Uncertainty Quantification in Multiscale Simulation of Materials: A Prospective
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phonons, thermal conductivity, nuclear (including ↗
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DOC-DICAM: Domain Aware One Class Defect Identification in Composite Aerostructure Material
Fiber-reinforced composites are a common material used in the design of aircraft structures due to their good tensile strength and resistance to compression. During the manufacturing process, these structures are thoroughly inspected for flaws and defects to ensure structural integrity during commercial use. Non-destructive testing (NDT) is a collection of inspection methods that allow inspectors to evaluate material without altering it. Due to the high safety standards in aerospace manufacturing, the NDT process is done manually and can be a significant bottleneck in the development workflow. In this paper, we develop an AI-based assistance tool to drastically reduce inspection time. Typical AI workflows require large amounts of annotated data, but defects rarely occur resulting in strong class imbalance. To overcome this, we formulate the problem of defect identification as an anomaly detection task in which our primary focus is learning non-defect characteristics. To do this, we develop a multi-task self-supervised learning framework that embeds problem specific domain knowledge into the deep learning model. We verify our method using fuselage data generated in a production environment. As a result, we show that our method can effectively identify defects and requires minimal training and inference time.
anomaly detection↗
Field Dislocation Mechanics for heterogeneous elastic materials: A numerical spectral approach
Spectral methods using Fast Fourier Transform (FFT) algorithms have recently seen a surge in interest in the mechanics of materials community. The present work addresses the critical question of determining accurate local mechanical fields using FFT methods without artificial fluctuations arising from materials and defects induced discontinuities. Precisely, this work introduces a numerical approach based on intrinsic discrete Fourier transforms for the simultaneous treatment of material discontinuities arising from the presence of dislocations and from elastic stiffness heterogeneities. To this end, the elasto-static equations of the field dislocation mechanics theory for periodic heterogeneous materials are numerically solved with FFT in the case of dislocations in proximity of inclusions of varying stiffness. An optimal intrinsic discrete Fourier transform method is sought based on two distinct schemes. A centered finite difference scheme for differential rules are used for numerically solving the Poisson-type equation in the Fourier space, while centered finite differences on a rotated grid is chosen for the computation of the modified Fourier–Green’s operator associated with the Lippmann–Schwinger-type equation. By comparing different methods with analytical solutions for an edge dislocation in a composite material, it is found that the present spectral method is accurate, devoid of any numerical oscillation, and efficient even for an infinite phase elastic contrast like a hole embedded in a matrix containing a dislocation. The present FFT method is then used to simulate physical cases such as the elastic fields of dislocation dipoles located near the matrix/inclusion interface in a 2D composite material and the ones due to dislocation loop distributions surrounding cubic inclusions in 3D composite material. In these configurations, the spectral method allows investigating accurately the elastic interactions and image stresses due to dislocation fields in the presence of elastic inhomogeneities.
36 MATERIALS SCIENCE↗
The bridge between the materials and devices of thermoelectric power generators
Beyond ZT , ( ZT ) eng was used as a direct indicator for thermoelectric performance to achieve a balance between material tuning and device reliability.
charge transport↗
Enhanced Thermoelectric Performance in 18-Electron Nb 0.8 CoSb Half-Heusler Compound with Intrinsic Nb Vacancies
Typical 18-electron half-Heusler compounds, ZrNiSn and NbFeSb, are identified as promising high-temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, is recently reported to also exhibit thermoelectric properties of a heavily doped n-type semiconductor. Here for the first time, it is experimentally demonstrated that the nominal 19-electron NbCoSb is actually the composite of 18-electron Nb 0.8 +δCoSb (0 ≤ δ < 0.05) and impurity phases. Single-phase Nb 0.8δ +CoSb with intrinsic Nb vacancies, following the 18-electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. The carrier concentration can be controlled by varying the Nb deficiency, and the optimization of the thermoelectric properties can be realized within the narrow pure phase region. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ≈100% and a maximum zT of 0.9 is achieved in Nb 0.83 CoSb at 1123 K. Here, this work expands the family of half-Heusler thermoelectric materials and opens a new avenue for searching for nominal 19-electron half-Heusler compounds with intrinsic vacancies as promising thermoelectric materials.
36 MATERIALS SCIENCE↗
ZnSb Polymorphs with Improved Thermoelectric Properties
Zn$_x$Sb$_{1–x}$ systems are one of the most promising materials for thermoelectric applications due to their low cost, low toxicity, and high thermoelectric figure of merit. We report the discovery and characterization of novel ZnSb polymorphs with improved thermoelectric properties from a systematic $ab initio$ structural search. The putative ground state at low temperature is a structure with $Pbca$ symmetry. Corresponding electron and phonon transport properties are computed, where the lattice thermal conductivity was accurately obtained through third-order interatomic force constants. At 500 K, both the reduced figure of merit $S^2/L$ as well as the lattice thermal conductivity are superior to any previously reported numbers on ZnSb polymorphs, rendering the $Pbca$ phase a promising candidate for an exceptionally good thermoelectric material.
