Scientific classification of diamonds The structural imperfections to be found in diamonds

10.4.6.4 Scientific classification of diamonds The structural imperfections to be found in diamonds

include crystalline inclusions, cracks, impurity atoms and vacant sites. Broadly, inclusions and cracks act as stress-concentrators and mainly affect the mechanical properties, whereas fine-scale defects, notably impu- rity atoms of nitrogen and boron, influence physi- cal properties such as optical absorption, electrical conductivity, etc. Inclusions in natural diamonds are particles of mineral matter. Inclusions in synthetic dia- monds derive from the metals used as catalysts (i.e. Ni, Co, Fe).

The generally-accepted classification of diamonds, which is shown in Table 10.2, recognizes four main categories. It is based upon absorption characteristics determined over the ultraviolet, visible and infrared regions of the electromagnetic spectrum. The choice of this approach is perhaps not surprising when one considers the visible response of cut diamonds to white light. Absorption spectra are highly structure-sensitive and have made it possible to classify different qualities of natural and synthetic diamond in terms of their content of fine defects, such as impurity atoms and vacant sites. Each type of defect provides so-called ‘optical centres’ which decide the specific manner in which components of incident radiation are absorbed and/or transmitted by the crystal structure. A diamond frequently contains more than one type of defect, hence interpretation of absorption spectra can sometimes be difficult and rather arbitrary.

The decrease in intensity for a particular wave- length, as a result of absorption, is expressed by

the classic exponential relation I D I 0 e , where I is the intensity of transmitted radiation, I 0 is the

intensity of incident monochromatic radiation and ˛ length. Long path lengths within the diamond increase the amount of optical interaction, producing a size

effect which explains why larger diamonds tend to be more colourful.

It will be seen that the classification first dis- tinguishes between Type I diamonds which contain nitrogen (say, up to 0.1–0.2%) and Type II diamonds which have an extremely low nitrogen content. Type

I diamonds are further sub-divided according to the spatial distribution of nitrogen atoms. In natural dia- monds, geologic periods of time at high temperatures and pressure have permitted nitrogen atoms to cluster, sometimes, apparently, into platelets. In most synthetic diamonds, the nitrogen atoms are more dispersed. Nitrogen, singly or in groups, can occupy substitu- tional or interstitial sites and are responsible for at least five types of optical centre (e.g. A, B, N, N3, platelet). (Thus, the designation Type IaB signifies that the dia- mond contains B centres wherein a few nitrogen atoms have clustered and replaced carbon atoms.)

From the distinction between Type IIa and Type IIb diamonds, it will be seen that the ability of diamond to act as either an insulator or a semiconductor is governed by its impurity content. In Type II diamonds, the concentration of impurity atoms is smaller than in Type I diamonds, usually being expressed in parts per million. In terms of the electron theory of conduction, pure diamond has a filled valence band that is separated from a partly filled conduction band by a substantial energy gap of 5.5 eV. The mean available thermal energy is approximately 0.025 eV and is therefore insufficient to transport electrons through the gap. Pure diamond is consequently an electrical insulator at room temperature. In a similar sense, photons of white light have associated wavelengths ranging from 400 nm (D 3.1 eV) to 730 nm (D 1.7 eV) and pass through the crystal without bringing about electronic transitions which bridge the gap. Absorption of energy from these incident photons is small. However, photons associated with shorter wavelengths in the ultraviolet range can exceed the energy requirement of 5.5 eV and the gap is bridged.

Moving on from the special case of pure dia- mond to Type I diamonds, the introduction of nitrogen atoms (Z D 7) into the carbon structure (Z D 6) allows tetrahedral bonding to be maintained but also adds extra electrons. Optical absorption now becomes likely because electrons can move to higher levels within the band gap. However, the presence of nitrogen atoms

Table 10.2 Classification of diamonds

Type I (nitrogen present) Type II (negligible nitrogen content) Type Ia

Type Ib Type IIa

Type IIb

Clustering of N atoms N atoms dispersed and

Contain boron

substituting for

C atoms

Non-conducting Semiconductivity possible in doped synthetic diamonds

Most natural stones Most synthetic

Rarely found in nature

diamonds; rarely natural stones

340 Modern Physical Metallurgy and Materials Engineering does not confer conductivity. In Type IIb diamonds, the

thicker layer of tungsten carbide. The latter is brazed principal impurity is boron (Z D 5). Measured atomic

onto the tool body. The standard quality of PCD is

C that is set Each boron atom can contribute only three electrons to

concentrations of boron are often less than 0.5 ð 10 6 .

subject to a temperature ceiling of 700 °

by two factors. First, cobalt expands more than the the surrounding four carbon atoms and therefore pro-

diamond phase and higher temperatures cause stress vides an acceptor site within the band gap, allowing

C, cobalt promotes ‘holes’ to form in the valence band. Semiconductiv-

cracks to form. Second, above 700 °

the conversion of diamond into graphite. These prob- ity then becomes possible. The two impurity elements

lems were solved by either removing the cobalt phase nitrogen and boron thus have contrasting effects upon

by acid leaching or by using silicon instead of cobalt. the diamond structure, providing donor and acceptor

This variety of PCD is thermally stable at tempera- sites, respectively. When both elements are present,

C and has been successfully used for some degree of compensation occurs. For instance, a

tures up to 1200 °

drilling bits in hard-rock mining. surfeit of nitrogen can neutralize the effect of boron atoms and the semiconducting characteristic is absent.

