WO2012166728A1

WO2012166728A1 - Materials from rice hull ash

Info

Publication number: WO2012166728A1
Application number: PCT/US2012/039856
Authority: WO
Inventors: Robert Larsen; Dimitris Katsoulis
Original assignee: Robert Larsen; Dimitris Katsoulis
Priority date: 2011-06-01
Filing date: 2012-05-29
Publication date: 2012-12-06
Also published as: TW201249853A

Abstract

A method of preparing a silicon carbide-containing composition comprises treating rice hull ash with a reducing agent in a reactor at a temperature of from about 700 °C to about 1400 °C. The treating forms a silicon carbide-containing composition comprising nanomaterials. The rice hull ash comprises a composition having the formula Si_xO_yC_z„ wherein x is from about 15 atomic % to about 49.9 atomic %, y is from about 0.1 atomic % to about 66.6 atomic %, z is from about 0.1 atomic % to about 49.9 atomic %, and x+y+z is about 100 atomic %. The reducing agent has a residence time of from about 5 seconds to about 1 hours.

Description

MATERIALS FROM RICE HULL ASH

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent

Application Serial No. 61/491,927, filed on June 1, 2011, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to a method of converting rice hull ash to other usable products, and in particular to a method of preparing a silicon carbide-containing composition by treating rice hull ash. The present disclosure also relates to a silicon carbide-containing composition prepared by the aforementioned method.

BACKGROUND OF THE INVENTION

[0003] The outer coatings of rice grains, referred to as rice hulls, are indigestible as grown, and are therefore a byproduct of the rice production industry that is often considered a waste material. Because rice hulls are primarily composed of cellulose, lignin, and silica, attempts have been made to use rice hulls as a raw material for the manufacture of silicon-containing products. For example, U.S. Patent No. 4,214,920 to Amick et al. and

International Application WO 2010/017364 A2 to Laine et al. each describes methods for producing solar cell-grade silicon from pyrolyzed rice hulls, also referred to as rice hull ash. SUMMARY OF THE INVENTION

[0004] The present disclosure relates generally to a method of preparing a silicon carbide-containing composition by treating rice hull ash.

[0005] The method of the present disclosure provides for the formation of silicon-carbide containing structures, such as nanomaterials, that may be used for the formation of lithium-ion battery anodes. In some examples, the method of the present disclosure provides for the surprising formation of nanomaterials without the use of a known added catalyst.

[0006] In one example, the present disclosure is directed to a method of preparing a silicon carbide-containing composition. The method comprises treating rice hull ash with a reducing agent. The reducing agent can be selected from (i) hydrogen gas and (ii) a mixture comprising hydrogen gas and hydrogen chloride. The treating takes place in a reactor at a temperature of from about 700 °C to about 1400 °C. The treating forms a silicon carbide-containing composition comprising nanomaterials. The rice hull ash has a carbon to silica weight ratio of between about 1 :0.5 and about 1 :20. The rice hull ash comprises a composition having the formula Si_xO_yC_z, wherein x is between about 15 atomic % and about 50 atomic %, y is between about 0.1 atomic % and about 66.6 atomic %, z is between about 0.1 atomic % and about 50 atomic %, and x+y+z is approximately 100 atomic %. The reducing agent has a residence time of between about 5 seconds and about 1 hour.

[0007] In another example, the present disclosure is directed to another method of preparing a silicon carbide-containing composition. The method comprises treating rice hull ash with a reducing agent in a reactor at a temperature of from about 700 °C to about 1400 °C. The treating forms a silicon carbide-containing composition comprising nanomaterials. The rice hull ash comprises a composition having the formula Si_xOyC_z„ wherein x is between about 15 atomic % and about 50 atomic %, y is between about 0.1 atomic % and about 66.6 atomic %, z is between about 0.1 atomic % and about 50 atomic %, and x+y+z is about 100 atomic %.The reducing agent has a residence time of between about 5 seconds and about 1 hour. [0008] In another example, the present disclosure is directed to yet another method of preparing a silicon carbide-containing composition. The method comprises pyrolyzing rice hulls in a substantially inert atmosphere comprising argon gas to form a rice hull ash having a carbon to silica weight ratio of between about 1 :0.5 and about 1 :2. The rice hull ash also comprises a composition having the formula Si_xO_yC_z, wherein x is between about 15 atomic % and about 50 atomic %, y is between about 0.1 atomic % and about 66.6 atomic %, z is between about 0.1 atomic % and about 50 atomic %, x+y+z is approximately 100 atomic %. The weight percentage of the composition having the formula Si_xO_yC_z in the rice hull ash is between about 1% weight/weight of rice hull ash and about 30% weight/weight of rice hull ash. The method further comprises treating the rice hull ash with a reducing agent in a reactor at a temperature of from about 700 °C to about 1400 °C to form a silicon carbide- containing composition. The reducing agent is selected from (i) hydrogen gas and (ii) a mixture comprising hydrogen gas and hydrogen chloride having a mole ratio of hydrogen gas to hydrogen chloride of between about 1 : 100 and about 1 :0.01. The reducing agent has a residence time of between about 5 seconds and about 1 hours. The resulting silicon carbide-containing composition comprises nanowires consisting of crystalline silicon carbide or nanowires comprising a first layer consisting of crystalline silicon carbide on a second layer consisting of amorphous silica.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a flow diagram of an example method of preparing a silicon carbide-containing composition.

[0010] FIG. 2 is a conceptual view of an example silicon carbide nanowire made by the example method of FIG. 1.

[0011] FIG. 3 is a conceptual view of an example silicon carbide on silica nanowire made by the example method of FIG. 1.

[0012] FIG. 4 is an image taken by transmission electron microscopy of a silicon carbide nanowire of Example 1. [0013] FIG. 5 is an X-ray diffraction image of the silicon carbide nanowire of Example 1.

[0014] FIG. 6 is an image taken by transmission electron microscopy of a silicon carbide on silica nanowire of Example 2.

[0015] FIG. 7 is another image taken by transmission electron microscopy of the silicon carbide on silica nanowire of Example 2.

[0016] FIG. 8 is an X-ray diffraction image of the silicon carbide on silica nanowire of Example 2.

