Key Engineering Materials Vols. 336-338

Abstract: The layered cobalt oxides as thermoelectric (TE) materials are introduced in detail on their developments, representative systems, some theories, unsolved problems and approaches for improving performances. TE performances of the layered cobalt oxides are discussed from structures, doped situations, processes and the dimensionless figure of merit. In contrast with other TE materials, the layered cobalt oxides show many promising applications. The theory of Seebeck coefficient and hopping conduction mechanism in the layered cobalt oxides are discussed. Heike formula explains that Seebeck coefficient origins from electronic spin states and proportions of different value states of Co ion. An unbalance of the spin and orbital degrees of freedom between Co3+ and Co4+ sites results in the large Seebeck coefficient. On the basis of the Boltzmann transport equation, Seebeck coefficient is decided by energy band structures. High state density near Fermi Energy band is responsible for the coexistence of large Seebeck coefficient and high electrical conductivity. Hopping conduction mechanism found in experiments is a main transport way of charge carriers at high temperature for the layered cobalt oxides. Through different materials systems and theories analysis, unsolved problems and new approaches for improving TE performances are put forward.

Abstract: We have investigated the modulated structure of the misfit-layered crystal Bi1.8Sr2.0Rh1.6Ox by means of electron diffraction and high-resolution electron microscopy. This compound consists of two interpenetrating subsystems of a hexagonal RhO2 sheet and a distorted four-layered rock-salt-type (Bi,Sr)O block. Both subsystems have common a-, c-axes and β-angles with a = 5.28 Å, c = 29.77 Å and β = 93.7º. On the other hand, the crystal structure is incommensurated parallel to the b-axes, among which b1 = 3.07 Å for the RhO2 sheet and b2 = 4.88 Å for the (Bi,Sr)O block. The misfit ratio, b1/b2 ~ 0.63, characterizes the structural analogue as [Bi1.79Sr1.98Oy]0.63[RhO2]. This compound has two modulation vectors, i.e., q1 = – a* + 0.63b1* and q2 = 0.17b1* + c*, and the superspace group is assigned as the Cc(1β0, 0μ1)-type from the electron diffraction patterns. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly show displacive as well as compositional modulations.

Abstract: A co-precipitation preparation method and the properties of an ultraviolet (UV)-attenuating agent are described in this paper. The composite particles of ultra-fine zinc oxide and titanium oxide are used to attenuate UV radiation. Preparation of TiOSO4, ZnCl2 solution and the co-precipitation of the composite particles by alkali are included during the process. Various types of surfactants have been used to modify the composite particles. Particle sizes are determined by laser particle analyzer, and reflectance and absorption coefficient are determined by UV-VIS spectrophotometer. Results show that particle size of the composite particles as well as total reflectance and absorption coefficient depend on the surfactants, pH value, and carline temperature. The average sizes of zinc oxide and titanium oxide ultra-fine particles range from39 to 65 nm. Attenuation capability of ultraviolet radiation becomes stronger when the particle average sizes becomes smaller. Composites less than 40 nm with titanium oxide of rutile phase attenuate most effectively the ultraviolet radiation ranging from 280 nm to 390 nm.

Abstract: Structural characteristics of the high performance cobaltite thermoelectric semiconductors have been studied intensively by means of X-ray and neutron diffraction measurements and high resolution electron microscopy (HREM). These cobaltites consists of CoO2 triangular conducting sheets and several different types of block layers, i.e., Na, Ca, Sr single layers, three or four layered rock-salt layers, where Co-O2 square lattices are situated at their middle, and Bi-O or Tl-O double layers plus alkaline oxygen layers. Cold neutron scattering technique is employed to search possible low-energy excitation modes, being unique for nearly 1D and 2D crystals, and phonon density of states, DOS, of several high performance cobaltites at temperatures in the range from 10K to the ambient. Low energy, i.e., less than 2meV, excitation modes were found in the three different thermoelectric ceramics, i.e., γ -Na0.7CoO2, [Ca2CoO3]pCoO2, and [Ca2(Cu,Co)2O4]pCoO2. Possible origin of these low energy excitations are discussed in terms of low-energy corrugation mode generated due to weak chemical bondings, for which Van-der-Waals force dominates, between the CoO2 conduction sheets. These characteristics could be the key to realize low thermal conductivity and high-ZT of these ceramics.

