High-dielectric-constant capacitors for hybrid electric vehiclesBy Angela Hardin • June 1, 2010
The DC bus capacitors are currently the largest component of inverters in hybrid electric vehicles. They comprise a significant fraction of the inverter’s volume (≈35%), weight (≈23%), and cost (≈23%), and present a significant barrier to meeting DOE’s targets for advanced high-temperature inverters that can be cooled by engine coolant at 105°C.
By improving the volumetric efficiency, ceramic-film capacitors with high dielectric constant (k) can substantially reduce the weight, size, and cost of inverters and thereby bridge a significant technology gap.
Our group has developed a new film-on-foil approach to fabricate high k (>1100) ceramic-film capacitors on inexpensive base metal foils that can later be stacked on, or embedded directly into, printed wire boards. This approach significantly reduces the size and weight of the capacitors, and the ceramic nature of the films allows them to operate at higher temperatures.
To realize this technology, superior dielectric materials must be made by engineering their microstructure through control of critical process parameters. Another challenge is to form large area or aspect ratio (diameter/thickness, d/t) film capacitors with very good dielectric properties. Our immediate goals for this fiscal year are to make capacitors with very large aspect ratios >105 and the desired dielectric properties by controlling parameters of the chemical solution deposition method and to understand the major factors affecting the performance of capacitors with a large aspect ratio.
Achievements so far are:
- Exponentially improved fabrication of capacitors with higher d/t. Achieved d/t >104 and established stringent process controls to consistently fabricate capacitors with aspect ratios >104 successfully.
- Measured capacitance values >1.6 microFarad and dielectric loss < 10% for d/t >104. Working to scale the single element capacitance to 10 microFarad.
- Measured energy density values of 85 J/cm3.
- Thermally cycled (about 1000 cycles) film capacitors between -50°C and +150°C with no measurable degradation in dielectric properties.
- Published more than 15 papers, made over 10 conference presentations, and filed 2 patent applications.
Authors: Beihai Ma, Manoj Narayanan, Steve Dorris, U. (Balu) Balachandran