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Ultrasonic nozzle

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Ultrasonic nozzles are a type of spray nozzle that uses high frequency sound waves produced by piezoelectric transducers acting upon the nozzle tip that will create capillary waves in a liquid film. Once the amplitude of the capillary waves reach a critical height (due to the power level supplied by the generator), they become too tall to support themselves and tiny droplets fall off the tip of each wave resulting in atomization.[1]

Picture of an ultrasonic nozzle

The primary factors influencing the initial droplet size produced are frequency of vibration, surface tension, and viscosity of the liquid. Frequencies are commonly in the range of 20-180 KHz, beyond the range of human hearing, where the highest frequencies produce the smallest drop size.[2]

History

The phenomenon referred to as ultrasonic atomization has its roots in late 19th century acoustical physics, specifically in the works of John William Strutt, 3rd Baron Rayleigh, author of the definitive work The Theory of Sound.[3] In 1962 Dr. Robert Lang followed up on this work, essentially proving a correlation between his atomized droplet size relative to Rayleigh's liquid wavelength.[4] Ultrasonic nozzles were first brought to market commercially by Dr. Harvey L. Berger. US A 3861852, "Fuel burner with improved ultrasonic atomizer", published Jan 21, 1975, assigned to Berger Harvey . The first application of Dr. Berger's work was in home heating oil burners, in an attempt to create a more efficient burner and conserve fuel during the 1970s energy crisis in the United States.[5]

Subsequent uses of the technology include coating blood collection tubes, spraying flux onto printed circuit boards, coating implantable drug eluting stents and balloon/catheters, Float glass manufacturing coatings,[6] anti-microbial coatings onto food,[7] precision semiconductor coatings and alternative energy coatings for solar cell and fuel cell manufacturing, among others.

Ultrasonic nozzle applications

Drug Eluting Stents and Drug Eluting Balloons

Pharmaceuticals such as Sirolimus (Rapamycin) and Paclitaxel used with or without a polymer is coated on the surface of drug eluting stents (DES) and drug eluting balloons (DEB). These devices benefit greatly from ultrasonic spray nozzles for their ability to apply coatings with little to no loss. Medical devices such as DES and DEB because of their small size, require very narrow spray patterns, a low-velocity atomized spray and low-pressure air.[8]

Fuel Cells

File:How Ultrasonic Spray Nozzles Work.oggtheora.ogv
How ultrasonic spray nozzles work.

Research has shown that ultrasonic nozzles can be effectively used to manufacture Proton exchange membrane fuel cells. The inks typically used are a platinum-carbon suspension, wherein the platinum acts as a catalyst inside the cell. Traditional methods to apply the catalyst to the proton exchange membrane typically involve screen printing or doctor-blades. However, this method can have undesirable cell performance due to the tendency of the catalyst to form agglomerations resulting in non-uniform gas flow in the cell and prohibiting the catalyst from being fully exposed and running the risk that the solvent or carrier liquid may be absorbed into the membrane, both of which impeded proton exchange efficiency.[9] When ultrasonic nozzles are used, the spray can be made to be as dry as necessary by the nature of the small and uniform droplet size, by varying the distance the droplets travel and by applying low heat to the substrate such that the droplets dry in the air before reaching the substrate. Process engineers have finer control over these types of variables as opposed to other technologies. Additionally, because the ultrasonic nozzle imparts energy to the suspension just prior to and during atomization, possible agglomerates in the suspension are broken up resulting in homogenous distribution of the catalyst, resulting in higher efficiency of the catalyst and in turn, the fuel cell.[10][11]

Carbon Nanotubes

CNT thin films are used as alternative materials to create transparent conducting films (TCO layers)[12] for touch panel displays or other glass substrates, as well as organic solar cell active layers. [13]

Photoresist Spray onto MEMs Wafers

Microelectromechanical Systems (MEMs)[14] are small microfabricated devices that combine electrical and mechanical components. Devices vary in size from below one micron to millimeters in size, functioning individually or in arrays to sense, control, and activate mechanical processes on the micro scale. Examples include pressure sensors, accelerometers, and microengines. Fabrication of MEMs involves despositing a uniform layer of photoresist[15] onto the Si wafer. Photoresist has traditionally been applied to wafers in IC manufacturing using a spin coating technique.[= 1] In complex MEMs devices that have etched areas with high aspect ratios, it can be difficult to achieve uniform coverage along the top, side walls, and bottoms of deep grooves and trenches using spin coating techniques due to the high rate of spin needed to remove excess liquid. Ultrasonic spray techniques are used to spray uniform coatings of photoresist onto high aspect ratio MEMs devices and can minimize usage and overspray of photoresist.[16]

Printed Circuit Boards

The non-clogging nature of ultrasonic nozzles, the small and uniform droplet size created by them and the fact that the spray plume can be shaped by tightly controlled air shaping devices make the application quite successful in wave soldering processes. The viscosity of nearly all fluxes on the market fit well within the capabilities of the market. In soldering, "no-clean" flux is highly preferred. But if excessive quantities are applied the process will result in corrosive residues on the bottom of the circuit assembly.[17]

Solar Cells

Photovoltaic and dye-sensitized solar technology both need the application of liquids and coatings during the manufacturing process. With most of these substances being very expensive, any losses due to over-spray or quality control are minimized with the use of ultrasonic nozzles. In efforts to reduce the manufacturing costs of Solar cell, traditionally done using the batch-based Phosphoryl chloride or POCl3 method, it has been shown that using ultrasonic nozzles to lay a thin aqueous-based film onto silicon wafers can effectively be used as a diffusion process to create N-type layers with uniform surface resistance.[18]