10.4.6.6 Baked carbons and graphitized carbons If a diamond is colourless and electrically conductive one might reasonably infer that it contains traces of

Crucibles made from natural graphite bonded with fire- nitrogen and uncompensated boron and is classifiable

clay have been used in metal-melting operations for as Type IIb.

centuries. Although their strength is not exceptional, they can withstand temperatures of up to 1200 Specific types of diamond have proved successful ° C.

in highly specialized applications. For instance, Type Other applications of natural graphite include lubri- Ia and Type IIa diamonds have very high thermal con-

cants, brake linings, bearings, foundry facings and pen- cil ‘leads’. Crystallinity is well developed and they are

ductivities (1000 and 2000 W m 1 , K 1 , respectively,

relatively unreactive.

at room temperature) and have been used as heat sinks Most of the great demand for manufactured car- in electronic devices that release much thermal energy

bons is met by synthesis. Applications are extremely within a confined space. Detector ‘windows’ of Type II

diverse: brushes for electric motors and generators, diamond are virtually transparent to infrared radiation

rocket nozzles and nose cones for space vehicles, and have been used in conditions as disparate as those

refractory linings for furnaces, moulds for metal- of diamond synthesis anvils and space probes. Type IIb

founding and hot-pressing, moderators and reflectors diamonds have interesting semiconductive prospects,

for nuclear reactors, electrodes for arcwelding, etc. possibly being capable of operating at higher temper-

There are two main types of bulk carbon products for atures than silicon devices, but developments in this

industrial applications. Neither melting nor sintering area have been restricted by the difficulty of develop-

are involved in their manufacture. ing n-type devices to complement the p-type devices

Baked carbons, the first category, are produced by described above.

extruding/moulding a mix of carbonaceous filler or grist (e.g. petroleum coke, anthracite, pitch coke, etc.)

10.4.6.5 Polycrystalline diamond (PCD) and binder (e.g. coal-tar pitch, thermosetting resin). Polycrystalline aggregates of diamonds rarely occur in

The shaping method determines the general anisotropy nature. It is now possible to synthesise a fine-grained

of the final product because the coke particles usu- aggregate by sintering diamond crystallites (2–25 mm

ally have one axis longer than the other, consequently in size) at a high temperature and pressure in the pres-

extrusion orients the particles parallel to the extrusion ence of a metal such as cobalt. The resultant blanks

direction whereas moulding orients particles perpen- of PCD are typically about 50 mm diameter and can

dicular to the compression forces. (Accordingly, the

be electric discharge machined (EDM) to the required final product usually has a so-called ‘grain’.) The shape and size, thus overcoming the problems associ-

‘green’ shape is heated (baked) in a reducing atmo- ated with producing large single crystals. During sin-

C. During this tering, the randomly-oriented grains deform plastically

sphere at a temperature of 1000–1200 °

critical operation, the pitch decomposes or ‘cracks’ and and interlock. Molten cobalt acts as a solvent/catalyst

provides carbon linkages between the particles of filler. and also fills the interstices between the grains. This

Copious amounts of volatiles are evolved, producing structure has a better wear resistance than either natu-

about 20–30% porosity. Porosity weakens the struc- ral diamond or tungsten carbide. It is also appreciably

ture, influences electrical conductivity and, because of tougher than single crystals of diamond because the

its connected nature, makes the structure permeable to random orientation of grains forces any propagating

gases and liquids. It may be reduced by impregnating crack to take an erratic, transgranular path which con-

the baked carbon with more pitch and baking again. sumes strain energy.

Further heat treatment at substantially higher temper- PCD is used to make metal-working dies for draw-

C causes the structure to graphi- ing the larger sizes of metallic wire (e.g. >0.2 mm).

atures of 2500–3000 °

tize. An electric resistance furnace is necessary for (With smaller sizes of wire, a natural single crystal of

this purpose, hence the products are sometimes called diamond gives a better surface texture.) When used

electrographites. (Most designs of electric furnaces for for cutting tools, a thin layer of PCD is supported by a

larger sizes of product are modifications of the original

Ceramics and glasses 341 Acheson design of 1895, a furnace which has a special

place in the history of the carbon industry.) Final shapes, whether baked or graphitized, can be readily and accurately machined. The final proper- ties of the carbon product depend upon the interplay of variables such as initial grain size, type of filler or binder, purity, furnace temperature, etc. Thus, the higher processing temperatures that are used for elec- trographites eliminate strain, heal imperfections and promote graphitization within the structure. As a result, electrographites have much higher electrical and ther- mal conductivities than baked carbons, making it pos- sible to halve the cross-sectional area of an electrode and gain valuable savings in mass and size. Accord- ingly, electrographites are favoured for large steel- melting furnaces. However, both baked carbons and the more costly electrographites find general appli- cation in a wide variety of electric furnaces. Their availability and continuous development have made

a vital contribution to the electric are processes used to mass-produce steel, aluminium, carbides of silicon and calcium, alkali metals and their hydroxides, phos- phorus, magnesium, chlorine, etc. In addition to being refractory, electrodes must withstand chemical attack, mechanical damage during charging of the furnace, severe gradients in temperature and thermal shock. In the last respect, carbon electrodes possess a valu- able combination of properties; that is, low thermal expansivity, low modulus of elasticity, good high- temperature strength and high thermal conductivity. The low electrical resistivity minimizes resistive heat- ing and thus helps to restrict the temperature of the electrode.

The chemical industry uses a variety of impregnation techniques to combat the problem of intrinsic porosity so that corrosive liquids such as caustic solutions and sulphite liquors can be handled. Thermoset resins are used to render the carbon impervious to fluids but, being organic, tend to decompose if the service temperature rises above 170 °

C. Surfaces can also be sealed with more stable compounds such as carbides, silicides, borides and nitrides; for example, impregnation with molten silicon at a temperature of 2000 °

C forms silicon

carbide in situ and makes the surface impervious.