Theoretical Bulk Research Articles

Effect of Cu substitution on magnetic and photocatalytic properties of Mn–ZnFe2O4 nanoparticles

The citrate approach was employed to successfully synthesize Mn0.4Zn0.6−xCuxFe2O4 (MZCF) with a nanostructured shape. A thorough examination was conducted on the microstructural, magnetic, and optical properties of the nanoparticles obtained. The X-ray diffraction (XRD) patterns revealed that all of the compositions exhibit a high degree of single-phase spinel formation. The experimental findings indicate that the introduction of copper ions leads to a reduction in both lattice constant and porosity, while concurrently resulting in an increase in the theoretical X-ray and bulk densities. The analysis of UV–Vis spectroscopy indicated that MZCF exhibited a direct band gap falling within the range of 1.61–1.78 eV. The magnetic characteristics were assessed through the analysis of hysteresis plots. The introduction of copper (Cu) content leads to notable improvements in the saturation (Ms) and remanent magnetization (Mr) values, which range from 40.21 to 58.78 and 4.46–10.54 emu/g, respectively. The assessment of the microwave frequency response has verified the appropriateness of MZCF nanoferrites for application within the frequency spectrum spanning from 8 to 13 gigahertz. The nanoparticles exhibit improved magnetic properties, suggesting their potential appropriateness for applications involving high-frequency microwave absorption and switching. Furthermore, the nanoparticles Mn0.4Zn0.6Fe2O4 and Mn0.4Zn0.2Cu0.4Fe2O4 were subjected to analysis utilizing a solar light simulator in order to induce degradation of MB dye.

Experimental and theoretical bulk phase diagram and interfacial tension of ouzo.

Ouzo is a well-known drink in Mediterranean countries, with ingredients water, alcohol and trans-anethole oil. The oil is insoluble in water, but completely soluble in alcohol, so when water is added to the spirit, the available alcohol is depleted and the mixture exhibits spontaneous emulsification. This process is commonly known as the louche or ouzo effect. Although the phase boundaries of this archetypal ternary mixture are well known, the properties of coexisting phases have not previously been studied. Here, we present a detailed experimental investigation into the phase behaviour, including tie-lines connecting coexisting phases, determination of the critical point (also called the plait point in ternary systems) and measurements of the surface tension and density for varying alcohol concentrations. Additionally, we present a theory for the thermodynamics and phase diagram of the system. With suitable selection of the interaction parameters, the theory captures nearly all features of the experimental work. This simple model can be used to determine both bulk and non-uniform (e.g. interfacial) properties, paving the way for a wide range of future applications of the model to ternary mixtures in general. We show how our accurate equilibrium phase diagram can be used to provide improved understanding of non-equilibrium phenomena.

Ultrahigh Strength and Plasticity Mechanisms of Si and SiC Nanoparticles Revealed by First-Principles Molecular Dynamics.

It is now well established that materials are stronger when their dimensions are reduced to the submicron scale. However, what happens at dimensions such as a few tens of nanometers or lower remains largely unknown, with conflicting reports on strength or plasticity mechanisms. Here, we combined first-principles molecular dynamics and classical force fields to investigate the mechanical properties of 1-2nm Si and SiC nanoparticles. These compression simulations unambiguously reveal that the strength continues to increase down to such sizes, and that in these systems the theoretical bulk strength can be reached or even exceeded in some cases. Most of the nanoparticles yield by amorphization at strains greater than 20%, with no evidence of the β-tin phase for Si. Original and unexpected mechanisms are also identified, such as the homogeneous formation of a dislocation loop embryo for the ⟨111⟩ compression of SiC nanoparticles, and an elastic softening for the ⟨001⟩ compression of Si nanoparticles.

First-principles molecular dynamics compression of small metallic nanoparticles

Whether the strength of materials keeps increasing, saturates or decreases when characteristic dimensions are reduced down to the nanometer scale remains an open question. To bring new insights, first principles molecular dynamics calculations are performed to model the compression of 1–2 nm aluminum nanoparticles with different shapes. We find that the plastic deformation occurs at high strains and stresses by a mechanism combining diffusive and displacive atomic displacements. This deformation mode is characterized by a slow dynamic and the partial or even complete recovery of crystalline order. The strength depends on the nanoparticles shape, and is in all cases significantly higher than the theoretical bulk strength. It is however substantially lower than values extrapolated using reported classical molecular dynamics investigations. Our results suggest that the strength saturates at low scales.

