Characterization of Silicon-based Ultrasonic Nozzles

Transcription

1 Tamkang Journal of Science and Engineering, Vol. 7, No. 2, pp (24) 123 Characterization of licon-based Ultrasonic Nozzles Y. L. Song 1,2 *, S. C. Tsai 1,3, Y. F. Chou 4, W. J. Chen 1, T. K. Tseng 1, C. S. Tsai 1,5, J. W. Chen 2, Y. D. Yao 6, C. H. Yang 7, M. F. Huang 7 and Y. A. Lai 7 1 Institute for Applied Science & Engineering Research Academia nica Taipei, Taiwan 115, R.O.C. 2 Department of Physics National Taiwan University Taipei, Taiwan 16, R.O.C. 3 Department of Chemical Engineering California State University Long Beach, CA 984, U.S.A. 4 Department of Mechanical Engineering National Taiwan University Taipei, Taiwan 16, R.O.C. 5 Department of Electrical Engineering & Computer Science University of California Irvine, CA 92697, U.S.A. 6 Insititute of Physics Academia nica Taipei, Taiwan 115, R.O.C. 7 Department of Mechanical Engineering Chang Gung University Taoyuan, Taiwan 333, R.O.C ylsong@gate.sinica.edu.tw Abstract This paper presents the design and characterization of micro-fabricated.5 MHz silicon-based ultrasonic nozzles. Each nozzle is made of a piezoelectric drive section and a silicon-resonator consisting of five Fourier horns, each with half wavelength design and twice amplitude magnification. Results of impedance analysis and measurement of longitudinal vibration confirmed the simulation results with one pure longitudinal vibration mode at the resonant frequency in excellent agreement with the design value. Furthermore, at the resonant frequency, the measured longitudinal vibration amplitude gain at the nozzle tip of the 5-horn nozzle is in good agreement with the theoretical values of 2 5. Using this design, very high vibration amplitude gain at the nozzle tip can be achieved with no reduction in the tip cross sectional area for contact of liquid to be atomized. Therefore, the required electric drive power should be drastically reduced, thus avoiding transducer failure in ultrasonic atomization using the 5-horn ultrasonic nozzle. Key Words: Ultrasonic Nozzle, Fourier Horn, High Frequency Nozzle

2 124 Y. L. Song et al. 1. Introduction Ultrasonic nebulizers are commercially available at frequencies as high as 2.5 MHz; however, they require very high electric drive power, which severely limits atomization rates in order to avoid transducer failure. Moreover, because the transducer must be in direct contact with the precursor solution, performance degrades quickly over time. In contrast, an UMTF atomizer [1,2] is made of an annulus for airflow and an ultrasonic nozzle with a central for liquid flow. Its piezoelectric transducers are not in direct contact with the precursor liquids and, thus, avoid interference with transducer operation as in the case of conventional ultrasonic atomization using a nebulizer. For example, arrays of such high frequency silicon ultrasonic nozzles can be fabricated using the MEMS-based technology and used for mass production of nanoparticles of advanced functional materials by spray pyrolysis, a chemically flexible and ambient pressure process. In fact, uniform spherical particles of yttria stabilized zirconia 68 nm in diameter have been produced by spray pyrolysis of 5 8 µm precursor (yttrium-containing zirconium hydroxyl acetate) drops, generated by an ultrasonic nebulizer at 2.5 MHz [3]. Thus, an ultrasonic nozzle with resonant frequency significantly higher than currently available maximum frequency of 12 khz is needed. This paper reports on the design, measurement, and characterization of.5 MHz ultrasonic nozzles fabricated using MEMS technology in (1) silicon wafers. To the best of our knowledge, no ultrasonic nozzle at such a high frequency has ever been reported. As shown in Figure 1, the nozzle is composed of a transducer drive section and a silicon resonator with a central for liquid flow. The drive section consists of a pair of PZT piezoelectric transducer plates with a rectangular section of silicon bonded in between. The silicon resonator is made of five Fourier horns in cascade, with half-wavelength design and vibration amplitude magnification of two. The nozzle is geometrically configured such that excitation of the PZT transducer plates creates a standing wave through the nozzle with maximum longitudinal vibration at the nozzle tip. As the liquid issues from the nozzle tip, a capillary wave is generated on the liquid surface with initial amplitude proportional to the amplitude of the longitudinal vibration of the nozzle tip. Traveling in temporally unstable liquid capillary wave even- *Corresponding author Liquid in 3.78 mm Z Y X 8.5 mm Nodes mm Amplitude Gain in Longitudinal Vibration: 27 Figure 1. mulation results of a 5-Fourier horn ultra - sonic nozzle with.5 MHz design frequency. tually collapses into drops with a diameter inversely proportional to the ultrasonic frequency [4]. A unique advantage of the multiple-horn nozzle is that its tip cross sectional area that provides contact loading of liquid to be atomized remains the same as the individual horn while the longitudinal vibration amplitude gain at the nozzle tip is greatly increased to a level that cannot be achieved by any single-horn nozzle at a much reduced tip cross sectional area. 2. Experimental Transducer Drive Section Resonance Frequence: 491 khz Resonator Section 1.25 mm Liquid out The aforementioned Fourier horn profile obtained by Fortran 9/77 simulation was used in the layout of the mask for fabrication of 5-Fourier horn nozzles. Major fabrication steps for the silicon resonator halves (including the base sections where PZT transducer plates are to be bonded) using MEMS techniques are illustrated in Figure 2. For simplicity, the horn profile is not shown in the figure. Inductive Coupled Plasma (ICP) etching, instead of wet etching, was employed because of its capability to produce a smooth and precision finish in cutting through the 53 µm-thick wafer for the making of resonator halves with a rectangular trough 7 µm in depth and 2 µm in width. Subsequently, two resonator halves were glued together to form a central rectangular (14 x 2 µm) for liquid flow. Two PZT plates, one on each side, were then bonded to the resonator at the base section using silver paste; the central line of the PZT plates was aligned with the nodes of the resonator base section. Finally, the PZT plates were connected electrically

