Toshiki Tsubota

High‐temperature thermoelectric properties of (Zn1−xAlx)O

A mixed oxide (Zn1−x Al x )O exhibits promising thermoelectricproperties attaining a dimensionless figure of merit ZT of 0.30 at 1000 °C, which value is much superior to other oxides and quite comparable to conventional state‐of‐the‐art thermoelectricmaterials. The addition of a small amount of Al2O3 to ZnO results in a large power factor of 10–15×10−4 W/mK2, showing a marked increase in the electrical conductivity while retaining moderate thermoelectric power. A large product of the carrier mobility and density of states would be responsible for the favorable electrical properties of the present oxide. A figure of merit Z=0.24×10−3 K−1 is attained by (Zn0.98Al0.02)O at 1000 °C, even with a high thermal conductivity. A predominant proportion of the phononthermal conductivity promises a further improvement in the thermoelectric performance by selective enhancement of phonon scattering.

Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion

The thermoelectric properties of a mixed oxide (Zn 1-x Al x )O (x=0, 0.005, 0.01, 0.02, 0.05) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, σ, of the oxide increases on Al-doping by more than three orders of magnitude up to ca. 10 3 S cm -1 at room temperature, showing metallic behaviour. The Seebeck coefficient, S, of (Zn 1-x Al x )O (x>0) shows a general trend in which the absolute value increases gradually from ca. -100 µV K -1 at room temperature to ca. -200 µV K -1 at 1000 °C. As a consequence, the power factor, S 2 σ, reaches ca. 15×10 -4 W m -1 K -2 , the largest value of all reported oxide materials. The thermal conductivity, κ, of the oxide decreases with increasing temperature, owing to a decrease in the lattice thermal conductivity which is revealed to be dominant in the overall κ. In spite of the considerably large values of κ, the figure of merit, Z=S 2 σ/κ, reaches 0.24×10 -3 K -1 for (Zn 0.98 Al 0.02 )O at 1000 °C. The extremely large power factor of (Zn 1-x Al x )O compared to other metal oxides can be attributed to the high carrier mobility revealed by the Hall measurements, presumably resulting from a relatively covalent character of the Zn–O bond owing to a fairly small difference of the electronegativities of Zn and O. The dimensionless figure of merit,ZT, of 0.30 attained by (Zn 0.98 Al 0.02 )O at 1000 °C demonstrates the potential usefulness of the oxide.

Heteroepitaxial growth of diamond on an iridium (100) substrate using microwave plasma-assisted chemical vapor deposition

Abstract An iridium (100) layer was epitaxially coated on a MgO (100) plate by sputtering at 1123 K, and was then utilized in the formation of diamond by microwave plasma-assisted chemical vapor deposition (MPCVD) using methane as the carbon source. The electric contact between the substrate and holder was confirmed by coating the entire MgO surface with iridium. The iridium substrate was then treated by bias-enhanced nucleation under optimized conditions. It was found that diamond particles formed by MPCVD were essentially oriented to the iridium substrate. The diamond particles were then grown to the 〈100〉 and further to the 〈111〉, and a smooth diamond film was obtained. The full width at half maximum of the (400) rocking curve of the diamond film was 0.16°, which was close to that of a diamond single crystal.

Chemical modification of diamond surface using a diacyl peroxide as radical initiator and CN group-containing compounds for the introduction of the CN group

The introduction of the CN group onto a diamond surface was attempted using diacyl peroxides with CN group-containing compounds. When acetonitrile was used as solvent for the radical reaction process, the CN group was introduced onto the diamond surface, however when benzonitrile was used no CN group was introduced. The mechanism for introduction of the CN group onto the diamond surface is proposed to be as follows: Acetonitrile includes an alkyl group in its structure. The radical species derived from the diacyl peroxide appears to abstract the hydrogen atom from the alkyl group, with the radical species generated from acetonitrile then reacting with the diamond surface activated by the radical species derived from the diacyl peroxide. However, for benzonitrile, no hydrogen abstraction reaction can proceed as it has no alkyl group.

Chemical reaction of hydrogenated diamond surface with peroxide radical initiators

Abstract The reactivity of hydrogenated diamond surfaces with peroxide radical initiators such as benzoyl peroxide, lauroyl peroxide, dicumyl peroxide and di- t -butyl peroxide was investigated in this study. With benzoyl peroxide as the radical initiator, the intensity of the peaks assigned to the aliphatic CH stretching vibration decreased with increasing reaction time, as well as with increasing amount of benzoyl peroxide. Moreover, peaks assigned to the aromatic CH and CO stretching vibrations appeared following the reaction. For the reaction with lauroyl peroxide, the area under the peaks assigned to the CH bond increased with treatment time and amount of lauroyl peroxide, and a CO stretching vibration peak appeared. The shape of the IR spectra of diamond powder treated with dicumyl peroxide or di- t -butyl peroxide is virtually the same as that of hydrogenated diamond powder surface.

