Single phase a-plane MgZnO epilayers for UV optoelectronics: substitutional behaviour of Mg at large contents

High quality 1 μm thick a-plane MgxZn1−xO layers were produced by molecular beam epitaxy with Mg contents higher than 50%. Resonant Rutherford backscattering spectrometry combined with ion channeling revealed a uniform growth in both composition and atomic order. The lattice-site location of Mg, Zn and O elements was determined independently, proving the substitutional behaviour of Mg in Zn-sites of the wurtzite lattice. X-Ray diffraction pole figure analysis also confirms the absence of phase separation. Optical properties at such high Mg contents were studied in Schottky photodiodes.

Estudo da seletividade de catalisadores a base de cobre e paládio em transformações de hidrocarbonetos insaturados

Compreender a correlação entre as características de um catalisador particular e seu desempenho catalítico tem sido um dos principais objetos da pesquisa em catálise heterogênea a fim de usar esse conhecimento para o desenho racional de catalisadores mais ativos, seletivos e estáveis. A seletividade é um dos fatores mais importantes a ser controlado pelo desenho de catalisadores, podendo ser alcançada de diversas maneiras, levando-se em consideração mudanças do tipo estrutural, química, eletrônica, de composição, de cinética e de energia. O trabalho descrito nessa tese de doutorado compreende a síntese e caracterização de catalisadores compostos de nanopartículas de óxido de cobre, paládio e cobre-paládio e seu estudo em reações de hidrogenação e oxidação seletivas de hidrocarbonetos insaturados. Os catalisadores foram preparados através da deposição de nanopartículas dos metais cataliticamente ativos sobre suportes magneticamente recuperáveis compostos de nanopartículas de magnetita revestidas por sílica com superfícies funcionalizada com diferentes grupos orgânicos. A natureza magnética do suporte permitiu a fácil separação do catalisador do meio reacional pela simples aproximação de um ímã na parede do reator. O catalisador pôde ser completamente separado da fase líquida, fazendo com que a utilização de outros métodos de separação como filtração e centrifugação, comumente utilizados em sistemas heterogêneos líquidos, fossem completamente dispensados. Os catalisadores foram inicialmente testados em reações de hidrogenação de alquenos e alquinos. As reações de hidrogenação foram realizadas utilizando hidrogênio molecular como agente redutor, dispensando a utilização de agentes redutores mais agressivos. Os catalisadores compostos de NPs de Pd mostram excelente atividade e capacidade de reutilização na hidrogenação de cicloexeno, podendo ser utilizados em até 15 ciclos sem perda de atividade. Nas reações de hidrogenação de alquinos, os catalisadores que contêm cobre mostraram maior seletividade para a obtenção dos produtos de semi-hidrogenação, com destaque para o catalisador composto de NPs de CuPd, que não apresenta nem traços do produto de hidrogenação completa na amostra final. Esse catalisador bimetálico alia as características do paládio (elevada atividade) e do cobre (elevada seletividade) para fornecer um catalisador ativo e seletivo para a transformação desejada. Além disso, os grupos funcionais presentes na superfície do suporte catalítico mostraram influência na atividade e seletividade para a hidrogenação de alquenos e alquinos. Os catalisadores sintetizados também foram testados na reação de oxidação de cicloexeno e mostraram seletividade para a produção do composto carbonílico α,β-insaturado, cicloex-2-en-1-ona, que é um reagente de partida de grande interesse para a síntese de diversos materiais na indústria química. As reações de oxidação foram realizadas utilizando-se apenas O2 como oxidante primário, dispensando o uso de oxidantes tóxicos como cromatos, permanganatos ou compostos halogenados, que não são recomendados do ponto de vista ambiental. Os catalisadores sintetizados puderam ser reutilizados em sucessivos ciclos de oxidação, mostrando seletividade para a formação dos produtos alílicos em todos os ciclos. Os catalisadores foram estáveis sob as condições reacionais e não apresentaram problemas de lixiviação da espécie ativa para o meio reacional, que é comum na catálise heterogênea. Um estudo cinético mostrou que, mesmo no início da reação, o catalisador tem seletividade para a ocorrência de oxidação alílica em detrimento da reação de oxidação direta que dá origem ao epóxidos correspondente, e se mostrou condizente com o mecanismo proposto na literatura para a reação de oxidação de alquenos via radicalar.

