Method and apparatus for producing silicon nanoparticles

Abstract

A method for producing porous silicon particles includes providing a rotary tubular furnace having a tube extending between a first opening and a second opening opposite the first opening. The method includes providing a mixture of a silica precursor, a metal reducing agent, and a thermal modifier into an interior cavity of the tube through the first opening. The method includes rotating the tube containing the mixture. The method includes subjecting the mixture in the tube to a heat treatment to produce a reaction product including porous silicon particles. The method further includes collecting the reaction product at the second opening, wherein providing the mixture, rotating the tube, subjecting the heat treatment, and collecting the reaction product are performed simultaneously so that porous silicon particles are continuously produced.

Description

Related Applications

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 63 / 403,654, entitled "METHODS AND APPARATUS OF PRODUCING SILICON NANOPARTICLES," filed September 2, 2022, which is incorporated herein by reference in its entirety. [Technical Field]

[0002] The present disclosure relates generally to methods and apparatus for producing nanoparticles, and specifically to producing porous silicon particles. [Background technology]

[0003] Porous silicon particles (alternatively referred to in the following disclosure as "porous silicon nanoparticles," "porous silicon nanostructures," or "porous silicon nanotubes," or more generally as "silicon particles" or "silicon crystallites") are promising anode materials for lithium-ion batteries (LIBs), with a theoretical capacity of approximately 3600 mAh / g, compared to the theoretical capacity of graphite, a conventional anode material with a theoretical capacity of approximately 372 mAh / g. The significantly higher capacity of silicon can lead to increased energy densities in LIBs. In addition, porous silicon particles exhibit other advantages, including, for example, fast charging. Furthermore, porous silicon particles can also be utilized in other applications, including, for example, hydrogen gas production, fuel cells, drug delivery, catalyst support, electronics, photovoltaics, photoluminescence, photocatalysis, and the like.

[0004] However, during battery operation, the porous silicon particles undergo repeated volume expansion and contraction due to the reversible lithiation of silicon. In some cases, this volume expansion can cause the porous silicon particles to expand to three times their original volume. This contrasts with graphite, which expands to approximately 10% of its original volume. The repeated volume expansion and contraction leads to structural degradation of the silicon material during cycling and a decrease in its reversible capacity.

[0005] To mitigate the degradation of porous silicon particles during lithiation in LIB anode electrodes, conventional practices include reducing the size of the porous silicon particles below a critical threshold and introducing pores into the porous silicon particles. At sizes below the threshold (e.g., less than about 150 nm), the porous silicon particles typically do not shatter upon expansion. Furthermore, the porous silicon particles can expand to their own pore volume, reducing stress on themselves and surrounding particles.

[0006] Current technologies for producing porous silicon particles suitable for LIB applications include, for example, top-down chemical vapor deposition (CVD) of silicon-containing gases (e.g., silane) onto carbon-based materials or other substrates (e.g., copper foil). However, the production of silane gas has not yet been sufficiently scalable, which has impacted the scalable production of porous silicon particles and significantly affected industries that utilize such particles (e.g., the LIB anode industry). In addition, top-down CVD processes produce porous silicon particles with micrometer-scale sizes, rather than nanometer-scale sizes, which are larger than the threshold particle size for porous silicon particles mentioned above. Additionally, CVD methods require a substrate within which silicon can be integrated, resulting in limited volumes of silicon composites compared to the ability to produce 100% silicon nanoparticles with maximum theoretical capacity.

[0007] Another technique for producing porous silicon particles involves using metallothermic reduction reactions in a top-down synthesis process. This method utilizes nanoscopic silica precursors and converts them into nanoscopic porous silicon particles in a reduction process. When performed correctly, this reduction reaction occurs below the melting points of both silicon and silica, thereby preserving the nanoscopic structure. However, metallothermic reduction is highly exothermic, and special precautions are always taken to avoid runaway reactions that could destroy the nanoscopic structure and properties of the porous silicon particles. In many studies, such reduction reactions have been carried out in batch processes. However, such batch processes generally have three major drawbacks. First, conventional reactors are typically designed to perform these reduction reactions on a small scale (e.g., batch sizes of approximately 5 g). At small scales, the overall exothermic energy can be kept low enough to be dissipated from the reactor, but the purity of the porous silicon particles is reduced. Second, in batch processes, the upper reaction purity of the silicon is typically determined by the reaction time. This reaction time ranges from 1 to 10 hours, typically 6 hours, which reduces the efficiency of the production process. Third, when the purity of the produced porous silicon particles is low, (HF) acid is required to remove unreacted precursor materials (e.g., silica) from the porous silicon particles. Any one or more of these factors can reduce the economic viability of producing porous silicon particles. While scaling up the batch process can address some aspects of these drawbacks, the question of how to dissipate the exothermic energy released during the metallothermic reduction reaction remains.

[0008] Therefore, for at least these reasons, it would be desirable to improve the scalable production of silicon, such as porous silicon particles. Summary of the Invention

[0009] The present disclosure provides an apparatus and method for producing porous silicon particles.

[0010] In one aspect, the present disclosure provides a method for producing nanoparticles. The method can include providing a rotary tubular furnace including a tube extending between a first opening and a second opening opposite the first opening. The method can include providing a mixture of a silica precursor, a metal reducing agent, and a thermal modifier through the first opening into an interior cavity of the tube. The method can include rotating the tube containing the mixture. The method can include subjecting the mixture in the tube to a heat treatment to produce a reaction product including porous silicon particles. The method can further include collecting the reaction product at the second opening, where providing the mixture, rotating the tube, subjecting the heat treatment, and collecting the reaction product are performed simultaneously to continuously produce porous silicon particles.

[0011] In another aspect, the present disclosure provides a method for producing silicon particles. The method can include providing a rotary tubular furnace including a tube extending between a first opening and a second opening opposite the first opening. The method can include receiving a mixture through the first opening into an interior cavity of the tube, the mixture including a silica precursor, a metal reducing agent, and a salt. The method can include rotating the tube containing the mixture. The method can include subjecting the mixture in the tube to a heat treatment, resulting in a reaction product including silicon particles. The method can include sintering the silicon particles. The method can further include collecting the sintered silicon particles at the second opening, where receiving the mixture, rotating the tube, subjecting the mixture to a heat treatment, and collecting the reaction product are performed sequentially.

[0012] In yet another aspect, the present disclosure provides a rotary tube furnace for producing nanoparticles. The rotary tube furnace can include a material inlet configured to receive a reactant. The rotary tube furnace can include a material outlet configured to discharge a product. The rotary tube furnace can include a tube having an internal cavity, the tube extending along a longitudinal axis between a first opening connected to the material inlet and a second opening connected to the material outlet. The rotary tube furnace can include a mixing module configured to continuously rotate the tube about the longitudinal axis. The rotary tube furnace can include a heating module configured to heat the internal cavity of the tube to a selected temperature. The rotary tube furnace can include a tilting module connected to the tube and configured to elevate the first opening of the tube relative to the second opening of the tube. The rotary tube furnace can further include a vacuum module connected to the tube and configured to maintain the internal cavity of the tube at a selected vacuum level.

[0013] In yet another aspect, the present disclosure provides a porous silicon reaction product produced by a method that can include providing a rotary tubular furnace including a tube extending between a first opening and a second opening opposite the first opening. The method can include providing a mixture of a silica precursor, a metal reducing agent, and a thermal modifier into an interior cavity of the tube through the first opening. The method can include rotating the tube containing the mixture. The method can include subjecting the mixture in the tube to a heat treatment to produce a reaction product including porous silicon particles. The method can further include collecting the reaction product at the second opening, where providing the mixture, rotating the tube, subjecting the heat treatment, and collecting the reaction product are performed simultaneously so that porous silicon particles are continuously produced.

[0014] According to the disclosed method, the metallothermic reduction of silica can be carried out continuously, for example, using an apparatus including an unsealed rotary furnace. The apparatus is configured to continuously heat and mix the reactants of the metallothermic reduction reaction while providing controlled temperature and atmospheric conditions. Advantageously, the production of porous silicon particles using the disclosed apparatus and method exhibits higher throughput (e.g., the same or greater amount of silicon particles produced in a shorter production period) and higher product purity than the batch processes commonly used in conventional silicon particle production. In contrast, batch processes typically result in incomplete silica reduction, subsequently necessitating the use of toxic HF acid as a secondary step to remove unreacted silicon. Additionally, the disclosed apparatus and method can reduce the residence time of the reactants in the rotary furnace to less than one hour, which is much less than the 1 to 10 hours (e.g., 6 hours) involved in typical batch processes.

[0015] Aspects of the present disclosure are best understood from the following detailed description when viewed in conjunction with the accompanying drawings. It should be noted that, according to standard practice in the industry, the various features have not been drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or decreased for clarity of illustration. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a perspective view of an embodiment of an apparatus for producing porous silicon particles, according to some embodiments of the present disclosure. [Figure 2] FIG. 1 is a perspective view of an embodiment of an apparatus for producing porous silicon particles, according to some embodiments of the present disclosure. [Figure 3] 1 is a flowchart illustrating a method of producing porous silicon particles, according to some embodiments of the present disclosure. [Figure 4] 1 is a flowchart illustrating a method of performing a heat treatment in producing porous silicon particles, according to some embodiments of the present disclosure. [Figure 5]1 is a plot comparing X-ray diffraction spectra of porous silicon particles produced by a batch process (spectrum A) and a continuous process (spectrum B), according to some embodiments of the present disclosure. [Figure 6] 5 is a plot illustrating an example temperature profile during an intermediate stage of the method shown in FIGS. 3 and 4, according to some embodiments of the present disclosure. [Figure 7] 5 is a plot illustrating an example temperature profile including spikes at an intermediate stage of the method shown in FIG. 4, according to some embodiments of the present disclosure. [Figure 8A] 1 is a nitrogen adsorption isotherm for a sample of porous silicon particles produced according to some embodiments of the present disclosure. [Figure 8B] 1 is a nitrogen adsorption isotherm for a sample of porous silicon particles produced according to some embodiments of the present disclosure. [Figure 9A] 8B is a table showing physical properties of a sample of porous silicon particles corresponding to the sample of FIG. 8A, according to some embodiments of the present disclosure. [Figure 9B] 8C is a table showing physical properties of a sample of porous silicon particles corresponding to the sample of FIG. 8B, according to some embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following disclosure provides many different embodiments, or examples, for implementing various functions of the provided subject matter. To simplify the disclosure, specific example components and configurations are described below. These are, of course, merely examples and are not intended to be limiting. For example, in the description that follows, the formation of a first feature above or on a second feature may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which an additional feature is formed between the first and second features such that the first and second features are not in direct contact. Additionally, the present disclosure may repeat reference numerals and / or characters in various examples. This repetition is for simplicity and clarity and does not, in itself, affect the relationship between the various embodiments and / or configurations described.

