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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Anomalous elastic behavior of tantalum at high pressures: Experimental and theoretical studies

Elastic wave velocities of polycrystalline tantalum have been measured up to 13.6 GPa at room temperature using ultrasonic interferometry technique in a multi-anvil apparatus. The bulk and shear moduli, as well as their pressure derivatives are obtained using Eulerian 3rd order finite strain equations, yielding K S0 = 193.9(29) GPa, G 0 = 69.3(10) GPa, K S0' = 3.18(5) and G 0' = 0.88(1). Here the Steinberg-Guinan yield stress model is examined using the current shear modulus and its pressure derivative, where a slightly lower A 0 = 0.0127 GPa –1 compared to previous studies is obtained. First-principles calculations on the elasticity of tantalum have also been performed up to 200 GPa. Anomalous softening of the elastic shear constant C 44 and shear wave velocity V S is observed between 80– 180 GPa, whereas the anisotropy shows a softening-stiffening behavior with increasing pressure and reaches a minimum at around 150 GPa. Such anomalies might commonly originate from the nesting of Fermi surface for Group VB metals.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Primary solidification of ternary compounds in Al-rich Al–Ce–Mn alloys

Primary solidification of ternary compounds Al 20 Mn 2 Ce and Al 10 Mn 2 Ce were analyzed through the coupling of the thermodynamic modeling and classic nucleation theory. Thermodynamic models of Al 20 Mn 2 Ce and Al 10 Mn 2 Ce were developed using the CALPHAD approach based on first-principles calculated enthalpy of formation and experimental data obtained from this work and the literature. The analysis suggested that despite the larger thermodynamic driving force for nucleation of Al 10 Mn 2 Ce, nucleation is preferred for the Al 20 Mn 2 Ce phase in the highly undercooled liquid due to its smaller interfacial energy. Therefore, manufacturing methods with rapid cooling rates will favor primary solidification of Al 20 Mn 2 Ce for Al-rich Al–Ce–Mn alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Exploring the elastic and electronic properties of chromium molybdenum diboride alloys

We perform first-principles calculations to study the structural, mechanical, thermal, electronic, and magnetic properties of Cr 1-x Mo x B 2 for x = 0.25, 0.33, 0.50, 0.67 and 0.75. Based on structural search methods, we determine the ground-state structure for each concentration. The ternaries are either monoclinic (x = 0.25, 0.75) or trigonal (x = 0.33, 0.50, 0.67). The calculated mechanical properties reveal that the strength of Cr 1-x Mo x B 2 is maximized for x = 0.50. Cr 0.5 Mo 0.5 B 2 exhibits excellent mechanical properties (B = 298 GPa, Y = 558 GPa, G = 235 Gpa, ν = 0.19, H v =27 GPa), surpassing those of β-MoB 2 at a lower cost. All of these ternaries are hard alloys with Vickers hardness greater than 24 GPa. Chemical bonding analysis demonstrates that the strength of the new compounds is related to the alternating planar and buckled B-B layers, as well as the strong TM-B bonds. Finally, the enhanced strength of Cr 0.5 Mo 0.5 B 2 is a consequence of the high density of strong interlayer Cr-Mo metallic bonds around the Fermi level.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Strain induced electronic structure, and magnetic and structural properties in quaternary Heusler alloys ZrRhTiZ (Z = Al, In)

In this work, electronic structure, and magnetic and structural properties of quaternary Heusler alloys ZrRhTiZ (Z = Al, In) have been analyzed from first principles calculations. The ferromagnetic ground state and thermodynamical stability of these compounds are supported by relative total energies, and by derived formation and cohesive energies, respectively. The investigated Curie temperatures of the compounds exceed room temperature indicating that these compounds are promising candidates for beyond room temperature spintronics and magneto-electronics applications. Both compounds follow the Slater-Pauling 18 electron rule and exhibit half-metallic ferromagnetic behavior with magnetic moments of 2 μB with 100% spin polarization in one spin channel at the Fermi-level. The effect of uniform strain on half-metallicity of these compounds inverts the band gap from one spin channel to another spin channel with a mixed spin (magnetic moment) regime that can be mapped to an exchange bias like effect observed in some of the magnetic Heusler compounds. The half metallic ferromagnetic behavior is retained within –2 to 2% strain contrasting with the anomalous spin flip in –1% and 1% strain in ZrRhTiZ (Z = Al, In).