Bulk and Surface Contributions to Ionisation Potentials of Metal Oxides

AbstractDetermining the absolute band edge positions in solid materials is crucial for optimising their performance in wide‐ranging applications including photocatalysis and electronic devices. However, obtaining absolute energies is challenging, as seen in CeO2, where experimental measurements show substantial discrepancies in the ionisation potential (IP). Here, we have combined several theoretical approaches, from classical electrostatics to quantum mechanics, to elucidate the bulk and surface contributions to the IP of metal oxides. We have determined a theoretical bulk contribution to the IP of stoichiometric CeO2 of only 5.38 eV, while surface orientation results in intrinsic IP variations ranging from 4.2 eV to 8.2 eV. Highly tuneable IPs were also found in TiO2, ZrO2, and HfO2, in which surface polarisation plays a pivotal role in long‐range energy level shifting. Our analysis, in addition to rationalising the observed range of experimental results, provides a firm basis for future interpretations of experimental and computational studies of oxide band structures.

Bulk and Surface Contributions to Ionisation Potentials of Metal Oxides.

Determining the absolute band edge positions in solid materials is crucial for optimising their performance in wide-ranging applications including photocatalysis and electronic devices. However, obtaining absolute energies is challenging, as seen in CeO2 , where experimental measurements show substantial discrepancies in the ionisation potential (IP). Here, we have combined several theoretical approaches, from classical electrostatics to quantum mechanics, to elucidate the bulk and surface contributions to the IP of metal oxides. We have determined a theoretical bulk contribution to the IP of stoichiometric CeO2 of only 5.38 eV, while surface orientation results in intrinsic IP variations ranging from 4.2 eV to 8.2 eV. Highly tuneable IPs were also found in TiO2 , ZrO2 , and HfO2 , in which surface polarisation plays a pivotal role in long-range energy level shifting. Our analysis, in addition to rationalising the observed range of experimental results, provides a firm basis for future interpretations of experimental and computational studies of oxide band structures.

On the rising extra storage capacity of ultra-small Fe[formula omitted]O[formula omitted] particles functionalized with HCS and their potential as high-performance anode material for electrochemical energy storage

We report a nanocomposite material exhibiting ultra-small Fe3O4 nanoparticles uniformly decorated on hollow carbon spheres (Fe3O4@HCS). The unique hierarchically-structured material displays excellent electrochemical cycling performance that exceeds the theoretical bulk capacity of Fe3O4 when used as an anode in lithium-ion batteries. In particular, it features increasing reversible capacity upon cycling yielding 1050 mAh g−1at 0.1 A g−1 in cycle 250. Comprehensive scanning and transmission electron microscopy images combined with detailed electrochemical analysis demonstrate that the outstanding electrochemical performance can be traced back to the formation and decomposition of a capacitative surface layer during dis/charging.

Atomistic simulation of FCC and HCP Ni-Gd core–shell nanosystem

This research is focused on simulations of Gd, Ni, and Ni-Gd core–shell magnetic nanosystem using VAMPIRE software and the Landau-Lifshitz-Gilbert-Heun approach. The generation of temperature dependent magnetization curves allows the estimation of the corresponding Curie temperature. Simulations at various Ni and Gd particle diameters provided critical values below which the Tc value differs from the theoretical bulk Tc value, being 5 and 7 nm for Ni and Gd, respectively. It was demonstrated that both FCC and HCP structure configurations yield undistinguishable behaviors at large particle sizes. After fitting, the values of the phenomenological shift exponent and the microscopic length of Ni and Gd corresponded to spherical nanosystem. The Tc values have been tuned by varying the core diameter of Ni-Gd core–shell nanosystem. Moreover, the antiferromagnetic coupling was seen to play a significant role in the M-T curves and spin distributions of cores and shells. The hysteresis loops produced coercivity field values driven by inter-domain coupling, as well as anisotropy decreases caused by Ni% increments and rising temperatures. These magnetic properties disclosed here are promising for future biomedical and spintronic applications.

