Patent Publication Number: US-10332726-B2

Title: Microwave chemical processing

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/351,858, filed Nov. 15, 2016, and entitled “Microwave Chemical Processing,” which is incorporated fully herein by reference. 
    
    
     BACKGROUND 
     Microwave plasmas are used in the industrial chemical processing of gases. This is typically accomplished by flowing the gases to be reacted through an elongated vessel while microwave radiation is coupled into the vessel to generate a plasma. The plasma cracks the gas molecules into component species. Microwave chemical processing systems are effective because microwave plasmas operate at relatively high power coupling efficiencies at low ion energies, and are capable of supporting various gas reactions, such as the conversion of methane into hydrogen and carbon particulates, the conversion of carbon dioxide into oxygen and carbon, and coating particulates and other seed materials with other layers for functionalization and complex layered materials and aggregates processing. 
     Typical systems for chemical gas processing include a quartz reaction chamber through which process gases flow, and a microwave magnetron source coupled to the reaction chamber through a waveguide. The input microwave radiation can be continuous wave or pulsed. Systems are designed to control the effective coupling of the microwave radiation into the reaction chamber, and the gas flow within the reaction chamber to improve the energy absorption by the flowing gas. Often the systems include a wedge located where the microwave waveguide intersects the quartz reaction chamber, to concentrate the electric field within a small area, and the waveguide conductive walls are not exposed to the gases to be processed. 
     One example of chemical processing is the microwave processing of methane to produce hydrogen. Methane can be cracked by a plasma into CH x  radicals and H-atoms. When such systems are operated in continuous mode, the H-atom density is mainly controlled by the gas temperature, which is directly related to the microwave power density, and in some cases by diffusion processes. The CH x  radical density, likewise, is controlled by the gas temperature and H-atom concentrations. Alternatively, when such systems are operated in pulsed mode, H-atom and CH x  radical production is controlled by in-pulse power density and its associated higher plasma kinetic energy, which controls gas temperature and thermal dissociation. Typically, during the time the plasma is off the H-atoms recombine and are consumed. Short duty cycles are used to increase the in-pulse power for a constant time-averaged power, and short off-plasma times are used to limit H-atom recombination. Therefore, pulsed systems crack the methane into hydrogen and other hydrocarbon radicals more efficiently (i.e., using less time-averaged input power) than continuous wave systems. 
     SUMMARY 
     In methods of the present embodiments, pulsed microwave radiation is supplied through a waveguide having a length, where the microwave radiation propagates in a direction along the waveguide. A pressure within the waveguide is at least 0.1 atmosphere. A supply gas is provided into the waveguide at a first location along a length of the waveguide, where a majority of the supply gas flows in the direction of the microwave radiation propagation. A plasma is generated in the supply gas in at least a portion of the length of the waveguide, and a process gas is added into the waveguide at a second location downstream from the first location. A majority of the process gas flows in the direction of the microwave propagation at a rate greater than 5 slm. An average energy of the plasma is controlled to convert the process gas into separated components, by controlling at least one of i) a pulsing frequency of the pulsed microwave radiation, where the pulsing frequency is greater than 500 Hz; and ii) a duty cycle of the pulsed microwave radiation, where the duty cycle is less than 90%. 
     In gas processing systems of the present embodiments, the systems include a waveguide having a first gas inlet, a second gas inlet downstream of the first gas inlet, and a length. The first inlet is configured to receive a supply gas, and the second inlet is configured to receive a process gas. A pulsed microwave radiation source is coupled to the waveguide to generate a plasma in the supply gas, where the microwave radiation propagates in a direction along the length of the waveguide to react with the process gas. The microwave radiation source is configured to pulse microwave radiation on and off at a frequency from 500 Hz to 1000 kHz and with a duty cycle less than 90%, the majority of the flow of the supply gas and the majority of the flow of the process gas are parallel to the direction of the microwave propagation. The flow of the process gas is greater than 5 slm, and the waveguide is configured to accommodate pressures of at least 0.1 atmosphere. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-section of a conventional microwave chemical processing system. 
         FIG. 2  is a vertical cross-section of a microwave gas processing system in accordance with some embodiments of the present disclosure. 
         FIG. 3  is a vertical cross-section of a microwave gas processing system in accordance with further embodiments of the present disclosure. 
         FIG. 4  is a graph of time variation of plasma temperature within a reaction chamber in accordance with embodiments of the present disclosure. 
         FIG. 5  is a graph of time variation of plasma temperature within a reaction chamber, where the pulsing period is shorter than that of  FIG. 4 . 
         FIG. 6  is a vertical cross-section of a microwave gas processing system with precursor gas input, in accordance with embodiments of the present disclosure. 
         FIG. 7  is a vertical cross-section of a microwave gas processing system having a filament, in accordance with embodiments of the present disclosure. 
         FIG. 8  is a vertical cross-section of a microwave gas processing system in which an electron source and an electrode are depicted, in accordance with embodiments of the present disclosure. 
         FIG. 9  is an example flow chart of methods for microwave processing of a gas in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made to embodiments of the disclosed invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the present technology, not as a limitation of the present technology. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present technology without departing from the scope thereof. For instance, features illustrated or described as part of one embodiment may be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter covers all such modifications and variations within the scope of the appended claims and their equivalents. 
     The present disclosure relates to microwave plasma chemical processing of hydrocarbon gases. More specifically, the present disclosure relates to microwave plasma chemical processing of hydrocarbon gases using various techniques including pulsing of the microwave radiation to control the energy of the plasma. The ability to control the energy of the plasma enables the selection of one or more reaction pathways in conversion of the hydrocarbon gases into specific separated components. Pulsed microwave radiation can be used to control the energy of the plasma because the short-lived high-energy species that are created when a plasma ignites can be re-generated at the start of each new pulse. The plasma energy is controlled to have a lower average ion energy than conventional techniques, but at a high enough level to enable the targeted chemical reactions to occur at high gas flows and high pressures. 
