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In the absence of periodicity, the structure of glass is ill-defined, and a large number of structural states are found at similar energy levels. However, little is known about how these states are connected to each other in the potential energy landscape. We simulate mechanical relaxation by molecular dynamics for a prototypical [Formula: see text] metallic glass and follow the mechanical energy loss of each atom to track the change in the state. We find that the energy barriers separating these states are remarkably low, only of the order of 1 meV, implying that even quantum fluctuations can overcome these potential energy barriers. Our observation of numerous small ripples in the bottom of the potential energy landscape puts many assumptions regarding the thermodynamic states of metallic glasses into question and suggests that metallic glasses are not totally frozen at the local atomic level.
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The movement of the construction industry towards sustainable development has drawn attention to the revision of concrete. In addition to reducing pollution, the use of nano-materials should lead to the provision of higher quality concrete in terms of regulatory items (workability, resistance characteristics, durability characteristics, microstructure). The present study investigates 15 key characteristics of concrete modified with nano-CaCO3, nano-clay, nano-TiO2, and nano-SiO2. The results of the study showed that nanomaterials significantly have a positive effect on the hydration mechanism and the production of more C-S-H gel. The evaluation of resistance characteristics also indicates the promising results of these valuable materials. The durability characteristics of nano-containing concrete showed significant improvement despite high dispersion. Concrete in coastal areas (such as bridges or platforms), concrete exposed to radiation (such as hospitals), concrete exposed to impact load (such as nuclear power plants), and concrete containing recycled aggregate (such as bricks, tiles, ceramics) can be effectively improved by using nanomaterials. It is hoped that the current review paper can provide an effective image and idea for future applied studies by other researchers.
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Developing microscopic understanding of the thermal properties of liquids is challenging due to their strong dynamic disorder, which prevents characterization of the atomic degrees of freedom. There have been significant research interests in the past few decades to extend the normal mode analysis for solids to instantaneous structures of liquids. However, the nature of normal modes that arise from these unstable structures is still elusive. In this paper, we explore the instantaneous eigenmodes of dynamical matrices of various Lennard-Jones argon liquid and gas systems at high temperatures and show that the normal modes can be interpreted as an interpolation of Tâ∞ (gas) and T=0 (solid) mode descriptions. We find that normal modes become increasingly collisional and translational, recovering atomistic gaslike behavior rather than vibrational with increase in temperature, suggesting that normal modes in liquids may be described by both solidlike and gaslike modes.
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Biochar is commonly used for soil amendment, due to its excellent water-holding capacity. The Cr(VI) contamination of water is a current environmental issue in industrial regions. Here, we evaluated the effects of two-step modifications on boosting biochar's performance in terms of the removal of aqueous hexavalent chromium (Cr(VI)), along with investigating the alterations to its surface properties. The first modification step was heat treatment under air at 300 °C, producing hydrophilic biochar (HBC). The resulting HBC was then impregnated with zero-valent iron nanoparticles (nZVI), creating an HBC/nZVI composite, adding a chemical reduction capability to the physical sorption mechanism. Unmodified biochar (BC), HBC, and HBC/nZVI were characterized for their physicochemical properties, including surface morphology and elemental composition, by SEM/EDS, while functional groups were ascertained by FTIR and surface charge by zeta potential. Cr(VI) removal kinetic studies revealed the four-time greater sorption capacity of HBC than BC. Although unmodified BC showed faster initial Cr(VI) uptake, it rapidly worsened and started desorption. After nZVI impregnation, the Cr(VI) removal rate of HBC increased by a factor of 10. FTIR analysis of biochars after Cr(VI) adsorption showed the presence of Cr(III) oxide only on the used HBC/nZVI and demonstrated that the carbonyl and carboxyl groups were the main groups involved in Cr(VI) sorption. Modified biochars could be considered an economical substitute for conventional methods.
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Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.
