Magnetotransport properties of Zn90Mn7.5Cu2.5O100 films

Zn₉₀Mn_7.5Cu_2.5O₁₀₀ films have been prepared on a-plane sapphire substrates by pulsed laser deposition. Paramagnetism mainly caused by Mn²⁺ ions was observed in the films from room temperature down to 2 K. Magnetotransport properties (magnetoresistance (MR) and Hall effect) were studied from 5 K to 290 K up to a field of 6 T. Negative MR was observed at temperature above 100 K. Low field positive MR and high field negative MR was observed at 5 K. Clear anomalous Hall effect with a kink at low field was observed below 20 K, indicating that there exist two different scattering mechanisms for the Mn²⁺ and Cu²⁺ ions.     

Electrical characterization on l-tyrosine doped triglycine sulfate single crystals

Dielectric, hysteresis, and scanning electron microscope studies on l-tyrosine doped triglycine sulfate (TyTGS) are reported. Increase in the relative permittivity compared to triglycine sulfate is observed on the doped crystal. Low frequency dielectric relaxation process is found to be modified in TyTGS crystals. No significant change in the spontaneous polarization compared to pure triglycine sulfate is observed. Increase in the coercive field in TyTGS compared to pure triglycine sulfate suggests that the dopant hinders the domain wall motion. Lens shaped domains, typical of triglycine sulfate are also observed on TyTGS crystal.     

Sensitized monomer fluorescence and excitation energy transfer in perylene-doped phenanthrene in crystalline and in polymer matrix

Sensitized monomer fluorescence and excitation energy transfer in crystalline and spin cast polymer films was investigated at room temperature. The fluorescence spectra of perylene doped phenanthrene reveals the characteristic monomeric emission of perylene and partial quenching of phenanthrene emission. The excimer formation of perylene is not observed in mixed crystalline luminophors and in spin cast films of the phenanthrene luminophors. The observed quenching of phenanthrene emission indicates the excitation energy transfer from phenanthrene to perylene in crystalline as well as in polymer matrix. Energy transfer is not observed in the experiments when phenanthrene and perylene were physically mixed where the components exist separately. The overlap between the excitation spectrum of perylene and emission spectrum of the phenanthrene supports the fact that the perylene molecules accept the excitation energy from phenanthrene. The energy transfer was found to depend upon the perylene concentration.     

Bio-inspired laminate design exhibiting pseudo-ductile (graceful) failure during flexural loading

Discontinuous reinforcement phases are often observed in high toughness natural materials, for example, nacre. The aim of this study is to introduce a degree of ‘pseudo-ductility’ to fibre reinforced polymer materials by exploiting such discontinuities. The work presented aims to take a simple concept of discrete material sections and apply it in the form of ply cuts in a carbon fibre reinforced polymer. A variety of specimen types which encompass the principles inspired by the architecture of nacre were tested in four point bend flexure and the failure processes investigated. Finite element analysis was also carried out to understand stress conditions around ply cuts and their role in the observed failure. It was observed that ply cut spacing and ply cut density were important parameters in achieving ‘pseudo-ductile’ failure.     

Emission wavelength trimming of self-assembled InGaAs/GaAs quantum dots with GaAs/AlGaAs superlattices by rapid thermal annealing

The effects of rapid thermal annealing on InGaAs quantum dots grown by atomic layer molecular beam epitaxy to the structural transformation and optical properties are investigated. No misfit dislocation was observed from either the as-grown or annealed dots. The size and composition of the quantum dots become more uniform upon annealing mainly from the height fluctuation as predicted by the theoretical model. Large bandgap blue shifts, resulted from the In and Ga interdiffusion, were observed with the preservation of three-dimensional carrier confinement. The GaAs/AlGaAs superlattice was found to minimize the defect diffusion and dot interdiffusion during the high-temperature epitaxial overgrowth.     

X-ray diffraction topography investigation of the core in Bi12SiO20 crystals

The core in a large Bi₁₂SiO₂₀ crystal as well as the regions free of optical inhomogenities was examined by X-ray double-crystal diffraction topography. It was established that the observed defects in the central core are two-dimensional. The absorption of some impurities as well as the composition changes observed in this area by other authors, should be considered as a consequence of the formation of these defects.     

Flow-induced crystallization of polypropylene in the presence of graphene nanoplatelets and relevant mechanical properties in nanocompsoite fibres

