In this term paper we will discuss about:- 1. Introduction to Ceramic Materials 2. Classification of Ceramics 3. Properties 4. Structure 5. Advantages 6. Applications.
Term Paper Contents:
- Term Paper on the Introduction to Ceramic Materials
- Term Paper on the Classification of Ceramics
- Term Paper on the Properties of Ceramic Materials
- Term Paper on the Structure of Crystalline Ceramics
- Term Paper on the Advantages of Ceramic Materials
- Term Paper on the Applications of Ceramics
Term Paper # 1. Introduction to Ceramic Materials:
Ceramic materials are defined as those containing phases that are compounds of metallic and non-metallic elements.
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The science of ceramics, nearly as old as mankind, is the processing of earthly materials by heat. The crude cooking utensils of early man were the first application of the materials now used in jet engines and atomic reactors. All the early ceramic products were made from clay because the ware could be easily formed.
It was then dried and fired to develop the permanent structure. Because the other ceramic materials lacking plasticity also have desirable properties, other methods of forming and processing have been developed. Other forming methods used for ceramic materials are injection moulding, sintering and hot pressing. In other cases the formed materials are allowed to harden on the job by the addition of water, as in case of cements.
Term Paper # 2. Classification of Ceramics:
A. Classification of Ceramic Materials:
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Ceramic materials are classified as follows:
1. Functional Classification:
(i) Abrasives- Alumina, carborundum
(ii) Pure oxide ceramics- MgO, Al2O3, SiO2
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(iii) Fire-clay products- Bricks, tiles, procelain etc.
(iv) Inorganic glasses- Window glass, lead glass etc.
(v) Cementing materials- Portland cement, lime etc.
(vi) Rocks- Granites, sandstone etc.
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(vii) Minerals- Quartz, calcite, etc.
(viii) Refractories- Silica bricks, magnesite, etc.
2. Structural Classification:
(i) Crystalline Ceramics – Single-phase like MgO or multi-phase from the MgO to Al2O3 binary system.
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(ii) Non-Crystalline Ceramics – Natural and synthetic inorganic glasses.
(iii) “Glass-bonded” Ceramics – Fire clay products-crystalline phases are held in glassy matrix.
(iv) Cements – Crystalline or crystalline and non-crystalline phases.
B. Classification of Ceramic Products:
A general classification of ‘ceramic products’ is difficult to make because of the great versatility of these materials, but the following list includes the major groups:
1. Whitewares.
2. Bricks and tiles.
3. Chemical stonewares.
4. Cements and concretes.
5. Abrasives.
6. Glass.
7. Insulators.
8. Porcelain enamel.
9. Refractories.
10. Electrical porcelain.
11. Mineral ores.
12. Slags and fluxes.
Term Paper # 3. Properties of Ceramic Materials:
1. Mechanical Properties:
The ceramic materials possess the following mechanical properties:
(i) The compressive strength is several times more than the tensile strength.
(ii) Non-ductile/brittle. Stress concentration has little or no effect on compressive strength.
(iii) The ceramic materials possess ionic and covalent bonds which impart high modulus of elasticity. The modulus decreases with increase in temperature (due to increase in interatomic distance at elevated temperature).
(iv) As compared to pure metals, more force is required to cause slip in diatomic ceramic materials, because diatomic material consists of a mixture of positively and negatively charged ions which have strong forces of attraction between them.
(v) Below recrystallisation temperature, non-crystalline ceramics are fully brittle. The cleavage failure occurs along crystallographic planes and propagation of the crack takes place at high speed.
(vi) At high temperature rigidity is high.
(vii) In case of alloy consisting of two or metals, each phase may have appreciable difference of coefficient of thermal expansion which generate stress. This stress may then cause the metal to fail.
2. Electrical Properties:
The electrical properties of ceramic products vary from the low loss, high frequency dielectrics to semiconductors. Electrical insulators fall into two general classifications, the classical electrical porcelain for both high and low tension service and the special bodies such as steatite, rutile, cordierite, high alumina, and clinoestatite for high frequency insulation.
Dielectric Constant:
Dielectric constant is the ratio of the capacitance of a dielectric compared to the capacitance of air under the same conditions.
A low dielectric constant contributes to low power loss and low loss factor; a high dielectric constant permits small physical size.
The dielectric constant for electrical porcelain varies between 4.1 and 11.0. Some special bodies have reported values of several thousands.
Porcelain has large positive temperature coefficient.
Rutile bodies have large negative coefficients.
By combining capacitor dielectrics having different temperature coefficients it is possible to reduce effect of the temperature change.
Dielectric Strength:
The dielectric strength of a material is defined as the ability of a material to withstand electrical breakdown.
