High-temperature power electronics have become a vital aspect of future designs of compact power converters for applications including power conditioning and distributed motor/actuator controls. But the development of high-temperature capacitors had lagged far behind other system components (e.g., semiconductor switches that can operate at temperatures >200°C).
A new family of high volumetric efficiency, high-temperature dielectrics has been developed based on high Curie temperature relaxor-ferroelectric ceramics. The dielectrics operate at temperatures far beyond conventional X7R and X8R formulations (125 to 150°C). These new higher temperature (>300°C) materials are suited for advanced power electronics based on emerging solid-state switching technologies such as IGBTs and SiC.
Capacitors used in these circuits must operate at high frequency (10 to 100 kHz) with voltages ranging from 200 to 600 V. They must also be able to handle high ac ripple currents, implying the need for low dielectric loss, low equivalent series resistance (ESR), and high insulation resistance. For applications on spacecraft, electric vehicles, supersonic aircraft, and ships, the capacitors must have a high volumetric efficiency to minimize volume and weight and, therefore, a high dielectric constant and/or very low dielectric layer thickness.
High-temperature capacitors have been a small, highly specialized market based on low permittivity materials (mica, Teflon, rerated NPO, and X7R ceramics). However, emerging high-power-density solid-state switching technologies represent substantially larger markets that, particularly for spacecraft and vehicles, are poorly served by existing capacitors. Thus, the development of a high-permittivity, high-temperature dielectric with excellent power handling capability is a breakthrough that will enable development and commercialization of advanced power electronics.
The goal of researchers at TRS Technologies was to develop a dielectric optimized for use with SiC-based electronics that had a high permittivity and low dielectric loss at 300°C. This was accomplished by developing one of the highest temperature ferroelectric relaxor dielectrics ever reported. Normal ferroelectrics are characterized by very sharp peaks in their dielectric constant vs. temperature curves that correspond to ferroelectric-to-paraelectric phase transitions (i.e., the Curie temperature). Below the Curie temperature these materials are ferroelectric and often have dielectric constants on the order of 500 to 5000 over a broad temperature range.
Ferroelectrics, however, often exhibit very high losses under ac driving conditions due to reorientation of the ferroelectric domains. On the other hand, relaxor ferroelectrics have broad, diffuse ferroelectric-to-paraelectric transitions. Near the transition temperature, the ferroelectric domains are nano-sized with unstable polarization and low loss under high ac driving conditions. The researchers’ strategy, therefore, was to take a normal high-temperature ferroelectric composition and modify it into a relaxor ferroelectric for high-temperature power capacitors.
In general, a material that is initially ferroelectric can be modified into a relaxor by doping with cations that perturb the translational symmetry of the lattice. Dopants can have larger sizes, have different valences, or create associated ionic vacancies on either the cation or anion sites. If the local strain or electric fields associated with these point defects are sufficiently strong, they interact with the spontaneous dipolar polarization of the ferroelectrics and fundamentally alter the polarization mechanism.
The long-range dipolar ordering, which characterizes the normal ferroelectric material and produces domains with a specific crystallographic relationship, is broken down by these local interactions, and frustrated dipolar microregions exist instead. Along with this radical change in polarization distribution, dynamic fluctuations in the orientations of these microregions are also introduced. The net result in terms of properties—and highly attractive to capacitor applications—is a broad or “diffuse” transition that has a frequency dependence to what is now called the Curie maximum (Tmax).
The performance of this new dielectric—referred to as HT-300—was demonstrated in the form of prototype multilayer ceramic capacitors (MLCCs). The prototypes were designed to be rated at 200-500 V with values ranging from 100 nF to 10 µF at 300°C, including 0.1 µF/500 V, 1.5 µF/500 V, 1 µF/200 V, and 10 µF/200 V.
Standard lifetime testing and highly accelerated lifetime testing (HALT) were used to modify the device designs to maximize lifetime. Newly developed NPO-type capacitors (referred to as WT-2) with 10 pF to 50 nF capacitance and application temperature from cryo condition and up to 500°C were demonstrated, as well as high Tc material (referred to as WT-1 and WT-3) with 10 nF to 50 µF at 360°C and application temperature of 200-450°C.
One of the attractive characteristics of WT-1 and WT-3 compositions is that they have low temperature dependence below 200°C compared to HT-300. This property makes these compositions an attractive choice when little capacitance variation with temperature is required.
WT-2 composition has excellent temperature stability and very low loss in wide range up to 450°C. This material is also useful in cryogenic conditions.
HT-300 was designed for operation near 300°C as required for specific applications but was shown to operate well at temperatures ranging from room temperature to over 400°C. The performance of the 500 V/100 nF parts has been outstanding: Lifetime, operating voltage, and resistivity of these capacitors are all significant improvements over available rerated X7R/X8R/NPO and other high-temperature formulations.
The HT-300 dielectric, operating at 500 V, outperforms the commercially available component (operating at only 200 V) by 10 times at 200°C and over 100 times at 300°C. Lifetime tests have also shown that the COTS components only survive about 4 h at 300°C with 200 V dc applied, while no failures where observed for the HT-300 prototypes after 12 days at 400°C with 500 V dc applied.
This article is based on SAE International technical paper 2009-01-3124 by S. Kwon, W.S. Hackenberger, and E.F. Alberta of TRS Technologies.