Patent Publication Number: US-11639545-B2

Title: Methods for chemical vapor infiltration and densification of porous substrates

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of, and claims priority to and the benefit of, U.S. Ser. No. 15/725,620, filed Oct. 5, 2017, now U.S. Pat. No. 10,648,075 and entitled “SYSTEMS AND METHODS FOR CHEMICAL VAPOR INFILTRATION AND DENSIFICATION OF POROUS SUBSTRATES.” The &#39;620 application is a continuation-in-part of, and claims priority to and the benefit of, U.S. Ser. No. 15/078,882, filed Mar. 23, 2016, now U.S. Pat. No. 9,938,618 and entitled “METHOD FOR RAPID AND EFFICIENT CHEMICAL VAPOR INFILTRATION AND DENSIFICATION OF CARBON FIBER PREFORMS, POROUS SUBSTRATES AND CLOSE PACKED PARTICULATES.” The &#39;882 application claims priority to and the benefit of U.S. Provisional Application No. 62/137,214, filed Mar. 23, 2015 and entitled “METHOD FOR RAPID AND EFFICIENT CHEMICAL VAPOR INFILTRATION AND DENSIFICATION OF CARBON FIBER PREFORMS, POROUS SUBSTRATES AND CLOSE PACKED PARTICULATES.” All the aforementioned applications are hereby incorporated by reference in their entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates to chemical vapor infiltration and densification, and more specifically, to systems and methods of chemical vapor infiltration and densification employing recirculated hydrocarbon gases. 
     BACKGROUND 
     Carbon fiber/carbon matrix (C/C) composites are used in the aerospace industry for aircraft brake heat sink materials, among other applications. Silicon carbide (SiC) based ceramic matrix composites (CMCs) have found use as brake materials and other components in automotive and locomotive industries. These composites may be typically produced using, for example, chemical vapor infiltration (CVI) or chemical vapor deposition (CVD). Such processes generally include placing porous preforms into a reactor and introducing a gaseous precursor to form silicon carbide depositions within the pores of the preform. The SiC may be deposited as a coating or series of coatings wherein a porous sample or preform may be densified with carbon then SiC, or with SiC then carbon. 
     However, conventional infiltration and/or or deposition processes tend to result in byproduct deposits accumulating within system components of the manufacturing system, such as the exhaust piping. The byproduct deposits may be reactive and even pyrophoric, and thus precautions are warranted to promote a safe manufacturing environment. For example, conventional manufacturing systems are often shut-down for periods of time while users manually clean the components and piping of the manufacturing system to remove the byproduct deposits. This cleaning procedure increases the downtime of the manufacturing system and thus decreases the capacity and throughput of conventional ceramic matrix composite manufacturing systems. Buildup of condensable hydrocarbon tars from conventional carbon CVI processes, although not pyrophoric in nature tends to cause reductions in throughput of the plumbing systems (similar to deposits of cholesterol in a person&#39;s arteries). 
     SUMMARY 
     A system for chemical vapor infiltration and densification is disclosed, in accordance with various embodiments. The system may comprise a reaction chamber and a plurality of conduits fluidly coupled to an exhaust outlet of the reaction chamber. A first set of conduits of the plurality of conduits may define a first flow path and a second set of conduits of the plurality of conduits may define a second flow path. The second flow path may be fluidly coupled to an inlet of the reaction chamber. A roughing pump may be fluidly coupled to the first set of conduits, A first valve may be in operable communication with the first set of conduits. A second valve may be in operable communication with the second set of conduits. A hydrogen extraction component may be in fluid communication with a least one of the first set of conduits or the second set of conduits. The hydrogen extraction component may comprise at least one of a cryogenic-cooler or a pressure swing absorption unit. 
     In various embodiments, the cryogenic-cooler may be configured to condense hydrocarbon molecules comprised of six or more carbon atoms. A first pump may be downstream from the second valve. The first pump may comprise a constant speed turbo pump, a variable speed turbo pump, a constant speed dry pump, or a variable speed dry scroll pump. A second pump may be downstream of the first pump. The second pump may comprise at least one of a diaphragm pump, a turbo pump, or a dry pump. 
     In various embodiments, an electric arc may be located proximate the exhaust outlet of the reaction chamber. The electric arc may be configured to breakdown hydrocarbon gases comprised of six or more carbon atoms. A stage may be disposed within a reaction zone of the reaction chamber. The stage may be electrically isolated from a wall of the reaction chamber. The cryogenic-cooler may comprise at least one of a helium cryogenic cooler or a liquid nitrogen condenser. 
     A method of chemical vapor infiltration and deposition is disclosed, in accordance with various embodiments. The method may comprise disposing a porous substrate within a reaction chamber, establishing a sub-atmospheric pressure within the reaction chamber, introducing a hydrocarbon reaction gas into a reaction zone of the reaction chamber to densify the porous substrate, and withdrawing unreacted hydrocarbon reaction gas from the reaction chamber. The unreacted hydrocarbon reaction may comprise hydrocarbon molecules having six or more carbon atoms. The method may further comprise removing at least a portion of the hydrocarbon molecules having six or more carbon molecules from the unreacted hydrocarbon reaction gas by causing at least a portion of the hydrocarbon molecules having six or more carbon atoms to condense, and recirculating at least a portion of the unreacted hydrocarbon reaction gas back into the reaction zone. 
     In various embodiments, the method may further comprise applying an electrical voltage to the porous substrate. In various embodiments, removing the portion of the hydrocarbon molecules having six or more carbon molecules from the unreacted hydrocarbon reaction may comprise flowing the unreacted hydrocarbon reaction gas through a trap including one or more sets of rotating blades. 
     In various embodiments, the method may further comprise extracting hydrogen from the unreacted hydrocarbon reaction gas. In various embodiments, extracting hydrogen may comprise flowing the unreacted hydrocarbon reaction gas through at least one of a cryogenic-cooler or a pressure swing absorption unit. The method may further comprise applying an electric arc to the unreacted hydrocarbon reaction gas withdrawn from the reaction chamber. The method may further comprise heating at least one of the hydrocarbon reaction gas or the portion of the unreacted hydrocarbon reaction gas recirculated into the reaction zone using a charged coil located proximate an inlet of the reaction chamber. The charged coil may provide an electrical conduction path to at least one of charge or ground an interior wall of the reaction chamber. 
