Investigations of silicon nitride and boron carbide monolayers and Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/B<Subscript>4</Subscript>C multilayers

Many different coating concepts for improving mechanical properties have been worked out. One of the advanced coating concepts is the multilayer and superlattice concept, mainly with one or two metallic components. Previous work has shown that the mechanical properties of the covalent-bonded Si₃N₄ and SiC could be improved when combining them in a multilayer system. In the present work the silicon nitride monolayer from the earlier work was combined with boron carbide instead of silicon carbide. First, the boron carbide thin films deposited at different substrate temperatures were examined. Then the number of monolayers in the multilayer system with a constant layer thickness was varied in order to investigate the influence of the interfaces on film properties of the multilayer system. PACS 81.05.Je; 81.15.Cd; 82.80.Pv; 87.64.Je     

On the toughening mechanisms of MoSi<Subscript>2</Subscript> reinforced Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

The toughness increment occurring in Si₃N₄-based composites due to the addition of MoSi₂ particles was compared to the predictions of theoretical models based on the combination of residual stresses and crack deflection toughening mechanisms. A direct application of theoretical models led to a substantial discrepancy between predicted and observed values. For this reason, the basic parameters of the theoretical models were experimentally evaluated. The residual stresses were assessed by measuring the strain in the reinforcing particles by X-ray diffraction. Moreover, the MoSi₂ interparticle distance was calculated by image analysis and the crack paths were analyzed in order to check the actual extent of crack deflection. The overall toughness increase recalculated as the sum of the newly estimated values of residual stresses and crack deflection contributions, was shown to be in good agreement with the experimental results. PACS 81.05.Je; 81.40.Np     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.     

Magnetic properties and the magnetoimpedance effect of nanostructured Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons with induced magnetic anisotropy

The structure and magnetic properties and the magnetoimpedance effect of nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons, obtained from the amorphous state by annealing under different conditions, were comparatively analyzed. Despite the similarity of the samples’ structural states and the processes of their quasi-static magnetization reversal, the features of the magnetoimpedance effect are indicative of significant differences in the processes of their dynamic magnetization.     

Effect of nanocrystallization on the mechanical and magnetic properties of finemet-type alloy (Fe<Subscript>78.5</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript>)

The kinetics of primary crystallization and the effect of structural parameters of the precipitating nanocrystalline α-phase Fe-Si on changes in microhardness, coercive force, and saturation magnetization in an amorphous Finemet-type 5BDSR alloy (Fe_78.5Si_13.5B₉Nb₃Cu₁) obtained by melt quenching are studied. It is found that both an increase in bulk density and an increase in the average nanoparticle size contribute to the hardening of the amorphous/nanocrystalline alloy.     

Properties of ferromagnetic resonance in Fe<Subscript>73.5</Subscript>CuNb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> nanocrystalline alloys

The structural properties and parameters of ferromagnetic resonance have been studied for Fe_73.5CuNb₃Si_13.5B₉ nanocrystalline alloys produced from the initial amorphous state via annealing under different conditions. The dependence of the linewidth of the ferromagnetic resonance on the grain size ΔH ∼ D ⁶ has been found. The result is discussed within the framework of the random magnetic anisotropy model.     

Chemical modification of the surface of Fe<Subscript>75</Subscript>Si<Subscript>25</Subscript> alloy particles with stearic acid under high-energy ball-milling conditions

The morphology, phase composition and surface structure of Fe₇₅Si₂₅-alloy particles are studied by electron microscopy, X-ray diffraction analysis, and Mössbauer, Auger, IR (infrared) and X-ray photoelectron spectroscopy. The alloy particles used as fillers for the polyethylene matrix are produced by high-energy ball milling in an organic medium with the addition of stearic acid. The addition of stearic acid is shown to promote plasticization of the brittle Fe₇₅Si₂₅ alloy and the formation of a surface layer of no more than 1.5 nm thick, consisting of oxides based on iron and silicon, responsible for the chemisorption of stearic acid on the surface. Chemical modification of the surface of filler particles with an amphiphilic surfactant is carried out to enhance their adhesion in the polymer matrix.     

