Patent Publication Number: US-2015069646-A1

Title: Carbon-carbon composites formed using pyrolysis oil and methods of forming the same

Description:
TECHNICAL FIELD 
     The present disclosure describes carbon-carbon composite materials and techniques for forming carbon-carbon composite materials. 
     BACKGROUND 
     Some carbon-carbon composite bodies, such as some carbon-carbon composite brake disks that are used in the aerospace industry, may be manufactured from porous preforms. The porous preforms may be densified using one of several processes, for example, chemical vapor deposition/chemical vapor infiltration (CVD/CVI), vacuum/pressure infiltration (VPI), high pressure impregnation/carbonization (PIC), or resin transfer molding (RTM), which may introduce carbon into the porous preform. 
     SUMMARY 
     The present disclosure describes carbon-carbon composite materials and techniques for forming carbon-carbon composite materials from porous preforms. In some examples, a technique includes infusing pyrolysis oil into a porous preform and polymerizing at least some components of the pyrolysis oil to form a phenolic resin. The pyrolysis oil may include a phenolic compound and at least one of an aldehyde or ketone compound, which may be polymerized to form the material including the phenolic resin. Polymerization of the phenolic compound and the at least one of the aldehyde or ketone may occur by heating the pyrolysis oil at relatively low temperatures over relatively short time periods, without the need for additional reagents, catalysts or special atmospheres. Polymerization of the at least some components of the pyrolysis oil may form the phenolic resin, which may have increased viscosity compared to pyrolysis oil, and may substantially prevent (e.g., prevent or nearly prevent) leaching, volatilization, and foaming before or during later processing of the infused preform, such as pyrolyzation of the material including the phenolic resin. 
     In some examples, the techniques described herein may include additional or alternative steps. For example, before infusion of pyrolysis oil or after pyrolyzation of the polymerized pyrolysis oil, a porous preform or partially densified preform may be infiltrated with a densifying agent using processes such as RTM, VPI, CVD/CVI or PIC. The densifying agent may include, for instance, a high carbon yielding, high viscosity resin or pitch, or gaseous carbon. In some examples, a technique may additionally or alternatively include infusing a porous preform with a mixture of pyrolysis oil and, for example, pitch or resin. In some examples, pyrolysis oil may be treated, prior to being infused in the porous preform, to reduce the water, ash, metal and/or metal ion content in the pyrolysis oil. 
     In one example, the disclosure describes a method including infusing a pyrolysis oil into a porous preform. The method also may include polymerizing at least some components of the pyrolysis oil infused in the porous preform to convert the at least some components of the pyrolysis oil to a material including a phenolic resin. Additionally, the method may include pyrolyzing the phenolic resin to convert the phenolic resin to carbon and form a partially densified preform. 
     In another example, the disclosure describes an article comprising a porous preform including pores, and a pyrolysis oil is disposed in at least some of the pores of the porous preform. The pyrolysis oil may include a phenolic compound and at least one of an aldehyde or a ketone compound. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flow diagram illustrating an example technique for forming a partially densified preform. 
         FIG. 2  is a perspective view of an example porous preform that may be infused with pyrolysis oil. 
         FIG. 3  is a lateral cross-sectional view of an example porous preform. 
         FIG. 4  is a lateral cross-sectional view of an example porous preform infused with pyrolysis oil. 
         FIG. 5  is a lateral cross-sectional view of an example porous preform after polymerization of pyrolysis oil. 
         FIG. 6  is a chart showing viscosity of an example pyrolysis oil as a function of temperature and time. 
         FIG. 7  is a lateral cross-sectional view of an example partially densified preform, after pyrolyzation of the polymerized pyrolysis oil. 
         FIG. 8  is a flow diagram illustrating an example technique for forming a carbon-carbon composite body from a porous preform, 
         FIG. 9  is a lateral cross-sectional view of an example carbon-carbon composite body, after densification. 
         FIG. 10  is a chart showing an example infrared spectroscopy absorbance spectrum of an example material including a phenolic resin and an example infrared spectroscopy absorbance spectrum of a reference sample of cresol resol. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes composite materials and techniques for forming composite materials from porous preforms. Example composite materials include carbon-carbon composite materials, such as carbon-carbon composite brake discs. In some examples, techniques described herein include infiltrating a porous preform (e.g., a porous carbon preform or a porous preform including a carbon precursor material) with pyrolysis oil. When used to infiltrate the porous preform, the pyrolysis oil may be in liquid form with a relatively low viscosity, such that the pyrolysis oil may infiltrate pores of the porous preform. After the infiltration, the infiltrated preform may be heated at a temperature that facilitates polymerization of at least some components of the pyrolysis oil. The polymerization may form a material with an increased viscosity compared to the pyrolysis oil. 