[0018] Additionally, spatially relative terms such as "beneath," "below," "lower," "above," and "upper" may be used herein to simplify the description to describe the relationship of one element or feature to another, as illustrated. Spatially relative terms are intended to encompass various orientations of the device in use or operation in addition to the orientation depicted. The device may also be oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0019] The present disclosure provides an apparatus and method for producing porous nanoparticles. The apparatus of the present disclosure is configured to produce porous silicon particles. The method of the present disclosure produces porous silicon particles. In some embodiments, the apparatus of the present disclosure may perform or implement the methods disclosed herein. In some embodiments, the method of the present disclosure represents a scalable method for producing porous silicon particles based on a metallothermic reduction reaction. In some embodiments, the apparatus of the present disclosure performs the method continuously rather than in a batch-driven (e.g., batch-by-batch) process, and the apparatus is configured to continuously sense and control the temperature, atmosphere, residence time, and throughput of the porous silicon particle production. In some embodiments, the sensing and control of the atmosphere, residence time, and throughput of the porous silicon particle production are performed independently of one another. In some embodiments, the apparatus of the present disclosure can be scaled up to an industrial scale (e.g., greater than about 100 feet in length) for throughput of porous silicon particles at the scale of, for example, 10,000 s metric ton (MT) / Pa.

[0020] With reference to FIGS. 1 and 2 , the present disclosure provides a rotary tubular furnace (hereinafter referred to as the “furnace” for simplicity) 100 configured to produce nanoparticles. In particular, the present disclosure is directed to the furnace 100 designed to involve a metallothermic reaction (alternatively referred to as a “metallothermic reduction reaction,” “exothermic reaction,” or “reduction reaction” in the following disclosure) between a silica precursor and a metal reducing agent (alternatively referred to as a “metal reductant” or “metal reactant” in the following disclosure) to produce porous silicon particles. Note that FIGS. 1 and 2 collectively illustrate non-limiting example embodiments of the furnace 100 from various perspectives. Components of the furnace 100 illustrated herein may be omitted or replaced, and additional components may be introduced in accordance with embodiments of the present disclosure. FIG. 1 illustrates a frontal overview of the furnace 100, while FIG. 2 illustrates the furnace 100 in a perspective side view.

[0021] In this embodiment, the furnace 100 includes a platform 101 that structurally supports the various components of the furnace 100. The furnace 100 includes a material inlet 102 connected (or coupled) to a tube 106 at a first opening 106A. The tube 106 is connected to a material outlet 108 at a second opening 106B opposite the first opening 106A. The furnace 100 further includes a reactant supply hopper 104 coupled to the material inlet 102, through which reactants for the reaction stored in the reactant supply hopper 104 may be provided to the tube 106. The furnace 100 may further include a product hopper 110 configured to store products (and by-products) once the reaction is complete. As described in more detail below, the platform 101 is configured to tilt to increase the rate at which the reactants move through the tube 106 from the material inlet 102 toward the material outlet 108.

[0022] In one example embodiment, the reactants stored in reactant feed hopper 104 may include a silica precursor and / or a metal reducing agent for a metallothermal reaction to produce porous silicon particles, which may become the product stored in product hopper 110. In some cases, reaction by-products, such as metal oxides from the metallothermal reaction, may be stored in product hopper 110 before being removed. In embodiments where the reactants are solid-phase, the reactants may be loaded into reactant feed hopper 104 and fed through material inlet 102. In some embodiments, a thermal moderator, such as magnesium chloride (MgCl), sodium chloride (NaCl), and / or other suitable salts, is fed through material inlet 102 before, during, or after feeding the reactants. This is because the melting point of the thermal moderator is below the melting point of silica / silicon and helps absorb the exothermic energy of the reaction. In some embodiments, the thermal moderator is first dried in an oven at approximately 110°C before being fed through material inlet 102.

[0023] In some embodiments, furnace 100 further comprises a feed rate controller 105 coupled to material inlet 102. In some examples, feed rate controller 105 is disposed between material inlet 102 and tube first opening 106A. Feed rate controller 105 is configured to adjust the rate at which reactants in reactant feed hopper 104 are fed through material inlet 102 into tube 106.

[0024] The tube 106 includes an annular wall surrounding an interior cavity configured to contain reactants received through the material inlet 102. In some embodiments, the tube 106 extends continuously along a longitudinal axis AA′ between a first opening 106A and a second opening 106B. In some embodiments, the interface between the material inlet 102 and the first opening 106A of the tube 106 is separated by a magnetic fluid sealing element 112, and the interface between the material outlet 108 and the second opening 106B of the tube 106 is separated by a magnetic fluid sealing element 114 similar to the magnetic fluid sealing element 112. The magnetic fluid sealing elements 112 and 114 are configured to seal the interior cavity of the tube 106 and can be maintained at elevated temperatures and / or reduced pressures relative to the ambient environment. In some embodiments, the furnace 100 further includes a cooling device 116 coupled to the magnetic fluid sealing elements 112 and 114. Chiller 116 is configured to provide a cooling fluid to keep magnetic fluid sealing elements 112 and 114 below 25° C. to reduce or prevent thermal damage. Chiller 116 provides cooling by continuously circulating water at approximately 21° C. within magnetic fluid sealing elements 112 and 114.

[0025] In this embodiment, still referring to FIGS. 1 and 2 , the furnace 100 includes a gas module 120 configured to provide a gas (e.g., an inert gas, such as argon (Ar), nitrogen (N), other suitable gases, or a combination thereof) to the interior cavity of the tube 106 and subsequently monitor the pressure and gas flow rate within the interior cavity. In this regard, the gas module includes at least a gas inlet 122 coupled to the material inlet 102 via a fluid connection. In the present disclosure, a “fluid connection” may refer to a physical point-to-point connection between two components. Alternatively, a “fluid connection” may refer to two components connected through a third component (e.g., a segment of tubing) such that a fluid (e.g., a gas or liquid) can travel between the two components. The present disclosure does not limit the location of the gas inlet 122, as long as the gas inlet 122 is located between the material inlet 102 and the first opening 106A of the tube 106. The gas module 120 further comprises a pressure gauge 124 coupled to the interior cavity of the tube 106 and configured to measure the pressure exerted by the gas. In some examples, the pressure gauge 124 may be a Pirani gauge, although other types of pressure measurement devices are also applicable. The gas module 120 may further comprise one or more transmission lines (e.g., tubing; not shown) connecting the gas inlet 122 to a gas tank (not shown) and a digital gas flow meter (not shown). In one example embodiment, one or more reactants in the gas phase may be supplied from a gas tank through the gas inlet 122 to the interior cavity of the tube 106.

[0026] In this embodiment, furnace 100 includes a mixing module 130 configured to continuously rotate the tube about longitudinal axis AA′, thereby mixing and forcing the reactants forward along the length of tube 106 between first opening 106A and second opening 106B, either in unison or in parallel. In some embodiments, mixing module 130 includes a screw feeder 132 extending partially into the interior cavity of tube 106; a motor / bearing 134 coupled to an end of screw feeder 132 and / or a segment of tube 106 and configured to rotate screw feeder 132 and / or tube 106; a rotational speed controller 138 coupled to the motor and configured to monitor the rotational speed of motor / bearing 134; and a mass flow readout 140 coupled to tube 106 and configured to display calculated (or estimated) mass flow data of the reactants as they move through tube 106. In some embodiments, the screw feeder 132 is configured to spiral (i.e., rotate) forward toward the material outlet 108 to transport the reactants from the material inlet 102 to the tube 106. In some examples, the screw feeder 132 may be an auger screw. In some embodiments, the mixing module 130 further includes a cooling device 142 coupled to the motor / bearing 134 and configured to provide cooling against overheating.

[0027] In this embodiment, furnace 100 includes a heating module 150 coupled to tube 106. In some embodiments, heating module 150 includes one or more heating elements 152 configured to surround tube 106 and provide heating from various locations around tube 106. Heating elements 152 may radiate heat by any suitable method, such as by resistive heating, conductive heating, IR heating, combustion-based heating, other heating methods, or combinations thereof. Heating module 150 further includes a thermometer 146 coupled to tube 106 and configured to monitor the temperature within or near tube 106. Thermometer 146 may be any suitable device, such as a K-type thermocouple. In some embodiments, heating element 152 is configured to heat tube 106 to a selected temperature (or temperatures over a range of temperatures), which may be monitored using thermometer 146.

