Theoretical Bulk Research Articles

Synthesis and characterization of single-phase epitaxial Cr2N thin films by reactive magnetron sputtering

Cr2N is commonly found as a minority phase or inclusion in stainless steel, CrN-based hard coatings, etc. However, studies on phase-pure material for characterization of fundamental properties are limited. Here, Cr2N thin films were deposited by reactive magnetron sputtering onto (0001) sapphire substrates. X-ray diffraction and pole figure texture analysis show Cr2N (0001) epitaxial growth. Scanning electron microscopy imaging shows a smooth surface, while transmission electron microscopy and X-ray reflectivity show a uniform and dense film with a density of 6.6 g cm−3, which is comparable to theoretical bulk values. Annealing the films in air at 400 °C for 96 h shows little signs of oxidation. Nano-indentation shows an elastic–plastic behavior with H = 18.9 GPa and Er = 265 GPa. The moderate thermal conductivity is 12 W m−1 K−1, and the electrical resistivity is 70 μΩ cm. This combination of properties means that Cr2N may be of interest in applications such as protective coatings, diffusion barriers, capping layers and contact materials.

Innovative Reserve Miniature Cylindrical Battery Based on Li-FeS2 System

In this paper, natural pyrite (FeS2) was used as the cathode material of a reserve small-sized cylindrical battery Li - FeS2. Li-FeS2 cylindrical reserve battery size: h = 55 mm; d = 6.3 mm.The theoretical specific discharge capacity of FeS2 is 890 mA·h/g. and the theoretical bulk energy is 6675 W·h/cm3 For comparison for sulfur electrode the theoretical bulk energy is 6400 W·h/cm3 As the anode, for this Li - FeS2 battery a lithium foil was used, the surface of which was modified with a LiAl alloy. An innovative method of electrochemical activation for increasing the specific energy of battery was developed. After battery activation the discharge voltage of the reserve battery rises, and as a result, the battery energy rises by about 30 %. The surface modification of the lithium anode with the LiAl alloy allows to decreased in 3 times complex impedance of the system at a temperature of + 25 °C, in 5-8 times the complex impedance of the system during long-term storage at temperature of + 60 °C and avoid surface deformation of the lithium electrode during long-term storage. After battery activation the battery has very low self - discharge. The developed reserve battery can be operated at the temperature range from plus 60 oC to minus 25 oC.

Liquid jets produced by an immersed electrical explosion in round tubes

Liquid jets produced by an electrical explosion in water in round, centimeter-wide tubes are investigated experimentally and theoretically. Several jet flow patterns including sharp conical forms and annular forms are observed through high-speed photography. The general features of these patterns are presented and examined herein. It is found that the jet pattern is influenced mainly by the tube diameter, with narrow tubes producing conical jets and wide tubes producing annular jets. The jet maintains a nearly constant velocity, which increases with the explosion energy and decreases with the tube diameter and the standoff distance. A quasi-one-dimensional theoretical model that includes the bubble dynamics is proposed. With the model proposed herein, the effective explosion energy is calibrated first by matching the maximum bubble size. The results show that the confinement imposed by the tube tends to reduce the effective energy and that tubes with smaller diameters collect less effective energy. A theoretical analysis of the jet velocity indicates that the jet has to overcome a threshold energy level to obtain a constant velocity and that in narrower tubes, the jet velocity is amplified. The amplification factor, which is defined as the ratio of the actual jet velocity to the theoretical maximum bulk velocity, generally decreases with the tube diameter. It approaches unity when the jet is annular or blunt, indicating that the kinematic focusing effect with this type of jet is negligible. A parametric analysis of the calibrated explosion energy and the amplification factor provides a relation that comprehensively represents the experimental data. The relation is found to be different from those obtained in previous investigations, and the reasons for this difference are discussed.

