Carbide
carbide,
any of a class of chemical compounds in which carbon is combined with a
metallic or semimetallic element. Calcium carbide is important chiefly as a
source of acetylene and other chemicals, whereas the carbides of silicon, tungsten,
and several other elements are valued for their physical hardness, strength,
and resistance to chemical attack even at very high temperatures. Iron carbide
(cementite) is an important constituent of steel and cast iron.
Preparation of carbides :
Carbides: Carbides
are binary compounds of carbon with elements of lower or about equal
electronegativity.
Preparation : Carbides
are generally prepared by heating the elements orits oxide with carbon or
hydrocarbon at very high temperatures.
Ca + 2C ——→ BaC2;
2Li + 2C ——→ Li2C2
CaO + 3C ——→ CaC2 + CO
4Li + C2H2 ——→
Li2C2 + LiH
Carbides are classified into three
types on the basis of chemical bonding.
Carbide
Classification of carbides :
Classification of carbides based on structural
type is rather difficult, but three broad classifications arise from general
trends in their properties. The most electropositive metals form ionic or
saltlike carbides, the transition metals in the middle of the periodic table
tend to form what are called interstitial carbides, and the nonmetals of
electronegativity similar to that of carbon form covalent or molecular
carbides.
Ionic carbides
Ionic carbides have discrete carbon anions of the
forms C4−, sometimes called methanides since they can be viewed as
being derived from methane, (CH4); C22−,
called acetylides and derived from acetylene (C2H2); and
C34−, derived from allene (C3H4).
The best-characterized methanides are probably beryllium carbide (Be2C)
and aluminum carbide (Al4C3). Beryllium oxide (BeO) and
carbon react at 2,000 °C (3,600 °F) to produce the brick-red beryllium carbide,
whereas pale yellow aluminum carbide is prepared from aluminum and carbon in a
furnace. Aluminum carbide reacts as a typical methanide with water to produce
methane.Al4C3 + 12H2O
→ 4Al(OH)3 + 3CH4
There are many acetylides that are well known and
well characterized. In addition to those of the alkali metals and the
alkaline-earth metals mentioned above, lanthanum (La) forms two different
acetylides, and copper (Cu), silver (Ag), and gold (Au) form explosive
acetylides. Zinc (Zn), cadmium (Cd), and mercury (Hg) also form acetylides,
although they are not as well characterized. The most important of these
compounds is calcium carbide, CaC2. The primary use for calcium
carbide is as a source of acetylene for use in the chemical industry. Calcium
carbide is synthesized industrially from calcium oxide (lime), CaO, and carbon
in the form of coke at about 2,200 °C (4,000 °F). Pure calcium carbide has a
high melting point (2,300 °C [4,200 °F]) and is a colourless solid. The
reaction of CaC2 with water yields C2H2 and a
significant amount of heat, so the reaction is carried out under carefully
controlled conditions.CaO + 3C → CaC2
+ CO CaC2 + 2H2O → C2H2 + Ca(OH)2
Calcium carbide also reacts with nitrogen gas at elevated temperatures
(1,000–1,200 °C [1,800–2,200 °F]) to form calcium cyanamide, CaCN2.CaC2 + N2 → CaCN2 +
C This is an important industrial reaction because CaCN2
finds extensive use as a fertilizer owing to its reaction with water to produce
cyanamide, H2NCN. Most MC2 acetylides have the CaC2
structure, which is derived from the cubic sodium chloride (NaCl) structure.
The C2 units lie parallel along the cell axes, causing a distortion
of the cell from cubic to tetragonal.
Salt-like (saline) carbides :
Salt-like
carbides are composed of highly electropositive elements such as the alkali
metals, alkaline earths, and group 3 metals including scandium, yttrium and lanthanum.
Aluminum from group 13 forms carbides, but gallium, indium and thallium do not.
