【作者】 王伟超；

【导师】 郑海务；

【作者基本信息】 河南大学 ， 凝聚态物理， 2010， 硕士

【摘要】 由于铋层状钙钛矿结构的铁电材料具有高的剩余极化、低的矫顽场强以及高的居里温度、低的介电损耗等优点,因而在非挥发性铁电随机存储器和高温高频压电器件方面具有广阔的应用空间。在铋层状钙钛矿结构的铁电材料中,Bi4Ti3O12和CaBi4Ti4O15的高居里温度的特性而受材料科学工作者的高度关注。而稀土元素独特的电子层结构,表现出许多优异的光、电功能,尤其是稀土元素具有一般元素所无法比拟的光谱性质,有关稀土发光离子掺杂铋层状铁电材料的研究已引起广泛的关注和兴趣。本文主要研究内容如下:1.采用溶胶-凝胶法在Pt/Ti/SiO2/Si衬底上制得Eu掺杂的CaBi4Ti4O15 :Nd铁电薄膜。分别在不同的退火温度和Eu掺杂浓度下研究了薄膜的光致发光性和铁电性。所制得的CaBi3.75-xEuxNd0.25Ti4O15薄膜都为多晶结构,且有正交相和四方相共存的准同型相界存在。当铕的掺杂浓度不超过x=0.2时,薄膜的晶粒大小及分布均匀,且表面致密光滑。而当铕的掺杂浓度超过x=0.2时,薄膜的表面开始变得粗造。薄膜的光致发光强度明显地依赖于退火温度和Eu的掺杂浓度。当铕离子掺杂浓度分别为0,0.2,0.5时,其薄膜所对应的剩余极化值2Pr分别为13.2,30.7和25.3μC/cm2。2.采用溶胶-凝胶法成功在导电玻璃FTO衬度上制备了具有高度c轴取向的Bi4Ti3O12薄膜,并对其表面光伏响应进行了研究。Au/BiT/FTO结构的剩余极值2Pr和矫顽场分别为98.8μC/cm2和212 kV/cm。在偏置电压从-1V到+1V中,其最大的表面光电压值为1.8 mV,是在零偏置电压下值的三倍。薄膜的表面光电压的强度随着正电场的增加而增强,随着负电场的增强而减弱,甚至其方向发生一个偏转,这表明Bi4Ti3O12薄膜的表面光电压强度明显的依赖铁电薄膜的极化状态。3.采用溶胶-凝胶法成功制备了不同Eu3+离子掺杂浓度的CaBi4Ti4O15 :Nd(CBENT)材料。分别用XRD、拉曼谱和扫描电镜对其结构和表面形貌进行测量。结果表明在CaBi3.75-xEuxNd0.25Ti4O15中,x在0.3和0.6之间,存在正交相和四方相共存的准同型相界。我们用350 nm激发光对Eu3+掺杂CBENT陶瓷时行了光致发光性质的研究,结果表明Bi3+可以作为Eu3+发光离子的敏化剂;其光致发光是由于Bi3+可以有效地把能量传递给Eu3+发光离子和Eu3+离子所处局部对称位置的改变;并且材料在准同型相界处有最好的光致发光性。随着Eu3+离子的增加,陶瓷的居里温度逐渐从785°C减小到765°C。此外,和CBNT陶瓷相比,Eu离子的掺入退化了CBENT陶瓷的铁电性。4.用溶胶-凝胶法制备了Er3+掺杂的Bi4Ti3O12粉末,利用X射线衍射仪和扫描电子显微镜对样品进行表征,发现Eu3+的引入并没有改变基质Bi4Ti3O12的晶体结构,制备得到的样品仍为纯的正交相结构。对掺杂Er3+的Bi4Ti3O12材料实验中观测到了在520、540和662 nm波长的上转换发射,这些发射峰来自于激发态2H11/2、4S3/2、4F9/2到基态4I15/2的电子跃迁。并证实了绿色和红色上转换发射都为双光子过程。Er3+单掺Bi4Ti3O12材料体系的上转换发光表明铋层状铁电材料也能够提高Er3+的上转换发光强度。且亦有较好的铁电性。更多还原

