Correlation between film thickness and zinc defect distribution along the growth direction in an isotopic multilayer ZnO thin film grown by pulsed laser deposition analyzed using the internal diffusion method

Zinc self-diffusion along the growth direction was analyzed for the isotopic multilayer ZnO thin film ((⁶⁴ZnO/⁶⁸ZnO)₆⁶⁴ZnO) deposited by pulsed laser deposition. The isotopic distribution was measured using a secondary ion mass spectrometry. The amplitude of the ⁶⁴Zn abundance in the depth profile was reduced by annealing at 993 K for several hours due to interdiffusion between the ⁶⁴ZnO and ⁶⁸ZnO layers. The diffusion profiles at the isotopic interfaces were analyzed using a periodic equation. The obtained zinc self-diffusion coefficients at several isotopic interfaces along the growth direction showed that the self-diffusion coefficients increased towards the film/substrate interface. A similar trend was also found in the lateral direction. The variation among the self-diffusion coefficients was related to the film thicknesses at the analysis positions. Since zinc self-diffusion is controlled by a vacancy-mediated mechanism, the variation in zinc diffusivity along the growth direction can be attributed to the effect of compressive biaxial stress. These findings are useful for producing high-quality ZnO devices.     

Diffusion in thermotropic liquid crystals

Diffusion coefficients in mesophases of thermotropic liquid crystals have been measured to a considerable extent only during the last twelve years. Some theories on diffusion in mesophases have been developed too. Measurements in nematic, smectic A, smectic B, and smectic C mesophases have been carried out by mass transport techniques, detected by radio-tracers or optically, by NMR spin-echo techniques, and by quasi-elastic neutron scattering. The diffusion is anisotropic in most of the cases. In nematics self-diffusion parallel to the molecular director (D₆) is somewhat faster than perpendicular (D_⊥). Both diffusion coefficients show about the same activation energy. Impurity diffusion (small molecules dissolved in mesophases) demonstrates the same behaviour with less anisotropy. In smectics A and C the diffusion coefficients D₆ and D_⊥ can be nearly equal. The anisotropy is now shown in their respective activation energies with E₆>E_⊥. This effect is more pronounced in impurity than in self-diffusion. In addition impurity diffusion shows a strong anisotropy of the diffusion coefficients (D_⊥?D₆). In smectics B the activation energies E₆ and E_⊥ of self-diffusion seem again to be equal and the diffusion coefficients show a small anisotropy (D_⊥≥D₆).     

Spectroscopic Detection of Oxygen in Nitrogeneous Gas Mixtures

A pulsed high-frequency discharge was used to excite a non-LTE (local thermal equilibrium) plasma with a high specific power density sufficiently high to dissociate oxygen and gaseous oxides. If traces of hydrocarbons were present in the gas mixture, in the spectrum the Violet system of CN appeared with a long radiation decay time. The CN bands are excited preferentially by inelastic collisions with metastable nitrogen molecules (A³σ). These levels can be efficiently depopulated by competing impact with atomic oxygen. By measuring the radiation signal during the radiation decay a dynamic range of 6 was achieved for a variation of the oxygen content between 1 and 100 ppm. The measured radiation decay of the violet CN 0–0-bands at 388 nm was simulated by a simple model of rate equations valid in the time region where diffusion processes may be neglected.     

Production and loss of N2 + ions during the decay period of plasmas produced in helium-nitrogen mixtures

The time dependence of the number density of N₂ ⁺ ions during the decay period of plasmas produced in helium containing 0.05, 0.17 and 0.5 percent nitrogen was studied in the pressure range from about 0.3 to 7 Torr by means of mass spectrometer techniques. During the early part of the afterglow period the time dependence of N₂ ⁺ is controlled by ambipolar diffusion loss towards the plasma container walls. The product of the ambipolar diffusion coefficientD _a and the reduced pressurep ₀ wasD _ap₀=900± 50 cm² Torr/sec. The production of N₂ ⁺ by collisions between metastable nitrogen molecules determines the temporal behavior of the N₂ ⁺ density during the late afterglow for extremely pure discharge conditions. From the data it follows that the metastable molecules involved are de-excited by collisions with ground state helium atoms with a rate constant of 3.4 × 10^?15 cm³ sec^?1, while the radiative lifetime of these metastable molecules is at least 20 msec. The surface catalytic efficiency for de-excitation upon striking the molybdenum covered plasma container walls was estimated to be smaller than 10^?3. Energy and radiative lifetime requirements suggest that N₂ ⁺ is produced during the plasma decay period by the process N₂ (a′¹ Σ _u ^? )+N₂ (a′¹σ _u ^? →N₂ ⁺ (X²Σ _u ⁺ )+N₂(X)+e.     

