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采用溶胶凝胶法制备Bi1-xCaxFeO3 (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, Bi1-xCaxFeO3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. Bi1-xCaxFeO3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, Bi0.9Ca0.1FeO3 的介电常数达到最大值, 是BiFeO3的7.5倍, 而Bi0.8Ca0.2FeO3的介电常数达到最小值, 仅仅是BiFeO3的十分之一. Bi1-xCaxFeO3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Bi1-xCaxFeO3 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TN略有变化而TM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.Multiferroic Bi1-xCaxFeO3 (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 Bi1-xFexO3 samples can be indexed according to the crystal structure of pure BiFeO3. The characteristic diffraction peaks of Bi1-xCaxFeO3 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 Bi1-xCaxFeO3 samples range from 0.5 to 2 μm.#br#The dielectric behaviors of Bi1-xCaxFeO3 ceramics change with content x and frequency. The dielectric constant measured at 1 kHz reaches a maximum value of εr=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 Bi0.8Ca0.2FeO3 (εr=57) is less than one-tenth that of BiFeO3 (εr=629.9). The dielectric losses of Bi1-xCaxFeO3 samples become smaller than that of BiFeO3 ceramic. This dramatic change in the dielectric properties of Bi1-xCaxFeO3 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 Bi1-xCaxFeO3 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 Bi1-xCaxFeO3samples occur at 878 K from M-T curve and the phase transition of BiFeO3 happens at 878 K by DSC measurement. The change in TN of Bi1-xCaxFeO3 depends mainly on the Fe-O-Fe super-exchange strength and magnetic structure of relative stability.
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Keywords:
- multiferroic/
- dielectric properties/
- magnetic hysteresis loops/
- magnetic phase transition temperature
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