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Predicting mesoscale spectral thermal conductivity using advanced deterministic phonon transport techniques
We present a review, demonstration and simulation of phonon transport for the purposes of predicting materials performance at the mesoscale. We focus primarily on the development and implementation of a unified methodology to enable predictive heat transport. We report on the current state of the art as it pertains to deterministic phonon transport methodologies, discussing various topics concerning phonons. In application, we focus on the Self-Adjoint Angular Flux (SAAF) formulation of the Boltzmann transport equation for phonons, and develop the spatial, angular, and material property discretization required to accurately simulate the predictive physics of heat transport in dielectrics. We discuss thermal interfacial resistance and present our formulation of the diffuse mismatch model for simulating phonon interactions at internal boundaries. We have recently developed a deterministic, spectral phonon transport method for predicting effective thermal conductivity ($\kappa_{\textrm{eff}}$), using Bose-Einstein source terms coupled through an average material temperature. In this method, we introduce a closure term to the phonon transport system which acts as a redistribution function for the total energy of the system, and serves as a guide for the amount of non-equilibrium behavior occurring in the system. This method predicts thermal conductivity and equilibrium temperature distributions in homogeneous and heterogeneous materials using data generated by ab initio density functional theory methods. We employ polarization, density of states and full dispersion spectra to resolve thermal conductivity with numerous angular and spatial discretizations. Our implementation utilizes a Richardson iteration on a modified version of the phonon scattering source. The equations associated with this method are solved via a modification of traditional source iteration. We compare the performance of source iteration applied to an existing uncoupled, traditional SAAF method to our new method and comment on the iterative performance of each. We observe ballistic and diffusive phonon scattering as acoustic thickness of the domain changes, and are able to make comparisons between the accuracy and efficiency of both methods.
Accelerated radiation transport modeling techniques for pencil beam computed tomography using gamma rays
Monte Carlo radiation transport modeling studies were performed for a compact, and high-resolution gamma-ray computed tomography system designed for imaging irradiated nuclear fuel. The system comprises a 60 Co source – chosen for its highly penetrating 1173 keV and 1332 keV gamma rays – a pair of high-aspect-ratio pencil beam collimators, and an inorganic scintillator detector. Here two acceleration methods are proposed to rapidly model a transmission type gamma-ray tomography system. The first, a variance reduction technique, is based on performing Monte Carlo simulations with a monodirectionally-biased source, sampled from a characteristic sub-volume of the full source volume. The second acceleration method is based on the deterministic calculations using the Beer–Lambert law and detector response characteristics. Comparison of simulations using acceleration approaches with analog simulations of the fully isotropic, full-volume equivalent, show that the Monte Carlo variance reduction technique gives quantitatively accurate predictions for large collimator aspect ratios while the deterministic calculations are semi-quantitative but converge close to the correct result as the collimator aspect ratio increases. As such, these techniques can be used to reduce the computational cost in generating simulated radiographs and tomographs by several orders of magnitude. Experimental validation efforts are currently underway and will be demonstrated in future work.
Processable High Electron Mobility {pi}-Copolymers via Mesoscale Backbone Conformational Ordering
The synthesis and experimental/theoretical characterization of a new series of electron-transporting copolymers based on the naphthalene bis(4,8-diamino-1,5-dicarboxyl)amide (NBA) building block are reported. Comonomers are designed to test the emergent effects of manipulating backbone torsional characteristics, and density functional theory (DFT) analysis reveals the key role of backbone conformation in optimizing electronic delocalization and transport. The NBA copolymer conformational and electronic properties are characterized using a broad array of molecular/macromolecular, thermal, optical, electrochemical, and charge transport techniques. All NBA copolymers exhibit strongly aggregated morphologies with significant nanoscale order. Copolymer charge transport properties are investigated in thin-film transistors and exhibit excellent electron mobilities ranging from 0.4 to 4.5 cm(2) V-1 s(-1). Importantly, the electron transport efficiency correlates with the film mesoscale order, which emerges from comonomer-dependent backbone planarity and extension. These results illuminate the key NBA building block structure-morphology-bulk property design relationships essential for processable, electronics-applicable high-performance polymeric semiconductors.