Self-diffusion of water and benzene molecules adsorbed in synthetic opal samples

The self-diffusion of adsorbed molecules of water and benzene in porous synthetic opals has been studied by pulsed field gradient nuclear magnetic resonance. The study indicates two “phases” of water molecules differing by the self-diffusion coefficients, indicating two types of porosities existing in the synthetic opals. The smallest self-diffusion coefficient characterizes water in ultramicropores found on the surface of nanospheres. The form of the diffusion decay depends on temperature in the region of high temperatures as a result of exchange between pores of different sizes. The ultramicropores are inaccessible for the largest benzene molecules. Water adsorption and self-diffusion data confirm that annealing of the opal samples at 1293 K caused the collapse of ultramicropores.     

Ionic and molecular Self-Diffusion in Ion-Exchange materials for fuel energetics studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to investigate the self-diffusion mechanism of water, alcohol molecules and Li⁻ counterions in sulfocation exchangers with different structures of the polymeric matrix. It was shown that in the homogeneous perfluorinated sulfocation exchange membranes the ionic and water translation motions are controled by the hydrogen bond network forming in ionogenic channels at the high water content. At the low solvent content, the self-diffusion coefficients of methanol and ethanol are higher than the water self-diffusion coefficients. The influence of non-ion-exchange sorbed electrolyte on Li⁺ self-diffusion coefficients was observed in the heterogeneous sulfocation exchanger KU-23.     

Elektronenverluste in einem abklingenden Wasserstoffplasma

The time dependence of the electron density in the afterglow period of an electrodeless discharge in hydrogen was measured by means of 4- and 8-mm microwave interferometry. An exponential decay was observed in the late afterglow permitting the evaluation of a time constant in the density range from 10¹²–10¹⁰ electrons per cm³. The decay time of the plasma was influenced by the discharge conditions. Electron losses could be explained by ambipolar diffusion and attachment to impurities. Two diffusion coefficients were found correlated to the discharge duration. The mobility value H₀=10.1±1.0 cm²/V · sec calculated from the diffusion coefficient found for short discharge pulses agrees with Saporoschenkos mobility value μ₀=10.2 cm²/V · sec for the H ₃ ⁺ -ion. A second mobility value μ₀=14.8±1.2cm²/V · sec found for longer discharge pulses might refer to the H⁺-ion.     

Ionic conductivity of anhydrous zirconium bis(monohydrogen orthophosphate) and its sodium ion forms

Conductivities have been measured by the vector impedance method for the layered compounds anhydrous α-zirconium phosphate, Zr(HPO₄)₂, and several of its sodium ion phases. The values of the ionic diffusion coefficients are of the order of 10^?6-10^?8 cm²/s at 200°C and activation energies range from 15.6 cal/mol for Zr(HPO₄)₂ to 18.6 cal/mol for ZrHNa(PO₄)₂. Comparisons are made with literature values of conductivity and self-diffusion coefficients for Zr(NaPO₄)₂·3H₂O and kinetically determined diffusion coefficients for the phases containing both sodium and hydrogen ions.     

A probe-based method for measuring the transport coefficient in the tokamak edge region

A new method for measuring the diffusion coefficient in the edge plasma of fusion devices is presented. The method is based on studying the decay of the plasma fluctuation spectrum inside a small ceramic tube having its mouth flush with a magnetic surface and its axis aligned along the radial direction. The plasma fluctuations are detected by an electrode, radially movable inside the tube. In the experiment described herein, which was performed in the edge region of the CASTOR tokamak, the electrode measured the floating potential. The experimental arrangement is the same used for the direct plasma potential measurements according to the “Ball-pen probe” [1], the design of which is based on the Katsumata probe principle. When the electrode protrudes from the tube, the measured signal shows the floating potential fluctuations of the plasma. Retracting the electrode into the tube, the signal power spectrum displays a decay. This decay is different for different frequencies, and is exponential. Assuming a mainly diffusive behaviour of the plasma inside the tube, the spectrum decay length can be used to derive a value of the diffusion coefficient. The measurement were performed at different radial positions in the CASTOR edge region, so that a radial profile of the diffusion coefficient was obtained. Typical values ofD are of (2–3) m²/s, consistent with expectations from the global particle balance. The radial profile shows a tendency of the diffusion coefficient to increase going deeper into the plasma.     