These materials feature isolated carbon centers, often described as "C4−",
in the methanides or methides; two atom units, "C22−"
in the acetylides ; and three atom units "C34−"
in the sesquicarbides.[ The graphite intercalation compound KC8,
prepared from vapour of potassium and graphite, and the alkali metal
derivatives of C60 are not usually classified as carbides.
Methanides
Tungsten carbide drill bits.
Carbides
of this class decompose in water producing methane. Two such examples are aluminium
carbide Al4C3 and beryllium
carbide Be2C.
The
reaction of transition metal carbides with water is very slow and is usually
neglected. For example, depending on surface porosity, 5–30 atomic layers of titanium
carbide are hydrolyzed within 5 minutes at ambient conditions, following by
saturation of the reaction.
Acetylides
Sesquicarbides
:
The polyatomic
ion C34–, sometimes called sesquicarbide, is found
in Li4C3 and Mg2C3. The ion is
linear and is isoelectronic with CO2. The C-C distance in Mg2C3
is 133.2 pm. Mg2C3 yields methyl acetylene, CH3CCH,
and propadiene , CH2CCH2, on hydrolysis which was the
first indication that it may contain C34–.
Covalent carbides
The carbides
of silicon and boron are described as "covalent carbides", although
virtually all compounds of carbon exhibit some covalent character. Silicon
carbide has two similar crystalline forms, which are both related to the
diamond structure. Boron carbide, B4C, on the other hand has an
unusual structure which includes icosahedral boron units linked by carbon
atoms. In this respect boron carbide is similar to the boron rich borides. Both
silicon carbide, SiC, (carborundum) and boron carbide, B4C are very
hard materials and refractory. Both materials are important industrially. Boron
also forms other covalent carbides, e.g. B25C.
Interstitial carbides :
The
carbides of the group 4, 5 and 6 transition metals (with the exception of
chromium) are often described as interstitial compounds. These carbides have metallic properties and
are refractory. Some exhibit a range of stoichiometries, e.g. titanium carbide,
TiC. Titanium carbide and tungsten carbide are important industrially and are
used to coat metals in cutting tools.
The
longheld view is that the carbon atoms fit into octahedral interstices in a
close packed metal lattice when the metal atom radius is greater than
approximately 135 pm:
- When the metal atoms are cubic close packed,
(ccp), then filling all of the octahedral interstices with carbon achieves
1:1 stoichiometry with the rock salt structure.
- When the metal atoms are hexagonal close
packed, (hcp), as the octahedral interstices lie directly opposite each
other on either side of the layer of metal atoms, filling only one of
these with carbon achieves 2:1 stoichiometry with the CdI2
structure.
Intermediate transition metal carbides :
In
these carbides, the transition metal ion is smaller than the critical 135 pm,
and the structures are not interstitial but are more complex. Multiple
stoichiometries are common, for example iron forms a number of carbides, Fe3C,
Fe7C3 and Fe2C. The best known is cementite,
Fe3C, which is present in steels. These carbides are more reactive
than the interstitial carbides, for example the carbides of Cr, Mn, Fe, Co and
Ni all are hydrolysed by dilute acids and sometimes by water, to give a mixture
of hydrogen and hydrocarbons. These compounds share features with both the
inert interstitials and the more reactive salt-like carbides
Molecular carbides :
The complex [Au6C(PPh3)6]2+,
containing a carbon-gold core.
Some
metals, such as lead and tin, are believed not to form carbides under any
circumstances. There exists however a mixed titanium-tin carbide, which is a
two-dimensional conductor. (In 2007, there were two reports of a lead carbide
PbC2, apparently of the acetylide type; but these claims have yet to
be published in reviewed journals.)
Calcium carbide :
Calcium
carbide is a chemical
compound with the chemical formula of CaC2. Its main use
industrially is in the production of acetylene and calcium cyanamide.
The
pure material is colorless, however pieces of technical-grade calcium carbide
are grey or brown and consist of only 80-85% of CaC2 (the rest is
CaO, Ca3P2, CaS, Ca3N2, SiC, etc.).
