Hiroki Akasaka

Work Function of Amorphous Carbon Nitride with Various Functional Groups

The work function of the amorphous phase of hydrogenated carbon nitride (a-CNx:H) was calculated using Fowler–Nordheim field emission. First, metal films were deposited on conductive ZnO:Al whiskers to determine the geometric field enhancement factor at the tip of the field emitter. The radius of curvature at the whisker tip after the deposition of metal films was measured. Next, the geometric field enhancement factor of ceramic whiskers was calculated using the work function of the metals and the radius of curvature of the emitter tip. Finally, the work function of a-CNx:H was introduced using the Fowler–Nordheim equation with the geometric field enhancement factor. The work function of a-CNx:H is dependent upon the functional group.

Detection of Protein Adsorption on Silica Surface Using Surface Plasmon Resonance Sensor

Rapid screening for exploring biocompatible ceramics is important. We demonstrate the detection of lysozyme (Lyz) on a silica surface using a custom-made surface plasmon resonance (SPR) sensor that can detect the interaction between ceramics and biomolecules. The results expressed as shifts of the resonance angle using the SPR sensor followed the expected theory when silica film was synthesized on the sensor chip. Moreover, the adsorption and desorption behaviors of Lyz on the hydrophilic or hydrophobic silica surface in phosphate buffer solution were investigated. Relatively large amounts of Lyz molecules aggregated on the hydrophobic silica surface.

Detailed Structural Analyses of KOH Activated Carbon from Waste Coffee Beans

The relationship of the detailed structural change of KOH activated carbon and hydrogen storage ability was investigated in activated carbon materials fabricated from waste coffee beans. The specific surface area of porous carbon materials calculated from N2 adsorption isotherms stood at 2070 m2/g when the weight ratio of KOH to carbon materials was 5:1, and pore size was in the range of approximately 0.6 to 1.1 nm as micropores. In the structural analysis, X-ray diffraction analysis and Raman spectroscopy indicated structural change in these carbon materials through KOH activation. The order of the graphite structure changed to a smaller scale with this activation. It is theorized that specific surface area increased using micropores provided by carbon materials developed from the descent of the graphite structure. Hydrogen storage ability improved with these structural changes, and reached 0.6 wt % at 2070 m2/g. These results suggest that hydrogen storage ability is conferred by the chemical effect on graphite of carbon materials.

Synthesis of Metal Oxide Particles Using Reaction Route from Rare-Earth Metal-EDTA Complexes

Highly dense, spherical yttria (Y2O3) and erbia (Er2O3) particles were synthesized from their corresponding metal-ethylenediaminetetraacetic (EDTA) complexes. The EDTA·Y·H and EDTA·Er·H complexes were prepared in powdered form. These complexes were used as the staring materials for synthesis of the Y2O3 and Er2O3 particles. The particles were synthesized using an H2-O2 flame produced with a commercial flame spray apparatus. Crystalline structure, surface and cross-sectional morphologies, and elemental distribution of the synthesized particles were investigated. It was confirmed that the crystalline phases of the Y2O3 and Er2O3 particles were homogeneous. In addition, the elemental distribution of the particles was uniform. These results indicate that dense, spherical particles of Y2O3 and Er2O3 have been synthesized with EDTA·Y·H and EDTA·Er·H complexes, respectively.

Structure and Mechanical Properties of Diamond-Like Carbon Films Prepared from C2H2 and H2 Mixtures by Pulse Plasma Chemical Vapor Deposition

Diamond-like carbon (DLC) films were prepared by dc pulse plasma chemical vapor deposition (CVD). The hydrogen flow ratio [H2/(C2H2+H2)] was varied from 0 to 0.75, and the pulse frequency was varied from 1 to 20 kHz. Then, the relationship between film structure and hardness was investigated. At H2/(C2H2+H2)=0, films prepared at 10 kHz have the greatest hardness because a further increase in pulse frequency leads to an increase in the number of regular sixfold ring structures in the film and excessive ion peening on the growth face. The hardness at H2/(C2H2+H2)=0.5 is greater than that at H2/(C2H2+H2)=0 because the effect of ion peening and the number of regular sixfold ring structures at H2/(C2H2+H2)=0.5 are suitable for fabricating hard films. In contrast, at H2/(C2H2+H2)=0.75, the hardness is lower than that at H2/(C2H2+H2)=0 because of the reduced effect of ion peening; thus, the effect of an increase in the number of polymer bonds becomes significant.