     A system for chemical vapor infiltration and densification is disclosed, in accordance with various embodiments. The system may comprise a reaction chamber and a plurality of conduits fluidly coupled to an exhaust outlet of the reaction chamber. A first set of conduits of the plurality of conduits may define a first flow path and a second set of conduits of the plurality of conduits may define a second flow path. The second flow path may be fluidly coupled to an inlet of the reaction chamber. A hydrogen extraction component may be in fluid communication with a least one of the first set of conduits or the second set of conduits. The hydrogen extraction component may comprise at least one of a cryogenic-cooler or a pressure swing absorption unit. 
     In various embodiments, the cryogenic-cooler or the pressure swing absorption unit may be configured to condense hydrocarbon molecules comprised of six or more carbon atoms. An electric arc may be located proximate the exhaust outlet of the reaction chamber. A trap may be in fluid communication with at least one of the first set of conduits or the second set of conduits. The trap may include one or more sets of rotating blades configured to trap and condense hydrocarbon molecules having six or more carbon atoms. A plasma conduit may be located upstream from the trap. The plasma conduit may be configured to breakdown a first hydrocarbon molecule having six or more carbon atoms into a plurality of second hydrocarbon molecules each having less than six carbon atoms. 
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, the following description and drawings are intended to be exemplary in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a schematic diagram of a system for chemical vapor infiltration and densification comprising a recirculation path, in accordance with various embodiments; 
         FIG.  2    illustrates a schematic diagram of a system for chemical vapor infiltration and densification comprising a recirculation path, in accordance with various embodiments; 
         FIG.  3    illustrates a schematic diagram of a system for chemical vapor infiltration and densification comprising a recirculation path and cryogenic cooling stage, in accordance with various embodiments; 
         FIG.  4    is a graph comparing densification of porous structures in CVI/CVD systems employing recirculated effluent gas to densification of porous structures in CVI/CVD systems employing an increased virgin reaction gas flow rate, in accordance with various embodiments; 
         FIGS.  5 A and  5 B  illustrate a method of chemical vapor infiltration and deposition; 
         FIG.  6 A  illustrates a retort having a frustoconical retort lid, in accordance with various embodiments; and 
         FIG.  6 B  illustrates a frustoconical retort lid, in accordance with various embodiments. 
     
    
    
     The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. A more complete understanding of the present disclosure, however, may best be obtained by referring to the detailed description and claims when considered in connection with the drawing figures, wherein like numerals denote like elements. 
     DETAILED DESCRIPTION 
     The detailed description of exemplary embodiments herein makes reference to the accompanying drawings, which show exemplary embodiments by way of illustration. While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the exemplary embodiments of the disclosure, it should be understood that other embodiments may be realized and that logical changes and adaptations in design and construction may be made in accordance with this disclosure and the teachings herein. Thus, the detailed description herein is presented for purposes of illustration only and not limitation. The steps recited in any of the method or process descriptions may be executed in any order and are not necessarily limited to the order presented. Furthermore, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option. Additionally, any reference to without contact (or similar phrases) may also include reduced contact or minimal contact. 
     Throughout the present disclosure, like reference numbers denote like elements. Accordingly, elements with like element numbering may be shown in the figures, but may not be necessarily be repeated herein for the sake of clarity. 
     Provided herein, according to various embodiments, are manufacturing systems and associated methods for fabricating C/C composite components. The systems and associated methods may provide for rapid and efficient deposition of carbon on and within carbon fiber preforms, porous substrates, and/or close packed particulates by pyrolytic carbon in the structures of isotropic, anisotropic, graphitic, amorphous, lonsdaleite, and diamond. The preforms, porous substrates, and/or close packed particulates are placed into a vacuum-tight reaction chamber as a shape, form, or mold and may be electrically isolated from the surrounding system. The reaction chamber may include a reaction zone defined by a retort within the reaction chamber. After loading the samples (i.e., the carbon fiber preforms, porous substrates, and/or close packed particulates) in the reaction zone, the reaction chamber may be evacuated to remove atmospheric gases. The samples can be grounded, charged with a positive or negative direct current (DC) voltage, or frequency oscillated using an alternating current (AC) voltage (that can be biased positively or negatively) independent of the whole system. The interior walls of the retort may be either grounded or electrically isolated from the surrounding system and the sample. The retort may be brought to an isothermal or gradient temperature condition above room temperature in a vacuum, with or without an inert gas. Once the set point temperature and pressure are achieved and controlled, one or more dry recycling/recirculating pumps may be activated in a circuit extending between the reaction chamber&#39;s exhaust to the reaction chamber&#39;s inlet/input. One or more hydrocarbon (CxHy) reaction gases, and/or inert carrier gases, and/or hydrogen gas are input into the vacuum tight reaction chamber/retort, either as a mixture or independently. The hydrocarbon reaction gases, and/or inert carrier gases, and/or hydrogen gas may be brought up to the reaction chamber temperature through a series of tubing coils. 
     The system may be under constant recycling/recirculating through the duration of the densification process by constant or variable speed dry pumps. One or more valve(s), in fluid communication with the pumps, may be configured to control a recycling/recirculating of effluent gas exhausted from the reaction zone. Translation (e.g., opening or closing) of the valves may pulsate the system and/or cause a cessation of the recycling/recirculating of the effluent gases. The internal system vacuum pressure may be controlled by one or more mass flow/pressure controllers and/or throttle valves located upstream and downstream of the reaction zone. On the exhaust side (downstream side) of the reaction zone, where the exhaust temperature begins to reduce just before the heavy hydrocarbons condense, variable or constant plasma may be created, using an electric arc method, such that the remnant hydrocarbon gases (i.e., the effluent hydrocarbon gases exiting the reaction zone) cross through the plasma when exiting the reaction zone, thereby reducing the heavier hydrocarbons (e.g., hydrocarbons molecules comprised of six or more carbon atoms) to simpler hydrocarbons (e.g., hydrocarbons molecules comprised of five or less carbon atoms). Depending on the particular gas or gas mixture input, additional plasma reactions may be performed near the reaction zone&#39;s exhaust point to further reduce the heavy hydrocarbons to simpler hydrocarbons. 
     At various points in the system plumbing, the downstream exhaust gas (also referred to herein as effluent gas) may be monitored to ascertain the type and/or concentration of hydrocarbon molecules within the exhaust gas. The system may comprise one more valves which may fine tune the process (e.g., adjust the volume, speed, hydrocarbon molecule size, or other feature(s) of the exhaust gas being recycled) to minimize the hydrocarbon gases exiting the system completely. During the process, a mass of the sample(s) (i.e., a mass of the preforms, porous substrates, and/or close packed particulates) may be measured continuously using an in-situ measurement device such as a precision load cell. 
