X. Y. Zhao

Synthesis of YbyCo4Sb12∕Yb2O3 composites and their thermoelectric properties

Composites containing Yb-filled CoSb3 and well-distributed Yb2O3 particles are synthesized by in situ reaction method. The structural, chemical, and transport properties of the composites are studied. Some Yb2O3 particles with microsize locate at the grain boundaries of matrix and others distribute within YbyCo4Sb12 grains as nanoscale inclusions. The combination of the “rattling” of Yb ions inside the voids of CoSb3 and the phonon scattering of the oxide defects results in a remarkable reduction in the lattice thermal conductivity. The thermoelectric performance of the composites is significantly improved, and the maximum figures of merit reach 1.3 for the Yb0.25Co4Sb12∕Yb2O3 and 1.2 for the Yb0.21Co4Sb12∕Yb2O3 composites at 850K.

Synthesis and thermoelectric properties of KyCo4Sb12

Polycrystalline K-filled CoSb3 are synthesized successfully. The uplimit for K filling is at least 0.45, being higher than those of either alkaline-earth (AE) or rare-earth (RE) metals but being in consistent with our earlier theoretical prediction. The measured transport properties (300–800K) show that K filling does not lower thermal conductivity much in comparison with AE or RE filling due to the relatively low mass of K atom. However, it improves electrical conductivity, retains large Seebeck coefficient, and leads to a reasonably good thermoelectric performance for the filled skutterudites. The maximum figure of merit ZT reaches 1 at 800K for K0.38Co4Sb12.

Synthesis and thermoelectric properties of Sr-filled skutterudite SryCo4Sb12

Strontium-filled skutterudites SryCo4Sb12 have been synthesized by a melting method. The filling fraction of Sr in CoSb3 skutterudite is up to y=0.40, closely consistent with the calculated value by density functional theory methods. The lattice parameters increase linearly with the increase of Sr content, and the relative change in lattice parameters is in agreement with theoretical prediction. Hall effect measurements have been performed by Van de Pauw method at room temperature. All samples filled with Sr atom exhibit n-type conduction. The thermal and electrical transport properties have been measured in the temperature range of 300–850K. The lattice thermal conductivity of SryCo4Sb12 is significantly depressed as compared to that of the unfilled CoSb3. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature and reaches a maximum value of 0.9 for Sr0.28Co4Sb12 at 850K.

Influence of fullerene dispersion on high temperature thermoelectric properties of BayCo4Sb12-based composites

BayCo4Sb12-based composites with dispersed fullerene or barium fulleride have been synthesized by the solid state reaction and spark plasma sintering technique. When fullerene was used as a dispersed phase, a part of barium in the filled skutterudite reacted with fullerene to form barium fullerides. Most of the extended impurity aggregated to form large defects at the grain boundaries of the matrix. The thermoelectric properties of the composites were measured from room temperature to 850 K. Both the electrical conductivity and thermal conductivity decreased with increasing impurity contents. We attributed these significant reductions to enhanced grain-boundary scattering of charge carriers and phonons. A large ZT value of 1.3 was obtained in a composite with an appropriate content of fullerene dispersants.

Soft phonon columns on the edge of the Brillouin zone in the relaxor PbMg1/3Nb2/3O3

which is similar to the onset temperature of the zone-center diffuse scattering, indicating a competition between ferroelectric and antiferroelectric distortions. We propose a model for the atomic displacements associated with these phonon modes that is based on a combination of structure factors and group theoretical analysis. This analysis suggests that the scattering is not from tilt modes rotational modes of oxygen octahedra, but from zone-boundary optic modes that are associated with the displacement of Pb 2+ and O 2� ions. Whereas similar columns of scattering have been reported in metallic and less commonly molecular systems, they are unusual in insulating materials, particularly in ferroelectrics; therefore, the physical origin of this inelastic feature in PMN is unknown. We speculate that the underlying disorder contributes to this unique anomaly.

Monoclinic MC phase in (001) field cooled BaTiO3 single crystals

We report here the finding of a new phase in BaTiO3 single crystals. High precision x-ray diffraction investigations of (001) field cooled crystals have shown the presence of a monoclinic MC phase for temperatures below 300 K. This MC phase is shown to be stable upon removal of the electric field. The results demonstrate the presence of structurally bridging low symmetry phases that lie between tetragonal and rhombohedral ones in this classic perovskite system: indicating a means to achieve high piezoelectricity in Pb-free systems via domain engineering.

Time- and temperature-dependent domain evolutions in poled (111)-cut (Pb(Mg1∕3Nb2∕3)O3)0.7(PbTiO3)0.3 single crystal

Ferroelectric domain evolution in poled (111)-cut (Pb(Mg1∕3Nb2∕3)O3)0.7(PbTiO3)0.3 single crystal has been studied by means of piezoresponse force microscopy (PFM). A time-dependent development of lamellar ferroelectric domains from a single domain structure of the just-poled sample has been observed, and it reveals that the formation of the lamellar macrodomains is via the accumulation of well-aligned speckle-shaped nanodomains grown from polar nanosized regions (PNRs). The domain evolutions from macrodomain to microdomain, and from ferroelectric to paraelectric phase at different temperatures, have been revealed in temperature-dependent PFM imaging, and the results are consistent with temperature-dependent relative permittivity measurement. PNRs are believed to play a key role in the domain evolution of depolarization process.

Synthesis and Thermoelectric Properties of (Sr,Yb)yCo4Sb12 Double Filled Skutterudites

(Sr,Yb)_yCo₄Sb₁₂ double filled skutterudites are synthesized by a melting method and sintered by Spark Plasma Sintering (SPS) technique. Small amount of Yb₂O₃ and YbSb₂ as impurity phases are found with high ytterbium content in the X-ray diffraction patterns of sintered materials. The thermal conductivity, electrical conductivity and Seebeck coefficient are measured from room temperature to 850 K. The influence of different filling fractions of ytterbium on the thermoelectric properties of (Sr,Yb)_yCo₄Sb₁₂ is studied. The lattice thermal conductivity of double filled skutterudites is reduced dramatically as compared to that of Sr single-filled skutterudites with similar filling fractions. An enhancement in power factor and dimensionless thermoelectric figure of merit (ZT) is observed in the double filled skutterudites

Synthesis and thermodynamics properties of alkaline metals filled CoSb 3 skutterudites

Alkaline metal (AM) atoms filled CoSb₃ skutterudites are synthesized by melting method and sintered by spark plasma sintering. The maximal filling fraction or filling fraction limit (FFL) of AM (AM=Li, Na, K, Cs) in CoSb₃ are estimated experimentally and show good agreement with our previous calculations based on density functional method. The effect of thermodynamics properties on FFL, crystallographic properties and microstructure are investigated. Among all the AM-filled CoSb₃ systems, Na shows the highest experimental FFL of ~65% due to the relatively stable chemical properties of Na and NaSb_x binary compounds. Furthermore, Na_yCo₄Sb₁₂ shows the best crystallization and purity under the same synthesis approach. In the cases of the end members Li and Cs, both our theoretical and experimental results show that the filling levels in CoSb₃ are infinitesimal due to the extremely low formation enthalpies of LiSb_x and CsSb_x compounds.