Alumina and Alumina Fibres - Properties and Applications - AZoM

Background

Minerals containing Alumina represent some 15% of the earth’s crust. It is therefore an abundant material and virtually inexhaustable, unlike raw materials for many alloys developed for special applications.

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Properties

The combination of high thermal conductivity, low thermal expansion and high compressive strength leads to good thermal shock resistance, so Alumina is suited to furnace use as crucibles, tubes and thermocouple sheaths.

High purity Alumina can be used up to °C and is gas tight up to °C. Few chemicals attack Alumina. It also shows good electrical insulation at high temperatures, good wear resistance and high hardness, making it suitable for components such as ball valves, piston pumps and deep drawing tools. Diamond tools are needed to machine or grind alumina.

Alumina Fibres

Remarkably, continuous filament yarns have been made from alumina with reasonable but not complete success and two, similar, versions are available from Goodfellow. They are both much less flexible than normal continuous filament yarns and are rather “hairy” i.e. have broken filaments - especially the FP version.

Applications

As mentioned above, alumina is used for applications including ball valves, piston pumps and deep drawing tools.

Typical Properties

The typical properties of alumina are tabulated below.

Table 1. Key properties of alumina

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Chemical Resistance Acids - concentrated Good Acids - dilute Good Alkalis Good Halogens Good Metals Good Electrical Properties Dielectric constant 9.0-10.1 Dielectric strength ( kV.mm-1 ) 10-35 Volume resistivity ( Ohm.cm ) > @ 25 °C Mechanical Properties Compressive strength ( MPa ) - Hardness - Knoop ( kgf.mm-2 ) Hardness - Vickers ( kgf.mm-2 ) - Shear strength ( MPa ) 330 Tensile modulus ( GPa ) 300-400 Tensile strength ( MPa ) 260-300 Physical Properties Apparent porosity ( % ) 0 Density ( g.cm-3 ) 3.9 Water absorption - saturation ( % ) 0 Thermal Properties Coefficient of thermal expansion ( x10-6 K-1 ) 8.0 @ 20- °C Melting point ( °C ) Specific heat ( J.K-1.kg-1 ) 850-900 @ 25 °C Thermal conductivity ( W.m-1.K-1 ) 26-35 @ 20 °C Upper continuous use temperature ( °C ) Properties Alumina Fibre Property Value Coefficient of thermal expansion - Longitudinal x10-6 K-1 5.7 Density g.cm-3 3.9 Modulus GPa 380 Tenacity GPa 1.4

About Goodfellow

Goodfellow supplies metals, ceramics and other materials to meet the research, development and specialist production requirements of science and industry worldwide.

The Goodfellow group consists of four companies. The main administration, research laboratories and workshops are located at the Company's headquarters in Cambridge, England. The subsidiary offices in America, France and Germany provide an additional service.

This information has been sourced, reviewed and adapted from materials provided by Goodfellow.

Introduction

In this lecture we are going see some more advanced fibres. Further, we will see their key features, applications and fabrication processes.

Alumina Fibre

• These are ceramics fabricated by spinning a slurry mix of alumina particles and additives to form a yarn which is then subjected to controlled heating.
• Fibers retain strength at high temperature.
• It also shows good electrical insulation at high temperatures.
• It has good wear resistance and high hardness.
• The upper continuous use temperature is about  .
• Fibers of glass, carbon and alumina are supplied in the form of tows (also called rovings or strands) consisting of many individual continuous fiber filaments.
• Du Pont has developed a commercial grade alumina fibre, known as Alumina FP (polycrystalline alumina) fibre. Alumina FP fibres are compatible with both metal and resin matrices. These fibres have a very high melting point of . They can withstand temperatures up to without any loss of strength and stiffness properties at this elevated temperature. They exhibit high compressive strengths, when they are set in a matrix.
• The Alumina whiskers are available and they exhibit excellent properties. Alumina whiskers can have the tensile strength of MPa and the tensile modulus of 427 GPa.

What are the applications of Alumina fibres?

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• The Alumina has a unique combination of low thermal expansion, high thermal conductivity and high compressive strength. The combination of these properties gives good thermal shock resistance. These properties make Alumina suitable for applications in furnace use as crucibles, tubes and thermocouple sheaths.
• The good wear resistance and high hardness properties are harnessed in making the components such as ball valves, piston pumps and deep drawing tools.