[0028] In some embodiments, as shown in FIG. 1, the heating element 152 includes a first heating zone HZ1, a second heating zone HZ2, and a third heating zone HZ3, which are arranged in this order along the length of the tube 106 between the first opening 106A and the second opening 106B. The heating zones HZ1 to HZ3 are independently programmed by a controller 153 connected to a control panel 180 (detailed below) to perform different heat treatments on different portions of the inner cavity of the tube 106. In some embodiments, the heating zones HZ1 to HZ3 correspond to multiple portions of the inner cavity of the tube 106 having different temperatures. For example, the first portion of the tube 106 corresponding to the first heating zone HZ1 has a first temperature T1, the second portion of the tube 106 corresponding to the second heating zone HZ2 has a second temperature T2, and the third portion of the tube 106 corresponding to the third heating zone HZ3 has a third temperature T3, where T1 < T2 < T3. In some embodiments, the temperatures T1 to T3 are programmed according to different heat treatments applied to the mixture of reactants in the reduction reaction. In this regard, the mixture of reactants moves continuously through different portions of the tube 106 and can undergo different heat treatments without frequently adjusting the heating element 152 (e.g., successive temperature ramping and cooling) and / or without stopping the manufacturing process. As a result, the overall throughput of the manufacturing process can be improved. In some embodiments, the heating zones HZ1 to HZ3 are programmed to the same temperature in the inner cavity of the tube 106.

[0029] The furnace 100 may further include a slide seal element 148 configured to seal a thermometer 146 (e.g., a thermocouple) in place, and the thermometer 146 can measure the temperature inside the inner cavity of the tube 106 in-situ during the reduction reaction.

[0030] In this embodiment, the furnace 100 further comprises an insulated chamber 154 that houses the heating element 152 and the tube 106. The insulated chamber 154 is configured to isolate the tube 106, maintained at a selected temperature, from the temperature of the surrounding environment (e.g., outside the insulated chamber 154). In some embodiments, the insulated chamber 154 comprises a chamber door 155 that can be secured to the body of the insulated chamber 154 by one or more locks 156. Such locks can improve the seal of the chamber door 155 for increased thermal insulation.

[0031] In this embodiment, the furnace 100 includes a tilt module 160 including a tilt element 162 that attaches the body of the insulated chamber 154 to the platform 101 and a tilt controller 164 coupled to the tilt element 162. The tilt element 162 is configured to extend or retract in response to commands received from the tilt controller 164 and determined by a user. The tilt element 162 may be driven by an actuator or other suitable mechanical means. In this embodiment, the body of the insulated chamber 154 is coupled to the platform 101, thereby allowing the insulated chamber 154 to tilt as shown. For example, the attachment point between the platform 101 and the body of the insulated chamber 154 serves as a pivot point about which the insulated chamber 154 tilts. By extending the tilting element 162, a first end of the tube 106 connected to the material inlet 102 is elevated or raised relative to a second end of the tube 106 connected to the material outlet 108, thereby moving material within the interior cavity of the tube 106 toward the material outlet 108. This, combined with the motor-driven rotation of the tube 106, contributes to a shorter residence time of the reactants within the tube 106, facilitating an increase in overall reaction throughput. Figure 1 shows the furnace 100 with the tilting element 162 extended, and Figure 2 shows the furnace 100 with the tilting element 162 retracted.

[0032] In this embodiment, furnace 100 further comprises a vacuum module 170 having a vacuum pump 172 coupled to tube 106 and a vacuum gauge 174 configured to measure the level of vacuum (i.e., pressure) in the interior space of tube 106. Vacuum pump 172 may be any suitable pump, such as a mechanical pump or a diffusion pump, in fluid communication with tube 106. In some examples, vacuum gauge 174 may be located above material outlet 108 as shown.

[0033] 1 and 2, the furnace 100 may further include a control panel 180 configured to receive instructions from a user and implement the received instructions by controlling one or more of the mixing module 130, the heating module 150, the tilt module 160, and the vacuum module 170. The furnace 100 may further include a display module (not shown) coupled to the control panel 180 and configured to output operational data related to one or more of the mixing module 130, the heating module 150, the tilt module 160, and the vacuum module 170. In some embodiments, the feed rate controller 105 and the rotation speed controller 138 may each include a control module and a display module for coordinating and monitoring their respective operation. Still further, the furnace 100 may include an emergency stop 190 configured to stop operation of the furnace 100 when switched to the “on” position.

[0034] In some embodiments, the furnace 100 may be powered by electricity, combustion, microwave, or other suitable source.

[0035] As detailed above, the present disclosure provides a scalable method for producing porous silicon particles based on a metallothermic reduction reaction. Advantageously, the apparatus (i.e., furnace 100) provided herein performs the method continuously, improving the throughput of porous silicon particle production. In one example, a reactant mixture (e.g., batch size) weighing approximately 700 g is passed through tube 106 in one hour, the mixture including a metal reducing agent (e.g., Mg), a silica precursor (e.g., SiO), and a thermal moderator (e.g., NaCl). Before being removed from tube 106, the mixture is heated from room temperature to approximately 750°C (suitable for initiating the metallothermic reduction reaction) using a set temperature profile with a residence time (or reaction time) of approximately 15 minutes. The amount of resulting porous silicon particles collected weighs approximately 30 g. Compared to this continuous reaction size of approximately 700 g, conventional small-scale batch processes are typically performed with batch sizes not exceeding approximately 5 g, such as from about 1 g to about 3 g. In this regard, to obtain porous silicon particles of similar purity using a conventional small-scale batch process, the reaction time may be on the scale of several hours, such as about 1 to 10 hours, much longer than the 15 minutes achieved by the continuous process disclosed herein. Note that furnace 100 may be scaled up (or down) depending on the amount of porous silicon particles desired. For example, furnace 100 may be configured to process more than 700 g of reactants (and thermal moderator) by expanding the diameter and / or length of tube 106. The configuration of other components of furnace 100 may also be adjusted accordingly.

[0036] Additionally, the metallothermal reduction reaction carried out by furnace 100 allows the reactant particles and thermal modifier to continuously mix and react with each other, resulting in a more complete reaction and potentially higher purity of the silicon product (i.e., porous silicon particles) compared to that obtained by batch processes. Correspondingly, the amount of unreacted silicon precursor is reduced or minimized, eliminating the need for removal processes involving corrosive acids such as HF.

[0037] 3 is a flow chart illustrating an embodiment of a method 200 for producing porous silicon particles, according to some embodiments of the present disclosure. Method 200 may, in some embodiments, be performed in / by or using the embodiments of furnace 100 detailed above. Method 200 may also be performed by other embodiments of furnaces. Method 200 is merely an example and is not intended to limit the present disclosure. Thus, it should be understood that additional steps may be provided before, during, and after method 200 of FIG. 3.

[0038] In step 202, the method 200 may include providing a furnace, an example of which is shown as furnace 100 in Figures 1 and 2 and described in detail above.

[0039] In step 204, the method 200 receives (or provides) a mixture of reactants (alternatively referred to as a “mixture” or “reactants”), including, for example, a silica (SiO) precursor and a metal reducing agent, and an optional thermal moderator, into the interior cavity of the tube 106 through the material inlet 102. The mixture may first be stored in the reactant supply hopper 104 and fed through the material inlet 102 at a rate controlled by the feed rate controller 105. In some embodiments, the reactant supply hopper 104 may be evacuated, for example, using a vacuum pump through an opening at its top, and heated with a jacket to thoroughly degas the reactants and remove residual water before passing them through the tube 106. In this embodiment, the silica precursor and the metal reducing agent are mixed and subsequently processed in the furnace 100 to form porous silicon particles.

[0040] In some embodiments, the silica precursor may be obtained from halloysite, a naturally occurring aluminosilicate having the chemical formula SiAlO(OH). Halloysite may contain aluminosilicate in its nanotube structure. The silica precursor may be obtained from halloysite through a series of purification steps. In this embodiment, a sample of the halloysite raw material is dehydrated to remove structural water and dealuminated to remove alumina (AlO) to produce a pure or substantially pure sample of silica nanotubes. In some examples, the dealuminated halloysite may then be dried in an oven at about 110°C. The halloysite raw material from which the silica precursor is obtained may be in the form of small pieces obtained directly from halloysite mining sites. Silica precursors obtained from other sources, either naturally occurring or synthetic, are also applicable in this embodiment. In some cases, the removed alumina may be recovered and resold as an important mineral for various applications. In some examples, the halloysite raw material includes about 50 weight percent (wt%) AlO and about 50 wt% SiO, and after dealumination, the amount of AlO may be reduced to less than about 50 wt%. In further examples, the amount of AlO may be reduced to between about 5 wt% and about 15%, such as about 10 wt%.

[0041] Advantageously, silica nanotubes obtained from halloysite are typically porous, and such porous structure can be maintained during subsequent metallothermal reactions to produce porous silicon particles suitable for a variety of applications. In some embodiments, the dealumination process is carried out to various extents to control the portion of alumina remaining in the silica precursor. In other words, silica precursors obtained from halloysite can also contain aluminum in the form of alumina.

[0042] In some cases, the by-products of the dealumination process are used in specialized applications, improving the economic value of the overall manufacturing process. By controlling various aspects of the dealumination process, such as the acid used, the temperature, and the pressure, the properties of the halloysite can be tailored, which in turn can affect the properties, surface area, porosity, and morphology of the resulting final porous silicon particles.

[0043] In some embodiments, the silica precursor obtained from halloysite is spray-dried to increase its bulk density and subsequently increase the throughput and efficiency of the subsequent metallothermal reaction in furnace 100. In some embodiments, the silica precursor obtained from halloysite has a particle size ranging from about 50 nm to about 500 nm in diameter and about 1 μm (e.g., less than about 400 mesh) to about 6 mm (e.g., about 3 mesh) in length. In some embodiments, the silica precursor has a particle size ranging from 10 nm to about 44 μm (e.g., about 325 mesh). In some embodiments, the particle size of the silica precursor is typically maintained in, or at least in relation to, the resulting porous silicon particles. In this regard, the particle size of the porous silicon particles can be controlled by adjusting the particle size and shape of the silica precursor. The specific particle size of the silica precursor can be controlled during the dehydration (e.g., spray-drying) and / or dealumination process to improve throughput. Drying of the dealuminated halloysite is not limited to spray drying, but may include any suitable bulk powder drying process.

[0044] In some cases, halloysite may contain small amounts of iron oxide that can be removed through a water immersion process. According to some embodiments, the iron oxide remaining after metallothermic reduction can be easily removed due to the abrupt change in the chemistry of the halloysite.

