Radio Frequency Induction Thermal Plasma Research Articles

Facile Synthesis of Single-Phase Alpha-Tungsten Nanopowders from Ammonium Paratungstate by RF Induction Thermal Plasma and Thermochemical Reduction

Facile, economic methods of preparing tungsten (W) nanopowder are critically needed to meet industrial demand. Herein, we report a method of preparing single-phase alpha-W (α-W) nanopowders using ammonium paratungstate (APT) as a starting material and the optimum synthesis conditions. The process involves two stages: i) the radio-frequency (RF) induction thermal plasma treatment of APT, followed by ii) thermochemical reduction at 600-900 <sup>o</sup>C. The crystallographic phase and morphological evolution of all products were systematically investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM), and the effects of the annealing temperature on the phase and particle size of the obtained powders were also evaluated. When the RF induction thermal plasma treatment was conducted with and without H<sub>2</sub>, the XRD and FESEM results showed the formation of mixed-phase α- and beta-W (β-W) nanopowder and WO<sub>3</sub> nanopowder, respectively. Single-phase α-W nanopowder was achieved by annealing the WO<sub>3</sub> nanopowder in an H<sub>2</sub> reductive atmosphere at 700 <sup>o</sup>C for 10 min, resulting in homogenous nanoparticles with a small particle size (d50) of 21.16 nm without any aggregation.

Comparison of different tungsten precursors for preparation of tungsten nanopowder by RF induction thermal plasma

Tungsten nanopowders were synthesized using three different precursors: commercial tungsten (W), tungsten trioxide (WO3), and ammonium paratungstate (APT) by radio-frequency (RF) induction thermal plasma in the presence and absence of H2. The crystalline structure, morphology, and particle size of the synthesized tungsten nanopowders were analyzed by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The W nanopowder obtained using APT precursor was found to be the most homogeneous and smallest nanoparticles and showed the largest α-W phase (stable) fraction, which is the preferred phase for tungsten because of its superior properties to those of the β-W phase (metastable).

Radio-frequency thermal plasma-induced novel chainmail-like core-shell MoO2 as highly stable catalyst for converting syngas to higher alcohols

A novel core-shell catalyst with crystalline MoO2 core and chainmail-like amorphous MoOX shell (noted as MoO2-Pla) was prepared by radio-frequency induction thermal plasma method, and its catalytic performance for higher alcohols synthesis from syngas (CO and H2) was compared with that of the MoO2 catalysts synthesized by hydrothermal method and temperature programed reduction method (noted as MoO2-Hyd and MoO2-TPR). It was found that the chainmail-like shell formed under such extreme condition was both active to the reaction and inert to the thermal sintering, which was very fascinating and significantly different from those of the previous reported core-shell catalysts. The unusual amorphous species in the shell increased the CO conversion of MoO2-Pla catalyst by 109 and 213.5%, respectively, compared with MoO2-Hyd and MoO2-TPR. More importantly, the shell formed under such ultra-high temperature (over 3000 °C) and ultra-fast quenching process exhibited ultra-stable chainmail-like particle surface, which endowed the catalyst with high catalytic stability within 300 h of experimental determination. This work provided a new alternative strategy and method for the design of high activity and high stability catalysts.

Design-Oriented Modelling of Different Quenching Solutions in Induction Plasma Synthesis of Copper Nanoparticles

The aim of this paper is to compare the effects of different mechanisms underlying the synthesis of copper nanoparticles using an atmospheric pressure radio-frequency induction thermal plasma. A design oriented modelling approach was used to parametrically investigate trends and impact of different parameters on the synthesis process through a thermo-fluid dynamic model coupled with electromagnetic field equations for describing the plasma behaviour and a moment method for describing nanoparticles nucleation, growth and transport. The effect of radiative losses from Cu vapour on the precursor evaporation efficiency is highlighted, with occurrence of loading effect even with low precursor feed rate due to the decrease in plasma temperature. A method to model nanoparticle deposition on a porous wall is proposed, in which a sticking coefficient is employed to model particle sticking on the porous wall used to carry a quench gas flow into the chamber. Two different reaction chamber designs combined with different quench gas injection strategies (injection through a porous wall for “active” quenching; injection of a shroud gas for “passive” quenching) are analysed in terms of process yield and size distribution of the synthetized nanoparticles. Conclusion can be drawn on the characteristics of each quenching strategy in terms of throughput and mean diameter of the synthesized nanoparticles.

