Ceramic Materials are inorganic and non-metallic materials that are commonly electrical and thermal insulators. They are brittle and composed of more than one element.
Contents
Crystal Structures of Ceramic Materials
Ceramic bonds are mixed -ionic and covalent – with a proportion that depends on the particular ceramics. The ionic character is given by the difference of electro-negativity between the cations (+) and anions (-). Covalent bonds involve sharing of valence electrons.
The building criteria for the crystal structure are to
– maintain neutrality
– make charge balance which dictates the chemical formula
– achieve the closest packing
The parameter that is important in determining contact is the ratio of cation to anion radii.
Silicate Ceramics
Oxygen and silicon are the most abundant elements in Earth’s crust. Their combinations occur in rocks, soils, clays and sand.
Carbon
Carbon is not really a ceramic, but it is an allotropic form of diamond which may be considered as a type of ceramic. Diamond has a host of interesting and even unusual properties, which are given below.
– Diamond-cubic structure
– Covalent C-C bonds
– Highest Hardness of any material known
– very high thermal conductivity ( unlike ceramics )
– Transparent in the visible and infrared, with high index of refraction
– Semiconductor ( can be doped to make electronic devices )- Metastable ( transforms to carbon when heated )
Imperfections in Ceramic Materials
Imperfection include point defects and impurities. Their formation is strongly affected by the condition of charge neutrality ( creation of unbalanced charges requires the expenditure of a large amount of energy).
Brittle Fracture of Ceramics
Application of ceramics are limited due to brittle nature of the fracture. It occurs due to the unavoidable presence of microscopic flaws. The flaws cannot be closely controlled in manufacturing and this leads to a large variability in the fracture strength of ceramic materials.
The compressive strength of ceramics is ten times higher than their tensile strength. They have very little plastic deformation before fracture.
Application of Ceramic Materials
Ceramics properties that are different from those of metals lead to different uses. In structures, designs must be done for compressive loads. The transparency to light of many ceramics leads to optical uses, like in windows, photographic cameras, telescopes and microscopes. Good thermal insulation leads to use in ovens, the exterior tiles of the shuttle orbitor, etc. Good electrical insulation is used to support conductors in electrical and electronic applications.
Ceramic Materials Processing
Ceramics have traditionally been based on oxide minerals, or other minerals that can be decomposed to yield oxides, like hydroxides, etc. The primary raw materials for traditional ceramics include clays, silica, and feldspars (K, Na) AlSi3O8, along with some other industrial chemicals.
Minerals Processing – Modern technical and advanced ceramics require high purity powders which are well beneficiated and have well defined characteristics.
Behaviour of Ceramic Powders during Compaction – Dry pressing of ceramic powders is still the most common shaping operation in ceramic industry. The powder preparation for industrial dry pressing includes either dry milling, followed by granulation, or wet milling followed by spray drying.
Glass Property
A special characteristic of glasses is that solidification is gradual, through a viscous stage, without a clear melting temperature. The specific volume does not have an abrupt transition at a temperature but rather shows a change in slope at the glass -transition temperature.
Heat Treating Glasses
Similar to the case of metals, annealing at elevated temperatures is used to remove stressed, like those caused by inhomogeneous temperatures during cooling.
Strengthening by glass tempering is done by heating the glass above the glass transition temperature but below the softening point and then quenched in an air jet or oil bath. The interior, which cools later than the outside, tries to contract while in plastic state after the exterior has become rigid. This causes residual compressive stresses on the surface and tensile stresses inside. To fracture, a crack has first to overcome the residual compressive stress, making tempered glass less susceptible to fracture. This improvement leads to use in automobile windshields, glass doors,etc.
Refractory Materials
In general, Products applied at temperatures greater than 600*C are referred to as refractories. In particular, the materials which withstand a temperature up to 1500* C, it is called a refractory material. If the temperature is more than 1800*C, it is designated as highly refractory.
Thermal Expansion of Refractory Material
The refractory material is exposed not only to mechanical and chemical stress, but also to thermal stress. Temperature differences of more than 1000*C are no exception in the wall of a unit. This causes massive thermal stresses in an area of just a few centimeters, which need to be withstood by the refractory material.
Thermal expansion using magnesia can be considered as an example with a range of +1.3%/1000*C
Types of Refractory Materials
1. Basic refractory
2. Acid refractory
The most important raw materials for basic refractories are given below.
– Magnesia caustic, sintered and fused magnesia
– Dolomite sintered and fused dolomite
– Chromite, that is, main part of chrome ore
– Olivine solid solution series forsterite to fayalite
– Magnesia chromite sintered or melted products of magnesia and chrome ore
Some of the acid refractory materials used are as follows.
– Clays as binding agent or calcined as chamotte
– Silica quartz; silica
– Mullite sintered or melted
– Aluminium oxide-rich raw materials bauxite; calcined, sintered and melted alumina
Electrical Properties of Ceramic Materials
Ceramics exhibit the largest possible diversity in electrical conductivity, in terms of the type and magnitude of the conductivity, the details of which are given below.
Charged carries must be present in the material
The carriers must be mobile in the applied electric field Ee.
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