Low Temperature Shoulder Research Articles

The oxidation of Pd(111)

The oxidation of Pd(111) was characterized using scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). Initial exposure of Pd(111) to O 2 at temperatures between 300 and 575 K resulted in a (2×2) structure observable by both LEED and STM. The maximum coverage achieved by O 2 exposure at 300 K was 0.25 ML. By increasing the temperature above 500 K, the oxygen coverage could be increased to 0.37 ML. To increase the oxygen coverage further, NO 2 was used as the oxidant. On Pd(111), NO 2 dissociatively adsorbs, with NO going to the gas phase below 500 K, leaving oxygen on the surface. Above 500 K, initial exposure of NO 2 to Pd(111) also produced the (2×2) structure. Increasing the oxygen coverage to between 1.0 and 2.2 ML resulted in a complicated LEED pattern. This pattern could be explained as the superposition of three equivalent domains of two surface structures: one with a square surface lattice rotated 15° with respect to Pd[110], the other with a rectangular surface lattice with the short sides of the rectangles parallel to Pd[110]. In STM movies, ad-islands and peninsulas were observed to nucleate and grow as the oxygen coverage reached this regime. The rectangular structure was observed on the original Pd(111) terraces, and the square structure on the islands and peninsulas. This suggested that when the oxygen coverage exceeds 0.25 ML, oxygen atoms penetrate the surface creating a rectangular structure with a lower Pd atom density than the clean surface; the liberated Pd atoms, along with oxygen, then form the islands and peninsulas. The lattice constants obtained from the STM images were 0.679±0.012 nm for the square structure and 0.394±0.008 nm and 0.638±0.022 nm for the rectangular structure, both consistent with LEED observations. Neither of these structures can be simply related to any crystal orientation of Pd or PdO, indicating that there are states intermediate between Pd and PdO. After further increasing the oxygen coverage, the complicated LEED patterns became faint and a low temperature shoulder attributed to PdO decomposition developed in TPD traces. The results indicate that Pd(111) oxidation proceeds through three stages involving four distinct surface states.

Internal friction associated with δ′ precipitation in Al–Li alloys

Abstract The anelastic behaviour of Al–Li alloys is studied in the temperature range between room temperature and 400 K. The internal friction technique is shown to be very sensitive to the microstructural changes that take place at these temperatures. A low-temperature shoulder (LTS) that appears from 270 to 400 K in the first heating run, is attributed to the growth of δ ′ precipitates during heating. The experimental behaviour of the internal friction spectra under different microstructural and experimental conditions allows one to attribute this LTS to the stress-induced Li–Li reorientation process, similar to the Zener effect, but in an unstable microstructural situation where small δ ′ precipitates are present.

Effect of powder granulometry and pre-treatment on sintering behavior of submicron-grained α-alumina

Two kinds of commercial, submicron-grained, nondoped α-alumina powders were cold-isostatically pressed and then sintered at a constant heating rate. Before the sintering, pre-treatments were executed at 820 or 920 ° C for 50 h. The effects of the pretreatments on the densification behavior and the microstructural development are discussed. The powders contained some nano-sized alumina particles about 10 nm in diameter as well as sub-micron-sized primary particles about 0.1 μm in diameter, which agglomerated in packs of 0·2–0·4 μm diameter. The densification rate curves showed a low temperature shoulder during sintering attributed to these nano-sized particles. After pretreatments, the nano-particles disappeared, resulting in disappearance of the shoulder in the subsequent densification rate curves. These treatments, however, caused little change in the apparent activation energy in the intermediate stage sintering. For the weakly agglomerated powders, the bodies sintered with pre-treatments had coarser final microstructures.

Vibrating reed study of the flux line dynamics of κ-(ET)2Cu[N(CN)2]Br

Vibrating reed measurements were performed on single-crystal samples ofκ-(ET)2- Cu[N(CN)2]Br in an applied magnetic fieldH oriented either parallel (“longitudinal”) or perpendicular (“transverse”) to the highly conducting ac-plane. Field-cooling data taken for the longitudinal orientation andH<0.7 T revealed a peak with a low-temperature shoulder in the reed dissipation 1/Q located at temperatureT Q below the superconducting transition temperatureT c (≈11.6 forH=0). The shoulder disappeared nearH≈0.7 T, accompanied by an abrupt change in the slope ofT Q (H), corresponding to a similar change in the slope of the upper critical magnetic fieldH c2 measured by Kwoket al. The existence of the shoulder in the dissipation peak bears on a number of current explanations for the exotic superconducting properties ofκ-(ET)2Cu[N(CN)2]Br.T Q data taken for the transverse orientation fell far below estimates ofH c2., indicating the existence of a substantial region of flux line (FL) mobility belowH c2(T). The location of a peak in NMR relaxation observed by De Sotoet al. lies close toT Q for the transverse orientation.

