Modeling and Simulation of a High Frequency MEMS-Fabricated Ultrasonic Nozzle
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Modeling and Simulation of a High Frequency MEMS-Fabricated Ultrasonic Nozzle S. C. Tsai1, T.K. Tseng2, Y. L. Song2, Y. F. Chou3, C. S. Tsai,2,4 and P.Z. Chang5 1
Dept. of Chemical Engineering, California State Univ., Long Beach, CA Institute of Applied Science and Engineering Research, Academia Sinica, Taipei, Taiwan 3 Dept of Mechanical Engineering, National Taiwan University, Taipei, Taiwan 4 Dept. of Electrical and Computer Engineering, Univ. of California, Irvine, CA 5 Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan 2
ABSTRACT This paper reports on the design and simulation of micro-fabricated 0.5 MHz Si-based ultrasonic nozzles that consist of multiple sections of Fourier horns of half wavelength design. Such high frequency ultrasonic nozzles should produce 10-15µm-diameter uniform precursor drops for nanoparticle synthesis by means of spray pyrolysis at ambient pressure. Results of 3-D simulation using a commercial FEM program, ANSYS, show existence of one resonant frequency of pure longitudinal vibration, which is very close to the design value. The results also show that at this resonant frequency increase in vibration amplitude at the nozzle tip is very close to the theoretical values of 2n, where n is the number of horn section. Therefore, the required electric drive power should be drastically reduced and the transducer failure in ultrasonic atomization can be more readily avoided. INTRODUCTION A silicon-based ultrasonic nozzle has a number of advantages over conventional metal-based ultrasonic nozzles [1]. Silicon has stronger electro-mechanical coupling strength, higher acoustic velocity, and potential for mass production of any resonator profile by semiconductor fabrication technology. In addition, ultrasound-modulated two-fluid (UMTF) atomization (spray) was found to be capable of producing more uniform and smaller drops than the conventional ultrasonic atomization at the same fundamental frequency [2]. In fact, the drop diameter obtained by UMTF atomization was found to equal to the wavelength of the capillary waves generated by the third harmonic frequency [2]. In other words, an UMTF atomizer operating at 0.5 MHz fundamental should produce uniform drops the same size as those produced by a conventional ultrasonic nozzle operating at 1.5 MHz, but it requires much lower ultrasonic power to operate because part of the atomization energy comes from the co-flowing air. An UMTF atomizer is made of an annulus for airflow and an ultrasonic nozzle with a central channel for liquid flow. Its piezoelectric transducers are not in direct contact with the precursor liquids and this avoids interference with transducer performance as in the case of conventional ultrasonic atomization using a nebulizer. Therefore, arrays of such high frequency ultrasonic nozzles can be used for mass production of nanoparticles of advanced functional materials by spray pyrolysis, a chemically flexible and continuous ambient pressure process. In fact, uniform spherical particles of yttria stabilized zirc
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