Method for producing highly pure and finely dispersed nanoparticles by local induction heating

Abstract

The purpose of this paper is to introduce a new method for producing high-purity aerosol nanoparticles of well-defined size and chemical composition for use as building blocks in nanomaterial synthesis for various applications. The method employs induction heating to generate metal vapor from a target electrode, which is then removed by a gas flow and cooled, producing atomic clusters and nanoparticles (singlets and aggregates) through nucleation and subsequent growth by condensation and solidification. The novelty of the method relies on the fact that the only material in the heating zone is the target electrode, avoiding any holders that may unintentionally release impurities when heated either directly by the induction heater or indirectly by the heated target electrode. This is highly advantageous because it ensures that only vapor from the target material is generated in the carrier gas, resulting in the formation of atomic clusters and, subsequently, extremely pure nanoparticles. Furthermore, the magnetic field generated by the inductor coil is concentrated at the sharp tip of the electrode, preventing turbulence in the surrounding carrier gas. The concentrated magnetic field ensures very localized heating at the electrode tip and hence the ejection of metal vapor from the electrode tip, while maintaining the laminar nature of the flow around the electrode tip ensures that the resulting downstream vapor has the same history and therefore produces particles of the same size (monodisperse).

Description

[Technical Field]

[0001] The present invention relates to a method for producing highly pure and finely dispersed aerosol nanoparticles, and more particularly to a method for producing aerosol nanoparticles of well-defined size and chemical composition for use as building blocks in the synthesis of nanomaterials for a variety of applications.

[0002] Specifically, the method employs an electromagnetic heater to locally supply heat to the target material to achieve a desired purity level of the produced atomic clusters and nanoparticles. [Background technology]

[0003] The synthesis of nanoparticles of well-defined size and chemical composition is of great importance in the field of nanotechnology, as these nanoparticles are composed of the building blocks of any nanomaterial. The nanoparticles produced have found numerous applications in fields such as electronics, biomedicine, textile production, etc.

[0004] Nanoparticles can be prepared by gas phase synthesis, where saturated dense gases are cooled under specific conditions, leading to the formation of atomic clusters and nanoparticles by nucleation, followed by growth by condensation and aggregation / solidification.

[0005] Specifically, the vapor phase synthesis method for producing nanoparticles has many advantages, including realizing the following: Continuous production of atomic clusters and nanoparticles Good control over the size and composition of atomic clusters and nanoparticles Highly reproducible production process The technique is highly versatile, as it can produce nanoparticles with a wider range of compositions. Minimal or no waste Relatively low operating costs

[0006] For example, aerosol nanoparticles can be produced by different gas-phase synthesis techniques: 1. electrohydrodynamic processes such as electrospraying or atomization, whereby a high-intensity electric field is applied to a liquid precursor solution (i.e., a protein solution), which is used, for example, to create protein-based nanoparticles and nanofibers; the main advantage of this technique is that it avoids any organic solvents and heat treatment; 2. flame techniques such as flame aerosol synthesis of nanoparticles, which can be used, for example, to produce nanoparticles, such as inorganic submicron particles, with tightly controlled morphology and composition; 3. evaporation-condensation techniques using heating or ablation, whereby vapor is produced from a target material by heating, laser ablation, or spark discharge, which is then carried away by a gas flow and cooled to produce atomic clusters and nanoparticles.

[0007] Within these categories, the evaporative condensation technique has significant advantages that may have several industrial applications.

[0008] Furthermore, the aerosol-based evaporation-condensation synthesis method is very attractive because it can produce nanoparticles of extremely high purity, and at the same time, it is a very environmentally friendly method since no by-product waste is formed. These two features make the evaporation-condensation aerosol-based method particularly attractive for industrial applications in general, as evidenced by the fact that several manufacturers in the field of nanotechnology (and nanoelectronics in particular) are already considering its adoption.

[0009] Of the evaporation-condensation methods, the use of a tubular furnace reactor, in which a vapor is generated from the target material and then cooled by a passing gas stream to form aerosol nanoparticles, is the least preferred, primarily because the entire tubular furnace reactor containing the target material must be heated, introducing impurities into the gas stream and, consequently, into the synthesized particles. Even inert materials, such as glass and ceramic, commonly employed in tubular furnaces, have been shown to introduce impurities into the carrier gas and, therefore, the resulting nanoparticles, constituting a major limitation in nearly all applications of nanotechnology.

