S. Q. Bai

Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes

20 mm long ropes consisting of soundly aligned single-walled carbon nanotube (SWNT) ropes, synthesized by the catalytic decomposition of hydrocarbons, were employed for direct tensile strength measurements. The average tensile strength of SWNT rope composites is as high as 3.6 +/-0.4 GPa, similar to that of carbon fibers. The tensile strength of SWNT bundles was extrapolated from the strength of the composites to be 2.3 +/-0.2 to 14.2 +/-1.4 GPa after simply taking into account the volume fraction of SWNT bundles in the minicomposite, and the tensile strength of single SWNTs was estimated to be as high as 22.2 +/-2.2 GPa. The excellent mechanical properties of SWNTs will make them an ideal reinforcement agent for high performance composite materials

Dual-frequency resonant phonon scattering in BaxRyCo4Sb12 (R= La, Ce, and Sr)

Low temperature transport properties of polycrystalline dual-element-filled skutterudites BaxRyCo4Sb12 (R=La, Ce, and Sr) are reported. Remarkably the combination of Ba and La or Ba and Ce is much more effective in reducing lattice thermal conductivity (κL) than Ba and Sr. The density-functional theory calculations and experimental data suggest that multiple-filled skutterudites using filler elements of different chemical natures, such as the rare earths, the alkaline earths, or the alkalines provide a broader range of resonant phonon scattering. The thermoelectric figure of merit of filled skutterudites can likely be improved by means of such multiple-element void filling.

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.

Lattice thermal transport in BaxREyCo4Sb12 (RE=Ce, Yb, and Eu) double-filled skutterudites

We use the Debye model to investigate the phonon scattering mechanisms in BaxREyCo4Sb12 double-filled skutterudites. Filling two types of guest atoms (barium and rare-earth elements) into the voids of skutterudite introduces additional phonon point defect scattering from the extra mass fluctuation at the void site, and additional phonon resonant scattering by the fillers with different vibrational frequencies, leading to significant reduction in the lattice thermal conductivity.

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.