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共生掺杂及磁性离子基团植入对铋层状材料电磁性能的影响
【作者】 王伟；
【导师】 陈小兵；
【作者基本信息】 扬州大学 ， 物理化学， 2009， 博士
【摘要】 铋系层状钙钛矿结构的铁电材料在非易失性铁电存储器和高温压电陶瓷方面具有潜在的应用前景。这种材料反转速度快,抗疲劳性能好,具有较高的铁电居里温度(Tc)和不含铅污染的优点。然而,由于自发极化被限制在ab平面内,典型的铋层状材料如B4Ti3O12 (BIT, 2Pr = 15μC/cm2)和SrBi4Ti4O15 (SBTi, 2Pr = 6.2– 13.0μC/cm2)存在着剩余极化小、压电活性低等不足,尚不能取代目前工业上广泛使用的PZT来满足工业化生产高密度存储器和压电传感器的需要。为了改善铋层状材料的性能,人们想出来许多办法来改善它的性能。本论文主要工作是基于铋层状材料的改性研究,包括共生掺杂和磁性离子基团植入两大部分,具体说来可以分为下面几个部分:(1) A位Nd离子掺杂:我们用传统固相反应法制备了系列Nd掺杂BIT-SBTi陶瓷样品,并研究了其相应的铁电、压电、介电行为。借助于XRD和Raman散射,研究了Nd掺杂对其晶体结构和离子占位的影响。研究显示,当Nd离子掺杂量为0.5时,材料的剩余极化和压电系数同步达到最大值,分别为33.2μC/cm2和14 pC/N。同比不掺杂的BIT-SBTi陶瓷,其增幅分别达到1.7倍和75%,进一步增加Nd掺杂量,材料的性能出现下降趋势。借助于材料的复阻抗谱,发现材料的传导激活能随掺杂量的增加而而增加,这可以被看成是材料中氧空位浓度下降的直接证据。介电常数和介电损耗的温度谱显示,随Nd掺杂量的增加,介电峰出现介电弥散的现象,但是在损耗的温度谱中没有能发现明显的介电驰豫峰。和La掺杂相比,Nd掺杂并没有引起所谓的驰豫铁电体的特征,并初步给出了解释。(2) B位高价掺杂:本章研究了Nb、V和W掺杂Bi4Ti3O12-MBi4Ti4O15(BIT-MBTi, M = Ca, Sr)陶瓷材料的电性能。研究结果显示,这样的高价施主掺杂显著增强了BIT-MBTi材料的铁电剩余极化和压电系数。其中W掺杂对BIT-SBTi材料的铁电剩余极化提升最大,而Nb掺杂BIT-SBTi材料的压电性能提升最为明显。Nb掺杂同样也提升了BIT-CBTi的剩余极化和压电系数,但没有对BIT-SBTi的影响来得显著。BIT-CBTi薄膜也表现出了较高的铁电剩余极化值39μC/cm2。电性能的增强是由于材料内部氧空位浓度的降低和动性的减弱所引起。材料内部传导激活能随掺杂量的增加直接给出了氧空位浓度减低的证据。在V掺杂BIT-SBTi的研究中发现了剩余极化的最大值和压电系数的最大值不出现在同一掺杂量上,这被认为是V掺杂对材料内部畴壁的结构和密度的影响要比对氧空位浓度的影响来得更为显著所致。BIT-CBTi的变温介电谱出现了双介电峰,分别位于658°C和728°C。对应于低温的介电峰可能和材料中BIT组元的规整化有关,而高温的介电峰则于材料的铁电顺电相变有关。(3)用湿化学方法制备了含磁性离子基团的铋层状材料Bi5FeTi3O15(BFTO)。用XRD谱和Raman谱分析了其晶体结构。用AFM和FESEM观察了其表面和断面形貌。研究发现,在~570 kV/cm外电场下,材料的铁电剩余极化(2Pr)和矫顽场(Ec)分别达到35.5μC/cm2和171 kV/cm。剩余极化在285 kV/cm外场下经过5.2×109反转后降为66%,BFTO和SBTi材料在Raman谱和铁电响应方面进行了对比分析。漏电流机制的研究发现,在~50 kV/cm到~200 kV/cm的外场范围内主要是Schottky发射机制起主导作用,在高于~200 kV/cm的外场时,漏电流机制可能是多种混合机制共同起作用。制备了基于传统固相烧结工艺制备的Bi5Fe0.5Co0.5Ti3O15(BFCT)陶瓷,XRD和HRTEM显示其层状钙钛矿Aurivillius相已经形成。其在230 kV/cm电场下的电滞回线显示样品的2Pr约为~13μC/cm2,矫顽场约为~140 kV/cm。这可能是局域Fe-O-Co基团的形成从而导致了它们之间的铁磁耦合相互作用形成了BFCT较强的磁性能。这样一种局域铁磁耦合也是材料中出现自旋玻璃态的原因之一。
【Abstract】 The Aurivillius family of bismuth-containing oxides encompasses many promising materials of potential use in non-volatile ferroelectric random-access memory and high-temperature piezoelectric devices due to their well-known fast switching speed, fatigue-free properties, relatively high Curie point (Tc) and lead-free chemical composition. However, as the spontaneous polarization movements are confined to the a(b) plane, the relatively smaller remanent polarization (2Pr) and lower piezoelectric activity of pure Aurivillius members such as B4Ti3O12 (BIT, 2Pr = 15μC/cm2) and SrBi4Ti4O15 (SBTi, 2Pr = 6.2– 13.0μC/cm2) are not so satisfactory when compared with PZT film in high-density integration of memory cell and pressure sensors applications. The main purposes of this dissertation are aimed at the modification of such Aurivillius phases and, briefly speaking, it can be summarized into following three parts:(1) Effect of Nd modification on ferro-, piezo- and dielectric properties of intergrowth Bi4Ti3O12-SrBi4Ti4O15 ceramics were investigated. X-ray diffraction and Raman scattering were used to identify the crystal phase and to distinguish the doping sites. With increasing Nd content up to 0.50, both remanent polarization (2Pr) and piezoelectric coefficient (d33) were found to increase and reach the maximum value of 33.2μC/cm2 and 14 pC/N, respectively, which gained an enlargement over 1.7 times in 2Pr and a desirable 