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At least 1,135 records · Page 63

Exploiting the fraternal twin nature of thermoelectrics and topological insulators in Zintl phases as a tool for engineering new efficient thermoelectric generators

This review article presents a radical overview of the rich chemistry and physics of Zintl phases as it relates to their interesting structure–property relationships. In particular, it unveils the evolution of topological insulator (TI) and thermoelectric (TE) properties in Zintl materials as having a common origin and as such are driven by the same materials properties ranging from a narrow band gap to a strong spin-orbit coupling effect. To this end, excellent TE properties are ubiquitous in many TIs and because of the ever-increasing interest in the studies of topological properties of matter, several robust theoretical frameworks now exist to provide us with additional intuitive nature and understanding of this exotic phase of matter. Here, this invariably has also led to the screening of tens of thousands of materials for the presence of nontrivial topology using the combined predictive power of machine learning and ab-initio first-principles calculations. On the other hand, studies on TE materials are still largely based on some sort of hit-and-miss approach. Here we leveraged on the similar materials features and intersection between TE and TIs and review several Zintl materials that are already identified as TIs and present how exploiting their bulk and surface states properties can be harnessed for developing highly efficient TE materials for practical purposes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Anharmonic thermodynamic properties and phase boundary across the postperovskite transition in MgSi O 3

To address the effects of lattice anharmonicity across the perovskite to postperovskite transition in MgSiO 3 , we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on abinitio molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit (N → ∞) are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation and the generalized gradient approximation calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Grüneisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope (dP/dT) that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

36 MATERIALS SCIENCE↗

Evolving physical and mechanical rock properties during exposure at Earth's surface

Rocks at Earth's surface experience progressive damage due to exposure to environmental conditions. Although environmental damage to rocks is recognized as being time-dependent, our quantitative understanding of its effects on physical and mechanical properties remains limited by field and laboratory studies constrained to short timescales. In this study, we aim to rigorously quantify how these properties evolve under longer natural environmental exposure spanning from the Holocene to the late Pleistocene. We conducted a ‘time series’ analysis of key physical and mechanical rock properties using multiple interior samples from 10 cogenetic granitoid boulders exposed for different amounts of time at two sites in Eastern California, USA. We find that over the late Pleistocene (~30 ka), rocks exhibit porosity increases of 40 ± 4 % and seismic velocity decreases of 7 ± 1 % at both sites. Similarly, over the same time period, rock tensile and compressive strengths declined by an average of 11 ± 4 %, and rock permeability increased on average by 104 ± 50 %. Microscopic analysis of thin sections of the same rocks indicates that microfracture density and intensity also progressively increased over the same timescales. These results suggest that time-dependent fracture propagation due to environmental exposure is likely a central contributor to the observed changes in rock properties. This study provides quantitative documentation that pronounced changes in the physical and mechanical properties of rock progressively occur within thousands of years of exposure.

Critical zone↗

Probing spin waves in Co 3 O 4 nanoparticles for magnonics applications

The magnetic properties of spinel nanoparticles can be controlled by synthesizing particles of a specific shape and size. The synthesized nanorods, nanodots and cubic nanoparticles have different crystal planes selectively exposed on the surface. The surface effects on the static magnetic properties are well documented, while their influence on spin waves dispersion is still being debated. Our ability to manipulate spin waves using surface and defect engineering in magnetic nanoparticles is the key to designing magnonic devices. We synthesized cubic and spherical nanoparticles of a classical antiferromagnetic material Co 3 O 4 to study the shape and size effects on their static and dynamic magnetic proprieties. Using a combination of experimental methods, we probed the magnetic and crystal structures of our samples and directly measured spin wave dispersions using inelastic neutron scattering. We found a weak, but unquestionable, increase in exchange interactions for the cubic nanoparticles as compared to spherical nanoparticle and bulk powder reference samples. Interestingly, the exchange interactions in spherical nanoparticles have bulk-like properties, despite a ferromagnetic contribution from canted surface spins.

36 MATERIALS SCIENCE↗

Enhanced Thermal Stability of Conductive Mercury Telluride Colloidal Quantum Dot Thin Films Using Atomic Layer Deposition

Colloidal quantum dots (CQDs) are valuable for their potential applications in optoelectronic devices. However, they are susceptible to thermal degradation during processing and while in use. Mitigating thermally induced sintering, which leads to absorption spectrum broadening and undesirable changes to thin film electrical properties, is necessary for the reliable design and manufacture of CQD-based optoelectronics. Here, low-temperature metal–oxide atomic layer deposition (ALD) was investigated as a method for mitigating sintering while preserving the optoelectronic properties of mercury telluride (HgTe) CQD films. ALD-coated films are subjected to temperatures up to 160 °C for up to 5 h and alumina (Al 2 O 3 ) is found to be most effective at preserving the optical properties, demonstrating the feasibility of metal–oxide in-filling to protect against sintering. HgTe CQD film electrical properties were investigated before and after alumina ALD in-filling, which was found to increase the p-type doping and hole mobility of the films. The magnitude of these effects depended on the conditions used to prepare the HgTe CQDs. With further investigation into the interaction effects of CQD and ALD process factors, these results may be used to guide the design of CQD–ALD materials for their practical integration into useful optoelectronic devices.

