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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

The Effects of Plasma Pre-Cracking on Drilling of Hard Rocks: A Single Insert Cutting Experiment

Abstract This paper studies the effects of plasma-induced cracks on rock cutting to support the concept of a plasma-integrated drag bit for accelerated geothermal drilling through hard rocks. For this, a single polycrystalline diamond compact (PDC) drilling technique is used in cutting granite to compare thrust and cutting forces between plasma-treated and untreated rock samples. The cracks are produced using underwater plasma at 80 J per pulse. This energy level does not produce visible damage to the rock. The cutting tests are conducted at a cutting speed of 12.7 m/min and four feed rates of 0.127, 0.201, 0.267, and 0.414 mm/s to represent actual drilling scenarios. The results show a general trend of reduction in both thrust and cutting forces for these feed rates, but the magnitude of reduction highly depends on the feed rate. The maximum force reduction of around 50% is found at the 0.267 mm/s feed rate with statistical significance. Cases with a higher force reduction are also found to have rougher surface topography, which indicates more excessive fracturing and, thus, a cracks-accelerated material removal process. This study provides initial evidence of using underwater plasma to increase the downhole drilling rate of hard rocks.

Engineering↗
DOE OSTI2019

Solid–state synthesis of multicomponent equiatomic rare–earth oxides

Phase formation in multicomponent rare–earth oxides is determined by a combination of composition, sintering atmosphere, and cooling rate. Polycrystalline ceramics comprising various combinations of Ce, Gd, La, Nd, Pr, Sm, and Y oxides in equiatomic proportions were synthesized using solid–state sintering. The effects of composition, sintering atmosphere, and cooling rate on phase formation were investigated. Single cubic or monoclinic structures were obtained with a slow cooling of 3.3°C/min, confirming that rare–earth oxides follow a different structure stabilization process than transition metal high–entropy oxides. In an oxidizing atmosphere, both Ce and Pr induce a cubic structure, while only Ce plays that role in an inert or reducing atmosphere. Samples without Ce or Pr develop a single monoclinic structure. The structures formed at initial synthesis may be converted to a different one, when the ceramics are annealed in an additional atmosphere. Phase evolution of a five–cation composition was also studied as a function of sintering temperature. In conclusion, the binary oxides used as raw materials completely dissolve into a single cubic structure at 1450°C in air.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Understanding the asymmetric orientations and stress states in polycrystalline NiTi SMA by in-situ synchrotron-based high-energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) dramatically affect the recoverable strain and mechanical response of polycrystalline NiTi. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. Here, in this work, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading and unloading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial deformation, the asymmetry of SIMTs is attributed to the inhomogeneous strain field caused by the Lüders-type mechanism. The asymmetrical SIMT starts with the forward Lüder band and disappears along with the backward Lüder band. The austenite with the favored orientation of $\langle$110$\rangle$ A //loading direction (LD) transformed and recovered back at a higher rate compared to other orientations during both loading and unloading.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Understanding and Mitigating the Contamination of Intrinsic poly-Si Gaps in Passivated IBC Solar Cells

We investigate factors that are critical for the performance of interdigitated back contact (IBC) solar cells based on polycrystalline silicon (poly-Si) passivated contacts. During patterning of doped lines using direct plasma deposition through a shadow mask, we show that the intrinsic poly-Si gap becomes contaminated with dopants, leading to shunting. Possible contamination mechanisms during masked deposition and high- temperature annealing are tested. Strategies developed to mitigate the contamination, such as reactive ion etching after deposition and amorphous to poly-Si crystallization in oxygen, will lead to improved IBC fabrication methods.

14 SOLAR ENERGY↗
DOE OSTI2021

Gas Bubble Evolution in Polycrystalline UMo Fuels Under Elastic-Plastic Deformation: A Phase-Field Model With Crystal-Plasticity

