Masaya Shigeta

Thermal plasmas for nanofabrication

In this paper, we review the recent progress in nanofabrication by thermal plasmas, and attempt to define some of the most important issues in the field. For synthesis of nanoparticles, the experimental studies in the past five years are briefly introduced; the theoretical and numerical modelling works of the past 20 years are reviewed with some detailed explanations. Also, the use of thermal plasmas to produce nanostructured films and coatings is described. A wide range of technologies have been developed, ranging from chemical vapour deposition processes to new plasma spraying processes. We present an overview of the different techniques and the important physical phenomena, as well as the requirements for future progress.

Growth mechanism of silicon-based functional nanoparticles fabricated by inductively coupled thermal plasmas

An experimental and computational study is conducted for the Si-based functional nanoparticle fabrication in an inductively coupled thermal plasma reactor. In the computational study, the improved multi-component co-condensation model with nodal discretization is proposed to clarify the nanoparticle growth mechanism in the consideration of coagulation and thermophoresis as well as simultaneous co-condensation. The nanoparticle growth by nucleation and co-condensation completes approximately in 12.6 ms for the Mo–Si system and in 5.0 ms for the Ti–Si system. Mo nanoparticles grow in advance, and then Si vapour condenses on the Mo nanoparticles in the Mo–Si system, while vapours of Si and Ti simultaneously co-condense following Si nucleation in the Ti–Si system. A smaller number of larger nanoparticles are created with an increase in the powder feed rate. When the silicon content in the feed powders is 66.7%, nanoparticles of MSi2 (M = Mo, Ti) are fabricated as the main product. Nanoparticles of Ti5Si3 are mainly synthesized in the case of the silicon content 33.0%. In the experiment, the nanoparticles are successfully fabricated and examined by x-ray diffractometry and transmission electron microscopy. The experimental and computational results show good agreement in the size distribution and the composition.

Growth model of binary alloy nanopowders for thermal plasma synthesis

A new model is developed for numerical analysis of the entire growth process of binary alloy nanopowders in thermal plasma synthesis. The model can express any nanopowder profile in the particle size-composition distribution (PSCD). Moreover, its numerical solution algorithm is arithmetic and straightforward so that the model is easy to use. By virtue of these features, the model effectively simulates the collective and simultaneous combined process of binary homogeneous nucleation, binary heterogeneous cocondensation, and coagulation among nanoparticles. The effect of the freezing point depression due to nanoscale particle diameters is also considered in the model. In this study, the metal–silicon systems are particularly chosen as representative binary systems involving cocondensation processes. In consequence, the numerical calculation with the present model reveals the growth mechanisms of the Mo–Si and Ti–Si nanopowders by exhibiting their PSCD evolutions. The difference of the materials’ saturation ...

Numerical investigation of cooling effect on platinum nanoparticle formation in inductively coupled thermal plasmas

A mathematical model is developed to simulate the comprehensive systems of platinum nanoparticle synthesis using an argon inductively coupled thermal plasma flow with forced cooling portions. Numerical investigation using the model is conducted to clarify and discuss the effects of several cooling methods on the formation mechanisms of nanoparticles in distinctive thermofluid fields with strong two dimensionality. The computational results show that cooling by a radial gas injection, and a counterflow, engenders the remarkable promotion of nanoparticles.

Numerical Analysis of Metallic Nanoparticle Synthesis Using RF Inductively Coupled Plasma Flows

A thermal plasma flow is regarded as a multifunctional fluid with high energy density, high chemical reactivity, variable properties, and controllability by electromagnetic fields. Especially a radio frequency inductively coupled plasma (RF-ICP) flow has a large plasma volume, long chemical reaction time, and a high quenching rate. Besides, it is inherently clean because it is produced without internal electrodes. An RF-ICP flow is, therefore, considered to be very useful for nanoparticle synthesis. However, nanoparticle synthesis using an RF-ICP flow includes complicated phenomena with field interactions. In the present study, numerical analysis was conducted to investigate the synthesis of metallic nanoparticles using an advanced RF-ICP reactor. An advanced RF-ICP flow is generated by adding direct current (DC) discharge to a conventional RF-ICP flow in order to overcome the disadvantages of a conventional one. The objectives of the present work are to clarify the formation mechanism of metallic nanoparticles in advanced RF-ICP flow systems and to detect effective factors on required synthesis. A two-dimensional model as well as a one-dimensional model was introduced for nanoparticle growth to investigate effects of spatial distributions of thermofluid fields in RF-ICP flows on synthesized nanoparticles. In an advanced RF-ICP flow, a characteristic recirculation zone disappears due to a DC plasma jet. Larger numbers of nanoparticles with smaller size are produced by using an advanced RF-ICP flow. Thermofluid fields in RF-ICP flows can be controlled by applied coil frequency by means of skin effect. Larger numbers of nanoparticles with smaller size are produced near the central axis. Dispersion of particle size distributions can be suppressed by higher applied coil frequency through control of RF-ICP flows. Applied coil frequency can be a remarkably effective factor to control nanoparticle size distribution.

