THE OXIDATION-KINETICS OF THIN POLYCRYSTALLINE SILICON FILMS

The oxidation dynamics and morphology of undoped and heavily phosphorus-doped polycrystalline silicon films oxidized at a wide temperature and time range in dry and wet O2 atmosphere have been investigated. It is shown that the oxidation rates of polycrystalline silicon films are different from that of single-crystal silicon when the oxidation temperature is below 1000-degrees-C. There is a characteristic oxidation time, t(c), under which the undoped polysilicon oxide is not only thicker than that of (100)-oriented single-crystal silicon, but also thicker than that of (111)-oriented single-crystal silicon. For phosphorus-doped polycrystalline silicon films, the oxide thickness is thinner not only than that of (111)-oriented, single-crystal silicon, but also thinner than that of (100)-oriented, single-crystal silicon. According to TEM cross-sectional studies, these characteristics are due to the enhanced oxidation at grain boundaries of polycrystalline silicon films. A stress-enhanced oxidation model has been proposed and used to explain successfully the enhanced oxidation at grain boundaries of polycrystalline silicon films. Using this model, the oxidation linear rate constant of polysilicon (B/A)poly has been calculated and used in the modeling of the oxidation dynamics. The model results are in good agreement with the experimental data over the entire temperature and time ranges studied.

A mathematical model to describe fat oxidation kinetics during graded exercise.

PURPOSE: The purpose of this study was to develop a mathematical model (sine model, SIN) to describe fat oxidation kinetics as a function of the relative exercise intensity [% of maximal oxygen uptake (%VO2max)] during graded exercise and to determine the exercise intensity (Fatmax) that elicits maximal fat oxidation (MFO) and the intensity at which the fat oxidation becomes negligible (Fatmin). This model included three independent variables (dilatation, symmetry, and translation) that incorporated primary expected modulations of the curve because of training level or body composition. METHODS: Thirty-two healthy volunteers (17 women and 15 men) performed a graded exercise test on a cycle ergometer, with 3-min stages and 20-W increments. Substrate oxidation rates were determined using indirect calorimetry. SIN was compared with measured values (MV) and with other methods currently used [i.e., the RER method (MRER) and third polynomial curves (P3)]. RESULTS: There was no significant difference in the fitting accuracy between SIN and P3 (P = 0.157), whereas MRER was less precise than SIN (P < 0.001). Fatmax (44 +/- 10% VO2max) and MFO (0.37 +/- 0.16 g x min(-1)) determined using SIN were significantly correlated with MV, P3, and MRER (P < 0.001). The variable of dilatation was correlated with Fatmax, Fatmin, and MFO (r = 0.79, r = 0.67, and r = 0.60, respectively, P < 0.001). CONCLUSIONS: The SIN model presents the same precision as other methods currently used in the determination of Fatmax and MFO but in addition allows calculation of Fatmin. Moreover, the three independent variables are directly related to the main expected modulations of the fat oxidation curve. SIN, therefore, seems to be an appropriate tool in analyzing fat oxidation kinetics obtained during graded exercise.

Oxidation kinetics of nickel nano crystallites obtained by controlled thermolysis of diaquabis(ethylenediamine) nickel(II) nitrate

The metal complex, [Ni(en)2(H2O)2](NO3)2 (en = ethylenediamine), was decomposed in a static furnace at 200 C by autogenous decomposition to obtain phase pure metallic nickel nanocrystallites. The nickel metal thus obtained was studied by XRD, IR spectra, SEM and CHN analysis. The nickel crystallites are in the nanometer range as indicated by XRD studies. The IR spectral studies and CHN analyses show that the surface is covered with a nitrogen containing species. Thermogravimetric mass gain shows that the product purity is high (93%). The formed nickel is stable and resistant to oxidation up to 350 C probably due to the coverage of nitrogen containing species. Activation energy for the oxidation of the prepared nickel nanocrystallites was determined by non-isothermal methods and was found to depend on the conversion ratio. The oxidation kinetics of the nickel crystallites obeyed a Johnson–Mehl–Avrami mechanism probably due to the special morphology and crystallite strain present on the metal.

Ozonolysis of methyl oleate monolayers at the air–water interface: oxidation kinetics, reaction products and atmospheric implications

Ozonolysis of methyl oleate monolayers at the air–water interface results in surprisingly rapid loss of material through cleavage of the C[double bond, length as m-dash]C bond and evaporation/dissolution of reaction products. We determine using neutron reflectometry a rate coefficient of (5.7 ± 0.9) × 10−10 cm2 molecule−1 s−1 and an uptake coefficient of [similar]3 × 10−5 for the oxidation of a methyl ester monolayer: the atmospheric lifetime is [similar]10 min. We obtained direct experimental evidence that <2% of organic material remains at the surface on atmospheric timescales. Therefore known long atmospheric residence times of unsaturated fatty acids suggest that these molecules cannot be present at the interface throughout their ageing cycle, i.e. the reported atmospheric longevity is likely to be attributed to presence in the bulk and viscosity-limited reactive loss. Possible reaction products were characterized by ellipsometry and uncertainties in the atmospheric fate of organic surfactants such as oleic acid and its methyl ester are discussed. Our results suggest that a minor change to the structure of the molecule (fatty acid vs. its methyl ester) considerably impacts on reactivity and fate of the organic film.

Oxidation Kinetics of Nicrofer-6025HT for Use in Elevated Temperature Electrochemical Devices

Recent advances in high temperature electrochemical devices have prompted research into potential materials for component fabrication.