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Flexible thermoelectric (TE) materials, which are devices that convert thermal gradients to electrical energy, have attracted interest for practical energy-harvesting/recovery applications. However, as compared with p-type materials, the progress on the development of n-type TE flexible materials has been slow due to difficulties involved in n-type doping techniques. This study used high mobility carbon nanotubes (CNTs) to a uniformly mixed hybrid-composite, resulting in an enhanced power factor by increasing electrical conductivity. The energy filtering effect and stoichiometric composition of the material used, bismuth telluride (Bi2Te3) correlated to a significant enhancement in TE performance, with a power factor of 225.9 µW m-1K-2 at room temperature: a factor of 65 higher than as-fabricated composite film. This paper describes a simplified synthesis for the preparation of the composite film that eliminates time-intensive and cost-prohibitive processing, traditionally seen during extrusion and dicing inorganic manufacturing. The resulting post-annealed composite film consisting of Bi2Te3 nanowire and CNTs demonstrate a promising candidate for material that can be used for an n-type TE device that has improved energy conversion efficiency.
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Thermal transport in multilayers (MLs) has attracted significant interest and shows promising applications. Unlike their single-component counterparts, MLs exhibit a thermal conductivity that can be effectively engineered by both the number density of the layers and the interfacial thermal resistance between layers, with the latter being highly tunable via the contrast of acoustic properties of each layer. In this work, we experimentally demonstrated an ultralow thermal conductivity of 0.33 ± 0.04 W m(-1) K(-1) at room temperature in MLs made of Au and Si with a high interfacial density of â¼0.2 interface nm(-1). The measured thermal conductivity is significantly lower than the amorphous limit of either Si or Au and is also much lower than previously measured MLs with a similar interfacial density. With a Debye temperature ratio of â¼3.9 for Au and Si, the Au/Si MLs represent the highest mismatched system in inorganic MLs measured to date. In addition, we explore the prior theoretical prediction that full phonon dispersion could better model the interfacial thermal resistance involving materials with low Debye temperatures. Our results demonstrate that MLs with highly dissimilar Debye temperatures represent a rational approach to achieve ultralow thermal conductivity in inorganic materials and can also serve as a platform for investigating interfacial thermal transport.
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We experimentally studied the thermoelectric power factor of hole gas in individual Ge-Si core-shell nanowires with Ge core diameters ranging from 11 to 25 nm. The Ge cores are dopant-free, but the Fermi level in the cores is pinned by surface and defect states in the epitaxial Si shell thereby doping the cores into the degenerate regime. This doping mechanism avoids the high concentration of dopants usually encountered in bulk thermoelectric materials and provides a unique opportunity to enhance the carrier mobility with suppressed ionized impurity scattering. Moreover, the carrier concentration in small diameter nanowires has also been effectively modulated by field effect, allowing one to probe the electrical conductivity and thermopower within a wide range of carrier concentrations, which is crucial to understand the thermoelectric transport behavior. We found that the thermopower of nanowires with Ge core diameters down to 11 nm still follows the behavior of bulk Ge. As a result, the power factor is found to be closely correlated with the carrier mobility, which is higher than that of bulk Ge in one of the core-shell nanowires studied here.
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Customized engineered fibers are currently being used extensively in the aerospace and automobile industries due to the ability to "design in" specific engineering characteristics. Understanding the thermal conductivity of these new fibers is critical for thermal management and design optimization. In the current investigation, a steady-state dc thermal bridge method (DCTBM) is developed to measure the thermal conductivity of individual poly(ether ketone) (PEK)/carbon nanotube (CNT) fibers. For non-conductive fibers, a thin platinum layer was deposited on the test articles to serve as the heater and temperature sensor. The effect of the platinum layer on the thermal conductivity is presented and discussed. DCTBM is first validated using gold and platinum wires (25 µm in diameter) over a temperature ranging from room temperature to 400 K with ±11% uncertainty, and then applied to PEK/CNT fibers with diverse CNT loadings. At a 28 wt. % CNT loading, the thermal conductivity of fibers at 390 K is over 27 Wm(-1)K(-1), which is comparable to some engineering alloys.
