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Table of contents
- RT-Chemical technologies and composite materials
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- The Mesophase Concept in Composites | SpringerLink
The elastic moduli of fiber-reinforced materials. Journal of Applied Mechanics Vol. Elastic properties of fiber reinforced composite materials. The influence of random filament packing on the transverse stiffness of unidirectional composites. Journal of Composite Materials ; Vol. Environmental factors in composite materials. Sih P. Hilton, R. Badaliance, P. Schenberger, and G. Composite properties for e-glass fibers in a room temperature curable epoxy matrix.
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Composites Vol. The nature of petroleum and coal-tar pitches is such that their susceptibilities to oxidation vary widely. Although the above tests offer strong indications of the susceptibility of an impregnant to oxidation stabilization, it is not possible to establish an absolute criterion for stabilizability short of manufacturing sample composites, oxidizing the sample composites under conditions indicated by the above tests, and conducting the carbonization tests. Visual observations to prove the matrix does not exude from the composite may be supplemented by micrographic examination of the matrix microstructure.
The weight gain in oxidation, relative to the matrix content of the impregnated composite, is plotted as a function of the square root of time in minutes to permit the full range of exposure times to be given in a single plot. A thermogravimetric curve for mesophase pitch powder also prepared from Ashland A petroleum pitch is included to show that the matrix pitch oxidizes approximately as rapidly as the powdered pitch. The carbonization tests showed that all specimens oxidized for more than one day, or to a weight gain of more than eight percent, had been stabilized.
It is clearly evident that major improvements in carbon yield, relative to the unoxidized matrix, can be attained by using oxygen exposure times just sufficient to attain full stabilization. General guidelines by which the process parameters are applied are as follows. Increasing the oxygen content of the oxidizing atmosphere around the impregnated preform is effective in increasing the depth and rate of oxygen absorption.
Pure oxygen is recommended but may be substituted for, perhaps for economic reasons, by air. The optimum temperature for oxidation must represent tradeoffs between minimum processing time, favored by higher temperature, and maximum carbon yield, favored by lower temperature.
RT-Chemical technologies and composite materials
Greater depths of oxygen penetration in the solid impregnant can be attained at lower oxidation temperatures. The process of oxidation stabilization is particularly well suited to thin-walled 2D composites and thus opens this class of composites to pitch-based processing. Heavy-walled or massive composites may require the use of air or more dilute oxidizing atmospheres to avoid spontaneous combustion or thermal excursions in the depth of the composite.
This effect is of concern when the composite is massive and constructed of fiber of low thermal conductivity. The most significant potential of oxidation stabilization lies in opening the field of pitch-based composite fabrication to a wider range of processing concepts than could be applied as long as thermal methods were the only means available to try to fix a graphitizable matrix in place within the fiber preform.
Mesophase fluidity no longer appears as part of the problem of matrix bloating, but as a property that may be exploited for its potential in improving densification methods and producing favorable matrix microstructures. By incorporating oxidation stabilization, composite fabrication can be pursued by room-pressure methods, thus eliminating the cost of high-pressure autoclaves, their control systems, and the safety precautions essential to their use. By eliminating the need for autoclave systems, very large structural composites can be economically densified by pitch-based methods.
By eliminating the expulsion of matrix by pyrolysis gases, each cycle of densification is made more efficient and fewer cycles are required to reach desired density levels. Further time economies come from the fact that lengthy steps of low heating rate are unnecessary, in contrast to the conventional processing of preforms impregnated with either pitch-based or thermosetting resin matrices.
Finally, the process of oxidation stabilization can be applied to both isotropic pitch and mesophase, thus opening another microstructural variable to explore in optimizing the properties of the finished composites. The only essential requirements are that the matrix be susceptible to oxidation at temperatures where it remains solid, and that sufficient porosity forms by anisotropic shrinkage to give oxygen access throughout the composite body.
All rights reserved. A SumoBrain Solutions Company. Login Sign up. Search Expert Search Quick Search. Method for oxidation stabilization of pitch-based matrices for carbon-carbon composites. United States Patent An oxidation process for stabilization of pitch-based matrices in the fabrication of carbon-carbon composites.
The invention can also be practiced to increase the carbon yield upon carbonization. Sufficient access porosity must exist in the matrix to permit oxidation throughout the composite body; this is normally formed in a preform due to the mismatch in thermal expansivities of fiber and matrix. Also, the softening point of the pitch must be above the oxidation temperature; this need can be met for most mesophase pitches.
The process of oxidation stabilization can also be applied to isotropic pitches if these requirements are met, and to pitch derived from coal tar or other sources if the pitch is susceptible to oxidation. This invention may be practiced to reduce the time and cost of pitch-based composite fabrication, to eliminate the need for high-pressure carbonization, and thus to make pitch-based fabrication practical for large composite structures.
