Abstract:
A method of densifying porous substrates, such as brake performs, using a liquid precursor, in which the rate at which “fresh” or “new” liquid precursor is consumed is reduced by maintaining the liquid precursor being used for the densification at a purity level that is less that pure but still chemically suitable for obtaining the desired densification product.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 60/821,596, filed on Aug. 7, 2006, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and apparatus for densifying porous articles with a desirably high rate of production, particularly, but not necessarily only, with respect to the field of friction braking articles, such as aircraft brakes. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the field of friction materials, it is generally known to use porous materials to manufacture friction members, such as friction brake disks. 
         [0004]    The manufacture of such friction members generally begins with the construction of a porous preform. For example, in many friction brake applications, annular preforms are used. 
         [0005]    The annular preforms can be constructed using several different known methods. For example, carbon fiber fabric plies can be needled together and annular preforms can be cut from the stacked material. 
         [0006]    Also, near net shape preforms can be formed, for example, by weaving carbon fibers, or by braiding the carbon fiber into a desired shape. Certain carbon fiber fabrics are known having a weave that facilitates laying the fabric in a spiral form. In this context, “near-net” refers to forming structures having a form close to a desired shape of the final article, such as an annular brake disk. 
         [0007]    Oxidized polyacrylonitride (“PAN”) fibers or pitch-based fibers are common examples of starting fibers used in this type of application. Subsequently, these fibers may be carbonized in a high temperature treatment step. In another conventional approach, the starting fibers are formed using a resin or pitch, and the resultant mass is later cured with a reactive gas, such as nitrogen gas. The thusly cured mass is then carbonized to obtain a semi-rigid preform. 
         [0008]    In any event, it is desirable to further densify the resulting porous preform (especially, but not necessarily only, with a carbonaceous material) so as to obtain desired friction and mechanical properties. 
         [0009]    Chemical vapor infiltration (“CVI”) is a widely used conventional technique in this regard for obtaining carbon/carbon composite materials. CVI uses a hydrocarbon-containing gas to infiltrate a porous preform. The CVI gas is then cracked under high temperatures so as to leave a carbon coating on the fiber structure of the preform. 
         [0010]    Conventional CVI typically requires several hundred hours of processing in order to obtain a carbon/carbon (“C/C”) structure having a desired density and mechanical properties. By way of example, a typical conventional CVI process includes a first infiltration cycle performed, for example, over approximately 300-500 hours or more. 
         [0011]    However, conventional CVI frequently causes rapid blockage of the surface porosity of the preform before interior portions of the preform are adequately densified. In order to “reopen” the surface porosity to permit further densification, an intermediate machining step becomes necessary. In general, this intermediate machining (using a known method, such as milling) removes surface layers of the preform having carbon-blocked pores to expose open pores of the preform so that the hydrocarbon gas can again infiltrate the preform structure. Taking into account that several hundred preforms are densified in a typical densification process, the intermediate machining step can add as much as 48 hours to the overall conventional CVI densification process. 
         [0012]    Once the intermediate machining of the partially densified articles is completed, a second CVI process is performed to make use of the reopened surface porosity of the preforms. This second CVI process step can last, for example, another 300-500 hours or more. This generally completes the conventional densification process using CVI. 
         [0013]    Another approach to densifying porous preforms uses a liquid instead of gaseous hydrocarbon precursor. This method of densification is sometimes referred to in the art as “film boiling” or “rapid densification.” 
         [0014]    Film boiling densification generally involves immersing a porous preform in a liquid hydrocarbon so that the liquid substantially completely infiltrates the pores and interstices of the preform. Thereafter, the immersed preform is inductively heated by appropriately placed electrical elements, such as induction coils, to a temperature above the decomposition temperature of liquid hydrocarbon (typically 1000° C. or more). More particularly, the liquid hydrocarbon adjacent to the inductively heated preform structure dissociates into various gas phase species within the preform porosity. Further thermal decomposition of the gas phase species results in the formation of pyrolitic carbon on interior surfaces in the open regions of the porous material. The use of liquid precursors for densification is discussed in, for example, U.S. Pat. Nos. 4,472,454, 5,389,152, 5,397,595, 5,733,611, 5,547,717, 5,981,002, and 6,726,962. Each and every one of these documents is incorporated herein by reference in its entirety. 
         [0015]    The concept of inductive heating in this field is generally known, including as described in the aforementioned references. However, heating a preform to high temperatures (at least 1000° C. and as much as 1400° C.) while it is literally immersed in highly volatile hydrocarbon liquids (such as, for example, cyclohexane) raises very important safety issues. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention relates to a process of densifying porous substrates using a liquid precursor, in which the volume of “new” or “fresh” liquid precursor used is usefully reduced by maintaining the liquid precursor used for densification at a level of purity that is less than pure, but still chemically suitable for the densification process. In effect, the present invention uses an artificially “aged” (in terms of the impurities present therein) liquid precursor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will be even more clearly understood with reference to the drawings appended hereto, of which: 
           [0018]      FIG. 1  is a schematic representation of a facility for performing densification using a liquid precursor according to the present invention; 
           [0019]      FIG. 2  and  FIG. 3  are partially cutaway side and top view of a reaction chamber for densifying porous articles according to the present invention; 
           [0020]      FIG. 4  illustrates an experimentally derived relationship between densification run cycles and liquid precursor purity, quantified as concentration of a principal precursor component; 
           [0021]      FIG. 5  illustrates an experimentally obtained progression of impurity levels in a liquid precursor over several cycles; 
           [0022]      FIG. 6  illustrates calculated versus experimental data over several cycles where precursor purity is periodically restored; and 
           [0023]      FIGS. 7 and 8  illustrate calculated versus experimental data over several cycles where precursor purity is periodically partly restored. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The feature and details of the method and apparatus of the invention with the reference to the accompanying drawings are shown by way of illustration and not as limitations of the invention. 
