Evolution Of Constants Research Articles

Crystallization dynamics and interface stability of strontium titanate thin films on silicon.

Different physical vapor deposition methods have been used to fabricate strontium titanate thin films. Within the binary phase diagram of SrO and TiO2 the stoichiometry ranges from Ti rich to Sr rich, respectively. The crystallization of these amorphous SrTiO3 layers is investigated by in situ grazing-incidence X-ray diffraction using synchrotron radiation. The crystallization dynamics and evolution of the lattice constants as well as crystallite sizes of the SrTiO3 layers were determined for temperatures up to 1223 K under atmospheric conditions applying different heating rates. At approximately 473 K, crystallization of perovskite-type SrTiO3 is initiated for Sr-rich electron beam evaporated layers, whereas Sr-depleted sputter-deposited thin films crystallize at 739 K. During annealing, a significant diffusion of Si from the substrate into the SrTiO3 layers occurs in the case of Sr-rich composition. This leads to the formation of secondary silicate phases which are observed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.

Influence of Heat Treatment on the Microstructure of a High Co-Ni Secondary Hardening Steel

The aim of this study was to analyze the influence of combined heat and cryogenic treatment on the evolution of martensite and reverted austenite in a high Co-Ni steel. To this end the evolution of phase fractions and lattice constants after thermal treatments were characterized by using X-ray diffraction (XRD) investigations and dilatometer analyses. The arrangement of phases and the element distribution in martensite, austenite and carbides were analyzed by using energy filtered transmission electron microscopy (EFTEM).

Metal substitution effect on superconductivity in barium intercalated ternary metal nitride chlorides

We systematically studied barium cointercalated ternary metal nitride chloride superconductors of Ba(NH3)yZrHfxNCl (). X-ray diffraction and Raman spectra measurements show a systematical evolution of lattice constants and lattice dynamics upon Hf replacement for Zr. The superconducting transition temperature () increases monotonically from 11.7 K to 20.2 K with increasing the hafnium content in metal-nitrogen bilayers from 0 to 1. The upper critical field Hc2(0) for Ba(NH3)yHfNCl is estimated to be about 13.2 T. Accompanied by the remarkably increased , the highest energy phonon mode coupled most strongly with electronic system shows a moderate hardening with Hf substitution for Zr. This result suggests the reinforcement of pairing interaction with Hf substitution arises mainly from the evolution of electronic structure of metal d-band rather than from phonon modes.

Hydrogenation of AB5 and AB2 metal hydride alloys studied by in situ X-ray diffraction

The evolution of lattice constants and abundances of metal (α) and metal hydride (β) phases during the hydrogenation process of an AB5 alloy with a CaCu5 crystal structure, an AB2 alloy with a predominating C14 structure, and a C14/C15-mixed AB2 alloy were reported. The preferred hydrogen insertion sites at different states of charge in both the α and β phases were studied based on the lattice parameter changes during hydrogenation. During the hydrogenation of the AB5 alloy, the ratio between lattice parameters a and c (a/c ratio) in the α phase decreases, stabilizes, and then decreases again while that in the β phase decreases and then stabilizes. The trends in unit cell volume changes are increasing, plateauing, and increasing again in the α phase and increasing followed by plateauing in the β phase as the hydrogenation level increases. In the C14-predominant AB2 alloy, the a/c ratio in the α phase increases at the beginning and then stabilizes while that in the β phase remains about the same and then increases during the addition of hydrogen. Moreover, the unit cell volume in the α phase increases slightly during hydrogenation, comparing to the increasing, decreasing, and then increasing trend in the β phase. In the C14/C15 mixed AB2 alloy, hydrogenation of the C15 phase starts at the beginning and promotes the hydrogenation of the C14 phase. The C14 phase, as the main hydrogen storage phase, completes hydrogenation first compared to the C15 phase, which is about 50% hydrided at 1MPa H2.

