very small lattice spacing near the inner regions of the CNT. Carbon nanotubes(CNTs) have some remarkable properties, such as better electrical conductivity than copper, exceptional mechanical strength, and very high flexibility( with futuristic potential for use in even earthquake- resistant buildings and crash-resistant cars). There is already considerable interest in industry in using CNTs in chemical sensors, field emission elements, electronic interconnects in integrated nanotube circuits, hydrogen storage devices, temperature sensors and thermometers, and others Because of the exceptional properties of CNTs(e. g, Youngs modulus of CNT'is 1-4 TeraPascals, TP Pa), there has been some interest in incorporating CNTs in polymers, ceramics, and metals Owing to CNT's metallic or semiconducting character, incorporating CNT in polymer matrice permits attainment of an electrical conductivity sufficient to provide an electrostatic discharge at very low CNT concentrations. Similarly, extremely hard/and wear-resistant metal-matrix composites and tough ceramic-matrix composites are being developed. Since the discovery of CNTs in 1991, similar nanostructures were formed in other layered compounds such as BN, BCN and WS2, etc. For example, whereas CNTs are either metallic or semiconducting(depending on the shell helicity and diameter), bn nanotubes are insulating and could possibly serve as nanoshields for nanoconductors. Also, bn nanotubes are thermally more stable in oxidizing atmospheres than are CNTs and have comparable modulus. The strength of nanotubular materials be increased by assembling them in the form of ropes, as has been done with CNt and Bn anotubes, with ropes made from single-walled CNTs being the strongest known material. The spacing between the individual nanotube strands in such a rope will be in the subnanometer range; for example, this spacing is 0. 34 nm in a rope made from multiwalled BN nanotubes which is on the order of the(0001)lattice spacing in the hexagonal BN cell Organic Fibers. Because the covalent C-C bond is very strong, linear-chain polymers such as olyethylene can be made very strong and stiff by fully extending their molecular chains. A wide range of physical and mechanical properties can be attained by controlling the orientation of these lymer chains along the fiber axis and their order or crystallinity. Allied Corporations Spectra 00 and Du Pont' s aramid fiber Kevlar are two successful organic fibers widely used for com posite strengthening. Aramid is an abbreviated name of a class of synthetic organic fibers that are aromatic polyamide compounds. Nylon is a generic name for any long-chain polyamide. Many highly sophisticated manufacturing techniques have been developed to fabricate the organic fibers for use in composites. These techniques include: tensile drawing, die drawing, hydrostatic extrusion, and gel spinning. A wide range of useful engineering properties is achieved in organic fibers depending on the chemical nature of the polymeric material, processing technique, and the control of process parameters. For example, high modulus polyethylene fibers with a modulus of 200 GPa, and Kevlar fibers with a modulus of 65-125 GPa and tensile strength of 2.8 GPa have been developed Kevlar fibers have poor compression strength and should be used under compressive loading only as a hybrid fiber mixture, that is, as a combination of carbon fiber and Kevlar. One limitation of most organic fibers is that they degrade (lose color and strength) when exposed to visible or ultraviolet radiation, and a coating of a light-absorbing material is used overcome this problem. Metallic Fibers. Metals such as beryllium, tungsten, titanium, tantalum, and molybdenum, and alloys such as steels in the form of wires or fibers have high and very consistent tensile strength values as well as other attractive properties. Beryllium has a high modulus(300 GPa) and low density (1.8 g/cc)but also low strength(1300 MPa). Fine(0. 1-mm)diameter steel wires with a high carbon(0.9%)content have very high strength(5 GPa). Tungsten fibers have a 402 MATERIALS PROCESSING AND MANUFACTURING SCIENCEvery small lattice spacing near the inner regions of the CNT. Carbon nanotubes (CNTs) have some remarkable properties, such as better electrical conductivity than copper, exceptional mechanical strength, and very high flexibility (with futuristic potential for use in even earthquake￾resistant buildings and crash-resistant cars). There is already considerable interest in industry in using CNTs in chemical sensors, field emission elements, electronic interconnects in integrated nanotube circuits, hydrogen storage devices, temperature sensors and thermometers, and others. Because of the exceptional properties of CNTs (e.g., Young's modulus of CNT is 1-4 TeraPascals, TPa), there has been some interest in incorporating CNTs in polymers, ceramics, and metals. Owing to CNT's metallic or semiconducting character, incorporating CNT in polymer matrices permits attainment of an electrical conductivity sufficient to provide an electrostatic discharge at very low CNT concentrations. Similarly, extremely hard/and wear-resistant metal-matrix composites and tough ceramic-matrix composites are being developed. Since the discovery of CNTs in 1991, similar nanostructures were formed in other layered compounds such as BN, BCN, and WS2, etc. For example, whereas CNTs are either metallic or semiconducting (depending on the shell helicity and diameter), BN nanotubes are insulating and could possibly serve as nanoshields for nanoconductors. Also, BN nanotubes are thermally more stable in oxidizing atmospheres than are CNTs and have comparable modulus. The strength of nanotubular materials can be increased by assembling them in the form of ropes, as has been done with CNT and BN nanotubes, with ropes made from single-walled CNTs being the strongest known material. The spacing between the individual nanotube strands in such a rope will be in the subnanometer range; for example, this spacing is --~0.34 nm in a rope made from multiwalled BN nanotubes, which is on the order of the (0001) lattice spacing in the hexagonal BN cell. Organic Fibers. Because the covalent C-C bond is very strong, linear-chain polymers such as polyethylene can be made very strong and stiff by fully extending their molecular chains. A wide range of physical and mechanical properties can be attained by controlling the orientation of these polymer chains along the fiber axis and their order or crystallinity. Allied Corporation's Spectra 900 and Du Pont's aramid fiber Kevlar are two successful organic fibers widely used for com￾posite strengthening. Aramid is an abbreviated name of a class of synthetic organic fibers that are aromatic polyamide compounds. Nylon is a genetic name for any long-chain polyamide. Many highly sophisticated manufacturing techniques have been developed to fabricate the organic fibers for use in composites. These techniques include: tensile drawing, die drawing, hydrostatic extrusion, and gel spinning. A wide range of useful engineering properties is achieved in organic fibers depending on the chemical nature of the polymeric material, processing technique, and the control of process parameters. For example, high modulus polyethylene fibers with a modulus of 200 GPa, and Kevlar fibers with a modulus of 65-125 GPa and tensile strength of 2.8 GPa have been developed Kevlar fibers have poor compression strength and should be used under compressive loading only as a hybrid fiber mixture, that is, as a combination of carbon fiber and Kevlar. One limitation of most organic fibers is that they degrade (lose color and strength) when exposed to visible or ultraviolet radiation, and a coating of a light-absorbing material is used to overcome this problem. Metallic Fibers. Metals such as beryllium, tungsten, titanium, tantalum, and molybdenum, and alloys such as steels in the form of wires or fibers have high and very consistent tensile strength values as well as other attractive properties. Beryllium has a high modulus (300 GPa) and low density (1.8 g/cc) but also low strength (1300 MPa). Fine (0.1-mm) diameter steel wires with a high carbon (0.9%) content have very high strength (~5 GPa). Tungsten fibers have a 402 MATERIALS PROCESSING AND MANUFACTURING SCIENCE