Thermoplastic interpenetrating polymer networks based on a poly(styrene‐b‐butadiene) copolymer and an ionically‐terminated polybutadiene ionomer

AbstractThermoplastic interpenetrating polymer networks (IPN) are mixtures of two physically crosslinked polymers. Thermoplastic IPNs were prepared by blending an SBS triblock elastomer with a 1,2‐polybutadiene that was ionically‐terminated at both ends. The morphologies of these IPNs were studied using differential scanning calorimetry and dynamic mechanical thermal analysis. It was concluded that the ionomer was incompatible with the SBS elastomer, since the Tgs of both the 1,2‐polybutadiene from the ionomer and the essentially 1,4‐polybutadiene from the SBS component were observable at temperatures that were close to those of the individual components. The addition of the polybutadiene material had, however, an influence on the relaxation processes of the polystyrene blocks. The polystyrene glass transition in the pure SBS copolymer is broadened by the interfacial region between polystyrene and polybutadiene. The low temperature shoulder was much more pronounced when the ion‐terminated polybutadiene was present, indicating it has a preference to be located in these interfacial regions. © 1994 John Wiley &amp; Sons, Inc.

Ionic effects on the dielectric behaviour of liquid and glassy states of butane-1, 3-diol and glycerol

The effects of electrolytes on the H-bond association in glycerol and butane-1,3-diol have been studied by measurement of their dielectric properties over a temperature range from their glassy to supercooled fluid states. The presence of ions decreases the equilibrium permittivity. This with the ion type and is not attributable entirely to the kinetic depolarization effects. The decrease in the degree of intermolecular H bonding or dipolar orientational correlation also contributes to the effects observed. The sub-Tg relaxation process appears as a low-temperature shoulder to the main peak. This shoulder becomes better resolved at high concentrations, mainly as result of less efficient packing of shorter, H-bonded chains and the reorientation of an increased number of dipolar ion-pairs. The permittivity of solutions at 77 K increases with the increase in their concentration owing to the increase in both the optical and infrared polarizabilities. The nature of H-bond association in pure liquids can be investigated by observing the effects of electrolytes on their electrical properties.

The effect of hydrogen on the crystallization behavior of amorphous PdSi alloys

Differential scanning calorimetry, X-ray diffractometry and Optical metallography were used to study the effect of hydrogen atmosphere and surface treatment on the crystallization process in PdSi metallic glasses. In Pd 80Si 20 metallic glasses, the surface treatment by mechanical polishing with 1200 SiC induces surface crystallization in which preferred nucleation at the surface occurs, while homogenous crystallization occurs in as-quenched samples without surface grinding. In amorphous Pd 85Si 15, the crystallization occurs simultaneously from the sample surface and in the bulk. Surface treatment by mechanical polishing does not change the crystallization morphology. These results were not affected by the atmosphere of argon or hydrogen. It is shown that the surface crystallization, which is dominant in the initial stages of the transformation, is revealed by the low temperature shoulder of the main peak of crystallization in a DSC trace. The process of crystallization of amorphous Pd 80Si 20 and Pd 85Si 15 is found to be impeded by absorbed hydrogen, this having been measured by the increase of both crystallization temperature and activation energy for crystallization process in a hydrogen environment. Such crystallization behavior can be explained by assuming that hydrogen reduces the free volume available for rearrangements of host atoms of the metallic glasses.

Adsorption of hydrogen on thin fcc-iron films grown on Cu(100)

The desorption kinetics and surface structure of hydrogen adsorbed at 180 K on ultrathin (1–12 ML) iron films grown at 290 K on the (110) face of copper are reported. Below 3 ML, the results indicate that film growth is not epitaxial, and a desorption peak appears at 310 K accompanied by a low temperature shoulder. From 3–5 ML of iron, a (4×1) LEED structure is observed, which persisted on hydrogen adsorption; the main desorption peak begins to shift down and the shoulder becomes more prominent. From 6–10 ML of iron, a p(2×2)-p4g superstructure is obtained which has systematic absences of the ( h + 1 2 , 0) and (0, k + 1 2 ) spots, from the p(2×2), and is highly stabilised by the adsorption of hydrogen. This pattern appears in conjunction with the observation of only a single desorption peak at 290 K and is consistent with alternate displacements of adjacent iron atoms in a structure analogous to that of carbide formation on Ni(100). Above 10 ML of iron the surface structure begins to break down to form another structure. The heat of adsorption of hydrogen on the 8 ML film is 71 kJ mol −1 (±1) which is low compared to that on bcc iron (88–109 kJ mol −1).