[0010] In this regard, ablation methods using sparks, arcs or lasers can produce nanoparticles of higher purity because they heat only the target material where the nanoparticles are synthesized, rather than the entire reactor.

[0011] However, a drawback of ablation methods is that they usually require high performance systems (eg, high power lasers or circuits for generating sparks and arcs), which increases manufacturing costs.

[0012] A cost-effective alternative to ablation is to use a wire-shaped target material that is heated by passing a high electric current through it. This method is widely used for research purposes, but has the disadvantage that the wire breaks easily and frequently when heated, which can disrupt nanoparticle production.

[0013] Furthermore, because the release of vapor from the wire is not localized but can occur from many points along the heated section of the wire, the particles in the resulting aerosol do not have the same history and, as a result, the particles are not monodisperse.

[0014] A newer alternative to using electric current to vaporize materials from a conductive target is to use induction heating, in which the target material is heated by an induction heater while a particle-free stream of air flows around the target, sweeping away the resulting vapor and cooling it to form nanoparticles that essentially have the same composition as the target material.

[0015] Compared with nanoparticle generators using electrical heating, the Inductive Heating Nanoparticle Generator (IHNG) has the advantage that it cannot be interrupted, and at the same time, by controlling the operating conditions (i.e., heating and quenching flow rates), a very stable nanoparticle synthesis process can be achieved.

[0016] In U.S. Patent No. 8,362,407 B2 (Apparatus for particle synthesis, 2013), an example of a hot-wall tubular reactor uses induction heating elements to heat the reactor walls. In this case, induction heating is used to heat the reactor walls and provide heat for particle / nanoparticle synthesis. As a result, the device is actually a tubular furnace reactor, which heats more than just the target material, potentially introducing impurities such as those described above into the resulting particles. This places significant limitations on the use of this device for nanomaterial synthesis.

[0017] According to a method claimed in Chinese Patent No. 102,762,492A (Method and Apparatus for Producing Nanoparticles, 2018), for example, an inert gas is supplied to a glass tube with a ceramic high-temperature heat shield mounted on top of a ceramic support structure. A gasification vessel made of heat-resistant metal or graphite is placed inside the heat shield. Outside the glass tube at the vessel location, an induction coil heats the gasification vessel. In addition to the heat shield, a cool inert air flow moving through the tube prevents other parts of the device from overheating. The method disclosed in the above patent application uses a holder for placing the material to be gasified, such as a heat shield mounted on top of a ceramic support structure, so that impurities can be directly induced by the induction heating coil or indirectly induced by heating the target material to be gasified.

[0018] Generally, in methods for producing nanoparticles that use a holder to hold a target material from which vapor is produced, the holder is heated along with the target material and, because they are attached together, vapors are formed that can mix with the vapor of the target material, thereby contaminating the resulting particles.

[0019] Even inert materials such as glass and ceramics, which are often employed in tube furnaces, have been shown to introduce impurities into the carrier gas and, therefore, the resulting nanoparticles, which constitutes a major limitation in almost all applications of nanotechnology.

[0020] In order to avoid as much as possible the contamination and impurities in the carrier gas and therefore obtain the maximum purity of the nanoparticles produced, it is not necessary to evaporate any components other than the target material in the heating zone, and therefore it is desirable to avoid placing any support or holder in the heating zone and place only the material that needs to be evaporated in the glass tube.

[0021] Furthermore, to obtain a laminar aerosol nanoparticle flow, it is necessary to suppress the aggregation of nanoparticles.

[0022] In accordance with the above, a nanoparticle production method based on evaporation-condensation aerosol-based IHNG is highly desirable, as it overcomes the above technical problems and can produce nanoparticles with extremely high purity and as uniform as possible in shape and size. Summary of the Invention

[0023] A specific object of the present invention is to provide an evaporation-condensation method for producing finely dispersed aerosol nanoparticles, which involves heating a target material in a localized manner to produce a vapor, and then cooling the vapor to form atomic clusters and extremely pure nanoparticles.

[0024] According to an aspect of the present invention, a method for generating an aerosol of highly pure and finely dispersed nanoparticles is disclosed, said method comprising providing an inlet region into which a carrier gas is introduced, an outlet region through which the generated aerosol nanoparticles are discharged, a glass tube containing a target material in the form of a conductive electrode, and locally heating the target material by induction heating to generate a vapor which is removed from the electrode in the region through a carrier gas flow, and thereafter forming atomic clusters, singlets, and aggregated nanoparticles.

