What is Aluminum Oxide - the Complete Guide

Alumina is one of the most popular ceramics on the market. According to market research, the global alumina market size has reached 44.73 billion yuan in 2024, and is expected to grow to 46.83 billion US dollars in 2025, and is expected to exceed 70.8 billion US dollars in 2034. This shows the importance of alumina as a material.

In this guide, you will learn all about alumina. From manufacturing processes, properties, types to applications, you can find a variety of information about it here.

 

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Quick Links

1. What is Aluminum Oxide?
2. Chemical Structure of Aluminum Oxide
3. Alumina Production Process
4. Properties of Aluminum Oxide
5. Types of Aluminum Oxide
6. Uses of Aluminum Oxide

 

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What is Aluminum Oxide?

Alumina is a precious compound extracted from bauxite. It is composed of two elements, oxygen and aluminum. Natural alumina exists in the form of a crystalline solid. The physical state of processed alumina is a white powder. It is insoluble in water and other common solvents such as propanol and ethanol (this property applies to most metal oxides). In addition, alumina is classified as an amphoteric compound because it can react with both bases and acids.

Reaction with strong acid: Al2O3 + 6HCl → 2AlCl3 + 3H2O

Reaction with strong base: Al2O3 + 2NaOH → 2NaAlO2 + H2O

Other names for aluminum oxide include：Alumina, aluminum (III) Oxide, aloxite as well as aloxide

Industrial terms: Aloxite, Alundum

Mineralogical name: Corundum

 

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Chemical Structure of Aluminum Oxide

Chemical Structure and Molecular Formula

The chemical formula of aluminum oxide is Al2O3. As an important inorganic substance, the molecular structure of aluminum oxide has unique characteristics. The molecular weight of alumina is 101.96 g/mol, and the stoichiometric ratio is Al:O = 2:3.

Its electronic configuration is expressed as:
Al³⁺: [Ne]
O²⁻: [Ne]2s²2p⁶

Compounds are usually represented by chemical formulas, which help show the proportions of individual atoms present in a particular compound’s elements.

Take aluminum oxide as an example. We express the chemical formula of aluminum oxide as Al2O3. In this case, the subscripts 2 and 3 indicate the number of atoms of the two elements that are exchanged, which explains why aluminum oxide has ionic bonds.

Alumina Crystal Structure

Alumina has many crystal forms, the most common of which is α-Al2O3 (corundum), which is arranged in hexagonal close-packed oxygen ions. The hexagonal structure looks like a crystal, which is why alumina is very hard.

The α-Al2O3 structure has the following characteristics:

• Hexagonal close-packed oxygen ion array;
• Aluminum ions occupy 2/3 of the octahedral voids;
• Unit cell parameters: a = 4.758 Å, c = 12.991 Å;
• Space group: R3c

In addition to α-Al2O3, alumina also has other important crystal phases, as follows:

• γ-Al2O3: cubic spinel structure
• θ-Al2O3: monoclinic system
• δ-Al2O3: tetragonal or orthorhombic structure
• κ-Al2O3: orthorhombic structure

Each crystal phase has its own unique properties!

 

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Alumina Production Process

Are you curious about how to make alumina? This section will explain the manufacturing process of alumina in detail. The Bayer process is one of the most important methods for industrial production of alumina. It was invented by Karl Josef Bayer in 1887.

Since aluminum metal easily reacts with oxygen in the air to form a compound - alumina, the Bayer process is an ideal purification method. This method uses bauxite as raw material. The main purpose is to separate alumina from impurities such as iron, titanium and silicon dioxide. After a series of chemical treatments, high-purity alumina can be obtained.

Digestion stage

During the digestion stage, you mix the alumina with a concentrated sodium hydroxide solution. Once the components of the bauxite have dissolved into the sodium hydroxide, they can be separated by filtering.

Raw material processing: crush the bauxite to less than 200 mesh, mix with sodium hydroxide solution, and process at 160-180℃ and 3.5-4.5MPa pressure.