Separation and fractionation of order and disorder in highly polydisperse systems

We study a polydisperse soft-spheres model for colloids by means of microcanonical Monte Carlo simulations. We consider a polydispersity as high as 24%. Although solidification occurs, neither a crystal nor an amorphous state are thermodynamically stable. A finite size scaling analysis reveals that in the thermodynamic limit: a the fluid-solid transition is rather a crystal-amorphous phase-separation, b such phase-separation is preceded by the dynamic glass transition, and c small and big particles arrange themselves in the two phases according to a complex pattern not predicted by any fractionation scenario.

Vapor-liquid equilibria using the Gibbs energy and the common tangent plane criterion

Phase thermodynamics is often perceived as a difficult subject that many students never become fully comfortable with. The Gibbsian geometrical framework can help students to gain a better understanding of phase equilibria. An exercise to interpret the vapor-liquid equilibrium of a binary azeotropic mixture, using the equilibrium condition based on the common tangent plane criterion (the Gibbs stability test), is presented in this paper. From a T-composition phase diagram for the mixture, the temperature is set at different values: above, intermediate to, and below the boiling temperatures of the pure components, to intersect different regions of the system. Students prepare an Excel spreadsheet where the Gibbs energy of mixing of the vapor and liquid mixtures are calculated and represented over the whole range of compositions and then, apply the Gibbs stability test to ascertain the aggregation state of the system and to calculate the VL phase equilibrium compositions. Finally, Matlab is used to generate the 3D Gibbs energy of mixing surfaces for both phases over the whole range of temperatures which facilitates the geometrical interpretation of the vapor-liquid equilibrium.

Screen-printed electrode-based electrochemical detector coupled with in-situ ionic-liquid-assisted dispersive liquid–liquid microextraction for determination of 2,4,6-trinitrotoluene

A novel method is reported, whereby screen-printed electrodes (SPELs) are combined with dispersive liquid–liquid microextraction. In-situ ionic liquid (IL) formation was used as an extractant phase in the microextraction technique and proved to be a simple, fast and inexpensive analytical method. This approach uses miniaturized systems both in sample preparation and in the detection stage, helping to develop environmentally friendly analytical methods and portable devices to enable rapid and onsite measurement. The microextraction method is based on a simple metathesis reaction, in which a water-immiscible IL (1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [Hmim][NTf2]) is formed from a water-miscible IL (1-hexyl-3-methylimidazolium chloride, [Hmim][Cl]) and an ion-exchange reagent (lithium bis[(trifluoromethyl)sulfonyl]imide, LiNTf2) in sample solutions. The explosive 2,4,6-trinitrotoluene (TNT) was used as a model analyte to develop the method. The electrochemical behavior of TNT in [Hmim][NTf2] has been studied in SPELs. The extraction method was first optimized by use of a two-step multivariate optimization strategy, using Plackett–Burman and central composite designs. The method was then evaluated under optimum conditions and a good level of linearity was obtained, with a correlation coefficient of 0.9990. Limits of detection and quantification were 7 μg L−1 and 9 μg L−1, respectively. The repeatability of the proposed method was evaluated at two different spiking levels (20 and 50 μg L−1), and coefficients of variation of 7 % and 5 % (n = 5) were obtained. Tap water and industrial wastewater were selected as real-world water samples to assess the applicability of the method.

Screen-printed electrode based electrochemical detector coupled with ionic liquid dispersive liquid–liquid microextraction and microvolume back-extraction for determination of mercury in water samples

