复合层状Bi7Ti4NbO21铁电陶瓷的结构与介电和压电性能研究

制备了Bi₇Ti₄NbO_{21，Bi4Ti3O12及Nb掺杂Bi4Ti3O12(Nb-Bi4Ti3O12)层状结构铁电陶瓷材料.结合Nb-Bi 4Ti3O12的介电温谱和 退极化实验结果，研究了Bi7Ti4NbO21的晶体结构 对其介电、压电性能的影响 .高分辨透射电镜结果表明，在Bi7Ti4NbO21中， 沿着c轴方向，(Bi2Ti3O10)^2-和(BiTiNbO7)^2-两个类钙钛矿层分别 与(Bi2O2)²⁺层叠加堆积而成.这种晶体结构决定了Bi7Ti4NbO21的 介电温谱在668℃和845℃出现介 电双峰.结合极化样品的退化实验分析，说明材料在这两个温度附近发生了铁电—铁电相变 、铁电—顺电相变，分别是(Bi2Ti3O10)^{2-< /sup>和(BiTiNbO7)2-层状 结构发生微观结构相变的结果.在退极化过程中，由于受热时钙钛矿层内空位引起的缺陷偶 极子的定向排列受到破坏，引起材料部分退极化，表现为300℃热处理后Bi7Ti 4NbO 21的压电活性降低了10%，显示了室温下材料的压电性能来源于自发极化的固有电 偶极子和缺陷偶极子的共同贡献.}}     

Mn4+掺杂对BiFeO3陶瓷微观结构和电学性能的影响研究

利用传统的固相反应法制备了BiFe_1-xMn_xO₃ (x= 0-0.20)陶瓷样品, 研究了不同Mn⁴⁺掺杂量对BiFeO₃陶瓷密度、物相结构、显微形貌、 介电性能和铁电性能的影响.实验结果表明:所制备的BiFe_1-xMn_xO₃ 陶瓷样品的钙钛矿主相均已形成,具有良好的晶体结构, 且在掺杂量x=0.05附近开始出现结构相变.随着Mn⁴⁺添加量的增加, 体系的相结构有从菱方钙钛矿向斜方转变的趋势,且样品电容率大幅度增大, 而介电损耗也略有增加;在测试频率为10⁴ Hz条件下, BiFe_0.85Mn_0.15O₃ (ε_r=1065)的 ε_r是纯BiFeO₃ (ε_r=50.6)的22倍; 掺杂后样品的铁电极化性能均有不同程度的提高,可能是由于Mn⁴⁺稳定性优于 Fe³⁺,高价位Mn⁴⁺进行B位替代改性BiFeO₃陶瓷, 能减少Bi³⁺挥发,抑制Fe³⁺价态波动,从而降低氧空位浓度,减小样品的电导和漏电流.     

Sr2Bi4Ti5O18的压致结构相变

 使用4∶1的甲醇-乙醇混合液作传压介质，利用金刚石对顶砧装置（DAC），研究了层状铁电体Sr₂Bi₄Ti₅O₁₈的在位高压拉曼光谱和压致结构相变（0～27 GPa）。发现在8.31 GPa，Sr₂Bi₄Ti₅O₁₈经历了一次结构相交。在23.13 GPa以上，Sr₂Bi₄Ti₅O₁₈似乎出现了压致非晶相交的先兆。并使用内模方法对Sr₂Bi₄Ti₅O₁₈的内模进行了指认。     

Al2O3Hx(x=1—3)分子团簇的结构与光谱研究

用密度泛函理论(DFT)的B3lyp方法在6-311++g(d,p)水平上对Al₂O₃H_x(x=1—3)分子的几何构型, 电子结构, 振动频率等性质进行了系统研究. 并给出了它们可能基态结构的总能量(E_T), 零点能(E_z), 摩尔热容(C_v), 标准熵(S), 原子化能(ΔE_m), 垂直电离能(IP)及垂直电子亲和能(E_A). Al₂O₃H和Al₂O₃H₂分子可能的基态的几何构型都为平面结构. Al₂O₃H₃的两个可能为基态的几何构型都是在立体Al₂O₃(D_3h)的几何结构基础上加三个氢原子构成. 这三个分子的能量最低结构为Al₂O₃H(²A′)C_s, Al₂O₃H₂(¹A′) C_s, Al₂O₃H₃ (²A) C₁.     

