Key Engineering Materials Vols. 602-603

Abstract: The composite microspheres with Ni cores and amorphous TiO₂ shells (Ni@TiO₂) were synthesized by a one-pot solvothermal method at 180°C for 15 h, which used nickel chloride hexahydrate as Ni source andtetrabutyl orthotitanate as Ti source. The Ni/TiO₂ core/shell composites were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX). A possible formation mechanism of core-shell Ni/TiO₂ was proposed. Furthermore, the microwave absorption properties of these microspheres were investigated in terms of complex permittivity and permeability. The minimum reflection loss is-29.5 dB at 14.4 GHz for a layer of 4.0 mm thickness. Such phenomenon is attributed to the synergy effect between magnetic loss of Ni core and dielectric loss of amorphous TiO₂ shell. The enhanced microwave absorption properties are also believed to be due to the unique coreshell structure. All results indicate that these Ni@TiO₂ microspheres may be attractive candidate materials for electromagnetic wave absorption applications.

Abstract: Dielectric properties and microstructures of ZrO₂ thin films prepared by the sol-gel method at different annealing temperatures have been investigated. All films exhibited ZrO₂ (111) highly orientation perpendicular to the substrate surface and the grain size increased with increasing annealing temperature. Dielectric properties of the Ag/ZrO₂/Ag/Si structure were measured by using HP 4192 Impedance Analyzer and Agilent 4155 Semiconductor Parameter Analyzer. The dependence of the microstructure and dielectric characteristics on annealing temperature was also investigated.

Abstract: Alumina ceramics can be widely used in electro vacuum area and microwave communication industry due to its excellent dielectric properties. However, the effects of the Y₂O₃ addition on dielectric properties of alumina ceramics were rarely investigated. In this paper, alumina ceramics was prepared using Y₂O₃ as additive, via both heterogeneous precipitation method (HP) and mechanical milling method (MM). The effect of Y₂O₃ on phase transformation during sintering and dielectric properties of alumina ceramics were investigated, and the influencing mechanism was discussed. The results showed that different adding way influenced phase transformation during sintering, and polarization relaxation existed in alumina ceramics during 10³~10⁷Hz. Besides, the universal polarization relaxation relation could be applied to alumina ceramics.

Abstract: In this study, we investigated the structure and ferroelectric properties of the as-deposited (Bi_3.25Nd_0.75)(Ti_2.9V_0.1)O₁₂ ferroelectric thin films on ITO substrate fabricated by rf magnetron sputtering method. The electrical, ferroelectric and physical characteristics of as-deposited (Bi_3.25Nd_0.75)(Ti_2.9V_0.1)O₁₂ thin films were developed under different conditions to find the optimal deposited parameters. The crystalline structure of the prepared (Bi_3.25Nd_0.75)(Ti_2.9V_0.1)O₁₂ thin films was analyzed by X-ray diffraction (XRD). Field emission scanning electron microscopy (FESEM) was used to observe the film thickness and the surface morphology including grain size and porosity. Additionally, the remnant polarization of the as-deposited ferroelectric thin films was improved by neodymium and vanadium elements doped in this study. The remanent polarization of as-deposited ferroelectric thin films was 11 μC/cm² as the measured frequency of 1kHz. Finally, the polarization of as-deposited ferroelectric thin film capacitor was decreased by 9% after the fatigue test with 10⁹ switching cycles.

Abstract: PLMNT3/75/25 ceramics were prepared by uniaxial hot-press sintering method and conventional sintering method, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance analyzer were used to measure the properties of the PLMNT ceramics. PLMNT ceramic with pure perovskite phase was fabricated by uniaxial hot-press sintering method, while, a secondary pyrochlore phase was found in PLMNT ceramic prepared by conventional sintering method. It was observed that the PLMNT ceramic fabricated by uniaxial hot-press sintering showed better densification and more uniform grain size than that of the PLMNT ceramic prepared by conventional sintering. The dielectric responses showed that the samples prepared by two methods both exhibited diffuse phase transition characteristic, and the values of the diffusivity parameter were calculated according to the linear fitting of the modified Curie-Weiss law. The relaxor ferroelectric characteristics of PLMNT ceramics accorded well with the Vogel-Fulcher relationship. The P-E hysteresis loops of the samples from uniaxial hot-press sintering were apparently slight than that of conventional sintering samples, while the value of the P_s (saturation polarization) was significantly higher.