     A chemical vapor infiltration system  100  for rapidly and efficiently densifying carbon fiber preforms, porous substrates, and close packed particulates by pyrolytic carbon in the structure of one or more of isotropic, anisotropic, graphitic, amorphous, lonsdaleite, and diamond and mixtures of said structures, is shown schematically in  FIG.  1   , in accordance with various embodiments. 
     The parts of the system shown in  FIG.  1    include:
           1 . roughing pump     2 . turbo pump     3 . 3 way valve     4 . throttle valve     5 . sampling port/valve     6 . valves     7 ,  8 ,  9 . Turbo pumps and/or scroll pumps     10 . flow controller     11 . pressure measurement     12 . valve     13 . optional diaphragm pump/plasma chamber     14 . sampling port     15 . gas inlet tube     16 . sample stage     17 . pressure measurement     18 . high voltage input for plasma     19 . retort     20 . plasma conduit     21 . gas feed     22 . load cell/rotary, motor     23 . element power feedthrough     24 . voltage input for plasma conduits (optional for high efficiency)     25 . inert gas input     26 . valve     27 . inert gas for power feeds     28 . gas sampling port     29 . insulated water cooled chamber with enclosed element and electrically non-conductive retort     30 . reaction zone     31 . retort lid/Venturi cone     35 . bleed line     36 . one way flow valve     38 . pressure swing absorption unit     39 . mass flow control     40 . virgin gas source     41 . conduit     62 . outlet of reaction zone.       

     System  100  may comprise a fluid cooled reaction chamber  29 . The chamber  29  may be generally cylindrical. In various embodiments, the chamber  29  has an internal diameter of 10.0 inches (25.4 cm), and a height of 16.0 inches (40.6 cm). However, the chamber  29  can be scaled up or down, and the sizes of the other components, such as the power supplies, scaled accordingly. The gas flow rates may also be adjusted to optimize the recirculation and densification process. System  100  may be controlled by engineering software. For example, a suitable systems engineering software may control the system and collect data in real time, and is available from National Instruments, 11500 Mopac Expressway, Austin, Tex. (U.S.A.) under the tradename Labview®. 
     In various embodiments, a sample may be loaded onto an electrically isolated sample stage/support  16  inside a reaction zone  30  defined by a retort  19  located within the chamber  29 . Stated differently, reaction zone  30  may be located within retort  19 . In various embodiments, chamber  29  may comprise a vacuum tight oven/furnace. In various embodiments, stage  16  may comprise an in-situ electrically isolated sample balance. 
     Referring to  FIGS.  6 A and  6 B , retort  19  may comprise a retort lid  31  coupled to a generally cylindrical retort body  33 . Retort lid  31  may be located proximate an outlet  62  of reaction zone  30 . Retort lid  31  may be configured to have a Venturi effect at the outlet  62  of reaction zone  30 . In various embodiments, retort lid  31  may comprise a Venturi cone. Stated differently, retort lid  31  may comprise a generally frustoconical shape. 
     Atmospheric gases may be evacuated with a roughing vacuum pump  1  and/or turbo vacuum pump  2  to a pressure below 2 Torr (266.6 pascal (Pa)). Inert gas may be input into reaction zone  30  to force out any remaining atmospheric gases in the reaction zone  30  and/or chamber  29 . The chamber  29  is heated to the desired temperature set point with a heater, and the pressure is maintain below 100 Torr (13,332.2 Pa). Inert gas may be provided to the chamber  29  through the power feedthroughs  23 / 25  at flows less than or equal to 150 standard cubic centimeter per minute (sccm or cm 3 /min) (i.e., less than or equal to 9.15 in 3 /min) at the location of the stage  16 . When the set point temperature (isothermal or gradient), is reached, one or more dry recycling/recirculating pumps  7 ,  8 ,  9 , and/or  13  may be turned on. Pumps  7 ,  8 ,  9 , and  13  may comprise one or more constant or variable speed turbo pumps, constant or variable speed dry pumps, diaphragm pumps, or other suitable pump and/or combinations thereof. 
     The exhaust to inlet circuit is opened and the flow and make-up of the recirculated gas can be measured and analyzed at a point (e.g., at sampling port  14 ) before the recirculated gases are input into reaction zone  30 . For example, the recirculated gas may be analyzed proximate to a low pressure one way flow valve  36  located just prior to (i.e., upstream from) the point where the recirculated gas and virgin gas are combined and input into reaction chamber  29 . As used herein, “virgin gas” refers to gas that has not yet flowed through reaction zone  30 . As used herein, “recirculated gas” refers to gas that has flowed through reaction zone  30  at least one time. In various embodiments, prior to being input into reaction zone  30 , the virgin gas and the recirculated gas may be combined in a single conduit (e.g., gas inlet tube  15 ). In various embodiments, the virgin gas and the recirculated gas may be input into reaction zone  30  via separate conduits. Stated differently, a first conduit may be coupled between virgin gas source  40  and reaction zone  30 , and a second conduit, discrete from the first conduit, may be coupled between one way flow valve  36  and reaction zone  30 . In various embodiments, a mass flow control (MFC)  39  may be in operable communication with virgin gas source  40  and may measure and control a flow rate of virgin reaction gas input into reaction zone  30 . In various embodiments, the flow of virgin gas may be adjusted in response to data obtained at sampling port  14 . 
