WO2024119262A1 - Methods and systems for preparing multimetal alloy nanoparticles - Google Patents

Abstract

The present disclosure relates to a method and system for preparing multimetal alloy nanoparticles (MMA NPs) such as high entropy alloy nanoparticles (HEA NPs). The method comprises: introducing a metal source comprising a plurality of metals into a thermal zone of a plasma torch to obtain a metal vapour comprising the plurality of metals; substantially homogenizing the metal vapour by plasma expansion and turbulence; cooling the metal vapour to co-nucleate and co-condense the MMA NPs; and collecting the MMA NPs. The system comprises a reactor comprising a reaction chamber; a plasma torch; an inlet for introducing a multimetal source to the plasma torch; the reaction chamber configured to substantially homogenize metal vapour by creating one or more of plasma expansion or turbulence; the reactor configured to cool the metal vapor to co-nucleate and co-condense the MMA NPs; and a collecting device.

Description

METHODS AND SYSTEMS FOR PREPARING MULTI METAL ALLOY NANOPARTICLES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present disclosure claims the benefit of priority from U.S. patent application no. 63/431 ,614, filed December 9, 2022, the contents of which are incorporated herein by reference in their entirety.

FIELD

[0002] The present disclosure relates to multimetal alloy nanoparticles (MMA NPs) such as high entropy alloy nanoparticles (HEA NPs). The present disclosure further relates to methods and systems for preparing multimetal alloy nanoparticles.

INTRODUCTION

[0003] Multimetal alloy nanoparticles (MMA NPs) have been useful in a broad range of applications such as catalysis, sensing, energy storage and structural alloys. More specifically, nano-sized particles have emerged as a new class of multifunctional materials for catalysis, plasmonics, nanoeletronics, chemical sensors, and drug delivery [Koo 2020], However, in conventional alloy design, the composition space has been limited to at most three principle elements in orderto avoid phase segregation orformation of harmful intermetallic phases. The multi-component alloy, for example having five or more principal elements with a near equimolar ratio, were first demonstrated in 2004 [Yeh 2004], Despite the tendency to segregate or order with similar elements, such alloys can be stabilized by their high configurational entropy and are named high entropy alloys (HEAs) [Tsai 2014], Homogenous mixing of large number of elements in HEAs causes an internal structure discontinuity and results in unusual combinations of functional properties appealing to broad range of applications such as catalysis, sensing, energy storage and structural alloys [Wang 2021], When the particle size reaches the nanoscale, properties of HEAs can be further enhanced by the high surface-volume ratio and quantum confinement effects, in concert with the four core effects proposed by Yeh [Tsai 2014],

[0004] Most micron-sized MMA powders are currently manufactured by either atomization or mechanical alloying technique. However, both processes produce MMA powders of a size of a few tens of microns [Ding 2017], Other methods of synthesizing alloy nanoparticles include wet chemical synthesis by co-reduction of metal salts loaded onto a support that hinders continuous particle growth and aggregation. However, only a limited number of HEAs have been explored by this approach as additional constraints are imposed on the synthesis such as ultrafast heating/cooling rates, in order to avoid segregation of elements at the atomic scale. Carbothermal shock (CTS) technique has been used to produce an MMA NP containing up to eight elements dispersed on a conductive carbon support [Yao 2018], Nevertheless, the CTS technique is limited to electrically conductive supports and operates in a batch mode. Another method explored in the production of alloy NPs is vapor-solid (VS) transformation, where metal vapour containing multiple elements is quenched to form crystal solids. DC arc discharge [Mao 2019], oscillatory spark discharge [Feng 2020], and laser ablation [Waag 2019] have been used as heat sources to for vaporisation of metals in the production of alloy NPs using VS transformation. These VS transformation methods are limited to evaporations of pellets or targets and can only operate in batch mode. Thus, VS transformation has not been used beyond lab-scale synthesis.

SUMMARY

[0005] Successful development of MMA NPs and their commercialization strongly rely on broader accessibility of various MMA NPs in different forms of bulk, thin film or powders. Scalable and economically viable synthetic methods for MMA NPs especially HEA NPs are of particular interest, yet the controlled incorporation of multiple elements into a small particle (< 100 nm) remains a significant challenge due to the elements’ different atomic sizes and unfavorable valence electron configurations.

[0006] Phase-segregation favored by thermodynamics can be avoided by controlling the diffusion kinetics of species in a particle (e.g. rapid heating, followed by fast quenching). The present specification provides a continuous high temperature process based on the thermal plasma jet technology used to produce MMA NPs such as HEA NPs comprising a plurality of metals (e.g. 5 or more metals). Rapid cooling (for example at a cooling rate of above about 10³ K/second) of well mixed, vaporised metal sources can produce MMA NPs in the nanometer range in a continuous fashion. When at least 5 metals are used, HEA NPs can be produced. The uniformity of the produced MMA NPs can be further increased by separation and filtration for example using a cyclone separator and filter unit.

[0007] Accordingly, in one aspect, the present disclosure includes a method for producing multielement metal alloy nanoparticles (MMA NPs) comprising introducing a metal source comprising a plurality of metals into a thermal zone of a plasma torch under conditions to vapourise at least a portion of the metal source to obtain a metal vapour comprising the plurality of metals, the thermal zone of the plasma torch having a temperature about 1000 K or more than about WOOK; substantially homogenizing the metal vapour by plasma expansion and turbulence; cooling the metal vapour at a rate of about 10³ K to about 10⁷ K per second to conucleate and co-condense the MMA NPs; and collecting the MMA NPs.

[0008] In some embodiments, the plurality of metals comprises at least 2 metals, at least 4 metals, at least 5 metals, at least 7 metals, at least 9 metals, about 2 to about 10, about 2 to about 6, about 5 to about 10 metals, 2 metals, 3 metals, 4 metals, 5 metals, 7 metals, 9 metals, or 10 metals. In some embodiments, the plurality of metals comprises at least 5 metals. In some embodiments, the MMA NPs are high entropy alloy nanoparticles (HEA NPs).

[0009] In some embodiments, the plurality of metals is selected from metals of Groups 1-15 of the Periodic Table of the Elements. In some embodiments, the plurality of metals is selected from refractory metals, optionally the refractory metal is selected from Nb, Mo, Ta, W, V, and mixtures thereof. In some embodiments, the plurality of metals is selected from Ni, Co, Cr, Fe, Mn, a refractory metal, and mixtures thereof. In some embodiments, the plurality of metal is selected from Ni, Co, Cr, Fe, Mn, Mo, and mixtures thereof. In some embodiments, the plurality of metal comprises Ni, Co, Cr, Fe and Mo. In some embodiments, the plurality of metal comprises Ni, Co, Cr, Fe and Mn.

[0010] In some embodiments, the plasma torch is a DC plasma torch, microwave plasma torch, RF inductively coupled plasma torch, or hybrid thereof. In some embodiments, the plasma torch is an inductively coupled plasma torch.

[0011] In some embodiments, the temperature of the thermal zone of the plasma torch is about 3000K to about 10000K, about 5000K to about 10000K, about 3000K to about 9000K, about 3000K to about 8000K, about 3000K, about 4000K, about 5000K, about 6000K, about 7000K or about 8000K.

[0012] In some embodiments, the temperature of the thermal zone of the plasma torch is maintained by heating a plasma gas. In some embodiments, the plasma gas comprises argon, and optionally at least one of hydrogen, helium or mixtures thereof. In some embodiments, the plasma gas comprises argon, and at least one of hydrogen, helium or mixtures thereof.

[0013] In some embodiments, the plasma gas comprises a central gas and a sheath gas. In some embodiments, the plasma gas is supplied at about 50 slpm to about 250 slpm, about 100 slpm to about 200 slpm, about 125 slpm to about 225 slpm, or about 150 slpm. In some embodiments, the central gas is supplied at about 20 slpm to about 50 slpm. In some embodiments, the sheath gas is supplied at about 30 slpm to about 230 slpm, about 100 slpm to about 150 slpm, or about 120 slpm.

[0014] In some embodiments, the metal source is introduced at a feed rate of about 0.5 g/min to about 3 g/min, about 1 g/min to about 2.5 g/min, or about 1 .2 g/min to about 2 g/min.