DETAILED DESCRIPTION

[0017] The present disclosure is directed to a method of preparing a silicon carbide-containing composition by treating rice hull ash and the resulting silicon carbide-containing compositions made therefrom. In particular, the present disclosure is directed to a method of preparing a silicon carbide- containing composition, the method comprising treating rice hull ash with a reducing agent to form a silicon carbide-containing composition comprising nanomaterials. In some examples, the rice hull ash that is treated with a reducing agent can include a composition having the formula Si_xO_yC_z, wherein x is between about 15 atomic % (at. %) and about 50 at. %, y is between about 0.1 at. % and about 66.6 at. %, z is between about 0.1 at. % and about 50 at. %, and x+y+z is equal to about 100 at. %.

[0018] In some examples, rice hull ash can be treated with hydrogen gas as a reducing agent at a temperature of between about 700 °C and about 1400 °C which has been found to result in the formation of nanowires comprising crystalline silicon carbide (SiC). In other examples, the rice hull ash can be treated with a mixture comprising hydrogen gas and hydrogen chloride gas at a temperature of between about 700 °C and about 1400 °C, which has been found to result in a nanowire comprising crystalline silicon carbide (SiC) on amorphous silica (Si(¾) with a clear SiC/Si02 interface running axially along the nanowire. The SiC or SiC on S1O2 nanowires can be used for various applications, including in the formation of anodes for lithium-ion batteries. [0019] FIG. 1 shows a flowchart of an example method 10 of converting rice hulls 12 to a silicon-carbide containing composition comprising nanowires 30, such as silicon carbide nanowires or silicon carbide on silica nanowires. The example method 10 can include pyrolyzing rice hulls 12 in a pyrolysis reactor 14 which converts the rice hulls 12 to rice hull ash (RHA) 16. The RHA 16 can be treated with a reducing agent 18 in a reactor 20 at an elevated temperature, e.g., between about 700 °C and about 1400 °C. The resulting product stream 22 can comprise silicon-carbide containing structures, such as the nanowires 30, that can be separated from other compositions in the product stream 22, such as via one or more separation operations 24. Separation 24 of the product stream 22 can isolate the silicon-carbide containing the nanowires 30 for use in subsequent processing, for example in the formation of anodes for lithium-ion batteries.

[0020] The rice hulls 12 (sometimes referred to as rice husks) can be a form of agricultural biomass that is a byproduct of the rice milling industry. In some examples, the rice hulls 12 can be obtained from any rice source because, in general rice hulls, no matter where they are obtained from, can have relatively similar impurity levels. The major constituents of the rice hulls 12 can be silica (between about 20% and about 25% by weight), cellulose, and lignin, with trace amounts of many other constituents, such as boron, calcium, phosphorous, aluminum, magnesium, manganese, potassium, or iron.

[0021] In some examples, the rice hulls 12 can be treated or cleaned prior to being pyrolyzed in the pyrolysis reactor 14 in order to extract impurities from the rice hulls 12. For example, the rice hulls 12 can be washed or leached of impurities. The term "impurities," as used herein when referring to rice hulls 12, can refer to chemical components other than silica or other than chemical components that can generate silicon or silicon compounds (such as, for example, silica, or silicon oxycarbides) by pyrolysis or to chemical components other than those that may generate carbon or carbon compounds (such as, for example, silicon oxycarbides) by pyrolysis, such as cellulose, hemicellulose, or lignin. In some examples, the rice hulls 12 are washed with an aqueous acid solution. Examples of aqueous acid solutions that may be used include, but are not limited to, aqueous hydrochloric acid (HQ), sulfuric acid (H₂SO₄), or the like, followed by removal of the aqueous acid solution from the rice hulls 12, such as by washing with water to rinse away the aqueous acid solution. Further description of methods of removing impurities from rice hulls can be found in U.S. Patent No. 4,214,920 to Amick et al, issued on July 29, 1980, the disclosure of which is incorporated in its entirety as if reproduced herein. In other examples, the rice hulls 12 can be washed with an aqueous base solution, such as aqueous sodium hydroxide (NaOH) and the like, followed by removal of the aqueous base solution, such as by washing the with water to rinse away the aqueous base solution.

[0022] In an example, pyro lysis of the rice hulls 12, such as in the pyro lysis reactor 14, can be performed at a pyro lysis temperature of from about 400 °C to about 1200 °C, for example from about 700 °C to about 900 °C, such as at about 700 °C, for from about 0.5 hours to about 1.5 hours, for example about 1 hour. In some examples, pyro lysis of the rice hulls 12 can be performed in an inert atmosphere, for example in the presence of flowing argon gas (Ar) 32. In an example, a flow rate of from about 0.5 liters per minute (LPM) and about 2 LPM, such as about 1 LPM of Ar gas 32 is flowed through the pyro lysis reactor 14 during pyrolysis of the rice hulls 12. In an example, the rice hulls 12 can be preheated in order to drive off water, such as by heating the rice hulls 12 to a temperature that is near or above the vaporization temperature of water, but less than the pyrolysis temperature. In one example, water is removed from the rice hulls 12 by preheating the pyrolysis reactor 14 to a temperature of about 150 °C for between about 0.5 hours and about 1.5 hours, such as for about 1 hour.

[0023] As noted above, the rice hulls 12 predominantly comprise silica, cellulose, hemicelluloses, and lignin. Upon being pyrolyzed, cellulose, hemicelluloses, and lignin generally yield carbon ash such that the resulting rice hull ash (RHA) 16 can primarily comprises silica (S1O₂) and solid carbon.

However, as described below, in some examples, the RHA 16 can also comprise silicon oxycarbide compounds having the general chemical formula Si_xO_yC_z. In an example, the carbon to silica weight ratio within the RHA 16 can be from about 1 :0.5 to about 1 :2, for example from about 1 :0.6 to about 1 : 1 such as about 1 :0.65. The ratio of carbon to silica within the RHA 16 can affect the composition and/or structure of the nano wires 30 that are formed via treatment of the RHA 16 with the reducing agent 18 at an elevated temperature within the reactor 20.

