Effect of solution concentration on droplet size in ultrasonic aerosol generator

Abstract

INTRODUCTIONWith the progresses in semiconductor technology, multicomponent thin films such as Pb(ZrxTi1-x)O3, (BaxSr1-x)TiO3 are getting more attention. Recently, liquid source misted chemical vapor deposition (LSMCVD) that takes advantage of the ultrasonically atomized precursor droplets was appeared (Huffman, 1995). The film quality was reported to be good. But it has been remained unknown about the characteristics of the produced droplets which are crucial for the preparation of good quality film. There have been a few reports about the size of ultrasonically atomized droplets since 1960s (Lang, 1962, Peskin et al. 1963). All of those results were obtained for pure solvents or at fixed concentration. The surface tension, density and the atomization frequency were identified as the primary variables that affects the size of the produced droplets. In this study, the size and size distribution of the ultrasonically atomized droplets were measured and the effect of the concentration on the droplet size was examined based on the preexisting theory.METHODSDroplets were produced by a domestic humidifier and carried by carrier gas to a measurement unit as shown in Fig. 1. Malvern 260C (He-Ne laser, wavelength of 6.382A) was used to measure the size and size distribution of produced droplets. Lead acetate was dissolved into two different solvents; 2-methoxyethanol and methanol. The concentrations were varied and the density, surface tension and the size and size distribution were measured at each concentration.RESULTSAs can be seen in Fig. 2, the mean diameter of the droplets decreased as the concentration increased and the mean size was smaller than those predicted by models proposed by Peskin et al. (1963) and Hinds (1982). The general form of these two models is as follows: where - surface tension; - density; 0 - atomization frequency; C1 - proportionality constant.These models predict that the mean size is inversely proportional to the power of two third of the atomization frequency and proportional to the one third of the ratio of surface tension to density. The proportionality has not yet been tested experimentally when the surface tension and density vary with the solution concentration. Fig. 3 shows the change of mean size as a function of the ratio of surface tension to density for the two solutions. The slopes of the regression line are 0.33 and 0.66 for 2-methoxyethanol and methanol solution, respectively. If the model equation (1) is correct, these slopes should be all 23.Therefore, we have to further investigate the effect of solution concentration on the droplet size more carefully.