Dissociative Excitation of C2H2 in the Electron Cyclotron Resonance Plasma of Ar: Production of CH(A2Δ) Radicals and Formation of Hydrogenated Amorphous Carbon Films

The dissociative excitation reaction of C2H2 with the electron-cyclotron resonance plasma of Ar was investigated based on the electrostatic-probe measurements and on the optical emission spectroscopy of the CH(A2Δ–X2Π) transition. The density, ne, and the temperature, Te, of free electrons were controlled by adding H2O molecules externally into the reaction region, and the dependence of the CH(A2Δ–X2Π) emission intensity on the addition of H2O was observed to compare with the evaluated dependencies based on ne and Te. The mechanism of production of CH(A2Δ) was found, predominantly, to be the electron impact with the contribution of 10–20% of the electron-impact dissociation of C2H radicals; the contribution of the ion–electron recombination was negligible. Hydrogenated amorphous carbon films were fabricated using the same reaction system. The atomic compositions, Raman spectra, and the hardness of films were discussed in terms of the variations of ne and Te upon the addition of H2O molecules.

New Violet Phosphor Sr7Al12O25:Eu2+ Synthesized from Sr–Al–O:Eu Powder Mounted on Polycrystalline Alumina

A new violet phosphor was synthesized by sintering under the reduction condition from the Sr–Al–O:Eu powder oxidized from an ethylenediaminetetraacetic (EDTA) complex on a polycrystalline alumina plate. The crystalline phase of violet phosphor was determined to be Sr7Al12O25 by the change in the crystallinity of the product with a change in the composition of the raw material. The emission wavelength of the phosphor was 410 nm for Sr7Al12O25:Eu2+. For the composition of Sr:Al:Eu = 6.86:8:0.14 in the EDTA complex with sintering at 1400 °C, Sr7Al12O25:Eu2+ was obtained only in the crystalline phase of Sr7Al12O25 and exhibited the highest emission intensity of 410 nm in our experiment. The sintering temperature at sintering and composition of metals in the EDTA complex affected the emission wavelength change from 410 to 460 nm because of change in crystallinity. These changes indicate that the emission color in the range from violet to green is controlled by composition change or sintering temperature.

Roles of Ions and Free Electrons in the Synthesis of Mechanically Hard Amorphous-CNx Films Using a Dissociative Excitation Reaction of BrCN with the Ar Electron Cyclotron Resonance Plasma

A dissociative excitation reaction of BrCN with the Ar electron cyclotron resonance (ECR) plasma was applied to synthesize mechanically hard amorphous carbon nitride (a-CNx) films. The hardness of the films has been measured by a nanoindenter to be at most 15 GPa under the maximum loading force of 50 mN. The self-bias voltage, -Vself, and the I–V characteristics have been measured to discuss the mechanism of hardening of the a-CNx films. It was confirmed that -Vself causes the Ar ion bombardment on the surface of the films, leading to the high hardness value of the films without applying an external bias voltage.

Tribological and thermal stability study of nanoporous amorphous boron carbide films prepared by pulsed plasma chemical vapor deposition

Abstract In this work, the thermal stability and the oxidation and tribological behavior of nanoporous a-BC:H films are studied and compared with those in conventional diamond-like carbon (DLC) films. a-BC:H films were deposited by pulsed plasma chemical vapor deposition using B(CH3)3 gas as the boron source. A DLC interlayer was used to prevent the a-BC:H film delamination produced by oxidation. Thermal stability of a-BC:H films, with no delamination signs after annealing at 500 °C for 1 h, is better than that of the DLC films, which completely disappeared under the same conditions. Tribological test results indicate that the a-BC:H films, even with lower nanoindentation hardness than the DLC films, show an excellent boundary oil lubricated behavior, with lower friction coefficient and reduce the wear rate of counter materials than those on the DLC film. The good materials properties such as low modulus of elasticity and the formation of micropores from the original nanopores during boundary regimes explain this better performance. Results show that porous a-BC:H films may be an alternative for segmented DLC films in applications where severe tribological conditions and complex shapes exist, so surface patterning is unfeasible.

Analysis of Amorphous Alumina Thin Film Etching by Phosphate Buffer Using Surface Plasmon Resonance

The optical surface plasmon resonance (SPR) phenomenon was used to estimate the etching rate for inorganic films. The etching rate for amorphous alumina films deposited by atmospheric chemical vapor deposition using a phosphate buffer (PB) solution was estimated by the SPR method using a multilayer structure of alumina/gold/chromium/optical glass. To demonstrate that the SPR technique can be used to measure film thickness, the decrease in the thickness of a film as a result of etching was confirmed by measuring the thickness by the X-ray reflectivity (XRR) technique. The alumina/silicon samples were immersed in PB solution, and the film thicknesses obtained by the XRR technique changed from 46 to 35 nm when etched by the 0.3 M PB solution for 5 h. In SPR experiment, the same concentration of PB was used for the multilayer structure of alumina, and it was observed that the thickness changed from 26.3 to 9.8 nm in 5 h. It was found that the etching rates determined by both techniques for each PB concentration were in agreement. These results indicate that the SPR technique, which is an easy and safe technique compared with the XRR technique, could be used to estimate the etching rate for multilayer structures.