Chemical reaction:  Al2O3·xH2O + 2NaOH → 2NaAlO2 + (x+1)H2O

Key parameter control: temperature accuracy controlled at ±2℃; 300-350g/L Na2O solution concentration; 1-2 reaction time

Filtration stage:

The filtration stage is to remove impurities from the filtrate, leaving pure alumina, but please note that this method is not the most efficient and it does not remove all impurities.

Solid-liquid separation: Use high-pressure filters to separate red mud and sodium aluminate solution

Process control: Filtration temperature is controlled at 95-105℃, pressure is controlled at 0.4-0.6MPa, and filtration speed is controlled at 8-12m3/(m2·h)

Red mud treatment: Wash and recover alkali solution, and make comprehensive use of red mud.

Precipitation stage:

This process requires cooling the aluminum oxide to precipitate. When small aluminum oxide crystals are formed, the cooling is stopped. You can also speed up this process by using aluminum hydroxide as a catalyst.

Seed addition: Add 60-80μm seed at a ratio of 30-50g/L

Precipitation conditions: Precipitate at 45-55℃ for 24-36 hours, pH value controlled between 13.5-14.0

Chemical reaction: 2NaAlO2 + 4H2O → 2Al(OH)3↓ + 2NaOH

Calcination stage:

This process requires heating the alumina until all the water of crystallization is removed. This step ensures that the hydrated alumina becomes anhydrous alumina.

Calcination in high temperature air at 1000-1200℃ for 1-2 hours

Chemical reaction: 2Al(OH)3 → Al2O3 + 3H2O

 

The above are the simple process steps for preparing alumina by the Bayer process. In addition to the Bayer process, there are many other processes, such as sintering process, leaching process, hydrothermal process, etc.

 

 

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Properties of Aluminum Oxide

Aluminum oxide exhibits various chemical and physical properties. Below are some of common notable properties of aluminum oxide.

Attached is the performance parameter table of alumina for your simple reference.

Alumina material properties
Name			Alumina (Al2O3)		Mechanical properties	Vickers hardness load 500g		（GPa）	13.7
Color			White			Flexural strength		MPa	350
Key Features			High temperature resistance			Compressive strength		MPa	–
			High insulation			Young’s modulus of elasticity		GPa	320
			Corrosion resistance			Poisson’s ratio		–	0.23
			High mechanical strength			Fracture toughness		MPa·√m	–
Bulk density			(Kg/m³)	3.7×103	Electrical characteristics	Dielectric strength		V/m	15 × 10⁶
Water absorption			%	0		Volume resistivity	20℃	Ω·cm	>10¹⁴
Thermal properties	Linear expansion coefficient	40-400℃	×10⁻⁶/°C	7.2			300℃		10¹⁰
		40-800℃		7.9			500℃		10⁸
	Thermal conductivity		W/m·K	24		Dielectric Constant (1MHz)		–	9.4
	Specific Heat		J/Kg·K	0.78 × 10³		Dielectric loss tangent (1MHz, ×10⁻⁴)		(×10⁻⁴)	4
	Thermal shock resistance (in water)		℃	200		Loss coefficient		(×10⁻⁴)	38

Mechanical Properties

Aluminum oxide has excellent hardness as opposed to other metal compounds. Therefore, this property makes it a suitable compound for numerous industrial applications.

You can use aluminum oxide in;

1. Manufacturing industrial abrasive
2. Industrial manufacture of cutting and grinding tools.

Additionally, aluminum oxide is also a suitable element in metalwork industries, for making finishing as well as shaping materials.

Hardness and strength

Vickers hardness:

• 99.5% purity: 15-17 GPa
• 99.9% purity: 18-20 GPa

Flexural strength:

• Room temperature: 300-400 MPa
• 1000°C: 150-200 MPa

Fracture toughness:

• 3.5-4.5 MPa·m½
• The higher the purity of alumina, the better the toughness

Boiling Point

The boiling point of Aluminum oxide differs from that of other metal oxides significantly. Under normal circumstances, the boiling point of aluminum oxide is 2977 degrees Celsius. As a result of this, you can use aluminum oxide in applications that require high temperatures.