Adsorption of hydrogen on thin fcc-iron films grown on Cu(100)

The desorption kinetics and surface structure of hydrogen adsorbed at 180 K on ultrathin (1–12 ML) iron films grown at 290 K on the (110) face of copper are reported. Below 3 ML, the results indicate that film growth is not epitaxial, and a desorption peak appears at 310 K accompanied by a low temperature shoulder. From 3–5 ML of iron, a (4 × 1) LEED structure is observed, which persisted on hydrogen adsorption; the main desorption peak begins to shift down and the shoulder becomes more prominent. From 6–10 ML of iron, a p(2 × 2)-p4g superstructure is obtained which has systematic absences of the (h + 1 2 , 0) and (0, k + 1 2 ) spots, from the p(2 × 2), and is highly stabilised by the adsorption of hydrogen. This pattern appears in conjunction with the observation of only a single desorption peak at 290 K and is consistent with alternate displacements of adjacent iron atoms in a structure analogous to that of carbide formation on Ni(100). Above 10 ML of iron the surface structure begins to break down to form another structure. The heat of adsorption of hydrogen on the 8 ML film is 71 kJ mol −1 ( ± 1) which is low compared to that on bcc iron (88–109 kJ mol −1).

The shape of the gel to liquid crystalline phase transition of dielaidoylphosphatidylethanolamine is markedly dependent on the method of sample preparation

Phosphatidylethanolamines are known to exhibit asymmetric phase transitions with a low temperature shoulder. However, in this work we demonstrate that suspensions of dielaidoylphosphatidylethanolamine can be prepared which exhibit very sharp and only slightly asymmetric phase transitions. Such preparations are made either by isolating a rapidly sedimenting fraction of a vortexed suspension of this lipid or by dialyzing a suspension which had been hydrated at pH 9.2 to pH 7.2. Smaller aggregates of the lipid can be isolated from the supernate of a vortexed suspension of dielaidoylphosphatidylethanolamine after removal of the rapidly sedimenting fraction or it can be produced by sonication of a sample at pH 9.2 followed by dialysis to pH 7.2 Such preparations exhibit very broad transitions and the transition temperature is shifted to lower values. These results demonstrate that the shape of the phase transition of dielaidoylphosphatidylethanolamine is particularly sensitive to the method of sample preparation. Furthermore, an asymmetric phase transition with a low temperature shoulder is not necessarily an intrinsic property of phosphatidylethanolamines.

Maturity of organic matter and migration of hydrocarbons in two Alaskan North Slope wells

Plots of T max and production index (P.I.) for seven Alaskan North Slope wells suggest that samples from the Ikpikpuk well are optimal for examination of thermal maturity and the Seabee well for migrational geochemical trends at all maturity levels. Vitrinite reflectance profiles for the two wells are identical and reach maximum values of 2.5% in the deepest rock units. Pyrolysis P1/org C, T max, and P.I. values all behave “normally” in Ikpikpuk but for Seabee show generally lower (by about 60°C) T max values and higher and more scattered P1/org C and P.I. values. Sorbed light hydrocarbon ratios including C2/C3; ic4/ nC4; iC5/ nC5; 22DMB/ nC6; MCP/benz; benz/tol; paraffin indexes 1 and 2 and C1/(C1 + C2 + C3) are compared for both wells. Light hydrocarbon data for Ikpikpuk generally shows smooth changes expected for maturation across the Kingak shale and Shublik formation with increasing paraffin indexes 1 and 2; decreasing MCP/enz; increasing benz/tol; increasing C1/(C1 + C2 + C3); and a spike in 22DMP/ nC6 at the bottom of the over-mature Sadlerochit sandstone. These ratios are consistent with some localized lateral migration at the bottom of the Sadlerochit Group sandstone and at the top of the pebble shale unit. In contrast, Seabee ratios were very different and showed no particular trends with depth, although some increase in proportions of methane and benzene are observable in the deepest sections of the well. The ratios of pyrolyzable C1/(C1 + C2 + C3) for the Ikpikpuk well were similar to those for the generated sorbed gases within for the catagentic and post mature sections of the well. Seabee showed pyrolyzable C1/(C1 + C2 + C3) ratios similar to those from Ikpikpuk except for a less pronounced bottom hole increase in the ratio. Migration phenomena with preferential retention of polar asphalts or pyrobitumens in strata through which or from which migration has occurred are postulated to be the cause of a low temperature P2 shoulder often observed in and below the catagenic zones of Ikpikpuk. GC/MS data of pyrolyzable material shows that maturation causes a decrease in alkane and alkene pyrolysis products as compared to aromatic and heterocyclic compounds. This low temperature shoulder appears to become the predominant P2 peak in sections of other wells, such as Seabee, which are extensively influenced by migration.