A novel approach is presented, whereby gold nanostructured screen-printed carbon electrodes (SPCnAuEs) are combined with in-situ ionic liquid formation dispersive liquid–liquid microextraction (in-situ IL-DLLME) and microvolume back-extraction for the determination of mercury in water samples. In-situ IL-DLLME is based on a simple metathesis reaction between a water-miscible IL and a salt to form a water-immiscible IL into sample solution. Mercury complex with ammonium pyrrolidinedithiocarbamate is extracted from sample solution into the water-immiscible IL formed in-situ. Then, an ultrasound-assisted procedure is employed to back-extract the mercury into 10 µL of a 4 M HCl aqueous solution, which is finally analyzed using SPCnAuEs. Sample preparation methodology was optimized using a multivariate optimization strategy. Under optimized conditions, a linear range between 0.5 and 10 µg L−1 was obtained with a correlation coefficient of 0.997 for six calibration points. The limit of detection obtained was 0.2 µg L−1, which is lower than the threshold value established by the Environmental Protection Agency and European Union (i.e., 2 µg L−1 and 1 µg L−1, respectively). The repeatability of the proposed method was evaluated at two different spiking levels (3 and 10 µg L−1) and a coefficient of variation of 13% was obtained in both cases. The performance of the proposed methodology was evaluated in real-world water samples including tap water, bottled water, river water and industrial wastewater. Relative recoveries between 95% and 108% were obtained.

Thermosetting Blends of Polybenzoxazine and Poly(ε-caprolactone): Phase Behavior and Intermolecular Specific Interactions

Polybenzoxazine (PBA-a)/poly(epsilon-caprolactone) (PCL) blends were prepared by an in situ curing reaction of benzoxazine (BA-a) in the presence of PCL. Before curing, the benzoxazine (BA-a)/PCL blends are miscible, which was evidenced by the behaviors of single and composition-dependant glass transition temperature and equilibrium melting point depression. However, the phase separation induced by polymerization was observed after curing at elevated temperature. It was expected that after curing, the PBA-a/PCL blends would be miscible since the phenolic hydroxyls in the PBA-a molecular backbone have the potential to form inter- molecular hydrogen-bonding interactions with the carbonyls of PCL and thus would fulfil the miscibility of the blends. The resulting morphology of the blends prompted an investigation of the status of association between PBA-a and PCL under the curing conditions. Although Fourier-transform infrared spectroscopy (FT-IR) showed that there were intermolecular hydrogen-bonding interactions between PBA-a and PCL at room temperature, especially for the PCL-rich blends, the results of variable temperature FT-IR spectroscopy by the model compound indicate that the phenolic hydroxyl groups could not form efficient intermolecular hydrogen-bonding interactions at elevated temperatures, i.e., the phenolic hydroxyl groups existed mainly in the non-associated form in the system during curing. The results are valuable to understand the effect of curing temperature on the resulting morphology of the thermosetting blends. SEM micrograph of the dichloromethane-etched fracture surface of a 90:10 PBA-a PCL blend showing a heterogeneous morphology.

Phase behavior, crystallization, and nanostructures in thermoset blends of epoxy resin and amphiphilic star-shaped block copolymers

This article reports thermoset blends of bisphenol A-type epoxy resin (ER) and two amphiphilic four-arm star-shaped diblock copolymers based on hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO). 4,4'-Methylenedianiline (MDA) was used as a curing agent. The first star-shaped diblock copolymer with 70 wt% ethylene oxide (EO), denoted as (PPO-PEO)(4), consists of four PPO-PEO diblock arms with PPO blocks attached on an ethylenediamine core; the second one with 40 wt% EO, denoted as (PEO-PPO)(4), contains four PEO-PPO diblock arms with PEO blocks attached on an ethylenediamine core. The phase behavior, crystallization, and nanoscale structures were investigated by differential scanning calorimetry, transmission electron microscopy, and small-angle X-ray scattering. It was found that the MDA-cured ER/(PPO-PEO)(4) blends are not macroscopically phase-separated over the entire blend composition range. There exist, however, two microphases in the ER/(PPO-PEO)(4) blends. The PPO blocks form a separated microphase, whereas the ER and the PEO blocks, which are miscible, form another microphase. The ER/(PPO-PEO)(4) blends show composition-dependent nanostructures on the order of 10-30 nm. The 80/20 ER/(PPO-PEO)(4) blend displays spherical PPO micelles uniformly dispersed in a continuous ER-rich matrix. The 60/40 ER/(PPO-PEO)(4) blend displays a combined morphology of worm-like micelles and spherical micelles with characteristic of a bicontinuous microphase structure. Macroscopic phase separation took place in the MDA-cured ER/(PEO-PPO)(4) blends. The MDA-cured ER/(PEO-PPO)(4) blends with (PEO-PPO)(4) content up to 50 wt% exhibit phase-separated structures on the order of 0.5-1 mu m. This can be considered to be due to the different EO content and block sequence of the (PEO-PPO)(4) copolymer. (c) 2006 Wiley Periodicals, Inc.

