Material s Engineering Branch Fall 2013

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1 Material s Engineering Branch Fall 2013 Department of Civil, Environmental and Architectural Engineering Piazzale J.F.Kennedy 1, Pad D, 16129, Genoa, Italy Fabrizio Barberis

2 SERP CHEM: Introduction to Material science MATERIALS ANISOTROPIC MATERIALS

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4 SERP CHEM: Introduction to Material science

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7 Now a higher softening point will require a higher spinning temperature, which is associated with the increased rate of cooling and higher tensions, explaining why it is more difficult to spin a mesophase pitch. Elevating the temperature of the quench air will slow down the cooling and reduce stress. Increasing the spin temperature will reduce the stress level but too high a temperature can cause thermal breakdown of the pitch, so too high a temperature cannot be used. Increasing the spinning temperature does increase the molecular orientation, so a careful balance has to be chosen.

8 Essentially, the higher modulus carbon fibers will not bond easily in the absence of a surface treatment. In some composites where the failure strain of the matrix is smaller than the failure strain of the fibers (as it is for ceramics or carbon), poor bonding is an asset since the largest fiber-matrix bond strength is not required. Conversely, if the failure strain of the matrix is larger than that of the fibers, a strong bond is desired.

9 The modulus increases steadily with temperature, but the strength peaks at about 1575C. The smaller the carbon fiber diameter, the greater is the strength

10 Young s modulus is an intrinsic property and is governed by the orientation of the graphitic crystallites relative to the fiber axis. The lower this angle, greater is the modulus. La is a measure of the crystallite basal planes and increases with temperature and the modulus. The orientation for HM fiber is 25 o in the core and o in the skin, with circumferential orientation.

11 POLYMER COMPOSITES : 1 Composite Materials generally suffer the followings: medium to low level impact resistance (inferior to that of metallic materials). subject to humidity (epoxy resin can absorb water by diffusion up to 6% of its mass; the composite of reinforcement/resin can absorb up to 2%) and heat The construction using only glass fibers is less and less favored in comparison with a combination of Kevlar fibers and carbon fibers for weight saving reasons: If one would like to have maximum strength, use Kevlar. If one would like to have maximum rigidity, use carbon. Kevlar fibers possess excellent vibration damping resistance.

12 Now a higher softening point will require a higher spinning temperature, which is associated with the increased rate of cooling and higher tensions, explaining why it is more difficult to spin a mesophase pitch. Elevating the temperature of the quench air will slow down the cooling and reduce stress. Increasing the spin temperature will reduce the stress level but too high a temperature can cause thermal breakdown of the pitch, so too high a temperature cannot be used. Increasing the spinning temperature does increase the molecular orientation, so a careful balance has to be chosen. Essentially, the higher modulus carbon fibers will not bond easily in the absence of a surface treatment. In some composites where the failure strain of the matrix is smaller than the failure strain of the fibers (as it is for ceramics or carbon), poor bonding is an asset since the largest fiber-matrix bond strength is not required. Conversely, if the failure strain of the matrix is larger than that of the fibers, a strong bond is desired. Processing speeds up to 1000 m/min are quoted and the fiber is drawn down to 8 14 m diameter. Since there is a mass loss in subsequent carbonization, to obtain a carbon fiber with a final diameter of 10 m would require an as-spun diameter of about 12 m. As with PAN fiber, a smaller diameter pitch fiber will produce a carbon fiber with higher tensile strength. Nippon Carbon have introduced a 6 m diameter pitch based carbon fiber (PBCF) with a strength of about 4 GPa.

13 2 1. Washing It is necessary to remove all the solvent from the fiber, usually carried out by counter current washing with hot water in conjunction with fiber stretching. 2. Stretching process Stretching or drawing aligns the chains of molecules, imparting strength but. reducing the elongation. Initially, the temperature is maintained above the wet Tg( about 65C) and as the solvent content is gradually removed in the washing process, the temperature is increased, permitting application of more stretch. Crystalline regions become oriented in the direction of the fiber axis and the void regions with another and align parallel, enabling crosslinks to form by dipole-dipole interaction, forming a two dimensional rod structure.

