The Mathematical Theory of Ito Diffusions on Hypersurfaces, with Applications to NMR Relaxation Problems

A mathematical framework for translational Brownian motion on hypersurfaces is presented, using an imbedding of the surface and Ito diffusions in the ambient space. This includes a survey of Ito calculus and differential geometry. Computational methods for time correlation functions relevant to spin relaxation studies on curved interfaces are given, and explicit calculations of time correlation functions and order parameters for a Rippled surface are presented.     

Combining thermal analysis with other techniques to monitor the decomposition of poly(m-phenylene isophthalamide)

The pyrolysis behaviour of Nomex, poly(m-phenylene isophthalamide) fibres under argon has been investigated up to a temperature of 1173 K with different methods to get direct information on the progressive changes taking place in the solid material and its carbon fibre residues. The main stages of the pyrolytic degradation of the fibres were determined by thermal analysis (TG and DTA) and their chemical and morphological evolution through the different steps was subsequently followed by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) measurements, respectively, on samples treated to various temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pyrolysis Analysis and Kinetics of Crude Oils

This research presents the results of an experimental study on the determination of pyrolysis behaviour and kinetics of six crude oils by differential scanning calorimetry (DSC) and thermogravimetry (TG/DTG). Crude oil pyrolysis indicated two main temperature ranges where loss of mass was observed. The first region between ambient to 400°C was distillation. The second region between 400 and 600°C was visbreaking and thermal cracking. Arrhenius-type kinetic model is used to determine the kinetic parameters of crude oils studied. It was observed that as crude oils gets heavier (°API decreases) cracking activation energy increases. Activation energy of cracking also show a general trend with asphaltene content. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diffusion of Cesium Ions in Agar Gel Containing Alkali Metal Bromides

The present paper gives an account of different aspects of the tracer diffusion of Cs⁺ ions in alkali metal bromides. We have measured the diffusion coefficients, D, of cesium ions in 1% agar gel medium at 25 ^∘C using a zone-diffusion technique over a concentration range of 5 × 10⁻⁵ to 0.1 mol,dm⁻³. The values of the diffusion coefficients were found to deviate from theory, which are explained on the basis of different types of interactions occurring in the ion-gel-water system. The study is also focused on the effect of alkali metal bromides on the obstruction effect and activation energy for the tracer-diffusion of cesium ions in agar gel medium. It is observed that both parameters, extent of obstruction, ∝, and activation energy, E, decrease with increasing charge density of the cation of the supporting electrolyte. The influence of these trends is explained on the basis of competitive hydration between the ions and agar molecules, and the relative distortion in the water structure that is brought about by these different ions and agar molecules.     

Cathodoluminescence Depth Profiling in SiO2:Ge Layers

 For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary electron energy E ₀ = 2…30 keV. Applying a constant incident power regime (E ₀·I ₀ = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx. Thermally grown SiO₂ layers of thickness d = 500 nm have been implanted by Ge⁺-ions of energy 350 keV and doses (0.5–5)10¹⁶ ions/cm². Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of d_m≅240 nm are expected. Afterwards the layers have been partially annealed up to T _a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm.     

Methane production from synthesis gas using a mixed culture ofR. rubrum M. barkeri,and M. formicicum

The components of synthesis gas, CO, H₂, and CO₂, may be converted into CH₄ biologically through either acetate or H₂/CO₂ as intermediates. Of these two routes, conversion through H₂/CO₂ is preferred. This paper presents results of mixed-culture studies employing the photosynthetic bacteriumR. rubrum for converting CO to CO₂ and H₂ by the water gas shift reaction and two methanogens,M. formicicum andM. barkeri, for converting CO₂ and H₂ into CH₄. Results are presented for triculture operation in two types of reactors, the packed bubble column and the trickle-bed reactor.     