     In some examples, the polymerization of at least some components of the pyrolysis oil takes place at relatively low temperatures and pressures and over relatively short time periods (without an extended stabilization period), and may not utilize any additional reagents. For example, the polymerization may be performed at a temperature between about 80° C. and about 300° C., at standard atmospheric pressure, and may occur over a time period of less than about 48 hours. This may be a lower temperature and a shorter time period than used when forming carbon in a porous preform using CVD/CVI, which, in some examples, may utilize high temperatures (e.g., above 800° C.), vacuum, and/or multiple cycles of up to 30 days each, to achieve desirable densities. Utilization of CVD/CVI processes also typically requires large capital investments in the chambers in which the CVD/CVI is performed. 
     In some examples, the polymerized components of the pyrolysis oil include a phenolic resin. As noted, the polymerized components of the pyrolysis oil may remain disposed substantially within the preform until a high temperature carbonization step converts at least the polymerized components of the pyrolysis oil to carbon. In some examples, after carbonization of at least the polymerized components of the pyrolysis oil, the partially densified preform may be infused with pyrolysis oil, components of the pyrolysis oil polymerized, and at least the polymerized components of the pyrolysis oil carbonized to further increase the density of the porous preform. Additionally or alternatively, in some examples, one or more resins or pitches may be mixed with the pyrolysis oil prior to infusion of the pyrolysis oil into the porous preform. 
       FIG. 1  is a flow chart illustrating an example technique for forming a partially densified preform. In some examples, the partially densified preform may subsequently be used to form a carbon-carbon composite material, such as a carbon-carbon composite brake disk. The technique of  FIG. 1  will be described with reference to the conceptual diagrams of  FIGS. 2-5  and  7 , which show different views of an example porous preform at different steps during the technique of  FIG. 1  and a partially densified preform resulting from the technique of  FIG. 1 . Although an example porous preform  20  is depicted in  FIGS. 2 and 3 , the technique of  FIG. 1  can be used with different preforms of other types (e.g., shapes or material compositions), and can be used to form partially densified preforms having different shapes or material compositions. 
     The technique of  FIG. 1  includes infusing (e.g., injecting) a pyrolysis oil into a porous preform ( 10 ).  FIG. 2  is a perspective view of an example porous preform  20  that may be used in the technique of  FIG. 1 . Examples of preforms that may be used as porous preform  20  include, but are not limited to: a fibrous preform, such as a woven fiber preform, a nonwoven fiber preform, a chopped-fiber and binder preform, a binder-treated random fiber preform, a carbon fiber preform, a ceramic fiber preform, a foam preform, a porous carbon body preform, or a porous ceramic body preform. In some examples, porous preform  20  includes a plurality of mechanically bound layers, which can be, for example, a plurality of fibrous layers, such as a plurality of woven or nonwoven fabric layers, connected together, e.g., bound by a binder, such as a resin binder, or via needle-punching of the plurality of layers. Alternatively, porous preform  20  does not include predefined layers, but, rather, can be formed from a bundle of fibers that are mechanically bound together, e.g., via needling. A combination of any of the aforementioned types of performs can be used. 
     In some examples, porous perform  20  is formed using carbon fibers. In others, porous preform  20  may be formed using a carbon fiber precursor material, such as polyacrylonitrile (PAN) fibers, which are subsequently pyrolyzed to form carbon fibers. In some examples, the carbon fiber precursor material may be pyrolyzed before infusing the pyrolysis oil into porous preform  20  ( 10 ). In other examples, the carbon fiber precursor material may not be pyrolyzed before infusing the pyrolysis oil into porous preform  20  ( 10 ), and the carbon fiber precursor may be converted to carbon fiber when the material including a phenolic resin (formed from polymerizing components of the pyrolysis oil) is pyrolyzed to carbon ( 14 ). 
     In some examples, as shown in  FIG. 2 , porous preform  20  may have a generally disc-shaped geometry so that a carbon-carbon composite material formed from porous preform  20  may be generally disc-shaped, e.g., for use as a brake disc. In other examples, porous preform  20  may define a different geometry. For example, the geometry of porous preform  20  may be similar to or substantially the same as the desired geometry for the finished part formed by the carbon-carbon composite material. 
