Browsing by Author "Acharya, Megha"

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    High sound pressure piezoelectric micromachined ultrasonic transducers using sputtered potassium sodium niobate
    (Springer Nature, 2024) Xia, Fan; Peng, Yande; Yue, Wei; Luo, Mingze; Teng, Megan; Chen, Chun-Ming; Pala, Sedat; Yu, Xiaoyang; Ma, Yuanzheng; Acharya, Megha; Arakawa, Ryuichi; Martin, Lane W.; Lin, Liwei; Rice Advanced Materials Institute
    This work presents air-coupled piezoelectric micromachined ultrasonic transducers (pMUTs) with high sound pressure level (SPL) under low-driving voltages by utilizing sputtered potassium sodium niobate K0.34Na0.66NbO3 (KNN) films. A prototype single KNN pMUT has been tested to show a resonant frequency at 106.3 kHz under 4 Vp-p with outstanding characteristics: (1) a large vibration amplitude of 3.74 μm/V, and (2) a high acoustic root mean square (RMS) sound pressure level of 105.5 dB/V at 10 cm, which is 5–10 times higher than those of AlN-based pMUTs at a similar frequency. There are various potential sensing and actuating applications, such as fingerprint sensing, touch point, and gesture recognition. In this work, we present demonstrations in three fields: haptics, loudspeakers, and rangefinders. For haptics, an array of 15 × 15 KNN pMUTs is used as a non-contact actuator to provide a focal pressure of around 160.3 dB RMS SPL at a distance of 15 mm. This represents the highest output pressure achieved by an airborne pMUT for haptic sensation on human palms. When used as a loudspeaker, a single pMUT element with a resonant frequency close to the audible range at 22.8 kHz is characterized. It is shown to be able to generate a uniform acoustic output with an amplitude modulation scheme. In the rangefinder application, pulse-echo measurements using a single pMUT element demonstrate good transceiving results, capable of detecting objects up to 2.82 m away. As such, this new class of high-SPL and low-driving-voltage pMUTs could be further extended to other applications requiring high acoustic pressure and a small form factor.
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    Size-Induced Ferroelectricity in Antiferroelectric Oxide Membranes
    (Wiley, 2023) Xu, Ruijuan; Crust, Kevin J.; Harbola, Varun; Arras, Rémi; Patel, Kinnary Y.; Prosandeev, Sergey; Cao, Hui; Shao, Yu-Tsun; Behera, Piush; Caretta, Lucas; Kim, Woo Jin; Khandelwal, Aarushi; Acharya, Megha; Wang, Melody M.; Liu, Yin; Barnard, Edward S.; Raja, Archana; Martin, Lane W.; Gu, X. Wendy; Zhou, Hua; Ramesh, Ramamoorthy; Muller, David A.; Bellaiche, Laurent; Hwang, Harold Y.
    Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high-energy-density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead-free NaNbO3 membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric-to-ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single-phase below 40 nm, as well as a mixed-phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First-principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size-driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead-free oxides with the membrane platform.