Structure And Properties Of Filled Tungsten Bronze Niobate Ceramics

In the present thesis, the structures, dielectric characteristics and ferroelectric transition were systematically investigated for the filled tungsten bronze niobate ceramics, the effects of A-site cations and introduction of magnetic cations on the structure and characteristics were especially emphasized, and the variation tendency and structure origins of the ferroelectric transition and the dielectric relaxations were discussed.In the Sr4(La1-xSmx)2Ti4Nb603o (x=0.25,0.5,0.75) system, all compounds formed the tetragonal tungsten bronze structure in space group P4bm. The stability of the filled tungsten bronze structure decreased with increasing Sm content which could be confirmed by calculating the tolerance factor and electro-negativity difference of the system. Three low-temperature dielectric relaxations were observed for the present ceramics. With increasing x, the radius difference between the Al and the A2 site cations (△R=RA2-RA1) increased, and the dielectric nature of Sr4(La1_xSmx)2Ti4Nb6O30 ceramics varied from the relaxor ferroelectric towards the diffuse and finally the normal ferroelectric. The Tm-f data of the relaxor ferroelectric transition well obeyed the Vogel-Fulcher relationship, which suggested that the relaxor behavior was analogous to a spin glass with polarization fluctuations above a static freezing temperature. The relaxation caused by the B-site disordering had larger active energy Ea, while the micro polar region originated from the incommensurate tilting of oxygen octahedral has smaller Ea.The tetragonal tungsten bronze structure in space group P4bm was determined for Ba5NdFe1.5Nb8.5O30 ceramics. Two obvious dielectric relaxations were observed in the temperature range from 163 to 623 K, and there was a dielectric constant plateau with weak temperature dependence between them. A Debye-type dielectric relaxation in temperature range from 163 to 410 K was identified to originate from the combined effect of the reorientation of the off-center Fe/Nb ions coupling with charge carrier hopping. The high temperature (550-623 K) relaxation could be attributed to the Maxwell-Wagner effects. Moreover, another weak dielectric relaxation was determined below 200 K. Both the non-central symmetric space group and the P-E hysteresis loop confirmed the ferroelectric nature of Ba5NdFe1.5Nb8.5O30 with Pr= 1.0μC/cm2. The linear magnetic hysteresis curve was observed in Ba5NdFe1.5Nb8.5O30 ceramics at 10 K, indicating the material was paramagnetic.The filled tungsten bronze structure in space group P4bm was confirmed for Ba5SmFe1.5Nb8.5O30 ceramics. Two obvious dielectric relaxations were observed in the temperature range from 174 to 623 K, and there was a dielectric constant plateau with weak temperature dependence between them. A Debye-type dielectric relaxation in temperature range from 174 to 450 K was identified to originate from the combined effect of the reorientation of the off-center Fe/Nb ions coupling with charge carrier hopping. The high temperature (550-623 K) relaxation could be attributed to the Maxwell-Wagner effects. Both the non-central symmetric space group and the P-E hysteresis loop confirmed the ferroelectric nature of Ba5SmFe1.5Nb8.5O30 with Pr= 2.0μC/cm. The linear magnetic hysteresis curve was observed in Ba5SmFe1.5Nb8.5O30 ceramics at 10 K, indicating the material is paramagnetic.The temperature stable high dielectric constant and ferroelectrictiy at room temperature made Ba5NdFe1.5Nb8.5O30 and Ba5SmFe1.5Nb8.5O30 very promising in the potential applications in variety of electronic devices such as the temperature-compensated capacitors.