【作者】 郑鹏；

【导师】 张家良；

【作者基本信息】 山东大学 ， 凝聚态物理， 2010， 博士

【摘要】 压电材料和介电材料是两类重要的功能电子材料。压电材料是实现机械能与电能相互转换的一类功能材料,在传感器、驱动器、超声换能器、蜂鸣器、电子点火器等各种电子元件和器件方面有着广泛的应用。目前广泛使用的压电材料主要是锆钛酸铅(Pb(Zr,Ti)O3,简称PZT)基陶瓷材料。但PZT的制备需要使用大量的含铅氧化物作为原料,在生产、使用和废弃后处理过程中都会给人类及生态环境带来严重的影响,发展无铅环境协调性压电陶瓷是一项紧迫且具有重大现实意义的课题。介电材料是一类利用材料的介电性质来制造电容性器件的电子材料,被广泛的应用在电容器、谐振器、滤波器、存储器等重要的电子器件中。近年来,随着电子器件向高性能化和尺寸微型化方向的发展,高介电材料受到越来越多的关注。在前述背景下,本论文主要开展了钛酸钡(BaTiO3)基压电陶瓷材料和钛酸铜钙(CaCu3Ti4O12,简称CCTO)高介电陶瓷材料的制备、物性及相关机理的研究。BaTiO3陶瓷是历史上最早发现的一种多晶压电材料,在B. Jaffe等于1954年发现PZT之前曾被广泛地应用。目前,虽然以BaTiO3为基体的陶瓷材料仍被广泛地用于制造各种电容器和PTC电阻等,但作为压电陶瓷材料的应用已很少见,主要的一个原因是由于通常制备的BaTiO3基陶瓷材料的压电活性太低(d33≤190 pC／N)。值得关注的是,最近几年日本研究者相继报道了以水热法合成的BaTi03超微粉为原料制备出压电活性非常高的BaTiO3陶瓷材料的研究结果。利用微波烧结、分段烧结或TGG技术烧结制备的BaTiO3陶瓷的d33值分别达到了360、460和788 pC／N。作者所在的课题组在前期的工作中采用普通的BaCO3和Ti02粉体为原料、通过固相反应方法也制备出了d33值高达419 pC／N的钛酸钡陶瓷。这些结果启示人们需要对钛酸钡基陶瓷作为无铅压电材料应用的潜能进行重新思考。然而,目前人们对于BaTiO3陶瓷呈现如此高的压电活性的物理机理还不是很明确。另外,考虑到BaTiO3在相对较窄的温度范围内具有多个相变点,强压电活性的BaTiO3陶瓷的温度稳定性也是一个值得关注的问题。CCTO是一种具有钙钛矿型衍生结构的氧化物,该材料不论是单晶形态还是多晶陶瓷形态都呈现异常高的介电常数,并且其静介电常数在很广的温度范围内几乎不随温度变化。对于CCTO高介电性的起源,有人认为起源于材料内在的晶格结构,也有人认为起因于内部阻挡层电容效应,还有人归结为与样品电极有关的耗尽层效应,因此在机制解释方面存在着很大的争议。另一方面,CCTO的晶粒是半导化的,这一点已经被广大研究者所接受,但目前人们对于半导化的起源及相应的电学输运机制的理解上还存在着较大的分歧。这对于全面理解CCTO的物理特性、研发新型高介电材料是十分不利的,还需要进一步对其研究。本论文以传统固相反应法制备的BaTiO3基压电陶瓷和CCTO高介电陶瓷为研究对象。考察了BaTiO3陶瓷的压电晶粒尺寸效应并探讨了相关的物理机理。讨论了BaTiO3陶瓷的温度稳定性问题,并制备了强压电活性高温度稳定性的BaTiO3基压电陶瓷。考察了CCTO陶瓷的微观组织结构、介电、复阻抗以及直流电阻率方面的电学性质,探讨了相关的高介电性的物理机理和晶粒半导化的起源。一、以普通碳酸钡和二氧化钛粉体为原料,利用传统的固相反应工艺制备了不同晶粒尺寸的高致密度钛酸钡陶瓷样品,研究了其压电介电物性随晶粒尺寸的变化关系。晶粒尺寸为0.94μm的精细晶粒钛酸钡陶瓷的相对介电常数约为4700,压电常数为340 pC／N,这些结果表明可以通过控制晶粒尺寸来获得较大的介电压电活性。通过比较介电常数和压电常数随晶粒尺寸的变化关系,发现了钛酸钡陶瓷的强压电活性和高介电活性有着共同的起源。但射线分析的结果表明精细晶粒钛酸钡陶瓷中晶格结构的变化不能解释高介电压电活性的起源,因此可以推断钛酸钡的高介电压电活性主要来自于非本征的贡献。内应力对陶瓷介电压电活性有着明显的影响。精细晶粒陶瓷中的内应力会导致相变点的变化,但内应力造成的相变点变化不足以完全解释当前不同晶粒尺寸的钛酸钡陶瓷的室温介电常数和d33的巨大差别,当然也就无法解释精细晶粒钛酸钡陶瓷室温高介电常数和高d33的起源。通过对陶瓷电畴构型的表征,发现了钛酸钡的强介电压电活性是和90°电畴结构密切相关的。钛酸钡陶瓷的介电压电活性随90°电畴宽度减小呈现先增加后减小的变化规律。经过分析,我们认为,随着晶粒尺寸的减小,90°电畴密度的增加和90°畴壁的有效质量的减小是导致介电压电晶粒尺寸效应的主要原因。二、考察了强压电活性BaTiO3陶瓷的温度稳定性问题。研究发现,尽管我们可以通过调节制备工艺来获得室温条件下压电活性非常良好的BaTiO3陶瓷,但是其较差的压电活性温度稳定性仍会严重的影响BaTiO3陶瓷作为强压电活性无铅压电材料的应用前景。然而,通过比较四方相和正交相的压电活性的温度稳定性,我们发现BaTiO3基压电材料在正交相具有更好的压电活性温度稳定性,这为我们改善BaTiO3基压电材料的温度稳定性提供了重要依据。通过适量的Zr的Ti位取代,我们成功的把BaTiO3的正交相移动到室温附近,有效地改善BaTiO3陶瓷的温度稳定性。但随着Zr的添加,Ba(Ti,Zr)O3材料的室温压电活性下降,除了Zr取代Ti引起的相移的影响,大晶粒中过多的人字形电畴结构导致的畴壁有效质量的增大也是一个十分重要的因素。减小Ba(Ti,Zr)O3陶瓷的晶粒大小可能是提高其室温压电活性的一个有效方法。研究发现,少量的CuO添加可以有效地降低Ba(Ti,Zr)O3的烧结温度,抑制晶粒生长。小晶粒中较小的畴壁有效质量可能是导致CuO掺杂后Ba(Ti,Zr)O3陶瓷室温压电性能升高的主要原因。此外,CuO的掺杂还有助于电畴结构的稳定,提高材料的抗经时老化特性。并且CuO的掺杂扩展了正交相的温度范围,扩展了相变点附近的两相共存区,进一步改善了Ba(Ti,Zr)O3陶瓷的温度稳定性。其中,1 mol% CuO改性的Ba(Ti0.9625Zr0.0375)O3陶瓷的室温d33高达300 pC／N,k33在-60℃-85℃的温度范围内均大于50%,并且在-30℃-55℃的温度范围内几乎不随温度变化,表现出较好的压电活性和温度稳定性。三、考察了高介CCTO陶瓷的微观结构以及电学性质。研究发现,随着烧结时间的延长,CCTO陶瓷的平均晶粒尺寸增大、介电常数增高。在对CCTO室温以上的高温区域的介电性的研究中,发现除了已知的在低温和常温下可观察到的100 kHz以上的类德拜弛豫色散之外,在100 Hz-100 kHz的频率范围内还有一个新的类弛豫性介电色散存在。因此,CCTO陶瓷的高温介电谱上包含两个类德拜型弛豫和一个巨大的低频介电响应。