36 MATERIALS SCIENCE↗
DOE OSTI2024

Additively manufactured Ni-20Cr to V functionally graded material: Computational predictions and experimental verification of phase formations

Throughout this work, a database for the Cr-Ni-V system was constructed by modeling the binary Cr-V and ternary Cr-Ni-V systems using the CALPHAD approach aided by density functional theory (DFT)-based first-principles calculations and ab initio molecular dynamics (AIMD) simulations. To validate this new database, a functionally graded material (FGM) using Ni-20Cr and elemental V was fabricated using directed energy deposition additive manufacturing (DED AM) and experimentally characterized. The deposited Ni-20Cr was pure fcc phase, while increasing the amount of V across the gradient resulted in the formation of sigma phase, followed by the bcc phase. The experimentally measured phase data was compared with computational predictions made using a Cr-Ni-V thermodynamic database from the literature as well as the database developed in the present work. The newly developed database was shown to better predict the experimentally observed phases due to its accurate modeling of binary systems within the database and the ternary liquid phase, which is critical for accurate Scheil calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Angle-resolved polarized Raman study of layered b-As x P x-1 alloys: Identification of As-P vibrational modes

In this study, the polarization-resolved Raman spectra of b-As x P x-1 flakes with varying arsenic concentration (x = 0, 0.4, and 0.8) are systematically investigated. A clear polarization dependence is observed in the Raman intensity of all the P-P, As-P, and As-As modes and confirmed by the first-principles calculations. The observed angle-dependence can be related to the structure of the Raman tensors of each peak. In particular, the Raman intensities corresponding to As-P vibration modes are studied in detail. The out of plane As-P mode ($A$$^{1}_{g}$) is clearly identified, but the butterfly-shaped behavior observed in the polar plot around 350 cm -1 is interpreted as the combination of the two in-plane vibrations, one along the armchair and the other along the zigzag direction. This work allows unambiguous assignment of the $A$$^{1}_{g}$, B 2g , and $A$$^{2}_{g}$ modes of the As-P vibrational modes and the importance of the consideration of the superposition of the in-plane B 2g and $A$$^{2}_{g}$ modes. Such study can be utilized to further characterize the anisotropic structural properties and analyze the local distribution of the As and P in b-As x P x-1 alloys which will play key roles in electronic and optical properties. Furthermore, this methodology can be further extended to alloyed two-dimensional material systems with in-plane anisotropy in general.

2D materials↗
DOE OSTI2020

Investigation of the reaction mechanism of the hydrodeoxygenation of propionic acid over a Rh(1 1 1) surface: A first principles study

Microkinetic models based on first principles calculations have been used to study the vapor and liquid phase hydrodeoxygenation of propionic acid on a Rh(111) surface. Calculations suggest that both decarboxylation and decarbonylation do not occur at an appreciable rate under all reaction environments. Propanol and propionaldehyde are the main products on this surface and they are produced at similar rates in both vapor and liquid phase environments. While a condensed phase can shift the reaction rate, the dominant pathways and selectivity towards the various products are hardly affected. At 473 K, the turnover frequency is increased by about a factor of 1.5 in liquid water relative to the gas phase. In liquid 1,4-dioxane, the turnover frequency is also slightly increased relative to the gas phase. Here, given the uncertainty in Rh cavity radius in the liquid phase calculations, computations with different cavity radii have been performed. With larger Rh cavity radius, the promotional effect of the solvents on the turnover frequency becomes more significant, while practically no changes are observed for a smaller cavity radius.

