Giant Dielectric Responses In Sm_1.5Sr_0.5NiO₄-Based Ceramics And Their Modification

In recent years, materials with giant dielectric constants have been concentrated by more and more researchers, because of their stable high permittivity and unique physical properties. Sm1.5Sro.sNio4 ceramics with K2NiF4 structure have a stable giant dielectric constant in a wide range of frequency and temperature combining with the merits of easy preparation, low dielectric losses and high dielectric constant up to high frequency, and these make their promising application for new type and miniaturization of microelectronic devices. In this thesis, the effects of the composition, crystal structure, microstructures in Sm1.5Sr0.5NiO4 based ceramics upon the dielectric properties were investigated systematically, and the origin of giant dielectric response were deeply discussed. The modification of the dielectric, magnetic properties were discovered in Sm1.5Sr0.5NiO4 based ceramics.At room temperature, a single orthorhombic phase was obtained in Sm1.5Sro.5Nio.7AIo.3O4 ceramics characterized by X-ray diffraction, while minor impurities existed in Sm1.5Sro.5Nio.5Alo.5O4 ceramics. Temperature stable giant dielectric response was observed in these two ceramics, and only one dielectric relaxation was found. After comparing the activation energies of dielectric relaxations and electrical conductions, the giant dielectric response should be attributed to the adiabatic small polaronic hopping process. The dielectric behaviour of the samples with different aluminium contents confirmed the correlation between the dielectric response and electrical conduction.The structure evolution of Smi.5Sro.5Ni1-χMnχo4(χ=0,0.03,0.05,0.1) has been investigated through the analysis of Rietveld refinement of X-ray diffraction data. With increasing the content of manganese ions, the orthorhombic phase strain parameters and the distortions of oxygen octahedron were decreased. A magnetic characteristic of a spin glass was found in Sm1.5Sr0.5Ni04 below 54 K, while an antiferromagnetic behavior at low temperature and a weak ferromagnetic behavior at room temperature were observed in x=0.1 samples. This complicated magnetic phenomenon may be caused by the competition between Ni2+-O-Ni2+. Mn4+-O-Mn4+ antiferromagnetic superexchange and Mn4+-O-Ni2+ferromagnetic superexchange. After comparing the activation energies of dielectric relaxations and electrical conductions, the low-temperature giant dielectric response should be attributed to the adiabatic small polaronic hopping process, while the low frequency relaxation at high-temperature was mainly attributed to the grain boundary effect. The suppression of dielectric loss should be benefited from the various polar region caused by Mn-substitution(Sml-χNdχ)i.5Sro.5Ni04 solid solution ceramics with K2NiF4-type structure can be synthesized in the entire range of 0.2≤χ≤0.8. All compounds formed the orthorhombic phase in space group B mab(64). With increasing x, the crystal structure has a tendency of transforming from orthorhombic phase to tetragonal phase. The relationship between the dielectric relaxations of all components and the adiabatic small polaronic hopping is comfirmed. Because of the compositional and structural inhomogeneity and fluctuation, the above room temperature dielectric relaxations are originated from extrinsic Maxwell-Wanger effect in x=0.4,0.6. Room temperature giant dielectric observed in x=0.8 is attributed to grain boundary effect.