钛酸钡的光学性质及其体积效应

钛酸钡(BaTiO_3, BTO)是铁电物理学和材料学领域具有代表性的研究对象.本文基于准粒子相互作用的GW方法研究BTO材料的光学性质,并研究了等应变情况下的体积效应.第一性计算结果表明,考虑了激子效应的GW (格林函数(G)-库仑势(W))方法得到的激发态性质更接近实验结果.引入等应变调控,发现体积膨胀会导致光学吸收峰红移,体积压缩则光学吸收峰蓝移.同时,探究了体积变化对BTO块材的自发极化和电子结构的影响,发现体积膨胀会使钛原子的d轨道和氧原子的p轨道杂化更显著,进一步导致材料自发极化的增大,而体积压缩对自发极化和dp杂化的影响正好相反.通过比较研究,还发现等应变的体积效应对极化的影响不如等应力体积效应明显.     

Phase transition and dielectric characteristics of nano-grained BaTiO3 ceramics synthesized from surface-coated nano-powders

Nano-grained BaTiO₃ (BTO) ceramics were synthesized by pressureless sintering BTO nano-powders coated with Mn materials. The BTO ceramics, which were prepared at 800 °C from nano-powders of 40 nm size, are of a cubic structure with a small fraction of tetragonal phase, similar to that of the starting BTO nano-powders of the same size. On the other hand, the BTO ceramics with about grains of 1300 nm size, which were prepared at sintering temperature ranging from 1100 to 1200 °C, have a mainly tetragonal phase. By increasing the sintering temperature and/or the amount of the coating materials, the crystallinity of the tetragonal phase was increased. The relation of the dielectric features of the nano-grained BTO ceramics with the structural variation is discussed in terms of process parameters.     

四方相BaTiO3铁电性的第一性原理研究

在广义梯度近似下，利用超软赝势对立方相和四方相BaTiO3晶胞中Ti原子沿c轴位移时体系的能量、原子间电子云重叠布局数和各原子上的净电荷等进行了自洽计算.结果显示，当Ti原子沿c轴位移0012nm时，四方相BaTiO3体系能量最低，其自发极化强度为0261C/m2，该结果与实验数据相符合；同时表明，O原子的2p轨道和Ti原子的3d轨道的杂 化是BaTiO3晶体出现铁电性的重要原因. 关键词： 四方相BaTiO3 铁电性 位移 态密度     

Anisotropic conical scattering in photorefractive BaTiO3: diffraction efficiency measurements

The simultaneous presence of beam fanning and anisotropic conical scattering hinders the exact measurements on anisotropic conical diffraction. In this paper, a method is described for the measurement of diffraction efficiency of the gratings responsible for anisotropic conical scattering in a photorefractive BaTiO₃ crystal. Dependence of diffraction efficiency on the angle of incidence of the incident beam and the build-up processes of beam fanning and anisotropic scattering have been studied.     

BaTiO3/p-Si异质结的整流特性和光诱导特性的研究

利用脉冲激光沉积法成功制备了BaTiO₃/p-Si异质结, 该异质结在80–300 K 显示出了良好的整流特性和光诱导特性. 开启电压随着温度的升高而逐渐降低. 利用不同频率的光子辐照样品, 观察到明显的光电导效应. 且随着照射光子能量的增大, 结电流也相应变大, 光诱导效应越明显. BaTiO₃薄膜电阻-温度(R-T) 曲线显示氧缺陷条件下BaTiO₃薄膜具有良好的半导体特性. 关键词： 异质结 光诱导效应 3薄膜')" href="#">BaTiO₃薄膜     

钛酸钡纳米颗粒铁电性临界尺寸的理论分析

从铁电体的Eular-Lagrange方程出发, 取贝塞尔方程级数解的形式, 得到了钛酸钡陶瓷颗粒的总极化强度表达式, 分析了各系数对总极化强度的影响. 根据总极化强度表达式, 采用MATLAB软件对尺寸在100 nm以下的钛酸钡纳米颗粒的铁电性进行了仿真分析. 结合实际数据探讨了尺寸效应对陶瓷颗粒铁电性的影响, 获得了与实验数据相符的数值解和极小值, 从而预测了钛酸钡纳米颗粒铁电性存在的临界尺寸为6 nm.     

