System and method for preparing pure metallic nanoparticles using a direct electric arc

Abstract

A method for synthesizing pure metallic nanoparticles (MNPs) uses a direct electric arc process. The method involves arranging a pair of tungsten filament electrodes within a reaction chamber, connected via a graphite rod passed through a quartz tube. An aqueous solution of a metal nitrate precursor, preferably Fe(NO 3 ) 3 ·9H 2 O, is introduced into the quartz tube. An inert gas, preferably argon, is supplied to maintain an oxygen-free atmosphere. A high voltage is applied across the electrodes to generate an electric arc, which creates a localized plasma, vaporizing the metal ions in the solution. The vaporized metallic species are rapidly cooled and condensed in the inert atmosphere, forming metallic nanoparticles with high purity. The resulting nanoparticles are then collected and washed with water to remove residual contaminants. This method provides a scalable, efficient, and environmentally friendly approach to produce MNPs with controlled size and morphology, suitable for various industrial and scientific applications.

1 . A method for preparing pure metallic nanoparticles using a direct electric arc, comprising:
 arranging a pair of electrodes within a reaction chamber, wherein at least one of the electrodes is a filament connected to a graphite rod and passed through a quartz tube, wherein said electrodes comprise tungsten filaments;   introducing an aqueous solution of a metallic precursor into the quartz tube within the reaction chamber, wherein said metallic precursor is a metal nitrate salt preferably 0.05-0.25 M Fe(NO 3 ) 3 ·9H 2 O is prepared using deionized water;   introducing an inert gas into the reaction chamber at a flow rate of approximately 10-15 mL/min;   applying a high voltage between the electrodes to generate an electric arc, and creating a plasma by applying a high voltage across the electrodes to vaporise the metal ions and to convert them into MNPs and maintaining the electric arc with a continuous electric current, thereby localized vaporizing the aqueous solution, wherein the electric arc is initiated with a breakdown voltage of approximately 240 V and is maintained with a sustaining voltage of approximately 80-90 V;   cooling and condensing vaporized metallic components escaping the electric arc into an inert atmosphere upon subjecting the vapor to rapid thermal quenching in a cooler argon-filled region, thereby inducing homogeneous nucleation, thereby forming metallic nanoparticles, wherein said inert gas is argon; and   collecting the formed metallic nanoparticles and washing the nanoparticles with water to remove any residual contaminants, wherein the tungsten filament electrodes are arranged such that the inter-electrode gap is precisely maintained at 2.0±0.2 mm using a micrometer-controlled actuator coupled with a servo motor, the actuator being automatically regulated through a closed-loop feedback system receiving real-time arc length measurements from a laser displacement sensor, wherein adjustments are made every 0.2 seconds during discharge to counter electrode erosion, thereby ensuring plasma column stability, uniform arc temperature of 4800-5200 K, and reproducible vaporization of the aqueous precursor droplets; and   wherein the Fe(NO 3 ) 3 ·9H 2 O solution of 0.05-0.25 M concentration is introduced via a peristaltic pump at a controlled rate of 0.35-0.45 mL/min into the quartz tube through a stainless-steel nozzle having a bore diameter of 0.30±0.02 mm and positioned at an angle of 42°±2° relative to the electrode axis, the nozzle being heated externally by a nichrome coil maintained at 90-95° C. to achieve partial evaporation, thereby reducing droplet diameter to below 20 μm prior to entry into the arc discharge zone.   
     
     
         2 . The method of  claim 1 , further comprising:
 cleaning the collected metallic nanoparticles multiple times with water; wherein the electric arc is generated for a duration of approximately 20 minutes; and   adjusting at least one parameter selected from the group consisting of: the cooling rate, the electric arc parameters (current, voltage, duration), the composition of the aqueous solution, and the inert gas flow rate, to control the average diameter of the synthesized Fe NPs.   
     
     
         3 . The method of  claim 2 , wherein initiation of the electric arc is accomplished by applying a high-voltage pulse of 240-250 V for 0.2-0.4 seconds to achieve breakdown, followed by stabilization of the discharge at a sustaining voltage of 80-90 V with a current of 85-95 A, wherein the current is modulated by a pulse-width modulation driver operating at 70±10 Hz with a duty cycle of 50-60%; and wherein the discharge chamber is operated at 0.9-1.1 atm pressure regulated by a back-pressure control valve connected to a downstream vacuum pump, wherein synchronization between precursor feed rate, arc current, and argon flow is achieved by a programmable logic controller that dynamically tunes these parameters every 500 ms, thereby maintaining steady vaporization conditions and preventing incomplete decomposition of precursor droplets. 
     
