Yoshitaka Mitsuda

The growth of diamond in microwave plasma under low pressure

The synthesis of diamond particles in a low pressure plasma has been studied, with emphasis on the investigation of the substrate effect and the plasma conditions. It was found that a special pre-treatment of silicon substrate made it possible to form dense films, and a thickness of about 15μm could be reached by 20 h discharge. Unfortunately, however, the prepared films had poor adhesion. Observations by scanning electron microscope (SEM) showed that the poor adhesion was due to the fact that the film consisted of large particles with a diameter of about 10μm, and each particle had contacted to the substrate only by a point, not by a face. In addition, the plasma diagnostics of optical and ultraviolet emission spectroscopy (OES, 200–750 nm) revealed that CH and H radicals have come to be criteria for the formation of diamonds, and the ratio of radicals drastically affected the characteristics of the deposits. Nucleation and growth mechanism are also discussed.

Titanium powder production by preform reduction process (PRP)

Abstract To develop an effective process for titanium powder production, a new preform reduction process (PRP), based on the calciothermic reduction of preform containing titanium oxide (TiO 2 ), was investigated. The feed preform was fabricated from slurry, which was made by mixing TiO 2 powder, flux (e.g. CaCl 2 ) and binder. Various types of preforms in the form of plates, spheres, or tubes were prepared using a conventional technique, and the fabricated preform was sintered at 1073 K before reduction in order to remove the binder and water. The sintered solid preform containing TiO 2 was then placed in a stainless steel container, and reacted with calcium vapor at a constant temperature ranging between 1073 and 1273 K for 6 h. Titanium powder was recovered from the reduced preform by leaching it with acid. As a result, pure titanium powder with 99 mass% purity was obtained. This process was found to be suitable for producing a homogeneous fine powder when the composition of flux and the size of the preform are controlled.

Measurement of Young’s modulus of carbon nanotubes by nanoprobe manipulation in a transmission electron microscope

A method for quantifying the nanomechanics of nanomaterials was developed using a nanoprobe manipulator fitted into a transmission electron microscope. Apparent Young’s moduli of various carbon nanotubes (CNTs) were measured using this method. The apparent Young’s modulus of an arc-grown CNT is as large as approximately 3.3TPa, which is close to the theoretical Young’s modulus (5.5TPa) of the single-walled CNT simulated using molecular dynamics. The relationship between the apparent Young’s modulus and the crystallinity of CNTs is demonstrated using the crystallinity parameter ID∕IG derived by Raman spectroscopic analysis. The apparent Young’s modulus is higher for better crystallinity of CNT.

Development of a new microwave plasma torch and its application to diamond synthesis

We have developed a new microwave plasma torch (MW plasma jet), which can generate a pure H2 plasma jet at atmospheric pressure from the end of a center electrode with an input power above 2 kW. The electric field has maximum strength at the end of the electrodes, so the plasma is initially ignited by the electric breakdown. The plasma jet seems to be powered and sustained by the electromagnetic field generated between the electrodes and/or in the chamber. Accordingly, the mechanism to sustain the plasma is considered to be different from that to initially ignite the plasma. In order to confirm the effectiveness of this plasma for CVD processes, the MW plasma jet has been applied to the synthesis of diamond from Ar‐H2‐CH4 system. Diamond particles or films were successfully synthesized at a higher rate (30 μm/h) on a larger area (25×25 mm2), compared with the usual synthesis methods. The growth rate was in proportion to substrate temperature and/or CH4/H2 ratio up to 1200 °C and/or 10 vol %, respectively....

Dynamic measurement of electrical conductivity of carbon nanotubes during mechanical deformation by nanoprobe manipulation in transmission electron microscopy

The effect of mechanical deformation on the electrical characteristics of individual multiwall carbon nanotubes (MWCNTs) was investigated by a nanoprobe manipulation technique in a transmission electron microscope (TEM). The electrical conductivity was measured during the deformation of the MWCNT in TEM. The electrical conductivity of the MWCNTs was sensitive to structural variation. The electromechanical characteristics were reversible within the elastic limit. However, when lattice defects were formed due to deformation, the electrical conductivity was not restored to the original state, even when the applied stress was released.

