烧结工艺对B2O3掺杂Ba1-xSrxTiO3梯度陶瓷介电性能的影响

研究了烧结温度及升温速率对氧化硼(B2O3)掺杂钛酸锶钡梯度陶瓷(Ba1-xSrxTiO3,x=0～0.4,步长0.02)的致密化、晶粒尺寸及介电性能的影响.结果表明,随着烧结温度的升高,在氧化硼挥发的同时致密化程度提高,从而居里峰提高且变得尖锐;随着氧化硼含量的增加,晶粒尺寸均匀长大、介电常数和介电损耗都增加;升温速率适中时,掺杂物的挥发、致密化进程及晶粒长大同步完成,梯度陶瓷介电性能才有效提高.此外,钛酸锶钡梯度陶瓷掺杂适量氧化硼明显降低烧结温度,比未掺杂相同成分的陶瓷烧结温度至少降低150℃,且介电损耗明显减小;梯度陶瓷的居里峰温度区间显著展宽,大大降低了该温区的介温系数,可望提高该系列陶瓷元器件精度及稳定性.     

BaTiO3-SrTiO3 layered dielectrics for energy storage

BaTiO₃-SrTiO₃ (BST) thick films (~ 250-390 μm) with layered structures were fabricated by tape-casting and lamination process. Layered composites with various Ba/Sr ratios were obtained by lamination of BaTiO₃ (BT) and SrTiO₃ (ST) tapes in different spatial configurations (2-2). As-prepared BST ceramics showed much improved sinterability over the laminates of pure BT or pure ST tapes. Dielectric properties of materials were measured in the temperature range of 25 °C to 200 °C. The method of utilizing of layered structures offered flexibility to maximize the energy storage capability at specific operating conditions: (temperature and electric field) by tailoring the dielectric properties through varying the spatial configurations of BT and ST films.     

Low temperature solid-phase synthesis of tetragonal BaTiO3 powders and its characterization

We report an alternative synthesis route for tetragonal and submicron barium titanate particles through a low temperature isothermal heat treatment of a precursor powder obtained from the BaO₂-TiO₂-C mixture utilizing combustion synthesis technique. Effort has been concentrated on the investigation and optimization of the synthesis process including combustion temperature and velocity, heat treatment procedure, and the characteristics of the powder obtained. It was shown that precursor powder prepared by the combustion method easily transformed to the tetragonal BaTiO₃ starting from 700 °C. Characterization of the products was performed by powder X-ray diffraction, scanning electron microscopy (SEM) and BET. From the results, it has been found that the formation of tetragonal BaTiO₃ powder at low temperature is conditioned by high chemical activity and specific characteristics of combustion products. The BaTiO₃ specimens were found to possess high dielectric constant and low dissipation factor at room temperature.     

Effects of ZnNb2O6 addition on BaTiO3 ceramics for energy storage

BaTiO₃ ceramics were prepared using solid state reaction with addition of ZnNb₂O₆, to investigate the effects of ZnNb₂O₆ addition on structure and properties. The results show that the ZnNb₂O₆ addition lowers sintering temperature, decreases grain size, while introduces second phase (Ba₂Ti₅O₁₂) for x ≥ 7.26 wt%. The dielectric breakdown strength is enhanced with the increasing doping level of ZnNb₂O₆ and reaches a maximum value at x = 7.26 wt%, exhibiting a maximum energy storage capability.     

Microstructure and dielectric characteristics of Ni ion doped BaTiO3 nanoparticles

In this article, we report the effect of nickel oxide (NiO) addition on the microstructure and the dielectric properties of nano-barium titanate (BaTiO₃). Nickel ion doped barium titanate nanoparticles have been synthesized through a chemical route using polyvinyl alcohol (PVA). The concentration of Ni²⁺ varies from 0 to 1.6 mol% in the specimens. The particle diameters in the specimen lie in the range of 24 nm to 35 nm. It is seen that the dielectric permittivity in doped specimens is enhanced compared to that of undoped BT ceramics. The dielectric permittivity shows maxima in the specimen containing 0.6 mol% of dopant concentration. The results are explained in terms of the change in lattice parameter c and a of the specimens.     