[0015] In some embodiments, the plasma gas has a high thermal conductivity. In some embodiments, the plasma comprises argon and hydrogen, or argon and helium. In some embodiments, the plasma gas comprises about 5% v/v to about 40%v/v, or about 10% v/v to about 20% v/v of hydrogen. In some embodiments, the plasma gas comprises about 5% v/v- about 95% v/v, or about 50% v/v to about 75% v/v of helium.

[0016] In some embodiments, the metal vapour is cooled at a rate of about 10³ K to about 10⁶ K per second, or about 10⁵ K to about 10⁶ K per second. In some embodiments, the metal vapour is substantially maintained at a pressure of less than 2 atm in the thermal zone. In some embodiments, the pressure is greater than 0.2 atm.

[0017] In some embodiments, the introducing of the metal source is into a reactor that is water-cooled. In some embodiments, the introducing of the metal source comprises injecting the metal source.

[0018] In some embodiments, the metal source is a solid, a liquid or a gas, optionally the metal source is a metal powder. In some embodiments, the metal source comprises substantially pure elemental metals, alloys and/or metal salts. In some embodiments, the metal source is introduced with a carrier gas, optionally the carrier gas comprises argon.

[0019] In some embodiments, the method is continuous. [0020] In some embodiments, the metals of the plurality of metals are distributed substantially homogenously in the MMA NPs.

[0021] In some embodiments, the MMA NPs have an average diameterof less than about 1000 nm, less than about 800 nm, less than about 600 nm, less than about 400 nm, less than about 300 nm, less than about 200 nm, less than about 100 nm. In some embodiments, the MMA NPs have a diameter of more than about 2 nm, more than 5 nm, more than 10 nm, more than 15 nm, or more than 20 nm.

[0022] In some embodiments, the homogenizing is carried out by plasma expansion and/or turbulence.

[0023] In some embodiments, the collecting of the MMA NPs is carried out by using a cyclone separator. In some embodiments, the collecting of the MMA NPs comprises separating the MMA NPs from any unvaporised metal source if present. In some embodiments, the collecting of the MMP NPs comprises separating the MMA NPs from impurities or side products. In some embodiments, the impurities comprise any unvaporised metal source if present. In some embodiments, the separating of the MMA NPs from any unvaporised metal source if present is carried out by using a cyclone separator.

[0024] In some embodiments, the method further comprises filtering the collected MMA NPs.

[0025] In another aspect, the present disclosure includes a system for producing multielement metal alloy nanoparticles (MMA NPs) comprising a reactor comprising a reaction chamber; a plasma torch coupled to an inlet end of the reaction chamber, the plasma torch being configured to maintain a thermal zone having a temperature of about 1000K or more than 1000K; an inlet configured for introducing a metal source comprising a plurality of metals into the thermal zone of the plasma torch; and a plasma gas inlet configured for receiving a plasma gas into the thermal zone of the plasma torch; the plasma torch being configured for vaporising at least a portion of the metal source to obtain a metal vapour comprising the plurality of metals; the reaction chamber being configured to substantially homogenize the metal vapour by creating one or more plasma expansion or turbulence; the reactor being configured to allow cooling of the metal vapour at a rate of about 10³ K to about 10⁷ K per second to co-nucleate and co-condense the MMA NPs; a collecting device in communication with the outlet end of the reaction chamber, the collecting device configured for collecting the MMA NPs.

[0026] In some embodiments, the collecting device is a separator, optionally a cyclone separator, wherein the separator is configured for separating any unvaporised metal source if present from the MMA NPs.

[0027] In some embodiments, the reaction chamber has a diameter that increases longitudinally from the inlet end to an outlet end of the reaction chamber. In some embodiments, the reaction chamber has a diameter at the inlet end that is larger than a diameter of the plasma torch.

[0028] In some embodiments, the system further comprises a filter unit in fluid communication with the collecting device, the filter unit being configured for filtering the collected MMA NPs.

[0029] In some embodiments, the reactor is water-cooled.

[0030] In some embodiments, the plasma torch is a DC plasma torch, a microwave plasma torch, RF inductively coupled plasma torch, or hybrid thereof.

[0031] In some embodiments, the thermal zone of the plasma torch is maintained by heating the plasma gas. In some embodiments, the plasma gas comprises argon, and optionally at least one of hydrogen, helium or mixtures thereof. In some embodiments, the plasma gas comprises argon, and at least one of hydrogen, helium or mixtures thereof.

[0032] In some embodiments, the plasma gas has a high thermal conductivity. In some embodiments, the plasma comprises argon and hydrogen, or argon and helium. In some embodiments, the plasma gas comprises about 5% v/v to about 40%v/v, or about 10% v/v to about 20% v/v of hydrogen. In some embodiments, the plasma gas comprises about 5% v/v- about 95% v/v, or about 50% v/v to about 75% v/v of helium.

[0033] In some embodiments, the plasma torch is configured to maintain the thermal zone at a temperature of about 3000K to about 10000K, about 5000K to about 10000K, about 3000K to about 9000K, about 3000K to about 8000K, about 3000K, about 4000K, about 5000K, about 6000K, about 7000K or about 8000K.

[0034] In some embodiments, the diameter of the reaction chamber at the outlet end is about 2 times, about 3 times, or about 4 times the diameter of the reaction chamber at the inlet end. In some embodiments, the diameter of the reaction chamber at the inlet end is about 2 times, about 3 times, or about 4 times a diameter of the plasma torch.

[0035] In some embodiments, the reactor is configured to allow cooling of the metal vapour at a rate of about 10³ K to about 10⁶ K per second, or about 10⁵ K to about 10⁶ K per second.

[0036] In some embodiments, the inlet is an injection probe configured for injecting the metal source into the thermal zone.

[0037] In another aspect, the present disclosure includes multiplemetal alloy nanoparticles (MMA NPs) prepared by a method of the present disclosure. In another aspect, the present disclosure includes a MMA NPs prepared using a system of the present disclosure.

DRAWINGS

[0038] The embodiments of the disclosure will now be described in greater detail with reference to the attached drawings in which:

[0039] Figure 1 shows in panel (a) a flowchart illustrating an example of a method of the present disclosure. Figure 1 Panels (b1) and (b2) show schematics of examples of the systems of the present disclosure. Figure 1 Panel (c) shows an illustrative schematic of an example system of the present disclosure. Figure 1 Panel (d) shows a picture of an exemplary set up of a system of the present disclosure. Figure 1 panel (e) shows a schematic illustrative of an example method of the present application compared to conventional methods of producing alloy NPs.

[0040] Figure 2 shows a picture of the HEA NPs prepared using a method of the present disclosure. The jar on the left shows a sample of the HEA NPs after being filtered using a filter unit. The jar on the right shows a sample of the HEA NPs collected using a cyclone separator.

[0041] Figure 3 shows TEM images of CrFeCoNiMo HEA NPs prepared using a method of the present disclosure at different magnifications showing the size of the NPs of the present disclosure.

[0042] Figure 4 shows annular dark-field images in a scanning TEM mode (ADF- STEM) at different magnifications of CrFeCoNiMo HEA NPs prepared using a method of the present disclosure, showing that the contrast of ADF-STEM in the particles is uniform.

[0043] Figure 5 shows high resolution TEM (HR-TEM) images with Local Fast Fourier Transform (FFT) patterns of CrFeCoNiMo HEA NPs prepared using a method of the present disclosure showing the crystallinity of the particles.

[0044] Figure 6 shows Electron Loss-Energy Spectroscopy (EELS) and Energy Dispersive X-Ray Spectroscopy (EDS) mapping of CrFeCoNiMo HEA NPs prepared using a method of the present disclosure, showing distribution of the various metals in the particles.

[0045] Figure 7 shows scanning electron microscopy (SEM) images of the MMA NPs (CrFeCoNiMo) produced using methods of the present disclosure with different plasma gases of Ar-H2 (H2: 8.3%) and helium Ar-He (He: 77.4%).

