Search

Intensifying the phonon scattering via point defect engineering has been demonstrated to be particularly effective in minimizing the lattice thermal conductivity for enhancing thermoelectric performance. In this work, significant phonon scattering has been realized by alloying YbMg2Bi2 with Mg3Bi2. The substantial mass difference between the host atom Yb and the alloying atom Mg leads to an intense phonon scattering effect that significantly reduces the lattice thermal conductivity. The room-temperature lattice thermal conductivity decreases from ∼2.7 W m−1 K−1 for YbMg2Bi1.96 to ∼0.8 W m−1 K−1 for Yb0.7Mg0.3Mg2Bi1.96, a reduction of ∼70%. Benefiting from the greatly reduced thermal conductivity, the average ZT has been effectively improved from ∼0.46 for YbMg2Bi1.96 to ∼0.61 for Yb0.8Mg0.2Mg2Bi1.96, an enhancement of ∼33%. In addition, the predicted maximum heat-to-electricity conversion efficiency can be increased from ∼7% for YbMg2Bi1.96 to ∼10% for Yb0.8Mg0.2Mg2Bi1.96.

We report $^{59}$Co, $^{93}$Nb, and $^{121}$Sb nuclear magnetic resonance (NMR) measurements combined with density functional theory (DFT) calculations on a series of half-Heusler semiconductors, including NbCoSn, ZrCoSb, TaFeSb and NbFeSb, to better understand their electronic properties and general composition-dependent trends. These materials are of interest as potentially high efficiency thermoelectric materials. Compared to the other materials, we find that ZrCoSb tends to have a relatively large amount of local disorder, apparently antisite defects. This contributes to a small excitation gap corresponding to an impurity band near the band edge. In NbCoSn and TaFeSb, Curie-Weiss-type behavior is revealed, which indicates a small density of interacting paramagnetic defects. Very large paramagnetic chemical shifts are observed associated with a Van Vleck mechanism due to closely spaced $d$ bands splitting between the conduction and valence bands. Meanwhile, DFT methods were generally successful in reproducing the chemical shift trend for these half-Heusler materials, and we identify an enhancement of the larger-magnitude shifts, which we connect to electron interaction effects. The general trend is connected to changes in $d$-electron hybridization across the series., Comment: 22 pages, 6 figures

Proper contacts between thermoelectric (TE) materials and electrodes are critical for TE power generation or refrigeration. The Bi-rich n-type Zintl material Mg3+δBi2-xSbx exhibits very good TE performance near room temperature, which makes Mg3+δBi2-xSbx-based compounds highly promising candidates to replace the Bi2Te3-ySey alloys, but ideal contacts that can match their TE performance have not yet been well studied. Here we investigate different metal (Ni and Fe) and metal alloy (NiFe, NiCr, NiCrFe, and stainless steel) contacts on n-type Mg3+δBi1.5Sb0.5. It is first shown that the low Schottky barrier and narrow depletion region resulting from the band degeneracy and high carrier concentration of a heavily doped TE material are beneficial for the formation of a low-resistivity ohmic contact with a metal or a metal alloy. Most fully optimized TE materials can take advantage of this. Second, it is found that the NiFe/Mg3+δBi1.5Sb0.5 contact exhibits excellent thermal stability and the lowest ohmic contact resistivity among those studied after aging for over 2100 h, which is attributed to the formation of metallic NiMgBi between the NiFe and Mg3+δBi1.5Sb0.5 layers. As a buffer phase, NiMgBi can effectively prevent elemental diffusion without negatively affecting the electron transport. Benefiting from such low contact resistance, a Mg3+δBi1.5Sb0.5/Bi0.4Sb1.6Te3 unicouple exhibits competitive conversion efficiency, 6% with a 150 K temperature difference and a hot-side temperature of 448 K.

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

The growth of Bi 2 Se 3 nanostructures by physical vapor deposition in presence or absence of Au catalysts on Si and mica substrates is investigated. In the presence of Au nanodroplets on substrate surfaces, the growth of Bi 2 Se 3 nanostructures in diverse crystallographic orientations is initiated via a catalyst-assisted process at low source flux condition. At elevated flux supply, nearly free-standing vapor-solid growth dominates and an array consisting of dense inclined standing nanoribbons are achieved. The morphologies of the obtained Bi 2 Se 3 nanoribbons can be easily manipulated by changing growth temperature.

Acidic potassium dichromate solutions (K2Cr2O7–H2SO4 and K2Cr2O7–HCl) are applied for patterning single crystalline Bi2Se3 thin films on Si(111) substrates. In solutions with appropriate component proportions, vertical walls and mesa-shaped structures on the etching profiles of (001) Bi2Se3 films can be achieved. Stoichiometric etching behavior is noted for Bi2Se3 in K2Cr2O7–H2SO4 etchant, while incongruently dissolution of Bi2Se3 in K2Cr2O7–HCl is observed which leaves a Se deficient layer on the etched film surface. The chemical reaction kinetics of Bi2Se3 in the two different etchants are also discussed.

