Acoustic transducer based on ZnO nanorods

we report on ZnO nanorods based acoustic transducer for sound pressure level sensing applications. The sensing element is a circular diaphragm (diameter: 27 mm) of Stainless steel (SS-304L) substrate, which also acts as a bottom electrode for the developed transducer. Prior to synthesis of nanorods, ZnO seed layer (∼100 nm) film was deposited by DC magnetron sputtering technique. ZnO nanorods were synthesized by hydrothermal method at lower temperature of 90 °C for 4 hrs growth duration. The average length and diameter of as- synthesized ZnO nanorods were 2 µm and 140 nm respectively. As-synthesized nanorods were used for the fabrication of acoustic transducer. The ZnO nanorods have been sandwiched between seed layer deposited on SS substrate and top electrode of silver epoxy (thickness: ∼200 nm). The developed transducer is used for acoustic pressure sensing at different frequencies (31.5–2000 Hz) with different sound pressure levels (SPL) in the range of 120–160 dB. The acoustic sensing study was performed using the acoustic calibrator (Model NO: MAC 1001H) set-up. It was found that, the maximum peak to peak response of the transducer was 226 mV at 1 KHz excitation frequency with the SPL of 160dB. This is the first exploratory study on the applicability of ZnO nanorods based acoustic transducer. The possible potential applications are in consumer products (telephone circuits, speech amplifiers, tape recorder and hearing aids/microphones etc.), space, medical, strategic, automotive and industrial applications.

I. Introduction

MEMS/NEMS devices utility has increased significantly in various fields such as medical, remote patient monitoring, environmental monitoring, long-range asset tracking, national security, chemical industry and aerospace applications [1]. Also, these MEMS/NEMS devices are playing a key role in the development of physical sensors (pressure, strain, temperature and acoustic) for monitoring health care applications as well as sensing of industrial parameters [2]. Detection of sound pressure levels (SPL) is also very important parameter for aerospace engineers to sense fatigue failure of metal panels and structures in large booster rockets. Availability of environmental energy (sound) also is used for detecting the sound pressure levels by using acoustic sensors/transducers [3]–[5]. The presently available literature reported mostly on bulk crystals [3] and thin film [6] based piezoelectric acoustic sensors for different applications like chemical gas sensing, biological sensing and microphones etc,. But, the conventional piezoelectric materials (quartz and lithium niobate and lithium tantalate) are incompatible with monolithic integration technology. Thin film based piezoelectric devices (ZnO and AIN) are having some advantage of compatibility with electronic integration [7]. However, it was found that, the output response, detection limit, signal to noise ratio and sensitivity of above sensors are relatively low. One of the ways for improving the response of the above sensors is by changing the operational frequency of the device. Uses of nanostructures (nanorods/nanowires) are an alternative way for improving the performance characteristics of sensors due to their excellent mechanical, electrical, chemical properties, sufficient surfaces to volume ratio and nanoscale dimensionality.