Diamonds
Chemistry
of Diamond:
Properties
& Types
PROPERTIES
OF DIAMONDS
Diamond
is the hardest natural material.
The Mohs hardness scale, on
which diamond is a '10' and corundum (sapphire) is a '9', doesn't adequately
attest to this incredible hardness, as diamond is exponentially harder than
corundum.
Diamond is also the least
compressible and stiffest substance.
It is an exceptional thermal
conductor - 4 times better than copper - which gives significance to diamonds
being called 'ice'.
Diamond has an
extremely low thermal expansion, is chemically inert with respect to most acids
and alkalis, is transparent from the far infrared through the deep ultraviolet,
and is one of only a few materials with a negative work function (electron
affinity).
One consequence of the
negative electron affinity is that diamonds repel water, but readily accept
hydrocarbons such as wax or grease.
Diamonds do not conduct
electricity well, although some are semiconductors.
Diamonds can burn if
subjected to a high temperature in the presence of oxygen.
Diamond has a high specific
gravity; it is amazingly dense given the low atomic weight of carbon.
The brilliance and fire of a
diamond are due to its high dispersion and high refractive index.
Diamond has the highest
reflectance and index of refraction of any transparent substances.
Diamond gemstones are
commonly clear or pale blue, but colored diamonds, called 'fancies,’ have been
found in all the colors of the rainbow.
Boron, which lends a
bluish color, and nitrogen, which adds a yellow cast, are common trace
impurities.
Two volcanic rocks
that may contain diamonds are kimberlite and lamproite.
Diamond crystals
frequently contain inclusions of other minerals, such as garnet or chromite.
Many diamonds
fluoresce blue to violet, sometimes strongly enough to be seen in daylight.
Some blue-fluorescing
diamonds phosphoresce yellow (glow in the dark in an afterglow reaction).
TYPE OF DIAMONDS
Natural
Diamonds
Natural
diamonds are classified by the type and quantity of impurities found within
them.
·
Type Ia - This is the most common type of natural diamond,
containing up to 0.3% nitrogen.
·
Type Ib - Very few natural diamonds are this type (~0.1%), but
nearly all synthetic industrial diamonds are. Type Ib diamonds contain up to
500 ppm nitrogen.
·
Type IIa - This type is very rare in nature. Type IIa diamonds
contain so little nitrogen that it isn't readily detected using infrared or
ultraviolet absorption methods.
·
Type IIb - This type is also very rare in nature. Type IIb
diamonds contain so little nitrogen (even lower than type IIa) that the crystal
is a p-type semiconductor.
Synthetic Industrial Diamonds
Synthetic
industrial diamonds have produced the process of High-Pressure High-Temperature
Synthesis (HPHT).
In HPHT synthesis, graphite
and a metallic catalyst are placed in a hydraulic press under high temperatures
and pressures.
Over the period of a few
hours, the graphite converts to diamond. The resulting diamonds are usually a
few millimeters in size and too flawed for use as gemstones, but they are
extremely useful as edges on cutting tools and drill bits and for being
compressed to generate very high pressures.
(Interesting side note:
Although used to cut, grind, and polish many materials, diamonds aren't used to
machine alloys of iron because the diamond abrades very quickly, due to a
high-temperature reaction between iron and carbon.)
Thin Film
Diamonds
A
process called Chemical Vapor Deposition (CVD) may be used to deposit thin
films of polycrystalline diamond.
CVD technology makes it
possible to put 'zero-wear' coatings on machine parts, use diamond coatings to
draw the heat away from electronic components, fashion windows that are
transparent over a broad wavelength range, and take advantage of other
properties of diamonds.
ANNE MARIE HELMENSTINE, PH.D.
Anne Helmenstine,
Ph.D., is an author and consultant with a broad scientific and medical
background.
EXPERIENCE
Anne has
taught chemistry, biology, and physics at the high school, college, and
graduate level. In her doctoral work, Anne developed ultra-sensitive chemical
detection and medical diagnostic tests. She has worked abstracting/indexing
diverse scientific literature for the Department of Energy. She presently works
as a freelance writer and scientific consultant. She enjoys adapting lab-based
science projects so that they can be performed safely at home.
EDUCATION
Dr.
Helmenstine has bachelor of arts degrees in physics and mathematics with a
minor in chemistry from Hastings College in Nebraska and a doctorate of
philosophy in biomedical sciences from the University of Tennessee at
Knoxville.
Chemistry
is part of everyone's life, from cooking and cleaning to the latest computer
chip technology and vaccine development. It doesn't have to be intimidating and
it doesn't have to be hard to understand.
You can
read more about Anne's current and past work on her Google Profile: Anne Helmenstine. Find
Anne's printable periodic tables and science projects at Science Notes.
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