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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Thermodynamics of Reactions Affected by Medium Reorganization

We present a thermodynamic analysis of the activation barrier for reactions which can be monitored through the difference in the energies of reactants and products defined as the reaction coordinate (electron and atom transfer, enzyme catalysis, etc). The free-energy surfaces along the reaction coordinate are separated into the enthalpy and entropy surfaces. For the Gaussian statistics of the reaction coordinate, the free-energy surfaces are parabolas, and the entropy surface is an inverted parabola. Its maximum coincides with the transition state for reactions with zero value of the reaction free energy. Maximum entropic depression of the activation barrier, anticipated by the concept of transition-state ensembles, can be achieved for such reactions. From Onsager’s reversibility, the entropy of equilibrium fluctuations encodes the entropic component of the activation barrier. The reorganization entropy thus becomes the critical parameter of the theory reducing the problem of activation entropy to the problem of reorganization entropy. Standard solvation theories do not provide reorganization entropy sufficient for the barrier depression. Complex media, characterized by many relaxation processes, need to be involved and proteins provide several routes for achieving large entropic effects related to incomplete (nonergodic) sampling of the complex energy landscape and active role of water in the reaction mechanism.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

On the upper bound of entropy production rate from particle multiplicity in heavy ion collisions

We provide a simple derivation for particle production in heavy-ion collisions that is proportional to the rate of entropy production. We find that the particle production depends only on the power of the centre-of-mass collision energy $\sqrt{s_{\rm NN}}$ and the effective phase-space/volume (e.g. geometry of the collision approximated by the number of nucleons participating in the collision $N_{\rm part}$). We show that at low-energies the pseudo-rapidity density of particles per participating nucleon pair scales linearly with $\sqrt{s_{\rm NN}}$ while at high-energies with $\sqrt{s_{\rm NN}}^{1/3}$. The $\sqrt{s_{\rm NN}}^{1/3}$ region is directly related to sub-nucleon degrees of freedom and creation of a quark-gluon plasma (QGP). This picture explains experimental observation that the shape of the distributions of pseudorapidity-density per nucleon pair of charged particles does not depend on $\sqrt{s_{\rm NN}}$ over a large span of collision energies. We provide an explanation of the scaling and connect it with the maximum rate per unit time of entropy production. We conclude with remarks on the hadron-parton phase transition. In particular, our considerations suggest that the pseudo-rapitidy density of the produced particles per $N_{\rm part}/2$ larger than approximately 1 (excluding particles from jet fragmentation) is a signature of a QGP formation.

Ploskon, Mateusz↗
DOE OSTI2013

New orthorhombic derivative of CaCu{sub 5}-type structure: RNi{sub 4}Si compounds (R=Y, La, Ce, Sm, Gd–Ho), crystal structure and some magnetic properties

The crystal structure of new YNi{sub 4}Si-type RNi{sub 4}Si (R=Y, La, Ce, Sm, Gd–Ho) compounds has been established using powder X-ray diffraction. The YNi{sub 4}Si structure is a new structure type, which is orthorhombic derivative of CaCu{sub 5}-type structure (space group Cmmm N 65, oC12). GdNi{sub 4}Si and DyNi{sub 4}Si compounds order ferromagnetically at 25 and 19 K, respectively whereas YNi{sub 4}Si shows antiferromagnetic nature. At 15 K, DyNi{sub 4}Si shows second antiferromagnetic-like transition. The magnetic moment of GdNi{sub 4}Si at 5 K in 50 kOe field is ∼7.2 μ{sub B}/f.u. suggesting a completely ordered ferromagnetic state. The magnetocaloric effect of GdNi{sub 4}Si is calculated in terms of isothermal magnetic entropy change and it reaches the maximum value of −12.8 J/kg K for a field change of 50 kOe near T{sub C} ∼25 K. - Graphical abstract: The RNi{sub 4}Si (R=Y, La, Ce, Sm, Gd–Ho) compounds crystallize in new YNi{sub 4}Si-type structure which is orthorhombic derivative of the basic CaCu{sub 5}-type structure. GdNi{sub 4}Si and DyNi{sub 4}Si compounds show the ferromagnetic-like ordering, whereas.YNi{sub 4}Si has the antiferromagnetic nature. The GdNi{sub 4}Si demonstrates the big magnetocaloric effect near temperature of ferromagnetic ordering. The relationship between initial CaCu{sub 5}-type DyNi{sub 5} and YNi{sub 4}Si-type DyNi{sub 4}Si lattices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Magnetic order and crystal structure study of YNi{sub 4}Si-type NdNi{sub 4}Si