These include, manufacturing boiler parts, used as an abrasive in sandpaper manufacturing, and furnace production.

Boiling point: 2977°C ±10°C

Phase transition temperature:

• γ→α phase transition: ~1000°C
• θ→α phase transition: ~1150°C

Melting Point

Al2O3 has a melting point of about 2072 degrees Celsius. The high melting point is attributed to strong ionic bonds that exists within the molecules.

You can use aluminum oxide in various industrial applications that require high melting points. Such applications include, glass making, kilns, construction of furnaces, among others.

• Melting point: 2072°C ±5°C

Further reading: Melting points of more ceramic materials

Density

Aluminum oxide exhibit a higher density compared to other metal oxides. Usually, the density ranges from 3.90 to 4.2 g/cm3. The variation of density in Aluminum oxide depends on the type of impurities as well as the structure of the metal.

Crystal Structure

The crystal structure of atoms in aluminum oxide exhibits a hexagonal structure. Furthermore, the proportion of oxygen anions and aluminum cations in the compound is in ratio 3:2. This means that in every three atoms of oxygen, there are two atoms of aluminum.

Chemical Stability

Chemical stability of aluminum oxide refers to the degree of its reaction with other elements. In this case, aluminum oxide is a highly reactive metal oxide. This is because aluminum metal readily reacts with atmospheric oxygen to form a stable compound (Aluminum oxide).

Insulation

The insulating property of aluminum oxide makes it a perfect insulator in many industrial applications.  For instance, most electrical insulations depends on aluminum oxide e.g. capacitors, as well as other integrated circuits.

On the contrary, despite having an excellent electrical insulation, aluminum oxide is a poor conductor of heat.

Volume resistivity:

• Room temperature: >10¹⁴ Ω·cm
• 1000°C: ~10⁸ Ω·cm

Breakdown strength:

• 10-15 kV/mm (room temperature)
• Will decrease with increasing temperature

 

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Types of Aluminum Oxide

There are various types of aluminum oxides. Each individual type has its unique properties. Below are some of the notable types of aluminum oxides:

Corundum

This is a special type of aluminum oxide that appears as a crystal. As such, corundum is an impure form of alumina. It contains other components such as chromium, and iron among others.

Such impurities determine the type of colors of the aluminum oxide. For instance, red color in alumina validates the presence of chromium.

Additionally, aluminum oxide rich in chromium is categorized in ruby variety. On the other hand, alumina may possess a variety of colors as opposed to the ruby variety. This types are classified as sapphires.

Hardness and toughness are dominant properties possessed by corundums. These properties makes corundum suitable in most abrasive applications such as manufacture of sandpapers.

Basic characteristics:

• Main component: α-Al2O3
• Crystal system: trigonal system
• Color: colorless (pure) and various colors (containing impurities such as iron, chromium, etc.)

Performance characteristics:

• Mohs hardness: 9
• Density: 3.95-4.1 g/cm³
• Very high chemical stability
• Excellent wear resistance

Main applications:

• High-end abrasives, sandpaper
• Optical materials
• Gemstone decoration (ruby, sapphire)
• Precision ceramic applications

 

Boehmite

Boehmite is commonly referred to as aluminum hydroxide. It appears as a mixture of different colors ranging from brown, yellow, white and red.

The color differentiation is attributed to the composition of impurities in the metal ore.

Boehmite is relatively less hard and strong as compared to corundum.

Due to this reason, boehmite is not suitable for industrial manufacturing of abrasive materials.

Basic characteristics:

• Main component: γ-AlO(OH)
• Crystal structure: orthorhombic system
• Appearance: white or light brown

Performance characteristics:

• Good thermal stability
• High specific surface area
• Controllable pore structure
• Excellent dispersibility

Main applications:

• Catalyst carrier
• Adsorbent
• Coating

Flame retardant material

 

Diaspore

Diaspore aluminum oxide is sometimes referred to as diasporite. Diasporite appears as white crystals that are characteristically same sized.

