PMN-PT单晶淬火处理对介电性能的影响

通过淬火处理可以使弛豫型PMN-PT单晶中一些应力集中的区域产生应力场致相变,在Td处产生宏畴,从而在温谱上Td处的介电峰强化.研究表明,经淬火处理单晶谐振极化所对应的低温介电行为受到压制,主介电峰变窄,峰值变高并稍向高温方向移动;频率弥散减弱,弛豫性变小.     

细晶粒BaTiO3陶瓷的微结构及介电性能测试

采用放电等离子烧结技术,在９００℃下得到细晶粒尺寸的、均匀的、纯ＢａＴｉＯ_３陶瓷。对样品的高分辨电镜分析表明,晶粒中只有１８０°电畴,没有９０°电畴,晶界宽度相当于两倍的晶格常数．同时观察到有部分孪晶出现．对细晶粒陶瓷介电性能的测量表明,介电常数对晶粒尺寸有明显的依赖关系,认为李晶的存在是导致小晶粒陶瓷介电常数没有迅速降低的原因之一．对这一效应,用通常的串、并联模型不能很好地拟合。     

CaCu3Ti4O12陶瓷高介电性的起因机制

为了探索CaCu3Ti4O12(CCTO)高介电性的起因机制,利用固相反应工艺制备了CCTO陶瓷样品,对其电学性质进行了研究.在40 Hz～100 MHz测量范围内,其室温下的介电频谱在1 MHz附近呈现一个类Debye型弛豫,而高温介电频谱分别在1 kHz以下和1 MHz附近呈现两个类Debye型弛豫.抛除表面层的同一个样品分别溅射金电极和烧渗银电极后升温测量其介电频谱,发现低频介电弛豫对电极的金属类型高度敏感,而高频介电弛豫与电极的金属类型无关,与材料微观结构存在着密切的关系.因此推断:CCTO低频介电弛豫起源于样品与电极之间的耗尽层效应,而高频介电弛豫起源于高阻态的晶界与半导化的晶粒形成内部阻挡层电容效应.     

La替代对Bi2O3-ZnO-Nb2O5陶瓷介电性能的影响

研究了A位La^3＋替代对Bi2O3-ZnO-Nb2O5（BZN）陶瓷结构和介电性能的影响。当La替代量X〈0．5时,陶瓷相结构为单一的立方焦绿石相。随着La替代量的增加,陶瓷样品的晶粒尺寸和密度逐渐减小。低温下的介电弛豫现象随着La替代量的增加也发生有规律的变化,介电常数逐渐减小,弛豫峰峰形逐渐宽化,峰值温度向低温方向移动。与La替代量为0．1、0．15、0．3和0．5相对应的弛豫峰的峰值温度分别为-95℃、-99℃、-109℃和-112℃。     

驰豫型铁电体的宏观介电性能与极化机制研究

驰豫型铁电体是很重要的功能材料,其介电性能和极化机制一直是研究的热点。作者通过对掺钛铌镁酸铅系列铁电陶瓷介电、铁电、热电等宏观性能和微观结构详细、系统的实验研究,提出铁电微畴为三方相畸变结构散布于立方钙钛矿母相中,通过类马氏体预相变的位移型形核过程,实现顺电—驰豫型铁电态的弥散相变,进一步通过类马氏体相变,完成驰豫型铁电态无序态分布的微畴向正常铁电态长程序铁电宏畴转变;提出T_1可调整A(B′B″)O_3型复合钙钛矿铁电体B′和B″离子价态和离子半径等不同所产生的内电场和弹性场,使立方母相所承受切应力发生变化,诱发极性微区、铁电微畴随组成变化表现不同活性。对弛豫型铁电体介电谱分析与计算表明,其在介电常数峰值温度Tm附近的介电弛豫过程连续变化,不对应结构突变的相变过程,较接近偶极介质。通过铌锌酸铅基系列铁电陶瓷在准同型相界附近异常特性的研究,进一步揭示弛豫型铁电体中结构特征决定宏观行为的必要性,反映出铁电材料由微观到介观、再到宏观的有趣规律性。在此基础上,对复合钙钛矿弛豫型铁电体偏压介电行为、电致伸缩特性、介电老化行为等进行了系统研究,合理解释了一系列新的实验现象和结果;并对钨青铜结构铌酸锶钡弛豫型铁电陶瓷复介电响应进行了详细的对比分析和讨     