         [0025]    Solely by way of example and/or illustration, mention is made hereinbelow of porous preforms, such as preforms for manufacturing friction brake disks. It is expressly noted, however, that the present invention is more generally applicable to densifying other kinds of porous substrates in the manner described. 
         [0026]    A highly schematic representation of a facility for performing densification using liquid precursors is illustrated in  FIG. 1 . The system can include a remote liquid precursor storage site  100  (including, for example, a fluid transfer system) for managing the transport of new and used precursor liquid. One example of a precursor liquid used according to the present system is a liquid hydrocarbon, such as cyclohexane (C 6 H 12 ). For example, a tank “farm” of one or more remote storage tanks (collectively indicated at  100 ) for the liquid precursor may be provided. The tank farm could also include one or more tanks for at least initially storing used liquid precursor. It may be desirable or even required (depending on applicable industrial requirements) to keep at least the tank farm  100  at some distance from the rest of the facility for safety reasons. For example, some local and/or national ordinances require separation on the order of hundreds of feet. 
         [0027]    The facility may optionally include a relatively smaller local storage tank  105  for keeping a relatively small quantity of new precursor liquid close to the processing equipment, if desired. 
         [0028]    The piping system (including pumps and the like) used to interconnect various parts of the facility is conventional and may be of any construction and arrangement appropriate for the transport of liquid precursor being used, particularly, but not necessarily only, liquid hydrocarbons. The fluid transfer system is preferably but not necessarily computer-controlled. Commercially available computer-controlled systems (for example and without limitation, those commercially available from the company OPTO  22 ) can be used for monitoring and controlling this type of fluid transfer system, including loading of new liquid precursor from an outside supplier. 
         [0029]    The liquid precursor is supplied to one or more reaction chambers (collectively indicated at  110 ) from the local precursor storage tank  105 . Preferably, sufficient liquid precursor is provided to substantially immerse the one or more preforms being densified therein, as well as the induction heating coils associated therewith. 
         [0030]    As mentioned before, the film boiling process creates gaseous species that in part causes the formation of pyrolitic carbon on interior surfaces of the preform porosity. Precursor vapor is captured to the extent possible and condensed at the conventional condenser unit  115  for possible recycling in the process. A commercial cooling tower  140  is available to maintain an adequate water temperature for the cooling of the condenser unit  115 . 
         [0031]    Effluent gas that is still left over is preferably conveyed to a thermal oxidizer  120  of a known configuration to burn off residual hydrocarbons in the effluent gas. 
         [0032]    The power from the power supply  125  is transferred to the induction coils  25  by metal bus bars  30  constructed according to a given arrangement of elements in a facility and according to appropriate desired dimensional considerations. The bus bars may be made from copper, for example, and are preferably but not necessarily water-cooled by water cooling networks  50 . (See  FIG. 2 .) 
         [0033]    Each power supply  125  may have remote PID loop control capability and can be monitored and controlled from a computer control terminal. Power density control, voltage control, current control, frequency control, and/or temperature control of the densification process by known methods is also within the scope of the contemplated arrangement. 
         [0034]      FIG. 2  illustrates a reaction chamber  110  constructed and arranged to process two (for example) porous preforms therein. The reaction chamber  110  has two induction heating coil sets  25  corresponding to each preform  35  that is to be processed. The induction coils sets  25  are preferably mounted in position using non-reactive thermally stable supports made from, for example, an electrically non-conductive glass composite material  45  (such as that known in the art as “G-10”) that can withstand, for example, a liquid hydrocarbon environment. The induction heating coils  25  are preferably water-cooled pancake spiral coils, and may be made from copper metal. For this reason, it is desirable to provide a heat exchanger  135  (see  FIG. 1 ) dedicated to the water-cooling system for the induction coils  25 . In use, a preform (or worn brake)  35  being densified is heated by direct coupling from the induction coil  25 . 
         [0035]    In an example of loading and unloading coil/reactor chamber  110 , a top cover panel  15  is provided with a conventional locking mechanism for sealing the chamber  110 . Each reactor chamber  110  (as many as are provided) is provided with a common liquid precursor supply line connection  20 , and a common exhaust line  10  operably connected to the condenser  115  and thermal oxidizer  120 . 
         [0036]    Each reactor chamber  110  can be desirably filled, drained, and monitored from the computer control system. The exhaust liquid precursor vapor from the densification process is condensed and fed back to the reactor chambers  110 , whereas residual effluent gas is then taken to the thermal oxidizer  120  and burned. 
         [0037]    Because volatile liquid hydrocarbons are a particular example of the liquid precursor used in the present invention, it is desirable (but not obligatory) to provide a nitrogen (N 2 ) gas supply system (not shown) to, for example, flush out the piping systems and generally fill voids in the system with an inert gas (instead of oxygen-containing air) so as to decrease the risk of combustion. In a particular example, empty spaces in both the remote and local liquid precursor storage tanks are maintained at a slight, continuously supplied, overpressure of nitrogen (or other conventionally known inert) gas so as to prevent potentially dangerous accumulations of volatile vapors. Hydrocarbon species mixed with the exhausted nitrogen gas are sent to the thermal oxidizer  120  so that the hydrocarbons can be burned off before the gas is exhausted to the exterior. 
         [0038]    Also, because the system uses a “wet” process, it is useful to provide a drying oven  130  in the system to dry off densified preforms following densification. The exhaust from such a drying oven  130  is preferably also connected to the thermal oxidizer  120  in order to process heavy and light aromatics entrained in the resultant effluent gas. With respect to safety considerations, it is useful to use an oven structure that is structurally resistant to failure in the event of an explosion therein, given the presence of volatile gases in the oven during drying. The drying process can be, for example, computer controlled in order to simplify process control. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Aged Cyclohexane- eight consecutive runs 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 #1A 
                 #1B 
                 #2A 
                 #2B 
                 #3A 
                 #3B 
                 #4A 
                 #4B 
               