     Microwave plasma chemical processing systems using pulsed microwave radiation have been developed that control the energy of the plasma and have very high cracking efficiency, in excess of 90%. These conventional systems, however, use low flow rates, below 1 standard liter per minute (slm), and small gas volumes within the plasma, with a consequence that the production rate is low and the production cost is high. These conventional systems cannot increase the gas flow rate and the gas volume within the plasma while using high frequency microwave pulsing (e.g., above roughly 100 Hz) because the plasma cannot ignite fast enough to keep up with the pulses when a large volume and high flow of gas is used. 
     Microwave Gas Processing Systems 
     In the present disclosure, a microwave plasma can be generated in a supply and/or process gas, and the energy in the plasma is sufficient to form separated components from process gas molecules. In some embodiments, a source of microwave radiation is coupled to a reaction chamber, the plasma is generated along a first portion of the length of the reaction chamber, and the process gas is separated into components along a second portion of the length of the reaction chamber. In some embodiments, the microwave radiation is coupled directly into the plasma and not through a dielectric wall as in conventional methods. 
       FIG. 1  illustrates a conventional microwave chemical processing system. As shown in  FIG. 1 , a microwave chemical processing system  100  generally includes a reaction chamber  101 , one or more gas inlets  102  configured to receive process gas flowing into the reaction chamber, one or more outlets  103  configured to collect separated products out of the reaction chamber  101 , and a source of microwave radiation  104  that is coupled to the reaction chamber through a waveguide  105 , among other elements not shown for simplicity. The microwave radiation  109  creates a microwave plasma  106  in the reaction chamber  101 , and provides energy for reactions to occur. A microwave emitter circuit  107  can control the microwave radiation  109  emitted from the microwave radiation source  104  to be either continuous wave or pulsed. Given the right conditions, the energy in the plasma will be sufficient to form separated components from the process gas molecules. 
     Parallel Propagation Microwave Gas Processing Systems 
       FIGS. 2 and 3  show embodiments of microwave gas processing systems of the present disclosure, in which a waveguide coupled to a microwave source serves as the reaction chamber. The direction of propagation of the microwave radiation is parallel to the majority of the flow of the supply gas and/or the process gas, and the microwave radiation enters the waveguide upstream of the portion of the waveguide where the separated components are generated. 
     As shown in  FIG. 2 , a microwave chemical processing system  200 , in accordance with some embodiments, generally includes a waveguide  205 , one or more gas inlets  202  configured to receive supply gas and/or process gas  208   a  flowing into the waveguide  205 , and a microwave radiation source  204  that is coupled to the waveguide  205 , among other elements not shown for simplicity. 
     In some embodiments, microwave circuit  207  controls a pulsing frequency at which microwave radiation  209  from microwave radiation source  204  is pulsed. In some embodiments, the microwave radiation  209  from microwave radiation source  204  is continuous wave. 
     The waveguide  205  has a length L. In a portion Lo prior to where the plasma is generated, the cross-sectional area of the waveguide  205  decreases along the path of the microwave propagation. This decrease in area serves to concentrate the electric field, thus increasing the microwave energy density while still providing a significant amount of area in which plasma can be formed compared to conventional systems. For example, the decreased cross-sectional area of portion L 0 , where the gas inlet  202  is located, may have a rectangular cross-section of dimensions 0.75 inches by 3.4 inches when using a microwave radiation frequency of 2.45 GHz. This cross-sectional area is much greater than conventional systems where the plasma generation area is generally less than one square inch. The dimensions of the waveguide  205  are set according to the microwave frequency, in order to properly function as a waveguide. 
     In conventional gas processing systems, the limited region in which plasma can form, such as less than one square inch as described above, constrains the volume in which gas reactions can occur. Also, in conventional systems the microwave radiation enters the reaction chamber through a window (typically quartz). In these systems, dielectric materials (e.g., particulate carbon) are coated on the window during processing leading to less and less power delivery over time. This can be highly problematic if these separated components absorb microwave radiation because they can prevent the microwave radiation from coupling into the reaction chamber to generate the plasma. Consequently, a rapid build-up of by-products, such as carbon particles that are produced from the gas reactions, occurs and limits the run-time of the processing equipment. In the present embodiments, the system  200  and other embodiments described below are designed without the use of a window; that is, using a parallel propagation/gas flow system where the radiation enters upstream from the reaction. As a result, more energy and power can be coupled into the plasma from the microwave radiation source. The greater volume within the waveguide  205 , compared to limited reaction chamber volumes in conventional systems, greatly reduces the issue of particle build-up causing limited run-times, thus improving production efficiency of the microwave processing system. 
     The microwave radiation  209  in  FIG. 2  creates a microwave plasma  206  in the supply gas and/or process gas within a first portion Li of the length of the waveguide  205 . In some embodiments, a supply gas that is different from the process gas is used to generate the microwave plasma  206 . The supply gas may be, for example, hydrogen, helium, a noble gas such as argon, or mixtures of more than one type of gas. In other embodiments, the supply gas is the same as the process gas, where the process gas is the gas from which separated components are being created. In some embodiments, L 1  extends from a position along the waveguide downstream from the position where the supply and/or process gas  208   a  enters the waveguide  205 , to the end of the waveguide  205  or to a position between the entrance of the supply and/or process gas and the end of the waveguide  205 . In some embodiments, L 1  extends from where the supply and/or process gas  208   a  enters the waveguide  205 , to the end of the waveguide  205  or to a position between the entrance of the supply and/or process gas and the end of the waveguide  205 . The generated plasma  206  provides energy for reactions to occur in process gas  208   b  within a second portion  201  of the waveguide  205 , the second portion  201  having a length L 2 . In some embodiments, L 2  extends from where the process gas  208   a  enters the waveguide  205 , to the end of the waveguide  205  or to a position between the entrance of the process gas and the end of the waveguide  205 . Given the right conditions, the energy in the plasma  206  will be sufficient to form separated components from the process gas molecules. One or more outlets  203  are configured to collect the separated products out of the waveguide  205  downstream of the portion  201  of the waveguide where reactions occur in the process gas  208   b . In the example shown in  FIG. 2 , the propagation direction of the microwave radiation  209  is parallel with the majority of the supply and/or process gas flow  208   b , and the microwave radiation  209  enters the waveguide  205  upstream of the portion  201  of the waveguide where the separated components are generated. 
