Valence and spin states in perovskites LaCo0.95M0.05O3 (M=Mg, Ga, Ti)

The samples LaCoO₃ with dilute substitutions on cobalt sites have been studied using the resistivity, thermopower and magnetic susceptibility measurements over the temperature range up to 900 K. The Co-site substitution does not affect the magnetic transition at 100 K and the onset of massive population of hole carriers at 500 K, characteristic for undoped LaCoO₃. On the other hand, the low-temperature transport and magnetism is markedly distinct for samples with extra charge on cobalt ions introduced by the heterovalent dopants (Mg²⁺, Ti⁴⁺) compared to samples with minor non-stoichiometry (LaCoO₃, Ga³⁺-doped sample). Magnetic properties suggest that these extra charges create thermally stable magnetic polarons of total S2–3. Common features of Co-site doped and La-site doped samples (La_1−xSr_xCoO₃) are discussed.     

Effect of substitution of Co ions on the structural and electrical properties of nanosized magnesium aluminate

Cobalt substituted nanosized magnesium aluminates having a nominal composition MgAl_2−xCo_xO₄ where x = 0.0, 0.5, 1.0, 1.5, 2.0 were synthesized by the chemical co-precipitation method. Aluminium (Al³⁺) ions were completely and successfully substituted by Co²⁺ ions, which yielded an electron rich terminal compound MgCo₂O₄. All the samples were characterized by means of X-ray diffraction (XRD), thermogravimetry and differential thermal analysis (TG/DTA) and dc electrical resistivity measurements. The investigated samples were found to be spinel single phase cubic close packed crystalline materials as demonstrated by XRD data. Using the Debye Scherer formula, the calculated crystallite size was found Co²⁺ concentration dependent and varied between 7 and 19 nm. The lattice constant, X-ray density and bulk density were found to increase while percentage porosity decreases on increasing the Co²⁺ concentration. The dc electrical resistivity was found to decrease as a function of temperature as expected for a typical semiconductor. The doped Co²⁺ ions are believed to form small polarons and hopping of these small polarons between the adjacent sites seems to be partially responsible for conduction in the system. The activation energy of hopping of small polarons was also calculated.     

Elastic properties of nanostructured materials with layered grain boundary structure

Atomistic calculations of the elastic constants for a bulk nanostructured material that consists of a layered structure where alternating layers meet along high angle grain boundaries and where atoms interact via a Lennard-Jones potential are presented. The calculations of the elastic constants were performed in the frame of homogeneous deformations for a wide range of layer widths ranging from 2.24 up to 74.62 nm. The results showed that the relaxation of the atomic structure affects the elastic constants for the cases where more than 5% of atoms are located in the GB region. Also it was found that the way that external stresses are applied on the system affects the values of the obtained elastic properties, with the elastic constants related to the characteristic directions of the grain boundary being the most affected ones. The findings of this work are of interest for the fabrication methods of nanostructured materials, the measurement methods of their elastic properties as well as multiscale modeling schemes of nanostructured materials.     

Nonlinear behavior of V–I curves at low temperatures in nanoparticles of La2/3B1/3MnO3 with B=Ca,Sr

We measure voltage–current curves at low temperature in La_2/3B_1/3MnO₃ samples (B=Ca,Sr). The powder oxides were prepared by different wet-chemical routes and low calcination temperatures were used for nanoparticles formation. We studied samples prepared by three different synthesis methods: gel-combustion, urea sol–gel and liquid-mix. Although the average particle size was almost the same for all the samples (30 nm), we found different behaviors in the V–I curves depending on the synthesis route. The obtained data are analyzed in the electronic tunneling picture.     

Improved initial epitaxial growth of superconducting YBa2Cu3O7 thin films on Y–ZrO2 substrates with a La1.85Sr0.15CuO4 buffer layer

Epitaxial thin films of YBa₂Cu₃O₇ (YBCO) have been deposited on Y–ZrO₂ (YSZ) substrates by means of the pulse laser deposition technique. It has been found that the initial epitaxy of YBCO thin films grown on YSZ can be significantly improved by using La_1.85Sr_0.15CuO₄ (LSCO) as a buffer layer. X-ray diffraction measurements show that the epitaxial YBCO films have single in-plane orientation with YBCO [100]LSCO [100] and LSCO [100]YSZ [110]. The real-time resistance measurements reveal that with LSCO buffer layers the initial formation of the YBCO ultra-thin films changes from the island growth to the layer-by-layer growth.     

