Carbon comes in many forms and there is no other element that can take on so
many difference appearances and properties and morphologies. We are all
familiar with many synthetic polymers, for example "Nylon", which can be
found as a yarn in a sweater, as a yarn in a tire cord, as a molded part or
even as an extruded film. We are familiar with carbon as diamond and
carbon as graphite, again two forms with dramatically different properties.
Carbon can also be prepared in the glassy state, sometimes called the
vitreous state. The glassy carbon products offered by SPI Supplies contain carbon
in this form is completely amorphous and shows no
signs of crystallinity whatsoever; by x-ray diffraction one sees only an
"amorphous halo". When the glassy carbon offered by SPI Supplies is fractured,
it exhibits the fracture face that is characteristic of a glassy material.
There is question that
glassy carbon has a range of properties and applications
that make it a truly new and exciting material for research and production applications.
From our perspective, the glassy carbon offered by SPI
Supplies exhibits properties that are nearly as dramatically different from
the properties of graphite as diamond. Indeed the glassy carbon offered by SPI
Supplies is a unique type of material with very unusual properties.
From an analytical standpoint, graphite gives strong x-ray diffraction
peaks, indicating a high degree of "order" and a high level of x-ray
diffraction determined crystallinity. But the SPI Supplies Brand of glassy
carbon exhibits no diffraction peaks and exhibits only an amorphous halo.
Most graphites (excluding
HOPG and
pyrolytic graphite) exhibit considerable
porosity whereas the SPI Supplies brand of glassy carbon is quite dense,
certainly much more so than graphite. This might sound like a contradiction,
but when one looks at the density of glassy carbon in
comparison to the theoretical density of the graphite crystal, its density
is less, specifically ~ 1.5 vs. 2.25. But since graphite normally has
considerable porosity, the ~ 1.5 density is higher than the typical graphite
(excluding HOPG and pyrolytic graphite). As a consequence of this high
density and the lack of porosity that plagues ordinary graphite, the SPI
Supplies glassy carbon can be polished off to a mirror finish. Indeed it
does take on the appearance of a "black glass".
Another important property of the SPI Supplies Brand of glassy carbon is
good thermal conductivity, which imparts to it a high level of resistance to
thermal shock (making possible its application for crucibles).
Theoretically, glassy carbon should behave like any other form of carbon,
such as graphite, but in fact it is far less reactive because of the higher
density relative to the normally more porous graphite. Its resistance is
more akin to HOPG and high density pyrolytic graphite. While there are several
reasons for it, the main reason for the impressive chemical resistance is a
consequence of the disordered structure and therefore the inability to form
intercalation compounds. This gives rise to high resistance to corrosion by
acid and alkaline agents and melts. SPI Supplies offers four different grades
of glassy carbon, the main difference between them being their density.
The higher the temperature "rating", the higher the density, and since it
requires longer treatment at high temperatures during the production, the
energy costs are considerably higher and that is one of the reasons for the
dramatically higher prices for the highest density material. And because of the
high purity, glassy carbon is attractive for applications in chemical analysis and
semiconductor and ultrapure materials technology. Glassy carbon shows no memory effects and
is therefore applicable for ultratrace analysis.
Depending on grade, the material can be taken as high as 2500°C without any significant devitrification.
However above that temperature, some devitrification will occur, and there
will be a nucleation and growth of a graphite phase. Remember the higher the density,
the higher the temperature will be that this transition will occur.
Note: that the SPI Glas glassy carbon products can be heated in air,
depending on grade selected, up to 600°C without undergoing any reaction
but above this temperature, any heating should be done strictly either
under vacuum or in an inert environment such as argon. If vacuum is not used for
higher temperatures, there will be a reaction and the formation of CO and
CO
2. A similar kind of reaction will occur above 600°C with
water or water vapor, resulting in the evolution of CO and H
2.
The standard product in general, exhibits an "as produced" surface and has
not, as of that point in time been subjected to any metallographic type
polishing. We can, upon request, offer highly polished surfaces for an
additional cost. We believe that in general our customers can do this kind of final
finishing operation far more cost effectively than we can since they know
just how far the finishing has to be taken for their own particular
application.
SPI Supplies can harness its capabilities to produce custom shapes for
customers. Keep in mind that the glassy carbon is "substrate grown", that
is, it is grown from a surface and there are limits to how thick of a piece
that can be made and still retain the high quality of the SPI Supplies
product. That maximum thickness is presently ~ 2.8 mm, and as we are asked
to come closer and closer to that upper limit, the cost does tend to
increase dramatically because of a drop off in yield. The best yields are
possible at 2 mm or less. Just remember that the
production of glassy carbon is an extremely expensive and long drawn out process
carried out over a period of 3-4 months. For special production items,
one has to be prepared to have lots of
patience......
SPI Supplies offers its glassy carbon in four different grades, each
with its own set of properties
and which are based on the final heat treatment temperature of the glassy carbon:
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