Aluminium oxide is a chemical compound of aluminium and oxygen with the chemical formula
Al2O3. It is the most commonly occurring of several aluminium oxides, and specifically
identified as aluminium(III) oxide. It is commonly called alumina, and may also be called
aloxide, aloxite, or alundum depending on particular forms or applications. It commonly
occurs in its crystalline polymorphic phase α-Al2O3, in which it comprises the mineral
corundum, varieties of which form the precious gems ruby and sapphire. Al2O3 is significant
in its use to produce aluminium metal, as an abrasive owing to its hardness, and as
a refractory material owing to its high melting point.
Natural occurrence Corundum is the most common naturally occurring
crystalline form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum,
which owe their characteristic colors to trace impurities. Rubies are given their characteristic
deep red color and their laser qualities by traces of chromium. Sapphires come in different
colors given by various other impurities, such as iron and titanium.
Properties
Al2O3 is an electrical insulator but has a relatively high thermal conductivity for a
ceramic material. Aluminium oxide is insoluble in water. In its most commonly occurring crystalline
form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as
an abrasive and as a component in cutting tools.
Aluminium oxide is responsible for the resistance of metallic aluminium to weathering. Metallic
aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of aluminium
oxide forms on any exposed aluminium surface. This layer protects the metal from further
oxidation. The thickness and properties of this oxide layer can be enhanced using a process
called anodising. A number of alloys, such as aluminium bronzes, exploit this property
by including a proportion of aluminium in the alloy to enhance corrosion resistance.
The aluminium oxide generated by anodising is typically amorphous, but discharge assisted
oxidation processes such as plasma electrolytic oxidation result in a significant proportion
of crystalline aluminium oxide in the coating, enhancing its hardness.
Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals
lists in 1988. Aluminium oxide is on EPA's Toxics Release Inventory list if it is a fibrous
form. Amphoteric nature
Aluminium oxide is an amphoteric substance, meaning it can react with both acids and bases,
such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with
an acid, neutralising the other and producing a salt.
Al2O3 + 6 HF → 2 AlF3 + 3 H2O Al2O3 + 6 NaOH + 3 H2O → 2 Na3Al(OH)6
Structure
The most common form of crystalline aluminium oxide is known as corundum, which is the thermodynamically
stable form. The oxygen ions nearly form a hexagonal close-packed structure with aluminium
ions filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral.
In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space
group of R-3c. The primitive cell contains two formula units of aluminium oxide.
Aluminium oxide also exists in other phases, including the cubic γ and η phases, the
monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase
that can be tetragonal or orthorhombic. Each has a unique crystal structure and properties.
Cubic γ-Al2O3 has important technical applications. The so-called β-Al2O3 proved to be NaAl11O17.
Molten aluminium oxide near the melting temperature is roughly 2/3 tetrahedral, and 1/3 5-coordinated,
very little octahedral Al-O is present. Around 80% of the oxygen atoms are shared among three
or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing,
with the remaining 10–20% being edge-sharing. The breakdown of octahedra upon melting is
accompanied by a relatively large volume increase, the density of the liquid close to its melting
point is 2.93 g/cm3. Production
Aluminium hydroxide minerals are the main component of bauxite, the principal ore of
aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite3), boehmite),
and diaspore), along with impurities of iron oxides and hydroxides, quartz and clay minerals.
Bauxites are found in laterites. Bauxite is purified by the Bayer process:
AlO(OH) + H2O + NaOH → NaAl(OH)4 Al(OH)3 + NaOH → NaAl(OH)4
Except for SiO2, the other components of bauxite do not dissolve in base. Upon filtering the
basic mixture, Fe2O3 is removed. When the Bayer liquor is cooled, Al(OH)3 precipitates,
leaving the silicates in solution. NaAl(OH)4 → NaOH + Al(OH)3
The solid Al(OH)3 Gibbsite is then calcined to give aluminium oxide:
2 Al(OH)3 → Al2O3 + 3 H2O The product aluminium oxide tends to be multi-phase,
i.e., consisting of several phases of aluminium oxide rather than solely corundum. The production
process can therefore be optimized to produce a tailored product. The type of phases present
affects, for example, the solubility and pore structure of the aluminium oxide product which,
in turn, affects the cost of aluminium production and pollution control.
