- 1 Introduction
- 2 Defects in Crystal
- 3 Phase Diagram
- 4 Mechanical Properties of Materials
- 5 Heat Treatment
- 6 Thermal Properties
- 6.1 Specific Heat Capacity
- 6.2 Specific Latent Heat
- 6.3 Thermal Expansion Coefficient
- 6.4 Thermal Conductivity (l)
- 6.5 Currently Used Techniques to measure thermal conductivity
- 6.6 Thermal Sensor
- 6.7 Thermal Properties of Materials
- 6.8 High Temperature Materials
- 6.9 Super-alloys
- 6.10 Materials for Cryogenic Applications
- 6.11 Insulating Materials
- 6.12 R-value
- 7 Ceramic Materials
- 7.1 Glass Property
- 7.2 Refractory Materials
- 8 Polymers
- 8.1 Thermoplastic and Thermosetting Polymers
The combined knowledge of materials from materials science and materials engineering enable engineers to convert materials into products needed by the society.
Classification of Materials
Solid materials have conveniently been grouped into three basic classifications, namely, i) metallic materials ii) polymeric materials, and iii) ceramics materials: based primarily on chemical make-up and atomic structure.
1. Metallic Materials
Metallic materials are normally combination of one or more metallic elements and may also contain some non-metallic elements like C, N, O, etc. Metals have a crystalline structure in which the atoms are arranged in an orderly manner. They are quite strong, yet deformable, which accounts for their extensive use in structural applications. Metals are commonly divided into two classes namely, i) Ferrous Metals and ii) Non-Ferrous Metals.
2. Polymeric Materials
Polymeric materials include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other non-metallic elements. They are mostly non-crystalline, poor conductors of electricity.
3. Ceramic Materials
Ceramics are compounds of metallic and non-metallic elements. They are most frequently oxides, nitrides, and carbides. Ceramic materials can be crystalline, non-crystalline or mixture of both. They are typically insulative and have resistant to high temperatures.
Composite materials are mixtures of two or more materials. Usually the components do not dissolve in each other and can physically be identified by an interface between components. Two outstanding types of modern composite materials used for engineering application are fiber-glass-reinforcing material in a polyester or epoxy matrix and carbon fibers in an epoxy matrix.
Electronic Materials are extremely important types of materials for advanced engineering applications. The most important electronic material is pure silicon. Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms, where concentration may be controlled over very small spatial regions.
Bio-materials are employed in components implanted into the human body for replacement of diseased of damaged body parts. These materials must not produce toxic substances and must be compatible with body tissues.
Defects in Crystal
In reality, crystals are never perfect and contain various types of imperfection and defects, which affect many of their physical mechanical properties. The classifications of crystal imperfections are frequently made according to the geometry or dimensionality of the defects in crystal, the details of which are summarized below.
Types of Defects in Crystal
1. Point Defects
2. Line Defects
3. Surface Defects
1. Point Defects
Point defects are localized disruptions in otherwise perfect atomic or ionic arrangements in crystal structure. These imperfection may be introduced by movement of atoms or ions.
This is the simplest point defect. In this system, an atom is missing from its regular atomic site. It formed during solidification as a result of atomic vibrations and during recovery as a result of local rearrangement of atoms. Vacancies are also introduced during plastic deformation.
ii) Interstitialcy or Self-interstitial
Self-interstitial defect in a solid is obtained when an atom in a crystal occupies an interstitial site between surrounding atoms in normal atom sites.
Impurities in Solid
A pure metal consisting of just only one type of atom is not possible; impurity or foreign atoms will always be present, and some will exist as crystalline point defects.
The simplest type of alloy is that of the solid solution. A solid solution is a solid that consists of two or more elements atomically dispersed in a single-phase structure. Impurity point defects are found in two types of solid solutions, namely, i) interstitial and ii) substitutional.
In interstitial solid solution, impurity atoms fill the voids or interstices among the host atoms. For metallic materials that have relatively high atomic packing factors, these interstitial positions are relatively small. Consequently, the atomic diameter of an interstitial impurity must be substantially smaller than that of the host atoms.
