Does an overlayer/underlayer transition occur for HRu(001)?

The reflectivity for low energy electrons (8–35 eV) has been studied from clean, hydrogen and deuterium covered Ru(001) faces. This technique is sensitive to hydrogen adsorbed at room temperature, which is nearly invisible in UPS. Explanations for the difference between low temperature and room temperature hydrogen layers are examined in the light of these results; a previously suggested overlayer/underlayer transition model can be ruled out.     

Specific heat,electrical resistance,and other properties of superconducting Pd-H alloys

Specific heat and electrical resistance measurements have been carried out in the ～1.2 to 4.2 K range for a series of Pd-H alloys having H/Pd atomic ratios in the ～0.82 to 0.88 range. Superconductive transitions in the resistance were observed; the specific heat data exhibited broad, yet very pronounced peaks with characteristics that depended strongly upon hydrogen concentration. A fairly successful theoretical fitting of the specific heat data was carried out on the basis of the assumption that Pd-H alloys are simply inhomogeneous BCS superconductors. Data for the dependence of the superconducting transition temperature upon hydrogen concentration were derived from both the specific heat and electrical resistance measurements. An interesting scaling feature of the specific heat data is discussed. A search for a possible interrelationship between the superconductive properties and an exothermic process which occurs in Pd-H alloys yielded negative results. A marked suppression of superconductive effects was observed in Pd-H alloys based on one particular Pd ingot; this was probably due to Fe impurities.     

入射激波诱导环氧丙烷爆炸特征研究

用自行设计激波管点火测试技术,实验研究了温度范围760-1380K间入射激波诱导下环氧丙烷的点火机理。利用激波管压力传感器测定了H*(486.1) 和O (470.5nm)随激波诱导强度变化的点火时间特征。实验结果表明：在低马赫数下氢氧自由基出现时间较接近,1.5-2.5马赫间随激波诱导强度增大而线性减小;而马赫大于2.5后,氧自由基的出现时间迅速减小,是由于高活化能的氧自由基的点火时间对强激波较敏感,而诱导强度大于3.5马赫后对两者点火影响区别就下明显了。实验数据将有益于含能材料点火时间的研究。     

入射激波诱导环氧丙烷爆炸特征研究

用自行设计激波管点火测试技术,实验研究了温度范围760-1380K间入射激波诱导下环氧丙烷的点火机理。利用激波管压力传感器测定了H*(486.1) 和O (470.5nm)随激波诱导强度变化的点火时间特征。实验结果表明：在低马赫数下氢氧自由基出现时间较接近,1.5-2.5马赫间随激波诱导强度增大而线性减小;而马赫大于2.5后,氧自由基的出现时间迅速减小,是由于高活化能的氧自由基的点火时间对强激波较敏感,而诱导强度大于3.5马赫后对两者点火影响区别就下明显了。实验数据将有益于含能材料点火时间的研究。     

The importance of the activation volume for the description of the molecular dynamics of glass-forming liquids

High pressure dielectric measurements were carried out on hydrogen bonded d-glucose and two van der Waals peracetyl saccharides, i.e. α pentaacetyl glucose and α octaacetyl maltose. In this study we found that after removing H bonds, the molecular dynamics of both modified saccharides is very sensitive to pressure, as reflected by a large value of the pressure coefficient of the glass transition temperature, equal to 270 K GPa(-1) and 280 K GPa(-1) for α pentaacetyl glucose and α octaacetyl maltose, respectively. On the other hand, dT(g)/dP for d-glucose is much lower, equal to 67 K GPa(-1). Our result confirms the general rule that the hydrogen bonding glass-forming liquids exhibit much lower values of dT(g)/dP compared to the van der Waals systems. Additionally, on the basis of results reported herein and also recent literature data for polyalcohols, we point out that the activation volume correlates fairly well with the molecular volume in the case of hydrogen bonding liquids. On the other hand, much larger values of the activation volumes at T(g) with respect to the molecular volumes were found for both peracetyl saccharides.     

Collisional broadening of the J = 1 ← 0 transition of HC15N by nitrogen,oxygen, helium,and air

Nitrogen-broadened linewidths have been measured for the J = 1 ← 0 rotational transition in the fundamental vibrational state of the hydrogen cyanide isotopic species ¹H¹²C¹⁵N, located at 88 055.01 MHz. The data have been recorded for four temperature levels in the range 234–295 K. The collisional half-widths derived from these measurements allowed us to determine, to our knowledge for the first time, the temperature dependence of the foreign gas broadening parameter. Linewidth measurements at room temperature with oxygen, air, and helium as broadening gases are also reported.     