     Porous preform  20  comprises a disc-shaped porous body  22  with a central bore  24  extending through an axial thickness of disc-shaped porous body  22 . In one example, porous preform  20  has an inner diameter ID at bore  24 , an outer diameter OD at an outer edge  26 , and an axial thickness T, which is measured in a direction substantially orthogonal to the direction in which inner diameter ID and outer diameter OD are measured. 
       FIG. 3  is a conceptual diagram illustrating a lateral cross-sectional view of an example of porous preform  20  shown in  FIG. 2 , which is taken along a diameter of porous preform  20 . Porous body  22  of porous preform  20  may include, for example, a plurality of fibers  28  that define a plurality of pores  30  within porous body  22 . The porosity of porous body  22  extends substantially throughout the thickness T of porous body  22 . As described above, fibers  28  may be bound together by a binder, fibers  28  may be formed into a plurality of fibrous layers (not shown) that are bound or needle-punched together, or fibers  28  may be mechanically joined, e.g., using needle-punching, without previously being formed into distinct fibrous layers. Fibers  28  and pores  30  are not necessarily shown to scale, but rather are shown conceptually in order to illustrate aspects of the present disclosure. 
     As described above, the technique of  FIG. 1  includes infusing a pyrolysis oil into porous preform  20  ( 10 ). In some examples, the pyrolysis oil of this disclosure includes low-viscosity, complex organic liquid materials. Pyrolysis oil may be free-flowing when fresh, and, in some examples, may be infused into porous preform  20  ( 10 ) without heating. In other examples, aged pyrolysis oil with increased viscosity may be heated to aid infusion into porous preform  20  ( 10 ). Pyrolysis oil may be produced by pyrolysis of biomass material. For example, pyrolysis oil may be formed by pyrolyzing woody biomass, such as hardwood, softwood, angiospermae trees, gymnospermae trees, hardwood bark, or softwood bark. As other examples, pyrolysis oil may be formed by pyrolyzing other biomass, such as corn fiber, bagasse, other lignocellulosic feedstock, straw, nuts, seeds, algae, miscanthus, switchgrass, or sorghum. Pyrolysis oil may also be produced by the pyrolysis of cellulosic residuals from the forestry, paper and pulp, construction and demolition, and agricultural sectors, such as bark, sawdust, lignin material, wood, waste, or straw. 
     Pyrolysis oil may include a mixture of components (e.g., organic liquid components), including at least one phenolic compound and at least one aldehyde and/or ketone. In some examples, pyrolysis oil includes a mixture of phenolic, aldehyde, and ketone compounds. The phenolic compound may include, for example, phenol or a polyphenol, and, in some examples, may include one or more substituents bonded to the non-hydroxyl group positions of the aromatic hydrocarbon. 
     In some examples, the organic liquid materials may be composed of organic acids (including phenols) and other oxygenates (such as aldehydes and/or ketones), traces of alkalis, and up to 25% water. For example, pyrolysis oil may include a complex mixture of water, guaiacols, catecols, syringols, vanillins, furancarboxaldehydes, isoeugenol, pyrones, acetic acid, formic acid, and other carboxylic acids. Pyrolysis oil also may contain other compounds, including hydroxyaldehydes, hydroxyketones, sugars, dehydrosugars, and phenolic compounds. In some examples, pyrolysis oil may also include oligomeric species, which may be derived from lignin or cellulose. Pyrolysis oil also may be referred to as, for example, py oil, pyrolysis liquid, biofuel, bio-oil, bio-crude oil (BCO), wood oil, wood distillates, wood liquids, liquid wood, liquid smoke, and pyroligneous acid. 