Growth and optical activity of Ca<Subscript>3</Subscript>NbGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> single crystal

Crystals of Ca₃NbGa₃Si₂O₁₄ (CNGS) with ordered langasite structure were grown using the Czochralski method along the Cartesian X axis [110]. The as-grown crystals exhibit high optical quality and structure perfection. Optical activities were obtained by measuring polarised transmission at various wavelengths between crossed polarisers using a TU-1900 spectrophotometer and we found that CNGS crystals showed very large values of . PACS 81.10.-h; 42.79.Ci; 78.20.Ek     

Experimental and theoretical studies of interactions between Si<Subscript>7</Subscript> clusters

The possibility of using magic Si₇ clusters to form a cluster material was studied experimentally and theoretically. In experiments Si₇ clusters were deposited on carbon surfaces, and the electronic structure and chemical properties of the deposited clusters were measured using X-ray photoelectron spectroscopy (XPS). A non bulk-like electronic structure of Si₇ was found in the Si 2p core level spectra. Si₇ is suggested to form a more stable structure than the non-magic Si₈ cluster and Si atoms upon deposition on carbon surfaces. Theoretically it was possible to study the interaction between the clusters without the effect of a surface. Density functional theory (DFT) calculations of potential curves of two free Si₇ clusters approaching each other in various orientations hint at the formation of cluster materials rather than the fusion of clusters forming bulk-like structures.     

Effect of uniaxial tension and hydrostatic compression on the geometry and morphology of amorphous Fe<Subscript>77</Subscript>Ni<Subscript>1</Subscript>Si<Subscript>9</Subscript>B<Subscript>13</Subscript> alloy ribbon surface

The effect of hydrostatic pressure and uniaxial compression on the relief of an amorphous Fe₇₇Ni₁Si₉B₁₃ alloy ribbon surface was studied using scanning tunneling and atomic-force microscopy. The fracture surfaces of samples were also studied. It is found that both the initial surfaces and the surfaces of samples subjected to hydrostatic compression or tension, as well as fracture surfaces, are fractal or multifractal, but their fractality parameters are different. Hydrostatic pressure decreases the surface roughness and the average fractal dimension of the surface on both sides of the ribbons. The dependence of the surface fractal characteristics on tension is more complex. Prior to the occurrence of a “critical event” on the surface (formation of a deformation band or a through crack), the Hölder index and the half-width of the singularity spectrum decrease. The correlation is discussed between the fractal characteristics of the ribbon surface and those of a fracture surface, and the role of an excess free volume in the initiation of fracture of amorphous alloys is analyzed.     

Effect of calcination conditions on lithium conductivity in Li<Subscript>1.3</Subscript>Ti<Subscript>1.7</Subscript>Al<Subscript>0.3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> prepared by sol-gel route

In order to study the influence of powder calcination temperature on lithium ion conductivity, synthesized Li_1.3Ti_1.7Al_0.3(PO₄)₃ (LATP) was calcined at temperatures between 750 and 900 °C. The shape and size of the particles were characterized employing scanning electron microscopy (SEM), and specific surface area of the obtained powder was measured. The crystallinity grade of different heat-treated powders was calculated from XRD spectra. Posteriorly, all powders were sintered at 1100 °C employing field-assisted sintering (SPS), and the electrical properties were correlated to the calcination conditions. The highest ionic conductivity was observed for samples made out of powders calcined at 900 °C.     

N+BF<Subscript>2</Subscript> and N+C+BF<Subscript>2</Subscript> high-dose co-implantation in silicon

Nitrogen and boron BF₂, and nitrogen, carbon, and boron BF₂ high-dose (6×10¹⁶–3×10¹⁷ cm^-2) co-implantation were performed at energies of about 21–77 keV. Subsequent high-temperature annealing processes (600, 850, and 1200 °C) lead to the formation of three and two surface layers respectively. The outer layer mainly consists of polycrystalline silicon and some amorphous material and Si₃N₄ inclusions. The inner layer is highly defective crystalline silicon, with some inclusions of Si₃N₄ too. In the N+B-implanted sample the intermediate layer is amorphous. Co-implantation of boron with nitrogen and with nitrogen and carbon prevents the excessive diffusivity of B and leads to a lattice-parameter reduction of 0.7–1.0%. Received: 10 January 2002 / Accepted: 30 May 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/3974895; E-mail: Lucia.Barbadillo@uam.es     

Electronic excitations in BeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, and Be<Subscript>3</Subscript>Al<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>18</Subscript> crystals

Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl₂O₄, phenacite Be₂SiO₄, and beryl Be₃Al₂Si₆O₁₈ crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.     