PMZNT弛豫铁电陶瓷直流偏压下的介电非线性的研究

在一定温度范围内，对PMN－PZN－PT系弛豫铁电陶瓷直流偏压下的介电非线性进行了研究。结果表明，在较小的直流偏压场下，介电常数存在一个具有温度依赖性的最大值；在较大的直流偏压场下，介电常数大幅度降低。分别采用偶极子玻璃模型和多畴态模型对上述非线性现象进行了定性解释。     

钛酸钡-铌酸钾钠弛豫铁电储能陶瓷的合成和表征

综合储能性能(充电能量密度、放电能量密度和储能效率)较低是储能陶瓷领域亟待解决的关键科学问题。同时提高陶瓷的极化差(ΔP)和击穿场强(BDS)，是提高陶瓷综合储能性能的重要方法。以BaTiO₃(BT)为主晶相，K_0.5Na_0.5NbO₃(KNN)为包覆剂、助烧剂和添加剂，合成了晶粒尺寸为100 nm和200 nm的BT-KNN陶瓷。结果表明：BT-KNN陶瓷具有明显的纳米畴、弛豫行为和介电温度稳定性，且兼具高ΔP和高BDS。相比晶粒尺寸为100 nm的BT-KNN陶瓷，晶粒尺寸为200 nm的BT-KNN陶瓷具有更加优异的综合储能性能，包括较高的充电能量密度W (2.50 J·cm^-3)、放电能量密度Wrec (2.08 J·cm^-3)和储能效率η (83.2%)。该研究可为高综合储能性能陶瓷的合成提供一定的理论依据。     

BaNb2O6掺杂Na1/2Bi1/2TiO3无铅压电陶瓷的介电弛豫

采用传统陶瓷制备方法,制备了无铅新压电陶瓷材料Na1/2Bi1/2TiO3-xBaNb2O6.研究了BaNb2O6掺杂对(Na1/2Bi1/2)TiO3陶瓷晶体结构、介电性能与介电弛豫行为的影响.XRD分析表明,在所研究的组成范围内陶瓷材料均能够形成纯钙钛矿固溶体.材料的介电常数-温度曲线显示陶瓷具有两个介电反常峰Tf和Tm,修正的居里一外斯公式较好的描述了陶瓷弥散相变特征,弥散指数随BaNb2O6掺杂量的增加而增加.该体系陶瓷表现出与典型弛豫铁电体明显不同的弛豫行为,低掺杂量的陶瓷仅在低温介电反常峰Tf附近表现出明显的频率依赖性,而高掺杂的陶瓷材料在室温和Tf之间都表现出明显的频率依赖性.并根据宏畴一微畴转变理论探讨了该体系陶瓷产生介电弛豫的机理.     

高密度CaCu_3Ti_4O_(12)陶瓷的介电性质

利用热压方法制备了高密度的CaCu3Ti4O12陶瓷,研究了其微观结构、晶格结构和介电性质。实验发现,短时间热压方法制备的陶瓷的相对密度就能达到98.3%,并且微观结构呈现晶粒尺寸双峰分布的特点。XRD显示热压方法制备的陶瓷中含有少量的Cu2O第二相,而经过退火处理的热压陶瓷则只含有CuO第二相。热压方法制备的陶瓷和经过退火处理的热压陶瓷的室温介电频谱上有两个介电弛豫,而常规方法制备的陶瓷只有一个。并且热压方法制备的陶瓷的低频介电常数高达2×105,经过退火处理的热压陶瓷的低频介电常数更是达到1×106。本研究进一步讨论了其微观结构和介电性质之间的关系。     

Sr、Nb掺杂(Bi0.5Na0.5)0.94Ba0.06TiO3陶瓷的介电性能研究

采用固相反应法制备Sr、Nb掺杂BNBT无铅压电陶瓷,利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和介电谱仪表征了样品的晶体结构,表面形貌及介电性能,研究了陶瓷的相结构,微观形貌和介电响应。结果表明,BNBT陶瓷为单一的钙钛矿结构,而BNBT-3SN、BNBT-3SN-3Sr和BNBT-3SN-1Nb陶瓷除了主相钙钛矿结构,存在少量第二相。形貌结构显示出样品相对比较致密,晶粒尺寸分布均匀。SrNb₂O₆的引入使介电峰T_s向低温方向移动,增强弛豫相变程度从而提高了陶瓷基体的介电温度稳定性,BNBT-3SN陶瓷在40～406℃内表现出良好的温度稳定性。通过阻抗图谱分析可知,所有样品主要为晶界传导机制。而且研究表明Sr、Nb掺杂可通过对BNBT-3SN陶瓷氧空位浓度的作用,实现对电导率的调控。随着Sr²⁺的增加,样品的氧空位浓度增加,电导率增大;而Nb⁵⁺的增加,填充了氧空位导致氧空位浓度减小促使电导率降低。     