36 MATERIALS SCIENCE↗

Tuning electrical and interfacial thermal properties of bilayer MoS2 via electrochemical intercalation

Layered two-dimensional (2D) materials such as MoS 2 have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to modulate material properties of such layered 2D films reversibly. We probe both the electrical and thermal properties of Li-intercalated bilayer MoS 2 nanosheets by combining electrical measurements and Raman spectroscopy. Furthermore, we demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8 × 10 12 to 1.5 × 10 14 cm -2 ), and we simultaneously obtain the thermal boundary conductance between the bilayer and its supporting SiO 2 substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MW m -2 K -1 before intercalation to 1.8 ± 0.9 MW m -2 K -1 when the MoS 2 is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Ectomycorrhizal effects on decomposition are highly dependent on fungal traits, climate, and litter properties: A model-based assessment

It has been proposed that competition between ectomycorrhizal (ECM) fungi and free-living saprotrophs for resources like nitrogen (N) slows decomposition and increases the soil carbon storage in ECM ecosystems compared to arbuscular (AM) ecosystems. However, empirical evidence for the generality of such ECM effects is equivocal, and confounding mechanisms have been proposed that affect the magnitude and direction of ECM effects on soil carbon. Here, for this work, we conduct a theoretical modeling experiment, where we explicitly incorporate mycorrhizal processes into the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (CORPSE) model. We use the model to explore the conditions under which ECM N acquisition processes can induce stronger saprotrophic N limitation and result in slower decomposition rates and greater soil organic carbon accumulation compared to AM processes. We found that the ECM fungi more strongly inhibited decomposition when litter inputs were N-depleted and relatively recalcitrant and when ECM fungi possessed a strong capacity to mine N from both recalcitrant soil organic matter and microbial necromass. Climate and seasonality also played a role as the ECM competition effect was strongest at low mean annual temperatures and when litterfall peaked seasonally. Priming effects driven by high root exudation rates in ECM-dominated systems could overwhelm the competition effect and reduce soil carbon under some circumstances. The ECM effect on decomposition in our simulations was highly context dependent. Based on our model results, we expect to see a strong ECM competition effect in temperate deciduous and boreal forests with relatively recalcitrant litter inputs, and with ECM fungi that produce oxidases and necromass-degrading enzymes. However, even a relatively strong ECM competition effect on decomposition only increased soil organic carbon accumulation by ~10%.

59 BASIC BIOLOGICAL SCIENCES↗

Origins of the Pressure-Induced Phase Transition and Metallization in the Halide Perovskite (CH 3 NH 3 )PbI 3

Hybrid halide perovskites are promising for applications because of their favorable optoelectronic properties and low cost. Here we investigate the effects of hydrostatic pressure on the structural and electronic properties of (MA)PbI 3 (MA = CH 3 NH 3 + ) using first-principles density functional theory calculations. Our calculations predict that at a pressure of 0.23 GPa, the orthorhombic Fmmm phase becomes unstable with respect to a cubic Im$\bar{3}$ phase, in good agreement with room-temperature experiments (~0.3 GPa). At higher pressures, about 6 GPa, we predict the onset of pronounced intra- and interoctahedral distortions. This symmetry lowering leads to the introduction of I 5p–I 5p* antibonding and Pb 6p–Pb 6p bonding character into the valence band maximum (VBM) and the conduction band minimum (CBM) states, respectively. We find this change in bond character explains the evolution of the VBM and CBM states under compression, trends that ultimately lead to metallization at significantly higher pressures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Semi-classical approximation description of static properties of nuclei

In this report, we present recent development in semi-classical description of static properties of nuclei. By employing the Wigner transform, we derive simple semiclassical approximations for evaluating properties of nuclear system. We present results of calculations of static properties, using the energy density functional associated with Skyrme effective nucleon–nucleon interaction. In particular, we consider properties of the Wigner distribution function, line of beta stability, Coulomb and symmetry energies, deformation energies, equation of state, nuclear radii, and single-particle level density.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Effect of lithium diffusion into Ga 2 O 3 thin films

