The Japanese research team elucidated the microscopic mechanism in which amorphous silica(1) becomes negatively charged as a vibrational energy harvester(2), which is anticipated to achieve self-power generation without charging, as it is needed for IoT that is garnering attention in recent years with its "trillion sensors" that create a large-scale network of sensors. Unlike wind power and solar power generation, vibrational power generation, which utilizes natural vibration for power generation, is not affected by weather.

Vibrational energy harvester that uses potassium ion electret, which the research group had previously developed, is of interest since it can operate semi-permanently. The potassium ion electret(3) is a vibrational energy harvester that uses introduction of potassium atoms in amorphous silica to create a negative charge on the amorphous silica. However, its microscopic mechanism was unknown, making it difficult to improve its performance.

Through quantum mechanics calculations, the research group discovered that when potassium atoms are inserted in amorphous silica, electrons are provided from the potassium atom to the silicon atom. This causes the silicon atom to behave like a phosphorus atom. Silicon atoms form 5 covalent bonds with oxygen atoms instead of the usual 4, creating a SiO5 structure. We discovered that this structure is what accumulates negative charge.

This result provides a design guidance toward improving reliability and longevity of vibrational energy harvesters. This would allow sensors that do not require charging, to become widely available, and contribute toward actualization of the internet of things (IoT)(4).

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