Nanoscale Mechanics of Carbon Nanotube Evaluated by Nanoprobe Manipulation in Transmission Electron Microscope

We investigated the nanoscale mechanics of individual multiwalled carbon nanotubes (MWCNTs) by a nanoprobe manipulation technique in a transmission electron microscope (TEM). The force applied to individual MWCNTs was measured using a commercially available Si cantilever installed in a manipulator. It was clearly observed that this force is released by buckling like deformation. The average Youngs modulus of the MWCNTs estimated using a conventional mechanical theory was approximately 1.1 TPa. Although the MWCNTs exhibited a high flexibility, the deformation of the MWCNTs above the elastic limit led to structural defects, which resulted in a local plastic deformation. Nanomechanics measurements in the TEM revealed that the structural defects cause stiffness deterioration.

New recovery process for rhodium using metal vapor

Abstract Rhodium (Rh) is an essential element as a catalyst in automotive catalytic converters, and the recovery of Rh from scrap is an important issue. However, the chemical inertness of Rh makes it difficult to recover it from scrap. This study focused on a new process for recovering platinum group metals (PGM) from scrap with the purpose of improving Rh dissolution in acid. Reactive metal vapors such as magnesium (Mg) and calcium (Ca) were reacted with powdered Rh in a closed stainless steel reaction vessel at a constant temperature ranging from 873 to 1173 K. Mg and Ca deposited on Rh at temperatures above 973 and 1073 K, respectively. Dissolution experiments of the obtained compounds were conducted at 323–333 K for 1 h using an aqueous HCl solution or aqua regia. After reactive metal treatment 99% of Rh was dissolved by aqua regia leaching. Under the same leaching condition only 8% of the untreated pure Rh powder was dissolved. Rh was recovered from the solution obtained by leaching Mg reacted Rh compound using precipitation or cementation methods, demonstrating the feasibility of such recovery. These results show the possibility of developing a new Rh recovery process utilizing reactive metal vapor treatment.

In situ emission and mass spectroscopic measurement of chemical species responsible for diamond growth in a microwave plasma jet

Chemical species responsible for diamond growth were examined by in situ methods using a microwave plasma jet from Ar‐H2‐CH4/C2H2. Irrespective of the reactants, diamond was successfully deposited, and morphology was likely to depend only upon the carbon equivalent concentration in the gas phase. Emission spectroscopy revealed that the C2 radical was the main emissive species in the plasma, and that its vibration temperature was estimated to be about 5000 K with little dependence on the axial position. Moreover, emission from just above the substrate showed a considerably lower concentration ratio of H atom to C, including radicals, than the case of low‐pressure microwave plasma. On the other hand, mass spectroscopy mainly detected CH4 and C2H2 in the species impinging on a substrate. The importance of kinetic processes in a boundary layer was strongly emphasized.

Effect of high supersaturation at the initial stage on diamond nucleation phenomena

Abstract The surface of diamond film deposited by chemical vapor deposition is usually too rough for it to be used as optical coatings for UV or visible light. The surface roughness is due primarily to the low density of diamond nuclei. To enhance the nucleation density of diamond, we investigated the effect of high supersaturation during an initial stage of deposition on the diamond nucleation phenomena. The deposition process was performed in two sequential steps: first, carbonic material was deposited onto the substrate in a CH 4 H 2 plasma for a few minutes; second, diamond particles were grown in a CH 4 O 2 H 2 plasma for 30 min. The high CH 4 concentration in the initial stage increased the number density of diamond particles by nearly two orders of magnitude. When the high concentration treatment was limited to a few minutes, the deposition of other carbon allotropes was found to be suppressed.

Electron field emission from undoped polycrystalline diamond particles synthesized by microwave-plasma chemical vapor deposition

Electron emission from polycrystalline diamond particles (PDPs) was obtained at low electric fields in the absence of intentional doping. The PDPs were synthesized on a silicon substrate using microwave-plasma chemical vapor deposition accompanied by bias-enhanced nucleation. Polycrystalline diamond particles of two different sizes, i.e., ∼500 nm and 2 to 5 μm, were obtained, the surfaces of which were covered with small crystal grains composed of fine facets. Electron emission from the PDPs was characterized by Fowler-Nordheim tunneling with low turn-on-field values (0.8 – 2.0 V/μm) and a low barrier height of 0.02 eV. An emission current greater than 5 μA was maintained for over 24 h in a cathode based on the developed PDPs. In contrast, single-crystalline diamond particles prepared for comparative purposes exhibited no emission up to 2.5 V/μm. Auger electron spectroscopy revealed that the surface oxygen content modified by annealing in air did not affect the emission properties. The macroscopic spatial...