Proton magnetic resonance of ethane in the liquid and gaseous states

The proton chemical shift of ethane is reported, covering for gaseous ethane the density range 0 to 0·5 g cm^-3 and the temperature range 0 to 110°C, and for liquid ethane from the critical temperature down to -55°C. The results are discussed in terms of the van der Waals interaction term σ _w , using a virial expansion including both binary and ternary collisions. A parallel is drawn between the medium shifts and the equation of state for non-ideal gases. Some comments are made regarding (i) the uniqueness of the parameter B for protons in a C-H bond in non-polar molecules, (ii) the neighbour anisotropy contribution σ _a in molecules such as ethane and ethylene, and (iii) the van der Waals gas-to-liquid medium shift, and its temperature dependence, in larger molecules.     

Monte Carlo study of the two-step spin transition in [FeXZn1∓X(2-pic)3]Cl2 · EtOH

The Monte Carlo method has been applied to reproduce the two-step spin transition in [Fe_xZn_1?x(2-pic)₃]Cl₂ · EtOH (2-pic = 2-picolylamine). The partition function of the Hamiltonian has been factorized in two parts describing the inner degrees of freedom of the spin transition molecules and the interaction between them. The factorization is equivalent to an effective Hamiltonian which can be mapped on an Ising system in an applied magnetic field. The interaction consists of an infinite range and a short range interaction part. Qualitatively the two-step transition under pressure and the Zn diluted system have been well reproduced with infinite range and the next nearest neighbour interaction of ferromagnetic type and the nearest neighbour interaction of antiferromagnetic type in a simple cubic lattice.     

Analytical solution of 1D Ising-like systems modified by weak long range interaction

It is well-known that 1D systems with only nearest neighbour interaction exhibit no phase transition. It is shown that the presence of a small long range interaction treated by the mean field approximation in addition to strong nearest neighbour interaction gives rise to hysteresis curves of large width. This situation is believed to exist in spin crossover systems where by the deformation of the spin changing molecules, an elastic coupling leads to a long range interaction, and strong bonding between the molecules in a chain compound leads to large values for nearest neighbour interaction constants. For this interaction scheme an analytical solution has been derived and the interplay between these two types of interaction is discussed on the basis of experimental data of the chain compound which exhibits a very large hysteresis of 50 K above RT at 370 K. The width and shape of the hysteresis loop depend on the balance between long and short range interaction. For short range interaction energies much larger than the transition temperature the hysteresis width is determined by the long range interaction alone. Received 26 November 1998     

Cems study of the carbon distribution in austenite

CEMS resolution allows a careful study of Fe−C pure austenite spectra with high carbon content. Three Fe environments are detected which are ascribed to Fe atoms with one carbon first nearest neighbour and zero carbon second nearest neighbour and two environments with no carbon first nearest neighbour but zero carbon second nearest neighbour and one to four carbon second nearest neighbours respectively. This confirms the repulsive interaction between carbon interstitials and the tendency towards Fe₈C ordering is suggested.     

Characteristics of heat treated polyparaphenylene for lithium-ion secondary batteries

In order to clarify the mechanism of Li-ions insertion/deinsertion into/from polyparaphenylene (PPP)-based carbons, the PPP carbonization has been analyzed. The weight loss of the PPP carbonized samples increased at a faster rate in the temperature range 600–680 °C. Hydrocarbons such as benzene and biphenyl were mainly detected by gas chromotography–mass spectrometry at a temperature of 710 °C during thermo-gravimetric analysis of PPP. The microtexture and the structure of heat treated PPP samples were characterized by high resolution transmission electron microscopy and investigated by image processing. The resistivity measurements of heat treated PPP showed a drastic decrease in the 680–730 °C HTT range. We related the microtexture evolution, the hydrocarbons release and the resistivity decrease of PPP carbonized samples to Li-ion battery capacity.     

Surface diffusion and formation of ordered state

Surface diffusion and formation for an ordered phase is investigated using the lattice gas model in Monte-Carlo simulation. Interaction up to the second nearest neighbour between particles is considered. The coverage is calculated as a function of a normalized coordinate by the square root of the diffusion time, starting from the initial step coverage. When the temperature T is lower than T_c, a shoulder appears in the coverage curve at near half coverage. It is shown that this shoulder corresponds to the formation of a c(2 × 2) phase, in which the existence of long range order is confirmed. On the other hand, when T is higher than T_c, the shoulder in the coverage curve disappears, but the short range order of the first and second nearest neighbour correlation still remains and the formation of islands of c(2 × 2) pattern is locally observed. The microscopic growth mechanism of the c(2 × 2) phase is considered. The observed growth rate is explained phenomenologically through the diffusion coefficient which is concentration-dependent.     

Role of buffer layer in electronic structures of iron phthalocyanine molecules on Au(111)

We investigate the electronic structures of one and two monolayer iron phthalocyanine (FePc) molecules on Au(111) surfaces. The first monolayer FePc is lying flat on the Au(111) substrate, and the second monolayer FePc is tilted at～15° relative to the substrate plane along the nearest neighbour [101ˉ] direction with a lobe downward to the central hole of the unit cell in the first layer. The structural information obtained by first-principles calculations is in agreement with the experiment results. Furthermore, it is demonstrated that the electronic structures of FePc molecules in one-monolayer FePc/Au(111) system are perturbed significantly, while the electronic structures of FePc molecules in the second monolayer in two-monolayer FePc/Au(111) system remain almost unchanged due to the screening of the buffer layer on Au(111).     