     The process of pyrolyzing biomass may include, for example, converting organics in the biomass to solids, liquids, vapors, aerosols, and/or gases by heating biomass in the absence or near absence of oxygen, then rapidly cooling and condensing the vapor, aerosol and/or gas which produces the pyrolysis oil. Pyrolysis oil may be formed using a variety of pyrolysis techniques. The residence time of the pyrolysis, the heating rate, and temperature to which the biomass is heated may vary depending on the particular process. Additionally, the precise mixture of components in the pyrolysis oil may at least partially depend upon the residence the heating rate, and temperature. Example pyrolysis techniques include conventional pyrolysis, which may include a residence time of between about 5 and about 30 minutes, a low heating rate, and a temperature of about 600° C.; fast pyrolysis, which may include a residence time on the order of seconds, a very high heating rate, and a temperature of about 650° C.; flash-liquid pyrolysis, which may include a residence time of less than about 1 second, a high heating rate, and a temperature of less than about 650° C.; flash-gas pyrolysis, which may include a residence time of less than about 1 second, a high heating rate, and a temperature of less than about 650° C.; ultra pyrolysis, which may include a residence time of less than about 0.5 second, a very high heating rate, and a temperature of about 1000° C.; vacuum pyrolysis, which may include a residence time of between about 2 seconds and about 30 seconds, a medium heating rate, and a temperature of about 400° C.; hydro-pyrolysis, which may include a residence time of less than about 10 seconds, a high heating rate, and a temperature of less than about 500° C.; and methano-pyrolysis, which may include a residence time of less than about 10 seconds, a high heating rate, and a temperature of greater than about 700° C. Further details regarding some types of pyrolysis oil may be found in Mohan; Pittman, Jr.; Steele. Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review,  Energy  &amp;  Fuels , Vol. 20, No. 3 (American Chemical Society 2006), the entire content of which is incorporated herein by reference. In some examples, the concentration of any water, ash, metals and/or metal ions in the pyrolysis oil, for example alkali metals, alkaline earth metals or transition metals, can be reduced prior to infusion of the pyrolysis oil in the porous preform  20  ( 10 ), as described with reference to the technique of  FIG. 8 . 
     In some examples, infusing the pyrolysis oil precursor into porous preform  20  ( 10 ) includes infusing the pyrolysis oil at least to a predetermined depth within porous preform  20 . As used herein, “predetermined depth” may refer to an absolute or relative depth from outer edges of porous body  22 , such as from outer edge  26 , from inner edge  32  (e.g., an inner edge  32  that defines bore  24 ), from upper edge  34 , and from lower edge  36 . In some examples, infusing the pyrolysis oil into porous preform  20  ( 10 ) includes infusing the pyrolysis oil to a relative depth of at least about 15% from each of the edges  26 ,  32 ,  34 , and  36  (e.g., at least about 15% of the difference between inner diameter ID and outer diameter OD infused from each of outer edge  26  and inner edge  32 , and at least about 15% of thickness T infused from each of upper edge  34  and lower edge  36 ). In other examples, infusing the pyrolysis oil into porous preform  20  ( 10 ) includes infusing the pyrolysis oil to a relative depth of at least about 25% from edges  26 ,  32 ,  34 ,  36 , such as at least about 30% from edges  26 ,  32 ,  34 ,  36 ; or at least about 40% from edges  26 ,  32 ,  34 ,  36 ; or about at least about 45% from edges  26 ,  32 ,  34 ,  36 . 
     In some examples, infusing the pyrolysis oil into porous preform  20  ( 10 ) may include infusing the pyrolysis oil substantially throughout the entire porous preform body  22  so that an inner portion  38  of porous preform body  22  is substantially filled with the pyrolysis oil. The term “inner portion” may refer to a generally geometrically-centered region within body  22 , wherein the inner portion has a volume that is a predetermined percentage of the total volume of the preform, such as between about 15% and about 50% of the volume of the preform, such as between about 20% and about 30% of the volume of the preform. 
     As an example, for a generally annular preform, such as the example preform  20  shown in  FIG. 2 , a geometric center of preform body  22  will generally be an annulus  25  ( FIG. 2 ) that is generally axially centered within porous preform body  22 , e.g., generally centered within thickness T, and that is generally radially centered within porous preform body  22 , e.g., generally centered between inner diameter ID at inner edge  32  and outer diameter OD at outer edge  26 . By way of example, an “inner portion” may be defined as a predetermined percentage of the radius of porous preform body  22  on either side of center annulus  25 , such as at least about 50% of the radius (e.g., at least about 25% of the radius of center annulus  25  on either side of center annulus  25 ), for example at least about 30% of the radius (e.g., at least about 15% of the radius on either side of center annulus  25 ), such as at least about 20% of the radius (e.g., at least about 10% of the radius on either side of center annulus  25 ), for example at least about 15% of the radius (e.g., at least about 7.5% of the radius on either side of center annulus  25 ). In another example, preform body  22  comprises a plurality of fibrous layers bonded, laminated, or needle-punched together, and inner portion  38  may comprise a predetermined number of inner layers of the plurality of layers on either side of a generally axially centered layer. In one example, the precursor is infused substantially throughout the entire porosity of porous body  22 . 
