Mean Atomic Weight Research Articles

DFT based computational investigations of the physical properties of perovskites-structure SrXO3 (X = Si, Tb, Th) for optoelectronic and thermo-mechanical applications

Density Functional Theory (DFT) method is employed for investigating the structural, elastic, electronic, optical, and thermal properties of Perovskite structure SrXO3 (X = Si, Tb, and Th). The studied lattice parameters show good agreement with the previous theoretical and experimental results. The elastic stiffness constants, (Cij) of SrSiO3, SrTbO3, and SrThO3 satisfied the born stability criteria which confirm the mechanical stability of these phases. It is observed that when Tb or Th is replaced with Si, the brittleness behaviours of SrTbO3 or SrThO3 change to ductility, which improves its machinability for industrial applications. The investigation of anisotropic factors of SrSiO3, SrTbO3, and SrThO3 ensured that SrSiO3 is more anisotropic than SrTbO3 and SrThO3. By evaluating the electronic properties, it can be advantageous for applying the topological insulators (TI), solar cells, or PV (Photovoltaic) cell fabrication. Furthermore, the optical properties are studied and detail discussed with SrXO3 (X = Si, Tb, and Th). The reason of low Debye temperature (ΘD) and low minimum thermal conductivity (Kmin) has been perfectly explicated through the consideration of the mean atomic weight (M/n) of the compounds. Temperature-dependent of heat capacities (Cv, Cp) and linear thermal expansion coefficient (α) are also estimated using the quasi-harmonic Debye model and discussed. The SrTbO3 and SrThO3 compounds can be employed as potential thermal barrier coating (TBC) materials for high-temperature applications, according to the low values of Kmin, ΘD and α.

Arepo white dwarf merger simulations resulting in edge-lit detonation and run-away hypervelocity companion

ABSTRACT We present a series of high-resolution simulations generated with the moving-mesh code arepo to model the merger of a 1.1-M⊙ carbon-oxygen primary white dwarf (WD) with an outer helium layer and a 0.35-M⊙ secondary helium WD. Our simulations lead to detonations that are consistent with the edge-lit scenario, where a helium detonation is ignited at the base of the helium layer of the primary WD, which triggers an off-centre carbon detonation. This produces an asymmetric ejecta pattern and differences in line-of-sight observables (e.g. mean atomic weight). The ejecta that are flung into space are dominated by 56Ni, 4He, 28Si, and 32S. Our simulations result in a surviving degenerate companion of mass 0.22–0.25 M⊙ moving at >1700 $\mathrm{km}\, \mathrm{s}^{-1}$, consistent with the observational findings of hypervelocity WDs. The secondary’s surface layers are enriched by heavy metals, with 56Ni making up approximately 0.8 per cent of the remaining mass. We also analyse the sensitivity of the outcome on simulation parameters, including the ‘inspiral time’, which defines a period of accelerated angular momentum loss. We find that the choice of ‘inspiral time’ qualitatively influences the simulation result, including the survival of the secondary. We argue that the shorter inspiral cases result in qualitatively and quantitatively similar outcomes. We also investigate the sensitivity of our results on the primary’s chemical profile by comparing simulations using isothermal, constant composition models with the same mass and central composition and characterized by either a bare carbon-oxygen core (no helium) or a carbon-oxygen core enveloped by a thick helium layer.

Influence of Se doping on recently synthesized NaInS2-xSex solid solutions for potential thermo-mechanical applications studied via first-principles method

In the present work, the structural and hitherto unexplored thermal and mechanical properties of NaInS2-xSex (x = 0, 0.5, 1.0, 1.5 and 2.0) compounds have been studied using the density functional theory. Besides, the elastic anisotropy indices and hardnesses of NaInS2-xSex have been investigated as Se content is varied. The mechanical stability of all the compounds under study has been confirmed. The ratio of shear to bulk modulus (G/B) is low suggesting that the NaInS2-xSex (x = 0.5 and 1.5) compounds exhibit damage tolerant (ductility) properties while rest of the compositions are brittle in nature. The predicted hardness (H) values are also influenced with the Se content in the following order: H (NaInSSe) >H (NaInS2) >H (NaInSe2) >H (NaInS1.5Se0.5) >H (NaInS0.5Se1.5). All the anisotropic indices under study indicate that NaInS2-xSex compounds are anisotropic in nature. The Mulliken bond population analysis suggests that the degree of covalency of In-S/Se bonds decreases when S is substituted by Se. The origin of low Debye temperature (ΘD) and low minimum thermal conductivity (Kmin) have been successfully explained by considering the mean atomic weight (M/n) and average bond strength of the compounds. Temperature dependence of heat capacities (Cv, Cp) and linear thermal expansion coefficient (α) are also estimated using the quasi-harmonic Debye model and discussed. The low values of Kmin, ΘD and α and damage tolerant behavior clearly indicate that the NaInS2-xSex (x = 0.5 and 1.5) compounds can be used as promising thermal barrier coating materials for high temperature applications.

