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Material properties of diamond

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

   Diamond
   An octahedral diamond crystal in matrix
   General
   Category Native Nonmetal, Mineral
   Chemical formula Carbon, C
   Identification
   Colour Most often colorless to yellow or brown. Rarely pink, orange,
   green or blue.
   Crystal habit Octahedral, spherical or massive
   Crystal system Isometric
   Cleavage Octahedral; perfect and easy
   Fracture Conchoidal
   Mohs Scale hardness 10
   Luster Adamantine to greasy
   Refractive index 2.417
   Pleochroism None
   Streak None
   Specific gravity 3.516 - 3.525
   Fusibility Burns above 800°C, melting point 3550 °C
   Solubility Resistant to acids, but melts in hot steel
   Major varieties
   Ballas Spherical, radial structure, cryptocrystalline, opaque black
   Bort Poorly-formed, cryptocrystalline, shapeless, translucent
   Carbonado Massive, microcrystalline, opaque black

   Diamond is transparent to opaque, optically isotropic crystalline
   carbon. It is the hardest naturally-occurring material known—owing to
   its strong covalent bonding—yet its toughness is only fair to good due
   to important structural weaknesses. Diamond has a high refractive index
   (2.417) and moderate dispersion (0.044), properties which are
   considered carefully during diamond cutting and which (together with
   their hardness) give cut diamonds their brilliance and fire. Scientists
   classify diamonds into two main types and several subtypes, depending
   on the nature of crystallographic defects present. Trace impurities
   substitutionally replacing carbon atoms in a diamond's crystal lattice,
   and in some cases structural defects, are responsible for the wide
   range of colors seen in diamond. Most diamonds are electrical
   insulators but extremely efficient thermal conductors. The specific
   gravity of single-crystal diamond (3.52) is fairly constant. Contrary
   to a common misconception, diamond is not the most stable form of solid
   carbon; graphite has that distinction.

Hardness and crystal structure

   Known to the Ancient Greeks as adamas ("untameable" or "unconquerable")
   and sometimes called adamant, diamond is the hardest known naturally
   occurring material, scoring 10 on the old Mohs scale of mineral
   hardness. The material boron nitride, when in a form structurally
   identical to diamond, is nearly as hard as diamond; a currently
   hypothetical material, beta carbon nitride, may also be as hard or
   harder in one form. Furthermore, it has been shown 1 2 that ultrahard
   fullerite (C^60) (not to be confused with P-SWNT Fullerite) when
   testing diamond hardness with a scanning force microscope can scratch
   diamond. In turn, using more accurate measurments, these values are now
   known for diamond hardness. A Type IIa diamond (111) has a hardness
   value of 167 GPa (±6) when scratched with an ultrahard fullerite tip. A
   Type IIa diamond (111) has a hardness value of 231 GPa (±5) when
   scratched with a diamond tip which leads to hypothetically inflated
   values.
   The diamond crystal bond structure gives the gem its hardness and
   differentiates it from graphite.
   The diamond crystal bond structure gives the gem its hardness and
   differentiates it from graphite.

   Cubic diamonds have a perfect and easy octahedral cleavage, which means
   that they have four planes—directions following the faces of the
   octahedron where there are fewer bonds and therefore points of
   structural weakness—along which diamond can easily split (following a
   blunt impact), leaving smooth surfaces. Similarly, diamond's hardness
   is markedly directional: the hardest direction is the diagonal on the
   cube face, 100 times harder than the softest direction, which is the
   dodecahedral plane. The octahedral plane, followed by the axial
   directions on the cube plane, are intermediate between the two
   extremes. The diamond cutting process relies heavily on this
   directional hardness, as without it a diamond would be nearly
   impossible to fashion. Cleavage also plays a helpful role, especially
   in large stones where the cutter wishes to remove flawed material or to
   produce more than one stone from the same piece of rough.

   Diamonds typically crystallize in the cubic crystal system ( space
   group Fd\bar{3}m ) and consist of tetrahedrally, covalently bonded
   carbon atoms. A second form called lonsdaleite with hexagonal symmetry
   is also found, but it is extremely rare and is believed to form only
   when meteoric graphite falls to Earth. The local environment of each
   atom is identical in the two structures. In terms of crystal habit,
   diamonds occur most often as euhedral (well-formed) or rounded
   octahedra and twinned, flattened octahedra known as macles (with a
   triangular outline). Other forms include dodecahedra and (rarely)
   cubes. There is some evidence that interstitial nitrogen impurities
   play an important role in the formation of euhedral crystals—the
   largest diamonds found, such as the Cullinan Diamond, have been
   shapeless or massive. These diamonds are Type II and therefore contain
   little if any nitrogen (see Composition and colour).

