Negative-charge-storing mechanism of potassium-ion SiO2-based electrets for vibration-powered generators

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 atoms, 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 1000 cm−1.

Power enhancement of MEMS vibrational electrostatic energy harvester by stray capacitance reduction

“…Fundamental characterizations of the control device are reported elsewhere [37]. Detailed processes of the electret charging and the fabrication process are documented in [44] and [45], respectively.…”

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

“…The calculation procedure we used is similar to our previous work. 13) All the calculations were performed using the Vienna ab initio Simulation Package, which is based on density functional theory. [15][16][17][18] In the structural optimization, the calculations were stopped when the maximum force on each atom was less than 5.0 × 10 −2 eV Å −1 .…”

“…The overall model formation procedure is similar to our previous study. 13) First, we prepared a supercell of alpha quartz (144 atoms) and inserted a single potassium atom and a single hydrogen atom into each supercell. We inserted one hydrogen atom because we used steam oxidation and we believe many hydrogen atoms exist in the oxide film.…”

“…[29][30][31][32][33] The temperature history used in these MD calculations was as follows. To obtain the amorphous models, the systems are heated up to 5000 K and equilibrated for 10 ps, then quenched to 0 K. The quenching rate is a key factor to obtain plausible amorphous structures since it has been reported that a-SiO 2 with a SiO 5 structure can be formed at a cooling rate of 50 K ps −1 in our previous work, 13) we adopted the same rate in the present study.…”

“…12) In a recent study, the origin of the negative charge accumulating in a potassium ion electret was investigated by first-principles calculations. 13) The models used for the calculations were designed to simulate the fabrication processes for potassium-ion electrets and the observed characteristic structures. As a result, we found that a fivefoldcoordinated Si atom which is not observed in normal amorphous SiO 2 is responsible for accumulating the negative charge (Fig.…”