Field assisted and flash sintering of alumina and its relationship to conductivity and MgO-doping

We show that flash-sintering in MgO-doped alumina is accompanied by a sharp increase in electrical conductivity. Experiments that measure conductivity in fully dense specimens, prepared by conventional sintering, prove that this is not a cause-and-effect relationship, but instead that the concomitant increase in the sintering rate and the conductivity share a common mechanism. The underlying mechanism, however, is mystifying since electrical conductivity is controlled by the transport of the fastest moving charged species, while sintering, which requires molecular transport or chemical diffusion, is limited by the slow moving charged species. Joule heating of the specimen during flash sintering cannot account for the anomalously high sintering rates. The sintering behavior of MgO-doped alumina is compared to that of nominally pure-alumina: the differences provide insight into the underlying mechanism for flash-sintering. We show that the pre-exponential in the Arrhenius equation for conductivity is enhanced in the non-linear regime, while the activation energy remains unchanged. The nucleation of Frenkel pairs is proposed as a mechanism to explain the coupling between flash-sintering and the non-linear increase in the conductivity.     

Chemical redox recovery of giant, small-gap and other fullerenes

A simple method for extracting otherwise insoluble fullerenes into organic solvents is presented. The fullerenes are reduced to anionic charge states by contact with zinc in the presence of an alkylphosphonium salt in tetrahydrofuran. The anionic fullerenes become soluble, and the non-fullerene carbon matrix is separated by filtration. The anionic fullerenes can then be precipitated from solution by the action of a chemical oxidant such as iodine. It is also shown that there are approximately as many fullerenes left behind in carbon arc soot after solvent extraction as are extracted by the solvent. In contrast, more than twice as many fullerenes are left behind in combustion-produced fullerene soot as are solvent-extracted. Thus, approximately 70% of the total recovered carbon product from large-scale combustion synthesis is fullerenes. Conversely, no detectable amount of fullerenes can be recovered from conventional carbon blacks under the same conditions. Analysis by single photon ionization mass spectrometry suggests that C₆₀ and C₇₀ account for almost half of the fullerenes that were not solvent-extractable, suggesting that they are readily incorporated into toluene-insoluble fullerene polymers. The redox process presented here is easily up-scaled, and we routinely recover 100 g of heretofore insoluble fullerenes in 8 h.     

Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature

Despite significant progress in carbon nanotube (CNT) synthesis by thermal chemical vapor deposition (CVD), the factors determining the structure of the resulting carbon filaments and other graphitic nanocarbons are not well understood. Here, we demonstrate that gas chemistry influences the crystal structure of carbon filaments grown at low temperatures (500 °C). Using thermal CVD, we decoupled the thermal treatment of the gaseous precursors (C₂H₄/H₂/Ar) and the substrate-supported catalyst. Varying the preheating temperature of the feedstock gas, we observed a striking transition between amorphous carbon nanofibers (CNFs) and crystalline CNTs. These results were confirmed using both a hot-wall CVD system and a cold-wall CVD reactor. Analysis of the exhaust gases (by ex situ gas chromatography) showed increasing concentrations of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) that correlated with the structural transition observed (characterized using high-resolution transmission electron microscopy). This suggests that the crystallinity of carbon filaments may be controlled by the presence of specific gas phase precursor molecules (e.g., VOCs and PAHs). Thus, direct delivery of these molecules in the CVD process may enable selective CNF or CNT formation at low substrate temperatures. The inherent scalability of this approach could impact many promising applications, especially in the electronics industry.     

Modeling of particle deposition in a vertical turbulent pipe flow at a reduced probability of particle sticking to the wall

Particle deposition on the wall in a dilute turbulent vertical pipe flow is modeled. The different mechanisms of particle transport to the wall are considered, i.e., Brownian motion, turbulent diffusion and turbophoresis. The Saffman lift force, the electrostatic force, the virtual mass effect and wall surface roughness are taken into account in the model developed. A boundary condition that accounts for the probability of particle sticking to the wall is suggested. An analytical solution for deposition of small Brownian particles is obtained. A particle relaxation time range, where the model developed is reliably applicable, is evaluated. Computational results obtained at different particle-wall sticking probabilities in the wide particle relaxation time range are presented and discussed.     

Hexafluorozirconic acid based surface pretreatments: Characterization and performance assessment

A new phosphate-free pretreatment from Henkel Corp. named TecTalis^®, was investigated. The treatment bath is composed of dilute hexafluorozirconic acid with small quantities of non-hazardous components containing Si and Cu. The corrosion resistance of treated steel was compared to samples treated in a phosphate conversion coating bath, in simple hexafluorozirconic acid and in TecTalis without the addition of the Cu-containing component. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used to characterize the coating surface morphology, structure and composition. A Quartz Crystal Microbalance (QCM) was used for studying film growth kinetics on thin films of pure Fe, Al and Zn. Electrochemical Impedance Spectroscopy (EIS) was performed on treated and painted steel for studying long-term corrosion performance of the coatings. The phosphate-free coating provided long-term corrosion performance comparable to that of phosphate conversion coatings. The coatings uniformly cover the surface in the form of 10–20 nm sized nodules and clusters of these features up to 500 nm in size. The coatings are usually about 20–30 nm thick and are mostly composed of Zr and O with enrichment of copper at randomly distributed locations and clusters.     