Variations of microhardness with solidification parameters and electrical resistivity with temperature for Al-Cu-Ag eutectic alloy

Al-Cu-Ag eutectic alloy was directionally solidified upwards with different growth rates (1.83-498.25 μm/s) at a constant temperature gradient (8.79 K/mm) and with different temperature gradients (3.99-8.79 K/mm) at a constant growth rate (8.30 μm/s) by using a Bridgman type directional solidification apparatus. The dependence of microhardness (HV) on the growth rate (V), temperature gradient (G) and microstructure parameter (λ) were found to be HV = k₁ V^0.10, HV = k₂ G^0.13 and HV = k₃ λ^−0.22, respectively. The electrical resistivity of the Al-Cu-Ag eutectic cast alloy increases linearly with the temperature in the range of 300-780 K. The enthalpy of fusion and specific heat change during melting for same alloy were also determined to be 223.8 J/g, and 0.433 J/g K, respectively by a differential scanning calorimeter from heating curve during the transformation from eutectic solid to eutectic liquid.     

The characteristics of chromized 1020 steel with electrical discharge machining and Ni electroplating pretreatments

A uniform and continuous chromized coating on AISI 1020 steel is produced by low-temperature pack chromization (LTPC) with electrical discharge machining and Ni electroplating pretreatments. The anticorrosive performance of the chromized steels is investigated in a 0.5 M H₂SO₄ solution at room temperature. The testing results indicate that the chromized specimen with electrical discharge machining and Ni electroplating pretreatments exhibits the lowest corrosion current density, 2.16 × 10⁻⁸ A cm⁻², among the tested specimens. The corrosion resistance of all tested specimens are in the order of bare 1020 < 1020-Cr(700-2) < 1020-Ni-Cr(700-2) < 1020-EDM-Ni-Cr(700-2). Moreover, the 1020-Ni-Cr(700-2) specimen have the best conductivity as a result of the less amount of oxides in the superficial coating.     

Investigation on simultaneous doping of Sn and F with ZnO nanopowders synthesized using a simple soft chemical route

ZnO nanopowders simultaneously doped with Sn and F are synthesized by employing a simple soft chemical route. The effect of simultaneous doping on the structural, optical and surface morphological properties are investigated in detail and reported. The structural, FTIR and Raman studies revealed the proper Sn and F incorporation into the ZnO matrix. The synthesized nanoparticles exhibit the Raman bands at 335, 386, 423, 440 and 553 cm⁻¹ which were assigned to wurtzite-type ZnO. The blue shift in the absorption spectrum, caused by the doping process suggests an increase in the optical band gap. The PL studies showed the occurrence of energy transition from ZnO to dopant sites. The surface morphological studies confirmed the nanosize of the obtained powder particles. The EDAX profiles confirmed the presence of expected elements in the final product. The characteristics of the synthesized nanopowders showed that they are potentially significant for several technological applications.     

Magnetic properties of iron-based soft magnetic composites with MgO coating obtained by sol-gel method

Soft magnetic composites with a thin MgO insulating layer were produced by a sol-gel method. Energy dispersive X-ray spectroscopy, X-ray analysis, Fourier transform infrared spectroscopy, density measurement and compositional maps confirmed that thin layers of MgO covered the iron powders. Coercivity measurement showed that the stress relaxation and reduction of hysteresis loss efficiently occurred at 600 °C. At this temperature, the phosphate insulation of commercial SOMALOY^TM samples degrade and their electrical resistivity, magnetic permeability and operating frequency decreases noticeably. The results show that the MgO insulation has a greater heat resistance than conventional phosphate insulation, which enables stress-relief at higher temperatures (600 °C) without a large increase in eddy current loss. The results of annealing at 600 °C show that the electrical resistivity and ferromagnetic resonance frequency increased from 11 μΩ m and 1 kHz for SOMALOY^TM samples to 145 μΩ m and 100 kHz for the MgO insulated composites produced in this work.     

Thermoelectric performance in pseudo-ternary (PbTe)0.95-x(Sb2Se3)x(PbS)0.05 system with ultra-low thermal conductivity via multi-scale phonon scattering

Among the intermediate temperature (500–900 K) thermoelectric materials, both the p- and n-type lead telluride (PbTe) compounds have attracted extensive interests. Till date various approaches were adopted to enhance the thermoelectric performance of n-type PbTe-based materials as they show greater potential space for further improvement compared to p-type ones. Herein, a pseudo-ternary n-type (PbTe)_0.95-x(Sb₂Se₃)_x(PbS)_0.05 system was designed and a large value of ZT = 1.61 at 850 K in case of x = 0.01 was achieved. Two factors are responsible for the improved thermoelectric performance. The incorporation of lead sulfide is the key factor to maintain a high level of power factor above 700 K and the multi-scale hierarchical architectures yield ultra-low thermal conductivity.     