[0025] According to the method, the step of locally heating the target material includes a step of providing a coil-shaped induction heater concentrically surrounding the glass tube, concentrating the magnetic field generated by the induction heater mainly in a region of the target material, said region being configured, for example, in the shape of an electrode tip, and determining heating of said region of the electrode.

[0026] According to another aspect of the present invention, a method for producing monodisperse aerosol nanoparticles at high purity levels is disclosed, comprising a glass tube including an inlet region including a laminarizer into which a carrier gas is taken and an outlet region through which the produced aerosol nanoparticles are discharged, a target material in the form of a conductive electrode configured to ensure that the carrier gas flow remains laminar as it passes around it, and locally heating the target material by induction heating to produce a vapor cloud which is removed in a laminar flow through the carrier gas flow and maintains the same history as it flows further downstream to produce particles of the same size.

[0027] According to another aspect of the present invention, a method for producing monodisperse aerosol nanoparticles with high purity levels is disclosed, wherein the formation of agglomerated nanoparticles is suppressed by diluting the vapor cloud flow immediately downstream of the target electrode.

[0028] According to an embodiment of the present invention, dilution of the vapor flow immediately downstream of the target electrode is achieved by providing a specific opening in the glass tube in the region immediately downstream of the target electrode through which the dilution gas flow is introduced.

[0029] According to another embodiment of the present invention, a method for producing monodisperse aerosol nanoparticles with high purity levels is disclosed, which suppresses the formation of nanoparticle agglomerates by heating the resulting agglomerated nanoparticles to melt them into spherical particles.

[0030] According to a further embodiment of the invention, heating the resulting aggregated nanoparticles to melt them into spherical particles is achieved by adding a second induction heating zone downstream of the target electrode, said second induction heating zone having the effect of heating the aggregated nanoparticles.

[0031] A major advantage of this method is that the flow is restricted using materials such as glass or ceramic that are not heated by induction and do not come into contact with the heated target material, thus avoiding the formation of impurities in the carrier gas and producing nanoparticles of extremely high purity.

[0032] Another advantage of the disclosed nanoparticle production method is its ability to produce highly monodisperse spherical particles with sizes spanning a very wide range (i.e., from less than 1 nm to several microns). This is achieved by maintaining a very localized emission point on the target material and the laminar nature of the carrier gas flow as it circles the electrode on the target material, thus ensuring that the resulting vapor cloud has the same history and produces particles of the same size as it flows further downstream.

[0033] All features of the disclosed method according to the present invention are defined in the appended claims.

[0034] It should be understood that the specification provided in conjunction with the foregoing and following detailed description and drawings is merely exemplary of the invention and is intended to provide an overview for understanding the nature and character of the invention as claimed. [Brief explanation of the drawings]

[0035] For a better understanding of the present invention, preferred embodiments will now be described with reference to the accompanying drawings, which are intended purely by way of example and are not to be construed as limiting. [Figure 1] 1A-1C are cross-sectional side views illustrating the operating principle of a method according to an embodiment of the present invention. [Figure 2]10A-10C are cross-sectional side views illustrating how the shape of an electrode tip can ensure laminar flow of the carrier gas as it moves around the electrode tip, in accordance with an aspect of the present invention. [Figure 3] 10 graphically depicts nanoparticle size distribution measurements as a function of operating flow rate (i.e., 1, 10, and 30 lpm). [Figure 4] 5A and 5B are cross-sectional side views illustrating the operating principle of the method according to a second embodiment of the present invention; [Figure 5] 10A and 10B are cross-sectional side views illustrating the operating principle of a method according to a third embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0036] The following discussion is presented to enable any person skilled in the art to make and use the present invention. Various modifications to the embodiments will be readily apparent to those skilled in the art without departing from the scope of the invention as claimed. Thus, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims.

[0037] As used herein, the term "nanoparticle" refers to particles having a diameter smaller than 300 nm (nanometers), while "aerosol nanoparticle" refers to nanoparticles synthesized by aerosol-based techniques and therefore suspended in gas for a sufficiently long time.

[0038] As used herein, the term "induction heating" (referred to herein as IH) refers to the process of heating an electrically conductive material by the induction of eddy currents.

[0039] As used in the present invention, the term "inductively heated nanoparticle generator" (referred to herein as IHNG) refers to a device that generates aerosol nanoparticles by using IH to vaporize a substance from a conductive target electrode.

