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Decomposition Rate Constant (decomposition + rate_constant)
Selected AbstractsThermal Decomposition of NTO: An Explanation of the High Activation EnergyPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 4 2007Valery Abstract Burning rate characteristics of the low-sensitivity explosive 5-nitro-1,2,4-triazol-3-one (NTO) have been investigated in the pressure interval of 0.1,40,MPa. The temperature distribution in the combustion wave of NTO has been measured at pressures of 0.4,2.1,MPa. Based on burning rate and thermocouple measurements, rate constants of NTO decomposition in the molten layer at 370,425,°C have been derived from a condensed-phase combustion model (k=8.08,1013,exp(,19420/T) s,1. NTO vapor pressure above the liquid (ln P=,9914.4/T+14.82) and solid phases (ln P=,12984.4/T+20.48) has been calculated. Decomposition rates of NTO at low temperatures have been defined more exactly and it has been shown that in the interval of 180,230,°C the decomposition of solid NTO is described by the following expression: k=2.9,1012,exp(,20680/T). Taking into account the vapor pressure data obtained, the decomposition of NTO in the gas phase at 240,250,°C has been studied. Decomposition rate constants in the gaseous phase have been found to be comparable with rate constants in the solid state. Therefore, a partial decomposition in the gas cannot substantially increase the total rate. High values of the activation energy for solid-state decomposition of NTO are not likely to be connected with a sub-melting effect, because decomposition occurs at temperatures well below the melting point. It has been suggested that the abnormally high activation energy in the interval of 230,270,°C is a consequence of peculiarities of the NTO transitional process rather than strong bonds in the molecule. In this area, the NTO molecule undergoes isomerization into the aci -form, followed by C3-N2 heterocyclic bond rupture. Both processes depend on temperature, resulting in an abnormally high value of the observed activation energy. [source] Kinetic study of the free-radical polymerization of vinyl acetate in the presence of deuterated chloroform by 1H-NMR spectroscopyJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008Mohammad Ali Semsarzadeh Abstract The free-radical polymerization of vinyl acetate was performed in the presence of deuterated chloroform (CDCl3) as a chain-transfer agent (telogen) and 2,2,-azobisisobutyronitrile as an initiator. The effects of the initiator and solvent concentrations (or equivalent monomer concentration) and the reaction temperature on the reaction kinetics were studied by real-time 1H-NMR spectroscopy. Data obtained from analysis of the 1H-NMR spectra were used to calculate some kinetic parameters, such as the initiator decomposition rate constant (kd), kp(f/kt)1/2 ratio (where kp is the average rate constant for propagation, f is the initiator efficiency, and kt is the average rate constant for termination), and transfer constant to CDCl3 (C). The results show that kd and kp(f/kt)1/2 changed significantly with the solvent concentration and reaction temperature, whereas they remained almost constant with the initiator concentration. C changed only with the reaction temperature. Attempts were made to explain the dependence of kp(f/kt)1/2 on the solvent concentration. We concluded from the solvent-independent C values that the solvent did not have any significant effect on the kp values. As a result, changes in the kp(f/kt)1/2 values with solvent concentration were attributed to the solvent effect on the f and/or kt values. Individual values of f and kt were estimated, and we observed that both the f and kt values were dependent on the solvent (or equivalent monomer) concentration. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Metallocene Combinations in Ethylene Polymerization: A Cyclic and Differential Pulse Voltammetry StudyMACROMOLECULAR REACTION ENGINEERING, Issue 3 2008Fernando Silveira Abstract A series of metallocenes, namely [Cp2ZrCl2], [(MeCp)2ZrCl2], [(nBuCp)2ZrCl2], [(iBuCp)2ZrCl2], [(tBuCp)2ZrCl2], [Cp2TiCl2], [Et(Ind)2ZrCl2], [Et(IndH4)2ZrCl2] and [MeSi2(Ind)2ZrCl2)], were combined in a 1:1 molar ratio within a reactor for ethylene polymerization, with MAO as the cocatalyst. The catalysts were characterized by cyclic and differential pulse voltammetry. The combined systems that showed the highest and lowest activities were combined in 1:3 and 3:1 molar ratios. The catalyst activity in the ethylene polymerization reaction is discussed in terms of the estimated consumption rate, decomposition rate constant and half-life of the metallocene species formed with MAO in an ethylene atmosphere. [source] The interaction of plant genotype and herbivory decelerate leaf litter decomposition and alter nutrient dynamicsOIKOS, Issue 1 2005Jennifer A. Schweitzer We examined how plant genetic variation and a common herbivore (the leaf-galling aphid, Pemphigus betae) influenced leaf litter quality, decomposition, and nutrient dynamics in a dominant riparian tree (Populus spp.). Based on both observational studies and a herbivore exclusion experiment using trees of known genotype, we found four major patterns: 1) the quality of galled vs non-galled or gall-excluded litter significantly differed in the concentration of condensed tannins, lignin, nitrogen and phosphorus; 2) the difference in litter quality resulted in galled litter decomposing at rates 34 to 40% slower than non-galled litter; 3) plant genotype and herbivory had similar effects on the magnitude of decomposition rate constants; and 4) plant genotype mediated the herbivore effects on leaf litter quality and decomposition, as there were genotype-specific responses to herbivory independent of herbivore density. In contrast to other studies that have demonstrated accelerated ecosystem properties in response to arthropod herbivory, our findings argue that herbivore-induced secondary compounds decelerated ecosystem properties though their "after-life" effects on litter quality. Furthermore, these data are among the first to suggest that genotype-specific responses to herbivores can have a major impact on decomposition and nutrient flux, which likely has important consequences for the spatial distribution of nutrients at the landscape level. Due to the magnitude of these effects, we contend that it is important to incorporate a genetic perspective into ecosystem studies. [source] Extracellular Enzyme Activities and Carbon Chemistry as Drivers of Tropical Plant Litter DecompositionBIOTROPICA, Issue 3 2004Steven D. Allison ABSTRACT Litter quality parameters such as nitrogen and lignin content correlate with decomposition rates at coarse scales, but fine-scale mechanisms driving litter decomposition have proven more difficult to generalize. One potentially important driver of decomposition is the activity of extracellular enzymes that catalyze the degradation of complex compounds present in litter. To address the importance of this mechanism, we collected 15 Hawaiian plant litter types and decomposed them in fertilized and control plots for up to two years. We measured litter nutrient content and carbon chemistry prior to decomposition, as well as extracellular enzyme activities, mass loss, and litter nutrient content over time. We found that water-soluble carbon content, cellobiohydrolase activities, and polyphenol oxidase activities were significantly correlated with mass loss. Enzyme activities and decomposition rate constants both varied significantly by litter type, and fertilization increased mass loss rates in five litter types. Some litter types that decayed faster under fertilization also showed time-dependent increases in carbon-degrading enzyme activities, but others decayed faster independent of enzyme changes. These results suggest that extracellular enzyme activities partially determine litter decomposition rates, but high soluble carbon content may circumvent the requirement for enzyme-catalyzed decomposition. [source] | |||||||||||||