The specific values of dielectric strength vary from 100 V per mil for low-tension electrical porcelain to 500 V per mil for some special bodies.
Rutile bodies show higher breakdown strength at higher frequencies.
Volume and Surface Resistivity:
A volume resistivity of 106 ohms/cm3 is considered the lower limit for an insulating material. At room temperature practically all ceramic materials exceed this lower limit. As the temperature of ceramic materials is raised, the volume resistivity decreases; the volume resistivity of soda-lime glasses decreases rapidly with temperature, whereas some special bodies are good insulators (above 106 ohm/cm3) at 700°C. Crystallised alumina has a volume resistivity of 500 ohms/cm3 at 1600°C.
Surface resistivity for dry, clean surface is 1012 ohms/cm2. At 98% humidity, the surface resistivity may be 1011 ohms/cm2 for a glazed piece or 109 ohms/cm2 for an unglazed piece. The presence of dissolved gases and other deposits also tends to decrease the surface resistivity of ceramic materials.
3. Thermal Properties:
Since the ceramic materials contain relatively few electrons, and ceramic phases are transparent to radiant type energy, their thermal properties differ amply from that of metals.
The following are the most important thermal properties of ceramic materials (which vary from material to material and from condition to condition):
(i) Thermal capacity.
(ii) Thermal conductivity.
(iii) Thermal shock.
(i) Thermal Capacity:
The specific heats of fine clay bricks are 0.25 and 0.297 at 1000°C and 1400°C respectively.
Carbon bricks possess specific heats of about 0.812 at 200°C and 0.412 at 1000°C.
(ii) Thermal Conductivity:
The ceramic materials possess a very low thermal conductivity since they do not have enough electrons (for bringing about thermal conductivity). The conduction of heat takes place by phonon conductivity and the interaction of lattice vibration, while at elevated temperatures conduction takes place by the transfer of radiant energy.
The impurity content, porosity and temperature decrease the thermal conductivity.
In order to have maximum thermal conductivity, it is imperative to have maximum density which most of the ceramic materials do not possess.
(iii) Thermal Shock:
“Thermal shock resistance” is the ability of a material to resist cracking or disintegration of the material under abrupt or sudden changes in temperature.
Thermal shock is developed primarily because of thermal expansion or contraction, which is largely a function of internal structure particularly the inter-atomic bonding. Loosely packed structures can provide internal expansion. Thus the coefficient of expansion is low.
Lithium compounds are used in many ceramic compounds to reduce thermal expansion and to provide excellent thermal shock resistance.
Common ceramic materials graded in order of decreasing thermal shock resistance are given below:
(i) Silicon nitride
(ii) Fused silica
(iii) Cordierite
(iv) Zircon
(v) Silicon carbide
(vi) Beryllia
(vii) Alumina
(viii) Porcelain, and
(ix) Steatite.
4. Chemical, Optical and Nuclear Properties:
Chemical Properties:
a. Several ceramic products are highly resistant to all chemicals except hydrofluoric acid and to some extent, hot caustic solutions. They are not affected by the organic solvents.
b. Oxidic ceramics are completely resistant to oxidation, even at very high temperatures.
c. Ziconia, magnesia, alumina, graphite etc., are resistant to certain molten metals and are thus employed for making crucibles and furnace linings.
d. Where resistance to attack from acids, bases and salt solutions is required, ceramics like glass are employed.
Optical Properties:
a. Several types of glasses have been employed for the production of windows, subjected to high temperatures and optical lenses.
b. Special glasses, in large number, have also been used for selective transmission or absorption of particular wavelengths such as infrared and ultraviolet.
Nuclear Properties:
As ceramics are refractory, chemically resistant and because different compositions offer a wide range of neutron capture and scatter characteristics, they are finding nuclear applications and are being used as- Fuel elements, moderators, controls and shielding.
Term Paper # 4. Structure of Crystalline Ceramics:
Most ceramic phases, like metals, have crystalline structure. Ceramic crystals are formed by either a pure ionic bond, a pure covalent bond or by bonds that possess the ionic as well as covalent characteristics.
Ionic bonds give ceramic materials of relatively high stability. As a class, they have a much higher melting point, on the average, than do metals or organic materials. Generally speaking, they are also harder and more resistant to chemical reaction.
Covalent crystals usually also possess high hardness, high melting point and low electrical conductivity at room temperature.
The ceramics crystals structures are, however, invariably more complex as compared to those of metals, since atoms of different sizes and electronic configurations are assembled together.
Common crystal structures found in crystalline ceramics particularly those of oxide type include the following:
(i) Rock salt structure
(ii) Cerium chloride structure
(iii) Zinc blende structure
(iv) Wurzite structure
(v) Spinel structure
(vi) Fluorite structure, and
(vii) Ilmenite structure.