             
          
           
               
                   
                 &lt;% Area&gt; 
               
             
          
           
               
                 Chemical Name 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
               
               
                   
               
               
                 Unknown Gas 
                 0.02 
                 0.07 
                 0.12 
                 0.08 
                 0.12 
                 0.10 
                 0.14 
                 0.09 
               
               
                 Cyclopentadiene 
                 0.02 
                 0.08 
                 0.07 
                 0.10 
                 0.10 
                 0.13 
                 0.13 
                 0.13 
               
               
                 Hexane 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                 Methylcyclopentane 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                 Benzene 
                 0.20 
                 0.73 
                 0.61 
                 1.15 
                 0.97 
                 1.55 
                 1.41 
                 1.89 
               
               
                 Cyclohexane 
                 99.57 
                 98.34 
                 98.60 
                 97.44 
                 97.82 
                 96.58 
                 96.89 
                 95.81 
               
               
                 Cyclohexene 
                 0.03 
                 0.07 
                 0.06 
                 0.11 
                 0.09 
                 0.14 
                 0.13 
                 0.17 
               
               
                 Heptane 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.00 
               
               
                 Methycyclohexane 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Toluene 
                 0.03 
                 0.12 
                 0.09 
                 0.18 
                 0.15 
                 0.24 
                 0.21 
                 0.31 
               
               
                 Ethylbenzene 
                 0.00 
                 0.01 
                 0.00 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                 Phenylacetylene 
                 0.01 
                 0.03 
                 0.02 
                 0.04 
                 0.03 
                 0.05 
                 0.05 
                 0.07 
               
               
                 Styrene 
                 0.03 
                 0.11 
                 0.08 
                 0.17 
                 0.13 
                 0.22 
                 0.19 
                 0.29 
               
               
                 Nonane 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.01 
                 0.00 
                 0.01 
               
               
                 Indene 
                 0.01 
                 0.05 
                 0.04 
                 0.08 
                 0.06 
                 0.11 
                 0.09 
                 0.14 
               
               
                 Naphthalene 
                 0.04 
                 0.17 
                 0.13 
                 0.27 
                 0.21 
                 0.36 
                 0.31 
                 0.47 
               
               
                 Methylnaphthalene 
                 0.00 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.02 
                 0.01 
                 0.02 
               
               
                 Acenaphthalene 
                 0.00 
                 0.03 
                 0.02 
                 0.04 
                 0.03 
                 0.05 
                 0.04 
                 0.06 
               
               
                 Fluorene 
                 0.00 
                 0.01 
                 0.00 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.02 
               
               
                 Total 
                 99.99 
                 99.86 
                 99.88 
                 99.72 
                 99.77 
                 99.61 
                 99.65 
                 99.51 
               