Experimental measurements of soot formation in fuel-rich homogeneous mixtures using an optical rapid compression machine

Advanced combustion strategies are necessary for the use of more environmentally sustainable fuels than traditional diesel. Alcohol fuels and alcohol/gasoline blends are of particular interest as they are readily available in the marketplace. Heavy-duty engines typically use compression ignited, conventional diesel mixing controlled combustion. Mixing controlled combustion features a non-premixed diffusion flame with a wide range of local equivalence ratios, leading to potentially high rates of soot formation. This work studies the sooting behavior of iso-octane and ethanol as a function of equivalence ratio. Measurements are carried out in a rapid compression machine (RCM) and are reported for pre-ignition conditions of 10–30 bar and temperatures of 650–800 K. Theoretical equilibrium and bulk gas temperatures are calculated for both fuels. These data are used to identify the critical equivalence ratio, the lowest equivalence ratio where soot is detected with a single-pass laser extinction diagnostic. The critical equivalence ratio for iso-octane varies between 1.82 and 1.77 for compressed pressures of 10 and 20 bar, respectively. Ethanol, sometimes considered sootless, had a critical equivalence ratio between 2.37 and 2.12 for compressed pressures of 20 and 30 bar, respectively. When characterizing soot formation by oxygenated equivalence ratio, the critical equivalence ratios for ethanol approach those of iso-octane. This suggests the oxygenated nature of alcohol fuels reduces sooting tendency, but other factors such as fuel molecular structure and morphology may play a role. It was observed that ethanol will form soot at equivalence ratios only slightly higher than iso-octane, which could have implications in mixing controlled combustion. It was seen for both fuels that soot formation is pressure sensitive, with the critical equivalence ratio being inversely proportional to compressed pressure and the rate of soot formation. Future work will investigate the sooting behavior of gasoline/ethanol blends.

Analysis of the magnetization for Fe and Ni ferromagnetic nanoparticles with variable geometry using VAMPIRE software

The purpose of this research is to have a quantitative analysis on the temperature dependence magnetization of ferromagnetic nanoparticles (NPs), i.e., the Curie temperature (Tc), was systematically studied. The atomistic simulations were carried out using the VAMPIRE 5.0 software based on the Landau-Lifshitz-Gilbert-Heun method. The variable parameters of the simulations were damping, time step, particle geometry (spherical, cubic, and cylindrical) and particle size. We have calculated λ for Fe and Ni, in addition we have found different Tc values for each nanogeometry studied following a finite-size effect. We have found ν values for cubic NPs close to the reported values. In particular, it was observed that the Tc values for the studied geometries in the case of Fe and Ni decreases from their theoretical bulk values, for a critical particle size diameter less than 5 nm. Hence, the presented results (optimized atomistic parameters such as simulation time step, damping, and critical exponents) are the basis for advanced simulations of hybrid and complex nanostructures with perspective in biomedical and environmental applications.

Behavior of copper under high pressure: Experimental and theoretical analyses

The physical properties of high-purity copper under high pressure were investigated with X-ray diffraction(XRD) using the 3rd generation synchrotron radiation and a diamond anvil cell(DAC) and First-principles calculation using ab-initio simulation program. And they differ 15 % from those reported in the past. The previously reported experimental isothermal bulk moduli for polycrystalline copper and single crystalline copper are 140.2 ± 3.9 GP and 137.6 ± 0.2 GPa respectively, and the theoretical isothermal bulk modulus of copper is 134.6 GPa [1–20]. However, the recently measured bulk moduli of copper are 120.8 ± 4.4 GPa for polycrystal and 120.7 ± 2.1 GPa for single crystal respectively. The difference might mainly come from the purity of copper owing to the development of scientific technology, and the 3-dimensional effect of defects in nearly pure(perfect) crystalline materials was first observed by using DAC and XRD.

High energy product of isotropic bulk Sm-Co/α-Fe(Co) nanocomposite magnet with multiple hard phases and nanoscale grains

Nanocomposite magnets consisting of hard and soft magnetic phases have potential applications to be the next generation of permanent magnets with very high energy product and less expensive rare-earth elements. But it is still a big challenge to obtain bulk magnets with ideal microstructure and high performance. In this work, two - step warm processing at relative low temperatures had been adopted to obtain nearly theoretical density bulk nanocomposite magnets from amorphous/nanocrystalline powder precursors. Novel nanostructures consisting of multiple Sm-Co hard phases (SmCo5 as main phase, SmCo3, SmCo7, Sm2Co17 as minor phases) and 25 wt% α-Fe(Co) soft phase, nanoscale grain size below 20 nm for both the hard phase and soft phase, and the diffusion of Fe and Co compositions had been obtained in bulk isotropic magnets. Besides the ideal nanostructures, a high coercivity of 5.9 kOe, Mr/Ms value of 0.78 and a high square degree of demagnetization curve S = 0.47 were obtained. All of these factors together brought a new record-high energy product (BH)max of 23.6 MGOe. These results make an important step toward fabricating novel nanostructure with high performance.