The conventional doping strategy for thermoelectric materials generally focuses on a shallow donor/acceptor model with the energy level close to the band edge as for electronic devices. However, thermoelectric devices operate over a large temperature difference, and the optimal carrier concentration increases with increasing temperature. A shallow level cannot meet the requirement over a large temperature range. Here, an innovative strategy of introducing an intermediate level is proposed. Such an intermediate level introduces more carriers with increasing temperature, consistent with the trend of increasing optimal doping concentration with temperature, enabling larger power factor over a broader temperature range. Furthermore, the intermediate level typically requires more impurities, leading to increased phonon scattering. This strategy allows simultaneous optimization of carrier concentration over a wide temperature range and suppression of thermal conductivity via stronger point-defect phonon scattering. Experimental results from heavily-doped ZrCoSb employing shallow, intermediate, and deep levels successfully corroborate this strategy, where simultaneously improved power factor and figure of merit are obtained by introducing an intermediate level. Our work indicates that the performance of known thermoelectric materials should be reevaluated by introducing an intermediate level to unleash their full potential.

Thermal transport in superlattices is governed by various phonon-scattering processes. For extracting the phonon-scattering contribution of hetero-interfaces in chalcogenide superlattices, single-crystalline Bi2Se3/In2Se3 (BS/IS) superlattices with minimized defects are prepared on fluorophlogopite mica by molecular beam epitaxy. The cross-plane heat-conducting properties of the BS/IS superlattices are demonstrated to depend precisely on the period thicknesses and constituents of the superlattices, where a minimum in the thermal conductivity indicates a crossover from particle-like to wave-like phonon transport in the superlattices. The thermal-conductivity minimum of the BS/IS superlattices is nearly one order of magnitude lower than that of intrinsic BS film.

To investigate the electronic behavior and magnetic properties of NbFeSb, we have performed 93Nb NMR, specific heat and magnetic measurements on NbFeSb samples heat treated at high temperatures. Magnetic measurements combined with an observed Schottky anomaly and changes in the NMR line width indicate the presence of a 0.2% concentrated native magnetic defect in stoichiometric NbFeSb samples. The origin of these native defects is believed to be due to Fe antisites on Nb sites. In addition, NMR shift and spin-lattice relaxation results below 200 K reveal a Korringa-like response indicating heavily-doped p-type behavior due to native defects. Above 280 K, this converts to an activated behavior, indicating the presence of an impurity band, empty at low temperatures, which is located around 0.03 eV above the valence band maximum.

Because graphene oxide is so important for many applications and also the precursor for large quantity of graphene, a low cost and environmentally friendly method using much less acid is essential to the future of graphene oxide and graphene. It is a great challenge to reduce the acid usage to avoid the safety risk, but tremendous benefits could be achieved if this were made possible for the industrial-scale synthesis of graphene oxide and graphene. In this work, we report a fast and simple method that uses much less acid than the state of the art to obtain nanoscale graphene oxide by using the heat released by the reaction. With the introduction of dry ice, the reaction temperature can be well controlled, thus stabilizing the highly oxidative and explosive Mn2O7 intermediate in order to avoid safety risk.

International audience; In this paper a series of di-block copolymers of L-lactide and (meth)acrylate [(M)A, representing methyl methacrylate, tert-butyl acrylate and 2-ethylhexyl acrylate] were synthesized by varying the molecular weight of the polylactide (PLLA) macroinitiator and the structure of the (meth)acrylate monomers. The glass transition temperature, crystallinity and thermal stability of copolymers with different poly(meth) acrylate [P(M)A] blocks were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results indicated that the glass transition temperature of the copolymers could be tuned by changing the chain structure and chain length of the P(M)A blocks. Besides, the crystallization of the copolymers was inhibited by the introduction of P(M)A blocks, and the toughness of the copolymers could be tuned. It is noted that the thermal stability of the copolymers depended on the type of P(M)A blocks and the PLLA/ P(M)A blocks ratio. Furthermore, the microphase separation of copolymers in thin films was observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and the results showed that the composition of copolymers significantly affected the surface morphology of the block copolymer thin films.

Poly (lactic acid)/anatase-titanium dioxide (PLA/A-TiO2) and PLA/rutile-TiO2 (PLA/R-TiO2) composites were prepared by spin coating technique (thin film) and extrusion-injection processing (thick film), respectively. The ultraviolet (UV) photoresponse effects of the two oxide structures were investigated mainly by measuring the UV absorbance or mechanical properties. For the thin films, the UV absorbance results exhibit a UV shielding effect of R-TiO2 and a degradation acceleration effect of A-TiO2 when the particles are embedded in PLA matrix. But for the thick films, the mechanical property results only exhibit a UV shielding effect of both R-TiO2 and A-TiO2 particles and improve the anti-UV performance of PLA when the particles are encapsulated in PLA matrix.

