Patent Description:
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 CHX 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 CHX 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 CHX 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. <CIT> discloses a tubular casing into which a gas and a microwave are introduced for stably generating a plasma at atmospheric pressure. <CIT> discloses a method for processing a gas and a device for performing the method. <CIT> discloses high volume production of nanostructured materials. <CIT> discloses low power density microwave discharge plasma excitation induced chemical reactions.

The present invention is defined in and by the appended claims.

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 <NUM>%. These conventional systems, however, use low flow rates, below <NUM> 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 <NUM>) because the plasma cannot ignite fast enough to keep up with the pulses when a large volume and high flow of gas is used.

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. 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> illustrates a conventional microwave chemical processing system. As shown in <FIG>, a microwave chemical processing system <NUM> generally includes a reaction chamber <NUM>, one or more gas inlets <NUM> configured to receive process gas flowing into the reaction chamber, one or more outlets <NUM> configured to collect separated products out of the reaction chamber <NUM>, and a source of microwave radiation <NUM> that is coupled to the reaction chamber through a waveguide <NUM>, among other elements not shown for simplicity. The microwave radiation <NUM> creates a microwave plasma <NUM> in the reaction chamber <NUM>, and provides energy for reactions to occur. A microwave emitter circuit <NUM> can control the microwave radiation <NUM> emitted from the microwave radiation source <NUM> 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.

<FIG> and <FIG> 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>, a microwave chemical processing system <NUM> generally includes a waveguide <NUM>, one or more gas inlets <NUM> configured to receive supply gas and/or process gas 208a flowing into the waveguide <NUM>, and a microwave radiation source <NUM> that is coupled to the waveguide <NUM>, among other elements not shown for simplicity.

In some embodiments, microwave circuit <NUM> controls a pulsing frequency at which microwave radiation <NUM> from microwave radiation source <NUM> is pulsed. In some embodiments, the microwave radiation <NUM> from microwave radiation source <NUM> is continuous wave.

The waveguide <NUM> has a length L. In a portion Lo prior to where the plasma is generated, the cross-sectional area of the waveguide <NUM> 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<NUM>, where the gas inlet <NUM> is located, may have a rectangular cross-section of dimensions <NUM> inches (<NUM>) by <NUM> inches (<NUM>) when using a microwave radiation frequency of <NUM>. This cross-sectional area is much greater than conventional systems where the plasma generation area is generally less than one square inch (<NUM><NUM>). The dimensions of the waveguide <NUM> 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 (<NUM><NUM>) 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 <NUM> 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 <NUM>, 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 <NUM> in <FIG> creates a microwave plasma <NUM> in the supply gas and/or process gas within a first portion L<NUM> of the length of the waveguide <NUM>. In some embodiments, a supply gas that is different from the process gas is used to generate the microwave plasma <NUM>. 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<NUM> extends from a position along the waveguide downstream from the position where the supply and/or process gas 208a enters the waveguide <NUM>, to the end of the waveguide <NUM> or to a position between the entrance of the supply and/or process gas and the end of the waveguide <NUM>. In some embodiments, L<NUM> extends from where the supply and/or process gas 208a enters the waveguide <NUM>, to the end of the waveguide <NUM> or to a position between the entrance of the supply and/or process gas and the end of the waveguide <NUM>. The generated plasma <NUM> provides energy for reactions to occur in process gas 208b within a second portion <NUM> of the waveguide <NUM>, the second portion <NUM> having a length L<NUM>. In some embodiments, L<NUM> extends from where the process gas 208a enters the waveguide <NUM>, to the end of the waveguide <NUM> or to a position between the entrance of the process gas and the end of the waveguide <NUM>. Given the right conditions, the energy in the plasma <NUM> will be sufficient to form separated components from the process gas molecules. One or more outlets <NUM> are configured to collect the separated products out of the waveguide <NUM> downstream of the portion <NUM> of the waveguide where reactions occur in the process gas 208b. In the example shown in <FIG>, the propagation direction of the microwave radiation <NUM> is parallel with the majority of the supply and/or process gas flow 208b, and the microwave radiation <NUM> enters the waveguide <NUM> upstream of the portion <NUM> 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 <NUM> or the gas inlet(s) <NUM>. 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 Lo and L<NUM>, or in portion Lo upstream of portion L<NUM> and downstream of the gas inlet(s) <NUM> (in an embodiment where gas inlet <NUM> is within portion Lo) and the microwave radiation source <NUM>.

