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

Abstract

The invention aims to introduce a novel method for generating high-purity aerosol nanoparticles which are clear in size and chemical composition and can be used as a footstone for the synthesis of nanomaterials for various applications. The method employs induction heating to produce metal vapors from a target electrode, followed by being taken away by a gas stream and cooled, generating atomic clusters and nanoparticles (singlet and agglomerated) through nucleation and subsequent condensation and solidification growth. The method is novelty in that the sole material in the heating zone is the target electrode, thereby avoiding any holder that may unexpectedly release impurities when heated directly by an induction heater or indirectly by the heated target electrode. This is very advantageous because it is ensured that only the vapor from the target is generated in the carrier gas, thereby forming atomic clusters and subsequently forming very high purity nanoparticles. In addition, the magnetic field generated by the induction coil is focused at the tip of the electrode, and the tip of the electrode is shaped to be pointed, so that carrier gas flowing around the tip of the electrode does not form turbulent flow. The focused magnetic field ensures local heating of the electrode tip, thereby releasing metal vapors. Meanwhile, by maintaining the laminar flow around the electrode tip, it is ensured that the steam produced downstream has the same trajectory, thereby producing particles of the same size (monodisperse).

Description

Technical Field

[0001] The present invention relates to a method for generating high-purity and finely dispersed aerosol nanoparticles, and more particularly to a method for generating aerosol nanoparticles with well-defined size and chemical composition for use as building blocks for the synthesis of nanomaterials for various applications.

[0002] Specifically, the method uses electromagnetic heaters to locally deliver heat to the target material to achieve the desired purity level of generated atomic clusters and nanoparticles. Background Art

[0003] In the field of nanotechnology, the synthesis of nanoparticles with well-defined size and chemical composition is crucial because they are the building blocks of any nanomaterial. The prepared nanoparticles have a wide range of applications in fields such as electronics, biomedicine, and textile production.

[0004] Nanoparticles can be prepared by gas-phase synthesis, which involves cooling a saturated, high-density gas under specific conditions to form atomic clusters and nanoparticles through nucleation, followed by growth through condensation and agglomeration / coagulation.

[0005] Specifically, the method of preparing nanoparticles by gas phase synthesis has many advantages, including providing:

[0006] Continuous production of atomic clusters and nanoparticles;

[0007] Excellent control over the size and composition of atomic clusters and nanoparticles;

[0008] The production process is highly repeatable;

[0009] The technology is highly versatile, as it can produce nanoparticles with a wider range of compositions;

[0010] Little to no waste generation; and

[0011] Relatively low operating costs.

[0012] Different gas-phase synthesis techniques can produce aerosol nanoparticles, for example:

[0013] 1. Electrohydrodynamic processes, such as electrospraying or atomization, are used to create protein-based nanoparticles and nanofibers and involve applying a high-intensity electric field to a liquid precursor solution (i.e., a protein solution); the main advantage of this technique is that it avoids any organic solvents and thermal treatment;

[0014] 2. Flame techniques, such as flame aerosol synthesis of nanoparticles, which can be used to produce nanoparticles, such as inorganic submicron particles with strictly controlled morphology and composition;

[0015] 3. Evaporation-condensation technology using heating or ablation, which generates vapor from the target material through heating, laser ablation or spark discharge. The generated vapor is then carried away by air flow and cooled to generate atomic clusters and nanoparticles.

[0016] Among these technology categories, evaporation-condensation technology has important advantages and can have various industrial applications.

[0017] Furthermore, aerosol-based evaporation-condensation synthesis methods are attractive because they can produce nanoparticles of extremely high purity and are environmentally friendly, as they do not generate waste byproducts. These two characteristics make aerosol-based evaporation-condensation methods particularly attractive for industrial applications in general, as evidenced by the fact that many manufacturers in the field of nanotechnology, particularly nanoelectronics, are already considering adopting these methods.

