Preparation And Physical Properties Of Giant Dielectric And Room-Temperature Multiferroic Materials

Giant dielectric constant materials (GDCMS) and multiferroics are two classes of important materials in the field of functional material. Generally, GDCMS refer those dielectrics which dielectric constants exceed1000. GDCMS (CCTO) have a broad application prospect in the field of microelectronic device and play an important role for device miniaturization and high speed. Among numerous GDCMS, CaCu3Ti4O12has attracted much attention, due to its giant dielectric constant, good temperature stability and nonlinear Ⅰ-Ⅴ behavior. It is necessary to study on the origin of giant dielectric effect and to modify the properties of GDCS. Multiferroics refer those materials which process two or more primary ferroics in the same phase. The so-called primary ferroics refer ferroelectricity, ferromagnetism and ferroelasticity. Multiferroics can control magnetization by electric field and control polarization by magnetic field, and provide possibility for designing multiple-state storages. It processes huge application prospect in the fields of multiple-state storages, filter, magnetoelectric (or electromagnetic) sensors. Most of the singe-phased Multiferroics discovered recently are below room temperature, limiting the practical use of them. Therefore, study on room temperature multiferroic materials is the hotspot in academia at present. This dissertation carried out a series of studies on the GDCMS CCTO and room temperature multiferroic Bi2Fe4O9ceramics. The main contents are as follows:(1) A series of CaCu3Ti4O12(CCTO) ceramics are prepared by citric acid sol-gel method. Effect of sintering temperature and durations on the phase, microstructure, dielectric properties and nonlinear Ⅰ-Ⅴ behavior is investigated in detail. Systematical characterizations by both X-ray diffraction and Raman spectra identify component phases of the sintered products, verifying the formation of single phased CCTO for the sample sintered at1020℃for5h or1080℃for2h. Through the dielectric spectrum at room temperature and high temperature regions, we found that CCTO is composed of semiconducting grains and insulating grain boundaries. On the basis of the internal barrier layer capacitor (IBLC) model, an interpretation of giant dielectric behavior is presented. Meanwhile, all the samples show nonlinear Ⅰ-Ⅴ effect.(2) A series of Bi1/2Na1/2Cu3Ti4O12(BNCTO) ceramics are prepared by citric acid sol-gel method. Through the results of different sintering temperatures on the phase and dielectric effect, we found that the sample sintered at1000℃is single-phase and behaves the best dielectric properties. A series of BNCTO samples with different grain sizes are synthesized by sintering at1000℃for2h,5h,10h and20h. The results show that all the samples behave giant dielectric effect and non-linear Ⅰ-Ⅴ behavior. The dielectric constant increases with the increasing of grain sizes, it increases monotonically from14110(for1.4μm sample) to36183(for4.3μm sample) at1kHz, in accompaniment with the breakdown voltage reducing from112.5to43.2V/mm and the nonlinear coefficient reducing from4.9to3.4. The dielectric constant remains almost below the frequency range off<105Hz. However, it decreases dramatically near1MHz, which shows Debye-like relaxation behavior. Dielectric spectrum at high temperature region shows a thermal activated relaxation process. We explained the giant dielectric behavior by IBLC model. Based on IBLC model of Schottky-type potential barrier and brick model, an interpretation of the nonlinear electrical behaviors is presented.(3) A series of Bi1/2-xNa1/2-xBa2xCu3Ti4O12(x=0,0.025,0.05and0.075) ceramics are prepared by citric acid sol-gel method and effect of Ba doping on the phase, microstructure, giant dielectric behavior and nonlinear Ⅰ-Ⅴ properties are investigated. The results show that the sample is single phase for x=0.025, further increasing Ba content, the second phase appears. Since Ba doping suppresses the growth of grain, the grain size decreases with the increase of Ba content. All the samples show giant dielectric effect and non-linear Ⅰ-Ⅴ behavior. Compared with BNCTO ceramics, the dielectric constant decreases and the breakdown voltage drops down by Ba doping. The dielectric constant changes from20,004to7,006at1kHz. The breakdown voltage, nonlinear coefficient and the electrostatic barrier height decrease to10.8V/mm,1.5and0.66eV. Giant dielectric response and nonlinear electrical behavior can be interpreted in terms of IBLC model. These results imply that BNCTO is a promising candidate for low voltage varistor materials.Meanwhile, a series of Bi1/2-xNa1/2-xLa2xCu3Ti4O12(x=0,0.025,0.05and0.075) ceramics are prepared by citric acid sol-gel method and effect of La doping on the phase, microstructure and giant dielectric behavior are investigated. The results show that all the samples are single phase. With the increase of doping content, the grain size decrease gradually. The dielectric constant decreases quickly by La doping. Impedance spectroscopy results show that both the grain and grain boundary resistances increase as the grain size increases, which lead to the collapse of IBLC microstructure.(4) Phase pure Bi2Fe4O9ceramics are successfully synthetised by solid state reaction method. The samples sintered at850℃for2h show weak ferromagnetism and ferroelectric properties at room temperature, with Neel temperature TN=263K. The results show that relatively low presintering temperature and sintering with rapid annealing can prepare single-phase Bi2Fe4O9ceramics with multiferroic properties at room temperature. A series of Bi2Fe4O9ceramics with different grain size (60-2000run) are prepared by citric acid sol-gel method and effect of grain size on the magnetic and ferroelectric properties is investigated. The results show that both the magnetic and ferroelectric properties are strongly dependent on the grain size. XRD results show that all the samples are single phase, indicating that phase pure sample can be synthesized by this method.60nm sample shows weak ferromagnetic behavior at room temperature, with the increase of grain size, ferromagnetic properties weaken gradually and antiferromagnetic properties become dominant. Meanwhile, the Neel temperature TN increases. Except for60nm sample, ferroelectric hysteresis loops are observed for other samples at room temperature.900nm sample shows the best ferroelectric properties, due to the lowest leakage current density.(5) A series of Ti doped Bi2Fe4O9ceramics are prepared by citric acid sol-gel method and effect of Ti doping on the microstructure, magnetic and ferroelectric properties is investigated. XRD results show that all the samples are phase pure, and no other second phases are observed. Compared with Bi2Fe4O9compound, Ti doped samples show weak ferromagnetism at room temperature, which is related to the collapse of the frustrated antiferromagnetic spin structure. Meanwhile, appropriate Ti doping can reduce electric leakage and lead to the enhancement of electrical polarization. Among all the samples, x=0.15sample shows the best multiferroic properties, with remanent magnetization and remanent polarization equal to0.0188emu/g and0.262μC/cm2, respectively.Then, effect of Cr doping on the microstructure, ferromagnetic and ferroelectric properties of Bi2Fe4O9ceramics is investigated. The results show that all the samples exhibit ferromagnetic and ferroelectric properties at room temperature. With the increasing of Cr content, both the ferromagnetic and ferroelectric properties are strengthened. The collapse of spin frustration structure and Fe-O-Cr ferromagnetic interactions by Cr doping contributes to weak ferromagnetic properties at room temperature. The drop down of leakage current and lattice distortion leads to the enhancement of ferroelectric properties. For x=0.08sample, the remanent magnetization and remanent polarization are0.025emu/g and0.53μC/cm2, respectively.