Eu2+, Cr3+共掺杂的MAl12O19 (M=Ca, Sr, Ba) 的发光性质及能量传递

三基色荧光粉中, 红色荧光粉性能较差, 为获得性能优良的红色荧光粉, 本文采用高温固相法合成了Eu²⁺, Cr³⁺单掺杂及共掺杂的碱土金属多铝酸盐MAl₁₂O₁₉ (M =Ca, Sr, Ba) 发光体. 实验表明, 在以上三种基质中均存在Eu²⁺→Cr³⁺的能量传递, 利用能量传递可以有效将Eu²⁺的蓝光或绿光转换为红光. 三种碱土金属多铝酸盐基质的晶体结构相似,但Eu²⁺, Cr³⁺发光受晶体场影响,导致在不同的基质中Eu²⁺, Cr³⁺间能量传递效率不同.通过光谱分析及能量传递效率计算发现, 相同掺杂浓度下,CaAl₁₂O₁₉中Eu²⁺→Cr³⁺的能量传递效率最高,SrAl₁₂O₁₉次之, BaAl₁₂O₁₉最低.红光转换率在CaAl₁₂O₁₉中最高.     

Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.     

SiO2分子的基态(X1A1)结构与分析势能函数

应用群论及原子分子反应静力学方法推导了SiO₂分子的电子态及其离解极限，采用B3P86方法，在6-311G^**水平上,优化出SiO₂基态分子稳定构型为单重态的C_2V构型，其平衡核间距R_e=R_Si—O=0.1587 nm,∠OSiO＝111.2°,能量为-440.4392 a.u..同时计算出基态的简正振动频率:对称伸缩振动频率ν(B₂)=945.4cm^-1，弯曲振动频率ν(A₁)=273.5 cm^-1和反对称伸缩振动频率ν(A₁)=1362.9cm^-1.在此基础上，使用多体项展式理论方法，导出了基态SiO₂分子的全空间解析势能函数，该势能函数准确再现了SiO₂(C_2V)平衡结构.     

PbTiO3/PbZr0.3Ti0.7O3/PbTiO3夹心结构铁电薄膜子晶层的优化

用Sol-Gel法制备了Pb_(1+x)TiO₃/PbZr_03Ti_07O₃/Pb_(1+x)TiO₃(PT/PZT/PT)夹心结构及PZT铁电薄膜，为了获得高质量的PT/PZT/PT夹心结构铁电薄膜，使用不同过量Pb配比(x)的PbTiO₃ (PT)层进行制备，以获得优化的PT子晶层.X射线     

Gd2O3:Eu3+纳米晶的燃烧合成及光致发光性质

采用柠檬酸作燃烧剂用燃烧合成法制备了Gd₂O₃:Eu³⁺纳米晶.用X射线衍射仪(XRD)、高分辨透射电子显微镜(HRTEM)和荧光分光光度计等对Gd₂O₃:Eu³⁺纳米晶的结构、形貌和发光性能进行了分析.结果表明：不同柠檬酸与稀土离子配比(C/M)制备的样品经800℃ 退火1 h后，均得到了纯立方相的Gd₂O₃:Eu³⁺纳米晶，晶粒尺寸约为30 nm，尺寸分布较窄，其中以C/M=1.0时制备的纳米晶结晶性最好，发光强度最大.Gd₂O₃:Eu³⁺纳米晶主发射峰位置均在612 nm处 (⁵D₀→⁷F₂跃迁)，激发光谱中电荷迁移态发生红移，观察到Gd³⁺向Eu³⁺的有效能量传递.对柠檬酸与稀土离子配比(C/M)对结晶度、发光性质等的影响也进行了分析和讨论.     

