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Nanosintering
Sintering of nanomaterials offers the possibility to produce parts with the versatility that powder technology offers but still keeping the physical features that a nanometer-range grain structure offers, such as unusually high strength, hardness, wear resistance or superplasticity. There are however many challenges in sintering of nanomaterials, such as powder agglomeration and also high contamination due to the particles high surface energy. It has also been shown that nanoparticles sinters at lower temperatures which could hinder the intergranular bonding since sintering time has to be minimized to keep the nanoregime properties of the material.

Driving force
Nanopowders are very unstable due to their high surface area per unit volume which gives them a high surface energy. The sintering process is driven by this surface energy because the system as a whole tries to minimize its energy. In classical sintering theory the surface energy for the particles are assumed to be isotropic(i.e. a spherical particle with the same surface energy over the whole surface) but when particles are nanosized they can have preferential geometrical shapes due to the crystal structure of the material (for more information see Nanoparticles). These differences gives the powder compact different properties during sintering compared to a similar powder compact but with particles of greater size, differences such as sintering at lower temperatures and a faster sintering rate.

In classical sintering theory the procedure of sintering a powder compact is divided into three stages, initial, intermediate and final stage sintering. In these stages there are different mechanisms that are active, for example during the initial stage surface diffusion is considered most active during the formation of the sintering necks. The grain/particle size has different effect on the different mechanisms of material transportation(e.g. more grain boundary area will give more grain boundary diffusion and more surfaces will give more surface diffusion).

A difference from normal sintering is for example the argument that there is no densification from surface diffusion (see sintering mechanisms) one could expect that when the grains get smaller (i.e. more surface) there would be no or less densification for nanopowders compared with regular sized particles.. But as it turns out most of the time the opposite is observed. The difference in results could be due to the nanopowders tendency to attract contamination and to agglomerate which affects the surface energy. Also the anisotropic surface could play a major role in affecting the surface diffusion.

Compaction
The final density of the specimen after sintering strongly depends on how the green piece is compacted. Thus making the compaction step very important and generally it can be said that a good dense compact of nanopowders harder to press a compact of conventional powder. Large pores within the green compact is a common problem and elimination of these is achieved with high temperatures which as stated above promotes grain growth and thus the loss of the desired nanofeatures.

During cold compaction the particles are experiencing different stages like sliding and rearrangement, elastic compression at contact points and plastic yielding for metals. The main difference for nanoparticles are the large surface area which gives rise to more friction between particles, also the small particle size increases the amount of interparticle contact points which gives rise to more friction. The extra forces which becomes greater with a smaller particle size is due to mechanical, van der Waals force and surface adsorption phenomena. Irregular particle shapes also prevents particle rearrangement and promotes agglomeration. The green density of the compacts can be improved with the help of lubricants and coatings on the particles.

Contamination
Due to the high surface energy of the nanoparticle it has a tendency to attract impurities that attach so the particle can lower its overall energy and become more stable. Sintering is highly controlled by the surfaces of the particles so the process is sensitive to impurities that can collect at the particle surfaces. Because the surface to volume ratio of nanopowders are so high the surface properties during sintering of nanopowder become even more significant. These impurities affect the sinterability of the powder but are not necessarily bad since they can (if finely dispersed) inhibit grain growth. On the other hand it has been shown that it is hard to keep a small grain size with high-purity powder under controlled conditions.