Numerical analysis for co-condensation processes in silicide nanoparticle synthesis using induction thermal plasmas at atmospheric pressure conditions

Numerical analysis is conducted to clarify the formation mechanisms of silicide nanoparticles synthesized in an induction thermal plasma maintained at atmospheric pressure. The induction thermal plasma is analyzed by an electromagnetic fluid dynamics approach, in addition to a multi-component co-condensation model, proposed for the silicide nanoparticle synthesis. In the Cr–Si and Co–Si systems, silicon vapor is consumed by homogeneous nucleation and heterogeneous condensation processes; subsequently, metal vapor condenses heterogeneously onto liquid silicon particles. The Mo–Si system shows the opposite tendency. In the Ti–Si system, vapors of silicon and titanium condense simultaneously on the silicon nuclei. Each system produces nanoparticle diameters of around 10 nm, and the required disilicides, with the stoichiometric composition, are obtained. Only the Ti–Si system has a narrow range of silicon content. The numerical analysis results agree with the experimental findings. Finally, the correlation chart, predicting the saturation vapor pressure ratios and the resulting silicon contents, is presented for estimation of nanoparticle compositions produced in the co-condensation processes.

Numerical simulation of a potassium-seeded turbulent RF inductively coupled plasma with particles

Abstract Seeding a small amount of vaporized potassium with low ionization potential into plasma is one of the effective methods for the enhancement of the electrical conductivity of plasma, which strongly affects the thermofluid fields. In the present study, the thermofluid fields and the particle behavior in the radio frequency inductively coupled plasma (RF-ICP) with electrical enhancement by seeding vaporized potassium are numerically investigated for laminar flow and turbulent flow. The behaviors of four kinds of particles injected into the electrically enhanced plasma were obtained with a Lagrangian method. Particle phase change with melting and the particle diameter variation with evaporation, non-continuum effect for smaller particle diameter, turbulent dispersion and interference with eddies were also taken into account. It is shown that, the electrons diffuse widely and the temperature region less than 2000–8000 K shifts downstream in seeding potassium. In the turbulent flow, electrons diffuse more upstream, which results in the expansion of the temperature region to upstream. The injected particles are heated more rapidly and evaporate more intensively in the turbulent flow.

Mixing and magnetic effects on a nonequilibrium argon plasma jet

Effects of gas admixture and of an applied magnetic field on a nonequilibrium Ar plasma jet are investigated experimentally. The maximum gas temperature and gas velocity are observed in the core region of an Ar/He plasma due to the small density, the large thermal conductivity and Mach number effect of He. Furthermore, their radial gradients become very steep in the applied magnetic field. In contrast, the gas temperature and gas velocity of an Ar/N2 plasma jet are not so high and their radial gradients are moderate, because of the energy loss needed to dissociate N2 gas in the core region and of the active thermal diffusion in the radial direction. Electron number density is larger in the case of an Ar/Ar plasma jet or an Ar/He plasma jet compared with that of an Ar/N2 plasma, since N2 gas dissociates before ionization. It is shown that the admixing of He or N2 gas influences controllably the flow and temperature fields of a nonequilibrium Ar plasma jet also in an applied magnetic field.

Three-dimensional flow dynamics of an argon RF plasma with dc jet assistance: a numerical study

Time-dependent three-dimensional numerical simulation based on a large-eddy simulation approach is conducted to ascertain the complicated thermofluid dynamics of an argon radio-frequency (RF) inductively coupled plasma with a direct-current (dc) plasma jet assistance, considering non-uniform densities and properties in time and space as well as turbulence generation and suppression. Using a combination of numerical schemes suitable to capture vortices, the present simulation successfully shows unsteady behaviour of the plasma as well as wave-like interfaces between a high-temperature flow and a low-temperature flow as a result of the balance of fluid-dynamical instability and a viscous diffusion effect. Small cold vortices generated near a dc jet injector are entrained into and merged with vortices generated around the dc jet. Subsequently, they interact with large vortices in an RF induction coil region, which causes a much more complex vortex structure.

Simple equations to describe aerosol growth

A simple set of one algebraic equation and two ordinary differential equations is proposed to describe an aerosol growth process simultaneously involving nucleation, condensation/evaporation and coagulation using a monodisperse approach. With the assumption of spherical particles in a gas-volume-based environment, this method is applicable to both the free molecular size and the continuum size regimes, although it is not of use in the transition regime because it does not take into account the dynamics of transition. Despite its much simpler mathematical formulation and lower computational costs, this method gives reasonable numerical results for the particle number concentration and the particle size. These results practically coincide with those obtained with a more complex and elaborate method using the moments of a size distribution function. Modification of the equations to express different probabilities for particle fusion on impact is also suggested.