Scintillation properties of Ce-doped LuLiF4 and LuScBO3

The crystals of 1 mol% Ce-doped LuLiF₄ (Ce:LLF) grown by the micro-pulling down (μ-PD) method and 1 mol% Ce-doped LuScBO₃ (Ce:LSBO) grown by the conventional Czochralski (Cz) method were examined for their scintillation properties. Ce:LLF and Ce:LSBO demonstrated ∼80% transparency at wavelengths longer than 300 and 400 nm, respectively. When excited by ²⁴¹Am α-ray to obtain radioactive luminescence spectra, Ce³⁺ 5d-4f emission peaks were detected at around 320 nm for Ce:LLF and at around 380 nm for Ce:LSBO. In Ce:LSBO, the host luminescence was also observed at 260 nm. By recording pulse height spectra under γ-ray irradiation, the absolute light yield of Ce:LLF and Ce:LSBO was measured to be 3600±400 and 4200±400 ph/MeV, respectively. Decay time kinetics was also investigated using a pulse X-ray equipped streak camera system. The main component of Ce:LLF was ∼320 ns and that of Ce:LSBO was ∼31 ns. In addition, the light yield non-proportionality and energy resolution against the γ-ray energy were evaluated.     

Dielectric, ferroelectric and piezoelectric properties of Bi0.5Na0.5TiO3-(Ba0.7Ca0.3)TiO3 ceramics at morphotropic phase boundary composition

(1 − x)Bi_0.5Na_0.5TiO₃-x(Ba_0.7Ca_0.3)TiO₃ (BNT-xBCT, 0 ≤ x ≤ 0.15) solid solutions have been synthesized by a conventional solid state sintering method for obtaining a morphotropic phase boundary (MPB) with good piezoelectric properties. X-ray diffraction patterns reveal that a MPB of rhombohedral and tetragonal phases is formed at compositions 0.09 ≤ x ≤ 0.12. Addition of BCT into BNT greatly lowered coercive field E_c without degrading remanent polarization P_r. The specimen with x = 0.09 has the good piezoelectric properties: d₃₃ = 125 pC/N and k_p = 0.33. A modified Curie-Weiss law was used to fit the dielectric constant of BNT-xBCT ceramics, and a frequency dispersion was observed during the phase transitions from antiferroelectric to paraelectric in specimens with x exceeding 0.06.     

Influence of rare-earth addition on microstructure and dielectric behavior of Ba0.6Sr0.4TiO3 ceramics

Ba_0.6Sr_0.4TiO₃ (BST) ceramics with 0.5 mol% various trivalent rare-earth additions prepared by a solid-state route are investigated. A strong correlation is observed between the microstructure, dielectric properties and rare-earth element dopant. The results display that comparing with the lattice constants of undoped and doped rare-earth BST, the structure transforms from cubic to tetragonal structure. In addition, the dopant improves the tetragonal distortion with the ionic radius of rare earth decreasing, and then deteriorates it with further decreasing. Large ions rare-earth additions effectively suppress the grain growth of BST. It is found that the temperature-permittivity characteristics for the BSTR (R, namely, rare earth) system could be controlled using various rare-earth elements. Especially, such as Sm, Eu, Gd dopants are effective to satisfy the tunable microwave devices application due to the decrease of permittivity and the improvement of dissipation factors of BST ceramic with the accompanying high-tunability.     

Growth and dielectric properties of BaTiO3 thin films prepared by the microwave-hydrothermal method

Polycrystalline BaTiO₃ thin films were grown on Ti-covered polymer substrates at 80 °C using the microwave-hydrothermal technique. Onset of BaTiO₃ formation occurred almost instantaneously at 80 °C and complete film coverage was achieved within 2 min. Longer reaction time was necessary for extensive grain growth to achieve dense films. Good quality capacitor films were only achieved at 4 h reaction time but loss tangents were high. Film dielectric constant and dielectric loss values of as-grown M-H films decreased with longer reaction time. Oxygen plasma treatment improved loss tangents to 4% by removal of both absorbed moisture and lattice hydroxyls.     

High-capacity superconducting current leads of Y1Ba2Cu3O7?x

We have fabricated and measured a high-capacity superconducting current lead composed of a Y₁Ba₂Cu₃O_7–x cylinder, 20 cm long and 0.9 cm² cross section. A steady-state, d.c., critical current of 225 A at a temperature of 77 K was measured in this sample, using a voltage criterion of 2×10^–7 V/cm (p = 8×10^–10 ohm-cm). This current was limited by the currentinduced, self magnetic field. To our knowledge this is the largest d.c. critical current so far reported in a Y₁Ba₂Cu₃O_7–x sample and demonstrates the possibility of using hightemperature superconducting HTS materials for current leads to low-temperature superconducting LTS magnets or in power distribution systems.     

