Temperature-dependent Processes Research Articles

Thermally activated carrier transfer processes in InGaN∕GaN multi-quantum-well light-emitting devices

We have studied the temperature-dependent carrier transfer processes in InGaN∕GaN multi-quantum-well light-emitting devices using various optical techniques such as photoluminescence, electroluminescence, and photoluminescence excitation spectra. The role of the defects in the GaN barrier neighboring to the InGaN region was demonstrated clearly in capturing carriers only at low temperatures. The physical origin of the defects was most possibly attributed to the stacking faults at the interface according to the high-resolution transmission electron spectroscopy pictures.

Hydrochemical-isotopic tendencies to define hydraulic mobility of formation water at the Samaria-Sitio Grande oil field, Mexico

The chemical (major elements) and isotopic ( 2H, 3H, 13C, 18O) composition of formation water is presented for the Sitio Grande oilfield, SE-Mexico, extracted from 28 production wells from the carbonate reservoir at a depth between 3585 and 4545 m.b.s.l. The linear δ 18O/δD-trend explains the formation of reservoir water as part of three subsequent stages: a) the evaporation of marine water at the surface, causing enrichment of both, δ 18O and δD-values, b) a reversal trend with decreasing δ 18O-and δD-ratios by the extreme evaporation of brines and, c) the subsequent mixing with isotopically depleted meteoric water. A SW-NE directed flow direction of deep groundwater systems is indicated by parallel-oriented isoline trends of stable isotope ratios and conservative elements, supported by the dominance of parallel directed microfractures and extensional faults, and by tracer test results. The arrival of artificial tritium, three years after tracer injection in the well SG-85, reflects a) the importance of long-term monitoring of tracer tests, and b) an estimated flow velocity of 2.2×10 −5 m/s. As the arrival of the tracer was detected exclusively in three production wells towards the SW of the injection site, lateral migration of groundwater occurs mainly along defined, channel-like conducts with a narrow dispersion angle of 39 °. Surface water, injected as part of an enhanced oil recovery program, was not detected in the production zone by chemical and isotopic methods. This fact is not a proof for lacking hydraulic conductivity, as the large reservoir size and the termination of the injection program in 1996 could have caused a complete dilution of the injected fluids. Restricted vertical flow movement between reservoir layers and insignificant temperature-dependent reaction processes is indicated by the heterogeneous distribution of δ 18O-and Cl-values throughout the groundwater column of most reservoir zones. Little influence of water-rock interaction, such as Ca-Mg exchange processes between formation water and host rock is reflected by the heterogeneous abundance of calcium in the liquid phase in both, dolomite-free and dolomitized carbonate host rock layers.

Digestive state influences the heart rate hysteresis and rates of heat exchange in the varanid lizardVaranus rosenbergi

To maximize the period where body temperature (Tb) exceeds ambient temperature (Ta), many reptiles have been reported to regulate heart rate (fH) and peripheral blood flow so that the rate of heat gain in a warming environment occurs more rapidly than the rate of heat loss in a cooling environment. It may be hypothesized that the rate of cooling, particularly at relatively cool Tbs, would be further reduced during postprandial periods when specific dynamic action (SDA) increases endogenous heat production (i.e. the heat increment of feeding). Furthermore, it may also be hypothesized that the increased perfusion of the gastrointestinal organs that occurs during digestion may limit peripheral blood flow and thus compromise the rate of heating. Finally, if the changes in fh are solely for the purpose of thermoregulation, there should be no associated changes in energy demand and, consequently, no hysteresis in the rate of oxygen consumption (V(O2)). To test these hypotheses, seven individual Varanus rosenbergi were heated and cooled between 19 degrees C and 35 degrees C following at least 8 days fasting and then approximately 25 h after consumption of a meal (mean 10% of fasted body mass). For a given Tb between the range of 19-35 degrees C, fh of fasting lizards was higher during heating than during cooling. Postprandial lizards also displayed a hysteresis in fh, although the magnitude was reduced in comparison with that of fasting lizards as a result of a higher fh during cooling in postprandial animals. Both for fasting and postprandial lizards, there was no hysteresis in V(O2) at any Tb throughout the range although, as a result of SDA, postprandial animals displayed a significantly higher V(O2) than fasting animals both during heating and during cooling at Tbs above 24 degrees C. The values of fh during heating at a given Tb were the same for fasting and postprandial animals, which, in combination with a slower rate of heating in postprandial animals, suggests that a prioritization of blood flow to the gastrointestinal organs during digestion is occurring at the expense of higher rates of heating. Additionally, postprandial lizards took longer to cool at Tbs below 23 degrees C, suggesting that the endogenous heat produced during digestion temporarily enhances thermoregulatory ability at lower temperatures, which would presumably assist V. rosenbergi during cooler periods in the natural environment by augmenting temperature-dependent physiological processes.