利用特征弛豫频率与温度的关系,求出了两个弛豫频率的激活能分别为0.086 eV和0.632 eV。通过对CCTO复阻抗谱的分析,发现高温下CCTO的复阻抗谱包含三个阻抗半圆弧,而不是以前报道的两个。进一步的分析发现,这三个阻抗圆分别代表了三种不同的电学机制。通过对不同电极样品的室温介电谱和高温介电谱的分析,我们发现高频介电弛豫是和电极效应无关的,而中频介电弛豫则主要是电极效应的贡献。据此我们推论高频下的阻抗半圆弧主要起源于晶粒,而中频和低频下的半圆弧主要起源于晶界和电极效应的贡献。根据前面的实验结果分析,我们提出了一个在三个不随频率变化的RC并联电路(RgCg, RgbCgb和RxCx)中加入一项与空间电荷输运行为相关的、随频率变化的阻抗的新的等效电路模型,其中RgCg,RgbCgb和RxCx分别代表来自晶粒、晶界和电极效应的贡献。利用该等效电路模型,成功地对实验数据进行了拟合处理,并得到了描述三种不同机制效应的相应的特征电阻的激活能分别为0.107 eV、0.627 eV和0.471 eV。最后,经过理论推导证明,介电谱上的两个特征频率主要是由RbCgb和RgbCx的大小决定的。四、研究了CCTO陶瓷不同温度下的Ⅰ-Ⅴ曲线和直流电阻率随温度的变化关系。研究发现,在高温强电流条件下,CCTO陶瓷晶界和电极处的势垒处于击穿态,测量得到的电阻主要来自于晶粒的贡献。通过对CCTO陶瓷不同温度条件下的直流电阻率的测量,我们发现高温下CCTO的晶粒电阻率随温度的变化规律符合绝热近似的小极化子跳跃传导的行为,而非半导体能带传导的Arrhenius定律。据此我们推测CCTO晶粒中存在着小极化子。样品还原气氛处理的结果表明氧缺陷并不是导致CCTO晶粒半导化的原因,可能的传导机制来自于Cu或Ti的变价。考虑到我们测量得到的负的Seebeck值,我们认为CCTO陶瓷晶粒内的电荷输运主要通过Ti3+/Ti4+的小极化子跳跃传导。更多还原

【Abstract】 Piezoelectric and dielectric materials are two important classes of electronic materials. Piezoelectric materials are a class of functional materials that realize the conversion between mechanical energy and electrical energy and thus are popularly utilized to fabricate sensors, actuators, transducers and other electronic devices. Currently, Pb(Zr,Ti)O3 (PZT)-based piezoelectric ceramics take the predominated position in the market of practical piezoelectric materials because of their excellent electrical properties. However, due to the toxicity of lead oxide that is largely used during the production process, there is an increasing demand to replace PZT with the environment-benign lead-free alternatives. Dielectric materials are widely used to form capacitive devices such as capacitance, resonators and filters. Those dielectric materials with high dielectric permittivity have attracted considerable interest in recent years since they might offer the opportunity to enhance the performance or shrink the dimensional sizes of the microelectronic device. Under these circumstances, this thesis concentrates on the studies of material preparations, physical properties and the related mechanisms for BaTiO3-based piezoelectric ceramics and CaCu3Ti4O12 (CCTO) high-dielectric ceramics.BaTiO3 ceramics is historically the first polycrystalline piezoelectric material and had been once widely used as a piezoelectric material before the discovery of PZT. Nowadays, however, its main technical applications are no longer as a piezoelectric but as a dielectric material, largely because of its poor piezoelectric properties (usually, d33≤190 pC/N) compared with PZT. Nevertheless, surprisingly high d33 values (350, 460 and 788 pC/N, respectively) were reported recently for those BaTiO3 ceramics that were prepared from hydrothermally synthesized fine BaTiO3 powders by some special fabrication techniques like microwave sintering, two-step sintering and templated grain growth (TGG). More importantly, we have recently succeeded in obtaining BaTiO3 