Chen Zou

Electrical breakdown and ultrahigh electrical energy density in poly(vinylidene fluoride-hexafluoropropylene) copolymer

This paper investigates the electrical breakdown of a polar fluoropolymer, poly(vinylidene fluoride-hexafluoropropylene) which exhibits an exceptionally high discharged electrical energy density (>25 J/cm3). It is shown that above room temperature, the breakdown strength decreases with temperature. It is further shown that such a temperature dependence of breakdown strength is consistent with the electromechanical breakdown model by taking into consideration of the plastic deformation of semicrystalline polymers.

Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites

Work on dielectric spectroscopy of epoxy resin filled with nano-SiO2 at different relative humidities and temperatures is reported. Above the glass-transition temperature (Tg), dc-like imperfect charge transport (QDC or LFD) dominates the low frequency dielectric spectrum. Another mid-frequency relaxation process was found in the non-dried composites. Water also induces glass-transition temperature decreases, which can be measured both by dielectric spectroscopy and DSC. Both theory and experiment demonstrated that a higher water content could exist in nanocomposites than unfilled epoxy suggesting a bigger free volume when nanostructured. In our system, the hydrophilic surface of silica is likely to cause water to surround and lead to delamination of the epoxy from SiO2. This is a potential mechanical and dielectric weakness in the nanocomposites, which may lead to an ageing phenomenon. Hydrophobic surface group may reduce the water adsorption in nanocomposites.

Semicrystalline polymers with high dielectric constant, melting temperature, and charge-discharge efficiency

The dielectric properties of semicrystalline poly(tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride) (THV) terpolymers with different compositions are investigated in a broad temperature range and under high voltage for potential applications in high energy density film capacitors with high thermal stability. It was found that the capacitor charge-discharge efficiency under high voltage (>;200 MV/m) and the melting temperature can be significantly improved by increasing the tetrafluoroethylene (TFE) content. Melting temperature above 220°C can be achieved in THV terpolymers with high TFE content. Although the THV terpolymers have lower dielectric constant than polyvinylidene fluoride (PVDF), their high melting temperature, low leakage current, and high charge-discharge efficiency represent significant advantages for capacitors operated under high charge-discharge repetition rate and at high ambient temperatures.

Wide temperature polyimide/ZrO2 nanodielectric capacitor film with excellent electrical performance

In this letter, wide temperature dielectric properties and corona resistance of Upilex-S® polyimide (PI) films filled with Zirconium dioxide (ZrO2) nanoparticles were investigated. ZrO2/PI nanodielectrics exhibited the stable dielectric properties, high energy density and high charge-discharge efficiency below 300 °C. Testing of corona resistance showed even a small amount of nanofillers can improve the lifetime of PI significantly. Scanning electron microscopy with x-ray microanalysis (SEM-EDS) analysis suggested the higher thermal conductivity and evaporation of ZrO2 nanoparticles may induce this improvement. These high performance features make polyimide nanocomposites attractive for high energy density capacitor applications at high temperature.

The influence of physical and chemical linkage on the properties of nanocomposites

It has been shown by several groups that the mechanical and electrical behavior of composites changes quite substantially, and often beneficially, when the filler particle size is less than 100 nm in diameter. There is also good reason to believe that the interface between the embedded particulates and the polymer matrix holds the key to understanding the bulk phenomena observed. Materials based on an SiO/sub 2/-polyolefin system have been formulated with functionalized particulates so as to affect the physical and chemical linkages. The agents used to achieve this include amino-silane, hexamethyl-disilazane and triethoxyvinylsilane. The emerging picture of the interface is supported by detailed dielectric spectroscopy and internal space charge assessment. The nature of the internal structure has been related to the bulk properties observed such as the breakdown strength, voltage endurance, and the measurement of internal charges resulting from interfacial polarization.