Because of presence of PH3, NH3, and H2S,
technical-grade calcium carbide has a distinctive smell which some find
unpleasant.
Production
Calcium
carbide is produced industrially in an electric arc furnace from a mixture of lime
and coke at approximately 2000 °C. This method has not changed since its
invention in 1888:
CaO + 3 C → CaC2 + CO
The
high temperature required for this reaction is not practically achievable by
traditional combustion, so the reaction is performed in an electric arc furnace
with graphite electrodes. The carbide product produced generally contains
around 80% calcium carbide by weight. The carbide is crushed to produce small
lumps that can range from a few mm up to 50 mm. The impurities are
concentrated in the finer fractions. The CaC2 content of the product
is assayed by measuring the amount of acetylene produced on hydrolysis. As an
example, the British and German standards for the content of the coarser
fractions are 295 L/kg and 300 L/kg respectively. Impurities present in the
carbide include phosphide, which produces phosphine when hydrolysed.
Crystal
structure :
Pure
calcium carbide is a colourless solid. The common crystalline form at room
temperature is a distorted rock-salt structure with the C22−
units lying parallel.
Applications :
Production of acetylene
The
reaction of calcium carbide with water, producing acetylene and calcium
hydroxide, was discovered by Friedrich Wöhler in 1862.
CaC2 + 2 H2O → C2H2
+ Ca(OH)2
This
reaction is the basis of the industrial manufacture of acetylene, and is the
major industrial use of calcium carbide.
In China,
acetylene derived from calcium carbide remains a raw material for the chemical
industry, in particular for the production of polyvinyl chloride. Locally
produced acetylene is more economical than using imported oil. Production of
calcium carbide in China
has been increasing. In 2005 output was 8.94 million tons, with the capacity to
produce 17 million tons.
Carbide lamps
Lit carbide lamp
Calcium
carbide is used in carbide lamps, in which water drips on the carbide and the
acetylene formed is ignited. These lamps were usable but dangerous in coal
mines, where the presence of the flammable gas methane made them a serious
hazard. The presence of flammable gases in coal mines led to miner safety lamps
such as the Davy lamp, in which a wire gauze reduces the risk of methane
ignition. However, carbide lamps were still used extensively in slate, copper,
and tin mines, where methane is not a serious hazard, but most miner's lamps
have now been replaced by electric lamps.
Carbide
lamps are still used for mining in some less wealthy countries, for example in
the silver mines near Potosí, Bolivia.
Other uses
In the
ripening of fruit, calcium carbide is sometimes used as source of acetylene
gas, which is a ripening agent similar to ethylene.
Calcium
carbide is used in toy cannons such as the Big-Bang Cannon, as well as in bamboo
cannons.
Calcium
carbide, together with calcium phosphide, is used in floating, self-igniting
naval signal flares, such as those produced by the Holmes' Marine Life
Protection Association.
In the Netherlands,
calcium carbide is still used in a traditional New Year's Eve custom called Carbidschieten
(Carbide Shooting). To create an explosion, carbide and water are put in a milk
churn with a lid, with ignition usually done with a torch. Some villages in the
Netherlands fire multiple milk churns in a row. The custom may be derived from
an old pagan religious practice intended to chase off spirits.
tremely hard like diamond and
possess very high melting points.
Boron
Carbide :
Boron Carbide is one of the hardest materials
known, ranking third behind diamond and cubic boron nitride. It is the hardest
material produced in tonnage quantities. Originally discovered in mid 19th
century as a by-product in the production of metal borides, boron carbide was
only studied in detail since 1930.
Boron carbide powder (see figure 1) is mainly
produced by reacting carbon with B2O3 in an electric arc
furnace, through carbothermal reduction or by gas phase reactions. For
commercial use B4C powders usually need to be milled and purified to
remove metallic impurities.