Self-selecting vapor growth of transition-metal-halide single crystals
Transition-metal halides can host a large variety of novel phenomena, such as magnetism in the monolayer limit, quantum spin liquid and spiral spin liquid states, topological magnons, and chiral phonons. Sizable high-quality single crystals are necessary for investigations of magnetic and lattice excitations by, for example, inelastic neutron scattering. In this paper, we review a less well-known vapor transport technique, self-selecting vapor growth, and report our growths of transition-metal halides using this technique. We report the growth and characterizations of sizable single crystals of α–RuCl 3 , CrCl 3 , Ru 1–x Cr x Cl 3 , and CrBr 3 . In order to expedite the conversion of starting powder to single crystals, we modified the technique by cooling the growth ampoule through an appropriate temperature range. Our work shows that the self-selecting vapor transport technique can provide large single crystals of transition-metal halides, demonstrating its potential for providing high-quality single crystals of quantum materials.
Development of a series of Korean pediatric and adult head computational phantoms and application to nuclear medicine dosimetry - Paper 26
Over the past decades, advances in Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) imaging techniques and an increasing number of new radiopharmaceuticals have resulted in a surge of neuroimaging applications in nuclear medicine on pediatric and adult patients. It is not feasible to directly measure the absorbed dose to the organs of the human body from nuclear medicine procedures. The formalism of the Medical Internal Radiation Dose (MIRD) committee of the Society of Nuclear Medicine has been adopted as the calculation method to estimate radiation dose to target organs from radionuclides distributed in source organs. The absorbed dose to a target organ from different types of radionuclides distributed within source organs can be estimated by the products of the time-integrated activity in each source region and organ dose conversion coefficients, called S values (Gy/MBq-s). S values are computed from a comprehensive set of Specific Absorbed Fraction (SAF), which are calculated from computational human phantoms with realistic anatomical structures coupled with Monte Carlo transport technique. The current SAF data for pediatric and adult head and brain dosimetry is based on a series of stylized computational phantoms. However, dosimetric errors caused by the unrealistic anatomy of stylized phantoms in internal dosimetry calculations have been reported by several studies. In the current study, we developed a series of realistic computational head phantoms representing 6, 9, 12, 15, and 22-year-old standard Korean males, and computed a comprehensive set of SAFs using the phantoms coupled with Monte Carlo transport technique. (authors)
Some techniques for transport in radiation hydrodynamics [Slides]
Summary: Non-relativistic IMC/SN with Lagrangian hydrodynamics; IMC-DDMC momentum deposition; Post-ALE IMC particle remap and SN remap; collapse of multi-material state per zone.
Flux growth in a horizontal configuration: An analog to vapor transport growth
Flux growth of single crystals is normally performed in a vertical configuration with an upright refractory container holding the flux melt. At high temperatures, flux dissolves the charge, forming a homogeneous solution before nucleation and growth of crystals takes place under proper supersaturation generated by cooling or evaporating the flux. In this paper, we report flux growth in a horizontal configuration with a temperature gradient along the horizontal axis: a liquid transport growth analogous to the vapor transport technique. In a typical liquid transport growth, the charge is kept at the hot end of the refractory container and the flux melt dissolves the charge and transfers it to the cold end. Once the concentration of charge is above the solubility limit at the cold end, the thermodynamically stable phase nucleates and grows. Compared to the vertical flux growth, the liquid transport growth can provide a large quantity of crystals in a single growth since the charge/flux ratio is not limited by the solubility limit at the growth temperature. This technique is complementary to the vertical flux growth and can be considered when a large amount of crystals is needed but the yield from the conventional vertical flux growth is limited. Finally, we applied this technique to the growth of IrSb 3 , Mo 3 Sb 7 , and MnBi from self-flux, and the growth of FeSe, CrTe 3 , NiPSe 3 , FePSe 3 , CuInP 2 S 6 , RuCl 3 , and OsCl 4 from a halide flux.
Density estimation via measure transport: Outlook for applications in the biological sciences
Abstract One among several advantages of measure transport methods is that they allow or a unified framework for processing and analysis of data distributed according to a wide class of probability measures. Within this context, we present results from computational studies aimed at assessing the potential of measure transport techniques, specifically, the use of triangular transport maps, as part of a workflow intended to support research in the biological sciences. Scenarios characterized by the availability of limited amount of sample data, which are common in domains such as radiation biology, are of particular interest. We find that when estimating a distribution density function given limited amount of sample data, adaptive transport maps are advantageous. In particular, statistics gathered from computing series of adaptive transport maps, trained on a series of randomly chosen subsets of the set of available data samples, leads to uncovering information hidden in the data. As a result, in the radiation biology application considered here, this approach provides a tool for generating hypotheses about gene relationships and their dynamics under radiation exposure.