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Heterostructure core-shell semiconductor nanowires (NWs) have attracted tremendous interest recently due to their remarkable properties and potential applications as building blocks for nanodevices. Among their unique traits, thermal properties would play a significant role in thermal management of future heterostructure NW-based nanoelectronics, nanophotonics, and energy conversion devices, yet have been explored much less than others. Similar to their electronic counterparts, phonon spectrum and thermal transport properties could be modified by confinement effects and the acoustic mismatch at the core-shell interface in small diameter NWs (<20 nm). However, fundamental thermal measurement on thin core shell NWs has been challenging due to their small size and their expected low thermal conductivity (κ). Herein, we have developed an experimental technique with drastically improved sensitivity capable of measuring thermal conductance values down to â¼10 pW/K. Thermal conductivities of Ge and Ge-Si core-shell NWs with diameters less than 20 nm have been measured. Comparing the experimental data with Boltzmann transport models reveals that thermal conductivities of the sub-20 nm diameter NWs are further suppressed by the phonon confinement effect beyond the diffusive boundary scattering limit. Interestingly, core-shell NWs exhibit different temperature dependence in κ and show a lower κ from 300 to 388 K compared to Ge NWs, indicating the important effect of the core-shell interface on phonon transport, consistent with recent molecular dynamics studies. Our results could open up applications of Ge-Si core shell NWs for nanostructured thermoelectrics, as well as a new realm of tuning thermal conductivity by "phononic engineering".
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Mitral inflow filling pattern usually consists of 2 forward flow velocities in sinus rhythm: early rapid filling (E) and late filling with atrial contraction (A). However, additional mid-diastolic flow velocity may be present resulting in triphasic mitral inflow filling pattern. When mitral inflow is triphasic, mitral annulus velocity recorded by tissue Doppler imaging (TDI) frequently demonstrates a mid-diastolic component (L'). The significance of L' has not been explored previously. The purpose of this study was to explore possible mechanisms and clinical implications of triphasic mitral inflow with or without L' using TDI and proBNP. Of 9004 patients who underwent transthoracic echocardiography from March to November 2003, 83 (0.9%) patients (33 male, 50 female; mean age, 63+/-10 years) with a triphasic mitral inflow velocity pattern, including mid-diastolic flow velocity of at least 0.2m/s, and sinus rhythm were prospectively identified in our clinical echocardiography laboratory. Peak velocity of E, mid-diastolic (L), and A, and deceleration time (DT) of the E wave velocity were measured. Diastolic mitral annular velocities were measured at the septal corner of the mitral annulus by TDI from the apical 4-chamber view. ProBNP was measured at the time of echocardiogram using a quantitative electrochemiluminescence immunoassay. Mean heart rate was 54+/-6 beats/min (range, 40-67). Mean left ventricular (LV) ejection fraction (EF) was 64+/-13% and LV systolic dysfunction (EF<40%) was present in only 6 (7%). Patients were classified into 2 groups: group 1 (n=47) included those who had L' and group 2 (n=36) included those without L'. Group 1 patients had significantly higher peak velocity (35+/-14 vs 26+/-6 cm/s, p=0.0002) and TVI (35+/-14 vs 26+/-6 cm/s, p=0.0002) of L, E/E' (18+/-8 vs 14+/-6, p=0.02), and left atrial volume index (42+/-14 vs 34+/-10 ml/m(2), p=0.0037). E' (4.7+/-1.3 vs 6.2+/-2.3 cm/s, p=0.001) and A' (6.2+/-2.0 vs 8.6+/-3.4 cm/s, p=0.0006) were significantly lower in group 1 compared with those of group 2. ProBNP was significantly higher in group 1 (847+/-1461 vs 438+/-1039 pmol/l, p=0.0012) and it was above normal in all except in 1 patient of group 1. In conclusion, the presence of L' in subjects with triphasic mitral inflow velocity pattern with mid-diastolic flow is associated with higher E/E', elevated proBNP and enlarged left atrium indicating advanced diastolic dysfunction with elevated filling pressures. This unique mitral annular velocity pattern should be helpful in identifying the patients with advanced diastolic dysfunction and increased LV filling pressures.