含铋层状结构铁电体PbBi4Ti4O15和SrBi4Ti4O15的X射线衍射数据的测定

用衍射仪和Guinier聚焦相机收集了含秘层状结构铁电体PbBi₄Ti₄O₁₅SrBi₄Ti₄O₁₅的X射线粉末衍射图谱(Cu K_a),给出了d值大1.13?的88和76条衍射线的衍射数据和指标。它们都属正交晶系,空间群Bb2₁m(C_2v¹²),点阵常数和X射线理论密度为PbBi₄Ti₄O₁₅α=5.431?,b=5.459?,c=41.36?; z=4;D_x=7.986g/cm³.SrBi₄Ti₄O₁₅:α=5.428?,b=5.438?,c=40.94?;Z=4;D_x=7.447g/cm³.     

Temperature‐dependent Raman scattering in ferroelectric Bi4−xNdxTi3O12(x = 0, 0.5, 0.85) single crystals

The temperature‐dependent Raman spectra of ferroelectric Bi_4−xNd_xTi₃O₁₂(x = 0, 0.5, 0.85) single crystals were recorded from 100 to 800 K. It was found that there is a critical Nd content x₀ between 0.5 and 0.85. The Nd³⁺ ions prefer to replace Bi³⁺ ions in pseudo‐perovskite layers when x < x₀, while they might begin to incorporate into (Bi₂O₂)²⁺ layers when x ≥ x₀. Nd substitution leads to a decrease in the ferroelectric–paraelectric transition temperature (T_c). A monoclinic distortion of orthorhombic structure occurs in Bi₄Ti₃O₁₂ crystals at temperatures below 200 K. Copyright © 2009 John Wiley & Sons, Ltd.     

Compositional dependence of ferroelectric properties of highly c-axis oriented Bi4−xNdxTi3O12 film capacitors

Highly c-axis oriented neodymium-modified bismuth titanate (Bi_4−xNd_xTi₃O₁₂) films having a variety of neodymium (Nd) contents were successfully grown on Pt/TiO₂/SiO₂/Si(100) substrates using metal-organic sol decomposition. After systematically examining ferroelectric properties of the c-axis oriented Bi_4−xNd_xTi₃O₁₂ film capacitors as a function of the Nd-content, we concluded that the capacitor with x=0.85 had the largest remanent polarization. The Bi_3.15Nd_0.85Ti₃O₁₂ capacitor fabricated using a top Pt electrode showed well-saturated polarization-electric field (P-E) switching curves with the remanent polarization (P_r) of 51 μC/cm² and the coercive field (E_c) of 99 kV/cm at an applied voltage of 10 V. More importantly, the Pt/Bi_3.15Nd_0.85Ti₃O₁₂/Pt capacitor exhibited fatigue-free behavior up to 4.5×10¹⁰ read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.     

Dy掺杂对Sr2Bi4Ti5O18铁电陶瓷性能的影响

用传统固相烧结法制备了Sr₂Bi_4-xDy_xTi₅ O₁₈(SBDT-x, x=0—0.20)陶瓷样品. x射线衍射分析表明, 微量的Dy掺杂没有影 响Sr₂Bi₄Ti₅O₁₈(SBTi) 原有的层状钙钛 矿结构. 通过研究样品的介电特性, 发现Dy掺杂减小了材料的损耗因子, 降低了样品铁电- 顺电相转变的居里温度. 铁电性能测量结果表明, 随Dy含量的增加, SBDT-x系列样品的剩余 极化先增大, 后减小. 当Dy掺杂量为0.01时, 剩余极化达到最大值, 约为20.1 μC·cm^-2. 掺杂引起剩余极化的变化, 与材料中缺陷浓度、内应力以及晶格畸变程度等因 素有关, 是多种作用机理相互竞争的结果. (Bi₂O₂)²⁺ 层通常被看作是绝缘层和空间电荷库, 对材料的铁电性能起关键作用. 掺杂离子进入(Bi2O₂)²⁺层会导致铁电性能变差. 关键词： 2Bi₄Ti₅O₁₈陶瓷')" href="#">Sr₂Bi₄Ti₅O₁₈陶瓷 Dy掺 杂 铁电性能 居里温度     

