Michitaka Ohtaki

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.

Enhancement of the Thermoelectric Performance of p‐Type Layered Oxide Ca3Co4O9+δ Through Heavy Doping and Metallic Nanoinclusions

By converting heat directly into electricity, thermoelectric (TE) generation offers a promising technology to recover waste heat emitted from industrial sectors and energy consumption processes. [ 1 ] The key to realize an effi cient TE generator lies, however, in fi nding good materials with high TE performance, a good durability at high temperature, and preferably robustness to operating in air. The performance of a TE material is evaluated by the dimensionless fi gure-of-merit ZT ( = S 2 T / ρ κ , where S , T , ρ , and κ are the Seebeck coeffi cient, absolute temperature, electrical resistivity, and thermal conductivity, respectively). By far the most widely used TE materials are alloys of Bi 2 Te 3 , PbTe, and SiGe, which often suffer from poor durability at high temperature, are harmful or scarce, and have costly constituting elements. Metal oxides have been considered as an alternative to overcome these problems. Metal oxide-based materials have been attracting continuous interest as TE materials over the years since the discovery of large TE power in p-type NaCo 2 O 4 single crystals by Terasaki et al. in 1997. [ 2 ] However, practical application of this oxide for power generation from waste heat has never been realized because of the volatility of Na and the instability of the compound against humidity. Another Co-based oxide p type material Ca 3 Co 4 O 9 + δ has also been intensively investigated because of its good TE performance ( ZT = 0.83 at 973 K for the single crystal) [ 3 ] and its high thermal and chemical stabilities even up to 1200 K in air. [ 4–7 ] An incommensurate character in the crystal structure of this compound is explicitly described as [Ca 2 CoO 3 ] b 1/ b 2 [CoO 2 ], where b 1 and b 2 are two different periodicities along the b axis for the rock salt-type Ca 2 CoO 3 subsystem and the CdI 2 -type CoO 2 subsystem, respectively. [ 4 ] Single crystals are less likely to be applied for fabricating practical TE devices, because they will be too expensive. It is hence highly desirable to achieve suffi cient TE properties in a polycrystalline form of these oxides. Although the diffi culty of discovering novel high performance

Preparation and photocatalytic activities of a semiconductor composite of CdS embedded in a TiO2 gel as a stable oxide semiconducting matrix

A semiconductor composite of CdS particles embedded in a TiO2 gel after heat treatment at 250°C in air is revealed to show catalytic H2 evolution from an EDTA aqueous solution with light of λ⩾450 nm, in contrast to bare CdS which is stoichiometrically photocorroded. A significant improvement in the activity even with considerable losses of CdS in the gel during the heat treatment suggests that the TiO2 gel crystallized into anatase can prevent photocorrosion of CdS embedded therein, and serve as a semiconducting matrix to facilitate charge transfer from the embedded CdS to Pt at the surfaces of the gel.

Synthesis of TiO2/CdS nanocomposite via TiO2 coating on CdS nanoparticles by compartmentalized hydrolysis of Ti alkoxide

Hydrolysis of Ti alkoxide in the presence of inverse micelles containing CdS nanoparticles in their hydrophilic interior results in formation of TiO2/CdS nanocomposites in which the CdS nanoparticles are embedded in a TiO2 matrix with a thickness of ≤10 nm at the surface of the particles. The primary hydroxyl groups introduced by 2-mercaptoethanol as a capping agent used for preparation of the CdS nanoparticles are considered to play an important role for successful adhesion and growth of the TiO2 layer on the CdS surfaces. TEM observation strongly supports formation of semiconductor-in-semiconductor heteronanostructure by compartmentalized hydrolysis of Ti alkoxide within the inverse micelles in which the surface-capped CdS nanoparticles coexist.

Solid Oxide Fuel Cell with Composite Electrolyte Consisting of Samaria‐Doped Ceria and Yttria‐Stabilized Zirconia

A composite oxide containing samaria-doped ceria [SDC, (CeO 2 ) 0.8 (Smo 1.5 ) 0.2 ] and yttria-stabilized zirconia [YSZ, (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 ] was developed as an electrolyte for a solid oxide fuel cell (SOFC). A SOFC with a YSZ electrolyte was characterized by a high open-circuit voltage (OCV) and a steep decrease in voltage, V, with increasing current, I, whereas that with a SDC electrolyte exhibited a low OCV and a gradual decrease of its V-I curve. A biphasic electrolyte microstructure with SDC and YSZ was proposed in which SDC grains are dispersed in a YSZ matrix. In this composite structure, the electronic conduction caused by reduction of the SDC was effectively blocked by the YSZ matrix. An SOFC with this composite electrolyte offered the advantages of YSZ with its high OCV and SDC with its gradual decrease of the V-I curve. The SDC content and the particle size significantly affected the power-generation characteristics of a cell with this composite electrolyte. The electrode polarization was significantly lowered by using the SDC or the composite electrolyte.

