Preparation And Properties Of CaTiSiO₅ Based High Temperature Stable Dielectric Materials

Multilayer ceramic capacitors?MLCC?with high reliability are growing demanded for power electronics and for electronics operating in harsh environments.For example,the electronics for the controlling and sensing systems in aviation and aerospace or downhole drilling systems need to operate at temperature greater than200oC or even higher than 400oC.For ferroelectric ceramics applied at 200oC or above,there is a large dielectric constant at room temperature.When the temperature is higher than the Curie point,the sharp decreases in the dielectric constant and breakdown strength are obtained by the dielectric loss from conductivity and hysteresis.In contrast to ferroelectric materials,the dielectric constant of a linear dielectric is independent of the external electric field,and its energy storage density?U?is proportional to the dielectric constant??_r?and the square of the breakdown strength?E_b?.Linear dielectrics generally have relatively lower dielectric constant,but they can maintain stable and excellent dielectric properties,breakdown strength and energy storage efficiencies in a wide range of temperature,and are widely used in high temperature stable MLCCs.As a new type of linear dielectric,doped CaTiSiO₅based dielectric materials have attracted much attention.This thesis focuses on the preparation and properties of(Al³⁺,Nb⁵⁺)co-doped CaTiSiO₅ based ceramics and MLCCs.The main content and conclusions of this thesis are as follows:The CaTi_1-x(Al_0.5Nb_0.5)_xSiO₅?CTANSx?ceramics were prepared by solid-state reaction method.Al³⁺and Nb⁵⁺are incorporated into the monoclinic CaTiSiO₅ crystal lattice,and substitute Ti⁴⁺ions partially.The crystal structure was twisted because of the difference of ionic radius,which destroys the coherence of the off-centering of octahedra in the CaTiSiO₅,inhibits the transition of the high temperature paraelectric phase A2₁/a to the low temperature antiferroelectric phase P2₁/a.From room temperature to 350oC,CTANSx?x?0.03?ceramics show a stable high temperature CaTiSiO₅ phase.At room temperature,CTANS_0.03 ceramic shows the dielectric constant?_r⁵3,dielectric loss tan?<0.001,insulation resistance⁹×10¹³?·cm,breakdown strength>1000 kV/cm,theoretical energy storage density².65 J/cm³,energy storage efficiency⁹6%.Moreover,CTANS_0.03 dielectric material has a good temperature stability:the dielectric constant is stable in the range of RT-300oC,the insulation resistance is¹×10¹¹?·cm and the temperature coefficient of capacitance is-29 ppm/oC at 300oC,and the energy storage efficiency can still reach 89%at 180oC.CaTi_0.97(Al_0.5Nb_0.5)_0.03SiO₅-based MLCCs are prepared by a series of steps of casting,punching,printing,laminating and cutting.The dielectric properties and breakdown strength of MLCCs with precious metal internal electrodes were investigated.MLCC with good stability from room temperature to 300oC could be obtained,and the breakdown voltage at room temperature could be up to 4130V.Preliminary work on MLCCs with Ni as the internal electrode were carried out,and the effect of sintering process and MnO₂-doping on the resistance of the material were studied.