Properties

 

Diamonds are one of the most studied substances because of their unique properties. When they were first discovered it was the mineral's exceptional hardness for which it was noted. Since then numerous other special properties have seen diamond used in industrial, medical and scientific applications. The following table shows the main physical properties of diamond.

  • Refractive index 2.42
  • Dispersion 0.044
  • Transparency 0.225 – 1000 microns
  • Density 3.51 g/cm3
  • Young's modulus 0.8 – 1.2 x 1012 dyn/cm2
  • Vickers hardness 10 – 15 x 104 kg/mm2
  • Compressibility 1.8 x 107 cm3/kg
  • Coefficient of friction 0.05 – 0.1
  • Thermal expansion coefficient 1 x 10-6/°C (@ 20°C)
  • Thermal conductivity 10 – 20 watts/cm °K (@ 20°C)
  • Specific heat 1 cal /gm atom°K
  • Electrical resistivity 104 -1016 ohm-cm
  • Atomic weight 12.01
  • Atomic radius 0.77 Angstroms

Industrial diamonds are those which are unable to be used as gem or near-gem by virtue of their shape, colour or quality. Their application depends on their form. Those with sharp tips or edges may be used in their natural state as a dressing tool for grinding wheels or for large rock drills. Industrial diamonds of 1 mm – 3 mm in diameter may be processed to produce rounded stones for rock drilling bits. While heavily cracked or included diamonds of any size may be crushed to produce grit for saws or powder for polishing.

Besides hardness, diamonds find application in the machining industry on account of their very low friction, generating less heat at any cutting surfaces. Also, their extremely low coefficient of thermal expansion means a diamond cutter will maintain its dimensions despite any thermal changes. While diamonds are extremely hard, they are correspondingly brittle and can be readily shattered by impact.

Not all non-jewellery applications of diamond use industrial diamonds, with gem qualities needed for some demanding applications. The very low friction of diamonds and its hardness has seen the material fashioned into a scalpel blade and used for delicate surgery, particularly ocular surgery. The low friction means it slices through tissue easily while the high hardness ensures the edge does not deteriorate. In addition, diamond is hydrophobic, meaning it repels water, so thin slices of tissue do not stick to the blade.

Dallas Diamonds diamond propertiesDiamonds have the lowest specific heat and the highest thermal conductivity of any solid. These attributes make diamonds the ideal candidate for heat sinks such as used for electronic chips. Such electronic applications are normally satisfied by synthetic diamonds, where better control of impurities and dimensions is possible.

Optically a diamond is special, giving it its distinctive place in jewellery. Its high refractive index (2.42) provides it with a steep angle (24 degrees off normal) for total internal reflection. The material's dispersion (0.044) which is responsible for the 'fire' of a facetted gem, is not extraordinary with numerous other substances exhibiting higher values.

The spectral transmission of diamonds is notable as a colourless sample is transparent from the ultra-violet through to the mid infrared. It is this characteristic which has seen diamonds used as camera windows on a Venus probe and also on infrared seeking missile noses which have to withstand erosion from rain drops. The former use was also exploiting the diamonds' resistance to acids.

Electrically, diamond is an exceptional insulator. However in the presence of trace amounts of boron, the substance becomes a semiconductor. The electrical resistance of such conducting diamonds has an extremely high temperature dependence, which has seen the substance used as a highly sensitive thermometer detecting temperature changes as low as a millionth of a degree centigrade.

The presence of trace impurity elements in diamond is also largely responsible for the colours in diamond. A perfect diamond crystal comprises only carbon atoms arranged in a cubic lattice structure. Such a diamond is colourless (also termed 'white'). Variations from this perfect structure in the form of impurities or structural deformations can impart colour to a diamond.

The most common impurity in diamond is nitrogen which, being of a similar size to carbon, can be integrated into the crystal lattice. These nitrogen-bearing diamonds are termed type 1 diamonds and are by far the most abundant in nature. The nitrogen can be manifest in several forms, from isolated single N atoms (type 1b diamonds) to clusters of 2 (type1aA), 3 or 4 (type1aB) atoms. It is the presence of nitrogen that is responsible for the yellow colouration in off-white diamonds. Diamonds that are totally of either 1aA or 1aB type are colourless. All synthetic, but only a few natural diamonds are type 1b. The Tiffany diamond is a noted example of a natural fancy 1b.

Boron is also of similar size to carbon and is often present in diamonds, but its presence is only noticeable when the nitrogen level is extremely low. The boron impurity, as an acceptor, transforms the lattice into a semi-conductor which also absorbs light in the red region of the spectrum. A blue colour results, for which the most famous example is the Hope diamond. Argyle blue or violet diamonds are not the result of boron impurities. Nickel or high concentrations of hydrogen are believed to be responsible for this colouration in these diamonds.

The presence of colour in brown and pink diamonds remains a mystery. It has been established that lattice misalignments and ruptured bonds are present in all brown and pink diamonds. Levels of nitrogen are also important, with lower levels favouring pink colours. A significant feature of Argyle's pink diamonds is their chroma- and thermo-chromatic properties, by which the intensity of colour can be temporarily changed by either exposure to UV which bleaches the colour or exposure to heat which intensifies it. Such behaviour is indicative of charge transfers between two defect centres believed to be single nitrogen and another structure.

Smaller scale lattice disturbances are generated by radioactive particles which dislodge carbon atoms from their positions. The resulting centre (GR1) gives rise to a green colouration. Because of the limited penetration of radioactive particles into a diamond, the green colour is usually confined to the outer layers, which will be mostly removed on polishing. Natural green diamonds are not widespread, the most notable example being the 'Dresden Green'.

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