The integration of lithium based compounds (e.g., Li:NiO/Ga 2 O 3 , LiGa 5 O 8 /Ga 2 O 3 ) in Ga 2 O 3 based pn-heterojunctions raises concerns about interface stability, i.e., Li diffusion effects on Ga 2 O 3 properties. In this work the ex-situ diffusion of Li is investigated in three different Ga 2 O 3 epilayers systems [(001) κ-Ga 2 O 3 and (−201) β-Ga2O3 heteroepitaxy on (001) α-Al 2 O 3 , and (010) β-Ga 2 O 3 homoepitaxy] at relevant temperatures for the synthesis / processing of Li-based epilayers. It is here experimentally demonstrated and quantified the Li diffusion in all the investigated Ga 2 O 3 epilayers systems and Li bulk (D Li,bulk ) and 2D defects (D Li,2D ) diffusion coefficients are provided. In the case of the (010) β-Ga 2 O 3 homoepitaxial layer (nominally free of structural defects), hybrid functional theory calculations foresee a diffusion mechanism mediated by Ga vacancies (VGa). Moreover, in the (010) β-Ga 2 O 3 homo-layer a significant effect on its functional properties (e.g., additional Raman vibrational modes, induced conductivity in an otherwise insulating sample) upon the Li-diffusion process is experimentally highlighted and tentatively related to the passivation of acceptor defects (i.e., formation of V Ga -nLi complexes).

Defects↗

High strain rate deformation of aged TRIP Ti-10V-2Fe-3Al (wt.%) examined by in-situ synchrotron X-ray diffraction

Transformation Induced Plasticity (TRIP) is a promising avenue for tailoring the work hardening response of metastable $β$ titanium (Ti) alloys. Here we show that aged TRIP Ti-10V-2Fe-3Al (wt.%) maintains higher elongations and flow stresses as strain rate increases, if phase stability and microstructural characteristics are tuned. Low temperature aging influences the matrix $β$ phase stability by $ω$ phase precipitation, which affords a promising way to impact the TRIP effect and obtain desirable mechanical properties, ranging from high damping capacity to good strength/ductility combinations. Although TRIP is active during quasi-static and dynamic testing up to 2000 s –1 , increasing aging time and/or strain rate reduces the overall propensity for the TRIP effect and extent of transformation, which occurs rapidly just at the onset of yielding. Importantly, TRIP with $ω$ phase precipitation provides interesting alloying, microstructure, and property design strategies for engineering applications like lightweight protective structures, where high strains and the need for energy absorption are encountered.

36 MATERIALS SCIENCE↗

Recycling Epoxy by Vitrimerization: Influence of an Initial Thermoset Chemical Structure

Epoxy/anhydride cured thermosets are widely used in aerospace and wind energy structures. Recycling the enormous amounts of epoxy thermoset waste remains a global challenge. Vitrimerization, a practical, low-cost, eco-friendly and scalable method was developed for closed-loop recycling of the crosslinked epoxy by converting the permanent network into a vitrimer type dynamic network. Here we study the effect of virgin epoxy network properties on network reforming efficiency and the activation energy of the dynamic network by varying the epoxy/anhydride ratio. The results point out that the permanent network crosslink density and glass transition temperature have a more pronounced effect on vitrimerization efficiency by comparison with the concentration of free hydroxyl groups in the system. Furthermore, the result can be potentially used to better control vitrimerization efficiency in recycling epoxy thermoset polymers and tailor the vitrimerized products properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Impact of Materials Properties on Higher-Temperature Engine Operation

Here, we examine the effects on materials temperatures and engine efficiency via simulations of engines operating at temperatures which exceed the thermal limits of today’s materials. Potential focus areas include high-speed, high-load operation (in the fuel-enrichment zone) as well as conditions of selective cooling at lower speeds and loads. We focus on a light-duty DISI and a heavy-duty CI engine using GT-Power. Temperature distributions within the head, block, piston, and valves were obtained from 3D FEA simulations coupled with 1D GT-Power representations of the engine’s gas flow and combustion regions. We use experimentally measured thermal properties of current commercial alloys for specific engine components, as well as candidate developmental alloys with improved temperature tolerance, to gauge the effects of materials properties on engine performance, particularly focusing on operating areas where materials which can withstand higher temperatures can enable intensified combustion conditions and increased specific output. The outcome of this work is guidance on materials selection targets and benefits as OEMs meet engine-performance needs in the coming decades, to include Class 8 line-haul freight vehicles with reduced emissions, as well as design of future near net zero-carbon light-duty range extenders for more rapid electrification of medium- and heavy-duty vehicles.