Two-photon polarization spectroscopy applied for quantitative measurements of atomic hydrogen in atmospheric pressure flames

Two-photon – polarization spectroscopy has been applied for the first time to determine absolute number densities and kinetic temperatures of atomic hydrogen via the 1 S–2 Stransition in flames at atmospheric pressure. This sensitive laser spectroscopic technique, so far mainly developed and applied for quantitative plasma diagnostics, is also well suited for real combustion processes, because the signal is directly related to the two-photon absorption itself and therefore not limited by quenching or photo-ionization. It can be applied with high spatial and temporal resolution in a wide range of pressure and temperature. Furthermore, a well-established calibration procedure allows for a precise determination of the absolute ground-state density of atomic hydrogen. The measurements also demonstrate that a certain potential of this method comes especially into its own in conjunction with laser radiation of highest spectral quality, i.e. pulsed single-longitudinal mode UV radiation.     

Observation of two isotropic-nematic phase transitions near a surface

Using specified conditions, we succeeded in observing the isotropic-nematic (Iso-N) liquid crystal phase transition at surfaces followed by that in bulk for the first time. An additional heat anomaly peak was found at a higher temperature side of a main phase transition peak using highly sensitive differential scanning calorimetry (HS-DSC). The peak is pronounced particularly in the cooling process, since the transition starts at surfaces on cooling. The temperature dependence of retardation allows us to safely conclude that the higher temperature peak that appeared in HS-DSC is attributed to the Iso-N transition at surfaces. These measurements also indicate that the surface transition is of first order. These behaviors were theoretically explained by generalized Maier-Saupe theory.     

Melting line of hydrogen at high pressures

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature versus pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure and temperature by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and find a peak in the melting line at P=64.7+/-4 GPa and T=1055+/-20 K.     

Use of non-aqueous electrochemical cells to investigate air- and water-sensitive hydrogen storage materials

Electrochemical methods have been used to study hydrogen in vanadium, titanium and three binary magnesium alloy systems. These materials are all sensitive to air and/or water, which prohibits the use of aqueous electrolytes. The measurements have involved the use of alkali halide and lower temperature organic-anion molten salts saturated with an alkali hydride so that they conduct charge by the transport of hydride ions, rather than protons. Alkali metals, or their alloys, can be used as indirect reference electrodes for hydrogen in these electrolytes.     

Metal–insulator transition in boron-doped amorphous carbon films

Boron-doped amorphous graphite-like carbon (GLC) films have been prepared with different boron concentrations. Electrical transport measurements in the temperature range 1.3–300?K on the films shows a doping-induced metal–insulator (MI) transition. On the metallic side of the transition, the experimental data are interpreted in terms of weak localization and the effect of electron–electron interactions. Data on the insulator side of transition are analyzed in terms of hopping conduction. Critical behaviour is observed near the transition, with the resistivity obeying a power-law temperature dependence.     

Measurement of ultraviolet Argon(II) transition probabilities

The transition probabilities of twelve Ar(II) lines around 300nm have been measured in a stationary arc experiment at 1 atm. The reported data are based on absolute intensities, calibrated by the continuum radiation of a high temperature hydrogen plasma. For two lines the maximum emission coefficient at 26500 K has been used for an immediate determination of the respective A_mn-values. These data serve for temperature measurements at lower arc currents, where the other lines are investigated. The results, the uncertainty of which should be below ±15%, agree satisfactorily with coulomb approximation calculations, but show severe discrepancies as compared with more recent NBS tables.     

Thermal and dielectric studies of self-assembly systems formed by hydroquinone and alkyloxy benzoic acids

Self-assembly systems formed by hydroquinone and alkyloxy benzoic acids are isolated and characterized. Hydroquinone formed double hydrogen bonds with p-n-alkyloxy benzoic acids. Various hydrogen bonded complexes have been synthesized with hydroquinone and pentyloxy to dodecyloxy benzoic acid, respectively. FTIR studies confirm the hydrogen bond formation in the complex. Polarizing Optical Microscopic (POM) studies revealed the textural information while the transition and enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. Phase diagram has been constructed from the POM and DSC data. The members of the present homologous complexes are characterized by POM, DSC, optical tilt angle and dielectric studies. Odd-even effect has been evinced in enthalpy values and transition temperatures corresponding to the isotropic to nematic phase transition. Optical tilt angle in Smectic C phase data has been fitted to a power law and it has been observed that the temperature variation of the tilt angle follows Mean Field theory prediction. Results of Fourier Transform Infrared Spectra (FTIR), POM, DSC, tilt angle and dielectric studies are discussed.     

Diffuse phase transition of Fe doped lead ytterbium tantalate ceramics

The effect of different concentration of Fe on the phase transition behavior of Lead ytterbium tantalate is investigated by dielectric and differential scanning calrimetry measurements. The samples are prepared through solid state reaction method and it has been found that the sintering temperature significantly lowered when the proportion of Pb(Fe_1/2Ta_1/2)O₃ increased. It has been observed that the doping in small amounts (0≤x≤0.2) of Fe could meliorate the dielectric and ferroelectric properties. The diffuseness in the mode of phase transition increases and the phase transition temperature decreases as a function of Fe content. It is revealed that the dielectric data and heat capacity data follow a similar trend in the variation of the mode of phase transition and phase transition temperatures. The phase transition temperature values obtained from the heat capacity measurement well agreed with the values obtained from dielectric measurement.     

