High Performance Synthetic Fibers for Composites (1992) / Chapter Skim
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3 Fiber-Forming Processes: Current and Potential Methods
3 Fiber-Forming Processes: Current and Potential Methods
Pages 49-102
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From page 49... ...
Pyrolytic conversion of precursor fibers. Chemical conversion of precursor fibers.
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From page 50... ...
~ Wind up ~ Spinnerehe V1 ~ Vo I' Heat or Chemical Treatment V2 ~` `( v2 ~ Packaging 1 m Q CO r FIGURE 3.1 General processing steps for converting bulk materials to fibers.
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From page 51... ...
The commercial processes for high-strength, high-modulus fibers are based on two physical concepts: melt or dry jet wet spinning from a nematic liquid crystalline phase in which the already rod-like molecules are uniaxially ordered, and melt or gel spinning and drawing of conventional, random- co i 1 p o lymers under condi tions that permi t extreme ly high e longat tonal forces (high draw ratios) to mechanically elongate and orient the component molecules (cf.
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From page 52... ...
The finished polymer is washed to remove the solvent, and the equipment required for the mixing and isolation makes this an expensive process. Both aramids and the aromatic heterocyclics are redissolved and converted to highly oriented fiber through dry jet wet spinning (cf.
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From page 53... ...
draw ratios. The elongational forces cause the molecular chains to be extended and highly oriented in the fiber direction, resulting in high levels of tensile modulus and tensile strength.
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From page 54... ...
This indicates that limited government resources should be applied to the more specialized inorganics. FIBER FORMATION BY PYROLYTIC CONVERSION OF PRECURSOR FIBERS Introduction The use of precursors that can be pyrolyzed to form continuous inorganic filaments has provided a route to the manufacture of synthetic inorganic fibers of many different compositions.
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More complete details on processing, current status, performance, and needs in the area of high-performance synthetic fibers are ~roviaea in She f~1 1 m~.~;~= ~; ~- An nonoxide and oxide fibers. - rig Low -~= ~ BILL VL1 Carbon Fibers Even though high-performance carbon fibers were first introduced in the 1960s, the physical properties of these reinforcing fibers have improved dramatically over the past decade.
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From page 56... ...
Figure 3.3 gives a flow diagram for the process, which is well documented in the literature. The precursor fibers used by the major PAN-based carbon fiber suppliers differ significantly from the acrylic fibers used for textile acrylic apparel and industrial applications.
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57 _ a, ~ ~ o ~ _ ~ 5: Is m ~ ~_ ~ u, In < o 1' N ~ I ~ O :~= t~ o In C, o Harp .~ , Hi\ _ .= ._ Q Ct ~ ~ ' 1 ~ I
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In fact, over the past 7 to 8 years improvements in precursor as well as carbon fiber technology have increased the tensile strength of this class of fibers from approximately 3.45 GPa (500 Ksi)
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From page 59... ...
In short, this new process can easily produce the current standard high-quality precursor fiber, but it also has the versatility to modify fiber composition and even fiber cross-sectional shape, offering the potential to create a new family of carbon fibers with unique properties. Carbon Fiber Technology.
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From page 60... ...
Such processes have been demonstrated in the laboratory, but none have yet been commercialized on a large scale. As a result of the above efforts, it is projected that commercial carbon fibers with tensile strength well in excess of 6.9 GPa (1000 Ksi)
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From page 61... ...
Pi Itch- based Carbon Fibers Technical Description and Present Status. The production of highmodulus, pitch-based carbon fibers begins by heat treating a petroleum or coal tar pitch feedstock to produce a liquid crystal precursor, termed "mesophase." The liquid crystal material is melt spun into a precursor fiber that is converted into a carbon fiber in a process somewhat analogous to that used for PAN-based carbon fiber.
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From page 62... ...
62 Process Outline Chemistry Generalized Process | Pitch Pu;tication ~·E ~Schematic ~ ' |MQsophase Formation , ~ , ~ | Spinning | Step A ~ ~ , At? ~ Pitch Precursor Fiber O | Stabilization | l / \ Step B / \ 1 ~ Carbonization ~ ~ Stabilization ~ \ ~ | Surface Treatment ~ ~(Graphit~Like Structure)
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From page 63... ...
In the area of pitch-based carbon fibers, the technical programs will be directed toward higher tensile strength products, fibers with improved compressive properties, improved routes to mesophase production, fibers with improved conductivity, and fibers that have improved processability. As indicated earlier, there is a substantial amount of interest in pitch-based fibers for their excellent conductivity, which offers opportunities in military, industrial electronic, and structural applications.
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From page 64... ...
If high molecular weight polymers such as polyphenylene can be produced commercially, they may replace PAN and possibly pitch as carbon fiber precursors and significantly alter the process economics. Supercritical extraction is typical of pitch-separation techniques which could significantly change the economics of pitch-based carbon fiber processes.