In substitutional solid solution, solute or impurity atoms replace the host atoms. The crystal structure of the parent element or solvent is unchanged.
Schottky Imperfections – When two oppositely charged ions are missing from an ionic crystal, a cation-anion vacancy is created which is known as a Schottky imperfection.
Frenkel Imperfection – If a positive cation moves into an interstitial site in an ionic crystal, a cation vacancy is created in the normal ion site. This vacancy-interstitialcy pair is called Frenkel Imperfection.
2. Line Defects
Line imperfections or defects in crystalline solids are defects that cause lattice distortion centered around a line. The main two types of dislocations are edge dislocations and screw dislocation, a combination of these two gives the mixed dislocations.
i) Edge Dislocation
An Edge Dislocation is created in a crystal by the intersection of an extra half plane of atoms. The inverted ‘tee’ indicates a positive edge dislocation, whereas upright ‘tee’ indicates a negative edge dislocation. The edge dislocation has a region of compressive strain where the extra half of plan is present and region tensile strain below the extra half plane of atoms
ii) Screw Dislocation
The Screw Dislocation can be formed in a perfect crystal by applying upward and downward shear stresses to regions of a perfect crystal, which have been separated by a cutting plane.
iii) Mixed Dislocation
Most dislocations in crystal are of mixed type having the edge and the screw components with a transition region between them.
Burgers vector characterizes a dislocation line. It indicates whether the dislocation line is an edge, screw or mixed dislocation. The Burgers vector is determined by carrying out a conventional procedure by tracing out a Burgers circuit. Burger vector is perpendicular to the edge dislocation.
3. Surface Defects
i) External Surface – One of the most obvious boundaries is the external surface, along which the structure terminates. Surface atoms are not bonded to the maximum number of nearest neighbors and therefore in a higher energy state than the atoms at interior position. To reduce this energy, materials tend to minimize.
ii) Grain Boundary and Interface – In this defect boundary separates two small grains or crystals having different crystallographic orientation in polycrystalline materials. In metals, grain boundaries are created during solidification, It is a narrow region of about two to five atomic diameters in width. Atomic packing in grain boundaries is lower than within the grains because of the atomic mismatch.
iii) Twin Boundary – Twin boundary is a mirror reflection of the atomic arrangement on the other side. Twin boundaries occur in pairs, such that the orientation change introduced by one boundary is restored by the other. The region between twin boundaries is called the twined region. Twins which form during the process of re-crystallization are called annealing twins, Whereas, twins form during plastic deformation are called deformation twins, commonly observed in HCP and BCC crystals.
iv) Stacking Fault – Stackin Faults are also planar surface imperfections created by a fault in stacking sequence of atoms planed in crystals.
If A is missing the stacking sequence become ABCABCABC. It is called HCP stacking.
v) Bulk or Volume Defects – These include pores, cracks, foreign inclusion and other phases.
Phase Diagram – A phase in a material in terms of its micro-structure is a region that differs in structure and composition from another region. Phase diagrams are graphical representations of what phases are present in a materials system at various temperatures, pressures and compositions.
Necessity of a Phase Diagram- Following points shows you the why it is necessary to use phase diagram.
1. To show what phases are present at different compositions and temperatures under slow cooling conditions.
2. To indicate equilibrium solid solubility of one element in another.
3. To indicate temperature at which an alloy cooled under equilibrium conditions starts to solidify and the temperature range over which solidification occurs.
4. To indicate the temperature at which different phases start to melt.
In single component systems, there is no composition variable and the only other variables present are temperature and pressure. Such as Water can exist in solid, liquid or vapour phases depending on temperature and pressure.
Gibbs Phase Rule
J.W. Gibbs derived an equation from thermodynamic consideration that enables the number of phases that can coexist in equilibrium in chosen system to be computed.
P + F = C + 2
where, P is the number of phases which coexist in a chosen system; C is the number of components in the system and F is the degrees of freedom.