<FIG> shows another embodiment of a microwave chemical processing system <NUM> in which a supply gas and a process gas are injected at different locations. The microwave chemical processing system <NUM>, in accordance with some embodiments, generally includes a waveguide <NUM>, one or more supply gas inlets <NUM> configured to receive supply gas 308a flowing into the waveguide <NUM>, one or more process gas inlets <NUM> configured to receive process gas 311a, and a source of microwave radiation <NUM> that is coupled to the waveguide <NUM>, among other elements not shown for simplicity. The location of process gas inlet <NUM> is downstream of the location of supply gas inlet <NUM>, where downstream is defined in a direction of the microwave propagation.

In some embodiments, microwave circuit <NUM> controls a pulsing frequency at which microwave radiation <NUM> from microwave radiation source <NUM> is pulsed. In some embodiments, the microwave radiation from radiation source <NUM> is continuous wave.

The waveguide <NUM> has a length L. The length Lo of the waveguide where microwave radiation <NUM> enters has a decreasing cross-sectional area along the direction of the microwave propagation, as described above in relation to <FIG>. The microwave radiation <NUM> creates a microwave plasma <NUM> in the supply gas 308b within a first portion L<NUM> of the length L of the waveguide <NUM>. In some embodiments, Li extends from a position along the waveguide <NUM> downstream from the position where the supply gas 308a enters the waveguide <NUM>, to the end of the waveguide <NUM> or to a position between the entrance of the supply gas and the end of the waveguide <NUM>. In some embodiments, L<NUM> extends from where the supply gas 308a enters the waveguide <NUM>, to the end of the waveguide <NUM> or to a position between the entrance of the supply gas and the end of the waveguide <NUM>. One or more additional process gas inlets <NUM> are configured to receive process gas flowing into the waveguide at a second set of locations downstream of the supply gas inlet(s) <NUM>. The generated plasma <NUM> provides energy for reactions to occur within a second portion <NUM> of the waveguide <NUM>, the second portion <NUM> having a length L<NUM>. In some embodiments, L<NUM> extends from where the process gas 311a enters the waveguide <NUM>, to the end of the waveguide <NUM> or to a position between the entrance of the process gas and the end of the waveguide <NUM>. 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 <NUM> are configured to collect the separated products out of the waveguide <NUM> downstream of the portion <NUM> where reactions occur. In the example system <NUM> shown in <FIG>, the propagation direction of the microwave radiation <NUM> is parallel with the majority of the supply gas flow 308b and the process gas flow 311b, and the microwave radiation <NUM> enters the waveguide <NUM> upstream of the portion <NUM> of the waveguide where the separated components are generated.

As described above, the waveguide (e.g., <NUM> in <FIG>, and <NUM> in <FIG>) has a total length L, a portion of the total length L<NUM> along which the plasma is generated, and a portion of the total length L<NUM> along which the process gas is converted into the separated components. In some embodiments, the total length L of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> of the waveguide is from <NUM> to <NUM>. In some embodiments, the total length L of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> of the waveguide is from <NUM> to <NUM>. In some embodiments, length L<NUM> is more than <NUM>%, or more than <NUM>%, or more than <NUM>% or more than <NUM>% or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>% of the length of the waveguide, L. In some embodiments, length L<NUM> is more than <NUM>%, or more than <NUM>%, or more than <NUM>% or more than <NUM>%, or more than <NUM>% or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or more than <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>% of the length of the waveguide, L.

