Preparation and characterization of BaTiO3 powders and ceramics by sol-gel process using hexanoic and hexanedioic acid as surfactant

The BaTiO₃ xerogels, powders and ceramics were prepared through the sol-gel process using hexanedioic acid as surfactant. The xerogels, powders and ceramics were characterized by methods of thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, and transmission electron microscope. The dielectric properties of the ceramics were also measured. The results indicated that the powders calcined at 900 °C for 2 h were pure BaTiO₃ phase partly consisted of the tetrahedron BaTiO₃. The powders were nanometer scale particles. After sintering, the ceramics mainly consisted of the tetrahedron BaTiO₃ phase. Compared with the powders prepared using hexanoic acid as surfactant, the particle size of the powders prepared using hexanedioic acid obviously increased, and as well as the grain size, the relative density and the whole permittivity of the ceramics increased. Furthermore, the forming process of the powders with better dispersibility and the influence of the carboxyl number on the particle size of the powders can be explained using the “organic acid micro-capsules” model. Relatively, the ceramic prepared using hexanedioic acid as the surfactant had higher relative density (96.6%), room temperature permittivity (3089) and lower dielectric loss (0.015).     

Investigation of electrically active defects in amorphous barium titanate thin films

DC and AC electrical properties of amorphous barium titanate thin film capacitors have been investigated as a function of temperature. A clear correlation is found between the temperature dependence of DC leakage currents and the temperature variation of the AC loss peaks, showing that these measurement techniques are probing the same electrical defects. Using either of these two techniques in amorphous barium titanate, we were able to detect oxygen vacancies diffusion with activation energy around 1 eV, and electron traps at 0.3 and 0.4 eV.     

Relaxation of defects in amorphous barium titanate thin films

Relaxation of electrical defects in amorphous barium titanate thin films was studied by the thermally stimulated current (TSC) technique. We were able to detect at least three relaxation peaks in the TSC spectra. One is associated with activation energy of 0.65 eV and is possibly related to electronic trap levels below the conduction band. Another one is associated with activation energy close to 1 eV corresponding to the migration of positively charged oxygen vacancies within the films.     

掺杂钛酸钡薄膜的合成及性能研究

通过化学溶液沉积法制备的BiFeO3-BaTiO3薄膜在室温下能够同时显现铁电性和铁磁性。在600℃至700℃的条件下,以Pt/TiOx/SiO2/Si为载体,能够成功得到钙钛矿单相0.7BiFeO3-0.3BaTiO3薄膜。随着结晶温度上升,晶粒持续增长,最终在700℃时到达更高的结晶度。由于0.7BiFeO3-0.3BaTiO3薄膜的绝缘电阻较低,它所显现的极化(P)-电场(E)磁滞回线较弱。尽管如此,在0.7BiFeO3-0.3BaTiO3薄膜铁的位置上添加锰,高作用场的漏电流有效地减少,最终铁电性质得到了提高。在室温下,添加了摩尔分数5%的锰的0.7BiFeO3-0.3BaTiO3薄膜同时显现铁电极化和铁磁磁化磁滞回线。     

High-Performance barium titanate capacitors with double layer structure

High-performance barium titanate (B_aTiO₃) capacitors with excellent electrical and dielectric properties have been made by a two step deposition scheme using reactive rf magnetron sputtering. A novel double layer structure has been developed to reduce the pinholes and improve the electrical properties, such as higher dielectric constant, lower dissipation factor, higher breakdown fields and low leakage currents. Films deposited on a cooled substrate are amorphous whereas those deposited on a heated substrate are poly crystalline. Both polycrystalline and amorphous natures are verified by x-ray diffraction and scanning electron microscopy. Amorphous films have a low leakage current, a high breakdown voltage up to 2.5 x 10⁶ V/cm, and a dielectric constant less than 20. Polycrystalline films yield a high dielectric constant of 330. However, these films also have large leakage currents. The capacitors with the two layer structures,i.e. amorphous layer on top of polycrystal layer, have been shown to be much superior to those prepared by either polycrystal or amorphous layer alone for practical applications. The dielectric constant and breakdown voltage of capacitors with a double layer are found to be as high as 220 and 1.2 x 10⁶ V/cm, respectively. The leakage current is reduced to the same order as the amorphous films alone.     