     The pyrolysis oil may be infused into porous preform body  22  using any suitable technique. In some examples, the pyrolysis oil is infused into porous preform body  22  using vacuum pressure infiltration (VPI). VPI may include immersing porous preform body  22  in the pyrolysis oil and subjecting porous preform  20  and the pyrolysis oil to a vacuum, such that the liquid precursor wicks into porous preform body  22 . In one example, vacuum infiltration may be performed at room temperature (e.g., between about 20° C. (about 68° F.) and about 25° C. (about 77° F.)). In another example, porous preform body  22  may be heated, e.g., up to a temperature of about 40° C. (about 105° F.), to aid infusion of the pyrolysis oil into pores  30  within inner portion  38 , or in some examples, throughout pores  30  of substantially the entire porous preform body  22 . 
     In other examples, the pyrolysis oil is infused into porous preform body  22 , using resin transfer molding (RTM), in which pyrolysis oil is infused into porous preform body  22  through application of heat and pressure. In some examples, porous preform body  22  may be rigidized by one or more CVD/CVI cycles prior to infusion of pyrolysis oil using RTM. 
     In some examples, the pyrolysis oil includes a material or materials with a viscosity that is sufficiently low so that the pyrolysis oil can be infused substantially throughout the entire volume of porous preform body  22 , e.g., so that the pyrolysis oil is infused at least to a predetermined depth within preform  20  or so that the pyrolysis oil is infused substantially into inner portion  38  of body  22 . The pyrolysis oil may be, for example, relatively non-viscous. In one example, the viscosity of the pyrolysis oil is between about 4 centipoise and about 100 centipoise, for example between about 4 centipoise and about 50 centipoise, such as about 4.5 centipoise. 
     After infiltration, the pyrolysis oil is disposed in at least some of pores  30 .  FIG. 4  is a lateral cross-sectional view of an example infiltrated porous preform  50  infused with pyrolysis oil. As shown in  FIG. 4 , in some examples, pores  30  ( FIG. 3 ) substantially throughout (e.g., throughout or nearly throughout) the volume of porous preform body  22  are at least partially filled with pyrolysis oil  52 . In some examples, substantially all (e.g., all or nearly all) pores  30  within porous preform body  22  may be substantially filled (e.g., filled or nearly filled) with pyrolysis oil  52 . In other examples, at least some of pores  30  may be at least partially filled with pyrolysis oil  52 , and at least some of pores  30  may remain unfilled with pyrolysis oil. 
     After infusing pyrolysis oil into porous preform  20  to form infiltrated porous preform  50  ( 10 ), infiltrated porous preform  50  may be heated at a temperature that facilitates polymerization of one or more components of the pyrolysis oil  52  ( 12 ). For example, the at least one phenolic compound and the at least one aldehyde and/or ketone compound may react to form a phenolic resin. Reactions 1 and 2 illustrate two example types of reactions between phenolic compounds and aldehyde/ketone compounds to form phenolic resins. 
     
       
         
         
             
             
         
       
     
     As shown in Reaction 1, a phenol and a ketone (where R 1  and R 2  are hydrocarbons) or aldehyde (where at least one of R 1  or R 2  is hydrogen) may react to form a linear phenolic resin. Although not shown in Reaction 1, in some examples, Reaction 1 alternatively or additionally may form a crosslinked phenolic resin, in which additional sites on the phenol ring react with the aldehyde and/or ketone. As another example, Reaction 2 illustrates a set of reactions in which a substituted phenol and a ketone or aldehyde react to form a crosslinked phenolic resin. Potential crosslinking sites are indicated by the carbon groups extending from the phenol rings. Reactions of other phenolic compounds and an aldehyde and/or ketone may be similar to or substantially the same as the reactions shown in Reactions 1 and 2, depending on the particular chemical structure of the phenolic compound and aldehyde and/or ketone, 
       FIG. 5  is a lateral cross-sectional view of an example porous preform after polymerization of the one or more components of pyrolysis oil  52  ( 12 ). As shown in  FIG. 5 , in some examples, pores  30  ( FIG. 3 ) substantially throughout (e.g., throughout or nearly throughout) the volume of porous preform body  22  are at least partially filled with phenolic resin  62 . In some examples, substantially all (e.g., all or nearly all) pores  30  within porous preform body  22  is substantially filled (e.g., filled or nearly filled) with phenolic resin  62 . In other examples, at least some of pores  30  is at least partially filled with phenolic resin  62 , and at least some of pores  30  may remain unfilled with phenolic resin  62 . In some examples, the pores  30  at least partially filled with phenolic resin  62  may substantially correspond to (e.g., correspond to or nearly correspond to) the pores  30  in which pyrolysis oil  52  was disposed. 