     The vacuum pressure may be set to a desired level and exhausted through a flow controller such as a throttle valve  4  at a point immediately after the first recycle/recirculating turbo pump  7  connected by conduits and separated by a valve  26  that can be opened or closed at any time for the process. The diameter/orifice size of the conduits and valves may be varied throughout system  100  to control flow output to the throttle valve  4 . The output may pass through a secondary turbo pump  2  just prior to the roughing vacuum pump  1 . Virgin hydrocarbon reaction gases such as for example, methane 0-100 liter per minute (L/min) (0-26.4 gal/min), propane 0-50 L/min (0-13.2 gal/min), natural gas mixtures 0-150 L/min (0-39.6 gal/min), or any desired hydrocarbon reaction gas can be introduced right after the one way flow valve  36  on the inlet/upstream side of reaction zone  30 . Hydrogen can be included or excluded from the process depending on the microstructure desired. For example, a pressure swing absorption swing (PSA)  38  or magnetite filter may be located upstream of one way flow valve  36 , and may extract hydrogen from the recirculated gas. The pressure within system  100  may be maintained below 100 Torr (13,332.2 Pa), either at a constant pressure or pulsed from a low pressure to a higher pressure 100 Torr or less (13,332.2 Pa or less) 
     The sample(s), which may be located on stage  16 , can either be grounded or charged by a constant positive or negative DC voltage or an AC voltage that can be frequency varied and/or positively/negatively biased. The walls of the retort  19  can be grounded, or positively or negatively charged, but are isolated from the sample(s). In various embodiments, the sample may be negatively charged, and the walls of the retort  19  may be configured to be insulating. The contact between the reaction gases and the heating elements may be minimized. The Venturi effect may be utilized at the exhaust side of the chamber  29 /retort  19  (i.e., at lid  31 ) to separate the phases of the reaction gas. The Venturi effect may also be utilized at the first recirculating turbo pump  7 . At a point on the exhaust side of reaction zone  30  (i.e., after outlet  62 ) and before the first recycle/recirculating pump  7 , a bleed line  35  may be fed to a precision flow controller  10  (e.g., a throttle valve), which bypasses the throttle valve  4 , and to the primary exhaust turbo pump  2  before the roughing pump  1 . In this regard, a portion of the gas downstream of valve  26  may be directed through bleed line  35  to roughing pump  1 . Bleed line  35  may also fluidly coupled to a point after (i.e., downstream from) the recycle/recirculating pumps  7 ,  8 ,  9 . In this regard, a portion of the gas downstream from pump  9  may be directed through bleed line  35  to roughing pump  1 . When the main throttle valve  4  is closed, the precision flow controller  10  may be engaged to control the pressure within system  100 . In various embodiments, the conduits of bleed line  35  may have a diameter that is equal to or less than ¼th the diameter of the main recirculating path plumbing (i.e., conduits). The exhaust gas can be analyzed (e.g., at sampling port  28 ) and the recirculating flow rate adjusted to increase the process efficiency. For example, in response to the data output from sampling port  28 , a larger or a smaller portion of the recirculated gas may be directed through bleed line  35  and roughing pump  1  to maintain a desired pressure and/or recirculation flow rate within system  100 . Because the system is continuously recycled/recirculated, the main upstream flow inlet can be shut off simultaneously with the exhaust and recirculated to extract most of the carbon in the hydrocarbon gases. 
     During the process described above, an electric arc variable frequency or fixed de corona plasma  20  can be implemented at the Venturi point (i.e., the retort lid  31 ) at the outlet  62  of reaction zone  30 , near where the exhausting gas begins to condense out the heavy hydrocarbons as tars in order to breakdown and form simpler hydrocarbon reaction gases for the purpose of recycling/recirculating back into reaction zone  30 . Additional electric arc corona plasmas can be used prior to the bleed line  35  before the first recycling/recirculating pump  7 . To extract excess hydrogen on the exhaust side a magnetite filter can be used after the plasma treatment. In addition, the entire recirculating plumbing, before and after (upstream of and downstream of) the turbines of pumps  7 ,  8 ,  9 , can be high temperature plasma transfer lines. The recirculating plumbing may be designed to incorporate a solenoidal electromagnet system surrounding the plasma tubing thereby enhancing the partially/fully ionized plasma and allowing for further adjustment of stream density and direction. 
     Pumps  7 ,  8 ,  9 , may comprise a controllable variable speed turbo/turbine vacuum pump with either an in-line or 90 degree exit port that can operate at vacuum pressures above 3 Torr (400.0 Pa) with the turbine blades made from stainless steel, CMC or other high strength, high temperature alloys or composites. The entrance and/or exit ports of the pumps  7 ,  8 ,  9  may utilize the Venturi effect to enhance gas flow. The pumps  7 ,  8 ,  9  can be air powered, electric motor powered, fluid powered, etc. with or without magnetic bearings. The higher the rpm the better as far as flow capability. Though there may be no upper limit, a lower limit of 10,000 rpm may be reasonable for the system. 
     Pumps  7 ,  8 ,  9 , may comprise one or more constant or variable speed inline turbo pump(s) or 90 degree turbo pump(s), or one or more constant or variable speed dry pump(s). Pumps  7 ,  8 ,  9  may be used to recycle/recirculate the reaction gases not consumed in the first pass through the reaction zone  30  (i.e., the gases output at outlet  62 ). In various embodiments, system  100  may employ pumps  7 ,  8 ,  9  without plasma circulation. 
     In various embodiments, partially or fully ionized hydrocarbon, argon, helium, hydrogen or combinations and mixtures of gases including a hydrocarbon gas, may use plasma conduits to flow and recirculate said gases with or without the use of one or more the pumps  7 ,  8 ,  9  to aid in the recirculation. 
     A dry diaphragm pump  13  can be used either alone or in combination with one or more of the pump(s)  7 ,  8 ,  9  to recycle/recirculate the hydrocarbon gases in the CVI/CVD process to improve efficiency and the rate of carbon deposition (i.e. preform densification). 
     In various embodiments, hydrocarbon gases, including heavy hydrocarbons, are recycled/recirculated and/or modified to improve the rate of densification and the efficiency of the CVI/CVD process. 
     In various embodiments, an electric arc corona plasma may be used to breakdown heavy hydrocarbon gases by either constant or variable DC voltages and/or constant or variable frequency AC voltages that can be biased negatively or positively. For increased efficiency, the plasma may be contained within the complete recirculation circuit such that the recirculated gases are dissociated and kept at a high temperature. 
     The sample(s) to be densified may be electrically isolated from the system and either grounded or negatively or positively charged with DC voltage or by an AC voltage which can be frequency modulated and biased positively or negatively to increase the rate of deposition and the efficiency. 
     The retort  19  may be electrically isolated from the sample(s). The retort may be grounded, or charged with DC or AC voltages. 
     A turbo vacuum pump  7  can be followed by a dry diaphragm pump  13  or multiple turbo pumps  8 ,  9  to aid in building up a pressure higher than the reaction chamber  29  pressure at the reaction gas entrance point to open an alternative one way flow valve  36  directed toward the reaction chamber/retort to facilitate continuous or pulsed recirculation. 
     The efficiency of the CVI/CVD process in depositing carbon may be greater than 5% as measured by the total mass of carbon contained in the total volume of the hydrocarbon gases used when compared to the mass gain of the preform, substrate or particulate mold. The sample(s) may be weighed using an in-situ measurement process and device in the CVI/CVD system. 
     An electrically isolated coiled tube in or near the electrically isolated retort  19  can be provided to preheat the incoming reaction gases and provide an electrical conduction path to charge or ground the interior walls of the electrically isolated retort. 
