| Nowadays, mono-sized particles with the size between millimeter and nanometer are now drawing greater attention in a number of applications, such as spherical solar cells, semiconductor integrated circuits, electronic packaging, precise instrument such as small gears, rapid solidification and so on. As one of the drop-on-demand (DOD) method, the principle of Pulsated Orifice Ejection Method (POEM) is that a micro column of melt around the orifice is forced to eject away under the action of pressure difference and the reciprocating motion of the rod driving by the piezoelectric actuator. The column necks into a droplet due to its surface tension. One droplet is generated by one reciprocating motion of the rod in the same condition, so the particles prepared by POEM have the characteristics of identical cooling history, identical and controllable size, high sphericity and the same microstructure.In this paper, theoretical models of droplet formation and vibration were established based on self-developed POEM system and with the aid of computational fluid dynamics software Fluent to discuss the mechanism of droplet necking process and droplet vibration. The influences of physical properties and technological parameters on the necking process and droplet vibration were investigated systematically by simulation. Furthermore, the model of solidification&conduction is established to analysis the critical forming condition of Fe-Co based metallic glass particles. Based on the results and discussion, some conclusions can be drawn as follows:POEM is one of the DOD methods, a micro liquid column is forced to eject away by downward movement of the rod, and then it necks into a droplet due to its surface tension. Physical properties, such as contact angle between melt and orifice, surface tension and viscosity of melt, show great effects on necking process.90°can be defined as the critical angle, droplets can be successfully ejected only when contact angle is around or larger than90°. When surface tension is small, some satellite droplets will be obtained and with the surface tension being increased to extremely high, no droplet could be obtained. Viscosity describes a fluid’s internal resistance to flow, necking time will be postponed with the increasing of viscosity. As for extremely high viscosity, necking failed because of energy consumption. Technological parameters also have a close relationship with necking process. The smaller the orifice is, the stronger the effects of surface tension on the liquid surface around orifice are. So preparation smaller particles by POEM with small orifice, it should increase the applied pressure appropriately. The experimental results and simulation results have the same tendency. Droplet will vibrate from irregular spheroidicity to spherical during falling after necking. The physical properties also have significant effects on droplet vibration. With the increase of surface tension, the vibration frequency increases and the vibration time decreases; with the increase of viscosity, vibration frequency changes little, but vibration time decreases greatly. Different sizes of [(Fe0.5Co0.5)0.7sB0.2Si0.05]96Nb4metallic glass particles with the characteristics of high sphericity and narrow size distribution were successfully prepared by POEM in the atmosphere of He or50%Ar+50%He, respectively. With the droplet size increasing, the content of glassy phase decreased and crystalline phase occurred. The temperature inside the droplet and the temperature gradient decrease as the distance to the center increasing. As for smaller droplets, it is easy to form fully glassy phase because of the irregular arrangement of atoms caused by larger temperature gradient. Critical cooling rate for fully glassy phase of this composition metallic glass particles prepared by POEM is estimate to be within the range of2200-2400K/s independent of the atmosphere. |
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