Problem of Preparing Nano Particles
2.2.2. Problem of Preparing Nano Particles
The preparation of nano-crystalline α-silicon carbide for studying the sintering mechanism is also a very difficult problem. The properly distributed nano crystalline particles within the very narrow range by grinding necessitates a proper understanding of the process of grinding of silicon carbide. At the initial stage, large cracks in original particles propagate, generating smaller particles with very fine cracks. Thus, the probability of particles having cracks progressively decreases with grinding leading to the generation of newer stronger particles. Hence, continuously higher and higher fracture stress is required to grind the particle. During the grinding, it becomes progressively difficult to obtain further reduction in particle size in nano crystalline region [4].
In the nano crystalline region, the nano particles develop the tendency to agglomerate and the physical equilibrium between the aggregates and fragmentation is established, resulting in a decrease in ‘storing of stress energy’. Consequently, it results in an increased stress for initiating fracture and the generation of nano particles. There are two principal factors which need to be controlled to obtain nano particles :
1. Ratio of the weight of the metal balls of grinding media to the total weight of the particles to
be processed by the grinding route.
2. Semi-fluid/viscous characteristic of the grinding media containing fine particles.
SILICON CARBIDE
71 Another factor of importance is the total time of grinding, which also needs to be controlled in
order to optimize the process of grinding to obtain nano particles [4, 5] The purity of the nano particles is also very important. The commercially available α-silicon
carbide is made by Acheson process, in which a mixture of sand and crushed coke with common salt and saw dust is electrically heated in a ‘resistance furnace’ above 2600°C. The reduction reaction of high- grade silica results in a-silicon carbide grains. The overall reaction is :
SiO 2 + 3C = SiC + 2CO
The sawdust is added to increase the porosity of the furnace charge, thus increasing the circula- tion of the reacting gases, and facilitating the removal of carbon dioxide.
As these raw materials always have some impurities, the Acheson α-silicon carbide also contains impurities. Some of these impurities have deleterious effect on the sintering of silicon carbide. Moreo- ver, the product of silicon carbide obtained from Acheson (resistance) furnace are lumps and grits. These are ground to different sizes by various mechanical milling processes, thereby further adding the impurities to the ground particles. This available powder does not sinter. This powder needs to be cleaned. Moreover, after purification, this powder does not give a high density. This needs to be further ground. So, further impurities are incorporated into the nano crystalline powder during the preparation from fine particles by the grinding route.
Finally, the very method of manufacture of silicon carbide keeps some excess carbon in it. Thus, the purification is very much essential to remove all these impurities to make the nano-crystalline silicon carbide particles sinter.
The purpose of any investigation should be to develop a scientific basis and a better understand- ing of the details of the grinding process by ‘attrition milling’ in order to be able to prepare nano- crystalline particles of both α- and β-silicon carbides by optimizing different parameters of the process of grinding. In this way, the optimized ‘grinding parameters’ can be used to prepare the nano particles by attrition grinding route for the purpose of sintering [2-5].