Yoshinori Tokoi

Particle Size Controllability of Ambient Gas Species for Copper Nanoparticles Prepared by Pulsed Wire Discharge

Copper nanoparticles have been prepared by pulsed wire discharge in several ambient gases. The experimental results of the particle size distribution and discharge behavior are discussed in terms of ambient gas species selection for particle size control. Scanning electron microscopy and voltage and current measurements were performed for the evaluation. It was found that temperature diffusivity and the breakdown voltage of the ambient gas are important factors for the availability of ambient gas species selection for particle size control.

Synthesis of TiO2 Nanosized Powder by Pulsed Wire Discharge

Nanosized powders of TiO2 have been synthesized by pulsed wire discharge (PWD) using titanium wire in oxygen gas. The relative energy (R, R = Ei/Ev), which is the ratio of the charged energy of the capacitor (Ei) to the vaporization energy of the wire (Ev), was changed from R = 1.9 to 52.1. The X-ray diffraction patterns of nanosized powders synthesized at various relative energies showed that all the samples had peaks of rutile and anatase. The rutile content increased with the increase in the relative energy, and changed from 6 to 84 vol % at 100 kPa. The median particle diameter decreased from 30.9 to 20.4 nm. Control of the rutile content and particle size was possible by varying the relative energy used for PWD.

Densely Packed Linear Assembles of Carbon Nanotube Bundles in Polysiloxane-Based Nanocomposite Films

Linear assemblies of carbon nanotubes (LACNTs) were fabricated and controlled in polysiloxane-based nanocomposite films and the effects of the LACNTs on the thermal and electrical properties of the films were investigated. CNTs were dispersed by mechanical stirring and sonication in a prepolymer of polysiloxane. Homogeneous suspensions were cast on polyamide spacers and oriented by linear-assembly by applying DC and switching DC electric fields before the mixture became cross-linked. Densely packed LACNTs that fixed the composite film surfaces were fabricated with various structures and thicknesses that depended on the DC and switching DC conditions. Polymer nanocomposites with different LACNT densities exhibited enhanced thermal and electrical conductivities and high optical transmittances. They are considered promising structural materials for electronic sectors in automotive and aerospace applications.

Oxidation of nanodiamonds and modulation of their assembly in polymer-based nanohybrids by field-inducement

Linear assembly of densely packed oxidized nanodiamonds (OxNDs) was performed in polymer-based nanohybrid films. A homogeneous suspension of the pre-polymer polyepoxide and OxNDs was placed onto a polyamide spacer and subjected to an electric field in order to induce the relocation and assembly of the fillers before the mixture became cross-linked. The OxNDs suspended readily, forming linear assemblies of OxNDs (LAOxNDs) of varying thicknesses, aligned perpendicular to the film surfaces. Nanohybrid films with linear assemblies of LAOxNDs exhibited a moderate increase in thermal conductivity while maintaining the electrical insulation properties of the polyepoxide. Mechanisms for the field-induced fabrication and the structural variation of LAOxNDs in the pre-polymer matrix are elucidated in relation to the variations in physical properties. The present air oxidation process and field-induced application are simple but effective in enhancing the physical properties of polymer-based hybrids, and hence, has the potential for applying in the fabrication and modulation of nanocomposite materials.

Particle Size Determining Equation in Metallic Nanopowder Preparation by Pulsed Wire Discharge

Copper nanopowders are prepared by pulsed wire discharge (PWD) using copper wires in nitrogen gas. From experimental results obtained in the present study and published literature, a relationship to predict the particle size of powders prepared by PWD is proposed. A theoretical plasma/vapor density (Dth), which is the most important factor for controlling the particle size is defined as mPEc-1, where m, P, and Ec are the weight of the wire, pressure of nitrogen gas, and charged energy in the capacitor, respectively. From high-speed photographs obtained during PWD, the relationship between the measured Dexp and Dth is shown by Dexp∝Dth0.6. The relationship between the median particle diameter d50 and Dth is found to be d50∝Dexp∝(Dth0.6)0.4 by transmission electron microscopy observations. This empirical relationship is in agreement with that expected from the formation and growth of particles via Brownian coagulation of free molecules. From the above relationship, it is possible to predict and control the particle size of powders prepared by PWD.