[0045] In this embodiment, the metal reducing agent includes magnesium (Mg), aluminum (Al), or a combination thereof. The metal reducing agent may additionally or alternatively include zinc (Zn), lithium (Li), sodium (Na), potassium (K), other suitable metals, or a combination thereof. For illustrative purposes, Mg is utilized as the metal reducing agent in this description of the metal thermal reaction. In this embodiment, the metal reducing agent is utilized in powder form.

[0046] In some embodiments, a metal reducing agent such as Mg may be provided as Mg vapor in the gas phase, rather than in the more cost-effective form of Mg metal (ingot). Benefits of this may include increased furnace 100 capacity, since only the silica precursor is provided in the solid phase. Advantageously, reacting the metal reducing agent in the vapor phase is beneficial for maintaining the morphology of the silica precursor during the reduction reaction and may also help preserve the nanostructure of the resulting silicon product. In some embodiments, the metal reactant is provided as a powder. For example, Mg may be provided as a powder having a particle size ranging from about 300 mesh (e.g., about 50 μm) to about 6 mesh (e.g., about 3 mm).

[0047] If the particle size of the metal reducing agent (e.g., Mg) is too large (e.g., larger than about 3 mm or 6 mesh), the surface area-to-volume ratio of the metal reducing agent may be small. In one example, during a reduction reaction using Mg as the metal reducing agent, Mg-containing gas is generated from the surface of the Mg powder and subsequently reacts with the silica precursor. Therefore, large Mg particles may slow the reduction reaction, requiring an extended reaction time. For example, a reduction reaction using Mg particles larger than about 6 mesh (e.g., about 3 mm) may take approximately 6 to 8 hours to complete without adjusting other parameters, such as pressure. In contrast, with 300 mesh-sized (or smaller) Mg particles, the reaction time may be as short as 15 minutes, thereby significantly improving the throughput of porous silicon particle production.

[0048] In some embodiments, by controlling one or more of the parameters provided herein, the metal reducing agent can be provided with a particle size greater than 6 mesh. In some embodiments, one or more parameters of the metal thermal reduction process, such as reaction time, application of thermal moderator, reaction temperature, and reaction pressure, can be adjusted (or adapted) according to one or more given particle sizes (or size ranges) of the metal reducing agent and thermal moderator, thereby making the manufacturing process more economical and versatile. For example, applying negative pressure within tube 106 can increase the vaporization rate of larger Mg particles, thereby improving reaction time compared to when negative pressure is not applied. On the other hand, when smaller Mg particles of 300 mesh size (or smaller) are used, the reaction time can be reduced to about 15 minutes without applying pressure within tube 106.

[0049] In some embodiments, thermal moderators such as MgCl and NaCl are provided to the furnace 100 along with the silica precursor and metal reducing agent because the melting point of the thermal moderator is below that of silica / silicon and helps absorb the excess exothermic energy of the reaction. For example, NaCl melts at approximately 801°C. Other suitable salts that can provide sufficient latent heat energy of fusion (i.e., energy released during the melting process) for the reduction reaction can also be used as thermal moderators in this embodiment.

[0050] In some embodiments, the thermal moderator (e.g., salt) has a particle size ranging from about 325 mesh (e.g., about 44 μm) to about 80 mesh (e.g., about 177 μm). In some embodiments, the particle size of the thermal moderator is less than about 325 mesh. If the particle size of the thermal moderator is too large (e.g., greater than about 177 μm or 80 mesh), the surface area and packing density of the salt particles in the reactant mixture (or feed mixture) may be reduced, limiting the heat absorption during the reduction reaction and resulting in large spikes in the temperature profile (discussed in more detail below). If the particle size of the thermal moderator is too small (e.g., less than about 44 μm or 325 mesh), the bulk density of the feed mixture may be reduced, and finer salt particles may be trapped between the silica and Mg particles, slowing the reaction rate and increasing the residence time of the feed mixture in the furnace 100.

[0051] In some embodiments, the particle sizes of one or more of the metal reducing agent, thermal modifier, and silica precursor are adjusted to control aspects of the metal thermal reaction (described in more detail below). For example, the thermal modifier can include a distribution of particle sizes, with the relative amounts of the various particle sizes set according to the desired temperature profile for heating the reactant mixture in the furnace 100. In some embodiments, the amount of thermal modifier particles having smaller particle sizes is less than the amount of thermal modifier particles having larger particle sizes. For example, the thermal modifier may include a first amount of particles having a size of about 325 mesh, a second amount of particles having a size greater than about 325 mesh but less than about 80 mesh, and a third amount of particles having a size of about 80 mesh, where the first amount is less than the second amount and the second amount is less than the third amount.

[0052] The metallothermic reduction described herein is typically carried out at relatively low temperatures, e.g., from about 500° C. to about 900° C. This is because this temperature range is below the melting points of silicon, which is about 1414° C., and the silica precursor, which is about 1600° C., and is favorable for maintaining the morphology (e.g., shape) of the silica precursor in the silicon product formed after the reaction is complete.

[0053] The chemical reaction scheme for the metallothermic reduction of silica to produce silicon and magnesium oxide (MgO) is shown below as Scheme I. The reaction is typically carried out under an inert atmosphere such as Ar, N2, or under vacuum. Similarly, other metals, including Al, can also be used as reductants to form, for example, silicon and alumina. [ka]

[0054] Because metallothermal reduction reactions are typically highly exothermic (i.e., they generate heat during the reaction), the released thermal energy can increase the local temperature (at or near the reactants) beyond the melting point of silica / silicon, potentially damaging the nanostructure of the resulting silicon product (e.g., porous silicon particles). For at least this reason, conventional silicon production based on silica reduction relies on very small batch-scale reactors to control such production. In the present disclosure, factors including the rotation of the tube 106, the pressure applied within the tube 106, the rate of mass flow of the inert gas within the tube 106, the throughput of the reactants (e.g., silica precursor, metal reducing agent, and / or thermal moderator), and continuous mixing of the reactants in the tube 106 can help dissipate the excess heat generated by the reduction reaction. By controlling the excess heat of the reduction reaction, production of high-purity porous silicon particles can be achieved within a relatively short reaction time, such as about 15 minutes. In some embodiments, control of the temperature, pressure, and constant mixing of the reactants enables the controlled synthesis of porous silicon particles. In some embodiments, controlled temperature spikes during the reduction reaction (as described in more detail below) improve the purity of the resulting porous silicon particles to greater than about 80% within a relatively short reaction time.

[0055] For example, Figure 5 compares X-ray diffraction (XRD) spectrum A corresponding to a sample of porous silicon particles produced by a static batch process in a muffle furnace with spectrum B corresponding to a sample of porous silicon particles produced by the disclosed continuous process in furnace 100 described herein. Both samples have the same parameters, including a molar ratio of metal reducing agent (e.g., about 2.07:1 amount of metal reducing agent to amount of silica precursor), a weight ratio of thermal moderator (e.g., about 4:1 amount of thermal moderator to amount of silica precursor), an upper reaction temperature (e.g., about 750°C), and a reaction (or residence) time of about 15 minutes.

[0056] Comparison of spectra A and B shows that the intensity of each silicon peak is significantly higher for the sample produced by the continuous process than for the sample produced by the batch process, indicating that the purity of the porous silicon particles present in the sample produced by the continuous process is higher than that of the sample produced by the batch process. In addition, the broad peaks present in spectrum A but absent in spectrum B also suggest the presence of unreacted silica precursor remaining in the sample produced by the batch process but absent in the sample produced by the continuous process. For the sample corresponding to spectrum A, an acid bath containing HF and / or other acids is required to selectively remove unreacted silica from the low yield of silicon produced by the batch process. This significantly impacts the cost of the process. First, HF is an expensive and highly toxic acid, and its scalability in bulk synthesis processes has not been proven. Second, the low silicon yield after the HF acid bath increases production costs due to increased reaction time and wasted reactants (e.g., wasted metal reductant and silica precursor raw material). In some cases, samples produced by batch processes can be converted to obtain silicon particles with higher purity, for example, by reactions according to Schemes II and III detailed below. However, in this case, the metal reducing agent, such as Mg, and the silica precursor are wasted, increasing the cost of the production process. In addition, an acid bath with HF may still be required to obtain porous silicon particles after the conversion process.

[0057] In contrast, due to the relatively high purity of silicon, samples corresponding to spectrum B do not need to undergo acid bath cleaning, thereby reducing the cost of production and avoiding the use of corrosive acids such as HF. In some embodiments, the purity of silicon in samples corresponding to spectrum B ranges from about 75% to about 80%, while the purity of silicon in samples corresponding to spectrum A ranges from about 10% to about 15%. While the purity of samples produced by batch processes can be higher than 15%, the reaction time required to achieve such a level of purity will be much longer than a continuous process. For example, a batch process may take several hours, such as 6 hours, to achieve a similar level of purity as the continuous processes disclosed herein.

[0058] In some embodiments, for example, a mixture including a silica precursor, a metal reducing agent, and a thermal modifier, all provided in powder form, is first homogenized in a mixing process to thoroughly combine and also reduce the particle size of the salt (e.g., thermal modifier). In one such example, the salt may be ground from table salt (e.g., NaCl) grade to a much finer size.

[0059] In some embodiments, the molar ratio of the amount of metal reducing agent (e.g., Mg) to the amount of silica precursor (e.g., dealuminated halloysite) is about 1.5 to about 2.5, e.g., about 2.07:1. In some embodiments, the weight ratio of the amount of thermal modifier (e.g., NaCl) to the amount of silica precursor (e.g., dealuminated halloysite) and metal reactant (e.g., Mg) may be less than about 5:1, such as about 4:1. In one such example, 1 g of dealuminated halloysite may be mixed with 7.4 g of NaCl and 0.85 g of Mg, each having a particle size not exceeding 300 mesh. In some examples, no thermal modifier may be used in the mixture.