Insertion of lithium ion in anatase TiO 2 nanotube arrays of different morphology

Abstract Anatase TiO 2 nanotube arrays of different morphology were prepared by a two-step process: anodic oxidation at voltages 20–60 V and subsequent annealing at 400 °C. By amplifying anodization voltage the inner diameter of nanotubes increased. At 60 V nanotubes changed the shape from cylindrical tube to truncated cone with elliptical opening. Electrochemical insertion of Li-ion in nanotubes was studied by cyclic voltammetry and galvanostatic charge-discharge experiments. The cyclovoltammetric response was fast for all nanotube arrays. The galvanostatic areal charge/discharge capacity of nanotube arrays increased with increasing anodizaton voltage. Although the mass of nanotubes prepared at 45 V was larger, the gravimetrical capacity was much higher for nanotubes prepared at 60 V because of the larger surface area exposed to the electrolyte. Gravimetrical capacity values exceed theoretical bulk capacity of anatase due to the surface storage of Li-ion. Diffusion coefficient of Li-ion was calculated to be between 5.9·10 −16 and 5.9·10 −15 cm 2 s −1 .

Tunable optical properties of some rare earth elements-doped mayenite Ca12Al14O33 nanopowders elaborated by oxalate precursor route

Rare earth (RE) ions-doped mayenite Ca12Al14−xRExO33 nanopowders (where RE = La and Gd and x = 0–1.0) were synthesized using the oxalate precursor technique. The as-prepared precursors were calcined at 800 °C for 2 h. Obviously, all RE-doped Ca12Al14−xRExO33 possessed a well-crystalline cubic mayenite phase till RE content of 0.8. The crystallo-chemical aspects including crystallite size, lattice parameters, theoretical X-ray density and bulk density were robustly on RE nature and ratio. The microstructure and the average grain size were significantly influenced by the RE kind and content. The high transparency of Ca12Al14−xRExO33 over 80% was found to be evinced in the visible wavelength range of 400–800 nm. Besides, the incorporation of RE cation minimized the direct band gap energy from 4.42 eV for pure mayenite to 3.85 and 3.59 eV with x value 1.0 of La3+ and Gd3+ ions. The photoluminescence spectra of pure mayenite nanoparticles showed that the band edge emission (λexc = 248 nm) with an intense visible emission band at 360 nm was detected. Otherwise, the band edge emission showed a slight shift toward short wavelength due to the substitution Al3+ by RE3+ ions. Such results open a new avenue for application of mayenite as a good candidate for transparent low-temperature electron conductor for optoelectronics applications.

T-C8B2N2: A potential superhard material

A potential superhard material C8B2N2 with I-4m2 space group is found and confirmed to be stable with first-principles calculations. The results show that its structure is highly incompressible with bulk modulus of 383.4GPa and shear modulus of 383.0GPa. It shows that this material is nearly isotropy with universal anisotropy index of 0.056, and its fractional anisotropy ratio of shear modulus and bulk modulus are 0.0055 and 0.0, respectively. Interestingly, its theoretical bulk modulus, shear modules, Young’s modulus, Poisson’s ratio and Vickers hardness are almost same to those of well-known superhard material c-BN.

ScVO4 under non-hydrostatic compression: a new metastable polymorph

The high-pressure (HP) behaviour of scandium vanadate (ScVO4) is investigated under non-hydrostatic compression. The compound is studied by means of synchrotron-based powder x-ray diffraction (XRD) and optical-absorption techniques. The occurrence of a non-reversible phase transition is detected. The transition is from the zircon structure to the fergusonite-type structure and takes place around 6 GPa with nearly 10% volume discontinuity. XRD measurements on the pressure cycled sample confirm for the first time that the fergusonite-type ScVO4 can be recovered as the metastable phase at ambient conditions. Raman spectroscopic measurements verify the metastable phase to be of a fergusonite-type phase. Theoretical calculations also corroborate the experimental findings. The fergusonite phase is found to be stiffer than the ambient-pressure zircon phase, as indicated by the observed experimental and theoretical bulk moduli. The optical properties and lattice-dynamics calculation of the fergusonite ScVO4 are discussed. At ambient pressure the band gap of the zircon (fergusonite)-type ScVO4 is 2.75 eV (2.3 eV). This fact suggests that the novel metastable polymorph of ScVO4 can have applications in green technologies; for instance, it can be used as photocatalytic material for hydrogen production by water splitting.