For its application as an electrical insulator in integrated circuits, where conformal growth
of thin film is a prerequisite and the preferred growth mode is atomic layer deposition, Al2O3
films were prepared by the chemical exchange between trimethylaluminum Al(CH3)3 and H2O:
2 Al(CH3)3 + 3 H2O → Al2O3 + 6 CH4 H2O in the above reaction was subsequently
replaced by ozone as the active oxidant and the following reaction took place:
2 Al(CH3)3 + O3 → Al2O3 + 3 C2H6 The Al2O3 films prepared using O3 show 10–100
times lower leakage current density compared with those prepared by H2O.
Known as alundum or aloxite in the mining, ceramic, and materials science communities,
aluminium oxide finds wide use. Annual world production of aluminium oxide is approximately
45 million tonnes, over 90% of which is used in the manufacture of aluminium metal. The
major uses of specialty aluminium oxides are in refractories, ceramics, and polishing and
abrasive applications. Large tonnages are also used in the manufacture of zeolites,
coating titania pigments, and as a fire retardant/smoke suppressant.
Applications The great majority of aluminium oxide is consumed
for the production of aluminium, usually by the Hall–Héroult process.
Filler Being fairly chemically inert and white, aluminium
oxide is a favored filler for plastics. Aluminium oxide is a common ingredient in sunscreen
and is sometimes present in cosmetics such as blush, lipstick, and nail polish.
Catalysis Aluminium oxide catalyses a variety of reactions
that are useful industrially. In its largest scale application, aluminium oxide is the
catalyst in the Claus process for converting hydrogen sulfide waste gases into elemental
sulfur in refineries. It is also useful for dehydration of alcohols to alkenes.
Aluminium oxide serves as a catalyst support for many industrial catalysts, such as those
used in hydrodesulfurization and some Ziegler-Natta polymerizations.
Purification Aluminium oxide is widely used to remove water
from gas streams. Other major applications are described below.
Abrasive Aluminium oxide is used for its hardness and
strength. It is widely used as an abrasive, including as a much less expensive substitute
for industrial diamond. Many types of sandpaper use aluminium oxide crystals. In addition,
its low heat retention and low specific heat make it widely used in grinding operations,
particularly cutoff tools. As the powdery abrasive mineral aloxite, it is a major component,
along with silica, of the cue tip "chalk" used in billiards. Aluminium oxide powder
is used in some CD/DVD polishing and scratch-repair kits. Its polishing qualities are also behind
its use in toothpaste. Aluminium oxide can be grown as a coating on aluminium by anodising
or by plasma electrolytic oxidation. Both its strength and abrasive characteristics
originate from the high hardness of aluminium oxide.
Paint
Aluminium oxide flakes are used in paint for reflective decorative effects, such as in
the automotive or cosmetic industries. Composite fiber
Aluminium oxide has been used in a few experimental and commercial fiber materials for high-performance
applications. Alumina nanofibers in particular have become a research field of interest.
Other Aluminum oxide is used as a catalytic "fuel"
which decomposes hydrazine in Chemical-reaction Monopropellant rockets.
In lighting, transparent aluminium oxide is used in some sodium vapor lamps. Aluminium
oxide is also used in preparation of coating suspensions in compact fluorescent lamps.
In chemistry laboratories, aluminium oxide is a medium for chromatography, available
in basic, acidic and neutral formulations. Health and medical applications include it
as a material in hip replacements. As well, it is used as a dosimeter for radiation
protection and therapy applications for its optically stimulated luminescence properties.
Aluminium oxide is an electrical insulator used as a substrate for integrated circuits
but also as a tunnel barrier for the fabrication of superconducting devices such as single
electron transistors and superconducting quantum interference devices.
Insulation for high-temperature furnaces is often manufactured from aluminium oxide. Sometimes
the insulation has varying percentages of silica depending on the temperature rating
of the material. The insulation can be made in blanket, board, brick and loose fiber forms
for various application requirements. Small pieces of aluminium oxide are often
used as boiling chips in chemistry. It is also used to make spark plug insulators.
Using a plasma spray process and mixed with titania, it is coated onto the braking surface
of some bicycle rims to provide abrasion and wear resistance.
See also
References
External links Media related to Aluminium oxide at Wikimedia
Commons CDC - NIOSH Pocket Guide to Chemical Hazards