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Transformation Mechanism (transformation + mechanism)
Selected AbstractsTransformation Mechanism of the Dehydration of DiasporeJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2003Lars Löffler The dehydration of diaspore to corundum was investigated by means of X-ray powder diffraction at reaction temperatures (400° and 600°C) as well as by transmission electron microscopy (TEM). The TEM studies were performed at the reaction interface of partially dehydrated natural diaspore crystals. The corundum produced consisted of thin dense regions which were separated by nanometer-sized pores forming lamellae with a periodicity of 3.7 nm. At the reaction front a transition phase (D,) could be detected in electron diffraction patterns. The phase D, is isotypic to diaspore, but with a larger spacing of the close-packed (100) oxygen planes (aD, from 0.475 to 0.480 nm). The expansion with respect to diaspore is explained by breaking of the hydrogen bonds of diaspore, considered to be the initial step of the transformation. The spacing of the lamellar pore system in corundum is explained by the misfit of the (100) planes of D, and the (0003) planes of corundum. We conclude that at well-fitting regions of the closed-packed planes at the D,/C interface, dense corundum is formed, while at misfitting regions, the formation of corundum is not favored and the pores are produced. Hence, the transformation of the solid phases takes place as a two-step process, i.e., D , D,, C. [source] Fabrication of High-Strength Continuous Zirconia Fibers and Their Formation Mechanism StudyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004He-Yi Liu Continuous zirconia fibers with a nanometer ceramic structure and a tensile strength up to 2.8 GPa were fabricated by pyrolyzing polyacetylacetonatozirconium precursor fibers through a special atmosphere heat treatment. DSC-TGA, GC-MS, IR, SEM, and TEM were used to study the fiber transformation mechanism during heating. Results showed that special atmosphere heat treatment could make the organics in the fibers come out directly without carbonization and remove them almost entirely under 400°C, and the obtained zirconia fibers had few defects, good continuity, and high strength. [source] A food-grade site-directed mutagenesis system for Streptococcus thermophilus LMG 18311LETTERS IN APPLIED MICROBIOLOGY, Issue 3 2010T. Blomqvist Abstract Aims:, To develop a general method for site-directed mutagenesis in the dairy starter strain Streptococcus thermophilus LMG 18311 which does not depend on antibiotic-resistance genes or other selection markers for the identification of transformants. Methods and Results:, In a previous study, we demonstrated that Strep. thermophilus LMG 18311 can be made competent for natural genetic transformation by overexpression of the alternative sigma factor ComX. In the present study, we wanted to investigate whether the natural transformation mechanism of Strep. thermophilus LMG 18311 is efficient enough to make it feasible to perform site-directed mutagenesis in this strain without the use of a selection marker. Competent bacteria were mixed with a DNA fragment engineered to contain a nonsense and a frameshift mutation in the middle of the target gene (lacZ) and subsequently seeded on agar plates. By performing colony-lift hybridization using a digoxigenin-labelled oligonucleotide probe, we succeeded in identifying transformants containing the sought after mutation. Conclusions:, By exploiting the natural transformability of Strep. thermophilus LMG 18311 and standard molecular methods, we have demonstrated that the genome of this bacterium can be altered at preselected sites without introduction of any foreign DNA. Significance and Impact of the Study:, A food-grade site-directed mutagenesis system has been developed for Strep. thermophilus LMG 18311 that can be used by the dairy industry to construct starter strains with novel and/or improved properties. [source] Nucleation, growth, and phase transformation mechanism of Ge2Sb2Te5 thin filmsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2009Eun Tae Kim Abstract The nucleation, growth, and metastable-to-stable phase transformation behavior of Ge2Sb2Te5 thin film were studied by means of transmission electron microscopy. The results confirm that nucleation begins at the surface of the film and proceeds by growth of grains through the thickness of film. Also, the result provides further grounds to the suggestion that some of the Ge2Sb2Te5 grains grow with an elongated grain shape to reduce interface energy between crystalline and