Determination of Pitch in a Cholesteric DSCG-Water Lyomesophase by NMR Techniques

Nuclear Magnetic Resonance (NMR) techniques are used to determine the pitch in a cholesteric lyomesophase prepared by adding of l-alanine to a solution of disodiumcromoglycate (DSCG) in water. The method, which is particularly suitable for the study of bulk samples, consists of a combination of two independent types of measurements: (i) The effective reorientational correlation time of the director, due to molecular translational diffusion, is determined by a lineshape analysis of the deuterium resonances of the solvent D₂O and/or deuterated l-alanine. (ii) The translational self-diffusion coefficients of both these constitutents is determined by a pulsed field-gradient spin-echo experiment. From the results of these measurements, the pitch and its dependence on the chiral dopant concentration is determined.     

Non-Arrthenius Behaviour and Divacancy Contribution in Self-Diffusion of 64Cu in Cu

The self-diffusion of ⁶⁴Cu in Cu was investigated in the high temperature range (1078 to 1348 K), applying the tracer-standard sectioning technique. For the frequency factor D₀ and the activation energy Q₀ we found D₀ = (0.68)cm² · s⁻¹ and Q₀ = (2.17 ± 0.02) eV. A slight bending of the Arrhenius plot suggests a divacancy contribution to the self-diffusion of copper. Data sets used in an numerical analysis are in good accord with published results.     

On the Quantum Theoretical Calculation of Activation Energies for the Self-Diffusion of Single Atoms and the Diffusion of Adatoms on Metal Surfaces (I)

By application and modification of the equation given by A. Einstein for calculating the internal oscillation energy of crystals, the activation energies for the self-diffusion of single atoms on the metal surfaces of Rh, Pt, and for the surfaces diffusion of adatoms on metal surfaces were calculated on the basis of quantum theory from the surface Debye frequency and the bulk Debye frequency, neglecting the frequency distribution. A comparison of the activation energies calculated by quantum theory with the experimental values of the literature obtained by the field-ion microscope shows a relatively good agreement for the single crystal surfaces and temperature ranges. The calculation showed a strong increase of the activation energies for the self-diffusion at higher temperatures and a relatively great dependence on the respective crystal surface of the metal. Analogous results were also obtained for the surface diffusion of adatoms on the metals.     

Investigations for Sn diffusion in Pb1−xSnxTe and PbTe

By annealing Pb_1−xSn_xTe and PbTe isothermally in a quartz ampoule Sn diffused from Pb_1−xSn_xTe into PbTe. The profiles obtained have been investigated by means of an electron beam microanalyser, and the coefficients of diffusion have been determined at various temperatures. The diffusion of Sn can be explained by the expressions: D_PbSnTe = 1.5 · 10⁻¹ exp (−1.8 eV/kT) cm² s⁻¹ (0,14 < x < 0,18) D_PbTe = 5,5 · 10⁻⁴ exp (−1.5 eV/kT) cm² s⁻¹. N-type layers are observed at the surface of Pb_1−xSn_xTe specimens.     

On the effect of an Si3N4 barrier layer over GaAs1−xPx during Zn diffusin: A photoluminescence analysis

An Si₃N₄ barrier layer on top of a GaAs_1−xP_x layer is applied to diminish the out-diffusion of matrix atoms during the formation of p-n junctions by Zn diffusion. An extended photoluminescence analysis of protected and nude but otherwise identical samples is carried out in order to compare the effects introduced by the barrier layers. Samples obtained from two different diffusion processes establishing the p region of a low and high impurity concentration, are studied. Spectra evolution with temperature is obtained from 10 to 300 K. Differences between spectra corresponding to samples diffused with and without an Si₃N₄ barrier layer indicate that nonradiative center concentrations are lowered in the presence of the barrier layer.     

Untersuchung der Korngrenzendiffusion von Zn-65 in α-Aluminium-Zink-Legierungen

The diffusion of Zn-65 in large angle grain boundaries in Al and in 8-Al Zn alloys was investigated. p_K = γδD_K rises exponentially up to a Zn content of 7 at-%, then at a lower rate with concentration. Above 40 at-% Zn p_K is almost independent of concentration. At the solidus temperature, D_K is of the order of 10⁻⁵ cm²/sec. A simple vacancy model for the diffusion in large angle grain boundaries is being discussed. From the model, the following parameters for the grain boundary self diffusion in Al and Cu that have not been investigated so far can be estimated: The activation energies for the electromigration in thin polycrystalline Al films and for the superplasticity at the eutectoid composition are compatible with the results of this investigation. The kinetics of the discontinuous precipitation and the eutectoid transformation in Al Zn alloys is governed by the phase boundary diffusion.     