ceramics with high piezoelectric properties through conventional solid-state reaction route with starting raw materials of ordinary BaCO3 and TiO2 powders. These results indicate that BaTiO3-based ceramics possess a high possibility to become a good lead-free piezoelectric material. However, related mechanism for the excellent piezoelectric properties remains unclear. Furthermore, considering BaTiO3 undergoes three polymorphic phase transitions in a relatively narrow temperature region, the piezoelectric temperature dependence of the recently obtained high piezoelectric constant BaTiO3 ceramics another concern.CCTO is an oxide that has a cubic perovoskite-related crystal structure and exhibits an enormously large dielectric permittivity (ε’) at low frequencies in both forms of single crystals and ceramics. The dielectric permittivity keeps almost constant in the low frequency range below 100 kHz at room temperature and is nearly independent of temperature over the wide temperature region. So far, several models have been proposed to explain the dielectric behavior but are quite controversial, including both intrinsic and extrinsic mechanism explanations from the viewpoint such as crystal structure, internal barrier layer capacitance (IBLC) effect and contact-electrode depletion effect. Besides, though it is widely accepted that the grains of CCTO ceramics are semiconductive, the origin of the semiconductive and relevant conduction behavior are still disputable. This situation is extremely unfavorable for a full understanding of the unusual dielectric property and its related mechanism of CCTO and further research needs to be done.This thesis takes the BaTiO3-based piezoelectric ceramics and CCTO high-dielectric ceramics prepared by the conventional solid-state reaction as research objects. For BaTiO3-based piezoelectric ceramics, the grain size effect on the piezoelectric properties of BaTiO3 ceramics is investigated. The piezoelectric temperature dependence of BaTiO3 ceramics is discussed and ceramics with high piezoelcectric activities and stable temperature dependence are successfully obtained. For CCTO high-dielectric ceramics, the effects of microstructure and electrode on the dielectric and electrical properties are investigated and the high temperature conduction behavior is discussed.1. BaTiO3 ceramics with high piezoelectric coefficient (d33) have been successfully obtained through the conventional solid-state reaction route starting from ordinary BaCO3 and TiO2 powders. The BaTiO3 ceramic with an average grain size about 0.94μm is found to have the excellent piezoelectric properties of d33= 340 pC/N andε’= 4700. This result suggests that it is possible to obtain very high piezoelectric activities and permittivity by the grain size control. By carefully analyzing the variations of permittivity and piezoelectric activities