Biomedical Engineering Reference
In-Depth Information
grain sizes in the nanometer range becomes a critical processing
step. Fully dense specimens with nanosize features are most
important for structural, magnetic, electric, electronic, and medical
applications. The focus in the consolidation of out-of-equilibrium
powders has been the retention of the metastable condition of the
initial structures.
The sintering process of powder materials with particle in the
nanometer range was studied in many laboratories [13, 14, 33].
For example, high-energy ball milled and heat treated Nd
2
(Fe, Co,
Zr)
14
B/α-Fe and Nd(Fe, Mo)
12
N
x
/α-Fe magnetic nanopowders have
been compacted by hot pressing to form bulk magnets [23]. Studies
of nanopowder densiication have lead to a better understanding of
numerous sintering issues such as powder agglomeration, surface
condition or contamination, pore role in sintering, and grain
growth. Thermodynamic and kinetic aspects of metastable powder
densiication, sintering mechanisms, and scaling laws applicable to
nanopowder sintering were examined, as well [13, 58].
4.2.1 ConsolidationMethods
When powder particle size decreases below the micron range, the
consolidation efforts are faced with additional problems related to
powder agglomeration, interparticle friction, and contamination.
In spite of these new challenges, all known consolidation methods
have been applied for full densiication of both ceramic and metal
nanopowders, including conventional sintering. Non-conventional
sintering methods include: microwave sintering, shock or dynamic
consolidation, and ield-assisted sintering. The main purpose of using
these methods is to enhance densiication and prevent grain growth.
The very short high temperature exposure during consolidation
provides the best means to retain ine grain size or out of equilibrium
conditions such as amorphous structures or supersaturated solid
solutions [46]. Examples of dense materials that have retained
nanosize grains <100 nm by conventional sintering are provided in
Table 4.1 [33, 41].
A large variety of pressure assisted methods has been applied
for consolidation of nanopowders ranging from low (<100 MPa) to
high pressure methods (>0.5 GPa) such as piston cylinder, diamond
anvil, and torsion-pressure method (sometimes referred to as
severe plastic deformation consolidation — SPDC). To illustrate the