               
                   
               
               
                   
                 #5A 
                 #5B 
                 #6A 
                 #6B 
                 #7A 
                 #7B 
                 #8A 
                 #8B 
               
             
          
           
               
                   
                 &lt;% Area&gt; 
               
             
          
           
               
                 Chemical Name 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
               
               
                   
               
               
                 Unknown Gas 
                 0.19 
                 0.10 
                 0.10 
                 0.10 
                 0.15 
                 0.10 
                 0.07 
                 0.02 
               
               
                 Cyclopentadiene 
                 0.18 
                 0.16 
                 0.14 
                 0.14 
                 0.15 
                 0.14 
                 0.12 
                 0.18 
               
               
                 Hexane 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                 Methylcyclopentane 
                 0.01 
                 0.01 
                 0.01 
                 0.02 
                 0.01 
                 0.02 
                 0.01 
                 0.02 
               
               
                 Benzene 
                 1.89 
                 2.19 
                 2.00 
                 2.33 
                 2.24 
                 2.55 
                 2.26 
                 2.75 
               
               
                 Cyclohexane 
                 95.90 
                 95.26 
                 95.68 
                 94.98 
                 95.22 
                 94.59 
                 95.23 
                 94.27 
               
               
                 Cyclohexene 
                 0.16 
                 0.19 
                 0.17 
                 0.20 
                 0.19 
                 0.22 
                 0.20 
                 0.23 
               
               
                 Heptane 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Methycyclohexane 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Toluene 
                 0.29 
                 0.35 
                 0.31 
                 0.38 
                 0.35 
                 0.42 
                 0.37 
                 0.44 
               
               
                 Ethylbenzene 
                 0.01 
                 0.02 
                 0.01 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
               
               
                 Phenylacetylene 
                 0.06 
                 0.08 
                 0.07 
                 0.08 
                 0.07 
                 0.08 
                 0.07 
                 0.09 
               
               
                 Styrene 
                 0.26 
                 0.32 
                 0.28 
                 0.35 
                 0.32 
                 0.38 
                 0.33 
                 0.39 
               
               
                 Nonane 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                 Indene 
                 0.12 
                 0.15 
                 0.13 
                 0.16 
                 0.14 
                 0.17 
                 0.15 
                 0.16 
               
               
                 Naphthalene 
                 0.42 
                 0.53 
                 0.45 
                 0.57 
                 0.51 
                 0.62 
                 0.54 
                 0.62 
               
               
                 Methylnaphthalene 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
                 0.03 
                 0.02 
                 0.02 
               
               
                 Acenaphthalene 
                 0.05 
                 0.06 
                 0.05 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
               
               
                 Fluorene 
                 0.01 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
                 0.02 
               
               
                 Total 
                 99.59 
                 99.48 
                 99.46 
                 99.45 
                 99.49 
                 99.44 
                 99.49 
                 99.31 
               
               
                   
               
             
          
         
       
     
         [0039]    Table 1 provides the analysis of aged C 6 H 12  and the major impurities produced from the densification process. In this study, eight densification cycles in three reactors were performed with carbonized preforms using a dedicated power curve. In particular, an objective was to reduce the usage of new or “fresh” liquid precursor (which can be relatively costly) to replenish the precursor being used during production. 
         [0040]    Sixteen aged C 6 H 12  samples from eight consecutive runs were collected and sent for analysis. The goal was to correlate individual contaminant peak value to the aged C 6 H 12 . Samples were collected before and after each densification cycle and identified as A and B. 
         [0041]    The C 6 H 12 % concentration at the end of 8th cycle was still above 94%. The major resulting impurity peaks were Benzene, Naphthalene, Toluene, Styrene, Cyclohexene, Cyclopentadiene, and Indene. Six out of the seven measured impurities followed a steady, generally increasing trend throughout the eight cycles. A gas chromatography (GC) analysis made from the aging study indicates all major contaminants and Cyclohexane concentrations are very predictable. All impurity peaks are consistently repeated from one cycle to the next cycle. 
         [0042]    Simulation of aged liquid precursors by doping a liquid precursor with various impurities is also contemplated. In this manner, it may be possible to beneficially extend the usable life of a liquid precursor and/or reduce the need for highly pure (and, therefore, relatively expensive) liquid precursors. In other words, on the one hand, a “minimum” level of precursor purity can be identified, such that the precursor can be used at purity levels above that minimum (before replacement with highly pure precursor). On the other hand, a lower acceptable level of precursor purity can be identified such that the contemplated densification process would use that lower purity precursor on an ongoing basis. Either way, the usage of high purity precursor liquid is desirably reduced. 
         [0043]    Aged liquid precursors were simulated by doping a liquid precursor with various impurities mentioned above in order to approximate the gradual “deterioration” of a highly pure liquid precursor after several process cycles. A model was developed to predict Cyclohexane concentrations at various cycles. Extrapolation was performed based on experimental data from the eight consecutive runs previously described.  FIG. 4  illustrates the predicted Cyclohexane concentrations versus number of consecutive process runs. It is noted that the calculated Cyclohexane concentration substantially reaches steady state after about 40 runs, mainly because new Cyclohexane is added to the process tank prior to the densification cycle. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 500 Gallon Batch Calculations 
               