[0024] In an example, the pyrolysis conditions can be selected so that the

RHA 16 comprises silicon oxycarbide compounds. Silicon oxycarbides have the general chemical formula Si_xO_yC_z, wherein x, y, and z are atomic percentages with respect to the total number of silicon, oxygen, and carbon atoms of the silicon oxycarbide compound, and correspondingly x+y+z is approximately equal to 100 atomic %. In an example, silicon oxycarbides formed in the RHA 16 by pyrolysis of the rice hulls 12 can result in x (e.g., silicon atomic percentage) being from about 15 at. % to about 50 at. %, for example from about 20 at. % to about 40 at. %, such as about 25 at. %. In an example, silicon oxycarbides formed in the RHA 16 by pyrolysis of the rice hulls 12 can result in y (e.g., oxygen atomic percentage) being from about 0.1 at. % to about 66.6 at. %, for example from about 10 at. % to about 60 at. %, such as about 50 at. %. In an example, silicon oxycarbides formed in the RHA 16 by pyrolysis of the rice hulls 12 can result in z (e.g., carbon atomic percentage) being from about 0.1 at. % to about 50 at. %, for example from about 1 at. % to about 30 at. %, such as about 25 at. %. In an example, the weight percentage of silicon oxycarbides having the general formula Si_xO_yC_z is from about 1 % weight/weight of the RHA 16 to about 30 % weight/weight of the RHA 16, for example from about 5 % weight/weight of the RHA 16 to about 20 % weight/weight of the RHA 16, such as about 10 % weight/weight of the RHA 16. The formation of silicon oxycarbides is believed to be beneficial in the formation of silicon carbide- containing structures such as nano wires.

[0025] The formation of silicon oxycarbides in the RHA 16 can be achieved under particular pyrolysis conditions. For example, it has been found that an optimal temperature range during pyrolysis of the rice hulls 12 for the formation of silicon oxycarbides can be from about 700 °C to about 900 °C. The concentration of silicon oxycarbides in the RHA 16 appears to decrease as the temperature is increased from about 900 °C to about 1 100 °C. It is also believed that the residence time within the pyro lysis reactor 14 can affect the

concentration of the silicon oxycarbides in the RHA 16. In an example, a residence time of the rice hulls 12/RHA 16 in the pyrolysis reactor 14 can be from about 5 minutes to about 100 hours and a residence time of gasses passing over the rice hulls 12 and the RHA 16 in the pyrolysis reaction 14 of from about 5 seconds to about 1.5 hours have been found to provide for a preferred concentration of silicon oxycarbides in the RHA 16. It is also believed that pressure within the pyrolysis reactor 14 or pretreatment of rice hulls 12, or both, such as with an acid or base solution, can also affect the concentration of silicon oxycarbides formed in the RHA 16.

[0026] In some examples, the RHA 16 that is formed by pyrolysis of the rice hulls 12 can be washed or cleaned to remove impurities, similar to the removal of impurities from the rice hulls 12 described above. For example, the RHA 16 can be washed or leached of impurities using an aqueous acid solution. The term "impurities," as it is used herein with respect to the RHA 16, can refer to chemical components that can be present in the RHA 16 that can provide for the formation of silicon carbide-containing structures, such as nanowires 30, when treated with a reducing agent. For example, "impurities" can refer to chemical components other than silica, carbon, or silicon oxycarbide

compositions, which are believed to contribute to the formation of the silicon carbide containing nanowires 30. Examples of aqueous acid solutions that can be used include, but are not limited to, hydrochloric acid (HQ), sulfuric acid (H₂SO₄), methanesulfonic acid (HCH₃SO₂OH), trifluoromethanesulfonic acid (HCF₃SO₂OH), formic acid (HCOOH), acetic acid (CH₃COOH), propanoic acid (C₂H₅COOH), or the like, followed by removal of the aqueous acid solution from the RHA 16, such as by washing with water to rinse away the acid solution. In an example, impurities can be washed or leached from the RHA 16 via an aqueous base solution, such as aqueous NaOH, followed by removal of the aqueous base solution from the RHA 16, such as by washing with water to rinse away the aqueous base solution. When using a base solution to remove impurities from the RHA 16, care can be taken to avoid dissolving silica in the base solution, which can remove some of the silicon that is desired to be used to form the silicon-carbide containing composition. Therefore, if a base solution is used, it is preferred that the base solution be configured, e.g., by adjusting the base concentration or flow rate of base over the RHA 16, to avoid dissolution of silica into the base solution. Washing the RHA 16 with an acid solution or base solution after pyro lysis may be performed instead of or in addition to washing the rice hulls 12 with an acid solution or a base solution before pyrolysis.

Further description of methods of removing impurities from rice hulls can be found in International Application WO 2010/017364 A2 to Laine et al, published on February 11, 2010, the disclosure of which is incorporated in its entirety as if reproduced herein.

[0027] In an example, pyrolysis of the rice hulls 12 to form the RHA 16 can be performed by one party, while treatment of the RHA 16 with the reducing agent 18 to form the nano wires 30 can be performed by another party. For example, a vendor that specializes in the production of rice hull ash can pyrolyze the rice hulls 12 to produce RHA 16. The vendor can also wash the rice hulls 12 or the RHA 16 with an aqueous acid solution or an aqueous base solution to reduce impurities in the rice hulls 12 or the RHA 16. The vendor can then deliver the resulting RHA 16 to a manufacturer that then performs the process of treating the RHA 16 with the reducing agent 18 in the reactor 20, and, if needed, separating or isolating 24 the resulting silicon-carbide containing product, such as the nanowires 30, from the product stream 22 from the reactor 20. The manufacturer can also wash the RHA 16 that it receives from the vendor with an aqueous acid solution or an aqueous base solution to reduce impurities in the RHA 16.

[0028] The RHA 16 can be treated with a reducing agent 18 at an elevated temperature, e.g., from about 700 °C to about 1400 °C, to lead to the formation of silicon carbide-containing compositions comprising nanomaterials. In an example, the reducing agent 18 can have a residence time within the reactor 20 of from about 5 seconds to about 90 minutes, for example from about 0.5 minutes to about 5 minutes, such as about 2 minutes. In an example, the RHA 16 can have a residence time within the reactor 20 of from about 5 minutes (about 0.1 hours) to about 100 hours, for example about 30 minutes. The residence time of the RHA 16 within the reactor 20 can provide for the size of nanomaterials formed, e.g., with a longer residence time resulting in longer nanomaterials or nanomaterials having a larger aspect ratio.