In monolithic UMo fuels, the interaction between the Al cladding and large gas bubble volumetric swelling causes both elastic-plastic and creep deformation. In this work, a phase-field model of gas bubble evolution in polycrystalline UMo under elastic-plastic deformation was developed for studying the dynamic interaction between evolving gas bubble/voids and deformation. A crystal plasticity model, which assumes that the plastic strain rate is proportional to resolved shear stresses of dislocation slip systems on their slip planes, was used to describe plastic deformation in polycrystalline UMo. Xe diffusion and gas bubble evolution are driven by the minimization of chemical and deformation energies in the phase-field model, while evolving gas bubble structure was used to update the mechanical properties in the crystal plasticity model. With the developed model, we simulated the effect of gas bubble structures (different volume fractions and internal gas pressures) on stress-strain curves and the effect of local stresses on gas bubble evolution. The results show that 1) the effective Young’s modulus and yield stress decrease with the increase of gas bubble volume fraction; 2) the hardening coefficient increases with the increase of gas bubble volume fraction, especially for gas bubbles with higher internal pressure; and 3) the pressure dependence of Xe thermodynamic and kinetic properties in addition to the local stress state determine gas bubble growth or shrinkage. The simulated results can serve as a guide to improve material property models for macroscale fuel performance modeling.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Polyhexatic and polycrystalline states of skyrmion lattices

We report Monte Carlo studies of lattices of up to 10 5 skyrmions treated as particles with negative core energy and repulsive interaction obtained from a microscopic spin model. Temperature dependence of translational and orientational correlations has been investigated for different experimental protocols and initial conditions. Cooling the skyrmion liquid from a fully disordered high-temperature state results in the formation of a skyrmion polycrystal. A perfect skyrmion lattice prepared at T = 0, on raising temperature undergoes a first-order melting transition into a polyhexatic state that consists of large orientationally ordered domains of fluctuating shape. On the further increasing temperature, these domains decrease in size, leading to a fully disordered liquid of skyrmions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Why Increased CdSeTe Charge Carrier Lifetimes and Radiative Efficiencies did not Result in Voltage Boost for CdTe Solar Cells

After a focused effort over the last decade, order-of-magnitude improvements in doping and electro-optical characteristics (radiative efficiency, carrier lifetime, and passivation) have been reported for polycrystalline CdSeTe solar cells. Surprisingly, this did not result in higher solar cell voltages regardless of device contacting layers, absorber grading profiles, and other changes in device architecture. From detailed evaluation of radiative emission and carrier dynamics in CdSeTe heterostructures and devices, it is shown that the complexity introduced to the absorber to achieve lifetime and passivation metrics resulted in charge carrier trapping, which now negatively affects CdSeTe absorbers.

CdTe↗
DOE OSTI2021

Highly Efficient Surface Charge Transfer in Fe 2 TiO 5 Epitaxial Thin Film Photoanodes

Pseudobrookite (Fe 2 TiO 5 ) has attracted significant attention as an emerging photoanode for water oxidation due to reports of enhanced performance in polycrystalline heterostructures with α-Fe 2 O 3 or TiO 2 . However, the specific properties and contribution of Fe 2 TiO 5 remain unknown. Here, we present the first photoelectrochemical characterization of epitaxial thin film Fe 2 TiO 5 , an ideal platform for probing the inherent response of this earth-abundant photoanode for solar water splitting. Moreover, by using an electrolyte containing a hole scavenger, we find highly efficient charge transfer at the Fe 2 TiO 5 –electrolyte interface. This notable surface property is exploited in the form of an Fe 2 TiO 5 /α-Fe 2 O 3 heterostructure photoanode, for which we observe a photocurrent density increase by 1 order of magnitude and an onset potential improvement by ~300 mV. Establishing these fundamental properties elucidates the nature of reaction mechanisms on Fe 2 TiO 5 and informs the design of highly efficient water photo-oxidation devices by incorporating this material with other photoanodes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Ultrathin Ruthenium Films on Graphene Buffered SiO 2 via Quasi Van der Waals Epitaxy

In this study, we demonstrate a quasi van der Waals epitaxy approach to prepare single crystalline Ru ultrathin film on large scale, monolayer graphene. Physical and epitaxial properties of bulk, near surface and surface of ultrathin Ru films were comprehensively studied using various structural, morphological, compositional, and electrical characterization techniques. Here, we confirm that Ru can epitaxially grow on single, monolayer graphene using magnetron sputtering at elevated temperature of 600 °C. The epitaxial Ru films with film thickness ranging from 94.2 nm down to 3.9 nm show the (0001) out-of-plane orientation. The epitaxial relationships between Ru and graphene are out-of-plane Ru(0001) || graphene(0001) and in-plane Ru[$11\bar20$] || graphene[$11\bar20$]. All the Ru films show smooth surfaces with root-mean-square roughness less than 0.8 nm and have negligible oxide layer on the surfaces. The Ru films on graphene demonstrate significantly reduced electrical resistivity comparing to their counterpart grown on bare SiO 2 , which show polycrystalline nature. For 7.1 to 3.9 nm film thicknesses, the resistivity of Ru on graphene shows 38 to 45% resistivity decrease from that of Ru film on bare SiO 2 without graphene. Our observations suggest the existence of the above-classical van der Waals interaction between Ru and graphene. On the other hand, graphene is capable of effectively blocking the inter-diffusion/interaction between Ru and SiO 2 during a 1000 °C annealing process.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Collapse of the waiting time effect in a spin glass