The degrees of freedom are the number of variables which can be changed independently without changing the state of the phase or phases in equilibrium in a chosen system.
Binary Phase Diagrams
Two component systems are usually presented in binary phase diagrams. Therefore, three-dimensional diagram is needed to plot the variations in temperature, pressure and composition. In order to simplify the presentation of phase relationships on paper binary phase diagrams are usually drawn at constant (atmospheric) pressure.
F = C – P + 1
Application of Phase Diagrams
Three kind of information are available, namely, i) the phases that are present, ii) the compositions of these phases, iii) the percentage or fraction of the phases.
Phases Present – One just locates the temperature-composition point on the diagram and note the phase with which the corresponding phase field is labelled.
Determination of Phase Compositions – The first step in the determination of phase composition is to locte the temperature-composition point on the phase diagram.
In some binary alloy systems, the two elements are completely soluble in each other in both the liquid and the solid states. In these systems, only a single type of crystal structure exists over all compositions range of components and therefore, is called the isomorphous system.
Mechanical Properties of Materials
Materials are subjected to force or loads during their services. In such situations, it is necessary to know the characteristics of materials and to design the member from which it is made of such that any resulting deformation will not be excessive and fracture will not occur.
One of the most common mechanical properties is tensile property.
Elastic and plastic Deformation
When a piece of metal is subjected to a uni-axial tensile force, deformation of the metal occurs. If the metal returns to its original dimensions when the force is removed, the metal is said to have undergone elastic deformation.
If the metal deformed to such an extent that it cannot fully recover its original dimensions, it is said to have undergone plastic deformation.
Engineering Stress = F/A
Engineering Strain = Change in length/original length
Anelastic Behavior of Materials
Elastic Recovery is the time dependent. After load release some finite time is required for complete recovery of elastic strain with reversal of relaxation process. This time dependent elastic behavior is known as Anelasticity.
A viscoelastic material can be thought of as a material whose response is between that of a viscous material and an elastic material.
Several relaxation processes take place within a material in response to an externally applied stress. If the time scale of a relaxation process is too fast or very slow as compared to the time interval over which the stress is applied, the stress-strain relationship is essentially independent of time.
Conditions in which stress taken place:
(A) The C-atoms along the two contracting axes jump to occupy vacant positions along the elongated axis, as there is more space available there. This reduces the total distortional energy around the interstitial atoms. This jumping results in an additional stretching in the direction of the applied stress.
(B) If Fe with small quantity of C in solution be subjected to a number of alternating cycles of loading and unloading within the elastic region. If the time taken for each cycle is very small as compared to the relaxation time given above, the C-atoms will not have enough time to jump from the contracted axis to the elongated axis before the stress reversal takes place. Under such conditions, the C-atoms do not jump at all. The stress-strain curve simply corresponds to bond stretching.
(C) Finally consider the situation where the time taken for each cycle of loading and unloading is about the same as the relaxation time. The c jumping will continue to occur, as loading is done. The strain due to C Jumping will somewhat lag behind the strain due to bond stretching, which is instantaneous. Even after the maximum load has reached, the strain due to C jumping will continue to occur, resulting in further strain as function of time. Due to this effect, the streess-strain curve during loading does not coincide with the curve during unloading.
Fatigue Behavior of Materials
Fatigue is the lowering of strength of failure of a material due to repetitive stress, which may be above or below the yield strength. In many types of service applications, metal parts are subjected constantly to repetitive of cyclic stresses in the form of tension, compression, bending, vibration, thermal expansion and contraction, etc which will fail at a much lower stress than that which the part can withstand under the application of single static stress. These failures, which occure under repeated or cyclic stress, are called fatigue failures.
Types of Fatigue Failure
1. A tiny crack initiates or nucleates typically at surface, often at a time well after loading begins.
2. The crack gradually propagates as the load continues to cycle. During this stage of fatigue process, branch marks or striations are created as shown in.
3. A sudden fracture of the material occurs when the remaining cross section of the material is too small the applied load.
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In iron-carbon system, carbon is present either in the form of free carbon or as iron carbide.