The low-loss EELS spectra from radiation damaged gold nanoparticles

The advantage of the EELS spectra in transmission electron microscopy is the ability to obtain plasmon information of the structure of individual nanoparticles. The EELS analyses of the plasmonic information of gold nanoparticles were obtained before and after irradiation by the convergent beam electron diffraction (CBED) technique. The results indicate that before irradiating the gold nanoparticles (35 nm in diameter) by the CBED technique, only the EELS spectrum of carbon is obtained. Nevertheless, the EELS spectra of gold were obtained after CBED irradiation of the nanoparticles. The spectrum of gold shows five plasmonic modes at 15.9, 24.8, 32.8, 45.4, and 63.2 eV, but theoretical surface and bulk plasmons of gold are at 6.378 and 9.020, respectively. When compared with the experimental plasmon peaks, we determined that peaks at energies above 9.026 eV are multiples of the surface and bulk (valence) plasmons of the gold nanoparticles.

Elastic anisotropy of novel YAlC from an ab initio study

First principles calculations are performed to investigate the structural, elastic and electronic properties of orthorhombic YAlC, the first aluminide carbide with a polyacetylene-related geometry. The computed equilibrium structural parameters are consistent with the available experimental data. On the basis of the computed elastic constants, theoretical bulk modulus as well as shear modulus, the elastic anisotropy is evaluated in terms of shear anisotropic factors and directional-dependent bulk modulus and directional-dependent Young's modulus. In conjunction with the unique atomic arrangements, the density of states and electron density analysis reveal that the mixed character of the ionic, covalent and metallic chemical bonding among the ingredients is the main contribution to the remarkable elastic anisotropy.

Electronic structure of palladium in the presence of many-body effects

Including on-site electronic interactions described by the multi-orbital Hubbard model we study the correlation effects in the electronic structure of bulk palladium. We use a combined density functional and dynamical mean field theory, LDA+DMFT, based on the fluctuation exchange approximation. The agreement between the experimentally determined and the theoretical lattice constant and bulk modulus is improved when correlation effects are included. It is found that correlations modify the Fermi surface around the neck at the $L$-point while the Fermi surface tube structures show little correlation effects. At the same time we discuss the possibility of satellite formation in the high energy binding region. Spectral functions obtained within the LDA+DMFT and $GW$ methods are compared to discuss non-local correlation effects. For relatively weak interaction strength of the local Coulomb and exchange parameters spectra from LDA+DMFT shows no major difference in comparison to $GW$.

Multi-layered elastic shell metamaterial elements for improved transmission in sonic crystals

A metamaterial-based sonic crystal, comprised of a lattice arrangement of cylindrical elastic shells, can be tuned to provide wave steering or phononic band filtering. The effective acoustic properties of such sonic crystals are determined by the geometric and material properties of each individual shell and the lattice spacing between shells. By matching the effective acoustic properties to those of the surrounding fluid, the sonic crystal can be made acoustically transparent at low frequencies, leading to improved transmission over that of non-matched shells. Bi-layered shells provide additional design parameters to broaden the region of acoustic transparency provided by a simple shell. In this effort, three-layered shells are employed to improve transmission further. Transformation acoustics analysis is applied to determine theoretical density and bulk modulus distributions, and then real-world materials are selected which best match these theoretical distributions. Finally, shell geometry is optimized to minimize scattering while constraining the effective acoustic properties to match those of the external fluid.

Spin disorder in maghemite nanoparticles investigated using polarized neutrons and nuclear resonant scattering

The manuscript reports the investigation of spin disorder in maghemite nanoparticles of different shape by a combination of polarized small-angle neutron scattering (SANSPOL) and nuclear forward scattering (NFS) techniques. Both methods are sensitive to magnetization on the nanoscale. SANSPOL allows for investigation of the particle morphology and spatial magnetization distribution and NFS extends this nanoscale information to the atomic scale, namely the orientation of the hyperfine field experienced by the iron nuclei. The studied nanospheres and nanocubes with diameters of 7.4 nm and 10.6 nm, respectively, exhibit a significant spin disorder. This effect leads to a reduction of the magnetization to 44% and 58% of the theoretical maghemite bulk value, observed consistently by both techniques.