Preparation and Characteristics of Porous Silica Films by a Modified Base-Catalyzed Sol-Gel Process Containing PVA: II. Film Preparation

Porous SiO₂ films were successfully deposited on silicon substrates by a modified base-catalyzed Sol-Gel process (MBCP) containing polyvinyl alcohol (PVA). The process conditions, such as the gelation time, the synthesis temperature, the stabilizing agent of the precursor solution and the spin coating speed, the heat-treatment, the annealing temperature of the film on the microstructure and porosity of porous SiO₂ films were systematically investigated by SEM, XRD and ellipsometry techniques. This study provides a novel preparation technique for the porous SiO₂ film. Using this process, the resultant film can reach a thickness of 3.6 m for one layer, a porosity of 25–50%, a low thermal conductivity of 0.11 W/m·K. This film will be used as a low dielectric layer, an thermal-insulating layer and a low refractive index layer.     

Characterization of plant oils on a monolithic silica column by high-performance liquid chromatography-atmospheric pressure chemical lonization-mass spectrometry

Summary Five plant oils (peanut, pumpkin seed, sesame seed, soybean, and wheat germ) have been analyzed by high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (HPLC-APCI-MS). Gradient elution was performed with acetone-acetonitrile mobile phases on a short monolithic silica column (SilicaROD, RP-18e, 50 mm×4.6 mm). Identification of plant oil triacylglycerols (TAG) was based on the pseudomolecular ion [M+H]⁺ and the diacylglycerol [M−RCO₂]⁺ fragments. Positional isomers of triacylglycerols were identified from the relative intensities of the [M-RCO₂]⁺ fragments. Principal-component analysis, used to find similarities and differences between the different oils, indicated that the different plant oils could be clearly differentiated according to their triacylglycerol composition. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001     

Azocalixarenes: Synthesis, Characterization, Complexation, Extraction, Absorption Properties and Thermal Behaviours

Azocalixarenes, which have a conjugated chromophore azo (–N=N–) group in p-positions, are synthesized in “one-pot” procedures in satisfactory yields. Their structures in the solid and solution are elucidated by UV–vis, FT-IR, ¹H and ¹³C NMR spectroscopic methods as well as elemental analysis techniques. Some of them are complexed with metal ions (alkali, alkaline-earth and transition metal ions) under neutral conditions. A wide variety of applications is expected by the functionalization of the side arms. The selective liquid–liquid extraction of various alkali, alkaline-earth and transition metal cations from the aqueous phase to the organic phase is carried out by using azocalixarenes. Furthermore, the synthesized azocalixarenes are utilized for selective extraction of Fe³⁺ cations from the aqueous phase to the organic phase. The effects of varying pH and solvent upon the absorption ability of azocalixarenes substituted with electron-donating and electron-withdrawing groups at their o-, m-, p-positions are examined. The thermal behaviours of azocalixarenes containing upper rim functionalized groups are investigated by means of thermogravimetry (TG), differential thermal analysis (DTA) and differential thermogravimetry (DTG).     

Computer simulation of the steady state currents at enzyme doped carbon paste electrodes

The plate-gap model of porous enzyme doped electrode has been proposed and analyzed. It was suggested that reaction diffusion conditions in pores of bulk electrode resemble particular conditions in thin gap between parallel conducting plates. The model is based on the diffusion equations containing a nonlinear term related to the Michaelis–Menten kinetic of the enzymatic reaction inside gap. Steady state current was calculated for the wide range of given parameters and substrate concentrations. All dependences of current on substrate concentration were approximated by hyperbolas in order to obtain “apparent” parameters (maximal currents and apparent Michaelis constants) of modelled biosensors. Simple approximate relationships between given and apparent parameters were derived. The applicability of theoretical plate-gap model was tested for the case of carbon paste electrodes which were doped with PQQ – dependent glucose dehydrogenase. It was found, that soluble glucose dehydrogenase based biosensors exhibit characteristic features of the theoretical plate-gap biosensors.