钛酸钡单晶沿垂直解理面方向的超高压电响应的研究

研究了钛酸钡单晶沿着解理面的压电响应. 首先发现在钛酸钡单晶中存在着解理面，当垂直 这一解理面方向施加电场时，得到了超过2000pC/N的超高压电常量d33. 而这一d 33值比沿着自发极化方向〈001〉方向施加电场得到的压电常量（d33〈001 〉=87pC/N）高20多倍. 而且，沿此方向在很低电场强度（低于1kV/mm）下得到了高达0 6%的应变值，这一应变值是同等场强下PMN_PT单晶的10倍之多. 这一结果无疑对发展无铅压 电材料驱动器提供了一种新可能. 关键词： 压电响应 解理面 钛酸钡单晶     

Enhancement of positive temperature coefficient resistance effect of BaTiO3-based semiconducting ceramics caused by B2O3 vapor dopants

The positive temperature coefficient resistivity (PTCR) effect of semiconducting BaTiO₃ ceramics can be remarkably influenced by both dopants and ceramic processing. The behavior of B₂O₃ as vapor dopants was studied in this paper. It was found that the PTCR effect was enhanced distinctly, while the room temperature resistance also increased.     

外延压应变对BaTiO$lt;sub$gt;3$lt;/sub$gt;铁电体抗辐射性能影响的分子动力学研究

采用基于壳模型的分子动力学模拟方法, 研究了存在外延压应变时BaTiO₃铁电体的辐射位移效应, 以O原子作为初冲原子(primary knock-on atom, PKA), 能量为1 keV, 方向为[001], 分别计算了外延压应变为0, 0.4%, 0.8%, 1.2%, 1.6%, 2.0%时体系的缺陷数量、分布, 以及辐射前后的极化强度, 比较了压应变为2%以及无应变下损伤区域、缺陷离位距离和反向外电场下PKA的迁移距离. 结果表明, 随外延压应变增加体系极化近似线性增加, 辐射后极化降低幅度降低、缺陷产生的数量有所减小, 2% 压应变存在时缺陷原子的离位距离、PKA在反向外电场作用下的迁移距离和损伤区域都小于无应变的情况, 说明外延压应变的存在对辐射造成的晶格损伤具有抑制作用, 对辐射损伤具有改善作用, 可以通过引入外延压应变来调控BaTiO₃的辐射损伤. 关键词： 应变 3')" href="#">BaTiO₃ 辐射损伤 分子动力学模拟     

积分球腔内BaTiO3：Ce晶体的自泵浦相位共轭增强现象的观测

报道了聚四氟乙烯积分球腔内光折变晶体ＢａＴｉＯ３：Ｃｅ在Ｈｅ－Ｎｅ激光作用下产生自泵浦相位共轭的增强现象，并对实验结果进行了分析。     

小尺寸NaLuF4:Yb3+/Tm3+纳米晶的生长及上转换发光

为了详细地探究NaLuF₄纳米晶的生长过程,利用自主研发的可以精确控制实验参数的自动纳米合成仪制备了不同系列的NaLuF₄：Yb³⁺/Tm³⁺纳米材料。对不同反应温度下（285,295,305℃）制备的样品进行物相分析,发现随着反应时间的增加,NaLuF₄纳米晶均遵循相似的生长规律,即α-相→α-相+β-相→均匀的β-相→聚集的β-相。在不同温度下,均有一个时间段可以获得小尺寸（小于50 nm）、单分散、粒径分布窄的纯β-NaLuF₄纳米晶。另外,测试了不同温度下制备的β-NaLuF₄：Yb³⁺/Tm³⁺纳米材料的上转换发射光谱,结果表明随着反应温度的升高,样品的发光先增强后减弱。出现这种光谱规律可能是受晶体尺寸和结晶性两方面因素的影响。此外,样品在紫外区的高阶多光子发光很强。例如,361 nm发射峰强度大约是800 nm发射峰强度的2倍。     

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO₃ content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO₃, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO₃. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.     

Structural and electronic properties of Fe-doped BaTiO3 and SrTiO3

We have performed first principles calculations of Fe-doped BaTiO3 and SrTiO3. Dopant formation energy, structure distortion, band structure and density of states have been computed. The dopant formation energy is found to be 6.8eV and 6.5eV for Fe-doped BaTiO3 and SrTiO3 respectively. The distances between Fe impurity and its nearest O atoms and between Fe atom and Ba or Sr atoms are smaller than those of the corresponding undoped bulk systems. The Fe defect energy band is obtained, which mainly originates from Fe 3d electrons. The band gap is still an indirect one after Fe doping for both BaTiO3 and SrWiO3, but the gap changes from Γ-R point to Γ-X point.     

