What is Aluminum Oxide - the Complete Guide

Aluminum oxide is one of the most popular ceramics available in the market. In this guide, we take a look at some fundamental aspects on aluminum oxide.

From manufacturing process, properties, types to applications, you will find all information about this important compound right here.

 

What is Aluminum Oxide?

Aluminum oxide refers to a precious compound extracted from bauxite. It is made up of individual elements such as oxygen and aluminum. Aluminum oxide exists as a white powder in its physical state.

Furthermore, it is categorized as an amphoteric compound. This is because it reacts with bases and acids at the same time. Additionally, it is insoluble in both water as well as other universal solvents such as propanol and ethanol. This property cuts across the majority of metal oxides.

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

 

Formula for Aluminum Oxide and Aluminum Oxide Structure

Chemical compounds are normally represented by chemical formulas. Such chemical formulas help to show the proportion of individual atoms present in the element of a particular compound.

For instance, you can represent the chemical formulae of aluminum oxide as Al2O3.  In this circumstance, the subscript 2 and 3 represents the number of atoms exchanged by the two elements. This explains why aluminum oxide has an ionic bond.

Since aluminum is a metal, it   loses three electrons to gain stability whilst oxygen need to gain two electrons to attain stability. This explains the exchange in valence electrons hence the chemical formulae Al2O3.

Oxygen and aluminum atoms in Al2O3 forms a hexagonal structure. Similarly, the hexagonal structure appears to be crystalline, which explains why Aluminum oxide is hard.

 

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

·       Hardness

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.

·       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 3000 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.

·       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.

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.

 

How to Produce Aluminum Oxide

Industrial production of aluminum oxide entails a complex process known as Bayer process.

Bayer process is an industrial technique developed by Bayer Carl Joseph that aims at purifying aluminum ores.

Since aluminum metal readily reacts with atmospheric oxygen to form a compound (aluminum oxide), the Bayer process is ideal for purification.

This method separates aluminum oxide from other impurities such as titanium, iron as well as silica. The Bayer process begins by heating the aluminum ore (bauxite) in presence of caustic soda.

Afterwards, the heated aluminum ore is dissolved in a suitable solvent such as sodium aluminate. Thereafter, filtration follows, which removes compounds from the bauxite.

Stages of Aluminum Oxide Production

1.       Digestion Stage

This is the immediate stage after extracting aluminum oxide. In this stage, you will mix crushed aluminum oxide powder with concentrated sodium hydroxide solution.

By doing so, the components present in bauxite dissolve in sodium hydroxide making it possible to separate them. You can achieve this through filtrartion.

2.       Filtration Stage

After dissolving aluminum ore in sodium hydroxide, the next process that follows is filtration. The filtrate is discarded out since it contains impurities thus leaving behind pure Aluminum oxide.

However, you should note that this method is not the most effective. Usually, it does not remove all impurities.

3.       Precipitation

In this process, you allow the filtered aluminum oxide to cool at a temperature of about 65 degrees Celsius.

Cooling stops after the formation of small crystals of aluminum oxide. Additionally, you can speed up this process using a catalyst aluminum hydroxide.

4.       Calcination

Calcination process entails heating the aluminum oxide until all water of crystallization is removed. This ensures that hydrated aluminum oxide turns to anhydrous.

Other ways of producing aluminum oxide are:

·       Sintering Process

Apart from the Bayer process, sintering process is also an ideal method in the purification of aluminum oxide. This process significantly differs from the Bayer process in terms temperatures involved.

For instance, the aluminum ore is heated under high temperatures ranging from 1250-1550 degrees Celsius. Additionally, you can add additives to aluminum ore which helps to dissolve aluminum impurities.

Leaching Process

This refers to the use of acids in dissolving metals. In this case, acid leaching can also be used in aluminum oxide purification. The most suitable acid used in this process is normally concentrated sodium hydroxide.

Hydrothermal Processing

Hydrothermal processing similarly requires high temperatures and water. At high temperatures, aluminum oxide is reacted with water in a vacuum until it forms a vicious liquid.

Next, it involves heating the slurry under very high pressure and temperatures. This method is however preferable since it guarantees production of alumina with high quality.

 

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.

·       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.

·       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.

·       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.

·       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.

 

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.

·       Biomaterials

Biomaterials helps in replacement of damaged tissues in bodies of human beings. Some of these tissues may include organs include limbs, hands, bones and joints.

Similarly, you can apply the knowledge in biomaterials in manufacturing of light emitting instruments for treatment in hospitals.

For instance, you can use this knowledge in curing cancerous cells which require light to cure.

·       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.

·       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.

·       Medical Industry

The field of medicine has also benefited from the production of medical equipment made of alumina. Among them include human engineered organs made from aluminum that help save lives of patients.

Aluminum oxide metal does not interfere with the general working and physiological processes of the body hence making it suitable.

Extended reading: More uses of alumina

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.

 

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.