Capacitance dilatometry for the in-situ controlled expansion process of cellular polymer-filler composites (ferroelectrets)

Ferroelectrets are unusual composites, consisting of a closed-cell polymer-filler foam structure. Ferroelectrets behave like ferroelectric materials during and after internally charging the voids of the cellular structure. In order to optimize the electromechanical properties of ferroelectrets, the voids within the cellular polymer composite are expanded in a controlled manner in a high pressure gas chamber. Thereby, the gas pressure is first increased, subsequently released and finally the expanded structure is stabilized by a heat setting treatment at elevated temperatures. Capacitance dilatometry provides an easy means for the in-situ monitoring of the multi-step foaming process, and therefore allows for the control of the degree of foaming. The mechanical stabilization of the expanded cellular polymer composite structure is explained in terms of the enhanced crystallinity as a result of the heat setting process.