Preparation And Properties Of TiO₂ Based Dielectric Ceramics Of High Dielectric Breakdown Strength

Pulse forming line is a very important part of pulsed power system.It is a very important research topic of pulsed power technology.With the development of compact and miniaturization of pulse power system,higher requirements are imposed on the pulse forming line.Because dielectric ceramic processes excellent dielectric properties,it becomes the preferred material for pulse forming line,but its current electrical strength is difficult to meet the application demand of the next generation of pulsed power system.Therefore,how to improve the electrical strength of ceramics has become the research focus.In this work,CaO-MgO-Al₂O₃-SiO₂-TiO₂?ASTO?ceramics with good dielectric properties were studied,considering the decomposition of Ni₂O₃ at high temperature,Ni₂O₃ was introduced into the ASTO ceramics for the first time.x wt%Ni₂O₃ doped ASTO?x=0?0.5?1.0?2.0?2.5?ceramics have been synthesized by conventional solid state reactions and the effects of Ni₂O₃ on the phase structures,microstructure,electric properties,dielectric properties and breakdown strength of the composites have been investigated.The results show that with the increase of x,the optimum sintering temperatures of the samples decrease from 1280?to 1230?and the bulk density increases from 3.122 g/cm³ to 3.175 g/cm³.The distribution of the crystal phase of the ceramic is more uniform and the microstructure become denser for the increase of Ni₂O₃.The resistivity of the ceramic first increases,then decreases,and reaches the highest value of 2.82×10¹⁴?·cm at x=2.0.And meantime its electrical strength is also increased to 82.1 kV/mm?x=2.0?from the 40.9 kV/mm?x=0?.The dielectric constant of the ceramics has little change,and the dielectric loss is reduced from 0.0028?x=0?to 0.0016?x=2.0?.In order to understand the physical mechanism to improve the dielectric strength of Ni₂O₃ doped ASTO ceramics,complex impedance spectroscopy was used to analysis the electrical structures of the samples.The results indicate that the relaxation and conductance of Ni₂O₃ doped ASTO ceramics are attributed to the contributions of grain,grain boundary and amorphous phase.For x=0,0.5,the complex impedance spectroscopy show two semicircles,the high frequency semicircle is representative of the response from the grain and the low frequency semicircle is related to the grain boundary.When x?1.0,a new semicircle appears which may be the response of amorphous phase.The activation energy of grain firstly increases and then decreases,while the activation energy of amorphous phase increases gradually.When x=2.0,the activation energy of grain and grain boundary reach their maximums,which means the insulation performance of the ASTO ceramics reaches its best,and so the highest dielectric breakdown strength is obtained.In addition,IR analysis shows that Ni₂O₃doping can enhance Al-O and Si-O bonds.When x=2.0,the vibrational dipole moments of Al-O and Si-O bonds are the smallest,and the chemical properties are the most stable,which explains the reason of the best dielectric breakdown strength of ceramic at this time from the chemical point of view.On the basis of the ceramics prepared by solid state reaction,to further improve the dielectric breakdown strength of the ceramic,core-double shell structure powders have been synthesized by sol-gel method and the microstructure of the ASTO ceramics has been controlled.The results indicate that pH=10 has the better coating effect and the ceramic has denser and more uniform microstructure compared to that of pH=7.The dielectric breakdown strength of the ceramic by sol-gel is 1.4 times compared to that by conventional solid state reaction,which means that the powder coating by sol-gel can indeed improve the microstructure of the ceramics and improve the breakdown strength.