[0046] Figure 8 shows a graph illustrating the size distribution of the MMA NPs (CrFeCoNiMo) produced using methods of the present disclosure with different plasma gases of Ar-H₂ (H2: 8.3%) and Ar-He (He: 77.4%).

[0047] Figure 9 shows in panel (a) X-ray diffraction (XRD) patterns of the feedstock mixture (Cr-Fe-Co-Ni-Mo) and the MMA NPs (CrFeCoNiMo) produced using methods of the present disclosure with different plasma gases of Ar-H2 (H2: 8.3%) and Ar-He (He: 77.4%). The XRD patterns confirm the in-situ alloying of the pure element metals using a method of the present disclosure. Panel (b) shows XRD patterns of the feedstock mixture and various MMA NP samples collected from different locations of the cyclone separator and the filter unit.

[0048] Figure 10 shows transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images of the MMA NPs (CrFeCoNiMo) produced using a method of the present disclosure with Ar-H2 (H2: 8.3%) (Panel (a)) or Ar-He (He:77.4%) (Panel (b)) as plasma gas. The corresponding energy dispersive X-ray spectroscopy (EDS) elemental maps show homogenous distribution of the five metals in the particles.

[0049] Figure 11 shows the elemental composition of the MMA NPs (CrFeCoNiMo) produced with different plasma gases of Ar-H2 (H2: 8.3%) and Ar-He (He: 77.4%).

[0050] Figure 12 shows in Panel (a) atomically-resolved HAADF-STEM image and the corresponding Fast Fourier Transform (FFT) analysis of the MMA NPs (CrFeCoNiMo) produced using a method of the present disclosure with Ar-H2 (H2: 8.3%) as plasma gas, confirming their single Face-Centered Cubic (FCC) structure. Panel (b) shows phase stability calculations by Density-functional Theory (DFT) simulations for the MMA NPs (CrFeCoNiMo), showing a higher stability of an FCC structure over a Body-Centered Cubic (BCC) structure.

[0051] Figure 13 shows an optical emission spectrum (OES) measured at Z = 0.23 m from the bottom of the plasma torch during the synthesis of MMA NPs (CrFeCoNiMo) produced with a method of the present disclosure with Ar-H2 (H2: 8.3%) (Panel (a)) or Ar- He (77.4%) (Panel (b)) as plasma gas, indicating that the plasma jet temperature is high enough to vaporize the metal powders injected.

[0052] Figure 14 shows in Panel (a) thermofluid simulations showing the effect of the reactor geometry (D reactor = D torch and D reactor = 3D torch) on the turbulence intensity. The enhanced turbulence (D reactor = 3D torch) improves intermixing of the vapors produced from the vaporization of the feedstock mixture; in Panel (b), temperature and thermal conductivity fields calculated for different plasma gases of Ar (100%), Ar-H2 (H2: 8.3%), and Ar-He (He: 77.4%), showing enhanced cooling rates of the plasma jet in the presence of hydrogen or helium; and in Panels (c) and (d), axial temperature profiles with local heating and cooling rates calculated for different plasma gases of Ar-H2 (H2: 8.3%) (Panel (c)) and Ar-He (He: 77.4%) (Panel (d)), showing high heating and cooling rates up to 10⁶ K/s.

[0053] Figure 15 shows X-ray diffraction (XRD) patterns of the feedstock mixture (Cr-Mn-Fe-Co-Ni) and the MMA NPs (CrMnFeCoNi) produced using methods of the present disclosure with different plasma gases of Ar-H2 (H2: 8.3%) and Ar-He (He: 77.4%). The XRD patterns confirm the in-situ alloying of the pure element metals using a method of the present disclosure. [0054] Figure 16 shows high-angle annular dark-field scanning TEM (HAADF- STEM) images of the MMA NPs (CrMnFeCoNi) produced using a method of the present disclosure with Ar-H2 (H2: 8.3%) (Panel (a)) or Ar-He (He:77.4%) (Panel (b)) as plasma gas. The corresponding energy dispersive X-ray spectroscopy (EDS) elemental maps show homogenous distribution of the five metals in the particles.

[0055] Figure 17 shows energy dispersive X-ray spectroscopy (EDS) line scan along the black line across a single MMA NP (CrMnFeCoNi) produced with different plasma gases of Ar-H2 (H2: 8.3%) (Panel (a)) and Ar-He (He: 77.4%) (Panel (b)). The scans show that five metals are homogenously distributed in the particles.

[0056] Figure 18 shows the elemental composition of the MMA NPs (CrMnFeCoNi) produced with different plasma gases of Ar-H2 (H2: 8.3%) (panel(a)) and Ar-He (He: 77.4%) (panel (b)).

[0057] Figure 19 shows in panel (a) a STEM image of Co-Mo binary nanoparticles (Co0.48Mo0.52) synthesized with Ar-H2 (H2: 8.3%) using a method of the present disclosure. The corresponding EDS elemental maps in Panel (b) show homogenous mixing of two elements in the particles.

[0058] Other features and advantages of the present disclosure will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the disclosure, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the description as a whole.

DESCRIPTION OF VARIOUS EMBODIMENTS

I. Definitions

[0059] Unless otherwise indicated, the definitions and embodiments described in this and other sections are intended to be applicable to all embodiments and aspects of the present disclosure herein described for which they are suitable as would be understood by a person skilled in the art.

[0060] The term “and/or” as used herein means that the listed items are present, or used, individually or in combination. In effect, this term means that “at least one of” or “one or more” of the listed items is used or present. [0061] As used in the present disclosure, the singular forms “a”, “an” and “the” include plural references unless the content clearly dictates otherwise. For example, an embodiment including “a compound” should be understood to present certain aspects with one compound, or two or more additional compounds.

[0062] In embodiments comprising an “additional” or “second” component, such as an additional or second compound, the second component as used herein is chemically different from the other components or first component. A “third” component is different from the other, first, and second components, and further enumerated or “additional” components are similarly different.

[0063] As used in this disclosure and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "include" and "includes") or "containing" (and any form of containing, such as "contain" and "contains"), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.

[0064] The term “consisting” and its derivatives as used herein are intended to be closed terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, and also exclude the presence of other unstated features, elements, components, groups, integers and/or steps.

[0065] The term “consisting essentially of”, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and/or steps as well as those that do not materially affect the basic and novel characteristic(s) of these features, elements, components, groups, integers, and/or steps.

[0066] The present description refers to a number of chemical terms and abbreviations used by those skilled in the art. Nevertheless, definitions of selected terms are provided for clarity and consistency.

[0067] The terms "about", “substantially” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies or unless the context suggests otherwise to a person skilled in the art. [0068] The term “high entropy alloy nanoparticles”, “HEA NPs” or the like as used herein means alloys that contain at least 5 elements with concentrations between 5 and 35 atomic percent.

[0069] The term “plasma expansion” as used herein means the phenomenon where the plasma undergoes expansion due to a change in the available volume of the surrounding. For example, upon entering the reaction chamber, the plasma jet can expand due to a diameter change in the reaction chamber creating more volume as the plasma jet travels through the reaction chamber.

[0070] The term “turbulence” as used herein means the presence of recirculation eddies in a gas flow.

[0071] The term “in communication with” as used herein means allowing the passage of substance including substance in the form of liquid, gas, solid or mixtures thereof. For example, a component being in communication with another component includes the component being in fluid communication with the other component. For example, a component being in communication with another component allows the passage of substances in different forms such as a gas stream containing or carrying solid particles.

[0072] The term “homogenizing” as used herein means mixing the plurality of metals in the metal vapour to increase homogeneity or increase evenness of distribution, but does not necessarily require that the plurality of metals in the metal vapour is in a perfectly homogenous state or completely evenly distributed.

II. Methods and Systems of the Disclosure

[0073] In one aspect, the present disclosure includes a method for producing multielement metal alloy nanoparticles (MMA NPs) comprising introducing a metal source comprising a plurality of metals into a thermal zone of a plasma torch under conditions to vapourise at least a portion of the metal source to obtain a metal vapour comprising the plurality of metals, the thermal zone of the plasma torch having a temperature about 1000 K or more than about WOOK; substantially homogenizing the metal vapour by plasma expansion and turbulence; cooling the metal vapour at a rate of about 10³ K to about 10⁷ K per second to conucleate and co-condense the MMA NPs; and collecting the MMA NPs.