     As the phenolic compound and aldehyde and/or ketone react and form a phenolic resin  62 , the viscosity of the material in pores  30  may increase, e.g., become more viscous than pyrolysis oil  52 ,  FIG. 6  is a chart showing viscosity of an example pyrolysis oil as a function of temperature and time. The chart displays trends of increased viscosity of pyrolysis oil (measured in millipascal-seconds, or centipoise) with both the passage of time (measured in days) and increased temperature (measured in degrees Celsius). As shown in  FIG. 6 , separate samples of example pyrolysis oil were maintained at temperatures of 20° C., 35° C., 50° C., and 80° C., respectively, for periods of days. The respective representative viscosities of the samples of pyrolysis oil were measured at distinct points in time at a temperature of 50° C. The change in viscosity of the samples of example pyrolysis oil as a function of temperature and time is set forth the chart, which was obtained from Oasmaa; Peacoeke, A Guide to Physical Property Characterisation of Biomass-Derived Fast Pyrolysis Liquids, VTT Publications, Vol. 450 (Espoo 2001). The increased viscosity of the phenolic resin  62  compared to the pyrolysis oil  52  may reduce the tendency of material to leak, foam, and/or volatilize before or during subsequent handling and processing of the preform. 
     In some examples, polymerization of at least some components of pyrolysis oil  52  may occur without the introduction of additional reagents, catalysts, or special atmospheres (e.g., vacuum or an inert atmosphere), allowing for a more efficient and economical cost of production compared to methods that require a special atmosphere and/or additional reagents or catalysts. In some examples, polymerization of pyrolysis oil  52  may utilize only heating of infiltrated porous preform  50 , and may utilize relatively low temperatures. For example, infiltrated porous preform  50  may be heated at a temperature of between about 80° C. and about 300° C., depending, for example, on the composition of the pyrolysis oil, such as between about 150° C. and about 200° C., to form phenolic resin  62 . In some examples, infiltrated porous preform  50  may be heated at the temperature for less than about 48 hours, such as less between about 1 hour and about 48 hours, less than about 12 hours, or about 12 hours, to facilitate polymerization of the phenolic compound and the aldehyde and/or ketone compound. Further, in some cases, heating infiltrated porous preform  50  at a temperature within these ranges includes heating the infiltrated porous preform to the temperature at which it is being heated (i.e., in some examples, infiltrated porous preform  50  may reach a substantially similar temperature as the temperature of its surroundings during the heating process). 
     As pyrolysis oil  52  may include other components in addition to the phenolic compound and the aldehyde and/or ketone, the other components may also undergo reactions and/or may vaporize and be removed from infiltrated porous preform  50  during the heating process. For example, any aqueous phase may boil off. Because at least some of the other components in pyrolysis oil  52  may remain in pores  30  after heating of the infiltrated porous preform  50  ( 12 ), pores  30  may contain a material including a phenolic resin  62  after the heating step ( 12 ). In some examples, polymerization of pyrolysis oil  52  in infiltrated porous preform  50  prior to pyrolyzation may result in higher densities and yields of the final pyrolyzed product, as compared to pyrolyzation of pyrolysis oil  52  without prior polymerization to form a material including a phenolic resin. 
     The technique of  FIG. 1  further includes pyrolyzing phenolic resin  62  to form carbon ( 14 ). To pyrolyze phenolic resin  62 , infiltrated porous preform  50  including phenolic resin  62  may be heated at a temperature between about 200° C. and about 800° C. to form a partially densified preform  70  ( 14 ). The temperature at which infiltrated porous preform  50  including phenolic resin  62  is heated may depend upon, for example, the composition of the phenolic resin. In some examples, infiltrated porous preform  50  including phenolic resin  62  may be heated at a temperature between about 500° C. and about 800° C., such as about 700° C. to form a partially densified preform  70  ( 14 ). Further, in some examples, heating infiltrated porous preform  50  including phenolic resin  62  at a temperature within these ranges includes heating the infiltrated porous preform  50  including phenolic resin  62  to the temperature at which it is being heated (i.e., in some examples, infiltrated porous preform  50  including phenolic resin  62  may reach a substantially similar temperature as the temperature of its surroundings during the heating process). 