[0029] The reducing agent 18 preferably has a reduction potential that is sufficiently strong to separate the oxygen atoms from the silicon in silica (Si(¾) or in a silicon oxycarbide compound (Si_xO_yC_z) to allow for the formation of SiC from silica and carbon, or to allow a sub-stoichiometric amount of oxygen in the S1O2 (e.g., less than 2: 1 oxygen to silicon) to be displaced by carbon facilitating the formation of a silicon oxycarbide. Reduction potential is a measure of a material's potential to donate electrons to another material. Reduction potential can be measured in volts, with a lower number corresponding to a stronger reducing agent. In an example, at least a portion of the reducing agent 18 has a reduction potential, with respect to a standard hydrogen electrode, of 0 V or lower (e.g., with a reduction potential of either 0 V or a negative reduction potential), such as between about 0 V and about -3 V. In an example, the reducing agent 18 comprises hydrogen gas (H₂), which has a reduction potential of about 0 V. It has been found that applying a hydrogen gas flow to the reactor 20 at temperatures of from about 700 °C to about 1400 °C can result in the formation of SiC nanomaterials from the RHA 16.

[0030] In an example, the reducing agent 18 can comprise a mixture of hydrogen gas and hydrogen chloride gas (HC1). It has been found that using a mixture of ¾ gas and HC1 gas as the reducing agent can result in a structure comprising silicon carbide on a silica base (see FIG. 3, discussed in more detail below), with a SiC/Si02 interface running axially along the nanowire. Without being bound to any one theory, it is believed that the SiC on S1O2 nanowire is formed because the mixture of ¾ and HC1 is a weaker reducing agent than ¾ gas alone, such that the formation of a nanowire consisting essentially of only SiC does not occur. In an example, the molar ratio of ¾ gas to HC1 gas can be from about 1 to 0.01 to about 1 to 100, for example from about 1 to 0.5 to about 1 to 2, such as about 1 to 1.

[0031] Examples of reducing agents that can be applied to the RHA 16 include, but are not limited to, any suitable reducing agent known to one of skill in the art; for example, hydrogen gas 0¾); a mixture of hydrogen gas and hydrogen chloride (HCl_(g)); compounds comprising a Group I metal, also referred to as alkali metals, e.g., compounds comprising lithium, sodium, potassium, and the like; and compounds comprising Group II metals, also referred to as alkaline earth metals, e.g., compounds comprising beryllium, magnesium, calcium, and the like. In an example, the reducing agent 18 can be in the gaseous state such that it may be easily flowed over the RHA 16.

[0032] Treatment of the RHA 16 with the reducing agent 18 can be performed at an elevated temperature in order to drive the formation of silicon carbide. In an example, the RHA 16 can be treated with the reducing agent 18 at an elevated temperature of from about 700 °C to about 1400 °C, for example from about 1000 °C to about 1400 °C, such as at about 1200 °C. Factors that may affect the temperature that can be sufficient to provide for the formation of silicon carbide-containing compositions from the treatment of the RHA 16 can include the strength (e.g., reduction potential) of the reducing agent 18, the concentration of the reducing agent 18, e.g., the partial pressure of the reducing agent 18 in the gases that are passed over the RHA 16 in the reactor 20, the presence and concentration of an added catalyst, such as iron, the addition of an agent involved in the reaction, such as elemental silicon (Si). These same factors can also affect the morphology of silicon carbide that is produced by treatment of the RHA 16 with the reducing agent 18.

[0033] The growth of nanomaterials, such as nanotubes, nanofibers, and nanowires, has been described previously. However, nanostructures are generally only described as being formed in the presence of a catalyst that is known to facilitate the formation of nanostructures, such as iron (Fe), iron/molybdenum (Fe/Mo), iron/platinum (Fe/Pt), nickel (Ni), cobalt (Co), iron/nickel (Fe/Ni), and iron/cobalt (Fe/Co). In an example, treatment of the RHA 16 with the reducing agent 18 in the reactor 20 can be performed in the absence of a catalyst such that the nano wires 30 are surprisingly formed without an added catalyst, e.g., without the addition of a catalyst that is known to provide for the formation of nanostructures.

[0034] FIG. 2 shows an example nanowire 40 that can be formed by treating the RHA 16 with hydrogen gas (H₂) as the reducing agent 18 at an elevated temperature of from about 700 °C to about 1400 °C, wherein the resulting structure formed is a silicon carbide (SiC) nanowire 40. As shown in FIG. 2, in some example, the SiC nanowire 40 can include an elongated structure having a length Lsic that is substantially longer than its lateral width Wsic- In an example, the nanowire 40 formed by treating the RHA 16 with hydrogen gas at an elevated temperature of from about 700 °C to about 1400 °C can comprise crystalline SiC in the form of a nanowire 40. In an example, the nanowire 40 formed by such a process can consist essentially of crystalline SiC, wherein "consisting essentially of is used herein to mean that the mechanical or electrical properties of the resulting nanowire 40 are not substantially affected by the presence of compounds other than crystalline SiC. In an example, the nanowire 40 formed by treating the RHA 16 with hydrogen gas at an elevated temperature of from about 700 °C to about 1400 °C can consist of crystalline SiC.

[0035] FIG. 3 shows an example nanowire 50 that can be formed by treating the RHA 16 with a mixture of hydrogen gas (¾) and hydrogen chloride (HC1) at an elevated temperature of from about 700 °C to about 1400 °C, wherein the resulting nanowire 50 can comprise a layer 52 of silicon carbide

(SiC) on a layer 54 of silica (S1O₂), with a SiC/Si0₂ interface 56 running axially along the nanowire 50. In an example, the SiC/Si0₂ interface 56 can run along at least about 10% of the axial length of the nanowire 50, for example at least about 25% of the axial length of the nanowire 50, such as at least about 50% of the axial length of the nanowire 50, for example at least about 75% of the axial length of the nanowire 50, such as at least about 90% of the axial length of the nanowire 50. In an example, the SiC layer 52 and the silica layer 54 with the interface 56 can be formed substantially along the entire axially length (e.g., about 100% of the axial length) of the nanowire 50. In an example, the SiC layer 52 and the S1O2 layer 54 can be formed such that one side of the nanowire 50 is crystalline SiC and the other side of the nanowire 50 is amorphous S1O2 with an interface 56 of intimate contact between the two layers 52, 54 running axially along the nanowire 50.

