Effects Of Oxygen Content On Giant Dielectric Responses In Charge-ordering Nicklate Ceramics And Their Modification

The charge-ordered nicklates powders were synthesized by a sol-gel process, and Nd2NiO4+8and Sm1.5Sr0.5NiO4.δ ceramics with different oxygen contents were obtained by sintering them in air and post-annealing in the flow of nitrogen. In this paper, we discussed the effects of excess oxygen or oxygen vacancies to the giant dielectric responses in Nd2NiO4+δand Sm1.5Sr0.5NiO4-δ ceramics, respectively. Also, the La1.5Sr0.5NiO4-SiO2composite powders were prepared to improve the dielectric properties of the nickelate ceramics. The following results were obtained in the thesis:1. The content of excess oxygen decreases after post-annealing the Nd2NiO4+δ ceramics in the flow of nitrogen. Rietveld analysis shows the part of main phase changes from orthorhombic to tetragonal phase after post-annealing the as-sintered ceramics in the flow of nitrogen. The temperature-stable giant dielectric responses are observed in these two ceramics, and a low temperature dielectric relaxation is determined in the annealed ceramics. By comparing the activation energies of low temperature dielectric relaxation and electric conductivity of grain interior, the giant dielectric response should be primarily attributed to the small polaronic hopping process. From the present work, the correlation between giant dielectric response and the small polaronic hopping process is confirmed in the oxygen-excess nickelate ceramics.2. The concentration of oxygen vacancies in Sm1.5Sr0.5NiO4-δ ceramics increases after post-annealing in a flow of nitrogen. The dielectric constant decreases at low temperature, while increases at high temperature after post densification annealing in a flow of nitrogen, while the dielectric loss increases in the overall temperature range. The activation energy for the low temperature dielectric relaxation decreases with increasing the concentration of oxygen vacancy. The bulk electrical conductivities should be related to the holes located around Ni3+ions, while that of the grain boundary is linked to electrons ionized from oxygen vacancies. The giant dielectric response in the as-sintered Sm1.5Sr0.5NiO4-δceramics should be mainly attributed to the small polaronic hopping process, while that of the annealed ceramics should be directly linked to the oxygen vacancies. Though the oxygen vacancies may contribute to the giant dielectric response in the present ceramics, they are harmful to the temperature stability of dielectric constant, especially the negative dielectric constant and loss at high temperature may induced by excessive oxygen vacancies. 3. The La1.5Sr0.5NiO4-SiO2particles were successfully prepared by a sol-gel process. Compared with La1.5Sr0.5NiO4ceramics, the dielectric constant of La1.5Sr0.5Ni04-SiO2composite ceramics slightly decreases, while the dielectric loss significantly decreases in La1.5Sr0.5NiO4-SiO2composite ceramics. The best dielectric response, i.e. the moderate dielectric constant with the lowest dielectric loss, is obtained in the ceramics with Ni:Si=1.25:1. The ac conductivity of La1.5Sr0.5Ni04-SiO2composite ceramics decreases more than two orders of magnitude comparing with that of La1.5Sr0.5Ni04ceramics. Based on the impedance analysis, the activation energies of both the bulK and grain boundary increase in La1.5Sr0.5NiO4-SiO2composite ceramics, especially the activation energy of the grain boundary increases a lot.