     The Venturi effect can be used to separate the phases of the hydrocarbon gases exiting the retort  19  for increasing efficiency using a plasma at the point prior to or at the heavy hydrocarbon condensation point within the reaction chamber exit tube. 
     In various embodiments, the recirculating/recycling may occur after the roughing vacuum pump  1 , which may be a dry vacuum pump or a liquid vacuum pump. For example, in various embodiments, at least a portion of the effluent gas exiting roughing pump  1  may be recirculated into reaction zone  30  via a conduit  41 . In various embodiments, after the roughing pump  1 , the effluent gas may go through a pressure swing unit  38  or magnetite filter to extract hydrogen before circulating back into the reaction zone  30 . 
     In system  100 , the effluent gases output from reaction chamber  29  (also referred to herein as unreacted hydrocarbon gas), at a pressure below atmospheric (e.g., less than 760 Torr (i.e. less than 101,325.0 Pa)), are recirculated in a semi-closed loop. In various embodiments, the effluent gases may be recirculated through a condensing/plasma/“whipper” system configured to remove condensable hydrocarbons of C6 variation or larger (i.e., hydrocarbons comprising six or more carbon atoms). In various embodiments, the effluent gases may be recirculated through a condensing/plasma/“whipper” system configured to remove condensable hydrocarbons of C5 variation or larger (i.e., hydrocarbons comprising five or more carbon atoms). The recirculated effluent gases may then be input into the reaction zone  30 . System  100  comprising a semi-closed loop allows the effluent gases to remain within the system plumbing such that a pressure throughout the system  100  remains fairly constant (e.g., the pressure remains under 100 Torr (13,332.2 Pa)). The closed system may allow for increased efficacy and lower energy costs as compared to conventional recirculation systems, wherein the effluent gasses are removed from the system plumbing, filtered, and re-pressurized in an external tank before being added back into the higher pressure input stream. In system  100 , the recirculated effluent gases may increase the number of moles of small carbon molecules flowing through reaction zone  30  and available for deposition within the porous substrate, without increasing the flow and/or amount of initial (i.e., virgin) reaction gas input into the system. Stated differently, recirculating the effluent gases may increase the densification rate, as more molecules are passing through the reaction zone, which increases the number of molecules that will make collisions (i.e., bond) with the porous substrate. Accordingly, the disclosed CVI/CVD systems and methods may allow for faster densification of carbon fiber preforms and/or densification to greater than 1.7 g/cc. The disclosed systems and methods of may further alleviate a need to machine the substrate to reopen closed surface pores during densification. In this regard, the substrate densification may be completed in a single CVI/CVD cycle. 
     With reference to  FIG.  2   , a system  200  for chemical vapor infiltration and densification is illustrated, in accordance with various embodiments. System  200  may comprise a reaction chamber  129 . Reaction chamber  129  may comprise a retort  119  defining an internal reaction zone  130 . Stated differently, reaction zone  130  is located in the interior of retort  119 . 
     Retort  119  may be similar to retort  19  in  FIG.  6 A . Retort  119  may comprise a retort lid  131 , similar to retort lid  31  in  FIG.  6 B . Retort lid  131  may be configured to have a Venturi effect. In various embodiments, retort lid  131  may comprise a Venturi cone. Stated differently, retort lid  131  may comprise a generally frustoconical shape. Retort lid  131  may be located proximate an outlet  162  of reaction zone  130 . 
     A stage  116  is located within reaction zone  130 . Stage  116  may be electrically isolated from various other components of system  200  (e.g., from the heating elements  153  and or walls of retort  119 ). During CVI/CVD, porous preform(s)  132  may be disposed on stage  116 . The porous preform(s)  132  may be grounded, charged with a positive or negative DC voltage, or frequency oscillated using an AC voltage that can be biased positively or negatively. 
     A virgin gas source  140  may be fluidly coupled an inlet  152  of reaction chamber  129  via a conduit  142 . As used herein, “virgin gas” refers to gas that has not yet been sent through the reaction zone  130 . Virgin gas source  140  may supply methane, ethane, propane, butane, natural gas mixtures, or any desired hydrocarbon reaction gas to reaction zone  130 . An MFC  146  may be in operable communication with virgin gas source  140  and may measure and control a flow rate of virgin reaction gas input into reaction zone  130 . For example, MFC  146  may cause virgin gas source  140  to output between 0-50 liter standard per minute (L/min) of virgin reaction gas into reaction zone  130 . In various embodiments, MFC  146  may cause virgin gas source  140  to output between 0-100 liters/min (L/min) of virgin reaction gas into reaction zone  130 . In various embodiments, MFC  146  may cause virgin gas source  140  to output between 0-150 L/min (0-39.6 gal/min) of virgin reaction gas into reaction zone  130 . System  200  may comprise a load cell  148  configured to measure a mass of porous substrate  132  at various points, or continuously, throughout the CVI/CVD process. MFC  146  may adjust the flow of virgin gas input into reaction zone  130 , in response to data output from one or more pressure or other type sensors  192  located within system  200 . In various embodiments, a pressure within system  200  is maintained at or below 100 Torr (13,332.2 Pa). 
     One or more inert gas inlets  154  may be in fluidly coupled to reaction zone  130 . Inert gas (e.g., argon and nitrogen) may be input into reaction zone  130  to force atmospheric gases from reaction zone  130 . One or more heating elements  153  may be employed to heat reaction zone  130  to a desired temperature (isothermal or gradient). 
     A pre-heat zone  150  may be located between reaction zone  130  and inlet  152  of reaction chamber  129 . Inlet  152  may comprise an inlet for inputting the virgin reaction gases and the recirculated gases, as discussed in further detail below, into reaction chamber  129 . Pre-heat zone  150  may comprise heated and/or charged coils  151  configured to increase a temperature of the reaction gases (e.g., the virgin recirculated gases) entering reaction zone  130 . In various embodiments, coils  151  provide an electrical conduction path to charge or ground an interior wall of retort  119 . In various embodiments, a high voltage and inert gas feedthrough  193  may be located proximate inlet  152 . 
     One or more conduits  163  may fluidly couple a roughing pump  160  an exhaust outlet  162  located at retort lid  131 . In that regard, effluent gases  164  may be drawn out of reaction zone  130  by roughing pump  160 . The effluent gases  164  may exit reaction zone  130  via outlet  162 . 