[0036] In an example, the SiC layer 52 can comprise crystalline SiC. In an example, the SiC layer 52 can consist essentially of crystalline SiC, wherein "consisting essentially of is used herein to mean that the mechanical or electrical properties of the layer 52 are not substantially affected by the presence of compounds other than crystalline SiC. In an example, the SiC layer 52 can consist of crystalline SiC. In an example, the Si(¾ layer 54 can comprise amorphous Si(¾. In an example, the S1O2 layer 54 can consist essentially of amorphous Si(¾, wherein "consisting essentially of is used herein to mean that the mechanical or electrical properties of the resulting layer 54 are not substantially affected by the presence of compounds other than amorphous S1O2. In an example, the S1O2 layer consists of amorphous Si(¾.

[0037] As shown in FIG. 3, the SiC layer 52 of the nanowire 50 can have a thickness Tsic and the S1O2 layer 54 can have a thickness Tsio2- In an example, the thickness Tsic of the SiC layer 52 can be from about 5 nanometers to about 100 nanometers, for example from about 10 nanometers to about 50 nanometers, and the thickness Tsio2 of the S1O2 layer 54 can be from about 5 nanometers to about 100 nanometers, for example from about 10 nanometers to about 50 nanometers, such that the overall lateral width Wsic/si₀2 of the nanowire 50 can be from about 10 nanometers to about 200 nanometers, for example from 20 nanometers to about 100 nanometers.

[0038] The structures that are formed by treating the RHA 16 with the reducing agent 18 in the reactor 20 can be referred to herein as "nanomaterials," and in certain examples as "nanowires." However, nanostructures other than nano wires can be formed during the treatment of the RHA 16 with the reducing agent 18, such as nanowhiskers, nanofibers, nanorods, nanotubes, and nanoparticles. In general, the term "nanomaterial," as it is used herein, can refer to a structure having at least one dimension that is less than about 1000 nanometers, for example less than about 200 nanometers. In an example, the term "nanomaterial" can refer to a structure having a length, e.g. Lsic of the nanowire 40 or Lsic/si02 of the nanowire 50, of from about 0.1 micrometers to about 1000 micrometers, such as from about 1 micrometers to about 100 micrometers. The term "nanomaterial" can also refer to a structure having a lateral width, such as a diameter for a generally circular cross-sectional nanowire, for example Wsic of the nanowire 40 or Wsic/si02 of the nanowire 50, of from about 5 nanometers to about 1000 nanometers, such as from about 10 nanometers to about 200 nanometers. In an example, in addition to having a length or lateral width as described above, the nanomaterials can have an aspect ratio (the ratio of the length of the nanomaterial to the lateral width of the nanomaterial) of from about 10 to 1 to about 10,000 to 1, for example from about 10 to 1 to about 5000 to 1, such as from about 10 to 1 and about 1000 to 1.

[0039] As noted above, after treating the RHA 16 with the reducing agent 18 at an elevated temperature in the reactor 20 to form the nanowires 30, such as the example nanowire 40 or the example nanowire 50, the nanowires 30 can be separated from the rest of the product stream 22 using one or more separation operations 24. Separation operations that may be used include, but are not limited to, centrifugation, flotation, elutriation, and filtration (e.g., using mesh of different sizes), reaction, e.g., by burning off carbon with oxygen gas (O₂), further reducing carbon, e.g., with hydrogen gas (H₂), and, in examples where silicon carbide only nanowires are produced, as in FIG. 2, hydrofluoric acid (HF) can be reacted with the treated ash to remove excess silica.

[0040] The silicon carbide-containing nanowires 30, 40, 50 described above can be useful for many applications. In an example, the resulting nanowires 30, 40, 50 can be used for the manufacture of electrochemical electrodes for lithium-ion batteries, and in particular for anodes of lithium-ion batteries. In general, lithium-ion batteries can comprise an anode, a cathode, and an electrolyte disposed between the electrodes that can allow for the flow of ions between the electrodes. The materials of the anode and cathode can be capable of allowing lithium ions to migrate into the electrode (referred to as

intercalation) and out of the electrode (referred to as deintercalation). When the lithium-ion battery is charging, lithium ions can deintercalate from the cathode and can intercalate into the anode. During discharging, the process can be reversed, with lithium ions deintercalating from the anode and intercalating into the cathode.

[0041] Performance of an anode material can depend on the overall capacity at which the material and anode structure can intercalate lithium ions as well as the rate at which the anode material and anode structure can intercalate lithium ions during charging and deintercalate lithium ions during discharging. Examples of parameters of the nanowires 30, 40, 50 that can affect the intercalation capacity and ability to intercalate or deintercalate lithium include porosity of the nanowires 30, 40, 50, aspect ratio of the nanowires 30, 40, 50, ability of the nanowires 30, 40, 50 to accommodate volumetric change (e.g., swelling or shrinking) that may occur during intercalation of lithium ions, overall size of the nanowires 30, 40, 50 (both lateral width and length), electrical conductivity of the nanowires 30, 40, 50, whether any or all of a particular nanowire 30, 40, 50 has been doped with impurities, the crystalline structure of silicon carbide (either alpha or beta) in the nanowires 30, 40, 50, and overall structure of the resulting anode made from nanowires 30, 40, 50. For example, the aspect ratio of the nanowires 30, 40, 50 can affect their ability to absorb further lithium ions because it has been found that when lithium ions are intercalated into the anode, the material of the anode can undergo a volumetric expansion, e.g., a swelling. It has been found that when the aspect ratio (e.g., the ratio of the length of the nanowire 30, 40, 50 to the lateral width of the nanowire 30, 40, 50) is sufficiently large, the nanowire 30, 40, 50 can be better able to accommodate the swelling of the nanowire structure in order to avoid damage to the structure, such as via pulverization. In an example, an anode for a lithium ion battery can be made from a mass of the nanowires 30, 40, 50, wherein the nanowires 30, 40, 50 can be interwoven, intertangled, or otherwise mechanically put together to allow for sufficient porosity of the anode to provide for adequate intercalation or deintercalation of lithium ions while still providing for a relatively small physical size of the anode. In an example, the nanowires 30, 40, 50 can remain attached to an underlying conductive particle, e.g., carbon that was present in the RHA 16, which can carry electrical charge away from the nanowires 30, 40, 50 during intercalation and deintercalation of the anode.

EXAMPLES

[0042] The present disclosure can be better understood by reference to the following examples which are offered by way of illustration. The present disclosure is not limited to the examples given herein.