Temperature Dependence of the Kinetics of Diffraction Gratings Formation in a Polymer Matrix Containing Azobenzene Chromophores: Monte Carlo Simulations and Experiment

We use the framework of recently formulated kinetic model of temperature-dependent processes accompanying the recording and erasure of diffraction gratings in a polymer matrix doped with azobenzene chrompohores under the illumination with spatially modulated and linearly polarized light to study general features of temperature dependence of time evolution of diffraction efficiency close to the glass transition temperature T g of the polymer matrix. Bond fluctuation Monte Carlo simulations are used. Basic kinetic results found in simulations are in agreement with preliminary experimental results obtained in degenerate two-wave mixing (DTWM) experiment. We discuss, using Monte Carlo results, the concept of temperature controlled writing of diffraction gratings in a temperature interval close to Tg.

Intermixing in self-assembled InAs quantum dot formation

The influence of growth parameters and substrate material on the basic processes during strain-induced InAs-quantum dot (QD) formation and the resulting structural properties of the QDs is studied with reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), and X-ray diffraction. The process of strain-induced QD formation is strongly influenced by the intermixing with substrate material and the desorption of indium at high temperatures. From the experimental results, we conclude that the strain-energy modification due to temperature-dependent intermixing has a larger influence on QD formation than temperature-dependent kinetic processes at the surface. Interestingly, we also find that the maximum growth temperature limited by desorption depends on the growth speed.

Efficient Near IR Photoluminescence from Gallium Nitride Layers Doped with Arsenic

Photoluminescence in the 1.2–1.4 eV spectral range from GaN:As layers grown on (0001) Al2O3 substrates was observed and studied. The photoluminescence is attributed to radiative recombination in GaAs nanocrystallites, self-organized in the GaN matrix during growth. The photoluminescence intensity attains a maximum at a growth temperature of ∼780°C, which is explained by the competition between several temperature-dependent processes that affect the formation of GaAs nanocrystallites. Sharp emission lines were observed at the high-energy edge of the photoluminescence band. These lines are caused by an emission of bound excitons in the GaAs nanocrystallites and by phonon replicas of the bound-exciton emission. The energies of the corresponding optical phonons are typical of GaAs. The photoluminescence-excitation spectra exhibit features related to resonantly excited free and bound excitons as well as to excitons formed simultaneously with the emission of optical phonons.

MONTE CARLO SIMULATIONS OF TEMPERATURE DEPENDENCE OF THE KINETICS OF DIFFRACTION GRATINGS FORMATION IN A POLYMER MATRIX CONTAINING AZOBENZENE CHROMOPHORES

A simple kinetic model of temperature-dependent processes accompanying the writing and decay of diffraction gratings in a polymer matrix doped with azo-dye chromophores under illumination with spatially modulated and linearly polarized light is introduced and studied using Monte Carlo simulations. The model generalizes a recently proposed athermal kinetic model based on three-channel photoizomerization cycles of trans–cis type of azobenzene molecules and orientational redistribution and diffusion. These processes are influenced by a polymer matrix in a thermal equilibrium. The Monte Carlo simulations show the important role of temperature on the kinetic processes during building and erasure of diffraction gratings. We discuss some interesting applications for photonic devices, related to all-optical processing and storage of information using holographic methods.

