- Materials Science, Materials Engineering, Environmental Engineering, Renewable Energy, Mechanical Engineering, Energy Engineering, and 9 morePhysics, Optics and liquid crystals, Polymers, Materials Science and Engineering, Engineering, Chemistry, Thermoelectric Materials, Ceramic Engineering, and Solid State Chemistryedit
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Many of the missions proposed and successfully completed by the National Aeronautics and Space Administration seek to scientifically investigate remote locations in our solar system, in particular to better understand the origin,... more
Many of the missions proposed and successfully completed by the National Aeronautics and Space Administration seek to scientifically investigate remote locations in our solar system, in particular to better understand the origin, evolution and structure of planetary systems. Long-lived, robust power systems are a fundamental capability for such missions, and radioisotope thermoelectric generators (RTGs) have proven to be a reliable power for exploration missions in deep space for the past 50 years. With increasing power needs for future missions, the improvement of thermoelectric materials’ conversion efficiency is necessary. In this paper, we show how compositing with inert metallic inclusions can be efficiently used to improve the electronic properties of Yb14MnSb11. In this study, we found that the power factor of the p-type high temperature material, Yb14MnSb11, increases from ∼8 to ∼11.5 μW cm−1 K−2 when composited with 5 vol. % W particles. At the same time, the composite samp...
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Yb 14 MnSb 11 and Yb 14 MgSb 11 are among the best p-type high-temperature (>1200 K) thermoelectric materials, yet other compounds of this Ca 14 AlSb 11 structure type have not matched their stability and efficiency. First-principles... more
Yb 14 MnSb 11 and Yb 14 MgSb 11 are among the best p-type high-temperature (>1200 K) thermoelectric materials, yet other compounds of this Ca 14 AlSb 11 structure type have not matched their stability and efficiency. First-principles computations show that the features in the electronic structures that have been identified to lead to high thermoelectric performances are present in Yb 14 ZnSb 11 , which has been presumed to be a poor thermoelectric material. We show that the previously reported low power factor of Yb 14 ZnSb 11 is not intrinsic and is due to the presence of a Yb 9 Zn 4+ x Sb 9 impurity uniquely present in the Zn system. Phase-pure Yb 14 ZnSb 11 synthesized through a route avoiding the impurity formation reveals its exceptional high-temperature thermoelectric properties, reaching a peak zT of 1.2 at 1175 K. Beyond Yb 14 ZnSb 11 , the favorable band structure features for thermoelectric performance are universal among the Ca 14 AlSb 11 structure type, opening the po...
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The Magnéli phase VO was synthesized in gram amounts from a powder mixture of VO/VO and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a... more
The Magnéli phase VO was synthesized in gram amounts from a powder mixture of VO/VO and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared VO has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K) while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m), a Seebeck coefficient of α = -(35 ± 2) μV K, which leads to a power factor of PF = 4.9 ± 0.8 × 10W (m K) at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as VO are involved. This study gives insight into the complicat...
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Engineering of nanoscaled structures allows to control the electrical and thermal transport in solids for thermoelectric applications where a combination of low thermal conductivity and low electrical resistivity is required.
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Liquid Crystalline Elastomers (LCEs) are very promising smart materials that can be made sensitive to different external stimuli, such as heat, pH, humidity and light, by changing their chemical composition. In this paper we report the... more
Liquid Crystalline Elastomers (LCEs) are very promising smart materials that can be made sensitive to different external stimuli, such as heat, pH, humidity and light, by changing their chemical composition. In this paper we report the implementation of a nematically aligned LCE actuator able to undergo large light-induced deformations. We prove that this property is still present even when the actuator is submerged in fresh water. Thanks to the presence of azo-dye moieties, capable of going through a reversible trans-cis photo-isomerization, and by applying light with two different wavelengths we managed to control the bending of such actuator in the liquid environment. The reported results represent the first step towards swimming microdevices powered by light.
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Tetragonal tungsten bronzes Nb8−xW9+xO47−δallow a continuous variation of the charge carrier concentration while fulfilling the concept of a “phonon-glass electron-crystal” through intrinsic nanostructure.
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Compounds with the Yb21Mn4Sb18 structure show local disorder as shown in the PDF model (left) and Cd solid solution provides the highest zT (right).