EXAMPLE 1

[0043] About 300 g of rice hulls were pyrolyzed to form rice hull ash in a quartz glass tube having a diameter of about 10.2 centimeters (about 4 inches) within a tube furnace having an inner diameter of about 10.2 centimeters (about 4 inches) at about 700 °C under about 1 liter per minute (LPM) purge flow of argon gas (Ar). The pyrolyzed rice hull ash was allowed to cool, after which about 25 g of the rice hull ash was placed in a quartz tube reactor having a diameter of about 5.1 centimeters (about 2 inches) in a tube furnace having a diameter of about 10.2 centimeters (about 4 inches). The quartz tube reactor containing the rice hull ash was purged with about 1 LPM Ar gas and heated to about 800 °C. Gas flow through the reactor was then switched to about 1 LPM ¾ and held for 1-2 hours. The reactor was then heated to about 1200 °C and held for 1-2 hours while continuing flow of the ¾ gas. The flow of ¾ was then switched back to about 1 LPM Ar and the reactor was allowed to cool.

[0044] Samples of the treated rice hull ash were taken for analysis by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS).For TEM, a powder sample of the treated rice hull ash of Example 1 was dissolved in distilled water and the solution was dropped on a holey C film coated Cu TEM grid and then dried on a hot plate at 50 °C for about two hours. The specimen was loaded into JEOL 21 OOF TEM (JEOL Ltd., Tokyo, Japan), and the morphology was observed at 200 KeV under bright field TEM mode using a high contrast objective aperture to enhance the image contrast. The digital images were taken using a Gatan CCD camera (Gatan, Inc., Pleasanton, CA) attached under the

TEM column with Digital Micrograph software. For elemental analysis, energy- dispersive X-ray spectroscopy (EDS) using a NORAN EDS detector (Thermo Scientific Inc., West Palm Beach, FL) coupled with TEM and Vantage data acquisition system was done under the condition of slow pulse process rate ( 10 eV per channel, ~ 15% dead time). For a Z-contrast image, scanning TEM

(STEM) with JEOL STEM control system was used. For bright field STEM and dark field STEM, 1 nm beam probe and #3 condenser aperture were used and the digital images were taken using a Gatan bright field/dark field detector. For quantitative analysis, electron energy loss spectroscopy (EELS, 0.5 eV/ch, 0.25 exposure, 40 micro aperture, 1 nm probe) was practiced for the selected sample region. For EELS, a Gatan GIF CCD (2k X 2k) detector (Gantan, Inc.,

Pleasanton, CA) was used with 40 micron objective aperture. To avoid hydrocarbon contamination on the nanowire surfaces, the TEM/EDS/EELS were done at -160C.

[0045] Results indicated that numerous nanowires consisting essentially of crystalline SiC had been produced. FIG. 4 shows a TEM image of a crystalline silicon carbide nanowire formed by the method of Example 1. FIG. 5 is an electron diffraction pattern for the silicon carbide nanowire formed in Example 1. The TEM image (FIG. 4) and the electron diffraction pattern (FIG. 5) confirm the formation of crystalline silicon carbide in the nanowires. EDS and the electron energy loss spectroscopy (EELS) revealed only Si and C in the nanowire, with the carbon to silicon ratio being calculated by EELS data to be about 1 : 1. There was no formation of a distinct amorphous surface layer observed by TEM on the nanowires of Example 1. EXAMPLE 2

[0046] About 300 g of rice hulls were pyrolyzed to form rice hull ash in a quartz glass tube having a diameter of about 10.2 centimeters (about 4 inches) within a tube furnace having an inner diameter of about 10.2 centimeters (about 4 inches) at about 700 °C under an about 1 LPM purge flow of argon gas (Ar). The pyrolyzed rice hull ash was allowed to cool, after which about 24 g of the resulting rice hull ash was placed in a quartz tube reactor having a diameter of about 5.1 centimeters (about 2 inches) in a tube furnace having a diameter of about 10.2 centimeters (about 4 inches). The quartz tube reactor containing the rice hull ash was purged with about 1 LPM Ar gas and heated to about 1200 °C. Gas flow through the reactor was then switched to a mixture comprising about 0.5 LPM H₂ and about 0.5 LPM HC1 and held for 1-2 hours. The flow of H₂ + HC1 was then switched back to 1 LPM Ar and the reactor was allowed to cool.

[0047] Samples of the treated rice hull ash were taken for analysis by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) using the same experimental procedures as described above for Example 1. . Results indicated that numerous microwires and nanowires had been produced. Some of these wires consisted entirely of crystalline SiC. Some consisted entirely of amorphous Si02. The formation of silica nanowire might be due to the mixing with silicon powder and its oxide formation at high temperature, Some of the nanowires consisted of both crystalline SiC and amorphous S1O2 with a clear interface that ran axially along the wire such that one side of the nanowire was clearly crystalline SiC and the other side of the wire was clearly amorphous S1O2 with an interface of intimate contact between the two.

[0048] FIG. 6 shows a TEM image of a crystalline silicon carbide on amorphous silica nanowire formed by the method of Example 2. FIG. 7 shows a TEM image of one of the crystalline silicon carbide on amorphous silica nanowires of Example 2 at a larger magnification than that shown in FIG. 6. The high resolution TEM images (FIGS. 6 and 7) and the electron diffraction pattern (FIG. 8) confirmed the formation of crystalline SiC nanowires. Most silica amorphous nanowires grew along with SiC nanocrystal wires, as shown in the TEM images of FIGS. 6 and 7. Large portion of SiC nanowires were separately formed without silica phase attachment at the surfaces. EDS and the EELS analysis show distinct S1O2 nanowire and SiC nanowire. In the SiC wire, only Si and C were detected while Si and O were detected in S1O2 amorphous phase in EDS and EELS. The atomic ratio of carbon to silicon calculated using EELS data was determined to be around 1.1 : 1 in the crystalline SiC nanowire. The atomic ratio of oxygen to silicon detected in the silica phase was around 2.2: 1. The detection of SiC crystal is consistent with the XRD results detecting the SiC crystal.