A Comparison of Angular Discretization Techniques for the Radiative Transport Equation [PowerPoint]
Abstract not provided.
Generalized Ellipsometry Measurements of Crystalline Thin Film and Bulk Tin Oxide
Here, several bulk and thin-film crystals of SnO2 are grown and examined using generalized ellipsometry techniques. The bulk samples are grown using the chemical vapor transport technique and thin films of SnO2 are grown using the pulsed laser deposition technique. The bulk samples are examined using the two-modulator generalized ellipsometry microscope (2-MGEM) at normal incidence and the spectroscopic two-modulator generalized ellipsometer (2-MGE). The spectroscopic optical functions of tin oxide are then obtained using the 2-MGE from 1.46 to 6.2 eV. The material is highly birefringent, and the ordinary bandgap is less than the extraordinary band edge. 2-MGE measurements are also made on thin-film samples of crystalline tin oxide grown on sapphire and rutile, showing no cross polarization. Because of the complicated morphology of the tin oxide films grown on sapphire, the ellipsometry data are simulated using the Tauc–Lorentz model. Films grown on rutile had the optic axis perpendicular to the sample surface, but the film is strained, resulting in a more complicated ellipsometric spectrum. These films are modeled using the air/surface roughness/tin oxide/interface/rutile model, where the roughness and interface are modeled using the incomplete Beta function.
High-fidelity Pebble Bed Reactor Depletion Based on Pebble Tracking Transport in Griffin
The pebble tracking transport (PTT) method is a high-fidelity, heterogeneous deterministic transport technique for pebble bed reactor analysis. It discretizes the broad-group neutron transport equation in space and angle with the discontinuous finite element and the discrete ordinates method, and utilizes various solving techniques, including mesh sweeping and diffusion acceleration, to provide pebble- wise reaction rates. This work presents the extension of the PTT method to enable fuel depletion capability in the Griffin code. We discuss the implementation details of the PTT-based high-fidelity depletion where isotope inventory of all individual pebbles is tracked through pre-determined pebble flow paths in the core. The implementation is verified with a generic pebble bed reactor model. Some preliminary equilibrium core results are included. Future works are also discussed.
Use of micro-photoluminescence as a contactless measure of the 2D electron density in a GaAs quantum well
We compare micro-photoluminescence (μPL) as a measure of the electron density in a clean, two-dimensional (2D) system confined in a GaAs quantum well (QW) to the standard magneto-transport technique. Our study explores the PL shape evolution across a number of molecular beam epitaxy-grown samples with different QW widths and 2D electron densities and notes its correspondence with the density obtained in magneto-transport measurements on these samples. We also measure the 2D density in a top-gated quantum well sample using both PL and transport and find that the two techniques agree to within a few percent over a wide range of gate voltages. We find that the PL measurements are sensitive to gate-induced 2D density changes on the order of 109 electrons/cm 2 . The spatial resolution of the PL density measurement in our experiments is 40 μm, which is already substantially better than the millimeter-scale resolution now possible in spatial density mapping using magneto-transport. Our results establish that μPL can be used as a reliable high spatial resolution technique for future contactless measurements of density variations in a 2D electron system.
Probabilistic Programming for Transportable Source Characterization and Uncertainty Quantification of the North Korean Nuclear Tests 2006–2017
Here, we introduce a transportable technique to determine the yield and depth of burial (DOB) from seismic source spectra of underground nuclear explosions. We demonstrate this technique on the six declared North Korean nuclear tests. This approach derives source spectra in absolute units from regional phase (Pg) amplitudes by correcting the observations for geometric spreading, attenuation, and site amplification. We couple the source spectra and explosion source models with a probabilistic programming framework that integrates deep learning techniques and Bayesian modeling. This approach permits the exchange of information across various data categories to quantify both the data and model uncertainty. This technique stands out as an innovative use of broad‐area propagation models, making it transportable across various geologic settings. This method proves to be effective in scenarios with diverse and/or limited observational data, even when the source depth is unknown. We present new independent estimates of absolute yield and DOB that are consistent with the prior assessments, underscoring the potential of this method in enhancing transportable nuclear explosion monitoring capabilities.