             
          
           
               
                 Chemicals 
                 Mass % 
                 Density 
                 Mass % × Den. 
                 kg/500 gal 
                 l/500 gal 
                 GALLONS 
               
               
                   
               
             
          
           
               
                 Cyclohexane 
                 90.00 
                 0.779 
                 0.701 
                 1291.500 
                 1657.895 
                 438.02 
               
               
                 Benzene 
                 5.46 
                 0.874 
                 0.048 
                 78.351 
                 89.646 
                 23.68 
               
               
                 Naphthalene 
                 1.25 
                 0.963 
                 0.012 
                 17.938 
                 N/A 
                 N/A 
               
               
                 Styrene 
                 0.78 
                 0.909 
                 0.007 
                 11.193 
                 12.314 
                 3.25 
               
               
                 Cyclohexene 
                 0.51 
                 0.810 
                 0.004 
                 7.319 
                 9.035 
                 2.39 
               
               
                 Cyclopentadiene 
                 0.44 
                 0.986 
                 0.004 
                 6.314 
                 6.404 
                 1.69 
               
               
                 Toluene 
                 0.86 
                 0.867 
                 0.007 
                 12.341 
                 14.234 
                 3.76 
               
               
                 Indene 
                 0.36 
                 0.997 
                 0.004 
                 5.166 
                 5.183 
                 1.37 
               
               
                 Phenylacetylene 
                 0.17 
                 0.930 
                 0.002 
                 2.440 
                 N/A 
                 N/A 
               
               
                 Acenaphthylene 
                 0.15 
                 0.899 
                 0.001 
                 2.153 
                 N/A 
                 N/A 
               
               
                 Total 
                 99.98 
                   
                 0.790 
                   
                   
                 474.16 
               
               
                   
               
               
                 Note: 
               
               
                 Calculated density for the mixture is: 0.790 g/cc. Allowable tank capacity is 480 gallons 
               
               
                 480 gallon batch weight: 0.790 × 480 × 3.785 = 1435 (kg) 
               
             
          
         
       
     
         [0044]    Nine chemicals were purchased from Alfa Aesar to dope the cyclohexane precursor. Table 2 provides the information about the chemicals used for the doping. The purity of the purchased chemicals ranged from 99.6% to 80%. 
         [0045]    Table 2 provides the quantity used for the individual chemicals to obtain a 90% Cyclohexane concentration. A 90% Cyclohexane concentration was used for the initial mix in order to simulate the model, although cyclopentadiene was not used in the mix for safety reasons because this gel-like material is extremely flammable in air. The effect on the test is believed however to be minimal. Six consecutive densification cycles were performed using the initial 90% mix. Samples of the liquid precursor were collected before and after each cycle and sent for GC analysis. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 500 Gallon Batch Calculations Vs GC measurements 
               
             
          
           
               
                   
                 Mass % 
                 GC % 
                   
                   
                   
                   
                   
               
               
                 Chemicals 
                 used in mix 
                 area 
                 Density 
                 Mass % × Den. 
                 kg/500 gal 
                 l/500 gal 
                 GALLONS 
               
               
                   
               
             
          
           
               
                 Cyclohexane 
                 90.000 
                 90.318 
                 0.779 
                 0.701 
                 1291.500 
                 1657.895 
                 438.02 
               
               
                 Benzene 
                 5.460 
                 5.255 
                 0.874 
                 0.048 
                 78.351 
                 89.646 
                 23.68 
               
               
                 Naphthalene 
                 1.250 
                 1.422 
                 0.963 
                 0.012 
                 17.938 
                 N/A 
                 N/A 
               
               
                 Styrene 
                 0.780 
                 0.870 
                 0.909 
                 0.007 
                 11.193 
                 12.314 
                 3.25 
               
               
                 Cyclohexene 
                 0.510 
                 0.536 
                 0.810 
                 0.004 
                 7.319 
                 9.035 
                 2.39 
               
               
                 Cyclopentadiene 
                 0.440 
                 0.010 
                 0.986 
                 0.004 
                 6.314 
                 6.404 
                 1.69 
               
               
                 Toluene 
                 0.860 
                 0.939 
                 0.867 
                 0.007 
                 12.341 
                 14.234 
                 3.76 
               
               
                 Indene 
                 0.360 
                 0.390 
                 0.997 
                 0.004 
                 5.166 
                 5.183 
                 1.37 
               