     Upon exiting outlet  162 , effluent gases  164  may be directed through conduits  163 . In various embodiments, conduits  163  may diverge at an intersect point  165  such that a first set of conduits  163   a  define, at least a portion of, a first fluid path  177 , and a second set of conduits  163   b  define, at least a portion of, a second fluid path  179 . In that regard, effluent gases  164  may flow along fluid path  177  (also referred to herein as an “exit path”), represented by arrows  168 , toward roughing pump  160  and/or effluent gases  164  may flow along a fluid path  179  (also referred to herein as a “recirculation path”), indicated by arrows  174 , toward inlet  152 . In various embodiments, at least a portion of the effluent gas  164  entering recirculation path  179  may be recirculated back into reaction zone  130  via conduits  163   b . A first valve  172  may be located between roughing pump  160  and the intersect point  165 . In various embodiments, a turbo pump  173  may be located between the first valve  172  and the roughing pump  160 . In various embodiments, a throttle valve  176  may located downstream of the first valve  172 . Stated differently, effluent gases  164  flow through throttle valve  176  after flowing though first valve  172 . The vacuum pressure within reaction zone  130  and throughout system  200  may be maintained at a desired level by opening and closing throttle valve  176  and first valve  172  and/or by increasing and decreasing the speed of roughing pump  160  and turbo pump  173 . 
     A pump  180  may be located along the recirculation path  179 . Pump  180  may include a turbo pump (e.g., a constant speed or a variable speed turbo pump), a dry pump (e.g., a constant speed or a variable speed dry scroll pump or a constant speed or a variable speed dry screw pump), or other pump capable of operating at vacuum pressures greater than 3 Torr and less than 100 Torr (i.e., greater than 400.0 Pa and less than 13,332.2 Pa). A second valve  182  may be located between pump  180  and intersect point  165 . First valve  172  and second valve  182  may regulate the flow and/or portion of effluent gases  164  within exit path  177  and recirculation path  179 . Pump  180  may comprise an in-line exit port or a 90 degree exit port. Pump  180  may comprise blades made from stainless steel, CMC or other high strength, high temperature alloys or composites. In various embodiments, a second pump  184  may be located downstream of pump  180  and second valve  182 . Second pump  184  may include a turbo pump, a dry pump, or other suitable pump. In various embodiments, second pump  184  may comprise a diaphragm pump. 
     Pump  180  and/or second pump  184  may be turned on and second valve  182  may be opened to direct at least a portion of effluent gases  164  into recirculation path  179 . At least a portion of gas effluent gases  164  within recirculation path  179  may be input into reaction zone  130 , thereby increasing the number of moles of carbon passing through reaction zone  130  and available for reaction with porous substrate  132 . In various embodiments, a volume flow meter  196  may be in operable communication with first valve  172 , second valve  182 , and/or a valve  190   a  to control the flow effluent gases  164  entering recirculation path  179 . Stated differently, volume flow meter  196  may be in operable communication with first valve  172 , second valve  182 , and/or valve  190   a  to control the flow effluent gases  164  entering reaction zone  130 . In various embodiments, prior to being input into reaction zone  130 , the virgin gas and the recirculated gas may be combined in a single conduit (as shown in  FIG.  1   ). In various embodiments, the virgin gas and the recirculated gas may be input into reaction zone  130  via separate conduits. Stated differently, a first conduit (e.g. conduit  142 ) may be coupled between virgin gas source  140  and inlet  152 , and a second conduit (e.g., conduit  163   b ), discrete from the first conduit, may be coupled between valve  190   a  and inlet  152 . 
     In various embodiments, additional valves  190  may be located along exit path  177  and/or recirculation path  179 . Valves  190  may be configured to regulate a flow of effluent gas  164  through system  200 . Valves  190 , in combination with roughing pump  160  and/or pump  180 , may also aid in maintaining the desired pressure level within system  200 . Opening or closing one or more valves  190  and/or adjusting a speed of roughing pump  160  and/or pump  180  may control the flow rate of effluent gas  164  through exit path  177  and recirculation path  179 . One or more pressure or other type sensor  192  may be located throughout system  200 . Valves  190  may be opened or closed and the speed of roughing pump  160  and/or pump  180  may be adjusted in response to data output from sensors  192 . 
     In various embodiments, one or more plasma conduits  202  may be located between effluent gas outlet  162  and second valve  182  and between second valve  182  and a trap  204 . Plasma conduits  202  may breakdown longer hydrocarbon chains. In various embodiments, plasma conduits  202  may breakdown a hydrocarbon molecules containing six or more carbon atoms into two or more hydrocarbon molecules containing less than six carbon atoms. For example, plasma conduits  202  may breakdown a hydrocarbon molecule containing six carbon atoms into two hydrocarbon molecules each containing three carbon atoms, or into one hydrocarbon molecule containing four carbon atoms and one hydrocarbon molecule containing two carbon atoms, or into three hydrocarbon molecules each containing two carbon atoms. A frequency of the plasma conduits maybe selected to breakdown a particular molecule size. In various embodiments, the frequency of the plasma conduits  202  may be selected to breakdown hydrocarbon molecules containing six or more carbon atoms. In various the frequency of the plasma conduits may be selected to breakdown hydrocarbon molecules containing five or more carbon atoms. In various embodiments, additional plasma conduits may be located downstream of trap  204 . 
     In various embodiments, trap  204  may be located downstream from second valve  182 . Trap  204  may comprise a one more sets of spinning blades. The rotating blades may be configured to condense hydrocarbons having six or more carbon atoms. In various embodiments, heavier hydrocarbon (e.g., hydrocarbons comprised of six or more carbon atoms) may condense in trap  204 , thereby leaving smaller hydrocarbon molecules in the gaseous state. Stated differently, the effluent gas output from trap  204  may comprise primarily smaller hydrocarbons (e.g., hydrocarbons comprised of fewer than six carbon atoms). 
     In various embodiments, a hydrogen extraction component  206  may be disposed along recirculation path  179 . Hydrogen extraction component  206  may be configured to separate hydrogen gas from the effluent gas. In various embodiments, hydrogen extraction component  206  comprises a PSA unit and/or magnetite filter configured to remove hydrogen from the effluent gas. The extracted hydrogen may exit recirculation path  179  via a conduit  208 . Removal of excess hydrogen may increase efficiency and densification rates as hydrogen may tend to inhibit carbon deposition (i.e., densification of porous substrate  132 ) in reaction zone  130 . 