In common with other non-oxide materials boron
carbide is difficult to sinter to full density, with hot pressing or sinter HIP
being required to achieve greater than 95% of theoretical density. Even using
these techniques, in order to achieve sintering at realistic temperatures (e.g.
1900 - 2200°C), small quantities of dopants such as fine carbon, or silicon
carbide are usually required.
Property
|
|
|
Density (g.cm-3)
|
2.52
|
|
Melting Point (°C)
|
2445
|
|
Hardness (Knoop 100g) (kg.mm-2)
|
2900-3580
|
|
Fracture Toughness (MPa.m-½)
|
2.9 - 3.7
|
|
Young's Modulus (GPa)
|
450 - 470
|
|
Electrical Conductivity (at 25°C) (S)
|
140
|
|
Thermal Conductivity (at 25°C) (W/m.K)
|
30 - 42
|
|
Thermal Expansion Co-eff. x10-6
(°C)
|
5
|
|
Thermal neutron capture cross section (barn)
|
600
|
As an alternative, B4C can be formed as
a coating on a suitable substrate by vapour phase reaction techniques e.g.
using boron halides or di-borane with methane or another chemical carbon
source.
Key Properties:
Boron carbide is characterised by its:
•
Extreme hardness
•
Difficult to sinter to high relative densities without the use of
sintering aids
•
Good chemical resistance
Table 1. Typical properties of boron
carbide
•
Good nuclear properties
•
Low density
Typical properties for boron carbide are listed in
table 1.
.
Applications :
Abrasives
Due to its high hardness, boron carbide powder is
used as an abrasive in polishing and lapping applications, and also as a loose
abrasive in cutting applications such as water jet cutting. It can also be used
for dressing diamond tools.
Nozzles
The extreme hardness of boron carbide gives it
excellent wear and abrasion resistance and as a consequence it finds
application as nozzles for slurry pumping, grit blasting and in water jet
cutters (see figure 2).
Nuclear applications
Its ability to absorb neutrons without forming
long lived radio-nuclides make the material attractive as an absorbent for
neutron radiation arising in nuclear power plants. Nuclear applications of
boron carbide include shielding, and control rod and shut down pellets.
Ballistic Armour
Boron carbide, in conjunction with other materials
also finds use as ballistic armour (including body or personal armour) where
the combination of high hardness, high elastic modulus, and low density give
the material an exceptionally high specific stopping power to defeat high
velocity projectiles.
Other Applications
Other applications include ceramic tooling dies,
precision toll parts, evaporating boats for materials testing and mortars and
pestles.
Aluminium carbide :
Aluminium
carbide, chemical formula
Al4C3, is a carbide of aluminium. It has the appearance
of pale yellow to brown crystals. It is stable up to 1400 °C. It decomposes in
water with the production of methane.
Structure
Aluminium
carbide has an unusual crystal structure that consists of two types of layers.
It is based on AlC4 tetrahedra of two types and thus two types of
carbon atoms. One is surrounded by a deformed octahedron of 6 Al atoms at a
distance of 217 pm. The other is surrounded by 4 Al atoms at 190–194 pm and a
fifth Al atom at 221 pm. Other carbides (IUPAC nomenclature: methides) also
exhibit complex structures.
Reactions
Aluminium
carbide hydrolyses with evolution of methane. The reaction proceeds at room
temperature but is rapidly accelerated by heating.
Al4C3 + 12 H2O →
4 Al(OH)3 + 3 CH4
Similar
reactions occur with other protic reagents:
Al4C3 + 12 HCl → 4 AlCl3
+ 3 CH4
Preparation
Aluminium
carbide is prepared by direct reaction of aluminium and carbon in an electric
furnace.
4 Al + 3 C → Al4C3
An
alternative reaction begins with alumina, but it is less favorable because of
generation of carbon monoxide.
2 Al2O3 + 9 C → Al4C3
+ 6 CO
Silicon
carbide also reacts with aluminium to yield Al4C3. This
conversion limits the mechanical applications of SiC, because Al4C3
is more brittle than SiC.