Synthesis of uniform α-Si3N4 nanospheres by RF induction thermal plasma and their application in high thermal conductive nanocomposites.

In this paper, single-crystalline α-Si3N4 nanospheres with uniform size of ∼50 nm are successfully synthesized by using a radio frequency (RF) thermal plasma system in a one-step and continuous way. All Si3N4 nanoparticles present nearly perfect spherical shape with a narrow size distribution, and the diameter is well-controlled by changing the feeding rate. Compact Si3N4/PR (PR = phenolic resin) composites with high thermal conductivity, excellent temperature stability, low dielectric loss tangent, and enhanced breakdown strength are obtained by incorporating the as-synthesized Si3N4 nanospheres. These enhanced properties are the results of good compatibility and strong interfacial adhesion between compact Si3N4 nanospheres and polymer matrix, as large amount of Si3N4 nanospheres can uniformly disperse in the polymer matrix and form thermal conductive networks for diffusion of heat flow.

Preparation of few-layer graphene nanosheets by radio-frequency induction thermal plasma

Graphene nanosheets (GNs) are successfully deposited via a radio frequency (RF) induction thermal plasma pyrolysis process using methane as the precursor. Products are characterized by X-ray powder diffractometer (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and BET measurements. The results illustrate that those few-layer GNs with the number of layer at about 5 and the size at 200–500nm are deposited. The H2 atmosphere in the RF induction thermal plasma is the key factor to fabricate the few-layer GNs. The direct conversion of CH4 to few-layer GNs through RF thermal plasma process suggests that the technique is simple, easy to operate, and is suitable for mass production of few-layer GNs in a continuous and scalable process.

Preosteoblasts behavior in contact with single-walled carbon nanotubes synthesized by radio frequency induction thermal plasma using various catalysts

The influence of single-walled carbon nanotubes (SWCNTs) produced by radio frequency (RF) induction thermal plasma with three catalyst mixtures (Ni-Co-Y2 O3 , Ni-Y2 O3 and Ni-Mo-Y2 O3 ) was evaluated on the behavior of murine MC3T3-E1 preosteoblasts. After analyzing SWCNTs properties, mitochondrial enzymatic (MTS) and lactate dehydrogenase (LDH) activities as well as neutral red (NR) uptake were measured to assess the cellular viability. To ascertain that the cytotoxicity was not merely as a result of the mechanical disturbance, either SWCNTs were added on the attached cells or cells were seeded on the SWCNT-covered plates. Regardless of the catalyst mixtures used for their production, SWCNTs added on the attached cells reduced cell viability drastically in a dose-dependent manner. However, the viability of cells seeded on SWCNTs even on those produced with Ni-Co-Y2 O3 was slightly decreased at 24 h and besides cells could proliferate within 48 h. Furthermore, cells were able to organize normal filamentous actin cytoskeleton and no apoptotic cells were detected in the cultures. Thus except mechanical disturbance, thermal plasma grown SWCNTs seem to induce no severe cytotoxicity on MC3T3-E1 preosteoblasts and therefore are considered promising CNTs to be studied more deeply for future applications in bone tissue engineering.

Detailed Numerical Simulation of Single-Walled Carbon Nanotube Synthesis in a Radio-Frequency Induction Thermal Plasma System

2D axisymmetric numerical calculations are conducted to model the thermo-fluid fields and chemical reactions leading to the formation of SWCNTs in an RF plasma system. A modified version of the SWCNT “reduced” chemical model is used to estimate the formation of SWCNT in an RF plasma system for the first time. The “reduced” model incorporates 14 species and 36 chemical reactions to predict the formation of metal and carbon clusters and SWCNTs. By combing the chemistry model into the RF plasma CFD code, the formation and development of carbon and metal catalyst clusters and their reactions which produce SWCNTs are shown. The chemistry model is shown to under-predict the yield rate of SWCNT. In order to better predict the yield rate, a sensitivity analysis is performed to modify the dominant reaction rates. The modified model predicts the yield of SWCNTs correctly within the range reported experimentally. However, more studies should be conducted to validate the accuracy of the model for different operating conditions.