[0018] The least popular of the evaporation-condensation methods is the tube furnace reactor, in which a target material generates vapor that is subsequently cooled by a passing gas stream to form aerosol nanoparticles. This is primarily due to the need to heat the entire tube furnace reactor—including the target material—which introduces impurities into the gas stream, leading to contamination of the resulting particles. Even inert materials commonly used in tube furnaces, such as glass and ceramics, have been shown to introduce impurities into the carrier gas, leading to contamination of the resulting nanoparticles, a major limiting factor for nearly all nanotechnology applications.

[0019] In this regard, ablation methods using sparks, arcs, or lasers can produce nanoparticles of higher purity because these methods heat only the target material where the nanoparticles are synthesized, rather than the entire reactor.

[0020] However, a drawback of ablation methods is that they generally require complex systems (e.g., high-power lasers or circuits for generating sparks and arcs), which in turn increases manufacturing costs.

[0021] A cost-effective alternative to ablation methods is to use a target in the form of a wire that is heated by passing a high current through it. While this method is widely used for research purposes, it has the disadvantage that nanoparticle production can be interrupted because the wire breaks easily and frequently when heated.

[0022] Furthermore, since the emission of vapor from the wire is not localized but occurs at many points along the heated portion of the wire, the particles in the generated aerosol have different trajectories and are therefore not monodisperse.

[0023] A novel alternative to evaporating material from a conductive target using electric current is to use induction heating. In this method, the target is heated by an induction heater while a particle-free gas flow around the target carries away the generated vapor and cools it, thus forming nanoparticles that, in principle, have the same composition as the target.

[0024] Compared with nanoparticle generators using electrical heating, the advantage of induction heating nanoparticle generators (IHNGs) is that they cannot be interrupted and a very stable nanoparticle synthesis process can be achieved by controlling the operating conditions (i.e., heating and quenching flow rates).

[0025] In US Patent 8,362,407 B2 (Particle Synthesis Apparatus, 2013), an example of a hot-wall tubular reactor is used, in which induction heating elements are used to heat the reactor walls. In this case, induction heating is used to heat the reactor walls, thereby providing heat for particle / nanoparticle synthesis. Therefore, the apparatus is effectively a tubular furnace reactor, in which components other than the target are also heated, potentially leading to the presence of impurities in the resulting particles, as described above. This poses a significant limitation to the use of this apparatus for nanomaterial synthesis.

[0026] According to the method claimed in CN 102,762,492A (Nanoparticle Preparation Method and Apparatus, 2018), an inert gas is supplied to a glass tube, in which a ceramic high-temperature heat barrier is provided, which heat barrier is provided, for example, on top of a ceramic support structure. A vaporization container made of high-temperature resistant metal or graphite is provided in the heat barrier. Outside the glass tube, at the location of the container, an induction coil heats the vaporization container. In addition to the heat barrier, the cold inert gas flow flowing in the tube prevents other components of the device from overheating. The method disclosed in the above patent application uses a holder for placing the material to be vaporized, similar to the heat barrier provided on top of the ceramic support structure, so that impurities can be introduced directly by an induction heating coil, or indirectly by heating the target material to be vaporized.

[0027] Typically, in a nanoparticle preparation method using a holder to support a target generating vapor, the holder is heated together with the target. Because the holder and target fit closely together, the generated vapor may mix with the target vapor, thereby contaminating the final particles.

[0028] Even inert materials commonly used in tube furnaces, such as glass and ceramics, have been shown to introduce impurities into the carrier gas and, consequently, into the final nanoparticles, limiting progress in nearly all applications of nanotechnology.

[0029] To avoid possible contamination and impurities in the carrier gas and achieve the highest purity nanoparticles, no components other than the target material need to be evaporated in the heating zone. Therefore, a method is needed to place only the material to be evaporated in the glass tube without any supports or holders in the heating zone.

[0030] Furthermore, in order to obtain a laminar aerosol nanoparticle flow, the agglomeration of nanoparticles must be suppressed.