Relationship between diffusion,self-diffusion and viscosity

We investigate the experimental limits of validity of the Stokes–Einstein equation. There is an important difference between diffusion and self-diffusion. The experimental evidences show, that in the case of self-diffusion the product Dη/T is constant and the Stokes–Einstein equation is still valid. On the other hand, comparison of existing experimental data, on viscosity η and diffusion coefficients D, of small, fast moving ions shows unambiguously that the product Dη/T depends strongly on temperature T. The temperature dependence of diffusion coefficient declines from that of viscosity. Therefore, the Stokes–Einstein equation is not valid in for fast moving ions.     

Defect-mediated self-diffusion in calcium aluminosilicate glasses: A molecular modeling study

The mechanism of self-diffusion in calcium aluminosilicate glasses is investigated at the atomistic level using molecular dynamics (MD) simulations. We study nine glass compositions having the fixed ratio R = [CaO]/[Al₂O₃] = 1 and the concentration of SiO₂ varied from 11.8 to 76.5 mol%. The diffusion coefficient is calculated for each composition at different temperatures from 300 to 6000 K in steps of 300 K. The self-diffusivities of the various elements are found to be close to each other in magnitude, signifying the cooperative nature of the atomic movement. Network “defects” such as miscoordinated cations, non-bridging oxygen, and oxygen triclusters are also studied as a function of temperature and composition. We find that the behavior of self-diffusion correlates well with the concentration of network defects. A model of self-diffusion in calcium aluminosilicate glasses is proposed where diffusion is considered as a defect-mediated process resulting from bond-switching reactions at high temperature.     

Solubility of CaO in CaF2 crystals

By means of conductivity investigations the solubility of CaO in CaF₂ crystals was estimated to be C_L(CaO) ≈︁ 3.7 exp (-0.68 eV/kT) = 3.7 exp (-7900 K/T) and the diffusion constant of O⁻ ions in CaF₂ D(O⁻) ≈︁ 3 exp (-2.2 eV/kT) cm²/s = 3 exp (-25 500 K/T) cm²/s.     

Water penetration mechanisms in nuclear glasses by X-ray and neutron reflectometry

To determine the water diffusion at the early stage of the alteration, X-ray and neutron reflectometry have been performed on altered simplified glasses and the SON68 glass (an inactive R7T7-type French nuclear glass). For the first experiment, the simplified and SON68 glasses were altered at pH 3 and pH 6 and characterized by X-ray reflectometry as a function of the alteration duration. The evolutions of the electron density profile obtained from the reflectivity curves simulations have allowed the determination of the layers compositions. At the beginning of the alteration and for pH 3, the altered surface layer is constituted of a dealkalized zone. Upon alteration progress, the water diffuses inside the layer and hydrolyzes the Si–O–B bonds. For the second experiment, glasses were altered in D₂O (pD 3) and analyzed in D₂O saturated cell. After a D₂O/H₂O substitution, the samples were characterized one more time in H₂O saturated cell. The evolution of the scattering length density shows that in the first stage of the alteration, the layer is constituted of two parts: a dealkalized glass and a dealkalized and boron depleted glass where water has diffused. According to the glass composition and after few hours of alteration, this dealkalized glass part can disappear.     

Cooling crystallization of aluminum sulfate in pure water modulated by sodium dodecylbenzenesulfonate

This study investigated the cooling crystallization of aluminum sulfate to explore the basic data for the recovery of aluminum resources from coal spoil. Cooling crystallization process of aluminum sulfate with sodium dodecylbenzenesulfonate (SDBS) was investigated experimentally. The effects of operating conditions, namely rotate speed and cooling rate on the crystal size (Median diameter, D_0.5) were studied. Based on single factor experimental results, the response surface method (RSM) with a Box–Behnken design (BBD) was used to determine the key operating conditions, from which a predictive equation was established to quantitatively describe the relationships of D_0.5 and there relative parameters. The optimum operating conditions for cooling crystallization of aluminum sulfate were as follows: rotate speed of 200–300 rpm, cooling rate of 4–5 °C /min and n (SDBS)/n (Al₂(SO₄)₃) of around 5E‐4. Molecular dynamics (MD) results reveal that SDBS decreases the diffusion coefficient (D) of Al³⁺ molecules, which inhibits nucleation and promotes crystal growth.     