Adlayer geometry and structural effects in the CO/Ni(110) system

The formation of ordered adlayers of CO on Ni(110) and the correlation between structure and adsorption energy, sticking coefficient and adsorbate induced change of work function was investigated. LEED, TDS and work function measurements served to monitor adsorption and desorption. Models are presented for the structures formed at intermediate coverages (0.5 < θ < 0.85) - identified as a c(8×2) and a c(4×2) structure - and the (2×1) formed close to saturation: The CO molecules are adsorbed on the Ni rows in the [110] direction, their separation is dominated by short range COCO repulsions rather than by the NiCO interaction. The repulsions in the [001] direction lead only to the formation of structures with staggered configurations. In the first two structures formed only below room temperature the CO stands upright and the repulsion is weak, leading to considerable disorder (antiphase domains) and a streaky LEED pattern. In the (2×1) structure which does not thermally disorder in the experimental temperature range, the high density of the adlayer results in a lateral tilt of the CO, and subsequently also to good correlation in the [001] direction. The repulsions become evident in TDS as a low temperature shoulder at the main peak (c(8×2) and c(4×2) structure) or as a distinct extra peak at 330 K ((2×1) structure). The adsorption kinetics can be modelled by a first order precursor model ( K = 0.95). The work function almost linearly increases with coverage to 1500 mV at saturation. Both quantities are not noticeably affected by the degree of order in the adlayer.

Adsorption of hydrogen on Pd(100)

The energetic, kinetic and structural properties of hydrogen chemisorbed on a Pd(100) surface were studied by means of thermal desorption, work function and LEED measurements. Under the applied conditions no interference with bulk dissolution occurs and dissociative adsorption gives rise to a continuous increase of the work function by up to 0.20 eV. The dipole moment of the adsorbate complex is constant up to θ ≈ 0.9 and then increases until saturation at θ ≈ 1.35 (at 170 K) is reached. The formation of a second adsorbed state at high coverages manifests itself also by a low-temperature shoulder in the thermal desorption spectra and in the variation of the isosteric heat of adsorption, E ad, with coverage: E ad remains practically constant ( 24.5 kcal mole ) up to θ ≈ 0.9 and then decreases. The sticking coefficient is initially rather high ( s 0 ≈ 0.5) and varies with coverage in a way which can be described by a precursor-state model. The preexponential factor for desorption is about 10 −2 cm 2 atom −1 s −1. Desorption follows second order kinetics only at very low coverages, at high θ it exhibits quasifirst order. This effect is attributed to the existence of lateral interactions between adsorbed hydrogen atoms which manifest themselves also in the appearance of a c(2 × 2) LEED pattern at low temperatures. The “extra” diffraction spots attain their maximum intensity at θ = 0.5, and a structural model is proposed whereafter in this phase the H atoms occupy next-nearest neighboring adsorption sites with local fourfold symmetry. Order-disorder transitions were followed by recording the intensity of the half-order spots as a function of temperature at various coverages. The resulting phase diagram exhibits a critical temperature T c = 260 K at θ = 0.5 and is slightly asymmetric with respect to this coverage. The data are analysed in terms of a lattice gas model and estimates for the pairwise interaction energies yield repulsion between nearest neighbors ( w 1 = 0.5 kcal mole ) and attraction between next-nearest neighbors ( w 2 = −0.3 kcal mole ). The additional operation of non-pair-wise interactions is made responsible for the asymmetric shape of the phase diagram. Whereas the adsorbed layer is obviously localized at T ⩽ 270 K, a detailed analysis of the adsorption entropy reveals that for T ⩾ 370 K a rather good description can be obtained with a model of delocalized two-dimensional translation.