     In some embodiments, a plasma backstop (not shown) is included in the system to prevent the plasma from propagating to the microwave radiation source  204  or the gas inlet(s)  202 . In some embodiments, the plasma backstop is a ceramic or metallic filter with holes to allow the microwave radiation to pass through the plasma backstop, but preventing the majority of the plasma species from passing through. In some embodiments, the majority of the plasma species will be unable to pass the plasma backstop because the holes will have a high aspect ratio, and the plasma species will recombine when they hit the sidewalls of the holes. In some embodiments, the plasma backstop is located between portion L 0  and L 1 , or in portion L 0  upstream of portion L 1  and downstream of the gas inlet(s)  202  (in an embodiment where gas inlet  202  is within portion L 0 ) and the microwave radiation source  204 . 
       FIG. 3  shows another embodiment of a microwave chemical processing system  300  in which a supply gas and a process gas are injected at different locations. The microwave chemical processing system  300 , in accordance with some embodiments, generally includes a waveguide  305 , one or more supply gas inlets  302  configured to receive supply gas  308   a  flowing into the waveguide  305 , one or more process gas inlets  310  configured to receive process gas  311   a , and a source of microwave radiation  304  that is coupled to the waveguide  305 , among other elements not shown for simplicity. The location of process gas inlet  310  is downstream of the location of supply gas inlet  302 , where downstream is defined in a direction of the microwave propagation. 
     In some embodiments, microwave circuit  307  controls a pulsing frequency at which microwave radiation  309  from microwave radiation source  304  is pulsed. In some embodiments, the microwave radiation from radiation source  304  is continuous wave. 
     The waveguide  305  has a length L. The length L 0  of the waveguide where microwave radiation  309  enters has a decreasing cross-sectional area along the direction of the microwave propagation, as described above in relation to  FIG. 2 . The microwave radiation  309  creates a microwave plasma  306  in the supply gas  308   b  within a first portion L 1  of the length L of the waveguide  305 . In some embodiments, L 1  extends from a position along the waveguide  305  downstream from the position where the supply gas  308   a  enters the waveguide  305 , to the end of the waveguide  305  or to a position between the entrance of the supply gas and the end of the waveguide  305 . In some embodiments, L 1  extends from where the supply gas  308   a  enters the waveguide  305 , to the end of the waveguide  305  or to a position between the entrance of the supply gas and the end of the waveguide  305 . One or more additional process gas inlets  310  are configured to receive process gas flowing into the waveguide at a second set of locations downstream of the supply gas inlet(s)  302 . The generated plasma  306  provides energy for reactions to occur within a second portion  301  of the waveguide  305 , the second portion  301  having a length L 2 . In some embodiments, L 2  extends from where the process gas  311   a  enters the waveguide  305 , to the end of the waveguide  305  or to a position between the entrance of the process gas and the end of the waveguide  305 . Given the right conditions, the energy in the plasma will be sufficient to form separated components from the process gas molecules. One or more outlets  303  are configured to collect the separated products out of the waveguide  305  downstream of the portion  301  where reactions occur. In the example system  300  shown in  FIG. 3 , the propagation direction of the microwave radiation  309  is parallel with the majority of the supply gas flow  308   b  and the process gas flow  311   b , and the microwave radiation  309  enters the waveguide  305  upstream of the portion  301  of the waveguide where the separated components are generated. 
     As described above, the waveguide (e.g.,  205  in  FIGS. 2, and 305  in  FIG. 3 ) has a total length L, a portion of the total length L 1  along which the plasma is generated, and a portion of the total length L 2  along which the process gas is converted into the separated components. In some embodiments, the total length L of the waveguide is from 1 cm to 1000 cm. In some embodiments, length L 0  of the waveguide is from 1 cm to 100 cm. In some embodiments, length L 1  of the waveguide is from 1 cm to 100 cm. In some embodiments, length L 2  of the waveguide is from 1 cm to 1000 cm. In some embodiments, the total length L of the waveguide is from 30 cm to 60 cm. In some embodiments, length L 0  of the waveguide is from 10 cm to 40 cm. In some embodiments, length L 1  of the waveguide is from 10 cm to 30 cm. In some embodiments, length L 2  of the waveguide is from 5 cm to 20 cm. In some embodiments, length L 1  is more than 10%, or more than 20%, or more than 30% or more than 40% or more than 50%, or more than 60%, or more than 70%, or more than 80%, or from 10% to 90%, or from 20% to 80%, or from 30% to 70% of the length of the waveguide, L. In some embodiments, length L 2  is more than 5%, or more than 10%, or more than 15% or more than 20%, or more than 25% or more than 30%, or more than 35%, or more than 40%, or more than 45%, or more than 50%, or more than 55%, or more than 60%, or from 1% to 90%, or from 1% to 70%, or from 1% to 50%, or from 10% to 50%, or from 10% to 40%, or from 20% to 40% of the length of the waveguide, L. 
