Initiation Period (initiation + period)

Distribution by Scientific Domains

Selected Abstracts

From monosilane to crystalline silicon, part II: Kinetic considerations on thermal decomposition of pressurized monosilane

J. O. Odden
Kinetic aspects of the thermal decomposition of monosilane at 690,830 K and initial pressures of 0.1,3.7 MPa in a free-space reactor are considered. Neglecting the preparatory initiation period for the reaction (which is difficult to evaluate under the present dynamic conditions), the onset temperature for the decomposition is stipulated to some 700,710 K, independent of the initial monosilane pressure. The overall reaction appears to be of first order throughout the progressing decomposition process. We observe considerably lower reaction rates under the high-pressure conditions than existing models in the literature suggest. A modified model is proposed that simulates the observed reaction rates within 1% and moreover predicts credible concentrations of the involved gaseous species. A key feature of the modified model is incorporation of two third-body assisted surface reactions, which generate monosilane from disilane and disilane from trisilane. 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 309,321, 2006 [source]

A New View of the Anionic Diene Polymerization Mechanism

A.Z. Niu
Abstract We investigated the anionic polymerization of butadiene in d-heptane solvent using tert -butyl lithium as initiator. Two complementary techniques were used to follow the polymerization processes: 1H NMR and small angle neutron scattering (SANS). The time resolved 1H NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The initiation event commences slowly and then progressively accelerates. This indicates an autocatalytic mechanism. The microstructure of the first monomer units attached is to a high extent 1,2. The disappearance of initiator --- at about 10% monomer conversion --- signals the onset of the normal ,6% vinyl content of the chain. Small angle neutron scattering was used to study the aggregation behavior of the carbon lithium head groups. It is well known that the polar head groups aggregate and form micellar structures. For dienes in non-polar solvents the textbook mechanism assumes the formation of only tetramers during the propagation reaction. By combining 1H NMR and SANS results we were able to determine quantitatively the aggregation number during all stages of the polymerization. Our measurements show the existence of large-scale structures during the initiation period. The initial degree of aggregation of more than 100 living polymer chains diminished as the polymerization progressed. In addition, even larger, giant structures with Nagg >>1000 and Rg , 1000 were found. [source]

Study of the metal dusting behaviour of high-temperature alloys

F. Di Gabriele
Metal dusting; Nickelbasis-Legierungen; Aufkohlung Abstract The corrosion phenomenon named metal dusting has been observed in many high-temperature industrial plants. An experimental research programme is being carried out into the degradation resistance of wrought and cast commercial and development high-temperature alloys in H2/CO gas mixtures at temperatures of 550C to 750C. Emphasis is placed on very high carbon activities, consistent with the next generation of steam-reforming and similar plants that are susceptible to metal dusting. The overall programme is concerned with the mechanisms of initiation and propagation of dusting and the sensitivity to damage of the more resistant alloys, as a function of environmental parameters. Initial tests have been carried out on a number of commercial alloys: Alloy 600, 693, 602CA, 601, 603 XL, 671, 617, 690 (wrought), and H46M (cast). The specimens were exposed to a gas mixture of high carbon activity at 650C for a total of 1000 hours. Many of the alloys showed at least the initial stages of metal dusting. Preliminary analysis using electron microscopy revealed that initiation of metal dusting is influenced by microstructure, stress state and composition. In some cases, attack was enhanced at stress points, such as corners and edges. Sample holders were found to influence strongly the length of the initiation period for the onset of the corrosion phenomenon. The reaction layers in the alloy beneath areas of damage were analysed by EDX and EPMA. Mechanical characterisation of such areas has been carried out using nanoindentation methods. These early results are discussed in terms of the effectiveness of oxide scales in inhibiting the onset of damage and presence of impurities in the ceramic holder in initiating the onset of damage. [source]

Fatigue crack initiation in naval welded joints: experimental and numerical approaches

D. Thevenet
This work deals with the fatigue behavior of welded joints. The proposed strategy can be split into two stages: the structure shakedown study and the fatigue crack initiation study. Firstly, stabilized elastic stress,strain cycles are obtained in any point of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. The second part of this work is the calculation of the fatigue crack initiation period. Under the assumption of a fast elastic shakedown, a recent approach, proposed by Lemaitre et al., based on damage mechanics, can be used to predict the fatigue crack initiation life in a structure subjected to variable and complex loadings. This strategy is used as a post,treatment of the shakedown finite element calculation for the studied welded specimen. As a validation, four,point bending fatigue tests under constant amplitude loading were carried out on welded specimens provided by DCNS group. Good correlations between experimental and calculated fatigue crack initiation lives have been established. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Mechanisms involved in the chemoprevention of flavonoids

BIOFACTORS, Issue 1-4 2000
Marie-HLNe Siess
Flavonoids, widespread in edible plants, have been studied extensively for their anticarcinogenic properties. However, only few studies have been done with these constituents being administered by the dietary route. In our research, the effects of feeding rats with flavone, flavanone, tangeretin, and quercetin were investigated on two steps of aflatoxin B1 (AFB1)-induced hepatocarcinogenesis (initiation and promotion). Nonpolar flavonoids such as flavone, flavanone and tangeretin administered through the initiation period, decreased the number of ,-glutamyl transpeptidase-preneoplastic foci. In the same conditions of administration, quercetin, a polyhydroxylated flavonoid, showed no protective effect. Moreover, feeding rats with flavanone during the phenobarbital-induced promotion step significantly reduced the areas of placental glutathione S-transferase preneoplastic foci. Quercetin, flavone, and tangeretin, administered in the same conditions, caused no significant effect. Therefore flavanone act as an anti-initiator as well as an anti-promotor. Several mechanisms were involved in the anti-initiating effects of flavone, flavanone, and tangeretin: enhancement of enzymes involved in the detoxication of AFB1 (glutathione S-transferase, UDP-glucuronyl transferase), increase of the formation of AFB1 -glutathione conjugates and inhibition of the binding of AFB1 to DNA. Although the relevance of these data to the human situation remains to be demonstrated, they confirm that several flavonoids administered by the dietary route possess promising chemoprotective effects. [source]