We demonstrate the living radical polymerization of tert-butyl acrylate (tBA) applying the SET mechanism, employing methyl 2-bromopropionate (MBP) as initiator in dimethyl sulfoxide (DMSO) at ambient temperature. It is observed that introducing copper bromide into the catalyst system is necessary for controlling on the SET-LRP polymerization of tBA. In this work, we make major investigation for the effect of the different stoichiometry quantity of copper bromide on the polymerization. Experiments show that the polymerization achieves better control with increasing the stoichiometry quantity of copper(II) deactivator. The structural analysis of the resulting polymers by 1 H NMR demonstrates the successful synthesis of poly(tBA)s by SET-LRP in DMSO. Moreover, this work is helpful to the SET-LRP of other monomers and is expected to expand the application of SET-LRP.

Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm−1 K−2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb‐based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm−2 is achieved based on a single‐leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.

The metastable α-In2Se3 thin film is epitaxially integrated on H-passivated Si (1 1 1) substrates to build a novel heterojunction solar cell by molecular beam epitaxy. The growth of In2Se3 on H–Si(1 1 1) at low temperature initiates as an amorphous layer then followed by re-crystalline of α phase film. Electronic transport properties of α-In2Se3/p-Si heterostructure are studied. Analysis of the temperature dependence of thermionic emission in reverse bias indicates a barrier height of ~0.21 eV at the α-In2Se3/p-Si interface. A photovoltaic conversion efficiency of 2% is measured for the heterojunction with optimized In2Se3 film thickness.

Diblock copolymers of poly(L-lactide)-block-poly(methyl methacrylate) (PLLA-b-PMMA) were synthesized through a sequential two-step strategy, which combines ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP), using a bifunctional initiator, 2,2,2-trichloroethanol. The trichloro-terminated poly(L-lactide) (PLLA-Cl) with high molecular weight (Mn,GPC = 1–12 × 104 g/mol) was presynthesized through bulk ROP of L-lactide (L-LA), initiated by the hydroxyl group of the double-headed initiator, with tin(II) octoate (Sn(Oct)2) as catalyst. The second segment of the block copolymer was synthesized by the ATRP of methyl methacrylate (MMA), with PLLA-Cl as macroinitiator and CuCl/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) as catalyst, and dimethyl sulfoxide (DMSO) was chosen as reaction medium due to the poor solubility of the macroinitiator in conventional solvents at the reaction temperature. The trichloroethoxyl terminal group of the macroinitiator was confirmed by Fourier transform infrared spectroscopy (FTIR) and 1H-NMR spectroscopy. The comprehensive results from GPC, FTIR, 1H-NMR analysis indicate that diblock copolymers PLLA-b-PMMA (Mn,GPC = 5–13 × 104 g/mol) with desired molecular composition were obtained by changing the molar ratio of monomer/initiator. DSC, XRD, and TG analyses establish that the crystallization of copolymers is inhibited with the introduction of PMMA segment, which will be beneficial to ameliorating the brittleness, and furthermore, to improving the thermal performance. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

In this work, we report the magnetostrictive properties of Sm1-xNdxFe2 compounds, which are free of (expensive) heavy rare earths. Anomalies are observed in the temperature dependence of both the magnetization and the magnetostriction of these compounds, which are ascribed to spin-reorientation transitions. At low fields, the quasi-static magnetostriction and the piezomagnetic coefficient d(33) of Sm0.88Nd0.12Fe2 are larger than those of SmFe2, which is due to the compensation of the anisotropy of the Sm3+ and Nd3+ ions. At a magnetic-field strength of 6 kOe, the static magnetostriction of Sm0.88Nd0.12Fe2 is -1244 ppm at 225 K and -1022 ppm at 290 K and, at a bias magnetic field of 700 Oe, the d(33) equals 0.55 ppm/Oe (6.8 nm/A). (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4801340]

The structural, magnetic, and magnetostrictive properties of Sm(Fe1-xMnx)(2) (0 to of Sm(Fe1-xMnx)(2) decreases from 200K for x = 0 to 140K for x = 0.15, indicating the increasing anisotropy of the Mn-substituted compounds. The magnetostriction coefficient lambda(111) of the Sm(Fe1-xMnx)(2) compounds slightly decreases with Mn substitution and maintains a value larger than 1900 ppm for 0

Mn(2)O(3)/Mn(3)O(4) nanoclusters were prepared by air oxidation of Mn(3)O(4) nanoparticles. The nanoparticles located at the surface of the Mn(2)O(3)/Mn(3)O(4) nanoclusters are oxidized to Mn(3)O(4) to form the special structure of the Mn(2)O(3)/Mn(3)O(4) nanoclusters. An exchange bias was observed in the Mn(2)O(3)/Mn(3)O(4) nanoclusters, which was induced by exchange coupling between ferromagnetic Mn(3)O(4) and antiferromagnetic Mn(3)O(4) phases. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3055349]