Spin conversion rates in solid CH4Kr mixtures

Spin conversion rates in solid CH₄Kr mixtures have been measured at Kr concentrations up to 33% and 0.4 < T < 2K. The conversion rate increases by increasing the Kr concentration at constant temperature and increases weakly by decreasing the temperature at constant concentration.     

Étude par rmn de la localisation et des mouvements des protons dans les hydrures MgH2 et Mg2NiH4

Wide line and pulsed NMR studies are reported for MgH₂; and Mg₂NiH₄ hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH₂, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgH_x, (x<0.04).Mg₂NiH₄ shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T₁ and T₂ relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10^?8 < D < 4.6 × 10^?8 cm²/s.     

Correlation and isotope effects for cation diffusion in magnetite

The simultaneous diffusion of ⁵²Fe and ⁵⁹Fe has been measured in Fe₃O₄ as a function of equilibrium oxygen partial pressure (10^?9 <p_o2 < 10^?4 atm) at 1200°C. The p_o2 dependence of D goes through a minimum near 10^?6 atm in agreement with earlier data of Dieckmann and Schmalzried. Comparison of the isotope effect results with correlation-factor calculations suggests that at p_o2 γ 10^?6, diffusion occurs predominantly by vacancy jumps between the normally occupied octahedral sites on the spinel lattice; jumps between tetrahedral sites probably play a lesser role. At p_o2< 10^?6 atm, diffusion occurs by an interstitialtype mechanism involving the simultaneous migration of two atoms. Five of the seven interstitialcy jumps considered in our correlation-factor calculations are consistent with the experimental results.     

Oxygen diffusion studies on (Y2O3)2(Sc2O3)9(ZrO2)89

The oxygen tracer diffusion coefficient (D?) has been measured for 9 mol% scandia 2 mol% yttria co-doped zirconia solid solution, (Y₂O₃)₂(Sc₂O₃)₉(ZrO₂)₈₉, using isotopic exchange and line scanning by Secondary Ion Mass Spectrometry, as a function of temperature. The values of the tracer diffusion coefficient are in the range of 10^? 8–10^? 7 cm² s^? 1 and the Arrhenius activation energy was calculated to be 0.9 eV; both valid in the temperature range of 600–900 °C. Electrical conductivity measurements were carried out using 2-probe and 4-probe AC impedance spectroscopy, and a 4-point DC method at various temperatures. There is a good agreement between the measured tracer diffusion coefficients (D?, Ea = 0.9 eV) and the diffusion coefficients calculated from the DC total conductivity data (D_σ, Ea = 1.0 eV), the latter calculated using the Nernst–Einstein relationship.     

Minimum metallic conductivity in a thin crystal

Electrical conductivities of thin crystals of Bi₂(Te,S)₃ measured from 4.2°K to 300°K fall into four regions: 1) σ < 1.3×10^?5 S with positive temperature coefficient of conductivity; 2) 1.3×10^?5 S < σ < 1.4×10^?5 S with temperature independent conductivity; 3) 1.4×10^?5 S σ < 4×10^?5 S with negative temperature coefficient of conductivity, and 4) σ > 4×10^?5 S with hardly any temperature dependence. A disproportionately high fraction of samples falls into the second range; 1.3×10^?5 S < σ < 1.4×10^?5 S.     

Vacancy-induced heat transfer in solid Kr and Ar

We have measured constant-volume thermal conductivities of solid krypton at six molar volumes between 28.84 and 29.54 cm³ mole^?1 and of solid argon at a molar volume of 24.30 cm³ mole^?1 at temperatures of order and above the respective Debye temperatures. An excess thermal conductivity above that following from the T^? law is ascribed to an additional energy flux carried over by vacancies. The estimated activation energy Q_P for self-diffusion derived from the constant-volume values Q_V as determined from the temperature dependences of the “excess” thermal conductivities are in good agreement with the available data of direct diffusion measurements in solid Ar and Kr.     

Self-diffusion in Kr compared with Van der Waals model

The predictions of the Van der Waals model for the self-diffusion in Kr are compared with the experimental results at 220 K and for 0.7ρ_c ? ρ ? 2.1ρ_c. The Van der Waals model fails to represent the Kr data while at the same temperature in the case of CH₄ this model works quite well for ρ_c < ρ ? 3ρ_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.     