Induction thermal plasma process modifies the physicochemical properties of materials used for carbon nanotube production, influencing their cytotoxicity

The effect of radio frequency induction thermal plasma (RFITP) process on the cytotoxicity of materials used for single-walled carbon nanotube production remains unknown. In this study, the influence of RFITP process on physicochemical and cytotoxic properties of commercial Co, Ni, Y2O3, Mo catalysts and carbon black was investigated. The cytotoxic assays (MTS, LDH, neutral red, TUNEL) revealed the strongest effect of commercial Co on murine Swiss 3T3 fibroblasts affecting their viability in a dose-dependent manner within 24 h. The cells contained also less actin stress fibres. Although RFITP affects the properties of each catalyst (size, morphology, chemistry), only cytotoxicity of Ni catalyst was increased. The plasma-treated Ni induced apoptosis. Comparing Ni particles before and after RFITP process with commercial nanoparticles of Ni revealed that the particles with similar surface area have different cytotoxicities. Interestingly, the observed toxicity of the catalysts was not mainly due to the release of ions.

Modelling for the optimization of the reaction chamber in silicon nanoparticle synthesis by a radio-frequency induction thermal plasma

The optimization of the reaction chamber for the silicon nanoparticle synthesis process by a radio-frequency induction thermal plasma is addressed using a plasma thermo-fluid-dynamic model coupled with electromagnetic field equations and with a moment model for nanoparticle transport. Various reaction chamber geometries composed of two parts—a conical top and a cylindrical bottom—are evaluated in terms of the yield of the synthesis process, the presence of recirculation flow patterns that may affect the uniformity of the produced nanoparticles and the size distribution of nanoparticles at the chamber outlet. Turbulent diffusion is suggested as the physical phenomenon that leads to nanoparticle deposition onto the walls of the reaction chamber. The injection of a suitable gas along the walls of the reaction chamber at the axial position, where the nanoparticle nucleation takes place, is proven to be effective in increasing the synthesis process yield.

Fluid-dynamic characterization of a radio-frequency induction thermal plasma system for nanoparticle synthesis

The fluid flow in a radio-frequency induction thermal plasma (RF-ITP) system for the synthesis of nanoparticles has been characterized using three- and two-dimensional modelling supported by enthalpy probe and calorimetric measurements in order to provide insights for the improvement of the process. The RF-ITP system is composed of a commercial inductively coupled plasma torch mounted on a reaction chamber that is equipped with viewports for diagnostics. The three-dimensional model predicted an almost axisymmetric temperature field in the reaction chamber in agreement with enthalpy probe measurements performed along two perpendicular scan axes, whereas recirculating flow patterns resulted in being strongly non-axisymmetric. Temperature profiles at two distances (60 mm and 100 mm) from the torch outlet have been calculated using two-dimensional modelling and compared with enthalpy probe measurements for different operating conditions with the aim of validating the predictive ability of the modelling tool. Calorimetric measurements have been performed in order to estimate the power coupled to the torch, which is usually an arbitrary input parameter for the models. Poor agreement was obtained between energy balances from modelling and from calorimetric measurements and, starting from this, a discussion on the uncertainties in the calculation of the radiative losses has been proposed. Finally, new insights for the improvement of the process of nanoparticle synthesis in the RF-ITP system are suggested.