Temperature dependence of self-diffusion coefficient in several liquid metals

The temperature-dependent self-diffusion coefficient D(T) in liquid metals between the melting temperature T_m and boiling point T_b is modeled in terms of the relationship among D, liquid viscosity η, and liquid–vapor surface tension γ_lv. The model predictions for D(T) correspond to available experimental and molecular dynamics (MD) simulations results for liquid alkali metals Li, Na, Rb and Cs, semi-metal Al, transition and noble metals Ni, Cu, Ag and Au.     

Determination of the diffusion coefficient of arsenic in Ga solution from growth rate measurements of LPG GaAs

On the basis of the LPE GaAs growth rate measurements in which we used current marks of time, the diffusion coefficient and the critical supercooling of arsenic in Ga solution by a semiempirical method was determined. Some knowledge of these parameters is indispensable for investigating the growth mechanism and kinetics of epitaxial layers from high temperature solutions. Taking into account the fact that the diffusion coefficient as a function of temperature is of the form D(T) = D₀ · exp (–B/T), the results were extrapolated in a range of temperatures 750–950°C typical for liquid epitaxy of GaAs.     

Effect of film thickness on the breakdown temperature of atomic layer deposited ultrathin HfO2 and Al2O3 diffusion barriers in copper metallization

We investigate the effect of thickness of HfO₂ and Al₂O₃ barrier films on the breakdown temperature of Cu/barrier film/Si structures. The HfO₂ and Al₂O₃ films are deposited using tetrakis-diethylamino hafnium and tris-diethylamino aluminum, respectively, as the metal precursors and ozone as the oxidizer at 250 °C. Interactions between the layers of Cu/barrier film (1 or 2 nm-thick)/Si structures due to high-temperature annealing in N₂ are probed using sheet resistance measurements. The crystallinity of the multilayer structure and the possible formation of any anneal-induced reaction products are investigated with X-ray diffraction (XRD). The formation of Cu-silicide phase(s) due to diffusion of Cu atoms through the barrier layer indicates the failure of the corresponding diffusion barrier. The surface morphology of Cu is examined using scanning electron microscopy (SEM) and elemental mapping is done with energy dispersive X-ray spectroscopy (EDS). Our results show that both 1 and 2 nm-thick HfO₂ and Al₂O₃ barrier films are capable of restricting the diffusion of Cu at high annealing temperatures; therefore, they could be used as effective diffusion barriers between Cu and Si.     

NMR in soft materials: A study of DMPC/DHPC bicellar system

The DMPC/DHPC bicellar system at the molar ratio of 2.8:1 has been characterised by measurements of self-diffusion coefficient (using PFGSE and PFGSTE NMR sequences), differential scanning calorimetry (DSC) and small angle scattering of synchrotron radiation (SAXS). The DSC curve shows only one endothermic peak characterised by the peak temperature T_peak = 295.7 K and the onset temperature T_onset = 290.1 K. This peak can be assigned to the nematic to smectic phase transition. Below the phase transition temperature, NMR diffusion experiments indicate a two-exponential decay of the spin echo amplitude allowing two diffusion coefficients D₁ and D₂ to be extracted from the experimental data. The maximum size (D_max) of the bicelle determined from SAXS data using the pair distance distribution function p(r) is 11.2 nm and the bilayer thickness is 5 nm.     

Isochronal Studies of α‐Si:H Crystallization and the Performance of it's Schottky Barriers

Hydrogenated amorphous silicon films (α‐Si:H) were crystallized employing a metal induced crystalline (MIC) technique. Structural changes during annealing these films at 300 °C for different periods (0‐300 minutes) were obtained by XRD. Al was used as a metal induced crystalline for α‐Si:H produced by ultra high vacuum (UHV) plasma enhanced chemical vapor deposition (PECVD). XRD shows that crystallization of the interacted α‐Si:H film underneath Al initiates at 300 °C for 15 minutes. A complete crystallization was obtained after annealing for 60 minutes. A gold dot was evaporated onto α‐Si:H films, which annealed for different periods to form Schottky barriers. Electrical properties of Au/α‐Si:H were calculated such as the ideality factor, n, barrier height, Φ_B, donor concentration, N_D, and the diffusion voltage, V_d, as a function of the annealing time of α‐Si:H films. All these parameters were carried out through the current voltage characteristics (J‐V) and the capacitance voltage measurements (C‐V). The results were presented a discussed on the basis of XRD performance and the thermionic emission theory.     

