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Crystal plasticity modeling of 3rd generation multi-phase AHSS with martensitic transformation
Abstract not provided
Observation of magnetic-field-induced transformation in MnCo 0.78 Fe 0.22 Ge alloys with colossal strain output and large magnetocaloric effect
The thermal, structural and magnetic properties were studied for the hexagonal MnCo0.78Fe0.22Ge alloys, which undergoes a first-order phase transformation from paramagnetic hexagonal phase into ferromagnetic orthorhombic martensite on cooling. Owing to the magnetostructural coupling, large magnetocaloric effect (Delta S-M= 10.97 J kg(-1) K-1) was obtained at 254 K. In-situ synchrotron high-energy X-ray diffraction experiments were conducted to reveal the detailed change in crystallographic structure of phases and the effect of applied magnetic field on phase transformation behaviors. An anomalously huge strain of 11.89% and volume expansion of 4.35% in unit-cell were obtained between martensite and parent phase across the transformation. Furthermore, the magnetic field-induced martensitic transformation was directly evidenced at 250 K, which eventually demonstrates the possibility to achieve magnetic-field-induced strain and large magnetocaloric effect simultaneously.
Solid matrix transformation and tracer addition using molten ammonium bifluoride salt as a sample preparation method for laser ablation inductively coupled plasma mass spectrometry
Solid sampling and analysis methods, such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), are challenged by matrix effects and calibration difficulties. Matrix-matched standards for external calibration are seldom available and it is difficult to distribute spikes evenly into a solid matrix as internal standards. While isotopic ratios of the same element can be measured to high precision, matrix-dependent effects in the sampling and analysis process frustrate accurate quantification and elemental ratio determinations. Here we introduce a potentially general solid matrix transformation approach entailing chemical reactions in molten ammonium bifluoride (ABF) salt that enables the introduction of spikes as tracers or internal standards. Proof of principle experiments show that the decomposition of uranium ore in sealed PFA fluoropolymer vials at 230 °C yields, after cooling, new solids suitable for direct solid sampling by LA. When spikes are included in the molten salt reaction, subsequent LA-ICP-MS sampling at several spots indicate that the spikes are evenly distributed, and that U-235 tracer dramatically improves reproducibility in U-238 analysis. Precisions improved from 17% relative standard deviation for U-238 signals to 0.1% for the ratio of sample U-238 to spiked U-235, a factor of over two orders of magnitude. These results introduce the concept of solid matrix transformation (SMT) using ABF, and provide proof of principle for a new method of incorporating internal standards into a solid for LA-ICP-MS. This new approach, SMT-LA-ICP-MS, provides opportunities to improve calibration and quantification in solids based analysis. Finally, looking forward, tracer addition to transformed solids opens up LA-based methods to analytical methodologies such as standard addition, isotope dilution, preparation of matrix-matched solid standards, external calibration, and monitoring instrument drift against external calibration standard
Electronic structure, magnetic properties, and magnetostructural transformations in rare earth dialuminides
We report electronic structure, magnetic properties, and magnetostructural transformations of selected rare earth dialuminides calculated by using local spin density approximation (LSDA), including the Hubbard U parameter (LSDA + U) approach. Total energy calculations show that CeAl{sub 2} and EuAl{sub 2} adopt antiferromagnetic (AFM) ground states, while dialuminides formed by other magnetic lanthanides have ferromagnetic (FM) ground states. The comparison of theoretical and experimental magnetic moments of CeAl{sub 2} indicates that the 4f orbital moment of Ce in CeAl{sub 2} is quenched. Theoretical calculations confirm that Eu in EuAl{sub 2} and Yb in YbAl{sub 2} are divalent. PrAl{sub 2} exhibits a tetragonal distortion near FM transition. HoAl{sub 2} shows a first order magnetostructural transition while DyAl{sub 2} shows a second order transformation below magnetic transition. The dialuminides formed by Nd, Tb, and Er are simple ferromagnets without additional anomalies in the FM state.
Phase transformation as the single-mode mechanical deformation of silicon
The mixture of the metastable body-centered cubic (bc8) and rhombohedral (r8) phases of silicon that is formed via nanoindentation of diamond cubic (dc) silicon exhibits properties that are of scientifc and technological interest. This letter demonstrates that large regions of this mixed phase can be formed in crystalline Si via nanoindentation without signifcant damage to the surrounding crystal. Cross-sectional transmission electron microscopy is used to show that volumes 6 μm wide and up to 650 nm deep can be generated in this way using a spherical tip of ~21.5 μm diameter. The phase transformed region is characterised using both Raman microspectroscopy and transmission electron microscopy. It is found that uniform loading using large spherical indenters can favor phase transformation as the sole deformation mechanism as long as the maximum load is below a critical level. We suggest that the sluggish nature of the transformation from the dc-Si phase to the metallic (b-Sn) phase normally results in competing deformation mechanisms such as slip and cracking but these can be suppressed by controlled loading conditions.
