Synthesis of Nanoscale Materials and Structures 267
that distinguishes machined products form those that are merely
molded or cast. What you don’t see is the swarf—the machined
shreds, chips, and tangles. These have suffered extreme deformation; their structure (like those of the last two processes) is refined
by it. There is an interesting scaling law here: The finer the cut, the
thinner the chip and the greater the shear it has suffered. Micromachining—machining on the scale of watch making, as in Figure
8.18—deforms the chips more than ordinary coarse machining.
Better yet is machining with a diamond tip with submicron radius.
Diamond nanomachining is designed for surface profiling, but the
swarf it produces can be useful too. As with particle milling, a way
must be found to consolidate the swarf into useful products. We
get to that next.
Bottom-up processes
consolidation of nanoclusters and milled powders
Most of the processes described thus far do not make solid objects;
they make clusters, powders, or chips. The long-established way to
consolidate powder is to press in a die that has the desired form,
then heat to a temperature at which diffusional bonding takes place.
This powder-pressing and sintering route to manufacturing products
(see Figure 8.19) is widely used to make engine parts for cars and
components for household appliances such as washing machines.
The difficulty in using it to consolidate nano structured particles is
that sintering takes time, and at the sintering temperature, the structure coarsens. It is not an easy problem to solve; during consolidation there will always be some coarsening. The question is how to
minimize it. One way is to compact the powder in such a clean
environment that the particles bond, even at room temperature,
but that is seldom possible. An alternative is to sinter so fast that
there is little time for coarsening. Figure 8.20 shows one way to do
this—that of flash sintering. The powder (or swarf) is compressed in
a die through which a bank of capacitors is discharged. The blast of
heat, generated by the resistance of the packed powder, is enough
to create good bonding without leaving enough time for serious
coarsening.
Methods for nanoprofiling
Often it is not a nanomaterial that is sought; it is nanofeatures
on the surface of something much bigger. These are created by
micromachining (cutting material away) or by microlithography
(putting material where you want it).
Steel or
ceramic
balls
Component A
Component B
Rotating drum
Repeated
extreme
deformation
and cold welding
Figure 8.17
Powder milling with mechanical alloying. The
heavy steel or tungsten carbide balls trap, deform,
weld, and break up the powder particles, mixing
them so completely that they become alloys with a
nanoscale structure.
that distinguishes machined products form those that are merely
molded or cast. What you don’t see is the swarf—the machined
shreds, chips, and tangles. These have suffered extreme deformation; their structure (like those of the last two processes) is refined
by it. There is an interesting scaling law here: The finer the cut, the
thinner the chip and the greater the shear it has suffered. Micromachining—machining on the scale of watch making, as in Figure
8.18—deforms the chips more than ordinary coarse machining.
Better yet is machining with a diamond tip with submicron radius.
Diamond nanomachining is designed for surface profiling, but the
swarf it produces can be useful too. As with particle milling, a way
must be found to consolidate the swarf into useful products. We
get to that next.
Bottom-up processes
consolidation of nanoclusters and milled powders
Most of the processes described thus far do not make solid objects;
they make clusters, powders, or chips. The long-established way to
consolidate powder is to press in a die that has the desired form,
then heat to a temperature at which diffusional bonding takes place.
This powder-pressing and sintering route to manufacturing products
(see Figure 8.19) is widely used to make engine parts for cars and
components for household appliances such as washing machines.
The difficulty in using it to consolidate nano structured particles is
that sintering takes time, and at the sintering temperature, the structure coarsens. It is not an easy problem to solve; during consolidation there will always be some coarsening. The question is how to
minimize it. One way is to compact the powder in such a clean
environment that the particles bond, even at room temperature,
but that is seldom possible. An alternative is to sinter so fast that
there is little time for coarsening. Figure 8.20 shows one way to do
this—that of flash sintering. The powder (or swarf) is compressed in
a die through which a bank of capacitors is discharged. The blast of
heat, generated by the resistance of the packed powder, is enough
to create good bonding without leaving enough time for serious
coarsening.
Methods for nanoprofiling
Often it is not a nanomaterial that is sought; it is nanofeatures
on the surface of something much bigger. These are created by
micromachining (cutting material away) or by microlithography
(putting material where you want it).
Steel or
ceramic
balls
Component A
Component B
Rotating drum
Repeated
extreme
deformation
and cold welding
Figure 8.17
Powder milling with mechanical alloying. The
heavy steel or tungsten carbide balls trap, deform,
weld, and break up the powder particles, mixing
them so completely that they become alloys with a
nanoscale structure.