   The faces of diamond octahedrons are highly lustrous due to their
   hardness; growth defects in the form of trigons or etch pits are often
   present on the faces, the former being triangular pits whose points are
   aligned with the faces of the octahedron. A diamond's fracture may be
   step-like, conchoidal (shell-like, similar to glass) or irregular.
   Diamonds which are nearly round due to the stepping tendency of
   octahedrons are commonly found coated in nyf, a gum-like skin; the
   combination of stepped faces, growth defects, and nyf produces a
   "scaly" or corrugated appearance, and such diamonds are termed
   crinkles. A significant number of diamonds crystallize anhedrally: that
   is, their forms are so distorted that few crystal faces are
   discernable. Some diamonds found in Brazil and the Democratic Republic
   of the Congo are cryptocrystalline and occur as opaque, darkly-colored,
   spherical, radial masses of tiny crystals; these are known as ballas
   and are important to industry as they lack the cleavage planes of
   single-crystal diamond. Carbonado is a similar opaque microcrystalline
   form which occurs in shapeless masses. Like ballas diamond, carbonado
   lacks cleavage and its specific gravity varies widely, from 2.9–3.5.
   Bort diamonds, found in Brazil, Venezuela, and Guyana, are the most
   common type of industrial-grade diamond, also cryptocrystalline or
   otherwise poorly crystallized, but possessing cleavage, translucency,
   and lighter colors.

   Due to its great hardness and strong molecular bonding, a cut diamond's
   facets and facet edges are observably the flattest and sharpest. A
   curious side effect of diamond's surface perfection is hydrophobia
   combined with lipophilia. The former property means a drop of water
   placed on a diamond will form a coherent droplet, whereas in most other
   minerals the water would spread out to cover the surface. Similarly,
   diamond is unusually lipophilic, meaning grease and oil readily collect
   on a diamond's surface. Whereas on other minerals oil would form
   coherent drops, on a diamond the oil would spread. This property is
   exploited in the use of so-called "grease pens," which apply a line of
   grease to the surface of a suspect diamond simulant.

   Diamond is so strong because of the shape the carbon atoms make. It's a
   very strong 3d shape, each carbon atom having four joined to it with
   covelent bonds.

Toughness

   Unlike hardness, which only denotes resistance to scratching, diamond's
   toughness or tenacity is only fair to good. Toughness relates to the
   ability to resist breakage from falls or impacts: due to diamond's
   perfect and easy cleavage, it is vulnerable to breakage. A diamond will
   shatter if hit with an ordinary hammer.

   Ballas and carbonado diamond are exceptional, as they are
   polycrystalline and therefore much tougher than single-crystal diamond;
   they are used for deep-drilling bits and other demanding industrial
   applications. Particular cuts of diamonds are more prone to
   breakage—such as marquis or other cuts featuring tapered points—and
   thus may be uninsurable by reputable insurance companies. The culet is
   a facet (parallel to the table) given to the pavilion of cut diamonds
   designed specifically to reduce the likelihood of breakage or
   splintering. Extremely thin, or very thin girdles are also prone to
   much higher breakage.

   Solid foreign crystals are commonly present in diamond—these and other
   inclusions, such as internal fractures or "feathers"—can compromise the
   structural integrity of a diamond. Cut diamonds that have been enhanced
   to improve their clarity via glass infilling of fractures or cavities
   are especially fragile, as the glass will not stand up to ultrasonic
   cleaning or the rigors of the jeweler's torch. Fracture-filled diamonds
   may shatter if treated improperly.

Optical properties

   The lustre of a diamond is described as adamantine, which simply means
   diamond-like. It is the highest luster possible bar that of metal
   (metallic), and is due to diamond's superlative hardness. Reflections
   on a properly cut diamond's facets are undistorted, due to their
   flatness. The refractive index of diamond (as measuried via sodium
   light, 589.3 nm) is 2.417; because it is cubic in structure, diamond is
   also isotropic. Its high dispersion of 0.044 (B-G interval) manifests
   in the perceptible fire of cut diamonds. This fire—flashes of prismatic
   colors seen in transparent stones—is perhaps diamond's most important
   optical property from a jewelry perspective. The prominence or amount
   of fire seen in a stone is heavily influenced by the choice of diamond
   cut and its associated proportions (particularly crown height),
   although the body colour of fancy diamonds may hide their fire to some
   degree.