     In some embodiments, the waveguide (e.g.,  205  in  FIGS. 2, and 305  in  FIG. 3 ) is configured to maintain a pressure from 0.1 atm to 10 atm, or from 0.5 atm to 10 atm, or from 0.9 atm to 10 atm, or greater than 0.1 atm, or greater than 0.5 atm, or greater than 0.9 atm. In many conventional systems, the microwave chemical processing is operated at vacuum. However, in the present embodiments with the plasma being generated within the waveguide, operating in a positive pressure environment assists in preventing the generated plasma from feeding back into the microwave emitter source  204 ,  304 . 
     The waveguide (e.g.,  205  in  FIGS. 2, and 305  in  FIG. 3 ) may have a rectangular cross-section within length L 1  of dimensions 0.75 inches by 3.4 inches, to correspond to a microwave radiation frequency of 2.45 GHz. Other dimensions are possible for other microwave frequencies, and dependent upon waveguide mode this can be from 3-6 inches. The waveguide may be made of any innately conductive material or a material with a sufficient conductive coated layer to propagate greater than 90% of incoming power. Some examples of waveguide materials are stainless steel, stainless steel coated with a conductive layer (e.g., Al, Ni, Au, or a Ni/Au alloy), stainless steel with an aluminum liner, or a ceramic materials coated with a conductive layer. Notably, the waveguide serves as the chamber in which the plasma is generated and the process gas reactions to occur, rather than having a separate waveguide and quartz reaction chamber as in conventional systems. Having the waveguide serve as the reactor chamber provides a much larger volume in which gas reactions can occur (e.g., up to 1000 L). This enables high flow rates of process gas to be processed, without being limited by a build-up of carbon particulate as occurs in conventional systems. For example, process gas flow rates through the inlet (e.g.,  202  in  FIGS. 2, and 310  in  FIG. 3 ) into the waveguide (e.g.,  205  in  FIGS. 2, and 305  in  FIG. 3 ) may be from 1 slm (standard liters per minute) to 1000 slm, or from 2 slm to 1000 slm, or from 5 slm to 1000 slm, or greater than 1 slm, or greater than 2 slm, or greater than 5 slm. Supply gas flow rates through the inlet (e.g.,  202  in  FIGS. 2, and 302  in  FIG. 3 ) into the waveguide (e.g.,  205  in  FIGS. 2, and 305  in  FIG. 3 ) may be, for example, from 1 slm to 1000 slm, or from 2 slm to 1000 slm, or from 5 slm to 1000 slm, or greater than 1 slm, or greater than 2 slm, or greater than 5 slm. Dependent upon the gas plasma properties that result in sufficient plasma density (e.g., secondary electron emission coefficient) the flows can be from 1 slm to 1000 slm and with pressures up to 14 atm. 
     In some embodiments, the process gas is a hydrocarbon, such as C 2 H 2 , C 2 H 4 , C 2 H 6 . In some embodiments, the process gas is methane, and the separated components are hydrogen and nanoparticulate carbon. In some embodiments, the process gas is carbon dioxide with water, and the separated components are oxygen, carbon and water. In some embodiments, the process gas is H 2 S and the separated components are hydrogen gas and sulfur. In some embodiments, the process gas does not contain carbon dioxide. In some embodiments, the process gas is a complex gas-based material, for example SiH 4 , trimethylaluminum (TMA), trimethylgallium (TMG), glycidyl methacrylate (GMA), SF 6 , and other materials used in the semiconductor industry for the deposition and etching of metals and dielectrics. 
     In some embodiments, one of the separated components is nanoparticulate carbon such as, but not limited to, carbon black, carbon nano-onions (CNOs), necked CNOs, carbon nanospheres, graphite, pyrolytic graphite, graphene, graphene nanoparticles, graphene platelets, fullerenes, hybrid fullerenes, single-walled nanotubes and multi-walled nanotubes. One or more of these nanoparticulate carbons may be produced during a particular process run. 
     Tuning Microwave Energy In Microwave Gas Processing Systems 
     Different process gases require different amounts of energy to react into different separated components. In the present disclosure, the available reaction pathways can be selected by changing the average energy of the plasma. In some embodiments, the microwave radiation coupled to the plasma is pulsed, and the average energy of the plasma, and therefore the reaction pathways, are selected by controlling the microwave radiation pulse duration and frequency, duty cycle, shape, and time-averaged output power level.  FIGS. 4 and 5  illustrate time variations of gas temperature in the systems and methods of the present disclosure that occur due to pulsing of the microwave radiation. 
       FIG. 4  illustrates a typical time variation of the plasma temperature within a reaction chamber when microwave radiation that is coupled into the reaction chamber is pulsed. In  FIG. 4 , time t 1  indicates the start of a first pulse period, and time t 4  indicates the end of a first pulse period and the beginning of a second pulse period. Within the first pulse period, time t 1  to t 3  indicates a first duration of the pulse period where the microwave radiation is on, and time t 3  to t 4  indicates a second duration of the pulse period where the microwave radiation is off or at a lower power than during the duration when the microwave power is on. The duty cycle in this example is (t 3 -t 1 )/(t 4 -t 1 )×100, expressed as a percentage. For example, a duty cycle of 30% indicates that the microwave radiation is on for 30% of each pulse period. Upon the initiation of the first pulse, the plasma temperature quickly rises due to the excitation of the high energy species in the plasma. However, the high energy species created at the start of the pulse are relatively short lived, and the plasma temperature decreases until a steady state is reached within the plasma. Time t 2  indicates the time where equilibrium is reached within the plasma in the reaction chamber. 