Ultrasonic Spray Pyrolysis

Ultrasonic Spray pyrolysis is a Chemical vapor deposition (CVD) method utilized in the formation of a variety of materials in thin film or nanoparticle form. Precursor materials are often fabricated through sol-gel methods and examples include the formation of aqueous silver nitrate,[19] synthesis of zirconia particles,[20] and fabrication of solid oxide fuel cell SOFC cathodes.[21]

 
High Temperature Ultrasonic Nozzle

An atomized spray produced from an ultrasonic nozzle is subjected to a heated substrate typically ranging from 300-400 degrees C[22]. Due to the high temperatures of the spray chamber, extensions to the ultrasonic nozzle (as pictured and labeled - High Temperature Ultrasonic Nozzle) such as a removable tip (tip is hidden under the vortex air shroud labeled #2) have been designed to be subjected to high temperatures while protecting the body (labeled #1) of the ultrasonic nozzle that contains temperature sensitive piezoelectric elements, typically outside of the spray chamber or by other means of isolation.

References

  1. ^ Lang, Robert (1962). "Ultrasonic Atomization of Liquids". The Journal of the Acoustical Society of America. 34: 6.
  2. ^ Berger, Harvey (1998). Ultrasonic Liquid Atomization Theory and Application. Hyde Park, NY: Partridge Hill Publishers. p. 44. ISBN 0-9637801-5-8.
  3. ^ Rayleigh, J. W. S. (1877,1894). The Theory of Sound Vol. I. London: Macmillan. ISBN 978-1-108-03220-9. {{cite book}}: Check date values in: |date= (help)
  4. ^ Lang, Robert (1962). "Ultrasonic Atomization of Liquids". The Journal of the Acoustical Society of America. 34: 6.
  5. ^ Berger, Harvey. "Fuel burner with improved ultrasonic atomizer".
  6. ^ Davis, Nancy (Feb 2005). "Ultrasonic Spray for Glass Manufacturing" (PDF). Glass Magazine.
  7. ^ DiNapoli, Jessica (2013-10-10). "Sono-Tek targets food safety". Times Herald-Record.
  8. ^ Berger, Harvey. "Director of Technology". European Medical Device Technology. Retrieved 7 February 2014.
  9. ^ Wheeler, D (March 2008). "Status of Manufacturing: Polymer Electrolyte Membrane (PEM) Fuel Cells" (PDF). Technical Report. NREL/TP-560-41655: 6. {{cite journal}}: |volume= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ Engle, Robb (2011-08-08). "MAXIMIZING THE USE OF PLATINUM CATALYST BY ULTRASONIC SPRAY APPLICATION" (PDF). PROCEEDINGS OF ASME 2011 5TH INTERNATIONAL CONFERENCE ON ENERGY SUSTAINABILITY & 9TH FUEL CELL SCIENCE, ENGINEERING AND TECHNOLOGY CONFERENCE. ESFUELCELL2011-54369.
  11. ^ Millington, Ben (2011-10-15). "A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique". Journal of Power Sources. 196 (20): 8500–8508. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Majumder, Mainak; et al. (2010). "Insights into the physics of spray coating SWNT films". Chemical Engineering Science. 65: 2000–2008. {{cite journal}}: Explicit use of et al. in: |first= (help)
  13. ^ Steirer, K. Xerxes; et al. (2009). "Ultrasonic spray deposition for production of organic solar cells". Solar Energy Materials & Solar Cells. 93: 447–453. {{cite journal}}: Explicit use of et al. in: |first= (help)
  14. ^ "Microelecromechanical Systems (MEMS)".
  15. ^ "Pattern Transfer".
  16. ^ "Process for Coating a Photoresist Composition onto a Substrate".
  17. ^ Rathinavelu, Umadevi. "Effect of No-Clean Flux Residues on the Performance of Acrylic Conformal Coating in Aggressive Environments" (PDF). IEEE.
  18. ^ Voyer, Catherine (June 7, 2004). "Evaluation of Dopant Sources and Deposition Methods Suitable for In-Line Diffusion in the PV Industry" (PDF). 19th European Photovoltaic Energy Conference: 848.
  19. ^ Kalyana C. Pingali, David A. Rockstraw, and Shuguang Deng, 2005, "Silver Nanoparticles from Ultrasonic Spray Pyrolysis of Aqueous Silver Nitrate," Aerosol Science and Technology, 39:1010-1014
  20. ^ Y. L. Song, S. C. Tsai, C. Y. Chen, T. K. Tseng, C. S. Tsai, J. W. Chen, and Y. D. Yao, 2004, "Ultrasonic Spray Pyrolysis for Synthesis of Spherical Zirconia Particles" Journal of the American Ceramic Society, Vol.87, No. 10
  21. ^ Hoda Amani Hamedani, 2008, Investigation of Deposition Parameters in Ultrasonic Spray Pyrolysis for Fabrication of Solid Oxide Fuel Cell Cathode, Georgia Institute of Technology
  22. ^ Nakaruk, A (Nov 6, 2010). "Influence of Deposition Temperature on Titania Films Deposited by Ultrasonic Spray Pyrolysis". The AZo Journal of Materials Online. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Berger, Harvey L. Ultrasonic Liquid Atomization: Theory and Application. 2nd ed. Hyde Park: Partrige Hill, 2006. 1-177.

Lefebvre, Arthur, Atomization and Sprays, Hemisphere, 1989, ISBN 0-89116-603-3


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