As opposed to boehmite, diasporites are relatively harder with stronger tensile strength. However, despite their high tensile strength, diasporites have poor tenacity. This explains why they are normally very brittle in nature. The same way, they are insoluble in water as well as in other universal solvents.

Basic characteristics:

• Main component: α-AlO(OH)
• Crystal structure: orthorhombic system
• Color: white, gray or light brown

Performance characteristics:

• Good fire resistance
• High mechanical strength
• Stable chemical properties
• Low thermal expansion coefficient

Main applications:

• Various refractory materials
• High temperature ceramic applications
• Special cement
• Grinding materials

 

Gamma- Alumina

It refers to a special type of aluminum oxide commonly used in the petroleum industry. Naturally, it occurs as white crystals thus resembling pure alumina.

A notable property of gamma-alumina is its ability to readily dissolve in both bases and acids. Furthermore, this type of alumina also readily dissolves in water to form a solution.

Basic characteristics:

• Chemical formula: γ-Al2O3
• Crystal structure: cubic spinel type
• Specific surface area: 150-300 m²/g

Performance characteristics:

• High specific surface area
• Strong catalytic activity
• Good thermal stability
• Porous structure

Main applications:

• Catalyst carrier
• Adsorbent
• Desiccant
• Surface treatment

 

Alpha- Alumina

It varies with gamma- alumina in terms of porosity, heat conductivity, as well as density. Generally, alpha- alumina are denser, have good thermal conductivity and are solid as opposed to gamma-alumina.

Basic characteristics:

• Chemical formula: α-Al2O3
• Crystal structure: Hexagonal close-packed
• Purity: Usually >99.5%

Performance characteristics:

• The most stable alumina phase
• Excellent mechanical properties
• High temperature stability
• Chemically inert

Main applications:

• High-performance ceramics
• Electronic substrates
• Bioceramics
• Optical devices

 

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Uses of Aluminum Oxide

Aluminum oxide has numerous industrial applications. Here are some of the most common applications:

Ceramics

Aluminum oxide is a significant raw material commonly used in the industrial manufacturing of ceramics. For instance ceramic coating for cars is made from aluminum oxide.

Technical ceramics
In the field of technical ceramics, the main application of alumina is concentrated in the manufacture of high-performance components. Such ceramics usually require 99.5% or even higher purity alumina ceramics to achieve optimal performance.
For example, in semiconductor manufacturing equipment, alumina ceramics can be used to make wafer carriers and other components, thanks to the excellent high temperature and corrosion resistance of alumina.
In precision instrument manufacturing, alumina is also used to make measuring probes and sensor components, because of its dimensional stability and wear resistance.

Structural ceramics
The primary uses of structural ceramics are load-bearing and protection. In mechanical engineering, aluminum oxide bearings and seals show you its unique wear resistance, and its superior service life also makes it effective in high-speed rotating equipment.
In the field of protection, alumina is used in military equipment. Alumina armor has extremely high hardness and good toughness, and is an important part of military protective equipment.

 

Biomedical Applications

Alumina can be used as a biomaterial to help replace damaged tissue in the human body. This includes organs such as limbs, hands, bones and joints. Similarly, you can apply your knowledge of biomaterials to light-emitting devices for hospital treatments, such as using this knowledge to treat cancer cells that require light to heal.

Artificial Organs
The application of alumina in the biomedical field is mainly reflected in artificial joints and dental implants. Alumina has good biocompatibility and can be an ideal medical implant material.
In some hip replacement surgeries, alumina ceramic ball heads can provide a lower friction coefficient and good wear resistance. In recent years, alumina-based composite materials have also achieved good breakthroughs in the field of dental restoration.

Medical Devices
In addition to human implants, alumina is also widely used to manufacture surgical instruments and diagnostic equipment parts. These devices usually require excellent cleaning and sterilization properties, and the chemical stability and non-toxicity of alumina just meet these needs. In addition, in medical imaging equipment, alumina ceramic windows are also widely used because of their good X-ray transmittance.