   Some diamonds exhibit fluorescence of various colors and intensities
   under long wave (LW) ultra-violet light (365 nm): Cape series stones
   (Type Ia; see composition and colour) usually fluoresce blue, and these
   stones may also phosphoresce yellow. (This is a unique property among
   gemstones). Other LW flurescence colors possible are green (usually in
   brown stones), yellow, mauve, or red (Type IIb). In natural diamonds
   there is typically little if any response to shortwave (SW)
   ultraviolet, but the reverse is true of synthetics. Some natural Type
   IIb diamonds may phosphoresce blue after exposure to SW ultraviolet. In
   naturals, fluorescence under X-rays is generally bluish-white,
   yellowish or greenish. Some diamonds, particularly Canadian diamonds,
   show no fluorescence.

   Cape series diamonds have a visible absorption spectrum (as seen
   through a direct-vision spectroscope) consisting of a fine line in the
   violet at 415.5 nm—however, this line is often invisible until the
   diamond has been cooled to very low temperatures. Colored stones show
   additional bands: brown diamonds show a band in the green at 504 nm,
   sometimes accompanied by two additional weak bands also in the green.
   Type II diamonds may absorb in the far red, but otherwise show no
   observable visible absorption spectrum.

   Gemological laboratories, such as the Adamas Gemological Laboratory ,
   make use of spectrophotometer machines that can distinguish natural,
   artificial, and colour- enhanced diamonds. The spectrophotometers
   analyze the infrared, visible, and ultraviolet absorption spectrums of
   diamonds cooled with liquid nitrogen to detect tell-tale absorption
   lines that are not normally discernable.

Electrical properties

   Except for most natural blue diamonds—which are semiconductors due to
   substitutional boron impurities replacing carbon atoms—diamond is a
   good electrical insulator. Natural blue diamonds recently recovered
   from the Argyle diamond mine in Australia have been found to owe their
   colour to an overabundance of hydrogen atoms: these diamonds are not
   semiconductors. Natural blue diamonds containing boron and synthetic
   diamonds doped with boron are p-type semiconductors. If an n-type
   semiconductor can be synthesized, electronic circuits could be
   manufactured from diamond. Worldwide research is in progress, with
   occasional successes reported, but nothing definite. In 2002 it was
   reported in the journal Nature that researchers have succeeded in
   depositing a thin diamond film on a diamond surface which is a major
   step towards manufacture of a diamond chip. In 2003 it was reported
   that NTT developed a diamond semiconductor device . In April of 2004
   Nature reported that below the superconducting transition temperature
   4 K, boron-doped diamond synthesized at high temperature and high
   pressure is a bulk, type-II superconductor . In October of 2004
   superconductivity was found to occur in heavily boron-doped microwave
   plasma-assisted chemical vapor deposition (MPCVD) diamond below the
   superconducting transition temperature of 7.4 K .

Thermal properties

   Unlike most electrical insulators, diamond is a good conductor of heat
   because of the strong covalent bonding within the crystal. Most natural
   blue diamonds contain boron atoms which replace carbon atoms in the
   crystal matrix, and also have high thermal conductance. .999-^12C
   monocrystalline synthetic diamond has the highest thermal conductivity
   of any known solid at room temperature: 2000–2500 W·m/m^2·K (200–250
   W·mm/cm^2·K), five times more than copper. Because diamond has such
   high thermal conductance it is already used in semiconductor
   manufacture to prevent silicon and other semiconducting materials from
   overheating. At lower temperatures conductivity becomes even better as
   its Fermi electrons can match the phononic normal transport mode near
   the Debye point, and transport heat swifter, to overcome the drop of
   specific heat with the fewer quantal microstates, to reach 41,000
   W·m/m^2·K at 104 K. The same diamond at .99999-^12C is predicted to
   200,000 W·m/m^2·K (20 kW·mm/cm^2·K).

   Diamond's thermal conductivity is made use of by jewellers and
   gemologists who may employ an electronic thermal probe to separate
   diamonds from their imitations. These probes consist of a pair of
   battery-powered thermistors mounted in a fine copper tip. One
   thermistor functions as a heating device while the other measures the
   temperature of the copper tip: if the stone being tested is a diamond,
   it will conduct the tip's thermal energy rapidly enough to produce a
   measurable temperature drop. This test takes about 2–3 seconds.
   However, older probes will be fooled by moissanite, an imitation of
   diamond introduced in 1998 which has a similar thermal conductivity.

