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Firstly, the coating materials to be deposited are prepared into liquids such as solution, sol or suspension, and then these liquids are atomized into droplets in micron scale through a special ultrasonic atomization device called ultrasonic nozzle. These micro droplets are evenly deposited on the surface of the substrate by a certain amount of carrier gas, so as to form a thin-film coating layer. For some specific materials, ultrasonic coating can obtain high-quality thin film coatings with only a small part of the cost of thin film deposition equipment such as vapor deposition or sputtering. Therefore, ultrasonic coating is regarded as an economic and excellent thin film deposition process. Compared with traditional single fluid or two fluid spray coating (air spray coating), ultrasonic coating has significant advantages, such as very soft and uniform atomization, non-clogging spray nozzle, highly controllable flow rate, etc., so as to obtain much higher utilization rate of raw material (up to more than 4 times) than air spray coating, higher uniformity and accuracy, and lower maintenance cost. The ultrasonic spray nozzle which is the core component of ultrasonic coating system is introduced in detail as below.
Ultrasonic nozzle, also known as ultrasonic spray nozzle or ultrasonic spray gun, is an ultrasonic atomization device based on the principle of Langevin transducer. The maximum amplitude of ultrasonic vibration is designed at the front tip of the ultrasonic spray nozzles. When the liquid is delivered from the rear of the nozzle to the front tip, it is shattered into small droplets by the high-frequency ultrasonic oscillation at the nozzle tip to form atomization. Then, the liquid mist can be sent to the substrate or specific space through a certain amount of carrier gas. Under the condition of not exceeding the limit, the liquid atomization amount is only determined by the flow rate of liquid delivery and the operation frequency of the ultrasonic spray nozzle. The atomization of ultrasonic nozzle still conforms to the principle of ultrasonic atomization. The atomized droplets size can be determined by the following equation,
where, f is ultrasonic frequency, D0.5 is mean droplet diameter, ρ is the liquid density, σ is the liquid tension. Because the liquid tension has a certain relationship with the liquid viscosity, and the viscosity of the liquid that can be atomized by ultrasound is very limited, generally less than 30cps, so the range of liquid tension can be changed is very small. Similarly, the liquid density will not be much different. Therefore, the main factor determining the size of atomized droplets is ultrasonic frequency. It can be seen from the equation that the higher the ultrasonic frequency gives the smaller the atomized droplets. Consequently, we can control the size of atomized droplets by adjusting the ultrasonic frequency of ultrasonic nozzle. The distribution of atomized droplets size can be measured by laser particle sizer. Figure 2 shows the distribution of atomized droplet size of siansonic ultrasonic nozzle and air nozzle. We can see that the atomized droplet of ultrasonic nozzle is smaller and more uniformly distributed.
A variety of chemical reagents, lubricating oil, particle suspension, etc. can be atomized by ultrasonic nozzle. However, the viscosity, miscibility and solid concentration of the liquid need to be considered. For the best atomization effect, the liquid viscosity should be less than 30cps and the solid concentration should be less than 30%. Because the atomization process is affected by the motion effect produced by the liquid film. Because the atomization process is affected by the motion effect produced by the liquid film, high viscosity results in low atomization, which brings difficulties to the application. Due to the high cohesion of high polymer molecules, atomizing liquids containing high polymer molecules is problematic even when diluted. Most of the time, the solid particle mixture can be atomized. Because the solid particles will be dispersed in the atomized droplets. Even coarse mud can be atomized when it is delivered to the nozzle at low speed.
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