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NON-OXIDE REFRACTORIES - Coggle Diagram
NON-OXIDE REFRACTORIES
Overview:
- Refractories are ceramic materials capable of withstanding high temperatures. The bulk of refractory materials consist of single or mixed high melting point oxides such as silicon, aluminum, magnesium, calcium and zirconium.
- Non-oxide refractories also exist and include materials such as carbides, nitrides, borides and graphite. The actual composition of a refractory material is dependent on operating factors such as temperature, atmosphere and the materials it will be in contact with.
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Refractoriness
The refractoriness of a material is a measure of its ability to withstand exposure to elevated temperatures without undergoing appreciable deformation. It is generally measured using “Seger Cones”, which are conical-shaped ceramic objects of differing compositions.
When heated to various temperatures, these cones will slump as they soften in response to the temperature, with the degree of slumping being dependent on the composition. A similar cone-shaped object is made from the material to be measured and heated along with standard Seger Cones.
After the conclusion of the heating cycle, the sample material is compared to the Seger Cones to gain a comparative measure of its refractoriness. It is important to note that if a material is subjected to a mechanical load during the test, it may well soften at well below the temperature indicated by the Seger Cone test.
Operating Conditions
The atmosphere in which a refractory is to operate can dictate what materials can be used in that application. For example, graphite refractories can operate at temperatures of up to several thousand degrees Celsius under reducing conditions or oxygen-free conditions such as a vacuum. However, they may begin to sublime at approximately 1000 °C under oxidizing conditions.
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The type of materials that a refractory comes into contact with can also dictate the materials suitable to use. For instance, in steel making, basic refractories are used because the refractories often encounter slags containing magnesium and calcium oxides. If the refractory lining was made from acidic refractories, it would be eroded quickly by the chemical interaction of the slag and the acidic lining (e.g. silica) forming low melting point compounds.
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Porosity
Refractories also come in a range of different densities and porosities. Generally, low porosity refractories display higher thermal conductivity compared to high porosity materials. The latter are usually strong insulators due to the high volume of air they envelop since air is a very poor thermal conductor.
However, high porosity materials do not cope as well with higher temperatures and direct flame impingement and tend to shrink under these conditions. Thus, the low porosity materials are used in the hotter zones, while more porous materials are used as thermal backup materials.
Key Properties
- The ability to withstand high temperatures and confine heat within a limited area such as a furnace.
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- To maintain sufficient dimensional stability at elevated temperatures and after/during repeated thermal cycling.
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- To maintain sufficient mechanical properties (e.g. compressive strength) at elevated temperatures.
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Applications
Refractory Carbides
- Hard, wear-resistance, chemically inert, chemically resistant and nearly impervious to hydrogen at high temperatures, refractory carbides can be made from chemical vapour deposition at temperature as low as 10% of their melting point,
- Example: Zirconium Carbide (ZrC), Hafnium Carbide (HfC) and Titanium Carbide (TiC).
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Refractory Nitrides
- The refractory nitrides can be divided into two major types: the interstitial nitrides and the covalent nitrides. The refractory nitrides are in many respects similar to the refractory carbides. They are hard and wear-resistant with high melting points and good chemical resistance.
- Example: Iron Nitrides (Fe2N).
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