Abstract: Barium zirconate titanate ceramics were prepared by microwave sintering. Effects of microwave sintering time at 2.5kW on microstructure, dielectric and ferroelectric properties of barium zirconate titanate ceramics have been investigated. The result shows that the ceramic samples sintered at 2.5kW for 15~30min are single phase perovskite structure and there is no secondary phase observed. The degree of crystallinity increases with the increase of microwave sintering time. As the microwave sintering time increases, barium zirconate titanate ceramics become more uniform and the grain size increases. The Curie temperature of the samples sintered at 2.5kW for 15min, 20min and 30min is-20°C, -10°C and-15°C, respectively. As the microwave sintering time increases, the dielectric constant of barium zirconate titanate ceramics decreases initially and then increases, and the dielectric loss decreases. Moreover, the remnant polarization of the sample increases initially and then decreases, and the coercive electric field decreases as the microwave sintering time increases.

Abstract: Extending the investigations on (Na_0.5K_0.5)NbO₃-based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Na_0.5K_0.5)NbO₃Bi_0.5(Na_0.90K_0.10)_0.5TiO₃ [NKN-BNK. (Na_0.5K_0.5)NbO₃ with 1 ~ 5 mol% Bi_0.5(Na_0.90K_0.10)_0.5TiO₃ has been prepared following the conventional mixed oxide process. It can be concluded that the NKN-BNKT ceramics have orthorhombic structures in the case x 0.03. With increasing BNKT content (x=0.04 to 0.05), however, the structure changes from orthorhombic to rhombohedral phase. Above results demonstrated that the MPB between orthorhombic and rhombohedral phases exits in the solid solution with the BNKT content of x=0.03. At the MPB composition, the cryctalline structure of ceramics is considered to be a coexistence of orthorhombic and rhombohedral phase. Owing to the phase coexistence at the phase boundary, there exists a different symmetry regions (DSR) near the MPB. The DSR boundary motion increases the dielectric permittivity and piezoelectric coefficients. The electromechanical coupling factor and dielectric constant are higher for compositions near the MPB. The dielectric constant (K^T₃₃), planar coupling coefficient (k_p), thickness coupling coefficient (k_t) and piezoelectric constant (d₃₃) of 0.98NKN-0.02BNKT ceramics were 1180, 30%, 58%, and 180, respectively.

Abstract: nanoscale lead-free perovskite powders with a composition of (Na_0.52K_0.4425Li_0.0375)(Nb_0.9125Ta_0.0375Sb_0.05)O₃ (KNN-LTS) have been synthesized via a low-cost water-based sol-gel method, to reduce the sintering temperature and enhance the electrical properties. KNN-LTS nanopowders with average particle size of about 20 nm are obtained by citrate precursor sol-gel process, where Nb (OH)₅ and Sb₂O₃ are used to replace the costly mental alkoxides. The sol-gel derived nanopowders can be densified at lower temperature of 940 °C and exhibited excellent electrical properties after sintering at 1020 °C (d₃₃ = 396 pC/N, k_p = 50.1% = 1882 and tanδ = 0.02), providing a tremendous potential method for high-performance lead-free ceramics preparation.

Abstract: Lead-free potassium sodium niobate ceramic thin films were synthesized using rf magnetron sputtering technology for MFIS structures. The optimal sputtering parameters of the as-deposited KNN thin films for depositing times of 1h were obtained. Regarding the measured physical properties, the micro-structure and thickness of as-deposited KNN thin films for different oxygen concentration were obtained and compared by XRD patterns and SEM images. The surface roughness of KNN thin film was also observed by AFM morphology. The average grain size and root mean square roughness were 250 and 7.04 nm, respectively. For KNN thin films in the MFIS structure, the capacitance and leakage current density were 280 pF and 10^-8A/cm², respectively. We investigated that the leakage current density and the memory window increased, the capacitance critically increased as the oxygen concentration increased from 0 to 40%. However, the excess oxygen concentration process was decreased the electrical and physical of as-deposited KNN thin film. The effect of oxygen concentration on the physical and electrical characteristics of KNN thin films was investigated and determined.

Abstract: High Curie-temperature (T_c) polycrystalline 0.2Bi (Ni_1/2Ti_1/2)O₃-0.8PbTiO₃ (0.2BNT-0.8PT) thin films were fabricated on Pt (111)/Ti/SiO₂/Si substrates via an aqueous chemical solution deposition (CSD) technique. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 55 nm. The dielectric, piezoelectric and ferroelectric properties of the films was investigated. The permittivity peak appeared at 485 °C, which was 100 °C higher than that of Pb (Zr,Ti)O₃ thin films. The local effective piezoelectric coefficient d₃₃ was 45 pm/V at 3V. Moreover, a large remnant polarization with 2P_r up to 92 uC/cm² and a small leakage current of 2.2×10^-5 A/cm² under an electric field of 400 kV/cm were obtained. The magnitude of the measured polarization and the high Curie temperature make the 0.2BNT-0.8PT films promising candidates for application in high-temperature ferroelectric and piezoelectric devices.