Short range orders in molten Al: An ab initio molecular dynamics study

Short range orders (SROs) in molten Al from 943 to 1523K have been studied through ab initio molecular dynamics simulations. Pair-correlation functions, bond-angle distribution functions, Honeycutt and Andersen analysis were employed to study the local structure of molten Al. A set of indices (l2,l3,l4…lj) were defined to differentiate the SROs. FCC- and HCP-SROs do not exist in equilibrium molten Al generally. Small amount of icosahedral or icosahedron-like SROs are found in molten Al and the structures of the most SROs are more complex than an icosahedron. The non-monotonous or non-classical evolution for the amount of the specific SROs, with five and six 15xx+1431 (sum of 15xx and 1431) bonded-pairs packing around the central atom, induce that the evolution of self-diffusion constants obeys different Arrhenius relationships in temperature ranges lower and higher than 1073K.

Mechanical and thermodynamic properties of α-UH3 under pressure

The mechanical and thermodynamic properties of α-UH3 under pressure have been examined by first-principles pseudopotential plane-wave calculations based on the density functional theory. In order to describe the strong on-site Coulomb repulsion among the localized 5f electrons, the generalized gradient approximation and local density approximation plus a Hubbard parameter (GGA+U and LDA+U) formalisms have been adopted for the exchange correlation term. The pressure evolution of elastic constants of α-UH3 has been revealed, and α-UH3 is found to be mechanically stable at least up to 20GPa. Under pressure, the low-frequency vibration modes of α-UH3 change slightly, while the high-frequency vibration modes shift upward. Our study is expected to facilitate the understanding of uranium hydrides and their applications.

Ab initiostudy of the structural phase transitions of the double perovskitesSr2MWO6(M=Zn, Ca, Mg)

We study the interplay of structural distortions in double perovskites ${\text{Sr}}_{2}M{\text{WO}}_{6}$ ($M=$ Zn, Ca, Mg) by means of first-principles calculations and group theoretical analysis. Structure relaxations of the cubic, tetragonal, and monoclinic phases show that the ground states of the three compounds are monoclinic, although the energy difference between the monoclinic and tetragonal structures is very small in the case of ${\text{Sr}}_{2}\text{Mg}{\text{WO}}_{6}$. The symmetry analysis of the distortions involved in the experimental and calculated low-temperature structures shows that the amplitude of two primary distortions associated to rigid rotations of the $MX$${}_{6}$ and WO${}_{6}$ octahedra are dominant, although the amplitude of a third mode related to deformations of the $MX$${}_{6}$ groups can not be neglected. The energy maps of the space spanned by the three relevant modes are calculated, and the couplings among the modes are evaluated, showing that the role of a hard secondary mode (in the Landau sense) coupled trilinearly to the two primary instabilities is crucial to stabilize the monoclinic ground state. Results suggest that the key role of the trilinear coupling among three modes could be rather common. A phenomenological theory including the effects of the chemical pressure is also developed. We find that the evolution of the stiffness constants in terms of the atomic substitution follows an accurate linear dependence and that the influence of quantum saturation of the order parameters could stabilize the tetragonal phase of ${\text{Sr}}_{2}\text{Mg}{\text{WO}}_{6}$.

Oxygen Incorporation in ZnTeO Alloys via Molecular Beam Epitaxy

Highly mismatched ZnTeO alloys were grown by molecular beam epitaxy under various oxygen partial pressures (10−7 Torr, 10−6 Torr, and 10−5 Torr). Despite this large variation in oxygen partial pressure, there was almost no change in the concentration of incorporated oxygen. However, increasing the oxygen partial pressure significantly enhanced the photoluminescence of the oxygen-related transition at 1.9 eV. The evolution of lattice constants, as determined by high-resolution x-ray diffraction, appeared to follow Vegard’s law for ideal substitutional alloys for ZnTeO films prepared at 10−7 Torr and 10−6 Torr, while a pronounced deviation occurred at 10−5 Torr. Channeling nuclear reaction analysis further revealed the complexity of oxygen incorporation, as no significant shadowing was observed from the epitaxial films. The evidence suggests that the oxygen location in ZnTeO is more complicated than O-Te substitutional defects in a homogeneous solid solution, and that the configuration of the oxygen dopant might play an important role in the band structure and optical properties of the alloys.