Systematic selection of solvents for the fabrication of 3D combined macro- and microporous polymeric scaffolds for soft tissue engineering

In this study, we investigate the fabrication of 3D porous poly(lactic-co-glycolic acid) (PLGA) scaffolds using the thermally-induced phase separation technique. The current study focuses on the selection of alternative solvents for this process using a number of criteria, including predicted solubility. toxicity, removability and processability. Solvents were removed via either vacuum freeze-drying or leaching, depending on their physical properties. The residual solvent was tested using gas chromatography-mass spectrometry. A large range of porous, highly interconnected scaffold architectures with tunable pore size and alignment was obtained, including combined macro- and microporous structures and an entirely novel 'porous-fibre' structure. The morphological features of the most promising poly(lactic-co-glycolic acid) scaffolds were analysed via scanning electron microscopy and X-ray micro-computed tomography in both two and three dimensions. The Young's moduli of the scaffolds under conditions of temperature, pH and ionic strength similar to those found in the body were tested and were found to be highly dependent on the architectures.

Particle formation during soy protein precipitation

The current success of soy foods is driving soy ingredient manufacturers to develop innovative products for food manufacturers. One such innovation is separating the soy proteins glycinin and beta-conglycinin to take advantage of their individual functional and nutritional properties. Precipitation by acidification is a low-cost method for separating these two proteins. Separation is achieved by preferentially precipitating glycinin at pH ~ 6 while beta-conglycinin remains in solution. Understanding the particle formation during protein precipitation is important as it can influence the efficiency of the precipitation process as well as subsequent downstream processes such as the particle-liquid separation step, usually achieved by centrifugation. Most of the previous soy protein precipitation studies are limited to precipitation at pH 4 as this is the pH range most commonly used in the commercial manufacturing of soy protein isolates. To date, there have been no studies on the particle formation during precipitation at pH > 5.Precipitation of soy protein is generally thought to occur by the rapid formation of primary particles in the size range of 0.1 - 0.3 microns followed by aggregation of these particles via collision to aggregates of size about 1 - 50 microns. The formation of the primary particles occurs on a time scale much shorter than that of the overall precipitation process (Nelson and Glatz, 1985). This study shows that precipitation of soy protein is indeed rapid. At high pH levels, binary liquid-liquid separation occurs forming a protein-rich heavy phase. The protein-rich phase appears as droplets which can be coalesced to form a uniform bulk layer under centrifugation forces. Upon lowering the pH level by the addition of acid, further protein is precipitated as amorphous material which binds the droplets together to form aggregates of amorphous precipitates. Liquid-liquid separation has been observed in many protein solutions but this phenomenon has only scarcely been reported in the literature for soy proteins. It presents an exciting opportunity for an innovative product. Features of the liquid-phase protein such as protein yield and purity will be characterized.

Interaction of liquid copper with sintered iron compacts

Interaction of liquid copper with sintered iron is important in brazing, liquid phase sintering and infiltration. In brazing, the penetration of liquid copper into the pores is to be `avoided', whereas in infiltration processes it is `encouraged', and in liquid phase sintering it should be `controlled' so that optimum mechanical properties are achieved. The main objective of the research is to model the interaction by studying the effect of the process variables on the mechanisms of copper interaction in Fe-Cu and Fe-Cu-C systems. This involves both theoretical and experimental considerations. Dilatometric investigations at 950, 1125 and 1200oC, together with metallographic analyses were carried out to clarify the copper growth phenomenon. It is shown that penetration of liquid copper into the iron grain boundaries is the major cause of dimensional changes. Infiltration profiles revealed that copper penetration between the iron interparticle contact points and along iron grain boundaries is a rapid process. The extent of copper penetration depends on the dihedral angle. Large dihedral angles hinder, and small angles promote copper penetration into the grain boundaries. Dihedral angle analysis shows that the addition of 0.6wt.% graphite reduces the number of zero dihedral angle from 27 to 3o and increases the mean dihedral angle from 9.8 to 41.5o. The dihedral angle was lowest at 1125oC and then increased to higher values as the system approached its equilibrium condition. Elementally mixed (E.M.) Fe-Cu compacts showed a rapid expansion at the copper melting point. However, graphite additions reduced compact growth by increasing the mean dihedral angle. In order to reduce the copper growth phenomenon, iron powder was coated with a thin layer of copper by an immersion coating (I.C.) technique. The dilatometric curves revealed an overall shrinkage in the I.C. compacts compared to their corresponding E.M. compacts. Multiple regression models showed that temperature had the most effect on dimensional changes and density had the most contributing effect upon the copper penetration area in the infiltrated powder metallurgy compacts.

