GAS-PHASE PRODUCTION OF ALIGNED METAL NANOPARTICLES USING EXTERNAL MAGNETIC FIELDS

A method and system are disclosed of assembling metal particles into nanoparticles. The method includes electromagnetically levitating the metal particles; inductively heating the electromagnetically levitated metal particles beyond their melting point into metal droplets; and wherein an evaporation flux achieved at a surface of the metal droplets result in a supersaturation of metal atoms around the metal droplets leading to nucleation and growth of the nanoparticles.

TECHNICAL FIELD

The present disclosure generally relates to a method and system of assembling metal particles into nanoparticles.

BACKGROUND

Metal nanoparticles and their assemblies are being explored as functional components in sensors, plasmonics, energetic composites, electronics, and catalytic materials. Demands for devices and composites based on metal nanoparticle systems have fostered the development of scalable synthetic approaches that can assemble metal nanoparticles into well-defined structures and arrangements. The structure and arrangement of metal nanoparticles in organized assemblies show collective properties that depend on size, shape, and surface properties of aggregates having applications in imaging, sensing, and photocatalysis. These properties are also the deciding factors in modulation of material properties such as packing density, porosity, and mechanical strength in composite structural materials such as aerogels. Aligned nanoparticle chains have high aspect ratios and high surface area-volume ratio characteristics that have shown to have important applications in surface-sensitive applications such as catalysis and sensors. Therefore, particle production techniques that are scalable and capable of morphological control of aggregate architecture and arrangement are highly desirable. Currently, controlled assembly has been limited to traditional colloidal phase routes that employ surface capping to stabilize metal nanoparticles and prevent irreversible, random aggregation. These stabilized primary particles can be used as building blocks to engineer the formation of complex aggregates with desired architecture in a stepwise manner. However, such multi-step control is impossible in high purity gas-phase synthesis as the particles aggregate instantaneously after nucleation and formation of primary particles. As a result, while directed assembly of particles is common in colloidal chemistry, it has rarely been explored in aerosol-based synthesis. In addition, solution phase syntheses use multi-step processes that involve ligands, surfactants, and hazardous solvents, that require additional purification steps, thereby limiting their scalability.

In this regard, gas-phase synthesis approaches are particularly attractive as they not only allow for continuous particle production but also circumvent the need for surfactants or ligands, thus enabling direct, scalable production of high purity metal nanoparticles. However, a major limitation of gas-phase synthesis is the associated difficulty in directing the assembly of metal nanoparticles in a controlled manner, as Brownian forces cause random aggregation. As a result, commercially available nanoparticles generated by gas-phase synthesis such as laser ablation and sputtering are randomly aggregated without any well-defined microstructural features.

Levitation-flow technique is an alternative gas-phase technique for metal nanoparticle synthesis, however, so far particle characterization has been mostly limited to synthesis and bulk-characterization of nanoparticles, while the control on aggregate architecture and controlled assembly have been vastly neglected.

SUMMARY

In accordance with an exemplary embodiment, a method is disclosed of assembling metal particles into nanoparticles, the method comprising: electromagnetically levitating the metal particles; inductively heating the electromagnetically levitated metal particles beyond their melting point into metal droplets; and wherein an evaporation flux achieved at a surface of the metal droplets result in a supersaturation of metal atoms around the metal droplets leading to nucleation and growth of the nanoparticles.

In accordance with another exemplary embodiment, a system is disclosed for assembling metal particles into nanoparticles, the system comprising: an electromagnetic levitation coil, the electromagnetic levitation coil configured to electromagnetically levitate the metal particles and inductively heat the electromagnetically levitated metal particles beyond their melting point into metal droplets; and wherein an evaporation flux achieved at a surface of the metal droplets result in a supersaturation of metal atoms around the metal droplets leading to nucleation and growth of the nanoparticles.

DETAILED DESCRIPTION

In accordance with an exemplary embodiment, an electromagnetic levitation technique is disclosed, which uses magnetic fields to levitate and inductively heat metal pieces, that result in metal evaporation and formation of nanoparticles in the gas phase. In addition, the applied field has an additional aligning effect, for example, on ferromagnetic metals, such as Fe and Ni. For Fe and Ni, the magnetic field interacts with the generated particles to form chain assemblies, for example, composed of less than 20 nm particles, which effect has not been observed for non-ferromagnetic materials. Thus, employing an external magnetic field during particle formation leads to controlled formation of chain aggregates with a relatively high aspect-ratio and surface area.

In accordance with an exemplary embodiment, since the process is a continuous, gas-phase technique, the process can be scaled up and nanochains can be produced in a scalable manner. In addition, bulk powders can be generated, which retain their chainlike morphology, which can allow for the commercial manufacturing of nanopowders composed of high surface-area metal nanochains, and which materials can be used in optoelectronics, biomedical imaging, sensing, catalysis, and as filtration and purification materials.

