edited by MARYVIRGINIA ORNA,0.S.U College 01 New Rochelle New Rochelle. NY 10801
Diamond: Some Interesting Physical Properties Franklin R. Bickford Tompkins Cortland Community College Dtyden. NY 13503
Diamond is an unusual suhstance. In addition to the fact that i t is the hardest known suhstance, it also has extreme values for several other fundamental physical properties: thermal conductivity, melting point, and entropy. At 25'C, the diamond has the highest thermal conductivity and the lowest molar entropy of any substance. In addition, diamond has the highest melting point of any element, 3550°C. Thus, aside from its attractiveness as a prized gem stone, diamond is interestine and useful from a scientific and technoloeical standpoint. The Moh's scale of hardness used hv eeoloeists is a ten noint scale ranging from a value of 1for the sokest kineral to a h u e of 10 for diamond, the hardest. This scale is nonlinear and in fact it is almost logarithmic from 1to 9, with each step on the scale being approximately 1.2 times harder than the preceding step. However, the increase in hardness from Moh's 9 which corresponds to corundum (AlzOa), to Moh's 10 (diamond) is much greater than the preceding step from 8 to 9, since diamond is about five times harder than corundum. A synthetic material, cubic boron nitride, exceeds the hardness of corundum hut is still only ahout half as hard as diamond. The major industrial use of diamond is as a cutting tool, i.e., diamond-studded drill bits and saws, employs diamond for its truly exceptional hardness. The thermal conductivity of a diamond a t 25'C is greater than that of any other suhstance; a perfect diamond crystal conducts heat six times better than silver, the best metallic conductor. The values in watts m-' K-' a t 0°C for diamond and silver, respectively, are 2620 and 429 (1).This is an interesting property . . since diamond is a noncouductor of electricity a i d generally solids that are nonconductors of electricity are not good heat conductors. The use of diamond in cutting tools iH also dependent on diamond's high thermal conductivity. The heat created by friction in cutting is quickly carried away. This is especially important for diamond since it is oxidized to CO or C02 when heated to 700°C in pure oxygen (2). Another technological application of diamond that makes use of its exceptional heat transfer property is its use in some electric circuits as a heat sink. Here diamond is used solely as a heat transfer medium. The accumulation of heat
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in these circuits would render them nonfunctional and therefore the heat must be conducted away quickly. The molar entropy of diamond, 2.4 J mol-1 K-I (3), is the lowest of any suhstance a t room temperature, indicating that diamond has a very high degree of order. Ruby and sapphire, both varieties of corundum, derive their color from impurities and have entropy values ahove 50 J mol-' K-' a t room temperature (4). A ronipilriwn with graphite, tht! mure stable rrystnlltne allotruphic form of cnrl~unat 25'C and 1 atm, emphasizes the extreme nature of some of diamond's nhvsical nroierties. The hardness of graphite is similar to that f; talc, having a value between 1and 2 on the Moh's scale. makine eranhite . one of the softest minerals known. ~ r a p h i t eis a good conductor of electricitv, whereas diamond is a nonconductor. In terms of thermal ionductivity, diamond exceeds graphite by a factor of ten. At 5.7 J mol-' K-' (31. the molar entroov ."of eranhite is more than twice that of diamond. The fundamental explanation of these exceptional nronerties oidiilmund 13 based on h e s t r u c t u r e oidiamonh i t n d the C-C' hond strength. In diumond, each cnrhon atum is covalently bonded t; four other carhbn atoms in a rigid, three-dimensional network. The bonding electrons are held rigidly in localized tetrahedrally-oriented honds and are not available to conduct electricity. This rigid structure, held together by four strong, symmetrically oriented, covalent bonds, is responsible for the hardness, high melting point, and low entropy of diamond. The hardness of a suhstance results not only from the strength of the honds holding the atoms together, hut also from the symmetry of the honds. In diamond, the four sp3 covalent bonds of each atom are oriented symmetricaily toward the vertices of a regular tetrahedron and this con. tributes to the hardness. The thermal conductivity of diamond depends on an entirely different mechanism than that which occurs in metals, where heat is conducted principally by mobile electrons. Although the exact process by which diamond conducts heat is quite complex, it is analogous to the way that sound is conducted in air; that is, by the transfer of kinetic energy of vibration from one particle to the next (4).
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Literature Clted (1) Weaat.RabertC.,(Edit04CRCHxxdbxxk of Chemistry Y Y ~ P ~ Y Y ~ ~ ~ ~ CIn