Photocatalytic Hydrogen or Oxygen Evolution from Water over S- or N-Doped TiO2 under Visible Light

S- or N-doping of powder having an anatase or rutile phase extended the photocatalytic activity for water oxidation and reduction under UV light and visible light irradiation. For the reduction of water, anatase-doped showed higher level of activity than that of doped having a rutile phase using ethanol as an electron donor. Furthermore, the activity level of S-doped for hydrogen evolution was higher than that of N-doped photocatalysts under visible light. Photocatalytic oxidation of water on doped having a rutile phase proceeded with fairly high efficiency when ions were used as electron acceptors compared to that on doped having an anatase phase. In addition, water splitting under visible light irradiation was achieved by construction of a Z-scheme photocatalysis system employing the doped having anatase and rutile phases for and evolution and the redox couple as an electron relay.

Photochemical modification of diamond powders with elemental sulfur and their surface-attachment behavior on gold surfaces

A useful method of modifying the surface of diamond powder with sulfur-containing functionalities was developed by the photolysis of elemental sulfur. The introduction of sulfur-containing functional groups on the diamond surfaces was confirmed by X-ray photoelectron spectroscopy (XPS), diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) and mass spectroscopy analyses. The sulfur-modified diamond powder attached to gold surfaces through sulfur-containing linkages. In brief, the exposure of the modified diamond powder to a gold colloid resulted in gold nanoparticles being attached to the diamond powder. The treatment of the modified diamond powder with thin gold film on a Si substrate resulted in the alignment of surface-attached diamond powder through sulfur linkages formed by self-assembly.

Improvement of visible light photocatalytic acetaldehyde decomposition of bismuth vanadate/silica nanocomposites by cocatalyst loading

Photocatalytic activity of bismuth vanadate (BiVO(4)) for acetaldehyde decomposition under visible light irradiation was improved by inclusion of a nanocomposition of silica as an adsorbent material and loading of platinum (Pt) or trivalent iron ion (Fe(3+)) as reduction cocatalysts. Addition of silica enhanced photocatalytic activity due to improvement of adsorption ability, but total decomposition of acetaldehyde was not observed within 24h of visible light irradiation. For further improvement of photocatalytic activity, BiVO(4) with an optimized amount of silica composition were modified with Pt or Fe(3+). Photodeposition of Pt greatly increased photocatalytic activity, and acetaldehyde was totally decomposed within 24h of visible light irradiation.

Reactivity of the hydrogen atoms on diamond surface with various radical initiators in mild condition

Abstract Radical reactions of various radical initiators in liquid phase were carried out to investigate the reactivity of hydrogenated diamond surface. Benzophenone, which is one of the most popular photosensitizers, and AIBN (α,α′-azobisisobutyronitrile), which is one of the most popular radical initiators of azo compounds, could not abstract hydrogen atoms on the diamond surface. On the other hand, benzoyl peroxide, which is one of the most popular radical initiators of peroxides, could abstract the hydrogen atoms on a diamond surface. From the results of this study, it was found that the reactivity for the hydrogen abstraction reaction with organic radical reagents depends on the kind of the radical species.

Surface morphology and electrical properties of boron-doped diamond films synthesized by microwave-assisted chemical vapor deposition using trimethylboron on diamond (100) substrate

Prime novelty: The smoothness of the synthesized boron-doped diamond was improved by the pre-treatment of a hydrogen plasma. Moreover, the Hall mobility also increased with this pre-treatment. Surface morphology and electrical properties, such as electrical conductivity, hole concentration and Hall mobility, were investigated for boron-doped diamond films, which were synthesized by microwave-assisted chemical vapor deposition (MPCVD) on a (100) diamond substrate. Trimethylboron (TMB) was used as a dopant source and methane (CH4) was used as a carbon source. The morphology of the synthesized diamond surface depended on the MPCVD conditions such as TMB and CH4 concentrations in the gas phase, and lower concentrations of TMB and CH4 lead to a smoother surface. When the substrate was treated in a hydrogen plasma, the electrical properties of the boron-doped diamond films, as well as the smoothness of the surface, were improved. After optimizing the synthesis conditions, Hall mobility reached to 2020 cm2 V−1 s−1 at 243 K for a diamond film with a hole concentration of 5×1012 cm−3.