We employ density functional theory based phonon transport methods to provide a rigorous understanding of the nature of thermal transport in coherent short -period AlN/GaN superlattices (SLs), with period lengths up to three unit cells of each, and compare these with properties of their bulk constituents. Increasing the period length leads to phonon band folding with frequency gaps and thus smaller phonon velocities and more phonon scattering than in bulk, both of which reduce lattice thermal conductivity (K). Contrary to expectations, we find that velocity variations among larger-period AlN/GaN SLs play only a minor role in cross-plane K reductions, while variations in intrinsic phonon scattering are strongly correlated with their transport behaviors. This work provides insights into the microscopic behaviors of technologically relevant nanostructured materials, which are likely applicable to a wider range of SL systems and other nanostructures.

Energetic carriers in semiconductors thermalize by impact-ionization, which generates electron–hole pairs (EHPs), and by energy losses to phonons. The average EHP creation energy is typically about three times the energy gap. In 1960, Shockley derived a simple equation for the average EHP creation energy with a single free parameter that fits experimental values for a wide range of materials, but the underlying assumptions, as stated, have been widely criticized as lacking justification. Modified expressions derived by improved approximations have been proposed but do not fare better. Here, we revisit the foundations of Shockley's equation and provide a robust justification for the kinetic-energy component as a model averaging procedure and then apply a similar procedure to the phonon component of the equation. The phonon result retains Shockley's form, but the interpretation and justification are now on par with those of the kinetic-energy term. The single-parameter fit to the data remains unchanged, i.e., the present analysis accounts for the exceptional applicability of Shockley's equation.

Using an automated optimization scheme, we generate a gallium projector augmented wave (PAW) data set with the 3d electrons treated as core electrons, in contrast to typical treatment of the 3d electrons as valence electrons. Treating the 3d electrons as core electrons reduces calculation sizes, enabling faster defect studies of GaN. We show that the results of defect calculations in GaN are not adversely affected by freezing the 3d electrons to create the new PAW. For all three point defects tested (gallium vacancy, gallium interstitial in the tetrahedral site, and gallium interstitial in the octohedral site), the defect energy levels do not deviate by more than 0.25eV from results with 3d electrons treated as valence. We show speedups in the defect calculations using the new PAW from 1.3 to 2.9, with increasing speedup for larger supercell size. These speedups would translate to similar speedups in Born-Oppenheimer molecular dynamics studies of GaN defects. In addition, we share some insight into the development of the optimized PAW including the use of the arctangent of the logderivatives—a superior accuracy metric for automated optimization of logderivatives/scattering properties.

We model the highly reduced thermal conductivity of nanostructured materials observed in nanoribbons. For highly scaled structures, such as wires with diameters on the order of 20 nm, physical effects beyond classical boundary scattering, including acoustic softening, become important. To date, work on acoustic softening has focused on reductions in group velocity. However, a reduction in the group velocity implies that the phonon dispersion is modified. Here, we investigate how changes in the phonon dispersion manifest in the mean free path, heat capacity, and group velocity. Including these effects in the modeling of thermal conductivity, we find that softening increases low-temperature thermal conductivity while reducing high temperature thermal conductivity. We further compare the model to experimental data.

There are a broad range of applications for narrowband long-wave infrared (LWIR) sources, especially within the 8-12 μm atmospheric window. These include infrared beacons, free-space communications, spectroscopy, and potentially on-chip photonics. Unfortunately, commercial light-emitting diode (LED) sources are not available within the LWIR, leaving only gas-phase and quantum cascade lasers, which exhibit low wall-plug efficiencies and in many cases require large footprints, precluding their use for many applications. Recent advances in nanophotonics have demonstrated the potential for tailoring thermal emission into an LED-like response, featuring narrowband, polarized thermal emitters. In this work, we demonstrate that such nanophotonic IR emitting metamaterials (NIREMs), featuring near-unity absorption, can serve as LWIR sources with effectively no net power consumption, enabling their operation entirely by waste heat from conventional electronics. Using experimental emissivity spectra from a SiC NIREM device in concert with a thermodynamic compact model, we verify this feasibility for two test cases: a NIREM device driven by waste heat from a CPU heat sink and one operating using a low-power resistive heater for elevated temperature operation. To validate these calculations, we experimentally determine the temperature-dependent NIREM irradiance and the angular radiation pattern. We purport that these results provide a first proof-of-concept for waste heat-driven thermal emitters potentially employable in a variety of infrared application spaces.