Effects of selected water chemistry variables on copper pitting propagation in potable water

The pit propagation behavior of copper (UNS C11000) was investigated from an electrochemical perspective using the artificial pit method. Pit growth was studied systematically in a range of HCO₃⁻, SO₄²⁻ and Cl⁻ containing-waters at various concentrations. Pit propagation was mediated by the nature of the corrosion products formed both inside and over the pit mouth (i.e., cap). Certain water chemistry concentrations such as those high in sulfate were found to promote fast pitting that could be sustained over long times at a fixed applied potential but gradually stifled in all but the lowest concentration solutions. In contrast, Cl⁻ containing waters without sulfate ions resulted in slower pit growth and eventual repassivation. These observations were interpreted through understanding of the identity, amount and porosity of corrosion products formed inside and over pits. These factors controlled their resistive nature as characterized using electrochemical impedance spectroscopy. A finite element model (FEM) was developed which included copper oxidation kinetics, transport by migration and diffusion, Cu(I) and Cu(II) solid corrosion product formation and porosity governed by equilibrium thermodynamics and a saturation index, as well as pit current and depth of penetration. The findings of the modeling were in good agreement with artificial pit experiments. Malachite, bronchantite, cuprite, nantokite and atacamite corrosion products were both observed in experiment and predicted by the model. Stifling and/or repassivation occurred when the resistance of the corrosion product layer became high enough to lower the pit bottom potential and pit current density such as 10^-5 A/cm² could be attained with thick and dense layer. The ramifications of these findings towards pit propagation characteristics in potable waters will be discussed with improved insight into the roles of Cl⁻ and SO₄²⁻ ions.     

CFD simulation of a chemical-looping fuel reactor utilizing solid fuel

A computational fluid dynamic (CFD) study has been carried out for the fuel reactor for a new type of combustion technology called chemical-looping combustion (CLC). CLC involves combustion of fuels by heterogeneous chemical reactions with an oxygen carrier, usually a granular metal oxide, exchanged between two reactors. There have been extensive experimental studies on CLC, however CFD simulations of this concept are quite limited. In the present paper we have developed a CFD model for the fuel reactor of a chemical-looping combustor described in the literature, which utilized a Fe-based carrier (ilmenite) and coal. An Eulerian multiphase continuum model was used to describe both the gas and solid phases, with detailed sub-models to account for fluid–particle and particle–particle interaction forces. Global reaction models of fuel and carrier chemistry were utilized. The transient results obtained from the simulations were compared with detailed experimental time-varying outlet species concentrations (Leion et al., 2008) and provided a reasonable match with the reported experimental data.     

Preparation and characterisation of a poly(acrylamidoglycolic acid-co-acrylamide) hydrogel for selective binding of Cu and application to diffusive gradients in thin films measurements

A poly(acrylamidoglycolic acid-co-acrylamide) [poly(AAGA-co-AAm)] hydrogel was prepared by copolymerising 2-acrylamidoglycolic acid (AAGA) with acrylamide (AAm). The copolymer hydrogel composition and structure was characterised by FTIR spectroscopy and elemental microanalysis and found to contain 3.5 AAGA monomer units for each AAm monomer unit. This was similar to the monomer ratios used in the synthesis. The metal ion binding properties of the hydrogel were characterised for a range of metal ions (Cu²⁺, Cd²⁺, K⁺, Na⁺, Mg²⁺ and Ca²⁺) under varying conditions of pH, ionic strength, metal concentration and time. The hydrogel was shown to bind Cu²⁺ and Cd²⁺ strongly under non-competitive binding conditions, with binding capacities of 5.3 and 5.1 μmol cm⁻², respectively. The binding capacity of each metal decreased, under competitive binding conditions (with a range of metal ions present at 17.8 μN), to 1.3 and 0.17 μmol cm⁻², respectively, indicating stronger selectivity for Cu²⁺. The metal ions were readily recovered (>94%) by eluting with 2 M nitric acid solution for 24 h. The binding capacities for Cu²⁺ and Cd²⁺ were also found to decrease with increasing ionic strength and at pH values <5. The copolymer was found to have an equilibrium swelling ratio (q_w) of over 500 at a maxima of pH 5.4 and at low ionic strengths. Finally, the copolymer hydrogel was tested as a binding phase with the diffusive gradients in thin films technique. A linear mass vs. time relationship was observed for Cu²⁺ in synthetic Windermere water with a recovery of approximately 100%.     

Chemical doping of single-wall carbon nanotubes

Single-wall carbon nanotubes can be doped, or intercalated, with electron donors or acceptors, similar to graphite and some conjugated polymers. The resulting materials show many of the same features: enhanced electrical conductivity, conduction electron paramagnetism, partial or complete reversibility, and cyclability. Reactions may be carried out in vapor or liquid phase, or electrochemically. Structural information is sketchy at best, due to the limited quality of currently available materials and solvent-related effects. Recent developments in coagulation-based fiber extrusion and partially aligned materials offer new opportunities for novel material modifications by chemical doping.     

Synthesis and structure-activity relationship of the isoindolinyl benzisoxazolpiperidines as potent,selective, and orally active human dopamine D4 receptor antagonists

A new class of potent dopamine D(4) antagonists was discovered with selectivity over dopamine D(2) and the alpha-1 adrenoceptor. The lead compound was discovered by screening our compound collection. The structure-activity relationships of substituted isoindoline rings and the chirality about the hydroxymethyl side chain were explored. The isoindoline analogues showed modest differences in potency and selectivity. The S enantiomer proved to be the more potent enantiomer at the D(4) receptor. Several analogues with greater than 100-fold selectivity for D(4) over D(2) and the alpha-1 adrenoreceptor were discovered. Several selective analogues were active in vivo upon oral or intraperitoneal administration. A chiral synthesis starting from either D- or L-O-benzylserine is also described.