【作者】 彭亮；

【导师】 胡志高；

【作者基本信息】 华东师范大学 ， 微电子学与固体电子学， 2016， 硕士

【摘要】 近年来,非填满型钨青铜结构铁电材料铌酸锶钡(SrxBa1-xNb2O6, SBN)作为一种无铅型材料,由于其优良的铁电性能,电光性能和热释电性能,以及其环境友好性等,是当前铁电材料研究领域的热点之一。目前,无铅型铁电材料的研究主要集中在三大领域：一、含铋层状结构的铁电体,如钛酸铋Bi4Ti3O12,铌酸锶铋SrBi2Nb209(SNBO)等。二、钙钛矿型钛酸盐系铁电材料,如钛酸钡BaTiO3(BT),钛酸锶SrTiO3(ST),钛酸锶钡(BaxSr1-x)TiO3(BST)等。三、钨青铜型的铌酸盐系铁电体,如铌酸锶钡SrxBa1-xNb2O6(SBN)等。其中SBN及其掺杂铁电材料具有较高电阻率、良好的抗疲劳特性和高介电常数等特点,在电光探测器、热释电探测器、铁电存储领域有很大的应用前景。非填满型钨青铜结构铁电体由于其非填满特性,不同组分的Sr、Ba以及掺杂能对其相变温度起到调控作用,降低其相变温度以改善相关器件的应用范围是当前科研工作者所致力于的研究目标之一。然而目前已有工作大多是基于微观结构XRD表征或者是以电学为主的宏观特性的规律探究等手段进行研究,对于其光谱学的研究还比较匮乏。通过光学手段,获得材料的介电函数等信息,在此基础之上,通过建立模型拟合得到材料的电子跃迁信息,并结合红外、拉曼声子振动,研究材料的相变机制具有十分重要的意义。因此,本文通过变温椭圆偏振光谱、拉曼声子散射等光学手段系统研究了铌酸锶钡及其掺杂铁电材料的相变过程,通过结合电子跃迁能量和声子振动模式随温度的变化,从光学角度探究了其相变温度的改善和相变过程中微结构的变化。一、 通过变温拉曼散射实验在150-750 K范围内研究铌酸锶钡及钙掺杂铌酸锶钡铁电陶瓷铁电相到顺电相的相变过程。通过对640cmq处声子模式的频率和半高宽进行拟和发现其斜率在居里温度点附近发生变化。随着Sr组分的增加,其居里温度从556 K下降到359 K,原因是离子半径较小的Sr占据了离子半径较大的Ba离子的位置。而随着离子半径更小的Ca离子的掺入其居里温度却维持在350 K左右不再降低,原因是Ca离子进入到氧离子的位置从而对铌氧八面体的振动特性没有明显影响。由此我们得到SBN的居里温度与Sr组分之间的关系：Tc(x)860-1000x。二、 通过变温椭圆偏振光谱实验在200-600 K范围内研究不同组分铌酸锶钡及掺钙铌酸锶钡铁电陶瓷的复介电函数,根据Tauc-Lorentz模型可以得到一个典型的带间跃迁能量,跃迁能量在5eV附近,并且随着温度的增加而降低。通过对跃迁能量随温度的变化规律的研究,发现在200-600 K范围内存在两个异常点,这个区间的温度是铌酸锶钡的弛豫相变过程。并且,由椭圆偏振光谱和拉曼散射光谱得到的相变温度基本一致,由此说明拉曼散射和椭圆偏振都是表征铁电材料相变温度的有效手段。更多还原

【Abstract】 In recent years, unfilled tungsten bronze lead-free ferroelectric material SrxBa1-xNb2O6(SBN) has attracted extensive research due to its excellent ferroelectric, electric-optics, and pyroelectricity properties. Currently, the research of lead-free ferroelectric materials has been focused on three areas:1) Bi-layer structure based ferroelectrics, such as Bi4Ti3O12, SrBi2Nb2O9(SNBO), etc.2) perovskite type titanate-based ferroelectrics, such as BaTiO3(BT), SrTiO3(ST), and (BaxSr1-x)TiO(BST), etc.3) tungsten bronze type niobate-based ferroelectrics, such as LiNbO3, Sr,Ba1-xNb2O6(SBN), etc. Among them, SBN has a great application prospect in electro-optic detector, pyroelectric detector, and ferroelectric nonvolatile memory because of its high resistivity, Good fatigue resistance characteristic, and high dielectric constant. The special frame of unfilled tungsten bronze ferroelectric make it possible to adjust its phase transition temperature with different Sr/Ba ratio and doping other metallic element. To reduce the phase transition temperature and promote the performance of relevant device have become one of the research objectives that researchers are devoted to. However, most of the existing work have adopted microstructure based XRD diffraction or electrical method to invest the macro characteristics. We lack investigations of its optical properties a lot. We can use optical methods to obtain materials’dielectric functions and fit the results to acquire the electron transition information. It would be of great significance to invest materials’mechanism of phase transition combined with infrared and Raman phonon vibration information. Thus, we investigate the lattice vibration and electronic properties of Cay(SrxBa-x)1-yNb2O6 ceramics with different Sr/Ba ratios and different Ca dopants by temperature-dependent Raman spectroscopy and spectroscopic ellipsometry experiment in this article. The phase transition from ferroelectric to paraelectric can be detected from the main mode variation with temperature. Moreover, the relationship between the energy band gap and phase transition of SBN ceramics is discussed in detail.(1) The phase transition of SBN and Ca doped SBN from ferroelectric to paraelectric has been investigated through temperature dependent Raman scattering in the temperature range of 150-750 K. The frequency of phonon mode locates at about cm" move to low frequency, full width at half maximum (FWHM) increase and vibration intensity die down with increasing the temperature. The temperature coefficient of the frequency and FWHM of cm" Raman mode modifies in the vicinity of Curie temperature after fitting procedure. The Curie temperature of SBN ceramics decreases from 556 K to 359 K with increasing Sr content. The phenomenon can be attributed to the A site substitution that originated in the smaller ionic radius of Sr, as compared to the Ba element. The difference in the Ba-O and Sr-O bond distance and the presence of vacancies can lead to a large distortion of the octahedron. As for CSBN ceramics, however, the Tc changes slightly and almost remains unchanged at about K, which is consistent with the results from the electrical method. This could be due to the reason that Ca+ ionic move into C site which is formally occupied by O atoms and contribute little to displacement along z axis. While the displacement along z axis is proportional to the Curie temperature. The relationship between Sr content and Curie temperature can be interpreted by:Tc(x)= 860-1000x.(2) The complex dielectric functions of SBN and Ca doped SBN ceramics have been investigated with temperature dependent spectroscopic ellipsometry (SE) in the temperature range of 200-600 K. We use Tauc-Lorentz model to fit it and find a typical interband transition at about 5 eV, and it decreases with increasing temperature. After investigation of the relationship between transition energy and temperature, we find two abnormal points between 200-600 K. This temperature region can be interpreted as the diffuse phase transition of SBN. Moreover, good agreement between both kinds of measurements is obtained, corroborating the validity of spectroscopic ellipsometry for measuring the Curie temperature of SBN ceramics. These results indicate that SE could be a possible method to identify the phase transition of relaxor ferroelectric materials.更多还原