Low temperature1H NMR study of electronic structure parameters (T 1e T) in zirconium dihydride

Measurements of the proton-spin-lattice relaxation times (T ₁) are reported for zirconium dihydrides: ZrH_1.87, ZrH_1.93 and ZrH_2.00 at nominal frequency of 41 MHz, with emphasis on the low temperature (4.2–25 K) behaviour of theT ₁. The dominant contributions are originated from the hyperfine interactions of the protons with the conduction electrons and the paramagnetic impurities. The (T _1e T)^–1/2 values determined from the low temperature range (4.2–25 K) agree with those obtained in the extended range (4.2–300 K) as well as with the values reported by the other researches. The paramagnetic impurity contribution is found to be small and teperature independent.     

A pulsed NMR study of hydrogen diffusion in hydrides of group IVa transition metals

Activation energies E_A for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by ¹H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T₁) and rotating (T_1p) frames. For the HfHx system both activation energies obtained from T₁ data, and the value of T₁ at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH₂ (where M = metal), there is excellent agreement between activation energies obtained from T₁ and T_1p data. Mean activation energies obtained were 0.51 eV for TiH_1.98 and 0.83 eV for ZrH_1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH_1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T₁ and T_1p respectively.     

Diffusion coefficients of hydrogen and deuterium in highly concentrated palladium hydride and deuteride phases

Fick's diffusion coefficients for hydrogen and deuterium were determined in palladium hydride and deuteride in the pressure range of gaseous hydrogen and deuterium from 10¹–10⁴ bar and the temperature range from 208 to 338K. This corresponds to the concentration range H, D/Pd from about 0.8 to near stochiometry. An electrical resistance relaxation method was used experimentally. Einstein's diffusion coefficients were calculated and the activation volumes and energies were evaluated. An attempt is made to interpret the anomalous behaviour of the diffusion coefficient in the low temperature range. The results are presented in terms of a compact formula.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

Transport properties of H–N2 mixtures

Transport coefficients (shear viscosity, volume viscosity, thermal conductivity, and mass and thermal diffusion coefficients) of H–N₂ mixtures in the dilute-gas limit have been calculated from the intermolecular potential in the temperature range 300–2000K using the classical trajectory method. The intermediate results pertaining to H–N₂ binary interactions are reported, mainly in terms of cross-section ratios. Cross-sections evaluated with the Mason–Monchick approximation yield very good results for this system, the largest deviations, about 2.5%, being observed for the thermal diffusion coefficient. The accuracy here of this approximation can primarily be attributed to a light H atom and a weakly non-spherical potential resulting in a high rotational collision number. Furthermore, we investigate to which H–N₂ cross-sections and their ratios the values of the mixture transport coefficients are most sensitive. Our results indicate that, for some cross-section ratios, reliance on universal correlations at high temperatures, often used in flame codes, can induce sizeable errors in the thermal conductivity coefficient and especially in the thermal diffusion coefficients. We also observed that the volume viscosity is particularly sensitive to the value of the cross-section for internal energy relaxation in H–N₂ collisions.     

Kinetic coefficients of the semiconducting phase of FeSi2 at low temperatures

The results of studying the kinetic coefficients of β-FeSi₂ in the temperature range 4.2–300 K are considered. The resistivity decreases upon heating in the entire temperature range under investigation. The temperature dependences of the resistivity and thermal conductivity exhibit a break at ～20 K. In the range of 4.2–20 K, the resistivity is a linear function of temperature. The thermo-emf increases rapidly upon cooling and attains values exceeding 15 mV/K. The temperature dependence of the thermo-emf exhibits a break at ～40 K. The observed set of temperature dependences of the kinetic coefficients apparently cannot be explained by a superposition of the known effects only. A new effect probably exists that is associated with a strong electron-phonon interaction in FeSi₂ and which requires a further investigation.     

Thermal deactivation of trapped excitons in 4,4′-dimethylbenzophenone

We present here the spectral and time analysis of the phosphorescence emission of pure 4,4′-dimethylbenzophenone in the temperature range 77–150 K. The values of trapping and detrapping rate constants are obtained from a kinetic model involving a trap located at 1000 cm^_1. An order of magnitude of the diffusion coefficient is deduced.     