The influence of hot pressing conditions on mechanical properties of nickel aluminide/alumina composite

The influence of hot pressing conditions on mechanical properties of nickel aluminide/alumina composite has been investigated in the present paper. In particular, effect of the process parameters, viz. compacting pressure, sintering temperature and sintering time on the evolution of density, elastic constants and tensile strength properties of the intermetallic-ceramic composite has been studied. Elastic constants, the Young's modulus and Poisson's ratio, have been evaluated using an ultrasonic testing method, and the tensile strength has been determined by a Brazilian-type splitting test. Microscopic observations of microstructure evolution complemented the experimental procedure. Experimental results have been confronted with theoretical models showing a good agreement between the data compared.

Influence of hydraulic retention time on heterotrophic biomass in a wastewater moving bed membrane bioreactor treatment plant

Wastewater treatment using moving bed membrane bioreactor technology was tested with real urban wastewater at a pilot plant, combining moving bed treatment as a biological process with hybrid biomass (suspended and fixed) and the advantages of a membrane separation system. The evolution of the kinetic constants of the hybrid biomass and organic matter removal were studied in a pilot plant under different operational conditions, by varying hydraulic retention time (HRT), mixed liquor suspended solids (MLSS) and temperature, and considering the attached biomass of the carrier and the dispersed biomass of the flocs to reproduce real treatment conditions. The rates of organic matter removal were 97.73 ± 0.81 % of biochemical oxygen demand (BOD5), 93.44 ± 2.13 % of chemical oxygen demand (COD), 94.41 ± 2.26 % of BOD5 and 87.62 ± 2.47 % of COD using 24.00 ± 0.39 and 10.00 ± 0.07 h of HRT, respectively. The influence of the environmental variables and operational conditions on kinetic constants was studied; it was determined that the most influential variable for the decay coefficient for heterotrophic biomass was HRT (0.34 ± 0.14 and 0.31 ± 0.10 days−1 with 10.00 ± 0.07 and 24.00 ± 0.39 h of HRT, respectively), while for heterotrophic biomass yield, this was temperature (0.61 ± 0.04 and 0.52 ± 0.06 with 10.00 ± 0.07 and 24.00 ± 0.39 h of HRT, respectively). The results show that introducing carriers in an MBR system provides similar results for organic matter removal, but with a lower concentration of MLSS.

Functional renormalization group for few-nucleon systems: SU(4) symmetry and its breaking

We apply the functional renormalization group to few-nucleon systems. Our starting point is a local effective action that includes three- and four-nucleon interactions, expressed in terms of nucleon and two-nucleon boson fields. The evolution of the coupling constants in this action is described by a renormalization group flow. We derive these flow equations both in the limit of exact Wigner SU(4) symmetry and in the realistic case of broken symmetry. In the symmetric limit we find that the renormalization flow equations decouple and can be combined into two sets, one of which matches the known results for bosons, and the other result matches the one for a single flavor of spin $1/2$ fermions. The equations show universal features in the unitary limit, which is obtained when the two-body scattering length tends to infinity. We calculate the spin-quartet neutron-deuteron scattering length and the deuteron-deuteron scattering lengths in the spin-singlet and spin-quintet channels.

Temperature evolution of optical constants and their tuning in silver

In this study, high temperature spectroscopic ellipsometry is employed to deduce the temperature dependent refractive indices of silver. The thermo-optic coefficients (thermo-refractive: dn/dT, thermo extinction coefficient: dk/dT) computed from the temperature dependent refractive indices, showed maximum dispersion and sign changes around the L (direct transition from the d band to the Fermi level i.e., L3–L′2(EF)) interband transition. The positive and negative dn/dT shows that tuning of refractive indices is possible by appropriate choice of incident wavelengths (or energies) and temperatures. The dispersion in dn/dT indicates the possibility of silver thin films being used as thermal lens for either converging or diverging purposes. A negative dk/dT for incident energy radiation around the L transition, indicates, increased transmission and decreased attenuation for these energies. The above dispersion of the thermo-optic coefficients are attributed to a natural outcome of the temperature dependent smearing in the Fermi distribution near the L transition and shift of energy levels due to the temperature dependent deformation potential. This paper also reports the experimental temperature dependence of refractive indices of silver corresponding to 632.8nm, which has hitherto been obtained from a theoretically based Drude model.