Covalently-attached 1-pyrenylmethyl groups as a probe of temperature-dependent relaxation processes in polyethylene films

1-Pyrenylmethyl lumophores have been attached covalently to chains of several types of polyethylene (crystallinities 37–74%). The temperature dependence of their fluorescence between 40 and 400 K, as well as that of ultra-high-molecular-weight polyethylene (85% crystallinity) containing covalently-attached 1-pyrenyl groups, has been combined with data from DSC measurements and information concerning the properties of films to determine the utility and limitations of the fluorescence changes to detect the onset temperatures of α -, β - and γ -relaxation processes in the polyethylenes. The relative sensitivities of fluorescence from covalently-attached pyrenyl and anthryl (with a much shorter excited singlet-state lifetime) probes to each of the relaxation processes are compared in the same films. The probe whose excited singlet-state lifetime better matches the rate associated with a relaxation process is better able to detect it.

Effect of elevating the skin temperature during topical ALA application on in vitro ALA penetration through mouse skin and in vivo PpIX production in human skin.

An approach to induce increased protoporphyrin IX (PpIX) production in aminolevulinic acid (ALA)-based photodynamic therapy (PDT) of skin lesions is to elevate the skin temperature during topical ALA application. Increased skin temperature may increase the (depth of) penetration of ALA into the skin, which may in turn increase PpIX production (in deeper layers). The effect of skin temperature on in vitro ALA penetration into mouse skin was determined in an in vitro percutaneous penetration model at two different temperatures. The effect of skin temperature on PpIX production in human skin during ALA application was measured with in vivo fluorescence spectroscopy in temperature-controlled areas (5 different temperatures). The data from the experiment with the in vitro percutaneous penetration model clearly show that the penetration of ALA into skin is temperature dependent. The penetration of ALA through the mouse skin was higher when its temperature was maintained at 37 [degree]C than through skin that was kept at 32 [degree]C. The fluorescence data from the in vivo experiment show that the PpIX fluorescence increases with increasing temperature of the skin during the application period. The overall activation energy (E(a)) for PpIX production was obtained for each hour of the ALA application period from the fluorescence data using the Arrhenius equation. The E(a) value in the first hour of ALA application was not significant, indicating that the PpIX production in that period is dominated by processes that are not temperature dependent, like the passive diffusion of ALA across the stratum corneum. In the second, third and fourth hours of ALA application, the E(a) for PpIX production proved to be significant, which indicates that the PpIX production in these time intervals is dominated by temperature-dependent processes. In conclusion, the data from the present study indicate that improving ALA-based PDT of skin lesions might be achieved by elevating the skin temperature during the ALA application.

Using infrared thermography to analyze substrate and adhesive effects in bonded structures

This study was focused on the application of infrared thermography for assessment of technological procedures used to improve adhesion in bonded structures. Infrared thermography is a remote imaging technique which can be fruitfully exploited either for mapping surface temperature in temperature-dependent manufacturing processes, or for nondestructive evaluation of end products. Basically, for nondestructive evaluation two different approaches are possible: traditional pulse thermography (PT) and lock-in thermography (LT) and the use of both techniques is discussed. Several specimens were fabricated to simulate different bonded structures that are used mainly in the aerospace industry. Such bonded structures included: adhesively-bonded aluminum joints with or without surface treatment before bonding, glass-reinforced epoxy composites and carbon-reinforced epoxy composites with or without plasma treatment before bonding. Pulse and lock-in thermographies are both capable of visualizing inhomogeneities in bulk materials, as well as disbonding, delamination, flaws and foreign inclusions in bonded structures. The results show that lock-in thermography is also capable of evaluating the effect of adhesive thickness and the effects induced in bonded structures by substrate surface treatments.

