Atomization Characteristics Of Vacuum Spray Jet Based On Conical And Fan-shaped Nozzles

As an intense research area, polymer thin films with well flexible, mechanical, low-cost and optoelectrical properties are being widely studied for promising applications in molecular electronics such as sensors, solar cells, field-effect transistors and light emitting diodes. Due to the problem of thermal decomposition, polymer films are difficult to be prepared by conventional thermal vapor, sputtering and e-beam deposition methods. Liquid phase fabrication technologies, such as spin-coating, ink-jet printing, screen printing and dip coating, are normally used to fabricate polymer film. However, such 'wet' methods are mostly carried out under ambient conditions, which can hardly avoid the influences of oxygen, solvent residue and dust on the as-fabricated coatings. Recently, vacuum spray methods have been developed for polymer thin films fabrication. In this case, polymer solutions with low-concentration (even<0.1 wt%) can be introduced into a vacuum chamber through high-pressure nozzle to deposit onto substrate via jet atomization. The as-fabricated polymer films have advantages of homogeneous thickness, smooth surface, less impurity and gradient controllability. Besides, effects of dust and solvent residue can be largely excluded.However, many researches in vacuum spray deposition mostly focus on morphology and characteristics of the as-fabricated polymer thin films. To obtain uniform and smooth polymer films, it is important to obtain large spray cone-angle, jet flow velocity and small droplet size, which are mainly dominated by the nozzle geometry and spray conditions. In this paper, the vacuum spray process for conical and fan-shaped nozzles based on cylinderical nozzle is theoretically studied by using the Gambit and Fluent software to learn the influence of simple direct-injection nozzle structures on vacuum jet atomization. It finds that conical nozzle has larger dynamic pressure at nozzle exit, jet flow velocity, penetration distance compared with fan-shaped nozzle. The structure of straight line has great effect on nozzle properties in converging and relaxing the liquid.The mathematical model of vacuum jet flow atomization is preliminarily established and improved based on cone-straight nozzle. The effect of spray parameters and structure parameters of cone straight nozzle on performances of nozzle properties and vacuum jet flow atomizing structures is investigated. The results show that shrink angle has great impact on nozzle properties and jet flow atomization, and there is an optimum value (about 90°) trends to the largest spray cone-angle. The flux outlet the nozzle decreases with increase of the square of injection pressure, a nozzle with smaller diameter has larger spray cone-angle.The paper further simulates and analyzes the vacuum spray process of combined nozzles, such as sector-straight-cone nozzle, sector-cone-straight nozzle, straight-cone-sector nozzzle, straight-sector-cone nozzle, cone-sector-straight nozzle and cone-straight-sector nozzle, respectively. Also, the effect of variable-area flow nozzle, SPZZ nozzle and separating nozzle on vacuum jet flow atomization is studied. The results show that composite nozzles with conical structure outlet have larger dynamic pressure, jet flow velocity, discharge coefficient and initial spray cone-angle. The link stopper with bigger gradient in SPZZ nozzle can impede the liquid flow, but playing a guiding role when gradient is smaller, and atomization is better with shorter distance from nozzle exit. The first form of separating nozzle has larger dynamic pressure, discharge coefficient, jet flow velocity and spray area, the jet flow velocity has an inverse relationship with initial spray cone-angle. The results may be heipful for the optimization of nozzle structure and the melioration in fabrication of large-area polymer thin films.