Improvement of bio-oil stability in wood pyrolysis process

Pyrolysis is one of several thermochemical technologies that convert solid biomass into more useful and valuable bio-fuels. Pyrolysis is thermal degradation in the complete or partial absence of oxygen. Under carefully controlled conditions, solid biomass can be converted to a liquid known as bie-oil in 75% yield on dry feed. Bio-oil can be used as a fuel but has the drawback of having a high level of oxygen due to the presence of a complex mixture of molecular fragments of cellulose, hemicellulose and lignin polymers. Also, bio-oil has a number of problems in use including high initial viscosity, instability resulting in increased viscosity or phase separation and high solids content. Much effort has been spent on upgrading bio-oil into a more usable liquid fuel, either by modifying the liquid or by major chemical and catalytic conversion to hydrocarbons. The overall primary objective was to improve oil stability by exploring different ways. The first was to detennine the effect of feed moisture content on bio-oil stability. The second method was to try to improve bio-oil stability by partially oxygenated pyrolysis. The third one was to improve stability by co-pyrolysis with methanol. The project was carried out on an existing laboratory pyrolysis reactor system, which works well with this project without redesign or modification too much. During the finishing stages of this project, it was found that the temperature of the condenser in the product collection system had a marked impact on pyrolysis liquid stability. This was discussed in this work and further recommendation given. The quantity of water coming from the feedstock and the pyrolysis reaction is important to liquid stability. In the present work the feedstock moisture content was varied and pyrolysis experiments were carried out over a range of temperatures. The quality of the bio-oil produced was measured as water content, initial viscosity and stability. The result showed that moderate (7.3-12.8 % moisture) feedstock moisture led to more stable bio-oil. One of drawbacks of bio-oil was its instability due to containing unstable oxygenated chemicals. Catalytic hydrotreatment of the oil and zeolite cracking of pyrolysis vapour were discllssed by many researchers, the processes were intended to eliminate oxygen in the bio-oil. In this work an alternative way oxygenated pyrolysis was introduced in order to reduce oil instability, which was intended to oxidise unstable oxygenated chemicals in the bio-oil. The results showed that liquid stability was improved by oxygen addition during the pyrolysis of beech wood at an optimum air factor of about 0.09-0.15. Methanol as a postproduction additive to bio-oil has been studied by many researchers and the most effective result came from adding methanol to oil just after production. Co-pyrolysis of spruce wood with methanol was undertaken in the present work and it was found that methanol improved liquid stability as a co-pyrolysis solvent but was no more effective than when used as a postproduction additive.