36 MATERIALS SCIENCE↗

Development and Characterization of a Sustainable Bio-Polymer Concrete with a Low Carbon Footprint

Polymer concrete (PC) has been used to replace cement concrete when harsh service conditions exist. Polymers have a high carbon footprint when considering their life cycle analysis, and with increased climate change concerns and the need to reduce greenhouse gas emission, bio-based polymers could be used as a sustainable alternative binder to produce PC. This paper examines the development and characterization of a novel bio-polymer concrete (BPC) using bio-based polyurethane used as the binder in lieu of cement, modified with benzoic acid and carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs). The mechanical performance, durability, microstructure, and chemical properties of BPC are investigated. Moreover, the effect of the addition of benzoic acid and MWCNTs on the properties of BPC is studied. The new BPC shows relatively low density, appreciable compressive strength between 20–30 MPa, good tensile strength of 4 MPa, and excellent durability resistance against aggressive environments. The new BPC has a low carbon footprint, 50% lower than ordinary Portland cement concrete, and can provide a sustainable concrete alternative in infrastructural applications.

36 MATERIALS SCIENCE↗

Fusion welding of refractory metals and ZrB 2 -SiC-ZrC ceramics

Molybdenum and a molybdenum alloy were fusion welded to ZrB2-based ceramics to determine if the electrical and thermal properties of the metals and ceramics affected their weldability. Commercial ceramic powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics to commercial metals using plasma arc welding. Weldability varied as indicated by the range of porosity observed within the fusion zones. Measured thermal and electrical properties appeared to have little to no effect on the weldability of metal-ceramic welds despite the large range of values measured across each property. Differences in melting temperatures between metal and ceramic coupons did affect weldability by changing the weld penetration depth into ceramic coupons. Finally, future studies on metal-ceramic welds are suggested to investigate the effect that work function, melt viscosity, wetting, or other properties have on weldability.

36 MATERIALS SCIENCE↗

Intraspecific variability in root traits and edaphic conditions influence soil microbiomes across 12 switchgrass cultivars

Microbial communities help plants access nutrients and tolerate stress. Some microbiomes are specific to plant genotypes and, therefore, may contribute to intraspecific differences in plant growth and be a promising target for plant breeding. Switchgrass (Panicum virgatum L.) is a potential bioenergy crop with broad variation in yields and environmental responses; recent studies suggest that associations with distinct microbiomes may contribute to variation in cultivar yields. We used a common garden experiment to investigate variation in 12 mature switchgrass cultivar soil microbiomes and, further, to examine how root traits and soil conditions influence microbiome structure. Here, we found that average root diameter varied up to 33% among cultivars and that they associated with distinct soil microbiomes. Cultivar had a larger effect on the soil bacterial than fungal community, but both were strongly influenced by soil properties. Root traits had a weaker effect on microbiome structure, but root length contributed to variation in the fungal community. Unlike the soil communities, the root bacterial communities did not group by cultivar, based on a subset of samples. Microbial biomass carbon and nitrogen and the abundance of several dominant bacterial phyla varied between ecotypes, but overall the differences in soil microbiomes were greater among cultivars than between ecotypes. Our findings show that there is not one soil microbiome that applies to all switchgrass cultivars, or even to each ecotype. These subtle but significant differences in root traits, microbial biomass, and the abundance of certain soil bacteria could explain differences in cultivar yields and environmental responses.

59 BASIC BIOLOGICAL SCIENCES↗

Initial development of a high temperature life prediction method directly accounting for variability in material properties

This report presents the initial development of a life prediction method that accounts for the variability of the material properties in Grade 91 steel. We account for material variability by both fitting a variable 3-parameter Weibull distribution to the rupture data of the material and by quantifying the effect that the creep deformation properties have on stresses via a Monte Carlo approach. To ensure a reasonable time frame when applying Monte Carlo to the creep stress, we propose a methodology that models the material as an extremely viscous Stokes flow with a non-Newtonian viscosity, therefore solving the problem in a single step. We use the final probabilistic model for structural failure developed in this document to evaluate a Flat Head Vessel designed with the Section III, Division 5 ASME Boiler Pressure and Vessel Code rules with a 100,000h design life and assess its probability of premature failure.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

Hybridization-driven strong anharmonicity in Yb-filled skutterudites

A high-pressure study of the structural and thermal transport properties is carried out on one of the most efficient filled skutterudites, $\mathrm{Yb_{0.3}Co_4Sb_{12}}$, to understand the relatively low thermal conductivity behavior in this family. By combining x-ray diffraction and Raman scattering measurements, we detect a phase transition at around 12.4 GPa. The mode Grüneisen parameters of the observed phonon modes are obtained from the determined bulk modulus and the phonon frequency shifts with pressure. The strong anharmonicity in this material is demonstrated by the obtained large average Grüneisen parameter. We also find the depressed group velocity within the low-frequency range, the flat guest mode avoided crossing with the acoustic-phonon mode, and the significant contribution of optical phonons. The hybridization of the guest atom and host lattice and the related enhanced anharmonicity are suggested to account for the low lattice thermal conductivity in the studied system. So, these findings provide new insight into how phonon-phonon interactions lower lattice thermal conductivity in this important thermoelectric family.

36 MATERIALS SCIENCE↗