Enhancement of polarization in bismuth titanate thin films co-modified by La and Nd for non-volatile memory applications

Bismuth titanate thin films co-modified by La and Nd, Bi_3.15(La_0.425Nd_0.425)Ti₃O₁₂ (BLNT) were grown on Pt/Ti/SiO₂/Si(100) substrates by metalorganic solution decomposition method. The co-substitution leads to structural distortion with enhanced remanent polarization. The BLNT film capacitor with a top Pt electrode showed a large remanent polarization (2P_r) of 90 μC/cm² at an applied voltage of 10 V, which is higher than that of highly c-axis oriented BLT, and comparable with BNdT thin films. The BLNT films exhibited a fatigue-free behavior up to 1×10⁹ switching cycles at a frequency of 1 kHz. These experimental results reveal that BLNT thin films can be used as capacitors in ferroelectric random access memory applications. PACS 73.40.Rw; 73.61.-r; 77.55. +F; 77.80.-E; 81.20.Fw     

Study on ferroelectric polarization induced resistive switching characteristics of neodymium-doped bismuth ferrite thin films for random access memory applications

The present work reports on resistive switching (RS) characteristics of Neodymium (Nd)-doped bismuth ferrite (BFO) layers. The Nd (2–10 at%) doped BFO thin film layers were deposited using a spray pyrolysis method. The structural analysis reveals that a higher Nd doping concentration in BFO leads to significant distortion of the prepared Nd:BFO thin films from rhombohedral to tetragonal characteristics. The morphological analysis shows that all the deposited Nd:BFO thin films have regularly arranged grains. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the prepared Nd:BFO thin films have a higher Fe ³⁺/Fe ²⁺ratio and less oxygen vacancy (V_O) defects which enriches the ferroelectric characteristics in Nd:BFO layers. The polarization-electric field (P-E) and RS characteristics of the fabricated Nd:BFO-based RS device were examined. It was observed that the Nd (7 at%) doped BFO RS device shows large remnant polarization (P _r) of 0.21 μC/cm² and stable RS characteristics.     

Multiferroic properties of solben gel derived Bi5Fe1-xCoxTi3O15 thin films

Co-doped Bi₅FeTi₃O₁₅ thin films (BFCT-x, Bi₅Fe_1-xCo_xTi₃O₁₅) were prepared using a sol—gel technique. XRD patterns confirm their single phase Aurivillius structure, and the corresponding powder Rietveld analysis indicates the change of space group around x=0.12. The magnetic hysteresis loops are obtained and ferromagnetism is therefore confirmed in BFCT-x thin films. The remanent magnetization (M_r) first increases and reaches the maximum value of 0.42 emu/cm³ at x=0.12 due to the possible Fe³⁺—O—Co³⁺ ferromagnetic coupling. When x = 0.25, the M_r increases again because of the dominant Fe³⁺—O—Co³⁺ ferromagnetic coupling. The remanent polarization (2P_r) of BFCT-0.25 was measured to be as high as 62 μC/cm², a 75% increase when compared with the non-doped BFCT-0 films. The 2P_r remains almost unchanged after being subjected to 5.2 × 10⁹ read/write cycles. Greatly enhanced ferroelectric properties are considered to be associated with decreased leakage current density.     

