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Saws made of carbon
You can’t saw without producing sawdust – and that can be expensive
if, for example, the “dust” comes from wafer manufacturing in the
photovoltaic and semiconductor industries, where relatively high kerf
loss has been accepted as an unavoidable, if highly regrettable, fact of
life. But now scientists from the Fraunhofer Institute for Mechanics of
Materials IWM in Freiburg together with colleagues from the Australian
Commonwealth Scientific and Industrial Research Organisation CSIRO have
developed a saw wire that is set to effect dramatic reductions in kerf
loss: in place of diamond-impregnated steel wires, the researchers use
ultra-thin and extremely stable threads made of carbon nanotubes coated
with diamond.
The potential of coated carbon nanotubes has long been understood:
possible applications include its use as a hard and tough composite
material or as a component of highly sensitive sensors and
thermoelectric generators. However, the new material is extremely
difficult to synthesize. Diamonds only grow under extreme conditions –
at temperatures of around 900 degrees Celsius in an atmosphere
containing hydrocarbons. Growing diamonds on nanotubes is a tricky
proposition, because carbon tends to form graphite. In order to catalyse
the formation of the diamond phase, it’s necessary to use reactive
hydrogen to prohibit the deposition of graphite. However, this process
also damages the carbon nanotubes.
But the IWM scientist Manuel Mee found a solution for protecting the
fine carbon nanotubes, which grow like forests on a substrate: “During
our first experiments, fused silica from the reaction chamber
accidentally came into contact with the coating plasma. It settled on
the substrate and protected it against the aggressive hydrogen.” And to
his surprise, diamonds actually grew on this layer. “What followed was
careful, painstaking work,” points out Mee. “We had to study the silicon
oxide layer, which was deposited in an undefined manner, and find a
method of controlling the deposition and optimizing the process.” Tests
with a transmission electron microscope at CSIRO’s lab in Australia
revealed that the nanotubes actually survived under their protective
layer.
A German-Australian success story
How exactly to proceed from there was the question that now faced the
scientists. If they found a way to coat with diamond the nanothreads
that the CSIRO specialists make from nanotubes, these diamond-coated
nanothreads could be used to manufacture ultra-thin saws capable of
cutting through silicon wafers for instance. The Australian team at
CSIRO is one of the principal global experts with the know-how to
manufacture yarns from carbon nanotubes. The manufacturing process
requires special carbon nanotube “forests”, which can be extracted as an
ultra-thin “felt” and twisted into a very thin yarn ten to twenty
micrometers in diameter. In principle, this diamond-coated yarn is the
ideal material on which to base a new generation of saws, which could be
used in the solar industry for example. As Mee explains: “The new saw
wires held out the promise of being far superior to traditional steel
wires. Because of their high tensile strength, they can be manufactured
much thinner than steel wires – and that means significantly less kerf
loss.”
In the meantime, the physicist has managed to implement his idea. A
joint patent application by Fraunhofer and CSIRO has already been filed
for the method and corresponding products. Mee and his colleagues are
currently carrying out sawing tests. “To be able to show our partners in
industry the potential the technology holds,” says Mee, “we have to
demonstrate how it can help solar companies to save material when
processing wafers.”
Press Release; Fraunhofer Institute; August 1, 2013
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