Diamonds:

23 April 2001  
 

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Diamond:  
A Story of Superlatives

23 April 2001  
 

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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

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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

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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)

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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

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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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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

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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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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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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 

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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.

 

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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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

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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

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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

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Any Questions??????