[0074] In another aspect, the present disclosure includes a system for producing multielement metal alloy nanoparticles (MMA NPs) comprising a reactor comprising a reaction chamber; a plasma torch coupled to an inlet end of the reaction chamber, the plasma torch being configured to maintain a thermal zone having a temperature of about 1000K or more than 1000K; an inlet configured for introducing a metal source comprising a plurality of metals into the thermal zone of the plasma torch; and a plasma gas inlet configured for receiving a plasma gas into the thermal zone of the plasma torch; the plasma torch being configured for vaporising at least a portion of the metal source to obtain a metal vapour comprising the plurality of metals; the reaction chamber being configured to substantially homogenize the metal vapour by creating one or more plasma expansion or turbulence; the reactor being configured to allow cooling of the metal vapour at a rate of about 10³ K to about 10⁷ K per second to co-nucleate and co-condense the MMA NPs; a collecting device in communication with the outlet end of the reaction chamber, the collecting device configured for collecting the MMA NPs.

[0075] Referring to Figure 1 (a), a flowchart is shown of an example method 100 of producing MMA NPs. As shown in Figure 1 (a), the method 100 of producing MMA NPs begins at step 102, in which a metal source is introduced into the thermal zone of a plasma torch. The metal source may comprise a plurality of metals.

[0076] At step 102 of the method 100, at least a portion of the metal source is vaporized in the thermal zone of the plasma torch to obtain a metal vapour. It is understood that when the metal source comprises a plurality of metals, the metal vapour will comprise the plurality of metals. The thermal zone of the plasma torch can have a temperature of about 1000 K or more than about 1000 K. The temperature of the thermal zone of the plasma torch can be selected based on the nature of the metals of the metal source as described below.

[0077] Still referring to Figure 1 (a), at step 104 as illustrated, the metal vapour is substantially homogenized by plasma expansion and/or turbulence. The metal vapour can be substantially homogenized as described in more detail herein. The substantially homogenized metal vapour is then cooled at step 106 to co-nucleate and co-condense the MMA NPs. The cooling of the metal vapour is described in more detail below. The substantially homogenized metal vapour can be cooled at a rate of about 10³ K to about 10⁷ K per second. The metal vapour can be cooled as described in more detail herein. The MMA NPs is then collected at step 108 of the method 100. In some embodiments, the collecting of the MMA NPs at step 108 comprises separating the MMA NPs from impurities and side products. For example, the impurities can comprise any unvaporized metal source if present. It is envisioned that step 108 can be carried out using a cyclone separator. The MMA NPs can be collected as described in more detail herein.

[0078] Referring now to Figure 1 (b1), a schematic illustration of an example of the systems for producing MMA NPs of the present disclosure is provided. The exemplary system 200 of the disclosure comprises a reactor 210 and a collecting device 220. As shown in Figure 1 (b1 ), the reactor 210 can comprise a reaction chamber 216. The reactor 210 comprises an inlet 212 and an outlet 218. The inlet 212 is for receiving the metal source (e.g. step 102 of method 100). The reactor 210 further comprises a plasma gas inlet 213. The plasma gas inlet 213 is for receiving a plasma gas as described below to supply to a plasma torch 214. It is envisioned that the reaction chamber 216 can have a diameter at the inlet that is larger than a diameter of the plasma torch 214 (as shown in Figure 1 (b1 )). As shown, the plasma torch 214 can be coupled to the inlet end of the reaction chamber 216. The plasma torch 214 may be operable to generate and/or maintain a thermal zone proximate to the inlet 212 for vapourizing at least a portion of the metal source within the reaction chamber 216 (e.g. step 102 of the method 100). After vapourisation, as the metal vapour travels along the reaction chamber 216, it is substantially homogenized by plasma expansion and/or turbulence (e.g. step 104 of the method 100). The reaction chamber 216 may have a diameter at the inlet end of the reaction chamber 216 that is larger than a diameter of the plasma torch 214 (as shown in Figures 1 (b1 )). This geometric design of the diameter difference between the plasma torch and the reaction chamber can increase plasma expansion and turbulence in the reaction chamber, improving the homogenizing of the plurality of metals in the metal vapour. It is also envisioned that the increased plasma expansion and/or turbulence can contribute to the cooling of the metal vapour (e.g. step 106 of the method 100). As shown in Figures 1 (b1), the reactor 210 is in communication with a collecting device 220. Accordingly, the MMA NPs may exit the reactor 210 through the outlet 218 to be collected by the collecting device 220. The collecting is further described below.

[0079] Referring to Figure 1 (b2), another embodiment of the system of the present disclosure is shown. The embodiment shown in Figure 1 (b2), system 200A, is similar to that of system 200 in Figure 1 (b1 ) with a difference being that the reaction chamber 216A has a diameter that increases longitudinally from the inlet end of the reaction chamber to the outlet end of the reaction chamber. This geometric design of the reaction chamber can increase plasma expansion and/or turbulence in the reaction chamber, improving the homogenizing of the plurality of metals in the metal vapour. This feature can work in combination with the diameter difference between the plasma torch and the reaction chamber at the inlet end to further improve the homogenizing.

[0080] Referring now to Figure 1 (c), a second exemplary system (300) for producing MMA NPs of the present disclosure is provided. The exemplary system 300 comprises a reactor 300A, a collecting device 319 in communication with the reactor 300A, and a filter unit 320 in communication with the collecting device 319. As in Figure 1 (c), showing the reactor 300A, a plasma torch 310 may be coupled to a reaction chamber 316 at an inlet end of the reaction chamber 316. The plasma torch can be a radio frequency (RF) inductively coupled plasma torch (ICP). The plasma torch 310 has a plasma gas inlet 312, through which a plasma gas is introduced into the plasma torch 310 to maintain a thermal zone of the plasma torch (ora plasma jet) 311. The the reactor 300A comprises an inlet 314 configured to receive the feedstock metal source 313 into the thermal zone of the plasma torch 311 . The metal source 313 is at least partially vapourised in the thermal zone of the plasma torch 311 to produce a metal vapour 315 comprising the plurality of metals. The metal vapour 315 is mixed by turbulence and/or plasma expansion as the metal vapour travels along the reaction chamber 316. The reaction chamber 316 can have a diameter that increases longitudinally from the inlet end of the reaction chamber to the outlet end of the reaction chamber, further increasing plasma expansion and turbulence. The reaction chamber 316 can have a diameter at the inlet end of the reaction chamber 316 that is larger than a diameter of the plasma torch 310 (as shown in Figures 1 (b1 ) and (b2)). The mixing substantially homogenizes the plurality of metals in the metal vapour 315. The metal vapour 315 is cooled as it travels along the reaction chamber. The reactor 300A can be a water-cooled reactor, in which case the reactor 300A can comprise a water inlet 317a located proximal to the outlet end of the reaction chamber and a water outlet 317b located proximal to the inlet end of the reaction chamber. The reactor 300A can also comprise a window for spectroscopic measurements (318) for example for optical emission spectroscopy. The collecting device 319 can be a cyclone separator. The filter unit 320 when present can comprise at least one filter 121 . The filter can be porous metal filter. The filter unit 320 can comprise a gas outlet 323 that is connected to a vacuum pump. The resulting MMA NPs exit the reactor 300A through the outlet end of the reaction chamber and enter the collecting device 319. The collected MMA NPs exits the collecting device 319 into the filter unit 320. The MMA NPs 322 is deposited on the at least one filter 321. The deposited MMA NPs can be collected from the at least one filter 321 to obtain the product MMA NPs 324.

[0081] In some embodiments, the plurality of metals comprises at least 2 metals, at least 4 metals, at least 5 metals, at least 7 metals, at least 9 metals, about 2 to about 10, about 2 to about 6, about 5 to about 10 metals, 2 metals, 3 metals, 4 metals, 5 metals, 7 metals, 9 metals, or 10 metals. In some embodiments, the plurality of metals comprises at least 5 metals. In some embodiments, the MMA NPs are high entropy alloy nanoparticles (HEA NPs).