     In various embodiments, an electric arc variable frequency or fixed DC corona plasma  210  may be implemented at the Venturi point at outlet  162  (i.e., proximate to retort lid  131 ). The electric arc variable frequency or fixed DC corona plasma may facilitate condensation of heavy hydrocarbons, which may lead to formation of lighter hydrocarbon reaction gases within the effluent gas  164  flowing through recirculation path  179  and into reaction zone  130 . 
     In system  200 , the effluent gases  164  output from reaction zone  130 , may be kept at a pressure below atmospheric (e.g., less than 760 Torr (101,325.0 Pa)), as the conduits  163  form a semi-closed loop. In various embodiments, the effluent gases  164  may be recirculated through a condensing/plasma/“whipper” system to remove condensable hydrocarbons of C5 variation or larger (i.e., hydrocarbons comprising 5 or more carbon atoms). The recirculated effluent gases  164  may then be input into the reaction zone  130 . In this regard, the recirculated effluent gases  164  remain within the system  200  plumbing such that a pressure throughout the system  200  remains fairly constant (e.g., the pressure remains under 100 Torr (13,332.2 Pa)). The closed system may allow for increased efficacy and lower energy costs as compared to conventional recirculation systems, wherein the effluent gasses are removed from the system plumbing, filtered, and re-pressurized in an external tank before being added back into the higher pressure input stream. 
     Recirculating effluent gases  164  may increase the number of moles of small carbon molecules (e.g., molecules having five or less carbon atoms or, for example, molecules having four or less carbon atoms) flowing through the reaction zone  130  and available for deposition within the porous substrate  132 , without increasing the flow and/or amount of virgin reaction gas input into the system. Stated differently, recirculating effluent gas  164  may increase deposition/densification rates, as more molecules are passing through the reaction zone  130 , which increases the number of molecules that will make collisions (i.e., bond) with the porous substrate  132 . Accordingly, the disclosed system and method of CVI/CVD may allow for faster densification of carbon fiber preforms and/or densification to greater than 1.7 g/cc. The disclosed system and method may also alleviate a need to machine porous substrate  132  to reopen closed surface pores. In this regard, system  200  may allow the densification of porous substrate  132  to be completed in a single CVI/CVD cycle. 
     With reference to  FIG.  3   , a system  300  for chemical vapor infiltration and densification comprising a recirculation path with cryogenic cooling stage is illustrated, in accordance with various embodiments. System  300  may comprise a reaction chamber  329  comprising a retort  319  that defines a reaction zone  330 . Stated differently, reaction zone  330  is located in the interior of retort  319 . Retort  319  may be similar to retort  19  in  FIG.  6 A . Retort  319  may comprise a retort lid  331 , similar to retort lid  31  in  FIG.  6 B . Retort lid  331  may be configured to have a Venturi effect. In various embodiments, retort lid  331  may comprise a Venturi cone. Stated differently, retort lid  331  may comprise a generally frustoconical shape. Retort lid  331  may be located proximate an outlet  362  of reaction zone  330 . 
     A stage  316  may be located within reaction zone  330  and may support one or more porous substrates  332 , during the CVI/CVD process. Virgin gas may be input, from virgin gas source  340 , into reaction zone  330  via an inlet  352  of reaction chamber  329 . An MFC  346  may be in operable communication with virgin gas source  340  and may measure and control a flow rate of virgin reaction gas input into reaction zone  330 . One or more inert gas inlets  354  may be in fluidly coupled to reaction zone  330 . Inert gas may be input into reaction zone  330  to force atmospheric gases from reaction zone  330 . Effluent gas  364  may exit reaction zone  330  via the outlet  362  located at retort lid  331 . Upon exiting outlet  362 , effluent gas  364  may be directed through a series of conduits (e.g., pipes)  363 . In various embodiments, an electric arc variable frequency or fixed DC corona plasma  310  may be employed downstream of exhaust outlet  362 . 
     In various embodiments, one or more plasma conduits  402  may be located between outlet  362  and a trap  404 . In various embodiments, trap  404  may comprise a “whipper” having one or more sets of rotating blades configured to breakdown (i.e., condense) larger hydrocarbons. For example, a temperature within trap  404  may cause hydrocarbon molecules comprising six or more carbon atoms to condense. 
     System  300  may comprise a cryogenic-cooler  410 . In various embodiments, cryogenic cooler  410  may be a helium (He) cryogenic cooler or a liquid nitrogen condenser. Cryogenic-cooler  410  may be employed to condense (i.e., liquefy) lighter hydrocarbon gases (e.g., hydrocarbons gases comprising hydrocarbon molecules having six or less carbon atoms, or, for example, hydrocarbon gases comprising hydrocarbon molecules having five or less carbon atoms). Condensing hydrocarbon gases may allow hydrogen, which remains in a gaseous state, to be readily extracted from cryogenic cooler  410 . In this regard, cryogenic-cooler  410  may comprise a hydrogen extraction component. A frustoconical shaped conduit  408  may couple trap  404  and cryogenic-cooler  410 . The frustoconical shape of conduit  408  may facilitate a Venturi effect. The gaseous hydrogen may exit cryogenic-cooler  410  along exit path  377  (i.e., indicated by arrows  381 ). The liquefied hydrocarbons may accumulate in a conduit  414 . A frustoconical shaped conduit  412  may be located at the exhaust outlet of cryogenic cooler  410  and may be fluidly coupled to cryogenic-cooler  410  and conduit  414 . The frustoconical shape of conduit  412  may allow for expansion of the hydrocarbons exiting cryogenic-cooler  410  and may facilitate a phase change of the hydrocarbons from the liquid state to the a gaseous state. Once most or all the hydrogen is removed, the liquefied hydrocarbons may be heated within conduit  414 . The heating returns the small hydrocarbons to a gaseous state. 
     A valve  384  may be opened and one or more pumps (e.g., a primary pump  380  and one or more secondary pumps  382 ) located along recirculation path  379  may be turned on such that the gaseous hydrocarbons will flow from conduit  414  through recirculation path  379  (indicated by arrows  474 ). The flow of the effluent gas  364  through recirculation path  379  may be controlled via valve  384 , primary pump  380 , and/or one or more secondary pumps  382 . In various embodiments, a volume flow meter  396  may be in operable communication with a valve  390   a  (e.g., a one way flow valve) to control the flow of recirculated gas entering reaction zone  330 . 
     The flow of effluent gas  364  and the hydrogen gas extracted in cryogenic cooler  410  may be controlled via a valve  372 , a throttle vale  376 , a roughing pump  360 , and/or a turbo pump  373 . In various embodiments, a portion  397  of the effluent gas output from roughing pump  360  may be recirculated back into reaction zone  330 . A compressor pump  399  may be located downstream of roughing pump  360 . In various embodiments, compressor pump  399  may comprise a second cryogenic-cooler, PSA, magnetite filter, or other hydrogen extraction component. 
