【作者】 范桂芬；

【导师】 吕文中；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2007， 博士

【摘要】 本论文选择了A位离子复合取代的钙钛矿型铁电体钛酸铋钠(Bi0.5Na0.5TiO3,简称BNT)作为研究的基材。针对BNT陶瓷作为压电陶瓷材料本身存在的一些缺点,借鉴传统压电陶瓷改性研究的思路,对BNT陶瓷进行了系统的改性研究,以期提高BNT陶瓷的压电性能,达到实用化的目的。为了提高BNT陶瓷的致密度及耐击穿场强,采用锂离子取代A位钠离子对BNT陶瓷进行改性研究;根据不同结构的钙钛矿铁电体固溶后,在准同型相界附近具有优异的压电性能的特点,本文将三方结构的BNT分别与四方结构的(Bi0.5K0.5)TiO3和正交结构的KNbO3铁电体复合形成二元系固溶体,为了进一步提高BNT基陶瓷的压电性能,在BNT与(Bi0.5K0.5)TiO3二元复合的基础上进行了La2O3和MnO2掺杂的改性研究。最后将BNT与(Bi0.5K0.5)TiO3和KNbO3进行三元复合,研究其组成、结构及性能的变化并期望得到最佳的性能配方。本文系统研究了A位Li取代对BNT陶瓷烧结特性、晶体结构和性能的影响,实验研究的结果表明:适量锂取代会提高BNT陶瓷的致密度,其相对密度达到95%以上,且在x=0.1时,陶瓷的绝缘电阻率达到最大,其值为1.56×1012Ω.cm。XRD结果显示,在固溶限内的锂取代没有改变BNT陶瓷的三方相结构。并且该体系在x=0.15处得到最佳的压电性能:压电常数d33的值为110pC/N,机电耦合系数kp的值为18%。该系陶瓷具有弛豫铁电体的特征并在居里温度以上表现出奇特的介电行为。同时,采用离子电导诱导的空间电荷极化理论详细分析了产生这种介电行为的机理。对于(1-x)Bi05Na0.5TiO3-xBi0.5K0.5TiO3陶瓷,XRD分析表明,该体系在x=0.16~0.22为三方-四方准同型相界,该体系在准同型相界略靠四方结构这一边,即x=0.22时得到最佳的压电性能,剩余极化强度Pr、机电耦合系数kp和压电系数d33都达到极大值,分别为47μC/cm2、0.33和174pC/N,是目前该体系得到的最好性能。采用La2O3对BNKT18陶瓷进行掺杂改性研究,在掺杂量为0.5wt%得到最佳的性能:kp=0.32,d33=157pC/N,εT33/ε0=963,Qm=139,tanδ=0.039。在La掺杂的基础上,进一步用Mn掺杂来改性,提高陶瓷的综合性能,在Mn掺杂量为0.3wt%时,最佳性能为:kp=0.34,d33=158pC/N,εT33/ε0=865,Qm=291,tanδ=0.02。对于(1-x)Bi0.5Na0.5TiO3-xKNbO3陶瓷,XRD测试结果表明,在研究的组成范围内,KNbO3的加入没有改变BNT陶瓷的三方相结构。只是随着KNbO3含量的增加,陶瓷的三方结构的畸变程度降低,在x≥0.08时,陶瓷转变成了对称性较高的假立方结构。性能的测试结果表明,在x=0.05时,得到最佳的压电性能:kp=0.23,d33=125pC/N,εT33/ε0=830。对于(1-x-y)BNT-xBKT-yKN三元系陶瓷,提出了该三元系陶瓷的三元相图。在三元相图中相界附近,得到最佳的压电性能:对于x=0.10,y=0.03的组成,机电耦合系数kp和压电常数d33分别为0.38和138pC/N,kp达到最大值;对于x=0.16,y=0.02的组成,机电耦合系数kp和压电常数d33分别为0.35和197pC/N,d33达到最大值。所研究的三元系压电陶瓷的某些配方其性能达到BNT基无铅压电陶瓷的先进水平。XRD、SEM及EDAX的测试结果表明,对于含钾的BNT基陶瓷,钾离子在烧结过程中起关键作用,钾离子含量的多少决定了烧结温度的高低以及烧结温区的宽窄,而与组成中具体的成分关系不大。一般是低含钾量的样品烧结的温区要宽,高含钾量的样品烧结的温区要窄。为了得到高含钾量的致密陶瓷样品,尽量通过降低烧结温度延长保温时间来实现。通过介电温谱和不同温度下的电滞回线,对BNT基材料的相变过程和介电行为进行了研究,发现研究的BNT陶瓷都具有弛豫铁电体的特征,且基本上确定在低温介电反常以上没有反铁电相产生。低温介电反常产生的原因主要是发生了铁电宏畴-铁电微畴的转变。更多还原

