【作者】 李旭东；

【导师】 陈祖煌；

【作者基本信息】 哈尔滨工业大学 ， 材料与化工（专业学位）， 2024， 硕士

【摘要】 铁电材料因存在自发极化及其在外加电场下极化方向可翻转的特性,被广泛应用于铁电存储器、电卡制冷设备等。钛酸锶铅(PbxSr1-xTiO3)是一种备受瞩目的铁电材料,具有可调的极化强度、介电常数以及热释电响应性能。一百多年来,铁电材料尤其是铁电薄膜的研究主要集中在匀质材料,但近年来,研究者开始探索宏观材料中成分梯度带来的新特性。成分梯度改变了材料的宏观反演对称性,引入了诸如电滞回线在电场方向上的移动、低介电温度依赖性、内建电场及顺电材料中的自极化现象等新特性。虽然有研究者对成分梯度钛酸锶铅薄膜的性质进行了理论计算研究,但到目前为止实验方面的研究还是一片空白。本研究旨在探究成分梯度钛酸锶铅(PbxSr1-xTiO3)薄膜的铁电性能、结构以及成分梯度对其性质产生的影响。通过对PbTiO3的生长条件窗口的探究,研究不同衬底上PbTiO3薄膜的性质,理解应变以及Sr的引入对PbxSr1-xTiO3性质的影响。在样品制备方面,本研究使用一种创新的成分梯度铁电薄膜制备方法,通过控制外延生长条件,在单胞厚度以下通过层状生长方式沉积PbTiO3,并在沉积到一个单胞厚度以下时切换至PbxSr1-xTiO3靶材进行层状沉积,通过调整脉冲激光的数量,精确控制PbxSr1-xTiO3成分。在材料表征方面,本研究通过XRD表征展示了成分梯度PbxSr1-xTiO3铁电薄膜结构的连续变化,并测量了其铁电性能。研究发现,内建电场的存在导致电滞回线在水平方向上发生位移,且成分变化不同的薄膜表现出不同方向的位移。此外,随着Sr含量增加到大约35%,薄膜的居里温度显著下降,铁电-顺电相变引起的介电异常使得薄膜展现出极高的介电常数。所有成分梯度的PbxSr1-xTiO3在外加电场作用下,原本对称的双电流峰中的一个发生劈裂,形成三电流峰现象,这一现象在先前研究中尚未报道。通过pund测试对未劈裂的电流峰进行了偶极子翻转分析,揭示了虽在Ⅰ-Ⅴ曲线中表现为单峰,实际翻转过程却分两步进行。对于该现象产生的原因,提出了猜想:成分梯度PbxSr1-xTiO3中不均匀的矫顽场与不均匀的内建电场共同作用。更多还原

【Abstract】 Ferroelectric materials,characterized by their spontaneous polarization and reversible polarization under an external electric field,are widely used in devices such as ferroelectric memories and electrocaloric cooling devices.Strontium lead titanate(PbxSr1-xTiO3,where 0≤ x≤1)is a notable ferroelectric material with adjustable polarization intensity,dielectric constant,and pyroelectric response performance.Over the past century,research on ferroelectric materials,particularly ferroelectric thin films,has predominantly focused on homogeneous materials.However,recent studies have begun to explore new properties brought about by compositional gradients in macroscopic materials.These gradients alter the material’s macroscopic inversion symmetry and introduce new features such as the movement of ferroelectric hysteresis loops in the direction of the electric field,low dielectric temperature dependence,inherent electric fields,and the phenomenon of self-polarization in paraelectric materials.Although theoretical calculations on the properties of compositional gradient strontium barium titanate films have been conducted,experimental research remains scarce to this date.This study aims to explore the ferroelectric performance,structure,and the impact of compositional gradients on the properties of strontium lead titanate(PbxSr1-xTiO3)films.By investigating the growth condition windows of PbTiO3,this research examines the properties of PbTiO3 films on different substrates and understands the effects of strain and the incorporation of Sr on the properties of PbxSr1-xTiO3.In terms of sample preparation,the study employs an innovative method for fabricating compositional gradient ferroelectric films.By controlling the epitaxial growth conditions and using a layer-by-layer growth method to deposit PbTiO3 at sub-unit cell thickness,the deposition switches to another PbxSr1-xTiO3 target material when the thickness is less than one unit cell.The component of PbxSr1-xTiO3 is precisely controlled by adjusting the number of laser pulses.In terms of material characterization,this study demonstrates the continuous change in the structure of compositional gradient PbxSr1-xTiO3 ferroelectric films through XRD characterization and measures their ferroelectric performance.The study finds that the existence of an internal electric field causes the displacement of ferroelectric hysteresis loops horizontally,and films with different compositional changes show displacements in different directions.Furthermore,as the Sr content increases to about 35%,the Curie temperature of the films significantly drops,and the ferroelectric-paraelectric phase transition causes a dielectric anomaly that results in a very high dielectric constant in the films.All compositional gradients of PbxSr1-xTiO3 under an external electric field cause one of the originally symmetrical dual current peaks to split,forming a unique triple current peak phenomenon,previously unreported in research.Through pund testing of the unsplit current peak,the dipole flipping process was analyzed,revealing that although it appears as a single peak in the I-V curve,the actual flipping process occurs in two steps.A hypothesis is proposed for the cause of this phenomenon:the uneven coercive field and uneven internal electric field within the compositional gradient(PbxSr1-xTiO3)act together.更多还原