Wataru Norimatsu

Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Abstract Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a ‘natural superlattice’ in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX2 structure which has a general formula of (MX)1+x(TX2)n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)1.2(TiS2)2 has better thermoelectric properties compared with pure TiS2 in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.

Enhanced thermoelectric performance of Nb-doped SrTiO3 by nano-inclusion with low thermal conductivity

Authors reported an effective path to increase the electrical conductivity while to decrease the thermal conductivity, and thus to enhance the ZT value by nano-inclusions. By this method, the ZT value of Nb-doped SrTiO3 was enhanced 9-fold by yttria stabilized zirconia (YSZ) nano-inclusions. YSZ inclusions, located inside grain and in triple junction, can reduce the thermal conductivity by effective interface phonon scattering, enhance the electrical conductivity by promoting the abnormal grain growth, and thus lead to the obvious enhancement of ZT value, which strongly suggests that, it is possible to not only reduce the thermal conductivity, but also increase the electrical conductivity by nano-inclusions with low thermal conductivity. This study will give some useful enlightenment to the preparation of high-performance oxide thermoelectric materials.

Nanoscale stacking faults induced low thermal conductivity in thermoelectric layered metal sulfides

Layered metal sulfides (MS)1+x(TiS2)2 (M = Pb, Sn, Bi) with alternative stacking of MS layers and TiS2 layers (a natural superlattice) have been proposed as thermoelectric materials. In this paper, various nanoscale stacking faults have been found in these materials, including the translational disorder in (SnS)1.2(TiS2)2 and the staging disorder in (BiS)1.2(TiS2)2. The lattice thermal conductivities along the layers are systematically and significantly reduced by these stacking faults which are only a few unit cells apart, without deteriorating the electron mobility, demonstrating a “phonon-blocking, electron-transmitting” scenario.

Nitrate-ion-selective exchange ability of layered double hydroxide consisting of MgII and FeIII

In this study, layered double hydroxide (LDH) consisting of Mg(II) and Fe(III) (Mg/Fe-LDH) was synthesized by using a combination of coprecipitation with hydrothermal aging, and its anion-exchange properties were investigated. Through various analyses, the chemical formula of the proposed Mg/Fe-LDH was determined to be [Mg(0.76)Fe(0.24)(OH)(2)](Cl(-))(0.21)(CO(3)(2-))(0.02)·0.76H(2)O. Furthermore, amorphous Fe(III) impurities were contained in the present Mg/Fe-LDH. The proposed Mg/Fe-LDH exhibited clear selectivity for nitrate ions dissolved in water. This selectivity for nitrate ions can be explained by an anion-sieve effect by the existence of amorphous Fe(III) impurities. Our findings suggest that it is possible to synthesize LDHs with high selectivity for various anions by effective hybridizing Fe(III) impurities.

Atom-by-atom simulations of graphene growth by decomposition of SiC (0001): Impact of the substrate steps

The atomic-scale carbon rearrangement into graphene by the thermal decomposition of SiC (0001) was simulated by the density-functional tight-binding technique. By decomposing the terrace of the SiC (0001) surface, the carbon chains formed a three-dimensional structure, because the carbon atoms are released by losing their original contacts to silicon atom. On the other hand, in the step model, the silicon atoms at the step-edge act as trapping sites for the released carbon atoms, and the carbon network effectively nucleated and expanded. After nucleation at the step, graphene can grow by the further decomposition together with retreat of the step.

Epitaxial growth of boron-doped graphene by thermal decomposition of B4C.

We grew graphene by thermal decomposition of B(4)C and investigated its features by high-resolution transmission electron microscope observations. At temperatures higher than 1600 °C in a vacuum, B(4)C decomposes and graphene forms epitaxially on its surface. The number and the morphology of the graphene layers depend on the surface orientation. An electron diffraction technique revealed the presence of a superstructure with a two-times larger unit cell, which is consistent with the structure of BC(3). We have directly confirmed boron in the graphene layers by electron energy loss spectroscopy measurements and boron-mapping experiments.