[0060] In some embodiments, the temperature at which the exothermic reaction occurs according to Scheme 1 is controlled by factors including the weight ratio of the amount of thermal moderator to the total amount of silica precursor and metal reducing agent, the chemical nature (or type) of the thermal moderator, the molar ratio of metal reducing agent to silica precursor, the size of the salt particles, or a combination thereof. For example, if the ratio of the amount of thermal moderator to the amount of silica precursor is decreased, the exothermic reaction may lead to a larger "spike" (as described in more detail below). If a different salt is used, i.e., if the chemical nature of the thermal moderator is changed, the appropriate degree of thermal energy during the exothermic reaction may therefore change based on the latent heat of fusion of the salt. In some embodiments, any salt that can control the thermal energy released during the exothermic reaction so as not to adversely affect the nanostructure of the porous silicon particles is applicable.

[0061] As described in more detail below, the present disclosure provides a method for forming porous silicon particles based on the metallothermic (e.g., exothermic) reduction of silica, which may be implemented in the furnace 100 provided herein. Advantageously, the furnace 100 and methods using the furnace 100 offer a scalable, bottom-up synthesis route to producing porous silicon particles having a porous structure. In this embodiment, such a scalable process may utilize reactants (e.g., silica obtained from halloysite) that are available on a larger scale (e.g., metric tons) suitable for various industries, such as the LIB anode industry. Furthermore, the furnace 100 and methods using the furnace 100 provided herein may directly translate to scalable production that is unaffected, or substantially unaffected, by challenges faced by small batch-driven processes in conventional technologies (e.g., top-down CVD of silane gas and small-scale batch reactions of metallothermic reduction according to Scheme I).

[0062] In step 206, the method 200 continuously rotates the tube 106 to mix the reactants, including the silica precursor, the metal reducing agent, and the thermal moderator (if included). The mixed reactants are hereinafter collectively referred to as the "mixture." In this embodiment, the continuous rotation of the tube 106 is performed by the mixing module 130.

[0063] In step 208, method 200 changes the gas environment within the interior cavity of tube 106. In some embodiments, step 208 applies a negative pressure to tube 106, a process performed by vacuum module 170. In some embodiments, step 208 fills the interior cavity of tube 106 with an inert gas, such as Ar or N, at a pressure similar to atmospheric pressure, which is about 1 atm.

[0064] In some embodiments, the gas environment within the interior cavity of tube 106 is first established by flowing an inert gas through tube 106 for about 30 minutes to replace or purge the interior ambient atmosphere. Such purging may be repeated three times to establish the inert gas environment. Once the inert gas environment is achieved, the flow rate of the inert gas applied to maintain pressure within tube 106 may be, for example, about 175 mL / min.

[0065] In step 210, the method 200 subjects the mixture to a heat treatment. In this embodiment, the heat treatment of the mixture in the tube 106 is performed and monitored by the heating module 150.

[0066] In some embodiments, referring to FIG. 4 , thermal treatment of a mixture is performed in furnace 100 according to method 300. Method 300 is merely an example and is not intended to limit the present disclosure. In this regard, it should be understood that additional steps may be provided before, during, and after method 300 of FIG. 4 . Intermediate stages of the thermal treatment described by method 300 may be further illustrated by plot 400 of FIG. 6 , which includes profiles 402, 404, and 406, each showing temperature change over time. Profiles 402 and 404 correspond to mixtures having the same composition, while profile 406 corresponds to a mixture having Mg with a larger particle size than the Mg in the mixtures corresponding to profiles 402 and 404.

[0067] In step 302, method 300 performs a degassing process by heating the mixture to a temperature below the initiation temperature at which an exothermic (e.g., metallothermic) reaction occurs according to Scheme I. In some embodiments, the degassing temperature is less than half the initiation temperature. In some examples, the degassing temperature may be less than about 200°C, such as about 160°C, and may be held at such a temperature for about 30 minutes.

[0068] In step 304, method 300 rapidly heats the mixture to reach an initiation temperature T4, which may be approximately 500°C, and the mixture may be held at this temperature for approximately 30 minutes, according to some examples. The initiation temperature is a temperature below which no, or substantially no, exothermic reaction occurs. Referring to FIG. 6, the heating in step 304 is illustrated by segment 410 in each of profiles 402-406. The rapidity of the heating performed in step 304 is represented by the relatively steep slope of segment 410 as compared to the other segments in each of profiles 402-406.

[0069] In step 306, method 300 heats the mixture at a rate of about 1°C / min over a range of temperatures, such as from about 500°C to a reaction temperature T5 of about 560°C, where an exothermic reaction occurs. The heating of the mixture performed in step 306 is represented by segment 420 in each of profiles 402-406 in FIG. 6.

[0070] In some embodiments, as shown in the expanded view of plot 400, portions 402S, 404S, and 406S of profiles 402-406 are shown as slight humps, respectively, indicating a slight temperature increase above an onset temperature of about 500°C, such as in the range of about 515°C to about 530°C, when an exothermic reaction occurs. Such a temperature increase may last for about 1-2 minutes.

[0071] Alternatively, in some embodiments, the exothermic reaction produces a "spike" in step 306, such as a peak temperature T7 of about 800°C in FIG. 7. The spike may be high in intensity and of a relatively short duration. For example, the spike may exceed a starting temperature of about 500°C, peak at a temperature of about 400°C, and may be greater than 900°C. It should be noted that the peak temperature T7 shown in FIG. 7 corresponds to a temperature slightly lower than about 800°C due to a lag in the sensing process by thermometer 146.

[0072] The exact temperature (or temperature range) at which the exothermic reaction occurs can be determined by factors including, for example, the molar ratio of metal reducing agent to silica precursor, the weight ratio of thermal moderator (e.g., NaCl and / or MgCl salts) to the combined silica precursor and metal reducing agent, the particle size of the thermal moderator, the degree of dealumination (e.g., as indicated by the amount of Al2O3 remaining in the silica precursor after dealumination), the size of the metal reducing agent powder, or a combination thereof. For example, by adjusting the amount (e.g., the ratios described above) and particle size of the thermal moderator, the exothermic reaction between the silica precursor and the metal reducing agent can be controlled. This is because the thermal moderator can absorb the sensible heat energy released by the exothermic reaction and melt the thermal moderator (e.g., at approximately 801°C for NaCl) until the phase change temperature reaches a temperature at which latent heat energy can also be absorbed (typically, the latent heat energy is an order of magnitude greater than the sensible heat energy). For example, referring to FIG. 6, profile 406, which corresponds to a mixture containing larger particle size Mg, shows a greater increase in temperature reached during the exothermic reaction than the mixtures corresponding to profiles 402 and 404.

[0073] In step 308, method 300 rapidly heats the reaction mixture to a temperature T6 after the exothermic reaction has occurred to remove certain reaction by-products from the reaction mixture, thereby improving the yield of the exothermic reaction. In some examples, the reaction mixture may be heated to at least a temperature in the range of about 560°C to about 800°C, such as about 750°C, and maintained at such a temperature for about 15 minutes to about 60 minutes. At such temperatures, no exothermic reaction occurs. As shown in FIG. 6 , the slope of segment 430 of each of profiles 402-406 is greater than the slope of segment 420, indicating that the rate of heating performed in step 308 is greater than the rate in step 306. The heating step in step 308 improves the yield and quality of the porous silicon particles by converting reaction by-products, such as magnesium silicide (MgSi).

[0074] The method 300 then cools the reaction mixture, which may be in powder form and include the porous silicon particles. The cooling process is illustrated by segment 440 in each of profiles 402-406, as shown in Figure 6. In some embodiments, cooling of the powder reaction mixture may also be accomplished by indirectly cooling the product hopper 110.

[0075] In some embodiments, heat treatment according to method 300 is performed in furnace 100 in a batch-driven process. However, heat treatment may additionally or alternatively be performed continuously in furnace 100 to achieve the same or substantially the same temperature profile as any of profiles 402-406. To accomplish this, the rotational speed of screw feeder 132, controlled by motor 134, and the feed rate of the mixture, controlled by feed rate controller 105, are adjusted, and pipe 106 is configured with three different heating zones, HZ1-HZ3, set to different temperatures corresponding to those described above with respect to the operation of method 300. For example, heating zone HZ1 may be set to a temperature corresponding to rapid heating of the mixture in step 304, heating zone HZ2 may be set to a temperature (or range of temperatures) high enough for the reduction reaction in step 306, and heating zone HZ3 may be set to a temperature corresponding to rapid heating of the reaction product in step 308. The rotational speed and feed rate determine the residence time of the mixture in each zone, thereby producing the same temperature profile (e.g., profiles 402-406) as the stepwise heat treatment described in method 300, but continuously. In some embodiments, the sequential implementation of the heat treatment provides a scalable solution for producing porous silicon particles at higher throughputs. The scalable solution is two-fold. First, indirectly heated rotary furnaces (i.e., where the flame is outside the rotating tube) are an established technology, commercially available, and produce throughputs in the kiloton (kT) range. Second, the combination of process parameters described herein and implemented in furnace 100 allows residence times to be reduced to less than an hour, such as 15 minutes, at higher temperatures (e.g., 750°C). This contrasts with the many hours typically required for small-scale batch processes involving metallothermic reduction reactions. A thermometer 146 (eg, a thermocouple) may be used to create a desired temperature profile along the longitudinal axis AA′ of the tube 106 .

[0076] For example, by controlling parameters including the molar ratio of metal reducing agent to silica precursor, the weight ratio of thermal modifier to the combination of silica precursor and metal reducing agent, the size of the thermal modifier particles, the degree of dealumination, or a combination thereof, many aspects of the thermal treatment of the mixture (e.g., according to method 300) can be tailored to produce silicon porous particles having a range of particle (e.g., crystallite) sizes suitable for applications including LIBs.