In some embodiments, the waveguide (e.g., <NUM> in <FIG>, and <NUM> in <FIG>) is configured to maintain a pressure from <NUM> atm to <NUM> atm (<NUM> Pa to <NUM> Pa), or from <NUM> atm to <NUM> atm (<NUM> Pa to <NUM> Pa), or from <NUM> atm to <NUM> atm (<NUM> Pa to <NUM> Pa), or greater than <NUM> atm (<NUM> Pa), or greater than <NUM> atm (<NUM> Pa), or greater than <NUM> atm (<NUM> Pa). 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 <NUM>, <NUM>.

The waveguide (e.g., <NUM> in <FIG>, and <NUM> in <FIG>) may have a rectangular cross-section within length L<NUM> of dimensions <NUM> inches(<NUM>) by <NUM> inches (<NUM>), to correspond to a microwave radiation frequency of <NUM>. Other dimensions are possible for other microwave frequencies, and dependent upon waveguide mode this can be from <NUM>-<NUM> inches (<NUM> - <NUM>). The waveguide may be made of any innately conductive material or a material with a sufficient conductive coated layer to propagate greater than <NUM>% 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 <NUM>). 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., <NUM> in <FIG>, and <NUM> in <FIG>) into the waveguide (e.g., <NUM> in <FIG>, and <NUM> in <FIG>) may be from <NUM> slm (standard liters per minute) to <NUM> slm, or from <NUM> slm to <NUM> slm, or from <NUM> slm to <NUM> slm, or greater than <NUM> slm, or greater than <NUM> sim, or greater than <NUM> slm. Supply gas flow rates through the inlet (e.g., <NUM> in <FIG>, and <NUM> in <FIG>) into the waveguide (e.g. , <NUM> in <FIG>, and <NUM> in <FIG>) may be, for example, from <NUM> slm to <NUM> slm, or from <NUM> slm to <NUM> slm, or from <NUM> slm to <NUM> slm, or greater than <NUM> slm, or greater than <NUM> slm, or greater than <NUM> slm. Dependent upon the gas plasma properties that result in sufficient plasma density (e.g., secondary electron emission coefficient) the flows can be from <NUM> slm to <NUM> slm and with pressures up to <NUM> atm (<NUM> Pa).

In some embodiments, the process gas is a hydrocarbon, such as C<NUM>H<NUM>, C<NUM>H<NUM>, C<NUM>H<NUM>. In some embodiments, the process gas is methane, and the separated components are hydrogen and nanoparticluate 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<NUM>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<NUM>, trimethylaluminum (TMA), trimethylgallium (TMG), glycidyl methacrylate (GMA), SF<NUM>, 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.

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. <FIG> and <FIG> 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> 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>, time t<NUM> indicates the start of a first pulse period, and time t<NUM> indicates the end of a first pulse period and the beginning of a second pulse period. Within the first pulse period, time t<NUM> to t<NUM> indicates a first duration of the pulse period where the microwave radiation is on, and time t<NUM> to t<NUM> 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<NUM>-t<NUM>)/(t<NUM>-t<NUM>) x <NUM>, expressed as a percentage. For example, a duty cycle of <NUM>% indicates that the microwave radiation is on for <NUM>% 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<NUM> 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<NUM> and t<NUM> is higher than the average energy in the plasma at equilibrium (between times t<NUM> and t<NUM>). 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> shows an example graph of a pulse with a period that is shorter than the example pulse depicted in <FIG>. The pulsing may be, for example, greater than <NUM>, such as greater than <NUM>, and up to <NUM>. 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<NUM>-t<NUM>)/(t<NUM>-t<NUM>) is larger for the example shown in <FIG> than it is for the example shown in <FIG>. 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 <NUM> nS to <NUM>, or from <NUM> nS to <NUM>.