Effect of temperature on the synthesis of silver nanoparticles with polyethylene glycol: new insights into the reduction mechanism

Polyethylene glycol (PEG) molecules act as a reducing and stabilizing agent in the formation of silver nanoparticles. PEG undergoes thermal oxidative degradation at temperatures over 70 °C in the presence of oxygen. Here, we studied how the temperature and an oxidizing atmosphere could affect the synthesis of silver nanoparticles with PEG. We tested different AgNO₃ concentrations for nanoparticles syntheses using PEG of low molecular weight, at 60 and 100 °C. At the higher temperature, the reducing action of PEG increased and the effect of PEG/Ag⁺ ratio on nanoparticles aggregation changed. These results suggest that different synthesis mechanisms operate at 60 and 100 °C. Thus, at 60 °C the reduction of silver ions can occur through the oxidation of the hydroxyl groups of PEG, as has been previously reported. We propose that the thermal oxidative degradation of PEG at 100 °C increases the number of both, functional groups and molecules that can reduce silver ions and stabilize silver nanoparticles. This degradation process could explain the enhancement of PEG reducing action observed by other authors when they increase the reaction temperature or use a PEG of higher molecular weight     

Structural Evolution of Compressing Amorphous Ice

Molecular dynamics simulation is employed to study structural evolution during compressing low density amorphous ice from one atmosphere to 2.5 GPa. The calculated results show that high density amorphous ice is formed under intermediate pressure of about 1.0GPa and O-O-O angle ranges from about 83°to 113° and O-H…O is bent from 112° to 160°The very high density amorphous ice is also formed under the pressure larger than 1.4 GPa and interstitial molecules are found in 0.3-0.4 A just beyond the nearest O-O distance. Low angle O-H… O disappears and it is believed that these hydrogen bonds are broken or re-bonded under high pressures.     

Pairwise thermal entanglement in a three-qubit Heisenberg XX model with a nonuniform magnetic field and Dzyaloshinski-- Moriya interaction

Pairwise thermal entanglement in a three-qubit Heisenberg XX model is investigated when a nonuniform magnetic field and the Dzyaloshinski-Moriya interaction are included. We find that the nonuniform magnetic field and Dzyaloshinski-Moriya interaction are the more efficient control parameters for the increase of entanglement and critical temperature. For both the nearest neighbour sites and the next nearest neighbour sites, the magnetic field can induce entanglement to a certain extent and the Dzyaloshinski-Moriya interaction can enhance the entanglement to a stable value. The steady value of the nearest neighbour site entanglement C 12 is larger than the next nearest neighbour site entanglement C 13 . An interesting phenomenon is that the entanglement curve of C 12 appears a peak value when the Dzyaloshinski-Moriya interaction is considered in a nonuniform magnetic field.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

The effect of pressure on the electric conduction in liquid benzene saturated with water

The pressure dependence of the current in liquid benzene saturated with water was studied up to 5 kbar in the temperature range from 25°C to 220°C. The results are compared with measurements in dried liquid benzene.     

High‐temperature tensile cell for in situ real‐time investigation of carbon fibre carbonization and graphitization processes

A new high‐temperature fibre tensile cell is described, developed for use at the Advanced Photon Source at Argonne National Laboratory to enable the investigation of the carbonization and graphitization processes during carbon fibre production. This cell is used to heat precursor fibre bundles to temperatures up to ～2300°C in a controlled inert atmosphere, while applying tensile stress to facilitate formation of highly oriented graphitic microstructure; evolution of the microstructure as a function of temperature and time during the carbonization and higher‐temperature graphitization processes can then be monitored by collecting real‐time wide‐angle X‐ray diffraction (WAXD) patterns. As an example, the carbonization and graphitization behaviour of an oxidized polyacrylonitrile fibre was studied up to a temperature of ～1750°C. Real‐time WAXD revealed the gradual increase in microstructure alignment with the fibre axis with increasing temperature over the temperature range 600–1100°C. Above 1100°C, no further changes in orientation were observed. The overall magnitude of change increased with increasing applied tensile stress during carbonization. As a second example, the high‐temperature graphitizability of PAN‐ and pitch‐derived commercial carbon fibres was studied. Here, the magnitude of graphitic microstructure evolution of the pitch‐derived fibre far exceeded that of the PAN‐derived fibres at temperatures up to ～2300°C, indicating its facile graphitizability.     

