Host (nanodimensional pores of mesoporous material)–guest (semiconductor nanoparticles) nanocomposite materials

An aluminosilicate with the MCM-41 structure (AlMCM-41) was used as a host for the synthesis of cobalt sulfide nanoparticles. Cobalt sulfide nanoparticles were introduced in host by ion exchange and hydrothermal methods. Products (CoSAlMCM-41) were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), EDX, IR, BET and UV–vis spectroscopy. The results show that CoS nanoparticles encapsulated into channels of AlMCM-41 material by hydrothermal synthesis and they grow outside the mesopore AlMCM-41 matrix by ion-exchange methods. Absorption peaks at higher energy than the fundamental absorption edge of bulk CoS indicate quantum confinement effect in nanoparticles as a consequence of their small size. The absorption spectra show that the optical band gap for CoS nanoparticles by hydrothermal and ion-exchange methods are 3.73 and 4.89, respectively.     

High pressure structural (B1–B2) phase transition and elastic properties of II–VI semiconducting Sr chalcogens

We evolve an effective interionic interaction potential (EIoIP) with long-range Coulomb interactions and the Hafemeister and Flygare type short-range overlap repulsion extended up to the second neighbor ions and the van der Waals (vdW) interaction to discuss the pressure induced structural aspects of NaCl-type (B1) to CsCl-type (B2) structure in alkaline earth chalcogenides (SrX; X = S, Se and Te). Particular attention is devoted to evaluate the vdW coefficients following the Slater–Kirkwood variational method, as both the ions are polarizable. The present calculations have revealed reasonably good agreement with the available experimental data on the phase transition pressures (P_t = 17.5, 14.5, 12.5 GPa) and the elastic properties. The calculated values of the volume collapses [ΔV(P)/V(0)] are also closer to their observed data. Further, the variations of the second- and third-order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other semiconducting compounds with rocksalt (B1) type crystal structure. Also the Born and relative stability criteria is valid in strontium monochalcogenides.     

Mechanics of three-dimensional braided structures for composite materials – part I: fabric structure and fibre volume fraction

This paper is the first part of a series on the mechanics of three-dimensional braided structures for composite materials which include fabric structure and fibre volume fraction models, prediction of mechanical properties, finite element analysis and simulation of deformations. In the present paper, the normalized pitch length is introduced as a key parameter of three-dimensional braided structures. It is demonstrated that the braiding angles and fibre volume fractions can be represented as functions of this key parameter. The structures of three-dimensional braids were simulated and the braided fabrics and preforms were designed and produced. Fibre volume fraction models were established. The predictions from the fibre volume fraction models are supported by experimental results.     

Numerical correlation of material structure weaknesses in anisotropic polycrystalline materials

Summary Repeated computer experiments were implemented to investigate material structure weaknesses in polycrystalline materials. The computational investigation relies on behaviors of mesoscopic stress responses in a simulated polycrystalline aggregate containing a fairly large number of constituent grains. A Kröner-Kneer structure-based model was adopted and refined to provide an efficient numerical solution to local mesoscopic stresses, calculated in terms of grain-averaged fields at a material scale of grain size, in arbitrarily polygon-shaped grains. Three criteria have been proposed for classifying material structure weaknesses in the simulated polycrystalline materials. It is found that material structure weakness of three types can be well correlated by a defined Orientation-Geometry Factor Ø and Relevance Parameter. Every correlated relation, incorporating effects of grain orientation and geometry, provides a base for discerning material structure weaknesses. The homogenization of an anisotropic material is also discussed.     

Structure characterization of switched Ag – metal oxide contact materials

The structural changes of several Ag ‐ metal oxide composites during switching operations were investigated. During the switching operation the contact material is melting and for a short moment (ca.10 ms) the metal oxide particles will be floating in the liquid silver resulting in structural changes. These processes affect the switching performance of contact materials and shall be described in this work. The low hardness of the Ag‐matrix requires careful sample preparation in order to avoid preparation artifacts and to reveal the actual structural features. This was achieved by a final step, in which the polished surface was ion beam etched. The samples will be analyzed by stereography, light microscopy, field emission scanning electron microscopy (FE‐SEM) and electron back scatter diffraction (EBSD). It was found that the structure of a switched material can be divided into four category regions: Firstly, the bulk material, secondly, a very small heat affected zone, thirdly, a broad fusion zone and finally, the surface layer. The fusion zone is characterized by large columnar grains, increasing porosity and oxide agglomeration towards the surface. These structural phenomena can only be revealed by ion beam etching, as will be illustrated by examples of selected contact material.     