Waiting time measurements are reported here for a bulk polycrystalline sample of the spin glass Cu 0.95 Mn 0.05 . We observe a suppression of aging as the glass temperature is approached from below. These results are interpreted in terms of a spin-glass correlation length approaching and reaching crystallite sizes. They provide a direct experimental connection between the correlation length scale and relaxation times in the spin-glass state. In particular, these measurements track the value of the dynamic critical exponent as the system transitions from the critical temperature regime to its low-temperature, quasistatic nonequilibrium phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Grain-resolved temperature-dependent anisotropy in hexagonal Ti-7Al revealed by synchrotron X-ray diffraction

Hexagonal metals have anisotropic coefficients of thermal expansion causing grain-level internal stresses during heating. High energy x-ray diffraction microscopy, a non-destructive, in situ, micromechanical and microstructural characterization technique, has been used to determine the anisotropic coefficients of thermal expansion (CTEs) for Ti-7Al. Two samples of polycrystalline α-phase Ti-7Al were continuously heated from room temperature to 850 °C while far-field HEDM scans were collected. The results showed a change in the ratio of the CTEs in the ‘a’ and ‘c’ directions which explains discrepancies found in the literature. The CTE additionally appears to be affected by the dissolution of α 2 precipitates. Analysis of the grain-resolved micromechanical data also shows reconfiguration of the grain scale stresses likely due to anisotropic expansion driving crystallographic slip.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Bridging microscale to macroscale mechanical property measurements of FeCrAl alloys by crystal plasticity modeling

FeCrAl alloys are candidates for accident tolerant fuel cladding of light water reactors. In this work, a microstructure- and temperature-dependent crystal plasticity model is employed to bridge microscale to macroscale mechanical property measurements of FeCrAl alloys. With the visco-plastic self-consistent (VPSC) polycrystal plasticity framework, a mechanism-based single crystal plasticity (MSCP) model adopts the Arrhenius type rate equation to describe the dependence of the critical resolved shear stress for dislocation slips on their temperature-dependent intrinsic frictional resistance and the microstructure-dependent irradiation hardening. The intrinsic frictional resistance associated with {110}<111> and {112}<111> slip systems were measured by in-situ micromechanical testing on unirradiated/irradiated samples at 25-500 °C. The irradiation hardening is estimated by the Bacon-Kocks-Scattergood (BKS) model with density and size of radiation-induced defects measured from microstructural characterization. Several features associated with thermo-mechanical behavior of unirradiated/irradiated polycrystalline FeCrAl alloys are captured. High density of deformation-induced dislocations and radiation-induced defects results in obvious hardening at room temperature, which is weakened at high temperature, and facilitates damage evolution during deformation. Moreover, both high temperature and radiation-induced defects, which facilitate dislocation multiplication, trigger large hardening rate. Finally, the proposed method together with application of accelerator-based ion irradiation technique is a surrogate approach to simulate neutron damage, improving the efficiency associated with evaluation of mechanical properties of FeCrAl alloys exposed to temperature, stress and radiation conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Defect annihilation in heavy ion irradiated polycrystalline gold

In this study, we explore the interaction of electron wind force (EWF) with defects originating from ion irradiation in-situ inside a transmission electron microscope. Nanocrystalline gold specimens were self-ion irradiated to a dose of 5 × 10 15 ions/cm 2 (45 displacement per atom) to generate a high density of displacement damage. We also developed a molecular dynamics simulation model to understand the associated atomic scale mechanisms. Both experiments and simulations show that the EWF can impart significant defect mobility even at low temperatures, resulting in the migration and elimination of defects in a few minutes. Overall, we propose that the EWF interacts with defects to create highly glissile Shockley partial dislocations, which makes the fast and low temperature defect annihilation possible.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Spectra-Orthogonal Optical Anisotropy in Wafer-Scale Molecular Crystal Monolayers

