The sodium diffusion in aluminium-oxide

The transport of Na through the polycrystalline ceramic arc tube of high intensity discharge lamps has been investigated. This complex process consists of several steps: solution in the ceramics, diffusion through the ceramics, leaving the bulk phase, evaporation from the surface. Among the listed processes the kinetics of the diffusion was examined in the temperature range 400-1200 °C, separately from other disturbing effects. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) were used to determine the concentration depth profiles. The obtained results confirmed that the grain boundary diffusion plays an important role in the transport process of sodium through the ceramic wall. The bulk and the grain boundary diffusion coefficients and the temperature dependencies of these transport processes have been determined. The activation energy of Na bulk diffusion is 56.5 ± 6.7 kJ/mol at 900-1200 °C, respectively the activation energies of Na grain boundary diffusion amount to 97.5 ± 21.6 kJ/mol in the temperature range 700-1100 °C and 7.7 ± 4.0 × 10⁻² kJ/mol at 400-700 °C. The preexponential factor of the bulk diffusion was found to be D_o = 5.1 × 10⁻¹⁵ ± 9.5 × 10⁻¹⁷ cm²/s in the temperature range 900-1200°C, whereas the preexponential factors of grain boundary diffusion are D_o = 1.1 × 10⁻¹⁰ ± 1.1 × 10⁻¹¹ cm²/s at 700-1100 °C and D_o = 7.5 × 10^-15 ± 1.5 × 10⁻¹⁷ cm²/s at 400-700 °C.     

EDXRF measurements on gold diffusion-doped Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy

Gold (Au) diffusion in superconducting Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y was investigated over the temperature range 500-800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D₁=6.7×10⁻⁵exp(−1.19 eV/k_BT) and D₂=9.7×10⁻⁴exp(−1.09 eV/k_BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm⁻², in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T_c phase to the low-T_c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.     

Measurement of diffusion coefficients of Ag in YBa2Cu3O7 by the EDXRF technique

The diffusion of Ag in superconducting YBa₂Cu₃O₇ ceramic was studied over the temperature range 700–850 °C by the energy‐dispersive X‐ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am‐241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of Ag diffusion coefficients in grains (D₁) and over the grain boundaries (D₂) are described by the equations D₁ = 1.4 × 10^?2exp[?(1.18 ± 0.10)/kT] and D₂ = 3.1 × 10^?4exp[?(0.87 ± 0.10)/kT]. Copyright © 2003 John Wiley & Sons, Ltd.     

Oxygen ion diffusion measurements in Bi0.775La0.225O1.5

The oxygen transport properties of the Bi_0.775La_0.225O_1.5 electrolyte material have been investigated. Isotopic exchange depth profiling (IEDP) technique with secondary ion mass spectrometry (SIMS) was used in order to measure the oxygen tracer diffusion coefficient D. The activation energy for oxygen tracer diffusion was found to be 115 ± 2 kJ/mol (1.19 ± 0.02 eV). The measured D values were converted using the Nernst-Einstein relationship to conductivity and compared with data from AC Impedance. The agreement of the two sets of data implies that the material is an oxygen ion conductor.     

Chemical diffusion in molybdenum disulphide

Ceramic molybdenum disulphide (MoS₂) was equilibrated at an ambient sulphur vapour partial pressure p(S₂), 10 Pa<p(S₂)<1000 Pa. After the step change of p(S₂) to a new value, the equilibration kinetics was monitored by measuring electrical conductivity. The application of the solution of Fick's second law (with the initial condition: no concentration gradient in specimen and the boundary condition: surface concentration constant) to the kinetic data gave the chemical diffusion coefficient. The chemical diffusion coefficient, D_chem, determined at 1273 K, was D_chem=(3.20±0.32)*10⁻⁷ cm² s⁻¹ and was found to be independent of sulphur vapour partial pressure. The usefulness of transient electrical conductivity method for determining real values of diffusion data was discussed in terms of defect structure of the studied material.     