[0082] In some embodiments, the plurality of metals is selected from metals of Groups 1-15 of the Periodic Table of the Elements. In some embodiments, the plurality of metals is selected from refractory metals, optionally the refractory metal is selected from Nb, Mo, Ta, W, V, and mixtures thereof. In some embodiments, the plurality of metals is selected from Ni, Co, Cr, Fe, Mn, a refractory metal, and mixtures thereof. In some embodiments, the plurality of metal is selected from Ni, Co, Cr, Fe, Mn, Mo, and mixtures thereof. In some embodiments, the plurality of metal comprises Ni, Co, Cr, Fe and Mo. In some embodiments, the plurality of metal comprises Ni, Co, Cr, Fe and Mn.

[0083] In some embodiments, the plasma torch is a DC plasma torch, microwave plasma torch, RF inductively coupled plasma torch, or hybrid thereof. In some embodiments, the plasma torch is an inductively coupled plasma torch. [0084] In some embodiments, the temperature of the thermal zone of the plasma torch is about 3000K to about 10000K, about 5000K to about 10000K, about 3000K to about 9000K, about 3000K to about 8000K, about 3000K, about 4000K, about 5000K, about 6000K, about 7000K or about 8000K. In some embodiments, the temperature of the thermal zone of the plasma torch is at least about 1000 K, at least about 2000 K, at least about 3000 K, or at least about 5000 K. In some embodiments, the temperature of the thermal zone of the plasma torch is less than about 12000 K, less than about 10000 K, less than about 8000K, or less than about 7000 K. It can be appreciated that the temperature of the thermal zone of the plasma torch can be selected depending on the metal source such that the temperature is sufficient to vapourise at least a portion of the metal source. For example, the metal salt form of a metal element may have a lower vapourisation temperature of the elemental form of the metal element. For example, it can be appreciated that different metal elements have different vapourisation temperatures. Thus, depending on the choice of metal to be included in the desired MMA NPs, the temperature of the thermal zone of the plasma torch can be determined. Traditional methods of preparing MMA NPs often use metal salts due to the technical difficulty of vapourising elemental metal at high temperature. Without wishing to be bound by theory, due to the presence of counterions such as chlorides in metal salts, using metal salts as feedstock can affect the purity of the final MMA NPs product. Since thermal plasma jets are partially ionized gases that can achieve high temperature (e.g. >8000K) and high speed (e.g. up to supersonic range) [Boulos 1994], they are capable of vapourising elemental metals with high vapourisation temperature in addition to metal salts with lower vapourisation temperature. The methods and systems of the present disclosure use thermal plasma jets, which are capable of achieving high temperature, allowing for the use of pure elemental metals and for higher purity MMA NPs.

[0085] In some embodiments, the temperature of the thermal zone of the plasma torch is maintained by heating a plasma gas. In some embodiments, the plasma gas comprises argon, and optionally at least one of hydrogen, helium or mixtures thereof. In some embodiments, the plasma gas comprises argon, and at least one of hydrogen, helium or mixtures thereof.

[0086] In some embodiments, the plasma gas comprises a central gas and a sheath gas. In some embodiments, the plasma gas is supplied at about 50 slpm to about 250 slpm, about 100 slpm to about 200 slpm, about 125 slpm to about 225 slpm, or about 150 slpm. In some embodiments, the central gas is supplied at about 20 slpm to about 50 slpm. In some embodiments, the sheath gas is supplied at about 30 slpm to about 230 slpm, about 100 slpm to about 150 slpm, or about 120 slpm.

[0087] In some embodiments, the metal source is introduced at a feed rate of about 0.5 g/min to about 3 g/min, about 1 g/min to about 2.5 g/min, or about 1 .2 g/min to about 2 g/min.

[0088] In some embodiments, the plasma gas has a high thermal conductivity. In some embodiments, the plasma comprises argon and hydrogen, or argon and helium. In some embodiments, the plasma gas comprises about 5% v/v to about 40%v/v, or about 10% v/v to about 20% v/v of hydrogen. In some embodiments, the plasma gas comprises about 5% v/v- about 95% v/v, or about 50% v/v to about 75% v/v of helium.

[0089] In some embodiments, the metal vapour is cooled at a rate of about 10³ K to about 10⁶ K per second, or about 10⁵ K to about 10⁶ K per second.

[0090] In some embodiments, the metal vapour is substantially maintained at a pressure of less than 2 atm in the thermal zone. In some embodiments, the pressure is greater than 0.2 atm.

[0091] In some embodiments, the introducing of the metal source is into a reactor that is water-cooled.

[0092] In some embodiments, the introducing of the metal source comprises injecting the metal source.

[0093] Without wishing to be bound by theory, the methods and systems of the present disclosure are capable of rapidly heating of the metal source (e.g. the feedstock) to produce an atomically mixed state of metal vapours and of rapidly cooling the mixed state with an ultra-high cooling rate to form a solid-solution particle. It is contemplated that the cooling of the metal vapour can be achieved through at least one of a number of design features of the methods or systems of the present disclosure. One or more of, or all of, the features of the methods or systems of the present disclosure can contribute to the rapid cooling of the metal vapour. For example, the cooling of the metal vapour can be at least partially achieved by rapid and thorough mixing of the metal vapour through plasma expansion and/or turbulence. In some embodiments, the plasma expansion and/or turbulence is achieved by reactor design, for example, where the reaction chamber has a diameter that increases longitudinally from the inlet end to an outlet end of the reaction chamber. For example, the reaction chamber can be water-cooled. In some embodiments, the cooling of the metal vapour is at least partially achieved by using a plasma gas having a high thermal conductivity such that as the metal vapour travels through the reaction chamber, it is rapidly cooled as heat is dissipated through the plasma gas. In some embodiments, a cooling gas can be introduced. For example, the reaction chamber can further comprise an inlet configured to introduce a cooling gas. In some embodiments, the cooling gas has high thermal conductivity. In some embodiments, the cooling gas comprises at least one of hydrogen or helium.

[0094] In some embodiments, the metal source is a solid, a liquid or a gas, optionally the metal source is a metal powder. In some embodiments, the metal source comprises substantially pure elemental metals, alloys and/or metal salts. In some embodiments, the metal source is introduced with a carrier gas, optionally the carrier gas comprises argon.

[0095] In some embodiments, the method is continuous.

[0096] In some embodiments, the metals of the plurality of metals are distributed substantially homogenously in the MMA NPs.

[0097] In some embodiments, the MMA NPs have an average diameter of less than about 1000 nm, less than about 800 nm, less than about 600 nm, less than about 400 nm, less than about 300 nm, less than about 200 nm, less than about 100 nm. In some embodiments, the MMA NPs have a diameter of more than 2 nm, more than 5 nm, more than 10 nm, more than 15 nm, or more than 20 nm.

[0098] In some embodiments, the homogenizing is carried out by plasma expansion and/or turbulence. In some embodiments, following the homogenizing, the metal vapour is substantially homogenized. In some embodiments, following the homogenizing, the metal vapour is homogenized. In some embodiments, the plurality of metals in the MMA NPs is distributed substantially homogenously. In some embodiments, the plurality of metals in the MMA NPs is distributed homogenously.

[0099] In some embodiments, the collecting of the MMA NPs is carried out by using a cyclone separator. In some embodiments, the collecting of the MMA NPs comprises separating the MMA NPs from any unvaporised metal source if present. In some embodiments, the collecting of the MMP NPs comprises separating the MMA NPs from impurities or side products. In some embodiments, the impurities comprise any unvaporised metal source if present. In some embodiments, the separating of the MMA NPs from any unvaporised metal source if present is carried out by using a cyclone separator.

[00100] In some embodiments, the method further comprises filtering the collected MMA NPs.

[00101] In some embodiments, the collecting device is a separator, optionally a cyclone separator, wherein the separator is configured for separating any unvaporised metal source if present from the MMA NPs.

