Clustering phenomena in Al-4.4 at. % Zn alloy with 0.03 and 0.08 at. % Ag additions

The influence of 0.03 and 0.08 at. % Ag additions on the clustering of Zn atoms in an Al-4.4 at. % Zn alloy has been studied by resistometry. The effect of quenching and ageing temperatures shows that the ageing-ratio method of calculating the vacancy-solute atom binding energy is not applicable to these alloys. Zone-formation in Al-Zn is unaffected by Ag additions, but the zone-reversion process seems to be influenced. Apparent vacancy-formation energies in the binary and ternary alloys have been used to evaluate the v-Ag atom binding energy as 0.21 eV. It is proposed that, Ag and Zn being similar in size, the relative vacancy binding results from valency effects, and that in Al-Zn-Ag alloys clusters of Zn and Ag may form simultaneously, unaffected by the presence of each other. © 1970 Chapman and Hall Ltd.

Electron-hole recombination in cobalt-doped p-type germanium

The recombination properties of cobalt centers in p-type germanium containing cobalt in the concentration range 1014 to 1016 atoms/cm3 have been investigated. The measurement of lifetime has been carried out by steady-state photoconductivity and photo-magneto-electric methods in the temperature range 145 to 300°K. The cross-sections Sno (electron capture cross-section at neutral centers). Sn- (electron capture cross-section at singly negatively charged centers) and their temperature variations have been estimated by the analysis of the lifetime data on the basis of Sah-Shockley's multi-level formula. The value of Sno is (15±5).10-16 cm2 and is temperature independent. The value of Sn- is ≈4·10-16 cm2 around 225°K and it increases with increase of temperature. The possible mechanisms for capture at neutral and repulsive centers are discussed and a summary of the capture cross-sections for cobalt centers is given. A comparison of the cross-section values of cobalt and their temperature variations with those of the related impurities-manganese, iron and nickel-in germanium has been made.

Potential functions for hydrogen bond interactions - III. Empirical Potential Function for the Peptide N-H...O=C Hydrogen Bond

A careful comparison of the distribution in the (R, θ)-plane of all NH ... O hydrogen bonds with that for bonds between neutral NH and neutral C=O groups indicated that the latter has a larger mean R and a wider range of θ and that the distribution was also broader than for the average case. Therefore, the potential function developed earlier for an average NH ... O hydrogen bond was modified to suit the peptide case. A three-parameter expression of the form {Mathematical expression}, with △ = R - Rmin, was found to be satisfactory. By comparing the theoretically expected distribution in R and θ with observed data (although limited), the best values were found to be p1 = 25, p3 = - 2 and q1 = 1 × 10-3, with Rmin = 2·95 Å and Vmin = - 4·5 kcal/mole. The procedure for obtaining a smooth transition from Vhb to the non-bonded potential Vnb for large R and θ is described, along with a flow chart useful for programming the formulae. Calculated values of ΔH, the enthalpy of formation of the hydrogen bond, using this function are in reasonable agreement with observation. When the atoms involved in the hydrogen bond occur in a five-membered ring as in the sequence[Figure not available: see fulltext.] a different formula for the potential function is needed, which is of the form Vhb = Vmin +p1△2 +q1x2 where x = θ - 50° for θ ≥ 50°, with p1 = 15, q1 = 0·002, Rmin = 2· Å and Vmin = - 2·5 kcal/mole. © 1971 Indian Academy of Sciences.

Clustering and solute-vacancy binding energies in Al-4.4% Zn alloy with 0.01% ternary additions

Isochronal and isothermal ageing experiments have been carried out to determine the influence of 0.01 at. % addition of a second solute on the clustering rate in the quenched Al-4,4 a/o Zn alloy. The influence of quenching and ageing temperatures has been interpreted to obtain the apparent vacancy formation and vacancy migration energies in the various ternary alloys. Using a vacancy-aided clustering model the following values of binding free energy have been evaluated: Ce-0.18; Dy-0.24; Fe-0.18; Li-0.25; Mn-0.27; Nb-0.18; Pt-0.23; Sb-0.21; Si-0.30; Y-0.25; and Yb-0.23 (± 0.02 eV). These binding energy values refer to that between a solute atom and a single vacancy. The values of vacancy migration energy (c. 0.4 eV) and the experimental activation energy for solute diffusion (c. 1.1 eV) are unaffected by the presence of the ternary atoms in the Al-Zn alloy.

