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News !

November 9, 2016
My Topical Review (Invited) has been published on Journal of Physics D: Applied Physics. --- The Page


I N D E X
 

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Three-dimensional flow dynamics of a DC-RF hybrid thermal plasma

  

A strong and useful plasma field is obtained by combining a radio-frequency (RF) inductively coupled thermal plasma and a direct current (DC) thermal plasma jet. However, a thermal plasma is a unique fluid with intense light emission, high temperature (over 10,000 K), a complicated flow caused by electromagnetic forces and thermal expansion. This feature prevents direct measurements in experiments; therefore, the details of the thermofluid field are still poorly understood.

A time-dependent 3-D simulation based on magnetohydrodynamics (MHD) has been attempted to clarify the thermofluid field of the plasma, which is governed by the conservation equations of mass, momentum (Navie-Stokes) and energy coupled with the electromagnetic equations (Maxwell). Simultaneously, the simulation takes into account the temperature-dependent large variations of the thermodynamic and transport properties as a plasma "fluid".

Right figure and Left figure show the dynamic behaviors of the thermofluid field and the vortex structure interacting with the electromangeitc field, respectively (Play speed of Left is 1/6 of Right). The colors indicate the temperatures. Such a complicated flow has been predicted from experimental studies; however, it has never been obtained by any axisymmetric 2-D simulations which have been carried out.

The present time-dependent 3-D simulation has first successfully obtained these realistic results and revealed the plasma flow dynamics.   (Note: Some careful treatments are required to capture vortex structures of thermal plasma flows by numerical simulation. See the next section.)

For more information, please see ...
Three-dimensional flow dynamics of an argon RF plasma with DC jet assistance: a numerical study,
Journal of Physics D: Applied Physics, Vol. 46, No. 1, (January, 2013) 015401 (12 pages).
Masaya Shigeta


 

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Fundamental problem in numerical simulation of thermal plasmas

  

As mentioned the above section, simulation of thermal plasma is generally difficult. The entire flow field, in which the plasma at a high temperature and a cold gas at room temperature co-exist, must be treated simultaneously. Widely varied temperatures of 300-12,000 K cause large variations of the transport properties and the density. Meanwhile, the Mach numbers are very small in and around the plasma. Consequently, when a numerical method for a compressible flow simulation is used, the computation takes an extremely long time to obtain a numerical solution for a practical time scale. Therefore, a thermal plasma is treated as an incompressible flow with the density as a temperatur-edependent variable. This condition, which is severe for numerical flow simulations, usually destabilizes the computation (= the computation easily diverges). That is why thermal plasma simulations have often used differencing schemes which suppress numerical instability effectively. However, those schemes also suppress the actual physical instability simultaneously. In consequence, the numerical result does not simulate any realistic flow with vortices as shown in Left figure. On the other hand, schemes that are effective for vortex capture often cause destabilization of computations. Although these two aspects mutually conflict, thermal plasma flows should also be calculated as "simulation" somehow using such schemes to obtain realistic results. As a result, the effort gives a more realistic flow as shown in Right figure.

Experiments have predicted that a thermal plasma jet entrains surrounding cold gas by Kelvin-Helmholtz instability for more than 20 years ago. Nevertheless, such a flow has never been simulated because of the numerically severe conditions. Meanwhile, the present effort broke through this problem and gave a successful result.

For more information, please see ...
Turbulence modelling of thermal plasma flows,
Journal of Physics D: Applied Physics, Vol. 49, No. 49, (November, 2016), pp. 493001 (18 pages).
Masaya Shigeta


 


Simple equations to describe aerosol growth

 
          often-used equations of moments                      our new equations

Both sets of equations give almost the same results for the time evolutions of the particle number density and mean size of aerosol.

Aerosol growth through nucleation, condensation/evaporation and coagulation has usually been described by the simultaneous equations of the moments of the particle size distribution function (PSDF) with its profile assumption (Left) in numerical calculations. This method solves the four complex ordinary differential equations.