     The energy in the plasma can be expressed as a plasma temperature (in units of eV), and describes the energy of the radical species in the plasma. The gas temperature in the reaction chamber or waveguide is also related to the energy of the plasma, since the energy from the plasma is transferred to the gas. Plasma efficiency is one property that affects the relationship between plasma energy and these temperatures, which is dominated by gas mixtures and types by innate gas plasma properties such as secondary electron emission coefficient, and pair production. Therefore, the average energy in the plasma between time t 1  and t 2  is higher than the average energy in the plasma at equilibrium (between times t 2  and t 3 ). As the total pulse period is shortened, there is a greater fraction of time where the high energy species exist within each pulse period. 
     In accordance with the present embodiments,  FIG. 5  shows an example graph of a pulse with a period that is shorter than the example pulse depicted in  FIG. 4 . The pulsing may be, for example, greater than 500 Hz, such as greater than 100 kHz, and up to 1000 kHz. Since the total pulse period is shorter, but the duration of time within the pulse period where the high energy species exist is the same, the fraction of time where high energy species exist, (t 2 -t 1 )/(t 4 -t 1 ) is larger for the example shown in  FIG. 5  than it is for the example shown in  FIG. 4 . The higher energy level resulting from high frequency pulsing of the microwave radiation enables more efficient cracking of the process gas molecules, by utilizing increased thermal cracking in addition to kinetic mechanisms. The increased contribution of thermal cracking results in less power input required, consequently also enabling higher flow rates of the process gas to be used. In some embodiments, the rise time and fall time of the pulse is from 5 nS to 1000 mS, or from 5 nS to 10 mS. 
     In some embodiments, the average energy in the plasma is controlled by changing the pulse period, by choosing a pulsing frequency to achieve a desired plasma energy. Additionally, in some embodiments, the average energy of the plasma is controlled by controlling the duty cycle. This can be understood by contemplating the situation where the time-averaged input power and the pulse period are both held constant and the duty cycle is varied. A shorter duty cycle will increase the magnitude of the power coupled into the chamber when the microwave radiation is on. That is because the time-averaged power is held constant and the duration that the power is on (i.e., the duty cycle) is shorter. In some embodiments, the higher power coupled into the reaction chamber during the first duration of the pulse period will increase the average temperature and average energy of the plasma. As previously described, controlling the energy of the plasma can be used to select given reaction pathways for the creation of separated components from a process gas. Therefore, in some embodiments, the reaction pathways can be selected by controlling the duty cycle of the microwave radiation coupled into the reaction chamber. This is advantageous because a relatively low amount of power (i.e., time-averaged power) can be used to generate reaction products from reaction pathways that would be impossible to facilitate at the same power in a continuous wave. 
     In some embodiments, the reaction pathways can be selected by controlling time-averaged power input into the plasma. For example, if the duty cycle and pulse frequency are held constant, and the power input into the microwave generator is increased, then the energy of the plasma will increase. By way of another example, if the duty cycle and pulse frequency are held constant, and the power is more effectively coupled into the reaction chamber, then the energy of the plasma will increase. 
     In some embodiments, the reaction pathways can be selected by controlling a shape of the microwave radiation pulse. In some embodiments, the microwave pulse is a rectangular wave, where the power is constant during the duration of the pulse period when the microwave is on. In some embodiments, the pulse power is not constant during the duration of the pulse period when the microwave power is on. In some embodiments, the microwave pulse is a triangular wave, or a trapezoidal wave. In some embodiments, the pulse quickly rises to a value E 1  (e.g., at time t 1  in  FIG. 5 ) and then increases over some period of time up to a value E 2  (e.g., at time from t 1  to t 2  in  FIG. 5 ). In some embodiments, the pulse quickly rises to a value E 1  (e.g., at time t 1  in  FIG. 5 ) and then linearly increases over some period of time up to a value E 2  (e.g., at time from t 1  to t 2  in  FIG. 5 ). In some embodiments, the pulse quickly rises to a value E 1  (e.g., at time t 1  in  FIG. 5 ), then increases over some period of time to a value E 2  (e.g., at time from t 1  to t 2  in  FIG. 5 ), and then quickly decreases to a low value E 0  (e.g., from time t 2  to t 3  in  FIG. 5 ) less than E 1  and greater than the value of the energy when the microwave power is off. The plasma can be referred to as diffuse during the time period when the high energy species exist in higher fractions (i.e., at the beginning of the pulse, before the plasma reaches equilibrium). In some embodiments, the microwave energy increases over the time period where the plasma is diffuse, which increases the time average fraction of high energy species in the plasma. 
     As described above, tuning the pulse frequency, duty cycle, and pulse shape can enable the creation of a higher fraction of higher energy species within the plasma for a given time-averaged input power. The higher energy species can enable additional reaction pathways that would otherwise not be energetically favorable. 
     The techniques above can be further understood by using methane (CH 4 ) as an example process gas, to be separated into hydrogen and nanoparticulate carbon. Typically, 4-6 eV is needed to dissociate methane (CH 4 ); however, the plasma energy typically settles at approximately 1.5 eV after an initial ignition energy spike. By pulsing the microwave, the average plasma energy (i.e. time-averaged plasma energy) is maintained at the higher levels, where the frequency and duration of the pulsing controls the average plasma energy. Specifically, pulsing parameters such as frequency and duty cycle can be controlled to provide an average plasma energy of 4-6 eV to select specific dissociation reactions of the methane. Another advantage of pulsing the microwave energy is that the energy is more distributed throughout the chamber in which microwave is being input. In conventional systems, at equilibrium the plasma forms a dense layer of ionized species in the chamber towards the location of the microwave input, which absorbs the incoming microwave radiation and consequently prevents further microwave radiation from penetrating deeper into the chamber. The high frequency pulsing of the present disclosure maintains the plasma in a non-equilibrium state for a greater fraction of time and the dense layer of ionized species is present for a smaller fraction of time, which allows the microwave radiation to penetrate deeper into the chamber and the plasma to be generated in a larger volume within the chamber. 