     
         4 . The method of  claim 1 , wherein the argon carrier gas having 99.999% purity is introduced into the chamber at a flow rate of 12±1 mL/min using a digital mass flow controller with ±1% accuracy, said gas being preconditioned by sequential passage through a silica gel dryer, a 3 Å molecular sieve, and a 0.2 μm PTFE particulate filter, wherein a stainless-steel diffuser plate installed at the inlet ensures laminar distribution across the plasma region. 
     
     
         5 . The method of  claim 1 , wherein vaporized metallic species are quenched in a double-jacketed quartz condensation chamber with inner wall water circulation at 15-20° C. regulated by a PID-controlled chiller, wherein the residence time of vapor within the cooling zone is adjusted to 160-180 ms by maintaining argon velocity at 0.12-0.14 m/s, and wherein the chamber incorporates a conical expansion section angled at 18°+2°; and wherein the condensation chamber inner surface is lined with fused quartz polished to Ra<0.2 μm to minimize heterogeneous nucleation, and wherein the expansion geometry prevents stagnation zones by gradually reducing flow velocity. 
     
     
         6 . The method of  claim 1 , wherein nanoparticle collection is performed by directing the cooled argon-nanoparticle stream onto a stainless-steel mesh collector having pore size 50-70 μm, the mesh being maintained at 5-10° C. using a thermoelectric Peltier cooler. 
     
     
         7 . The method of  claim 2 , wherein the collected nanoparticles are subjected to sequential washing cycles comprising dispersion in deionized water at a ratio of 0.1 g per 100 mL, ultrasonication at 40 kHz for 10-15 minutes to break soft agglomerates, and centrifugation at 8500±500 rpm for 10 minutes to separate metallic nanoparticles from ionic residues, said cycle being repeated at least three times until residual conductivity of wash supernatant is less than 10 μS/cm; and wherein the washed nanoparticles are redispersed in ethanol containing 0.05 wt. % polyvinylpyrrolidone (PVP), the dispersion being rotary evaporated at 50-55° C. under 100-150 mbar vacuum until dryness, followed by vacuum drying at 60° C. for 8-10 hours in a desiccator. 
     
     
         8 . The method of  claim 1 , wherein the electrodes are coated prior to use with a graphite film of 50-100 μm thickness deposited by brushing colloidal graphite suspension followed by drying at 80° C. for 2 hours, the coating reducing tungsten sputtering and preventing incorporation of tungsten impurities in the nanoparticles; and wherein the reaction chamber incorporates a dual-zone cooling jacket, the upstream loop maintaining electrode temperature at 40-50° C. using hot water circulation, and the downstream loop providing quenching at 10-15° C. using chilled water, each loop being independently PID-controlled to ±0.5° C. stability. 
     
     
         9 . The method of  claim 2 , wherein precursor solution is degassed by ultrasonication at 40 kHz for 15 minutes followed by vacuum evacuation at 100 mbar for 5 minutes prior to introduction, said degassing eliminating entrapped bubbles that otherwise destabilize atomization and arc stability; and wherein nanoparticles are washed alternately with ethanol and acetone, each cycle comprising 5 minutes ultrasonication and 10 minutes centrifugation at 10,000 rpm, the alternating polar-nonpolar solvent washing improving removal of carbonaceous impurities and enhancing nanoparticle surface passivation. 
     
     
         10 . The method of  claim 1 , wherein droplet injection into the arc zone is synchronized with plasma oscillations using a phase-locked loop control system that detects plasma light intensity via photodiodes and modulates peristaltic pump speed in-phase with high-energy intervals; and wherein nucleated nanoparticles are passed through a cyclone separator operated at inlet velocity 12-14 m/s and cut size of 15-20 nm, wherein oversized agglomerates are separated into the underflow, while primary nanoparticles are collected in the overflow stream. 
     
     
         11 . The method of  claim 1 , wherein a capping step is employed by dispersing washed nanoparticles in 0.1 wt. % sodium citrate aqueous solution followed by ultrasonication for 20 minutes at 50 kHz, said citrate molecules binding via carboxylate groups to nanoparticle surfaces, imparting electrostatic stabilization and preventing agglomeration in suspension; and wherein the stabilized suspension is freeze-dried by pre-freezing at −40° C. for 6 hours and sublimation under vacuum of 10 −3  mbar for 24 hours. 
     