     In some examples, the pyrolyzation is performed under partial vacuum conditions or an inert atmosphere.  FIG. 7  is a lateral cross-sectional view of an example partially densified preform  70 , after polymerization of pyrolysis oil  52  ( FIG. 4 ) and pyrolyzation of phenolic resin  62  ( FIG. 5 ). As shown in  FIG. 7 , carbon  72  may be disposed in pores  30  ( FIG. 3 ) in which the material including the phenolic resin was disposed prior to pyrolyzation of the material including the phenolic resin. Hence, in some examples, carbon  72  may be disposed substantially throughout or nearly throughout) the volume of partially densified preform  50 . In other examples, carbon  72  may be disposed at least partially throughout the volume of partially densified preform  70 . 
     Although not shown in  FIG. 1 , in some examples, partially densified preform  70  may be cured at a temperature between about 1200° C. and about 2450° C. following pyrolyzation. In some examples, curing partially densified preform  70  at a temperature within this range includes heating the partially densified preform to the temperature at which it is being heated (i.e., in some examples, partially densified preform  70  may reach a substantially similar temperature as the temperature of its surroundings during the curing process). In some examples, the curing step substantially prevents (e.g., prevents or nearly prevents) any structural change or chemical reaction within partially densified preform  70  at temperatures at least as high as this range after the curing step. 
     In some examples, carbon  72  formed by pyrolyzing phenolic resin  62  is porous. In some instances, after pyrolyzing phenolic resin  62  to form carbon  72 , the technique of  FIG. 1  may be repeated, and pyrolysis oil  52  may be infused in at least some of the pores of a porous carbon  72  ( 10 ), components of pyrolysis oil  52  may be polymerized to form phenolic resin  62 , ( 12 ), and phenolic resin  62  may be pyrolyzed to form additional carbon  72 , further increasing the density of partially densified preform  70 . 
     In other examples, the carbon  72  formed by pyrolyzing phenolic resin  62  is substantially non-porous. In some examples, the porosity of the carbon  72  may be affected by the pyrolyzation temperature, and a higher pyrolyzation temperature may result in a more porous carbon  72 . Hence, a porous preform may be at least partially densified using infiltrating of the preform with pyrolysis oil, polymerization of at least some components of the pyrolysis oil to form a phenolic resin, and pyrolyzation of the phenolic resin to form carbon in pores of the porous preform. 
       FIG. 8  is a flow diagram illustrating an example technique for forming a carbon-carbon composite body from a porous preform. The technique of  FIG. 8  incorporates the steps of the technique of  FIG. 1  for forming a partially densified porous preform, and includes additional, optional processing steps that may be used to form a carbon-carbon composite body or material, such as a carbon-carbon composite brake disk. The steps of  FIG. 8  can be performed in different orders than shown in  FIG. 8 , different combinations, and/or can be optional. For example, the rigidization ( 80 ) and densification ( 90 ) steps may or may not be performed in the specified order, or at all. 
     In some examples, in the technique of  FIG. 8  includes, prior to infusing porous preform  20  with pyrolysis oil ( 84 ), rigidizing porous preform  20  using CVD/CVI ( 80 ). CVD/CVI may rigidize porous preform  20  by depositing a thin layer of carbon on at least some of fibers  28  of porous preform  20 . 
     In some examples, the technique shown in  FIG. 8  may optionally include treating a pyrolysis oil to reduce metal or metal ion content in the pyrolysis oil ( 82 ), in addition to or as an alternative to rigidizing porous preform  20  ( 80 ). In some examples, pyrolysis oil  52  may contain metals or metal ions, such as alkali metals, alkaline earth metals, or transition metals. The amount of metals or metal ions in pyrolysis oil  52  may depend upon the feedstock from which pyrolysis oil  52  was formed and the technique used to form pyrolysis oil  52 . In some examples, the total metal concentration in biomass-derived pyrolysis oil may range from, for example, about 0.02 weight percent to about 0.5 weight percent, but may be present in larger or smaller amounts. 