Doping effect on the phase transition temperature in ferroelectric SrBi2−xNdxNb2O9 layer‐structured ceramics: a micro‐Raman scattering study

The temperature dependence of Raman spectra for SrBi_2−xNd_xNb₂O₉ ceramics (x from 0 to 0.2) has been studied in a wide temperature range from 80 to 873 K. It is found that the peak position of the A_1g[Nb] phonon mode at 207 cm^–1, which is directly associated with the distortion of NbO₆ octahedron, decreases with increasing Nd composition, while the A_1g[O] phonon mode at 835 cm^–1 increases. Moreover, both the peak position and intensity of the A_1g[Nb] phonon mode reveal strong anomalies around the ferroelectric to paraelectric phase transition temperature. It indicates that the phase transition temperature decreases from about 710 to 550 K with increasing Nd composition, which is due to the fact that the introduction of Nd ions in the Bi₂O₂ layers reduces the distortion extent of NbO₆ octahedron. Copyright © 2011 John Wiley & Sons, Ltd.     

微结构对Eu掺杂Bi4Ti3O12铁电薄膜铁电性能的影响

采用金属有机物分解法在Pt/Ti/Si(111)基底上制备了退火温度分别为600℃,650℃,700℃的Bi_3.15Eu_0.85Ti₃O₁₂(BET)铁电薄膜,并对其结构及铁电性能进行了测试,再使用扫描探针显微镜对BET薄膜的电畴翻转进行了实时观测.BET薄膜c畴发生180°畴变的最小电压为+6V,而r畴由于其高四方性,即使极化电压增至+12V也不会发生翻转.薄膜的铁电性主要源于c畴的极化,随着退火温度的升高,c畴的区域面积增加,BET薄膜的剩余极化强度随之增大.退火温度为700℃的BET薄膜剩余极化强度达到84μC/cm². 关键词： 铁电薄膜 电畴翻转 扫描探针显微镜     

La掺杂对Bi4Ti3O12薄膜铁电性能的影响

利用Sol-Gel法在Pt/Ti/SiO₂/Si衬底上制备出Bi₄Ti₃O₁₂和Bi_3.25La_0.75Ti₃O₁₂薄膜，研究了La掺杂对Bi₄Ti₃O₁₂薄膜的晶体结构、铁电性能和疲劳特性的影响，发现La掺杂没有改变Bi₄Ti₃O₁₂薄膜的基本晶体结构，并且提高了Bi₄Ti₃O₁₂铁电薄膜的剩余极化值和抗疲劳性能，对La掺杂改善Bi₄Ti₃O₁₂铁电薄膜性能的机理进行了讨论. 关键词： 铁电性能 4Ti₃O₁₂薄膜')" href="#">Bi₄Ti₃O₁₂薄膜 3.25La_0.75Ti₃O₁₂薄膜')" href="#">Bi_3.25La_0.75Ti₃O₁₂薄膜 sol-gel法 La掺杂     

Sr位Nd掺杂对SrBi$lt;sub$gt;2$lt;/sub$gt;Nb$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;9$lt;/sub$gt;性能的影响及机理研究

采用传统固相法制备了（Sr_1-3x/2A_x/2Nd_x）Bi₂Nb₂O₉（x=0,0.05,0.1和0.2）陶瓷,并系统研究了Nd离子取代Sr离子对SrBi₂Nb₂O₉性能的影响及其作用机理.研究结果表明：Sr_1-3x/2A_x/2Nd_xBi₂Nb₂O₉的介电常数和介电损耗随温度变化的行为具有明显的离子松弛极化特征.Nd³⁺对Sr²⁺的部分取代,导致Sr_1-3x/2A_x/2Nd_xBi₂Nb₂O₉剩余极化强度P_r稍有下降,但其压电系数d₃₃却有所增加,根据铁电热力学理论,这是Nd³⁺对Sr²⁺取代导致材料介电常数增大所致.Sr_1-3x/2A_x/2Nd_xBi₂Nb₂O₉的居里温度（T_C）没有随Nd含量的增加而变化,拉曼光谱技术分析表明这是其NbO₆八面体畸变程度没有发生变化所致.Nd³⁺取代Sr²⁺提高了材料的介电常数ε_r、压电系数d₃₃、机电耦合系数K_p,同时降低了机械品质因数Q_m,但是谐振频率温度系数C值没有改变. 关键词： 压电陶瓷 介电性能 压电性能 拉曼光谱