[0040] 1, an IHNG used to generate aerosol nanoparticles according to a first embodiment of the present method includes a target material in the form of a conductive electrode (1) with a locally heated tip (2), a glass tube (3), a coiled induction heater (4), a laminar flow device (5), and a region where vapor (6) generated by the electrode generates atomic clusters that then grow to form singlets (7) and ultimately aggregate nanoparticles (8). The glass tube (3) has an inlet region (9) for introducing a carrier gas and an outlet region (10) through which the generated nanoparticles (in the form of an aerosol) are discharged.

[0041] To achieve laminar flow throughout the system, a laminarizer (5) is employed after the carrier gas inlet region (9), consisting of a device containing multiple holes configured to allow carrier gas to flow through the holes.

[0042] FIG. 1 shows a schematic representation of the operating principle of the various steps employed in the method for synthesis of aerosol nanoparticles according to a first embodiment.

[0043] First, vapor is generated from a conductive target material that needs to be evaporated by induction heating. Referring to FIG. 1, the target material, configured in the form of an electrode (1), is contained within a glass tube (3) and locally heated by an induction heater (4), which is a coiled tube concentrically surrounding the glass tube (3) and the target material (1). As a result of the localized heating, the target material (1) generates a vapor cloud, which is removed by a pure carrier gas flow (9) introduced through a laminar flow device (5) on one side of the glass tube (3). The vapor cloud is then cooled and generates atomic clusters (6), singlet nanoparticles (7), and aggregated nanoparticles (8) in the opposite region of the glass tube (3).

[0044] According to this method, no support or holder for the target material is used, thus avoiding the production of impurities that could contaminate the produced nanoparticles.

[0045] The fact that the target material is heated locally is believed to be a unique advantage of this method for producing nanoparticles. In fact, because only the target material (electrode) is heated just below its melting point, the possibility of producing any impurities that may arise from other materials heated beyond the target electrode is avoided. As a result, the nanoparticles produced according to this method are produced solely from vapor generated from the target material, and we believe that the purity level of said nanoparticles is high.

[0046] An essential feature of the method employed in accordance with the present invention is the localized heating applied to the target material, which is achieved by configuring the IHNG apparatus as follows. 1. The target material is configured into the shape of an electrode (1) with a sharp tip (2). 2. The tip of the electrode (1) is placed in line with the center line of the induction heater (4), where the magnetic field is concentrated and strongest. 3. Avoid using other elements made of ferromagnetic materials inside the glass tube (3).

[0047] In this way, the vapors produced by the heated target material have the same origin and therefore upon natural cooling (brought about by the carrier gas) nanoparticles of the same size (i.e., with a fairly narrow size distribution) are produced, as shown by the measurements provided in Figure 3.

[0048] To further narrow the particle size distribution and therefore increase the size uniformity (monodispersity) of the resulting particles, the tip (2) of the target electrode (1) can be shaped to ensure laminar flow of the carrier gas around it.

[0049] Referring to FIG. 2, in accordance with an embodiment of the present invention, the electrode tip (2) is formed with a very smooth shape so that the carrier gas moves around it in smooth streamlines (11) and avoids any turbulence in the gas flow to ensure that the carrier gas flow around the electrode is laminar and remains laminar as it approaches the electrode tip.

[0050] In this way, all vapors are generated from the same point (i.e., the electrode tip) and therefore have the exact same history (which depends on the residence time in the IHNG), resulting in the production of highly monodisperse nanoparticles.

[0051] By varying the temperature of the target material and the carrier gas flow rate, nanoparticles of different sizes can be produced. This has been demonstrated in a series of preliminary experiments showing that the average particle size can vary from about 1 nanometer to over 100 nanometers depending on the temperature and carrier gas flow used. Measurements were performed using a scanning mobility particle size measurement system consisting of a nano-differential mobility analyzer and a condensation particle counter connected to the outlet of the INHG.

[0052] Figure 3 shows the results of experiments conducted by applicant, presenting three different curves showing particle concentration (expressed in arbitrary units) versus particle size (expressed in nanometers) at different operating carrier gas flow rates (i.e., 1, 10, and 30 liters per minute [lpm]). The particle concentration distribution exhibits a log-normal profile, with a peak corresponding to the mean value of the distribution, representing the mean particle size. As shown in Figure 3, the mean particle size varies from approximately 4 nanometers to 70 nanometers, corresponding to carrier flow rates ranging from 1 to 30 lpm.

[0053] According to another aspect of the present invention, a method for producing monodisperse aerosol nanoparticles at high purity levels is disclosed, which suppresses the formation of agglomerated nanoparticles, a typical phenomenon inherent in practically all evaporation-condensation methods of producing aerosol-based nanoparticles.