Mass transport in cuprous oxide

The chemical diffusion coefficient of Cu₂O has been obtained for an oxygen partial pressure near 5 10^?4 atm as a function of the temperature in the range 700–900°C D? = 1 62 10^?4 exp(?5140 ± 600 cal mol ^?1)/RT cm²s^?1 This was easily achieved according to the electrochemical method used for the preparation of gaseous mixtures whose Po₂; is lower than 10^?5 atm The slight difference observed with the previously published results by Maluenda, and obtained for Po₂ values which increase with T between 10^?4 and 0.21 atm, may be due to an oxygen partial pressure effect already observed in the case of CoO. An ambipolar treatment of the chemical diffusion, in the case of p-type semiconductor M_aO_b, oxides, has allowed us to express the chemical diffusion coefficient as a function of the concentration of the prevailing defects and of their diffusion coefficient In the case where the prevailing defects are cationic vacancies α times ionized we have shown that the expression D? = (1 + α)D_vα can be generalized to the A₂O compounds This set of results has allowed us, according to the copper self diffusion data obtained recently by Peterson etal, to estimate the apparent enthalpy of formation of the catiomc vacancies ΔH_f 23 ± 0 8 kcal mol^?1.     

Search for a logarithmic term in the density expansion of the diffusion coefficient of Kr in Xe

Measurements of the diffusion coefficient of Kr in Xe at T = 298 K and in the density range 0–200 amagat are reported. The data are analyzed in order to investigate whether a logarithmic divergence appears in the density expansion of the diffusion coefficient. This analysis shows that a power series expansion is more consistent with our data though the presence of the logarithmic term can not be excluded. The values of the first coefficients of the power series expansion are compared with the theoretical predictions.     

Criteria determination to choose piezoelectric materials for BAW resonator applications via colored picosecond acoustics

A great number of piezoelectric materials which could be used in the fabrication of BAW resonators were investigated via colored picosecond acoustics technique in order to study the required parameters for designing and fabricating improved devices. These parameters concerns acoustic longitudinal velocity, v_L, elastic stiffness constant, C^D₃₃, intrinsic mechanical loss, tanδ, and electromechanical coupling coefficient, k²_t. We first quantify the effect of the ratio between the wavelength pulse of a femtosecond laser and the period of Brillouin oscillations, λ_p/T. It is found that C^D₃₃ depends linearly on λ_p/T. Then, we deduced novel relations for stiffness constant and mechanical coupling coefficient. Moreover, the determination of different parameters (density, refractive index and v_L) of piezoelectric film is achieved, with good agreement with literature. The optimized conditions for the parameter choice of BAW resonators are found to be: 300?GPa?<?C^D₃₃?<?500?GPa and 1.7?GPa?<?e²₃₃/ε^s₃₃?<?28.5?GPa with λ_p/T?>?37 10³?m/s where e₃₃ and ε^s₃₃ are the piezoelectric constant and the materials permittivity in the direction 3, respectively.     

Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn₂O₅ (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature T_C. This can be explained as an effect of vanishing ferroelectric ordering above T_C. It was found that the reduced sound velocity increases with increasing V⁽³⁾ (the third-order atomic force constants of the anharmonic phonons) in the interval T?<?T_C, whereas the reduced sound velocity remains unchanged in the interval T_C?<?T?<?T_N. It was also found that the reduced sound velocity increases with the increase of V⁽⁴⁾ (the fourth-order atomic force constants of the anharmonic phonons) in the interval T?<?T_N. In addition, the ferroelectric phase transition temperature T_C decreases when V⁽⁴⁾ increases in the interval T?<?T_N. Those theoretical results are in agreement with the experimental data.     

Integral intensities of absorption bands of silicon tetrafluoride in the gas phase and cryogenic solutions: Experiment and calculation

The spectral characteristics of the SiF₄ molecule in the range 3100–700 cm^?1, including the absorption range of the band ν₃, are studied in the gas phase at P = 0.4–7 bar and in solutions in liquefied Ar and Kr. In the cryogenic solutions, the relative intensities of the vibrational bands, including the bands of the isotopically substituted molecules, are determined. The absorption coefficients of the combination bands 2ν₃, ν₃ + ν₁, ν₃ + ν₄, and 3ν₄ are measured in the solution in Kr. In the gas phase of the one-component system at an elevated pressure of SiF₄, the integrated absorption coefficient of the absorption band ν₃ of the ²⁸SiF₄ molecule was measured to be A(ν₃) = 700 ± 30 km/mol. Within the limits of experimental error, this absorption coefficient is consistent with estimates obtained from independent measurements and virtually coincides with the coefficient A(ν₃) = 691 km/mol calculated in this study by the quantum-chemical method MP2(full) with the basis set cc-pVQZ.     

