Key Engineering Materials Vol. 655

Abstract: Barium calcium titanate (Ba0.96Ca0.04)(Zr0.05Ti0.95)O3 ferroelectric ceramics were prepared by sol–gel technique. The ceramics were sintering at 1290°C-1370°C. X-ray diffraction was employed to investigate the microstructure, and the surface topography was investigated by SEM graphs. The dielectric constant vs temperature was measured by Temperature dielectric spectrometer, and TF Analyzer 2000 measured the ferroelectric properties. It can be found that all the ceramics show pure perovskite structure suggesting that solid solution were formed. SEM showed that the samples uniform in grain size at 1330 °C. Our study revealed that when the sintering temperature is 1330 °C the maximum dielectric constant was 29,600, the maximum piezoelectric coefficient d33 could reach 399pm/V and the remanent polarization (Pr) was 8.3 μc/cm2, respectively.

Abstract: In this study, the (Mn, Sb) co-tuned PbCa_0.01[(Mn_xZn_0.1-x)_1/3(Sb_xNb_0.1-x)_2/3 (Zr_0.505Ti_0.4895)_0.9]O₃ (PCZMNSZT-100x) ceramics were prepared near the morphotropic phase boundary (MPB) using the wolframite precursor method. The phase structure evolutions, electrical and temperature stability properties were systematically investigated. As the fraction of rhombohedral phase increases, the degree of disorder in the diffused phase transition (DPT) correspondingly increased and the “hard” piezoelectric characterization would be enhanced. Moreover, the temperature change rate of k, Q_m and f_r can be tuned by adjusting the x content. The specimens with x=0.08 exhibit the obtained values as k_p ~ 0.50, k_t ~ 0.52, Q_m ~ 1550, and the low temperature change rate of properties having Δf_r/f_r = -0.02% and ΔQ_m/Q_m = 10.5%, up to 100 °C. To evaluate the material feasibility, the proposed multi-bulk step-down transformers using a ring/dot structure are fabricated. Their obtained electrical properties have a maximum efficiency of 92% with an output power of 15 W, a voltage gain of 0.68 and a nearly zero temperature change rate of resonant frequency are found at the load of 100 Ω.

Abstract: The low dielectric loss ceramic-polymer composites were prepared by extrusion processing technique with high density polyethylene (HDPE) as matrix and high permittivity ceramic powder in BaO-Nd₂O₃-TiO₂ (BNT) system as filter. The dielectric response in microwave frequency range was characterized, and the experimentally observed dielectric properties were compared with the theoretical models. The relative permittivity of the composites is increased from 3 to 10 with the increase in the volume fraction of BNT filter from 10 to 40 vol%. The dielectric losses of composites are less than 0.001 for all compositions. For 40 vol% ceramic loaded HDPE, the relative permittivity of 10 and loss tangent of 0.0007 at 7 GHz were obtained. The BNT-HDPE composites are promising materials for microwave dielectric antenna applications.

Abstract: BaTiO₃ nanotube arrays are prepared by hydrothermal reaction using the TiO₂ nantotubes prepared by anodic oxidation reaction. The element, structure, elementary composition and ferroelectric properties are researched by SEM, XRD and TF Analyzer. The result shows that the prepared BaTiO₃ arranged orderly and have tetragonal structure after annealing. The diffracted intensity increased with the hydrothermal temperature rising up. With the increasing of Ba(OH)₂ concentration, remnant polarization intensity and coercive field climbed up and then decreased. With the increasing of hydrothermal temperature, remnant polarization intensity and coercive field increased. When the Ba(OH)₂ concentration is 0.3mol/L, the hydrothermal temperature is 260°C,the remnant polarization and coercive field of the samples reached the maximum and respectively reached 6.69mC/cm² and 1.32 kV/cm.