 

Refractory Materials

The manufacturing of refractory materials heavily depends on aluminum oxide as a raw material. This is attributed to the suitable physical and chemical properties of alumina such as,

• Excellent tensile and mechanical strength,
• Good heat (thermal) conductivity.
• High boiling and melting point.
• Corrosion and chemical resistance.
• Uses of Alumina in Refractory Applications

Alumina has been widely used in various refractory application. You can use refractory in cement industry, glass making and steel manufacturing.

Cement Industry
In the production of cement, alumina refractory materials are mainly used for the lining of rotary kilns. Alumina can withstand high temperatures above 1450°C and strong chemical corrosion, and is very handy in such harsh environments. If you use high-purity alumina bricks, you can greatly extend the service life of the kiln and reduce your downtime for maintenance.

Glass Manufacturing
Using alumina refractory materials in glass melting furnaces can bring you excellent corrosion resistance and good thermal stability. High-purity alumina can still maintain its complete structure at 1600℃, which effectively prevents the penetration and erosion of glass liquid. If alumina refractory materials are used in large quantities, it can effectively increase the service life of your furnace.

Iron and Steel Metallurgy
In the iron and steel smelting industry, alumina refractory materials can be used in pouring systems and high-temperature working areas. These areas usually need to withstand the erosion and thermal shock of molten metal, and high-purity alumina materials can withstand thermal shock well, ensuring the safe operation of smelting equipment.

 

Electronics

Aluminum oxide is an important raw material in the manufacturing of various electronic appliances. Normally, alumina offers good insulating property which is significant in avoiding electrical shock.

For this reason, it is commonly applied in circuit breakers. Other applications where aluminum oxide is used in electronics include resistors, and capacitors.

Circuit Components
In the electronics industry, alumina can be made into substrates. Its excellent heat dissipation and insulation properties can become an important material for integrated circuit packaging. Especially in some high-power LED and radio frequency devices, the superior properties of alumina substrates can be used for Their performance is significantly improved.

Insulating Materials
Alumina has insulating properties and can be used as an insulating material. It is widely used as insulators in high-voltage electrical equipment.

 

Abrasives

In the field of abrasives, aluminum oxide is the preferred material for various high-end grinding. Its excellent hardness and toughness make it an excellent grinding material. In some metal and wood surface treatment processes, aluminum oxide abrasives are almost your perfect choice.

Extended reading: Aluminum Oxide Abrasive vs Silicon Carbide Abrasive

Extended reading: More uses of alumina

 

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Conclusion

In summary, aluminum oxide is one of the most valuable metal oxide that naturally occur on the earth’s surface.

It contains various chemical and physical properties that makes them suitable for numerous industrial applications.

 

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FAQs

Below are frequently asked questions about aluminum oxide as a metal compound.

1.      Is Aluminum Oxide Toxic to Humans?

From a medical point of view, aluminum oxide is less toxic to the human body and can be considered non-toxic because ingestion of aluminum oxide does not cause serious health problems.

Daily contact with aluminum oxide is safe and secure, however, you cannot ingest aluminum oxide, which may cause minor health problems such as headaches, nausea, coughing, and vomiting. Therefore, aluminum oxide should not be ingested by the human body.

Further reading: Is aluminum oxide toxic?

2.      Why is Aluminum Oxide Expensive?

As compared to other metal oxides, aluminum oxide ranks among one of the most expensive metal compounds.

Producing aluminum oxide requires a lot of energy which significantly impacts on the cost.

 

This explains why aluminum is valued highly as opposed to other metals. Generally, the high costs compensates for the production process.

3.      Is Aluminum Oxide Safe?

The quest for the safety of aluminum oxide remains to be a mystery to many individuals. However, it has been proven that aluminum oxide is safe for human beings.

In some rare situations, there instances side effects for those using this aluminum compound.

For instance, lung failure as a result of inhalation of the aluminum powder, eye as well as skin irritation.