Molten salt synthesis of Li1 + x (Ni0.5Mn0.5)1 − xO2 as cathode material for Li-ion batteries

Li_1 + x(Ni_0.5Mn_0.5)_1 − xO₂ cathode material for Li-ion batteries has been prepared by a molten salt method using Li₂CO₃ salt. The influences of synthetic temperature and time have been intensively investigated. It is easy to obtain materials with a hexagonal α-NaFeO₂ structure except broad peaks between 20° and 25°. Nickel in Li_1 + x(Ni_0.5Mn_0.5)_1 − xO₂ is oxidized to a trivalent state while manganese maintained a tetravalent state. It is found that the discharge capacities of all samples increase with cycling. The sample prepared at 850 °C for 5 h has a discharge capacity of 130 mAh g^− 1 between 2.5 and 4.5 V versus V_Li+/Li at a specific current of 0.13 mA cm^− 2 after 50 cycles at 25 °C.     

Kinetics for the reaction of phenyl radical with phenylacetylene and styrene

The kinetics for the reactions of C₆H₅ with phenylacetylene and styrene have been measured by CRDS in the temperature range 297-409 K under an Ar pressure of 3.6 Torr. The total rate constants can be given by the following Arrhenius expressions (in units of cm³ mol⁻¹ s⁻¹): k₁(C₆H₅ + C₆H₅C₂H) = 10^13.0±0.1exp [−(2430 ± 150)/RT] and k₂(C₆H₅ + C₆H₅C₂H₃) = 10^13.3±0.1 exp [−(2570 ± 180)/T]. Additional DFT and MP2 calculations have been carried out to assist our interpretation of the measured kinetic data. The addition of C₆H₅ to the terminal CH_x (x = 1 or 2) sites is predicted to be the dominant channel for both reactions. The calculated bimolecular rate constants are in reasonable agreement with experimental values for the temperature range studied.     

Boron neutralization and hydrogen diffusion in silicon subjected to low-energy hydrogen implantation

Using the hydrogen neutralization of the boron acceptor, the diffusion of hydrogen is investigated in the temperature range 20 °–160 °C. The hydrogenation is performed by low-energy implantation. We observe a fast initial hydrogen migration, followed by a long-time diffusion phase that is described by an effective diffusion coefficientD _eff=D _{0 eff} exp(–E _a/kT) withD _{0 eff}–cm²s^–1 andE _a=(0.83±0.05) eV. No deeper hydrogen migration is detected for implantation times longer than – 30 min. Our data are explained by the build-up of a large amount of molecular hydrogen beneath the surface, which strongly hinders the transfer of the implanted hydrogen to the bulk. The thermal reactivation kinetics of the neutralized boron show a rapid initial step followed by a longtime thermally activated second order phase, which is limited by the recombination of hydrogen into molecules.     

Thermal stability, oxygen non-stoichiometry and transport properties of LaNi0.6Fe0.4O3

Thermal stability, oxygen non-stoichiometry and electrical conductivity of LaNi_0.6Fe_0.4O_3−δ were investigated in the temperature region of 20-1000 °C in Ar/O₂ gas flows at oxygen partial pressures between 0.5 and 21,000 Pa. Diffusion mobility was measured in Ar/O₂ gas flow at pO₂ = 18 Pa. Crystal structure of this compound was found to be stable at the mentioned experimental conditions. LaNi_0.6Fe_0.4O_3−δ is a p-type semiconductor with metallic type conductivity above 150 °C at the investigated pO₂ range. Two different (fast and slow) oxygen exchange areas on the temperature-pO₂ diagram were established, which are due to two different oxygen anion positions in the double B-site mixed perovskite structure. Oxygen non-stoichiometry in the fast oxygen exchange region reaches about 0.005 of oxygen atomic index. Chemical diffusion and oxygen surface exchange coefficients do not vary at 600-800 °C, but show visible increase above 800-850 °C.     