High-pressure Brillouin scattering of the single-crystal PbSc_{1/2}Ta_{1/2}O_{3} relaxor ferroelectric

The effect of pressure on the acoustic modes and adiabatic elastic constants of the perovskite-type ($AB$O${}_{3}$) relaxor compound PbSc${}_{1/2}$Ta${}_{1/2}$O${}_{3}$ have been studied by Brillouin spectroscopy at room temperature up to 9.2 GPa. It is shown that the elastic constants are very sensitive to the pressure-induced structural transformations that were established by diffraction and Raman scattering analysis. Changes in the evolution of the pseudocubic elastic constants ${c}_{11}$ and ${c}_{44}$ occur at the two phase-transition pressures ${p}_{c}$${}_{1}$ $=$ 1.9 GPa and ${p}_{c2}$ \ensuremath{\sim} 5.5 GPa. Changes in evolution of ${c}_{12}$ occur at the two characteristic pressures ${p}_{1}$${}^{*}$ $=$ 1.2 GPa and ${p}_{2}$${}^{*}$ \ensuremath{\sim} 3.0 GPa, which mark changes in the local structure that precede the corresponding phase transitions. The elastic anisotropy and the width of the transverse acoustic mode increase in the pressure range between ${p}_{c}$${}_{1}$ and ${p}_{c}$${}_{2}$ with a maximum near ${p}_{2}$${}^{*}$. A strong decrease in the Cauchy parameter is observed at pressures above ${p}_{2}$${}^{*}$, which indicates an enhancement of the covalent character of chemical bonding. Increasing pressure gradually suppresses the central peak typical of the relaxor ergodic state, and the quasielastic scattering vanishes at a pressure slightly above ${p}_{2}$${}^{*}$.

Standard model gauge coupling unification

We study the low energy evolution of coupling constants of the standard model and show that gauge coupling unification can be achieved at the electroweak scale with a suitable normalization. We choose the grand unification group to be the semidirect product of Spin(8) by S 3. In this case the three low energy gauge couplings and the two scalar self-couplings are determined in terms of two independent parameters. In particular, it gives a precise prediction for the mass of the Higgs boson.

Efficient measurement of the sign and the magnitude of long‐range proton‐carbon coupling constants from a spin‐state‐selective HSQMBC‐COSY experiment

A spin state-selective Heteronuclear Single-Quantum Multiple-Bond Connectivities (HSQMBC-COSY) experiment is proposed to measure the sign and the magnitude of long-range proton-carbon coupling constants ((n)J(CH); n > 1) either for protonated or for non-protonated carbons in small molecules. The simple substitution of the selective 180° (1)H pulse in the original selHSQMBC pulse scheme by a hard one allows the simultaneous evolution of both proton-proton and proton-carbon coupling constants during the refocusing period and enables a final COSY transfer between coupled protons. The successful implementation of the IPAP principle leads to separate mixed-phase α/β cross-peaks from which (n)J(CH) values can be easily measured by analyzing their relative frequency displacements in the detected dimension.

Compression behaviors of distorted rutile-type hydrous phases, MOOH (M = Ga, In, Cr) and CrOOD

X-ray diffraction measurements of distorted rutile-type oxyhydroxides β-GaOOH, InOOH, β-CrOOH, and β-CrOOD were taken at a maximum pressure of up to 35 GPa under quasi-hydrostatic conditions, at ambient temperature. Anomalies in the evolution of the relative lattice constants and the axial ratios of β-GaOOH, InOOH, and β-CrOOD suggest anisotropic stiffening along the a- and/or b-axes where the hydrogen bond is formed. The changes were observed at 15 GPa in β-GaOOH and InOOH and at 4 GPa in β-CrOOD. The pressures were higher in oxyhydroxides that have longer O…O distances of the hydrogen bond at ambient pressure. In contrast, such stiffening behavior was not observed in CrOOH, which has a significant short O…O distance and strong hydrogen bond. The stiffening behaviors observed in the present study can be attributed to the symmetrization of the hydrogen bonds in oxyhydroxides, as was previously found in δ-AlOOH(D).

