Aluminum oxide: physical and chemical characteristics and areas of application

Chemical compound with formula Al2O3

This article is about aluminum (III) oxide, Al2O3.
For other uses, see Alumina (compounds). Aluminum Oxide (Aluminum Oxide)

Aluminium oxide

It is a chemical compound of aluminum and oxygen with chemical formula 23. It is the most commonly occurring of several aluminum oxides, and is specifically designated as
aluminum(III) oxide
.
It is commonly called alumina
and may also be called
aloxide
,
aloxite
, or
alundum
depending on specific forms or applications. It occurs naturally in crystalline form. polymorphic phase α-Al2O3 as the mineral corundum, varieties of which are the precious gemstones Ruby and sapphire. Al2O3 is important in its use for the production of aluminum metal, as an abrasive due to its hardness, and as a refractory material due to its high melting point.[7]

Content

• 1 Natural phenomenon
• 2 Properties 2.1 Amphoteric character

Properties

Aluminum oxide in powder form.
Al2O3 is an electrical insulator but has a relatively high thermal conductivity (30 W−1K−1)[2] for a ceramic material. Aluminum oxide does not dissolve in water. In its most common crystalline form, called corundum or α-alumina, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.[7]

Aluminum oxide is responsible for the resistance of aluminum metal to weathering. Aluminum metal is highly reactive with atmospheric oxygen, and a thin passivation layer of aluminum oxide (4 nm thick) forms on any exposed aluminum surface within hundreds of picoseconds.[ need better source

][11] This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be improved through a process called anodizing. Number alloys such as aluminum bronzes take advantage of this property by incorporating some aluminum into the alloy to improve corrosion resistance. The alumina produced by anodizing is usually amorphous, but discharge oxidation processes such as plasma electrolytic oxidation produce a significant proportion of crystalline alumina in the coating, improving its hardness.

Aluminum oxide was removed from the US Environmental Protection Agency's list of chemicals in 1988. Aluminum oxide is included in the EPA. Toxic release inventory list if it is in fibrous form.[12]

Amphoteric character

Aluminum oxide is an amphoteric substance, meaning it can react with both an acid and a base, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralizing the other and forming a salt.

Al2O3 + 6 HF → 2 AlF3 + 3 hours 2O Al2O3 + 2 NaOH + 3 H2O → 2 NaAl (OH)4 (sodium aluminate)

How winged metal is produced

Metal production can be divided into two stages.

• The first is the extraction of bauxite, its crushing and separation of silicon using steam.
• Second stage: alumina is mixed with molten cryolite and the mixture is exposed to electric current. During the reaction, liquid aluminum settles at the bottom of the bath.

The resulting metal is cast into ingots; then it is sent to consumers or to the production of alloys and high-purity aluminum.

The method is energy-consuming and consumes a lot of electricity.

It can be technical or ultra-clean

The resulting aluminum is called technical or unalloyed. It contains at least 99% pure metal. It is consumed by the electronics industry and is needed in the production of heat exchange and heating devices and lighting equipment.

Some of this metal is sent for additional purification, “refining.” As a result, we have a high-purity metal with an aluminum content of at least 99.995%.

It is used in electronics and in the production of semiconductors. Cable production and chemical engineering now cannot do without ultra-pure aluminum.

Interesting: before the discovery of an industrial method for producing aluminum, it was rare and cost more than gold. Our great chemist, D.I. Mendeleev, the British honored him with a gift. These were analytical balances (an indispensable thing for a chemist), whose cups were made of gold and aluminum.

Metal for wings

Without a metal like aluminum, conquering the sky is impossible. People are not given wings, but man has wanted to fly since ancient times. It is not in vain that the myth of Icarus has lived since ancient times. Attempts to take off were made several times.

But the breakthrough came in 1903, when the Wright brothers, romantics of the sky and wonderful mechanics, took an airplane into the air. This plane opened the way to the sky.

Structure

Corundum from Brazil, measuring approximately 2 × 3 cm.
The most common form of crystalline alumina is known as corundum, which is a thermodynamically stable form.[13] The oxygen ions form a nearly hexagonal close-packed structure with aluminum ions filling two-thirds of the octahedral voids. Each Al3+ center is octagonal. Regarding its crystallography, corundum adopts a trigonal Bravais lattice with the cosmic group of p3c (number 167 in international tables). The primitive cell contains two formula units of aluminum oxide.

