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High density energy storage materials

Final Report Summary - HIDSOM (High density energy storage materials)

Brief summary on nonlinear dielectric materials for high density energy storage
Dielectric materials with high energy density 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 high power, small size, light weight and long cycling lifetime. The aim of this project is to develop new nonlinear dielectric materials with high energy density, in other words, high maximum polarization, low remnant polarization and high breakdown field, in either paraelectric, antiferroelectric or relaxor ferroelectric ceramics. Different material systems, such as (Ba1-xSrx)TiO3(BST), La doped Pb(ZrxSnyTi1-x-y)O3(PZST), AgNbO3(AN), (1-x)Bi(M1M2)O3-xBaTiO3(BM1M2-BT) and (1-x)Pb(Zn1/3Nb2/3)O3-xBaTiO3(PZN-BT) ceramics, were synthesized to achieve maximum energy density. Different techniques, such as glass addition, chemical doping, spark plasma sintering (SPS) and hot press sintering were employed to enhance the performances. A microstructural model, reverse boundary layer capacitor (RBLC), was proposed for structure optimization in composites.
(1) BST with soda lime glass and ZnO addition
Soda lime glass and ZnO addition 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-granular, 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 exhibit 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 optimized processing condition. Further, ZnO is an n-type semi-conductor, it is expected to increase the breakdown field of the ceramic composites at low addition level, according to the RBLC model. 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.
Glass is useful for preparing densified ceramics in many ceramic systems, therefore Soda Lime Glass (SLG) is employed as a sintering aid for (BaSr)TiO3 ceramics. Two series of BST ceramics were prepared with addition of SLG. The Ba/Sr ratios are 0.6/0.4 and 0.4/0.6 respectively. X-Ray results indicate the main phases are in perovskite structure for SLG doped BST ceramics. There is a second phase detected in 3wt% SLG added BST indexed as Ba2TiSi2O8.

Fig 1 XRD pattern of SLG added BST ceramics
a) b)
Fig 2 The fracture sections of SLG added BST ceramics
a) 3wt% SLG added B6S4 b) 3wt% SLG added B4S6

Fig 3 The P-E and I-E loops of 1wt% SLG added BST ceramics
With a maximum energy density of 1.01J/cc in this series

Two series of BST ceramics were prepared with addition of ZnO. The Ba/Sr ratios are 0.6/0.4 and 0.4/0.6 respectively. X-Ray results indicate all the compositions are in perovskite structure. There is no apparent peaks of possible Zinc riched second phase. The crystal reflections show slight trend moving to low angle with increasing Zinc content.

Fig 4 XRD patterns of ZnO added BST ceramics

BST 0.7wt%ZnO+BST 1.5wt%ZnO+BST

3wt%ZnO+BST 1wt%ZnO(Bi2O3, PbO)+BST
Fig 5 Fracture sections of ZnO added BST ceramics

Fig 6 The P-E and I-E loops of 1wt% ZnO added BST ceramics
With a maximum energy density of 1.08J/cc in this series

(2) Lead based antiferroelectric PZST
Antiferroelectrics are field switchable to ferroelectric phase at high field, which favors high energy storage than normal ferroelectric materials, especially for the antiferroelectric composition with high transformation field. Several lanthanum doped PZST compositions were studied to investigate the energy storage property using SPS. X-ray diffraction results indicate single phase lead containing perovskite can be sintered in the reducing atmosphere using SPS, at temperature 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.
pzst1: (Pb0.94La0.06)[(Zr0.70Sn0.30)0.92Ti0.08]O3 est Ef>10kV/mm act Ef>20kV/mm
pzst2: (Pb0.94La0.06)[(Zr0.60Sn0.40)0.92Ti0.08]O3 est Ef>14kV/mm act Ef>20kV/mm
pzst3: (Pb0.94La0.06)[(Zr0.60Sn0.40)0.94Ti0.06]O3 est Ef>16kV/mm act Ef>20kV/mm
pzst4: (Pb0.91La0.06)[(Zr0.60Sn0.40)0.84Ti0.16]O3 Ef~4kV/mm
pzst5=pzst2 Ef=12.5kV/mm

Fig 7 XRD patterns of PZST ceramics
pzst1 pzst2
pzst3 pzst4
Fig 8 Etched surface of PZST ceramics

Fig 9 PE loops of PZST ceramics
With the maxima energy densities of 1.13J/cc (pzst1) 1.57J/cc (pzst2) 1.03J/cc (pzst3) 2.06J/cc (pzst4) 1.15J/cc(pzst5)