Quantitative material analysis using secondary electron energy spectromicroscopy

This paper demonstrates how secondary electron energy spectroscopy (SEES) performed inside a scanning electron microscope (SEM) can be used to map sample atomic number and acquire bulk valence band density of states (DOS) information at low primary beam voltages. The technique uses an electron energy analyser attachment to detect small changes in the shape of the scattered secondary electron (SE) spectrum and extract out fine structure features from it. Close agreement between experimental and theoretical bulk valance band DOS distributions was obtained for six different test samples, where the normalised root mean square deviation ranged from 2.7 to 6.7%. High accuracy levels of this kind do not appear to have been reported before. The results presented in this paper point towards SEES becoming a quantitative material analysis companion tool for low voltage scanning electron microscopy (LVSEM) and providing new applications for Scanning Auger Microscopy (SAM) instruments.

The Effects of Constriction Factor and Geometric Tortuosity on Li‐Ion Transport in Porous Solid‐State Li‐Ion Electrolytes

Abstract3D focused ion beam tomography is used to analyze the microstructures of Li‐ion conducting Li6.75La2.75Ca0.25Zr1.5Nb0.5O12 (LLCZN) garnet porous electrolytes with different levels of porosity and the theoretical effective bulk conductivities of the electrolyte are calculated based on LLCZN volume fraction, constriction factor, geometric tortuosity, and percolation factor. The experimentally measured effective bulk conductivities are consistently lower than the theoretical values when assuming constant bulk conductivity, suggesting the bulk conductivity of the LLCZN decreased with increasing porosity. This work highlights the importance of understanding the full effects of altering the microstructure of solid‐state electrolytes, as this will play a key role in advancing Li‐ion battery technology to higher energy and power densities.

Sound Velocities in Shock‐Synthesized Stishovite to 72 GPa

AbstractStishovite (rutile‐type SiO2) is the archetype of dense silicates and may occur in postgarnet eclogitic rocks at lower‐mantle conditions. Sound velocities in stishovite are fundamental to understanding its mechanical and thermodynamic behavior at high pressure and temperature. Here, we use plate‐impact experiments combined with velocity interferometry to determine the stress, density, and longitudinal sound speed in stishovite formed during shock compression of fused silica at 44 GPa and above. The measured sound speeds range from 12.3(8) km/s at 43.8(8) GPa to 9.8(4) km/s at 72.7(11) GPa. The decrease observed at 64 GPa reflects a decrease in the shear modulus of stishovite, likely due to the onset of melting. By 72 GPa, the measured sound speed agrees with the theoretical bulk sound speed indicating loss of all shear stiffness due to complete melting. Our sound velocity results provide direct evidence for shock‐induced melting, in agreement with previous pyrometry data.

Ab initio study of the electronic, elastic, vibrational and dielectric properties of KBaBi

KBaBi is predicted to be a topological insulator recently, but most fundamental properties of it have not been studied. In this work, first-principles calculations were carried out to study its electronic, bonding, elastic, vibrational and dielectric properties. The results indicate that KBaBi has a band gap of 0.22 eV with the presence of spin orbital coupling. Weak p-d hybridization exists between the Ba 5d and the Bi 6p orbitals, so KBaBi has some covalent property, as revealed by the electron density difference. Studies imply that KBaBi is mechanically and dynamically stable, but it is mechanically anisotropic. KBaBi has a theoretical bulk modulus of 20.8 GPa and a shear modulus of 11.7 GPa. Group theory analysis indicates that there are six infrared- and four Raman-active modes at the Brillouin zone center of KBaBi. The vibrational frequencies and the corresponding eigenvectors were calculated and discussed. Calculation unveils that the electrons have more dielectric contribution to the total dielectric constants than the ions in the ab plane, but it is revised along the c axis. Investigations of the band structure, the Debye temperature and the phonon dispersion curves indicate that KBaBi is a promising thermalelectric material.

Single-channel bimodal interferometric sensor using subwavelength structures.

A novel configuration of photonic sensors based on a single-channel bimodal interferometer is proposed. The design consists of a subwavelength grating (SWG) periodic structure supporting two dispersive TE-like modes that interfere at the output to create fringes in the transmission spectrum. Dispersion relations of the bimodal periodic structures have been computed in order to study the sensing performance, obtaining a theoretical bulk sensitivity of ~1300nm/RIU and a surface sensitivity of ~6.1nm/nm. Finite-Difference Time Domain (FDTD) analysis has been also carried out in order to confirm the previously obtained sensitivity results, thus showing a perfect agreement between theoretical modelling and simulation.