Fabrication and characterization of epitaxial Sr0.6Ba0.4Nb2O6/La0.7Sr0.3CoO3 heterostructures

Epitaxial Sr_0.6Ba_0.4Nb₂O₆(SBN60)/La_0.7Sr_0.3CoO₃ heterostructures were fabricated on LAO(0 0 1) substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction. The θ-2θ scans showed single crystalline Sr_1−xBa_xNb₂O₆ (SBN) and La_xSr_1−xCoO₃ (LSCO) layers with a 〈0 0 1〉 orientations perpendicular to the substrate plane. Phi scans on the (2 2 1) plane of SBN layer indicated that the films have two in-plane orientations with respect to the substrate. The SBN unit cells were rotated in the plane of the film by ±18.4° as well as ±45° with respect to the LAO substrate. This rotation was explained by considering the lattice matching between films and substrate, and minimization of electrostatic energy. Spectroellipsometry (SE) was used to characterize the depth profile, the microstructural inhomogeneities, including voids and surface roughness, refractive indices and extinction coefficients of the films.     

Study of cation tracer diffusion in NaF

Cation tracer diffusion coefficients were measured in pure NaF crystals in the intrinsic ionic conductivity range (876–970 °C). The results can be rationalized satisfactorily in terms of contributions to the observed Na tracer diffusivities arising from both free vacancies and neutral vacancy pairs, the latter contribution amounting to about 53 per cent of the total Na diffusion at the highest measuring temperature. The best-fit defect parameters derived in an earlier conductivity study [21] from this laboratory on similar NaF crystals give for the free vacancy contribution D_v^*(Na) = 4·25 exp (?2·21 eV/kT) cm²s^?1. A combination of these D_v^*(Na) values with the present diffusion data yields for the vacancy-pair contribution D_p^*(Na) = 1·15 × 10⁸exp (?4·04 eV/kT) cm²s^?1. Comparison of the present findings with published values of the anion tracer diffusion coefficient in NaF showed that D_p^* (F) is 2·3 to 4·4 times larger than D_p^*(Na) over the temperature range of our observations, the difference between the two contributions increasing with decreasing temperature. When approximate account is taken of the temperature-dependence of the two pair correlation factors, this last result indicates that the anion jumps into the vacancy pair occur with a higher frequency, and increasingly so at lower temperatures, than do those involving the cations.     

Use of TiB2 diffusion barriers for Ni/Au ohmic contacts on p-GaN

The use of a TiB₂ diffusion barrier for Ni/Au contacts on p-GaN is reported. The annealing temperature (25-950 °C) dependence of ohmic contact characteristics using a Ni/Au/TiB₂/Ti/Au metallization scheme deposited by sputtering were investigated by contact resistance measurements and auger electron spectroscopy (AES). The as-deposited contacts are rectifying and transition to ohmic behavior for annealing at ≥500 °C . A minimum specific contact resistivity of ∼3 × 10⁻⁴ Ω cm⁻² was obtained after annealing over a broad range of temperatures (800-950 °C for 60 s). The contact morphology became considerably rougher at the higher end of this temperature range. AES profiling showed significant Ti and Ni outdiffusion through the TiB₂ at 800 °C. By 900 °C the Ti was almost completely removed to the surface, where it became oxidized. Use of the TiB₂ diffusion barrier produces superior thermal stability compared to the more common Ni/Au, whose morphology degrades significantly above 500 °C.     