Purification of iron to low carbon and nitrogen contents by annealing

Purification of iron specimens to low levels of carbon and nitrogen by annealing in dry hydrogen below 1180 K was investigated. The magnetic after-effect and strain ageing were used for estimating the carbon and nitrogen contents. Decarburization of a few mm thick specimens was found to be controlled by both surface reactions and volume diffusion. The residual carbon content was found to depend not solely on the method of hydrogen purification but also on the type and state of the purification apparatus and on the annealing procedure. Carbon and nitrogen contents smaller than 1 at ppm were reproducibly obtained in a closed ZrH₂ purification system which was baked at 500 K during annealing of specimens.     

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.     

Ion beam mixing and liquid interdiffusion

Abstract

A characteristic of ion mixing in the low temperature portion of the thermally assisted ion mixing regime is that the activation energy obtained from most experiments is in the range of 0.1 to 0.3 eV. These values are typically a factor of 4 to 10 lower than the vacancy migration energy of most elements. This discrepancy is maintained even when the ion mixing data is contrasted to the more comparable data from concentrated homogeneous alloys. It appears that an explanation of the ion mixing activation energy is not possible by radiation enhanced diffusion (RED) where, at the low temperature end of RED, defect annihilation is by direct vacancy-interstitial recombination and the predicted activation energy is Q = 0.5 E^M _v

In an attempt to understand the origin of the low activation energies obtained during ion mixing we have performed calculations of the mutual diffusion coefficients in binary liquid mixtures. A hard sphere fluid model based on the kinetic gas theory of Enskog was used. The model was corrected to agree with molecular dynamic calculations of liquid state diffusion and included thermodynamic driving forces. The calculations resulted in temperature dependent mixing curves which are in good agreement with ion mixing experiments and suggested that the temperature rise in the thermal spike approximately ranged between 1000 and 4000 K.     

Simultaneous detection of CO and CO2 at elevated temperatures using tunable diode laser absorption spectroscopy near 1570 nm

Simultaneous measurements of CO and CO₂ at elevated temperatures are demonstrated using a single semiconductor distributed-feedback (DFB) laser near 1570 nm. Wavelength modulation spectroscopy with second-harmonic detection is used to improve the detection sensitivity and accuracy. A proper line pair near 6368.086 and 6368.330 cm^?1 is selected using some line-selection criterions for the target temperature range of 300–1000 K. Normalization of the 2f signal with the 1f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. The CO and CO₂ concentrations measurements are within 2.21% and 2.55% of the expected values over the tested temperature range of 300–1000 K. The minimum detectable concentrations of CO and CO₂ at 1000 K are 80 ppm m and 153 ppm m, respectively.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

Thermal expansion and mean-square displacements of metal and boron atoms in dysprosium diboride DyB<Subscript>2</Subscript>

Dysprosium diboride is investigated by x-ray diffraction in the temperature range 4.2–300 K. The data obtained are used to determine the (301) and (104) interplanar distances and the corresponding intensities of the x-ray reflections. The calculated linear and bulk thermal expansion coefficients of the DyB₂ compound are characterized by a pronounced anomaly in the temperature range corresponding to the magnetic transformation. The estimates of the mean-square displacements for the Dy and B atoms (calculated from the reflection intensities) are satisfactorily described by the Debye dependences with the characteristic temperatures θ_Dy = 210 K and θ_B = 800 K.     

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.     

Ar-Broadening of isotropic Raman lines in the ν2 band of acetylene

Using a high resolution Raman spectrometer, we have measured Ar-broadening coefficients in the ν₂Q branch of C₂H₂ for 22 lines at 295 K, 20 lines at 174 K, and 16 lines at 134 K. These lines with J values ranging from 1 to 23 are located in the spectral range 1970.9-1974.3 cm⁻¹. The collisional widths are obtained by fitting each spectral line with a Rautian profile. The resulting broadening coefficients are compared with theoretical values arising from close coupling and coupled states calculations. A satisfactory agreement is obtained at room as well as at low temperatures, especially for odd J lines. By comparing broadening coefficients at 295, 174, and 134 K from a simple power law, the temperature dependence of these broadenings has been determined both experimentally, and theoretically.     