Zn-Mn共掺对BTO薄膜光学性质及铁电性能的影响

利用溶胶凝胶法在Si(100)和LNO/Si(100)衬底上成功制备了Zn-Mn共掺钛酸钡薄膜.为了更充分地研究掺杂量对薄膜的晶体微结构和铁电性的影响,利用同样的方法分别制备了不同掺杂量的掺Zn和掺Mn钛酸钡薄膜.X射线衍射和原子力显微镜测量的结果表明薄膜均匀致密,且平均晶粒尺寸在30 nm以内.通过比较400～700 nm范围内各钛酸钡薄膜的折射率和消光系数,可以得到,其禁带宽度随着掺Zn或掺Mn量的变化而变化.对薄膜的铁电性能进行研究表明,Zn-Mn钛酸钡具有良好的铁电性,其剩余极化值为11.26μC/cm2,说明微量的Zn-Mn共掺可以增强薄膜的铁电性.     

The electronic structures and optical properties of BaTiO3 and SrTiO3 using first-principles calculations

The electronic-energy band structures and total density of states (TDOS) for bulk BaTiO₃ and SrTiO₃ were calculated by the first-principles calculations using density-functional theory and local-density approximation. The calculated band structure of BaTiO₃ and SrTiO₃ show the energy band gaps of 1.81 and 1.92 eV at the Γ point in the Brillouin zone, respectively. The optical properties of the both perovskites in the core-level spectra are investigated by the first principles under scissor approximation. The optical constants like refractive index and extinction coefficient of both BaTiO₃ and SrTiO₃ were derived from the calculated real and imaginary parts of the dielectric function. The calculated spectra were compared with the experimental results for BaTiO₃, SrTiO₃ in good agreement.     

X-ray photoelectron spectroscopy and electrical properties studies of Nb-doped strontium titanate ceramics prepared from titanyl acylate precursors

Through a sol-gel or sol-precipitation process Nb₂O₅-doped SrTiO₃ was obtained from titanyl acylate precursors in acidic or alkaline solution, respectively. Nonstoichiometric powders were obtained by varying the Sr/Ti ratio in the starting materials. X-ray photoelectron spectroscopy (XPS) was used to investigate the surface properties of the powders and the sintered bulk ceramics. Excess Ti in the sol-gel process was converted to TiO₂ (rutile) after calcination. In contrast, in the sol-precipitation process, the excess Ti was converted to Magnéli phases, which benefited the powders sinterability, and also enhanced the formation of Ti³⁺ on the powder surface, resulting in an abnormally high dielectric constant. The excess Sr in the sol-gel process was converted to SrCO₃ and then to gaseous CO₂ during sintering, which resulted in some cavities in the microstructure. In the sol-precipitation process, a structure of Sr_n+1Ti_nO_3n+1 was established and some near rectangular shaped, high-density grains were obtained.     

钛酸钡陶瓷补偿缺陷分析

:研究了施主杂质 Nb的含量 ,x (Nb)在 1.0 %～ 7.0 %范围变化时对 Ba Ti O3半导陶瓷的晶粒尺寸、铁电相变和电特性的影响 ,x (Nb)在 4.0 %左右时观察到了掺杂效应的转变。阻抗行为揭示出高、低施主含量样品在晶粒边界和晶粒体内载流子导电的激发能不同 ,可由随施主含量增加 ,从纯钡空位到晶界处钡空位和晶粒内部钛空位相结合的补偿缺陷模式的转变来解释。     

Low temperature sintering properties of Y-doped BaTiO3 ceramics by BaB2O4 sintering aid

The sintering process of semiconducting Y-doped BaTiO₃ ceramics added with BaB₂O₄ as low temperature sintering aid were investigated. When the low temperature sintering aid BaB₂O₄ added Y-doped BaTiO₃ ceramics prepared by Sol-Gel method, the sintering temperature of BaTiO₃-based ceramics would be greatly decreased, and also widen sintering range. Y-doped BaTiO₃ ceramics with BaB₂O₄ addition can be obtained at 1050 °C. Ceramics samples with room temperature resistivity 60-80 Ω cm, ratio of the maximum resistivity to minimum resistance (Rmax/Rmin) 10⁴ and temperature coefficient of resistivity (α) 10%/°C were obtained.     