Magnetic properties of BiFe<Subscript>0.93</Subscript>Mn<Subscript>0.07</Subscript>O<Subscript>3</Subscript> powders obtained by ultrasonic spray pyrolysis

Samples of BiFe_0.93Mn_0.07O₃ with different specific surface area were synthesized for the first time by ultrasonic spray pyrolysis. The resulting powders consist of porous particles of a spherical shape of medium size ~0.5 μm and have record values of residual magnetization and coercive force. It is found that the magnetic properties of the porous powder particles are determined by the distortion of the crystal lattice and the presence of uncompensated magnetic moments of iron ions on the surface.     

Influence of semiconductor cap layer on the impurity-free vacancy disordering of the In<Subscript>0.2</Subscript>Ga<Subscript>0.8</Subscript>As/GaAs multiquantum-well structure by dielectric capping layers

The feasibility of normal GaAs, low-temperature-grown GaAs (LT-GaAs) and low-temperature-grown InGaAs (LT-InGaAs) as the capping layers for impurity-free vacancy disordering (IFVD) of the In_0.2Ga_0.8As/GaAs multiquantum-well (MQW) structure has been studied. The normal GaAs, LT-GaAs and LT-InGaAs layers were tested as the outermost capping layer and the intermediate cap layer underneath the SiO₂ or Si₃N₄ capping layer. The degree of quantum-well intermixing (QWI) induced by rapid thermal annealing was estimated by the shift of the photoluminescence (PL) peak energy. It was found that the IFVD of the In_0.2Ga_0.8As/GaAs MQW structure using LT-GaAs (LT-InGaAs) as the outermost capping layer was much smaller (larger) than that using a SiO₂ (Si₃N₄) capping layer. It was also observed that the insertion of the normal GaAs, LT-GaAs and LT-InGaAs cap layers below the SiO₂ or Si₃N₄ capping layer reduces the degree of QWI and the PL intensity after the QWI. A plausible explanation for the influence of normal GaAs, LT-GaAs and LT-InGaAs cap layers for the QWI of the InGaAs/GaAs structure is also discussed. PACS 68.55.Ln; 73.20.Dx; 78.55.-m     

Electronic structure and half-metallic property of Si<Subscript>3</Subscript>CaC<Subscript>4</Subscript>

The electronic structures and magnetic properties of Si₃CaC₄ in zinc-blende phase has been studied by employing the first-principles method based on density functional theory (DFT). The calculations predict stable ferromagnetic ground state in Si₃CaC₄, resulting from calcium substitution for silicon. The calculated total magnetic moment is 2.00 μ _B per supercell, which mainly arises from the Ca and neighboring C atoms. Band structures and density of states studies show half-metallic (HM) ferromagnetic property for Si₃CaC₄. The ferromagnetic coupling is generally observed between the Ca and C atoms. The ferromagnetism of Si₃CaC₄ can be explained by the hole-mediated double exchange mechanism. The sensitivity of half-metallicity of Si₃CaC₄ as a function of lattice constant is also discussed, and the half-metallicity can be kept in a wider lattice constant range.     

Synthesis of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>-reduced graphene oxide composite and its application for hybrid supercapacitors

We describe in this paper the synthesis and the characterization of Li₄Ti₅O₁₂-reduced graphene oxide (LTO-RGO) composite and demonstrate their use as hybrid supercapacitor, which is consist of an LTO negative electrode and activate carbon (AC) positive electrode. The LTO-RGO composites were synthesized using a simple, one-step process, in which lithium sources and titanium sources were dissolved in a graphene oxide (GO) suspension and then thermal treated in N₂. The lithium-ion battery with LTO-RGO composite anode electrode revealed higher discharge capacity (167 mAh g^?1 at 0.2 C) and better capacity retention (67%) than the one with pure LTO. Meanwhile, compared with the AC//LTO supercapacitor, the AC//LTO-RGO hybrid supercapacitor exhibits higher energy density and power density. Results show that the LTO-RGO composite is a very promising anode material for hybrid supercapacitor.     

