多铁材料Bi1-xCaxFeO3的介电、铁磁特性和高温磁相变

采用溶胶凝胶法制备Bi_1-xCa_xFeO₃ (x=0, 0.05, 0.1, 0.15, 0.2)陶瓷样品. X衍射图谱表明所有样品的主衍射峰均与纯相BiFeO₃相符合且具有良好的晶体结构. 随着x的增大, Bi_1-xCa_xFeO₃样品的主衍射峰由双峰(104)与(110) 逐渐重叠为单峰(110), 当x ≥0.15时, 样品呈现正方晶系结构; 扫描电镜形貌分析可知, 晶粒由原来的0.5 μm逐渐增大到2 μm. Bi_1-xCa_xFeO₃样品介电常数和介电损耗随着x 的增加先增大而后减小. 当f=1 kHz, Bi_0.9Ca_0.1FeO₃ 的介电常数达到最大值, 是BiFeO₃的7.5倍, 而Bi_0.8Ca_0.2FeO₃的介电常数达到最小值, 仅仅是BiFeO₃的十分之一. Bi_1-xCa_xFeO₃样品所呈现的介电特性是由偶极子取向极化和空间电荷限制电流两种极化机理共同作用的结果. 随着Ca²⁺ 的引入, BiFeO₃ 样品的铁磁性显著提高. X射线光电子能谱图表明Fe²⁺和Fe³⁺ 共存于Bi_1-xCa_xFeO₃ 样品中, Fe²⁺/Fe³⁺比例随着Ca²⁺ 掺杂量的增加而增大, 证明Ca²⁺掺杂增加了Fe²⁺的含量, 增强BiFeO₃的铁磁特性. 从M-T曲线观察到BiFeO₃样品在878 K附近发生铁磁相变, 示差扫描量热法测试再次证明BiFeO₃ 在878 K发生相变. Ca²⁺掺杂使BiFeO₃样品的T_N略有变化而T_M基本不变, 其主要原因是Fe-O-Fe反铁磁超交换作用的强弱和磁结构相对稳定.

Dielectric properties and high temperature magnetic behavior on multiferroics Bi1-xCaxFeO3 ceramics

Multiferroic Bi_1-xCa_xFeO₃ (x=0, 0.05, 0.1, 0.15, 0.2) ceramics are prepared by sol-gel method. The effects of Ca doping on the structure, delectrical, ferromagnetism properties and high temperature magnetic behavior of BiFeO₃ ceramics are studied. The structures of BiFeO₃ ceramics are characterized by X-ray diffraction (XRD). The results show that all the peaks for Bi_1-xFe_xO₃ samples can be indexed according to the crystal structure of pure BiFeO₃. The characteristic diffraction peaks of Bi_1-xCa_xFeO₃ samples become gradually wider and the (104) and (110) peaks of BiFeO₃ merge partially into a broadened peak (110) with Ca²⁺ doping increasing. XRD analysis reveals a phase transition in Ca-doped BiFeO₃ from rhombohedral to orthorhombic when x is larger than 0.15. The scan electron microscope images indicate that Ca²⁺ doping significantly increases the grain sizes of BiFeO₃ ceramic. The average grain sizes of Bi_1-xCa_xFeO₃ samples range from 0.5 to 2 μm.#br#The dielectric behaviors of Bi_1-xCa_xFeO₃ ceramics change with content x and frequency. The dielectric constant measured at 1 kHz reaches a maximum value of ε_r=4603.9 when x=0.1, seven times as big as that of pure BiFeO₃. With further increasing the Ca content (x=0.15, 0.2), the value of the dielectric constant is back to the level of pure BiFeO₃. The dielectric constant of Bi_0.8Ca_0.2FeO₃ (ε_r=57) is less than one-tenth that of BiFeO₃ (ε_r=629.9). The dielectric losses of Bi_1-xCa_xFeO₃ samples become smaller than that of BiFeO₃ ceramic. This dramatic change in the dielectric properties of Bi_1-xCa_xFeO₃ samples can be understood in terms of orientational relaxation of dipoles and the space charge limited conduction associated with crystal defects at low frequency.#br#The magnetic measurements show that all samples possess strong ferromagnetism at room temperature expect BiFeO₃ which is weakly ferromagnetic. The X-ray photoelectron spectroscopy spectrum indicates the coexistence of Fe²⁺ and Fe³⁺ in Bi_1-xCa_xFeO₃ samples. The ratios of Fe²⁺/Fe³⁺ are 21/79, 23/77, 27/73, 32/68, and 32/68, respectively. The ratio of Fe²⁺/Fe³⁺ increases with doping Ca content increasing, and the magnetic preparation of BiFeO₃ is enhanced.#br#It is evidenced that the ferromagnetic phase transitions of Bi_1-xCa_xFeO₃samples occur at 878 K from M-T curve and the phase transition of BiFeO₃ happens at 878 K by DSC measurement. The change in T_N of Bi_1-xCa_xFeO₃ depends mainly on the Fe-O-Fe super-exchange strength and magnetic structure of relative stability.