     In some examples, pyrolysis oil  52  may be treated prior to infusing pyrolysis oil  52  into porous preform  20  to reduce the concentrations of metals or metal ions in pyrolysis oil  52  ( 82 ). One technique of reducing metals in pyrolysis oil  52  includes contacting the metal- or metal ion-containing pyrolysis oil with an acidic ion-exchange resin having sulfonic acid groups to produce low metal pyrolysis oil and spent ion-exchange resin. The low metal pyrolysis oil in this instance may be removed from the spent ion-exchange resin. The spent acidic ion-exchange resin may also be washed with a solvent, such as a solvent selected from the group consisting of methanol, ethanol, acetone, and combinations thereof, to remove at least a portion of residual low metal pyrolysis oil from the spent acidic ion-exchange resin. In some examples, this technique may remove substantially all (e.g., all or nearly all) alkali metals (for example, sodium, potassium, and cesium) and alkaline earth metals (magnesium, calcium and strontium) from pyrolysis oil. Transition metals (for example, Fe, Ni, and Mn) and other metals also may be at least partially removed from the pyrolysis oil. Further details regarding treatment of pyrolysis oil to reduce the metal or metal ion content of pyrolysis oil may be found at U.S. Patent Application Publication No. 2011/0146135 by Brandvold, which is entitled, “LOW METAL BIOMASS-DERIVED PYROLYSIS OILS AND PROCESSES FOR PRODUCING THE SAME,” the entire content of which is incorporated herein by reference. 
     Alternatively or additionally, in some examples, pyrolysis oil  52  may be treated prior to infusing pyrolysis oil  52  into porous preform  20  to reduce the concentrations of water or ash in the pyrolysis oil. An example technique of removing or reducing the concentration of water from pyrolysis oil  52  includes vacuum drying the pyrolysis oil at temperatures below 100° C., for example, between about 40° C. and about 60° C., or at about 40° C. Pyrolysis oil  52  may alternatively or additionally be treated for removal or reduction of ash content prior to infusion into porous preform  20  by, for example, filtering out any ash by passing the pyrolysis oil through a metal or ceramic filter having pores of a size sufficient to remove or reduce the concentration of the ash content. 
     The technique of  FIG. 8 , like that of  FIG. 1 , includes infusing (e.g., injecting) a pyrolysis oil into porous preform  20  ( 84 ), heating infiltrated porous preform  50  at a temperature that facilitates polymerization of one or more components of the pyrolysis oil ( 86 ), and pyrolyzing the phenolic resin to form carbon ( 88 ). Details regarding these steps are described above with reference to steps ( 10 ), ( 12 ), and ( 14 ), respectively, of  FIG. 1 . 
     Additionally and optionally, the technique of  FIG. 8  may also include further densifying the partially densified preform to form a carbon-carbon composite body  100  ( 90 ).  FIG. 9  is a lateral cross-sectional view of an example carbon-carbon composite body  90 , after densification. Densifying the porous preform or partially densified preform ( 90 ) may include infiltrating one or more densifying agents  102  into pores of carbon formed by pyrolyzing the phenolic resin  72  ( 90 ). The infiltration may be accomplished using at least one or more of the processes as described herein, for example, CVD/CVI, VPI, PIC, or RTM, which may introduce carbon, additional carbon, or carbon precursors into the partially densified preform  70  ( FIG. 7 ). The densifying agent  102  may include at least one of gaseous carbon, a high carbon yielding, high viscosity carbon resin or pitch comprising at least one of a derivative of coal tar precursor, a derivative of a petroleum precursor, a derivative of a synthetic pitch precursor, a synthetic pitch, a coal tar pitch, a petroleum pitch, a mesophase pitch, or a high char-yield thermoset resin. Such materials when in the form of carbon precursors may be pyrolyzed to form carbon. In some examples, carbon-carbon composite body  100  may comprise a generally annular shaped carbon-carbon composite brake disk, although other shapes and applications may be utilized. 
     Example 
       FIG. 10  is a chart  110  showing an example infrared spectroscopy absorbance spectrum of an example material including a phenolic resin  112  and an example infrared (IR) spectroscopy absorbance spectrum of a reference sample of cresol resol  114 . As shown in  FIG. 10 , the units of the x-axis of the chart are wavenumbers, while the y-axis shows absorbance in generic units. The example material including a phenolic resin was a sample of polymerized pyrolysis oil from a reactor processing pyrolysis oil at an elevated temperature (about 250° C.). The reference sample of cresol resol was a sample of HPX #813, cresol resol (92-94% solid resin). Spectra  112  and  114  were taken using an IR spectrometer. The example material including the phenolic resin was formed by polymerization of a pyrolysis oil produced from sawdust of white oak from sawmill cutting. As shown in  FIG. 10 , the spectrum of the example material including a phenolic resin  112  is consistent with the spectrum of a reference sample of cresol resol  114 , and spectrum  112  displays peaks consistent with a phenolic resin compound. 
     Various examples have been described. These and other examples are within the scope of the following claims.