Silicate Structures:
The silicates are co-ordinate structures based upon large anions arranged about small cations. The dimensions of the lattice in general are controlled by the anions rather than cations because of the larger sizes of the former. Most important are the Si+4 and O-2 ions. In all silicates the basic unit is the SiO4 tetrahedron. This appears to remain essentially unaltered regardless of the other materials present.
Silicates are important constituents of most of the ceramic materials since they are plentiful, cheap and have certain distinct properties, necessary for certain engineering applications.
Portland cement is the most widely known silicate. It has the very advantage of forming a hydraulic bond.
Certain other construction materials made of silicates are brick, tile, glass, vitreous enamels etc.
Silicates are also used as reinforcing glass fibres, chemical wares and electrical insulators.
Types of Silicate Structures:
The various silicate structures are:
1. Silicon-oxygen tetrahedron (SiO4)4- structure.
2. Double and poly-tetrahedral structures.
3. Chain structures.
4. Sheet structures.
5. Framework structures.
6. Vitreous structures.
1. Silicon-Oxygen Tetrahedron (SiO4)4- Structure:
In this structure (primary structural unit of silicates) one silicon atoms fits interstitially among four oxygen atoms.
Example:
Forsterite (Mg2SiO4), a mineral which is a high temperature refractory.
2. Double and Poly-Tetrahedral Structures:
This type of structure results when three or more tetrahedral units link together (a ring type structure is produced). One of the oxygens is a member of two units.
The composition of a three polyhedral unit is Si3O9, which produces (Si3O9)6- ions.
Example:
Polysilicates (double tetrahedral structure).
3. Chain Structure:
A chain structure is formed when two corners of each tetrahedra are linked.
A single chain structure (Fig. 6.3) can be noticed in proxenes.
A double chain structure (Fig. 6.4) results when two parallel identical chains are polymerized by sharing oxygen to every alternate tetrahedranol.
Example:
Amphiboles.
Theoretically, the length of these chain structures can be almost infinite.
4. Sheet Structure:
When the double chain structure extends infinitely in a two dimensional plane, a sheet structure results.
This structural arrangement provides certain important properties, e.g., the lubricating characteristics of talc and plasticity of clay, the cleavage of mica etc.
The sheet structure is found in ceramic materials which are clays, micas and talc.
5. Framework Structure:
This type of structure is an extension of silicate tetrahedral unit into three dimensions.
A framework structure is generally hard, has low atomic packing factors and possesses relatively low densities.
Examples:
Quartz, feldspar, cristobalite etc.
6. Vitreous Structures:
Glass is a vitreous silicate, having a vitreous structures. Glass has a three-dimensional frame work structure containing covalent bonds.
Term Paper # 5. Advantages of Ceramic Materials:
The ceramic materials entail the following advantages:
1. The ceramics are hard, strong and dense.
2. They have high resistance to the action of chemicals and to the weathering.
3. Possess a high compression strength compared with tension.
4. They have high fusion points.
5. They offer excellent dielectric properties.
6. They are good thermal insulators.
7. They are resistant to high temperature creep.
8. Availability is good.
9. Good sanitation.
10. Better economy.
Term Paper # 6. Applications of Ceramics:
The applications of ceramics are listed below:
1. The whitewares (older ceramics) are largely used as/in:
a. Tiles;
b. Sanitary wares;
c. Low and high voltage insulators;
d. High frequency applications;
e. Chemical industry-as crucibles, jars and components of chemical reactors;
f. Heat resistant applications-pyrometers, burners, burner tips, and radiant heater supports.
2. Newer Ceramics (e.g., borides, carbides, nitrides, single oxides, mixed oxides, silicates, metalloid and intermetallic compounds) which have the high hardness values and heat and oxidation values are largely used as/in:
a. Refractories for industrial furnaces.
b. Electrical and electronic industries-as insulators, semiconductors, dielectrics, ferroelectric crystals, piezoelectric crystals, glass, porcelain alumina, quartz and mica etc.
c. Nuclear applications-as fuel elements, fuel containers, moderators, control rods and structural parts. Ceramics such as UO2, UC, UC2 are employed for all these purposes.
d. Ceramic metal cutting tools-made from glass free Al2O3.
e. Optical applications—Ytralox, a comparative newcomer in the ceramic material field, is useful since it is as transparent as window glass and can resist very high temperature.
3. Advanced Ceramics (e.g., SiC, Si3N4, ZrO2, B4C, SiC, TiB2 etc.):
The advanced ceramics are untilized as/in:
a. Internal combustion engines and turbines, as armor plate;
b. Electronic packaging;
c. Cutting tools;
d. Energy conversion, storage and generation.