Plastic Electroactive Polymers for Heating And Cooling

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Fraunhofer-Institut für angewandte Polymerforschung

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Electroactive polymers deform under electrical voltages or change their temperature. Targeted temperature changes are particularly interesting for heating and cooling systems. The key to this is electrocaloric polymers— researchers at Fraunhofer IAP have developed an electrocaloric polymer film with a thin layer.

The electrocaloric polymer films, with a thickness of only four micrometers, are to be used in various systems for heating and cooling in the future. For example, in heat pumps for climate control of vehicle interiors, battery modules, electronic components, control cabinets, or laser systems. "Low layer thicknesses are crucial to operate electrocaloric systems with voltages well below one kilovolt," explains Dr. Michael Wegener, head of the "Sensors and Actuators" department at Fraunhofer IAP. Together with his team, he develops and improves various electroactive polymers—EAP for short—for a wide range of applications. For example, as electromechanical sensors and actuators for use in soft robotics, automation, as sound and vibration detectors, as ultrasonic transducers, as pyroelectric layers for infrared sensors, or even as electrocaloric materials for heating and cooling.

Electrocaloric Polymers: The Temperature Converters Among Electroactive Materials

Electrocaloric polymers respond to changes in electrical voltage with temperature changes: when an electric field is suddenly applied, the temperature increases abruptly. The greater the change in the electric field, the larger the temperature change. The reason lies in polar structures within the material, which are forced into an ordered alignment by the electric field, releasing energy in the process. Conversely, they absorb energy once the electric field is switched off, resulting in an equally abrupt cooling of the material. For the technical utilization of electrocaloric materials in heating and cooling systems, these processes must be repeated at a high frequency and controlled in such a way that the heating and cooling occur in different environments. This is the only way to create a heat pump with usable, permanently warm and cold zones. Several material properties are crucial for high electrocaloric performance, including:

• a large change in electrical polarization

• a high dielectric strength

• low thermal losses and

• good mechanical stability

Material Development: Tailored Solutions for Electrocaloric Applications

The researchers at Fraunhofer IAP specifically enhance the properties of electrocaloric materials and tailor the components developed from them to individual applications. The focus is on chemical and physical modifications of PVDF terpolymers as well as the development of components consisting of multiple layers of thin electrocaloric polymer films, optimized for the highest possible temperature changes.

First, we optimize the materials through chemical modifications, thermal or radiation-induced post-treatment, or the addition of fillers. Then, we develop specialized manufacturing methods such as doctor blade, coating, or printing processes and optimize the production parameters to produce the thin films with the desired properties. In the final step, we focus on optimizing further processes, such as mechanically stretching the films or applying electrode systems with suitable thermal properties.

Dr. Michael Wegener

High-Precision Thin-Film Technology: The Key to Efficiency

Electroactive polymers place high demands on thin-film processing and the construction of multilayer systems. Challenges include the homogeneity of polymer films, maintaining high electrical breakdown strength, and preserving the desired functional properties—in this case, significant temperature changes. "First, we optimize the materials through chemical modifications, thermal or radiation-induced post-treatment, or the addition of fillers. Then, we develop specialized manufacturing methods such as doctor blade, coating, or printing processes and optimize the production parameters to produce the thin films with the desired properties. In the final step, we focus on optimizing additional processes, such as mechanically stretching the films or applying electrode systems with suitable thermal properties," Wegener summarizes the development phases for electrocaloric polymer films.

Stacking Processes: Multilayer Structures for Efficient Heating And Cooling Applications

The developed individual films can be stacked into multilayer structures, known as components. Layering multiple films with interlying electrodes increases the amount of electrocaloric polymer that interacts with the electric field without increasing the required operating voltage. Such a structure and the use of many such components provide the suitable thermal mass to efficiently utilize the electrocaloric polymers in heating and cooling systems.

The newly developed electrocaloric polymer films and components are the result of the Fraunhofer flagship project Electrocaloric Heat Pumps (ElKaWe). In this project, scientists from six Fraunhofer institutes collaborated to develop polymer-based and ceramic electrocaloric materials as well as high-performance control electronics.

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