Diamond:
A Story of Superlatives
23 April 2001
Doug Martin
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History
• Diamond has long been cherished for its value
as a gemstone
• It was discovered to be made of carbon in
1796 - another discovery younger than our
nation
• Finally synthesized in 1953 by a Swedish
team, but they did not publish their results
• A team at GE announced their success in 1955
23 April 2001
Doug Martin
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Properties
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Mechanical hardness ~98 GPa
Compressive Strength > 110 GPa
Highest bulk modulus- 1.2*1012 N/m2
Lowest Compressibility- 8.3*10-13 m2/N
Highest thermal conductivity- 2*103 W/m/K
Optically transparent from deep UV to far IR
Good electrical insulator- R~1016 Ω
Highest melting point- 3820K
Resistant to corrosion by acid or base
Negative electron affinity
23 April 2001
Doug Martin
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Chemistry Stuff
• The origin of all of diamond’s superior
capabilities is its covalent network of sp3
hybridized C atoms
• Crystal system is isometric: 4/m 3 2/m
• Graphite is actually slightly more stable
than diamond at standard conditions (by just
a few eV)
23 April 2001
Doug Martin
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Thermodynamics
• If graphite is more stable, why would diamond
ever form, and even if it did, how come diamond
rings don’t turn into graphite?
• Diamond is formed deep inside the earth at
extreme temperature and pressure
• It turns out that the activation energy for the
reaction is almost as large as the lattice energy of
diamond
• Diamond is metastable b/c it is kinetically stable,
not thermodynamically stable
23 April 2001
Doug Martin
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Phase Diagram for Carbon
• As you can see, at
room temperature
graphite is the natural
form of C
• The little boxes we
will get to in a minute
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Doug Martin
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Industrial Applications
• Excellent abrasive – hard and resists wear
• Scratchless Windows for optical sensors
- Used for IR sensors on cruise missiles
• Potentially useful as a semiconductor:
-It’s band gap= 5.4 eV
• Low friction, no wear hinges and bearings
-Used on the space shuttle
23 April 2001
Doug Martin
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More Industrial Applications
• Diamond anvil cell - more in a minute
• Used for cutting tools - Cannot be used to
cut Fe materials because iron carbide forms
• Also useful as a heat sink in electronics
• Can be used as an insulator for wires
-Wires are extremely stiff for their weight
• Has potential to replace LCDs in screens
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Doug Martin
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Diamond anvil cells
• Used to create extreme pressures
- ~ 4.5 million atmospheres
• Conditions are similar to planetary interiors
• Hydrogen changes to metal at this P
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Doug Martin
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Semiconductor
• Diamond can be doped to change it from an
insulator to a semiconductor
• Difficulties still remain:
– P-doping is okay, but the elements used for ndoping are to large to bond with C in the lattice
– Patterning diamond films is difficult
23 April 2001
Doug Martin
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One possibility . . .
• Schematic diagram of a
sandwich, called a multichip module, that has a
stack of 40 layers
consisting of CVD
diamond covered by an
electronic chip. It is 10 cm
square. This processor
would have the computing
capacity of the Cray 3, a
supercomputer designed
but never built.
23 April 2001
Doug Martin
http://www.amnh.org/exhibitions/diamonds/future.html
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Synthetic Diamonds
• So diamonds have all sorts of useful
properties for industry, right?
• There’s just one catch-$$$$$$$$$$$
• We need a way to make diamonds cheaply
if they are to be of any use.
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Doug Martin
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Methods of Synthesis
• HPHT:
– Apply high temperatures and pressures to graphite
– Uses liquid metal (Fe) to catalyze the reaction
• Chemical Vapor Deposition
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Diamond is grown on a Si substrate
Graphite in gas phase is activated by heat or plasma
Reaction occurs at 1000-1400K in excess H2 gas
Most economical method for industrial application
23 April 2001
Doug Martin
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But . . .
• HPHT synthesis is slow and expensive
• CVD has several limitations:
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Reaction rates
Temperature – limits the number of substrates
Crystal quality
Many applications require smooth layers of diamond,
not individual crystals
– However, CVD products are on the market and the
technology is maturing
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Doug Martin
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Alternatives to Diamond
• Boron Nitride
– It’s hardness of 9.8 on the Mohs scale makes it very
useful for cutting tools and abrasives
– BN is isoelectronic with diamond, so it shares many of
its properties
• Tungsten Carbide
– Can substitute for diamond in many places
– Actually used in HPHT synthesis
• But diamond is still the best
23 April 2001
Doug Martin
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Any Questions??????
23 April 2001
Doug Martin
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