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

Selective dehydra-decyclization of cyclic ethers to conjugated dienes over zirconia

ZrO 2 provides high selectivity (>90%) to conjugated pentadienes through the dehydra-decyclization of C-5 cyclic ethers, even at high conversions. 1,3-Pentadiene was the major product in both reaction of 2-methyltetrahydrofuran and tetrahydropyran over ZrO 2 . The reaction of 3-methyltetrahydrofuran produced nearly stoichiometric amounts of isoprene. Other catalysts, including TiO 2 , γ-Al 2 O 3 , and H-ZSM-5, were generally much less selective and produced a mixture of diene isomers. A combination of TPD and steady-state measurements revealed that both piperylenes are exclusively produced through primary catalytic pathways from 2-methyltetrahydrofuran, avoiding any isomerization once formed. First-principle calculations on ZrO 2 imply the presence of an energetically favored, surface isomerization of ring-opened intermediates to conjugated alkenolates that selectively dehydrate to conjugated dienes, providing high selectivity to the desired products. Finally, the stabilization of the conjugated alkenolate is key for understanding the ability of ZrO 2 to selectively produce conjugated dienes from cyclic ethers, without the need for the thermo-limited diene isomerization.

1,3 -pentadiene↗
DOE OSTI2025

Tunable broadband luminescence in lead-free hybrid copper halides

Metal halides are an important class of optoelectronic materials combining exceptional optical and electronic properties. An inherent advantage of metal halides is their solution synthesis and processability, which render them as low-cost and environmentally friendly materials for a range of applications from photovoltaics and photodetection to solid-state lighting (SSL). Here, in this study, we synthesized three previously unreported lead-free organic–inorganic hybrid copper halides: (OA) 4 CuX 5 (X = Br, I; OA + = C 8 H 17 NH 3+ , n-octylammonium cation) and (HA) 2 CuI 3 (HA + = C 6 H 13 NH 3 + , n-hexylammonium cation), all of which exhibit broadband emissions arising from self-trapped excitons (STEs). Among these compounds, (OA) 4 CuI 5 demonstrates tunable dual-band white-light emission with a high color rendering index value of 91 at room temperature. Temperature-dependent photoluminescence measurements and first-principles calculations reveal distinct behaviors between the two emission states in (OA) 4 CuI 5 . These findings highlight the potential of copper halide compounds for optoelectronic applications, particularly in the development of environmentally friendly solid-state lighting technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Reinvestigation of the intrinsic magnetic properties of (Fe 1–$x$ Co $x$ ) 2 B alloys and crystallization behavior of ribbons

In this paper, new determination of the magnetic anisotropy from single crystals of ( Fe 1 - x Co x ) 2 B alloys are presented. The anomalous temperature dependence of the anisotropy constant is discussed using the standard Callen-Callen theory, which is shown to be insufficient to explain the experimental results. A more material specific study using first-principles calculations with disordered moments approach gives a much more consistent interpretation of the experimental data. Since the intrinsic properties of the alloys with x = 0.3 - 0.35 are promising for permanent magnets applications, initial investigation of the extrinsic properties are described, in particular the crystallization of melt spun ribbons with Cu, Al, and Ti additions. Previous attempts at developing a significant hysteresis have been unsuccessful in this system. Our melt-spinning experiment indicates that this system shows rapid crystallization.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electronic, structural and magnetic properties of Mn (1+x) Pt (1-x) Sb

Electronic and magnetic properties of half-metallic Heusler alloys can be modified by tuning their chemical compositions. We have carried out a combined theoretical and experimental investigation of Mn (1+x) Pt (1-x) Sb (0 ≤ x ≤ 0.5) alloys. Our first-principles calculations indicate that the stoichiometric MnPtSb exhibits nearly half-metallic band structure, but a robust half-metallicity can be achieved in Mn-rich compositions Mn (1+x) Pt (1-x) Sb (0 ≤ x ≤ 0.5) with x = 0.25 and higher in their cubic structures. In addition, while MnPtSb exhibits ferromagnetic alignment, Mn (1+x) Pt (1-x) Sb are ferrimagnetic for all non-zero values of x. We have also synthesized cubic MnPtSb and Mn 1.25 Pt 0.75 Sb alloys using arc melting and annealing. The magnetic properties of these alloys are consistent with our theoretical predictions. Furthermore, these results indicate that the Mn-rich Mn (1+x) Pt (1-x) Sb alloys have potential for spin-transport-based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Laser-induced forces on atoms during ultrafast demagnetization

