【作者】 赵丽；

【导师】 董伟；

【作者基本信息】 大连理工大学 ， 材料学， 2014， 硕士

【摘要】 球形微米级粒子特指尺寸在纳米和毫米之间的球形粉末颗粒,目前在球形硅太阳能电池、硅锗半导体集成电路、电子封装领域、制备精密仪器乃至快速成型制造领域都具有广泛的应用前景。脉冲微孔喷射法(POEM)属于按需喷射,在压力差和压电陶瓷的共同作用下使微小液柱从小孔中喷出,在表面张力的作用下颈缩形成液滴。压电陶瓷往复运动,每向下运动一次产生一个液滴,每个液滴形成的条件完全相同,不受实验环境的影响,因此粒径分布窄、圆球度高、热履历和微观结构均趋于一致。本文基于自行研制的微米级均匀粒子制备设备—POEM,借助于流体动力学软件-Fluent,建立液滴形成和振动模型,探讨液滴颈缩和振动机理,研究液滴物性值和实验参数等对颈缩和振动的影响；此外,建立凝固-传热模型,分析Fe-Co基金属玻璃粒子形成的临界条件。通过研究,得到如下结论：POEM设备属于按需喷射,由向下运动的传动杆将微小液柱从小孔中喷出,在表面张力的作用下颈缩形成液滴；液体的物性值(例如液体与坩埚的润湿角、表面张力、粘度等)对液滴的颈缩行为具有重要影响；只有当液体与坩埚的润湿角大于等于90°时才能颈缩得到液滴；当表面张力较小时会产生附加的卫星滴,当表面张力特别大时由于不能提供表面能导致颈缩失败；粘度可以衡量流体的内摩擦力和机械能的损耗,随着粘度的增加颈缩时间会延迟,当粘度特别高时,由于能量消耗会导致颈缩失败；实验参数与液滴的颈缩行为也有密切关系；用不同孔径尺寸制备微粒子时,微孔尺寸越小,表面张力的作用越明显,因此需要适当增大差压,而随微孔尺寸增加时,为了获得稳定液滴需要增大传动杆的速度；颈缩后的液滴在下落过程中做阻尼振动,由最初的不规则椭球形逐渐振动为球形：材料本身的物性值(如表面张力、粘度等)对液滴的振动有很大影响：增大表面张力,振动频率加快,振动时间缩短；粘度增加,振动频率几乎不受影响,而振动时间显著降低；用POEM设备在纯He气氛和混合气氛(50％He+50％Ar)制备粒径均匀、圆球度高、具有不同粒径的[(Feo.5Co0.5)0.75B0.2Si0.05]96]Nb4金属玻璃粒子；随粒径增大,玻璃相含量降低,内部出现结晶相；随着距中心距离的增大,液滴温度逐渐降低,温度梯度也逐渐降低；粒子越小,平均温度梯度越大,邻近原子越难形成规则排列,容易形成玻璃相；用POEM法制备该成分的金属玻璃微粒子的临界冷却速率为2200-2400K/s,不随冷却气氛而改变。更多还原

【Abstract】 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.更多还原