Fabrication and characteristics of electret using fine silica powder and its application of ultrasonic sensor

Electrets were prepared by spin‐coating the dispersion of fine silica powders, and their charge retention characteristics were investigated. Finer silica aggregates were formed on the PTFE surface layer by spin‐coating the dispersed solution of fine silica powder than those formed by spray‐coating colloidal silica. Electrets using fine silica powder also showed better charge retention characteristics than those using colloidal silica. Such an improvement in the charge retention might be due to the suppression of discharge in the excessive gap caused by coarse silica aggregates. Then, ECSs were fabricated using the prepared electrets, and their transmitting and receiving characteristics of airborne ultrasound were also investigated. The peak sensitivity of the ECS using a spin coating of fine powdered silica dispersion was slightly improved from those using spray coating of colloidal silica. On the other hand, the peak frequency and frequency band were greatly improved using spin coating of fine powdered silica dispersion. Therefore, ECS using fine silica powder is expected to be applied as a broadband airborne ultrasonic sensor.