Mechanical response and multilevel structure of biomimetic hydroxyapatite/polygalacturonic/chitosan nanocomposites

Using an in situ mineralization process that is biomimetic we have synthesized new nanocomposites of chitosan/hydroxyapatite in 50–50 ratio(ChiHAP50), polygalacturonic acid/hydroxyapatite in 50–50 ratio (PgAHAP50) and Chitosan/hydroxyapatite/Polygalacturonic acid (ChiPgAHAP50). Polygalacturonic acid (PgA) is electrostatically complementary to chitosan, and thus is expected to provide stronger interfacial interactions and improve mechanical response. Atomic force imaging of fractured and polished surfaces suggests a multilevel organization in the hydroxyapatite/biopolymer nanocomposite. The AFM images of ChiPgAHAP50 nanocomposite display presence of chitosan rich and polygalacturonic rich domains. These chitosan rich and PgA rich domains are made of smaller globular shaped particles in which, hydroxyapatite nano-particles are embedded in the biopolymer matrix. The average size of the hydroxyapatite particles in PgAHAP50, ChiHAP50 and ChiPgAHAP50 were found to be 25, 42 and 34 nm respectively. The elastic moduli determined from nanoindentation of PgAHAP50, ChiHAP50 and ChiPgAHAP50 composites are 29.81, 17.56 and 23.62 GPa respectively. Hardness values of the three composites in the same order were found to be 1.56, 0.65 and 1.14 GPa respectively. Macro-mechanical tests showed significant enhancement in elastic moduli, strain to failure and compressive strength of ChiPgAHAP50 composites over ChiHAP50 and PgAHAP50.     

Generation and analysis of FCG data using a single specimen and Kmax–ΔK testing matrix

A custom method to generate fatigue crack growth (FCG) data requires testing of multiple specimens at different load ratios, R, and the application of a load shedding procedure from pre-cracking level to threshold. In this paper, a novel method of testing has been investigated which utilizing a single specimen and a testing matrix in terms of K_max and ΔK values corresponding to predetermined R-ratios for which FCG data are recorded. Automatic K-controlled tests on 2324-T39 Al alloy were conducted using both increasing and decreasing ΔK procedures while K_max was kept constant. Results show that the increasing ΔK procedure gives less scatter than decreasing ΔK procedure. Also, fatigue crack growth curves near the threshold region obtained from increasing ΔK are above the curves obtained from decreasing ΔK procedure. These differences are explained by means of interaction between cyclic plastic zones and their effect on fatigue damage. The procedure with increasing ΔK demonstrated minimal interaction effects and hence it is recommended for efficient FCG data generation. The proposed procedure reduces testing time, the overall scatter associated with multiple samples and eliminates possible uncertainty linked to the load shedding procedure and its effects on threshold.     

Dependence of optical properties on the composition in Er-doped xNaPO3–(80 − x)TeO2–10ZnO–10Na2O glasses

Transparent and uniform tellurite–phosphate glasses were prepared and the reason why the substitution of NaPO₃ for P₂O₅ can eliminate the coloration of tellurite–phosphate was discussed. The result of TDA indicated that introducing NaPO₃ into tellurite glasses can improve thermal stability of glass hosts. The compositional dependence of absorption cross-sections of ⁴I_13/2, ⁴I_11/2 and ²H_11/2 level, emission cross-section of ⁴I_13/2 level, host phonon energy, up-conversion and 1.5 μm optical emission intensity as well as and quantum yield for ⁴I_13/2 level in PTEr glasses were investigated too. By analyzing obtained data, authors believe that tellurite–phosphate glasses can be used as potential host material for developing optical amplifiers.     

Crystal structure of the orthorhombic U2-Al4Mg4Si4 precipitate in the Al–Mg–Si alloy system and its relation to the β′ and β″ phases

The atomic structure of a common precipitate in the Al–Mg–Si system has been determined. It is isotypic with TiNiSi (space group Pnma) and contains four units of MgAlSi in a unit cell of size a = 0.675 nm, b = 0.405 nm, c = 0.794 nm. EDS analyses support the composition. A model was based on the atomic structure of the β′ precipitate, electron diffraction and high-resolution transmission electron microscopy (HRTEM) images. A quantum mechanical refinement of the model removed discrepancies between simulated and experimental diffraction intensities. Finally, a multi-slice least square refinement confirmed the structure. The structural relation with β″ is investigated. A similar Mg–Si plane also existing in β″ and β′, can explain most coherency relations between the precipitate phases and with matrix.     

