POLYMERS CERAMICS AND COMPOSITES
1. INTRODUCTION TO CERAMICS
- Ceramics are inorganic, non-metallic materials that consists of metallic and non-metallic elements for which the interatomic bonds are either totally ionic, or predominantly ionic but having some covalent character. The term "ceramics" comes from the greek word keramikos, which means "burnt stuff', indicating that desirable properties of these materials are normally achieved through a high-temperature heat treatment process called firing.
- The properties of these materials also vary greatly due to differences in bonding. In general, these materials are typically hard and brittle with low toughness and ductility. Due to their desirable characteristics such as high hardness, wear resistance, chemical stability, high temperatures strength. and low coefficient of thermal expansion, advanced ceramics are being selected as the preferred material for many applications. These materials are usually good electrical and thermal insulators due to absence of conduction electrons.
- Ceramic materials normally have relatively high melting temperature and high chemical stability in many hostile environments, are indispensable for many engineering designs.
- In general, we can divide ceramic material used for engineering applications into two groups:
(i) traditional ceramic materials
(ii) engineering ceramic materials
- Typically, traditional ceramics are made from three basic components; clay, silica (flint), and feldspar. Bricks and tiles are familiar examples of traditional ceramics.
- The engineering ceramics, in contrast, typically consists of pure or nearly pure compounds such as aluminium oxide (Al2O3), silicon carbide (SiC) and silicon nitride (Si3N1) Examples of the use of the engineering ceramics in high technology are SiC in high temperature high areas of the experimental AGT-100 automotive gas turbine engine and aluminium oxide in the support base for integrated circuit chips in a thermal conduction module.
- Silicate are materials composed primarily of silicon and oxygen, the two most abundant element in the earth's crust; consequently, the bulk of soils, rocks, clays, and sand come under the silicate classification.
- Rather than characterizing the crystal structures of these materials in terms of unit cells, it is more convenient to use various arrangements of an tetrahedron as shown in figure 1
- Each atom of silicon is bonded to four oxygen atoms, which are situated at the corners of the tetrahedron; the silicon atom is positioned at the centre. Since this is the basic unit of the silicates, it is often treated as a negatively charged entity.
- When all four corners of SiO4-4tetrahedra share oxygen atoms, an SiO2 network called silica is produced.
- Crystalline silica exists in several polymorphic forms that correspond to different ways in which the silicon tetrahedra are arranged with all corners shared.
- Quartz, tridymite and cristobalite are three basic silica structures and each of these has two or three modifications.
- 2.2. FELDSPARS
- In the feldspar silicate structure network, some Al3+ ions replace some Si4+ions to form a network with a net negative charge. This negative charge is balanced with large ions of alkali and alkaline earth ions such as Na+, Ca2+ and Ba2+ which fit interstitial positions.
3. MATERIAL PREPARATION
- Several ceramic products are made by the agglomeration of particles. Ceramic products prepared by agglomerating particle may be formed by a variety of methods in the dry, plastic or liquid conditions.
In ceramic industry, the cold-forming processes are predominant, but hot forming processes are also used to some extent. Commonly used methods for ceramic formation are
(ii) slip casting
4.1. DRYING AND BINDER REMOVAL
- The main purpose of drying ceramics is to remove water from the plastic ceramic body prior it is fixed at higher temperatures.
- Usually drying to remove water is carried out at or below 100° and can take about 24 hours for a large ceramic pant.
- By heating in the range 200 to 300°C, the bulk of organic binders can be removed from ceramic parts although some hydrocarbon residues may require heating to much high temperatures.
- This is the process by which small particles of a material are bonded together by solid state diffusion. This thermal treatment in ceramic manufacturing results in the transformation of porous compact into a dense, coherent product. This process is commonly used to produce ceramic shapes made of e.g., alumina, ferrites, beryllia and titanates.
- Particles in this process are coalesced by solid state diffusion at very high temperatures but below the melting point of the compound being sintered, e.g., the alumina spark plug insulator is sintered at 1600°C (the melting point of alumina is 2050°C).
- There are some ceramic products, e.g., porcelain, structural clay products, and some electronic components contain a glass phase. This glass phase serves as a reaction medium by which diffusion can take place at a lower temperature than in the rest of the ceramic solid material. During the firing of these types of ceramic materials, a process termed as vitrification takes place whereby the glass phase liquefies and fills the pore spaces in the material.
5. ELECTRICAL-PROPERTIES OF CERAMICS
- The electrical and mechanical properties of ceramics make them especially suitable for many insulator applications in the electrical and electronic industries.
- The ionic and covalent bonding-in these-materials-restricts-electron and ion mobility and therefore-these materials are quite suitable as good insulators. These bonding make most ceramic materials strong but relatively brittle.