Features of Electromagnetic Levitation Synthesis:

Gas-phase synthesis techniques offer a scalable approach to production of metal nanoparticles, however, directed assembly has been challenging due to fast particle diffusion rates that lead to random Brownian aggregation. In accordance with an exemplary embodiment, a method and system are disclosed that allows for directionality and control of nanoparticle assembly in the gas phase, which can be achieved by employing an external magnetic field from the levitation coils during particle formation such that directional interactions with the H-field compete with random particle aggregation.

In accordance with an exemplary embodiment, an electromagnetic-levitation technique is disclosed in which the particle formation occurs in the presence of a relatively strong magnetic field. In addition to levitation and induction heating, the external magnetic field can be applied to compete with random Brownian forces, which enables the formation of stringy, chain structures as shown inFIGS.1,2a, and2b. As shown inFIG.1, the ferromagnetic metals (Fe, Ni) form chain-like aggregates120, while Cu forms compact nanoparticle aggregates122. In accordance with an exemplary embodiment, a method and system for selective, on-the-fly, gas-phase assembly of particles with morphologically different microstructural features is disclosed. In addition, the process is solvent and ligand-free, continuous, and allows for gas-phase fabrication of high-purity metal nanoparticles with tunable micro-structural features, thus opening various possibilities for density, mechanical, and optical property modulation in the final materials of interest.

Details of the System:

FIG.2ashows an illustrative system200for electromagnetic levitation and heating of metal droplets110(FIG.1) to generate metal nanoparticles120,122in accordance with an exemplary embodiment. As shown inFIG.2a, the system200includes a levitation coil210wrapped around a tubular member212. The tubular member212having an inlet211and an outlet213, and wherein the inlet211is configured to receive a carrier gas114. The tubular member212can be, for example, a quartz tube214. The system200can also include a pyrometer216, for example, a two-color pyrometer, which is configured to monitor a surface temperature of the heated metal droplets110within the levitation coil210. The metal nanoparticles120,122exit the tubular member212via the outlet213.

In accordance with an exemplary embodiment, the system200is used to superheat the metal droplets110around the melting points of the metal droplets110. The levitation coil210, for example, can be made from copper tubing. In accordance with exemplary embodiment, the copper tubing, for example, has an outer diameter (O.D.) of approximately ⅛″ with a wall thickness of 1/32″, and insulated, for example, with fiberglass tape. The levitation coil210is made to fit around the tubular member212(i.e., quartz tube214). The quartz tube214can have, for example, an outer diameter (O.D.) of approximately ⅝″ and the levitation coil210is tightly wound around the quartz tube214. For example, the levitation coil210can include 11 turns creating a field in an upward direction and 2 turns creating a field in a downward direction as further described in connection withFIG.6. In accordance with an embodiment, a 10 mm gap between the arranged between the 11 turns and the 2 turns, which separates the fields in the upward and the downward directions. The levitation coil210can be water cooled and powered, for example, by a 20-kW high frequency (8 MHz) generator.

In accordance with an embodiment, a titanium getter, heated at 800° C., can be employed to purify the carrier gas114. The carrier gas114, for example, can be He and Ar. The carrier gas114preferably has a purity of approximately 99.999% to help prevent oxidation of the superheated metal droplets220. The carrier gas114is then fed through the quartz tube216to carry the metal nanoparticles120,122through the tubular member212for collection at the outlet213of the tubular member212. In addition, a type of carrier gas114can be selected to help control the temperature of the metal droplets220within the levitation coil, and corresponding characterizations of the nanoparticles120,122.

In accordance with an exemplary embodiment, metal pieces, for example, bulk metal pieces can be cut and weighed according to the desired diameter of the metal droplet110suitable for the quarts tube216(e.g., a glass tube) housing in the levitating system200. In accordance with an embodiment, droplet diameters, for example, can be approximately 6 mm to 10 mm, with their corresponding mass of the metal pieces being approximately 2.58 g (Cu, Ni) and approximately 2.47 g (Fe). In accordance with an embodiment, the metal pieces can be ultrasonicated in acetone, for example, for approximately 15 minutes to remove surface impurities.

In accordance with an exemplary embodiment, the metal pieces are then introduced into the tubular member212with the levitation coil212, where the metal pieces are levitated and heated to temperatures beyond their melting point. For example, as shown inFIG.2b, the metal pieces250at temperatures in excess or beyond their melting point will assume a spherical shape due to surface tension. Fe droplets can assume a spherical shape at approximately 1750 K.

In accordance with an exemplary embodiment, the droplet temperature can be modulated by varying the field strength of the levitation coils and based on the type of the carrier gas114(for example, He or Ar), and wherein the carrier gases114also function as a cooling gas. In addition, the carrier gas114is preferably maintained at a relatively constant flow around the metal droplet110. A pyrometer216, for example, a two-color pyrometer can continuously monitor the surface temperature of the heated droplets110. The pyrometer216can be calibrated by levitation-heating and cooling of standard metal pieces (Cu, Mn, Fe, Ni, and Ti), and using the recalescence point at the known melting points of the metal pieces.