【Abstract】 Bismuth layer-structured ferroelectric (BLSF) ceramics are potential candidate lead-free materials in nonvolatile random access memories, especially at high temperatures and high frequencies device application due to their high remnant polarization, low coercive electric field and low dielectric constant, high Curie temperature etc. In Bi-layer structure ferroelectric material, Bi4Ti3O12 and CaBi4Ti4O15 have been widely investigated in view of the characteristic of high Curie temperature. Otherwise, rare-earth doped powders display many excellent properties based on photoluminescence (PL) and electrical properties due to the unique electron structure of rare-earth, especially photoluminescence properties. In this thesis, the main results are listed as follows.Chapter 2 reports ferroelectric and Eu-doped CaBi4Ti4O15: Nd thin films prepared on Pt/Ti/SiO2/Si substrates by a sol-gel method. All the CBENT films have a polycrystalline bismuth-layered perovskite structure, and a morphotropic phase boundary between orthorhombic phase and tetragonal phase was shown to exist. The thin films are dense and smooth with uniformly distributed grains when x is less than 0.2. When x is larger 0.2, a roughened surface with some pores was obtained. The PL intensity of the CBENT films are significantly dependent on annealing temperature and Eu concentration. The remnant polarization 2Pr values of the Eu-doped CaBi4Ti4O15: Nd thin films with Eu concentrations of x=0, 0.2, 0.5 were approximately 13.2, 30.7 and 25.3μC/cm2, respectively.In chapter 3, the surface photovoltage (SPV) responses was investigated in sol-gol derived Bi4Ti3O12(BiT) thin films of highly c-axis orientation. The remnant polarization (2Pr) and coercive electric field (Ec) of the Au/BiT/FTO capacitor were about 98.8μC/cm2 and 212 kV/cm, respectively. The maximal surface photovoltage value 1.8 mV of the device of FTO/BiT/ITO with dc bias voltage (from -1V to +1V) was observed at room temperature, and it is three times larger than that under the zero bias. The SPV intensity increased with the positive field increasing, and became weaker with a reverse response with the negative field increasing. It is suggested that the SPV with respect to the unpoled and poled films are largely depend on the ferroelectric polarization.Chapter 4 mainly introduces photoluminescence and dielectric properties of CaBi3.75-xEuxNd0.25Ti4O15 ferroelectric ceramics. Their structures and surface morphology were examined by X-ray diffraction, Raman spectrum and scanning electron microscopy, respectively. The morphotropic phase boundary (MPB) between orthorhombic and tetragonal was found located in the range of 0.3<x<0.6 for the ceramics. The photoluminescence properties of Eu3+ ions in these host materials were investigated upon excitation at 350 nm, indicating that Bi3+ ions could largely sensitize the emission of Eu3+ ions, which was attributed to the efficient energy transfer from Bi3+ to Eu3+ ions and the modification of the local symmetry of Eu3+ ions. The materials near MPB show a strong compositional dependence with the optimal properties of photoluminescence. Temperature-dependent dielectric spectrum shows the Curie temperature TC gradually increases from 785 to 765°C with increasing the (Nd, Eu) modification. In addition, Eu3+ dopant deteriorates the ferroelectric properties of the CaBi3.75-xEuxNd0.25Ti4O15 ceramics in comparison with the CBNT ceramics.Chapter 5 reports the Er-doped Bi4Ti3O12 ferroelectric powders prepared by a sol-gel method. Their structures and surface morphology were examined by X-ray diffraction, Raman spectrum, and scanning electron microscopy, respectively. The results suggest that Er doping have no change the crystal structure of Bi4Ti3O12 of orthorhombic phase. It can be observed that upconversion emission bands with the peak centred at 520 nm, 540 nm and 662 nm are due to the electron transitions of 2H11/2→4I15/2、4S3/2→4I15/2 and 4F9/2→4I15/2 of the Er-doped Bi4Ti3O12, respectively. Moreover, the upconversion green and red luminescence of the Er-doped Bi4Ti3O12 powder is the process of two photon absorption. The upconversion luminescence of the Er-doped Bi4Ti3O12 suggest that Bi-layer structure ferroelectric material can improve the intensity of upconversion luminescence of the Er3+ ions, which have a promising value of for applications to multifunctional ferroelectric materials.更多还原