               
                 Phenylacetylene 
                 0.170 
                 0.204 
                 0.930 
                 0.002 
                 2.440 
                 N/A 
                 N/A 
               
               
                 Acenaphthylene 
                 0.035 
                 0.010 
                 0.899 
                 0.000 
                 0.502 
                 N/A 
                 N/A 
               
               
                 Total 
                 99.87 
                 99.95 
                   
                 0.789 
                   
                   
                 474.16 
               
               
                   
               
             
          
         
       
     
         [0046]    A total of twelve aged precursors from six consecutive cycles were sent to a laboratory for gas chromatography analysis. Aged precursor samples were collected before and after the densification cycle from the 500 gallon tank. Table 3 provides the GC analysis for the simulated 90% mix precursor. It was observed in Table 3 that the reported concentrations from all chemicals closely followed the mass % used in the 90% mixture with the exception of Cyclopentadiene, which was not actually used due to its high flammability in air. It is however believed that the omission of cyclopentadiene from the actual samples has a minimal overall effect on the test. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Doping Study- six consecutive runs 
               
             
          
           
               
                   
                 #1A 
                 #1B 
                 #2A 
                 #2B 
                 #3A 
                 #3B 
                 #4A 
                 #4B 
                 #5A 
                 #5B 
                 #6A 
                 #6B 
               
             
          
           
               
                   
                 &lt;% Area&gt; 
               
             
          
           
               
                 Chemical Name 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
                 Qty % 
               
               
                   
               
             
          
           
               
                 Unknowns 
                 0.040 
                 0.410 
                 0.420 
                 0.540 
                 0.580 
                 0.770 
                 0.710 
                 0.830 
                 0.750 
                 0.840 
                 0.850 
                 0.970 
               
               
                 Cyclopentadiene 
                 0.010 
                 0.080 
                 0.094 
                 0.154 
                 0.131 
                 0.172 
                 0.164 
                 0.204 
                 0.252 
                 0.236 
                 0.203 
                 0.237 
               
               
                 Benzene 
                 5.255 
                 5.719 
                 5.533 
                 5.824 
                 5.617 
                 6.133 
                 5.783 
                 6.270 
                 5.895 
                 6.371 
                 6.026 
                 6.473 
               
               
                 Cyclohexane 
                 90.318 
                 89.569 
                 89.856 
                 89.398 
                 89.758 
                 88.871 
                 89.583 
                 88.728 
                 89.444 
                 88.472 
                 89.166 
                 88.429 
               
               
                 Cyclohexene 
                 0.536 
                 0.551 
                 0.529 
                 0.526 
                 0.507 
                 0.529 
                 0.497 
                 0.515 
                 0.481 
                 0.505 
                 0.474 
                 0.489 
               
               
                 Toluene 
                 0.939 
                 0.992 
                 0.962 
                 1.001 
                 0.963 
                 1.050 
                 0.978 
                 1.067 
                 0.985 
                 1.101 
                 1.017 
                 1.078 
               
               
                 Ethylbenzene 
                 0.002 
                 0.010 
                 0.010 
                 0.018 
                 0.018 
                 0.027 
                 0.025 
                 0.033 
                 0.030 
                 0.039 
                 0.036 
                 0.041 
               
               
                 Phenylacetylene 
                 0.204 
                 0.184 
                 0.180 
                 0.167 
                 0.161 
                 0.156 
                 0.145 
                 0.143 
                 0.132 
                 0.140 
                 0.128 
                 0.127 
               
               
                 Styrene 
                 0.870 
                 0.842 
                 0.820 
                 0.815 
                 0.782 
                 0.809 
                 0.750 
                 0.787 
                 0.722 
                 0.808 
                 0.739 
                 0.760 
               
               
                 Indene 
                 0.390 
                 0.311 
                 0.306 
                 0.276 
                 0.264 
                 0.247 
                 0.228 
                 0.223 
                 0.205 
                 0.229 
                 0.206 
                 0.209 
               
               
                 Naphthalene 
                 1.422 
                 1.284 
                 1.243 
                 1.214 
                 1.156 
                 1.163 
                 1.069 
                 1.109 
                 1.018 
                 1.162 
                 1.059 
                 1.085 
               
               
                 Methylnaphthalene 
                 0.000 
                 0.010 
                 0.010 
                 0.016 
                 0.015 
                 0.020 
                 0.018 
                 0.023 
                 0.021 
                 0.027 
                 0.025 
                 0.027 
               
               
                 Acenaphthalene 
                 0.010 
                 0.030 
                 0.030 
                 0.038 
                 0.037 
                 0.043 
                 0.039 
                 0.050 
                 0.046 
                 0.055 
                 0.049 
                 0.055 
               
               
                 Fluorene 
                 0.000 
                 0.005 
                 0.005 
                 0.010 
                 0.010 
                 0.014 
                 0.012 
                 0.015 
                 0.014 
                 0.019 
                 0.017 
                 0.018 
               