     The vacuum pressure within reaction zone  330  and system  300  may be maintained at the desired level by opening and closing throttle valve  376 , valve  372 , and/or valve  384  and by increasing and decreasing the speed of roughing pump  360  and/or pump  380 . In various embodiments, additional valves  390  may be located along the exit path  377  and/or recirculation path  379 . One or more pressure or other type sensor  392  may be located throughout system  300 . Valves  390  may be configured to regulate flow of effluent gas  364  through system  300 . Opening or closing one or more valve  390  may control the flow rate of effluent gas  364  through exit path  377  and/or recirculation path  379 . Opening or closing one or more valves  390  may also help maintain the desired pressure level within system  300 . 
       FIG.  4    is a graph  500  comparing the weight gain (i.e., densification) of a porous substrate sample  5  within a CVI/CVD system employing recirculated effluent gas (line  502 ), as disclosed herein, to the weight gain of a porous substrate sample  8  within a CVI/CVD system employing an increased virgin gas flow rate (line  504 ), to the weight gain of a porous substrate sample  6  within a control CVI/CVD system (line  506 ). Graph  500  illustrates that at 25 hours, an increased virgin gas flow rate increased the weight gain of the porous substrate, as compared to the weight gain of the control substrate, by approximately 4%. Whereas, at 25 hours recirculating effluent gas increased the weight gain of the sample substrate, as compared to the weight gain of the control substrate, by approximately 37%. Graph  500  illustrates that a higher efficiency and a more rapid densification may be achieved by recirculating effluent gas as compared to increasing the virgin gas flow. Accordingly, the disclosed systems and methods of CVI/CVD may allow for faster densification of a carbon fiber preform and/or densification to greater than 1.7 g/cc. The disclosed systems and methods of CVI/CVD may allow for densification of a carbon fiber preform without machining steps to re-open closed surface pores. In this regard, the carbon fiber preform may be densified during a single CVI/CVD cycle. 
     Referring to  FIG.  5 A , a method  600  of CVI/CVD is illustrated in accordance with various embodiments. Method  600  may comprise disposing a porous substrate within a reaction chamber (step  602 ), establishing a sub-atmospheric pressure within the reaction chamber (step  604 ), introducing a hydrocarbon reaction gas into a reaction zone of the reaction chamber to densify the porous substrate (step  606 ), and withdrawing unreacted hydrocarbon reaction gas from the reaction chamber (step  608 ). In various embodiments, the unreacted hydrocarbon reaction may comprise hydrocarbon molecules having six or more carbon atoms. Method  600  may further comprise removing at least a portion of the hydrocarbon molecules having six or more carbon molecules from the unreacted hydrocarbon reaction gas by causing the portion of the hydrocarbon molecules having six or more carbon atoms to condense (step  610 ). In various embodiment, step  610  may comprise flowing the unreacted hydrocarbon reaction gas through a trap including one or more sets of rotating blades. Method  600  may further comprise recirculating at least a portion of the unreacted hydrocarbon reaction gas back into the reaction zone (step  612 ). 
     Referring to  FIG.  5 B , a method  620  of CVI/CVD is illustrated in accordance with various embodiments. Method  620  may comprise disposing a porous substrate within a reaction chamber (step  622 ), applying an electrical voltage to the porous substrate (step  624 ), and establishing a sub-atmospheric pressure within the reaction chamber (step  626 ). In various embodiments, step  626  may comprise inputting inert gas into the reaction chamber. 
     Method  620  may further comprise introducing a hydrocarbon reaction gas into a reaction zone of the reaction chamber to densify the porous substrate (step  628 ), and withdrawing unreacted hydrocarbon reaction gas from the reaction chamber (step  630 ). In various embodiments, the unreacted hydrocarbon reaction may comprise hydrocarbon molecules having six or more carbon atoms. Method  620  may further comprise removing at least a portion of the hydrocarbon molecules having six or more carbon molecules from the unreacted hydrocarbon reaction gas (step  632 ). In various embodiment, step  632  may comprise flowing the unreacted hydrocarbon reaction gas through a trap including one or more sets of rotating blades. In various embodiment, step  632  may comprise applying an electric arc to the unreacted hydrocarbon reaction gas withdrawn from the reaction chamber. 
     Method  620  may further comprise extracting hydrogen from the unreacted hydrocarbon reaction gas (step  634 ). In various embodiment, step  634  may comprise flowing the unreacted hydrocarbon reaction gas through at least one of a cryogenic-cooler or a pressure swing absorption unit. Method  620  may further comprise recirculating at least a portion of the unreacted hydrocarbon reaction gas back into the reaction zone (step  636 ), and heating at least one of the hydrocarbon reaction gas or the portion of the unreacted hydrocarbon reaction gas recirculated into the reaction zone using a charged coil located proximate an inlet of the reaction chamber (step  638 ). In various embodiments, the charged coil may provide an electrical conduction path to either charge or ground an interior wall of the reaction chamber. 
     Benefits, other advantages, and solutions to problems have been described herein with regard to specific embodiments. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical system. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the disclosure. 
     The scope of the disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” It is to be understood that unless specifically stated otherwise, references to “a,” “an,” and/or “the” may include one or more than one and that reference to an item in the singular may also include the item in the plural. All ranges and ratio limits disclosed herein may be combined. 
     Moreover, where a phrase similar to “at least one of A, B, and C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C. Different cross-hatching is used throughout the figures to denote different parts but not necessarily to denote the same or different materials. 
     The steps recited in any of the method or process descriptions may be executed in any order and are not necessarily limited to the order presented. Furthermore, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Elements and steps in the figures are illustrated for simplicity and clarity and have not necessarily been rendered according to any particular sequence. For example, steps that may be performed concurrently or in different order are illustrated in the figures to help to improve understanding of embodiments of the present disclosure. 
     Any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option. Additionally, any reference to without contact (or similar phrases) may also include reduced contact or minimal contact. Surface shading lines may be used throughout the figures to denote different parts or areas but not necessarily to denote the same or different materials. In some cases, reference coordinates may be specific to each figure. 
     Systems, methods and apparatus are provided herein. In the detailed description herein, references to “one embodiment,” “an embodiment,” “various embodiments,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement the disclosure in alternative embodiments. 
     Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element is intended to invoke 35 U.S.C. 112(f) unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.