Structural Modifications and Clustering of Low-Density Lipoproteins in Solution Induced by Heating

This work presents a systematic study of low-density lipoprotein (LDL) in solutions subjected to subtle temperature changes, monitored by small angle X-ray scattering. From the data analysis, information about the equilibrium aggregation of the particles, as well as changes on the internal structure of the lipoproteins, were observed. The electron density profiles of the LDL particles were retrieved with a recently developed deconvolution method. Our results indicate that LDL particles keep their structure in the temperature range from about 22 °C up to 60 °C. Moreover, the formation of aggregates and their evolution as a function of time were monitored. Interestingly, when the temperature is raised to 80 °C, the results indicate the rupture of the particle and unspecific aggregation.     

Temperature induced asymmetric effects in the surface extended energy loss fine structure of Ni(100)

Electron energy loss spectra have been detected on a Ni(100) surface above the M_{2, 3} core edge at several different temperatures, between 80 and 1300K. We find a strong temperature dependence of the SEELFS signal. In particular we find a strong attenuation of the signal on increasing the temperature (especially the part originating from higher coordination shells). We find also an apparent contraction of the nearest neighbour distance obtained by the Fourier transform technique. We present a model calculation by which we tentatively interpret the apparent contraction as originating from an artifact introduced by the experimental integration limits of the Fourier transform. This in turn is due to an asymmetric pair distribution function of atoms in the surface region.     

Competing interactions in double-chains: Ising and spherical models

We investigate the thermodynamic properties of double chains of Ising and spherical spins with different first and a crossed second neighbour interaction in zero field. The interest is focussed on the region where different ground states are nearly degenerate due to competing interaction constants. The Ising system shows quasi-singular behaviour of the susceptibility for certain ratios of parameters. Moreover the nearest neighbour correlation function exhibits a sharp crossover from high-temperature “compensation-point” to low temperature ferro- or antiferromagnetic behaviour. An analogy is found between compensation points and tricritical points of higher dimensional systems.     

Infrared absorption of various isolated impurities in GaAs

We have calculated the infrared absorption at the local mode frequency for various isolated impurities such as B, Al, Mg, Si, C, and P in GaAs. To calculate the local mode frequency we take into account the effects from impurity mass defect and local force constant shifts and use the symmetry properties of the lattice. To calculate the absorption we consider contributions to the dipole moment from impurity and nearest neighbour charges and from charge migration effects. We suggest a physical model which accounts for impurity and nearest neighbour charges and polarizabilities.     

C60晶体中分子取向性的研究

本文用集团模型计算C₆₀晶体的结合能,考察晶体原胞中四个不等价分子之间的相对取向变化时晶体结合能的相应变化。认为低温下C₆₀晶体的有序化结构在能量上是不简并的,存在一个使能量最低的分子相对取向。 关键词：     

Flow patterns and heat-transfer characteristics of a top heat mode closed-loop oscillating heat pipe with check valves (THMCLOHP/CV)

The aim of this research was to investigate the flow patterns and heat transfer of a top heat mode closed-loop oscillating heat pipe with check valves (THMCLOHP/CV). In this study, the heat pipe was made of a high-quality glass capillary tube with an inner diameter of 2.4 mm bent into 10 meandering turns. The number of check valves was 2 and the tube was filled with R141b at a filling ratio of 50% of internal volume of the tube. The combined lengths of the evaporator, adiabatic and condenser sections were equal to 50 mm. The pipe was operated at the top heat mode, and the angles of inclination were 20°, 40°, 60°, 80°, and 90°. The heat applied at the evaporator section was controlled at 85°Cto 105°C, and 125°C. The results show that in the evaporator section, bubbles are produced and grow as a result of the continuous nucleate boiling. They coalesced and their volume expanded. Similarly, in the condenser section the vapor plug condensate caused the bubbles to collapse and accumulate as a liquid mass at the lower section of the U-bend tube. A new slug then developed and the bubbles coalesced in an upward flow. Heat flux increased when the evaporator temperature and inclination angle increased causing the average length of the vapor plug to decrease and the average velocity of vapor plug to increase. The maximum heat flux occurred at an evaporator temperature of 125°C and an inclination angle of minus 90°.     

Network topology of deeply supercooled water

Empirical potential structure refinements have been made to recent high-energy x-ray diffraction data, providing molecular models of deeply supercooled water. The average O-O coordination number is found to drop from 5.13 at 293?K to 4.85 at 244?K, within 3.5?Å. Triplet O-O-O bond angle distributions reveal a broad peak centred at 96.4° at 293?K which shifts to 100.0° at 244?K, indicative of the local geometry becoming increasingly tetrahedral with decreasing temperature. However, although the number of non-bonded interstitial molecules between the first and second shells is depleted upon cooling, the number of interstitial molecules forming triplets that are embedded within the hydrogen bonded tetrahedral network at θ_OOO?=?53°, remains constant. This is consistent with previous observations of an invariant O-O coordination number with temperature (4.24 out to 3.3?Å) and corresponds to non-bonded molecules positioned at close to half the ideal tetrahedral angle. Both -O-O-O- and hydrogen-bonded -O-H-O- ring length distributions show increases in 6 and 7-membered rings upon supercooling. This is concomitant with a shift and increase in intensity of peaks at r₄ ～8.7?Å and r₅ ～10.8?Å in the oxygen-oxygen pair distribution function, which in the models correspond to correlations between adjacent and next-nearest-neighbour hydrogen-bonded rings.