prepared by a two-step sintering process. The CCTO powders were pre-synthesized at 900oC by solid-state reaction and the effects of the amount of CuO on the formation of the CCTO powders were investigated. The CCTO ceramics were prepared by the second-step sintering. It was found that the abnormal grain growth and inhomogeneous microstructure are controlled by the amount of excessive CuO. The optimized CuO content in the composites is ~14 wt%. The maximum permittivity is 115,000 (1 kHz, 210oC).

Abstract: CaCu3Ti4O12 (CCTO) ceramics were prepared by cold isostatic press (CIP) forming combined with the conventional sintering method. The effects of calcining temperature on properties of CCTO ceramics were investigated. The dielectric properties as the function of temperature at typical frequencies were measured in the temperature range from 243 to 363 K. In addition, the dielectric properties and complex impedances at room temperature were analyzed in a frequency range from 20 Hz to 106 Hz. The results showed that CCTO ceramics with dielectric constant higher than 104 were obtained. Compared with the dry press forming, CCTO ceramics prepared by CIP had more homogeneous grains, less porosity and better dielectric properties.

Abstract: CaCu3Ti4O12 (CCTO) ceramic thick films have been prepared on Copper substrate, using ceramic powders by an aerosol deposition method (ADM). The ceramic powders are prepared by traditional solid state reaction processing at 1100 oC for 5 h. X-ray diffraction and scanning electron microscopy are used to investigate the microstructure and the phase composition of the deposited films. The results indicate that thick films are pure CCTO phase and homogenous. The dielectric impedance spectra indicate that the dielectric constant of CCTO thick film can reach 3×103.

Abstract: CaCu3(Ti1-xZrx)O12 ceramics (x = 0.02, 0.04, 0.06, 0.08) were prepared by the conventional solid-state reaction method. Phase compositions, microstructure and dielectric properties were examined by XRD, SEM and impedance spectroscopy, respectively. The experimental results indicated that there is no obvious effect on microstructure characteristic by substituting Zr4+ for Ti4+. It was found that the values of the dielectric constant for all of the samples dropped into the order of 102, which is significantly lower than the order of 105 that of normally reported in the literature. It was suggested that the presence of impurity phases played an important role for the degeneracy of dielectric properties.

Abstract: Ba5(Nb1-xVx)4O15 (0≤x≤0.1) ceramics doped with H3BO3 were prepared by solid-state reaction. The effects of H3BO3 additives on the sinterability and microwave dielectric properties of the ceramics were investigated. The formation of a single phase was identified with the X-ray powder diffraction patterns of Ba5(Nb1-xVx)4O15 ceramics in the composition x from 0 to 0.025. Minor phases including Ba3(VO4)2 were formed as the secondary phase for the compositions with x>0.025. The dielectric constants (εr) of the ceramics increased firstly from 38.7 to 39.69 with x increasing from 0 to 0.01, and, then decreased to 36.91 with x=0.1, meanwhile, the unloaded quality factor and temperature coefficient of resonance frequency (τf) decreased as the x value increased from 0 to 0.1. Microwave dielectric properties, εr=36.91, Q×f =16,167GHz and τf =35ppm/°C, were obtained for Ba5(Nb0.9V0.1)4O15 ceramics with 1wt% H3BO3 sintered at 900°C for 2h.

Abstract: In this study, it would be interesting to investigate ceramic material based on Sr0.4Ba0.6Nb2O6 composition, V2O5 was used to substitute Nb2O5 to form the compositions of Sr0.4Ba0.6Nb2-xVxO6, where 0 ≤ x ≤ 0.3. The prepared Sr0.4Ba0.6Nb2-xVxO6 ceramics were sintered from 1060oC to 1300oC, and the needed sintering temperatures were dependent on V2O5 content. The SEM micrographs showed that even 1220oC was used as the sintering temperature of Sr0.4Ba0.6Nb2O6 ceramics, the pores were still observed. The crystal intensities of some (h,k,l) planes, for example (3,2,0) and (5,5,0) planes, apparently increased with the increase of V2O5 content and slightly decreased with the increase of sintering temperatures. The temperature-dependent dielectric characteristics were measured from 25oC to 300oC at 1MHz as a function of sintering temperatures and V2O5 content. The studied results would show that the sintering temperatures and V2O5 content also had large influence on the maximum dielectric constants (revealed at Curie temperatures) and the temperature-dielectric curves of Sr0.4Ba0.6Nb2-xVxO6 ceramics.

Abstract: With citric acid as chelating agent, ethylene alcohol as etherification agent and water as solvent, NaNbO3 nanopowder was synthesized by citrate sol-gel method. The effects of amount of CA and EG, pH value on the stability of the precursor sol were investigated. The evolution of NaNbO3 crystal phase was also investigated by XRD and TG-DTA. The results showed that a homogeneous precursor sol was formed at in the case of n(CA) : n(metal ion)=3:1, n(CA) : n(EG)=1: 2 and 1:3 and pH=7.5. Sintering process largely influences the morphology of the prepared products. Grain-like nanoparticles could be obtained with a rapid temperature rising rate, while nanorods were obtained with a slow rate. The forming mechanism of different morphologies of the prepared nanoparticles was also discussed.

Abstract: Ca(Al1/2Nb1/2)O3 (CAN) and Ca(Fe1/2Nb1/2)O3 (CFN) were prepared by solid reaction route. With the sintering temperature increased, the Q×f value of CAN changes from 7 200 to 17 500 GHz due to the changing of the ordering degree, whereas, the dielectric constant and Q×f value of CFN decrease remarkably without structural changing. The reason of this phenomenon is that the difference in size of two B-site ions in CAN is larger than that in CFN. As a result, the ordering degree of CAN is more sensitive to the conditions of sample preparation, compared with CFN.

Abstract: (1–x)CaTiO3–xNdAlO3 (CTNA) microwave dielectric ceramics have a high Q value and a low temperature coefficient of resonant frequency ( τf ). CTNA ceramic powder was synthesized using a polymeric precursor derived from a mixed-metal ethylenediaminetetraacetic acid (EDTA) / ethylene glycol solution. The precursors and derived oxide powders were characterized by thermogravimetric analysis and X-ray diffraction. When x = 0.3, CTNA was synthesized at 900°C for 3 h in air. Compared with that prepared by the solid-state reaction method in literatures, the CTNA ( x = 0.3 ) ceramics with better microwave properties could be obtained at lower calcination and sintering temperatures by EDTA chelating method.

Abstract: The variations of dc conductivities (σ) of the Sr2-xLixFeMnO6 double perovskite system over a range of temperatures (100 ~ 300 K) and for several concentrations (0≤ x ≤ 0.4) of incorporated Li+ ions are well described by the equation σ = (B/T) exp(−Ea/kT) and show a decrease with decreasing temperature for a given doping. At a given temperature, σ tends to decrease with increasing x. The derived activation energy shows a gradual increase with x with a range of values that are lower than what is usually reported. The measured ac impedance is used to extract the relaxation frequencies as a function of composition and temperature.