Controlling the spectral and polarization response of two-dimensional (2D) crystals is vital for developing ultrathin platforms for compact optoelectronic devices. However, independently tuning optical anisotropy and spectral response remains challenging in conventional semiconductors due to the intertwined nature of their lattice and electronic structures. Here, we report spectra-orthogonal optical anisotropy─where polarization anisotropy is tuned independently of spectral response─in wafer-scale, one-atom-thick 2D molecular crystal (2DMC) monolayers synthesized on monolayer transition-metal dichalcogenide (TMD) crystals. Utilizing the concomitant spectral consistency and structural tunability of perylene derivatives, we demonstrate tunable optical polarization anisotropy in 2DMCs with similar spectral profiles, as confirmed by room-temperature scanning tunneling microscopy and cross-polarized reflectance microscopy. Additional angle-dependent analysis of the single-crystal and polycrystalline molecular domains reveals an epitaxial relationship between the 2DMC and TMD. In conclusion, our results establish a scalable, molecule-based 2D crystalline platform for unique and tunable functionalities unattainable in covalent 2D solids.

2D materials↗
DOE OSTI2023

A prediction of the thermodynamic, thermophysical, and mechanical properties of CrTaO 4 from first principles

It is reported that the self-forming CrTaO 4 oxide scale can protect refractory high-entropy alloys from oxidation, superior to Cr 2 O 3 . In this paper, the phase stability, mechanical, and thermal properties of three polymorphous phases of CrTaO 4 are systematically investigated from first-principles density functional theory calculations. The mechanical properties predicted using the strain–energy methods indicated that all three phases are mechanically stable. The temperature dependence of elastic constants and polycrystalline moduli of three phases demonstrated the thermal softening as temperature increase. The Helmholtz-free energies as a function of volume and temperature are derived from phonon dispersions within the quasi-harmonic approximation at six strained volumes. The calculated apparent bulk coefficients of thermal expansion of these three phases are evaluated, the highest value approximately 13.4× 10 –6 K –1 within a temperature range of 500–2000 K for the rutile I4 1 md phase. The lattice thermal conductivity calculated by the Debye–Callaway model suggested that the rutile type I4 1 md phase has the lowest value of approximately 2.1 W/m/K at 1800 K. The other two phases, C2/m and P2/c, exhibit higher values due to relatively lower Grüneisen parameters and larger phonon velocities. The melting point of CrTaO 4 is predicted to be between 1975 and 2449 K using ab initio molecular dynamics simulations. Finally, this work provides a comprehensive theoretical understanding of the thermodynamic, mechanical, and thermal properties for the new material CrTaO 4 and serves as an example of a viable computational design strategy for improved oxidation resistance of refractory alloys at high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Post-growth annealing effects on charge and spin excitations in ND 2-x Ce x CuO 4

We report a Cu K- and L 3 -edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd 2-x Ce x CuO 4 , with focus on post-growth annealing effects. For the parent compound Nd 2 CuO 4 (x = 0), a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of RE 2 CuO 4 (RE = rare earth) can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For x = 0.16, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering

36 MATERIALS SCIENCE↗
DOE OSTI2021

Reverse-Engineering Strain in Nanocrystallites by Tracking Trimerons

Although strain underpins the behavior of many transition-oxide-based magnetic nanomaterials, it is elusive to quantify. Since the formation of orbital molecules is sensitive to strain, a metal–insulator transition should be a window into nanocrystallite strain. Using three sizes of differently strained Fe 3 O 4 polycrystalline nanorods, the impact of strain on the Verwey transition and the associated formation and dissolution processes of quasiparticle trimerons is tracked. In 40 and 50 nm long nanorods, increasing isotropic strain results in Verwey transitions going from T V ≈ 60 K to 20 K. By contrast, 700 nm long nanorods with uniaxial strain along the (110) direction have T V ≈ 150 K—the highest value reported thus far. A metal–insulator transition, like T V in Fe 3 O 4 , can be used to determine the effective strain within nanocrystallites, thus providing new insights into nanoparticle properties and nanomagnetism.

36 MATERIALS SCIENCE↗