Solar slow magneto-acoustic-gravity waves: an erratum correction and a revisited scenario

ABSTRACT Slow waves are commonly observed on the entire solar atmosphere. Assuming a thin flux tube approximation, the cut-off periods of slow-mode magneto-acoustic-gravity waves that travel from the photosphere to the corona were obtained in Costa et al. In that paper, however, a typo in the specific heat coefficient at constant pressure cp value led to an inconsistency in the cut-off calculation, which is only significant at the transition region. Due to the abrupt temperature change in the region, a change of the mean atomic weight (by a factor of approximately 2) also occurs, but is often overlooked in analytical models for simplicity purposes. In this paper, we revisit the calculation of the cut-off periods of magneto-acoustic-gravity waves in Costa et al. by considering an atmosphere in hydrostatic equilibrium with a temperature profile, with the inclusion of the variation of the mean atomic weight and the correction of the inconsistency aforementioned. In addition, we measure the dominant periods near a particular active region (AR 1243) as observed by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamic Observatory (SDO) on 2011 July 3 and compare them to our analytical results. The cut-off periods obtained analytically are consistent with the corresponding periods measured in observations.

Structural, electronic and optical properties of bandgap-tunable Cu2ZnSixSn1−xSe4 alloys

Density functional theory calculations are performed to investigate the structural, electronic and optical properties of Cu2ZnSnSe4 alloys. The lattice constants linearly decrease with the increment of the Si concentration and the mean atomic weight. The Cu2ZnSixSn1−xSe4 alloys are miscible with low formation enthalpies. A tunable bandgap is obtained in a wide energy range (1.06–2.26 eV) and primarily due to the upward shift of the conduction band minimum. The alloy Cu2ZnSi0.5Sn0.5Se4 has an optimal bandgap (1.49 eV) and a higher optical absorption coefficient than that of the well-studied CuZnSnS4. The bandgap-tunable Cu2ZnSixSn1−xSe4 alloys can be promising for their photovoltaic applications.

The effects of La2O3 on the early stages of crystallization for MgO-Al2O3-SiO2-TiO2-La2O3 glass-ceramics

The early stages of crystallization for MgO-Al2O3-SiO2-TiO2-La2O3 glasses with different La2O3 concentrations were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The glass transition temperature (Tg) of the glass decreases at first and then increases again with increasing La2O3 concentration. This indicates that the structure of the glass becomes weaker at first and then stronger again. Lanthanum acts in glasses as network modifier and will usually decrease the network connectivity of the glass structure. Nevertheless, if the La2O3 concentration is high enough, the oxygen and other ions start to agglomerate around La, resulting in a more closely packed structure. Heat-treatment of the sample with x = 0.1 at 770–810 °C results in the precipitation of a droplet phase with higher mean atomic weight embedded in a matrix with lower mean atomic weight. The initial crystalline phase magnesium aluminum titanate (MAT) precipitates from the droplet phase. Nevertheless, for the sample with x = 0.4, dendrite-like structure could be observed after heat-treatment of the glass at 810 °C. Furthermore, the crystalline phase first precipitated is the lanthanum containing perrierite, which could be attributed to the rearrangement of the glass structure as an effect of La3+ incorporation.

On the Relationship between Structural-Elastic Properties of Co-Zn Ferrites at 300 K

The structural - elastic properties correlations have been studied for polycrystalline spinel ferrite system, ZnxCo1-xFe2O4, x = 0.0-0.6, at 300 K. The cation distribution formulae determined from X-ray diffraction line intensity calculations are used to calculate bulk modulus (Ko) in particular and Young′s modulus (E0), rigidity modulus (G0), longitudinal modulus (L0) and Lame′s constant (λL0) in general. The longitudinal wave velocity (Vl0) and transverse wave velocity (Vso) computed from empirical relation based on X-ray density and mean atomic weight is used to calculate L0 and G0 respectively. The applicability of the heterogeneous metal mixture rule for theoretical estimation of elastic constants has been tested. The results are compared with elastic moduli determined from conventional ultrasonic pulse transmission technique and causes for the observed difference between the two have been discussed.