Coarse-Grained Molecular Dynamics for Computer Modeling of Nanomechanical Systems

Unique challenges for computer modeling and simulation arise in the course of the development and design of nanoscale mechanical systems. Materials often exhibit unconventional behavior at the nanoscale that can affect device operation and failure. This uncertainty poses a problem because of the limited experimental characterization at these ultra-small length scales. In this Article we give an overview of how we have used concurrent multiscale modeling techniques to address some of these issues. Of particular interest are the dynamic and temperature-dependent processes found in nanomechanical systems. We focus on the behavior of sub-micron mechanical components of Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS), especially flexural-mode resonators. The concurrent multiscale methodology we have developed for NEMS employs an atomistic description of millions of atoms in relatively small but key regions of the system, coupled to, and run concurrently with, a generalized finite element model of the periphery. We describe two such techniques. The more precise model, Coarse-Grained Molecular Dynamics (CGMD), describes the dynamics on a mesh of elements, but the equations of motion are built up from the underlying atomistic physics to ensure a smooth coupling between regions governed by different length scales. In many cases the degrees of smoothness ofmore » the coupling provided by CGMD is not necessary. The hybrid Coupling of Length Scales (CLS) methodology, combining molecular dynamics with conventional finite element modeling, provides a suitable technique for these cases at a greatly reduced computation expense. We review these models and some of the results we have obtained regarding size effects in the elasticity and dissipation of nanomechanical systems.« less

Human intestinal absorption of imidacloprid with Caco-2 cells as enterocyte model

In order to assess the risk to mammals of a chronic exposure to imidacloprid (IMI), we investigated its absorption with the human intestinal Caco-2 cell line. Measurements of transepithelial transport revealed an apparent permeability coefficient of 21.6 × 10 −6 ± 3.2 × 10 −6 cm/s reflecting a 100% absorption. The comparison of apical to basal (A–B) and basal to apical (B–A) transports showed that the monolayer presents a basal to apical polarized transport. Studies of apical uptake demonstrated that the transport was concentration-dependent and not saturable from 5 to 200 μM. Arrhenius plot analysis revealed two apparent activation energies, E a(4–12 °C) = 63.8 kJ/mol and E a(12–37 °C) = 18.2 kJ/mol, suggesting two temperature-dependent processes. IMI uptake was equivalent when it was performed at pH 6.0 or 7.4. Depletion of Na + from the transport buffer did not affect the uptake, indicating that a sodium-dependent transporter was not involved. Decrease of uptake with sodium-azide or after cell surface trypsin (Ti) treatment suggested the involvement of a trypsin-sensitive ATP-dependent transporter. Investigations on apical efflux demonstrated that initial velocities paralleled the increase of loading concentrations. A cell surface trypsin treatment did not affect the apical efflux. The lack of effect when the efflux was performed against an IMI concentration gradient suggested that an energy-dependent transporter was involved. However, the inhibition of P-glycoproteins ( P-gp) and multidrug resistance-associated proteins (MRP) by taxol, vincristine, and daunorubicine had no effect on IMI intracellular accumulation suggesting the involvement of transporters distinct from classical ATP binding cassette transport (ABC-transport) systems. All results suggest that IMI is strongly absorbed in vivo by inward and outward active transporters.

Comparison of smectite- and illite-rich gouge frictional properties: application to the updip limit of the seismogenic zone along subduction megathrusts

Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ∼150°C is one of the key mineralogical changes thought to control the updip limit of seismicity. If correct, this hypothesis requires illite-rich gouges to exhibit frictionally unstable (velocity-weakening) behavior. Here, we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay-rich gouges. We sheared 5-mm-thick layers of commercially obtained pure Ca-smectite, a suite of smectite–quartz mixtures, and natural illite shale (grain size ranging from 2 to 500 μm) in the double-direct shear geometry to shear strains of ∼7–30 at room humidity and temperature. XRD analyses show that the illite shale contains dominantly clay minerals and quartz; within the clay-sized fraction (<2 μm), the dominant mineral is illite. Thus, we consider this shale as an appropriate analog for fine-grained sediments incoming to subduction zones, within which smectite has been transformed to illite. We observe a coefficient of friction (μ) of 0.42–0.68 for the illite shale, consistent with previous work. Over a range of normal stresses from 5 to 150 MPa and sliding velocities from 0.1 to 200 μm/s, this material exhibits only velocity-strengthening behavior, opposite to the widely expected, potentially unstable velocity-weakening behavior of illite. Smectite sheared under identical conditions exhibits low friction (μ=0.15–0.32) and a transition from velocity weakening at low normal stress to velocity strengthening at higher normal stress (>40 MPa). Our data, specifically the velocity-strengthening behavior of illite shale under a wide range of conditions, do not support the hypothesis that the smectite–illite transition is responsible for the seismic–aseismic transition in subduction zones. We suggest that other depth- and temperature-dependent processes, such as cementation, consolidation, and slip localization with increased shearing, may play an important role in changing the frictional properties of subduction zone faults, and that these processes, in addition to clay mineralogy, should be the focus of future investigation.

