Precipitation kinetics of W2B5 in (Ti0.4W0.5Cr0.1)B2 solid solutions

The isothermal precipitation kinetics of W₂B₅ secondary phase from supersaturated polycrystalline (Ti_0.4W_0.5Cr_0.1)B₂ solid solutions were investigated with X-ray diffractometry and scanning electron microscopy in the temperature range between 1500 and 1700°C. The precipitate formation is described by a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, where W₂B₅ particles nucleate preferentially at grain boundaries and subsequently grow into the volume by a two-dimensional process controlled by volume diffusion of the transition metals. Numerical calculations are used to describe quantitatively the time dependence of the precipitated fraction and to determine a differential JMAK exponent n _diff which gives information on the nucleation and growth modes. n _diff decreases during the precipitation process from 2 to about 0.8 for all temperatures investigated. The first limit corresponds to the classical JMAK model (two-dimensional diffusional growth and constant nucleation rate) and the decrease in n _diff is the consequence of an impingement of the nucleating and growing particles in the late stages of the process. Nucleation and growth rates are determined as functions of reciprocal temperature, where the first quantity shows a non-monotonic behaviour with a maximum at about 1650°C and the second quantity exhibits an Arrhenius behaviour with an activation enthalpy of 3.6?eV. From this it can be concluded that the overall precipitate formation is dominated by the kinetics of atomic motion at low temperatures and by the thermodynamics of nucleation at high temperatures.     

Experimental and theoretical investigation of oxygen diffusion in stabilised zirconia

Oxygen diffusion in stabilised zirconias is investigated by the simultaneous application of computer modelling and experimental techniques to yttria-stabilised zirconia. Using the Mott-Littleton method, migration pathways for oxygen ions have been calculated in perfect cubic zirconia. The oxygen migration occurs through a straight pathway, but not starting from the ideal lattice positions. The calculated activation energy of migration is about 0.2 v eV. Oxygen transport is investigated experimentally in YSZ containing 8-24 v mol% Y 2 O 3 as a function of stabiliser content by combining the stable isotope ( 18 O 2 ) method with ionic conductivity measurements. It was found that for a given temperature, diffusion and conductivity are highest for YSZ containing 8-10 v mol% yttria, but with differing activation energies which can be compared to the calculated values.     

Silica gel assisted reductive cyclization of 2-nitro-β-piperidinostyrenes,derived from 2-nitrotoluenes,to indoles

An efficient modification of the Leimgruber-Batcho method of indole synthesis has been devised that facilitates the synthesis of a variety of 2,3-unsubstituted indoles containing halogen, methoxy and benzyloxy groups, from 2-nitrotoluenes, in high yields. The modified method involves the condensation of 2-nitrotoluenes with tripiperidinomethane followed by the reductive cyclization of the intermediate 2-nitro-β-piperidinostyrenes with iron and acetic acid in refluxing toluene in the presence of silica gel (column chromatography grade, 60–200 mesh).     

The effect of implanted yttrium on the oxidation behaviour at high temperatures of a Fe-Cr-Al alloy containing zirconium

Summary The influence of implanted yttrium on the oxidation behaviour of a ferritic Fe-23Cr-5Al+0.2Zr alloy was studied between 1473 K and 1600 K under isothermal and thermal cycling conditions in oxygen and in air, respectively. The microstructure and morphology of the starting material as well as of the scales were studied by means of optical and scanning electron microscopy. The composition of the alloy and of the scales was determined using various X-ray techniques. The oxidation mechanism was studied at 1473 K using the two-stage oxidation method in ¹⁸O/¹⁶O atmospheres. The implanted yttrium was found to affect the scale growth rate and the oxidation mechanism as well as the scale morphology. No spalling of the scale under thermal cycling conditions occurred. The observed growth mechanism of the scales was related to their complex morphology. On leave from: Institute of Materials Science, Academy of Mining and Metallurgy, Krakow, Poland     

Kinetics and equilibria of cis/trans isomerization of backbone amide bonds in peptoids

The biological activities of N-substituted glycine oligomers (peptoids) have been the subject of extensive research. As compared to peptides, both the cis and trans conformations of the backbone amide bonds of peptoids can be significantly populated. Thus, peptoids are mixtures of configurational isomers, with the number of isomers increasing by a factor of 2 with each additional monomer residue. Here we report the results of a study of the kinetics and equilibria of cis/trans isomerization of the amide bonds of N-acetylated peptoid monomers, dipeptoids, and tripeptoids by NMR spectroscopy. Resonance intensities indicate the cis conformation of the backbone amide bonds of the peptoids studied is more populated than is generally the case for the analogous secondary amide bond to proline residues in acyclic peptides. Rate constants were measured by inversion-magnetization transfer techniques over a range of temperatures, and activation parameters were derived from the temperature dependence of the rate constants. The rate of cis/trans isomerization by rotation around the amide bonds in the peptoids studied is generally slower than that around amide bonds to proline residues and takes place on the NMR inversion-magnetization transfer time scale only by rotation around the amide bond to the C-terminal peptoid residue.