Raman spectra of crystalline double sulphates Part II. Ammonium double sulphates

Raman spectra of single crystals of (NH4)2M(SO4)2·6 H2O where M=Mg, Zn Ni or Co have been recorded using λ 2537 excitation. Interesting results concerning the substitution of the divalent atoms in the double sulphate lattice on the sulphate and ammonium frequencies are observed. The spectra of these double sulphates are discussed in the light of the known crystal structure details and in relation, to the spectra of the corresponding potassium double sulphates, reported recently by the author. The Raman spectrum of NaNH4SO4·2 H2O has also been recorded for the first time and the results obtained are also included.

Resistometric studies of solute-vacancy interactions and clustering kinetics in an FCC matrix [AlZn Alloy with Ag, Ce, Dy, Li, Nb, Pt, Sb, Y, or Yb]

Al-4.4 a/oZn and Al-4.4 a/oZn with Ag, Ce, Dy, Li, Nb, Pt, Y, or Yb, alloys have been investigated by resistometry with a view to study the solute-vacancy interactions and clustering kinetics in these alloys. Solute-vacancy binding energies have been evaluated for all these elements by making use of appropriate methods of evaluation. Ag and Dy additions yield some interesting results and these have been discussed in the thesis. Solute-vacancy binding energy values obtained here have been compared with other available values and discussed. A study of the type of interaction between vacancies and solute atoms indicates that the valency effect is more predominant than the elastic effect.

The Path to a Porous Semiconductor Multilayer

Thin films are the basis of much of recent technological advance, ranging from coatings with mechanical or optical benefits to platforms for nanoscale electronics. In the latter, semiconductors have been the norm ever since silicon became the main construction material for a multitude of electronical components. The array of characteristics of silicon-based systems can be widened by manipulating the structure of the thin films at the nanoscale - for instance, by making them porous. The different characteristics of different films can then to some extent be combined by simple superposition. Thin films can be manufactured using many different methods. One emerging field is cluster beam deposition, where aggregates of hundreds or thousands of atoms are deposited one by one to form a layer, the characteristics of which depend on the parameters of deposition. One critical parameter is deposition energy, which dictates how porous, if at all, the layer becomes. Other parameters, such as sputtering rate and aggregation conditions, have an effect on the size and consistency of the individual clusters. Understanding nanoscale processes, which cannot be observed experimentally, is fundamental to optimizing experimental techniques and inventing new possibilities for advances at this scale. Atomistic computer simulations offer a window to the world of nanometers and nanoseconds in a way unparalleled by the most accurate of microscopes. Transmission electron microscope image simulations can then bridge this gap by providing a tangible link between the simulated and the experimental. In this thesis, the entire process of cluster beam deposition is explored using molecular dynamics and image simulations. The process begins with the formation of the clusters, which is investigated for Si/Ge in an Ar atmosphere. The structure of the clusters is optimized to bring it as close to the experimental ideal as possible. Then, clusters are deposited, one by one, onto a substrate, until a sufficiently thick layer has been produced. Finally, the concept is expanded by further deposition with different parameters, resulting in multiple superimposed layers of different porosities. This work demonstrates how the aggregation of clusters is not entirely understood within the scope of the approximations used in the simulations; yet, it is also shown how the continued deposition of clusters with a varying deposition energy can lead to a novel kind of nanostructured thin film: a multielemental porous multilayer. According to theory, these new structures have characteristics that can be tailored for a variety of applications, with precision heretofore unseen in conventional multilayer manufacture.

Methyl 1-methyl-1H-1,2,3-triazole-4-carboxylate

The title molecule, C5H7N3O2, has an almost planar conformation, with a maximum deviation of 0.043 (3) angstrom, except for the methyl H atoms. In the crystal structure, intermolecular C-H center dot center dot center dot O hydrogen bonds link the molecules into layers parallel to the bc plane. Intermolecular pi-pi stacking interactions [centroid-centroid distances = 3.685 (2) and 3.697 (2) angstrom] are observed between the parallel triazole rings.

Crystal And Molecular Structure Of A Dimethyl Sulphoxide Complex With Lanthanum Nitrate, La(No3)34.(Ch3)2so

The crystal structure of the complex La(NO3)3.4(CH3)2SO has been solved by the heavy-atom method. The complex crystallizes in the monoclinic space group C2/e with four formula units in a unit cell of dimensions a= 14.94, b= 11.04, c= 15.54 A and fl= 109 ° 10'. The parameters have been refined by threedimensional least-squares procedures with anisotropic thermal parameters for all atoms except hydrogen. The final R index for 1257 observed reflexions is 0.094. The La 3 + ion is coordinated by ten oxygen atoms with La-O distances varying from 2.47 to 2.71 A. The geometry of the coordination polyhedron is described.

