The use of a diepoxide resin in the form of 1,4-butanediol diglycidyl ether as the epoxy resin, lithium perchlorate (20 wt.%) as the ionic salt, a hardener (4,7,10-trioxatridecane-1,13-diamine, 15 wt.%) as the curing agent, and a poling DC electric field of 720 V/m gives an electret that exhibits a maximum voltage of 3.4 V during poling (30 min) and a stabilized voltage of 0.67 V after depoling (7.0 h). An epoxy system that hardens slowly (such as one with less hardener) is preferred, due to the longer time during poling for the ions to remain mobile. The rate of hardening rather than that of curing is the governing factor. The lithium salt hastens the curing, but it provides the ions and stabilizes the electret voltage, particularly during the first 30 min of depoling. After the first 30 min of depoling, crosslinking significantly enhances the stability of the electret voltage. The time constant for depoling is 0.8 h during the first 30 min of depoling and is 9 h afterward. Decrease of the lithium salt proportion from 20 to 10 wt.% still provides an effective electret, although the performance is reduced. An epoxy resin produced from Bisphenol F and epichlorohydrin is ineffective due to the high viscosity and fast hardening.
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