[0031] As mentioned above, there is an urgent need for an IHNG-based aerosol-based evaporation-condensation method for preparing nanoparticles, which can overcome the above technical difficulties and produce nanoparticles with extremely high purity and as uniform shape and size as possible. Summary of the Invention

[0032] A specific object of the present invention is to provide an evaporation-condensation method for preparing finely dispersed aerosol nanoparticles, which method comprises heating a target material in a localized manner to generate vapor, and then cooling the generated vapor to form atomic clusters and extremely high-purity nanoparticles.

[0033] According to one aspect of the present invention, a method for preparing high-purity and finely dispersed aerosol nanoparticles is disclosed, the method comprising: providing a glass tube comprising an inlet region and an outlet region, wherein a carrier gas is introduced into the inlet region and the generated aerosol nanoparticles are discharged through the outlet region; providing a target material in the form of a conductive electrode; locally heating the target material by induction heating to generate vapor, which is carried away from the electrode by a flow of the carrier gas and then flows in a region where the vapor forms atomic clusters, singlet nanoparticles and agglomerated nanoparticles.

[0034] According to the method, the step of locally heating the target material includes: providing a wound coil induction heater coaxially surrounding the glass tube, focusing the magnetic field generated by the induction heater mainly on a certain area of the target material, for example, the area is configured in the form of an electrode tip, and determining to heat the electrode area.

[0035] According to another aspect of the present invention, a method for producing high-purity monodisperse aerosol nanoparticles is disclosed, the method comprising: providing a glass tube comprising an inlet region and an outlet region, the inlet region comprising a laminar flow device, a carrier gas being introduced from the inlet region, and the produced aerosol nanoparticles flowing out from the outlet region; providing a target material in the form of a conductive electrode, the conductive electrode being configured to ensure that the flow of the carrier gas remains laminar as it flows around the conductive electrode; locally heating the target material by induction heating to produce vapor clouds, which are carried away by the flow of the carrier gas in a laminar manner and maintain the same flow history as they flow downstream, thereby producing particles of the same size.

[0036] According to another aspect of the present invention, a method for producing high-purity monodisperse aerosol nanoparticles is disclosed, wherein the formation of agglomerated nanoparticles is suppressed by diluting the vapor cloud flow immediately downstream of a target electrode.

[0037] According to an embodiment of the present invention, the vapor flow immediately downstream of the target electrode is diluted by providing specific openings in the glass tube in the region immediately downstream of the target electrode, and a dilution gas flow is introduced through the specific openings.

[0038] According to another embodiment of the present invention, a method for producing high-purity monodisperse aerosol nanoparticles is disclosed, wherein agglomerate formation of nanoparticles is suppressed by heating and fusing the agglomerated nanoparticles into spherical particles.

[0039] According to another embodiment of the present invention, a second induction heating zone is added downstream of the target electrode to heat and melt the obtained agglomerated nanoparticles into spherical particles. The second induction heating zone has the function of heating the agglomerated nanoparticles.

[0040] A major advantage of this method is that it uses materials such as glass or ceramics to restrict the flow, which are not heated by induction. Because these materials do not come into contact with the heated target, the formation of impurities in the carrier gas is avoided, resulting in extremely high-purity nanoparticles.

[0041] Another advantage of the nanoparticle production method disclosed herein is that it can produce highly monodisperse spherical particles over a very wide size range (i.e., from less than 1 nanometer to several microns). This is achieved by maintaining a very localized emission point on the target and maintaining laminarity in the carrier gas flow as it flows around the target electrode, thereby ensuring that the generated vapor cloud follows the same trajectory as it flows downstream and produces particles of the same size.

[0042] All features of the method disclosed according to the invention are defined in the appended claims.