[00102] In some embodiments, the reaction chamber has a diameter that increases longitudinally from the inlet end to an outlet end of the reaction chamber. In some embodiments, the diameter of the reaction chamber at the outlet end is about 2 times, about 3 times, or about 4 times the diameter of the reaction chamber at the inlet end. Without wishing to be bound be theory, a reaction chamber having a diameter that increases longitudinally from the inlet end to an outlet end of the reaction chamber increases plasma expansion and creates added turbulence in the metal vapour as the metal vapour travels through the reaction chamber from the inlet to the outlet. In some embodiments, the reaction chamber has a diameter at the inlet end of the reaction chamber larger than a diameter of the plasma torch. In some embodiments, the diameter of the reaction chamber at the inlet end is about 2 times, about 3 times, or about 4 times the diameter of the plasma torch. Without wishing to be bound by theory, a reaction chamber having a diameter at the inlet end larger than the diameter of the plasma torch increases plasma expansion and creates added turbulence in the metal vapour as the metal vapour enters the reaction chamber. The plasma expansion and/or turbulence mixes the plurality of metals in the metal vapour and increases homogeneity of the metal vapour at the co-nucleation and co-condensation stage such that the plurality of metals is distributed more evenly in the resulting MMA NPs.

[00103] In some embodiments, the system further comprises a filter unit in communication with the collecting device, the filter unit being configured for filtering the collected MMA NPs.

[00104] In some embodiments, the reactor is water-cooled. [00105] In some embodiments, the plasma torch is a DC plasma torch, a microwave plasma torch, RF inductively coupled plasma torch, or hybrid thereof.

[00106] In some embodiments, the thermal zone of the plasma torch is maintained by heating the plasma gas. In some embodiments, the plasma gas comprises argon, and optionally at least one of hydrogen, helium or mixtures thereof. In some embodiments, the plasma gas comprises argon, and at least one of hydrogen, helium or mixtures thereof.

[00107] In some embodiments, the plasma gas has a high thermal conductivity. In some embodiments, the plasma comprises argon and hydrogen, or argon and helium. In some embodiments, the plasma gas comprises about 5% v/v to about 40%v/v, or about 10% v/v to about 20% v/v of hydrogen. In some embodiments, the plasma gas comprises about 5% v/v- about 95% v/v, or about 50% v/v to about 75% v/v of helium.

[00108] In some embodiments, the plasma torch is configured to maintain the thermal zone at a temperature of about 3000K to about 10000K, about 5000K to about 10000K, about 3000K to about 9000K, about 3000K to about 8000K, about 3000K, about 4000K, about 5000K, about 6000K, about 7000K or about 8000K.

[00109] In some embodiments, the reactor is configured to allow cooling of the metal vapour at a rate of about 10³ K to about 10⁶ K per second, or about 10⁵ K to about 10⁶ K per second.

[00110] In some embodiments, the inlet is an injection probe configured for injecting the metal source into the thermal zone.

[00111] In some embodiments, the MMA NPs are produced at a rate of more than 50 g/h, more than 60 g/h, more than 70 g/h, more than 80 g/h, more than 90 g/h, or more than 100 g/h. In some embodiments, the MMA NPs are produced at a rate of less than 3000 g/h, less than 2000 g/h, less than 1000 g/h, less than 900 g/h, less than 800 g/h, or less than 700 g/h.

[00112] The methods and systems of the present disclosure offer a number of advantages compared to the traditional methods of preparing MMA NPs. Unlike other methods that are limited by the lifetime of consumable electrodes (e.g. arc discharge method) or types of feedstock (e.g. solid targets for laser ablation), the methods of the present disclosure can use any types of feedstock (e.g. solid, liquid, gas) and use a plasma torch that is substantially maintenance-free without consumable electrodes. [00113] The traditional atomization method requires creation of a metal pool from a metal mixture; however, the pool temperature achievable in the atomization method is typically limited below 3,000 °C. Therefore, some refractory metals which have high melting temperatures (e.g., refractory metals: Mo, Nb, W, Ta, V) cannot be processed effectively by the atomization method. In the methods of the present disclosure, plasma core temperature can reach above 8,000 K or above 10,000 K and thus can vaporize any elements existing in the periodic table. The traditional mechanical alloying method can be conducted at room temperature; however, there are many challenges associated with contamination during high-energy ball milling and high propensity of oxidation. The traditional CTS and sol-gel combustion methods typically employ a mixture of metal salts or metal nitrides as feedstock as their processing temperatures and energy contents are not high enough for complete vaporization of pure metal powders in micron sizes. In those cases, chemical species other than pure metals can be released from the feedstock, being a potential source of contamination. Other high temperature processes such as arc discharge, oscillatory spark discharge, laser ablation methods are capable of melting and vaporizing pure metal powders; however, such processes require feedstock in a form of solid pellet or target, which greatly limits the flexibility of the process. In the methods of the present disclosure, any kinds (e.g., pure metals, alloys, metal salts) and any forms of feedstock (e.g., solid, liquid or gas) can be used and continuously vaporized for the efficient synthesis of MMA NPs. Thus, the methods and systems of the present disclosure can be used for large scale synthesis of MMA NPs, especially HEA NPs, of high purity.

[00114] In the methods of the present disclosure, the rapid heating ensures that the injected metal source evaporates in the plasma jet soon after being introduced, e.g. within about a few tens of milliseconds. The resulting MMA NPs can be continuously collected in-situ. Therefore, the production rate of the methods of the present disclosure can be high compared to that of other conventional processes. For example, a yield rate about 35 g/h was demonstrated. Other previous methods in the literature were operated in a batch mode, and production rates demonstrated were significantly lower, e.g. below 1 g per batch [Yao 2018], The conventional mechanical alloying method can be used for large scale production of HEA powders; however, the process requires a long processing time of over 10 hours to complete alloying by mechanical energy and also results in production of micron-sized particles. [00115] Without wishing to be bound by theory, the high cooling rate of the methods of the present disclosure can minimize phase segregation or other intermetallic formation during cooling period. Compared to other conventional methods (e.g., arc melting: about 10³ k), the thermal plasma jet offers a cooling rate of 10⁵-10⁶ K/s via strong plasma jet expansion, plasma gas of high thermal conductivity, and/or additional quenching or cooling gas injection. This cooling rate limits the diffusion of species in a particle during the cooling period, minimizing the formation of segregated or intermetallic phases.

EXAMPLES

[00116] The following non-limiting examples are illustrative of the present disclosure.

Example 1

Preparation of MMA NPs of Five Elements (HEA NPs)

[00117] A system for preparing MMA NPs was set up. The exemplary system is shown in Figure 1 (d). The exemplary system contained a reactor comprising a 2-5 MHz radio frequency (RF) inductively coupled plasma torch (e.g. a Tekna PL-50™ from Tekna Plasma Systems, Inc.) that can produce high temperature thermal plasma jet, and a 1-m long, water-cooled stainless steel chamber. A stable plasma was maintained by heating a central inert plasma gas (e.g. argon, 30 slpm) to a high temperature (e.g. about 8000 K). A sheath gas is introduced into the plasma zone through a sheath gas inlet, the sheath gas assisting in stabilizing the thermal plasma. The sheath gas comprised an inert gas (e.g. argon, 120 slpm) and/or a mixture of argon and hydrogen gases (e.g., 120114 slpm) to prevent potential oxidation of HEA NPs by residual oxygen in the reaction chamber. A stable plasma was generated at a plate power of 45 kW and the reactor pressure was maintained at 66.7 kPa or atmospheric pressure. A mixture of five elemental metal (e.g., Ni, Co, Cr, Mo and Fe) powders was continuously provided to the plasma jet produced. The powder mix was released from a powder feeder and carried by argon gas (5 slpm) to the plasma torch. The powder was injected into the plasma jet through an injection probe located on the top of the plasma torch. The feed rate was typically about 1.2-2.0 g/min. The final products were collected by a cyclone separator and/or a filter unit.

Characterisation of Five-Element HEA NPs [00118] The resulting nanoparticles were characterised using transmission electron microscopy (TEM) (Figures 3 to 5), electron loss-energy spectroscopy (EELS) and energy-dispersive X-Ray spectroscopy (EDS) mapping (Figure 6).

[00119] The TEM images show that the resulting MMA NPs have a diameter that is less than 200 nm. The MMA NPs of the present disclosure were also shown to be crystalline (Figure 5). As shown in the EELS and EDS maps (Figure 6), the five elements (Ni, Fe, Mo, Co, and Cr) are evenly distributed in the particle.