FIG.3shows the X-Ray diffraction patterns300of the nanoparticles120,122, for example, metal powders collected from the levitation system200showing that the nanoparticles120,122, emanating from the levitated droplets110are in a metallic phase.

FIG.4illustrates typical aggregates400and their corresponding fractal dimensions410(Df) for Fe, Ni, and Cu obtained from their respective metal droplets at 1940K, 1900K, and 1640K, respectively (scale bar: 100 nm), and wherein Fe and Ni nanoparticles120tend to form string-like aggregates with lower Df than that for the Cu nanoparticles122.

FIG.5ais a schematic illustration500of a levitated metal droplet110in the magnetic field130arising from the levitation coils120. As shown inFIG.5a, the levitation coils210include a plurality of coils220, for example, 11 turns creating a magnetic field in an upward direction, and a number of turns 222, for example, 2 turns creating a field in a downward direction. In accordance with an embodiment a gap224, for example, 10 mm, separates the upward direction and the downward direction.

FIG.5bis a chart510illustrating an estimated H-field interaction parameter, m, as a function of distance along coil-axis (z-axis) for different peak magnetic field strengths of the levitation coils, and wherein m=200 kBT0represents the transition from Brownian coagulation to magnetic-field interaction-dominated aggregation. Results from a Monte-Carlo simulation suggests a minimum value of magnetic field strength of approximately 217 kA/m for aligned aggregates to be formed, as shown inFIG.5b. The magnetic field strengths, for example, as shown inFIG.5b, ranges from approximately 110 kA/m to 340 kA/m. Therefore, interparticle forces can be induced by the high external magnetic field employed in the levitation system200, which are sufficient to compete with Brownian diffusion forces, resulting in elongated chain aggregates values for Fe and Ni, as disclosed. As shown inFIG.6, the levitation system600can consist of two sets of coaxial coils carrying currents with a 180-degree phase difference, resulting in magnetic fields along the coil z-axis in opposite directions. The upper coil610(c1) may be composed of a single coil (e.g., 2-turns) and generates a magnetic field in the downward direction630(towards gravity). The lower coil620may be composed of two coaxial coils622,624(e.g., c2-7 turns and c3-4 turns, respectively) and creates a field in the upward direction632(against gravity). By convention, the field in the upward z-direction is depicted as positive. The upper coil610and the lower coil620can be separated by a gap640, for example, a 10 mm gap. In order to achieve relatively high temperatures, the droplet110is levitated closer to the lower coil620than the upper coil610as shown.

FIGS.7a-7bshows SEM images700,710of two kinds of morphologically distinct materials generated from levitated metal droplets of Fe and Cu. As disclosed, due to the chainlike and elongated morphology of Fe aggregates induced by the magnetic field, the material deposited from Fe droplets also exhibit an open and porous structure, while the structure for copper appears rather compact and dense. In other words, the morphological features of the aggregates are mirrored in the bulk powder material at the micron-scale as well.

In accordance with an embodiment, the production rate of Fe nanochain aggregate powders can be estimated, for as follows. For example, for a 6 mm diameter droplet levitated at 2000° C., the mass of nanopowder collected can be approximately 110 mg/h. For a larger droplet, for example, a droplet having approximately 60 mm diameter in a commercial reactor, mass evaporation and production rate would scale according to the surface area be approximately 11 g/h. Assuming typical power consumption including electricity and a production time, a significant reduction in production times and manufacturing costs can be obtained.

Traditional colloidal techniques for synthesis of assembled nanoparticles use multi-step processes involving hazardous solvents and surfactants that require additional purification steps. The presented electromagnetic levitation system200, on the other hand, is single step, continuous, avoids use of hazardous solvents, and generates assemblies of high purity nanometals that are ligand or surfactant free. Thus, these characteristics make this technique for particle assembly more scalable and facile to generate large quantities of metal particle chain assemblies.

In accordance with an embodiment, a magnetic field can be applied during particle formation to form chain structures in other gas phase synthesis methods, such as horizontal evaporation process800as shown inFIG.8. As shown inFIG.8, the horizontal evaporation process800can include a tubular member810, for example, a quartz tube, a carrier gas820, a heating source830, induction coils840,842, and a collector850. The carrier gas820is preferably an inert gas, for example, He or Ar. The heating source830can be, for example, a laser. In accordance with an exemplary embodiment, the induction coils840,842, can provide both heating and an alignment field. In accordance with an alternative embodiment, the heating can be provided or augmented by other sources, such as lasers, followed by magnetic field for alignment, which can enable non-metallic vaporization.