14 2 3. Finish A finish is usually applied as an aqueous emulsion to act as a lubricant and antistat and penetrates into the interior of the fiber if applied before the collapsed stage. Typical finishes are sorbitan esters of long chain fatty acids, polyoxyethylene derivatives and silicones 4.. Drying, collapsing and relaxing Drying is undertaken to remove water from the surface of and within the fiber. The relaxation process significantly alters the stress/strain properties of the fiber and can be incorporated in the drying as a continuous stage, or can be carried out by a batch process where the fiber is subjected to a hot wet environment, normally under a slight pressure, in an autoclave, to elevate the boiling point.

15 CHEMICAL CHANGES DURING CARBONIZATION (controlled heating to 1500 C) The carbonization involves both stabilized PAN and Pitch The greatest weight loss (gases) occurs in the early stages of carbonization (before 1000C). Therefore, it is advantageous to apply an initial low temperature carbonization stage to avoid disruption of the fiber structure. Normally, about 30 s to 5 min is sufficient time, while suitable treatment would be about 0.5 min at 700C followed by 0.5 min at 900C in an inert N 2 atmosphere.

16 CHEMICAL CHANGES DURING CARBONIZATION (controlled heating to 1500 C) Up to l000c, carbonization leads the solid residue transforms from a viscoelastic into a brittle solid material. Continuing pyrolisis up to 1500 C completes the conversion of the PAN molecules into sheets of carbon that are appreciably anisotropic

17 5 Graphitization is the name of the process that involves heating the carbonized fiber to approximately 2500 C in times as short as a minute (US Patent ,1977). Graphitized pitch fibers exhibit a larger, more graphitic and better oriented crystal structure than PAN-based carbon fibers which are inherently non-graphitizable. Parallel to the fiber axes, pitch fibers have higher stiffness and thermal conductivity values and a reduced thermal expansion coefficient. These changes due to graphitization do not produce any significant increase in relative strength values. As a result of the extreme temperatures required to process them, graphitized pitch-based carbon fibers are more expensive and are fabricated for specialized applications.

18 5 Compared to PAN, the basic structural units in an original mesophasic pitch are much larger in area and length and are not twisted to the same degree. The resulting ease of graphitization and alignment of the graphene layers and the development of large crystallites, produces a large elastic modulus and electrical and thermal conductivity parallel to the fiber axes. Unfortunately, large graphitized regions tend to produce high local stress concentrations Thus, PITCH-BASED tend to exhibit high modulus and high electrical and thermal conductivities, but low strength. PAN-BASED tend to be of intermediate modulus and relatively high strength.

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20 High modulus fiber production The high modulus furnace operates at about 2500C and employs a graphite muffle operating in a carefully controlled inert atmosphere. The conditions must be quiescent, since even low volume gas flow over the hot graphite element is sufficient to continually remove a molecular layer of graphite and cause severe erosion leading to premature failure. The added heat treatment provides an improvement in the orientation of the graphite crystallites, giving carbon fibers with a high YM.

21 High modulus fiber production Line shrinkage in the carbonization stage is, nominally of the order 5 8%, A low shrinkage value signifies a high fiber tension. If the tension is too high, then the fiber will break, and if it is too low, then the increased fiber catenary will permit the fiber to drag along the furnace floors, degrading the fiber cosmetics.

22 Carbonization High modulus fiber production Increasing the heat treatment temperature results in a reduction of the interlayer spacing, a decrease in void space, a growth in thickness and area of the graphitic crystallites and an increase in the preferred orientation of the microstructure. All of these changes increase the elastic modulus and the electrical and thermal conductance. A corresponding reduction of the tensile strength also occurs

23 Graphitization A mesophase pitch based fiber can be further heat treated in a similar type of furnace, using highly controlled inert atmosphere, at temperatures in the range C, preferably C, producing fibers with a high degree of orientation, where the carbon crystallites are parallel to the fiber axis. These fibers are truly graphitic and have a structure characteristic of polycrystalline graphite with a three-dimensional order. A residence time of about 10 s 5 min may be employed.

