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Diamond: A Story of Superlatives

Diamond: A Story of Superlatives. 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

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Diamond: A Story of Superlatives

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  1. Diamond: A Story of Superlatives Doug Martin

  2. 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 Doug Martin

  3. 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~1016Ω • Highest melting point- 3820K • Resistant to corrosion by acid or base • Negative electron affinity Doug Martin

  4. 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) Doug Martin

  5. 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 Doug Martin

  6. 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 Doug Martin

  7. 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 Doug Martin

  8. 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 Doug Martin

  9. 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 Doug Martin

  10. 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 Doug Martin

  11. 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. Doug Martin http://www.amnh.org/exhibitions/diamonds/future.html

  12. 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. Doug Martin

  13. 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 Doug Martin

  14. 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 Doug Martin

  15. 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 Doug Martin

  16. Any Questions?????? Doug Martin

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