Brown adipose tissue (BAT) is a highly vascularized tissue that uptakes and oxidizes fatty acids from the circulation in response to a cold stimulus, resulting in thermogenesis. Tissue perfusion has been proposed as an approach to understand BAT metabolism, but current imaging techniques require invasive contrast or ionizing radiation. Power Doppler ultrasound imaging enables sensitive, high temporal resolution measures of the movement of blood as it perfuses a tissue without the need for contrast injections. The purpose of this study was to explore the utility of non‐contrast ultrasound perfusion imaging of human BAT during personalized cooling. Five healthy subjects [4 men; age: 31.8 ± 5.6 yrs.; body mass index: 22.5 ± 3.3 kg/m2; total body fat % (dual‐energy x‐ray absorptiometry scan): 22.9 ±7.0 %] underwent an individualized, perception‐based cooling protocol to stimulate BAT. Infrared thermography, a surrogate measure of BAT activity, and power Doppler ultrasound images were acquired over the right and left supraclavicular space, respectively, every five minutes in thermoneutrality (TN; duration: 15 min) and during cold exposure (CE) to the participant’s shiver threshold (duration: 58.3 ± 10.8 min; cooling dose: 319.7 ± 140.4 °C*min). Ultrasound images were post‐processed with a block‐wise, independent component analysis filter to analyze signal changes related to perfusion. BAT regions of interest were defined, and TN and CE conditions were compared as the mean ± standard deviation of the difference in the 95th percentile skin temperatures (infrared thermography) and the mean power Doppler signal (ultrasound). Supraclavicular skin temperature increased by 0.56 ± −0.21 °C (95% bootstrap confidence interval (CI): −0.57 to 1.72 °C), indicating a potential thermogenic response of BAT to individualized cooling (TN: 33.8 ± 1.1 °C vs. CE: 34.3 ± 0.9 °C). Similarly, the mean power Doppler signal increased by 11.6 ± 3.8 dB (95% CI: 3.2 to 19.4 dB) following cold exposure (TN: 46.8 ± 5.3 dB vs. CE: 58.3 ± 9.1 dB). These preliminary data demonstrate the feasibility of non‐contrast ultrasound perfusion imaging to detect the microvascular response of BAT to a cold stimulus in healthy adults. Power Doppler ultrasound imaging could prove useful when combined with existing noninvasive modalities (e.g. magnetic resonance imaging) to assess both the perfusion and metabolic substrate uptake response of BAT to potential obesity‐targeted therapies.Support or Funding InformationNIDDK/NIH R01‐DK‐105371, NCATS/NIH UL1‐TR000445

To improve first-principles calculations of materials properties, we introduce a new metric based on the arctangent of the logderivatives, and demonstrate its effectiveness for evaluating scattering properties of pseudopotentials and projector augmented wave (PAW) data sets. This metric is simpler and easier to obtain compared to modern metrics, accurately represents the agreement between pseudo and all-electron logderivatives, and reliably identifies ghost states without resorting to equation of state, density of states, or band structure calculations. Furthermore, we demonstrate that the arctangent metric can be used as a filter or screening metric during development of pseudopotentials, vastly reducing the development time for both hand-tuning and automated optimization. We found that screening by the arctangent metric reduces the search space of new pseudopotentials by at least 50% in the systems we studied.

Thermal transport in amorphous silicon dioxide (a-SiO2) is traditionally treated as random walks of vibrations owing to its greatly disordered structure, which results in a mean free path (MFP) approximately the same as the interatomic distance. However, this picture has been debated constantly and in view of the ubiquitous existence of thin a-SiO2 layers in nanoelectronic devices, it is imperative to better understand this issue for precise thermal management of electronic devices. Different from the commonly used cross-plane measurement approaches, here we report on a study that explores the in-plane thermal conductivity of double silicon nanoribbons with a layer of a-SiO2 sandwiched in-between. Through comparing the thermal conductivity of the double ribbon samples with that of corresponding single ribbons, we show that thermal phonons can ballistically penetrate through a-SiO2 of up to 5 nm thick even at room temperature. Comprehensive examination of double ribbon samples with various oxide layer thicknesses and van der Waals bonding strengths allows for extraction of the average ballistic phonon penetration depth in a-SiO2. With solid experimental data demonstrating ballistic phonon transport through a-SiO2, this work should provide important insight into thermal management of electronic devices.