Composition and colour

   Diamonds occur in a restricted variety of colors—steel gray, white,
   blue, yellow, orange, red, green, pink to purple, brown, and black.
   Colored diamonds contain crystallographic defects, including
   substitutional impurities and structural defects, that cause the
   coloration. Theoretically, pure diamonds would be transparent and
   colorless. Diamonds are scientifically classed into two main types and
   several subtypes, according to the nature of defects present and how
   they affect light absorption:

   Type I diamond has nitrogen (N) atoms as the main impurity, at a
   concentration of 0.1 percent. If the N atoms are in pairs they do not
   affect the diamond's colour; these are Type IaA. If the N atoms are in
   large even-numbered aggregates they impart a yellow to brown tint (Type
   IaB). About 98 percent of gem diamonds are type Ia, and most of these
   are a mixture of IaA and IaB material: these diamonds belong to the
   Cape series, named after the diamond-rich region formerly known as Cape
   Province in South Africa, whose deposits are largely Type Ia. If the N
   atoms are dispersed throughout the crystal in isolated sites (not
   paired or grouped), they give the stone an intense yellow or
   occasionally brown tint (Type Ib); the rare canary diamonds belong to
   this type, which represents only 0.1 percent of known natural diamonds.
   Synthetic diamond containing nitrogen is Type Ib. Type I diamonds
   absorb in both the infrared and ultraviolet region, from 320 nm. They
   also have a characteristic fluorescence and visible absorption spectrum
   (see Optical properties).

   Type II diamonds have very few if any nitrogen impurities. Type IIa
   diamond can be colored pink, red, or brown due to structural anomalies
   arising through plastic deformation during crystal growth—these
   diamonds are rare (1.8 percent of gem diamonds), but constitute a large
   percentage of Australian production. Type IIb diamonds, which account
   for 0.1 percent of gem diamonds, are usually a steely blue or grey due
   to scattered boron within the crystal matrix; these diamonds are also
   semiconductors, unlike other diamond types (see Electrical properties).
   However, an overabundance of hydrogen can also impart a blue colour;
   these are not necessarily Type IIb. Type II diamonds absorb in a
   different region of the infrared, and transmit in the ultraviolet below
   225 nm, unlike Type I diamonds. They also have differing fluorescence
   characteristics, but no discernable visible absorption spectrum.

   Certain diamond enhancement techniques are commonly used to
   artificially produce an array of colors, including blue, green, yellow,
   red, and black. Colour enhancement techniques usually involve
   irradiation, including proton and deuteron bombardment via cyclotrons;
   neutron bombardment via the piles of nuclear reactors; and electron
   bombardment via Van de Graaff generators. These high-energy particles
   physically alter the diamond's crystal lattice, knocking carbon atoms
   out of place and producing colour centers. The depth of colour
   penetration depends on the technique and its duration, and in some
   cases the diamond may be left radioactive to some degree.

   It should be noted that some irradiated diamonds are completely
   natural—one famous example is the Dresden Green Diamond. In these
   natural stones the color is imparted by "radiation burns" in the form
   of small patches, usually only skin deep. Additionally, Type IIa
   diamonds can have their structural deformations "repaired" via a
   high-temperature, high-pressure (HTHP) process, removing much or all of
   the diamond's colour.

   In the late 18th century, diamonds were demonstrated to be made of
   carbon by the rather expensive experiment of igniting a diamond (by
   means of a burning-glass) in an oxygen atmosphere and showing that
   carbonic acid gas (carbon dioxide) was the product of the combustion.
   The fact that diamonds are combustible bears further examination
   because it is related to an interesting fact about diamonds. Diamonds
   are carbon crystals that form deep within the Earth under high
   temperatures and extreme pressures. At surface air pressure (one
   atmosphere), diamonds are not as stable as graphite, and so the decay
   of diamond is thermodynamically favorable (δH = −2 kJ / mol). Diamonds
   had previously been shown to burn during Roman times.

   So, despite De Beers' 1948 ad campaign, diamonds are definitely not
   forever. However, owing to a very large kinetic energy barrier,
   diamonds are metastable; they will not decay into graphite under normal
   conditions.
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