Magnetic measurements and neutron powder diffraction investigation of the magnetic structure of the orthorhombic YNi{sub 4}Si-type (space group Cmmm) NdNi{sub 4}Si compound are presented. The magnetocaloric effect of NdNi{sub 4}Si is calculated in terms of the isothermal magnetic entropy change and it reaches the maximum value of –3.3 J/kg K for a field change of 50 kOe near T{sub C}=12 K. Below ∼12 K, NdNi{sub 4}Si exhibits a commensurate b-axis collinear ferromagnetic ordering with the Cmm′m magnetic space group in a zero magnetic field. At 1.5 K, the neodymium atoms have the magnetic moment of 2.37(5) μ{sub B}. The orthorhombic crystal structure and its thermal evolution are discussed in comparison with the CaCu{sub 5}-type compound. - Graphical abstract: The NdNi{sub 4}Si supplement the series of the orthorhombic derivative of the CaCu{sub 5}-type, namely the YNi{sub 4}Si-type, RNi{sub 4}Si compounds (R=Y, La, Ce, Sm, Gd–Ho). Below ∼12 K in a zero applied magnetic field, NdNi{sub 4}Si exhibits a commensurate b-axis collinear ferromagnetic ordering with the Cmm′m magnetic space group. Compared to the CaCu{sub 5}-type NdNi{sub 4}Si compound, the YNi{sub 4}Si-type counterpart has the relatively high ferromagnetic ordering temperature (9.2 K vs. 12 K), the small magnetocaloric effect (–7.3 J/kg K vs. –3.3 J/kg K for ∆H=50 kOe), and the large magnetic anisotropy at low temperatures. In contrast with CaCu{sub 5}-type NdNi{sub 4}Si, YNi{sub 4}Si-type NdNi{sub 4}Si shows distinct hysteresis loop at 2 K.We suggest that orthorhombic distortion may be used as a prospective route for optimization of permanent magnetic properties in the family of CaCu{sub 5}-type rare earth materials. - Highlights: • Below ∼12 K the YNi{sub 4}Si-type NdNi{sub 4}Si shows a ferromagnetic ordering. • MCE of NdNi{sub 4}Si reaches value of –3.3 J/kg K in 0–50 kOe near Curie point. • NdNi{sub 4}Si exhibits b-axis ferromagnetic order with the Cmm′m magnetic space group. • Contrary to CaCu{sub 5}-type, YNi{sub 4}Si-type NdNi{sub 4}Si shows hysteresis loop at 2 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Spectral splitting optimization for high-efficiency solar photovoltaic and thermal power generation

Utilizing the full solar spectrum is desirable to enhance the conversion efficiency of a solar power generator. In practice, this can be achieved through spectral splitting between multiple converters in parallel. However, it is unclear which wavelength bands should be directed to each converter in order to maximize the efficiency. We developed a model of an ideal hybrid solar converter which utilizes both a single-junction photovoltaic cell and a thermal engine. We determined the limiting efficiencies of this hybrid strategy and the corresponding optimum spectral bandwidth directed to the photovoltaic cell. This optimum width is inversely proportional to the thermal engine efficiency and scales with the bandgap of the photovoltaic cell. This bandwidth was also obtained analytically through an entropy minimization scheme and matches well with our model. We show that the maximum efficiency of the system occurs when it minimizes the spectral entropy generation. This concept can be extended to capture generalized non-idealities to increase the usefulness of this technique for a range of full solar spectrum utilization technologies.

14 SOLAR ENERGY↗
DOE OSTI2018

Effect of Sodium Doping on Magnetic and Magnetocaloric Properties of La{sub 0.65}Sr{sub 0.35}MnO{sub 3} Manganites

A series of Na-doped manganites La{sub 0.65}Sr{sub 0.35−x} Na{sub x}MnO{sub 3} (x = 0.20, 0.25, 0.30, and 0.35) were synthesized via sol-gel method. The structural, magnetic, and magnetocaloric properties of the samples were investigated by XRD, TEM, Raman spectroscopy, and magnetization measurement. Rietveld refinements of XRD patterns shown that samples were single phase with a space group R-3c. The variation of magnetization versus temperature in a magnetic applied field demonstrated a phase transition from ferromagnetic to paramagnetic. The value of T{sub C} decreased from 345 to 330 K when x increases from 0.20 to 0.35. Arrott plots shown that all samples exhibit a second-order magnetic phase transition. The maximum absolute values of the magnetic entropy change were found to be 0.91, 0.93, 1.12, and 1.10 J/kg for x = 0.2, 0.25, 0.3, and 0.35, respectively. Moreover, relative cooling power has been discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic and magnetocaloric properties of DyCo 2 C x alloys

