Biomedical Engineering Reference
In-Depth Information
challenges are unique to sintering of nano particles. For example, the
thermodynamic driving force for sintering of nano particles is extremely
large, calling into question the use of conventional sintering doctrines based
on linear diffusion theories.
In the context of engineering processes, sintering implies the bonding of
one solid particle to another. Sintering consists of two intertwined processes:
densification and grain growth. A unique issue of sintering of nano particles
is that nano particles almost always experience extremely rapid grain
growth, rendering the loss of nanocrystalline characteristics in fully sintered
states. With respect to manufacturing bulk nanocrystalline materials from
nanoscaled particles, the objective of sintering of nano particles is to achieve
maximum densification while retaining nanoscaled grain sizes. This goal,
however, has been very difficult to reach. Fundamentally, there are two
main reasons for this technological impasse. One is that the same factors
that result in densification also cause grain growth. In other words, both
densification and grain growth processes share the same driving force and
mass transport mechanisms. Furthermore, in many cases, grain growth is
required in order to break the local interfacial energy balance necessary to
sustain continuous elimination of pores.
1
Innovative sintering technologies include the two-step sintering technique
and various pressure-assisted sintering techniques. The two-step sintering
technique was designed to decouple the densification and grain growth
processes that occur during conventional pressureless sintering.
Conventional pressure-assisted processes including hot pressing and hot
isostatic pressing and unique processes such as microwave sintering and
spark plasma sintering (SPS) are all applied in the research of sintering of
nano particles.
Since the early 1990s, there have been a few comprehensive reviews that
deal with the topic of sintering of nano particles or processing of nanosized
particles.
2,3
The issues of sintering of nano particles and the difficulties of
manufacturing bulk nanocrystalline materials from nanoscaled powders are
also discussed in other reviews that cover broader topics of nano
materials.
4,5
These reviews collectively provide a strong foundation for
understanding the science and technology involved in sintering of nanosized
particles. This chapter, based on a comprehensive review by the authors,
6
focuses on the size-dependent properties and their effects on sintering. The
relationship between grain growth and densification is highlighted.
