The physics and basic properties of electrets are discussed, namely, what happens during corona charging of dielectrics, why the surface potential and trapped charge show certain limits, where the trapped charge is stored, why and how the charge is released from traps at high temperatures. The experiments have been conducted on single-layer SiO2 and Si3N4 and on multilayer combination of these materials. A strong lateral mobility of charge trapped near the SiO2/Si3N4 interface was observed at elevated temperatures. The positively and negatively charged electrets are compared to each other. The experiments on charge retention at elevated temperatures have shown the studied electrets are suitable for devices working at temperatures of up to 200–300°C. 1. Introduction Electrets could serve as a quasipermanent source of polarization in emerging applications like electrostatic energy harvesters [1]. This would allow autonomous devices powered by energy harvesters in applications, where temperatures of up to 200–300°C are observed and, thus, batteries cannot be used as a source of power. Electrets also used, or can be used, in other applications, for example, sensors, transducers, and electrostatic microbearings and micromotors [2, 3]. The inorganic electrets composed of at least one SiO2 layer and one Si3N4 layer show superior charge retention at elevated temperatures. However, the physics of electrets is not understood well enough at this moment. For example, attempts to pattern them result in instability of trapped charge at small feature size of a pattern [3]. It is obvious that better understanding of the processes occurring during and after electret charging, influence of atmospheric ions and fringing field [4], effects of temperature [4–7], and humidity and relevant surface conduction [3, 8, 9] on charge retention is necessary. It would allow designing better microdevices with electrets suitable for the market with its requirements for the long lifetime of electrets (and therefore the devices), small feature size of photolithographic pattern, and the extended temperature range in applications. This research is primarily targeted at development of electrostatic energy harvesters. They must be able to work for at least 10 years in applications with temperatures of 150–200°C and survive occasional short overheating to about 300°C. Therefore, organic electrets that discharge at much lower temperatures are not suitable for the application. The single-layer electrets dramatically discharge at 300°C either [4, 10]. Therefore, the only remaining option in this
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