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS₂, TlGaS₂, and TlGaSe₂ crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.     

Peculiarities of thermal expansion of layered crystals

Linear expansion coefficients parallel, α_□, and perpendicular, α_⊥, to layer plane of GaS, GaSe, and TlGaS₂ are investigated in the temperature range 5–300 K. Negative values of α_□ are observed in GaS and GaSe in the range of 30–50 K caused by negative Gruneisen parameters γ_□ in this interval. It is shown that negative α_□ in layer crystal can be due to TA_⊥ acoustic branch contribution to α_□.     

Kinetic analysis of n-decane–hydrogen blend combustion in premixed and non-premixed supersonic flows

Numerical analysis of ignition and combustion of an n-decane–hydrogen fuel blend in a premixed supersonic flow and in a model scramjet duct is performed using a reduced reaction mechanism built especially to describe the oxidation of blended n-C₁₀H₂₂–H₂ fuel in air at the temperature T₀ > 900–1000 K in the pressure range P₀ = 0.1–13 atm. The developed kinetic mechanism involves the principal reactions responsible for chain mechanism development both for n-decane and for hydrogen oxidation. It has been shown that using blended n-C₁₀H₂₂–H₂ fuel makes it possible to enhance the ignition and combustion both in premixed and in non-premixed supersonic fuel–air flows compared to burning pure hydrogen–air and n-decane–air mixtures. This allows high combustion completeness in the scramjet duct at the distance of ～1 m even at extremely low air temperature T₀ = 1000 K and pressure P₀ = 0.3 atm. This is due to the interaction of kinetics of the formation of highly reactive atoms and radicals, carriers of chain mechanism, in H₂–air and n-C₁₀H₂₂–air mixtures.     

氢化锆(ZrH1.7)和钯氢(PdH0.7)光学声子的温度效应

在本院重水堆旁的铍过滤探测器中子非弹性散射谱仪上,对氢化锆(ZrH_1.7)(含碳0.2％)和钯氢(PdH_0.7)两种金属氢化物在室温和低温(97K)两种温度下,分别测定了光学声子能谱。结果表明:氢化锆的光学声子能级是等间距的,能级宽度基本上不随温度变化,即光学声子的非谐性是微弱的,较好地遵从爱因斯坦的谐振模型;而钯氢的第二个光学声子能级间距大于第一个能级间距约8meV,并且光学声子能级的宽度从室温下的38meV变化到低温(97K)下的20meV,这表明对钯氢的超导性起决定作用的光学声子,存在较明显的非谐性。 关键词：     

NMR EVIDENCE OF HYDROGEN BOND IN 1-ETHYL-3-METHYLIMIDAZOLIUM-TETRAFLUOROBORATE ROOM TEMPERATURE IONIC LIQUID

The 1-ethyl-3-methylimidazolium-tetrafluoroborate (EMI-BF₄) room-temperature molten salt was investigated with NMR techniques. Diffusion coefficients measured at temperatures ranging from 300 to 360 K indicate phase-change occurred in the vicinity of 333 K, which is supported by ¹¹B quadrupolar relaxation rates. Combined with the sizes of diffusion particles obtained form the Stokes-Einstein equation, this phase change is ascribed to that the diffusion particle is transformed from “discrete ion-pair” to “individual ion” at temperatures above 335 K due to decomposition of the EMI-BF₄ ion pair. Analysis of the ¹³C dipole-dipole relaxation rates identifies the formation of hydrogen bond(C2H…F) between the counter ions, EMI⁺ and BF₄ ^?. That the values of viscosity in EMI-BF₄ are higher than those in EMI-AlCl₄ at corresponding temperatures is therefore understood according to the viscosity approach employed to investigate the H-bonding in molten salts