Perspective on ab initio phonon thermal transport
Coupling of the Peierls-Boltzmann equation with density functional theory paved the way for predictive thermal materials discovery and a variety of new physical insights into vibrational transport behaviors. Here, rapid theoretical and numerical developments have generated a wealth of thermal conductivity data and understanding of a wide variety of materials—1D, 2D, and bulk—for thermoelectric and thermal management applications. Nonetheless, modern ab initio descriptions of phonon thermal transport face challenges regarding the effects of defects, disorder, structural complexity, strong anharmonicity, quasiparticle couplings, and time and spatially varying perturbations. Highlighting recent research on these issues, this perspective explores opportunities to expand current ab initio phonon transport techniques beyond the paradigm of weakly perturbed crystals, to the wider variety of materials possible. Recent developments in phonon-defect interactions, complexity, disorder and anharmonicity, hydrodynamic transport, and the rising roles of molecular dynamics simulations, high throughput, and machine learning tools are included in this perspective. Lastly, as more sophisticated theoretical and computational methods continue to advance thermal transport predictions, novel vibrational physics and thermally functional materials will be discovered for improved energy technologies.
Doping and electronic properties of GaAs grown by close-spaced vapor transport from powder sources for scalable III–V photovoltaics
We report the use of a simple close-spaced vapor transport technique for the growth of high-quality epitaxial GaAs films using potentially inexpensive GaAs powders as precursors. The free carrier type and density (10 16 to 10 19 cm –3 ) of the films were adjusted by addition of Te or Zn powder to the GaAs source powder. We show using photoelectrochemical and electron beam-induced current analyses that the minority carrier diffusion lengths of the n- and p-GaAs films reached ~3 μm and ~8 μm, respectively. Hall mobilities approach those achieved for GaAs grown by metal–organic chemical vapor deposition, 1000–4200 cm 2 V –1 s –1 for n-GaAs and 50–240 cm V –1 s –1 for p-GaAs depending on doping level. We conclude that the electronic quality of GaAs grown by close-spaced vapor transport is similar to that of GaAs made using conventional techniques and is thus sufficient for high-performance photovoltaic applications.
Rapid assessment of REBCO CC angular critical current density J c (B, T = 4.2 K, θ) using torque magnetometry up to at least 30 tesla
Abstract Detailed design of REBa 2 Cu 3 O 7 − x (REBCO)-based magnets ideally relies on knowledge of the full angular and wide temperature range characterization of the critical current I c of the REBCO coated conductors (CC) at high magnetic fields. In practice, however, obtaining I c ( B , T , θ ) data by the commonly used electrical transport technique is expensive, tedious, and difficult, due to high critical current values that exceed 2000 A for B | | a b -plane ( θ = 90 deg). The conductors are often damaged during angular transport measurements at angles approaching the ab -plane. Therefore, so far, REBCO magnets have been designed without full I c ( B , T , θ ) data sets. Here, we present J c ( B , T , θ ) results for more than twenty samples of CC all made to the same advanced pinning specification produced by SuperPower Inc. For this, we employed torque magnetometry, benchmarking the results to transport I c measurements mostly made away from ab -plane, finding good agreement with scaling factors ≈1–1.3. What is striking is the huge variety of properties exhibited by the samples, even for CC made recently. Given the huge attention now being paid to the effects of screening currents and to the torques generated by offsets between the tape plane and the local magnetic field vector, our data set suggests some caution in detailed design of such magnets without having data of the type presented here.
Understanding Topological Pseudospin Transport in Van Der Waals' Materials
Because of symmetry, crystals often have degenerate valleys in the conduction or the valence band. This “valley” degree of freedom, similar to electronic charge and spin, has been proposed as an information carrier for new classes of electronic and optoelectronic devices. The basic requirement of information processing is to be able to create and manipulate a flow of valley pseudospin, i.e. valley pseudospin current. Valley pseudospin currents have been demonstrated in recent experiments in monolayer molybdenum disulfide and graphene on hexagonal boron nitride. However, the key fundamental questions regarding the nature of valley pseudospin transport remain open. This project will investigate the importance of Berry curvature effects in valley pseudospin transport and explore regimes for quantized valley Hall conductivity and pure valley pseudospin currents. The study will rely on combined optical and electrical transport techniques to create, manipulate and detect valley pseudospin currents in two-dimensional (2D) van der Waals’ materials of hexagonal structure. The insights provided by this research will deepen the understanding of valley pseudospin transport in 2D and help the development of novel valley-based technologies.