金属有机物化学气相淀积法生长的Si掺杂GaN薄膜的应力，结构与电学特性研究

The stresses, structural and electrical properties of n-type Si-doped GaN films grown by metaiorganic chemical vapor deposition （MOCVD） are systemically studied. It is suggested that the main stress relaxation is induced by bending dislocations in low doping samples. But for higher doping samples, as the Si doping concentration increases, the in-plane stresses in the grown films are quickly relaxed due to the rapid increase of the edge dislocation densities. Hall effect measurements reveal that the carrier mobility first increases rapidly and then decreases with increasing Si doping concentration. This phenomenon is attributed to the interaction between various scattering process. It is suggested that the dominant scattering process is defect scattering for low doping samples and ionized impurity scattering for high doping samples.     

Stress, structural and electrical properties of Si-doped GaN film grown by MOCVD

The stresses, structural and electrical properties of n-type Si-doped GaN films grown by metalorganic chemical vapor deposition (MOCVD) are systemically studied. It is suggested that the main stress relaxation is induced by bending dislocations in low doping samples. But for higher doping samples, as the Si doping concentration increases, the in-plane stresses in the grown films are quickly relaxed due to the rapid increase of the edge dislocation densities. Hall effect measurements reveal that the carrier mobility first increases rapidly and then decreases with increasing Si doping concentration. This phenomenon is attributed to the interaction between various scattering process. It is suggested that the dominant scattering process is defect scattering for low doping samples and ionized impurity scattering for high doping samples.     

Preparation and characterization of sphere-like Cu2SnS3 nanoparticles and their dropcasted thin films

Ternary sphere-like Cu₂SnS₃(CTS) semiconductor and 2D hexagonal sheets were synthesized via a simple solvothermal method using PVP as the surface ligand at two temperatures of 180 and 220℃. The structural, morphological, and chemical compositions as well as optical properties of as-synthesized CTS particles were characterized using X-ray diffraction (XRD), Raman spectroscopy, energy dispersive X-ray spectrometry (EDS), field emission scanning electron microscopy (FESEM), and UV-Vis spectroscopy. The size of sphere-like particles and the side length of hexagonal sheets were within the range of 120-140 nm and 500 nm-2 μm, respectively. FESEM, XRD, and EDS were analyzed to investigate the mechanism of the morphological evolution of CTS particles. CTS particles showed proliferation of Sn atomic ratio, which is strongly sensitive to reaction temperature and, highly affects the increase of band gap energy from 1.36 to 1.53 eV due to generation metal defects and formation SnS₂. The optical analysis via the transmittance and reflectance reveals that the band-gap energy of dropcasted CTS thin films decreases after annealing due to grain growth and change of chemical compositions. Photo-responses of CTS nanocrystal thin films indicated a considerable increase in the conductivity of the films under light illumination. All these results showed the potential of these films for solar cell applications.     

Effect of bismuth addition on sintering behavior and microwave dielectric properties of zinc titanate ceramics

The influences of Bi₂O₃ addition on the sintering behavior and microwave dielectric properties of ZnO-TiO₂ ceramics were investigated. ZnO-TiO₂ ceramics were prepared with conventional solid-state method and sintered at temperatures from 950°C to 1,100°C. The sintering temperature of ZnO-TiO₂ ceramics with Bi₂O₃ addition could be effectively reduced to 1,000°C due to the liquidphase effects resulting from the additives. A proper amount of Bi₂O₃ addition could effectively improve the densification and dielectric properties of ZnO-TiO₂ ceramics. The temperature coefficient of resonant frequency could be controlled by varying the sintering temperature and lead to a zero τ_f value. At 1,000°C, 1ZnO-1TiO₂ ceramics with 1 wt.% addition gave better microwave dielectric properties ɛ_r of 29.3, a Q × f value of 22,000 GHz at 8.36 GHz, and a τ_f value of +17.4 ppm/ °C.     

Temperature treatment of nano-scaled barium titanate filler to improve the dielectric properties of high-k polymer based composites

A commercially available nano-scaled barium titanate powder was compounded with a polyester-styrene reactive resin to create a high-k polymer-composite material. The influence of a temperature treatment on the filler was examined for treatment temperatures between 500 and 1200 °C. As a result grain and particle growth as well as lattice relaxation was induced into the powder particles. The resulting composite permittivity rises from 9.2 to 25.0 (at room temperature, 1 kHz, filler load 60 wt%/∼22 vol%) and the dielectric loss factor drops to values below 0.005 as a result of this treatment. The changes in lattice structure, powder morphology and composite viscosity were determined by XRD, SEM, BET, He-pycnometry and rheology measurements. The temperature dependency of the dielectric properties of the pure polymer and the composite were measured between −60 and 70 °C. The temperature dependency of the polymer matrix is superimposed with the temperature characteristic of the BaTiO₃.     