Formation mechanism of titanium boride nanoparticles by RF induction thermal plasma

The formation mechanism of titanium boride functional nanoparticles by the radio frequency induction thermal plasma was investigated by experiment and numerical analysis. The mixed powders of titanium and boron were injected into the plasma and evaporated immediately, and then nanoparticles were produced through the quenching process. The nanoparticle products were characterized by phase composition and crystalline diameter. TiB2 was easily synthesized with a low powder feed rate in the boron-rich condition, because the molar ratio of evaporated and nucleated boron in plasma was enhanced. The prepared titanium boride nanoparticles had the average crystalline diameter ranging from 10 to 30nm, and it was reduced with decreasing the powder feed rate and titanium content in the raw material due to abundant number of boron nuclei. This paper demonstrates the feasibility of the control of titanium boride nanoparticles’ crystalline mean diameter and phase composition with different raw material condition in the RF induction thermal plasma synthesis.

Synthesis of Pure, Crystalline (Ba,Sr)TiO3 Nanosized Powders in Radio Frequency Induction Thermal Plasma

Barium strontium titanate ((Ba,Sr)TiO3) nanosized powders were synthesized via spray pyrolysis of a liquid precursor mist in Ar–O2 radio frequency induction thermal plasma. To adjust the cation stoichiometry of the complex oxide, liquid precursors were prepared by mixing titanium butoxide (Ti(OBu)4) stabilized with diethanolamine and aqueous solutions of barium nitrate (Ba(NO3)2) and strontium nitrate (Sr(NO3)2) stabilized with citric acid. The molar ratios of titanium, barium, and strontium ions (Ba/(Ba+Sr) and (Ba+Sr)/Ti ratios) in the liquid precursor were varied. Pure, high-crystalline (Ba,Sr)TiO3 (Ba/(Ba+Sr)=0.0–1.0) powders of ∼45 nm in size were synthesized in this one-step process.

In-Flight-Melted Soda-Lime-Silica Glass by RF Induction Thermal Plasma

Granulated raw materials with a particle size of 20–80 μm were prepared from a slurry of Na2CO3, CaCO3, and SiO2 (quartz) by the spray-dry method, and injected with carrier gas into a radio-frequency induction thermal plasma. Spherical particles 5–60 μm in size were obtained and analyzed. Thermo-gravimetric analysis and X-ray diffraction analysis showed that during the short flight of the order of milliseconds, all carbonates were decomposed and >95% quartz was reacted into a noncrystalline state. Glass transition was clearly observed by differential thermal analysis. Increase of the carrier gas from 3 to 6 L/min led to a decrease in the volatilization ratio of Na2O from 46% to 18% with a slight decrease of the reaction ratio of quartz in trade balance. Electron probe microanalysis showed that the volatilization could be attributed to an excess heating of small particles <30 μm, and suppression by the increase of carrier gas.

Synthesis of functional TiO2-based nanoparticles in radio frequency induction thermal plasma

Abstract A method of synthesizing functional nanostructured powders by using reactive thermal plasma processing has been developed. Nanosized TiO2 powders have been synthesized by the plasma oxidation of solid and liquid precursors. Quench gases, either injected from the shoulder of the reactor or injected counter to the plasma plume from the bottom of the reactor, are used to affect the quench rate and hence the particle size of the resultant powders. The experimental results are well supported by numerical analysis of the effects of the quench gas on flow pattern and temperature field of the thermal plasma as well as trajectory and temperature histories of the particles. The plasma-synthesized TiO2 nanoparticles showed phase preferences different from those synthesized by conventional wet processes. Nanosized particles of high crystallinity and nonequilibrium chemical composition were formed in one step by reactive thermal plasma processing. The plasma-synthesized nanoparticles were spherical and hardly agglomerated. Applications of highly functional nanoparticles are thought to require nanoparticles structured in the forms of (1) dispersoids, (2) fillers, and (3) patterns. For these purposes, plasma-synthesized nanocrystallites seem to be suitable.

Generation of pulse-modulated induction thermal plasma at atmospheric pressure

The radio frequency induction thermal plasma of sufficiently high electric power for materials processing has been successfully generated with a pulsemodulated operating condition. A solid-state amplifier, which supplies the electric power with a nominal frequency of 1 MHz, was employed for the pulsing plasma generation. The Ar–H2 plasma was generated at a high power level of 17 kW at atmospheric pressure. Typically, the plasma remained stable until the pulse duty factor went down to 30%, when the period of the high power level was 5 ms and the low power level was about 6 kW.