Energy Storage Performance Of Calcium Titanate Based Linear Dielectric Ceramics

Dielectric capacitors,which store energy in the form of electric field,are widely used as core energy storage devices in pulsed power systems due to their high power density.However,the low energy storage density of dielectric capacitors no longer meets the demand for compact,low cost and safe devices.In the present thesis,the effects of microstructure and interface structure on dielectric strength and energy storage density in CaTiO3 linear dielectric ceramics have been systematically investigated.Meanwhile,the different breakdown behaviors are investigated to explore the microscopic mechanism of thermal breakdown in CaTiO3 ceramics.The following conclusions have been reached:The effects of crystal structure,electron structure and phonon structure on intrinsic dielectric strength are investigated by the first principle calculations for CaBO3(B=Ti or Zr).Compared to CaTiO3,CaZrO3 has a much higher band gap and a slightly lower phonon cutoff frequency,resulting in a higher dielectric strength,but the macroscopic dielectric constant is much lower that of CaTiO3.The intrinsic dielectric strength of CaTiO3 and CaZrO3 are predicted to be 4.5MV/cm and 9.0MV/cm,respectively,and the corresponding energy storage density is as high as 152J/cm3 and 107J/cm3,respectively。It is indicated that introducing a small amount of Zr4+ at the B site of CaTiO3 is expected to increase the intrinsic dielectric strength as well as energy storage density owing to the increased band gap.Fine microstructure and improved energy storage density have been achieved in CaZrxTi1-xO3(x=0.0,0.1,0.2,0.3,0.4,0.5)solid solution ceramics.With increasing x,the dielectric constant er decreases from 175 to 103,while the dielectric strength Eb increases from 435kV/cm to 756kV/cm.The best energy storage performance is achieved at x=0.4,and the corresponding dielectric constant,dielectric strength and energy storage density is 119,714kV/cm and 2.7J/cm3,respectively.The highest energy storage density has an 80%increase compared to that of CaTiO3 end-member.The energy storage density significantly increases with decreasing the thickness due to the increased dielectric strength.For the samples with x=0.5,the dielectric strength increases from 584kV/cm to 756kV/cm while the energy storage density increases from 1.6J/cm3 to 2.6J/cm3 when the thickness changes from 0.50mm to 0.15mm.Dense CaTiO3 ceramics with greatly enhanced dielectric strength and energy storage density are prepared by spark plasma sintering(SPS).Compared to the ceramics prepared by standard solid state reaction(CS),the SPS sample microscopically has a finer grain size and less porosity,and exhibits a lower electrical conductivity and a higher thermal conductivity on a macroscopic scale,resulting in its improved dielectric strength(910 kV/cm),and the energy density is as high as 6.9 J/cm3.The dielectric constant of CaTiO3 at 573K is still about 140 and the electrical conductivity is in the order of 10-10S/cm,which can be used as dielectric material for energy storage system at elevated temperature.An amorphous alumina thin film is introduced between the surface of CaTiO3 ceramic and electrode,forming a tri-layered structure microscopically.The dielectric constant keeps unchanged,while the dielectric loss increases with the increasing of the deposition time of thin film,and the best deposition time is about 120s with a thickness of 100nm.The alumina film pevents the space charge from migrating under high field and suppress the surface dischrage.The dielectric strength can be further improved to 1188kV/cm and the correspoding energy storage density is up to 11.8J/cm3.It is found that dielectric breakdown originated from the weak areas both on the surface and inside the ceramics,leaving a tubular molten channel after breakdown,and the traces of grain melting recrystallization and plastic deformation is found in the breakdown channel,indicating a multi-field coupling breakdown process.The development of a local temperature perbutation under electric field is explored by numerical methods,and combining with experimental result to investigate the dielectric breakdown process in detail.It is suggested that thermal breakdown should be the dominant mechnasim of dielectric breakdown in ceramics.