High-temperature thermoelectric properties of In2O3-based mixed oxides and their applicability to thermoelectric power generation

Thethermoelectric properties of mixed oxides In2O3·MOx(MOx= Cr2O3, Mn2O3, NiO, ZnO, Y2O3, Nb2O5 SnO2) are investigated in terms of the thermoelectric materials at high temperature. The Seebeck coefficients,S, of all the samples have negative values, and those of In2O3·SnO2 and In2O3·ZnO increase linearly with temperature, attaining values of –90 and –210 µV K–1 at 1000 °C, respectively. The electrical conductivities, σ, of these oxides are significantly high. The power factor S2σ of the oxides has constantly positive temperature coefficients up to over 1000 °C. Rather low thermal conductivities, κ, of the sintered bodies of the oxides,ca. 1.7 W m–1 K–1 at room temperature, lead to the largest value of the thermoelectric figure of merit Z= 0.4 × 10–4 K–1 for M = Sn at 1000 °C, and the Z value increases toward higher temperatures.

Synthesis and characterization of polyhedral Pt nanoparticles: Their catalytic property, surface attachment, self-aggregation and assembly

In this paper, we presented the preparation procedure of Pt nanoparticles with the well-controlled polyhedral morphology and size by a modified polyol method using AgNO(3) in accordance with the reduction of H(2)PtCl(6) in EG at high temperature around 160°C. The methods of UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution (HR) TEM measurements were used to characterize their surface morphology, size, and crystal structure. We have observed that the polyhedral Pt nanoparticles of sharp edges and corners were produced in the preferential homogenous growth as well as the formation of porous and large Pt particles by self-aggregation and assembly originating from as-prepared polyhedral Pt nanoparticles. It is most impressive to find that the arrangement of Pt nanoparticles was observed in their surface attachments, self-aggregation, random and directed surface self-assembly by the bottom-up approach. Their high electrocatalytic activity for methanol oxidation was predicted. The findings and results showed that the polyhedral Pt nanoparticle-based catalysts exhibited the high electrocatalytic activity for their potential applications in developing the efficient Pt-based catalysts for direct methanol fuel cells.

Chemical synthesis and characterization of palladium nanoparticles

This work presents the results of the successful preparation of Pd nanoparticles by the polyol method and the proposed techniques of controlling their size and shape. Polyvinylpyrrolidone (PVP) stabilized Pd nanoparticles of various shapes with the largest sizes in the forms of octahedrons (24 nm), tetrahedrons (22 nm) and cubes (20 nm) have been obtained by alcohol reduction in ethanol with the addition of a hydrochloric acid catalyst. Moreover, PVP–Pd nanoparticles of well-controlled spherical shapes have also been prepared by a modified polyol method. PVP–Pd nanoparticles of cubic, octahedral, tetrahedral and spherical shapes with well-controlled size achieved by using ethylene glycol (EG) as reductant and various inorganic species were also fabricated. In particular, Pd nanorods with sizes of 47 nm and 16 nm formed due to the anisotropic growth mechanism of Pd nanoparticles were found. At the same time, tetrahedral particles of sharp shapes of 120 nm and 70 nm sizes have been observed. A high concentration of inorganic species was used to control the size and shape of the Pd nanoparticles, leading to the appearance of various irregular sizes and shapes. There was evidence of the very sharp corners and edges of tetrahedral and octahedral Pd nanoparticles or others that were formed in the clustering and combination of the seeds of smaller particles.

Intramolecularly selective decomposition of surfactant molecules on photocatalytic oxidative degradation over TiO2 photocatalyst

Abstract Investigation on oxidative mineralization of various surfactants over TiO 2 photocatalyst reveals an anomalously retarded degradation of the trimethylammonium moiety in cationic surfactants. Model reactions using tetraalkylammonium chlorides as substrates confirm that the stepwise behavior of the mineralization time courses is ascribed to intramolecularly selective decomposition caused by a considerably slow degradation rate of the methyl groups directly bound to the quaternary nitrogen. A strong inhibition effect of bromide or iodide anions preventing the complete mineralization is also revealed.