     Continuing with methane as an example, at the lowest energy applied to the process gas, only one hydrogen atom would be removed, producing CH 3  radicals and free H atoms. When more energy is applied, the process gas including methane can be reacted into CH 3  radicals and free H atoms and into CH 2  plus H 2 . At even higher energies, the process gas including methane can be reacted into CH 3  radicals and free H atoms, and into CH 2  plus H 2 , and into CH radicals plus H 2 . At even higher energies, the process gas including methane can be reacted into CH 3  radicals and free H atoms, and into CH 2  plus H 2 , and into CH radicals plus H 2 , and into C plus H 2 . By controlling the amount of energy added to the plasma, different reaction pathways can be selected, and different products can be collected. 
     More generally, in various embodiments of the present disclosure the average energy of the plasma over the entire duration of the pulse period may be from 0.9 eV to 20 eV, or from 0.9 to 10 eV, or from 1.5 eV to 20 eV, or from 1.5 eV to 10 eV, or greater than 0.9 eV, or greater than 1.5 eV. The specific values to which the plasma energy is tuned will depend on the type of process gas being utilized. 
     In the microwave processing systems described above, the microwave radiation source is controlled by a microwave emitter circuit (e.g.,  207  in  FIGS. 2, and 307  in  FIG. 3 ), that can control the microwave radiation emitted from the source to be either continuous wave or pulsed. In some embodiments, the microwave emitter circuit produces microwave radiation through the use of a magnetron, e.g., at 915 MHz, 2.45 GHz, or 5.8 GHz. To control the output power of the microwave radiation, the microwave emitter circuit may pulse the magnetron at various frequencies and duty cycles. Each microwave emitter circuit is designed for a specific range of pulsing frequency, duty cycle, shape, and output power level, where the selection of specific values of these parameters is used to tune the chemical reaction pathways in the process gas. 
     In some embodiments, the microwave control circuit enables a pulse frequency from 500 Hz to 1000 kHz, or from 1 kHz to 1000 kHz, or from 10 kHz to 1000 kHz, or from 40 kHz to 80 kHz, or from 60 to 70 kHz, or greater than 10 kHz, or greater than 50 kHz, or greater than 100 kHz. In some embodiments, the microwave source emits continuous wave or pulsed microwave radiation with a time-average power from 1 to 100 kW. The pulse period has a first duration where the microwave power is on, and a second duration where the microwave radiation is off or at a lower power than during the first duration. In some embodiments, the second duration is longer than the first duration. The optimal duty cycle for a given system depends on many factors including the microwave power, pulse frequency, and pulse shape. In some embodiments, the duty cycle (i.e., the fraction of the pulse period where the microwave radiation is on, expressed as a percentage) is from 1% to 99%, or from 1% to 95%, or from 10% to 95%, or from 20% to 80%, or from 50% to 95%, or from 1% to 50%, or from 1% to 40%, or from 1% to 30%, or from 1% to 20%, or from 1% to 10%, or less than 99%, or less than 95%, or less than 80%, or less than 60%, or less than 50%, or less than 40%, or less than 30%, or less than 20%, or less than 10%. 
     Ion Energy Control In High Flow Microwave Gas Processing 
     In addition to tuning various parameters of the microwave radiation pulsing to control which chemical reaction pathways occur in the process gas, other techniques shall now be discussed that can also affect the plasma energy and therefore tune the chemical reactions. 
       FIG. 6  illustrates controlling ion energy of the plasma through the addition of one or more precursor gases, where the precursor gases are inserted upstream from where the process gas is flowed into the waveguide chamber. Precursor gases improve cracking efficiency by adding species of various ionization potentials. That is, different gases have different ionization energies, which is the amount of energy required to remove an electron from an atom or molecule is. In addition, various gases have different pair production (how many electrons per ion can be produced) and secondary electron emission properties (emission of electrons when the charged particles strike a surface). Thus, in the present disclosure the use of precursor gases is utilized to affect the energy of the plasma. 
     In  FIG. 6 , a microwave gas processing system  600  includes a microwave radiation source  604 , a waveguide  605 , and a microwave emitter circuit  607  similar to previous embodiments. The diagram of  FIG. 6  is a simplified drawing compared to the previous figures for clarity. A supply gas inlet  602  receives a precursor gas  620  which supplements the supply gas (not shown) to create the plasma in the waveguide. In various embodiments, the precursor gas  620  may include one or more of hydrogen, argon, helium, or various noble gases. Process gas inlet  610  is configured to receive the process gas that is to be reacted, similar to previous embodiments. For precursor gases that are not desired output products of the system (e.g., argon precursor gas in processing of methane), the precursor gases are removed from the separated components  630  and  632  that are output from outlet  603  in post-process steps. 
     In some embodiments, one or more of the separated components of the process gas are recycled back into the supply gas and/or process gas entering the waveguide  605 . As shown in  FIG. 6 , gas reactions in the waveguide  605  produce separated components  630  and  632 . For example, for methane as a process gas, first separated component  630  may be CH 3  and second separated component  632  may be atomic hydrogen H +  which recombines to form H 2  gas before being collected at the outlet  603 . Alternatively, first separated component  630  may be CH 2  and second separated component  632  may be hydrogen gas H 2 . The separated component  632  is recycled back into waveguide  605  through conduit  640 , back to supply gas inlet  602 . The recycled separated component  632  thus is used as a precursor gas  620 . Although it is counterintuitive to return the produced separated components back into the reaction system, recycling of the components adds energy to the plasma, and in some embodiments can also contribute to thermal cracking of the process gas since the recycled components have already been heated during the gas processing. In some embodiments, for example, separated component  632  can be 2-10 slm of H 2  that is recycled back into the waveguide  605 , for a process in which a total of 150-200 slm of H 2  is produced. Other amounts or portions of separated component  632  may be recycled, as determined by factors such as the flow rate of the process gas, and/or the amount of energy desired to be added to the process to initiate the targeted chemical pathways. 
