A Study Of New Dielectric Materials For MLCC In Wide Temperature Range And Mechanism Of Its Dielectric Property

A series new BaTiO3-Na0.5Bi0.5TiO3-Nb2O5-Mn2O3/Fe2O3/Co3O4/NiO/In2O3 X8R(-55~150oC, ?C/C25oC?±15%) and X9R(-55~200oC, ?C/C25oC?±15%) system were successfully prepared by using conventional solid-state method, with the aim of discovering new material systems of high dielectric constant and wider working temperature range which could satisfy the EIA-XnR criteria. The Core-Shell structure and high Curie temperature promise the BaTiO3-Na0.5Bi0.5TiO3-Nb2O5(BT-NBT-Nb) base system with high temperature stabitly and high ceiling working temperture. It can satisfy X8 R criteria by adding Ni into the system, with dielectric constant of 2060~2350 at room temperature and extremly low dielectric loss(?2.0%) within the whole measured temperature range. Using other varible-valence transition metal elements(Mn, Fe, Co) and fixed-valence element(In) to replace Ni can also meet the X8 R specification, while mainting the dielectric constant between 2100~2210(at 25oC). It was found that changing of the processing method can further improve the temperature stabiliy at high temperature end of BT-NBT-Nb-Co system, which including the replacement of BT powder, the control of pre-sintering temperature/time, sintering atomsphere, annealing atomsphere/annealing time. The obtained samples can meet the X9 R specification, with dielectric constant between 920~930 and 1490~1570, and dielectric loss less than 1.5%.There are some common effects of changing acceptor concentrations on the properties of BT-NBT-Nb-Mn/Fe/Co/Ni/In systems. For example, the Curie peak positions are moving downward and the activation energy of grain boundary are decreasing with increasing acceptor content. A series of experiments were designed to control the concentration of oxygen vacancy since the defect complex [MTi?-VO??] or [2MTi?-VO??] formed between oxygen vacancies and acceptor-defects are considered as the main reason of these phenomena, which including the adjustment of the sintering atomsphere and annealing atomsphere/time. The results demostrate the effects of oxygen vacancy and acceptor ions are the same on the structure and propery of these systems, thus supportting the view that it was oxygen vacancies which played decisive role on the dielectric, ferroelectric and conduction properties. XRD, SEM, TEM and EDS were used to characterize the crystal structure, microstructure and element distribution in the grains, combine with XPS and Raman spectrum to detect the valence state and local structure symmetry. The Core-Shell structure that was observed in the grains can be used to explain the dielectric property, the relationship between acceptor/oxygen vacancy concentration and corresponding dielectric/ferroelectric property were established. The impedance spectroscopy was fitted by using different equivalent circuits(4RC: core region, shell region, grain boudary, interface of ceramic/electrode; 3RC: grain, grain boundary, interface of ceramic/electrode), and the electrical structure and conduction mechanism in the BT-NBT-Nb-Mn/Fe/Co/Ni/In systems were analyzed.