Materials Literature
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Received: August 15, 1998; Revised: March 30, 1999 In this work it was evaluated the influence of the type and concentration of the acid catalyst and the gelation temperature on the structure of pores of silica gels obtained with TEOS. The values of the major structural parameters related to the pores such as volume f…
Bifunctional catalysts formed by metals (nickel, platinum as well as an association of both metals) supported in an ultrastabilized Y molecular sieve (USY) were prepared. The samples were obtained using a competitive ion exchange method, followed by calcination and reduction of the transition metal under hydrogen atmo…
Two cast g titanium aluminides, Ti47Al and Ti50Al (at. %), were obtained by arc melting under an argon atmosphere. The as-cast microstructure modification by annealing under protective argon atmosphere and hot isostatic pressing (HIP) was analysed by optical and scanning electron microscopy with microanalysis faciliti…
The aim of this work was to study the hydrothermal synthesis of Si and SiAlMCM-41 performed under both autogenic pressure and refluxing conditions. XRD data showed that the MCM-41 phase may be formed by both processes and that the synthesized material in the presence of Al and/or under reflux presents the hexagonally…
This approach assumes three functions independently acting on a set of microparticles. The first one, w1, concerns re-distribution of mass to decrease the surface energy. The second one, w2, concerns re-distribution of mass to increase the entropy of the microparticle set. The third one, w3, is a further re-distributi…
The temperature dependence of the yield stress T c of four bcc transition metals α-Fe, Nb, Mo and Ta, and an alkali metal K as well, follows a scaling relation of T c/T p against kT/b 3(TpKT)½, where T p is the Peierls stress, b the length of the Burgers vector and K T the line tension factor for a screw dislocation c…
The kinetics of the site exchange reaction of Al 3+ cations between tetrahedral and octahedral sites in MgAl 2 O 4 and ZnAl 2 O 4 spinel in natural and synthetic powdered samples was determined by 27 Al MAS‐NMR spectroscopy with a high‐speed magic‐angle‐spinning probe using samples heat‐treated for different time peri…
Many chemical and physical methods, such as plasma, e-beam, sputtering, CVD and others, have been used to modify the structure of polymer surfaces by depositing thin inorganic films. Most of these techniques are based upon the use of high energy sources that ultimately can damage either chemically or physically polyme…
We present a model for the motion of hard spherical particles on a two-dimensional surface. The model includes both the interaction between the particles via collisions and the interaction of the particles with the substrate. We analyze in detail the effects of sliding and rolling friction, which are usually overlooke…
A renewed interest in the Laue diffraction technique has been brought about by the development of new, more intense and brilliant synchrotron sources along with their insertion devices such as wigglers and undulators, and by the prospect of using these sources to study structural dynamics by time-resolved crystallogra…
The conventional 1 M NH 4 OAc‐exchangeable potassium (K + ) soil test is inadequate in soils where nonexchangeable K + contributes significantly to crop nutrition. Studies were conducted (i) to compare the abilities of the 1 M NH 4 OAc method with a modified NaBPh 4 method to estimate critical soil K + levels, (ii) to…
Due to the ubiquitous presence of a Berry phase, in most cases of dynamical Jahn-Teller systems the symmetry of the vibronic ground state is the same as that of the original degenerate electronic state. As a single exception, the linear H x h icosahedral model, relevant to the physics of C60 cations, is determined by…
The correlation between the spin polarization of the tunneling current in ferromagnet/Al2O3/Al tunnel junction experiments and the magnetic moment in ferromagnets has been a mystery [Phys. Rev. B 16, 4907 (1977) and Phys. Rep. 238, 173 (1994)]. In this study, an attempt is made to explain this correlation. By assuming…
The ab initio linear muffin-tin-orbital formalism and the semiempirical method of the intermediate neglect of the differential overlap based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of ${\mathrm{KNbO}}_{3}$ perovskite crys…
A resonant method based on a tunnel-diode oscillator for precision measurements of relative impedance changes in materials is described. The system consists of an effective self-resonant LC tank circuit driven by a forward-biased tunnel diode operating in its negative resistance region. Samples under investigation are…
Continued innovations in the polymer industry have made polymer surface modification methods a subject of intense research. The importance and necessity of surface modification of plastics are explained, and the advantages of physical surface treatments over the less-sophisticated chemical methods are outlined. Curren…
This article compares the cure behavior and properties of phthalonitrile polymers derived from three different monomers, namely, 4,4′-bis(3,4-dicyanophenoxy)biphenyl, 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane and 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane. Rheometric measurements with monomer melt in th…
A range of methods for the detection of barium carbonate contaminant in barium titanate powder has been assessed, namely: X‐ray diffraction (XRD), scanning electron microscopy (SEM), with EDS‐X‐ray microanalysis, Fourier transform infrared spectroscopy (FT‐IR), thermogravimetric analysis (TGA), gas chromatography (GC)…
Transparent and conductive thin films of multicomponent oxides composed of binary and/or ternary compounds with varied chemical compositions have been prepared by rf or dc magnetron sputtering. Transparent conducting oxide (TCO) films were prepared in multicomponent oxides composed of ZnO, SnO2, Ga2O3, or MgO combined…
Thermal decomposition of boehmite and bayerite has been studied by X-ray diffraction and by NMR and IR spectroscopies. Coordination and site distortion of Al polyhedra have been estimated by 27 Al NMR spectroscopy while IR optical properties of transitional aluminas, η-, γ-, δ-, and θ-Al 2 O 3 were derived from IR nea…