Anomalous Seismic Velocity Drop in Iron and Biotite Rich Amphibolite to Granulite Facies Transitional Rocks from Deccan Volcanic Covered 1993 Killari Earthquake Region, Maharashtra (India): a Case Study

65 Ma Deccan Volcanic Province of western India forms one of the largest flood basaltic eruptions on the surface of the earth. The nature of the concealed crust below this earthquake prone region, which is marked by several low velocity zones at different depths has hardly been understood. These low velocity zones have been invariably interpreted as fluid-filled zones, genetically connected to earthquake nucleation. While carrying out detailed geological and petrophysical studies on the Late Archean basement cores, obtained from a 617 m deep KLR-1 borehole, drilled in the epicentral zone of 1993 Killari earthquake region of the southern Deccan Volcanic Province, we came across several instances where we observed remarkable drop in measured P-wave velocity in a number of high density cores. We provide detailed petrographic and geological data on 11 such anomalous samples which belong to mid-crustal amphibolite to granulite facies transitional rocks. They are associated with a mean P-wave velocity of 6.02 km/s (range 5.82–6.22 km/s) conforming to granitic upper crust, but in contrast have a high mean density of 2.91 g/cm3 (range 2.75–3.08 g/cm3), which characterise mid to lower crust. This velocity drop, which is as much as 15 % in some cores, is primarily attributed to FeOT enrichment (up to about 23 wt%) during the course of mantle-fluid driven retrogressive metasomatic reactions, caused by exhumation of deep-seated mafic rocks. Presence of Iron content (mainly magnetite), widely seen as opaques in thin sections of the rocks, seems to have resulted into sharp increase in density, as well as mean atomic weight. Our study indicates that the measured Vp is inversely related to FeOT content as well as mean atomic weight of the rock.

The hard sphere view of the outer core

The hard sphere model for liquids attempts to capture the physical behavior of a real liquid in a simple conceptual model: a fluid of fixed size spheres that only interact repulsively when they come into contact. Is the model good enough to use for modeling internal planetary structure? To answer this question, I survey variants of hard sphere liquid theory by applying them to the Earth’s outer core to determine which of them explains wavespeeds in the outer core best. The variants explored here are the Carnahan-Starling hard sphere model, the Mansoori-Canfield extension to hard sphere mixtures, the transition metal hard sphere liquid, and the Lennard-Jones hard sphere liquid with attractive forces. With an empirical addition of a temperature dependence to the liquid’s hard sphere diameter, all of the variants explored can replicate wavespeeds in most of the radius range of the outer core. The hard sphere model for liquid transition metals explains the wavespeed best because it yields a mean liquid atomic weight of 48.8 g mol −1 at 10 wt% light element abundance in the core which is in good cosmochemical agreement with core light element models. Other variants also fit core wavespeeds but require implausibly low liquid mean atomic weight implying excessive incorporation of hydrogen or helium in the core. Applied to the detailed wavespeed structure of the Earth’s outermost outer core, the model suggests that the mean atomic weight could be reduced by up to 1.74% or the temperature could be increased by up to 400 K relative to an adiabatic profile, or there could be 8% fewer valence electrons in the liquid.

Fabrication and characterization of (Mg1−xFex)O (0.05≤x≤0.25) ceramics

Highly dense (Mg1−xFex)O (0.05≤x≤0.25) ceramics (relative density >98%) were prepared by ultrasonic assisted oxalate co-precipitation method together with spark plasma sintering. Mechanical measurements indicate that Young’s modulus decreases monotonously from 312 to 268.5±5.2GPa with increasing Fe content x from 0 to 0.25. In contrast, the Vickers hardness first increases from 6.8 to 8.50±0.11GPa as x increases from 0 to 0.1, and then decreases monotonously with further increasing x. The increase and decrease in the hardness with x can be mainly attributed to the solid solution strengthening and weakening of Peierls–Nabarro stress, respectively. Thermal analyses reveal that heat capacity Cp decreases (from 0.896±0.018 to 0.816±0.016(JK)−1g−1 at 323K, for instance) with increasing x (from 0.05 to 0.25) due to the lower Cp of FeO. Similarly, thermal conductivity κ of the samples decreases (say, from 24.8±1.0 to 3.7±0.1(WK)−1m−1 at 323K) with the increasing x (from 0.05 to 0.25), which could be ascribed to the decreasing melting point and increasing mean atomic weight resulting from substitution of Mg for Fe.