Kinetics of Oxygen Surface Exchange on Epitaxial Ruddlesden-Popper Phases and Correlations to First-Principles Descriptors.

Through alignment of theoretical modeling with experimental measurements of oxygen surface exchange kinetics on (001)-oriented La2-xSrxMO4+δ (M = Co, Ni, Cu) thin films, we demonstrate here the capability of the theoretical bulk O 2p-band centers to correlate with oxygen surface-exchange kinetics of the Ruddlesden-Popper oxide (RP214) (001)-oriented thin films. In addition, we demonstrate that the bulk O 2p-band centers can also correlate with the experimental activation energies for bulk oxygen transport and oxygen surface exchange of both the RP214 and the perovskite polycrystalline materials reported in the literature, indicating the effectiveness of the bulk O 2p-band centers in describing the associated energetics and kinetics. We propose that the opposite slopes of the bulk O 2p-band center correlations between the RP214 and the perovskite materials are due to the intrinsic mechanistic differences of their oxygen surface exchange kinetics and bulk anionic transport.

Structural and optical properties of nanocrystalline mayenite Ca12Al14O33 powders synthesized using a novel route

In this article, mayenite Ca12Al14O33 nanoparticles have been synthesized using oxalate precursor technique based on low cost starting materials for the first time. As a consequence, the effect of synthesis conditions on the crystal structure, crystallite size, microstructure, surface area SBET and optical properties was studied. The results confirmed that well crystalline cubic mayenite phase was formed at low annealing temperature 800 °C for 1 h. The crystallite size was strongly depending on the temperature and time. Completely, it was in the range from 70.3 to 131.5 nm. Besides, lattice constant and unit cell volume were decreased whereas theoretical bulk density was increased with increasing the temperature and time. Meanwhile, the microstructure of the formed mayenite sample annealed at 800 °C was exhibited as sliced- leaves shape which was transformed to platelet like structure with raising the temperature. Meanwhile, the N2 adsorption–desorption isotherm specific surface area (SBET) was 5.00 m2/g. Ultraviolet–visible-Near IR spectroscopy evinced that optical band gap energy using Kubelka–Munk function based on Tauc’s plot was found to decrease with evaluation temperature and time. Moreover, the room temperature photoluminescence spectra measurement at excitation band 248 nm of pure mayenite nanoparticles was displayed an intense emission band centred at 360 nm.

Structural, electronic and elastic properties of Y3AlC3, YAl3C3 and Y3AlC via first principles study

First principles calculations are preformed to investigate the structural, elastic and electronic properties of orthorhombic Y3AlC3, hexagonal YAl3C3 as well as cubic Y3AlC. The computed equilibrium structural parameters are consistent with the available experimental data. According to the theoretical bulk and shear modulus, the strength of hexagonal YAl3C3 is superior to orthorhombic Y3AlC3 and cubic Y3AlC. Density of states and electron density analysis unravel the existence of the directional, strong covalent bonding between Al and C atoms as the main contribution to the advantage. In addition, the elastic anisotropy is discussed by evaluating the elastic stiffness constants.

On the accuracy of commonly used density functional approximations in determining the elastic constants of insulators and semiconductors.

We have performed density functional calculations using a range of local and semi-local as well as hybrid density functional approximations of the structure and elastic constants of 18 semiconductors and insulators. We find that most of the approximations have a very small error in the lattice constants, of the order of 1%, while the errors in the elastic constants and bulk modulus are much larger, at about 10% or better. When comparing experimental and theoretical lattice constants and bulk modulus we have included zero-point phonon effects. These effects make the experimental reference lattice constants 0.019 Å smaller on average while making the bulk modulus 4.3 GPa stiffer on average. According to our study, the overall best performing density functional approximations for determining the structure and elastic properties are the PBEsol functional, the two hybrid density functionals PBE0 and HSE (Heyd, Scuseria, and Ernzerhof), as well as the AM05 functional.

