HIDSOM · High density energy storage materials

High density energy storage materials

High density dielectric energy storage materials and related devices are crucial for pulsed power applications, such as hybrid electrical vehicles, mobile medical electronics and high power source, as a secondary power supply, featured by small size, light weight and long cycling lifetime. The aim of this project is to develop new capacitive materials with high energy density, i.e., high maximum polarization, low remanent polarization and high breakdown field, in either paraelectric, antiferroelectric or relaxor ferroelectric ceramics and composites. Different techniques, such as glass addition, chemical doping, solid solution and spark plasma sintering (SPS) were employed to enhance the energy density of (Ba1-xSrx)TiO3(BST), La doped Pb(ZrxSnyTi1-x-y)O3(PZST) and Pb(Zn1/3Nb2/3)O3-BaTiO3(PZNBT) ceramics. A microstructural model, reverse boundary layer capacitor(RBLC), was proposed for structure optimization in energy storage composites. The main application orientation is to develop suitable material system for developing further thin layer capacitor (MLCC based) as shown in Fig 1. Fig 1 Comparison of dielectric energy storage materials (1) Soda lime glass and ZnO addition in BST Soda lime glass and ZnO addtion were used to enhance the densification and grain boundary strength of BST. With the addition of soda lime glass, the porosity and the sintering temperature of the SPS ceramics are significantly reduced. The fracture surfaces are mainly trans-grannualar, different with the pure BST indicating the enhanced boundary. The technical key point for glass ceramic preparation is to densify the ceramic with minimum interphase reaction and glass recrystallization. SPS sintering has advantages for densification due to its high pressure at sintering temperature, fine grain and minimum reaction/recrystallization due to fast sintering. The ceramics prepared are dense (pore free) and have high permittivity, high breakdown field. The BST with crystallized oxide sintering aids (ZnO and some mixture with Bi and Pb oxides) were prepared by using SPS. Similar with glass ceramics, those ceramics have pore free structure, high relative density, high breakdown field at optimised processing condition. It is found that a variety of oxide sintering aids are helpful for the densification, and thereafter, for the breakdown field, in the context of SPS sintering. Fig 2 Fracuture surface and hysteresis loop of BST ceramics with addition of soda lime glass or zinc oxide (2) Antiferroelectric PZST Antiferroelectrics are filed switchable to ferroelectric phase at high field, which favors high energy storage than normal ferroelectric materials, especially for the antiferroelectric compostion with high tranformation field. Several lanthanium doped PZST compositions were studied to investigate the energy storage property using SPS. X-ray diffraction results indicate single phase lead containning perovskite can be sintered in the reducing atmosphere using SPS, at temperture less than 950oC and soaking time less than 10min. Pore free and uniform fine grain ceramic body can be obtained inside the sintering body. The field induced phase transformations can be observed at 10-20kV/mm range with excellent energy storage properties. Fig 3 Etched surface and hysteresis loop of PZST ceramics (3) Relaxorferroelectric PZNBT PZNBT is a solid solution of Pb(Zn1/3Nb2/3)O3-BaTiO3. Although both end members could establish ferroelectric order in either A site (PZN) or B site (BT) of perovskite structure, their solid solution have an extrodinary phase transition behavior, showing a “U” shape dieletric maxima temperature variation with composition. The system can be regarded as a mismatched ferroelectric state in some intermediate compositions, where relaxorferroelectric behavior can be expected, i.e., low Pr, high Pmax and possible field induced phase transformation. (4) Reversal Boundary Layer Model Study For the composite structure, a model analysis were made to optimise the material and structural paprameters. Reverse Boundary Layer Capacitor (RBLC) configuration model, where the grain boundary has a higher electrical conductivity than the grain, is proposed in glass/ceramic composites for dielectric energy storage applications. By introducing glass additives as grain boundaries with electrical conductivity higher than ceramic grains, the steady electric field across grains can be larger than grain boundaries as desired due to the conductivity difference. The breakdown field is thus expected to increase in the RBLC-type brick wall model because of the field distribution. The equivalent circuit, grain boundary conductivity dependence of energy density, low-loss frequency range of the RBLC model are discussed. The simulation results suggests that the RBLC approach has advantages in overall energy density, compared with normal insulating glass phase composites. Fig 4 Calculated overall energy density and effective fields in ceramic grains and glass grain boundaries in series with grains, as functions of glass resistivity based on RBLC model The above work on ceramic energy storage materials using SPS is a promising start for the following research. Corresponding successive work on those material system and related energy storage thin layer devices, during the return phase and in other related projects, will undoubtly accelerate the capacitive energy stroage study.

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