Synthesis of Aluminum Nitride Nanopowder with Particle Size Less than 10 nm by Pulsed Wire Discharge in Nitrogen Gas

High purity aluminum nitride (AlN) nanopowders were synthesized by pulsed wire discharge (PWD) using aluminum wires in nitrogen gas rather than ammonia gas, which is harmful but traditionally considered mandatory for this reaction. The synthesis was carried out at various relative energies (K) of 24–289, where K was the ratio of the charged energy of the capacitor to the vaporization energy in the wire, and at nitrogen gas pressures (P) of 10–100 kPa. From the measurement of voltage and the current waveforms during PWD, it was determined that the deposited energy in the arc discharge (Ea) after wire heating increased with increasing K. Analysis of prepared nanopowders showed that an increase in AlN content (CAlN) and a decrease in median particle diameter (D50) resulted from an increase in K and/or Ea. The highest CAlN of 97 wt % with a D50 of 6 nm was obtained at K = 289 and P = 10 kPa. The arc discharge after wire heating was considered to generate active species from the nitrogen gas with higher decomposition energies than those observed with ammonia and to drive the nitriding process during PWD.

Phase Control of Ti–Fe Nanoparticles Prepared by Pulsed Wire Discharge

Ti–Fe nanoparticles were prepared by pulsed wire discharge (PWD) using iron (Fe) and titanium (Ti) twisted wire in Ar gas at a pressure of 100 kPa. The content of Fe (CFe), which was changed from 0 to 100 wt %, was controlled by adjusting the number of Ti and Fe wires in the twisted wire. From the X-ray diffraction, the phase of the prepared nanoparticles changed from α-Ti to β-Ti, FeTi, Fe2Ti, Fe(Ti), and Fe with increasing CFe. FeTi nanoparticles were obtained at approximately CFe = 30 wt %. From these results, the phases of the prepared Ti–Fe nanoparticles were controlled by adjusting the content of Fe in the twisted wire.

Determining factor of median diameter in intermetallic compound nanoparticles prepared by pulsed wire discharge

The preparation of NiAl intermetallic compound nanoparticles was carried out by pulsed wire discharge (PWD) using twisted pure Ni and Al wires in N2 ambient gas with varying number of turns of the wire (Nt), energy ratio (K), and ambient gas pressure (P). From the voltage and current waveforms during the wire heating, the energy deposition ratio up to the voltage peak (Kp) was calculated. It increased with an increase in Nt to 0.4 turns/mm and with increases in K and P. Under all the conditions, with an increase in Kp, the Ni composition ratio of the prepared particles (CNi) became closer to that of the wire (= 51.2 mol %). Furthermore, the collection rate (Rc) increased and the median particle diameter (d50) decreased. In particular, the change in d50 due to the change in Nt was not predicted by the relationship of d50 and Dth in our previous report. Single-phase NiAl intermetallic compound nanoparticles were successfully prepared under the condition in which Nt is 0.4 turns/mm, K is 3.4, and P is 100 kPa, where relatively high value of Kp was obtained. From these results, Kp was determined to be an important factor that determines the composition, collection rate, and median diameter of intermetallic compound nanoparticles synthesized by PWD using different kinds of twisted wires under various experimental conditions. This may be because of the selective wire heating in high-resistance parts that are associated with the introduction of lattice defects and/or necks by overwinding.

Measurement of metal vapor cooling speed during nanoparticle formation by pulsed wire discharge

Pulsed wire discharge(PWD) is one of nano-sized powder production methods. The object of this work is to study influence of the plasma/vapor/particle density using computer simulation and to establish temperature measurement method using a high-speed infrared thermometer in the PWD process. The temperature correction coefficient was obtained from geometric computer simulation results. Obtained correction coefficient was applied to the temperature measuring results. It was found from this result that obtained correction coefficient was appropriate. A temperature measurement method was established by using the high-speed infrared thermometer in PWD.

Preparation of palladium nanoparticles and a grain-size determining equation of pulsed wire discharge in N2, Ar, and He ambient gasses

Pulsed wire discharge (PWD) is one of the many nanoparticle preparation methods known for its high efficiency and high production rate. Particle size is controlled by gas pressure and input energy. However, the effect of ambient gas species on particle size is not well known. In this study, single-phase palladium (Pd) nanoparticles were prepared in N2, Ar, and He ambient gasses by PWD. The mean diameter of the prepared particles in Ar was smaller than those of the other nanoparticles because of the low ionization energy required to form an alternate current path and the partial wire evaporation. A novel equation for estimating the average grain size was proposed. By comparing the estimated and measured mean diameters, the validity of the equation was shown.