Study On Dielectric, Ferroelectric Properties Of Filled Tungsten Bronze Ceramics And Magnetization Of Ca₃Co₂O₆

As an important kind of functional materials, the applicable value of ferroelectric material especially the relaxor ferroelectrics have been well indicated. But due to some disadvantages of the applied ones, they still can not meet the applied requirement. For relaxor ferroelectrics, most of the applied ones are lead based single crystal or ceramics. Due to the toxicity and volatility of lead element, it brings great harm to environment and human health.The same condition, magnetic materials also have been used in many aspects of our modern life, such as used in computer, bank card and identity card. But for some specific magnetic system, such as the frustrated one may have large potential application due to the specific physical properties. However, because of the complex spin interaction the micro-mechanism is far from understood, it thus becomes one of the important topics of condensed matter physics.In these thesis we focused on Sr-based filled tetragonal tungsten bronze ceramics and low dimensional spin frustrated Ca3Co2O6; we did the following work:1.Sr4CaSmTi3Nb703o ceramics were prepared by solid state reaction method and the phase, dielectric and ferroelectric properties were investigated. Our experimental results confirmed that the ion valence of A1site and the ion distribution of B site affect the low temperature dielectric relaxor nature greatly but they affect the ferroelectric transition temperature weakly.2. Sr4CaBiTi3Nb703o were prepared by solid state reaction method and the structure, micrograph, dielectric and ferroelectric properties were investigated. The measuring results confirmed the strong relaxor property of Sr4CaBiTi3Nb7O30ceramics above room temperature, in addition the ceramics possessed some other superior properties:(1) high breakdown voltage;(2) low leakage current;(3) good ferroelectric properties. Thus, our experimental results suggest that Sr4CaBiTi3Nb7O30ceramics have large potential application value and can be considered as one kind of environmental friendly materials for replacing of lead based relaxor ferroelectric materials. For the phenomenon of the dielectric relaxation split into two relaxation regions, we also investigated it detailedly by variable temperature Raman spectrum technology and variable temperature ferroelectric technology. Our result confirmed that flipping and breathing of polar nanoregions are the origin of high temperature dielectric relaxation I and low temperature dielectric relaxation Ⅱ, respectively.3. Sr4BiSmTi4Nb6O3o ceramics were prepared by solid state reaction method and the structure, dielectric and ferroelectric properties were investigated. By comparing the phase transition characters and fitting parameters of Sr4CaBiTi3Nb7O30and Sr4BiSmTi4Nb6O30ceramics, our experimental results indicated that the influence of Al site ion valence and the distribution of B site ion on the ferroelectric properties of filled tetragonal tungsten bronze ceramics is weak. The leakage current-electric field curve and ferroelectric hysteresis loop confirmed the ferroelectric property of Sr4BiSmTi4Nb6O30ceramics.4. Sr4+xCa1-xBiTi3Nb7O30(x=0.25,0.5,0.75,1) ceramics were prepared by solid state reaction method and the structure, dielectric and ferroelectric properties were investigated. The results confirmed the relaxor ferroelectric nature of all the four samples. As the content of Sr2+ions increases in A1site, the transition temperature of Sr4+xCa1-xBiTi3Nb703o decrease. In addition the relationship between the permittivity maximum temperature Tm and Al site Sr2+ions content x is almost linear. Therefore, our experimental results suggest that according to the needed applied temperature, the transition temperature of relaxor Sr4+xCa1-xBiTi3Nb703o ceramics can be controlled successfully by changing the value of Al site Sr2+ions content, it thus has large guiding value for practical applications.5. Ca3Co2O6ceramics were prepared by solid state reaction method. In zero field cooling magnetization-temperature curves, as the applied magnetic field increases, the spin freezing peak around Tf=10K split into two and both of them move toward low temperature. This phenomenon was observed for the first time and it suggested that existence of two kinds of short-range ferromagnetic orderings with different characteristic magnetic correlation lengths in the frustrated spin chains. Based on this, a possible mechanism was proposed to explain the magnetization steps and the change in sign of relaxation rate in magnetization versus magnetic field curves.6. Ca3Co2-xHoxO6(x=0,0.05,0.1,0.15,0.2) ceramics were prepared by solid state reaction method and the magnetic variations with increasing Ho3+ion content were investigated. As Ho3+doping were introduced:(1) the magnetization steps become less well defined;(2) the spin freezing peak broadens;(3) antiferromagnetic interaction dominates over the ferromagnetic interaction;(4) the dynamics becomes slow;(5) the maximum of ac susceptibility move to low temperature. For these magnetic variations caused by introducing of Ho3+doping we have discussed detailedly and it would be helpful for understanding the complex magnetic interaction in Ca3Co2O6.