[0043] It must be understood that the description provided based on the foregoing and following detailed description and drawings is only intended to be exemplary of the present invention and to provide an overview for understanding the nature and characteristics of the invention as claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] For a better understanding of the present invention, preferred embodiments will now be described, which are by way of example only and are not to be considered limiting, with reference to the accompanying drawings, in which:

[0045] · Figure 1 is a cross-sectional side view illustrating the operating principle of a method according to an embodiment of the present invention.

[0046] · Figure 2 1 shows how the shape of the electrode tip according to aspects of the present invention ensures laminar flow of the carrier gas around the electrode tip.

[0047] · Figure 3 Nanoparticle size distribution measurements are graphically shown as a function of operating flow rate (ie, 1 lpm, 10 lpm, and 30 lpm).

[0048] · Figure 4 is a cross-sectional side view of the operating principle of the method according to the second embodiment of the present invention.

[0049] · Figure 5 FIG. 1 is a cross-sectional side view illustrating the operating principle of a method according to a third embodiment of the invention. DETAILED DESCRIPTION

[0050] The following discussion is intended to assist those skilled in the art in making and using the present invention. Those skilled in the art will readily appreciate that various modifications to the embodiments can be made without departing from the scope of the present invention. Therefore, 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 the appended claims.

[0051] For purposes of the present invention, the term "nanoparticles" refers to particles having a diameter of 300 nm (nanometers) or less, while "aerosol nanoparticles" refers to nanoparticles synthesized by aerosol-based techniques and thus can be suspended in a gas for a sufficiently long time.

[0052] As used herein, the term "induction heating" (herein referred to as IH) refers to the process by which a conductive material is heated by inducing eddy currents.

[0053] The term "induction heating nanoparticle generator" (herein abbreviated as IHNG) used in the present invention refers to a device that uses IH (ion heating) to evaporate materials from a conductive target electrode to prepare aerosol nanoparticles.

[0054] Reference Figure 1 According to a first embodiment of the present method, an IHNG for producing aerosol nanoparticles comprises: a target in the form of a conductive electrode (1) having a locally heated tip (2); a glass tube (3); a wound coil induction heater (4); a laminar flow device (5); and a region in which vapor (6) generated by the electrode forms atomic clusters, which subsequently grow to form singlet nanoparticles (7) and ultimately agglomerated nanoparticles (8). The glass tube (3) has an inlet region (9) for introducing a carrier gas and an outlet region (10) for discharging the produced nanoparticles (in the form of an aerosol).

[0055] In order to achieve laminar flow in the entire system, a flow laminarizing device (5) is used after the carrier gas flows into the inlet area (9), and the flow laminarizing device (5) includes a device containing multiple holes and is configured to allow the flow of the carrier gas to pass through.

[0056] Figure 1 The working principles of the various steps applied in the method for synthesizing aerosol nanoparticles according to the first embodiment are schematically represented.

[0057] First, the conductive target material to be vaporized is heated by IH to generate vapor. Figure 1 The target material is designed in the form of an electrode (1) and is accommodated in a glass tube (3). It is locally heated by an induction heater (4), which includes a wound coil tubular member coaxially surrounding the glass tube (3) and the target material (1). After local heating, the vapor cloud generated by the target material (1) is carried away by a flow (9) of a pure carrier gas, which is introduced through a laminar flow device (5) on one side of the glass tube (3), and the carried away vapor cloud is cooled in the other side area of the glass tube (3) to form atomic clusters (6), singlet nanoparticles (7) and agglomerated nanoparticles (8).

[0058] According to the present method, no support or holder for the target is required, thereby avoiding the generation of impurities that may contaminate the generated nanoparticles.

[0059] The localized heating of the target material provides a unique advantage for this method of nanoparticle production. Because only the target material (electrode) is heated below its melting point, impurities that could originate from heated materials other than the target material and electrode are avoided. Consequently, the nanoparticles produced according to this method are generated solely from the vapor generated by the target material and are therefore of high purity.