Phase equilibrium studies at normal pressures

A total pressure apparatus has been developed to measure vapour-liquid equilibrium data on binary mixtures at atmospheric and sub-atmospheric pressures. The method gives isothermal data which can be obtained rapidly. Only measurements of total pressure are made as a direct function of composition of synthetic liquid phase composition, the vapour phase composition being deduced through the Gibbs-Duhem relationship. The need to analyse either of the phases is eliminated. As such the errors introduced by sampling and analysis are removed. The essential requirements are that the pure components be degassed completely since any deficiency in degassing would introduce errors into the measured pressures. A similarly essential requirement was that the central apparatus would have to be absolutely leak-tight as any leakage of air either in or out of the apparatus would introduce erroneous pressure readings. The apparatus was commissioned by measuring the saturated vapour pressures of both degassed water and ethanol as a function of temperature. The pressure-temperature data on degassed water measured were directly compared with data in the literature, with good agreement. Similarly the pressure-temperature data were measured for ethanol, methanol and cyclohexane and where possible a direct comparison made with the literature data. Good agreement between the pure component data of this work and those available in the literature demonstrates firstly that a satisfactory degassing procedure has been achieved and that secondly the measurements of pressure-temperature are consistent for any one component; since this is true for a number of components, the measurements of both temperature and pressure are both self-consistent and of sufficient accuracy, with an observed compatibility between the precision/accuracy of the separate means of measuring pressure and temperature. The liquid mixtures studied were of ethanol-water, methanol-water and ethanol-cyclohexane. The total pressure was measured as the composition inside the equilibrium cell was varied at a set temperature. This gave P-T-x data sets for each mixture at a range of temperatures. A standard fitting-package from the literature was used to reduce the raw data to yield y-values to complete the x-y-P-T data sets. A consistency test could not be applied to the P-T-x data set as no y-values were obtained during the experimental measurements. In general satisfactory agreement was found between the data of this work and those available in the literature. For some runs discrepancies were observed, and further work recommended to eliminate the problems identified.

Thin film bearings with phase change of lubricant

The possible evaporation of lubricant in fluid film bearings has been investigated theoretically and by experiment using a radial flow hydrostatic bearing supplied with liquid refrigerant R114. Good correlation between measured and theoretical values was obtained using a bespoke computational fluid dynamic model in which the flow was assumed to be laminar and adiabatic. The effects of viscous dissipation and vapour generation within the fluid film are fully accounted for by applying a fourth order Runge-Kutta routine to satisfy the radial and filmwise transverse constraints of momentum, energy and mass conservation. The results indicate that the radial velocity profile remains parabolic while the flow remains in the liquid phase and that the radial rate of enthalpy generation is then constant across the film at a given radius. The results also show that evaporation will commence at a radial location determined by geometry and flow conditions and in fluid layers adjacent to the solid boundaries. Evaporation is shown to progress in the radial direction and the load carrying capacity of such a bearing is reduced significantly. Expressions for the viscosity of the liquid/vapour mixture found in the literature survey have not been tested against experimental data. A new formulation is proposed in which the suitable choice of a characteristic constant yields close representation to any of these expressions. Operating constraints imposed by the design of the experimental apparatus limited the extent of the surface over which evaporation could be obtained, and prevented clear identification of the most suitable relationship for the viscosity of the liquid/vapour mixture. The theoretical model was extended to examine the development of two phase flow in a rotating shaft face seal of uniform thickness. Previous theoretical analyses have been based on the assumption that the radial velocity profile of the flow is always parabolic, and that the tangential component of velocity varies linearly from the value at the rotating surface, to zero at the stationary surface. The computational fluid dynamic analysis shows that viscous shear and dissipation in the fluid adjacent to the rotating surface leads to developing evaporation with a consequent reduction in tangential shear forces. The tangential velocity profile is predicted to decay rapidly through the film, exhibiting a profile entirely different to that assumed by previous investigators. Progressive evaporation takes place close to the moving wall and does not occur completely at a single radial location, as has been claimed in earlier work.

High activity, templated mesoporous SO4/ZrO2/HMS catalysts with controlled acid site density for α-pinene isomerisation

Highly active mesoporous SO4/ZrO2/HMS (hexagonal mesoroporous silica) solid acid catalysts with tuneable sulphated zirconia (SZ) content have been prepared for the liquid phase isomerisation of α-pinene. The mesoporous HMS framework is preserved during the grafting process as evidenced by the X-ray diffraction (XRD) and porosimetry with all SO4/ZrO2/HMS materials possessing average pore-diameters ∼20 Å. XRD confirms the presence of a stabilized tetragonal phase of nanoparticulate ZrO2, with no evidence for zirconia phase separation or the formation of discrete crystallites, consistent with a uniform and highly dispersed SZ coating. The activity towards α-pinene isomerisation scales linearly with Zr loading, while the specific activities are an order of magnitude greater than attainable by conventional methodologies (∼1 versus 0.08 mol h−1 g Zr−1).