Structural Studies Of Analgesics And Their Interactions .1. Crystal And Molecular-Structure Of Antipyrine

The complex crystallizes in the space group P21/c with four formula units in a unit cell of dimensionsa= 12.747, b= 7.416, c= 17.894 A and/3= 90.2 °. The structure has been solved by the symbolic addition procedure using three dimensional photographic data and refined to an R value of 0.079 for 2019 observed reflexions. The pyramidal nature of the two hetero nitrogen atoms in the antipyrine molecule is inter:nediate between that observed in free antipyrine and in some of its metal complexes. The molecule is more polar than that in crystals of free antipyrine but less so compared with that in metal complexes. In the salicylic acid molecule, the hydroxyl group forms an internal hydrogen bond with one of the oxygen atoms in the carboxyl group. The association between the salicylic acid and the antipyrine molecules is achieved through an intermolecular hydrogen bond with the other carboxyl oxygen atom in the salicylic acid molecule as the proton donor and the carboxyl oxygen atom of the antipyrine molecule as the acceptor

1,3,6-Trimethylpyrano[4,3-b]pyrrol-4(1H)-one

All the non-H atoms of the title compound, C10H11NO2, are almost coplanar [maximum deviation = 0.040 (3) angstrom]. The crystal structure is stabilized by C-H center dot center dot center dot O hydrogen bonds.

Molecular Dynamics Simulations of Homogeneous Nucleation

Nucleation is the first step in a phase transition where small nuclei of the new phase start appearing in the metastable old phase, such as the appearance of small liquid clusters in a supersaturated vapor. Nucleation is important in various industrial and natural processes, including atmospheric new particle formation: between 20 % to 80 % of atmospheric particle concentration is due to nucleation. These atmospheric aerosol particles have a significant effect both on climate and human health. Different simulation methods are often applied when studying things that are difficult or even impossible to measure, or when trying to distinguish between the merits of various theoretical approaches. Such simulation methods include, among others, molecular dynamics and Monte Carlo simulations. In this work molecular dynamics simulations of the homogeneous nucleation of Lennard-Jones argon have been performed. Homogeneous means that the nucleation does not occur on a pre-existing surface. The simulations include runs where the starting configuration is a supersaturated vapor and the nucleation event is observed during the simulation (direct simulations), as well as simulations of a cluster in equilibrium with a surrounding vapor (indirect simulations). The latter type are a necessity when the conditions prevent the occurrence of a nucleation event in a reasonable timeframe in the direct simulations. The effect of various temperature control schemes on the nucleation rate (the rate of appearance of clusters that are equally able to grow to macroscopic sizes and to evaporate) was studied and found to be relatively small. The method to extract the nucleation rate was also found to be of minor importance. The cluster sizes from direct and indirect simulations were used in conjunction with the nucleation theorem to calculate formation free energies for the clusters in the indirect simulations. The results agreed with density functional theory, but were higher than values from Monte Carlo simulations. The formation energies were also used to calculate surface tension for the clusters. The sizes of the clusters in the direct and indirect simulations were compared, showing that the direct simulation clusters have more atoms between the liquid-like core of the cluster and the surrounding vapor. Finally, the performance of various nucleation theories in predicting simulated nucleation rates was investigated, and the results among other things highlighted once again the inadequacy of the classical nucleation theory that is commonly employed in nucleation studies.

Charge Ordering Induced Ferromagnetic Insulator: K2Cr8O16

Usually metallicity accompanies ferromagnetism. K2Cr8O16 is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state. Analyzing the electronic and magnetic properties within first principles electronic structure calculations, we find that the doped electrons due to K induce a charge-ordered and insulating ground state and interestingly also introduce a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering gives rise to a rare example of a charge-order driven ferromagnetic insulator.

Enantiospecific total synthesis of ent-5-senceioyloxy-10,11-epoxythapsan-10-ol

Enantiospecific total synthesis of optical antipode of the sesquiterpene 5-senecioyloxy-10,11-epoxythapsan-10-ol has been described. (R)-Carvone has been employed as the chiral starting material and a combination of intramolecular alkyation and Criegee fragmentation are employed for intramolecular stereospecific transfer of the chirality. An intramolecular diazoketone cyclopropanation and regioselective cyclopropane ring cleavage reactions have been employed or the creation of the three requisite contiguous quaternary carbon atoms.

(2-Chloro-8-methylquinolin-3-yl)methanol

The molecule of title compound, C11H10ClNO, is close to being planar (r.m.s deviation for the non-H atoms = 0.017 angstrom). In the crystal, molecules interact by way of O-H center dot center dot center dot O hydrogen bonds, generating C(2) chains propagating in [010]. The crystal structure is consolidated by C-H center dot center dot center dot pi interactions and aromatic pi-pi stacking interactions [centroid-centroid distance = 3.661 (2) angstrom].