4 Al + 3 SiC → Al4C3 + 3 Si
In
aluminium-matrix composites reinforced with silicon carbide, the chemical
reactions between silicon carbide and molten aluminium generate a layer of aluminium
carbide on the silicon carbide particles, which decreases the strength of the
material, although it increases the wettability of the SiC particles. This tendency can be decreased by coating the
silicon carbide particles with a suitable oxide or nitride, preoxidation of the
particles to form a silica coating, or using a layer of sacrificial metal.
An
aluminium-aluminium carbide composite material can be made by mechanical
alloying, by milling aluminium powder with graphite particles.
Occurrence :
Small
amounts of aluminium carbide are a common impurity of technical calcium carbide.
In electrolytic manufacturing of aluminium, aluminium carbide forms as a
corrosion product of the graphite electrodes.
In metal
matrix composites based on aluminium matrix reinforced with metal carbides (silicon
carbide, boron carbide, etc.) or carbon fibers, aluminium carbide often forms
as an unwanted product. In case of carbon fiber, it reacts with the aluminium
matrix at temperatures above 500 °C; better wetting of the fiber and inhibition
of chemical reaction can be achieved by coating it with e.g. titanium boride.
Applications :
Aluminium
carbide particles finely dispersed in aluminium matrix lower the tendency of
the material to creep, especially in combination with silicon carbide
particles.
Aluminium
carbide can be used as an abrasive in high-speed cutting tools. It has
approximately the same hardness as topaz.
Tungsten carbide :
Tungsten
carbide (WC) is an
inorganic chemical compound (specifically, a carbide) containing equal parts of
tungsten and carbon atoms. Colloquially, tungsten carbide is often simply
called carbide. In its most basic form, it is a fine gray powder, but it
can be pressed and formed into shapes for use in industrial machinery, tools, abrasives,
as well as jewelry. Tungsten carbide is approximately three times stiffer than steel,
with a Young's modulus of approximately 550 GPa, and
is much denser than steel or titanium. It is comparable with corundum (α-Al2O3)
or sapphire in hardness and can only be polished and finished with abrasives of
superior hardness such as cubic boron nitride and diamond amongst others, in
the form of powder, wheels, and compounds.
Chemical properties :
There
are two well characterized compounds of tungsten and carbon, WC and tungsten
semicarbide, W2C. Both compounds may be present in coatings and
the proportions can depend on the coating method.
WC can
be prepared by reaction of tungsten metal and carbon at 1400–2000 °C. Other methods include a patented fluid bed
process that reacts either tungsten metal or blue WO3 with CO/CO2
mixture and H2 between 900 and 1200 °C. Chemical vapor deposition
methods that have been investigated include: WC can also be produced by heating
WO3 with graphite in hydrogen at 670 °C following by carburization
in Ar at 1000 °C or directly heating WO3 with graphite at 900°C.
- tungsten hexachloride with hydrogen, as a reducing
agent, and methane, as the source of carbon at 670 °C (1,238 °F)
WCl6 + H2 + CH4 →
WC + 6 HCl
- reacting tungsten hexafluoride with hydrogen,
as reducing agent, and methanol, as source of carbon at 350 °C
(662 °F)
WF6 + 2 H2 + CH3OH
→ WC + 6 HF + H2O
- At high temperatures WC decomposes to
tungsten and carbon and this can occur during high-temperature thermal
spray, e.g. high velocity oxygen fuel (HVOF) and high energy plasma (HEP)
methods.
Oxidation
of WC starts at 500–600 °C. It is resistant to acids and is only attacked by hydrofluoric
acid/nitric acid (HF/HNO3) mixtures above room temperature. It
reacts with fluorine gas at room temperature and chlorine above 400 °C
(752 °F) and is unreactive to dry H2 up to its melting point.