(3) Lead based relaxor ferroelectric 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 extraordinary phase transition behavior, showing a “U” shape dielectric maxima temperature variation with composition. The system can be regarded as a mismatched ferroelectric state in some intermediate compositions, where weak polar state can be expected, i.e. low Pr, high Pmax and possible field induced phase transformation, similar materials including Bi(MgTi)O3-BaTiO3 and other bismuth based systems. Antiferroelectric like PE loop was observed in the intermediate composition of this material system, and the transformation field was observed increasing with BT content. The mechanism of the double loops observed here is not clear yet. It may be related with non-polar nano regions caused by the A and B sites co-occupancy.
PZNBT8: 0.8PZN-0.2BT
PZNBT7: 0.7PZN-0.3BT
PZNBT6: 0.6PZN-0.4BT

Fig 10 XRD patterns of PZNBT ceramics
pznbt8 pznbt7
Fig 11 Etched surface of PZNBT ceramics

Fig 12 PE loops of PZNBT ceramics
With a maximum energy density of 1J/cc in this series
(5) Lead free antiferroelectric AgNbO3
Lead free antiferroelectric AgNbO3 is in perovskite structure with complex symmetry variation, where it undergoes several transitions between orthorhombic, tetragonal and cubic phase with increasing temperature, similar to NaNbO3. Antiferroelectricity was claimed in the orthorhombic phase and double hysteresis loop was observed. The main obstacle in this materials system is how to prepare dense and uniform ceramics body since it requires an oxygen rich atmosphere. The solid solutions with LiTaO3 and Bi1/3NbO3 extend the antiferroelectricity to another axis with either rhombohedra ferroelectric or A site deficient perovskite structure. This may produce rich structure and property variation and related phenomena.

Fig 13 PE loops of AgNbO3-xBi1/3NbO3 ceramics
With a maximum energy density of 3J/cc for x=0.01
(6) Bismuth based relaxor ferroelectrics
The temperature dependence of permittivity 0.88BaTiO3-0.12Bi(Mg1/2Ti1/2)O3 (0.88BT-0.12BMT) ceramics shows relaxor-like behavior and weak temperature-dependence over a wide temperature range from −50 oC to 100 oC. The polarization-electric field hysteresis loops display a linear behavior at low fields, and slight saturated nonlinear behaviors at higher fields. A possible field-induced phase transition from weak polar state to ferroelectric state can be detected from the current-electric field curve. Optimal room temperature energy storage density of 1.81 J/cm3 was obtained. A temperature stable energy storage property of the 0.88BT-0.12BMT over a wide temperature range suggests its potential application in energy storage capacitors.

Fig 14 XRD patterns for 0.88BT-0.12BMT ceramic sintered at different temperatures

Fig 15 Dielectric constant and loss tangent as a function of temperature (−150 oC ~150 oC) at frequencies from 100 Hz to 100 kHz for 0.88BT-0.12BMTceramic.

Fig 16 P−E and I−E curves at different electric fields for 0.88BT-0.12BMT ceramic.
With a maximum energy density of 1.81J/cc.
Dense sintering body of (1-x)BaTiO3–xBi(Mg2/3Nb1/3)O3 (BTBMN1-4, x=0.05,0.10,0.15 and 0.20 respectively) ceramics were prepared using solid state sintering. XRD results show the ceramic exhibits a pure perovskite structure without the observation of second phase. SEM micrographs of the thermally etched surfaces of BTBMN ceramics show dense and uniform grain growth. BTBMN ceramics exhibited a significant frequency dispersion of the dielectric permittivity and a temperature stable plateau for x=0.20.

Fig 17 XRD patterns of BTBMN ceramics

Fig 18 SEM images of the thermally etched surfaces of the BTBMN ceramics: (a) BTBMN1; (b) BTBMN2; (c) BTBMN3; (d) BTBMN4.

Fig 19 Temperature dependence of dielectric constant and loss of (a) BTBMN1; (b) BTBMN2; (c) BTBMN3 and (d) BTBMN4 ceramics

Fig 20 PE loops of BTBMN ceramics
With a maximum energy density of 1.14J/cc for x=0.10

(7) Reversal Boundary Layer Model Study
For the composite structure, a model analysis were made to optimize the material and structural parameters. 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 21 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
(8) Thickness dependence
There is strong thickness dependence of the energy storage density and breakdown field. Thinner thickness corresponds to higher energy density and higher field. Regarding the practical device development, free standing films are required. Therefore the optimized methodology is to develop thin layer capacitor (MLCC based) to achieve high energy density and high total energy in devices, simultaneously.

Fig 22 Comparison of dielectric energy storage materials

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