[00120] The ADF-STEM images (Figure 4) show uninform contrast, suggesting that the five elements (Ni, Fe, Mo, Co, and Cr) are evenly distributed without segregation in the particles.

Example 2

Preparation of Quinary MMA NPs (CrFeCoNiMo) Using Argon-Hydrogen Plasma Jet

[00121] Chromium (Cr, <10 pm, 99.2%), iron (Fe, 6-10 pm, 99.5%), cobalt (Co, 1.6 pm, 99.8%), nickel (Ni, 3-7 pm, 99.9%), and molybdenum (Mo, 3-7 pm, 99.95%) powders were purchased (Alfa Aesar). The as-received elemental metal powders were mixed at an equal ratio (1 :1 :1 :1 :1 ), and the mixture was employed for the synthesis experiment without further treatment.

[00122] MMA NPs were synthesized by using RF thermal plasma. The exemplary synthesis system used comprised five parts: an induction plasma torch, a reaction chamber, a cyclone separator, a filtration chamber, and feedstock delivery.

[00123] For the plasma generation, a commercial RF induction plasma torch (Tekna PS-50™, Tekna Systems, Inc.) composed of a five-turn coil and a ceramic tube with an internal diameter of 50 mm was employed.

[00124] A 1-m long, double-walled stainless steel chamber was employed as the reaction chamber. Its diameter was designed to be at least 3 times larger (e.g., 150 mm) than that of the plasma torch in order to enhance the mixing of metal vapors by increased turbulence. The chamber walls were cooled by water to increase the cooling rate of the MMA NPs produced.

[00125] To selectively remove unvaporized feedstock particles, a cyclone separator was employed at the bottom of the reactor. The nano-size final products were collected from four porous metal filter units (surface area = 20 x 50 cm, 2.8 pm pore size) inside a filtration chamber connected to the end of the cyclone separator.

[00126] In the synthesis experiment, the plasma power was fixed at 45 kW at an RF frequency of -3 MHz (Lepel Co.) and a mixture of argon and hydrogen was employed as a plasma gas: 5 slpm of carrier gas (Ar), 30 slpm of central gas (Ar), and 120/14 slpm of sheath gas (Ar/H2).

[00127] The feedstock was continuously fed by a vibrating powder feeder (PFR200 feeder, Tekna Systems, Inc.) and delivered to an injection probe located on the top of the plasma torch by Ar carrier gas. The feed rate of the powder mix was about 1 .2-2.0 g/min. During the synthesis, the reactor pressure was kept constant as 66.7 kPa. To verify the formation of metal vapors from feedstock vaporization and investigate their spatial evolution, optical emission spectra were measured at Z = 0.23 m from the bottom of the plasma torch during the synthesis (QMMJ-55-UWIS-200/240-2PCBL-0.25, OZ Optics Ltd., with a core size of 200 pm).

[00128] After a 150-min operation under these conditions, about 200 g of powder was fed in total. The reaction products were collected from the cyclone separator (42 g) and the filter unit (84 g) in an open environment, and characterized without further purification or treatment.

Example 3

Synthesis of Quinary MMA NPs (CrFeCoNIMo) Using Argon-Helium Plasma Jet

[00129] The exemplary method described in Example 2 was used with the plasma gas modified to be a mixture of argon-helium to further improve the cooling rate of the plasma jet. (5 slpm of carrier gas (Ar), 30 slpm of central gas (Ar), and 120 slpm of sheath gas (He)). This resulted in production of MMA NPs with a smaller size (Ar-H2: 60.8 nm, Ar-He 39.7 nm) (Figures 7 and 8) and higher crystallinity (Figure 9).

[00130] Characterisation of the MMA NPs of Examples 2 and 3 are shown in Figures

7 to 12.

Example 4

Synthesis of Quinary MMA NPs (CrMnFeCoNi) Using Argon-Hydrogen Plasma Jet [00131] The exemplary method as described in Example 2 was used to produce a quinary MMA NPs with homogenous mixing of 5 elements in the particles.

[00132] Chromium (Cr, <10 pm, 99.2%, Alfa Aesar), manganese (Mn, <10 pm, 99.6%, Thermo Fisher), iron (Fe, 6-10 pm, 99.5%, Alfa Aesar), cobalt (Co, 1.6 pm, 99.8%, Alfa Aesar), and nickel (Ni, 3-7 pm, 99.9%, Alfa Aesar) powders were purchased. The as- received elemental metal powders were mixed at an equal ratio (1 :1 :1 :1 :1 ), and the mixture was employed for the synthesis experiment without further treatment.

Example 5

Synthesis of Quinary MMA NPs (CrMnFeCoNi) Using Argon-Helium Plasma Jet

[00133] The exemplary method described in Example 4 was used with the plasma gas modified to be a mixture of argon-helium to further improve the cooling rate of the plasma jet. (5 slpm of carrier gas (Ar), 30 slpm of central gas (Ar), and 120 slpm of sheath gas (He)).

[00134] Characterisation of the MMA NPs of Examples 4 and 5 are shown in Figures 15 to 18.

Example 6

Synthesis of Binary MMA NPs (CoMo) Using Argon-Hydrogen Plasma Jet

[00135] The exemplary method as described in Example 2 was used to produce a binary MMA NPs with homogenous mixing of two elements in the particles.

[00136] Cobalt (Co, 1.6 pm, 99.8%) and molybdenum (Mo, 3-7 pm, 99.95%) powders were chosen as feedstock (Alfa Aesar). The as-received elemental metal powders were mixed at an equal ratio (1 :1), and the mixture was employed for the synthesis experiments without further treatment.

[00137] In the synthesis experiment, the plasma power was fixed at 45 kW at an RF frequency of ~3 MHz (Lepel Co.) and a mixture of argon and hydrogen was employed as a plasma gas: 5 slpm of carrier gas (Ar), 30 slpm of central gas (Ar), and 120/14 slpm of sheath gas (Ar/H2).

[00138] EDS elemental maps of the resulting MMA NMPs show homogenous mixing of two elements in the particles collected from the filtration chamber. The STEM image of the resulting MMA NPs is shown in Figure 19. [00139] While the present disclosure has been described with reference to examples, it is to be understood that the scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. [00140] All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. Where a term in the present disclosure is found to be defined differently in a document incorporated herein by reference, the definition provided herein is to serve as the definition for the term .

FULL CITATIONS OF REFERENCES CITED IN THE DISCLOSURE

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5. Yao, Y., et al., Carbothermal shock synthesis of high-entropy-alloy nanoparticles, Science 359, 1489 (2018).

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Claims

CLAIMS:

1. A method for producing multielement metal alloy nanoparticles (MMA NPs) comprising introducing a metal source comprising a plurality of metals into a thermal zone of a plasma torch under conditions to vapourise at least a portion of the metal source to obtain a metal vapour comprising the plurality of metals, the thermal zone of the plasma torch having a temperature of about 1000 K or more than about WOOK; substantially homogenizing the metal vapour by plasma expansion and turbulence; cooling the metal vapour at a rate of about 10³ K to about 10⁷ K per second to conucleate and co-condense the MMA NPs; and collecting the MMA NPs.

2. The method of claim 1 , wherein the plurality of metals comprises at least 2 metals, at least 4 metals, at least 5 metals, at least 7 metals, at least 9 metals, about 2 to about 10, about 2 to about 6, about 5 to about 10 metals, 2 metals, 3 metals, 4 metals, 5 metals, 7 metals, 9 metals, or 10 metals.

3. The method of claim 1 or 2, wherein the plurality of metals comprises at least 5 metals.

4. The method of claim 3, wherein the MMA NPs are high entropy alloy nanoparticles (HEA NPs).

5. The method of any one of claims 1 to 4, wherein the plurality of metals is selected from metals of Groups 1 -15 of the Periodic T able of the Elements.

6. The method of any one of claims 1 to 5, wherein the plurality of metals is selected from refractory metals, optionally the refractory metal is selected from Nb, Mo, Ta, W, V, and mixtures thereof.