Hidden disorder in the [alpha];#8242;;#8594;[sigma] transformation of Pu-1.9 at. % Ga
Enthalpy and entropy are thermodynamic quantities critical to determining how and at what temperature a phase transition occurs. At a phase transition, the enthalpy and temperature-weighted entropy differences between two phases are equal ({Delta}H=T{Delta}S), but there are materials where this balance has not been experimentally or theoretically realized, leading to the idea of hidden order and disorder. In a Pu-1.9 at. % Ga alloy, the {delta} phase is retained as a metastable state at room temperature, but at low temperatures, the {delta} phase yields to a mixed-phase microstructure of {delta}- and {alpha}{prime}-Pu. The previously measured sources of entropy associated with the {alpha}{prime} {yields} {delta} transformation fail to sum to the entropy predicted theoretically. We report an experimental measurement of the entropy of the {alpha}{prime} {yields} {delta} transformation that corroborates the theoretical prediction, and implies that only about 65% of the entropy stabilizing the {delta} phase is accounted for, leaving a missing entropy of about 0.5 k{sub B}/atom. Some previously proposed mechanisms for generating entropy are discussed, but none seem capable of providing the necessary disorder to stabilize the {delta} phase. This hidden disorder represents multiple accessible states per atom within the {delta} phase of Pu that may not be included in our current understanding of the properties and phase stability of {delta}-Pu.
Cation and anion topotactic transformations in cobaltite thin films leading to Ruddlesden-Popper phases.
Topotactic transformations involve structural changes between related crystal structures due to a loss or gain of material while retaining a crystallographic relationship. The perovskite oxide La0.7Sr0.3CoO3 (LSCO) is an ideal system for investigating phase transformations due to its high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and strong coupling of the magnetic and electronic properties to the oxygen stoichiometry. While the transition between cobaltite perovskite and brownmillerite (BM) phases has been widely reported, further reduction beyond the BM phase lacks systematic studies. In this paper, we study the evolution of the physical properties of LSCO thin films upon exposure to highly reducing environments. We observe the rarely reported crystalline Ruddlesden-Popper phase, which involves the loss of both oxygen anions and cobalt cations upon annealing where the cobalt is found as isolated Co ions or Co nanoparticles. First-principles calculations confirm that the concurrent loss of oxygen and cobalt ions is thermodynamically possible through an intermediary BM phase. The strong correlation of the magnetic and electronic properties to the crystal structure highlights the potential of utilizing ion migration as a basis for emerging applications such as neuromorphic computing.
Transformation of double-half inverse Gaussian hollow beams into superposition of finite Airy beams using an optical Airy transform
In the present work, we investigate theoretically the transformation of a double-half inverse Gaussian hollow (DHIGH) beam into a superposition of finite Airy beams by using an optical Airy transform system. The analytical expression of the generated finite Airy beam is derived by means of the generalized Huygens–Fresnel integral diffraction. Numerical calculations show that the optical Airy transform system converts the central dark spot of the hollow incident beam into peak intensity. It is demonstrated that the characteristics of the transverse intensity distribution of the output finite Airy beam are determined by the phase pattern’s constants of the Airy transform system and the radius size of the incident DHIGH beam. Moreover, it is shown that the profile of the central peak intensity of the generated beam can be controlled by means of the radius size of the incident hollow DHIGH beam.
Phase Transformations and Formation of Ultra-Fine Microstructure During Hydrogen Sintering and Phase Transformation (HSPT) Processing of Ti-6Al-4V
The hydrogen sintering and phase transformation (HSPT) process is a novel powder metallurgy method for producing Ti alloys, particularly the Ti-6Al-4V alloy, with ultra-fine microstructure in the as-sintered state. The ultra-fine microstructure is obtained as a direct result of the use of H-2 gas during sintering. The refinement of the microstructure during HSPT is similar to that of thermal hydrogen processing (THP) of bulk Ti alloys. For both THP and HSPT of Ti-6Al-4V alloy, the mechanisms of the grain refinement depend on the phase equilibria and phase transformations in the presence of hydrogen, which are surprisingly still not well established to date and are still subjected to research and debate. In recent work by the present authors, a pseudo-binary phase diagram of (Ti-6Al-4V)-H has been determined by using in situ synchrotron XRD and TGA/DSC techniques. Aided by this phase diagram, the current paper focuses on the series of phase transformations during sintering and cooling of Ti-6Al-4V in a hydrogen atmosphere and the mechanisms for the formation of the ultra-fine microstructures obtained. Using experimental techniques, including in situ synchrotron XRD, SEM, EBSD, and TEM, the microstructural refinement was found to be the result of (1) the precipitation of ultra-fine alpha/alpha(2) within coarse beta grains during an isothermal hold at intermediate temperatures, and (2) the eutectoid transformation of beta -> alpha + delta d at approximately 473 K (200 degrees C). (C) The Minerals, Metals & Materials Society and ASM International 2015
Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides
Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II) aq ). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III) labile ) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III) labile throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2–1.0 mM Fe(II) aq at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution–reprecipitation mechanism mediated by Fe(III) labile that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. Further, a linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III) labile concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.