[0049] The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed can be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

Additional Embodiments

[0050] The present invention includes the following exemplary embodiments:

[0051] Embodiment 1 provides a method of preparing a silicon carbide- containing composition, the method comprises treating rice hull ash with a reducing agent selected from (i) hydrogen gas and (ii) a mixture comprising hydrogen gas and hydrogen chloride, in a reactor at a temperature of from about 700 °C to about 1400 °C to form a silicon carbide-containing composition comprising nanomaterials. The rice hull ash has a carbon to silica weight ratio of between about 1 :0.5 and about 1 :20. The rice hull ash comprises a composition having the formula Si_xO_yC_z, wherein x is between about 15 atomic % and about 50 atomic %, y is between about 0.1 atomic % and about 66.6 atomic %, z is between about 0.1 atomic % and about 50 atomic %, x+y+z is approximately 100 atomic %. The reducing agent has a residence time in the reactor of between about 5 seconds and about 1 hour.

[0052] Embodiment 2 provides the method of embodiment 1 , wherein the weight percentage of the composition having the formula Si_xO_yC_z in the rice hull ash is between about 1% weight/weight of rice hull ash and about 30% weight/weight of rice hull ash.

[0053] Embodiment 3 provides the method of either Embodiment 1 or 2 wherein the mole ratio of hydrogen gas to hydrogen chloride in the mixture (ii) is between about 1 :0.01 and about 1 : 100.

[0054] Embodiment 4 provides the method of any of Embodiments 1-3, wherein the rice hull ash is formed by pyrolyzing rice hulls at a temperature of between about 400 °C and about 1200 °C in a substantially inert atmosphere and, optionally, adjusting the weight ratio of carbon to silica to have a value of between about 1 :0.5 and about 1 :20.

[0055] Embodiment 5 provides the method Embodiment 4, wherein forming the rice hull ash and the treating the rice hull ash with a reducing agent are carried out separately.

[0056] Embodiment 6 provides the method of any of Embodiments 4 or 5, wherein the rice hulls are washed with an aqueous acid solution or an aqueous base solution to reduce the total level of impurities in the rice hulls.

[0057] Embodiment 7 provides the method of any of Embodiments 1-6, wherein the rice hull ash has a residence time in the reactor of between about 0.1 hours and about 100 hours.

[0058] Embodiment 8 provides the method of any of Embodiments 1-7, wherein the rice hull ash is washed with an aqueous acid solution or an aqueous base solution to reduce the total level of impurities in the rice hull ash before treating the rice hull ash with a reducing agent.

[0059] Embodiment 9 provides the method of any of Embodiments 1-8, wherein the nanomaterials comprise silicon carbide nanowires or silicon carbide on silica nanowires. [0060] Embodiment 10 provides the method of any of Embodiments 1-9, wherein the nanomaterials have a lateral width of between about 10 nanometers and about 1000 nanometers.

[0061] Embodiment 1 1 provides the method of any of Embodiments 1- 10, wherein the nanomaterials have an aspect ratio of between about 10 and about 10,000.

[0062] Embodiment 12 provides the method of any of Embodiments 1-

11 , wherein the nanomaterials are formed in the absence of an added catalyst.

[0063] Embodiment 13 provides a silicon carbide-containing

composition prepared by the method of any of Embodiments 1-12.

[0064] Embodiment 14 provides a battery, fuel cell, or semi-fuel cell, comprising the silicon carbide composition prepared by the method of any of Embodiments 1-12.

[0065] Embodiment 15 provides a method of preparing a silicon carbide- containing composition. The method of Embodiment 15 comprises treating rice hull ash with a reducing agent in a reactor at a temperature of from about 700 °C to about 1400 °C to form a silicon carbide-containing composition comprising nanomaterials. The rice hull ash comprises a composition having the formula Si_xO_yC_z, wherein x is between about 15 atomic % and about 50 atomic %, y is between about 0.1 atomic % and about 66.6 atomic %, z is between about 0.1 atomic % and about 50 atomic %, and x+y+z is about 100 atomic %. The reducing agent has a residence time in the reactor of between about 5 seconds and about 1 hour.

[0066] Embodiment 16 provides the method of Embodiment 15, wherein the reducing agent is selected from (i) hydrogen gas, (ii) a mixture comprising hydrogen gas and hydrogen chloride, (iii) a compound comprising a Group I metal, and (iv) a compound comprising a Group II metal.

[0067] Embodiment 17 provides the method of any of Embodiments 15 and 16, wherein the rice hull ash has a carbon to silica weight ratio of between about 1 :0.5 and about 1 :20. [0068] Embodiment 18 provides the method of any of Embodiments 15-

17, wherein the weight percentage of the composition having the formula Si_xO_yC_z in the rice hull ash is between about 1% weight/weight of rice hull ash and about 30% weight/weight of rice hull ash.

[0069] Embodiment 19 provides the method of any of Embodiments 16-

18, wherein the mole ratio of hydrogen gas to hydrogen chloride in the mixture (ii) is between about 1 :0.01 and about 1 : 100.

[0070] Embodiment 20 provides the method of any of Embodiments 15-

19, wherein the rice hull ash is formed by pyrolyzing rice hulls at a temperature of between about 400 °C and about 1200 °C in a substantially inert atmosphere and, optionally, adjusting the weight ratio of carbon to silica to have a value of between about 1 :0.5 and about 1 :20.

[0071] Embodiment 21 provides the method of Embodiment 20, wherein forming the rice hull ash and the treating the rice hull ash with a reducing agent are carried out separately.

[0072] Embodiment 22 provides the method of any of Embodiments 20 and 21, wherein the rice hulls are washed with an aqueous acid solution or an aqueous base solution to reduce the total level of impurities.

[0073] Embodiment 23 provides the method of any of Embodiments 15- 22, wherein the rice hull ash has a residence time in the reactor of between about 0.1 hours and about 100 hours.

[0074] Embodiment 24 provides the method of any of Embodiments 15-

23, wherein the rice hull ash is washed with an aqueous acid solution or an aqueous base solution to reduce the total level of impurities in the rice hull ash before treating the rice hull ash with a reducing agent.

[0075] Embodiment 25 provides the method of any of Embodiments 15-

24, wherein the nanomaterials comprise silicon carbide nanowires or silicon carbide on silica nanowires.

[0076] Embodiment 26 provides the method of any of Embodiments 15- 25, wherein the nanomaterials have a lateral width of between about 10 nanometers and about 1000 nanometers. [0077] Embodiment 27 provides the method of any of Embodiments 15-

26, wherein the nanomaterials have an aspect ratio of between about 10 and about 10,000.

[0078] Embodiment 28 provides the method of any of Embodiments 15- 27, wherein the nanomaterials are formed in the absence of an added catalyst.