               
                 Total 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
               
                   
               
             
          
         
       
     
         [0047]    Table 4 illustrates a progression of the major contaminants from the six consecutive cycles, or twelve precursor samples. The purity generally started at 90.3% and ended at 88.4% after six densification cycles. In most of the cases, contaminants reached steady state or slowly declined with the exception of Benzene. (See, for example, Cyclopentadiene, Cyclohexene, Toluene, Ethylbenzene, Phenylacetylene, Styrene, Indene, Naphthalene, Methylnaphthalene, Acenaphthalene, and Fluorene in Table 4) This indicates that continuation of chemical breakdown from Cyclohexane (C 6 H 12 ) to Benzene (C 6 H 6 ) is most likely to continue at lower C 6 H 12  concentration levels. (See, for example, the benzene concentration in  FIG. 5 .) 
         [0048]    In practice, several methods can be used to manage the precursor. Some of them are described hereafter. 
         [0049]    Several consecutive cycles could be run with the same liquid precursor bath, followed by periodically replacing the whole storage tank with new cyclohexane. In that manner, the cyclohexane concentration will vary between fresh cyclohexane concentration and a low concentration depending on the number of consecutive cycles run.  FIG. 6  is shown a case where the storage tank is drained every 8 cycles. 
         [0050]    Another way to manage the precursor is to periodically replace only a part of liquid precursor in the storage tank. This approach reduces the variations in concentrations and therefore permits better control of the pyrocarbon deposited from this precursor (microstructure, thermal-mechanical or friction properties). In this regard,  FIG. 7  illustrates how cyclohexane concentration is theoretically affected by replacing half the storage tank with fresh cyclohexane every 4 runs. 
         [0051]    According to these two methods, the storage tank is never totally drained. Multiple runs were performed according to the approaches corresponding with  FIGS. 6 and 7 , respectively, and the used cyclohexane samples were analyzed by gas chromatography. Tables 5a and 5b, respectively, provide the results of these analyses. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 5A 
               
             
             
               
                   
                   
               
               
                   
                 Run 
               
             
          
           
               
                   
                   
                 0 
                 1 
                 2 
                 3 
                 4 
                 6 
                 8 
                 8 
                 9 
               
               
                   
                 Sample 
                 0 
                 1-1 
                 1-2 
                 1-3 
                 2-0 
                 2-2 
                 2-4 
                 3-0 
                 3-1 
               
               
                   
                   
               
             
          
           
               
                 Chemical 
                 Methylnaphthalene 
                 0 
                 0.007 
                 0.013 
                 0.013 
                 0.008 
                 0.012 
                 0.014 
                 0.007 
                 0.009 
               
               
                   
                 Acenaphthylene 
                 0 
                 0.018 
                 0.026 
                 0.026 
                 0.015 
                 0.023 
                 0.025 
                 0.012 
                 0.018 
               
               
                   
                 Ethylbenzene 
                 0 
                 0.006 
                 0.013 
                 0.013 
                 0.009 
                 0.013 
                 0.02 
                 0.01 
                 0.012 
               
               
                   
                 Fluorene 
                 0 
                 0.006 
                 0.008 
                 0.008 
                 0.005 
                 0.007 
                 0.008 
                 0.01 
                 0.004 
               
               
                   
                 Phenanthrene 
                 0 
                 0.021 
                 0.031 
                 0.031 
                 0.023 
                 0.032 
                 0.032 
                 0.017 
                 0.02 
               
               
                   
                 Phenylacetylene 
                 0 
                 0.018 
                 0.037 
                 0.037 
                 0.024 
                 0.036 
                 0.052 
                 0.026 
                 0.032 
               
               
                   
                 Indene 
                 0 
                 0.043 
                 0.084 
                 0.084 
                 0.053 
                 0.078 
                 0.105 
                 0.051 
                 0.064 
               
               
                   
                 Cyclopentadiene 
                 0 
                 0.091 
                 0.181 
                 0.181 
                 0.115 
                 0.126 
                 0.162 
                 0.09 
                 0.134 
               
               
                   
                 Cyclohexene 
                 0 
                 0.056 
                 0.13 
                 0.13 
                 0.089 
                 0.125 
                 0.186 
                 0.095 
                 0.118 
               
               
                   
                 Styrene 
                 0 
                 0.085 
                 0.189 
                 0.189 
                 0.126 
                 0.189 
                 0.278 
                 0.138 
                 0.172 
               
               
                   
                 Naphthalene 
                 0 
                 0.14 
                 0.291 
                 0.291 
                 0.196 
                 0.294 
                 0.403 
                 0.199 
                 0.243 
               
               
                   
                 Toluene 
                 1.689 
                 3.209 
                 1.797 
                 1.797 
                 0.91 
                 0.835 
                 0.93 
                 0.476 
                 0.457 
               
               
                   