Effect of Ti<sup>4+</sup> Doping on Structural, Electrical and Magnetic Properties of Ni<sub>0.4</sub>Cu<sub>0.2</sub>Zn<sub>0.4</sub>Fe<sub>2-x</sub>Ti<sub>x</sub>O<sub>4</sub> Ferrites

Ti4+ doped Ni0.4Cu0.2Zn0.4Fe2-xTixO4 (x = 0.0, 0.02, 0.05, 0.07, 0.10) chemical compositions are prepared by conventional solid state reaction technique. The ferrite phase formation has been confirmed from the pattern of XRD. The theoretical density (ρth), bulk density (ρB), and porosity are calculated from the XRD data and using approximate formulas. Value of ρth is found to be greater than the value of ρB indicating the formation of pores inside the bulk specimens. The micro-structural investigation has been done using Field Emission Scanning Electron Microscope and it is found that the average grain size reduces with the increase of Ti content. Saturation magnetization (Ms) also reduces with the increase of Ti content, contrarily remanent magnetization (Mr) and coercivity (Hc) increases with the concentration of Ti in the composition due to the pinning effect. The real part of the initial permeability is found to be maximum for the x = 0.02 sample which could be due to the homogeneity and high density of the sample. For increasing frequency, the dielectric constant and dielectric loss are observed to decrease.

Structural and Physical Properties of ZrSi2 under High Pressure: Experimental Study and First-Principles Calculations.

Zirconium disilicide (ZrSi2) has been investigated experimentally and theoretically for its structural and physical properties at high pressures. In situ compression experiments demonstrate that at low pressure, ZrSi2 adopts the C49 structure (space group Cmcm), which persists up to 54.5 GPa at room temperature, and the unit cell of ZrSi2 along b-axis is at least twice as compressible as along a- and c-axis. A bulk modulus of 170.0 ± 0.7 GPa ( K'0 = 4) is derived from the compression experiment employing methanol-ethanol mixture as the pressure-transmitting medium. Diffraction line-width analysis suggests a yield strength of about 3.0 GPa for ZrSi2 under high pressures at room temperature. The first-principles calculations mostly agree with the experimental results, such as mechanical and dynamic stability and elastic anisotropy ( Kc > Ka ≫ Kb). However, predicted axial modulus Kb by modeling is significantly smaller than the experimentally determined value, resulting in a sizable discrepancy between experimental (170.0 GPa) and theoretical (121.0 GPa) bulk moduli.

Structural, morphological and electromagnetic properties of Sc3+ doped Ni-Cu-Zn ferrites

Abstract Various polycrystalline Ni0.3Cu0.2Zn0.5ScxFe2−xO4 (x = 0.00, 0.02, 0.04, 0.05 and 0.07) were synthesized by standard solid-state reaction technique. The composition was calcinated at 1173 K and the samples, prepared from these powders, were sintered at 1373, 1423, 1473 and 1523 K for 5 h in air. The result of compositional variation on structural, morphological and magnetic properties of Scandium (Sc3+) doped Ni-Cu-Zn ferrites was investigated. The X-ray diffraction measurements confirmed the single-phase cubic spinel structure of these compositions, and no traces of extra peaks corresponding to any unreacted ingredient were found. Lattice constant increases with substitution of Sc3+ in the place of Fe3+. Theoretical density and bulk density decrease with the increase of Sc3+ substitution. Bulk density of Ni0.3Cu0.2Zn0.5ScxFe2−xO4 reaches the maximum value for x = 0.05 sintered at 1423 K. The average grain sizes increase with increasing Sc3+ substitution up to x = 0.05 then it starts to decrease. The real part of initial permeability decreases for x = 0.02, then it increases for x values up to 0.05 after that it decreases again for x = 0.07. The doping of Sc3+ affects the magnetization may be by alteration of the cation distribution. The maximum saturation magnetization is observed 79 Am2/kg for x = 0.05 sintered at 1423 K. At lower frequency range (up to 105 Hz) dielectric constant decreases rapidly with increasing frequency but at higher frequencies (above 105 Hz) it becomes constant and show negligibly small values which is a good agreement with the Maxwell-Wagner model. Dielectric constant and AC resistivity show similar behavior at different frequency ranges. The resistivity mechanism has been explained on the basis of electron hoping between Fe2+ and Fe3+, and it is concluded that at higher concentrations some Sc3+ may possibly occupy A-sites besides a large portion of it going to B-sites, whereas for lower concentrations Sc3+ preferentially occupy B-sites in the Ni-Cu-Zn ferrites system.