Negative-charge-storing mechanism of potassium-ion electrets used for vibration-powered generators: Microscopic study of a-SiO2 with and without potassium atoms

TL;DR

This study reveals that a specific SiO5 structural motif in amorphous silica is responsible for its ability to store negative charge in potassium-ion electrets, crucial for vibration-powered generators.

Contribution

It identifies the atomic structure responsible for charge storage in potassium-ion electrets using first-principles calculations, providing new insights into their microscopic mechanism.

Findings

SiO5 structure is characteristic in potassium-doped a-SiO2

SiO5 remains negatively charged after potassium removal

SiO5 exhibits a Raman peak at 1000cm-1

Abstract

A potassium-ion electret, which is a key element of vibration-powered microelectromechanical generators, can store negative charge almost permanently. However, the mechanism by which this negative charge is stored is still unclear. We theoretically study the atomic and electronic structures of amorphous silica (a-SiO2) with and without potassium atoms using first-principles molecular-dynamics calculations. Our calculations show that a fivefold-coordinated Si atom with five Si-O bonds (an SiO5 structure) is the characteristic local structure of a-SiO2 with potassium atomsm, which becomes negatively charged and remains so even after removal of the potassium atoms. These results indicate that this SiO5 structure is the physical origin of the robust negative charge observed in potassium-ion electrets. We also find that the SiO5 structure has a Raman peak at 1000cm-1.