3 Characterization of licon-based Ultrasonic Nozzles 125 Create Horn Patterns on 1stde O 22 Coating of Al Film, Photolithography Coating and of Al Al Etching Film on Photolithography 1 st de and Al Etching on 1st de O O 2 2 O O 2 2 O 2 Align O 2 mark Align mark O 2 O 2 Create Flow Channels on 2 nd de ICP and Photo-resist ICP and Photo-resist Removal Removal Create Flow Channels on 2ndde O 22 mark Photo resist to an RF signal generator using coated copper wires 5 µm in diameter. The impedance of the micro-fabricated nozzles was measured using an Agilent Impedance Analyzer Model 4294A. A schematic diagram of the setup for measuring the longitudinal vibration at the nozzle tip is shown in Figure 3. The pair of PZT transducers of the -based ultrasonic nozzle is driven by the AC electrical signal from an Agilent Function Generator Model 3312A after amplification by Amplifier Research Model 75A25. An Agilent 2- O O 22 O O 22 Align O O 2 2 O 2 Wafer Bonding and ICP Wafer Bonding and ICP Align Al mark ( ) Al Etching and Photo-resist Removal Al Etching and Photo-resist Removal O 2 O 22 ( ) Al Photo resist Figure 2. Micro fabrication steps for manufacture of the silicon based ultrasonic nozzles Oscilloscope 54621A monitors the AC drive signal and also provides an external triggering of the Polytec Ultrasonics Vibrometer Controller Model OFV27. The Controller is part of the Polytec Laser Doppler Vibrometer (LDV) that also contains a Polytec Fiber Interferometer Model OFV 511. A He-Ne laser at nm wavelength in the Interferometer is divided into a reference beam and a probe beam. The probe beam traveling in a single fiber optic cable is focused and