Optical properties and weakening of elastic moduli with increasing glass transition temperature (T g) in (80–x)TeO2-xBaO-20ZnO glasses

Abstract BaO addition to ternary (80–x)TeO2-20ZnO-xBaO (x = 0–20 mol.%) glasses resulted in a decrease in ultrasonic velocities and independent elastic moduli; this result indicated that the rigidity of the glass network weakened possibly because non-bridging oxygen increased. Thermal analysis results showed that glass transition temperature increased as BaO content increased because of the stabilizing effect of Ba2+ on the glass network. Additional analyses using bulk compression and ring deformation models revealed that the ratio between theoretical bulk modulus and experimental bulk modulus increased; this result indicated that the compression mechanism mainly involved isotropic ring compression. Furthermore, the increase in non-bridging oxygen formation with BaO addition caused a decrease in optical energy gap and an increase in refractive index. An increase in Urbach energy indicated that the degree of disorder in the glass system also increased.

Structural, electronic, and elastic properties of CuFeS2: first-principles study

The structural, electronic, and elastic properties of CuFeS2 have been investigated by using the generalized gradient approximation (GGA), GGA + U (on-site Coulomb repulsion energy), the local density approximation (LDA), and the LDA + U approach in the frame of density functional theory. It is shown that when the GGA + U formalism is selected with a U value of 3 eV for the 3d state of Fe, the calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA + U calculations indicate that CuFeS2 is a semiconductor with a band gap of 0.552 eV and with a magnetic moment of 3.64 µB per Fe atom, which are well consistent with the experimental results. Combined with the density of states, the band structure characteristics of CuFeS2 have been analyzed and their origins have been specified, which reveals a hybridization existing between Fe-3d, Cu-3s, and S-3p, respectively. The charge and Mulliken population analyses indicate that CuFeS2 is a covalent crystal. Moreover, the calculated elastic constants prove that CuFeS2 is mechanically stable but anisotropic. The bulk modulus obtained from elastic constants is 87.1 GPa, which agrees well with the experimental value of 91 ± 15 GPa and better than the theoretical bulk modulus 74 GPa obtained from GGA method by Lazewski et al. The obtained shear modulus and Debye temperature are 21.0 GPa and 287 K, respectively, and the latter accords well with the available experimental value. It is expected that our work can provide useful information to further investigate CuFeS2 from both the experimental and theoretical sides.

A temperature microsensor for measuring laser-induced heating in gold nanorods.

Measuring temperature is an extensively explored field of analysis, but measuring a temperature change in a nanoparticle is a new challenge. Here, a microsensor is configured to measure temperature changes in gold nanorods in solution upon laser irradiation. The device consists of a silicon wafer coated with silicon nitride in which a microfabricated resistance temperature detector was embedded and attached to a digital multimeter. A polydimethylsiloxane mold served as a microcontainer for the sample attached on top of the silicon membrane. This enables laser irradiation of the gold nanorods and subsequent measurement of temperature changes. The results showed a temperature increase of 8 to 10°C and good correlation with theoretical calculations and bulk sample direct temperature measurements. These results demonstrate the suitability of this simple temperature microsensor for determining laser-induced heating profiles of metallic nanomaterials; such measurements will be essential for optimizing therapeutic and catalytic applications.

Correlation between acoustical and structural parameters in some oxide glasses

Correlation between acoustical and structural parameters has been carried out in TiO2–Na2O–CaO–P2O5, ZnO–PbO–B2O3 and BaO–TeO2 glass systems. The correlation is based on the recently presented Abd El-Moneim semi-empirical formulas, which relate the room temperature ultrasonic attenuation coefficient with oxygen density, average atomic ring size, first-order stretching force constant and experimental bulk modulus of the glass. Fractal bond connectivity and mean atomic volume have also been evaluated and interpreted in terms of composition to deduce changes in the structure of the glass. Furthermore, values of the theoretical bulk modulus have been calculated on the basis of Rocherulle et al. model and compared with the corresponding observed values. It has been found that, majority of the studied glass samples have two-dimensional structure and the network strength changes in the order BaO–TeO2 glasses≺TiO2–Na2O–CaO–P2O5 glasses≺ZnO–PbO–B2O3 glasses. Abd El-Moneim semi-empirical formulas can be applied successfully to predict changes of ultrasonic attenuation coefficient in these glasses at room temperature. Also, Rocherulle et al. model is valid for majority of the investigated glass samples.