Maria R. Soares

Evidence for the Aging of Wax Deposits in Crude Oils by Ostwald Ripening

Abstract The ageing of wax deposits is known to occur in paraffinic waxes forming in pipelines and storage tanks. It is characterized by a hardening of the deposit and an increase in paraffin content. A mechanism for the ageing of deposits, based on diffusion caused by temperature-concentration gradients within the wax, has been previously proposed. This work presents evidence indicating Ostwald Ripening to be another ageing mechanism of wax deposits. Rheology of paraffinic crudes, kept isothermally at temperatures in the neighborhood of the pour point, shows a kinetic of hardening of the oil samples. The X-ray diffraction and Cross Polar Microscopy indicate this phenomenon to be caused by an increase of the crystallites size with time. The DSC measurements support the idea that recrystallization takes place in the wax sample. The evidence gathered shows another ageing mechanism based on the recryztallization of the paraffins, such as Ostwald Ripening, to be responsible for the ageing of wax deposits besides the diffusion mechanism previously proposed

Polymorphism in tetra-butylammonium salts of Keggin-type polyoxotungstates

Abstract Some tetra-n-butylammonium salts of Keggin-type polyoxotungstates, namely α-[PW11O39]7−, α-[PW11M(H2O)O39]5−, MII=Co, Ni, α-[PW11CuO39]5−, α-[PW11Fe(H2O)O39]4− and α-[PW11Fe(OH)O39]5−, with the general formula [(C4H9)4N]4Hx[anion]·nH2O, were studied by powder X-ray diffraction. The salts α-[(C4H9)4N]4H3[PW11O39] and α-[(C4H9)4N]4H[PW11CuO39] were obtained in two crystalline phases, a behavior that has been scarcely documented in compounds with Keggin polyoxometalates. One of the phases corresponds to a body-centered cubic unit cell (Im 3 symmetry) and the other has a body-centered tetragonal unit cell (I41 or I4122 symmetry). For the compounds with Cu, the cell parameters of the cubic and tetragonal structures were a=17.675(2) and a=18.775(2), c=14.646(2) A, respectively. The cubic phase was obtained by precipitation from aqueous solution. The tetragonal phase was isolated by crystallization from acetonitrile. The remaining compounds were found to crystallize with one of these two crystal structures. α-[(C4H9)4N]4Hx[PW11M(H2O)O39]·nH2O, with M=Co, Ni, x=1, and M=Fe, x=0, n=1–2, were found to crystallize with the body-centered cubic structure. The method of synthesis used to prepare α-[(C4H9)4N]4H[PW11Fe(OH)O39] afforded the tetragonal phase. The conversion of the cubic into the tetragonal phase and the stability of each one are discussed.

Ab initio structure determination of novel small-pore metal-silicates: knots-and-crosses structures

Abstract Sodium chloride stannosilicate AV-13 (Na2.26SnSi3O9Cl0.26·xH2O) and zirconium and hafnium analogues of this material have been prepared and their structures solved from powder X-ray diffraction data using direct methods, and 23Na, 29Si and 119Sn solid-state NMR. AV-13 materials are small-pore solids, probably more adequately described as tunnel structures. The AV-13 framework consists of corner-sharing MO6 (M=Sn, Zr, Hf) octahedra and SiO4 tetrahedra. The latter form six-membered [Si6O18]12− rings, which are interconnected by MO6 octahedra. The structure is better understood by considering a three-dimensional knots-and-crosses lattice. In a given layer, successive distorted-cube M8 cages contain [Na6−x(H2O)x](H2O,Cl−) octahedra (knots) and cyclohexasilicate (crosses) units. While the former are extra-framework species, the six-membered rings are, of course, part of the framework. The cages are accessed via seven-membered [M3Si4O27]26− windows, with free aperture ca. 2.3×3.2 A, one per each pseudo-cube face. Pilling up layers generates the structure, with knots-and-crosses alternating. The non-framework five-coordinated Na cations are disordered.

