PMUT Array for Mid-Air Thermal Display

This paper presents a mid-air thermal interface enabled by a piezoelectric micromachined ultrasonic transducer (pMUT) array. The two-stage thermal actuating process consists of an ultrasound-transmission process via a pMUT array and an ultrasound-absorption process via porous fabric. The pMUT design employs sputtered potassium sodium niobate (K, Na)NbO3 (KNN) thin film with a high piezoelectric coefficient ( 31 ~ 8-10 C/m2) as piezoelectric layer for enhanced acoustic pressure. Testing results show that the prototype pMUT array has a resonant frequency around 97.6 kHz, and it can generate 1970 Pa of focal pressure at 15 mm away under the 10.6 Vp-p excitation. As a result, fabric temperature in the central focal area can rise from 24.2°C to 31.7°C after 320 seconds with an average temperature variation rate of 0.023°C /s. Moreover, thermal sensations on the human palms have been realized by the heat conduction through the fabric-skin contact. As such, this work highlights the promising application of pMUT array with high acoustic pressure for human-machine interface, particularly mid-air thermal display.