Investigation On The Dielectric Properties Of Niobate Glass-ceramic

The dielectric materials with high energy storage density which can effectively minimize the volume of the capacitor are urgently required by high-voltage capacitor. Because of the outstanding ferroelectric, the great potential of niobate glass-ceramic for certain applications has been recognized for many decades. Niobate is unique in that in order to obtain the ferroelectric properties and microstructure, it can change the combination of niobate crystal elements. In this thesis, the effect of different glass content, annealing temperature, nucleating agent and crystallization temperature on the microstructures and dielectric properties of 0.5 0.5 2 6Ba Sr Nb O and 3(Na, K)Nb O ferroelectric glass-ceramics were investigated. It aimed to optimize process conditions, prepare storage medium materials with high dielectric constant and high breakdown strength.In this paper, the glass-ceramic with 0.5 0.5 2 6Ba Sr Nb O as main crystallization were investigated firstly. The 2 5 2 3 2Ba O-Sr O-Nb O-Ca O-B O-Si O systems were researched,and the glass were prepared by melting method. The effect of different glass content on microstructure, crystal composition and dielectric property of 0.5 0.5 2 6Ba Sr Nb O glass-ceramics were investigated. The experimental results show that the crystalline phases and glass matrix each may have its own contribution to ferroelectric properties of glass-ceramic material as a whole. The glass phase is used to fill around the crystal which resulted in free of porosity, and thus, their electrical breakdown strength may reach a level that is an order of magnitude higher than that of the conventional ceramic dielectrics. Certainly, the addition of glass would decrease the crystallization trend. When the content of ferroelectric crystal is not adequate, the dielectric properties of glass-ceramic are poor, even worse without practical value. On the premise of the perfect glass phase content in experimental formula, In order to further improve the breakdown strength of the glass-ceramic, morphological control of tridymite in 0.5 0.5 2 6Ba Sr Nb O glass-ceramic was achieved by adjustment of annealing temperature. The experimental results show that annealed at 600oC-650 oC, glass-ceramic contain a little of tridymite crystal which as impure phase, and round tridymite crystal can be obtained. Because of more Si O2 keeping as amorphous phase, the breakdown strength of the ferroelectric glass-ceramic with round morphological feature of tridymite crystal can be increased. In order to further optimize performance, five different nucleating agents Cr2O3, Ba F2, Zr O2, Ca F2 and Ce O2 were chosen. The effect of different nucleating agent additions on crystallization temperature, crystallization behaviors, microstructures and dielectric properties of the glass-ceramics were studied. The experimental results show that different nucleating agent has different crystallization mechanism. The addition of nucleating agent decreases the crystallization temperature, accelerates the secondary phase elimination and promotes the uniform distribution of grain size. The sample with Ce O2 as the optimum nucleating agent achieves the dielectric constant of 301, breakdown strength of 425 k V/cm and the highest energy storage density of 2.41 J/cm3.Second, 3(Na, K)Nb O glass-ceramics were investigated in this paper. The glass was prepared by the melting method, and 2 2 2 5 2 3 2Na O-K O-Nb O-B O-Si O systems were researched. The effect of different glass content on crystallization temperature, crystal composition, microstructure and dielectric property of 3(Na, K)Nb O glass-ceramics were investigated. The experimental results show that dielectric constant of the glass-ceramic gradually increase with the content of glass phase decreasing, in addition the breakdown strength keep in a higher lecel and more stable range. The 3(Na, K)Nb O glass-ceramic with 80.0wt% glass phase obtained better dielectric properties. The correlation between the crystallization temperature and ferroelectric properties was studied by the measurements of impedance spectroscopy, activation energy, dielectric breakdown strength and dielectric constant in Na0.9K0.1Nb O3 glass-ceramic, while different nucleating agents were added to optimize performance. The experimental results show that all the DSC curves of the glasses show clearly two crystallization temperatures, the glass-ceramic crystallized at lower temperature has moderate crystal degree and weak interfacial polarization. The glass-ceramic with Ce O2 as the optimum nucleating agent and heated at the temperature of first exothermic peak based on DSC result achieves the dielectric constant of 291 and the highest energy storage density of 1.91J/cm3.