36 MATERIALS SCIENCE↗
A Chemical Understanding of the Band Convergence in Thermoelectric CoSb 3 Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions
N-Type skutterudites, such as Yb x Co 4 Sb 12 , have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conduction band closer to the primary conduction band and thus inducing band convergence.
36 MATERIALS SCIENCE↗
Engineering a Full Gamut of Structural Colors in All-Dielectric Mesoporous Network Metamaterials
Structural colors are a result of the scattering of certain frequencies of the incident light on micro- or nanoscale features in a material. This is a quite different phenomenon from that of colors produced by absorption of different frequencies of the visible spectrum by pigments or dyes, which is the most common way of coloring used in our daily life. However, structural colors are more robust and can be engineered to span most of the visible spectrum without changing the base material, only its internal structure. They are abundant in nature, with examples as colorful as beetle shells and butterfly wings, but there are few ways of preparing them for large-scale commercial applications for real-world uses. In this work, we present a technique to create a full gamut of structural colors based on a low-cost, robust, and scalable fabrication of periodic network structures in porous alumina as well as the strategy to theoretically predict and engineer different colors on demand. We experimentally demonstrate mesoporous network metamaterial structures with engineered colors spanning the whole optical spectrum and discuss their applications in sensing, environmental monitoring, biomimetic tissues engineering, etc.
36 MATERIALS SCIENCE↗
Stability, electronic structures and thermoelectric properties of binary Zn–Sb materials
We report first principles studies of the binary Zn–Sb phases in relation to thermoelectric properties and chemical stability. We identify the unknown structure of the Zn 3 Sb 2 phase using particle swarm optimization, finding a tetragonal structure different from the hexagonal Mg 3 Sb 2 and the hexagonal or cubic Ca 3 Sb 2 phases. All the phases are found to be semiconducting with bandgaps in the range of 0.06–0.77 eV. This semiconducting behavior is understood in Zintl terms as a balance between the Zn:Sb and Sb 3- :½(Sb 2 ) 4- ratios in the stable crystal structures. With the exception of Zn 3 Sb 2 , which has a small gap, all the compounds have electronic properties favorable for thermoelectric performance.
36 MATERIALS SCIENCE↗
Thermal transport in suspended silicon membranes measured by laser-induced transient gratings
Not Available
36 MATERIALS SCIENCE↗
A rapid method to extract Seebeck coefficient under a large temperature difference
The Seebeck coefficient is one of the three important properties in thermoelectric materials. Since thermoelectric materials usually work under large temperature difference in real applications, we propose a quasi-steady state method to accurately measure the Seebeck coefficient under large temperature gradient. Compared to other methods, this method is not only highly accurate but also less time consuming. It can measure the Seebeck coefficient in both the temperature heating up and cooling down processes. In this work, a Zintl material (Mg 3.15 Nb 0.05 Sb 1.5 Bi 0.49 Te 0.01 ) was tested to extract the Seebeck coefficient from room temperature to 573 K. Compared with a commercialized Seebeck coefficient measurement device (ZEM-3), there is ±5% difference between those from ZEM-3 and this method.
36 MATERIALS SCIENCE↗
p-type conductivity and damage recovery in implanted GaN annealed by rapid gyrotron microwave annealing
We demonstrate p-type activation of GaN doped by Mg ion implantation, and in situ during metalorganic chemical vapor deposition through sequential short-duration gyrotron microwave heating cycles at temperatures of 1200–1350 °C. GaN is implanted with 1019 cm−3 Mg ions, capped with AlN, and annealed under 3 MPa N2 overpressure in 5 s heating cycles for less than 60 s total using a high-power gyrotron microwave heating source. Through I–V characterization, photoluminescence spectroscopy, and Raman spectroscopy, we study the evolution of electrical properties, optically active point defects, and material strain in response to implantation and annealing. For Mg-implanted samples, increasing annealing temperature is characterized by an increase in the PL substitutional Mg-related peak (UVL) relative to the shallow-donor vacancy-related peak (GL2). Through comparison of implanted and in situ doped samples, it is demonstrated that the origin of compensating VN lies primarily in implantation rather than degradation from the annealing process. Transmission line measurements and diode I–V measurements show a sheet resistance of 1083 kΩ/□ and a hole concentration of 1.23 × 1015 cm−3, respectively, in the Mg-implanted material annealed at 1350 °C. We conclude that temperature-cycled gyrotron annealing at 1350 °C decreases implant-induced compensating point defects and activates Mg to obtain selective p-type conduction.