Electrochemical Activation of CO 2 through Atomic Ordering Transformations of AuCu Nanoparticles
Precise control of elemental configurations within multimetallic nanoparticles (NPs) could enable access to functional nanomaterials with significant performance benefits. This can be achieved down to the atomic level by the disorder-to-order transformation of individual NPs. Here, by systematically controlling the ordering degree, we show that the atomic ordering transformation, applied to AuCu NPs, activates them to perform as selective electrocatalysts for CO 2 reduction. In contrast to the disordered alloy NP, which is catalytically active for hydrogen evolution, ordered AuCu NPs selectively converted CO 2 to CO at faradaic efficiency reaching 80%. CO formation could be achieved with a reduction in overpotential of ~200 mV, and catalytic turnover was enhanced by 3.2-fold. In comparison to those obtained with a pure gold catalyst, mass activities could be improved as well. Finally, atomic-level structural investigations revealed three atomic gold layers over the intermetallic core to be sufficient for enhanced catalytic behavior, which is further supported by DFT analysis.
Progress and roadmap for electro-privileged transformations of bio-derived molecules
Biomass incorporates carbon captured from the atmosphere and can serve as a renewable feedstock for producing valuable chemicals and fuels. Here we look at how electrochemical approaches can impact biomass valorization, focusing on identifying chemical transformations that leverage renewable electricity and feedstocks to produce valorized products via electro-privileged transformations. First, we recommend that the field should explore widening the spectrum of platform chemicals derived from bio-feedstocks, thus offering pathways to molecules that have historically been derived from petroleum. Second, we identify opportunities in electrocatalytic production of energy-dense fuels from biomass that utilize water as the hydrogen source and renewable electricity as the driving force. Finally, we look at the potential in electrochemical depolymerization to preserve key functional groups in raw feedstocks that would otherwise be lost during harsh pre-treatments in traditional depolymerization routes. Finally, on the basis of these priorities, we suggest a roadmap for the integration of biomass and electrochemistry and offer milestones required to tap further into the potential of electrochemical biomass valorization.
Investigating phase transformation in the Li1.2Co0.1Mn0.55Ni0.15O2 lithium-ion battery cathode during high-voltage hold (4.5 V) via magnetic, X-ray diffraction and electron microscopy studies
This study is the first that provides evidence of phase transformation in a Li-rich Li1.2Co0.1Mn0.55Ni0.15O2 cathode material for lithium-ion batteries (LIBs) during constant voltage charging. Diffraction and magnetic measurement techniques were successfully implemented to investigate the structural transformation in this cathode material during holding a half-cell at 4.5 V in a charged state. The results from X-ray diffraction showed a decrease in c-lattice parameters during high-voltage hold. Magnetic data revealed an increase in average effective magnetic moments of transition metal (TM) ions at constant voltage corresponding to a change in electronic states of TM ions. Analysis showed the reduction of Ni4+ to Ni2+, which was attributed to charge compensation due to oxygen loss. The appearance of the strong {100} forbidden reflection in the single-crystal selected area electron diffraction (SAED) data was attributed to migration of transition metal ions to the octahedral vacancy sites in the lithium layer during high-voltage hold, which was in agreement with the magnetization results. After prolonged hold at 4.5 V, high-resolution transmission electron microscopy (TEM) images along with SAED results showed the presence of spinel phases in the particles, indicating a layered to spinel like phase transformation at constant voltage in agreement with the magnetic data. The results obtained from these magnetic and diffraction studies furnish the fundamental understanding of the structural transformation pathways in Li-rich cathodes at constant voltage and will be instrumental for modifying the parent structure to achieve greater stability.
Coupled-cluster representation of Green function employing modified spectral resolutions of similarity transformed Hamiltonians
In this paper we discuss a new formalism for producing an analytic coupled-cluster (CC) Green's function for an N-electron system by shifting the poles of similarity transformed Hamiltonians represented in N − 1 and N + 1 electron Hilbert spaces. Simple criteria are derived for the states in N − 1 and N + 1 electron spaces that are then corrected in the spectral resolution of the corresponding matrix representations of the similarity transformed Hamiltonian. The accurate description of excited state processes within a Green's function formalism would be of significant importance to a number of scientific communities ranging from physics and chemistry to engineering and the biological sciences. This is because the Green's function methodology provides a direct path for not only calculating properties whose underlying origins come from coupled many-body interactions but also provides a straightforward path for calculating electron transport, response, and correlation functions that allows for a direct link with experiment. As a special case of this general formulation, we discuss the application of this technique for Green's function defined by the CC with singles and doubles representation of the ground-state wave function.
Linear-algebraic bath transformation for simulating complex open quantum systems
In studying open quantum systems, the environment is often approximated as a collection of non-interacting harmonic oscillators, a configuration also known as the star-bath model. It is also well known that the star-bath can be transformed into a nearest-neighbor interacting chain of oscillators. The chain-bath model has been widely used in renormalization group approaches. The transformation can be obtained by recursion relations or orthogonal polynomials. Based on a simple linear algebraic approach, we propose a bath partition strategy to reduce the system-bath coupling strength. As a result, the non-interacting star-bath is transformed into a set of weakly coupled multiple parallel chains. Furthermore, the transformed bath model allows complex problems to be practically implemented on quantum simulators, and it can also be employed in various numerical simulations of open quantum dynamics.