Abstract Steam Assisted Gravity Drainage (SAGD) is used in West Canadian Sedimentary Basin (WCSB) for heavy oil recovery and numerical simulation is quantitative tool, which can be used to evaluate the SAGD performance and test developments options. This paper starts with the assumption that a single model is unlikely to be correct, and cannot represent the range in the outcomes due to the reservoir complexity and fluid heterogeneity. An alternative is to generate a number of models through an assisted history matching (AHM) process, with the intent of providing realistic estimates of the remaining uncertainty and quantifiable options to reduce the uncertainty. For a new piece of surveillance to add value, our working hypothesis is that the new surveillance should not be correlated with existing surveillance to avoid redundancy, and that the reduction in the range of acceptable models provides a way of quantifying the value. For our SAGD study, we are evaluating if temperature and pressure surveillance in observation wells has value, and how they should be included in the objective function. A synthetic two-dimensional SAGD model was used in the simulation, with a known truth case against which we can test the efficiency of different types of surveillance to recover the true reservoir performance. Three observation wells are distributed through the model with five stations for temperature and pressure measurements. To test the value of the temperature measurements and observer well, a blind searching algorithm was use initially to avoid introducing a search bias that could distort the correlation. Rate, pressure and, temperature misfits were calculated for each experiment. The model with less than 5% rate misfit were filtered and run for several years in prediction with the same depletion plan. The improvement in the lower confidence bound for the value of the field used as the benefit of the surveillance. A second test was to use an exploitative search technique on a simple arithmetic objective function. As expected, the global minimum did not represent the truth case performance, though the truth case was in the intersection of the surveillance constraints of matching pressure, rate and temperature within noise. The correlation between temperature and pressure surveillance show some correlation, such that there is some redundancy, yet the analysis of the models resulting from both parts showed that pressure and temperature surveillance have reduced the uncertainty in the future outcomes by 84% in cumulative oil production and 82% in the NPV. The proposed workflow used here will allow quantifying the potential of adding a new surveillance to the field and/or specifying the number and locations of the required observation wells which properly capture the steam chamber development and pressure propagation throughout SAGD reservoir.

The self-assembly of colloidal nanocrystals into ordered architectures has attracted significant interest enabling innovative methods of manipulating physicochemical properties for targeted applications. This study reports the self-assembly of CsPbBr3 perovskite nanocrystals (NCs) in one-dimensional (1D) superlattice chains mediated by ligand–solvent interactions. CsPbBr3 NCs synthesized at ≥170 °C and purified in a nonpolar solvent, hexane, self-assembled into 1D chains, whereas those purified in polar solvents, including toluene and ethyl acetate, were disordered or formed short-range two-dimensional (2D) assemblies. The NCs assembled into 1D chains showed red shifts in both the absorbance and photoluminescence spectra relative to those of disordered NCs purified in a 50/50 hexane/ethyl acetate mixture. Microscopy and X-ray diffraction results confirmed the formation of polymeric nanostrands in hexane followed by organization of the NCs into 1D chains along the nanostrands. Our results suggest that exce...

The interplay between mechanical strains and battery electrochemistry, or the tunable mechanochemistry of batteries, remains an emerging research area with limited experimental progress. In this report, we demonstrate how elastic strains applied to vanadium pentoxide (V2O5), a widely studied cathode material for Li-ion batteries, can modulate the kinetics and energetics of lithium-ion intercalation. We utilize atomic layer deposition to coat V2O5 materials onto the surface of a shapememory superelastic NiTi alloy, which allows electrochemical assessment at a fixed and measurable level of elastic strain imposed on the V2O5, with strain state assessed through Raman spectroscopy and X-ray diffraction. Our results indicate modulation of electrochemical intercalation potentials by ∼40 mV and an increase of the diffusion coefficient of lithium ions by up to 2.5-times with elastic prestrains of <2% imposed on the V2O5. These results are supported by density functional theory calculations and demonstrate how mechanics of nanomaterials can be used as a precise tool to strain engineer the electrochemical energy storage performance of battery materials.