[0077] In some embodiments, a sintering process is carried out in step 211 before cooling the reaction mixture containing the porous silicon particles and after rapidly heating the reaction mixture to a temperature T6 in step 308. Typically, the sintering process involves heating the porous silicon particles in an inert atmosphere to a sintering temperature much higher than the temperature (or temperature range) required for the exothermic reaction (i.e., reaction temperature T5) to increase the average crystallite size of the porous silicon particles. In some embodiments, the porous silicon particles may be sintered at about 900°C to about 1200°C for about 15 minutes to about 4 hours. In one example, the porous silicon particles may be sintered at about 1100°C for about 15 minutes. In this embodiment, the sintering temperature is higher than temperature T6 as shown in FIG. 6, similar to peak temperature T7 shown in FIG. 7.

[0078] The effect of the sintering process is shown in Figures 8A-9B. Figures 8A and 8B show nitrogen adsorption isotherms for samples of the resulting porous silicon particles that underwent different treatments in plots 500 and 550, respectively, and the physical properties of the same samples are shown in the tables of Figures 9A and 9B, respectively. The samples in Figures 8A and 9A were measured after being sintered in step 211, while the samples in Figures 8B and 9B were measured for the same samples as those in Figures 8A and 9A but before being sintered in step 211.

[0079] The nitrogen absorption isotherms presented in plots 500 and 550 show the change in the amount of nitrogen absorbed by samples of porous silicon particles before and after undergoing a sintering process at different relative pressures. The sintered samples of Figures 8A and 9A are shown to have crystallite sizes (or grain sizes) that are three times larger than the crystallite sizes of the unsintered samples. As a result, the sintered samples of Figures 8A and 9A had less surface area than the samples of Figures 8B and 9B.

[0080] In some embodiments, the crystallite size of the porous silicon particles produced by the methods of the present disclosure is seen as a primary driver of physical properties such as surface area and pore volume (or porosity), as shown in Figures 9A and 9B. For example, the samples shown in Figures 8A and 9A, which have larger crystallite sizes as a result of undergoing a sintering process (described below), are shown to have lower surface areas and pore volumes compared to the samples shown in Figures 8B and 9B, which have much smaller crystallite sizes. In some examples, the porous silicon particles produced by the methods disclosed herein exhibit such physical properties, resulting in improved performance in LIP applications, including maintaining a capacity of at least about 2500 mAh / g for at least 100 cycles of operation.

[0081] Typically, during the first few (e.g., 1–3) cycles of LIB operation, a portion of the electrolyte irreversibly disintegrates on the surface of the anode material (e.g., silicon), forming a passivation layer called the solid electrolyte interphase (SEI). It is usually acceptable for the SEI to develop uniformly and robustly in an experimental setting. However, in commercial applications, an excessive amount of SEI can be costly because it consumes expensive electrolyte materials and impairs LIB performance. The initial coulombic efficiency (ICE) indicates the ratio of charge (e.g., electrons) introduced to the anode to the charge removed from the anode. An ICE of 100% indicates that all electrons introduced to the anode can be removed. The ICE can be used to measure the amount of SEI formed during the first few cycles and should typically be greater than approximately 85% for porous silicon particles used in commercial LIB applications. This suggests that only about 15% or less of the charge decays in the first few cycles to form the SEI.

[0082] The samples with relatively high surface area produced without sintering (FIGS. 8B and 9B) exhibited an ICE of approximately 74%, which is less desirable for commercial applications. In comparison, the sintered samples of FIGS. 8A and 9A exhibited a lower surface area, which is the primary driver of SEI formation, and achieved an ICE of up to approximately 89%. This represents an unprecedented and significant improvement for porous silicon particles in LIB applications. Conventional techniques for reducing the SEI include, for example, modifying the surface of porous silicon particles with a carbon or Al2O3 coating or embedding the porous silicon particles within other carbon-containing particles. These techniques reduce the SEI by physically masking the surface of the porous silicon particles or isolating them from the electrolyte. In comparison, performing the sintering process in step 211 alone has been shown to achieve a similar effect in terms of reducing the surface area of ​​the porous silicon particles and increasing their ICE. This is crucial for improving the lifetime and / or performance of LIBs. In particular, by combining a surface modification process (eg, a coating or embedding process) with the sintering process, the life and properties of the porous silicon particles can be further improved significantly.

[0083] In some embodiments, instead of performing the sintering process in step 211, the effects of the sintering process, such as increasing the crystallite size and reducing the surface area of ​​the porous silicon particles, are achieved during the heat treatment of method 300, such as during the exothermic reaction performed in step 306. For example, by controlling factors such as the weight ratio of the thermal regulating material and the heating rate (e.g., the slope of segment 420 as shown in FIG. 6 ), the exothermic reaction can become rapid, as described above and shown in FIG. 7 , and the thermal energy released during the exothermic reaction can cause a chain reaction that can heat the mixture to a peak temperature T7 of at least about 900°C, similar to or exceeding the temperature of the sintering performed in step 211. In this regard, instead of recording a slight hump, as shown in each of portions 402S, 404S, or 406S of FIG. 6 , in the respective temperature profiles, thermometer 146 may record a temperature spike of at least about 800°C. This suggests that the mixture has locally reached a higher temperature, such as 1200°C, sufficient to sinter the porous silicon particles. In some embodiments, decreasing the weight ratio of thermal modifier in the mixture results in a larger spike in temperature during the exothermic reaction. In some instances, a weight ratio of thermal modifier to reactants of about 1:1 or greater results in a spike that can sinter the porous silicon particles. For example, the weight ratio can be about 2:1, 3:1, 4:1, or 5:1. As shown in FIG. 7 , by rapidly ramping the exothermic reaction to a peak temperature, T7, the thermal conditions for both the exothermic reaction and sintering are met, and porous silicon particles with enlarged crystallite size and reduced surface area can be produced.

[0084] In step 212, the method 200 collects the reaction product, including the porous silicon particles, from the tube 106 of the furnace 100 to the ambient environment (e.g., by exposure to air). In this embodiment, collection of the reaction product is performed by emptying the tube 106 using, for example, the tilting module 160. In the case of a continuous process performed in the furnace 100, the product hopper 110 can be isolated and disconnected using a manual control valve at the material outlet 108.

[0085] In some embodiments, steps 206, 208, 210, 211, and 212 can be performed simultaneously, simultaneously, or substantially simultaneously to pass the mixture through tube 106 while it is processed (e.g., heat treatment and / or sintering process) at high temperatures in a designated gas environment (e.g., an inert environment or vacuum) so that porous silicon particles are continuously produced. In this regard, the continuous reduction reaction, among other attributes, improves the throughput of the final product and offers a solution to scale-up production of porous silicon particles in contrast to traditional batch-driven processes (e.g., top-down CVD deposition processes and small-scale batch processes of metallothermic reduction reactions). In some embodiments, the configuration of furnace 100 also enables porous silicon particles to be produced in a batch-driven process through control of the feeding, mixing, rotation, heating, and tilting processes, which may be performed by one or more of mixing module 130, heating module 150, tilting module 160, other components of furnace 100, or a combination thereof.

[0086] Additionally, because furnace 100 provides in-situ monitoring of various aspects of the reduction reaction (e.g., reaction temperature, chamber / tube pressure, feed rate, rotation speed, tilt angle, etc.), any two or more of steps 206-212 may be performed in a coordinated manner, as described in more detail below.

[0087] In this embodiment, rotation of the heated section of tube 106 allows the mixture containing the metal reducing agent and silica precursor to be mixed (or further mixed) during the metallothermic reduction reaction. In some embodiments, the rotation is adjusted to redistribute the reactive mixture at a rate of about 0.1 rpm to about 20 rpm, such as about 1 rpm to about 20 rpm, spreading the exothermic energy of the reduction reaction throughout the entire volume of the reactive mixture and / or reaction products and over a broader time frame, thereby reducing spikes in local temperature. Continuous mixing of the reactants also results in more even distribution of the reactants (and, e.g., thermal moderators) within the mixture and closer proximity between the reactants (and, e.g., thermal moderators), thereby controlling the reaction and improving product yield and quality.

[0088] In some embodiments, the mixture of reactants (and, e.g., thermal modifier) ​​in the interior cavity of the tube 106 is agitated by including multiple spheres (or balls), such as stainless steel spheres, in the mixture. The spheres can improve mixing of the reactants within the tube 106, increase agitation during mixing, and prevent solidification of the reactants (e.g., in powder form) on the interior walls of the tube 106. In some embodiments, the spheres comprise a thermally conductive material, such as a metal or metal alloy, which can transfer heat from the interior walls of the tube 106 toward the center of the interior cavity, resulting in a more homogeneous heating regime, i.e., reducing temperature gradients within the interior cavity of the tube 106. Additionally, the spheres can remove heat from hot spots caused by exothermic energy released during the metal thermal reaction. Furthermore, the spheres may be used to clean the interior space of the tube 106. In some embodiments, the agitation by the spheres is controlled by rotation of the tube 106. In some instances, other types of agitators (e.g., different geometries and / or compositions) capable of conducting heat and distributing heat throughout the mixture within the tube 106 are also applicable in this embodiment.

[0089] In this embodiment, a controllable atmospheric pressure of about 10 Pa to about 100 kPa provided by vacuum module 170, and feed and exhaust rates controlled by feed rate controller 105, are applied to tube 106 during the reduction reaction. Additionally, the reaction temperature is controlled and monitored using thermometer 146, which may be an in-situ thermocouple, and the level of vacuum is controlled and monitored by vacuum gauge 174, allowing for control of the release of exothermic energy during the metallothermic reduction reaction.

[0090] In some embodiments, the heating module 150 and the gas module 120 may operate in coordination such that if the temperature of the interior cavity of the tube 106 rises above a predetermined set point, an inert gas, such as Ar, is released through the gas inlet 122. The inert gas may rapidly reduce the temperature by replacing the atmosphere within the interior cavity of the tube 106. In some embodiments, the coordinated operation may be monitored and controlled manually. In some embodiments, the coordinated operation may be performed by a computer-controlled feedback mechanism to reduce spikes in exothermic energy generated by the reduction reaction in the tube 106.