                 Benzene 
                 0.025 
                 0.659 
                 1.5 
                 1.5 
                 1.053 
                 1.557 
                 2.322 
                 1.196 
                 1.492 
               
               
                   
                 Cyclohexane 
                 98.275 
                 95.25 
                 95.136 
                 95.136 
                 97.062 
                 96.245 
                 94.887 
                 97.39 
                 96.723 
               
               
                   
                 Unknown 
                 0.011 
                 0.391 
                 0.564 
                 0.564 
                 0.312 
                 0.428 
                 0.576 
                 0.283 
                 0.502 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 5B 
               
             
             
               
                   
                   
               
               
                   
                 Run 
               
             
          
           
               
                   
                   
                 14 
                 15 
                 17 
                 19 
                 20 
                 22 
                 25 
                 33 
                 35 
               
               
                   
                 Sample 
                 3-6 
                 3-7 
                 4-2 
                 4-4 
                 5.1 
                 5.2 
                 6.2 
                 7.5 
                 8.2 
               
               
                   
                   
               
             
          
           
               
                 Chemical 
                 Methylnaphthalene 
                 0.011 
                 0.01 
                 0.008 
                 0.013 
                 0.007 
                 0.01 
                 0.01 
                 0.014 
                 0.006 
               
               
                   
                 Acenaphthylene 
                 0.022 
                 0.015 
                 0.021 
                 0.03 
                 0.013 
                 0.024 
                 0.02 
                 0.016 
                 0.007 
               
               
                   
                 Ethylbenzene 
                 0.017 
                 0.018 
                 0.012 
                 0.017 
                 0.011 
                 0.013 
                 0.014 
                 0.021 
                 0.012 
               
               
                   
                 Fluorene 
                 0.007 
                 0.005 
                 0.005 
                 0.009 
                 0.004 
                 0.007 
                 0.006 
                 0.008 
                 0.012 
               
               
                   
                 Phenanthrene 
                 0.032 
                 0.025 
                 0.025 
                 0.041 
                 0.021 
                 0 
                 0.027 
                 0.043 
                 0.017 
               
               
                   
                 Phenylacetylene 
                 0.046 
                 0.046 
                 0.035 
                 0.048 
                 0.028 
                 0.036 
                 0.033 
                 0.042 
                 0.022 
               
               
                   
                 Indene 
                 0.083 
                 0.078 
                 0.057 
                 0.084 
                 0.049 
                 0.067 
                 0.063 
                 0.082 
                 0.045 
               
               
                   
                 Cyclopentadiene 
                 0.147 
                 0.141 
                 0.119 
                 0.156 
                 0.083 
                 0.118 
                 0.142 
                 0.199 
                 0.085 
               
               
                   
                 Cyclohexene 
                 0.17 
                 0.172 
                 0.119 
                 0.16 
                 0.095 
                 0.122 
                 0.136 
                 0.206 
                 0.114 
               
               
                   
                 Styrene 
                 0.248 
                 0.247 
                 0.181 
                 0.248 
                 0.147 
                 0.191 
                 0.185 
                 0.246 
                 0.134 
               
               
                   
                 Naphthalene 
                 0.321 
                 0.296 
                 0.216 
                 0.329 
                 0.198 
                 0.266 
                 0.264 
                 0.359 
                 0.194 
               
               
                   
                 Toluene 
                 0.548 
                 0.547 
                 0.347 
                 0.422 
                 0.247 
                 0.3 
                 0.287 
                 0.394 
                 0.21 
               
               
                   
                 Benzene 
                 2.233 
                 2.242 
                 1.631 
                 2.253 
                 1.362 
                 1.737 
                 1.763 
                 2.576 
                 1.363 
               
               
                   
                 Cyclohexane 
                 95.636 
                 95.767 
                 96.804 
                 95.597 
                 97.458 
                 96.642 
                 96.541 
                 95.094 
                 97.491 
               
               
                   
                 Unknown 
                 0.479 
                 0.391 
                 0.42 
                 0.593 
                 0.277 
                 0.467 
                 0.509 
                 0.7 
                 0.288 
               
               
                   
               
             
          
         
       
     
         [0052]    The experimental data confirm the calculation is an accurate method to forecast the cyclohexane concentration versus the runs number (see  FIG. 8 .) It is noted that  FIGS. 7 and 8  also illustrate 8-cycle experimental data that further confirms the expected rate of cyclohexane degradation in terms of its general curvature. 
         [0053]    Moreover, the analyses show that the concentration of every chemical component remains substantially steady after several runs. 
         [0054]    Although the present invention has been described above with reference to certain particular examples for the purpose of illustrating and explaining the invention, it is to be understood that the invention is not limited solely by reference to the specific details of those examples. More specifically, a person skilled in the art will readily appreciate that modifications and developments can be made in the preferred embodiments without departing from the scope of the invention as defined in the accompanying claims.