A variety of crystals contain quasi-one-dimensional substructures, which yield distinctive electronic, spintronic, optical and thermoelectric properties. There is a lack of understanding of the ...lattice dynamics that influences the properties of such complex crystals. Here we employ inelastic neutron scatting measurements and density functional theory calculations to show that numerous low-energy optical vibrational modes exist in higher manganese silicides, an example of such crystals. These optical modes, including unusually low-frequency twisting motions of the Si ladders inside the Mn chimneys, provide a large phase space for scattering acoustic phonons. A hybrid phonon and diffuson model is proposed to explain the low and anisotropic thermal conductivity of higher manganese silicides and to evaluate nanostructuring as an approach to further suppress the thermal conductivity and enhance the thermoelectric energy conversion efficiency. This discovery offers new insights into the structure-property relationships of a broad class of materials with quasi-one-dimensional substructures for various applications.
We report a novel synthesis of Ti5Si3 nanoparticles (NPs) via the magnesio-reduction of TiO2 NPs and SiO2 in eutectic LiCl-KCl molten salts at 700 °C. The Ti5Si3 particle size (∼25 nm) is confined to ...the nanoscale due to the partial dissolution of Mg and silica in the molten salts and a subsequent heterogeneous reduction on the surface of the TiO2 NPs.
Two-phase PbTe–PbS materials, in which PbS is a nanostructured phase, are promising thermoelectric materials for the direct conversion of heat energy into electricity. In this study, a Vickers ...indentation mean hardness of 1.18
±
0.09
GPa was measured for hot pressed specimens Pb
0.95Sn
0.05Te–PbS 8% while the mean hardness of cast specimens was 0.68
±
0.07
GPa. The mean fracture toughness of the not pressed specimens was estimated as 0.35
±
0.04
MPa
m
1/2 via Vickers indentation. Resonant Ultrasound Spectroscopy (RUS) measurements on hot pressed specimens gave mean values of Young's modulus, shear modulus and Poisson's ratio of 53.1
GPa, 21.4
GPa and 0.245, respectively while for the cast specimens the Young's and shear moduli were about 10% lower than for the hot pressed, with a mean value of Poisson's ratio of 0.245. The differences between the hardness and elastic moduli values for the cast and hot pressed specimens are discussed.
Experimental evidence is given relevant to the temperature dependence of the valence band structure of PbTe and PbTe sub(1-x) S sub(x) alloys (0.04 < or = x < or = 0.19, and its effect on the ...thermoelectric figure of merit ZT. The x = 0.08 sample has ZT ~ 1.55 at 773 K. The magnetic field dependence of the high-temperature Hall resistivity of heavily p-type (> 10 super(19) cm super(-3)) Na-doped PbTe sub(1-x) S sub(x) reveals the presence of high-mobility electrons. This casts doubts on prior analyses of the Hall coefficient suggesting that temperature induces a rapid rise in energy of the "heavy" hole relative to the "light" hole bands. The electron-like behavior is likely induced by the topology of the Fermi surface when the L- and capital sigma -bands merge. Negative values for the low-temperature thermopower are also observed. The data show that PbTe continues to be a direct-gap semiconductor at temperatures where the ZT and S super(2)sigma of p-type PbTe are optimal, e.g., 700-800 K.
If nanostructured thermoelectric materials are to be used for future energy harvesting and power generation applications, scalable production of thermoelectric nanostructures must be developed. ...Herein we report a vapor phase conversion method to synthesize nanowire (NW) arrays of semiconducting higher manganese silicides (HMS, or MnSi1.75) for enhanced thermoelectric applications. Dense arrays of silicon NWs obtained by metal-assisted chemical etching were converted to single-crystalline HMS NW arrays with the original nanoscale morphology preserved by reacting with Mn vapor in a sealed stainless steel reactor at 950 °C. Structural characterization by X-ray and electron diffraction and high-resolution transmission electron microscopy confirm that the converted NWs are single-crystalline NWs of HMS phases such as Mn7Si12, Mn27Si47, and Mn39Si68. Investigations of the conversion process using in situ high resolution powder X-ray diffraction (HRPXRD) and mechanistic experiments reveal that the presence of excess Si substrate underneath the Si NWs, careful control of Mn precursor, and high reaction temperature are crucial to the selective formation of HMS phase. The electrical resistivity of these HMS NWs are similar to that of the bulk HMS.
