Aluminum oxide, more commonly referred to as alumina, boasts strong ionic interatomic bonds which give rise to its desirable material properties. Alumina exists in multiple crystalline phases that all irreversibly transition to alpha phase (close-packed hexagonal) at elevated temperatures.
Alumina is an insulator and resistant to acid attacks. Additionally, it forms an aluminium oxide passivation layer on any exposed surface for weathering protection.
Resistance
Alumina ceramic possess superior mechanical strength and thermal conductivity at both room temperatures and elevated temperatures, as well as being highly resistant to chemical corrosion. Alumina is one of the hardest substances — second only to diamond on Mohs scale — with superior abrasion resistance.
Pure alumina (Al2O3) is an insoluble solid at room temperature but becomes liquid at higher temperatures, melting with strong acids (such as hot sulfuric acid) or alkalis but is insoluble with water. Pure alumina’s highly chemical stability also makes it an excellent material choice for industrial engineering components.
Alumina can also be found as the base material in heat sinks, wires and cables for transformers (coils/windings), inductors and aluminium foil production. Furthermore, alumina serves as an excellent material for creating thermal insulation boards, panels and tiles to be used in thermal management solutions.
Alumina electrical conductivity can be affected by two primary factors, its volume fraction of conductive components and grain size. As volume fraction increases, electrical conductivity also improves. An increase in grain size also results in enhanced electrical conductivity.
Utilizing alumina as the hearth lining material of blast furnaces increases resistance against erosion by front slag and hot metal, as well as service life, while adding SiC to alumina-carbon composite bricks decreases thermal conductivity of the refractory and limits its ability to transfer heat rapidly.
Conductivity
Aluminum conducts electricity because of how its atoms are organized. Aluminum atoms contain 13 protons in their core and are surrounded by electrons which can move freely around their aluminum atoms — this movement of free electrons makes aluminum such an effective conductor of electricity.
B-alumina is an ideal material for solid electrolytes due to its superior ionic conductivity and low electronic transference number; this allows ions to migrate across its crystal structure easily — an essential factor when designing energy storage devices that convert ions.
Alumina naturally boasts a thin coating of aluminum oxide that protects it from reacting with oxygen in the atmosphere and causing metal corrosion. This layer can be further strengthened through anodizing, giving aluminum the extra protection it needs against corrosion while lasting longer — however this extra layer decreases conductivity significantly.
EC-grades of alumina with the highest electrical conductivity ratings are considered the highest quality grades, as these contain the most aluminum while being free from impurities like chromium, titanium, zirconia, tin and lead. Alumina’s conductivity may differ depending on its heat treatment process or paint/powder coating layer — and even factors like its size/shape could impact this quality rating.
Insulation
Aluminum oxide (Al2O3) is a hard-wearing technical ceramic with impressive mechanical and electrical properties, boasting chemical stability, high temperature resistance, bioinertness and bio-inertness, making it suitable for many different applications. Its thermal conductivity compares favourably with graphite but offers superior electrical insulation — making Al2O3 an excellent material choice as protective layer in thermocouples used in high temperature measurements.
Fine grain alumina comes in various purity grades, from 94% for easily metallizable materials up to 99.8% purity levels suitable for high performance applications. Strength, refractoriness and dielectric properties increase with increasing purity levels as does corrosion resistance at elevated temperatures as well as good corrosion resistance against strong acids.
Al2O3’s atomic structure makes it an insulator, with electrons tightly bound to its atoms rather than being free to move freely between them, leading to low electrical conductivity. However, coating it with other materials may increase conductivity.
Alumina is a strong material capable of withstanding high temperatures and impact damage, making it suitable for lining coal-fired power stations’ flue gas lines as well as protecting high wear areas in pulverized fuel lines. Alumina-zirconia composites also make a good choice as they offer both mechanical strength and electrical conductivity properties that could make life easier in these applications.
Surface
Alumina features strong ionic bonding between its constituent atoms, producing desirable properties such as hardness, chemical stability and corrosion resistance. Furthermore, it boasts high thermal conductivity for an oxide ceramic and high thermal conductivity at elevated temperatures — ideal for electrical applications due to its ability to withstand high temperatures without degrading over time. At lower temperatures its ionic bonding acts as an electronic insulator; at higher temperatures however it changes into an electronic conductor.
Ionic conductivity of alumina increases with temperature but decreases as particle size decreases due to an increase in distance between ions as particles shrink in size. Furthermore, its surface energy depends on particle size and surface roughness as well as being non-stoichiometric in nature.
Surface tension is an essential factor in transporting ions across alumina surfaces and is an indicator of dispersion. Vargaftik et al conducted studies measuring surface tension using pendant drop and capillary rise techniques with nanoparticles of alumina added to self-rewetting fluids using pendant drop techniques; their results revealed an increase in surface tension by adding Alumina to these fluids.
Results indicated that the concentration of alumina in self-rewetting fluids was important in controlling surface tension. Concentrations between 0.1 and 1.3 weight percent yielded results similar to distilled water (DW), with visible wavelengths being used as well as UV/IR absorbance measurements to ascertain concentration levels of alumina within self-rewetting systems.