Diffusion and aggregation of Alzheimer’s Aβ1–40 peptide in aqueous trifluoroethanol solutions as studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to measure the self-diffusion coefficient of Aβ_1–40 peptide in trifluoroethanol (TFE) and mixed solvent TFE-water (D₂O) buffer (pD 7.8) at 293 K. The data were analyzed on the basis of the Stokes model and the hardsphere approach was used to estimate self-diffusion coefficients. It was found that the extent of the Aβ_1–40 aggregation in TFE solutions depends on the concentration of the peptide and the sample preparation protocol. After soft mixing, i.e., without any additional mechanical pretreatment of the peptide, the peptide is present in the monomeric form in TFE solutions. However, the additional water-bath sonication of the sample during the dissolution of Aβ_1–40 in TFE enforces oligomerization of the peptide with the size of aggregates ranging from tetra- to hexamers. An increase of D₂O in the mixed TFE-D₂O solvent of up to 75% leads to the aggregation of the larger part of the peptide. However, the components of self-diffusion coefficients related to low-mass Aβ_1–40 oligomers (dimers and trimers) were not observed in the diffusion decay curves. The most probable explanation is that dimers and trimers are not the principal intermediate species in the aggregation of Aβ_1–40 peptide.     

Self-diffusion coefficients and orientational correlation times in aqueous NaCl solutions: Complementarity with structural investigations

The dynamical properties of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-6 m) at ambient conditions are characterized by molecular dynamics (MD) simulations. MD simulations are performed with a flexible SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. In this paper, attention has been focused on the self-diffusion coefficients (D_i) of ions and water molecules and on orientational correlation time of water molecules. It is found that the self-diffusion coefficients decrease with ion concentration. Moreover, the self diffusion coefficients of sodium and chloride at higher salt concentrations are very comparable which may be due to the formation of clusters of these ions. The deduced rotational dynamics speeds up as the salt concentration increases. Some complementarities between dynamical properties and structural ones, recently obtained, are carried out.     

Water behavior in perfluorinated ion-exchange membranes

The water molecules in acid and salt forms of perfluorinated sulfocation membranes (MF-4SK) have been investigated by employing nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) techniques. The mobility parameters, correlation time and activation energy of water molecules were estimated from the results of the temperature dependence of¹H relaxation time and compared with water self-diffusion coefficients obtained with pulsed field gradient NMR. The NMR data showed no frozen unbound water in membranes at low water content with an amount of water molecules per sulfonate groupn being comparable to the cationic hydration numberh _o, whereas DSC thermograms showed peaks which are usually interpreted as a water fusion phenomenon in the membranes. The diffusion mechanism of water molecules below 260 K is different from that above 300 K due to additional hydrogen bonds in water clusters at the low-temperature region.     

Self-diffusion of aluminium in the intermetallic compound Fe-48 at.% Al

The self-diffusion coefficient of Al in the B2-type intermetallic compound Fe-48 at.% Al has been determined using the intrinsic diffusion coefficients of Fe and Al and the self-diffusion coefficient of Fe with the help of the Darken-Manning relation. The self-diffusion coefficient of Al in Fe-48 at.% Al is estimated to be a factor of about 0.6 smaller than that of Fe, and the activation energy for the self-diffusion of Al is obtained to be 280 kJ mol^?1 which is a little larger than the value of 262 kJ mol^?1 for the self-diffusion of Fe, indicating that the diffusion mechanisms for both components are nearly equal.     

NMR determination of topological structure and self-diffusion of linear polymers

A general approach is proposed to calculate the attenuation of spin echo diffusion and the self-diffusion coefficients D of linear polymers with different average molecular weights M _w . It was shown that the changes in the topological structure of linear polymers resulted from the formation of a physical entanglement network at M _w > 10⁵ are responsible for the experimentally observed anomalous diffusion D(M _w ).     