Abstract: CoSb3 with nanoscale was synthesized by Cross-coprecipitation. A precursor consisting of antimony oxide and cobalt hydrate was prepared by the reaction of CoCl2, SbCl3 and both precipitators at room temperature. The precursor was reduced in thermal treatment under hydrogen atmosphere whereby the CoSb3 was thus obtained. The parameters especially reducing temperature and atmosphere (content of H2) influence the constituent phases and particle size of product significantly. The single phase CoSb3 with the average grain size of 60~70 nm was obtained after reduced at 723 K for 2 h with pure H2. Nanoscale CoSb3 powder was used as starting materials, and bulk CoSb3 compound was prepared by spark plasma sintering (SPS). The effect of grains size on thermal conductivity was investigated.

Abstract: Single-phase polycrystalline CoSb3 skutterudite was prepared through a new combination of mechanical alloying (MA) and spark plasma sintering (SPS). In order to investigate the influence of MA conditions on the microstructure and thermoelectric properties, MA synthesis were carried out under various conditions with different milling times. The powder sample MAed for 6h still consisted of metal Sb, and then transformed to CoSb3 with a little amount of metal Sb and CoSb2 phases after MA for 15h. Further prolonging the MA time resulted in the decomposition of CoSb3 to CoSb2 phase. The average grain size of the SPSed samples decreased from 650nm to 250nm as MA-time was prolonged from 6 to 24h. Lattice parameters estimated form XRD patterns increase with the increasing MA time. All samples SPSed at 600°C for holding 5 min show an n-type conduction. The electrical resistivity was 1030, 895, 410, 260 μm for the samples from the MA-derived powders with MA-time of 6, 15, 24 and 33h at room temperature, respectively, then reduced to 60 μm at 400°C for all samples. An optimum MA time is 24 h in which the sample shows the highest power factor 612μW/m*K2 at 150°C.

Abstract: Single-phase double atoms filling skutterudite compounds were synthesized by using melting reaction method. The effects of double atoms filling on the structure and lattice thermal conductivity of skutterudite compounds were investigated. The results of Rietveld refinement indicate that CamCenFexCo4-xSb12 compounds possess skutterudite structure and the Sb-icosahedron voids have been partially filled with filling atoms. With the same filling fraction, the lattice thermal conductivity of CamCenFexCo4-xSb12 is smaller than that of CamFexCo4-xSb12 and CenFexCo4-xSb12, furthermore, when the total filling fraction (m+n) is about 0.3 and respective filling fraction of Ca and Ce are approximately equal, the lattice thermal conductivity is the least.

Abstract: Sr-filled skutterudite compounds SryCo4Sb12 (y=0-0.20) were synthesized by melting method. XRD and EPMA results revealed that the obtained samples are single skutterudite phase with homogeneous chemical composition. The lattice parameters increase linearly with increasing Sr content in the range of y=0-0.20. The thermal conductivity, electrical conductivity and Seebeck coefficient were measured in the temperature range of 300-850K. The measurement of Hall effect was performed by Van de Pauw method at room temperature. The obtained Sr-filled skutterudite exhibits n-type conduction. The absolute value of the Seebeck coefficient of SryCo4Sb12 decreases with the increase of Sr content. The electrical conductivity increases with the increase of Sr content. The lattice thermal conductivity of SryCo4Sb12 is significantly depressed as compared with unfilled CoSb3. The maximum dimensionless thermoelectric figure of merit is 0.7 for Sr0.20Co4Sb12 at 850K. Further optimization of chemical composition would improve the thermoelectric performance.

Abstract: The direct extrusion process using the powder as raw materials was applied to prepare the thermoelectric materials. The mechanically alloyed powders of Ag added (Bi0.25Sb0.75)2Te3 were extruded by pulse discharge sintering method in the temperature range of 345°C ~ 425°C. High quality products were obtained by hot-extrusion method and their texture and thermoelectric properties were measured. The intensity of (110) plane increased with extrusion temperature up to 385°C and altered in the range of above 405°C
which coincided with the variation of power factor. The measured Power factor ranged from 3.5 ~ 4.0 × 10-3 W/K2·m. The figure of merit (Z) of the material extruded at 385°C was 3.1 × 10-3 /K, the highest value among the prepared materials.

Abstract: AgxPbmSbTe2+m thermoelectric materials were fabricated using a combined process of mechanical alloying (MA) and spark plasma sintering (SPS). The compound powder was synthesized by mechanical alloying (MA) from elemental powders using a planetary mill after a short time, and high-density bulk samples were fabricated by spark plasma sintering (SPS) at low temperature within a short time (12 minutes). The P-type materials were obtained with electrical properties comparable to the newly reported data. The properties of P-type AgxPbmSbTe2+m-based materials could be improved by optimizing the composition and the process.