[0091] Reducing the pressure within tube 106 promotes the production of metal (e.g., Mg) vapor. In the vapor phase, Mg reduction of the silica precursor can be carried out at lower temperatures (e.g., about 350°C to about 650°C) than reactions involving solid-phase Mg. Such a low initial reaction temperature reduces the likelihood of spikes resulting from exothermic reactions raising the reaction temperature near the melting points of silica or silicon. Additionally, because the metal reducing agent diffuses in the vapor phase, it can be more uniformly distributed throughout the silica precursor, increasing production yield and reducing reaction time. In some embodiments, magnetic fluid sealing elements 112 and 114 adequately seal the interior cavity of tube 106 to achieve and maintain the desired vacuum level / low pressure.

[0092] Furthermore, continuous mixing and dispersion of the vapor-phase metal reducing agent allows for the use of a more cost-effective form of metal powder without resorting to fine powder (e.g., 325 mesh or 280 mesh). In this regard, metal granules or chips (e.g., 10 mesh to 40 mesh) can be fed into furnace 100 along with the silicon precursor. Alternatively, as discussed above, a powder of the metal reducing agent may be employed in the processes of the present disclosure.

[0093] In some examples, the residence time of the mixture within tube 106 is controlled by adjusting screw feeder 132 (e.g., powder feed auger screw), the rotational speed (rpm) of tube 106, and the tilt angle provided by tilt module 160, or a combination thereof, to enable continuous production of porous silicon particles. Using in-situ pressure and temperature measurement devices (e.g., pressure gauge 124, vacuum gauge 174, and thermometer 146), the production of exothermic energy from the reduction reaction can be actively monitored and recorded. The rotational speed, gas flow rate, vacuum level, reactant feed rate, and vessel emptying (e.g., by tilting) can be adjusted accordingly to reduce or avoid inadvertent overheating and destruction of the nanostructure of the resulting porous silicon particles.

[0094] In some embodiments, a thermal moderator, such as NaCl and / or MgCl, or any salt with a large latent heat energy, is added to the mixture in tube 106 in an amount between 0 wt% and 500 wt% relative to the reactants. This amount is less than previously reported for similar applications. In some instances, the ability of the thermal moderator to regulate the temperature of the metal thermal reaction may be improved by more thorough continuous mixing.

[0095] In some embodiments, the metal thermal reaction provided herein forms porous, polycrystalline porous silicon particles, which may include primary particles having nanometer-scale sizes. In some cases, the primary particles may aggregate to form secondary particles having nanometer- to micrometer-scale sizes. The porous silicon particles may, in some instances, be nanotubular. The pores present in the halloysite precursor may be maintained within the porous silicon particles, even if the pore size varies. The pore size of the halloysite may vary based on the amount of alumina present (i.e., the degree of dealumination). Additionally, by varying one or more of the reaction temperature, vacuum level (i.e., pressure), and degree of dealumination within tube 106, the pore size of the final porous silicon particles may be tailored for various applications. For example, increasing the temperature of the metal thermal reaction may result in larger porous silicon particles with less surface area.

[0096] In some embodiments, collection of the porous silicon particles (and any accompanying metal oxides and / or metal silicides) is performed, at least in part, by tilting the tube 106 using the tilt module 160.

[0097] In step 214, method 200 may perform additional operations including, for example, removing primary reaction by-products and removing or collecting secondary reaction products.

[0098] In some embodiments, the reaction product containing porous silicon particles is washed (or leached) with an acid bath, such as a hydrochloric acid (HCl) bath, to remove unreacted silica precursor, secondary products such as MgO (see Scheme I), and / or by-products (e.g., MgSi), leaving a suspension of solid silicon product (i.e., porous silicon particles) to be collected. To control the acid washing process, factors including the concentration (e.g., molar amount) of the acid, the temperature of the acid, and the residence time of the reaction mixture in the acid can be adjusted. In one example, the acid wash may be carried out using an HCl bath with a concentration of about 3 M at about 80° C. for about 15 minutes. In another example, the acid wash may be carried out using an HCl bath with a concentration of about 1 M at about 80° C. for about 70 minutes. In some examples, the duration of the acid wash may be adjusted according to the concentration of the acid bath for a given acid used. In some embodiments, the concentration and duration of the acid washing process are adjusted based on the silicon yield in the reaction product.

[0099] In some embodiments, the continuous metallothermic reduction reactions carried out in furnace 100 result in silicon (i.e., porous silicon particles) with a purity of about 75% to about 90% in the reaction product. In other words, the continuous metallothermic reduction reactions can achieve a substantially complete metallothermic reduction reaction, leaving little or no unreacted precursor material, secondary products, or by-products. As such, the acid bath wash of the reaction product in step 214 can be omitted in some embodiments, reducing or minimizing the complexity, overall process cost, and operational hazards associated with handling corrosive and toxic acids.

[0100] In some embodiments, for example, on a 100 g scale, the collected porous silicon particles are subsequently dehydrated using Buckner filtration, rinsed multiple times (e.g., three times) with water slightly above room temperature (e.g., at about 40°C) to remove salts (e.g., MgO), and then dried in an oven at about 110°C. In some embodiments, for example, on a larger scale, a centrifuge and a spray dryer or paddle dryer may be used in place of the filtration method and oven, respectively. Other suitable means of carrying out the dehydration and drying processes are also applicable, so long as the porous silicon particles can be dried.

[0101] When the local distribution of the reactants is not uniform, such as during a batch process without continuous mixing of the reactants while heat treating according to method 300, in addition to the silica reduction reaction described above, a secondary reaction according to Scheme II below can occur to produce magnesium silicide (MgSi): [ka] The formation of Mg2Si is highly undesirable because its complete removal during the acid clean process removes some of the metal (Mg) from the reactant that could otherwise be used to reduce the silica precursor, inadvertently reducing the silicon yield in the reaction product and increasing reactant costs. During the acid clean process, decomposition of Mg2Si can produce silane gas, which is pyrophoric and therefore dangerous, and inhibits the scalability of the reduction reaction. Such secondary reactions occur when there are local stoichiometric instabilities in the metal reducing agent and silica precursor. The continuous mixing provided by furnace 100 during the heat treatment of method 300 reduces the occurrence of such secondary reactions, resulting in significantly improved silicon yields.

[0102] In contrast, continuous mixing of the reactants in the tubes 106 of the furnace 100 reduces the formation of MgSi by constantly and evenly distributing the reactants. Additionally, the use of a vacuum can significantly increase the conversion of MgSi to silicon products by further reacting with SiO. Furthermore, the furnace 100 may be used in a secondary "partial oxidation" step, which converts MgSi to silicon according to Scheme III below and can remove the MgSi before performing an acid cleaning process. In this regard, the furnace 100 may operate under ambient air or an oxygen environment instead of a vacuum or inert environment for the reduction of silica using a metal reactant. In some instances, the partial oxidation step may be performed in a separate furnace. [ka]

[0103] Conversion of Mg2Si in this manner consumes more Mg per gram of silicon in the reaction product, and therefore it is desirable to reduce or minimize its use. However, this represents a convenient and safe means of removing small amounts of Mg2Si that may be present in the reaction product. As detailed above, reducing the amount of secondary products and / or by-products improves silicon yield and eliminates the need for additional processing, such as acid bath washing.

[0104] This embodiment also provides a scalable 1 L to 1000 L heated tank with vacuum and inert gas capabilities. This heated tank can be used for the acid wash process to remove metal oxides from porous silicon particles. The heated tank can be advantageous for removing Mg2Si, for example, because pyrophoric gases can be collected under an inert atmosphere or vacuum and safely removed without sparks. The acid leaching process can be combined with heat, negative pressure, and homogenization (mixing / grinding) to improve leaching times and simultaneously dissipate fused porous silicon particles.

[0105] In some embodiments, prior to removing the metal oxide (e.g., MgO) and / or metal silicide (MgSi) by acid cleaning, the metal oxide and / or metal silicide powder may first be mixed with an aqueous slurry to generate hydrogen gas without creating dangerous flames or sparks. Such a slurry can then be more safely reacted with an acid (e.g., HCl) in an acid cleaning process to obtain the final product. Additionally, the generated hydrogen gas may be recovered and reused as a means to heat subsequent metal thermal reactions in the furnace 100.

[0106] In some embodiments, the silicon product obtained from the method 200 using the furnace 100 is mixed with a carbon material to form a silicon / carbon composite suitable for applications such as LIBs.

[0107] In one example, a silicon / carbon composite may be formed by mixing a carbonaceous material with a silica precursor (and a metal reactant) to form a mixture, such as during step 204 of method 200, and feeding the mixture into tube 106. The carbonaceous material will typically pass through furnace 100 unreacted, and the thermal energy generated by the metallothermic reduction may pyrolyze the carbonaceous material and subsequently react with the silicon product to form the silicon / carbon composite. Thus, the present disclosure provides the advantage of a one-step synthesis of silicon / carbon composites carried out in a single reactor (e.g., furnace 100).

[0108] In another example, a silicon / carbon composite may be formed by applying an acetylene CVD process during a metallothermal reduction process, such as during steps 206, 208, and / or 210 of method 200, thereby eliminating the need for a second CVD step. Advantageously, both the acetylene CVD process and the metallothermal reduction may be performed in furnace 100. For example, gas module 120 may be utilized to perform the acetylene CVD process by introducing gas-phase acetylene into tube 106 via gas inlet 122.

[0109] The present disclosure provides many advantages for the metallothermic reduction of silica, these advantages being merely examples and not intended to be limiting.

[0110] For example, in conventional practice, HF, which is highly toxic and corrosive, is typically used to remove unreacted silica, secondary products, and / or by-products. The silicon product obtained from the continuous process performed by furnace 100 is of high purity, eliminating the need for HF. The reduced reaction time and continuous process as demonstrated in this embodiment improves purity, eliminating the need for HF entirely.