75% increment in d33 value. However, further Nd modification starts to deteriorate the ferro- and piezoelectric behavior. Impedance spectroscopy shows the activation energy of conductivity increased with increasing Nd content, which can be regarded as direct proof of the restraint of oxygen vacancies. The thermal variations of dielectric permittivity and loss tanδwith Nd content show the characteristic of diffuse phase transformation while the convincible defect-related relaxation phenomenon was not found. Interestingly, in contrast to La modification, Nd modification does not induce the relaxor behavior even at very high doping content.(2) Electrical properties of Nb-, V-, and W-doped Bi4Ti3O12-MBi4Ti4O15 (BIT-MBTi, M = Ca, Sr) compounds were investigated. The remanent polarization (2Pr) and piezoelectric coefficient (d33) of BIT-MBTi are greatly increased by such donor doping. W-doped and Nb-doped BIT-SBTi exhibited the greatest enlargement in 2Pr and d33, respectively. Nb doping also increases 2Pr and d33 of BIT-CBTi though not that much as in BIT-SBTi case. BIT-CBTi thin film shows a high 2Pr value of 39μC/cm2. The enhanced properties are thought to stem from the reduced concentration and weakened mobility of oxygen vacancies. Increased activation energy of conduction further confirmed the restraint of oxygen vacancies. The different optimal V-doping content for 2Pr and d33 in BIT-SBTi might be related to the fact that V-doping could affect the structure and density of domain more apparently than the concentration of oxygen vacancies. The thermal variation of dielectric constant of BIT-CBTi shows a distinctive double anomaly at 658°C and 728°C. The first dielectric anomaly is related to the regulation of BIT parts and the second one corresponds to the ferro-paraelectric phase transition.(3) Thin films of Fe-containing Aurivillius phase Bi5FeTi3O15 (BFTO) were prepared using chemical solution deposition method. The structures of the films were analyzed using x-ray diffraction and Raman spectroscopy. The surface topography and crystal microstructure were characterized by AFM and FESEM. The remanent polarization (2Pr) and coercive field (Ec) of BFTO thin films under an electric field of ~570 kV/cm are determined to be 35.5μC/cm2 and 171 kV/cm, respectively. The normalized polarization of BFTO thin films under 285 kV/cm decreased to 66% after being subjected to 5.2×109 read/write cycles. Comparison between BFTO and SrBi4Ti4O15 in Raman spectra and ferroelectric behavior is also presented. The leakage current density measurement reveals the conduction mechanism of BFTO thin films in the intermediate electric field range from ~50 kV/cm to ~200 kV/cm is dominated by Schottky emission. With electric field higher than ~200 kV/cm, the leakage behavior is mingled with different conduction mechanisms.We have also synthesized Bi5Fe0.5Co0.5Ti3O15 ceramics by a multi-calcination-method. The Aurivillius structure with four-octahedra in between two Bi-O layers is confirmed by the XRD pattern and HRTEM image. The BFCT sample exhibits the coexistence of ferroelectric and ferromagnetic orders above room temperature, with its 2Pr and 2Mr at RT of 13μC/cm2 and 7.8 memu/g, respectively. The magnetic behavior below 275°C is spin-glass type and the Curie temperature is determined at ~ 345°C.