Growth and characterization of Nd:Bi12SiO20 single crystal

A Nd:Bi₁₂SiO₂₀ crystal was grown by the Czochralski method. The thermal properties of the crystal were systematically studied. The thermal expansion coefficient was measured to be α=11.42×10^?6 K^?1 over the temperature range of 295–775 K, and the specific heat and thermal diffusion coefficient were measured to be 0.243 Jg^?1 k^?1 and 0.584 mm²/s, respectively at 302 K. The density was measured to be 9.361 g/cm³ by the buoyancy method. The thermal conductivity of Nd:Bi₁₂SiO₂₀ was calculated to be 1.328 Wm^?1 K^?1 at room temperature (302 K). The refractive index of Nd:Bi₁₂SiO₂₀ was measured at room temperature at eight different wavelengths. The absorption and emission spectra were also measured at room temperature. Continuous-wave (CW) laser output of a Nd:Bi₁₂SiO₂₀ crystal pumped by a laser diode (LD) at 1071.5 nm was achieved with an output power of 65 mW. To our knowledge, this is the first time LD pumped laser output in this crystal has been obtained. These results show that Nd:Bi₁₂SiO₂₀ can serve as a laser crystal.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

The mechanism of surface heterodiffusion at elevated temperatures

Molecular dynamics simulations employing the Lennard-Jones potential have been performed on Kr/Ar(111) to investigate the nature of high-temperature surface heterodiffusion. These suggest that high activation energies and preexponential factors observed experimentally at temperatures above approximately 0.70 T_m arise from adatom-vacancy pair formation, which sharply increases the number of mobile species. Evidence is also obtained that at intermediate temperatures (0.45 T_m < T < 0.66 T_m), Arrhenius curves mass transfer diffusivity can depend on both coverage and adsorbate interactions. For adsorbates which exhibit islanding, low coverage diffusion is characterized by an activation energy equal to the enthalpy of migration, whereas at moderate coverage the activation energy contains an additional term due to the enthalpy of formation of mobile adatoms from immobile islands.     

Study of the hydrogen diffusion and segregation into Fe-C-Mo martensitic HSLA steel using electrochemical permeation test

Diffusion and trapping mechanisms are studied in order to improve Hydrogen embrittlement (HE) resistance of high yield strength steels. Investigations were carried on Fe-C-Mo model steel with a quenched and tempered martensitic microstructure. Hydrogen diffusion was studied by using the electrochemical permeation technique. The influence of the charging current densities in 1 M H₂SO₄ at ambient temperature shows a relation between the apparent diffusion coefficient D_app and the apparent subsurface concentration of hydrogen C_0app. Two domains can be separated and are mainly associated with a competition between two distinct processes: hydrogen trapping and hydrogen diffusion. These results are correlated to the quantities of reversible and irreversible traps into the membrane. Moreover, the experimental results revealed that the apparent diffusion coefficient increases and the total amount of trapped hydrogen decreases with temperature. The activation energy of the diffusion process (0.26 eV) and the trapping process (0.58 eV) are supposed to be, respectively, affiliated with lattice diffusion and with trapping on incidental dislocations and/or on martensitic lath interfaces due to misorientations (geometric necessary dislocations).     

Which mechanism underlies the water-like anomalies in core-softened potentials?

Using molecular dynamics simulations we investigate the thermodynamics of particles interacting with continuous and discrete versions of a core-softened (CS) intermolecular potential composed by a repulsive shoulder. Dynamical and structural properties are also analyzed by the simulations. We show that in the continuous version of the CS potential the density at constant pressure has a maximum for a certain temperature. Similarly the diffusion constant, D, at a constant temperature has a maximum at a density ρ D _max and a minimum at a density ρ D _min < ρD_max, and structural properties are also anomalous. For the discrete CS potential none of these anomalies are observed. The absence of anomalies in the discrete case and its presence in the continuous CS potential are discussed in the framework of the excess entropy.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Electrical conductivity and chemical diffusion coefficient measurements in single crystalline nickel oxide at high temperatures

Electrical conductivity measurements on nickel oxide have been performed at high temperatures (1273 K<T< 1673 K) and in partial pressures of oxygen ranging from Po₂ = 1.89 × 10^?4 atm to Po₂ = 1 atm. The $\text{po}{}_2{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$ dependence of the conductivity decreases from about $\text{1}\text{4}$ for Po₂ = 1 atm to smaller values for lower partial pressures of oxygen. The activation enthalpy for conduction increases for decreasing oxygen partial pressures (from 22.5 kcal mol^?1 at Po₂ = 1 atm to 26.0 kcal mol^?1 for Po₂ = 1.89 × 10^?4 atm). This behaviour can be explained by the simultaneous presence of singly and doubly ionized nickel vacancies, with different energies of formation.Furthermore, chemical diffusion coefficient measurements have been performed in the same temperature range, using the conductivity technique, and leading to the result:

$\text{D}\text{?}\text{= 0.244 exp (?}\text{36,600}\text{RT}\text{) cm}{}^2\text{s}{}^{\mathrm{?1}}$

.