2 1 Composite Material Structure and Processing Corrosion resistance Dimensional stability (can be designed for zero CTE) Vibration damping ability Low electrical resistivity High electromagnetic interference(EMI)shielding effectiveness High thermal conductivity. Another example of a composite is concrete,which is a structural composite obtained by combining (through mixing)cement (the matrix,ie.,the binder, obtained by a reaction-known as hydration-between cement and water),sand (fine aggregate),gravel(coarse aggregate),and optionally other ingredients that are known as admixtures.Short fibers and silica fume(a fine SiO2 particulate)are examples of admixtures.In general,composites are classified according to their matrix material.The main classes of composites are polymer-matrix,cement- matrix,metal-matrix,carbon-matrix and ceramic-matrix composites. Polymer-matrix and cement-matrix composites are the most common,due to the low cost of fabrication.Polymer-matrix composites are used for lightweight structures(aircraft,sporting goods,wheel chairs,etc.),in addition to vibration damping,electronic enclosures,asphalt(composite with pitch,a polymer,as the matrix),solder replacement,etc.Cement-matrix composites in the form of con- crete(with fine and coarse aggregates),steel-reinforced concrete,mortar(with fine aggregate,but no coarse aggregate)or cement paste(without any aggregate)are used for civil structures,prefabricated housing,architectural precasts,masonry, landfill cover,thermal insulation,sound absorption,etc. Carbon-matrix composites are important for lightweight structures (e.g.,Space Shuttles)and components(e.g.,aircraft brakes)that need to withstand high tem- peratures,but they are relatively expensive due to the high cost of fabrication. Carbon-matrix composites suffer from their tendency to be oxidized(2C +O2 2CO),thereby becoming vapor. Carbon fiber carbon-matrix composites,also called carbon-carbon composites, are the most advanced form of carbon,as the carbon fiber reinforcement makes them stronger,tougher,and more resistant to thermal shock than conventional graphite.With the low density of carbon,the specific strength(strength/density), specific modulus(modulus/density)and specific thermal conductivity(thermal conductivity/density)of carbon-carbon composites are the highest among com- posites.Furthermore,the coefficient of thermal expansion is near zero. Ceramic-matrix composites are superior to carbon-matrix composites in terms of oxidation resistance,but they are not as well developed as carbon-matrix composites.Metal-matrix composites with aluminum as the matrix are used for lightweight structures and low-thermal-expansion electronic enclosures,but their applications are limited by the high cost of fabrication and by galvanic corrosion. Metal-matrix composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion (CTE)and increase the strength and modulus.If a relatively graphitic kind of carbon fiber is used,the thermal conductivity can also be enhanced.The combination of low CTE and high2 1 Composite Material Structure and Processing ￾ Corrosion resistance ￾ Dimensional stability (can be designed for zero CTE) ￾ Vibration damping ability ￾ Low electrical resistivity ￾ High electromagnetic interference (EMI) shielding effectiveness ￾ High thermal conductivity. Another example of a composite is concrete, which is a structural composite obtained by combining (through mixing) cement (the matrix, i.e., the binder, obtained by a reaction – known as hydration – between cement and water), sand (fine aggregate), gravel (coarse aggregate), and optionally other ingredients that are known as admixtures. Short fibers and silica fume (a fine SiO2 particulate) are examples of admixtures. In general, composites are classified according to their matrix material. The main classes of composites are polymer-matrix, cement￾matrix, metal-matrix, carbon-matrix and ceramic-matrix composites. Polymer-matrix and cement-matrix composites are the most common, due to the low cost of fabrication. Polymer-matrix composites are used for lightweight structures (aircraft, sporting goods, wheel chairs, etc.), in addition to vibration damping, electronic enclosures, asphalt (composite with pitch, a polymer, as the matrix), solder replacement, etc. Cement-matrix composites in the form of con￾crete (with fine and coarse aggregates), steel-reinforced concrete, mortar (with fine aggregate, but no coarse aggregate) or cement paste (without any aggregate) are used for civil structures, prefabricated housing, architectural precasts, masonry, landfill cover, thermal insulation, sound absorption, etc. Carbon-matrix composites are important for lightweight structures (e.g., Space Shuttles) and components (e.g., aircraft brakes) that need to withstand high tem￾peratures, but they are relatively expensive due to the high cost of fabrication. Carbon-matrix composites suffer from their tendency to be oxidized (2C + O2 → 2CO), thereby becoming vapor. Carbon fiber carbon-matrix composites, also called carbon-carbon composites, are the most advanced form of carbon, as the carbon fiber reinforcement makes them stronger, tougher, and more resistant to thermal shock than conventional graphite. With the low density of carbon, the specific strength (strength/density), specific modulus (modulus/density) and specific thermal conductivity (thermal conductivity/density) of carbon-carbon composites are the highest among com￾posites. Furthermore, the coefficient of thermal expansion is near zero. Ceramic-matrix composites are superior to carbon-matrix composites in terms of oxidation resistance, but they are not as well developed as carbon-matrix composites. Metal-matrix composites with aluminum as the matrix are used for lightweight structures and low-thermal-expansion electronic enclosures, but their applications are limited by the high cost of fabrication and by galvanic corrosion. Metal-matrix composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion (CTE) and increase the strength and modulus. If a relatively graphitic kind of carbon fiber is used, the thermal conductivity can also be enhanced. The combination of low CTE and high