[0060] The main feature of the method according to the invention is the local heating of the target, which is achieved by configuring the IHNG apparatus in the following way:

[0061] 1. Constructing a target material into an electrode (1) with a tip (2);

[0062] 2. The tip of the electrode (1) is arranged to correspond to the center line of the induction heater (4), where the magnetic field is focused and has the highest intensity;

[0063] 3. Avoid using other components made of ferromagnetic materials in the glass tube (3).

[0064] In this way, the vapor generated by the heated target has the same origin, so nanoparticles of the same size (i.e., with a narrow size distribution) will be produced during natural cooling (induced by the carrier gas), e.g. Figure 3 The measurement results are shown in .

[0065] In order to further narrow the particle size distribution and thus improve the uniformity (monodispersity) of the resulting particle size, the tip (2) of the target electrode (1) can be shaped so that laminar flow of the carrier gas around the tip is ensured.

[0066] See also Figure 2 According to an embodiment of the present invention, in order to ensure that the carrier gas flowing around the electrode is laminar and remains laminar when approaching the electrode tip, the electrode tip (2) is shaped to be very smooth to ensure that the carrier gas flows around the electrode tip in smooth streamlines (11), thereby avoiding any turbulence in the gas flow.

[0067] In this way, since all vapor originates from the same point (ie, the electrode tip), it will have exactly the same flow trajectory (depending on the residence time in the IHNG), thus allowing the production of highly monodisperse nanoparticles.

[0068] By varying the target temperature and carrier gas flow rate, nanoparticles of varying sizes can be produced. This has been demonstrated in a series of preliminary experiments, which showed that the average particle size can vary from approximately 1 nanometer to over 100 nanometers, depending on the temperature and carrier gas flow rate used. These measurements were made by connecting a scanning mobility particle size spectrometer (SMS) to the INHG outlet, which consists of a nano-differential mobility analyzer and a condensed matter particle counter.

[0069] Figure 3The results of an experiment conducted by the applicant are shown, wherein three different curves show the relationship between particle concentration (expressed in arbitrary units) and particle diameter (expressed in nanometers) at different operating carrier gas flow rates (i.e., 1 liter / minute [lpm], 10 liters / minute [lpm], and 30 liters / minute [lpm]). The particle concentration distribution is log-normal, with the peak corresponding to the mean of the distribution, which represents the average particle diameter. Figure 3 As shown, when the flow rate of the carrier gas is changed from 1 lpm to 30 lpm, the average diameter of the particles changes from about 4 nm to 70 nm.

[0070] According to another aspect of the present invention, a method for producing high-purity monodisperse aerosol nanoparticles is disclosed, wherein the formation of agglomerated nanoparticles is suppressed. Agglomeration of nanoparticles is a typical phenomenon inherent in virtually all evaporation-condensation methods for producing aerosol-based nanoparticles.

[0071] The second and third embodiments of the method can be used for this purpose.

[0072] According to a second embodiment of the method, the agglomeration of the nanoparticles can be suppressed by diluting the vapor downstream of the target electrode, thereby forming monodisperse singlet nanoparticles with a diameter ranging from 10 nm to 15 nm, depending on the material used as the target.

[0073] refer to Figure 4 A flow of diluted carrier gas is introduced through a specific opening (12) in the glass tube located in the downstream area of the target electrode tip, thereby suppressing the agglomeration of nanoparticles and controlling the size of the generated singlet nanoparticles.

[0074] According to a third embodiment of the method, with reference to Figure 5 , using a first induction heater and a second induction heater, introducing a first coil (4) at the tip of the electrode in a manner coaxially surrounding a glass tube (3) for generating vapor from a target electrode, and introducing a second coil (13), the first coil (4) being arranged downstream of the first coil (4) for gently heating and melting the formed agglomerated nanoparticles, thereby forming partially confined agglomerates (14) and spherical solid particles (15) that become hot when subsequently cooled, and spherical solid particles (16) that are subsequently cooled.

[0075] Finally, it is obvious that the present invention is susceptible of numerous modifications and variations, all falling within the ambit of the invention as defined in the appended claims.