Structure of organometallic chemical vapor deposited BaTiO3 thin films on LaAIO3

BaTiO₃ thin films grown on LaA1O₃ by organometallic chemical vapor deposition were characterized with cross-sectional high resolution transmission electron microscopy. Epitaxy was confirmed for the films grown on (100) oriented substrates. The films displayed an aaxis orientation. The interface between the film and substrate was nearly atomically abrupt. Partial relaxation of the films was observed as a result of misfit dislocation formation.     

Interactions between ferroelectric BaTiO3 and Si

Ferroelectric BaTiO₃ thin films were deposited on single crystal Si substrates by radio-frequency magnetron sputtering. Bilayer structures of BaTiO₃ thin films, either amorphous on polycrystalline (A/P) or polycrystalline on microcrystalline (P/M), were utilized to reduce the leakage current and to enhance the dielectric constant of the films compared to single polycrystalline and amorphous layer structures, respectively. Relatively lower charge density, determined by the capacitance-voltage measurement on the capacitors with a configuration of Au/ BaTiO/p-Si/Al, was detected for the BaTiO₃ thin film with a structure of P/M. The current-voltage characteristics of the Al/SiO₂(~1.8 nm)/p-Si/Al diodes fabricated on the Si after removing BaTiO₃ layers gave direct evidence of the preservation of the Si surface crystal by using a P/M instead of a A/P structure. This was further confirmed by the Auger electron spectroscopy analysis on the samples.     

Preparation and characterization of ceramics laser alloyed with WO3 and CuO nanopowders

The surface of a ceramic substrate can be modified locally by introducing a second phase into a melt pool generated by a laser beam. CuO and WO₃ powders with nano-sized particles were used to laser-alloy alumina and a low-temperature co-fired ceramic (LTCC). A composite with complex multiphase microstructure was developed with particle agglomerates, small crystals as well as grains covered with reaction phase, in parts with typical length scales down to the submicron range. The geometry of the modified area could be controlled by the process parameters as well.A significant change of the thermal and electrical properties could be established for the laser-alloyed tracks in comparison with that of the ceramic substrate. The developed composites showed an electrical conductivity with a negative temperature coefficient for resistivity, i.e. resistivity decreases with increase in temperature, which is typical for semiconductors. An enhancement of thermal conductivity could be established as well. Especially for the WO₃ powder, it could be increased about 50% compared to the unmodified LTCC substrate.     

New dielectric material system of Nd(Mg1/2Ti1/2)O3–CaTiO3 at microwave frequency

The microwave dielectric properties of (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ (0.1  x  1.0) ceramics prepared by the conventional solid state method have been investigated. The system forms a solid solution throughout the entire compositional range. The dielectric constant decreases from 152 to 27 as x varies from 0.1 to 1.0. In the (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ system, the microwave dielectric properties can be effectively controlled by varying the x value. At 1400 °C, 0.1CaTiO₃–0.9Nd(Mg_1/2Ti_1/2)O₃ has a dielectric constant (ε_r) of 42, a Q × f value of 35 000 GHz and a temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 43 000 GHz for x = 0.9 is achieved at the sintering temperature 1500 °C.     

WO3/NiWO4复合薄膜的制备及其光电化学性能

采用两步水热法在导电玻璃(FTO)上制备了WO₃/NiWO₄复合薄膜。通过XRD,SEM表征了WO₃/NiWO₄复合薄膜的组成结构及微观形貌,利用UV-Vis、光电流测试、光电催化测试和交流阻抗测试分析了WO₃/NiWO₄复合薄膜的光电性能。结果表明:WO₃/NiWO₄复合薄膜相较于WO₃薄膜具有更好的光吸收特性、光电流密度和光电催化活性,其中水热反应3h的WO₃/NiWO₄复合薄膜的光电化学性能最佳。WO₃/NiWO₄-3h在1.4V(vs.Ag/AgCl)时的光电流密度为1.94mA/cm²,光电催化210min对亚甲基蓝溶液的降解效率为57.1%。交流阻抗图谱表明WO₃/NiWO₄薄膜的电荷转移电阻小于WO₃薄膜,光电化学性能更优。