Anomalous Temperature Effects Research Articles

Anomalous Temperature Effect in Weakly Coupled Superlattices: Carrier Transport in a THz Quantum Cascade Laser

Anomalous Temperature Effect in Weakly Coupled Superlattices: Carrier Transport in a THz Quantum Cascade Laser

Tension-compression asymmetry of temperature dependencies in Mg rare-earth alloys

To reveal the tension-compression asymmetry of temperature-dependency in Mg-RE alloys, uniaxial tensile and compression tests followed by microstructural characterizations were performed on a cast Mg-10Gd-3Y-0.5Zr (wt.%) alloy under various temperatures. The results show that this alloy exhibits anomalous temperature effects on mechanical properties (such as tensile strength and plasticity) during tension, while they are not observed during compression tests, demonstrating that the anomalous temperature effects exhibit loading path dependence. The tension-compression asymmetry of the anomalous temperature-dependency associated with variations in twinning. It is believed that these variations in twinning result from the interplay between the inhibitory effect of rare earth elements on twinning and the promoting effects of elevated temperature or a change in the loading path from tension to compression, with the loading path having a stronger promoting influence than the deformation temperature.

Atomistic study on the anomalous temperature-dependent dynamic tensile strength of ice under shock loading

ABSTRACT Although the compressive strength of ice under both quasi-static [M. Arakawa and N. Maeno, Mechanical strength of polycrystalline ice under uniaxial compression. Cold Reg. Sci. Tech 26 (1997), pp. 215–229.] and dynamic [X. Wu and V. Prakash, Dynamic compressive behavior of ice at cryogenic temperatures. Cold Reg. Sci. Tech 118 (2015), pp. 1–13.] loadings shows an anomalous temperature effect that the compression strength is insensitive to temperature in a specific temperature range below −100oC, it is still unclear whether the anomalous temperature exists for the tensile strength of ice at cryogenic temperatures. In this paper, the temperature-dependent dynamic tensile strength of ice 1 h under shock loading is investigated by molecular dynamics simulations. It is intriguing to see that the dynamic tensile strength of the ice exhibits a similar anomalous temperature effect, i.e. it is almost insensitive to temperature in the range 117 ∼ 163 K, which could be interpreted by the competitive mechanism between shock-induced pulverisation and melting. The evolution of the pentagonal-heptagonal defects and the ductile-to-brittle transformation are also observed with decreasing temperature, leading to the unique dynamic tensile behaviour of ice under shock.

The Effects of Stress Triaxiality, Temperature and Strain Rate on the Fracture Characteristics of a Nickel-Base Superalloy

In this work, to study the effects of stress triaxiality, temperature, and strain rate on the fracture behaviors of a single-crystal Nickel-base superalloy, a series of experiments over a temperature range of 293 to 1373 K, strain rate range of 0.001 to 4000/s, and stress triaxiality range of −0.6 to 1.1 are conducted. Anomalous peak of stress is noticed in the yield stress versus temperature curves, and strain rate effect on the anomalous peak of yield stress is analyzed. The anomalous peak shifts to higher temperature as the strain rate increases. Then the effects of stress triaxiality, temperature, and strain rate on its fracture behaviors, including strain to fracture, path of crack propagation, and fracture surface, are observed and analyzed. A valley of the fracture strain is formed in the fracture strain versus temperature curve over the selected temperature range. The micrograph of fracture surface is largely dependent on the temperature, stress triaxiality, and strain rate. Finally, the original Johnson-Cook (J-C) fracture criterion cannot describe the effect of stress triaxiality and temperature on the fracture behaviors of single-crystal Nickel-base superalloy. A modified J-C fracture criterion is developed, which takes the anomalous stress triaxiality and temperature effects on the fracture behaviors of single-crystal Nickel-base superalloy into account.

Reactions of atomic hydrogen with formic acid and carbon monoxide in solid parahydrogen I: Anomalous effect of temperature.