Flow-induced crystallization of polypropylene (PP) in the presence of graphene nanoplatelets (GnPs) was experimentally observed for the first time. Monofilaments of PP and PP/GnPs in the presence and absence of polypropylene grafted maleic anhydride (PP-g-MA) as a compatibilizer were produced via masterbatch dilution technique. Morphology, crystalline and super molecular structures, thermal and mechanical characteristics of the nanocomposite fibres were investigated by means of SEM, WAXD, DSC, DMTA, TGA, polarized light microscopy and tensile testing. The fractured cross-section images indicated a reasonably good GnPs dispersion in the compatibilized nanocomposite fibres since a few large GnPs aggregates were observed. Based on DSC, XRD, and polarizing optical microscopy results, it can say that GnPs acted as a nucleating agent and increased the crystallization temperature and simultaneously acted as a physical barrier, reducing crystallite growth of PP. The introduction of PP-g-MA into PP/GnPs nanocomposite fibre increased storage modulus. Upon inclusion of PP-g-MA, the most significant improvement of 48% was observed for tensile modulus with the incorporation of 0.5 wt.% GnPs. The observed results suggested that the presence of PP-g-MA changed the internal structure and morphology of PP/GnPs nanocomposite fibres rather than improving the interface of PP/GnPs. We concluded that using extensional flow and having proper dispersion of GnPs, flow-induced crystallization could be occurred in PP/GnPs nanocomposite fibres. About 20% increase in crystallinity was found for the compatibilized PP/GnPs nanocomposite fibres as compared to that of the virgin PP.     

Analytical prediction of damage due to large mass impact on thin ply composites

This article presents analytical models for predicting large mass impact response and damage in thin-ply composite laminates. Existing models for large mass impact (quasi-static) response are presented and extended to account for damage phenomena observed in thin-ply composites. The most important addition is a set of criteria for initiation and growth of bending induced compressive fibre failure, which has been observed to be extensive in thin ply laminates, while it is rarely observed in conventional laminates. The model predictions are compared to results from previous tests on CFRP laminates with a plain weave made from thin spread tow bands. The experiments seem to confirm the model predictions, but also highlight the need to include the effects of widespread bending induced fibre failure into the structural model.     

Transmission electron microscopy study of the initial growth stage of GaSb grown on Si (001) substrate by molecular beam epitaxy method

The microstructural properties at the initial growth stage of the GaSb heteroepitaxial growth on a silicon (Si) substrate were investigated using transmission electron microscopy. Well-separated and tall GaSb islands were observed when GaSb was directly grown on a Si substrate (sample A). On the other hand, GaSb was grown to the coalesced and flat islands when a low-temperature AlSb buffer (sample B) was introduced. The different morphologies of the GaSb islands were related to the microstructural properties of the interface between the GaSb and the Si substrate. The GaSb/Si interface was rough, and disordered atomic arrangements were observed at the interface in sample A. On the other hand, the GaSb/Si interface was flat, and well-ordered atomic arrangements appeared at the interface in sample B. Entirely different mechanisms for the relaxation of a misfit strain were demonstrated from a microstructural viewpoint.     

Possible Sound Mode Conversion in “Superfluid 4He-97% Open Aerogel” System

We have studied the acoustic properties of liquid helium filled in various aerogels. The longitudinal ultrasound velocity and attenuation were measured at the frequency of 10 MHz with aerogels that had porosity from 92 to 97%. The mode intermediate between first and fourth sound was observed. The attenuation of this mode decreased with decreasing temperature for dense aerogels. However, an attenuation maximum was observed around 1.6 K for 97% open aerogel at various liquid pressures. In the present work, we discuss the possibility of the sound modes conversion between first, second sound in superfluid and aerogel sound mode in this composite system.     

Influence of structural ordering on electrical conductivity of nanostructured manganese zinc ferrite

In the present study, nanostructured manganese zinc ferrite of 11 nm grain size was synthesized by co-precipitation technique and subsequently suitably heat treated to obtain higher grain sizes. The plot of temperature dependence of dc conductivity shows the semiconducting nature of samples. The observed changes in the electrical conductivity have been attributed with the influence of structural ordering upon annealing. The observed decrease in conductivity when the grain size is increased from 11 to 69 nm upon annealing is clearly due to the structural ordering which is evident from FESEM.     

Studies on wear rate and micro-hardness of the Al/Al2O3/Gr hybrid composites produced via powder metallurgy

Micro-structural characterization of the composites has revealed fairly uniform distribution and some amount of grain refinement in the specimens. Further, it was observed that the micro-hardness improve when increasing the milling time and the reinforcement content due to presence of hard Al₂O₃ particles. Was also observed a low wear rate exhibited by the Al/Al₂O₃/Gr hybrid composites due to presence of Al₂O₃ and Gr which they acted as load bearing elements and solid lubricant respectively. The observed wear rate and micro-hardness have been correlated with microstructural analyses.     

Nonlinear Transport of Wigner Solid on Superfluid 3He–A

Electrons trapped on the surface of liquid helium form the Wigner solid accompanied with the periodic surface deformation (dimple lattice). Because of the soft surface, the Wigner solid shows unique nonlinear transport properties. Here we present the results of the nonlinear transport measurements of the Wigner solid on the superfluid ³He A phase at temperatures down to 200 μK in a magnetic field of 0.363 Tesla. The transition from linear to nonlinear behavior is observed as increasing the driving voltage. This behavior is very similar to those previously observed in the B phase and normal phase, and attributable to the deformation of the dimple shape caused by the strong damping of liquid ³He.     