Alumina also exists in other, metastable phases, including cubic γ and η phases, monoclinic θ phase, hexagonal χ phase, orthorhombic κ phase, and δ phase, which can be tetragonal or orthorhombic.[13][14 ] Each has a unique crystal structure and properties. Cubic γ-Al2O3 has important technical applications. The so-called β-Al2O3 turned out to be NaAl11O17.[15]

The alumina melt near the melting point is approximately 2/3 tetrahedral (i.e. 2/3 Al surrounded by 4 oxygen neighbors) and 1/3 5-coordinated, with very little (octahedral Al-O present.[16] About 80% oxygen atoms are shared by three or more Al-O polyhedra, and most interpolyhedral bonds share angles, with the remaining 10–20% sharing edges.[16] The destruction of octahedra upon melting is accompanied by a relatively large increase in volume (~20%), density liquid near its melting point is 2.93 g/cm3.3.[17] The structure of molten aluminum oxide depends on temperature, and the proportion of aluminum increases 5 and 6 times upon cooling (and supercooling) due to tetrahedral AlO.4 units approaching to local structural patterns found in amorphous alumina.[18]

Characteristics of aluminum

• Aluminum density - 2.7*103 kg/m3;
• Specific gravity of aluminum - 2.7 g/cm3;
• The specific heat capacity of aluminum at 20oC is 0.21 cal/deg;
• The melting point of aluminum is 658.7oC;
• The specific heat capacity of aluminum melting is 76.8 cal/deg;
• The boiling point of aluminum is 2000oC;
• Relative change in volume during melting (deltaV/V) - 6.6%;
• Coefficient of linear expansion of aluminum (at a temperature of about 20oC): - 22.9 * 106 (1/deg);
• The thermal conductivity coefficient of aluminum is 180 kcal/m*hour*deg;

Aluminum elastic modulus and Poisson's ratio

Reflection of light by aluminum

The numbers given in the table show what percentage of light incident perpendicular to the surface is reflected from it.

Production

Aluminum hydroxide minerals are the main component of bauxite, the main ore of aluminum. The mineral mixture includes bauxite ores, including gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)) along with admixtures of iron oxides and hydroxides, quartz and clay minerals. [19] Bauxites occur in laterites. Bauxite is purified by the Bayer Process:

Al2O3 + H2O + NaOH → NaAl (OH)4 Al (OH)3 + NaOH → NaAl (OH)4

With the exception of SiO2, the remaining components of bauxite are not dissolved in the base. After filtering the main mixture, Fe2O3 was removed. When Bayer liquor cools, Al(OH)3 precipitates, leaving silicates in solution.

NaAl (OH)4 → NaOH + Al (OH)3

Solid Al(OH)3Gibbsite is then calcined (heated to over 1100 °C) to form aluminum oxide:[7]

2 Al (OH)3 → Al2O3 + 3 hours 2O

The resulting alumina tends to be multiphase, that is, composed of several alumina phases rather than just corundum.[14] In this way, the production process can be optimized to produce a customized product. The type of phases present affects, for example, the solubility and pore structure of the alumina product, which in turn affects the cost of aluminum production and pollution control.[14]

Minerals, deposits...and native aluminum?

Aluminum reserves in nature are enormous. Among metals, it holds first place in prevalence. But the “sociability” and activity of the element have led to the fact that the metal is practically absent in its pure form.

Aluminum production in millions of tons.

There are many minerals containing aluminum:

• bauxite;
• aluminas;
• feldspars;
• nephelines;
• corundum.

So extracting aluminum raw materials is not difficult.

If all reserves on Earth are depleted (which is doubtful), then aluminum can be extracted from seawater. There its content is 0.01 mg/l.

Anyone who wants to see native aluminum will have to descend into the craters of volcanoes.

The origin of such a metal is from the very depths of our planet.

Applications

Known as alpha alumina in the materials science community or alundum (in fused form) or aloxite[20] in the mining and ceramics community, alumina has many uses. Annual global production of aluminum oxide in 2015 was approximately 115 million tons, more than 90% of which is used in the production of aluminum metal.[7] The main applications of specialty aluminum oxides are refractories, ceramics, polishing and abrasives. Large volumes of aluminum hydroxide, from which alumina is obtained, are used in the production of zeolites, titanium coating pigments, and as a fire retardant/smoke suppressant.

More than 90% of the alumina produced, commonly called smelter alumina (SGA), is consumed in the production of aluminum, typically the Hall-Héroult Process. The remainder, commonly called specialty alumina, is used in a wide variety of applications, reflecting its inertness, heat resistance, and electrical resistance.[21]

Fillers

Aluminum oxide, being fairly chemically inert and white, is the preferred filler for plastics. Aluminum oxide is a common ingredient in sunscreen and is sometimes also present in cosmetics such as blush, lipstick and nail polish.

Glass

Many glass compositions have aluminum oxide as an ingredient.[22] Aluminosilicate glass is a commonly used type of glass that often contains 5% to 10% alumina.

Catalysis

Aluminum oxide catalyzes many reactions that are used industrially. In its largest scale application, alumina is a catalyst in the Claus Process for converting hydrogen sulfide waste gases into elemental sulfur in oil refineries. It is also useful in dehydrating alcohols into alkenes.