Low-temperature condensed phase reactions of atomic hydrogen with closed-shell molecules have been studied in rare gas matrices as a way to generate unstable chemical intermediates and to study tunneling-driven chemistry. Although parahydrogen (pH2) matrix isolation spectroscopy allows these reactions to be studied equally well, little is known about the analogous reactions conducted in a pH2 matrix host. In this study, we present Fourier transform infrared (FTIR) spectroscopic studies of the 193 nm photoinduced chemistry of formic acid (HCOOH) isolated in a pH2 matrix over the 1.7 to 4.3 K temperature range. Upon short-term irradiation the HCOOH readily undergoes photolysis to yield CO, CO2, HOCO, HCO and H atoms. Furthermore, after photolysis at 1.9 K tunneling reactions between migrating H atoms and trapped HCOOH and CO continue to produce HOCO and HCO, respectively. A series of postphotolysis kinetic experiments at 1.9 K with varying photolysis conditions and initial HCOOH concentrations show the growth of HOCO consistently follows single exponential (k = 4.9(7)x10(-3) min(-1)) growth kinetics. The HCO growth kinetics is more complex displaying single exponential growth under certain conditions, but also biexponential growth at elevated CO concentrations and longer photolysis exposures. By varying the temperature after photolysis, we show the H atom reaction kinetics qualitatively change at ∼2.7 K; the reaction that produces HOCO stops at higher temperatures and is only observed at low temperature. We rationalize these results using a kinetic mechanism that involves formation of an H···HCOOH prereactive complex. This study clearly identifies anomalous temperature effects in the reaction kinetics of H atoms with HCOOH and CO in solid pH2 that deserve further study and await full quantitative theoretical modeling.

Mechanistic studies of highly enantio- and diastereoselective aza-Petasis-Ferrier rearrangement catalyzed by chiral phosphoric acid.

The precise mechanism of the highly anti- and enantioselective aza-Petasis-Ferrier (APF) rearrangement of hemiaminal vinyl ethers catalyzed by a chiral phosphoric acid was investigated by undertaking experimental and theoretical studies. The APF rearrangement is characterized by the following unique mechanistic features: (i) efficient optical kinetic resolution of the starting racemic hemiaminal vinyl ether, (ii) enantioconvergent process from racemic hemiaminal vinyl ethers to optically active β-amino aldehyde products, and (iii) anomalous temperature effects on the enantioselectivity (enantioselectivity increases as reaction temperature increases). The following experiments were conducted to elucidate the unique mechanistic features as well as to uncover the overall scheme of the present rearrangement: (A) X-ray crystallographic analysis of the recovered hemiaminal vinyl ether to determine its absolute configuration, (B) rearrangements of enantiomerically pure hemiaminal vinyl ethers to validate the stereochemical relationship between the hemiaminal vinyl ethers and β-amino aldehydes, (C) theoretical studies on the transition states of the C-O bond cleavage and C-C bond formation steps to gain an insight into the optical kinetic resolution of the hemiaminal vinyl ether and the origin of the stereoselectivity, as well as to elucidate the overall scheme of the present rearrangement, and (D) crossover experiments of two hemiaminal vinyl ethers having different vinyl ether and aliphatic substituents to comprehend the mechanism of the anomalous temperature effect and the enantioconvergent process. The results of experiments and theoretical studies fully support the proposed mechanism of the present anti- and enantioselective APF rearrangement.

Organocatalytic enantioselective construction of multi-functionalized spiro oxindole dienes.

A diastereo- and enantio-selective domino Michael-cyclization-tautomerization reaction of isatylidene malononitriles with α,α-dicyanoalkenes catalyzed by a cinchona alkaloid-derived bifunctional thiourea catalyst has been developed. A series of multi-functionalized spiro oxindole diene derivatives have been obtained in good to excellent yields (up to 97%) with good to excellent enantioselectivities (up to 96%) as well as good diastereoselectivities (up to 7.9 : 1). In addition, an anomalous temperature effect on the enantioselectivity has also been studied for this transformation.

Anomalous Thermally Induced Pinning of a Liquid Drop on a Solid Substrate

The effect of substrate temperature on the wetting and spreading behavior of a UV ink monomer has been studied as a surrogate for the ink on four different substrates: DTC (digital top coat)-coated BOPP (biaxial oriented polypropylene), Flexo-coated BOPP, DTC-coated SGE (semigloss elite) paper, and Flexo-coated SGE paper. Results show that the dynamic contact angles of the monomer decrease exponentially over time after contacting the surface, and the rate of spreading is consistently higher at 95 °C than at 22 °C. This observation indicates that spreading is controlled by the viscosity of the monomer as it decreases with temperature. An anomalous temperature effect is observed for the static contact angle on the DTC-coated BOPP substrate. The static contact angle at 95 °C is significantly larger than that at 22 °C (52° versus 30°). This is counterintuitive, as the surface tension of the monomer is shown to decease with increasing temperature. Microscopy (SEM and AFM) studies suggest that there is little interaction between the DTC coating solution and the BOPP substrate during the fast-drying coating process. This results in a smooth coated surface and, more importantly, voids between the BOPP nanofibers underneath the DTC coating. As the DTC-BOPP substrate is heated to 95 °C, fiber expansions occur. Microscopy results show that nanosized protrusions are formed on the DTC surface. We attribute it to fiber expansions in the vertical direction. Fiber expansions in the lateral direction causes little surface morphology change as the expanded materials only fill the voids laterally between the nanofiber network. We suggest that the protrusions on the surface create strong resistance to the wetting process and pin the monomer drop into a metastable wetting state. This interpretation is supported by the sliding angle and sessile drop height experiments.