Mossbauer study of the effect of oxygen stoichiometry on the highTc superconductor Y1Ba2(Cu0.97Fe0.03)3O7?x

The Mossbauer effect of oxygen-depleted YBa₂Cu₃O_7-x and PrBa₂Cu₃O_7-x doped with 3%⁵⁷Fe has been investigated at room temperature. A manifold of quadrupole-split spectra has been found, whose parameters are in general agreement with those found by other workers. In addition, the Mossbauer spectra show that a fraction of the Fe sites develop magnetic order at room temperature when the oxygen content is reduced. It is demonstrated that the observed asymmetries in the Mossbauer spectra can be the result of a preferential alignment of the platelike crystallites that arises during the normal sample preparation process. The tendency to bond with the oxygen atoms is presumed to be responsible for the discreteness of the Mossbauer spectra as a function of oxygen depletion.     

Effects of Bi2O3 and Cr2Ti3O9 Co-doping on dielectric properties in BaTiO3-based ceramics

A microwave ceramic with general composition (1-x-y) BaTiO₃ + x Cr₂Ti₃O₉ + y Bi₂O₃ has been prepared by solid state synthesis at 1300-1400 °C. The phase composition, perovskite structural parameters and dielectric properties have been obtained by X-ray diffraction and dielectric measurements as a function of chemical composition and temperature. At low doping levels the formation of BaTiO₃-based solid solution has been found. The precipitation of BaCrO₃ has been detected at x = y = 2.0 mol%. A model of the incorporation of Cr³⁺ and Bi³⁺ ions into BaTiO₃-based crystal lattice has been proposed. Diffused phase transition in the temperature range 100-140 °C have been revealed by dielectric measurements for different ceramic composition. As high dielectric constant as 7311 and as low dielectric loss as 0.02 have been found for the composition of 0.98BaTiO₃-0.01Cr₂Ti₃O₉-0.01Bi₂O₃.     

Effects of Ho2O3 addition on defects of BaTiO3

Effects of Ho₂O₃ addition on defects of BaTiO₃ ceramic have been studied in terms of electrical conductivity at 1200 °C as a function of oxygen partial pressure (P_O2°) and oxygen vacancy concentration. The substitution of Ho³⁺ for the Ti site in Ba(Ti_1−xHo_x)O_3−0.5x resulted in a significant shift of conductivity minimum toward lower oxygen pressures and showed an acceptor-doped behavior. The solubility limit of Ho on Ti sites was confirmed less than 3.0 mol% by measuring the electrical conductivity and the lattice constant. Oxygen vacancy concentrations were calculated from the positions of P_O2° in the conductivity minima and were in good agreement with theoretically estimated values within the solubility limit. The Curie point moved to lower temperatures with increasing the oxygen vacancy concentration and Ho contents.     

Charge recombination reduction in dye-sensitized solar cells by depositing ultrapure TiO2 nanoparticles on “inert” BaTiO3 films

Ultrapure TiO₂ nanoparticles (∼5 nm in size) were supported on “inert” BaTiO₃ films by TiCl₄ treatment, which was used to fabricate dye-sensitized solar cells (DSSCs). The optimized electrode, designated as BaTiO₃/TiO₂(4), was obtained upon four cycles of TiCl₄ treatment. DSSC with BaTiO₃/TiO₂(4) electrode exhibits superior power conversion efficiency (PCE) compared to that with conventional anatase TiO₂ (∼25 nm in size) electrode. The interfacial charge recombination kinetics was investigated by electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). In contrast to DSSC with anatase TiO₂ electrode, the dramatically enhanced electron lifetime for DSSC with BaTiO₃/TiO₂(4) electrode could be attributed to the decrease of recombination reaction at the TiO₂ photoelectrode/electrolyte interface. It is proposed that the lower interfacial charge recombination can be related to the relatively shallower trap distributions in DSSC with BaTiO₃/TiO₂(4) electrode.     