AC Complex Impedance Analysis of Doped Strontium Titanate Multifunctional Ceramics

Doped SrTiO3 capacitor-varistor multifunctional ceramics were fabricated by a single sintering process.AC complex impedance analysis was performed to investigate electrical features of grains and grain boundaries for both as-reduced ceramic and reoxidized ceramics.The results showed that the as-reduced ceramic exhibited inductive response at high frequencies above 2MHz,which is attributed to the contribution of electron behavior in semiconducting grains.The high frequency inductive response disappeared in impedance plots of reoxidized ceramics.     

Study on electrical properties of Ni-doped SrTiO3 ceramics using impedance spectroscopy

The ceramics, SrTiO₃ (ST) and 0.4, 0.8 atom % Ni doped SrTiO₃, were prepared by solid state reaction route. The average grain size of undoped and doped samples was measured and found to be 1.2, 1.9 and 3.7 μm, respectively. The impedance measurements were conducted at 400–600°C to separate grain and grain boundary contributions. The grain and grain boundaries relaxation frequencies were shifted to higher frequency with temperature. Bulk resistance of doped and undoped ST ceramics was more or less the same. Single grain boundary resistance of doped sample was higher than that of undoped one, indicating that GB resistance increases with acceptor doping. Activation energies were calculated to confirm the same.     

Characterization of lead zirconate titanate ceramics for use in miniature high-frequency (20-80 MHz) transducers

The material properties of lead zirconate titanate (PZT) ceramics for operation in the thickness mode at frequencies as high as 80 MHz are reported. Each of the ceramics tested showed a reduction in k (t) with increasing frequency. In a fine-grained PZT, values of k(t) as high as 0.44 were measured at 80 MHz. The effects of grain size were also evident in the measurement of frequency dependent mechanical losses. Experimental and theoretical analysis of a 1 mmx1 mm, 45 MHz PZT transducer verified the validity of the measurements of the properties and demonstrated excellent insertion loss and bandwidth characteristics. The minimum insertion loss of -17.5 dB is in good agreement with theory and is a marked improvement over the performance of polymer devices. Details on the fabrication and testing of high frequency ceramic transducers are described.     

Superplasticity by internal frictional heat under biased cyclic loading

A new damping method was developed not only as a testing tool to investigate in situ deformation under stress, but also as a processing method to superplastically deform ceramics. The specific damping capacity (SDC) at low frequencies (<0.2 Hz) decreased with increasing frequencies, which matched previous internal friction results. However, at higher frequencies (0.2–5 Hz) SDC increased with frequencies, which was explained by a new internal frictional heat mechanism. Three different ceramics: a non-superplastic one and two superplastic ones with different activation energies, showed the same behavior at the high frequency damping tests (1–5 Hz). From these results, it was deduced that a cyclic load at high frequencies, superimposed on a static one, has a great potential to enhance superplasticity by specifically heating up grain boundaries from internal frictional heat.     

液相包覆制备Ce-TZP陶瓷的力学性能与稳定行为

采用XRD、SEM、TEM等分析手段对由液相包覆工艺制备的Ce—TZP陶瓷的微结构进行了研究，并和共沉淀粉体制备的Ce—TZP陶瓷进行了对比分析．力学性能表明，同共沉淀粉体制备的Ce—TZP陶瓷相比，由液相包覆工艺制备的Ce—TZP陶瓷虽硬度下降，但断裂韧性改善；液相添加少量Al2O3硬度随之增加、断裂韧性显著提高．电镜分析表明，液相包覆工艺制备的Ce—TZP陶瓷晶粒尺寸分布宽化，一部分晶粒尺寸较大但CeO2含量低、易发生马氏体相转变晶粒的存在是断裂韧性改善的主要原因．陶瓷体中单斜相大晶粒与四方相之间的残余应力、添加少量Al2O3在晶界上易形成薄的非晶包裹层，是增加可相变四方相数量，提高断裂韧性的其它机制。     