Neutron Larmor Diffraction study of the BaM2(XO4)2 (M = Co, Ni; X = As, P) compounds

Neutron Larmor Diffraction is a relatively new concept in neutron scattering which allows high resolution studies of subtle structural features in condensed matter. Several setups exist in various neutron facilities around the world. One of them is the Zero-field spin-Echo for Triple-Axis option (ZETA), available on the thermal triple-axis spectrometer IN22 (CEA-CRG beamline at Institut Laue Langevin). We have used ZETA to study thermal evolution of lattice constants in serveral systems of physical interest. Among them, the BaM2(XO4)2 family is a serie of compounds which have been intensively studied for three decades, mainly because of their fundamental though puzzling magnetic properties. In this paper, we aim to shed a new light on the physical understanding of those systems by investigating the link between structural and magnetic ordering. The magnetoelastic effect is found to be weak but can be resolved thanks to the high resolution provided by NLD.

Modelling of an EGSB treating sugarcane vinasse using first-order variable kinetics

An expanded granular sludge bed (EGSB) anaerobic reactor treating sugar cane vinasse was modelled using a simple model with two steps (acidogenesis and methanogenesis), two populations, two substrates and completely mixed conditions. A first-order kinetic equation for both steps with time-variant kinetic coefficients was used. An observer system was used to estimate the evolution of kinetic constants over time. The model was validated by comparing methane flow predictions with experimental values. An estimation of evolution of populations of microorganisms was also performed. This approach allows calculation of specific kinetic constants that reflect biological activity of microorganisms. Variation of specific kinetic constants reflects the influence of the fraction of raw vinasse in the feed. High salt concentrations in the reactor may have inhibited the process.

An elasto-plastic damage constitutive theory and its prediction of evolution of subsequent yield surfaces and elastic constants

Based on pair functional potentials, Cauchy–Born rule and slip mechanism, a material model assembling with spring-bundle components, a cubage component and slip components is established to describe the elasto-plastic damage constitutive relation under finite deformation. The expansion/shrink, translation and distortion of yield surfaces can be calculated based on the hardening rule and Bauschinger effect defined on the slip component level. Both kinematic and isotropic hardening are included. Numerical simulations and predictions under tension, torsion, and combined tension–torsion proportional/non-proportional loading are performed to obtain the evolution of subsequent yield surfaces and elastic constants and compare with two sets of experimental data in literature, one for a very low work hardening aluminum alloy Al 6061-T6511, and another for a very high work hardening aluminum alloy annealed 1100 Al. The feature of the yield surface in shape change, which presents a sharp front accompanied by a blunt rear under proportional loading, is described by the latent hardening and Bauschinger effect of slip components. Further, the evolution law of subsequent yield surfaces under different proportional loading paths is investigated in terms of their equivalence. The numerical simulations under non-proportional loading conditions for annealed 1100 Al are performed, and the subsequent yield surfaces exhibit mixed cross effect because the kinematic hardening and isotropic hardening follow different evolution tendency when loading path changes. The results of non-proportional loading demonstrate that the present model has the ability to address the issue of complex loading due to the introduction of state variables on slip components. Moreover, as an elasto-plastic damage constitutive model, the present model can also reflect the variation of elastic constants through damage defined on the spring-bundle components.

Superconductivity induced by doping Rh in CaFe2−xRhxAs2

In this paper, we report the synthesis of iron-based superconductors CaFe2−xRhxAs2 using a one-step solid state reaction method that crystallizes in the ThCr2Si2-type structure with a space group I4/mmm. The systematic evolution of the lattice constants demonstrates that the Fe ions are successfully replaced by the Rh. By increasing the doping content of Rh, the spin–density–wave (SDW) transition in the parent compound is suppressed and superconductivity emerges. The maximum superconducting transition temperature is found at 18.5 K with a doping level of x=0.15. The temperature dependence of dc magnetization confirms superconducting transitions at around 15 K. The general phase diagram was obtained and found to be similar to the case of the Rh-doping Sr122 system. Our results explicitly demonstrate the feasibility of inducing superconductivity in Ca122 compounds by higher d-orbital electron doping; however, different Rh-doping effects between FeAs122 compounds and FeAs1111 systems still remains an open question.