[0111] Additionally, as an improvement over conventional practice, metal oxides (e.g., MgO) can be recovered after the acid washing process and then sold as a by-product, improving the overall economic value of silicon production.

[0112] Additionally, according to some embodiments, by processing halloysite to form a silica precursor of a desired composition and controlling various aspects of the metallothermal reaction using the furnace 100 provided herein, the silicon:silica:alumina ratio in the final product can be customized. In one example, the reduction reaction can be controlled to adjust the amount of silicon product and the amount of unreacted silica in the final product. In another example, the dealumination process can be controlled to leave more or less alumina in the silica precursor.

[0113] Furthermore, it should be noted that the metallothermic reduction reaction and the method of performing the metallothermic reduction reaction using furnace 100 may be applied to reduce other silica precursors in addition to halloysite, such as fine gray kaolin clay, coarse white kaolin clay, diatomaceous earth, quartz, and sepiolite. Advantageously, the methods described herein (e.g., methods 200 and 300) and the apparatus for practicing such methods (e.g., furnace 100) can be applied to these and other similar silica-containing minerals to form porous silicon particles with physical and electrochemical properties suitable for forming anode electrodes in LIB applications.

[0114] For LIB applications, more silicon in the product will result in a higher reversible capacity as an anode material. However, in some cases, it may be desirable to control the amount of other materials (e.g., silica and / or alumina) in the product to mitigate the expansion of silicon in LIB applications. Additionally, the ability to tailor such ratios for different materials provides the opportunity to control the degree of silicon conversion and, therefore, produce products with different capacity and stability characteristics. Furthermore, porous silicon particles produced using the furnace 100 and methods provided herein may be mixed with graphite (e.g., from about 1 wt% to about 99 wt%). Furthermore, porous silicon particles can be used in fuel cells regardless of the cathode material they are paired with, such as nickel manganese cobalt (NMC) oxide or lithium iron phosphate (LFP) oxide.

[0115] Some additional uses of the porous silicon particles include splitting water molecules and producing hydrogen gas, which may be made more efficient with the large-scale production of silicon afforded by the methods and apparatus provided in this disclosure. Furthermore, the produced hydrogen gas may be recycled as a combustion source to provide power to the furnace 100 and the reduction process, thus improving the economics and energy intensity of the reduction process.

[0116] The features of some embodiments have been described above so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that the present disclosure may readily be used as a basis for designing or modifying other processes and structures to serve the same purposes and / or achieve the same advantages as the embodiments provided herein. Those skilled in the art should also realize that equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the present disclosure.

Claims

1. 1. A method for producing porous silicon particles, comprising: providing a rotary tube furnace having a tube extending between a first opening and a second opening opposite the first opening; providing a silica precursor, a metal reducing agent, and a thermal modifier as a mixture into an interior cavity of the tube through the first opening; rotating the tube containing the mixture; subjecting the mixture in the tube to a heat treatment to produce a reaction product comprising the porous silicon particles; collecting the reaction product at the second opening; and wherein providing the mixture, rotating the tube, applying the heat treatment, and collecting the reaction product are performed simultaneously such that the porous silicon particles are continuously produced.

2. 10. The method of claim 1, wherein the heat treating comprises heating the mixture to a first temperature to produce the porous silicon particles, the method further comprising heating the porous silicon particles to a second temperature to sinter the silicon particles, the second temperature being higher than the first temperature.

3. 10. The method of claim 1, wherein the heat treatment comprises heating the mixture to a first temperature while releasing thermal energy to produce the porous silicon particles, and the released thermal energy subsequently heats the porous silicon particles to a second temperature higher than the first temperature, thereby sintering the porous silicon particles.

4. The method of claim 1 further comprising establishing an inert gas environment within the tube prior to applying the heat treatment.

5. The method of claim 1 , further comprising applying a negative pressure within the tube prior to applying the heat treatment.

6. 10. The method of claim 1, wherein collecting the reaction product comprises tilting the rotary tube furnace so that the first opening is elevated relative to the second opening.

7. 10. The method of claim 1, wherein the metal reducing agent comprises at least one metal selected from the group consisting of magnesium, aluminum, sodium, potassium, zinc, and lithium.

8. 8. The method of claim 7, wherein the particle size of the metal reducing agent ranges from 300 mesh to 6 mesh.

9. Providing the silica precursor comprises: providing a sample comprising an aluminosilicate; subjecting the sample to a dehydration process; carrying out a dealumination process to remove aluminum from the sample, resulting in the silica precursor; The method of claim 1 , comprising:

10. 10. The method of claim 9, wherein performing the dealumination process comprises performing a spray drying process.

11. 10. The method of claim 1, further comprising applying an acid bath to remove at least one material selected from the group consisting of silica, metal oxides, and metal silicides.

12. 10. The method of claim 1, wherein the purity of the porous silicon particles is at least 75% such that the method does not include applying an acid bath.

13. 10. The method of claim 1, wherein the thermal regulating material comprises at least one substance selected from the group consisting of sodium chloride and magnesium chloride.

14. 14. The method of claim 13, wherein the thermal regulating material comprises a first quantity of particles having a first mesh size and a second quantity of particles having a second mesh size, the first mesh size being less than the second mesh size, and the first quantity being less than the second quantity.

15. 15. The method of claim 14, wherein the first mesh size is 325 and the second mesh size is 80.

16. 1. A method for producing silicon particles, comprising: providing a rotary tube furnace having a tube extending between a first opening and a second opening opposite the first opening; receiving a mixture through the first opening into an interior cavity of the tube, the mixture including a silica precursor, a metal reducing agent, and a salt; rotating the tube containing the mixture; subjecting the mixture in the tube to a heat treatment resulting in a reaction product comprising the silicon particles; sintering the silicon particles; and collecting the sintered silicon particles at the second opening; and wherein receiving the mixture, rotating the tube, applying the heat treatment, and collecting the reaction product are performed continuously.

17. The heat treatment includes: heating the mixture to a first temperature; heating the mixture to a second temperature, which is greater than the first temperature, resulting in an exothermic reaction forming the reaction product; subsequently heating the reaction product to a third temperature higher than the second temperature; 17. The method of claim 16, comprising:

18. 18. The method of claim 17, wherein sintering the silicon particles comprises, after subjecting the heat treatment, heating the silicon particles to a fourth temperature that is greater than the third temperature.

19. 18. The method of claim 17, wherein the exothermic reaction heats the reaction product to a fourth temperature, higher than the third temperature, thereby releasing thermal energy capable of sintering the silicon particles.

20. 17. The method of claim 16, wherein the mixture comprises the silica precursor and the metal reducing agent in a molar ratio of 1.5:1 to 2.5:

1.

21. 21. A plurality of porous silicon particles produced by the method of any one of claims 1 to 20, wherein the purity of the porous silicon particles is in the range of 75% to 90%.

22. A plurality of porous silicon particles, providing a rotary tube furnace having a tube extending between a first opening and a second opening opposite the first opening; providing a silica precursor, a metal reducing agent, and a thermal modifier as a mixture into an interior cavity of the tube through the first opening; rotating the tube containing the mixture; subjecting the mixture in the tube to a heat treatment to produce the porous silicon particles; collecting reaction products at the second opening; wherein providing the mixture, rotating the tube, applying the heat treatment, and collecting the reaction product are performed simultaneously such that the porous silicon particles are continuously produced; A plurality of porous silicon particles, wherein the purity of the porous silicon reaction product is in the range of 75% to 90%.

23. a material inlet configured to receive a reactant; a material outlet configured to release the product; a tube having an internal cavity, the tube extending along a longitudinal axis between a first opening connected to the substance inlet and a second opening connected to the substance outlet; a mixing module configured to rotate the tube about the longitudinal axis; a heating module configured to heat the interior cavity of the tube to a selected temperature; a tilt module coupled to the tube and configured to elevate the first opening of the tube relative to the second opening of the tube; a vacuum module coupled to the tube and configured to maintain the interior cavity of the tube at a selected vacuum level; A rotary tubular furnace comprising:

24. a gas inlet in fluid communication with the substance inlet or the first opening of the tube; a pressure gauge coupled to the gas inlet or the first opening of the tube and configured to measure the pressure of gas received through the gas inlet; 24. The rotary tubular furnace of claim 23, further comprising a gas module comprising:

25. The blending module comprises: a screw feeder extending through the interior cavity of the tube and configured to transport the reactants into the interior cavity of the tube; a motor coupled to the screw feeder and configured to rotate the screw feeder about the longitudinal axis; a controller coupled to the motor and configured to monitor a rotational speed of the motor; 24. The rotary tubular furnace of claim 23, comprising:

26. a platform configured to structurally support the rotary tube furnace; The tilt module comprises: a tilting element coupled to the platform and configured to lift a first portion of the platform supporting the first opening of the pipe relative to a second portion of the platform supporting the second opening of the pipe; a controller coupled to the platform or the tilting element and configured to monitor the tilt angle achieved by the tilting element; 24. The rotary tubular furnace of claim 23, comprising:

27. The vacuum module includes: a vacuum pump connected to the tube; a pressure gauge coupled to the tube and configured to measure the level of vacuum within the interior cavity of the tube; 24. The rotary tubular furnace of claim 23, comprising:

28. The heating module includes: one or more heating elements coupled to the tube; a thermometer coupled to the tube and configured to measure a temperature of the interior cavity of the tube; 24. The rotary tubular furnace of claim 23, comprising:

29. 24. The rotary tubular furnace of claim 23, further comprising a control panel configured to receive instructions from a user and implement the received instructions by controlling one or more of the mixing module, the heating module, the tilt module, and the vacuum module.

30. 24. The rotary tubular furnace of claim 23, further comprising a component configured to perform a chemical vapor deposition (CVD) process.

31. 24. The rotary tubular furnace of claim 23, further comprising a first ferrofluid sealing element disposed between the material inlet and the first opening of the tube, and a second ferrofluid sealing element disposed between the material outlet and the second opening of the tube.

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