TiO2和WO3复合掺杂金属陶瓷的致密化及抗高温氧化性

采用真空烧结技术制备TiO_2和WO_3复合掺杂17Ni/(NiFe_2O_4-10NiO)金属陶瓷试样,研究TiO_2和WO_3复合掺杂剂对试样物相组成、显微组织、烧结致密化和抗高温氧化性的影响。结果表明:TiO_2和WO_3复合掺杂后无新相生成,而是固溶到NiFe_2O_4晶格中,产生晶格畸变和空位,使致密度提高;TiO_2和WO_3复合掺杂有利于17Ni/(NiFe2O_4-10NiO)金属陶瓷形成更加致密的具有氧化保护性的致密层,阻碍金属Ni向外迁移,提高试样的抗高温氧化性,其中TiO_2和WO_3复合掺杂量为2%(比例为3∶1)和3%(比例为1∶3)时,试样的抗高温氧化性最好,其氧化30h后的氧化层厚度大约为80μm,比未掺杂试样降低了38.46%。     

Dielectric properties of sol-gel derived Ta2O5 thin films

The dielectric constant and the dielectric loss of tantalum pentoxide (Ta₂O₅) thin films, produced by sol-gel spin-coated process on Corning glass substrates, have been investigated in the frequency range of 20-10⁵ Hz and the temperature range of 183-403 K, using ohmic Al electrodes. The frequency and temperature dependence of relaxation time has also been determined. The capacitance and loss factor were found to decrease with increasing frequency and increase with increasing temperature. The activation energy values were evaluated and a good agreement between the activation energy values obtained from capacitance and dielectric loss factor measurements were observed.     

Microstructure and magnetic properties of BaTiO3-(Ni,Zn)Fe2O4 multiferroics

The microstructures of composite xBaTiO₃-(1−x)(Ni_0.5Zn_0.5)Fe₂O₄ (BT-NZF) multiferroics with various mixing ratios (x = 0.50, 0.60 and 0.70) are investigated by means of electron backscatter diffraction (EBSD) and magnetic force microscopy (MFM). The EBSD measurements reveal a change in the texture of the ferrite and the BaTiO₃ grains upon increasing the ferrite content in the sample. The sample with x = 0.70 exhibits the best ferrite texture, where only some directions are present. Furthermore, the resulting grain sizes vary from several µm (x = 0.50) to about 100 nm in the sample with x = 0.70. The MFM images reveal the presence of magnetic domains being extended over several adjacent grains, which according to the EBSD data may comprise different crystallographic orientations. In this way, we can explain the differences in the magnetic contrast obtained.     

Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure

BaTiO₃ films were epitaxially grown on SrTiO₃ (001) substrates buffered with SrRuO₃ films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO₃/SrRuO₃/SrTiO₃ multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO₃ films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm² and a coercive field of 29.5 kV/cm at room temperature.     

Preparation and humidity sensing properties of Ba0.8Sr0.2TiO3 nanofibers via electrospinning

Ba_0.8Sr_0.2TiO₃/Poly (vinylpyrrolidone) (BST/PVP) composite fibers were successfully synthesized via electrospinning. The ceramic nanofibers were obtained after calcining the composite at 800 °C for 2 h. The morphology and structure of the BST fibers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results reveal that the as-synthesized BST nanofibers show a diameter of 50-150 nm with the length over 0.1 mm, and a well-defined perovskite crystal structure. The electrical properties of the as-synthesized BST nanofibers were investigated through an impedance-type humidity sensor. The nanofibers exhibited excellent humidity sensing properties at room temperature. The possible sensing mechanism was proposed.     

Dependency of Flux Jump on Temperature and Field-Sweep Rate for Textured (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7- δ Bulk Superconductor with Gd-211 Particles

Anisotropy flux jumps in the mixed state of a textured (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_7-δ bulk superconductor with Gd-211 doping particles have been studied by means of magnetization measurements in the model of the magnetic field paralleling and perpendicular to the c axis. A typical anisotropy flux jump was observed at a temperature ranging from 2.0 to 3.0 K. Under the magnetic field perpendicular to c axis, no flux jump was found at whole temperature range until the sweep rate of 200 Oe/sec. For the magnetic field paralleling to the c axis, the number of flux jumps decreased with the increase of temperature, and the third quadrant of the M–H curve is the most flux-instability quadrant. The magnetic field sweep rate dependences of flux jumps were also studied and the influence of flux creep on flux jumps was also discussed.