Ask automotive engineers about what technology enhancements they could use to build better hybrid and electric vehicles (EVs) and you will hear a lot about higher-performance, lightweight batteries. But second on most of their wish lists will be improved power electronics, which today represents an emerging $61 billion global market. Power electronics account for about one-fifth of system material costs, according to the U.S. Department of Energy.
The power electronics in electric drive systems rely on inverters to help convert dc current from batteries into the three-phase ac current that traction motors need. One of the critical circuit components in EV inverters are dc bus capacitors, which play a key role in "alternating" the transferred power.
But dc bus capacitors are problematic; they can occupy around a third of a typical inverter’s volume, contribute a quarter to its weight, and add a quarter to its cost—mainly because they need to be cooled, the DOE says. Electrical resistance from the high current passing through the circuits makes the capacitors heat up significantly, so they must be cooled to prevent degradation, dry-out, or even thermal runaway.
Commercial polypropylene film dc capacitors—multilayer, high-surface area, electrolytic devices—can operate reliably only up to 105°C (221°F) even though engine coolant temperatures can rise as high as 125°C (257°F). Typically, a separate active-chilling loop operating at approximately 65°C (149°F) must be installed for the power inverters, which adds cost and complexity. Air-cooled radiator systems cannot provide sufficient chilling capability.
Scientists in the U.K. have developed a lead-free, high-temperature ceramic capacitor that could help solve the problem. The ceramic capacitor dielectric material, which they call Hiteca (HIgh TEmperature CApacitor), has a high energy density and operates stably at 200°C (392°F) and above, said Tatiana Correia, higher research scientist at the National Physical Laboratory, which is located outside London. “This is a completely new and most unusual material with high energy density,” she said.
“Electric vehicles are being held back by a few important technical issues,” Correia continued. “We believe this new, high-temperature capacitor can help solve one of those issues”—one that ultimately could help boost the range, lifetime, efficiency, and reliability of electric and hybrid vehicles.
A better capacitor
The project to create a better capacitor began several years ago when the U.K. government’s Technology Strategy Board program for Low Carbon Vehicles both targeted the need for a better capacitor material for EV inverters and recognized the nation’s technical capabilities in this area, Correia explained. In 2010 it funded NPL as well as several research collaborators—Queens University of Belfast, Queen Mary University, the capacitor firm Syfer, and French automotive component manufacturer Valeo—to conduct the necessary R&D effort.
The NPL group developed both the crystal structures and processing techniques that allow prototype capacitors to be made. The material has significantly higher energy density compared to other ceramic-based capacitors and contains no toxic lead, which faces future regulatory bans. Hiteca has an energy density of 25 J/cm³ at 1200 kV/cm (2 J/cm³ at 500 V).
The NPL-led team assembled prototype capacitors containing Hiteca that have a conventional multilayer design. It requires only relatively cheap raw materials and is processed using standard fabrication methods. The lab team made a paste (a granular slurry) of bismuth ferrite (BiFeO3) ceramic that had been doped with strontium-titanate (SrTiO3). They then employed tape-casting techniques to manufacture thin ceramic tapes from the slurry.
The Hiteca tape replaces the polymer dielectric films in an otherwise standard X7R-type (barium titanate, BaTiO3) capacitor design that East Anglia-based Syfer then followed when it produced more than 1800 test capacitors that are rated from 5 to 100 nF with operating voltages up to 1 kV. Correia believes that further optimization of the crystal structure could improve operating performance.
Beyond reducing the need for cooling and its associated weight, the new material’s high permittivity could help shrink down electronic devices. Hiteca also features less loss of capacitance with voltage, which could improve overall vehicle performance.
A little-studied ceramic
When the project began in May 2010, Correia and her team faced a long-standing technical issue. Conventional ferroelectric materials such as barium titanate have a high energy density, but they undergo a crystal phase transition with rising heat. “At 125°C the crystal unit cell changes shape, which alters the capacitance of the material from ferroelectric to paraelectric,” she explained. “We wanted to somehow shift the phase transition to a higher temperature.”
Correia said that the team tried out many substances before concentrating on a different class of ferroelectric ceramic—bismuth ferrites—which had been mostly ignored, having been described in only one scientific paper back in the 1960s. They began experimenting to try to adapt it for use in capacitors—a difficult task. “To increase energy density, we needed to tailor the composition; to change the structure, the crystalline lattice and the doping,” she said.
Although bismuth ferrite holds a charge well, it is less willing to release it. Adding strontium titanate helps the bismuth ferrite discharge more easily. “We thought that by mixing the two, we could get the best of each,” Correia noted. “We don’t know of any phase transition yet—and if there is one, it is above 200°C.”
The NPL group is now looking to integrate the capacitor technology into an EV power electronics system for further testing.
With its low dissipation factor, good energy density, and 200°C capacitance, they believe Hiteca could also find use in many systems that involve power conversion under extreme conditions—for example, in photovoltaic solar converters, space applications, or down-hole oil and gas extraction projects. The material’s properties may in addition be useful in pulse-power applications such as defibrillators and X-ray generators. NPL is looking to license this technology to its industry partners.
The adoption of high-temperature dc bus capacitors, together with other developmental high-temperature power electronics components, could lead to a potential cost savings of $188 per vehicle if the low-temperature cooling loop can be eliminated, according to papers published in recent years by the DOE's National Renewable Energy Laboratory and Oak Ridge National Laboratory.