Thermal evolution of vacancy defects induced in sintered UO2 disks by helium implantation

A slow positron beam coupled with Doppler broadening (DB) spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO₂ disks implanted with different 1 MeV ³He fluences and annealed in ArH₂ or in vacuum. The S(E) and W(E) behaviors indicate that for fluences in the range from 2 × 10¹⁴ to 2 × 10¹⁶³He cm⁻², the vacancy defects distribution evolves with the annealing temperature in the range from 264 to 700 °C under ArH₂. This evolution is found to be dependent on the ³He fluence implanted in the sintered UO₂ disks. For the lowest fluence of 2 × 10¹⁴³He cm⁻², the S(W) plot with positron energy as the running parameter suggests that only the concentration of vacancy defects decreases when annealing temperature increases. For the highest implantation fluences (from 5 × 10¹⁵ to 2 × 10¹⁶³He cm⁻²) the S(W) plot suggests that the nature of the vacancy defects changes in the annealing temperature range from 260 to 400 °C. Measurements performed in implanted UO₂ disks annealed in vacuum have revealed a partial recovery of the vacancy defects possibly due to their recombination with mobile oxygen interstitials. The role of the hydrogen infusion into the disk is also discussed.     

Surface mass diffusion and step stiffness on an anisotropic surface; Mo(0 1 1)

Results of step fluctuation experiments for Mo(0 1 1), using low-energy electron microscopy, are re-examined using recently developed procedures that offer accurate coefficients of surface mass diffusion. By these means, surface diffusion D_s is documented at T/T_m ∼ 0.5, while the crossover to relaxation driven by bulk vacancy diffusion is inferred for T/T_m ∼ 0.6. Here, T_m is the melting temperature T_m = 2896 K. We obtain D_s = 4 × 10⁻⁴ exp(−1.13 eV/k_BT) cm²/s for the temperature interval 1080-1680 K. Possible indications of diffusion along step edges appear for T/T_m ∼ 0.4. The same measurements of step fluctuation amplitudes determine also the step stiffness, which by symmetry is anisotropic on Mo(0 1 1). It is shown that three independent procedures yield mutually consistent step stiffness anisotropies. These are (1) step fluctuation amplitudes; (2) step relaxation rate anisotropies; and (3) the observed anisotropies of islands in equilibrium on the Mo(0 1 1) surface. The magnitude of the step stiffness obtained from step edge relaxation is consistent with earlier measurements that determine diffusion from grain boundary grooving.     

Collision-induced first overtone band of D2 in binary mixtures D2-X where X is Ar, Kr, and Xe

Enhancement spectra of the collision-induced absorption in the first overtone region 5500-6750 cm⁻¹ of D₂ in the D₂-Ar, D₂-Kr, and D₂-Xe binary mixtures were studied at 298 K for base densities of D₂ in the range 55-251 amagat and for partial densities of Ar, Kr, and Xe in the range 46-384 amagat. The observed spectra consist of the following quadrupolar transitions: O₂(3), O₂(2), Q₂ (J), J = 1-5 and S₂ (J), J = 0-5 of D₂. Binary and ternary absorption coefficients were determined from the integrated absorption coefficients of the band. Profile analyses of the spectra were carried out using the Birnbaum-Cohen (BC) lineshape function and characteristic lineshape parameters were determined from the analyses.     

Magnetic and Microwave Absorbing Properties of Electrospun Ba(1−x)LaxFe12O19 Nanofibers

Ba_(1−x)La_xFe₁₂O₁₉ (0.00≤x≤0.10) nanofibers were fabricated via the electrospinning technique followed by heat treatment at different temperatures for 2 h. Various characterization methods including scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and microwave vector network analyzer were employed to investigate the morphologies, crystalline phases, magnetic properties, and complex electromagnetic parameters of nanofibers. The SEM images indicate that samples with various values of x are of a continuous fiber-like morphology with an average diameter of 110±20 nm. The XRD patterns show that the main phase is M-type barium hexaferrite without other impurity phases when calcined at 1100 °C. The VSM results show that coercive force (H_c) decreases first and then increases, while saturation magnetization (M_s) reveals an increase at first and then decreases with La³⁺ ions content increase. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the M-type barium hexaferrites. The microwave absorption performance of Ba_0.95La_0.05Fe₁₂O₁₉ nanofibers gets significant improvement: The bandwidth below −10 dB expands from 0 GHz to 12.6 GHz, and the peak value of reflection loss decreases from −9.65 dB to −23.02 dB with the layer thickness of 2.0 mm.     