Microwave-hydrothermal synthesis of nanostructured Na-birnessites and phase transformation by arsenic(III) oxidation

Microwave-hydrothermal synthesis was employed to produce Na-birnessites. Crystalline, single-phase materials were obtained at temperatures as low as 120 °C and times as short as 1 min. X-ray diffraction and Raman spectroscopy were used to characterize the structural features of the nanostructured powders. Birnessites possessed a monoclinic structure in space group C2/m with nine Raman-active bands, all of which were observed for the first time due to optimized acquisition of the spectroscopic data. The highly reactive materials produced were submitted to sorption experiments with As(III). An oxidative precipitation occurred with the production of Mn(II) arsenate at higher arsenic concentrations. In addition, the formation of hausmannite (Mn₃O₄) was confirmed by X-ray diffraction and Raman analyses of the reacted solid phase. The observed 14 Raman-active modes were adjusted according to the tetragonal I4₁/amd space group for hausmannite. An additional band related to the breathing mode of the arsenate was observed, leading to the conclusion that adsorption onto hausmannite takes place in addition to the oxidative precipitation of manganese arsenate.     

Structural study and luminescence of TlSrLa(AsO4)2

This work presents the crystal structure and luminescent properties of TlSrLa(AsO₄)₂. In this phase Tl⁺ ions are located in large tunnels delimited by chains of alternating (AsO₄) and (Sr,La)O₈ polyhedra. Thallium atoms are eightfold coordinated with C₁ symmetry. Large TlO distances are observed revealing a low stereochemical activity of the 6s² lone pair. Excitation and emission spectra of Tl⁺ in TlSrLa(AsO₄)₂ showed broad bands at lower energy than those observed in previous works. Excitation spectra are decomposed into multiple Gaussian bands and a theoretical analysis is made to explain the number of observed components. Two Gaussian components are revealed for emission spectra.     

Dielectric and structural studies on sulphamic acid (SA) single crystal

Sulphamic acid single crystal of the size 14 × 8 × 6 mm³ has been grown by slow cooling solution growth method at 35.5 °C. Fermi resonance vibrations of N-S stretching and SO₃ rocking have been observed from micro-Raman analysis. Different shapes of etch patterns have been observed for etchants like water, ethanol and HCl for different etching periods. The dielectric constant and dielectric loss have been measured for different frequencies at different temperatures.     

Effect of Crystal Quality on HCP-BCC Phase Transition in Solid 4He

The kinetics of HCP-BCC structure phase transition is studied by precise pressure measurement technique in ⁴He crystals of different quality. An anomalous pressure behavior in bad quality crystals under constant volume conditions is detected just after HCP-BCC structure phase transition. A sharp pressure drop of 0.2 bar is observed at constant temperature. The effect observed can be explained if we suppose that microscopic liquid droplets appear on the HCP-BCC interphase region in bad quality crystals. After the interphase region disappearance, these droplets are crystallized with pressure reduction. It is shown that this effect is absent in high quality thermal-treated crystals.     

Effects of oxygen-plasma treatment in La0.7Ca0.3MnO3−δ thin films

Oxygen-plasma effects have been investigated for La_0.7Ca_0.3MnO_3−δ thin film samples. As a result, an increase in the metal-insulator transition temperature was observed in a thin film sample. On the other hand, an increase in the resistivity as well as a decrease of the metal-insulator transition temperature was observed in a sample showing considerable structural disorder.     

Raman probing of adhesion loss in carbon nanotube – reinforced composite

Raman spectroscopy of a carbon nanotube – reinforced phenolic resin is used to study the interaction of nanotubes with a host matrix. The observed sublinear dependence of the Raman G-band shift on the matrix strain, accompanied by inhomogeneous broadening of the spectral line, is interpreted as a gradual loss of adhesion between nanotubes and the polymer. An approach to simulate the ensemble-averaged Raman response of the nanotubes in composite is proposed, that takes into account nanotube orientation, angular dependence of the polarized Raman response of nanotubes, and adhesion loss between the nanotubes and the polymer. The comparison of the observed Raman line shapes and Raman shifts with simulation provides interesting insights into the micromechanics of nanotube interaction with polymer.     

Effects of annealing temperature on the structure and photoluminescence properties of ZnO films

Zinc oxide (ZnO) films with c-orientation were deposited on Si (1 1 1) substrates at room temperature (RT) by RF-magnetron sputtering. Violet (394 and 412 nm) and green (560 and 588 nm) photoluminescence (PL) were observed from the as-deposited and annealed samples. The PL intensity was increasing with increasing annealing temperature (T_a). The 412 nm violet peak shifted from 412 to 407 nm and the 394 nm violet peak shifted from 394 to 399 nm on increasing the temperature from 500 to 900 °C, whereas no shift in PL green peaks was observed over the whole range of temperature examined. The 412 nm violet luminescence is ascribed to radiative defects related to the interface traps existing at grain boundaries. With the increase of T_a, the stress in the films changed from compressive to tensile, which is believed to have resulted in the observed 412 nm violet emission peak shifts from 412-407 nm. The 394 nm violet luminescence observed is attributed to free excitonic emission, and the increase of the crystal size may result in the 394 nm violet emission peak shifts from 394 to 399 nm. The other two PL bands located at 560 and 588 nm are attributed to oxygen deficiency.