Lipid oxidation kinetics of ozone-processed shrimp during iced storage using peroxide value measurements

In this research, in situ generated ozone exposure/wash cycles of 1, 3, and 5 min applied to shrimp samples either before (BIS) or during iced storage (DIS) has been used to study the lipid oxidation kinetics using the peroxide values (PV). The induction period (IP) as well as PV at end of the IP (PVIP) have been obtained. The rate constants (k) as well as half-lives (t1/2) of hydroperoxides formation for different oxidation stages were calculated. The results showed that both IP and PVIP were lower with BIS (IP between 4.35±0.09 and 5.08±0.23 days; PVIP between 2.92±0.06 and 3.40±0.18 mEq kg−1) compared with DIS (IP between 5.92±0.12 and 6.14±0.09 days; PVIP between 4.49±0.17 and 4.56±0.10 mEq kg−1). The k value for DIS seemed to be the greater compared to BIS. In addition, whilst decreases and increases in t1/2 were found at propagation, respectively, for BIS and DIS, decreases and increases were only found at the induction of oxidation stage(s) for BIS. Further, the PV of ozone-processed samples would fit first order lipid oxidation kinetics independent of duration of ozone exposures. For the first time, PV measurements and fundamental kinetic principles have been used to describe how increasing ozone exposures positively affects the different oxidation stages responsible for the formation of hydroperoxides in ozone-processed shrimp.

Kinetics of the oxidation of MCRR by potassium permanganate

The occurrence of the microcystins in the water bodies, especially in drinking water resources, has received considerable attentions. In situ chemical oxidation is a promising cost-effective treatment method to remove MC from water body. This research investigated the reaction kinetics of the oxidation of MCRR by permanganate. Experimental results indicate that the reaction is second order overall and first order with respect to both permanganate and MCRR, and has an activation energy of 18.9 kJ/mol. The second-order rate constant ranges from 0.154 to 0.225 l/mg/min at temperature from 15 to 30 degrees C. The MCRR degradation rates can be accelerated through increasing reaction temperature and oxidant concentration. The reaction under acid conditions was slightly faster than under alkaline conditions. The half-life of the reaction was less than 1 min, and more than 99.5% of MCRR was degraded within 10 min. (c) 2005 Elsevier Ltd. All rights reserved.

Polymer supported metal complexes as catalysts for oxidation of thiosalts by molecular oxygen IV. Quaternised poly(4-vinyl pyridine) complexes with Cu2+ as template

Copper(II) complexes of quaternised poly(4-vinylpyridine) (PVP) of different degrees of quaternisation and copper content have been prepared by crosslinking the polymer with 1,2-dibromoethane in the presence of Cu2+ ion as template. The stability constant of the PVP---Cu(II) complexes is found to increase with the degree of crosslinking quaternisation of the resin, but the rate at which Cu2+ is adsorbed by the resin decreases. An optimum combination of both stability and rate can be achieved with a moderate degree (31%) of crosslinking. A kinetic study reveals that quaternisation increases significantly the catalytic activity of the complex for the oxidation of S2O2−3 by O2 compared with PVP----Cu(II) without quaternisation, but it deactivates the complex for the oxidation of both S3O2−6 and S4O2−6. The batch reactor oxidation kinetics at pH 2.16, where the rate is observed to be maximum, is well explained by the Langmuir—Hinshelwood model assuming the coordination of both O2 and thioanion to Cu(II) as a precursor to the oxidation reaction.

Kinetic study of the 2-methyl-3-methoxy-4-phenylbutanoic acid produced by oxidation of microcystin in aqueous solutions

Microcystins (MCs) are a family of related cyclic hepatotoxic heptapeptides, of which more than 70 types have been identified. The chemically unique nature of the C20 beta-amino acid, (2S, 3S, 8S, 9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca4,6-dienoic acid (Adda), portion of the MCs has been exploited to develop a strategy to analyze the entirety. Oxidation of MCs causes the cleavage of MC Adda to form 2-methyl-3-methoxy-4-phenylbutanoic acid (MMPB). In the present study, we investigated the kinetics of MMPB produced by oxidation of the most-often-studied MC variant, MC-LR (L = leucine, R = arginine), with permanganate-periodate. This investigation allowed insight regarding the influence of the reaction conditions (concentration of the reactants, temperature, and pH) on the conversion rate. The results indicated that the reaction was second order overall and first order with respect to both permanganate and MC-LR. The second-order rate constant ranged from 0.66 to 1.35 M/s at temperatures from 10 to 30 degrees C, and the activation energy was 24.44 kJ/mol. The rates of MMPB production can be accelerated through increasing reaction temperature and oxidant concentration, and sufficient periodate is necessary for the formation of MMPB. The initial reaction rate under alkaline and neutral conditions is higher than that under acidic conditions, but the former decreases faster than the latter except under weakly acidic conditions. These results provided new insight concerning selection of the permanganate-periodate concentration, pH, and temperature needed for the oxidation of MCs with a high and stable yield of MMPB.

ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODES IN ACETONITRILE

The anodic oxidation kinetics of hydrazine on glassy carbon electrodes in acetonitrile were examined by cyclic voltammetry, a rotating ring-disc electrode technique and chronoamperometry. The experimental results of the rotating ring-disc electrode prove that hydrazine is oxidized to HN=NH, which cannot be oxidized further in acetonitrile. Hydrazine molecules are adsorbed on the electrode surface. One-third of the adsorbed hydrazine molecules are oxidized to HN=NH and the other two thirds act as proton acceptors. A possible mechanism of hydrazine oxidation is proposed.

THE ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODE

The anodic oxidation kinetics of hydrazine (N2H4) on glassy carbon (GC) electrode was examined by cyclic voltammetry, rotating disk and ring-disc electrode techniques. The possible mechanisms of N2H4 oxidation in both aqueous and nonaqueous solutions are proposed.