Strain engineering has transformed applications in the semiconductor electronics industry, but has not been widely explored as a tool for electrochemical applications. Our early studies have demonstrated for the first time that mechanical strain applied across interfaces of engineered 2-D nanomaterials or externally applied to pseudocapacitive electrodes can result in deviations in the energetics and kinetics of Faradaic charge storage reactions [1-2]. Here, we present results that extend this idea to vanadium pentoxide (V2O5) due to its well-known capability to function as a cathode for the intercalation of lithium ions. To characterize the role of strain in modulating the lithium insertion properties, we use atomic layer deposition (ALD) to prepare ultrathin coatings of crystalline V2O5 on the surface of superelastic NiTi shape memory alloy surfaces. As a key challenge in characterizing the role of strain is the ability to assess electrochemical properties in a fixed strain state, we exploit the capability of NiTi to "lock-in" strain in the elastic regime, which extends up to ~ 15% strain. This enables us to directly measure the average strain transferred to the V2O5 material, and correlate this to the observed electrochemical properties. Through electrochemical tests, we observe over 50 mV shift in the electrochemical potential that is correlated to strain, and over 2X change in the diffusion coefficient of lithium ions. We further build a semi-quantitative model for strain modulated battery performance that is supported by density functional theory calculations. Overall, this work demonstrates new degrees of freedom to design or engineer electrodes for energy storage by using pre-strained or strain-engineered electrode materials. References: [1]. N. Muralidharan, R. Carter, L. Oakes, A.P. Cohn, and C.L. Pint*, "Strain engineering to modify the electrochemistry of energy storage electrodes," Sci. Rep. 6, 27542 (2016). [2]. L. Oakes, R. Carter, T. Hanken, A.P. Cohn, K. Share, B. Schmidt, and C.L. Pint*, "Interface strain in vertically stacked two-dimensional heterostructured carbon-MoS2 nanosheets controls electrochemical reactivity," Nat. Commun. 7, 11796 (2016).

Innovative Thermoelectric Materials, pp. 193-218 (2016) No AccessChapter 7: Modeling Thermoelectric MaterialsGreg WalkerGreg Walkerhttps://doi.org/10.1142/9781783266067_0007Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: The following sections are included: Introduction Continuum Considerations Noncontinuum Methods Wave versus Particle First-Principles Calculations Electronic Structure Methods References FiguresReferencesRelatedDetails Innovative Thermoelectric MaterialsMetrics History PDF download

We have used a multi-objective genetic algorithm to optimize pseudopotentials for force accuracy and computational efficiency. Force accuracy is determined by comparing interatomic forces generated using the pseudopotentials and forces generated using the full-potential linearized augmented-plane wave method. This force-based optimization approach is motivated by applications where interatomic forces are important, including material interfaces, crystal defects, and molecular dynamics. Our method generates Pareto sets of optimized pseudopotentials containing various compromises between accuracy and efficiency. We have tested our method for LiF, Si0.5Ge0.5, and Mo and compared the performance of our pseudopotentials with pseudopotentials available from the ABINIT library. We show that the optimization can generate pseudopotentials with comparable accuracy (in terms of force matching and equation of state) to pseudopotentials in the literature while sometimes significantly improving computational efficiency. For example, we generated pseudopotentials for one system tested that reduced computational work by 71% without loss of accuracy. These results suggest our method can be used to generate pseudopotentials on demand that are tuned for a user’s specific application, affording gains in computational efficiency.