amorphous phases at the initial stage of crystallization. We find edge dislocations in the metastable-to-stable phase transition region. We think these edge dislocations can explain the metastable-to-stable phase transformation mechanism as a result of vacancy diffusion. [source] Optical properties and transformation mechanism of oxygen centres and their aggregates in CaF2 crystalsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2005A.S. Mysovsky Abstract Oxygen-vacancy dipoles and dimers in CaF2 crystals have been studied ab initio at DFT level and with the shell model using pair potentials. The calculated dipole reorientation barrier is 0.64 eV and the activation energy for diffusion of the dipoles is 1.61 eV. Optical absorption of O2, -VA dipole have been calculated with TD DFT and identified with experimental absorption bands, which appeared to have complex structure. The photodissociation mechanism of the dipole is discussed. Several configurations of the dimer (O2, -VA)2 were calculated. The association energy for the most favourable one is 0.48 eV. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Coordination Chemistry of Conformation-Flexible 1,2,3,4,5,6-Cyclohexanehexacarboxylate: Trapping Various Conformations in Metal,Organic FrameworksCHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2008Jing Wang Abstract To study the conformations of 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (H6L), eleven new coordination polymers have been isolated from hydrothermal reactions of different metal salts with 1e,2a,3e,4a,5e,6a -cyclohexanehexacarboxylic acid (3e+3a, H6LI) and characterized. They are [Cd12(,6 - LII)(,10 - LII)3(,-H2O)6(H2O)6],16.5,H2O (1), Na12[Cd6(,6 - LII)(,6 - LIII)3],27,H2O (2), [Cd3(,13 - LII)(,-H2O)] (3), [Cd3(,6 - LIII)(2,2,-bpy)3(H2O)3],2,H2O (4), [Cd4(,4 - LVI)2(4,4,-Hbpy)4(4,4,-bpy)2(H2O)4],9.5,H2O (5), [Cd2(,6 - LII)(4,4,-Hbpy)2(H2O)10],5,H2O (6), [Cd3(,11 - LVI)(H2O)3] (7), [M3(,9 - LII)(H2O)6] (M=Mn (8), Fe (9), and Ni (10)), and [Ni4(OH)2(,10 - LII)(4,4,-bpy)(H2O)4],6,H2O (11). Three new conformations of 1,2,3,4,5,6-cyclohexanehexacarboxylate, 6e (LII), 4e+2a (LIII) and 5e+1a (LVI), have been derived from the conformational conversions of LI and trapped in these complexes by controlling the conditions of the hydrothermal systems. Complexes 1 and 2 have three-dimensional (3D) coordination frameworks with nanoscale cages and are obtained at relatively low temperatures. A quarter of the LI ligands undergo a conformational transformation into LII while the others are transformed into LIII in the presence of NaOH in 2, while all of the LI are transformed into LII in the absence of NaOH in 1. Complex 3 has a 3D condensed coordination framework, which was obtained under similar reaction conditions as 1, but at a higher temperature. The addition of 2,2,-bipyridine (2,2,-bpy) or 4,4,-bipyridine (4,4,-bpy) to the hydrothermal system as an auxiliary ligand also induces the conformational transformation of H6LI. A new LVI conformation has been trapped in complexes 4,7 under different conditions. Complex 4 has a 3D microporous supramolecular network constructed from a 2D LIII -bridged coordination layer structure by ,-, interactions between the chelating 2,2,-bpy ligands. Complexes 5,7 have different frameworks with LII/LVI conformations, which were prepared by using different amounts of 4,4,-bpy under similar synthetic conditions. Both 5 and 7 are 3D coordination frameworks involving the LVI ligands, while 6 has a 3D microporous supramolecular network constructed from a 2D LII -bridged coordination layer structure by interlayer N4,4,-HbpyH,,,O(LII) hydrogen bonds. 3D coordination frameworks 8,11 have been obtained from the H6LI ligand and the paramagnetic metal ions MnII, FeII, and NiII, and their magnetic properties have been studied. Of particular interest to us is that two copper coordination polymers of the formulae [{CuII2(,4 - LII)(H2O)4}{CuI2(4,4,-bpy)2}] (12,,) and [CuII(Hbtc)(4,4,-bpy)(H2O)],3,H2O (H3btc=1,3,5-benzenetricarboxylic acid) (12,,) resulted from the same one-pot hydrothermal reaction of Cu(NO3)2, H6LI, 4,4,-bpy, and NaOH. The Hbtc2, ligand in 12,, was formed by the in situ decarboxylation of H6LI. The observed decarboxylation of the H6LI ligand to H3btc may serve as a helpful indicator in studying the conformational transformation mechanism between H6LI and LII,VI. Trapping various conformations in metal-organic structures may be helpful for the stabilization and separation of various conformations of the H6L ligand. [source] Solid,Solid Phase Transitions: Interface Controlled Reactivity and Formation of Intermediate StructuresCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2007Stefano Leoni Dr. Abstract Finding new pathways to novel materials is an open challenge in modern solid-state chemistry. Among the reasons that still prevent a rational planning of synthetic routes is the lack of an atomistic understanding at the moment of phase formation. Metastable phases are, in this respect, powerful points of access to new materials. For the synthetic efforts to fully take advantage of such peculiar intermediates, a precise atomistic understanding of critical processes in the solid state in its many facets, that is, nucleation patterns, formation and propagation of interfaces, intermediate structures, and phase growth, is mandatory. Recently we have started a systematic theoretical study of phase transitions, especially of processes with first-order thermodynamics, to reach a firm understanding of the atomistic mechanisms governing polymorphism in the solid state. A clear picture is emerging of the interplay between nucleation patterns, the evolution of domain interfaces and final material morphology. Therein intermediate metastable structural motifs with distinct atomic patterns are identified, which become exciting targets for chemical synthesis. Accordingly, a new way of implementing simulation strategies as a powerful support to the chemical intuition is emerging. Simulations of real materials under conditions corresponding to the experiments are shedding light onto yet elusive aspects of solid,solid transformations. Particularly, sharp insights into local nucleation and growth events allow the formulation of new concepts for rationalizing interfaces formed during phase nucleation and growth. Structurally different and confined in space, metastable interfaces occurring during polymorph transformations bring about distinct diffusion behavior of the chemical species involved. More generally, stable structures emerge as a result of the concurrence of the transformation mechanism and of chemical reactions within the phase-growth fronts. [source] A comparative in situ Rietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxidesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2008Rune E. Johnsen Rietveld refinement based on in situ X-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107,s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl,NO3 LDH and spinel/zincite for a CoZnAl,NO3 LDH. Correlating these data with those from the TGA,MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. The a axis of the spinel formed by decomposition of the CoAl,NO3 LDH increases almost linearly from approximately 598 to 1163,K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl,NO3 shows a more complex temperature dependency. Between approximately 698 and 1073,K, the a axis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163,K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas. [source] A review of in situ measurement of organic compound transformation in groundwater,,PEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 4 2001Sharon K Papiernik Abstract Laboratory assessments of the rate of degradation of organic compounds in groundwater have been criticized for producing unrepresentative results. The potential for organic compounds to be transformed in groundwater has been measured using in situ methods, which avoid problems of attempting to duplicate aquifer conditions in the laboratory. In situ assessments of transformation rates have been accomplished using transport studies and in situ microcosms (ISMs); a review of these methods is given here. In transport studies, organic solutes are injected into an aquifer and the concentrations are monitored as they are transported downgradient. The change in mass of a solute is determined by the area contained under the breakthrough curve (plot of concentration versus time). ISMs isolate a portion of the aquifer from advective flow and act as in situ batch reactors. Experiments using ISMs involve removing water from the ISM, amending it with the solutes of interest, re-injecting the amended water, and monitoring the solute concentrations with time. In both transport and ISM studies, the loss of organic solutes from solution does not allow a distinction to be made between sorptive, abiotic and biotic transformation losses. Biological activity can be chemically suppressed in ISMs and the results from those experiments used to indicate sorption and abiotic loss. Transformation products may be monitored to provide additional information on transformation mechanisms and rates. Published in 2001 for SCI by John Wiley & Sons, Ltd [source] | |||||||||||||