The Anomalous Temperature Effect on the Ductility of Nanocrystalline Cu Films Adhered to Flexible Substrates

The effect of temperature, T, on the ductility of nanocrystalline (NC) Cu films of different thicknesses on flexible substrates is studied by uniaxial tension. It is found that the NC Cu films will fracture more easily with increasing T, exhibiting the anomalous T effect compared with that of bulk NC metals and freestanding films. Moreover, the T dependence of ductility becomes stronger for thinner films due to the larger decrement of the interfacial strength in thinner films. T-induced softening of the stretchable substrate reduces the interface constraint ability for suppressing strain localization, leading to the anomalous T effect.

Anomalous Temperature Effects of the Entanglement of Two Coupled Qubits in Independent Environments

We investigate the entanglement dynamical behavior of two coupled qubits via a Heisenberg XX interaction, which are connected with two independent finite temperature heat baths. By numerical simulations of the quantum master equation, it is found that the interesting phenomena of entanglement sudden death (ESD) as well as sudden birth (ESB) appear during the evolution process for particular initial states. We also show that two critical temperatures T1 (determining that the quantum state is entangled or separable) and T2 (where maximal stationary entanglement can be observed) exist, and stationary entanglement exhibits a non-monotonic behavior as a function of the finite temperature noise strength. These results enlarge the domain of the reasonable experimental temperature where stationary entanglement can be observable.

1.5‐µm emission of naturally‐oxidized InN crystals grown by MOVPE

AbstractPhotoluminescence (PL) spectrum of a naturally‐oxidized InN sample has been investigated. The PL‐peak wavelength locates at around 1.53 µm, which is by 250‐300 nm shorter than that of an intrinsic InN. In addition to this PL peak shift of the InN crystals (corresponding bandgap variation) their quality seems to be improved from the analysis of PL spectra. The naturally‐oxidized layer is found to have the original lattice structure of InN from X‐ray diffraction measurements. The 8‐20% of oxygen atoms has been evaluated to be incorporated in the InN layer by the energy dispersive X‐ray spectroscopy. The PL spectra have been affected by temperature. While a PL‐peak intensity increases with temperature decrease, a PL‐peak shifts toward the longer wavelength. Such red‐shift tendency is opposite to the case of other III‐V compounds. This anomalous temperature effect has been proved from variation of PL spectra with time after excitation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of dielectric polarization of the electrode on anomalous temperature effects in the electrical double layer

Monte Carlo simulations and a modified Poisson–Boltzmann (MPB) theory are used to investigate the temperature dependence of the capacitance (around the potential of zero charge) of an electric double layer in the presence of surface polarization due to a dielectric boundary. Within the context of the restricted primitive model planar double layer, whose solvent dielectric constant is ε2, the cases when the electrode is an insulator (ε1 = 1), when the electrode and the electrolyte have the same permittivity (ε1 = ε2, no polarization), and when the electrode is a conductor (ε1 → ∞) are studied for the case where the electrolyte concentration is 0.1 M. The simulations reveal a capacitance anomaly, that is, a positive temperature dependence of the capacitance at low temperatures for the former two situations. The MPB theory also shows this effect for these two situations and is in qualitative or better agreement with the simulation data. In these two cases, both the simulations and theory show a dramatic increase of the diffuse layer potential in the temperature regime where capacitance anomaly occurs. However, in the latter situation, where the electrode is metallic, the capacitance always has a negative temperature derivative for the MPB theory and probably also for the simulation data.

Near-infrared photoluminescence of vertically aligned InN nanorods grown on Si(111) by plasma-assisted molecular-beam epitaxy

