Tiejun Zhu

Bismuth telluride nanotubes and the effects on the thermoelectric properties of nanotube-containing nanocomposites

Nanotubes of quasilayered bismuth telluride compound were prepared by hydrothermal synthesis. Nanotubes have diameters smaller than 100nm and spiral tube-walls. The low-dimensional morphology and hollow structure enable bismuth telluride nanotubes to be a potential thermoelectric material with a high figure of merit due to the efficient phonon blocking effect. The experimental results show that the addition of nanotubes leads to a remarkable decrease in the thermal conductivity with the electrical conductivity much less affected and thus to an increase in the figure of merit of the Bi2Te3-based material.

Syntheses and thermoelectric properties of Bi2Te3∕Sb2Te3 bulk nanocomposites with laminated nanostructure

Nanocomposites with constituent sizes of <50nm are considered as a promising approach to enhance the figure of merit of bulk thermoelectric materials. A simple route involving hydrothermal synthesis and hot pressing was used in this work to prepare Bi2Te3∕Sb2Te3 bulk nanocomposites. It is shown that the composites have a laminated structure composed of Bi2Te3 and Sb2Te3 nanolayers with the thickness varying alternately between 5 and 50nm. The transport measurements indicate that the nanoscale laminated structure improves the thermoelectric performance with the maximal dimensionless figure of merit of 1.47 for the nanocomposite hot pressed from Bi2Te3 and Sb2Te3 nanopowders.

Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials.

Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron–phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm−2 at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability.

High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials

Mg2(Si,Sn) compounds have shown great promise for thermoelectric applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements. Herein we report on the thermoelectric properties and microstructure of high performance Mg2Si0.4−xSn0.6Sbx (0⩽x⩽0.015) alloys. The state-of-the-art ZT value of ∼1.1 has been attained in the samples with x=0.0075 due to the relatively low thermal conductivity. In light of the simple cubic structure and mostly light constituent elements, the reduction in lattice thermal conductivity has been discussed in connection with a fairly large amount of in situ formed nanostructures in these samples.

Band engineering of high performance p-type FeNbSb based half-Heusler thermoelectric materials for figure of merit zT > 1

We report new p-type FeNb1−xTixSb (0.04 ≤ x ≤ 0.24) half-Heusler thermoelectric materials with a maximum zT of 1.1 at 1100 K, which is twice that of the ZrCoSb half-Heusler alloys. The electrical properties are optimized by a tradeoff between the band effective mass and mobility via a band engineering approach. A high content of Ti up to x = 0.2 optimizes the power factor and reduces the lattice thermal conductivity. In view of abundantly available elements, good stability and high zT, FeNb1−xTixSb alloys could be promising materials for high temperature power generation.

Preferential c-Axis Orientation of Ultrathin SnS2 Nanoplates on Graphene as High-Performance Anode for Li-Ion Batteries

A SnS2/graphene (SnS2/G) hybrid was synthesized by a facile one-step solvothermal route using graphite oxide, sodium sulfide, and SnCl4·5H2O as the starting materials. The formation of SnS2 and the reduction of graphite oxide occur simultaneously. Ultrathin SnS2 nanoplates with a lateral size of 5-10 nm are anchored on graphene nanosheets with a preferential (001) orientation, forming a unique plate-on-sheet structure. The electrochemical tests showed that the nanohybrid exhibits a remarkably enhanced cycling stability and rate capability compared with bare SnS2. The excellent electrochemical properties of SnS2/G could be ascribed to the in situ introduced graphene matrix which offers two-dimensional conductive networks, disperses and immobilizes SnS2 nanoplates, buffers the volume changes during cycling, and directs the growth of SnS2 nanoplates with a favorable orientation.

Self-assembly of a CoFe2O4/graphene sandwich by a controllable and general route: towards a high-performance anode for Li-ion batteries

A CoFe2O4-nanocrystal/graphene-nanosheet (CFO/GS) nanohybrid has been synthesized by a facile in situ solvothermal route and has been investigated as a promising high-performance anode material for Li-ion batteries. The crystal size of CoFe2O4 can be controlled to 10–20 nm by pre-treating the precursors before the solvothermal reactions. The method for synthesizing the CFO/GS hybrid can be extended to synthesize MFe2O4/graphene (MFe2O4/G, M = Mn and Ni) hybrids. The CFO/GS hybrid exhibits superior cycling stability and rate capability compared to bare CoFe2O4. The improved electrochemical performance can be attributed to a combination of the conducting, confining and dispersing effects of graphene.

Nanostructuring and thermoelectric properties of bulk skutterudite compound CoSb3

Thermoelectric properties of nanostructured skutterudite CoSb3 have been reported. Nanosized CoSb3 powders were synthesized through a solvothermal route. The bulk materials with average grain sizes of 250 and 150nm were prepared by hot pressing and spark plasma sintering from the solvothermally synthesized CoSb3 powders. Both the samples show n-type conduction and the thermal conductivities are reduced compared with that of the sample prepared by the melt-annealing∕hot pressing method. A thermoelectric figure of merit of 0.61 has been obtained for the unfilled CoSb3 skutterudite by spark plasma sintering, which indicates that nanostructuring is an effective way to improve the thermoelectric properties of skutterudite compounds.Thermoelectric properties of nanostructured skutterudite CoSb3 have been reported. Nanosized CoSb3 powders were synthesized through a solvothermal route. The bulk materials with average grain sizes of 250 and 150nm were prepared by hot pressing and spark plasma sintering from the solvothermally synthesized CoSb3 powders. Both the samples show n-type conduction and the thermal conductivities are reduced compared with that of the sample prepared by the melt-annealing∕hot pressing method. A thermoelectric figure of merit of 0.61 has been obtained for the unfilled CoSb3 skutterudite by spark plasma sintering, which indicates that nanostructuring is an effective way to improve the thermoelectric properties of skutterudite compounds.

Double-shelled hollow microspheres of LiMn2O4 for high-performance lithium ion batteries

Double-shelled hollow microspheres of LiMn2O4 were prepared by a facile self-template method. The inner and outer shells of the obtained microspheres are composed of nanoparticles with diameters ranging between 200–400 nm. Galvanostatic charge/discharge cycling shows that this material delivers a discharge capacity of 127 mAh g−1 at C/10 rate and a capacity retainability of 80% after 800 cycles at 5 C rate, revealing a high reversible capacity, superior rate capability and excellent cycling stability under high rates. The improved performance is attributed to the short Li+ ion diffusion lengths in the nanobuilding blocks and the void core and space between the inner and outer shells to accommodate the volume expansion/contraction during Li+ ions insertion/extraction processes.

Recrystallization induced in situ nanostructures in bulk bismuth antimony tellurides: a simple top down route and improved thermoelectric properties

Recrystallization induced in situ nanostructure formation was used as a new means to obtain high performance polycrystalline thermoelectric materials, which was realized by a simple hot forging process to the coarse-grained Bi0.5Sb1.5Te3 alloys. The pole figure measurement showed that the oriented textures were weakened or eliminated after hot forging of the alloys with a quasi-layered crystal structure, implying the presence of deformation recrystallization. Transmission electron microscopy observation revealed the recrystallization induced in situ nanostructures and high density of defects in the hot forged samples. Transport property measurements indicated that the hot forged samples had both increased electrical power factor and reduced thermal conductivity, compare to the initial alloys without hot forging. The maximum ZT values of >1.3 at room temperature were reproducibly obtained for the hot forged samples, suggesting that the simple new method can be applied for large scale production of high performance polycrystalline thermoelectric materials with in situ nanostructures.