Theoretical study on the kinetics for OH reactions with CH3OH and C2H5OH

Kinetics and mechanisms for reactions of OH with methanol and ethanol have been investigated at the CCSD(T)/6-311 + G(3df, 2p)//MP2/6-311 + G(3df, 2p) level of theory. The total and individual rate constants, and product branching ratios for the reactions have been computed in the temperature range 200-3000 K with variational transition state theory by including the effects of multiple reflections above the wells of their pre-reaction complexes, quantum-mechanical tunneling and hindered internal rotations. The predicted results can be represented by the expressions k₁ = 4.65 × 10⁻²⁰ × T^2.68 exp(414/T) and k₂ = 9.11 × 10⁻²⁰ × T^2.58 exp(748/T) cm³ molecule⁻¹ s⁻¹ for the CH₃OH and C₂H₅OH reactions, respectively. These results are in reasonable agreements with available experimental data except that of OH + C₂H₅OH in the high temperature range. The former reaction produces 96-89% of the H₂O + CH₂OH products, whereas the latter process produces 98-70% of H₂O + CH₃CHOH and 2-21% of the H₂O + CH₂CH₂OH products in the temperature range computed (200-3000 K).     

The bΣ(b0) → XΣ(X10,X21) and aΔ(a2) → X21 transitions of AsBr

Emission spectra of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺,X₂1) and a¹Δ(a2) → X₂1 transitions of AsBr have been measured in the near-infrared spectral region with a Fourier-transform spectrometer. The arsenic bromide radicals were generated in fast-flow systems by reaction of arsenic vapor (As_x) with bromine and were excited by microwave-discharged oxygen. The most prominent features in the spectrum are the Δv = +1,0,−1, and −2 band sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺) transition in the range 11 700-12 700 cm⁻¹. With lower intensities, the Δv = 0 and −1 sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₂1) sub-system show up in the same range. Further to the red, between 6000 and 6700 cm⁻¹, the Δv = 0, +1, and −1 sequences of the hitherto unknown a¹Δ(a2) → X₂1 transition are observed. Analyses of medium- and high-resolution spectra have yielded improved molecular constants for the X₁0⁺, X₂1, and b0⁺ states and first values of the electronic energy and the vibrational constants of the a2 state.     

NaEu0.96Sm0.04(MoO4)2 as a promising red-emitting phosphor for LED solid-state lighting prepared by the Pechini process

NaEu_0.96Sm_0.04(MoO₄)₂ was prepared by the Pechini method (P phosphor) and as a comparison, also by solid-state reaction technique (S phosphor). The photo-luminescent properties, the morphology and the grain size were investigated. The phosphors show broadened excitation band around 400 nm, high intensity of Eu³⁺⁵D₀→⁷F₂ emission upon excitation around 400 nm, and appropriate CIE chromaticity coordinates. Intensive red light-emitting diodes (LEDs) were fabricated by combining the phosphor and a 400 nm InGaN chip for the first time, which confirm that the phosphor is a good candidate for near UV LED. The luminescent intensity of P phosphor prepared at 700 °C is near that of S phosphor prepared at 800 °C. In addition, P phosphor shows advantages of lower calcining temperature, shorter heating time, and smaller grain size. Considering all these factors, the suitable method for preparing the promising near UV LED phosphor NaEu_0.96Sm_0.04(MoO₄)₂ is recommended to be the Pechini process at 700 °C.     

Infrared emission spectrum of hot D2O

An emission spectrum of hot D₂O (1500 °C) has been analyzed in the 2077-4323 cm⁻¹ region. A considerable number of new vibration-rotation energy levels have been determined and two new vibrational levels identified. The new (0 4 1) and (0 2 2) vibrational levels have estimated band origins of 7343.93 ± 0.01 and 7826.38 ± 0.02 cm⁻¹, respectively.