Physical properties :
Tungsten
carbide is high melting, 2,870 °C (5,200 °F), extremely hard (8.5–9.0 Mohs
scale, Vickers hardness number = 2242) with low electrical resistivity (~2×10−7 Ohm·m),
comparable with that of some metals (e.g. vanadium 2×10−7 Ohm·m).
WC is
readily wetted by both molten nickel and cobalt. Investigation of the phase diagram of the
W-C-Co system shows that WC and Co form a pseudo binary eutectic. The phase
diagram also shows that there are so-called η-carbides with composition (W,Co)6C
that can be formed and the fact that these phases are brittle is the reason why
control of the carbon content in WC-Co hard metals is important.
Structure :
α-WC structure, carbon atoms are gray.
There
are two forms of WC, a hexagonal form, α-WC (hP2, space group P6m2, No. 187),
and a cubic high-temperature form, β-WC, which has the rock salt structure. The
hexagonal form can be visualized as made up of hexagonally close packed layers
of metal atoms with layers lying directly over one another, with carbon atoms
filling half the interstices giving both tungsten and carbon a regular trigonal
prismatic, 6 coordination. From the unit cell dimensionsthe following bond
lengths can be determined; the distance between the tungsten atoms in a
hexagonally packed layer is 291 pm, the shortest distance between tungsten
atoms in adjoining layers is 284 pm, and the tungsten carbon bond length is 220
pm. The tungsten-carbon bond length is therefore comparable to the single bond
in W(CH3)6 (218 pm) in which there is strongly distorted
trigonal prismatic coordination of tungsten.
Applications :
Cutting tools for machining
Sintered
tungsten carbide cutting tools are very abrasion resistant and can also
withstand higher temperatures than standard high speed steel tools. Carbide
cutting surfaces are often used for machining through materials such as carbon
steel or stainless steel, as well as in situations where other tools would wear
away, such as high-quantity production runs. Because carbide tools maintain a
sharp cutting edge better than other tools, they generally produce a better
finish on parts, and their temperature resistance allows faster machining. The
material is usually called cemented carbide, hardmetal or tungsten-carbide
cobalt: it is a metal matrix composite where tungsten carbide particles are the
aggregate and metallic cobalt serves as the matrix. Manufacturers use tungsten
carbide as the main material in some high-speed drill bits, as it can resist
high temperatures and is extremely hard.
Ammunition
Tungsten
carbide is often used in armor-piercing ammunition, especially where depleted
uranium is not available or is politically unacceptable. W2C
projectiles were first used by German Luftwaffe tank-hunter squadrons in World
War II. Owing to the limited German reserves of tungsten, W2C
material was reserved for making machine tools and small numbers of projectiles.
It is an effective penetrator due to its combination of great hardness and very
high density.
Tungsten
carbide ammunition can be of the sabot type (a large arrow surrounded by a
discarding push cylinder) or a subcaliber ammunition, where copper or other
relatively soft material is used to encase the hard penetrating core, the two
parts being separated only on impact. The latter is more common in
small-caliber arms, while sabots are usually reserved for artillery use.
Nuclear
Tungsten
carbide is also an effective neutron reflector and as such was used during
early investigations into nuclear chain reactions, particularly for weapons. A criticality
accident occurred at Los Alamos National Laboratory on 21 August 1945 when Harry
K. Daghlian, Jr. accidentally dropped a tungsten carbide brick onto a plutonium
sphere, causing the subcritical mass to go supercritical with the reflected neutrons.
Surgical instruments
It is
also used for making surgical instruments meant for open surgery (scissors,
forceps, hemostats, blade-handles, etc.) and laparoscopic surgery (graspers,
scissors/cutter, needle holder, cautery, etc.). They are much costlier than
their stainless-steel counterparts and require delicate handling, but give
better performance.
Jewelry
Tungsten
carbide, also called cemented carbide, has become a popular material in the
bridal jewelry industry due to its extreme hardness and high resistance to
scratching.