Time-Temperature-Transformation (TTT) Diagram
The time-temperature transformation curves correspond to the start and finish of transformations. The horizontal lines that run between the two curves, marks the beginning and end of isothermal transformations.
Constan-Cooling-Temperature (CCT) Diagram
The constant-cooling temperature diagrams, describe the transformations taking place at various constant cooling rates. The CCT Diagrams provide not only the information on the transformation start and finish conditions, but also on the resulting hardness and microstructures for each particular cooling curve.
Heat treatment is the controlled heating and cooling of metals to modify their physical and mechanical properties without changing the shape of the product. Heat treatment is associated with increasing the strength of materials. Heat treating is adopted by many group of industrial and metalworking processes used to alter the physical, properties of a material.
Types of heat treatment
i) Softening – Softening is done to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size or change the electromagnetic properties of steel.
ii) Hardening – Hardening of steels is done to improve the strength and wear properties.
Common Heat Treatments of Steels
A wide variety of heat treatments is generally used for improving the physical as well as mechanical properties of steels for effective applications.
Full annealing is the process of slowly raising the temperature about 50*C above the sustenitic temperature line A3 or line Acm in the case of hypo-eutectoid steels. It is held at this temperature for sufficient time for all the material to transform into austenite. Then slowly cooled at the rate of about 20*C/h.
Normalizing is the process of raising the temperature to over 60*C. It is held at this temperature to fully convert the structure into austenite, and then removed from the furnace and cooled at room temperature.
Tempering is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of martensite. This brittleness is removed by tempering. Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability.
The mechanism of tempering depends on the steel and the tempering temperature.
Tempering is done immediately after quench hardening. When the steel cools to about 40*C after quenching, it is ready to be tempered. The part is reheated to a temperature of 150*C to 400*C. In this region, softer and tougher structure Troostite is formed.
Austempering is a quenching technique. The part is not quenched thorough the martensite transformation. Instead the material is quenched above the temperature when martensite forms Ms, around 315*C. Bainite is tough enough so that further tempering is not necessary and the tendency to crack is reduced severely.
Martempering is similar to austempering except that the part is slowly cooled through the martensite transformation. The biggest advantage of martempering over rapid quenching is that there is less distortion and tendency to crack.
Quenching is the act of rapidly cooling the hot steel to harden the steel. A host of medium is used based on the desired properties of the steel.
Quenching by water – Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vapourizes, and there is no direct contact of the water with the steel.
By Salt water – It is more rapid quenching medium than plain water.
Quenching by Oil – Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of martensite formation to the finish is slow and this reduces the likelihood of cracking.
Carburizing is a process of adding carbon to the surface. This is done by exposing the part to a coarbon rich atmosphere at an elevated temperature and allows diffusion to transfer the carbon atoms into steel.
Pack Carburizing – Parts are packed in a high carbon medium, such as carbon powder or cast iron shavings and heated in a furnace for 12 h to 72 h at 900*C.
Gas Carburizing – Gas carburizing is conceptually the same as pack carburizing, except that CO is supplied to a heated furnace and the reduction reaction of deposition of carbon takes place on the surface of the part.
Liquid Carburizing – The steel parts are immersed in a molten carbon rich bath.
Nitriding is a process of diffusing nitrogen into the surface of steel. The parts are heat-treated and tempered before nitriding.
Carbonitriding – It is most suitable for low carbon and low carbon alloy steels. In this process, both carbon and nitrogen are diffused into the surface.
The strength of a material can significantly be improved by the precipitation of a finely dispersed second phase in the matrix. It is common phenomenon in case of some of the aluminium alloys.
Thermal properties of materials refer to the properties of a body or region of space that determine whether or not there will be a net flow of heat into it or out of it from a neighbouring body or region and also in which direction the heat will flow.
If there is no heat flow, the bodies or regions are said to be in thermal equilibrium and at the same temperature.
If there is a flow of heat, the direction of the flow is from the body or region of higher temperature to the body or region of lower temperature.
