Hitoshi Sakamoto

Carbon disc of high density and strength prepared from heat-treated mesophase pitch grains

Abstract Carbonaceous grains for binderless moulding were prepared by control of the preheat-treatment of the naphthalene-derived mesophase pitch. The carbonized and graphitized discs were evaluated in terms of physical properties and textures. Carbon disc of strikingly high bulk density, 2.04 g/cm3, high compressive strength, 1.94 tf/cm2, and large shrinkage ratio, 15.9%, was obtained through the grinding of the pitch preheat-treated at 753 K, their cold moulding, carbonization at a heating rate of 12 K/h, and successive graphitization at 2173 K. A homogeneous fine-mosaic texture of optical anisotropy was observed over the whole region in the artifact, indicative of sufficient adhesion among grains during the carbonization. X-ray analyses suggested a graphitizing nature of the grains in their moulds. Grains preheat-treated under 753 K exhibited excess fusibility for adhesion, leading to partial flow texture, where many cracks were easily induced during the calcination, decreasing the strength of the disc. In contrast, preheat-treatment over 753 K caused insufficient adhesion among grains, decreasing markedly both density and strength of the disc. A slight but significant presence of the pyridine-insoluble/quinoline-soluble fraction in the preheattreated pitch system is indicated to play a major role in adhesive ability of the grains preheat-treated at 753 K.

n-type high-conductive epitaxial diamond film prepared by gas source molecular beam epitaxy with methane and tri-n-butylphosphine

An n-type phosphorous (P) doped epitaxial diamond film with high conductivity was grown on a C(001) substrate by gas source molecular beam epitaxy using methane and tri-n-butylphosphine. The electrical conductivity of the diamond film was measured to be 0.33 (Ω cm)−1 at 23 °C with its activation energy to be 0.12 eV. The Hall measurements showed n-type conduction and a carrier concentration of 1.6×1018 cm−3 at 400 °C, which is comparable to the P concentration determined by secondary-ion-mass spectroscopy. These indicate the formation of shallow P donors with high electrical activation efficiency. A p-n junction diode was fabricated by growing a P-doped epitaxial film on a boron-doped C(001) substrate, which showed for the first time a rectification ratio of ∼103 at 10 V.

Self-adhesive carbon grains oxidatively prepared from naphthalene-derived mesophase pitch for mould of high density

Self-adhesive carbonaceous grains for binderless mould were prepared through the oxidation treatment of the ground mesophase pitches catalytically synthesized from naphthalene with HF/BF3. The oxidation conditions strongly influenced the carbonization behaviors of the mesophase pitch grains and the strength of the resultant carbon artifacts (diameter: 20 mm, thickness: 4 mm). The oxidation at 493 K for 1–2 h in air flow was optimum to balance the fusibility and the thermosetting property of the mesophase pitch grains. The properly oxidized grains were proven to show self-adhesive ability, shape stability, and higher carbon yield, providing a carbon artifact of strikingly high density, 1.88 g/cm3, high compressive strength, 2.9–2.6 tf/cm2, and large shrinkage ratio, 33–42 vol% by the calcination at 1573 K. The observation using a polarized light optical microscope and SEM indicated fine-mosaic texture, dense packing, and sufficient adhesion of the grains in the carbonized artifact.

Low-Temperature Si Selective Epitaxial Growth Using Electron-Beam-Induced Reaction

Low-temperature Si epitaxial growth has been achieved by irradiating electrons and Si2H6 molecules simultaneously onto Ge(100) and Si(100) surfaces. The growth takes place exclusively on the electron-irradiated area. The electron stimulation of the surface enables growth rate enhancement and improvement of crystallinity. Si growth is governed by surface reactions induced by electron beam irradiation.

Crystal structure and characterization of pyrroloquinoline quinone disodium trihydrate

BackgroundPyrroloquinoline quinone (PQQ), a tricarboxylic acid, has attracted attention as a growth factor, and its application to supplements and cosmetics is underway. The product used for these purposes is a water-soluble salt of PQQ disodium. Although in the past, PQQ disodiumpentahydrates with a high water concentration were used, currently, low hydration crystals of PQQ disodiumpentahydrates are preferred.ResultsWe prepared a crystal of PQQ disodium trihydrate in a solution of ethanol and water, studied its structure, and analyzed its properties. In the prepared crystal, the sodium atom interacted with the oxygen atom of two carboxylic acids as well as two quinones of the PQQ disodium trihydrate. In addition, the hydration water of the prepared crystal was less than that of the conventional PQQ disodium crystal. From the results of this study, it was found that the color and the near-infrared (NIR) spectrum of the prepared crystal changed depending on the water content in the dried samples.ConclusionsThe water content in the dried samples was restored to that in the trihydrate crystal by placing the samples in a humid environment. In addition, the results of X-ray diffraction (XRD) and X-ray diffraction-differential calorimetry (XRD-DSC) analyses show that the phase of the trihydrate crystal changed when the crystallization water was eliminated. The dried crystal has two crystalline forms that are restored to the original trihydrate crystals in 20% relative humidity (RH). This crystalline (PQQ disodium trihydrate) is stable under normal environment.

