Sintering of Ceramics

2.2.3. Sintering of Ceramics

The ceramic materials are refractory, i.e. it can withstand high temperature, and they are also brittle. Hence, they pose problems in their preparation, whereas the metals and polymers are melted, cast and sometimes machined to get the right type of dimensions. The preparation of ceramic materials mainly consists of grinding the raw materials, mixing with suitable binders and then pressing into differ- ent shapes and sizes to form the so-called ‘green bodies’, which are low in strength so that they cannot

be used for any application. The intial density of the green body is very important to achieve a higher sintered density.

After this stage, they have to be fired or sintered at sufficiently high temperature to complete different desirable ‘phase transformations’ and also develop necessary ‘atomic bonding or cohesion’ so that they have high strength in order to be suitable for a given application. This process of sintering is a subject by itself, so it cannot be discussed in details here. Moreover, there are various types of sintering,

i.e. gas-phase sintering (ZnO), liquid-phase sintering (ceramic whitewares and some refractories) and solid state sintering that consists of a host of ceramic materials with good mechanical and other proper- ties of importance. Here, only the latter will be discussed very briefly for the acquaintance on the sub- ject, which is only necessary for the understanding of sintering of the nano materials.

Generally speaking, the grain shape of ceramic raw materials is considered to be composed of spherical ensemble. When these ‘spheres’ are packed in a given volume, then there will be naturally a lot

NANO MATERIALS

of ‘pores’ between the grains of different sizes. The elimination of these ‘pores' in order to make the spherical particles come closer together with high cohesion is called sintering, when almost the theoreti- cal density of a given material can be achieved. The heating process involved in sintering gives the necessary energy, but there must be some driving force for ‘pore elimination’, with a consequent change in the surface dynamics.

What is the macroscopic driving force for sintering ?

The reduction of ‘excess energy’ associated with the surfaces is the main driving force. This reduction can take place mainly by two processes :

1. Total surface area is decreased by increasing the particle sizes leading to a ‘coarsening stage’,

2. Solid/Vapour interfaces are eliminated with the creation of ‘grain boundaries’, which is fol- lowed by the ‘grain growth’ leading to → ‘densification or sintering’.

Actually, in a given ceramic material undergoing sintering, the above two processes are in ‘com- petition’. However, our main goal is to reduce the former and optimize the latter, i.e. the densification. At a given sintering temperature, if the atomic processes leading to densification dominate, the pores start getting smaller and eventually disappear with sintering time, and the sintered compact shrink in size. However, if the atomic processes are such that the ‘coarsening’ is faster, both the pores and grains coarsen and get larger with time, i.e. excessive grain growth, that must be avoided at all cost.

The necessary condition for solid-state sintering to occur is that the grain boundary energy is less than twice the solid/vapour surface energy so that the ‘solid dihedral angle’ between the grains has to be less than 180° for the densification to occur. This is explained clearly in terms of an equation in the section - 2.4.1, with the details for sintering of nano particles of SiC.

For an effective sintering process in solid nano materials, the mass has to be transferred mainly by three different atomic diffusion mechanisms as :

1. Surface diffusion,

2. Volume diffusion, and

3. Grain-boundary diffusion. This obviously excludes the other processes like evaporation-condensation, and viscous and creep

flow in sintering. Now, it is important to know which of the above diffusion processes is operative in the sintering mechanism. This is described in terms of a ‘model’ that is relevant for the sintering of nano particles of SiC in the section - 2.7.3, along with the role of dopants in creating the necessary driving force for full densification. It has to be remembered that when the particles are small, say in the nano range, the total surface area is very large, which consequently gives a high surface energy for these nano particles. Hence, the driving force for reducing this surface is large enough to drive the sintering process to be completed faster.

Therefore, it is prudent to go ahead to full densification by the preparation of the nano particles of SiC, which is otherwise difficult for a covelent solid like SiC. This is what has been actually done in the present case. All the experimental data which have been presented here are interpreted with the help of such atomic diffusion processes in the entire discussion on sintering of nano particles of SiC in the following subsections. Moreover, in order to make our knowledge clear, the role of different dopants or additives, the role of adding carbon and the dependence of different atmospheres of sintering have all been delineated in order to elucidate the problem of sintering further. This should help the readers to understand the mechanism of solid state sintering through ‘practical’ data on an actual material like SiC.

SILICON CARBIDE