The magnetic and magnetocaloric properties of DyCo 2 C x (x = 0, 0.05, 0.1, and 0.15) alloys were investigated. The results show that the Curie temperature (T C ) of the DyCo 2 C x alloys increases with increasing C content, from 136 K (x = 0) to 152 K (x = 0.15), but the lattice parameter a of DyCo 2 C x exhibits a maximum at x = 0.05. The suppression of the ac susceptibility of DyCo 2 C x at low temperature indicates the enhancement of the domain wall pinning effect by carbon doping. The positive slops of the Arrott plots of the doped compounds indicate that the phase transition is second order for the carbon-doped alloys, and the maximum value of the isothermal magnetic entropy change (ΔS M ) for the magnetic field change of 50 kOe decreases from -13.9 J/kg· K (x = 0) to -7.8 J/kg·K (x = 0.15). Lastly, the relative cooling power (RCP) of DyCo 2 C x is nearly the same in all studied alloys, while the temperature-averaged entropy change over 10 K temperature span, TEC(10), indicates decreasing magnetocaloric performance of carbon doped materials.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Towards computational design of zeolite catalysts for CO 2 reduction

Carbon dioxide, an energy waste by-product with significant environmental consequences can be utilized and converted into useful chemical products such as formic acid, formaldehyde, methanol or methane, but more energy and cost efficient catalytic processes are required. Here we develop the methodology for the intelligent selection of porous zeolites for dual-adsorption of hydrogen and carbon dioxide as templates for preparing the optimal catalytic environment for carbon dioxide reduction. Useful zeolite catalysts were computationally screened from over 300 thousand zeolite structures using a combination of molecular simulation and machine-learning techniques. Several of the top candidates were very promising energy-efficient templates for catalysis with the potential to perform at 50% above conventional reactors. As a result, it is also found that an optimal cavity size of around 6 Å is required to maximize the change in entropy–enthalpy upon adsorption with a maximum void space >30% to boost product formation per volume of material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Critical behavior and magnetocaloric effect in Mn 3 Si 2 Te 6

Here, the critical properties and magnetocaloric effect of semiconducting ferrimagnet Mn 3 Si 2 Te 6 single crystals have been investigated by bulk magnetization and heat capacity around T c . Critical exponents β = 0.41 ± 0.01 with a critical temperature T c = 74.18 ± 0.08 K and γ = 1.21 ± 0.02 with T c = 74.35 ± 0.05 K are deduced by the Kouvel-Fisher plot, whereas δ = 4.29 ± 0.05 (3.40 ± 0.02) is obtained by a critical isotherm analysis at T = 74(75)K. The magnetic exchange distance is found to decay as J (r) ≈ r -4.79 , which lies between the mean-field and 3D Heisenberg models. Moreover, the magnetic entropy change -ΔS M features a maximum at T c , i.e., -ΔS$max\atop{M}$ ~ 2.53 (1.67) J kg -1 K -1 with in-plane (out-of-plane) field change of 5 T, confirming large magnetic anisotropy. The heat capacity measurement further gives -ΔS$max\atop{M}$ ~ 2.94 J kg -1 K -1 and the corresponding adiabatic temperature change ΔT ad ~ 1.14 K with out-of-plane field change of 9 T.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Universal Bound to the Amplitude of the Vortex Nernst Signal in Superconductors

A liquid of superconducting vortices generates a transverse thermoelectric response. This Nernst signal has a tail deep in the normal state due to superconducting fluctuations. Here, we present a study of the Nernst effect in two-dimensional heterostructures of Nb-doped strontium titanate (STO) and in amorphous MoGe. Additionally, the Nernst signal generated by ephemeral Cooper pairs above the critical temperature has the magnitude expected by theory in STO. On the other hand, the peak amplitude of the vortex Nernst signal below Tc is comparable in both and in numerous other superconductors despite the large distribution of the critical temperature and the critical magnetic fields. In four superconductors belonging to different families, the maximum Nernst signal corresponds to an entropy per vortex per layer of ≈kBln 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Correlation properties of entangled multiphoton states and Bernstein's paradox