Fluorescence properties of Eu3+-doped alumino silicate glasses

Alumino silicate glasses of a very broad range of molar compositions doped with 1⋅1020 Eu3+ cm−3 (about 0.2mol% Eu2O3) were prepared. As network modifier oxides Li2O, Na2O, K2O, MgO, CaO, SrO, BaO, ZnO, PbO, Y2O3 and La2O3 have been used. All glasses show relatively broad fluorescence excitation and emission spectra. For most glasses only a weak effect of the glass composition on the excitation and emission spectra is observed. Although the glasses should be structurally similar, notable differences are found for the fluorescence lifetimes. These increase steadily with decreasing mean atomic weight, decreasing refractive index and decreasing optical basicity of the glasses, which may be explained by local field effects. An exception from this rule are the strontium, barium and potassium containing glasses, which show significantly increased fluorescence lifetimes despite of their high refractive index, optical basicity and molecular weight. The non mono-exponential fluorescence decay curves as well as the fluorescence spectra indicate a massive change in the local surroundings of the doped rare earth ions for these glasses.

Effect of Sb and Sn additives on the activation energies of glass transition and crystallization in binary Se85Te15 alloy

Calorimetric measurements have been performed in glassy Se75Te15M10 (M=Sb, Sn) alloys to study the effect of Sb and Sn additives on the kinetics of glass transition and crystallisation in glassy Se85Te15 alloy. Activation energy of glass transition (E g) and the activation energy of crystallisation (E c) have been determined using different non-isothermal methods. An excellent agreement is observed in the values of E c and E g for all the non-isothermal methods used in the present study. The composition dependence of the activation energies of glass transition and crystallisation processes shows that the value of E g is higher in case of Sn additive while the value of E c is higher in case of Sb additive. The results are explained in terms of Pauling's concept of average heat of atomisation and mean atomic weights of ternary alloys, respectively.

Materials design for Cu-based quaternary compounds derived from chalcopyrite-rule

The stability of the Cu 2–II–IV–VI 4 and Cu–III–IV–VI 4 compounds (II=Zn, Cd; III=Ga, In; IV=Si, Ge, Sn; VI=S, Se, Te) as tetrahedral coordinated structures has been investigated by means of the electronegativity combination analysis of compositional elements. As compared with the results of the products synthesized from respective elemental mixtures, the possibility of crystallization in the form of tetrahedral coordinate compounds and the crystal structure are found to be expected by this semi-theoretical analysis. These melting points and lattice constants determined from the results of differential thermal analysis and powder X-ray diffraction, respectively, are found to vary linearly with increasing of mean atomic weight, and can be established from the empirical equations.

Structural, thermodynamical and optical properties of Cu2-II-IV-VI4 quaternary compounds

The Cu2-II-IV-VI4 compounds (II = Zn, Cd; IV = Si, Ge, Sn; VI = S, Se, Te) with a tetrahedral coordinated structure have been investigated, as compared with the results of the products synthesized from respective elemental mixtures. These crystal structures and melting points are determined, using differential thermal analysis and powder X-ray diffraction, respectively. The optical band gaps of these balk crystals grown by the horizontal gradient freezing or Iodine transport method are also measured by optical absorption. These melting points, lattice constants and band gaps are found to vary linearly with increasing of mean atomic weight, and can be established from the empirical equations.

Current–Voltage Characteristics of TlInX 2 –TlSmX 2 (X = S, Se, Te) Alloys

The steady-state current–voltage characteristics of TlInX2–TlSmX2 alloys were measured as a function of sample composition and thickness. The results demonstrate that the alloys studied exhibit threshold switching. The threshold voltage decreases with increasing mean atomic weight and can be stabilized by heat treatment. The most stable current–voltage characteristics are obtained for thin-film samples.

Elasticity of six polycrystalline silicate garnets at pressure up to 3.0 GPa

The elasticities of six polycrystalline silicate garnets (almandine, grossular, pyrope, uvarovite, andradite, and Prp 25 Alm 56 Spe 19 ) have been experimentally studied at pressures up to 3.0 GPa using a phase comparison method with an ultrasonic interferometer in a liquid cell piston-cylinder apparatus. Complete elasticity data sets (P- and S-wave velocities, bulk moduli K s , shear moduli G, and their first pressure derivatives K S ′ and G ′) have been obtained for all six garnets, and are used together with an up-to-date compilation of garnet elasticity data to examine composition-elasticity systematics of garnets. Our results suggest that pyralspite and ugrandite have different relationships between bulk sound velocity (V ϕ ) and mean atomic weight (M 0 ), between Poisson’s ratio (σ) and density (ρ), and between G / K s and K s /ρ ratios. A large error may occur when the systematics are applied across different garnet groups.