Laser-induced femtosecond demagnetization has attracted a broad attention as a possible candidate for information storage technology. However, whether or not lattice vibration directly participates in demagnetization has been highly controversial over a decade. A recent electron diffraction experiment attributed the demagnetization to the polarized phonon effect, but a similar x-ray diffraction experiment attributed it to the Einstein–de Haas effect. Common to both experiments is that neither the angular momentum of the lattice nor the rotation of the sample was directly probed. Here, we report our first first-principles calculation of forces on atoms induced by an ultrafast laser during ultrafast demagnetization. We employ two complementary methods: (i) the frozen lattice with electronic excitation and (ii) frozen excitation but moving the lattice. We find that the forces on atoms start at –50 fs and peak around 30 fs. The magnitude of the force is far smaller than the empirical estimates. Within the limit of our theory, our results suggest that the polarized phonon effect and the Einstein–de Haas effect are unlikely to be the main course of demagnetization. As a result, we expect that our finding has a profound impact on the future direction of laser-induced dynamics in magnetic and quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Tritium species diffusion on and desorption from γ-LiAlO 2 (100) surface: A first-principles investigation

γ-LiAlO 2 enriched in the 6 Li isotope is a good candidate for tritium (T, 3 H) production in nuclear reactor. In this work, to better understand its 3 H production performance, first-principles calculations are performed to study the 3 H-related species diffusion on and desorption from the γ-LiAlO 2 (100) surface. For the diffusion process, we investigate the pathways and energy barriers for the T and OT diffusion on the pristine and defective γ-LiAlO 2 (100) surfaces. Our results reveal that the O and Li/Al atoms of the surface layer play important roles for T and OT diffusion, respectively. By comparing with its diffusion barrier in bulk γ-LiAlO 2 , the order of energy barrier for T diffusion is: bulk < surface < defective surface, which is reversed for OT diffusion. Space is the prime factor for OT diffusion, while diffusion-mediating atom instead of space becomes the key factor for T diffusion. For the desorption process, we investigate the 3 H-related species (T, OT, T 2 , and T 2 O) desorption from the γ-LiAlO 2 (100) surface. The desorption energies we obtained are so high that the desorption behavior of all the 3 H-related species is nearly prohibited. We expect the possible T 2 or T 2 O release from the large accumulation of 3 H atoms on the surface.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Equilibrium distribution of point defects in Fe-Y-O as a typical representative of nanocluster-strengthened alloys

The statistical mechanic approach has been used to estimate the equilibrium defect distribution in a multiphase system by minimizing its free energy using microstructural characteristics and a finite set of defects obtained by first-principles calculations. Recently the approach was extended to take in consideration defects at the interface of the precipitate and matrix. Herein we apply the developed approach to the investigation of the stability of bcc Fe containing yttria, Y 2 O 3 , nanoclusters as a prototype of nanostructured ferritic alloys. It has been obtained that (100)Fe-O interface is unstable with respect to vacancies production. In a contrast to (100)Fe-O interface, the so called Klim interface is stable, i.e. local vacancy concentration at this interface at 600 K is below 10 -12 . It has been demonstrated that due to large defect formation energies the ODS particles are extremely stable and the main defect corresponds to Fe atoms substitute Y in Y 2 O 3 precipitate. Moreover, under thermodynamic equilibrium condition, the preexist vacancies in bulk Fe do not accumulate oxygen atom. The later observation not necessarily forbid the existence of a large amount of preexisting Fe vacancy - oxygen atom clusters at the initial stages of alloy formation far from equilibrium.