Aluminum oxide serves as a catalyst support for many industrial catalysts, such as those used in hydrodesulfurization and some Ziegler–Natta polymerization.

Gas purification

Aluminum oxide is widely used to remove water from gas streams.[23]

Abrasive

Aluminum oxide is used for its hardness and strength. Its naturally occurring form, Corundum, is a 9 on the Mohs Mineral Hardness Scale (just below a diamond). It is widely used as an abrasive, including as a much less expensive substitute for industrial diamond. Many types of sandpaper use aluminum oxide crystals. In addition, its low heat retention and low specific heat capacity make it widely used in grinding operations, especially cutting tools. As a powdered abrasive mineral, aloxite is the main component, along with silica, of the "chalk" tip used in billiards. Aluminum oxide powder is used in some CD/DVD polishing and scratch repair kits. Its polishing qualities are also the basis for its use in toothpaste. It is also used in microdermabrasion, both in a machine process available from dermatologists and estheticians, and as a manual dermal abrasive used according to the manufacturer's instructions.

Paint

Main article: Aluminum oxide effect pigment

Aluminum oxide flakes are used in paints to create reflective, decorative effects, such as in the automotive or cosmetics industries.[ citation needed

]

Composite fiber

Aluminum oxide has been used in several experimental and commercial fiber materials for high-performance applications (e.g., Fiber FP, Nextel 610, Nextel 720).[24] Alumina nanofibers in particular have become an area of ​​scientific research.

Body armor

Some armor uses aluminum oxide ceramic plates, usually combined with an aramid or UHMWPE backing, to ensure effectiveness against most weapon threats. Alumina ceramic armor is available to most civilians in jurisdictions where it is legal, but is not considered military.[25]

Abrasion protection

Aluminum oxide can be grown as a coating on aluminum by anodization or plasma electrolytic oxidation (see "Properties" above). Both the hardness and abrasion resistance characteristics of the coating are due to the high strength of the alumina, however the porous coating layer produced through conventional DC anodizing procedures is in the 60-70°C Rockwell hardness range. [26] which is comparable only to hardened carbon steel alloys, but is significantly inferior in hardness to natural and synthetic corundum. Instead, with plasma electrolytic oxidation, the coating is porous only on the surface oxide layer, while the underlying oxide layers are much more compact than with standard DC anodizing procedures and have a higher degree of crystallinity due to the oxide layers being remelted and densified to obtain α-Al2O3 clusters with much higher coverage. hardness values ​​around 2000 Vickers hardness.[ quote needed

]
Alumina production in 2005.
Alumina is used to produce tiles that are attached inside pulverized fuel lines and stacks in coal-fired power plants to protect high wear areas. They are not suitable for high impact areas as these tiles are fragile and prone to breakage.

Electrical insulation

Aluminum oxide is an electrical insulator used as a substrate (silicon on sapphire) 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 (SQIDs).

For its application as an electrical insulator in integrated circuits, where conformal thin film growth is a prerequisite and the preferred growth mode is atomic layer deposition, Al2O3 films can be prepared by chemical exchange between trimethylaluminum (Al(CH3)3) and H2O:[27 ]

2 Al (CH3)3 + 3 hours2O → Al2O3 + 6 channel 4

HAC2O in the above reaction can be replaced by ozone (O3) as the active oxidizing agent, and then the following reaction occurs:[28][29]

2 Al (CH3)3 + O3 → Al2O3 + 3 C2HAS6

Al2O3 films prepared using O3 show 10–100 times lower leakage current density compared to currents produced by H2O.

Aluminum oxide, being a dielectric with a relatively large band gap, is used as an insulating barrier in capacitors.[30]

Another

Lighting uses clear aluminum oxide in some sodium lamps.[31] Aluminum oxide is also used to prepare suspensions for coatings in compact fluorescent lamps.

In chemistry laboratories, aluminum oxide is a medium for chromatography, available in basic (pH 9.5), acidic (pH 4.5 in water) and neutral compositions.

Health and medical applications include it as a material in hip replacements[7] and birth control pills.[32]

It is used as a scintillator [33] and dosimeter for radiation protection and therapy for its optically stimulated luminescence properties.[ citation needed

]

High temperature furnace insulation is often made from aluminum oxide. Sometimes insulation contains varying percentages of silica depending on the temperature conditions of the material. Insulation can be manufactured in blanket, slab, brick and fiber form to suit different requirements.

Small pieces of aluminum oxide are often used as boiling chips in chemistry.

It is also used to make spark plug insulators.[34]

Using a plasma spray process and mixed with titanium, it is applied to the braking surface of some bicycle rims to provide resistance to abrasion and wear.[ citation needed

]

Most ceramic fishing rod eyes are circular aluminum oxide rings.[ citation needed

]

In its finest powder (white) form, called Diamantine, aluminum oxide is used as an excellent polishing abrasive in watchmaking and watchmaking.[35]

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