A normally ordered characteristic function (NOCF) of Bose operators is calculated for a number of discrete-variable entangled states (Greenberger-Horne-Zeilinger (GHZ) and Werner (W) qubit states and a cluster state). It is shown that such NOCFs contain visual information on two types of correlations: pseudoclassical and quantum correlations. The latter manifest themselves in the interference terms of the NOCFs and lead to quantum paradoxes, whereas the pseudoclassical correlations of photons and their cumulants satisfy the relations for classical random variables. Three- and four-qubit states are analyzed in detail. An implementation of an analog of Bernstein's paradox on discrete quantum variables is discussed. A measure of quantumness of an entangled state is introduced that is not related to the entropy approach. It is established that the maximum of the degree of quantumness substantiates the numerical values of the coefficients in multiqubit vector states derived from intuitive considerations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Direct and indirect measurements of the magnetic and magnetocaloric properties of Ni 0.895 Cr 0.105 MnGe 1.05 melt-spun ribbons in high magnetic fields

We report the magnetic and magnetocaloric properties of rapidly solidified Ni 0.895 Cr 0.105 MnGe 1.05 melt-spun ribbons studied by both direct (adiabatic temperature change) and indirect (isothermal magnetic entropy change) methods in intermediate and high magnetic fields up to 10 T. The maximum values of the adiabatic temperature changes (ΔT ad ) and magnetic entropy changes ( Δ S M ) were found to be ~2.6 K (µ o H = 10 T) and 4.4 J kg -1 K -1 (µ o ΔH = 5 T), respectively, near the Curie temperature (T C ). The ΔT ad curves and magnetization isotherms were found to be completely reversible, which indicates the high degree of reversibility of the MCEs in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ΔT ad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µ o H) 2/3 near T C , in accordance with the Landau theory of phase transitions.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2014

Negative to positive magnetoresistance and magnetocaloric effect in Pr 0.6 Er 0.4 Al 2

We report on the magnetic, magnetocaloric and magnetotransport properties of Pr 0.6 Er 0.4 Al 2 . The title compound exhibits a large positive magnetoresistance (MR) for H ≥ 40 kOe and a small but non negligible negative MR for H ≤ 30 kOe. The maximum positive MR reaches 13% at H = 80 kOe. The magnetic entropy and adiabatic temperature changes as functions of temperature each show two anomalies: a broad dome-like maximum below 20 K and a relatively sharp peak at higher temperature. As a result, observed behaviors are unique among other binary and mixed lanthanide compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2016

RNi{sub 8}Si{sub 3} (R=Gd,Tb): Novel ternary ordered derivatives of the BaCd{sub 11} type

The title compounds have been synthesized and characterized both from the structural and magnetic point of view. Both crystallize in a new monoclinic structure strictly related to the tetragonal BaCd{sub 11} type. The structure was solved by means of X-ray single-crystal techniques for GdNi{sub 8}Si{sub 3} and confirmed for TbNi{sub 8}Si{sub 3} on powder data; the corresponding lattice parameters (obtained from Guinier powder patterns) are a=6.3259(2), b=13.7245(5), c=7.4949(3) Å, β=113.522(3)°, V{sub cell}=596.64(3) Å{sup 3} and a=6.3200(2), b=13.6987(4), c=7.4923(2) Å, β=113.494(2)°, V{sub cell}=594.88(2) Å{sup 3}. The symmetry relationship between the tI48-I4{sub 1}/amd BaCd{sub 11} aristotype and the new ordered mS48-C2/c GdNi{sub 8}Si{sub 3} derivative is described via the Bärnighausen formalism within the group theory. The large Gd–Gd (Tb–Tb) distances, mediated via Ni–Si network, likely lead to weak magnetic interactions. Low-field magnetization vs temperature measurements indicate weak and field-sensitive antiferromagnetic ground state, with ordering temperatures of 3 K in GdNi{sub 8}Si{sub 3} and about 2–3 K in TbNi{sub 8}Si{sub 3}. On the other hand, the isothermal field-dependent magnetization data show the presence of competing interactions in both compounds, with a field-induced ferromagnetic behavior for GdNi{sub 8}Si{sub 3} and a ferrimagnetic-like behavior in TbNi{sub 8}Si{sub 3} at the ordering temperature T{sub C/N} of about (or slightly higher than) 3K. The magnetocaloric effect, quantified in terms of isothermal magnetic entropy change ΔS{sub m}, has the maximum values of –19.8 J(kg K){sup −1} (at 4 K for 140 kOe field change) and −12.1 J(kg K){sup −1} (at 12 K for 140 kOe field change) in GdNi{sub 8}Si{sub 3} and TbNi{sub 8}Si{sub 3}, respectively. - Graphical abstract: GdNi{sub 8}Si{sub 3} and TbNi{sub 8}Si{sub 3} compounds are isostructural, and crystallize in a new monoclinic type strictly related to the tetragonal BaCd{sub 11} structure. The large R–R interatomic distances mediated via Ni–Si network lead to a weak magnetism in both compounds. - Highlights: • Novel RNi{sub 8}Si{sub 3} (R=Gd, Tb) compounds have been synthesized and characterized. • GdNi{sub 8}Si{sub 3} and TbNi{sub 8}Si{sub 3} are isostructural compounds, belonging to a new monoclinic structure type. • The monoclinic GdNi{sub 8}Si{sub 3} type is an ordered derivative of the tetragonal BaCd{sub 11} type. • The large R–R interatomic distances mediated via Ni–Si network lead to a weak magnetism. • Both compounds GdNi{sub 8}Si{sub 3} and TbNi{sub 8}Si{sub 3} show antiferromagnetic-like order around 3 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Magnetic properties of Fe{sub 2}P-type Tb{sub 6}FeTe{sub 2}, Tb{sub 6}CoTe{sub 2}, Tb{sub 6}NiTe{sub 2} and Er{sub 6}FeTe{sub 2} compounds