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From page 65... ...
~ The family of pitch and PAN-based carbon fibers encompasses a broad range of thermal and electrical conductivity. This set of conductivity properties is useful for a number of applications in which electromagnetic properties are at least as important as the mechanical properties of the reinforcing fiber.
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In Si-based ceramics SiO2 is the thermodynamically most stable form and the compound least desirable in the final ceramic structure. ~ Carbon fiber is normally produced from high molecular weight, mechanically sound, environmentally stable precursor fibers amenable to fiberhandling technology.
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All polymers are: - Highly branched (C containing) - Low molecular weight - Low aspect ratio - Difficult to characterize Fibers from Preceramic Polymers ~ Preceramic I Spinning r Spun ~ Curing ~ Cured ~ Pyrolysis ~ eramic ~ I Polymer I L Fiber | ~Fiber ~ I Fiber I Step A ~ _~D FIGURE 3.5 Production of Si Ceramic Fibers from Polymeric Precursors.
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68 Tensile Strength (M pa)
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~ Development of effective diffusion boundary coatings for ceramic fibers to block reactive gases from entering the fiber structure and exacerbating the existing flaw populations. The present limitation to increased use of Si-based ceramic fibers is their very high cost.
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OXIDE FIBERS Introduction Ceramic oxide fibers can be prepared by any of the processes identified as polymeric, polymer-modified solutions or dispersions, fiber-forming salt solutions, or sol-gel systems, the latter sometimes being used as an all-inclusive term for all of these when applied to the preparation of nonvitreous ceramic fibers by pyrolytic conversion processes.9 Most of these fibers have been fabricated by processes that can be categorized generally under "pyrolytic conversion of precursor fibers." Although the information usually provided may disclose the general type of processing by which these fibers are prepared, for example, polymeric or sol-gel (sol is a colloidal dispersion in a liquid medium) , details are generally not sufficient for their immediate duplication.
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From page 71... ...
and the metals having atomic numbers 23-28 inclusive. Polymers were prepared from aqueous solutions of concentrated carboxylate salts by heat treatment in a closed vessel at 60° to 90°C.
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From page 72... ...
Precursor fibers were then fired to temperatures between 600° and 1700°C for conversion to the oxide fibers. Spinnabl e Sal ~ Sol utions Oxide precursor fibers may be prepared by spinning solutions such as aqueous solutions of carboxylate salts (e.g., aluminum formoacetate)
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Mi cros true Lure D evel opmen ~ Pyrolysis of precursor fibers to form oxide fibers overlaps with the early stages of crystallization. After removal of fugitive components, sintering continues.
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FIGURE 3.10. SEM of PRD-166 Surface Copyright ~ by the American Ceramic Society.
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New fibers of selected compositions will be necessary to satisfy these needs. The controlled pyrolysis of precursor fibers has been shown to be a useful process for the fabrication of both oxide and nonoxide high-performance fiber-e for structural applications.
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From page 76... ...
For example, BN fiber is made from B2O3 fiber, and TiN fiber can be prepared by reacting BN fiber with a titanium chloride and hydrogen mixture. The versatility of this fiber-making approach lies in the numerous combinations of gaseous reactants and precursor fibers that can be used.
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From page 77... ...
These product forms made thorough characterization of the fiber properties possible, as follows: Mechanical properties: Typical fiber diameter: Tensile strength: Young's modulus: Elongation at break: atmosphere. 4 to 6 microns 345 to 862 MPa (50 to 125 ksi)
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From page 78... ...
Fine-diameter precursor fiber favors the conversion kinetics. The diameter of the starting carbon fibers is about 8 microns.
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From page 79... ...
It must be pointed out that this technology is still in its early stage of development, the examples reported by early researchers having merely laid the groundwork for the technology. Efforts along the following lines may be fruitful: ~ Experimental work on various combinations of precursor fibers with a variety of reactants directed at making fibers of interest.
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From page 80... ...
A method for making potentially low cost carbon fibers via carbon CVD onto a carbon substrate grown in-situ also is under development. This process is being pursued mainly in two countries - in the United States by General Motors Research Laboratory and Hyperion International and in Japan by several companies and universities.
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From page 81... ...
The crystallite size, on the order of 20 A is so small that it is considered amorphous. The average tensile strength of high-quality boron fiber is the statistical result of many individual fiber tests; a typical histogram depicting these results is shown in Figure 3.13.
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From page 82... ...
Ultimate Tensile Strongth, 1 o6 psi A.: 0.400 o.5oo 0.6s0 0.800 Ultimate Tensile Strength 1 o6 psi ~1 1 1 0.400 o.mo 0.6s0 0.780 1 1 e Loo, t! 1 ~ 2.76 3.45 4.5 5.4 Ultimate Tensile Strength, GPa FIGURE 3.13 Histogram of boron fiber tonsil strength.