6. MECHANICAL PROPERTIES
- As a class of materials, ceramics are relatively brittle. The observed tensile strength of these materials varies greatly, ranging from very low values than 0.69 MPa to about 7 x 103 MPa for whiskers of ceramics such as Al2O3 prepared under carefully controlled conditions.
- The lack of plasticity in crystalline ceramics is due to their ionic and covalent bond.
- In covalent crystals and covalently bonded ceramics, the bonding between atoms is specific and directional involving the exchange of electron charge between pairs of electrons. Obviously, when covalent crystals are stressed to a sufficient extent, they exhibit brittle fracture to a separation of electron pair bonds without their subsequent reformation, clearly, covalently bonded ceramics brittle in both single crystal and polycrystalline states.
- The deformation of primarily bonded ceramics is different. Single crystals of ionically bonded solid e.g. (magnesium oxide and NaCl exhibit considerable plastic deformation under compressive at room temperature. Polycrystalline ionically bonded ceramics, however, are brittle, with cracks forming at the grain boundaries.
- In polycrystalline ceramics adjacent grains must change shape during deformation since there are limited slip systems in ionically bonded solids, cracking occurs at the grain boundaries and subsequent brittle fracture occurs, As most industrially important ceramics are polycrystalline most ceramic materials tend to be brittle .
- The high hardness of some ceramic materials makes them useful as abrasive materials for cutting, grinding and polishing other materials having lower hardness.
7. THERMAL PROPERTIES OF CERAMICS
- Due to strong ionic covalent bonding, most ceramic materials have low thermal conductivities. Ceramics are good thermal insulators.
- Ceramic materials due to their high heat resistance are used as refractories, which are materials that resist the action of hot environments, both liquid and gaseous. Refractories are used widely by the metals, chemical, ceramic and glass industries.
- Glass' is a ceramic which is' a mixture of substances with principal constituent of silica that has solidified from the liquid state without crystallization.
- Glass is an amorphous substance It is a vitreous silicate with a random arrangement of silicon oxygen tetrahedron (SiO4–4) units as shown in figure 7.
- A glass made of silica alone has many desirable properties but is require high temperature manufacture, thereby making its manufacturing difficult and expensive. In order to reduce the temperature, modifying ions are added to silicates which consists of sodium, potassium and calcium.The modifying ions help in loosening the Si-O bonds.
- The constituents of glass clued:
(i) silica which is the principal constituent
(ii) sodium or potassium carbonate to reduce the melting point of silica and to impart viscosity to the molten glass.
(iii) lime to impart durability
(iv) manganese dioxide to correct the colour of the glass.
(v) Colouring substance
- The glasses can be classified as
(i) Soda-lime (crown) glass having composition of 75% sand 12.5% lime and 12.5% soda. It is cheapest quality of glass and is used for window and container glass.
(ii) Flint glass which contains besides silica, varying proportions of lead oxide provide brilliance and high polish.
(iii) Pyrex (heat resistant) glass. Pyrex glass contains 80% silica and 14% boron oxide. It is used for cooking utensils and laboratory wares.
- The properties of glasses are:
(i) It has no definite silicate structure
(ii) It has no sharp melting point
(iii) It can absorb, refract and transmit light
(iv) It is excellent insulator at high temperature
(v) It is extremely brittle and hard
(vi) It is unaffected by air and water
(vii) It can take high polish
(viii) It can be produced in different colours
(ix) It is resistant to corrosion by chemical reagents
(x) It is weak in tension
(xi) It can withstand thermal shock without cracking.
- The applications and uses of glass include:
(i) The fibre glass reinforced with plastics can be used in making of furniture, bathroom fittings and parts of automobiles.
(ii) The optical glass is used for devices of science, astronomy and bacteriology.
(iii) The glass linings are used to provide corrosion resistance to valves, pipes and pump.
(iv) Many parts of guided missiles are made of glass.
(v) Deep driving vehicles have noses made of glass
(vi) The elevator cabin is made of glass.
(vii) The glass is used for windows of the buildings. Colour changing glasses are developed which can store solar energy during day and can gibe light at night.
- Carbon is an element that exists in various polymorphic forms, as well as in the amorphous state. This group of materials does not really fall within any one of the traditional metal, ceramic, polymer classification schemes. However, we choose to discuss these materials in this chapter since graphite, one of the polymorphic forms is sometimes classified as a ceramic, and in addition, the crystal structure of diamond, another polymorph, is similar to that of zinc blende.
- Diamond is metastable carbon polymorph at room temperature and atmospheric pressure. Its crystal structure is a variant of the zinc blende, in which carbon atoms occupy all positions (both Zn and S). Thus, each carbon bonds to four other carbons, and these bonds are totally covalent. This is appropriately called the diamond cubic crystal structure, which is also found for other Group IVA elements in the periodic table [e.g., germanium, silicon, and gray tin, below 13°C (55°F)].