Claims

1. A method for preparing high-purity and finely dispersed aerosol nanoparticles, the method comprising the following steps: - providing a glass tube (3), said glass tube (3) comprising an inlet region (9) and an outlet region (10), said inlet region (9) having a laminar flow device (5); - providing a wound coil induction heater (4) coaxially surrounding the glass tube (3); - providing a target in the form of a conductive electrode (1) having an electrode tip (2), the electrode tip (2) being located on a center line of the induction heater (4), the center line coinciding with a point of main intensity of the magnetic field generated by the induction heater (4); - a laminarizing device (5) for introducing a carrier gas into the flow passing through the inlet region (9) of the glass tube (3); - locally heating the target material to a temperature close to the melting point of the target material by focusing the magnetic field generated by the induction heater (4) on the electrode tip (2) of the conductive electrode (1), thereby generating vapor and atomic clusters (6); - generating singlet nanoparticles (7) and agglomerated nanoparticles (8) by naturally cooling the vapor and atomic clusters (6) while the vapor and atomic clusters (6) are carried by the flow of the carrier gas; - The singlet nanoparticles (7) and the agglomerated nanoparticles (8) are discharged in the form of aerosol through the outlet area (10).

2. The method for preparing high-purity and finely dispersed aerosol nanoparticles according to claim 1, wherein: The electrode tip (2) is shaped into a pointed shape to ensure that when the carrier gas flows close to the pointed electrode tip (2), the flow of the carrier gas is laminar and remains laminar.

3. The method for preparing high-purity and finely dispersed aerosol nanoparticles according to claim 1 or 2, wherein: The singlet nanoparticles (7) and the agglomerated nanoparticles (8) have an average size that varies from less than 1 nm to greater than 100 nm depending on the temperature and the flow rate of the carrier gas, and more particularly, when the flow rate of the carrier gas varies from 1 lpm to 30 lpm, the average size of the singlet nanoparticles (7) and the agglomerated nanoparticles (8) varies from 4 nm to 70 nm.

4. The method for preparing high-purity and finely dispersed aerosol nanoparticles according to claim 1 or 2, wherein: Any formation of agglomerated nanoparticles is suppressed, thereby producing monodisperse singlet nanoparticles (7) having a spherical shape with a diameter in the range of 10 nm to 15 nm.

5. The method for preparing high-purity and monodisperse aerosol nanoparticles according to claim 4, further comprising the step of diluting the flow of the carrier gas downstream of the target electrode tip (2).

6. The method for preparing high-purity and monodisperse aerosol nanoparticles according to claim 5, wherein: The step of diluting the flow of carrier gas is achieved by additionally introducing carrier gas through a specific opening (12) formed in the glass tube in a region immediately downstream of the electrode tip (2).

7. The method for preparing high-purity and monodisperse aerosol nanoparticles according to claim 4, wherein: The induction heater (4) comprises: a first induction coil (4) coaxially surrounding the glass tube (3) in the region of the electrode tip (2); and a second induction coil (13) coaxially surrounding the glass tube (3) in a region downstream of the first induction coil (4), thereby gently heating and melting the formed nanoparticle agglomerates and forming spherical solid particles after natural cooling.

8. A device for implementing the method according to claim 1, the device comprising: - a glass tube (3) comprising an inlet region (9) of a laminar flow device (5) with a flow, and an outlet region (10); - an induction heater (4) having at least one induction coil coaxially surrounding the glass tube (3); - a target in the form of a conductive electrode (1) having an electrode tip (2), the electrode tip (2) being located on the center line of the induction heater (4), the center line coinciding with the point of main intensity of the magnetic field generated by the induction heater (4).

9. An apparatus for implementing the method according to claim 4, further comprising a specific opening formed in the glass tube (4) in a region immediately downstream of the electrode tip (2) for diluting the flow of carrier gas, thereby suppressing the formation of agglomerated nanoparticles (8).

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