Silicon is an extremely important technological material, but its current industrial production by the carbothermic reduction of SiO
is energy intensive and generates CO
emissions. Herein, we ...developed a more sustainable method to produce silicon nanowires (Si NWs) in bulk quantities through the direct electrochemical reduction of CaSiO
, an abundant and inexpensive Si source soluble in molten salts, at a low temperature of 650 °C by using low-melting-point ternary molten salts CaCl
-MgCl
-NaCl, which still retains high CaSiO
solubility, and a supporting electrolyte of CaO, which facilitates the transport of O
anions, drastically improves the reaction kinetics, and enables the electrolysis at low temperatures. The Si nanowire product can be used as high-capacity Li-ion battery anode materials with excellent cycling performance. This environmentally friendly strategy for the practical production of Si at lower temperatures can be applied to other molten salt systems and is also promising for waste glass and coal ash recycling.
The high-performance PbTe–SnTe–PbS thermoelectric system forms a completely new composite PbTe–PbSnS2 with high n-type figure of merit. Electron diffraction and high-resolution electron microscopy ...characterization of the thermoelectric composite PbTe + 25% PbSnS2 reveals that the system is nanostructured, with PbSnS2nanocrystals in the range of 80 to 500 nm in size. In most of the cases, they are endotaxially grown within the PbTe matrix. Three independent crystal superstructures were observed for the PbSnS2 inclusions, originating from the same parent SnS-type structure. The presence of the parent structure is not excluded. Modified structural models for two of the superstructures observed in the PbSnS2 precipitates are proposed. Often, more than one of the structural phases are observed in the same nanocrystal, providing one extra phonon scattering factor in the system. Evidence was also found for the growth process of the nanocrystals, starting from PbS and followed by gradual dissolving of SnS. Our findings suggest that this nanostructured thermoelectric composite exhibits unique structural complexity, which contributes to the low lattice thermal conductivity reported for these nanocomposite materials.
Phase immiscibility in PbTe–based thermoelectric materials is an effective means of top‐down synthesis of nanostructured composites exhibiting low lattice thermal conductivities. PbTe1‐x Sx ...thermoelectric materials can be synthesized as metastable solid solution alloys through rapid quenching. Subsequent post‐annealing induces phase separation at the nanometer scale, producing nanostructures that increase phonon scattering and reduce lattice thermal conductivity. However, there has yet to be any study investigating in detail the local chemical structure of both the solid solution and nanostructured variants of this material system. Herein, quenched and annealed (i.e., solid solution and phase‐separated) samples of PbTe–PbS are analyzed by in situ high‐resolution synchrotron powder X‐ray diffraction, solid‐state 125Te nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy analysis. For high concentrations of PbS in PbTe, e.g., x >16%, NMR and IR analyses reveal that rapidly quenched samples exhibit incipient phase separation that is not detected by state‐of‐the‐art synchrotron X‐ray diffraction, providing an example of a PbTe thermoelectric “alloy” that is in fact phase inhomogeneous. Thermally‐induced PbS phase separation in PbTe–PbS occurs close to 200 °C for all compositions studied, and the solubility of the PbS phase in PbTe at elevated temperatures >500 °C is reported. The findings of this study suggest that there may be a large number of thermoelectric alloy systems that are phase inhomogeneous or nanostructured despite adherence to Vegard's Law of alloys, highlighting the importance of careful chemical characterization to differentiate between thermoelectric alloys and composites.
In addition to microstructural analysis, adherence to Vegard's law of alloys has long been the standard toward assessing thermoelectric materials as solid solutions as opposed to nano‐phase separated. Using infrared reflectivity and nuclear magnetic resonance spectroscopy, incipient phase separation may be observed for certain quenched “alloys” of PbTe1‐xSx that obey Vegard's law by in situ powder synchrotron X‐ray diffraction, demonstrating that careful chemical analysis is required to adequately demonstrate whether a thermoelectric material is truly phase homogeneous.
The traditional model of undergraduate research is less effective for engaging students who have little or no previous exposure to research, are unfamiliar with available research opportunities, or ...face financial or time constraints that prevent them from engaging in co- or extracurricular activities. Given today's changing student demographics, models such as course-embedded research need to be explored so that undergraduate research participation may be broadened across disciplines. This article describes how a community of practitioners was created to infuse research in courses at both two- and four-year campuses, with four examples of courses with embedded research activities. Discussed are strategies for implementing discipline-specific research activities at all levels of the undergraduate curriculum to expose a broader student population to the benefits of mentored research.
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