Ultrasonic Spray Pyrolysis for Nanoparticles Synthesis and Silicon-Based MHz Ultrasonic Nozzles

• Main Authors: Yu-Lin Song, 宋昱霖
• Other Authors: 鄭秀蘭
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/16483893052539741196

博士 === 國立臺灣大學 === 物理研究所 === 93 === Abstract This thesis includes two parts: (I) spray pyrolysis for nanoparticles synthesis and (II) silicone-based MHz ultrasonic nozzles. The MHz ultrasonic nozzles aim at producing uniform precursor droplets of the sizes needed for the nanoparticles synthesis. Part I presents new findings on ultrasonic spray pyrolysis of zirconium hydroxyl acetate precursor drops whose sizes were precisely measured using laser light diffraction technique. Precursor concentration plays a predominant role in determination of product particle size. At 0.01 wt% precursor concentration, conventional spray pyrolysis at 750 oC using precursor drops 5-8 μm in diameter, generated by an ultrasonic nebulizer at 2.66 MHz, yielded uniform spherical yttria stabilized zirconia (YSZ) particles 73 nm in diameter measured by scanning electron microscopy. The YSZ particle diameters were much smaller than those predicted by the one-particle-per-drop mechanism. Under similar reaction conditions, the high throughput ultrasound-modulated two-fluid (UMTF) spray pyrolysis of larger precursor drops (28 μm peak diameter) also yielded spherical dense particles; they were significantly smaller in size than those produced by the low throughput conventional ultrasonic spray pyrolysis of smaller drops (6.8 μm peak diameter). And these particles are much smaller than predicted by the conventional one particle per drop mechanism, suggesting that a gas-to-particle conversion mechanism may also be involved in spray pyrolysis. In addition, the uniform nanoparticles can be produced by spray pyrolysis at proper conditions using uniform drops of precursor salts with low vapor pressure. Part II presents the design, simulation, characterization and experimental results of micro-fabricated 0.5 MHz silicon-based ultrasonic nozzles. This thesis reports for the first time on successful ultrasonic atomization using micro-fabricated silicon Si-based high frequency ultrasonic nozzles of a novel design. Each nozzle is made of a piezoelectric drive section and a silicon-resonator consisting of multiple Fourier horns each with half wavelength design and twice amplitude magnification. Results of finite element 3-D simulation using the commercial ANSYS program predict existence of one resonant frequency of pure longitudinal vibration. Both 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 at the nozzle tip increases as the number of Fourier horns (n) increases in good agreement with the theoretical values of 2n. 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, decreasing the likelihood of transducer failure in ultrasonic atomization. Accordingly, the electric drive power required for atomization using the 3-horn nozzle is only 1/4 of that for a single horn nozzle. The drive voltage and power required for a 0.5 MHz 5-Fourier horn nozzle could be as low as 3 V and 0.1 Watt, respectively. The very low drive power requirement of the Si-base high frequency ultrasonic nozzles is highly desirable in their applications to nanoparticles synthesis and bio-dispersion spray.