Ce:BaTiO3晶体中两个深陷阱能级引起的光折变光栅暗衰减特性

对Ce:BaTiO3晶体中光折变光栅的暗衰减特性进行了实验研究，观测到了晶体禁带中的Fe离子和Ce离子两个深陷阱能级同时参与了光折变过程。在单深能级模型和浅－深能级模型的基础上，提出了简化的双深能级模型，并根据该模型定量地分析了Fe离子和Ce离子每个深陷阱能级在不同光栅波矢、不同光栅写入光强时对总空间电场以及光折变光栅的贡献。     

Structure and dielectric studies of nano-composite Fe2O3: BaTiO3 prepared by sol-gel method

Nano-structure pure barium titanate (BaTiO₃) and that was doped with iron oxide (Fe₂O₃), have been prepared by sol-gel method, using barium acetate (Ba(Ac)₂) and titanium butoxide (Ti(C₄H₉O)₄), as precursors. The as-grown prepared samples by sol-gel technique were found to be amorphous, which crystallized to the tetragonal phase after synthesized at 750 °C in air for 1 h as detected from the XRD patterns. The XRD data were confirmed by transmission electron microscope (TEM). The dielectric properties namely; dielectric constant (ε′) and loss tangent (tan δ) in the frequency range between 42 Hz and 1 MHz, at range of temperature 25-250 °C were investigated. The temperature dependence of ε′ and tan δ for the undoped and doped materials, at 1 kHz, was also investigated. As a result, tan δ increased rapidly with decreasing temperature below 125 °C (Curie temperature) while above this temperature, tan δ shows temperature independent. As a result, below and above Curie temperature, ferroelectric phase and paraelectric phase of BaTiO₃ can be obtained, respectively.     

Biaxial behaviour of an optical waveguide fabricated by ion implantation in a uniaxial BaTiO3 substrate

Biaxial behaviour has been observed in a waveguide fabricated by the technique of ion-beam implantation (H⁺ ions: 1 MeV at a rather high dose of 6 × 10¹⁶ ions/cm²) in a uniaxial BaTiO₃ single crystal. For the first time to our knowledge this phenomenon has been connected to the elasto-optic properties of the material. The index profile reconstructions both for TE and TM modes are led with two methods and we compare the results. The computation technique to connect the latter with the elasto-optic properties is presented. Then the calculations are successfully compared with the measured values.     

Chemical diffusivity of BaTiO3−δ: I. Experimental determination

The chemical diffusivity of ‘undoped' polycrystalline BaTiO_3−δ was determined via a conductivity relaxation technique, at elevated temperatures (800≤T/°C≤1100) as a function of the ambient oxygen partial pressure in the range of 10⁻¹⁶≤P_O2/atm≤1 including an n- to p-type transition regime. Mathematical formulation was developed to convert conductivity relaxation to the corresponding nonstoichiometry (δ) relaxation in the transition regime. It has been found that the chemical diffusivity appears to exhibit a maximum at the n-to-p transition point where the electronic minimum conductivity falls, and that surface reaction becomes more rate-determining than diffusion as the transition point is approached from both n-type and p-type branches. Experimental details are given and the results are exhaustively compared with those reported up to date on the ‘undoped' BaTiO₃.     

溶胶-凝胶法Er2O3薄膜的结构和光学特性

针对稀土Er掺杂Si光源中Er离子掺杂浓度低的问题,采用溶胶-凝胶（Sol-gel）法在Si(100)和SiO2/Si(100)基片上旋涂法制备Er₂O₃光学薄膜,Er离子浓度与以前掺杂方法相比提高了2个数量级.900 ℃热处理获得单一立方结构的Er₂O₃薄膜材料.光致发光（PL）特性研究表明在654 nm波长的激光泵浦下,Er₂O₃薄膜材料获得了1.535 μm的发光峰,并具有较小的温度猝灭1/5.在SiO₂/Si(100)基体上制备的Er2O3薄膜材料的光致发光强度比Si(100)基体上制备的薄膜提高2-3倍.研究结果表明具有强光致发光特性的Er₂O₃薄膜是一种有前景的硅基光源和放大器材料.     

Photoluminescence characteristics of ZnTiO3: Bi nanocrystals

The photoluminescence properties of the Bi³⁺ in sol-gel derived ZnTiO₃ nanocrystals have been investigated. An ultra-violet emission at 360 nm and a visible emission band at 506 nm have been observed, originating from two kinds of emission centers. The former is ascribed to the ³P₁-¹S₀ transition of Bi³⁺ and the latter to the recombination of the electrons with the photo-generated holes trapped in the zinc vacancies. In all cases the latter contribution is predominant.