Crystal structure and magnetic anisotropy of ludwigite Co<Subscript>2</Subscript>FeO<Subscript>2</Subscript>BO<Subscript>3</Subscript>

Co₃O₂BO₃ and Co₂FeO₂BO₃ single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co₂FeO₂BO₃ undergoes two-stage magnetic ordering at the temperatures characteristic of Fe₃O₂BO₃ (T _N1 ≈ 110 K, T _N2 ≈ 70 K) rather than Co₃O₂BO₃ (T _N = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.     

The magnetic and hyperthermia studies of bare and silica-coated La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> nanoparticles

The magnetic nanoparticles of La_0.75Sr_0.25MnO₃ perovskite manganite with a controlled size were prepared via sol–gel procedure, followed by thermal treatment and subsequent mechanical processing of the resulting raw product. The prepared materials were structurally studied by the XRD and TEM methods and probed by DC magnetic measurements. The nanoparticles of the mean crystallite sizes 11–40 nm exhibit T _C in the range of ≈310–347 K and the sample possessing 20-nm crystallites was identified as the most suitable for hyperthermia experiments. In order to obtain a colloidally stable suspension and prevent toxic effects, the selected magnetic cores were further encapsulated into silica shell using tetraethoxysilane. The detailed magnetic studies were focused on the comparison of the raw product, the bare nanoparticles after mechanical processing and the silica-coated nanoparticles, dealing also with effects of size distribution and magnetic interactions. The heating experiments were carried out in an AC field of frequencies 100 kHz–1 MHz and amplitude 3.0–8.9 kA m⁻¹ on water dispersions of the samples, and the generated heat was deduced from their warming rate taking into account experimentally determined thermal losses into surroundings. The experiments demonstrate that the heating efficiency of the coated nanoparticles is generally higher than that of the bare magnetic cores. It is also shown that the aggregation of the bare nanoparticles increases heating efficiency at least in a certain concentration range.     

X-ray diffraction studies of the structure of nanocrystals in Fe<Subscript>73.5</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> soft magnetic alloys before and after thermomechanical treatment

The structure of the Fe_73.5Si_13.5B₉Nb₃Cu₁ soft magnetic alloy has been investigated using X-ray diffraction in transmission geometry. The initial alloy prepared by rapid quenching from the melt has a short-range order (∼2 nm) in the atomic arrangement, which is characteristic of the Fe-Si structure with a body-centered cubic lattice. The alloy subjected to annealing contains Fe-Si nanocrystals with sizes as large as 10–12 nm. The annealing under a tensile load leads to an extension of the nanocrystal lattice so that, after cooling, a significant residual deformation is retained. This can be judged from the relative shifts of the (hkl) peaks in the X-ray diffraction patterns measured for two orientations of the scattering vector, namely, parallel and perpendicular to the direction of the load applied. The deformation is anisotropic: within the accuracy of the experiment, no distortions in the [111] direction are observed and the distortions in the [100] direction are maximum. It is known that crystals with a composition close to Fe₃Si exhibit a negative magnetostriction; i.e., their magnetization induced under a load (Villari effect) applied along the [100] direction is perpendicular to this direction along one of the easy magnetization ([010] or [001]) axes. In the alloy, the orientation of the nanocrystal axes is isotropic and the majority of the nanocrystals have a composition close to Fe₃Si. The direction of magnetization of these nanocrystals is determined by the residual deformation of their lattice and lies near the plane perpendicular to the direction of the tensile load applied during heat treatment. This is responsible for the appearance of transverse magnetic anisotropy of the easy-plane type in the Fe_73.5Si_13.5B9Nb₃Cu₁ alloy.