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 E1 (e.g., at time t<NUM> in <FIG>) and then increases over some period of time up to a value E2 (e.g., at time from t<NUM> to t<NUM> in <FIG>). In some embodiments, the pulse quickly rises to a value E1 (e.g., at time t<NUM> in <FIG>) and then linearly increases over some period of time up to a value E2 (e.g., at time from t<NUM> to t<NUM> in <FIG>). In some embodiments, the pulse quickly rises to a value E1 (e.g., at time t<NUM> in <FIG>), then increases over some period of time to a value E2 (e.g., at time from t<NUM> to t<NUM> in <FIG>), and then quickly decreases to a low value E0 (e.g., from time t<NUM> to t<NUM> in <FIG>) less than E1 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<NUM>) as an example process gas, to be separated into hydrogen and nanoparticulate carbon. Typically, <NUM>-<NUM> eV (<NUM>×<NUM>-<NUM>-<NUM>×<NUM>-<NUM> J) is needed to dissociate methane (CH<NUM>); however, the plasma energy typically settles at approximately <NUM> eV (<NUM>. 40x10 -<NUM> J) 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 <NUM>-<NUM> eV (<NUM>×<NUM>-<NUM>-<NUM>×<NUM>-<NUM> J) 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<NUM> radicals and free H atoms. When more energy is applied, the process gas including methane can be reacted into CH<NUM> radicals and free H atoms and into CH<NUM> plus H<NUM>. At even higher energies, the process gas including methane can be reacted into CH<NUM> radicals and free H atoms, and into CH<NUM> plus H<NUM>, and into CH radicals plus H<NUM>. At even higher energies, the process gas including methane can be reacted into CH<NUM> radicals and free H atoms, and into CH<NUM> plus H<NUM>, and into CH radicals plus H<NUM>, and into C plus H<NUM>. 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 <NUM> eV to <NUM> eV (<NUM>×<NUM>-<NUM> J to <NUM>×<NUM>-<NUM> J), or from <NUM> to <NUM> eV (<NUM>×<NUM>-<NUM> J to <NUM>×<NUM>-<NUM> J), or from <NUM> eV to <NUM> eV (<NUM>×<NUM>-<NUM> J to <NUM>×<NUM>-<NUM> J), or from <NUM> eV to eV (<NUM>×<NUM>-<NUM> J to <NUM>×<NUM>-<NUM> J), or greater than <NUM> eV (<NUM>×<NUM>-<NUM> J), or greater than <NUM> eV (<NUM>×<NUM>-<NUM> J). 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., <NUM> in <FIG>, and <NUM> in <FIG>), 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 <NUM>, <NUM>, or <NUM>. 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 <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or greater than <NUM>, or greater than <NUM>, or greater than <NUM>. In some embodiments, the microwave source emits continuous wave or pulsed microwave radiation with a time-average power from <NUM> to <NUM> 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 <NUM>% to <NUM>%, or from <NUM>%to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>%to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM> %, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%.

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> 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>, a microwave gas processing system <NUM> includes a microwave radiation source <NUM>, a waveguide <NUM>, and a microwave emitter circuit <NUM> similar to previous embodiments. The diagram of <FIG> is a simplified drawing compared to the previous figures for clarity. A supply gas inlet <NUM> receives a precursor gas <NUM> which supplements the supply gas (not shown) to create the plasma in the waveguide. In various embodiments, the precursor gas <NUM> may include one or more of hydrogen, argon, helium, or various noble gases. Process gas inlet <NUM> 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 <NUM> and <NUM> that are output from outlet <NUM> 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 <NUM>. As shown in <FIG>, gas reactions in the waveguide <NUM> produce separated components <NUM> and <NUM>. For example, for methane as a process gas, first separated component <NUM> may be CH<NUM> and second separated component <NUM> may be atomic hydrogen H+ which recombines to form H<NUM> gas before being collected at the outlet <NUM>. Alternatively, first separated component <NUM> may be CH<NUM> and second separated component <NUM> may be hydrogen gas H<NUM>. The separated component <NUM> is recycled back into waveguide <NUM> through conduit <NUM>, back to supply gas inlet <NUM>. The recycled separated component <NUM> thus is used as a precursor gas <NUM>. 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 <NUM> can be <NUM>-<NUM> slm of H<NUM> that is recycled back into the waveguide <NUM>, for a process in which a total of <NUM>-<NUM> slm of H<NUM> is produced. Other amounts or portions of separated component <NUM> 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<NUM> 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<NUM> provided by an external source, to which recycled H<NUM> is added. In such embodiments the generated plasma is a hydrogen plasma.