     In some embodiments, some or all of the supply gas contains one or more recycled separated components of the process gas. For example, the supply gas can be hydrogen, and the process gas can be methane, which is reacted to form hydrogen and carbon, and at least a portion of the hydrogen that is produced from the methane can be recycled and used as the supply gas. Recycling the produced hydrogen beneficially improves the efficiency of the overall gas processing because the plasma formed from the hydrogen is highly efficient at cracking hydrocarbon bonds in the process gas molecules. Additionally, in some embodiments, the recycled H 2  is already at a high temperature, and thus less energy input is need to achieve a thermal cracking energy. In some embodiments the supply gas is H 2  provided by an external source, to which recycled H 2  is added. In such embodiments the generated plasma is a hydrogen plasma. 
       FIG. 7  illustrates another technique for controlling the chemical pathways, through the use of filaments. In the embodiment of  FIG. 7 , the microwave processing system  700  includes a microwave radiation source  704 , a waveguide  705 , and a microwave emitter circuit  707  similar to previous embodiments. Microwave radiation  709  is supplied by the microwave radiation source  704 , to propagate in a direction down the length L of the waveguide  705 . In this embodiment, supply gas inlet  702  is placed near the entrance of the portion L 0 , rather than at the entrance to the portion L 1  as was illustrated in previous embodiments. One or more metal filaments  720  is placed within waveguide  705  to assist in the ignition of the plasma and/or the excitation of higher energy species within the plasma. In this embodiment, metal filament  720  is downstream of the first gas inlet  702 , near the entrance to the portion L 1  that has a constant cross-sectional area. In other embodiments, the filament  720  may be located at other locations within portion L 1  of the overall length L of the waveguide  705 , where L 1  is the region in the waveguide where the plasma is formed as described in relation to previous embodiments. In some embodiments, the filament  720  is located within portion L 1  and upstream of the process gas inlet  710 , so that it will be located outside of the portion L 2  (shown in  FIGS. 2 and 3 ) where reactions are taking place and which could coat the filament with reacted species. The presence of filament  720  can reduce the plasma ignition voltage by providing an ignition site, by focusing the electric field of microwave radiation  709 . Additionally, the filament  720  can become heated and emit electrons through thermionic emission, which further contributes to reducing the plasma ignition voltage. Although the filament  720  is illustrated as a single wire in this embodiment, filament  720  may take other configurations such as a coil or multiple filaments. In some embodiments, the filament  720  is tungsten. In some embodiments, the filament may be actively energized (powered) or may be passive. In some embodiments, the filament  720  is an osmium filament (e.g., configured as a plate, or coil, or other shape) adjacent to a heater coil. In some embodiments, the filament  720  is a ferrous material in the field of an inductive coil. In some embodiments, the filament  720  is actively heated where the active components (e.g. heating source components) are located outside of the waveguide  705  and the filament material that is being heated is inside of the waveguide  705 . 
       FIG. 8  illustrates yet further techniques for controlling the plasma energy, through the use of an electron source. Microwave processing system  800  includes a supply gas inlet  802 , a waveguide  805 , and a microwave radiation source  804  that supplies microwave radiation  809  as in previous embodiments. Microwave processing system  800  also includes one or more electron sources  820  to assist in the ignition of the plasma and/or the excitation of higher energy species within the plasma. The electron source  820  is configured to inject electrons into the waveguide  805 , thereby decreasing the amount of initial energy needed to ignite the plasma. The ignition level of the plasma can therefore be controlled by controlling the amount of electrons present. In some embodiments, the electrons are injected into the portion L 1  of the overall length L of the waveguide  805 , where L 1  is the region in the waveguide where the plasma is formed as described above. For example, in this embodiment the electron source  820  is configured to supply electrons into the waveguide  805  downstream of the first gas inlet  802 . In some embodiments, the electron source  820  is a field emission source. In some embodiments, the electron source  820  contains an osmium element adjacent to a heater coil. In some embodiments, the electron source  820  contains a ferrous material in the field of an inductive coil. In some embodiments, the electron source  820  contains a filament, as described above, and the generated electrons are injected into the portion L 1  using a high voltage electric field. In some embodiments, the electron source  820  is alternatively a source of ions. 
     An advantage of using a filament  720  and/or an electron source  820  within the waveguide is that they enable a plasma to form quickly enough to keep up with fast microwave pulsing frequencies (e.g., at frequencies greater than 500 Hz, or greater than 1 kHz), even with high gas flows (e.g., greater than 5 slm) and large gas volumes (e.g., up to 1000 L). This is particularly important at high pressures (e.g., greater than 0.9 atm, or greater than 1 atm, or greater than 2 atm), because the high energy species will extinguish quickly in a high pressure atmosphere, and if the plasma cannot ignite fast enough, then there will be a low fraction of high-energy species (i.e., integrated over time) in a pulsed plasma at high pressures. 
