Effect of carbon atoms on the reliability of potassium-ion electrets used in vibration-powered generators

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Abstract
Potassium-ion electrets, which are key elements in vibration-powered microelectromechanical generators, can store negative charge almost permanently. However, our experiments show that the charge storing capability of potassium ion electrets is severely degraded when a SiC plate instead of the usual SiO 2 plate is used in the fabrication process, indicating the negative influence of carbon incorporation. We theoretically studied the atomic and electronic structures of amorphous silica (a-SiO 2 ) with and without potassium and carbon atoms using first-principles molecular-dynamics calculations. Our calculations show that negative charge accumulates in fivefold-coordinated Si atoms without carbon contamination. However, carbon contamination of the potassium-ion electret causes oxygen deficiencies in SiO 2 , hindering the formation of fivefold-coordinated Si atoms. As a result, carbon contamination causes degradation of the negative charge storing capability of potassium-ion electrets.

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