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In general, a chemical vapor deposition (CVD) fiber being used to reinforce a particular metal structure will probably not be a monolithic multipurpose fiber, but will more likely be a specialized material having a surface composition specifically selected for the particular matrix.
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The driving force for development of this fiber has been the potential of lower manufacturing cost when compared to conventional carbon fiber manufacturing techniques. The use of these fibers is envisioned to be limited to secondary structural applications where discontinuous randomly oriented reinforcement can be employed.
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Single-Crystal Fibers One of the main deficiencies of currently available ceramic fibers is creep at high temperatures. Above ~1100°C the structure of polycrystalline A12O3 and SiC fibers changes, causing slippage along grain boundaries, which greatly impairs the physical properties of the fiber.36 Many of the mechanisms responsible for high-temperature creep, such as grain growth and slippage at grain boundaries, can be eliminated through the use of singlecrystal fibers.
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From page 86... ...
The individual fibers in this mass as prepared have been shown to exhibit mechanical properties comparable to those of medium-strength PAN-based carbon fibers .44 After high-temperature heat treatment, the fibers develop Young's modulus on the order of 600-800 GPa comparable to those of high-temperature, heat-treated, pitch-based carbon fibers. Some vapor-grown fibers when heat treated to intermediate temperatures have exhibited a benign sword-in-sheath mode of failure in which the central core of the filament pulls out of the outer CVD sheath.45 Fibers that have failed in this manner have been shown to recover a fairly large fraction of their modulus and load-carrying capability provided that the central core has not been completely removed from the outer sheath.
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From page 88... ...
The "relic" process, 2 wherein porous fugitive organic fibers are impregnated with inorganic solutions followed by drying and pyrolyzing so as to form ceramic fibers, should be recalled. Fibers made by this process are not generally considered useful for reinforcement of composites, but determined efforts to control the microstructure of the resultant fibers should not be automatically dismissed as an impossible task.
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From page 89... ...
89 Temperature Vapor Feed H2 CH4 VLS Liquid Catalyst Supersaturated with Si and C \/LS Solid Catalyst (e.g. Steel)
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From page 90... ...
Other methods include providing additional elements to the matrix that will either form precipitates that segregate at or react with the fiber surface during fabrication and in situ modification of the fiber surface during fiber manufacturing. In this section fiber coatings will be emphasized.
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From page 91... ...
For examples many plating solutions contain additional chemicals that modify or enhance the interaction of the depositing metal with the fiber surface; the scientific basis of understanding is not well developed. Electroplating is practiced commercially by American Cyanamid for coating Ni and, until recently, Cu onto carbon fibers.
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From page 92... ...
The major disadvantage of this process is the time required for the reactions to occur, which limits the throughput of the process. Chemical Vapor Deposi Lion Chemical vapor deposition (CVD)
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From page 93... ...
The major disadvantages of this coating method are that the coating thickness for a single pass is often too thin and thicker coatings put on using multiple passes often develop cracks at the interfaces between layers. Vacumn Deposi Lion 1 such a ~receramlc This category of fiber coatings includes sputtering, physical vapor deposition, e-beam evaporation, plasma-assisted COD, and ion-plating techniques.
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From page 94... ...
Considerations Involved in Scale-up of Fiber Processing The processing of bulk materials into fibers is a broad-based technology that is relatively well understood in principle. However, scale-up of fiber processing is more difficult than that of other materials because of the simultaneous heat, momentum, and mass transfers, combined with chemical reactions and unique, complex, flow-induced orientation effects.
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From page 95... ...
This process usually produces coatings that are 50 to 200 nm thick after the heat treatment stage. The process is capable of applying graded compositions by using multiple dipping stations, each having a slightly different composition.
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. Schematic of a continuous CVD fiber-coating line.
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The difficulties associated with control are exacerbated as the fiber forming process becomes more complicated, and the small-scale simulation of highly complex processes, such as the production of ceramic fiber from polymeric precursors, may be essentially impossible to evaluate on a very small (100-1000 g) scale.
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From page 98... ...
· The high cost of manufacturing remains a common problem from the standpoint of economics and utilization of many high-performance synthetic fibers. ~ Pyrolysis and chemical conversion of precursor fibers, CVD, and singlecrystal growth processes are promising routes to the fabrication of continuous high-performance synthetic inorganic fibers.
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From page 99... ...
D Hart, 'production of Continuous Ceramic Fibers," U.S.
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E Tressler, ''Strengths of Ceramic Fibers at Elevated Temperatures," J
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From page 101... ...
S Ban, "Chemical Vapor Deposition of Inorganic Thin Films," Thin Film Processes, Eds.
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From page 102... ...
S Chun, "The Deposition Characteristics and the Structural Natural Nature of the Deposit in the Chemical Vapor Deposition of Boron, " Thin Solid Films 131, p.
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