New insights into the temperature-dependent photoluminescence of Mg-doped GaAs nanowires and epilayers

The intentional introduction of impurities in semiconductor nanowires is very important in view of device applications. Doping affects the electronic energy level structure which in the case of III–V nanowires can also be strongly influenced by the simultaneous occurrence of two polytypes, zinc-blende (ZB) and wurtzite (WZ). In this work, we report a study on GaAs nanowires with different Mg-acceptor doping levels through temperature dependent photoluminescence. A comparable investigation is presented for Mg-doped GaAs epilayers. For the later, only a band is observed which is ascribed to the involvement of the Mg acceptor due to the observed bandgap energy narrowing effect with increasing the doping level, and the temperature dependent behaviour. A different behaviour is reported for nanowires: several radiative transitions are observed whose temperature dependence follows that of bulk GaAs, in accordance with spatially indirect recombination. Although the polytypic regions mask the role of doping in nanowires it favours the charge separation required for photovoltaic applications.

Growth of Lead Zirconate Titanate Single Crystals by the High Temperature Solution Method

To grow single crystals within the solid solutions between PbZrO3 and PbTiO3, PbZrxTi1- xO3 or PZT, is very difficult when compositions are around the morphotropic phase boundary (MPB). In this work, the PZT single crystals were grown by the high temperature solution method, using fluxes with a constant composition, different thermal profiles and different flux/PZT ratios. This approach was successful used to obtain PZT single crystals with compositions near the MPB, of 2 × 2 × 2 mm3 size and without second phases. The specific experimental conditions to obtain these crystals are analysed and discussed.

Substrate and Mg doping effects in GaAs nanowires

Mg doping of GaAs nanowires has been established as a viable alternative to Be doping in order to achieve p-type electrical conductivity. Although reports on the optical properties are available, few reports exist about the physical properties of intermediate-to-high Mg doping in GaAs nanowires grown by molecular beam epitaxy (MBE) on GaAs(111)B and Si(111) substrates. In this work, we address this topic and present further understanding on the fundamental aspects. As the Mg doping was increased, structural and optical investigations revealed: i) a lower influence of the polytypic nature of the GaAs nanowires on their electronic structure; ii) a considerable reduction of the density of vertical nanowires, which is almost null for growth on Si(111); iii) the occurrence of a higher WZ phase fraction, in particular for growth on Si(111); iv) an increase of the activation energy to release the less bound carrier in the radiative state from nanowires grown on GaAs(111)B; and v) a higher influence of defects on the activation of nonradiative de-excitation channels in the case of nanowires only grown on Si(111). Back-gate field effect transistors were fabricated with individual nanowires and the p-type electrical conductivity was measured with free hole concentration ranging from 2.7 × 1016 cm−3 to 1.4 × 1017 cm−3. The estimated electrical mobility was in the range ≈0.3–39 cm2 /Vs and the dominant scattering mechanism is ascribed to the WZ/ZB interfaces. Electrical and optical measurements showed a lower influence of the polytypic structure of the nanowires on their electronic structure. The involvement of Mg in one of the radiative transitions observed for growth on the Si(111) substrate is suggested.

Synthesis and Ab Initio Structure Determination from Powder Diffraction Data of K 4 Sn 2 Si 6 O 18

Abstract. The crystal structure of a new potassium stannosilicateK 4 Sn 2 Si 6 O 18 (AV -11) has been determined ab initio from powder -Xray diffraction data. The unit cell is trigonal, space group R 3 (no. 146), Z=3 with cell dimensions a = 10.1587, c = 14.8039 A, γ= 120o, V = 1323 3A . The structure is made up of SnO 6 octahedra and SiO 4 tetrahedra by sharing corners. The SiO 4 tetrahedra form a helix chain, periodically repeating every six tetrahedra. These chains extend along the [001] direction and are linked by isolated Sn-O octahedron giving a mixed framework. Introduction The synthesis of inorganic microporous materials with mixed framework of interconnected octahedral- and tetrahedral-oxide is presently raising considerable interest [1]. Among these mixed framework materials, more than twenty stannosilicates are known and a few tannosilicate phasess have been crystallised from hydrothermal conditions ([2] and ref. therein). In the course of our work on the synthesis of stannosilicates, we found a patent by the Exxon Research and Engineering Company, 1992 [3] where the synthesis of six stannosilicates are reported, but with no structural information given. One of these materials, phase X, presents a -ray pattern similar to the material Xreported here. Thus, we wish to report the synthesis and the structural determination by powder XRD data of a new potassium stannosilicate, named AV-11 (Aveiro microporous solid no.11).