The Effect of Gamma Irradiation on Space Charge Behaviour and Dielectric Spectroscopy of Low-density Polyethylene

The influence of gamma irradiation on low-density polyethylene (LDPE) has been studied through space charge measurement and dielectric spectroscopy. Commercial LDPE film with a thickness of 100¿m was irradiated with ¿-rays at a dose rate of approximately 10kGy/h to different doses up to 100kGy in an atmosphere of air, vacuum and nitrogen respectively. Space charge profiles vary depending on irradiation environments and radiation dose. The space charge profile of a sample after irradiation at 100kGy in air is more complicated than those irradiated in vacuum and nitrogen at same dose. More importantly, the space charge decay rate for the irradiated samples is slower than that of the virgin sample, indicating that the irradiation had changed the material by the generation of deep traps for space charge. Irradiation in air also produced changes in the dielectric spectrum in the frequency range between 10¿3Hz and 106Hz where the imaginary part of the permittivity was found to be about one order of magnitude higher than that of the virgin sample (~10¿4). This was attributed to oxidation as samples with the same (or lower) radiation dose in vacuum or nitrogen showed no pronounced difference to that of virgin samples.

PEN/Si3N4 bilayer film for dc bus capacitors in power converters in hybrid electric vehicles

High performance hybrid bilayer capacitor film has been developed by controlled deposition of silicon nitride (Si3N4) on polyethylene naphthalate (PEN). It was found that silicon nitride prepared with plasma-enhanced chemical vapor deposition can significantly increase the dielectric constant (K) and energy density of PEN by more than 20% and 50%, respectively. Moreover, the PEN/Si3N4 bilayer hybrid film also exhibits appropriate mechanical and thermal properties for capacitor winding. The high operating temperature and energy density of the PEN/Si3N4 bilayer dielectric film are very attractive for dc bus capacitors used in power inverters or converters found in hybrid electric vehicles, wind turbine generators, grid-tied photovoltaics, and smart grid, as well as pulsed power systems.

DC conduction mechanisms in epoxy nanocomposites under the humid environment

In this paper, time dependent direct current conduction characteristics were studied for both micro- and nano-filled epoxy composites at 0% and 75% relative humidity with temperature range of 298K to 343K. At low loading content of nanofillers, composites had similar conduction mechanisms as the matrix, which were ohmic conduction at low field or trap-filled limit conduction dominated at higher field. However, the higher concentrations of inorganic filler would shift the conduction mechanism to Schottky emission at higher field. Once the buildup of space charge was completed in composites, the quasi-steady state current deviated from the low-field linear relation with electric field (ohmic conduction). Moisture and the higher concentrations of nano-filler enhanced charge decay in composite and increased the threshold field of space charge accumulation, which was confirmed by previous pulsed electroacoustic (PEA) measurements.

Polymer film capacitors with high dielectric constant, high capacitance density, and high energy density

A series of high dielectric constant polymers have been developed with K from 10 to over 50. The high-K polymers have high dielectric breakdown strength above 700 V/μm and high energy density up to 27 J/cm3 in lab-scale small sample test. As thermoplastic semicrystalline polymers similar to polypropylene, these high-K polymers can be processed into thin capacitor film using melt-extrusion and biaxial orientation process and film thickness from 2 μm to 10 μm has been achieved. Metallized prototype capacitors show promising performance with energy density above 5 J/cm3. Future development will target at 10 J/cm3 energy density in the packaged capacitors. On the other hand, the capability to produce ultrathin capacitor film enables the manufacturing of film capacitors with high capacitance density above 10 μF/cm3. Potential applications include pulsed power capacitors for external and implantable medical defibrillators, and other industrial applications.

High temperature capacitors with high energy density

Power electronics are a key technology for hybrid and plug-in electric drive vehicles (EDV) and represent 20% of the material costs. DC bus capacitors are one of the critical components in EDV power inverters and they can occupy ~35% of the inverter volume, contribute to ~23% of the weight, and add ~25% of the cost. Current polypropylene (PP) film capacitors have dielectric constant K of 2.2 and temperature stability lower than 105°C. We recently developed a modified Polyvinylidene fluoride (PVDF) which combines high dielectric constant, low dielectric loss, low leakage current, high dielectric breakdown strength, and high temperature stability. The modified PVDF capacitor film also has graceful failure feature which is critical to applications demanding high reliability and long lifetime. The thermoplastic nature of the modified PVDF ensures that they can be processed into thin capacitor film using inexpensive melt extrusion and biaxial orientation process. In this report, DSC and dielectric spectroscope test results of the novel capacitor film will be presented.