Diffusionsuntersuchungen am System Cu–Ti

As a result of diffusion studies in the binary system of Cu–Ti within a temperature range of 750°C and 850°C the intermetallic phases Ti₂Cu, TiCu, Ti₃Cu₄, Ti₂Cu₃ and Ti₂Cu₇ have been detected by means of electron microprobe analysis. Up to now according to the equilibrium diagram of Zwicker the intermetallic phase Ti₃Cu₄ was reported to be instable only. By the present investigation a time dependence of the width of the total diffusion zone as well as of each of the phases, d∼ √t, has been verified, i.e. volume diffusion is dominating. The chemical diffusion coefficients of all phases have been measured for 750°C, 800°C and 850°C. The partial diffusion coefficients for Cu and Ti in Ti₂Cu₇ for 850°C have also been determined. The chemical diffusion coefficients follow the Arrhenius law, the activation energy Q and the frequence constant D₀ could be calculated.     

Modelling the growth of nitrides in ammonia‐rich environment

Surface properties and the principal processes at the growth of gallium nitride on GaN (0001) face in ammonia‐based are modeled using DFT (density functional theory – SIESTA code) ab initio calculations and 2‐d diffusion analysis. The GaN growth methods are: ammonia‐source MBE, MOVPE, and also HVPE. The adiabatic trajectories, calculated for hydrogen‐rich and hydrogen‐free state of the GaN(0001) surface, include the adsorption of NH₃, GaCl and HCl molecules and the desorption of Ga atoms. The adsorption of ammonia and GaCl has no energy barrier. Thus, in contrast to the results concerning Plasma‐Assisted Molecular Beam Epitaxy (PA MBE), proving that the GaN(0001) surface remains in metal‐rich state, these results indicate that, in the ammonia‐rich environment, typical for HVPE and MOVP growth, the GaN(0001) surface remains in the nitrogen‐rich state. In the case of HCl adsorption, the energy barrier depends on the surface coverage, and could reach 2.0 eV. The direct desorption of single Ga atom has the energy barrier, close to 7 eV. This indicates that Ga surface diffusion (growth controlling process) length is very large, leading to strong interaction of the step kinetics and the diffusion on the terraces. This interaction leads to double–step intertwined structures both in the case of dislocation‐mediated spiral growth and in the step flow growth mode. These morphologies, proposed by the geometric arguments, are observed in the atomic force microscopy (AFM) scans of the GaN(0001) surface. Additionally we have compared the interaction energy of two hydrogen atoms obtained in the DFT SIESTA and the high precision Gaussian in coupled cluster singles, double and perturbation triples CCSD(T) approximation. Both approaches yielded virtually identical interaction energy confirming the validity of DFT analysis of ammonia‐rich growth of GaN. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of the Surface Tilt Angle by Attenuated Total Reflection

Thin SiO₂ layers prepared by oxidizing an organo-silane in an Ar/O₂ rf plasma discharge produce parallel alignment for nematic liquid crystals with essentially zero tilt. In addition, if the rf plasma system is arranged to produce an Ar/O₂ beam, uniform parallel alignment can now be obtained with the director in the substrate plane at an angle perpendicular to the plane of incidence of the rf plasma beam. We have studied the aligning properties of such films using the evanescent wave generated by attenuated total reflection (ATR) on a thin gold film when a surface electromagnetic wave is excited. Liquid crystal cells were constructed on a high index glass prism with a 500 Å gold film and a 200 Å aligning layer. By following the ATR minimum as a function of applied electric field, the tilt at the interface of the liquid crystal and the aligning layer was calculated.     

A mössbauer study of hematite dissolution and devitrification in float glass

The effect of 5% hematite on the devitrification behaviour of float glass has been measured as functions of time and temperature by Mössbauer spectroscopy and X-ray diffraction (XRD). The devitrification products are devitrite (Na₂O·3CaO·6SiO₂), cristobalite, quartz and an iron-rich pyroxene, aegirine ((Na Ca)(Fe, Mg) Si₂O₆) for which XRD data are tabulated. The Mössbauer parameters for glassy and crystalline materials are interpreted in terms ofthe aegirine structure, whose site assignment is discussed in terms of next-nearest neighbour effecs. The bulk dissolution data obtained by Mössbauer and XRD are used to generate diffusion coefficients for Fe³⁺ in the glass melt, using a moving boundary model for diffusion controlled dissolution. The diffusion coefficients at 800°C are D_{Fe³⁺ (XRD)} = 3.52 × 10^?15 cm²/s and D_{Fe³⁺ (MS)} = 1.27 × 10^?15 cm²/s.