36 MATERIALS SCIENCE↗
DOE OSTI2021

First-principles study of the surface properties of LiAl 5 O 8 : Stability and tritiated water formation

LiAl 5 O 8 is an important secondary phase of γ-LiAlO 2 ceramics in tritium ( 3 H, T)-producing burnable absorber rods (TPBARs), present in small quantities in the as-fabricated condition and in larger quantities after irradiation due to Li burnup and mobility in the radiation-damaged lattice. In this work, we performed first-principles calculations to study the stabilities and structures of LiAl 5 O 8 surfaces the possibility of T 2 O formation on the (111) surface. By calculating the surface energies of all possible symmetrical stoichiometric and nonstoichiometric low-index surfaces, we identified the two most stable surfaces, one stoichiometric (001) surface and one nonstoichiometric (111) surface in an O-rich condition. The surface oxygen atoms on the nonstoichiometric (111) surface have remarkable displacements after relaxation, due to the large dangling O bonds. Considering a Li-poor and 3 H-rich condition relevant to TPBARs, we further studied these two stable surfaces with 37.5%, 75%, and 100% surface O atoms covered by 3 H atoms. Our results show that the 3H saturation is favorable for the nonstoichiometric (111) surface but not for the stoichiometric (001) surface. In addition, the structure of nonstoichiometric (111) surface is well stabilized when the surface O atoms are 100% saturated by 3 H atoms. Based on the fully 3H-saturated (111) surface, the 3 H 2 O desorption with an energy barrier of 0.69 eV is preferable in the 3 H-rich condition.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Back to functional hydrides: Effects of neutron-irradiated microstructure on hydrogen retention in yttrium hydride

Functional hydrides are promising candidates for advanced nuclear reactors, particularly in portable or transportable applications, due to their high hydrogen-retention capabilities, enabling efficient neutron moderation, and compact reactor design. However, hydrogen mobility in hydrides at elevated irradiation temperatures poses significant technological challenges, necessitating a comprehensive understanding of their irradiation behavior. Furthermore, this study investigated the microstructural and chemical stability of neutron-irradiated yttrium hydrides to assess their hydrogen-retention capacity. A targeted literature review was also conducted to contextualize neutron-irradiation effects on functional hydrides with regards to structural stability and hydrogen retention. Experimental characterizations revealed that, at high temperatures, irradiated hydrides retained their phase stability, which was likely enhanced by irradiation-induced microstructure evolution. Notably, an amorphous yttrium and oxygen -rich surface layer was present at the free surface of the hydride. Its thickness decreased while a continuous crystalline Y-O-rich layer was formed with increasing neutron damage. Additionally, the number density of dislocation loops and cavities generally increased as a function of neutron dose. First-principles calculations of hydrogen behavior within yttrium vacancy clusters in yttrium hydrides demonstrated vacancy-size-dependent hydrogen stability and configuration, highlighting the role of vacancy geometry in regulating hydrogen retention. Thus, the presence of irradiation-induced dislocation loops and cavities were hypothesized to improve hydrogen retention. Collectively, these findings advance the understanding of hydride behavior under neutron irradiation as well as their technological readiness for portable or transportable nuclear reactors.

Defect clusters↗
DOE OSTI2020

Magnetic oxygen in transition metal oxides: A case study of Ba 2 CoO 4

Transition metal oxides (TMOs) exhibit exotic magnetic properties in both naturally formed and artificially structured materials, often difficult to understand in conventional wisdom. Magnetic insulator Ba 2 CoO 4 has mystified the community, because the CoO 4 tetrahedron is completely isolated with the nearest Co atoms far apart (~5 Å), making it impossible to account for long-range magnetic ordering seen experimentally using only Co. By theoretically investigating magnetism and relating our findings to experimental observations in bulk Ba 2 CoO 4 , we illustrate for the first time that the magnetic moment on oxygen atoms are the origin of the unexpected long-range magnetic ordering and low magnetic dimensionality. We find that the magnetic moment is not only localized on Co atoms, as assumed in all conventional data analysis, but also distributed on its tetrahedrally-coordinated O atoms. The total magnetic moment of the CoO 4 building block is 4.63μ B with the magnetic moment on Co being only 3.08μ B . Therefore, the magnetic building block is CoO 4 not Co. Furthermore, our first principles calculations are capable of explaining the origin of the unique magnetic response, including the presence of long-range magnetic ordering with two-dimensional character, and a one-dimensional magnetoelastic behavior. Having oxygen contribute to the magnetic moment will undoubtedly be identified as a universal property of magnetic TMOs, which will require a fresh look at conventional models of magnetism in TMOs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