4 126 Y. L. Song et al. Polytec Vibrometer Controller OFV27 Polytec Fiber Interferometer OFV511 -Based Ultrasonic Nozzle Nozzle Holder Nodes Laser/ Photo- Detector Adapter Directional Coupler Amplifier Research 75A25 Agilent Function Generator 3312A Figure 3. Schematic diagram for measurement of longitudinal vibration at the nozzle tip. directed to the vibrating nozzle tip surface at normal incidence. The photo detectormeasures the time dependant intensity of the mixed light of the probe and the reference beams. The resulting (beat) frequency of the mixed light is just the Doppler frequency shift that is proportional to the tip vibration velocity along the axis of the probe beam. The Ultrasonics Vibrometer Controller converts the resulting frequency response into time dependant voltage output with a conversion factor of 125 mm/s/v. The maximum output voltage of the instrument is 3 V peak-to-peak corresponding to a peak-to-peak displacement of 7.5 µm at.5 MHz. Longitudinal vibration at the nozzle base was also measured to provide a reference for experimental determination of the vibration amplitude magnification (or gain) at the nozzle tip. 3. Results and Discussion Agilent 2- Oscilloscope 54621A 3-D simulation [4] displayed two resonant frequencies at 33 and 491 khz of pure longitudinal vibration modes. Indeed, the Agilent impedance analyzer measured these two resonant frequencies that shift to 334 and 496 khz, respectively, as shown in Figure 4. Also shown in the figure is the PZT mode at 534 khz. To further verify the longitudinal mode of vibration, the vibration velocity at the nozzle tip along the nozzle axis (longitudinal vibration) was measured using the Polytec Laser Doppler Vibrometer (LDV) at carefully tuned drive frequencies based on the impe- Impedance Z (Ω) Figure 4. Measured plots of electrical impedance and phase angle versus frequency for -based 5-horn ultrasonic nozzle with design frequency of.5 MHz. LDV Output Volt (V) Amplitude Impedance khz Phase KHz Frequency (khz) f (khz) Base Time (µs) Figure 5. Output signals of longitudinal vibration at the nozzle tip and base of a 5-Foureir horn ultrasonic nozzle measured by Laser Doppler Vibrometer (LDV) at peak-to-peak electrode drive voltage of 11.4 V and resonant frequency of 5 khz. dance analysis as shown in Figure 4. A strong time-dependent voltage output was detected only at the PZT drive frequency of 5 khz. As shown in Figure 5, the output signal at the nozzle tip is almost saturated even at a peak-to-peak electrode Tip Phase (θ )

5 Characterization of licon-based Ultrasonic Nozzles 127 LDV Output gnals Tip_Volt (V) M= Electrode Voltage V pp (V) Base _Volt (V) Figure 6. Output signals of longitudinal vibration at the tips and bases of another 5 Fourier nozzle measured by LDV. voltage as small as 11.4 V. The output signal was reduced to half when the drive frequency varied by ± 1 khz. Figure 6 shows the LDV output signal as a function of peak-to-peak electrode drive voltage for another 5-horn nozzle. Clearly, the output signal decreases as the drive voltage decreases. Also shown in Figure 6 are the measured output signals from the base of the 5-horn nozzle. Dividing the output signal from the tip by that from the base yields an overall amplitude gain of 24 as shown in the figure. Thus, the experimental results of resonant frequency and amplitude gain of longitudinal vibration are in excellent agreement with the simulation results Conclusion 3-D simulation has shown that silicon-based megahertz ultrasonic nozzles using five Fourier horns are capable of achieving a much larger amplitude gain at the nozzle tip with no reduction in the tip cross sectional area than single horn nozzles at the same resonant frequency. These theoretical predictions have been confirmed by both the impedance analysis and the longitudinal vibration measurement. Therefore, the required electric drive power of the 5-horn ultrasonic nozzle should be drastically reduced and the transducer failure in ultrasonic atomization should be readily avoided as well. References [1] Tsai, S.C., Luu, P., Childs, P. and Tsai, C.S., IEEE Transaction on Ultrasonics/ Ferroelectrics and Frequency Control, Vol. 46, pp (1999); Tsai, S.C., U.S. Patent #5,687,95 (1997). [2] Tsai, S. C., Luu, P., Childs, P., Teshome, A. and Tsai, C. S., Physics of Fluids, Vol. 9, pp (1997). [3] Song, Y. L., Tsai, S. C., Chen, C. Y., Tseng, T. K. and Tsai, C. S., Chen, J. W. and Yao, Y. D., (to be appeared in J. Amer. Ceramic Society, 24). [4] Tsai, S. C., Tseng, T. K., Song, Y. L., Chou, Y. F., Tsai, C. S. and Chang, P. Z., Mat. Res. Soc. Symp. Proc., Vol. 729, MEMS and Bio-MEMS, pp (22). Manuscript Received: Dec. 29, 23 Accepted: Jan. 19, 24