摘要
氮化硅是一种具备优异性能的陶瓷材料,其粉体的制备工艺有多种,其中,流化床中直接燃烧化合制取氮化硅粉技术比较容易实现工业化和大型化。由于硅粉原料的粒径较小,流化中易产生活塞流、沟流、团聚等现象,使得流化质量恶化。为改善流化状况,提出了外加声场强化流化的方法。
本文通过冷态实验和理论模型推导相结合的方式,研究硅粉在声场流化床中的流化特性。论文工作主要包括三部分:
(1)综述了国内外改善细颗粒流化质量方法的研究现状,分析各种方法的特点。声场流态化以其不受物料限制、结构简单、效果明显的优点,最适用于氮化硅粉制备工艺。通过研究声场流态化领域取得的进展,了解不同声场因素的作用规律。
(2)设计和搭建了声强化流化床冷态实验装置,包括流化床本体、声发生和声测试系统、压力测试系统。在该声强化流化试验台上进行了流化特性实验,研究声压级、声频率、声波形等声场参数和床高、粒径等物性参数对细颗粒流化质量的影响。得到如下结论:床高对临界流化速度几乎没有影响;声压级升高,临界流化速度下降;声频率存在一个界限,高于该界限时,对临界流化速度影响甚微,低于该界限时,临界流化速度陡降;相比三角波和方波,正弦波具有更好的作用效果;外加声场可以使难流化的C类颗粒流化,使易于流化的A类颗粒临界流化速度显著降低。综合比较得出实验条件下的最佳声参数为140dB、100Hz的正弦波。
(3)根据能量平衡模型,推导出细颗粒临界流化速度的计算公式,对A类颗粒在较宽的频率范围内适用。并可用来预测不同温度下的临界流化速度。
论文通过实验和理论研究,得到声场对流化质量的影响规律,不仅为硅粉氮化研究提供依据,还可为声场流态化的进一步研究和应用奠定基础。
Silicon nitride is a sort of ceramic material of excellent performance and has diverse preparations of its powder, of which the technology to generate silicon nitride powder directly through combustion in fluidized bed is more feasible to industrialize and be used in large scale.
Due to the tiny size of raw silicon powder, phenomena such as plug flow, channeling, and agglomeration are easy to occur during the fluidizing procedure, deteriorating the quality of fluidization. To improve the fluidization quality, sound assist method to enhance fluidization was proposed.
This paper is dedicated to study the fluidization characteristics of silicon powder in the fluidized bed of acoustic field through Cold state experiments. It consists of three parts:
(1) Status quo to improve the quality of fine particles flow at home and abroad was reviewed, and the characteristics of various methods was analyzed. Sound field stands out to be the most suitable for preparation of silicon nitride powder owing to its unconstrained admit to the fluidized material, simple structure, and the distinct effect. Through the progress of sound assist fluidization, behaviors of different sound field parameters could be understood.
(2) A device designed for the sound strengthen CFB Cold experiment was built, including fluidized bed body, a sound system and sound testing, pressure testing system. Experiments taken have studied the effect on fluidization quality for particles brought forth by acoustic parameters such as sound pressure level, sound frequency, sound wave and physical parameters such as bed height, particle size and so on.
Following conclusions were derived from these experiments: 1. the critical flow rate was hardly affected by the bed height; 2. the critical flow rate declined when the sound pressure level increased; 3. there is a threshold frequency, above which the critical flow rate was almost not influenced, while abrupt drop can be observed under the frequency; 4. compared to triangle wave and square wave, sine wave has better effect; 5. acoustic field could help to fiuidize group C which is difficult to fluidize under normal state and would significantly decrease the critical flow rate of group A which is easy to fluidize.
Comprehensive comparison showed that the best sound parameters for the test are 140 dB, 100 Hz, sine wave.
(3) According to the energy balance model, the formula for calculating the critical fluidization rate of fine particles could be derived, which is applicable for group A with a wide frequency range, and the critical flow rate under different temperature could be predicted.
This paper aims at the fundamental disciplines the sound field imposed on the critical fluidization rate. It has provided the basis for research on direct nitridation of silicon in fluidized bed, furthermore laying the foundation for research and application of sound field fluidization.
引文
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