[0054] The second and third embodiments of the method can be used for this purpose.

[0055] According to a second embodiment of the method, nanoparticle aggregation can be suppressed by diluting the vapor downstream of the target electrode, resulting in the formation of monodisperse singlet nanoparticles with diameters in the range of 10-15 nm, depending on the substance used as the target material.

[0056] Referring to Figure 4, a dilute carrier gas flow is introduced through a specific opening (12) made in the glass tube in the region just downstream of the target electrode tip, thus suppressing nanoparticle aggregation and controlling the size of the generated singlet nanoparticles.

[0057] According to a third embodiment of the present method, referring to FIG. 5, first and second induction heaters are employed, and a first coil (4) and a second coil (13) are introduced concentrically around the glass tube (3) at the tip of the electrode for generating vapor from the target electrode. The first and second induction heaters are disposed downstream of the first coil (4), and gradually heat and melt the formed aggregated nanoparticles to form partially confined aggregates (14), which then cool to form warm spherical solid particles (15) and then cool spherical solid particles (16).

[0058] Finally, it will be apparent that the present invention is susceptible to numerous modifications and variations, all of which are within the scope of the invention as defined in the appended claims.

Claims

1. 1. A method for producing highly pure and finely dispersed aerosol nanoparticles, comprising: providing a glass tube having an inlet region and an outlet region with a laminarizer; providing a coil-shaped induction heater concentrically surrounding the glass tube; providing a target material in the form of a conductive electrode having an electrode tip, the electrode tip being located on a centerline of the induction heater, the centerline coinciding with a point of main strength of a magnetic field generated by the induction heater; drawing a carrier gas through the laminarizer at the entrance region of the glass tube; locally heating the target material to a temperature close to but below its melting point by concentrating the magnetic field generated by the induction heater at the electrode tip of the conductive electrode to generate vapor and atomic clusters; generating singlet nanoparticles and aggregated nanoparticles by allowing the vapor and atomic clusters to cool naturally as they are carried by a carrier gas flow; Discharging the singlet nanoparticles and the aggregated nanoparticles in the form of an aerosol through the exit region.

1. A method for producing highly pure and finely dispersed aerosol nanoparticles, comprising:

2. 2. The method of producing highly pure and finely dispersed aerosol nanoparticles as described in claim 1, wherein the electrode tip is sharp to ensure that the carrier gas flow is laminar and remains laminar as the carrier gas flows near the sharp electrode tip.

3. 3. The method of producing highly pure and finely dispersed aerosol nanoparticles according to claim 1 or 2, wherein the singlet nanoparticles and the aggregated nanoparticles have an average size that varies from sub-nanometer dimensions up to 100 nanometers.

4. 3. A method for producing highly pure and finely dispersed aerosol nanoparticles according to claim 1 or 2, wherein any formation of aggregated nanoparticles is suppressed, resulting in the production of monodisperse singlet nanoparticles having a spherical shape with a diameter in the range of 10-15 nm.

5. 5. The method of producing highly pure and monodisperse aerosol nanoparticles of claim 4, further comprising the step of diluting the carrier gas flow downstream of the target electrode tip.

6. 6. The method for producing highly pure and monodisperse aerosol nanoparticles as described in claim 5, wherein the step of diluting the carrier gas flow is carried out by further introducing carrier gas through a specific opening made in the glass tube in the region immediately downstream of the electrode tip.

7. 5. The method for producing highly pure and monodisperse aerosol nanoparticles according to claim 4, wherein the induction heater comprises a first induction coil concentrically surrounding the glass tube in the region of the electrode tip, and a second induction coil concentrically surrounding the glass tube in a region located downstream of the first induction coil, and gradually heats and melts the formed nanoparticle aggregates, forming spherical solid particles upon natural cooling.

8. a glass tube having an inlet region and an outlet region with a laminarizer; an induction heater having at least one induction coil concentrically surrounding the glass tube; a target material in the form of a conductive electrode having an electrode tip, the electrode tip being located on the centerline of the induction heater, the centerline coinciding with the point of main strength of the magnetic field generated by the induction heater; 10. An apparatus for carrying out the method of claim 1, comprising:

9. 5. An apparatus for carrying out the method of claim 4, further comprising a specific opening made in the glass tube in a region immediately downstream of the electrode tip for diluting the carrier gas flow to suppress the formation of the agglomerated nanoparticles.

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