钛酸锶的宽频带介电谱

利用计算机自动控制的超低频介电谱测量装置和HP4192A阻抗分析仪，在10^-4Hz～10^5Hz宽这九个数量级的范围内测量了钛酸锶陶瓷的频域介电谱。其频域复介电常数可很好地拟合为两项之和。其中超低频项来自于晶界间空间电荷运动的贡献，高频项来自于晶粒内部空间电荷运动的贡献。高频项复介电常数的实部在频率增大时变为负值，这说明介电谱用频域方法来描述是有缺陷的，其测量结果无法用传统的介电极化加以解释。     

Dielectric properties of bismuth doped CaCu3Ti4O12 ceramics

Ca_1?3x/2Bi_xCu₃Ti₄O₁₂ (x = 0.0–0.3) ceramics were prepared by the conventional solid-state reaction method. X-ray powder diffraction analysis confirmed the formation of cubic CCTO phase except for subtle peaks of CuO. SEM micrographs suggested that the morphologies of doped CCTO ceramics had been sheet-like for high Bi-doping amount, and the dominant grain size decreasing could be seen for the small content of Bi-doping CCTO. Dielectric properties of pure and doped CCTO were investigated in a broad temperature range of 20–420 K. The results showed that bismuth doping could decrease the dielectric loss but suppress the dielectric temperature stability at the same time. Bi doped CCTO ceramics presented different relaxation properties. As to pure CCTO and BCCTO (x = 0.3) only one MW relaxation (Relaxation I) could be found, which moves to higher frequency with temperature increasing. However, two relaxation processes (Relaxation I and II) appear for BCCTO (x = 0.1–0.2). By means of complex impedance spectra analysis and Arrhenius fitting, we successfully separated the different conductive segments and explained the mechanisms of the two relaxation processes. Relaxation I appeared at low temperature could be attributed to the V_O doping energies inside CCTO grains which did not showed significant changing of activation energy after bismuth doping. For Relaxation II at higher temperature than Relaxation I, with activation energy obviously depending on the Bi-ion concentration, may be related with the V_O point defects at the grain boundaries.     

The abnormal grain growth and dielectric properties of (Nb,Ba) doped TiO2 ceramics

The effect of consolidation pressure and crystallite size of powders crystal phases of TiO₂ on sintered microstructure of TiO₂ ceramics doped with 0.25 mol % Nb and 1.0 mol % Ba were investigated. Also, the development sequence of abnormal grain growth of (niobium, barium) doped TiO₂ ceramics was proposed. The second phases of as-sintered surface were determined. The dielectric properties of Ag-electroded samples were correlated with the resistivity of the bulk (Nb, Ba) doped TiO₂ ceramics. Abnormal grain growth lowered the resistivity of bulk material of (Nb, Ba) doped TiO₂ ceramics, and moved the relaxation frequency of fan δ to high frequency region over 10⁵ Hz. Controlling the sintered microstructures can obtain reasonably good dielectric properties.     

Grain-size effects on thermal properties of BaTiO<Subscript>3</Subscript> ceramics

Dense nanocrystalline BaTiO₃ ceramics are successfully prepared by the high pressure assisted sintering. Microstructures are observed by scanning electronic microscopes. The grain sizes are estimated to be about 30 and 150 nm. In comparison, BaTiO₃ ceramics with the grain size of 600 nm and 1.5 μm are fabricated by conventional pressure-less sintering. The thermal properties of BaTiO₃ ceramics with different grain sizes are investigated by differential scanning calorimetry and thermal expansion. The results suggest that the enthalpy values for the tetragonal-cubic transition decreased and the thermal expansion values increased with decreasing grain size. Furthermore, the Curie temperature shifts to lower temperature with decreasing grain size.     

EFFECT OF GRINDING ON CRACKS AND THE STRENGTH OF CERAMICS

Abstract— Five kinds of silicon nitride samples with different microstructure were prepared in order to examine the effect of sintering condition on grain size and mechanical properties. The degradation in strength of ceramics caused by grinding is investigated in detail. It is shown that the critical process zone size is closely related to the grinding process. Futhermore, the crack size caused by grinding is estimated by the process zone size failure criterion and the Newman-Raju formula. It can be concluded that the process zone size failure criterion is useful when evaluating the sensitivity of ceramics to grinding.