For this problem, we derived a set of two ordinary differential equations and one algebraic equation (Right) without any profile assumption for the PSDF. In spite of its much simpler formulation and lower computational costs, it gives reasonable a numerical result which is almost the same as that obtained with a more complex set of equations (Left).

This mathematical model can be expected to be applied to numerical predictions for not only plasma-aided nanopowder syntheses but also water-droplet generation in a steam turbine (causing erosion), meteorological problems with cloud/fog generation, space design requiring humidity control, etc.

Note that the paper below presents the sets of equations applicable to the continuum size regime as well as the free molecular size regime shown above.

For more information, please see ...
Simple equations to describe aerosol growth,
Modelling and Simulation in Materials Science and Engineering, Vol. 20, No. 4, (May, 2012), pp. 045017 (11 pages).
Valerian A. Nemchinsky and Masaya Shigeta


 

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Silicide Nanopowder Growth in Thermal Plasma Synthesis

  

These movies show the time evolutions of the particle size-composition distributions of the silicide nanoparticles (nanopowder) synthesized in thermal plasma processing.

We have successfully clarified the formation mechanisms including binary nucleation and binary co-condensation of two components (Mo&Si, Co&Si, etc.) by our original mathematical model and solution algorithm "Two-Directional Nodal Method".

In Mo-Si system (Initial vapor ratio Mo:Si = 1:1), the molybdenum-rich nanoparticles first grow up and subsequently silicon condenses on the nanoparticles, which results in the significant growth of the nanopowder.
On the other hand, in Co-Si system (Initial vapor ratio Co:Si = 1:1), silicon-rich nuclei are first generated and immediately make a rapid growth into nanoparticles due to simultaneous co-condensation of cobalt and silicon.

These results show that the nanopowders synthesized in thermal plasma processing have widely ranging sizes and compositions inevitably even under a simple condition (Initial vapor ratio Metal:Si = 1:1).

These numerical results agree with the experiment results, which endorses the validity of our model.

In addition to molybdenum-silicide (Mo-Si) and cobalt-silicide (Co-Si), the nanoparitcles' formation mechanisms are being studied for titanium-silicide (Ti-Si), iron-silicide (Fe-Si), borides (boron-based intermetallic compound), and binary metal alloys (Fe-Co, Fe-Pt etc.).

For more information, please see ...
  • Growth model of binary alloy nanopowders for thermal plasma synthesis,
      Journal of Applied Physics, Vol. 108, Issue 4, (August, 2010), pp. 043306 (15 pages).
      Masaya Shigeta and Takayuki Watanabe
  • Effect of precursor fraction on silicide nanopowder growth under thermal plasma conditions: a computational study,
      Powder Technology, Vol. 288, (January 1, 2016), pp. 191-201.
      Masaya Shigeta, Takayuki Watanabe


  •  

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    Platinum nanopowder growth in the cooling counterflow region of thermal plasma

      

    It is possible to mass-produce nanoparticles (Right figure) by quenching a thermal plasma flow including material vapor (here, platinum vapor) with counterflow cooling (Left figure) promoting nucleation. These numerical results can be obtained by solving the mathematical models coupling the sequential physics of a plasma flow dynamics, material vaporization, and nanopowder growth.

    Nanoparticles are created through homogeneous nucleation and subsequent heterogeneous condensation growth. The nanoparticles simultaneously grow up by Brownian coagulation between themselves. However, it is still impossible to calculate this collective growth of many nanoparticles for a practical time scale by the "Molecular dynamics" approach even with powerful computers. Meanwhile, an "Aerosol dynamics" equation effectively expresses the growth. Although the equation cannot be solved even numerically yet, it can be calculated by combining with a statistical method. In addition, the calculation also takes into account diffusion, thermophoresis, and convection of nanoparticles as well as transport of material vapor.