24 Graphitization Surface treating a fiber which is graphitic in nature is difficult and great care would have to be taken to ensure that the fiber is not embrittled by the treatment process.

25 . Graphite: 1 Young s modulus varies appreciably with orientation and the material is extremely anisotropic 2 Specific heat increases with temperature and whilst graphite is considered a good conductor in the basal direction, it is highly anisotropic and is almost an insulator in the direction normal to the basal plane.. 3 The thermal expansion of an extruded rod in a radial direction can be up to three times the anisotropy in practical applications

26 . Graphite: 4 It is an excellent refractory material with a melting point of about 4473C, but it must be under a pressure of some 100 atm, otherwise it just simply sublimes. 5 Since graphite oxidizes slowly in air at about 400C, it should be protected in an inert atmosphere to operate successfully at elevated temperatures

27 The following trends usually go together: increase in the shear modulus increase in the degree of anisotropy decrease in the electrical resistivity increase in the thermal conductivity decrease in the coefficient of thermal expansion increase in density increase in thermal stability (oxidation resistance) increase in chemical stability increase in cost

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30 ATTRACTIVE PROPERTIES OF CARBON INCLUDE : Low density High tensile modulus and strength Low thermal expansion coefficient Thermal stability in the absence of oxygen to over 3000 C Excellent creep resistance Chemical stability, particularly in strong acids Biocompatibility High thermal conductivity Low electrical resistivity Good fatigue properties Availability in a continuous form Decreasing cost (versus time) No significant inhalation problem with filament diameters down to 5 mm

31 DISADVANTAGES OF CARBON INCLUDE THE FOLLOWING: Anisotropy (in the axial versus transverse directions) Low strain to failure Compressive strength is low compared to tensile strength Poor impact strength of composites Care required during handling carbon fiber, since it is electrically conducting and can cause havoc with electrical systems Oxidizes in air at temperatures above 450 C Tendency to be oxidized and become a gas (e.g., CO) upon heating in oxidation of carbon fibers is catalyzed by an alkaline environment Relatively high cost

32 Tensile Strength. The tensile strength is strongly influenced by flaws, so it increases with decreasing test (gage) length and with decreasing fiber diameter. There are two types of flaws, namely surface flaws and internal flaws. The surface flaws control the strength of carbon fibers that have not been heat-treated above C; The internal flaws control the strength of carbon fibers that have been heat- treated above C

33 Biocompatibility Carbon is more biocompatible than even gold or platinum, so carbon fibers are used as implants, which act as a scaffold for collagen in tendons, for the repair of abdominal wall defects, and for the growth of spinal axons in a spinal cord. The carbon fibers provide a favorable adhesive surface and a possible guiding function. Oxidation Resistance The oxidation resistance of carbon fibers increases with the degree of graphitization.

34 Electrical Properties The resistivity decreases with increasing temperature for each type of fiber. At a given test temperature, the resistivity decreases with increasing tensile modulus. This is because an increase in the tensile modulus is accompanied by a decrease in the concentration of defects, and defects cause carrier scattering. Thermal Conductivity In general, the thermal expansion coefficient of carbon fibers decreases with increasing tensile modulus. The thermal conductivities of P-100, P-120, and Kll00X fibers are all higher than that of copper, the thermal expansion coefficients and densities are much lower than those of copper. Thus, the specific thermal conductivity is exceptionally high for these carbon fibers.

35 THERMAL EXPANSION OF CARBON

36 SERP CHEM: Introduction to Material science ANISOTROPIC MATERIALS 3D WOVEN