- The physical properties of diamond make it an extremely attractive material. It is extremely hard (the hardest known material) and has a very low electrical conductivity, these characteristics are due to its crystal structure and the strong interatomic covalent bonds.
- Furthemore, it has an unusually high thermal conductivity for a nonmetallic material, is optically transparent in the visible and infrared region of the electromagnetic spectrum, and has a high index of refraction.
- Relatively large diamond single crystals are used as gem stones. Industrially, diamonds are utilized to grind or cut other softer materials.
- Techniques to produce synthetic diamonds have been developed, beginning in the mid-1950s, that have been refined to the degree that today a large proportion of the industrial quality materials are man-made, in addition to some of those of gem quality.
Fig.8: A unit cell for the diamond cubic crystal structure
- Another polymorph of carbon is graphite; it has a crystal structure distinctly different from that of diamond and is also more stable than diamond at ambient temperature and pressure. The graphite structure is composed of layers of hexagonally arranged carbon atoms within the layers, each carbon atom is bonded to three coplanar neighbor atom is bonded to three coplanar neighbor atoms by strong covalent bonds.
- The fourth bonding electron participates in a weak van der walls type of bond between the layers.
- As a consequence of these weak interplanar bonds, interplanar cleavage is facile, which gives rise to the excellent lubricative properties of graphite. Also, the electrical conductivity is relatively high in crystallographic directions parallel to the hexagonal sheets.
Fig.9: The sturcutre of graphite
- Other desirable properties of graphite include as high strength and good chemical stability at elevated temperatures and in nonoxidizing atmospheres, high thermal conductivity, low coefficient of thermal expansion and high resistance to thermal shock, high adsorption of gases, and good machinability
- Graphite is commonly used as heating elements for electric furnaces, as electrodes for arc welding in metallurgical crucibles, in casting molds for metal alloys and ceramics, for high-temperature refractories and insulations, in rocket nozzles, in chemical reactor vessels, for electrical contacts, brushes and resistors, as electrodes in batteries and in air purification devices.
Another polymorphic form of carbon was discovered in 1985. It exists in discrete molecular form and consists of a hollow spherical cluster of sixty carbon atoms; a single molecule is denoted by C60.
- Each molecule is composed of group of carbon atoms that are bonded to one another to form both hexagon (six-carbon atom) and pentagon (five-carbon atom) geometrical configurations. One such molecule shown in fig.10, is found to consist of 20 hexagons and 12 pentagons, which are arrayed such that no two pentagons share a common side; the molecular surface thus exhibits the symmetry of soccer ball.
- The material composed of C60 molecules is known as buckminsterfullerene, named in honor of R. Buckminster Fuller.
- Diamond and graphite are what may be termed network solids, in that all of the carbon atoms form primary bonds with adjacent atoms throughout the entirely of the solid. By way of contrast, the carbon atoms in buckminsterfullerene bond together so as to form these spherical molecules.
- In the solid state, the C60 units form a crystalline structure and pack together in a face-centered cubic array
- As a pure crystalline solid, this materials is electrically insulating. However, with proper impurity additions, it can be highly conductive and semiconductive.
Fig.10: The structure of C60 molecule
- INTRODUCTION TO POLYMER
- Polymers are materials consisting of giant or macromolecules, chain-like molecules having average molecular weights from 10,000 to more than 10,00,000 g/mole built by joining many mers or units through chemical bonding.
- Molecular weights is defined as the sum of atomic masses in each molecule.
- Nature has given us a number of polymers like proteins, carbohydrates, silk, wool, cotton, rubber, leather, etc.
- Man has made a variety of polymers with wide ranging properties such as softness like silk and wool and strong like steel.
- Most polymers, soids or liquids, are carbon based organic; however, they can be inorganic e.g., silicones based on Si-O network.
- This is defined as the chemical reaction in which a monomer is converted to the polymer under specific condition. However, monomer alone cannot undergo polymersation, but requires the presence of chemical called initator.
10.2. DEGREE OF POLYMERISATION
- The size of polymer depends on the number of repeating monomers constituting it. The degree of polymerization is determined by dividing the molecular weight of polymer by the mer weights.
10.3 MOLECULAR STRUCTURE OF POLYMERS
- The polymers can have structures which includes
10.3.1 LINEAR CHAIN STRUCTURE
The mers or simple molecules join together end to end in single chains to form linear chain structure or linear polymers. The linear polymer is shown in figure.12. The structure is simple and uniform and units are held together by relatively weak secondary bonds.
Fig: Linear chain structure
10.3.2 BRANCHED CHAIN STRUCTURE
- The polymer has also branched chains besides linear chains as shown in figure. The polymer is more stranger and less ductile due to the interlocking of the chains with each other. Branching is generally formed by removing a side atom form the main chain and replacing it by another C-C bonding. The chain packing efficiency is reduced due to the branching chains, thereby lowering the polymer density.