Abstract: Li₂ZnTi₃O₈ ceramics were prepared by reaction-sintering process (calcination free). The crystal phase and microstructure were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). A pure phase of Li₂ZnTi₃O₈ ceramics sintered at 1075 °C-1150 °C with cubic spinel structure was confirmed by XRD. The microwave dielectric properties (ε_r, Qxf) of Li₂ZnTi₃O₈ ceramics were strongly dependent on the densification and grain size. The τ_f of Li₂ZnTi₃O₈ ceramics was almost independent with the sintering temperatures. In particular, Li₂ZnTi₃O₈ ceramics by reaction-sintering method sintered at 1125 °C for 5 h exhibited good combination microwave dielectric properties of ε_r=21.7, Q×f=70 500 GHz (at 7.5 GHz) and τ_f=-13 ppm/°C.

Abstract: Single-crystalline BaTi₂O₅ nanowires were synthesized by a simple molten salt method, using BaC₂O₄•H₂O and TiO₂ powders as precursors. Electrical characterization was conducted with the as-synthesized BaTi₂O₅ nanowires. The current-time data of the nanowires obeyed the Curie-von Schweidler law. Steady-state I-V curve was obtained at room temperature from current-time data and it revealed that the leakage current of BaTi₂O₅ nanowires obeyed the ohmic law under low voltage, and obeyed the space-charge limited current (SCLC) law under higher voltage. The conductivity value of the BaTi₂O₅ nanowires is 2*10^-6 S/cm.

Abstract: In this paper, Cu/SiO₂ composites with different metal contents were successfully prepared by a selective reduction process. The phase composition and microstructure of composites were analyzed by XRD and SEM, respectively. The results show that spherical copper particles are uniformly distributed in silica matrix. The impedance and permittivity of composites were tested with RF impedance analyzer (0.1~1 GHz). It is indicated Cu/SiO₂ composites exhibit capacitive character. In addition, the dielectric constants get enlarged due to the enhancement of interface polarization.

Abstract: M-type barium ferrite (BaFe_l2O₁₉) receives much attention due to the remarkable properties of the bulk materials: large magnetic anisotropy, sufficient saturation magnetization, non-toxic, excellent chemical stability and good corrosion resistivity. Using stoichiometric amounts of iron nitrate, barium nitrate and sodium hydroxide as the starting materials, we have prepared BaFe_l2O₁₉ by hydrothermal synthesis method. The results show that when the n (Fe³⁺)/n (Ba²⁺) ratio of 6 and n (OH⁻)/n (NO₃⁻) ratio of 2, the morphology of BaFe₁₂O₁₉ is hexagonal platelets which approximately 800 nm in length and 50 nm in thickness. The coercivity of BaFe₁₂O₁₉ is 1286 Oe and the saturation magnetization is 23.7 emu/g. The BaFe₁₂O₁₉/phenolic resin composite has been prepared and the permeability of the BaFe_l2O₁₉/phenolic resin composite was measured.

Abstract: The phase structure, and electrical and magnetic properties of La_0.7Sr_0.3MnO₃ (LSMO)-x Ag (x is the mole ratio, x=0, 0.3, 0.5) composite were investigated. It is found that the sample with x=0 is single phase; the samples with x=0.3 and 0.5 present three phase composite structure of the manganese oxide and Ag. With the increasing of Ag content, the grain size of the samples increases and the grain boundaries transition from fully faceted to partially faceted. The permittivity of spectrum (10 MHz - 1 GHz) and the theoretical simulation reveal that the plasma frequency f_p increase with Ag content, due to the increasing of free electron concentration, which is further supported by the enhancement of conductivity. While for the permeability (μ_r'), the μ_r' decrease with the increasing of Ag content at low frequency range (f < 20 MHz), while at the relative high frequency range (f > 300 MHz), the μ_r' increased with Ag content. Therefore, the introduction of elemental Ag resulted in a higher μ_r' at the relative high frequency range.

Abstract: (001)-oriented LiNbO₃ (LN) ferroelectric films were grown on (100)-oriented n-type Si substrates using 15 nm-thick ZnO layers as buffers by pulse laser deposition technique and the LN/ZnO/n-Si heterojunctions were fabricated. Obvious photoswitching characteristics to white light were observed when the reverse voltages were applied on the LN/ZnO/n-Si heterojunction. High performance was exhibited, such as a large ON/OFF ratio, short photoresponse time, steady ON or OFF states, and well reversible. The results were discussed in terms of the band diagrams of the LN/ZnO/Si heterojunctions in this work.