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg₂Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg₂Sn and La-doped Mg₂(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg₂Sn.     

On grain boundary character distribution in materials with cubic structure

From analysis of numerous experimental data on grain boundary (GB) statistics in polycrystals it has been established that certain groups of materials with cubic structure reveal similar GB character distributions (GBCD) (distribution of GBs by reciprocal density of coincidence sites ). It has been shown that GBCD can be described with an empirical low with different parameters for various groups. Several criteria for classification of materials by these groups (the stacking fault energy value, hierarchy of GB energies and mechanism of replacement of high-energy GBs with low-energy ones) have been considered. It has been found that peculiarities of electronic structure of materials are correlated with the classification proposed.     

Microstructural analysis of hard amorphous carbon films deposited with high-energy ion beams

Hard amorphous carbon films produced using high-energy (ca. 30 keV) ion beam deposition of CH₃⁺ and CH₄⁺ on silicon wafers, have been investigated by Positron Annihilation Spectroscopy (PAS), the results are correlated with Raman Spectroscopy and Electrical Resistivity measurements. The microstructural modifications of the films as a function of the annealing temperature in the 300–600°C range have been studied. The evolution of the fractions of sp² and sp³ bonds is described and related to the changes of the open volume defect distribution and the graphitization process.     

A plasma frequency modulation model for constructing structure material with arbitrary cross-section thin metallic wires

Considering the fact that just the electrons confined in the region of the skin depth will actually affect the plasma frequency due to the skin effect, a model for constructing epsilon-near-zero (ENZ) metamaterials through the arranged thin metallic wires with arbitrary cross-section is developed, utilizing the perimeter approximation. With our model, plasma frequency can be freely modulated just by the variance of the metallic wire perimeter, irrespective of the cross-section shape of wires. The finite element method (FEM) and S-parameters retrieval method were employed to numerically simulate the plasma frequencies, which have verified the validity of the theoretical model.     

Sonication-assisted synthesis of a new cationic zinc nitrate complex with a tetradentate Schiff base ligand: Crystal structure,Hirshfeld surface analysis and investigation of different parameters influence on morphological properties

A nanostructured cationic zinc nitrate complex with a formula of [ZnLNO₃]NO₃ (where L = (N²E,N^2′E)-N¹,N^1′-(ethane-1,2-diyl)bis(N²-((E)-3-phenylallylidene)ethane-1,2-diamine)) was prepared by sonochemical process and characterized by single crystal X-ray crystallography, scanning electron microscopy (SEM), FT-IR and NMR spectroscopy and X-ray powder diffraction (XRPD). The X-ray analysis demonstrates the formation of a cationic complex that metal center is five-coordinated by four nitrogen atom from Schiff base ligand and one oxygen atom from nitrate group. The crystal packing analysis demonstrates the essential role of the nitrate groups in the organization of supramolecular structure. The morphology and size of ultrasound-assisted synthesized zinc nitrate complex have been investigated using scanning electron microscopy (SEM) by changing parameters such as the concentration of initial reactants, the sonication power and reaction temperature. In addition the calcination of zinc nitrate complex in air atmosphere led to production of zinc oxide nanoparticles.     

Micromagnetic study of magnetic domain structure and magnetization reversal in amorphous wires with circular anisotropy

In this work we present a detailed numerical investigation on the magnetic domain formation and magnetization reversal mechanism in sub-millimeter amorphous wires with negative magnetostriction by means of micromagnetic calculations. The formation of circular magnetic domains surrounding a multidomain axially oriented central nucleus was observed for the micromagnetic model representing the amorphous wire. The magnetization reversal explained by micromagnetic computations for the M-H curve is described in terms of a combined nucleation-propagation−rotational mechanism after the saturated state. Results are interpreted in terms of the effective magnetic anisotropy.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

Large-scale electronic structure calculation theory and applications to nanostructure materials

We review our recently developed methods in large-scale electronic structure calculations. The calculation is based on the tight-binding formalism of the Hamiltonian. First, the mathematical foundation of Krylov subspace method is focused. The density matrix can be calculated exactly and the numerical accuracy can be monitored during the iterative calculation. The key technique of the shifted-COCG (conjugate orthogonal conjugate gradient) method, collinear residual and seed switching, is explained in details. Second, several applications to nanostructure of semiconductors and metals, fracture propagation and surface reconstruction and formation process of gold nanowire, are explained.