Synthesis of copper nanoparticles within the interlayer space of titania nanosheet transparent films

We report the first in situ synthesis of copper nanoparticles (CuNPs) within the interlayer space of inorganic layered semiconductors (titania nanosheet films) through the following steps. A sintered titania nanosheet (s-TNS) film was synthesised, forming a transparent, layered semiconductor film (∼2 μm thick). A considerable amount of copper ions (ca. 68% relative to the cation exchange capacity of TNSs) was intercalated in the s-TNSs using the methyl viologen-containing s-TNSs as the intermediate. The resultant copper-containing s-TNS (TNS/Cu2+) film was treated with an aqueous solution of NaBH4, resulting in a colour change. Extinction spectra of NaBH4-treated films exhibited a wide extinction band at λmax (the extinction band maximum) = 683 nm. The spectral shapes and λmax were similar to those for copper nanoparticles on TiO2 surfaces. Transmission electron microscopy analysis demonstrated the wide distribution of electron dense particles on the titania sheet of NaBH4-treated TNS/Cu2+. XRD analysis and absorption/extinction analysis with different amounts of TNSs suggest that CuNPs were formed within the interlayer space rather than the surface of TNSs through NaBH4 treatment. Repeatable oxidation and reduction behaviour, i.e. colouration and decolouration cycles of the copper species within TNS films, was investigated.

Growth of graphene from SiC{0001} surfaces and its mechanisms

Graphene, a one-atom-layer carbon material, can be grown by thermal decomposition of SiC. On Si-terminated SiC(0001), graphene nucleates at steps and grows layer-by-layer, and as a result a homogeneous monolayer or bilayer can be obtained. We demonstrate this mechanism both experimentally and theoretically. On the C-face (000), multilayer graphene nucleates not only at steps, but also on the terraces. These differences reflect the distinct differences in the reactivity of these faces. Due to its high quality and structural controllability, graphene on SiC{0001} surfaces will be a platform for high-speed graphene device applications.

Structural features of epitaxial graphene on SiC {0?0?0?1} surfaces

We investigated the atomic-scale structural properties of graphene epitaxially grown on SiC {0?0?0?1} surfaces by high-resolution transmission electron microscope observations. In this review paper, we summarize our results about the interface structure, the growth mechanisms and the growth techniques. Graphene on the Si-terminated surface has a buffer layer that is strongly bonded to the substrate and has a low carbon atom density. Multilayer graphene on the Si-face exhibited an ABC-stacking selectively. Graphene on the Si-face nucleates at the step-edge, and grows layer-by-layer over the upper terrace. Based on the mechanism, we succeeded in growing high-quality and homogeneous monolayer and bilayer graphene on the Si-terminated surface with low-height and low-density steps using our original technique. Graphene on the C-face has weaker bonds with the substrate, and the resulting multilayer contains the rotational stacking disorder. The origin of the rotational stacking is closely related to the growth mechanism, where graphene layers nucleate on a terrace at a lower temperature and grow in all directions on the surface.

Thermoelectric properties of n-type Mn3−xCrxSi4Al2 in air

A silicide material with a good n-type thermoelectric property has been discovered. This silicide possesses a composition of Mn3−xCrxSi4Al2 (0 ≤ x ≤ 0.7) and hexagonal CrSi2 structure. The a- and c-cell parameters decrease with increasing the amount of Cr substitution. The absolute values of Seebeck coefficient and electrical resistivity increase by Cr substitution up to 573 K because of reduction of carrier density. The dimensionless thermoelectric figure of merit ZT reaches 0.21 at 773 K for a non Cr substituted sample at the Mn site and 0.30 at 573 K for a Cr substituted one with x = 0.3. Since oxide passive layer is formed around the surface, electrical resistivity measured at 873 K is constant for 2 days in air, which indicates good oxidation resistance in air of this material. The Mn3−xCrxSi4Al2 is a promising n-type material with a good oxidation resistance in the middle temperature range. A thermoelectric module consisting of 64 pairs of legs has been fabricated using MnSi1.7 and non Cr substitute...