    Many nuclei are generated at the interface between the plasma flow and the counterflow. Being transported downstream, the nuclei grow up into nanoparticles gaining the material vapor. The nanoparticles also increase their sizes by coagulation with each other, and consequently the number of nanoparticles decreases.

    For more information, please see ...
    Numerical investigation of cooling effect on platinum nanoparticle formation in inductively coupled thermal plasmas,
    Journal of Applied Physics, Vol. 103, Issue 7, (April, 2008), pp. 074903 (15 pages).
    Masaya Shigeta and Takayuki Watanabe



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    Dynamics of two non-neutral plasma rings in a unifrom magnetic field

      
            Fixed coordinate system                                                                   Rotating coordinate system

    A dynamic motion of two non-neutral plasma rings in a uniform magnetic field was simulated.
    Finite Larmor radius effect and Transient electric field effect were taken into account as well.
    Here, electron plasma or positron plasma was supposed as a non-neutral plasma.
    Colors indicate the speeds in each system




    News !

    November 9, 2016
    My Topical Review (Invited) has been published on Journal of Physics D: Applied Physics.

    August 10, 2016
    We received Best Paper Award for Thermal Engineering.

    August 2, 2016
    We received Award for Encouragement of Welding Physics and Technology.

    July 4, 2016
    I had an Invited talk at HTPP 14 held in Munich, Germany.

    April 13, 2016
    We received Best Paper Award of Japan Welding Society.

    March 28, 2016
    I attended the M6 meeting of "Nanodome" in EU's international project "Horizon 2020" as a member of External Advisory Board.

    January 19, 2016
    I had an Invited talk at ISN2A 2016 held in Caparica, Portugal.

    December 10, 2015
    I had an Invited talk at International Symposium C5 in MRS-J 2015.

    October 14, 2015
    I had an Invited talk at ICRP-9/GEC-68/SPP-33 sponsored by The Japan Society of Applied Physics & American Physical Society.

    August 4, 2015
    We received Award of Welding Arc Physics.

    July 22-23, 2015
    I had a lecture at Kyushu University.
    Title: Fundamentals and Applications of Computational Plasma Fluid Mechanics

    July 14, 2015
    I received Osaka University Presidential Award for Encouragement.

    May 29, 2015
    I had a Special lecture at Directors Meeting of Kansai Branch, Japan Welding Society.

    November 1, 2014
    I had a Plenary lecture at Japan Society of Mechanical Engineers, Kansai Branch, 15th Autumn Forum.

    October 10, 2014
    I had an invited talk at 81st Meeting for Japan Welding Society, Tokai Branch & Japan Thermal Spray Society, Chubu Branch.

    September 9, 2014
    I received Award for Encouragement of Research in The IUMRS International Conference in Asia 2014 (IUMRS-ICA 2014).

    July 29, 2014
    I had an invited talk at Gordon Research Conference (GRC), Plasma Processing Science, Smithfield (RI), USA.

    June 11, 2014
    I had an invited talk at International Conference on Microelectronics and Plasma Technology 2014 (ICMAP2014), Gunsan, Korea.

    June 4, 2014
    I had an invited talk at JWRI-KMUTT Workshop, Bangkok, Thailand.

    May 14, 2014
    I had an invited talk at Japan-Indonesia Welding Seminar 2014, Jakarta, Indonesia.

    April 15, 2014
    I received The Young Scientists' Prize, the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology.

    March 3, 2014
    I had a Review talk at 4th International Round Table on Thermal Plasmas for Industrial Applications: Challenges and Opportunities.

    December 10, 2013
    I had an invited talk at 23rd Annual Meeting of MRS-Japan.

    November 27, 2013
    I had an invited talk at 3rd China-Japan Workshop on Welding Thermo-Physics.

    August 1, 2013
    I have moved to Osaka University as an Associate Professor.

    July 17, 2013
    I had an invited talk at 31st International Conference on Phenomena in Ionized Gases (ICPIG-31).

    March 6, 2013
    I had an invited talk at Hokuriku Branch Symposium 2013 of The Institute of Electrical Engineers of Japan.