[0079] Embodiment 29 provides a silicon carbide-containing

composition prepared by the method of any of Embodiments 15-28.

[0080] Embodiment 30 provides a battery, fuel cell, or semi-fuel cell, comprising the silicon carbide composition prepared by the method of any of Embodiments 15-28.

[0081] Embodiment 31 provides a method of preparing a silicon carbide- containing composition. The method of Embodiment 31 comprise pyrolyzing rice hulls in a substantially inert atmosphere comprising argon gas to form a rice hull ash having a carbon to silica weight ratio of between about 1 :0.5 and about 1 :20. The rice hull ash comprises a composition having the formula Si_xO_yC_z, wherein x is between about 15 atomic % and about 50 atomic %, y is between about 0.1 atomic % and about 66.6 atomic %, z is between about 0.1 atomic % and about 50 atomic %, x+y+z is approximately 100 atomic %. The weight percentage of the composition having the formula Si_xO_yC_z in the rice hull ash is between about 1% weight/weight of rice hull ash and about 30% weight/weight of rice hull ash. The method of Embodiment 31 further comprises treating the rice hull ash with a reducing agent in a reactor at a temperature of from about 700 °C to about 1400 °C to form a silicon carbide-containing composition. The reducing agent is selected from (i) hydrogen gas and (ii) a mixture comprising hydrogen gas and hydrogen chloride having a mole ratio of hydrogen gas to hydrogen chloride of between about 1 :0.01 and about 1 : 100. The reducing agent has a residence time of between about 5 seconds and about 1 hours. The rice hull ash has a residence time in the reactor of between about 0.1 hours and about 100 hours. The silicon carbide-containing composition comprises nano wires consisting of crystalline silicon carbide or nanowires comprising a first layer consisting of crystalline silicon carbide on a second layer consisting of amorphous silica.

[0082] Embodiment 32 provides the method of Embodiment 31 further comprising washing the rice hulls with an aqueous acid solution or an aqueous base solution to reduce the total level of impurities in the rice hulls, and washing the rice hulls with water to remove the aqueous acid solution or the aqueous base solution prior to pyrolyzing the rice hulls.

[0083] Embodiment 33 provides the method of Embodiment 31 further comprising washing the rice hull ash with an aqueous acid solution or an aqueous base solution to reduce the total level of impurities in the rice hull ash, and washing the rice hull ash with water to remove the aqueous acid solution or the aqueous base solution prior to treating the rice hull ash with a reducing agent.

Claims

WHAT IS CLAIMED:

1. A method of preparing a silicon carbide-containing composition, the method comprising:

treating rice hull ash with a reducing agent selected from (i) hydrogen gas and (ii) a mixture comprising hydrogen gas and hydrogen chloride, in a reactor at a temperature of from about 700 °C to about 1400 °C to form a silicon carbide-containing composition comprising nanomaterials;

wherein the rice hull ash has a carbon to silica weight ratio of between about 1 :0.5 and about 1 :20,

wherein the rice hull ash comprises a composition having the formula Si_xO_yC_z, wherein x is from about 15 atomic % to about 50 atomic %, y is from about 0.1 atomic % to about 66.6 atomic %, z is from about 0.1 atomic % and about 50 atomic %, x+y+z is approximately 100 atomic %, and

wherein the reducing agent has a residence time in the reactor of between about 5 seconds and about 1 hour.

2. The method according to claim 1, wherein the weight percentage of the composition having the formula Si_xO_yC_z in the rice hull ash is from about 1 % weight/weight of rice hull ash to about 30% weight/weight of rice hull ash.

3. The method according to any of claims 1 and 2, wherein the mole ratio of hydrogen gas to hydrogen chloride in the mixture (ii) is from about 1 :0.01 to about 1 : 100.

4. The method according to any of claim 1-3, wherein the rice hull ash is formed by pyrolyzing rice hulls at a temperature of from about 400 °C to about 1200 °C in a substantially inert atmosphere and, optionally, adjusting the weight ratio of carbon to silica to have a value of from about 1 :0.5 to about 1 :20.

5. The method according to any of claims 1-4, wherein the

nanomaterials comprise silicon carbide nanowires or silicon carbide on silica nano wires.

6. The method according to any of claims 1-5, wherein the

nanomaterials have an aspect ratio of from about 10 to about 10,000.

7. A method according to any of claims 1-6, wherein the nanomaterials are formed in the absence of an added catalyst.

8. A method of preparing a silicon carbide-containing composition, the method comprising:

treating rice hull ash with a reducing agent in a reactor at a temperature of from about 700 °C to about 1400 °C to form a silicon carbide-containing composition comprising nanomaterials, wherein the rice hull ash comprises a composition having the formula Si_xO_yC_z, wherein x is from about 15 atomic % to about 50 atomic %, y is from about 0.1 atomic % to about 66.6 atomic %, z is from about 0.1 atomic % to about 50 atomic %, and x+y+z is about 100 atomic %, and wherein the reducing agent has a residence time of between about 5 seconds and about 1 hour.

9. The method according to claim 9, wherein the reducing agent is selected from (i) hydrogen gas, (ii) a mixture comprising hydrogen gas and hydrogen chloride, (iii) a compound comprising a Group I metal, and (iv) a compound comprising a Group II metal.

10. The method according to claim 9, wherein the mole ratio of hydrogen gas to hydrogen chloride in the mixture (ii) is from about 1 :0.01 to about 1 : 100.

1 1. The method according to any of claims 8-10, wherein the weight percentage of the composition having the formula Si_xO_yC_z in the rice hull ash is from about 1% weight/weight of rice hull ash to about 30% weight/weight of rice hull ash.

12. The method according to any of claim 8-1 1, wherein the rice hull ash is formed by pyrolyzing rice hulls at a temperature of from about 400 °C to about 1200 °C in a substantially inert atmosphere and, optionally, adjusting the weight ratio of carbon to silica to have a value of from about 1 :0.5 to about 1 :20.

13. The method according to any of claims 8-12, wherein the nanomaterials comprise silicon carbide nanowires or silicon carbide on silica nanowires.

14. The method according to any of claims 8-13, wherein the nanomaterials have an aspect ratio of from about 10 to about 10,000.

15. A method according to any of claims 8-14, wherein the

nanomaterials are formed in the absence of an added catalyst.