Study of self-focused piezoelectric transducer for liquid ejection

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Study of self-focused piezoelectric transducer for liquid ejection

 

Author: Hon, Sau-fong
Title: Study of self-focused piezoelectric transducer for liquid ejection
Degree: M.Phil.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Piezoelectric transducers.
Microelectronic packaging.
Department: Dept. of Applied Physics
Pages: 1 v. (various pagings) : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2351730
URI: http://theses.lib.polyu.edu.hk/handle/200/5409
Abstract: The main objective of the present work is to develop and fabricate focused acoustic ejectors for ejecting drops of viscous liquids in the drop-on-demand mode. To date, most of the ejecting systems for viscous liquids are large and complicated, in particular their electronic driving systems. The newly developed focused acoustic ejectors in the present work are more compact is size and can be operated using simple a electronic driving system. It is anticipated that the ejectors will find various applications in microelectronic packaging. Fresnel zone plates have been designed and fabricated using lead zirconate titanate piezoelectric plates as the self-focused piezoelectric transducers for the acoustic ejectors. Because of the annular structure of the electrodes, the acoustic waves generated by the piezoelectric plate of the Fresnel zone plate are in phase at the designed focal points. As a result, constructively interfere occurs and the intensity is increased. Our results of the distributions of wave intensity in glycerin (with a viscosity of 1400 cP) clearly reveal two focal points, at which the intensity is higher than the surroundings. The observed focal lengths are about 2.48 and 9.5 mm, which agree with the theoretical values. The observed focal spot is small, having a diameter close to the wavelength of the acoustic wave (0.448 mm). The Fresnel zone plates are operated at 4.28 MHz and the sound velocity of glycerin is 1920 m/s. Our results also reveal that after the milling of the un-electroded region of the piezoelectric plate, the vibration and hence the wave intensity is enhanced. The number of the annular electrodes does not have significant effects on the wave intensity. However, the acoustic wave is highly attenuated by the viscous liquids, causing the intensity at the principle focal point (9.5 mm) being smaller than that at the “harmonic focal point (2.48 mm). On the basis of the results, milled Fresnel zone plates with four annular electrodes are used for the fabrication of the focused acoustic ejectors. The ejectors are driven by a simple electrical signal, a series of tone bursts of sinusoidal wave. The ejection performances of the focused acoustic ejectors using glycerin as the medium have been evaluated in detail. Based on the results, the optimum operation parameters, including the driving voltage and duration of the tone burst, have been determined. Our results reveal that the ejector can eject glycerin in the downward orientation using a tone burse with a frequency of 4.28 MHz, a driving voltage of 35 V and a duration of 2 ms. The drop is small, having a diameter of 0.4 mm which is close to the wavelength of the acoustic wave. Using the same operation parameters, the ejector can eject drops in the drop-on-demand mode. The repetition frequency can be increased to 120 Hz, while the temperature remains at about 50°C. Besides glycerin, other viscous liquids, such as the pre-polymer of an epoxy (2000 cP) and a detergent, have been used for the evaluation of the ejection performance, and good results are obtained. Similar to glycerin, both the viscous liquids can be ejected effectively in the drop-on-demand mode using a similar tone-burst signal. It is suggested that since only a small volume of the liquid is deformed, the focused acoustic wave is strong enough to stretch the liquid significantly until the surface tension is overcome. As a result, a liquid drop is formed and ejected. Our results also demonstrate that viscosity is more important than the surface tension in controlling the drop ejection.

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