     
         12 . The method of  claim 1 , wherein the final nanoparticle powder is sieved through a 325-mesh stainless steel sieve to eliminate agglomerates>45 μm, and wherein the sieved powder is packaged in vacuum-sealed laminated aluminum pouches containing oxygen scavenger sachets rated at 50 mL O 2 /g, thereby ensuring stable storage with minimized oxidation; and wherein the condensation chamber is equipped with an acoustic standing-wave generator operating at 20-25 kHz and 0.5-1 W/cm 2 , wherein the acoustic field enhances Brownian dispersion, prevents chain-like agglomerate formation, and promotes spherical nanoparticle morphology. 
     
     
         13 . The method of  claim 2 , wherein the arc chamber walls are periodically passivated by flushing with 0.1 M nitric acid, rinsing with deionized water, and drying at 120° C. for 2 hours, such maintenance preventing contamination by deposited metallic residues and preserving nanoparticle purity over repeated runs; and wherein a neodymium permanent magnet of 0.8-1.0 T is applied externally to the washed nanoparticle suspension in ethanol, thereby magnetically retaining metallic particles while non-magnetic impurities are decanted, and wherein retained particles are re-dispersed and washed, yielding purified metallic nanoparticles with minimal oxide inclusions. 
     
     
         14 . The method of  claim 1 , wherein precursor concentration is dynamically varied between 0.05 M and 0.25 M by blending two reservoirs through a computer-controlled manifold, thereby producing oscillatory supersaturation within the plasma, inducing repeated nucleation pulses, and yielding narrower particle size distribution; and wherein nanoparticle formation is monitored in situ using a laser scattering probe positioned at the condensation outlet, said probe operating at 532 nm with multi-ange detection, wherein scattering signals are fed to a control algorithm that adjusts argon flow and precursor feed in real time, thereby maintaining targeted nanoparticle size distribution within ±10% of set value. 
     
     
         15 . The method of  claim 1 , wherein the annealed nanoparticles are transferred into an inert-atmosphere glove box having oxygen concentration less than 1 ppm and water vapor concentration less than 1 ppm, sealed in borosilicate ampoules under vacuum of 10 −3  mbar, and flame-sealed using a micro-oxygen torch while monitoring internal pressure with a Pirani gauge, thereby ensuring oxygen-free long-term storage; and wherein the plasma discharge is stabilized by introducing a secondary shielding flow of argon gas injected coaxially around the quartz tube at 2-3 mL/min, wherein the shielding flow prevents infiltration of atmospheric oxygen through micro-leaks, reduces convective losses at the plasma boundary. 
     
     
         16 . The method of  claim 2 , wherein the electrodes are cooled by an integrated water-cooled copper jacket surrounding the electrode holders, the cooling water being circulated at 200-250 mL/min and maintained at 25±1° C., wherein said cooling prevents thermal deformation of tungsten filaments during sustained discharge; and wherein an auxiliary hydrogen-containing reducing gas consisting of 95% argon and 5% hydrogen is introduced downstream of the plasma zone at 1-2 mL/min, wherein said reducing gas scavenges oxygen residues during condensation. 
     
     
         17 . The method of  claim 1 , wherein in-situ optical emission spectroscopy is performed during arc discharge by placing a fiber optic probe at 45° inclination to the plasma column, said probe transmitting real-time emission data to a spectrometer calibrated at 200-800 nm, wherein spectral line intensity ratios are monitored to estimate plasma temperature, and wherein precursor feed rate is dynamically adjusted based on said spectral data to maintain vaporization efficiency above 95%; and wherein nanoparticle growth suppression is further achieved by pulsing the precursor feed with an on-off cycle of 2-3 seconds at constant flow rate of 0.35-0.45 mL/min, wherein during the “off” interval, supersaturation in the plasma reduces. 
     
     
         18 . The method of  claim 1 , wherein the condensation chamber is configured with an internal helical flow-guiding insert fabricated from quartz with pitch of 20-25 mm, said insert imparting controlled swirl to the argon flow at Reynolds number below 2000. 
     
     
         19 . The method of  claim 1 , wherein the collected nanoparticles are subjected to low-energy plasma cleaning in a downstream RF plasma chamber operated at 13.56 MHz and 30-40 W power under argon pressure of 0.5-1.0 Torr for 5-10 minutes, wherein such treatment removes surface-bound organic contaminants and enhances metallic surface purity without altering nanoparticle size; and wherein nanoparticle collection efficiency is increased by applying an electrostatic field of 2-4 kV/cm across the condensation chamber, said field being generated by electrodes embedded along the chamber walls, wherein the charged metallic nanoparticles experience electrophoretic migration toward the collection substrate, thereby increasing deposition yield by 20-30%; and wherein prior to initiation of the electric arc, the entire reaction chamber is purged with argon at a high flow rate of 50-60 mL/min for 10-15 minutes, followed by stabilization at 12±1 mL/min during synthesis.