<FIG> illustrates another technique for controlling the chemical pathways, through the use of filaments. In the embodiment of <FIG>, the microwave processing system <NUM> includes a microwave radiation source <NUM>, a waveguide <NUM>, and a microwave emitter circuit <NUM> similar to previous embodiments. Microwave radiation <NUM> is supplied by the microwave radiation source <NUM>, to propagate in a direction down the length L of the waveguide <NUM>. In this embodiment, supply gas inlet <NUM> is placed near the entrance of the portion L<NUM>, rather than at the entrance to the portion L<NUM> as was illustrated in previous embodiments. One or more metal filaments <NUM> is placed within waveguide <NUM> to assist in the ignition of the plasma and/or the excitation of higher energy species within the plasma. In this embodiment, metal filament <NUM> is downstream of the first gas inlet <NUM>, near the entrance to the portion L<NUM> that has a constant cross-sectional area. In other embodiments, the filament <NUM> may be located at other locations within portion L<NUM> of the overall length L of the waveguide <NUM>, where L<NUM> is the region in the waveguide where the plasma is formed as described in relation to previous embodiments. In some embodiments, the filament <NUM> is located within portion L<NUM> and upstream of the process gas inlet <NUM>, so that it will be located outside of the portion L<NUM> (shown in <FIG> and <FIG>) where reactions are taking place and which could coat the filament with reacted species. The presence of filament <NUM> can reduce the plasma ignition voltage by providing an ignition site, by focusing the electric field of microwave radiation <NUM>. Additionally, the filament <NUM> can become heated and emit electrons through thermionic emission, which further contributes to reducing the plasma ignition voltage. Although the filament <NUM> is illustrated as a single wire in this embodiment, filament <NUM> may take other configurations such as a coil or multiple filaments. In some embodiments, the filament <NUM> is tungsten. In some embodiments, the filament may be actively energized (powered) or may be passive. In some embodiments, the filament <NUM> 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 <NUM> is a ferrous material in the field of an inductive coil. In some embodiments, the filament <NUM> is actively heated where the active components (e.g. heating source components) are located outside of the waveguide <NUM> and the filament material that is being heated is inside of the waveguide <NUM>.

<FIG> illustrates yet further techniques for controlling the plasma energy, through the use of an electron source. Microwave processing system <NUM> includes a supply gas inlet <NUM>, a waveguide <NUM>, and a microwave radiation source <NUM> that supplies microwave radiation <NUM> as in previous embodiments. Microwave processing system <NUM> also includes one or more electron sources <NUM> to assist in the ignition of the plasma and/or the excitation of higher energy species within the plasma. The electron source <NUM> is configured to inject electrons into the waveguide <NUM>, 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<NUM> of the overall length L of the waveguide <NUM>, where L<NUM> is the region in the waveguide where the plasma is formed as described above. For example, in this embodiment the electron source <NUM> is configured to supply electrons into the waveguide <NUM> downstream of the first gas inlet <NUM>. In some embodiments, the electron source <NUM> is a field emission source. In some embodiments, the electron source <NUM> contains an osmium element adjacent to a heater coil. In some embodiments, the electron source <NUM> contains a ferrous material in the field of an inductive coil. In some embodiments, the electron source <NUM> contains a filament, as described above, and the generated electrons are injected into the portion L<NUM> using a high voltage electric field. In some embodiments, the electron source <NUM> is alternatively a source of ions.