Meyers, V. (ORCID:0000000176731553)↗
Cleavable magnetic materials from van der Waals layered transition metal halides and chalcogenides
The study of exfoliated 2D magnetic materials is a vibrant and rapidly progressing field and is impacting many areas of condensed matter research including fundamental magnetism, spintronics and optoelectronics, and topological spin and electronic systems. The availability of promising candidate materials has enabled much of the progress in this field. Here, I present my perspective on the development of cleavable magnetic materials with a focus on transition metal halides and chalcogenides and an emphasis on crystal structure and magnetic order. To give proper context for these discussions, brief and incomplete surveys of recent work are included, focusing on specific aspects that I find most useful for guiding work on emerging materials and motivating expansion into other compounds and material families. Several instances of structural changes that can differentiate behaviors of bulk and ultrathin specimens are noted. Probing and understanding potential structural differences present a challenge but also an opportunity for material and device development, if they can be predicted and controlled. It is clear that careful investigations of structure, layer stacking, and defects in materials, how they may relate to the crystal chemistry, and how they may be different in crystals and mono- or few-layer specimens provide invaluable context for understanding the behavior of van der Waals layered magnetic materials.
36 MATERIALS SCIENCE↗
First-principles mode-by-mode analysis for electron-phonon scattering channels and mean free path spectra in GaAs
We present a first-principles framework to investigate the electron scattering channels and transport properties for polar materials by combining the exact solution of the linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from the polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature, and it makes a significant contribution to mobility. Furthermore, we achieved good agreement with experimental data for the mobility, and we identified that electrons with mean free paths between 130 and 210 nm provide the dominant contribution to the electron transport at 300 K. As a result, such information provides a deeper understanding of the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.
36 MATERIALS SCIENCE↗
Defect-driven anomalous transport in fast-ion conducting solid electrolytes
We report solid-state ionic conduction is a key enabler of electrochemical energy storage and conversion. The mechanistic connections between material processing, defect chemistry, transport dynamics, and practical performance are of considerable importance, but remain incomplete. Here, inspired by studies of fluids and biophysical systems, we re-examine anomalous diffusion in the iconic two-dimensional fast-ion conductors, the β- and β"-aluminas. Using large-scale simulations, we reproduce the frequency dependence of alternating-current ionic conductivity data. We show how the distribution of charge-compensating defects, modulated by processing, drives static and dynamic disorder, which lead to persistent sub-diffusive ion transport at macroscopic timescales. We deconvolute the effects of repulsions between mobile ions, the attraction between the mobile ions and charge-compensating defects, and geometric crowding on ionic conductivity. Our quantitative framework based on these model solid electrolytes connects their atomistic defect chemistry to macroscopic performance with minimal assumptions and enables mechanism-driven 'atoms-to-device' optimization of fast-ion conductors.
25 ENERGY STORAGE↗
Conical Intersections at the Nanoscale: Molecular Ideas for Materials
The capacity to predict and describe nonradiative processes in molecules via the identification and characterization of conical intersections is one of the greatest recent successes of theoretical chemistry. Only recently, nonetheless, has this concept been extended to materials science, where nonradiative recombination limits the efficiencies of materials for various optoelectronic applications. Here, we introduce recent advances in the theoretical study of conical intersections in semiconductor nanomaterials. After briefly introducing conical intersections, we argue that specific defects in materials can induce conical intersections between the ground and first excited electronic states, thus introducing pathways for nonradiative recombination. We present recent developments in theoretical methods, computational tools, and chemical intuition for the prediction of such defect-induced conical intersections. Through examples in various nanomaterials, we illustrate the significance of conical intersections for nanoscience. We also discuss challenges facing research in this area and opportunities for progress.
36 MATERIALS SCIENCE↗
Defect-Mediated Charge-Carrier Trapping and Nonradiative Recombination in WSe 2 Monolayers
Nonradiative charge-carrier recombination in transition-metal dichalcogenide (TMD) monolayers severely limits their use in solar energy conversion technologies. Because defects serve as recombination sites, developing a quantitative description of charge-carrier dynamics in defective TMD monolayers can shed light on recombination mechanisms. Herein we report a first-principles investigation of charge-carrier dynamics in pristine and defective WSe 2 monolayers with three of the most probable defects, namely, Se vacancies, W vacancies, and SeW antisites. We predict that Se vacancies slow down recombination by nearly an order of magnitude relative to defect-free samples by breaking the monolayer’s symmetry and thereby reducing the spectral intensity of the A 1g phonon mode that promotes recombination in the pristine monolayer. By contrast, we find W vacancies accelerate recombination by more than an order of magnitude, with half of the recombination events bypassing charge traps. The subsequent dynamics feature both charge trapping and charge-trap-assisted recombination. Although SeW antisites also slightly accelerate recombination, the predicted mechanism is different from the W vacancy case. First, a shallow energy level traps a photoexcited electron. Then, both shallow- and deep-trap-assisted recombination can occur simultaneously. Accelerated recombination arises for W vacancies and SeW antisites because they introduce new phonon modes that strongly couple to electron and hole dynamics. This work thus provides a detailed understanding of the mechanisms behind charge-carrier recombination in WSe 2 monolayers with distinct defects. Furthermore, materials engineering, particularly to avoid W vacancies, could advance this technology. The insights derived are important for future design of high-performance photoactive devices based on WSe 2 monolayers.
14 SOLAR ENERGY↗