Definition of Temperature Scale
Temperature is a measure of how hot a body is and a scale has to be needed to accurately determined its temperature.
First, a measurable physical property, X has to be selected that varies continuously with temperature.
Next, two standard temperature, called fixed points are chosen to create a linear scale. X is measure at upper fixed point, namely, boiling point of water and lower fixed point namely, freezing point of water.
Temperature values are them assigned to upper and lower fixed points. For celsius scale, a temperature value of 100 is given to boiling water and a value of 0 is assigned to the freezing point. To find unknown temperature, Ti, simply measure Xi and plug into the following equation.
This method hinges on the assumption that a physical property varies linearly with temperature.
Specific Heat Capacity
The Specific heat capacity of a solid or liquid is defined as the heat required to raise unit mass of substance by one degree of temperature.
It is the amount of heat energy required to raise the temperature of a unit mass by one unit. Cp is the specific heat capacity at constant pressure and Cv is specific heat capacity at constant volume.
Cp – Cv = R
where R is the gas constant.
Specific Latent Heat
Specific latent heat is the energy per unit mass absorbed or evolved when a substance changes its phase.
There are two types of latent heat and those are: (i) latent heat of fusion ( the heat given out when a liquid changes into a solid), and (ii) latent heat of vapourization (the heat absorbed when a liquid turns into a gas).
It may be noted that for a given substance, the liquid is in a higher energy state than the solid and the vapour is in a higher energy state than the liquid.
Thermal Expansion Coefficient
Thermal expansion coefficient is the percentage of dimensional change one can expect per unit increase in temperature. The coefficient is measure with a silatometer which records the change in a sample length over a range of temperature.
Thermal Conductivity (l)
Thermal conductivity is a property of materials that expresses the heat flux f(W/m2) that will flow through the material if a certain temperature gradient dT (K/m) exists over the material.
The thermal conductivity is usually expressed in W/m-K and denoted as l. The usual formula that expresses l can be represented as f = l x DT
Fourier’s law of heat conduction represented as
q = -k dT/dX
where q is the heat flux and is measure in energy/(time. area); T, the temperature (K); X, the distance in the x-direction; and k is the constant of proportionality.
Currently Used Techniques to measure thermal conductivity
There are a number of ways to measure thermal conductivity. In general, the steady-state techniques perform a measurement when the material that is analyzed is in complete equilibrium. The disadvantage is that it takes a long time to reach the required equilibrium.
In many physical phenomena heat is exchanged. Thermal sensor display some specific positive characteristics and those are given below.
No moving parts
Little or no energy consumption
The above mentioned characteristics will improve the reliability of measurement and control systems in which they are used. Below a common type of heat flux sensor, called thermopile, is used to explain some principles of measurement.
Measurements using Thermal Sensor
Various measurement like heat flux, radiation and mass flow rate can be determined using thermal sensors.
Heat flux is induced by a temperature gradient across the sensor. The temperature difference is measured by a thermopile.
The absorber absorbs radiation. The radiation is converted into heat. The heat flux to the heat sink is measured by the thermopile.
Mass Flow and Heat Transfer Coefficients
A certain amount of heat is generated by a resistor. The ratio between the heat that flows to the heat sink and the heat that was originally generated is a measure for the mass flow. This ratio can be determined using a thermopile.
Thermal Properties of Materials
Thermal properties of materials smoothen the calculation of the following properties.
Thermal conductivity – Thermal conductivity is the property of a material to conduct heat. Thermal conductivity can be defined as “the quantity of heat transmitted through a unit thickness of a material – in a direction normal to a surface of unit area – due to a unit temperature gradient under steady state conditions”
Thermal Diffusivity – In a sense, thermal diffusivity is the measure of thermal inertia. In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or ‘thermal bulk’.
High Temperature Materials
Materials at high temperature serve the needs of those who develop and use materials for high temperature applications. The industrial sectors covered include metal extraction, alloy manufacture, chemical processing, power engineering, engine and furnace industries, Within these sectors, the effect of high temperatures on materials performance, particularly creep, fatifue, strength, oxidation, corrosion and wear processes falls within the remit of the research publication.
