Masahito Tagawa

Local deviation in contact angles on heterogeneous fibrous solids

Wetting force measurements in solid/water/n-alkane systems were carried out using carbon fibers, boron fiber, fibrous quartz, and polymeric fibers by the Wilhelmy technique. Contact angles of water were independent of the velocity of the moving three-phase boundary on repyrolyzed PAN-based carbon fibers which did not show any contact angle hysteresis. Hysteresis in contact angles due to surface heterogeneity began to appear after irradiation with atomic oxygen which oxidized the carbon fibers. On atomic oxygen exposed carbon fibers, advancing contact angles increased and receding ones decreased with increasing velocity, and there was a region where both angles were constant. The range of deviations in contact angles due to surface heterogeneity was determined from local deviations in weight traces at the velocity of three-phase boundary of 0.3 mm min-1 where both angles were independent of the velocity. Deviations in contact angles, Δ, were larger in advancing than in receding for hydrophilic solids (e.g....

Contact angle hysteresis in carbon fibers studied by wetting force measurements

The surface free energy of polyacrylonitrile carbon fibers was investigated by using the Wilhelmy technique. The difference in surface free energy between immersion and emersion was observed for the carbon fiber pyrolyzed at 2500 °C.In contrast, the hysteresis disappeared with repyrolyzation of the carbon fibers at 3000 °C. Auger electron spectroscopic analysis indicated that the surface of the latter carbon fiber (repyrolyzed at 3000 °C) consisted of the basal planes of graphite. Rough surface topography of the carbon fiber repyrolyzed at 3000 °C, as observed by scanning electron microscope, did not affect the hysteresis. Therefore, the contact angle hysteresis was attributed to the chemical adsorbants on the activation sites of the fiber surfaces, as detected by Auger electron spectroscopy.

Formation of Thin Oxide Films on Room-Temperature Silicon (100) by Exposure to a Neutral Beam of Hyperthermal Atomic and Molecular Oxygen

Silicon (100) surfaces at room temperature were oxidized in a hyperthermal beam containing nearly equal fractions of atomic and molecular oxygen. An oxide layer with a terminal thickness of 4.5 nm was formed on the Si surfaces, and this layer was composed mostly of silicon dioxide (SiO2). However, a significant fraction of suboxide (SiOx, x<2) was formed at the Si/SiO2 interface, where the intrinsic stress was high. Post annealing in vacuum at 1000°C for 30 min decreased the amount of suboxide and stress at the interface.

A fast atomic oxygen beam facility with in situ testing/analysis capabilities

A fast atomic oxygen beam facility consisting of a beam source, a mass spectrometer, an Auger electron spectroscope, a scanning tunneling microscope, and a friction tester has been developed to investigate interaction of energetic atomic oxygen with solid surfaces. The fast atomic oxygen beam has been characterized by time of flight distribution, quadrupole mass spectrometry, and quartz crystal microbalance. The time of flight distribution of the beam has shown that the average translational energy of the atomic oxygen beam reaches 4.7 eV and that the full width at half-maximum is 5.5 eV. A flux of the atomic oxygen is calculated from the frequency shift of the quartz crystal microbalance with silver electrodes, and typical flux of the atomic oxygen beam being 4.0×1012u2009atoms/cm2u2009s. The flux of atomic oxygen of this source is fairly low, but is corresponding to that in the altitude of 500 km in low Earth orbit. The surface sensitive analysis methods equipped with the facility, such as Auger electron spectr...

Microtribological properties of ultrathin C60 films grown by molecular beam epitaxy

Abstract Molecular beam epitaxy (MBE) has been applied to a condensation of thin C 60 films with high crystallinity. In situ measurements with reflection high energy electron diffraction (RHEED) showed an epitaxial growth of C 60 film with a lattice constant of 1.0 nm on a MoS 2 substrate. Atomic force microscopy (AFM) and friction force microscopy (FFM) revealed a low friction coefficient of 0.012, the lowest value reported to date for C 60 .

Field‐stimulated exoelectron emission from 99.9999% pure Al

Exoelectrons were emitted from a 99.9999% pure Al tip by applying an electric field as a stimulus. This occurred at lower voltages than that necessary for field emission of electrons. Field‐stimulated exoelectron emission showed a characteristic storage effect in accordance with the time interval of applied voltage pulses. Based upon this storage effect and a computer calculation of the field strength at the apex of the tip, we show by analytical discussions that exoelectrons are emitted by tunneling, not by the Schottky effect.

Effect of Atomic Oxygen Exposures on the Tribological Properties of Molybdenum Disulfide Lubricants

In this paper, the effect of atomic oxygen exposures on the tribological properties of molybdenum disulfide (MoS2) is studied with a laser-induced breakdown type atomic oxygen facility. MoS2 single crystals as well as sputtered films are used as specimens to clarify both fundamental reaction and change in tribological properties of MoS2.

Tribo- and photo-stimulated exoelectron emission from graphite

Abstract Tribo- and photo-stimuated exoelectron emission from reactor graphite has been investigated, because an increase in tribo-stimulated exoelectron emission from graphite signifies large friction. Selective deformation of graphite fibrils was identified with transmission electron microscopy. The exothermic process during the preferred orientation caused emission of tribo-stimulated electrons. Photo-stimulated exoelectron emission from reactor graphite was also detected. However, the mechanisms of photo-stimulated exoelectron emission were different from that of tribo-stimulated exoelectron emission, and photo-stimulated exoelectron emission does not relate to a change in tribological characteristics.

Atomic configuration of carbon fibers studied by field ion microscopy

A field ion microscope has been used to investigate the atomic configurations of carbon fibers. A stable field ion micrograph was obtained by wing Ne as an image gas, and the aligned fibers were imaged as small net plane rings. As-received high-modulus fibers made from polyacrylonitrile showed the notable fibril alignment only at the periphery, while repyrolyzed fibers at 3000°C indicated the formation of aligned fibril at the central area as well as at the periphery. The pitch carbon fibers repyrolyzed at 3000°C showed a uniform distribution of aligned large fibrils, which correlated to the high Youngs modulus. A model based upon FIM observations was proposed in relating microscopic structures of fibrils to tribological design of carbon fiber composites. Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987

Electric field distribution of electron emitter surfaces

The electric field distribution of a tungsten field emitter surface and a LaB6 thermionic emitter surface has been studied. The computer simulation of electric field distribution on the emitter surface was carried out with a charge simulation method. The electric field distribution of the LaB6 thermionic emitter was experimentally evaluated by the Schottky plot. Two independent equations are necessary for obtaining local electric field and work function; the Fowler–Nordheim equation and the equation of total energy distribution of emitted electron being used to evaluate the electric field distribution of the tungsten field emitter. The experimental results agreed with the computer simulation.