The magnetic ordering of the Fe{sub 2}P-type Tb{sub 6}FeTe{sub 2}, Tb{sub 6}CoTe{sub 2} Tb{sub 6}NiTe{sub 2} and Er{sub 6}FeTe{sub 2} phases (space group P6-bar 2m) has been investigated through magnetization measurement and neutron powder diffraction. Tb{sub 6}FeTe{sub 2}, Tb{sub 6}CoTe{sub 2} and Tb{sub 6}NiTe{sub 2} demonstrate high-temperature ferromagnetic and low-temperature spin reorientation transitions, whereas Er{sub 6}FeTe{sub 2} shows antiferromagnetic transition, only. The Tb{sub 6}FeTe{sub 2} and Tb{sub 6}NiTe{sub 2} phases show same high-temperature collinear ferromagnetic structure, whereas Tb{sub 6}FeTe{sub 2} is the commensurate non-collinear ferromagnet and Tb{sub 6}NiTe{sub 2} is the canted ferromagnetic cone with K{sub 1}=[0, 0, {+-}3/10] and K{sub 2}=[{+-}2/9, {+-}2/9, 0] wave vectors at 2 K. The magnetic structure of Er{sub 6}FeTe{sub 2} is a flat spiral with K{sub 1}=[0, 0, {+-}1/10] at 2 K. The magnetic entropy change for Tb{sub 6}NiTe{sub 2} is {Delta}S{sub m}=-4.86 J/kg K at 229 K for the field change {Delta}{mu}{sub 0}H=0-5 T. In addition, novel Fe{sub 2}P-type Gd{sub 6}FeTe{sub 2}, Zr{sub 6}FeTe{sub 2}, Hf{sub 6}FeTe{sub 2}, Dy{sub 6}NiTe{sub 2}, Zr{sub 6}NiTe{sub 2} and Hf{sub 6}NiTe{sub 2} phases have been obtained. -- Graphical abstract: The novel Fe{sub 2}P-type Tb{sub 6{l_brace}}Fe, Co, Te{r_brace}Te{sub 2} and Er{sub 6}FeTe{sub 2} phases (space group P6-bar 2m) show the complex magnetic ordering below 18-228 K. The magnetocaloric effect for Tb{sub 6}NiTe{sub 2} in terms of the isothermal entropy change, -{Delta}S{sub m}, has the maximum value of 4.86 J/kg K at 229 K for the 0-5 T field change.

36 MATERIALS SCIENCE↗
NASA NTRS1983

High resolution schemes and the entropy condition

A systematic procedure for constructing semidiscrete, second order accurate, variation diminishing, five point band width, approximations to scalar conservation laws, is presented. These schemes are constructed to also satisfy a single discrete entropy inequality. Thus, in the convex flux case, convergence is proven to be the unique physically correct solution. For hyperbolic systems of conservation laws, this construction is used formally to extend the first author's first order accurate scheme, and show (under some minor technical hypotheses) that limit solutions satisfy an entropy inequality. Results concerning discrete shocks, a maximum principle, and maximal order of accuracy are obtained. Numerical applications are also presented.

Osher, S.↗