【Abstract】 In this dissertation, sodium bismuth titanate (abbr. BNT), which is a kind of A-site compound perovskite ferroelectrics, is investigated. Aiming at the shortcoming of the BNT and according to the modified method of traditional piezoelectric ceramic, the BNT ceramics was modified systemically to improve the piezoelectric properties and achieve the practical objective. To increase the density and breakdown field strength, Li+ substitution for Na+ in BNT was researched. According to the thought that the excellent properties would be obtained in the morphotropic phase boundary, the multi-component free-lead piezoelectric ceramics which were BNT-BKT, BNT-KN and BNT-BKT-KN, were formed respectively. To further improve the piezoelectric properties, La2O3 and MnO2 were doped to BNKT18 ceramics and the performance of this material was studied.Li substitution for Na in BNT ceramics which affects on the sintering characteristic, structure and performance of the BNT material was systemically studied. The experimental results indicate: high relative density of more than 95% is obtained from proper Li substitute which may improve the sintering process. When x=0.1, the resistivity reaches the maximum value, 1.56×1012Ω.cm. The results of XRD show that there are Li2O and Bi2Ti2O7 phases which result from the instability of perovskite at x=0.3. Li substitutes don’t change the structure symmetry of rhombohedra. When x=0.15, this composition expresses the optimal properties: d33=110pC/N, kp=18%, Pr=21μC/cm2. The diffuse phase and the dielectric dependence on frequency from the dielectric thermogram show that the Bi0.5(Na1-xLix)0.5TiO3 ceramics are the relaxor ferroelectrics. Above Tc, the relation of dielectric and frequency doesn’t accord with the Curie-Weiss law with the increase of the temperature, which arouses from the space charge polarization induced by ions conductance.For (1-x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 system: The patterns of XRD show that the range of x=0.16~0.22 is the morphotropic phase boundary with rhombohedral and tetragonal co-existed structure. When x=0.22, the piezoelectric properties which d33 and kp are 110pC/N and 18% respectively, are the best properties at the present time. La2O3 dopant can enhance the electromechanical coupling factor and piezoelectric constant but decrease the mechanical quality factor. At the La2O3 amount of 0.5wt%, the best properties are: kp=0.32, d33=157pC/N,εT33/ε0=963, Qm=139, tanδ=0.039. Based on 0.5wt% La2O3 doped BNKT18 ceramics, MnO2 dopant can enhance the depolarization temperature but decrease mechanical and dielectric loss. At the MnO2 amount of 0.3 wt%, the best properties are: kp=0.34, d33=158pC/N,εT33/ε0=865, Qm=291, tanδ=0.02. At the same time, the mechanics that the“soft or hard”adulterant influence the depolarization temperature was analyzed.(1-x)Bi0.5Na0.5TiO3-xKNbO3 system was prepared by the tradional ceramics fabrication, in range of the researched composition, XRD results show that KNbO3 don’t change the structure symmetry of rhombohedra and enhanced the structure symmetry to pseudo-cubic structure with the increase of KNbO3 content. At amount of x=0.05, the best properties are: kp=0.23, d33=125pC/N,εT33/ε0=830.For (1-x-y)BNT-xBKT-yKN ternary system, the phase diagram of the BNT-BKT-KN ternary system was put forward. Near the phase boundary of the phase diagram, the optimal piezoelectric properties are: for the composition of x=0.10 and y=0.03, kp and d33 are 38% and 138 pC/N, respectively. For the composition of x=0.16 and y=0.02, kp and d33 are 35% and 197pC/N, respectively. The d33 value reaches the maximum in the ternary system.The results of XRD, SEM and EDAX indicate: the K+ act as key function in the sintering process, the sintering temperature and the width of sintering temperature are decided by K+ content and independent of the component in the BNT based samples. Generally, the width of the sintering temperature is broad when the K+ content is little. For higher K+ content, the result is contrary. The density sample with higher K+ content can be gained by decreasing the sintering temperature and prolonging the sintering time.The results of dielectric thermogram and hysteresis loop at different temperature indicate: the BNT based ceramics are the relaxor ferroelectrics; above Td, there isn’t anti-ferroelectric in the temperature region of ferroelectric and paraelectric; the reason of dielectric abnormity at low temperature is the transition from ferroelectric macro-domain to ferroelectric micro-domain.更多还原