Polar nanodomains and relaxor behaviour in (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 crystals with x = 0.3–0.5

Phase transitions and dielectric properties of the (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ crystals with x = 0.3–0.5 are studied. The solid solutions in this composition range are shown to be relaxor ferroelectrics. The crystals with low x demonstrate a diffused maximum in the temperature dependences of the dielectric permittivity at T_m. T_m varies with frequency according to the Vogel–Fulcher law. The polarizing microscopy investigations reveal a first-order phase transition from the relaxor phase to the low-temperature ferroelectric phase at T_C, which is several degrees below T_m. The permittivity peak in the crystals with x = 0.5 is sharp, and T_m is equal to T_C and does not depend on frequency, as is typical of the transition from a ferroelectric to an ordinary paraelectric phase. Nevertheless, the relaxor, but not the paraelectric, phase is observed at T > T_m. This conclusion is confirmed by the observation of the temperature behaviour of complex dielectric permittivity at T > T_m, which is typical of relaxors and related to the existence of polar nanodomains.     

Si cross-polarization magic-angle spinning NMR spectroscopy––an efficient tool for quantification of thaumasite in cement-based materials

²⁹Si{¹H} cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy is a powerful and reliable tool for the quantification of thaumasite in cement-based materials. The most efficient method for quantifying thaumasite from ²⁹Si{¹H} CP/MAS NMR spectra is described and it is shown that the method allows detection of thaumasite contents below approximately 10 wt.% with a relative precision of 15% and contents above 10 wt.% with a relative precision of 10%. The applicability of ²⁹Si{¹H} CP/MAS NMR for quantification of thaumasite is demonstrated for different Portland cement pastes and shows that thaumasite contents as low as 0.2–0.5 wt.% can be detected in cementitious systems with low concentrations of paramagnetic impurities. For a Portland cement containing various amounts of limestone dust and stored at 5 °C in a MgSO₄ solution, large quantities of thaumasite have been detected. Furthermore, the quantity of thaumasite is found to be less sensitive to the amount of added limestone dust. For samples of a Portland cement with a fixed content of limestone dust but different quantities of added gypsum, the increased contents of gypsum are observed to result in larger quantities of thaumasite after prolonged hydration.     

Ni50Nb50/C amorphous multilayers for “water window” soft X-rays – structure and stability

Amorphous Ni₅₀Nb₅₀/C multilayers with a period of 2.4 nm for the water window soft X-rays were prepared by pulsed laser ablation deposition. The structural stability of these multilayers was studied by grazing incidence X-ray scattering. The multilayers were found to be highly stable at room temperature for periods as long as 3 years. However, the Ni₅₀Nb₅₀ layer was found to undergo an amorphous – crystalline disorder – order transformation due to annealing while the carbon layer remained unchanged. The crystalline phase after transformation could be identified as the NiNb phase. The vertical layered structure, however, was found to be highly stable even after annealing the multilayers at 473 and 593 K under vacuum. The multilayer period remains unchanged till 473 K and increases by 7% on annealing at 593 K.     

“aspect” – a new spectrometer for the measurement of the angular correlation coefficient a in neutron beta decay

The combination of the coefficient a of the antineutrino/electron angular correlation with the beta asymmetry of the neutron provides a sensitive test for scalar and tensor contributions to the electroweak Lagrangian, as well as for right-handed currents. A method is given for measuring a with high sensitivity from the proton recoil spectrum. The method is based on a magnetic spectrometer with electrostatic retardation potentials such as used for searches of the neutrino mass in tritium beta decay. The spectrometer can also be used for similar studies using radioactive nuclei.     

New ‘ηpl'' and ‘γ'' functions to evaluate J–R curve from cracked pipes and elbows. Part I: theoretical derivation

Experimental evaluation of J–R curve in a crack growth situation requires ‘η_pl' and ‘γ' functions that are specific to a cracked geometry and loading condition. All derivations of existing ‘η_pl' and ‘γ' functions are for specific cracked geometry and loading. In this paper, direct limit load based general equations of ‘η_pl' and ‘γ' functions have been derived. Subsequently, new ‘η_pl' and ‘γ' functions, which are not available in the literature, for pipe and elbow geometry with various crack configurations under different loading conditions have been derived. The derivations of ‘η_pl' and ‘γ' functions for throughwall circumferentially cracked elbow under in-plane bending moment uses the very recently proposed new limit load formulas.