We demonstrate that vertically aligned InN nanorods have been grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy (PA-MBE) at low and high growth temperatures (LT- and HT-InN nanorods). High-resolution scanning electron microscopy images clearly show that InN nanorods grown on Si(111) are hexagonal in shape, vertically aligned, well separated and densely distributed on the substrate. The size distribution of LT-InN nanorods is quite uniform, while the HT-InN nanorods exhibit a broad, bimodal distribution. The structural analysis performed by Raman scattering indicates that PA-MBE grown InN nanorods have the wurtzite-type InN single-crystal structure with the rod axis (growth direction) along the c-axis. In addition, both types of nanorods contain high concentrations of electrons (unintentionally doped). Compared to the HT-InN nanorods and the PA-MBE-grown InN epitaxial film, the LT-grown InN nanorods have a considerable number of structural defects. Near-infrared photoluminescence (PL) from LT- (∼ 0.77 eV) and HT-InN (∼ 0.70 eV) nanorods is clearly observed at room temperature. In comparison with the LT-InN nanorods, the PL efficiency of HT-InN nanorods is better and the PL peak energy is closer to that of InN-on-Si epitaxial films (∼ 0.66 eV). We also find that the PL band at low temperatures from nanorods is significantly weaker (compared to the InN film case) and exhibits anomalous temperature effects. We propose that these PL properties are results of considerable structural disorder (especially for the LT-InN nanorods) and strong surface electron accumulation effect (for both types of nanorods).

Meteorological singularities in the Czech Republic in the period 1961–2002

Based on the mean daily values of air temperature, air pressure, and precipitation totals at 13 climatological stations within the territory of the Czech Republic in the period 1961–2002, a statistical analysis of “meteorological singularities” (i.e., calendar-dependent deviations from the mean annual variation for selected meteorological elements) was performed. At the 13 stations analysed, a total of 45 meteorological singularities (37 singularities in air temperature, 35 in air pressure, and 30 in precipitation) were found. The singularities detected correlate well with cases traditionally recognised in the Czech Republic as well as with the results of analyses performed for Germany. Despite the considerable variability of singularities in time and space, most of them are found across the entire territory of the Czech Republic and can be observed for the most part in all three elements processed. The majority of the singularities detected may be explained on the basis of circulation mechanisms, by relating them to a significantly higher occurrence of certain groups of synoptic situations characterised by anomalous temperature or precipitation effects. Cases of “competition” between singularities, when different singularities may occur on the same calendar day, were found.

Near-infrared photoluminescence from vertical InN nanorod arrays grown on silicon: Effects of surface electron accumulation layer

We demonstrate that vertically aligned InN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence (PL) from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low-temperature grown InN nanorods) and strong surface electron accumulation effects.

Anomalous temperature and galvanomagnetic effects in a niobium-based superconducting tunnel junction

We report transport and phonon pulse experiments carried out to investigate the processes responsible for excess currents in superconducting tunnel junction (STJ) photon detectors. In an edge-anodised niobium device below 1.5 K anomalous temperature and galvano-magnetic effects were observed which correlated also to an increase in device responsivity, but with no significant change in loss rate, as determined by phonon pulses. The observations cannot be accounted for by any simple defect model and may be caused by the creation of a non-equilibrium qp stationary state.

Anomalous temperature effect on the hydrogen bond strength and phase transition in 2,4,6-trimethylpyridinium pentachlorophenolate.

The phase transition in 2,4,6-trimethylpyridinium pentachlorophenolate has been studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dielectric method as well as theoretical calculations. The crystal undergoes a first order phase transition of order-disorder type at 376 K. The transition to the high temperature phase causes anomalous hydrogen-bond shortening. Experimental and theoretical results show that the change in the mutual orientation of the phenol and pyridine rings is connected with the change of the hydrogen bond. Such an effect, which appears in the present simple hydrogen-bond complex, may be common also for the other hydrogen-bond complexes.

Influence of temperature on phytohormone interactions with monolayers obtained from phospholipids of wheat calli.

The effect of temperatures (15 and 5 degrees C) on adsorption parameters of phytohormones at monolayers prepared from a mixture of phospholipids extracted from non-embryogenic (NE) and embryogenic (E) winter wheat calli initiated from inflorescences (inf) and embryos (emb) was studied. The surface parameter values, i.e. limiting area and collapse pressure, were determined using the Langmuir method. Phytohormones 2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-acetic acid (IAA), kinetin, zeatin and zearalenone were investigated. The phytohormones, at a concentration of 0.2 microg/ml dissolved in water, were injected into the subphase. Phospholipids, at the concentration of 2 mg/ml, were spread at the water surface and the monolayer was compressed. The anomalous temperature effect was observed, especially, in non-embryogenic systems. In monolayers obtained from E phospholipids, the temperature effect was dependent on the kind of tissue from which the callus was initiated. Among all the examined phytohormones, the greatest changes (monolayer expansion) were found for IAA and zearalenone. However, this activity depended strongly on the kind of tissue from which the phospholipid mixture was extracted.

High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy

A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB2 particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed, above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature. The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature.

Anomalous temperature effect of nonlinearity of WO3 varistor doped with Al2O3

Anomalous temperature effect of nonlinearity of WO3 varistor doped with Al2O3