       FIG. 8  also illustrates an embodiment of an electrode  830  in the present systems, as another technique for controlling chemical pathways. The electrode  830  may be used independently of, or in combination with, the precursor gases  620  of  FIG. 6 , the filaments  720  of  FIG. 7  or the electron source  820  of  FIG. 8 . In some embodiments, the system  800  contains one or more sets of electrodes  830  to add energy to the plasma. The electrodes are configured to generate an electric field within the portion L 1  of the overall length L of the waveguide  805 , where L 1  is the region in the waveguide where the plasma is formed as described above. Electrode  830  is embodied in  FIG. 8  as a pair of coplanar electrodes of opposite charges, that are on the exterior of and on opposite sides of the portion of the waveguide  805  where the plasma  806  is generated. The electrodes can be energized to a particular voltage to accelerate the charged species within the plasma to a desired degree, thus controlling the plasma energy. The electrodes are particularly effective in combination with a pulsed microwave input. In conventional systems with electrodes and continuous microwave radiation, the plasma between electrodes will localize (e.g., near the electrodes) at equilibrium and screen the electric field from the electrodes, which limits the ability of the electrodes to add energy to the plasma. However, when the microwaves are pulsed, the plasma will exist in the non-equilibrium state for a larger fraction of time and will screen the electric field of the electrodes for a smaller fraction of time. 
     In some embodiments, the gas processing systems of the present disclosure will include magnets (not shown) to confine the plasma and reduce the ignition voltage. In some embodiments, the magnets are permanent or are electromagnets. The magnets can be positioned so the plasma density distribution can be controlled. In some embodiments, the magnets will increase the plasma density in the portion L 1 , which will improve the efficiency by which the process gas is separated by the plasma. 
     As previously described, the combination of pulsed microwave radiation, high gas flows (e.g., greater than 5 slm), large volumes of plasma (e.g., up to 1000 L), high pressures (e.g., greater than 0.1 atm or greater than 0.9 atm, or greater than 2 atm), either filaments or electron sources to assist in plasma ignition at the start of each pulse, and/or electrodes to further add energy to the plasma can enable cost-effective high-productivity chemical gas processing systems, with low energy input requirements. 
     The gas processing systems with the above features are configured in such a way that the plasma is generated and the process gas is separated into components within the waveguide itself, such as the examples depicted in  FIGS. 2, 3, 6, 7 and 8 . In such systems, microwave radiation enters the system upstream of the reaction generating the separated components, and therefore the problem of the separated components building up on a microwave entry window of a reactor and absorbing the microwave radiation before it can generate the plasma is alleviated. The portion of the waveguide where the separated components are generated acts as a reaction chamber, and the supply gas flow and/or the process gas flow through the reaction chamber is parallel to the propagation direction of the microwave radiation. The microwave radiation enters the reaction chamber upstream of the portion of the reaction chamber where the separated components are generated. 
     In some embodiments, gas recycling, filaments, and electron sources can be used in microwave gas processing systems utilizing continuous wave (CW) microwave radiation. In embodiments with CW microwave radiation, gas recycling, filaments, and electron sources would still be advantageous to improve the gas processing efficiency of the system, reduce the ignition voltage of the plasma, and control the density distribution of the plasma. 
     In some embodiments, the separated components can adhere to the walls of the waveguide downstream of the reaction generating the separated components, despite the large volume of the reaction volume in the waveguide. Although this does not prevent the plasma from being generated, it still represents a loss of production and a source of contamination in the system. Therefore, in some embodiments, the gas flow of the supply gas and the process gas can be designed to generate a plasma near the areas of deposition to remove the separated products that are deposited on the waveguide walls (or, reaction chamber walls). In some embodiments, additional inlets of supply gas and/or process gas can be configured to direct the gases to the areas of deposition to remove the separated products that are deposited on the waveguide walls (or, reaction chamber walls). 
     Methods Of Microwave Gas Processing 
       FIG. 9  is an example flow chart  900  representing methods for microwave processing of gas, using chemistry control in high efficiency gas reactions. In step  910 , microwave radiation is supplied through a waveguide having a length, where the microwave radiation propagates in a direction along the waveguide. The microwave radiation may be pulsed or continuous wave. In some embodiments, the microwave radiation is supplied into the waveguide at a power less than 100 kV. A pressure within the waveguide is at least 0.1 atmosphere, such as from 0.9 atm to 10 atm. In step  920 , a supply gas is provided into the waveguide at a first location along the length of the waveguide, where a majority of the supply gas is flowing in the direction of the microwave radiation propagation. In step  930 , a plasma is generated in the supply gas in at least a portion of the length of the waveguide. A process gas is added into the waveguide at step  940 , at a second location downstream from the first location. A majority of the process gas flows in the direction of the microwave propagation at a flow rate of greater than 5 slm. 
     In step  950 , an average energy of the plasma is controlled to convert the process gas into separated components. The average energy may be, for example, 0.8 eV to 20 eV. In some embodiments the pulsing frequency is controlled, where the pulsing frequency is greater than 500 Hz. For example, the pulsing frequency of the microwave radiation may be from 500 Hz to 1000 kHz. In some embodiments, the duty cycle of the pulsed microwave radiation is controlled in addition to or instead of the pulsing frequency, where the duty cycle is less than 50%. 
     In some embodiments, the process gas is methane, and the separated components comprise hydrogen and a nanoparticulate carbon. For example, the nanoparticulate carbon can include one or more forms of graphene, graphite, carbon nano-onions, fullerenes or nano-tubes. 
     In some embodiments, a precursor gas is added to the supply gas at the first location, the precursor gas comprising hydrogen or a noble gas. In some embodiments, the separated components comprise H 2 , and at least a portion of the separated component H 2  is recycled back to the first location. In such embodiments, the supply gas comprises H 2 , and the plasma comprises a hydrogen plasma. 
     In various embodiments, the methods include providing a metal filament in the waveguide, the metal filament serving to reduce an ignition voltage for generating the plasma. In various embodiments, the methods include providing a pair of electrodes coupled to the waveguide, wherein the electrodes are configured to add energy to the generated plasma. 
     While the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the scope of the present invention. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.