Raman Scattering Studies of CuInS2 Films Grown by RF Ion Plating

CuInS2 films were grown by rf ion plating and their crystallinity was characterized by Raman spectroscopy. Six Raman peaks were observed, most of which were assigned to the phonon modes of CuInS2, except for a peak at 307 cm-1 that was clearly observed in the films with a poor crystalline quality. The peak at 307 cm-1 was assigned to the totally symmetric mode by polarized Raman measurement. This peak was related to some kind of a localized mode with a smaller mean atomic weight of cations.

Mean atomic number and backscattered electron coefficient calculations for some materials with low mean atomic number

AbstractIn modelling electron backscattering from solids using Monte Carlo simulations, knowledge of mean atomic number, mean atomic weight, and density of the bulk material are required. We studied four different ways in common useforthe calculation of mean atomic number. An alternative and improved approach is to calculate the mean backscattered electron (BSE) coefficient, η, from a knowledge of the elemental composition and values of η for the elements. Again, we studied a number of formulae suggested for this averaging process. As it is not possible to measure η directly for a number of elements, the method used to interpolate between elements with known η was also examined. In addition, we obtained experimental backscattering relationships for topography‐free samples of poly (methylmethacrylate) (PMMA), carbon, aluminium, and a series of novel halogenated resins, all solids with relatively low mean atomic numbers, and calcified tissues. None of the methods for determining mean atomic number placed the materials of interest in the correct sequence of their experimentally determined BSE peaks: the data differed widely between the individual methods. The averaged BSE coefficient calculated by the Castaing formula gave the best agreement with the experimentally derived data.

In situ studies of ion irradiated zirconolite, pyrochlore and perovskite

The relative radiation resistance of the structures of zirconolite, pyrochlore and perovksite were investigated by in situ transmission electron microscopy using 1.5 MeV Kr + ions in the HVEM-Tandem User Facility, at Argonne National Laboratory. A suite of six fabricated zirconolites, one fabricated pyrochlore and one natural perovskite was used. Damage accumulation essentially occurs in the same way in all three phases and is revealed by the following changes in SAD patterns: weakening of superlattice Bragg diffraction maxima, appearance of diffuse rings which increase in intensity with dose, disappearance of superlattice or other specific classes of maxima, and disappearance of remaining sublattice maxima leaving only diffuse rings. The average critical doses for amorphisation ( D c) for all the zirconolites (undoped, Nd-doped, U-doped and Th-doped) and the pyrochlore in this study varied by a factor of ∼ 2 (from 3.5 to 6.1 × 10 18 ions m −2). No correlations were observed between D c and the atomic weight of dopants in zirconolite or the mean atomic weight of the sample. The D c value at room temperature of perovksite was found to be 1.8 × 10 19 ions m −2, 3–5 times the D c values for zirconolite. This observation is in line with what one expects from the topologic and chemical complexity of the two phases and is compared with the results of previous authors. Preliminary assessment was made of EELS as a tool for monitoring radiation damage.

Thermoelasticity of (Mg, Fe)SiO 3 perovskite and a comparison with the lower mantle

A re-examination of the classical theory of the thermal Grüneisen parameter, γ, for the case of a polyatomic anisotropic crystal yields a zero pressure value for silicate perovskite at high temperature, γ 0 = 1.30 ± 0.09. Corresponding values of volume expansion coefficient are α(290K) = 14.0 × 10 −6 K −1, α(1000 K) = 23.8 × 10 −6 K −1, α(2000 K) = 26.8 × 10 −6 K −1. These are used to calculate densities at high temperatures for comparison with the lower mantle. For this purpose lower mantle properties are extrapolated to the foot of the adiabat (temperature T 0) using the dK dP vs P K equation-of-stater, with the following results: K 0 = 204.5 ± 1.3 GPa, μ 0 = 129.0 ± 1.0 GPa, ϱ 0 = 3973 ± 6 kg m −3. Matching both density and incompressibility for a perovskite-magnesiowustite mixture to these values, it is shown that both minerals must have appreciable iron contents and that a substantial magnesiowustite fraction is required. The favoured solution requires about 6% Fe in the perovskite, 15% Fe in the magnesiowustite and a mineral mix with 28% magnesiowustite by volume (26.7% by mass), giving a mean atomic weight for the mixture of 20.97. Although the analysis is simplistic in the sense that additional minor elements and minerals must be present in the lower mantle, it gives no encouragement to the supposition that the lower mantle is enriched in either iron or silicon.