An advantage of using a filament <NUM> and/or an electron source <NUM> 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 <NUM>, or greater than <NUM>), even with high gas flows (e.g., greater than <NUM> slm) and large gas volumes (e.g., up to <NUM>). This is particularly important at high pressures (e.g., greater than <NUM> atm (<NUM> Pa), or greater than <NUM> atm (<NUM> Pa), or greater than <NUM> atm (<NUM> Pa)), 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> also illustrates an embodiment of an electrode <NUM> in the present systems, as another technique for controlling chemical pathways. The electrode <NUM> may be used independently of, or in combination with, the precursor gases <NUM> of <FIG>, the filaments <NUM> of <FIG> or the electron source <NUM> of <FIG>. In some embodiments, the system <NUM> contains one or more sets of electrodes <NUM> to add energy to the plasma. The electrodes are configured to generate an electric field within the portion L<NUM> of the overall length L of the waveguide <NUM>, where L<NUM> is the region in the waveguide where the plasma is formed as described above. Electrode <NUM> is embodied in <FIG> 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 <NUM> where the plasma <NUM> 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<NUM>, 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 <NUM> slm), large volumes of plasma (e.g., up to <NUM>), high pressures (e.g., greater than <NUM> atm (<NUM> Pa) or greater than <NUM> atm (<NUM> Pa), or greater than <NUM> atm (<NUM> Pa)), 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 <FIG>, <FIG>, <FIG>, <FIG> and <FIG>. 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).

<FIG> is an example flow chart <NUM> representing methods for microwave processing of gas, using chemistry control in high efficiency gas reactions. In step <NUM>, 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 <NUM> kV. A pressure within the waveguide is at least <NUM> atmosphere (<NUM> Pa), such as from <NUM> atm to <NUM> atm (<NUM> Pa to <NUM> Pa). In step <NUM>, 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 <NUM>, 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 <NUM>, 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 <NUM> slm.

In step <NUM>, an average energy of the plasma is controlled to convert the process gas into separated components. The average energy may be, for example, <NUM> eV to <NUM> eV (<NUM>×<NUM>-<NUM> J to <NUM>×<NUM>-<NUM> J). In some embodiments the pulsing frequency is controlled, where the pulsing frequency is greater than <NUM>. For example, the pulsing frequency of the microwave radiation may be from <NUM> to <NUM>. 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 <NUM>%.

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<NUM>, and at least a portion of the separated component H<NUM> is recycled back to the first location. In such embodiments, the supply gas comprises H<NUM>, 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.

Claim 1:
A gas processing system (<NUM>, <NUM>), comprising:
a microwave radiation source (<NUM>, <NUM>) configured to generate microwave radiation (<NUM>, <NUM>);
a waveguide (<NUM>, <NUM>) having a length (L) and being coupled to the microwave radiant source (<NUM>, <NUM>); and
one or more gas inlets (<NUM>, <NUM>) configured to provide supply gas and/or process gas (208a, 308a) into the waveguide (<NUM>, <NUM>);
wherein, in use, the waveguide (<NUM>, <NUM>) serves as a reaction chamber, wherein the microwave radiation source (<NUM>, <NUM>) is configured such that, in use, the direction of propagation of the microwave radiation (<NUM>, <NUM>) is parallel to the majority of the flow of the supply gas and/or the process gas (208a, 308a) in the waveguide, and the microwave radiation (<NUM>, <NUM>) generates a plasma in the supply gas and/or the process gas (208a, 308a), and wherein the microwave radiation source (<NUM>, <NUM>) is coupled to the waveguide (<NUM>, <NUM>) upstream of the portion (L<NUM>) of the waveguide (<NUM>, <NUM>) where separated components of a gas are generated.