Sheaffer, Patrick M. Del Mar, CA. Click for automatic bibliography generation. What is claimed is: 1. A process for the fabrication of carbon fiber reinforced carbon matrix composites to a desired shape comprising the steps of: a.
The process as defined in claim 2 wherein the oxidation of pitch based precursor is carried out for a time whereas the dimensional change of the shaped composite and carbon yield of the precursor after carbonization are optimized, said time being approximately ten hours. The process as defined in claim 1 wherein the lattice-work is impregnated by a series of partial impregnations and each of said series is oxidized in sequence; and the carbonization of the matrix precursor is performed after the last of said series of partial impregnations is oxidized.
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The process as defined in claim 1 wherein exposure to oxygen is controlled to maximize the carbon yield in carbonization. The process of claim 1 wherein the time and temperature of exposure to oxygen is controlled to minimize the dimensional change of the shaped composite during carbonization.
Description of the Prior Art Although carbon-carbon composites now find a variety of applications, ranging, for example, from prosthetic implants to components for braking systems and heat exchangers, the major impetus for their further development continues to come from space and defense needs, where the high costs and lengthy processing times of conventional fabrication methods can be justified by unique capabilities of thermal and mechanical performance in the final composite product.
The Mesophase Concept in Composites | SpringerLink
Objects of the Invention The immediate and direct object of the present invention is to provide a means of fixing pitch-based matrices in place within fiber preforms so that carbonization processes may be applied in composite fabrication without the risk of matrix expulsion from the preform by pyrolysis generated gases. First Test Measure the heat liberated, in the temperature range below that of burning, of a sample of powdered impregnant impregnant immersed in oxygen and slowly heated e.
Second Test Using a thermal gravimetric analyzer measure the weight changes on heating powdered impregnant slowly 1 in oxygen and 2 in an inert gas. It will also normally be subjected to additional conventional processing steps, including final machining and treatment with antioxidant solutions. The present invention makes use of processing modules which are known in themselves.
The advantages provided by the present invention lie in the selection and ordering of known processing modules to improve the friction and wear performance of the C-C composite brake discs prepared in accordance with this invention as compared with standard pitch-infiltrated brake discs. The present invention likewise improves the economics of disc manufacture. Heat treatment is employed to modify the mechanical, thermal, and chemical properties of the carbon in the preform.
The effect of such a treatment on graphitizable materials is well known. Higher temperatures increase the degree of order in the material, as measured by such analytical techniques as X-ray diffraction or Raman spectroscopy. Higher temperatures also increase the thermal conductivity of the carbon in the products, as well as the elastic modulus.
The preform is heated under inert conditions to well above the melting point of the impregnating pitch. Then, the gas in the pores is removed by evacuating the preform. Finally, molten pitch is allowed to infiltrate the part, as the overall pressure is returned to one atmosphere or above. In the VPI process a volume of resin or pitch is melted in one vessel while the porous preforms are contained in a second vessel under vacuum.
The molten resin or pitch is transferred from vessel one into the porous preforms contained in the second vessel using a combination of vacuum and pressure. The VPI process typically employs resin and pitches which possess low to medium viscosity. Such pitches provide lower carbon yields than do mesophase pitches. Accordingly, at least one additional cycle of pitch infiltration of low or medium char-yield pitch with VPI or RTM processing is usually required to achieve a final density of 1. The carbonization process is generally well known to those skilled in the art.
This process may be performed, for instance, by burying the foam preforms in a bed of activated carbon, enclosed in a superalloy retort with a sand seal. Carbonization of the infiltrated pitch can be carried out either in a furnace, a hot isostatic press, an autoclave, or in a uniaxial hot press. The higher the pressure, the higher the carbon yield achieved, although the biggest gains in carbon yield are achieved at moderate pressures up to psi. Standard machining processes, well know to persons skilled in the art of manufacturing carbon-carbon composite brake discs, are used in the manufacture of the carbon-carbon composite friction discs provided by the present invention.
Between densification processing steps, the surfaces of the annular discs are ground down to expose porosity in the surfaces. Once the final density is achieved, the annular discs are ground to their final thickness using standard grinding equipment to provide parallel flat surfaces, and then the inside diameter and outside diameter regions are machined, typically using a CNC computer numerical control Mill to provide the final brake disc geometry, including such features as rivet holes and drive lugs.
When the hydrocarbon gas mixture flows around and through the porous structures, a complex set of dehydrogenation, condensation, and polymerization reactions occur, thereby depositing the carbon atoms within the interior and onto the surface of the porous structures. Over time, as more and more of the carbon atoms are deposited onto the structures, the porous structures become more dense.
This process is sometimes referred to as densification, because the open spaces in the porous structures are eventually filled with a carbon matrix until generally solid carbon parts are formed.