The term “super-alloy” was first used shortly after the world war II to describe a group of alloys developed for use in turbo superchargers and aircraft turbine engines that required high performance at elevated temperatures.
Materials for Cryogenic Applications
The structural materials undergo a significant change in their mechanical properties at sub-zero temperatures particularly in their toughness. So, the materials suitable for cryogenic applications are those, which have less significant effect on their properties in that temperature range. Most of the aluminium alloys show good properties in cryogenic conditions.
Less dense materials are better insulators. A good insulator is obviously a poor conductor.
The R-value of material is its resistance to heat flow and is an indication of its ability to insulate. It is used as a standard way of telling how good a material will insulate. The higher the R-value, the better the insulation is.
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.
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.
Oxygen and silicon are the most abundant elements in Earth’s crust. Their combinations occur in rocks, soils, clays and sand.
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.
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.
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.
Polymers are common in nature which is generally seen in the form of wood, rubber cotton, leather, silk, proteins, anzymes, starches and cellulose. The key factor of polymer usage is its low production cost along with wide range of useful properties.
The enormous growth of synthetic polymers is due to the following reasons.
– Polymers are lightweight materials.
– Polymers act as insulators for electricity and heat
– Polymers cover a wide range of properties from soft packaging materials to stronger fibres than steel
– Polymers allow for relatively easy processing.
Most polymers are organic in nature and formed from hydrocarbon molecules. These molecules can have single, double or triple carbon bonds. A saturated hydrocarbon is one where all bonds between adjacent carbon atoms are single.
Isomers are molecules that contain the same molecules but in a different arrangement.
Polymer molecules are huge macromolecules that have internal covalent bonds. For most polymers, these molecules from very long chains. The backbone is a string of cargon atoms, often single bonded.
Polymers are composed of basic structures called mer units. A molecule with just one mer called a monomer.
Chemistry of Polymer Molecule
Chains are represented straight but in practice they have a three-dimensional zig-zag structure.
When all the mers are the same, the molecule is called a homo polymer. When there is more than one type of mer present the molecule is called a copolymer.
The mass of a polymer is not fixed, but is distributed around a mean value, since polymer molecules have different lengths. The average molecular weight can be obtained by averaging the masses with fraction of times they appear.
Polymers are usually no-linear; bending and rotation can occur around single C-C bonds. Random kings and coils lead to entanglement, like in the Spaghetti structure.
– Linear (end-to-end, flexible, like PVC, nylon)
The regularity and symmetry of the side-groups can affect strongly the properties of polymers. Side groups are atoms or molecules with free bonds called free-radicals, like H, O, methyl, etc.
– If the radicals are linked in the same order, the configuration is called isostatic.
– In a stereoisomer in a syndiotactic configuration, the radical groups alternative sides in the chain.
– In the atactic configuration, the radical groups are positioned at random.
In copolymers, polymers with at least two different types of mers can differ in the way mers are arranged.
Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density.
Thermoplastic and Thermosetting Polymers
Thermoplastic Polymers soften reversibly when heated (harden when cooled back). Thermoplastic are solids, which are formed using heat and pressure. Thermosetting resins usually begin as liquids, which are converted to solids by chemical reaction only.
Thermosetting polymers harden permanently when heated, as cross-linking hinder bending and rotations. Thermosetting polymers harden permanently when heated, as cross-linking hinder bending and rotaions. Thermosets are harder, more dimensionally stable, and more brittle than thermoplasts.
Plymerization is the synthesis of high polymers from raw materials like oil or coal. It may occur by
– addition polymerization, where monomer units are attached one at a time
– condensation polymerization, by step-wise intermolecular chemical reactions that produce the mer units.
The Mechanism of polymerization is divided into following two types :-
Chain Growth Reactions – In the growth reaction, vinyl polymerization initiates by radicals, cations, anions, or organometallic catalysts.
Step Growth Reactions – In this mechanism, reaction proceeds to normal functional group reactions of multi-functional monomers.