DiamondA Story of Superlatives

							                    Diamond:
                A Story of Superlatives




23 April 2001            Doug Martin      1
                 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              2
                    Properties
 •   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                 3
                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             4
                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                      5
          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




23 April 2001           Doug Martin   6
                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             7
       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

23 April 2001                   Doug Martin               8
                 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




23 April 2001                  Doug Martin        9
                 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 n-
        doping are to large to bond with C in the lattice
      – Patterning diamond films is difficult



23 April 2001             Doug Martin                   10
                One possibility . . .
• Schematic diagram of a
  sandwich, called a multi-
  chip 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                      11
     http://www.amnh.org/exhibitions/diamonds/future.html
                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.



23 April 2001          Doug Martin         12
                Methods of Synthesis
• HPHT:
      – Apply high temperatures and pressures to graphite
      – Uses liquid metal (Fe) to catalyze the reaction
• Chemical Vapor Deposition
      –   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                       13
                         But . . .
• HPHT synthesis is slow and expensive
• CVD has several limitations:
      – 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

23 April 2001               Doug Martin                       14
                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                     15
                Any Questions??????




23 April 2001          Doug Martin    16