These more recently introduced electrets with piezoelectric properties consist of an internally charged cellular dielectric, typically a polymer such as polypropylene (PP), with voids of heights in the order of a few µm and lateral dimensions of several tens of µm. Such voids can be generated by biaxially stretching a polymer film filled with small particles. During the stretching, microcracks form around the particles and develop into flat voids. A cross section of a cellular PP sample is shown in Fig. 1 (top) .
Charging of such a sample is achieved by exposure of one of its surfaces to a corona discharge. The deposited charges together with their counter charges on a back electrode generate a high internal field which causes breakdowns in the voids and thus deposition of charges of opposite sign on the upper and lower void surfaces, respectively. Finally, the sample is metallized on its two surfaces. The cross section of a charged and metallized ferroelectret film is shown in a schematic view in Fig. 1 (bottom). The films now exhibit large piezoelectric d33-coefficients which are very stable at room temperature. However, at temperatures above 50°C the charges and thus the d33-coefficients gradually decay in the PP samples.
The thermal stability has been considerably improved with structured fluoropolymer samples [2, 3]. These samples consist of layers of polytetrafluoroethylene (PTFE) and/or fluoroethylenepropylene (FEP) which are fused at high temperature such that small interfacial gas voids are formed between the layers. After charging and annealing, the fused multilayer films show large d33-coefficients thermally stable up to 90°C, as shown in Fig. 2. More recently, such layered samples were designed with air voids consisting of channels or tunnels arranged in a very regular way. Following this, such films were also made having large transverse (d31) coefficients .
Comparison of piezoelectric properties
The piezoelectric properties of such ferroelectrets have been analyzed by a simple model and experimental results can be explained by such calculations [1,2]. Typical d33- and- d31-coefficients for PP and PTFE/FEP ferroelectrets are compared in Table 1 with values for polyvinylidenefluoride (PVDF) and the conventional piezoelectric materials quartz and lead circonate titanate (PZT-5) . The large coefficients of the ferroelectrets are noteworthy. Since the ferroelectrets and PVDF are polymers, they have certain other properties such as mechanical softness and flexibility which distinguish them from the conventional piezoelectric materials and are advantageous for some applications (see, e. g., energy harvesting).
 G. M. Sessler and J. Hillenbrand, “Electromechanical Response of Cellular
Electret Films”, Appl. Phys. Letters 75, 3405 – 3407 (1999).
 X. Zhang, J. Hillenbrand, and G. M. Sessler, “Thermally stable fluorocarbon ferroelectrets with high piezoelectric coefficient”, Appl. Phys. A 84, 139-142 (2006); “Ferroelectrets with improved thermal stability made from fused fluorocarbon layers”, J. Appl. Phys. 101, 054114 (2007).
 R. A. C. Altafim et al, “Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films”, IEEE Trans. Dielectr. Electr. Insulat.13, 979 – 985 (2006).
 X. Zhang, P. Pondrom, L. Wu, and G. M. Sessler, “Vibration-based energy harvesting with piezoelectrets having high d31-activity”, Appl. Phys. Lett. 108, 193903 (2016).
 X. Zhang, P. Pondrom, G. M. Sessler, and X. Ma, “Ferroelectret nanogenerator with large transverse piezoelectric activity”, Nano Energy 50, 52 – 61 (2018).