Fig: Branched chain structure
10.3.3 CROSSLINKED STRUCTURE
The polymer has interlinking chains connecting adjacent linear chains as shown in figure. Due to the interlinking of individual molecular chains the movement of individual chain is restricted as the interlocking anchors, the adjacent chains together. Cross-linking provides increased strength and reduced plasticity to the polymer.
Fig: Crosslinked chain structure
- A composite is considered to be any multiphase material that exhibits a significant proportion of both properties of both constituent phases. This is known as principle of combined action.
- All composites have a matrix or binder which is combined with reinforcing material.
- In advance composites fibres are directionally oriented and continuous.
- Example: Laminates, Sandwich panels, Reinforced concrete
11.2. General characteristics of composites
- They are superior to all materials in terms of strength, stiffness, temperature, fatigue etc.
- They are complex in nature. There components differ very much from each other.
- Properties of composite depend on its basic components physical and chemical property and bonding with them.
- The matrix of composites may consist metal, ceramic, polymers, carbon. Matrix is responsible for binding and shaping of composite.
11.3. Important Composite Material
11.3.1 Particle reinforced composite
- It is particle-reinforced composites (large particle composite).
- Concrete is a composite material consist of aggregate of the cement particles bound together using binding medium.
- Most widely known concretes are Portland and Asphaltic cement.
- Asphaltic cement is used for paving material while Portland cement is used as structural building material.
2. Portland cement concrete
- It consists fine aggregate (sand), course aggregate (gravel) and water.
- Aggregate reduces the requirement of cement hence cost of construction decreases.
- Its tensile strength is 10 to 15 times lesser than compressive strength.
3. Reinforced concrete
- It contains fine aggregate, course aggregate, cement and reinforcement (steel).
- Addition of reinforcement increase the strength.
- Concrete has good compressive strength while steel has good tensile strength hence reinforced concrete is used widely. Sufficient reinforcement is maintained even when cracks develop in concrete.
- The coefficient of thermal expansion for steel is same as that of concrete hence steel is suitable material for reinforcement.
11.3.2. Fibre- Reinforced composite
- They are strong fibres imbedded in a softer matrix to produce products with high strength to weight ratios.
- Length to diameter ratio used for reinforcement effects the properties of material. As length to diameter ratio increases the strength also increases.
- Strength of these composite also depends on bond between fibres and matrix. If a proper bonding occurs between fibre and matrix good strength can be maintained and vice-versa.
- Long and continuous fibres are difficult to produce but they are better option against short fibres.
- In continuous and aligned fibres mechanical behaviour depends on direction of stress is applied. It also depends on stress- strain behaviour of fibre and matrix phase.
- continuous and aligned fibres properties are anisotropic in nature (varies with direction)
- Discontinuous and aligned fibres efficiency is lower than continuous and aligned fibres.
- Discontinuous and aligned fibres has more demand than continuous and aligned fibres.
11.3.3 Laminar composites
- Laminar composites are formed when multidirectional stress are imposed on single panel. Aligned layers that are fastened together one or top of another at different orientations are frequently utilized.
- They are light in weight yet provide high strength.
- Their cost of formation is low.
- Famous examples are plywood, safety glass.
11.3.4. Sandwich structure
- They have thin layers of facing materials over a low density material such as polymer foam or expanded metal structure.
- Their core is their source of strength.
- Core provides sheer rigidity along planes. Core separates faces and resists deformations perpendicular to face plane.
- Foamed polymers, synthetic rubber, cements are used for cores.
11.3.5. Polymer-Matrix composites
- These material consist resin as a matrix and fibres as the reinforcement medium.
- Due to their room temperature properties, ease of fabrication and cost, these PMC are used in great diversity of composite applications as well as in huge quantities.
18.104.22.168. Glass Fibre Reinforcement composites
- Fibre glass is composite consist of glass fibre, which way be continuous or discontinuous and contained with a polymer matrix.
- They have good strength and stiffness.
- They are used in automotive and marine bodies, plastic pipes, industrial floorings, containers.
22.214.171.124. Carbon Fibre Reinforced Polymer composites
- These consist of small crystallites of graphite. The atom in the basal planes are held together by very strong covalent bond.
- They have highest specific modulus and specific strength of all reinforcing fibre materials.
- They have high tensile modulus and high strength at elevated temperatures.
- They are not affected by moisture or a widely variety of solvents, acid and base at room temperature.
126.96.36.199. Aramid Fibre Reinforced polymer composites
- They have high strength, high modulus materials useful for outstanding strength to weight ratios, which are superior to metals,
- Two most famous materials Kevlar and Nomex.
- These composites have high longitudinal strength and tensile moduli and they are higher than than other polymeric fibre materials.
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