Diffusion in nanocrystalline material

The paper reports on first investigations of the diffusion in nanocrystalline materials. The self-diffusion of the radioisotope ⁶⁷Cu in nanocrystalline copper has been measured by serial sectioning with the aid of ion-beam sputtering. The values of the diffusion coefficients which were found at 353 K and 393 K are 2×10⁻¹⁸ m²/s and 1.7×10⁻¹⁷ m²/s, respectively, i.e., they are about 16 or 14 orders of magnitude larger than the bulk self-diffusion and about 3 orders of magnitude larger than the grain-boundary self-diffusion in copper. In comparison to the bulk, small values for the activation enthalpy, 0.64 eV, and the pre-exponential factor of the self-diffusion coefficient, 3×10⁻⁹ m²/s, have been observed.     

Tracer impurity diffusion in single-crystal rutile (TiO2−x)

By means of the radioactive-tracer sectioning technique, the tracer diffusion of the impurity ions, ⁴⁶Sc, ⁵¹Cr, ⁵⁴Mn, ⁵⁹Fe, ⁶⁰Co, ⁶³Ni and ⁹⁵Zr, in rutile single crystals was measured as functions of crystal orientation, temperature, oxygen partial pressure and Al impurity content. The diffusion coefficients are very sensitive to the electric charge of the impurity ions. Divalent impurities (e.g., Co and Ni) diffuse extremely rapidly in TiO₂, compared to cation self-diffusion, and exhibit an extreme anisotropy in diffusion behavior, divalent-impurity diffusion parallel to the c-axis is much larger than it is perpendicular to the c-axis. Trivalent impurity ions (Sc and Cr) and tetravalent impurity ions (Zr) diffuse similar to cation self-diffusion, both as functions of temperature and oxygen partial pressure. The divalent impurity ions Co and Ni apparently diffuse as interstitial ions along open channels parallel to the c-axis. The results suggest that Sc, Cr and Zr ions diffuse by an interstitialcy mechanism involving the simultaneous and cooperative migration of tetravalent interstitial titanium ions and the tracer-impurity ions. Iron ions diffuse both as divalent and as trivalent ions. The impurity diffusion as functions of oxygen partial pressure and Al-impurity content are consistent with calculations of point-defect concentrations in rutile.     

Measurement of iron self-diffusion in hematite single crystals by secondary ion-mass spectrometry (SIMS) and comparison with cation self-diffusion in corundum-structure oxides

Iron bulk self-diffusion coefficients were measured in Fe₂O₃ single crystals using original methodology based on the utilization of ⁵⁷Fe stable isotope as iron tracer and depth profiling by secondary ion-mass spectrometry (SIMS). The iron self-diffusion coefficients were measured along and perpendicular to the c-axis, between 900 and 1100°C, in an oxygen atmosphere. Along the c-axis, the coefficients can be described by D _{∥ c} (cm²/s)?=?5.2?×?10⁶?exp[?510 (kJ/mol)/RT], and are close to reliable data, available in the literature, obtained by means of radioactive techniques. Perpendicular to the c-axis, D _⊥c (cm²/s)?= 83?exp[?430 (kJ/mol)/RT], and the coefficients are smaller than coefficients along the c-axis. The data are compared with previously results of cation bulk self-diffusion in Cr₂O₃ and Al₂O₃ single crystals.     

An investigation of lithium transport properties in V6O13 single crystals

Lithium has been incorporated into single crystals of V₆O₁₃ by coulometric titration in solid state electrochemical cells. A single phase region was identified for Li_xV₆O₁₃ between x=0 and x=0.17 at 120°C. The lithium chemical diffusion coefficient, self-diffusion coefficient, ionic conductivity and partial molar entropy have been estimated at intervals over this composition range in two specific crystallographic directions. The chemical diffusion coefficients were found to be constant in a particular crystal direction with values of 3.5 × 10^-8 cm² s^-1 and 4.9 × 10^-9 cm² s^-1 perpendicular to the (0 1 0) and (0 0 1) planes respectively.     

Dynamic properties of water in silicalite-1 powder

Self-diffusion of D₂O in partially filled silicalite-1 crystals was studied at 25°C by ²H nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10⁻⁷ to ∼10⁻¹⁰ m²/s, which include values much higher and lower than that of bulk water (∼10⁻⁹ m²/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10–200 ms. A two-site Kärger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.