Lower phase transition temperature of solid hydrogen sulphide

The lower phase transition temperature of solid hydrogen sulphide is measured accurately using the polarizing microscope technique. A temperature difference is observed between one phase transition and its reverse. It is suggested that this may be due to hysteresis effect in the phase transition as a result of change of structure.     

Diffusion of muonium and hydrogen in diamond

Jump rates of muonium and hydrogen in diamond are calculated by quantum transition-state theory, based on the path-integral centroid formalism. This technique allows us to study the influence of vibrational mode quantization on the effective free-energy barriers DeltaF for impurity diffusion, which are renormalized with respect to the zero-temperature classical calculation. For the transition from a tetrahedral (T) site to a bond-center (BC) position, DeltaF is larger for hydrogen than for muonium, and the opposite happens for the transition BC-->T. The calculated effective barriers decrease for rising temperature, except for the muonium transition from T to BC sites. The calculated jump rates are in good agreement with available muon spin rotation data.     

The positive muon and the positron as probes of defects

Conclusions A brief but hopefully general comparison has been made between muons and positrons as probes for the study of defects in metals. Since muon experiments are not only more demanding in manpower, cost and availability than positron experiments, they should be carefully designed in light of the knowledge that the muon is extremely sensitive to both intrinsic and extrinsic defects. Initial experiments should provide estimates of the muon diffusion coefficients as a function of sample temperature. High temperature hydrogen diffusion measurements do provide quidelines, although so far most of the observations of muon trapping have been made at low sample temperatures where hydrogen diffusion data do not exist. Given that the diffusion constant is known as a function of temperature, high-purity Fe²⁶ after low temperature electron irradiation is therefore a good candidate to study with muons. Since the defect type and concentration can be controlled in electron irradiated samples, such investigations could confirm the stated values of the diffusion constants in Fe thereby providing a new method for evaluating diffusion coefficients below Stage III in different metals.All the samples should be initially characterized by other less costly techniques to obtain, where possible, the concentration and type of extrinsic and intrinsic defects. Both transverse and longitudinal measurements should be made to unravel the question of different diffusion mechanisms versus defect-, impurity- or self-trapping.Provided that the full capabilities of the muon are utilized and coupled with complementary techniques, i.e., positron annihilation, the muon will constitute a useful new probe in deepening our understanding or defects in metals.Work supported by the US Dept. of Energy under contract DE-AC02-76CH00016.     

Thermal expansion of the C15 Laves-phase materials TaV2 and TaV2H x

The thermal expansion of the C15 Laves-phase material TaV₂H _x was measured over the temperature range of 130–350?K for x?=?0, 0.34 and 0.53. A primary objective of the study was to see if the anomalous temperature dependence of elastic constants found in an earlier study is due to, or associated with, unusual thermal expansion. The expansion was typical of transition metals, and quite close to theoretical results for related Laves-phase materials. Grüneisen parameters were determined from the measurements, and these were also typical of transition metals, and close to calculated values for related materials. Both the thermal expansion and the Grüneisen parameters of TaV₂ decreased somewhat with the addition of hydrogen. No anomalous behavior of the expansion was found that might account for the highly anomalous temperature dependence of elastic moduli reported earlier. This absence is consistent with the attribution of the elastic constant behavior to an electronic structure effect.     

Electric and magnetic anomalies at the charge-density-wave transition in niobium-substituted vanadium diselenides

A systematic study has been made of the effect of niobium substitution on the CDW transition in VSe₂, especially in the low-doping region. Transitions were monitored by measurements of static magnetic susceptibility, which has been found to be particularly sensitive for both onset and lock-in discontinuities, and by measurements on resistivity and Hall effect. Results, discussed in terms of the model of Chan and Heine and in terms of McMillan's phenomenological model, suggest that structural changes due to the presence of the niobium dopant lead to enchanced electron-phonon coupling thus raising the CDW onset temperature.     

Pressure induced lattice instability and phase separation in the cuprates

High pressure structural studies using a synchrotron source and Raman measurements on various cuprates reveal several structural modifications. The data have shown strong deviations from the normal equation of state at characteristic pressures, hysteresis, and the appearance of additional peaks that can be attributed to a new phase. The combined data of synchrotron angle-dispersive experiments with the optical measurements indicate that at some critical pressures, at least for certain compounds, non-linear effects are observed together with phase separation that affect the distribution of the carriers and the transition temperature. The comparison of the data with those induced by an internal pressure by an atomic substitution indicates that the effect is related to the existence of carriers within the CuO₂ superconducting planes.