Kinetics of dissociative adsorption of dichlorosilane on Si(100)2 × 1

Abstract The temperature dependence of the dichlorosilane (SiH 2 Cl 2 ) dissociative adsorption kinetics on a Si(100)2 × 1 surface was investigated using ultraviolet photoelectron spectroscopy (UPS). By observing in situ the UPS intensity of the surface state originated from the dimer dangling bonds, the time evolutions of the surface chlorine and hydrogen coverage during SiH 2 Cl 2 exposure onto a Si(100) clean surface were obtained. They were successfully fitted with a rate equation assuming a competition between the SiH 2 Cl 2 adsorption and the desorption of H 2 , HCl, and SiCl. The reaction order and the reaction coefficient for the SiH 2 Cl 2 adsorption at RT were determined from the fitting to be 1.75 and 3.0 × 10 −2 ML/s, respectively, while they were changed to 1.80 and 9.0 × 10 −3 ML/s at 400°C or to 3.20 and 6.0 × 10 −3 ML/s at 600°C as the adsorption temperature was raised. Based on this strong temperature dependence of the parameters, we proposed a surface reaction model of the SiH 2 Cl 2 adsorption kinetics, which includes temperature dependent partial decomposition of the adsorption precursors.

First-order isothermal desorption kinetics of chlorine on SiH2Cl2-adsorbed Si(100) surface

Abstract The isothermal desorption kinetics of chlorine on Si(100) was investigated by observing the chloride coverage on the surface with ultraviolet photoelectron spectroscopy. The samples for desorption were prepared by dichlorosilane adsorption at 600°C under 1 × 10 -6 Torr. We found that the decrease of the chloride coverage showed an exponential dependence on the isothermal annealing time, indicating clearly that the chlorine desorption was a first-order reaction. From the Arrhenius plot of the first-order reaction coefficient, the activation energy of chlorine desorption was obtained to be 48.1 kcal/mol. Furthermore, we clarified by quadrupole mass spectroscopy that the dominant desorption species was the diatomic SiCl molecule. To interpret all these results, we proposed a chlorine desorption reaction model based on the collision between a chloride and a migrating Si adatom released from an atomic step.

Photoelectron intensity oscillation during chemical vapor deposition on Si(100) surface with Si2H6

We have found that the photoelectron intensity of the dimer‐dangling‐bond‐derived surface state on Si(100) shows a periodic oscillation during chemical vapor deposition with Si2H6 gas. The substrate temperature and Si2H6‐pressure dependence of the oscillation period was measured. By use of a selective‐growth method using SiO2‐patterned wafers, one oscillation period was clarified to correspond to the Si growth of one atomic layer of the Si(100) plane with a thickness of 1.36 A.

Methane adsorption and hydrogen isothermal desorption kinetics on a C(001)–(1×1) surface

Methane (CH4) adsorption and isothermal hydrogen (H2) desorption kinetics on a C(001)–(1×1) surface have been investigated by observing the surface hydrogen coverage θH which is estimated from the electron‐stimulated desorption yield of H+ ions. The time evolution of θH under CH4 exposure, in pressure and temperature ranges of 1×10−8–5×10−7 Torr and 600–800 °C, reveals that the CH4 adsorption is a first‐order reaction. The isothermal H2 desorption is conducted in a temperature range of 1095–1270 °C. The time evolution of θH shows that the isothermal H2 desorption is also a first‐order reaction. From Arrhenius plots of the reaction coefficients, the activation energies of CH4 adsorption and H2 desorption are evaluated to be 7.3±1.7 and 21±4.9 kcal/mol, respectively. Atomistic kinetics model for CH4 adsorption and H2 desorption are discussed, especially in connection with a recent finding of gas source molecular beam epitaxy of diamond (001) with pure CH4 without any hydrogen or oxygen radicals.

Diamond Epitaxial Growth by Gas-Source Molecular Beam Epitaxy with Pure Methane

Diamond epitaxial films with a thickness of 200-350 A have been successfully grown on C(100) surfaces by gas-source molecular beam epitaxy (GSMBE) with methane without addition of hydrogen or oxygen. The activation energy was measured by a selective growth method using Ta-patterned substrates to be 15 kcal/mol. This value is close to the activation energy of H2 desorption on C(100), suggesting that the diamond epitaxial growth rate by this GSMBE is limited by hydrogen desorption.