Materials Science Ceramics Materials Science Ceramics

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  • Materials Science Ceramics Materials Science Ceramics
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  • The word ceramic can be traced back to the Greek term keramos, meaning "a potter" or "pottery". Keramos in turn is related to an older Sanskrit root meaning "to burn". Thus the early Greeks used the term to mean "burned stuff" or "burned earth" when referring to products obtained through the action of fire upon earthy materials. Definition of ceramics The art and science of making and using solid articles formed by the action of heat on earthy raw materials. Conventional ceramics (Traditional ceramics) i.e. clay product, glasses, and cement The art and science of making and using solid articles with have as their essential components, and are composed in large part of inorganic nonmetallic materials. Conventional ceramics & New ceramics (ceramics which have either unique and outstanding properties, they have been developed in order to fulfill a particular need. Ex. electronic ceramics, bioceramics, etc.)
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  • Ceramic structures Two or more different elements Ionic and/or covalent bonds Ioniccovalent
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  • Ionic bonds most often occur between metallic and nonmetallic elements that have large differences in their electronegativities. Ionically-bonded structures tend to have rather high melting points, since the bonds are strong and non-directional. electronegativities. The other major bonding mechanism in ceramic structures is the covalent bond. Unlike ionic bonds where electrons are transferred, atoms bonded covalently share electrons. Usually the elements involved are nonmetallic and have small electronegativity differences.
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  • Electronegativity: The attraction of an atom for shared electrons.
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  • Ceramic structures Ceramic materials can be divided into two classes: crystalline and amorphous (noncrystalline). In crystalline materials, atoms (or ions) are arranged in a regularly repeating pattern in three dimensions (i.e., they have long-range order). In amorphous materials, the atoms exhibit only short-range order. Some ceramic materials, like silicon dioxide (SiO 2 ), can exist in either form. A crystalline form of SiO 2 results when this material is slowly cooled from a high temperature (T m >1723C). Rapid cooling favors noncrystalline (amorphous) formation since time is not allowed for ordered arrangements to form.
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  • Crystalline form of SiO 2 Amorphous form of SiO 2 The type of bonding (ionic or covalent) and the internal structure (crystalline or amorphous) affects the properties of ceramic materials.
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  • Classification of ceramics Ceramic materials Glasses Clay products Refractories Abrasives Cements Advanced ceramics Structural clay product Whitewares Glasses Glass ceramics
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  • Typical properties of ceramics Light weight Corrosion resistance Very brittle Low and variable tensile strengths High compressive strengths - generally much higher than tensile strength Very high hardness, high wear and abrasion resistance High heat capacity and low heat conductance Electrically insulating, semiconducting, or superconducting Nonmagnetic and magnetic
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  • Chemical Properties Chemical properties describe the chemical stability of materials. The high chemical durability of the great majority of ceramic products makes them resistant to almost all acids, alkalis, and organic solvents. Ceramics are more resistant to corrosion than plastics and metals are. Of further importance is the fact that ceramic materials are not affected by oxygen. Most ceramics have very high melting points, and certain ceramics can be used up to temperatures approaching their melting points. Ceramics also remain stable over long time periods.
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  • Mechanical Properties : Overview Mechanical properties describe the way that a material responds to forces, loads, and impacts. The following characteristics are commonly tested: Tensile strength - failure under tension Compression strength failure under compression Stiffness resistance to bending (elastic deformation) Hardness resistance to surface penetration or scratching Impact (Toughness) resistance to abrupt forces Fatigue failure resistance to continued usage (cyclic deformation)
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  • Deformation When materials are put into use, they undergo changes in dimensions in response to the forces they are exposed to. This is called deformation. Elastic deformation: the object reverts to its original size and shape when the load is removed. Plastic deformation: when load is removed, object has permanent change in shape Fracture occurs when the load causes the object to break into two or more pieces.
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  • Elastic Modulus (Youngs modulus) Stress-Strain diagrams for typical (a) brittle and (b) ductile materials
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  • Mechanical behavior is dependent on many factors: e.g. Temperature Temperature the ratio of the service or test temperature to the melting point is known as the homologous temperature. Composition Composition Microstructure Microstructure minuscule structural and fabrication flaws The ability to deform reversibly is measured by the elastic modulus. Materials with strong bonding require large forces to increase space between particles and have high values for the modulus of elasticity. Temp E
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  • Ceramics are strong, hard materials. The principal limitation of ceramics is their brittleness, i.e., the tendency to fail suddenly with little plastic deformation. - In ionic solids because ions of like charge have to be brought into close proximity of each other forming large barrier for dislocation motion, the slip is very difficult. Similarly, in ceramics with covalent bonding slip is not easy (covalent bonds are strong). High yield stress and hardness Brittle fracture occurs by the formation and rapid propagation of cracks. Tensile stress would be needed to break the bonds between atoms in a perfect solid and pull the object apart. Ceramics are weak in tension.
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  • Compressive (crushing) strength is important in ceramics used in structures such as buildings or refractory bricks. The compressive strength of a ceramic is usually much greater than their tensile strength. Ceramics are generally quite inelastic and do not bend like metals. The fracture toughness is the ability to resist fracture when a crack is present. Ceramics have low fracture toughness. Fracture of ceramics highly sensitive to the presence of defects e.g. pores. Highly resistant to wear and erosion (compression loading phenomena)
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  • Thermal Properties The most important thermal properties of ceramic materials are heat capacity, thermal expansion coefficient, and thermal conductivity. In solid materials at T > 0 K, atoms are constantly vibrating.atoms are constantly vibrating Thermal conductivity : The ability to carry thermal energy (heat). Thermal energy can be either stored or transmitted by a solid. The ability of a material to absorb heat from its surrounding is its heat capacity (The ability of a material to absorb heat). heat capacity Thermal expansion coefficient Thermal expansion coefficient Fractional change in length divided by change in temperature, a measure of a materials tendency to expand when heated.
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  • The potential energy between two bonded atoms is related to their bond length. Ceramics generally have strong bonds and light atoms. The result is that they typically have both high heat capacities and high melting temperatures.potential energy The conduction of heat through a solid involves the transfer of energy between vibrating atoms. The vibration of each atom affects the motion of neighboring atoms, and the result is elastic waves that propagate through the solid. Amorphous ceramics which lack the ordered lattice undergo even greater scattering, and therefore are poor conductors. Those ceramic materials that are composed of particles of similar size and mass with simple structures (such as diamond or BeO) undergo the smallest amount of scattering and therefore have the greatest conductivity.
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  • Thermal expansion Materials change size when heating. Atomic view: Mean bond length increases with T.
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  • The heat transmission is interrupted by imperfection of structure, i.e. grain boundaries and pores, so that more porous materials are better insulators.
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  • One of the most interesting high-temperature applications of ceramic materials is their use on the space shuttle. Ceramics are strong, hard materials that are also resistant to corrosion (durable). These properties, along with their low densities and high melting points, make ceramics attractive structural materials, e.g. automobile engines, armor for military vehicles, and aircraft structures.
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  • Electrical Properties Ceramics exhibit the largest possible diversity in electrical conductivity [ ( -cm )-1 ], in terms of the type and magnitude of the conductivity: insulators, < 10 -22 ( -cm )-1 (such as alumina) ionic conductors, ~ 10 -2 ( -cm )-1 (such as AgI) electronic semi-conductors, ~ 100 ( -cm )-1 (such as SiC) e