PTC materials for high-voltage heating systems in electric vehicles

Ceramic thermistor materials that exhibit a sudden increase in electrical resistance with increasing temperature are known as PTC (positive temperature coefficient) materials. They are used as overcurrent protection devices, temperature sensors or PTC heaters. In battery-driven electric vehicles, PTC components can be used both as air heaters and to control the temperature of the battery. In addition to a stable air conditioning of the interior, this also improves battery lifetime and range of the vehicle. Because of higher power consumption for heavy-duty applications compared with normal passenger transport, higher onboard voltages (≥ 800 V) are needed. This leads to new demands and challenges for the development of suitable PTC materials, particularly regarding their breakdown voltage.

According to the requirements resulting from such applications, Fraunhofer IKTS conducts extensive material research on barium titanate (BaTiO₃ )-based ceramics to produce suitable PTC components. Targeted modification of the material can be used to adjust the application or transition temperature as well as the steepness and height of the resistance jump (Fig. 1). To adapt the self-heating to the voltage, the basic conductivity of the material is also adjusted. Besides chemical modification, powder preparation and process technologies are developed to transform the materials into final components. This includes mixing and milling of raw materials, calcination, fine milling, shaping by powder pressing, sintering, cutting, finishing, and metallization. The material properties of the final PTC components are not only adjusted by adapting the chemical composition but also by varying the process variables during powder preparation or heat treatment to control the grain size in the component.

Improvement of breakdown voltage

The breakdown voltage of PTC components is the electrical voltage at which the electrical resistance drops abruptly. The subsequent sharp increase in electrical current will thus lead to overheating and component failure. For higher onboard voltages, the breakdown voltage of the PTC component must therefore be increased. This can be achieved by increasing the component thickness, which however results in a deterioration in heat dissipation and a higher weight. By changing the material composition and adjusting the manufacturing process, we were able to significantly increase the breakdown field strength from 560 V/mm to 1100 V/mm so far. Based on these results, it becomes possible to either double the operating voltage while the component thickness remains unchanged, or to cut the component thickness by half while the operating voltage remains constant. Current research aims to reduce PTC component thickness to 2 mm.

Acknowledgement

This project is supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) under the funding code 01MV22002A.