23 April 2001Doug Martin1 Diamond: A Story of Superlatives

23 April 2001Doug Martin2 History Diamond has long been cherished for its value as a gemstone It was discovered to be made of carbon in 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 2001Doug Martin3 Properties Mechanical hardness ~98 GPa Compressive Strength > 110 GPa Highest bulk modulus- 1.2*10 12 N/m 2 Lowest Compressibility- 8.3* m 2 /N Highest thermal conductivity- 2*10 3 W/m/K Optically transparent from deep UV to far IR Good electrical insulator- R~10 16 Ω Highest melting point- 3820K Resistant to corrosion by acid or base Negative electron affinity

23 April 2001Doug Martin4 Chemistry Stuff The origin of all of diamond’s superior capabilities is its covalent network of sp 3 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 2001Doug Martin5 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 2001Doug Martin6 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 2001Doug Martin7 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 2001Doug Martin8 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 2001Doug Martin9 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 2001Doug Martin10 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 2001Doug Martin11 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 2001Doug Martin12 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 2001Doug Martin13 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 K in excess H 2 gas –Most economical method for industrial application

23 April 2001Doug Martin14 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 2001Doug Martin15 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 2001Doug Martin16 Any Questions??????