with the changing of grain sizes, it is found that the high piezoelectric acitivities and the high permittivity have the same physical origins. XRD analyzing results show that changes of crystal structure in fine grain BaTiO3 ceramics can not account for the high permittivity and high piezoelectric constant. Some extrinsic contributions must exist in BaTiO3 ceramics with high dielectric permittivity and piezoelectric acitivities. The effect of internal stress on the dielectric permittivity and piezoelectric constant can not be ignored in BaTiO3 ceramics. Internal stress in fine grain BaTiO3 ceramics can lead to phase transition temperature shifts, but great differences of room-temperature dielectric permittivity and piezoelectric constant can not be fully ascribed to the shift of phase transition temperature. However, it is found that high dielectric permittivity and piezoelectric constant are closely related to domain configuration. The d33 values firstly increases and then decreases with the decrease of average domain width. A possible mechanism that results in the piezoelectric properties grain size effect in the present BaTiO3 ceramics is discussed. It is suggested that both density and effective mass of the 90°-domain wall in the BaTiO3 ceramics are considered to be important factors which significantly influence the d33 value.2. The piezoelectric temperature dependence of BaTiO3 ceramics is discussed. Though BaTiO3 ceramics with high piezoelectric activities can be obtained by choosing appropriate preparing parameters, the temperature unstability is a great obstacle to the application of BaTiO3 ceramics. However, by comparing the piezoelectric properties in tetragonal phase with that in orthorhombic phase, it is found that BaTiO3 ceramics exhibit more stable piezoelectric properties in the orthorhombic phase than in the tetragonal phase. This is a very important clue for us to gain the good piezoelectric temperature stability in BaTiO3-based ceramics. Partially substituting Ti with Zr can shift the phase transition temperature upward and is effective in reducing the piezoelectric temperature dependence. However, it is found that the piezoelectric activities decrease with the substituting Ti with Zr. Beside the effect of phase shift, more herringbone pattern domains in the large grain Ba(Ti,Zr)O3 ceramics is an important factor that lead to the decrease of piezoelectric activities. It is found that this could be overcome by incorporating a small amount of CuO additive. Furthermore, the CuO-modified BZT ceramics exhibit weaker long-time degradation and better temperature stability. CuO-modified Ba(Tio.9625Zro.o375)03 ceramics possess piezoelectric properties of d33= 300 pC/N, kp= 0.493, and k33= 0.651 with tanδ= 0.011, and its k33 remains larger than 0.50 in the broad temperature range from -60 to 85℃and is almost constant between -30 and 55℃. The results indicate that CuO-modified Ba(Ti,Zr)O3 ceramics are a promising low-cost lead-free material for practical applications.3. A series of CCTO ceramics are prepared by the conventional solid-state reaction method under various sintering conditions. Dielectric properties and complex impedance spectra are investigated within the frequency range of 40 Hz-110 MHz at room temperature and within the frequency range of 40 Hz-4 MHz at higher temperatures up to 350℃. The high dielectric constant is found to closely relate to the microstructure. A Debye-like relaxation appears above 75℃in the frequency range of 100 Hz-100 kHz, which shows the larger dielectric dispersion strength than that existing in the frequency region higher than 100 kHz. High-temperature dielectric dispersion exhibits a large low-frequency response and two Debye-like relaxations. Their characteristic frequencies follow the Arrhenius-law with the activation energy values of 0.086 eV and 0.632 eV, respectively. Furthermore, the existence of three semicircles in the complex impedance plane is disclosed in the present study, which differs in the reported number of two in literature. These semicircles are considered to represent different electrical mechanisms. The contact-electrode depletion effect is examined. From the analysis, we attribute the impedance semicircle in the high frequency region to the contribution of semiconducting grains and the other two to the contributions of the grain boundaries and electrode depletion effect respectively. An equivalent electrical circuit model is suggested to explain the dielectric and electrical properties, in which a frequency-dependent ZUDR term is included in parallel to one of three in-series connected RC elements. The model well fits simultaneously the data of dielectric dispersion and complex impedance. The activation energy values of the three resistances are calculated to be 0.471 eV,0.627 eV and 0.107 eV, respectively.4. The electrical property of CaCu3Ti4O12 ceramics was studied over the high temperature range of 300-800℃. The Seebeck coefficient S is negative with a large absolute value of～560μV/K at 300℃. The measuredⅠ-Ⅴresponses are highly linear, which indicates that CCTO ceramics as a whole are ohmic at high temperatures and that the measured p reflects essentially the electrical conduction property in the grains of ceramic polycrystalline structure. The change of p with T follows the rule of adiabatic hopping conduction of small-polaron rather than the one of thermally activated conduction. Possible mechanism for the small-polaron formation and transport is discussed. Oxygen vacancy is not the reason for the grain semiconductivity. Possible conduction mechanism may come from the aliovalence of Cu or Ti. A model that the small polaron originates from the aliovalence of Ti4+/Ti3+ is proposed.更多还原