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The Magnéli phase V6O11 was synthesized in gram amounts from a powder mixture of V6O11/V7O13 and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data... more
The Magnéli phase V6O11 was synthesized in gram amounts from a powder mixture of V6O11/V7O13 and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V6O11 has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K)−1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m)−1, a Seebeck coefficient of α = −(35 ± 2) μV K–1, which leads to a power factor of PF = 4.9 ± 0.8 × 10–5W (m K2)−1 at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V6O11 are involved. This study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.
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Thermoelectric materials are believed to play a fundamental role in the energy field over the next years thanks to their ability of directly converting heat into usable electric energy. To increase their integration in the commercial... more
Thermoelectric materials are believed to play a fundamental role in the energy field over the next years thanks to their ability of directly converting heat into usable electric energy. To increase their integration in the commercial markets, improvements of the efficiencies are needed. At the same time, cheap and non-toxic materials are required along with easily upscalable production cycles. Compounds of the tetragonal tungsten bronze (TTB) series Nb8−xW9+xO47 fulfill all these requirements and are promising materials. Their adaptive structure ensures glass-like values of the thermal conductivity, and the substitution on the cation side allows a controlled manipulation of the electronic properties. In this contribution we report the stability study of the two highly substituted samples of the series, Nb5W12O47 (x = 3) and Nb4W13O47 (x = 4), when subjected to thermal cycling. Moreover, we show the results of the thermoelectric characterization of these samples. The two compounds have not been affected by the thermal treatment and showed an improvement of the thermoelectric performances up to a zT = 0.2 above 1000 K.
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The thermoelectric properties of melt processed conductive nanocomposites consisting of an insulating polypropylene (PP) matrix filled with singlewalled carbon nanotubes (CNTs) and copper oxide (CuO) were evaluated. An easy and cheap... more
The thermoelectric properties of melt processed conductive nanocomposites consisting of an insulating polypropylene (PP) matrix filled with singlewalled carbon nanotubes (CNTs) and copper oxide (CuO) were evaluated. An easy and cheap route to switch p-type composites into n-type was developed by adding polyethylene glycol (PEG) during melt mixing. At the investigated CNT concentrations of 0.8 wt% and 2 wt% (each above the electrical percolation threshold of ~0.1 wt%), and a fixed CuO content of 5 wt%, the PEG addition converted p-type composites (positive Seebeck coefficient (S)) into n-type (negative S). PEG was also found to improve the filler dispersion inside the matrix. Two composites were prepared: P-type polymer/CNT composites with high S (up to 45 mV/K), and n-type composites (with S up to À56 mV/ K) through the addition of PEG. Two prototypes with 4 and 49 thermocouples of these p-and n-type composites were fabricated, and delivered an output voltage of 21 mV and 110 mV, respectively, at a temperature gradient of 70 K.
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Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low-cost materials with reasonable thermoelectric... more
Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low-cost materials with reasonable thermoelectric performances and cost-efficient preparation techniques are required. In this context metal oxides are an interesting class of materials because of their inherent high-temperature stability and relative high sustainability. Their thermoelectric performance, however, needs to be improved for wide application. Compounds with adaptive structures are a very interesting class of materials. A slight reduction of early transition metal oxides generates electrons as charge carriers and crystallographic shear planes as structure motif. The crystallographic shear planes lead to a reduction of intrinsic thermal conductivity. At the same time, the electronic transport properties can be tuned by the degree of reduction. So far only a few transition metal oxides with adaptive structures have been investigated with respect to their thermoelectric properties, leaving much room for improvement. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties.
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We report a method to fabricate polymer microstructures with local control over the molecular orientation. Alignment control is achieved on molecular level in a structure of arbitrary form that can be from 1 to 100 μm in size, by fixing... more
We report a method to fabricate polymer microstructures with local control over the molecular orientation. Alignment control is achieved on molecular level in a structure of arbitrary form that can be from 1 to 100 μm in size, by fixing the local boundary conditions with micro-grating patterns. The method makes use of two-photon polymerization (Direct Laser Writing) and is demonstrated specifically in liquid-crystalline elastomers. This concept allows for the realization of free-form polymeric structures with multiple functionalities which are not possible to realize with existing techniques and which can be locally controlled by light in the micrometer scale.