7. The method of any one of claims 1 to 5, wherein the plurality of metals is selected from Ni, Co, Cr, Fe, Mn, a refractory metal, and mixtures thereof.

8. The method of any one of claims 1 to 7, wherein the plasma torch is a DC plasma torch, microwave plasma torch, RF inductively coupled plasma torch, or hybrid thereof.

9. The method of any one of claims 1 to 8, wherein the temperature of the thermal zone of the plasma torch is about 3000K to about 10000K, about 5000K to about 10000K, about 3000K to about 9000K, about 3000K to about 8000K, about 3000K, about 4000K, about 5000K, about 6000K, about 7000K or about 8000K.

10. The method of any one of claims 1 to 9, wherein the temperature of the thermal zone of the plasma torch is maintained by heating a plasma gas.

11. The method of any one of claims 1 to 10, wherein the plasma gas has a high thermal conductivity.

12. The method of claim 11 , wherein the plasma comprises argon and hydrogen, or argon and helium.

13. The method of any one of claims 1 to 12, wherein the plasma gas comprises about 5% v/v to about 40%v/v, or about 10% v/v to about 20% v/v of hydrogen.

14. The method of any one of claims 1 to 12, wherein the plasma gas comprises about 5% v/v- about 95% v/v, or about 50% v/v to about 75% v/v of helium.

15. The method of any one of claims 1 to 14, wherein the metal vapour is cooled at a rate of about 10³ K to about 10⁶ K per second, or about 10⁵ K to about 10⁶ K per second.

16. The method of any one of claims 1 to 15, wherein the metal vapour is substantially maintained at a pressure of less than 2 atm in the thermal zone.

17. The method of claim 16, wherein the pressure is greater than 0.2 atm.

18. The method of any one of claims 1 to 17, wherein the introducing of the metal source is into a reactor that is water-cooled.

19. The method of any one of claims 1 to 18, wherein the introducing of the metal source comprises injecting the metal source.

20. The method of any one of claims 1 to 19, wherein the metal source is a solid, a liquid or a gas, optionally the metal source is a metal powder.

21. The method of any one of claims 1 to 20, wherein the metal source comprises substantially pure elemental metals, alloys and/or metal salts.

22. The method of any one of claims 1 to 21 , wherein the metal source is introduced with a carrier gas, optionally the carrier gas comprises argon.

23. The method of any one of claims 1 to 22, wherein the method is continuous.

24. The method of any one of claims 1 to 23, wherein the metals of the plurality of metals are distributed substantially homogenously in the MMA NPs.

25. The method of any one of claims 1 to 24, wherein the MMA NPs have an average diameter of less than about 1000 nm, less than about 800 nm, less than about 600 nm, less than about 400 nm, less than about 300 nm, less than about 200 nm, less than about 100 nm.

26. The method of any one of claims 1 to 25, wherein the homogenizing is carried out by plasma expansion and/or turbulence.

27. The method of any one of claims 1 to 26, wherein the collecting of the MMA NPs is carried out by using a cyclone separator.

28. The method of any one of claims 1 to 27, wherein the collecting of the MMA NPs comprises separating the MMA NPs from any unvaporised metal source is present.

29. The method of any one of claims 1 to 28, wherein the method further comprises filtering the collected MMA NPs.

30. A system for producing multielement metal alloy nanoparticles (MMA NPs) comprising a reactor comprising a reaction chamber; a plasma torch coupled to an inlet end of the reaction chamber, the plasma torch being configured to maintain a thermal zone having a temperature of about 1000K or more than 1000K; an inlet configured for introducing a metal source comprising a plurality of metals into the thermal zone of the plasma torch; and a plasma gas inlet configured for receiving a plasma gas into the thermal zone of the plasma torch; the plasma torch being configured for vaporising at least a portion of the metal source to obtain a metal vapour comprising the plurality of metals; the reaction chamber being configured to substantially homogenize the metal vapour by creating one or more plasma expansion or turbulence; the reactor being configured to allow cooling of the metal vapour at a rate of about 10³ K to about 10⁷ K per second to co-nucleate and co-condense the MMA NPs; a collecting device in communication with the outlet end of the reaction chamber, the collecting device configured for collecting the MMA NPs.

31 . The system of claim 30, wherein the collecting device is a separator, optionally a cyclone separator, wherein the separator is configured for separating any unvaporised metal source if present from the MMA NPs.

32. The system of claim 30 or 31 , wherein the reaction chamber has a diameter that increases longitudinally from the inlet end to an outlet end of the reaction chamber.

33. The system of any one of claims 30 to 32, wherein the system further comprises a filter unit in communication with the collecting device, the filter unit being configured for filtering the collected MMA NPs.

34. The system of any one of claims 30 to 33, wherein the reactor is water-cooled.

35. The system of any one of claims 30 to 34, wherein the plasma torch is a DC plasma torch, a microwave plasma torch, RF inductively coupled plasma torch, or hybrid thereof.

36. The system of any one of claims 30 to 35, wherein the thermal zone of the plasma torch is maintained by heating the plasma gas.

37. The system of any one of claims 30 to 36, wherein the plasma gas has a high thermal conductivity.

38. The system of claim 37, wherein the plasma gas comprises argon and hydrogen or argon and helium.

39. The system of any one of claims 30 to 38, wherein the plasma gas comprises about 5% v/v to about 40% v/v, or about 10% v/v to about 20% v/v of hydrogen.

40. The system of any one of claims 30 to 38, wherein the plasma gas comprises about 5% v/v to about 95% v/v, or about 50% v/v to about 75% v/v of helium.

41 . The system of any one of claims 30 to 40, wherein the plasma torch is configured to maintain the thermal zone at a temperature of about 3000K to about 10000K, about 5000K to about 10000K, about 3000K to about 9000K, about 3000K to about 8000K, about 3000K, about 4000K, about 5000K, about 6000K, about 7000K or about 8000K.

42. The system of any one of claims 30 to 41 , wherein the metal source is a solid, a liquid, or a gas.

43. The system of any one of 30 to 42, wherein the metal source is metal powder.

44. The system of any one of claims 30 to 43, wherein the metal source comprises substantially pure elemental metals, alloys and/or metal salts.

45. The system of any one of claims 30 to 44, wherein the plurality of metals comprises at least 2 metals, at least 4 metals, at least 5 metals, at least 7 metals, at least 9 metals, about 2 to about 10 metals, about 2 to about 6 metals, about 5 to about 10 metals, 2 metals, 3 metals, 4 metals, 5 metals, 7 metals, 9 metals, or 10 metals.

46. The system of any one of claims 30 to 45, wherein the plurality of metals comprises at least 5 metals.

47. The system of claim 46, wherein the MMA NPs are metal high entropy alloy nanoparticles (HEA NPs)

48. The system of any one of claims 30 to 46, wherein the plurality of metals is selected from metals of Groups 1 -15 of the Periodic T able of the Elements.

49. The system of any one of claims 30 to 47, wherein the plurality of metals comprises a refractory metal, optionally the refractory metal is selected from Nb, Mo, Ta, W, V, and mixtures thereof.

50. The system of any one of claims 30 to 47, wherein the plurality of metals is selected from Ni, Co, Cr, Fe, Mn, a refractory metal, and mixtures thereof, optionally the plurality of metal is selected from Ni, Co, Cr, Fe, Mn, Mo, and mixtures thereof, optionally, the plurality of metal comprises Ni, Co, Cr, Fe and Mo, and optionally the plurality of metal comprises Ni, Co, Cr, Fe and Mn.

51 . The system of any one of claims 30 to 50, wherein the injection probe is configured to inject the metal source in combination with a carrier gas.

52. The system of any one of claims 30 to 51 , wherein the diameter of the reaction chamber at the outlet end is about 2 times, about 3 times, or about 4 times the diameter of the reaction chamber at the inlet end.

53. The system of any one of claims 30 to 52, wherein the metal vapour is cooled at a rate of about 10³ K to about 10⁶ K per second, or about 10⁵ K to about 10⁶ K per second.

54. The system of any one of claims 30 to 53, wherein the inlet is an injection probe configured for injecting the metal source into the thermal zone.