    February 21, 2013
    Demonstration of dynamics of two non-neutral plasma rings in a unifrom magnetic field was added.

    January 6, 2013
    The explanation of simple equations to describe aerosol growth was added.

    January 3, 2013
    The explanations and animations of thermal plasma simulation were added.

    January 1, 2013
    The figure of vortex structure in my paper was adopted as the cover image of Journal of Physics D: Applied Physics, Vol.46, No.1.

    December 1, 2012
    I have come back from USA.

    November 30, 2012
    My paper "Three-dimensional flow dynamics of an argon RF plasma with DC jet assistance: a numerical study" has been published
    in Journal of Physics D: Applied Physics.

    October 3, 2012
    My paper "Time-Dependent 3-D Simulation of an Argon RF Inductively Coupled Thermal Plasma" has been published
    in Plasma Sources Science and Technology.

    September 30, 2012
    I have come to USA.

    June 29, 2012
    I had an invited talk at the 12th European Plasma Conference, High-Tech Plasma Processes (HTPP-12) which was held in Bologna, Italy.

    June 14, 2012
    I was invited to Celebratory reception to mark the 10th anniversary of the IOP Publishing Editorial Office in Japan at British Embassy Tokyo.
    because our collaborative review article with Dr. Murphy of CSIRO (Australia) is included in a collection of the most frequently downloaded papers published in 2011.
    (See also the section of "Journal of Physics D: Applied Physics" on this page.)

    May 28, 2012
    Our collaborative paper with Prof. Nemchinsky of Keiser University (USA) has been published in Modelling and Simulation in Materials Science and Engineering (IOP publishing).

    March 1, 2012
    Our collaborative paper with Prof. Colombo, Prof. Ghedini, Dr. Sanibondi and Mr. Gherardi of University of Bologna (Italy) has been published in Plasma Sources Science and Technology (IOP publishing).

    February 15, 2012
    Our collaborative paper with Prof. Watanabe, Dr. Choi and Ms. Cheng of Tokyo Institute of Technology has been published in Chemical Engineering Journal (Elsevier).

    November 21, 2011
    Web page of Plasma Research Group at University of Bologna is added on links.

    October 5, 7, 2011
    I gave two lectures at University of Bologna, Italy.

    August 17, 2011
    Our review article "Thermal plasmas for nanofabrication" (Invited paper) has been downloaded
    500 times for 4 months! = 3% of articles across all IOP journals!

    April 15, 2011
    Our review article "Thermal plasmas for nanofabrication" (Invited paper) has been published
    in Journal of Physics D: Applied Physics (Special issue on perspectives in plasma nanoscience).
    ( to see the quick review on IOP science -> Click! )

    March 11, 2011
    An incredible earthquake hit us.

    December 13, 2010
    I gave an invited talk at Second International Symposium on Plasma Nanoscience (iPlasmaNano-II) in Australia.

    November 11, 2010
    We won Best Paper Award at the International Symposium on Visualization
    in Joining & Welding Science through Advanced Measurements and Simulation.

    October 27, 2010
    I gave an invited talk at 2010 Workshop for Preparation of Nanoparticles by Thermal Plasmas in Korea.

    September 1, 2010
    An edited book "Nanomaterials: Properties, Preparation and Processes"
    has been published by NOVA Science Publishers, Inc. (New York).
    We wrote Chapter 3: Nanoparticle Synthesis by Thermal Plasmas.

    August 23, 2010
    Our original paper "Growth model of binary alloy nanopowders for thermal plasma synthesis"
    has been published in Journal of Applied Physics.

    July 23, 2010
    I gave an invited talk at the 97th Meeting of the 153rd Committee on Plasma Materials Science,
    the Japan Society of Applied Physics.

    April 13, 2010
    I gave an invited talk at WORKSHOP ON INDUSTRIAL APPLICATIONS OF THERMAL PLASMAS in Italy.


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