Phosphor thermometry measurements in turbine engine environments can be difficult because of high background radiation levels. To address this challenge, luminescence lifetime-based phosphor thermometry measurements were obtained using thulium-doped Y3Al5O12 (YAG:Tm) to take advantage of the emission wavelengths at 365 nm (D-1(2) -> H-3(6) transition) and at 456 nm (D-1(2) -> F-3(4) transition). At these wavelengths, turbine engine radiation background is reduced compared with emission from longer wavelength phosphors. Temperature measurements of YAG: Tm coatings were demonstrated using decay of both the 365 and 456 nm emission bands in a furnace environment up to 1400 degrees C. To demonstrate that reliable surface temperatures based on short-wavelength YAG: Tm emission could be obtained from the surface of an actual engine component in a high gas velocity, highly radiative environment, measurements were obtained from a YAG:Tm-coated Honeywell stator vane doublet placed in the afterburner flame exhaust stream of the augmenter-equipped General Electric J85 turbojet test engine at the University of Tennessee Space Institute (UTSI). Using a probe designed for engine insertion, spot temperature measurements were obtained by measuring luminescence decay times over a range of steady state throttle settings as well as during an engine throttle acceleration. YAG:Tm phosphor thermometry measurements of the stator vane surface in the afterburner exhaust stream using the decay of the 456 nm emission band were successfully obtained at temperatures up to almost 1300 degrees C. Phosphor thermometry measurements acquired with the engine probe using the decay of the 365 nm emission band were not successful at usefully high temperatures because the probe design allowed transmission of intense unfiltered silica Raman scattering that produced photomultiplier tube saturation with extended recovery times. Recommendations are made for probe modifications that will enable temperature measurements using the 365 nm emission band decay, which will be beneficial in environments with strong reflections of combustor radiation.

The thermal conductivities of two groups of silicon nanoribbons of ∼20 and ∼30 nm thickness and various widths have been measured and analyzed through combining the Callaway model and the Fuchs-Sondheimer (FS) reduction function. The results show that while the data for the ∼30 nm thick ribbons can be well-explained by the classical size effect, the measured thermal conductivities for the ∼20 nm thick ribbons deviate from the prediction remarkably, and size effects beyond phonon-boundary scattering must be considered. The measurements of the Young's modulus of the thin nanoribbons yield significantly lower values than the corresponding bulk value, which could lead to a reduced phonon group velocity and subsequently thermal conductivity. This study helps to build a regime map for thermal conductivity versus nanostructures' surface-area-to-volume ratio that clearly delineates two regions where size effects beyond the Casimir limit are important or not important.

The motion of large molecules in microfluidic flows is important because the trajectories of particles in shear flows do not always follow the local flow field. Therefore, a knowledge of the fluid dynamics is not sufficient to completely describe the motion of the particles. When a suspended particle does not track the flow, the particle is said to be active as opposed to passive. The dynamics of active particles are particularly interesting in microfluidic devices because the molecules of polymer chains can approach – and even exceed – the characteristic lengths of the device. Consequently, the deviations between the particle/molecular motion and the fluid motion can be significant. While active particle dynamics can address the motion of any suspended particle in a fluid, microfluidic researchers are usually interested in polymer chains because these structures in general do not follow the fluid flow, have a high degree of flexibility, and model important biological subsystems such as DNA and many types of proteins. If we want to track or manipulate different polymer chains in a microfluidic system, we need to understand their dynamics relative to the fluid flow. However, not only will the bulk motion deviate from what the fluid is doing, but also their conformation or shape can change, which alters the transport properties. In other words, as polymer chains travel in a fluid system, the inherent flexibility of the chains allows them to bunch up, stretch out, or tumble in the flow. As the shape of the chain changes, the fluid forces imposed on the chain will also change. As a result, the motion of a flexible chain will vary even if the flow field remains constant. For microfluidic systems designed to sequence or hybridize DNA, for example, the conformation can be as important as the trajectory of bulk motion of the molecule.

A multi-objective genetic algorithm (MOGA) was used to automate a search for optimized pseudopotential parameters. Pseudopotentials were generated using the atomPAW program and density functional theory (DFT) simulations were conducted using the pwPAW program. The optimized parameters were the cutoff radius and projector energies for the s and p orbitals. The two objectives were low pseudopotential error and low computational work requirements. The error was determined from (1) the root mean square difference between the all-electron and pseudized-electron log derivative, (2) the calculated lattice constant versus reference data of Holzwarth et al., and (3) the calculated bulk modulus versus reference potentials. The computational work was defined as the number of flops required to perform the DFT simulation. Pseudopotential transferability was encouraged by optimizing each element in different lattices: (1) nitrogen in GaN, AlN, and YN, (2) oxygen in NO, ZnO, and SiO4, and (3) fluorine in LiF, NaF, and KF. The optimal solutions were equivalent in error and required significantly less computational work than the reference data. This proof-of-concept study demonstrates that the combination of MOGA and ab-initio simulations is a powerful tool that can generate a set of transferable potentials with a trade-off between accuracy (error) and computational efficiency (work). (C) 2015 Elsevier B.V. All rights reserved.