Modeling temperature-dependent protein structural transitions by combined near-IR and mid-IR spectroscopies and multivariate curve resolution.

The combination of near- and midinfrared spectroscopies (NIR and MIR) is proposed to monitor temperature-dependent transitions of proteins. These techniques offer a high discriminating power to distinguish among protein structural conformations but, in temperature-dependent processes, present the drawback associated with the intense and evolving absorption of the deuterium oxide, used as a solvent in the protein solutions. Multivariate curve resolution-alternating least squares (MCR-ALS) is chosen as the data analysis technique able to unravel the contributions of the pure protein and deuterium oxide species from the mixed raw experimental measurements. To do so, MCR-ALS works by analyzing simultaneously experiments from MIR and NIR on pure deuterium oxide solutions and protein solutions in D2O. This strategy has proven to be effective for modeling the protein process in the presence of D2O and, therefore, for avoiding the inclusion of artifacts in the data stemming from inadequate baseline corrections. The use of MIR and NIR and MCR-ALS has been tested in the study of the temperature-dependent evolution of beta-lactoglobulin. Only the combined use of these two infrared techniques has allowed for the distinction of the three pure conformations involved in the process in the working thermal range: native, R-type state, and molten globule.

Study of photoluminescence from self-formed GaAs nanocrystallites in As-doped GaN grown by molecular beam epitaxy

We report on the results of the investigation of the photoluminescence (PL) from GaAs nanocrystallites embedded in a GaN host crystal and formed during the growth of GaN(As) by molecular beam epitaxy. The low-temperature PL spectra reveal two emission bands with maxima at 1.20 and 1.43 eV. The emission intensities of both PL bands show a characteristic 2/3 power-law dependence upon the optical excitation intensity, suggesting a strong contribution of Auger recombination in the total recombination rate of non-equilibrium carriers inside the GaAs nanocrystallites. The integral PL intensity demonstrates a sharp maximum in the growth temperature dependence at ∼780 °C. This can be explained by the competition of several temperature-dependent processes, which influence the formation of the GaAs nanocrystallites.

A simple analytical model of water

Water is an unusual liquid. It expands upon freezing, has minima in its volume, heat capacity, and isothermal compressibility with temperature, and shows signs of a first-order phase transition when supercooled. These anomalies disappear at high pressures. We review a recent analytical theory that predicts water's thermal properties and the main features of its phase diagram, including multiple crystalline phases and a fluid–fluid transition in the supercooled liquid. It also predicts a fragile-to-strong crossover in supercooled water's temperature-dependent relaxation processes. The theory is based on a simplified model for how triplets of waters interact via hydrogen bonds, steric repulsions, and dispersion attractions. It is designed to give simple insights into the microscopic origins of water's properties.

Thermodynamic Investigation into the Mechanism of the Chlorophyll Fluorescence Quenching in Isolated Photosystem II Light-harvesting Complexes

Chlorophyll fluorescence quenching can be stimulated in vitro in purified photosystem II antenna complexes. It has been shown to resemble nonphotochemical quenching observed in isolated chloroplasts and leaves in several important respects, providing a model system for study of the mechanism of photoprotective energy dissipation. The effect of temperature on the rate of quenching in trimeric and monomeric antenna complexes revealed the presence of two temperature-dependent processes with different activation energies, one between approximately 15 and 35 degrees C and another between approximately 40 and 60 degrees C. The temperature of the transition between the two phases was higher for trimers than for monomers. Throughout this temperature range, the quenching was almost completely reversible, the protein CD was unchanged, and pigment binding was maintained. The activation energy for the low temperature phase was consistent with local rearrangements of pigments within some of the protein domains, whereas the higher temperature phase seemed to arise from large scale conformational transitions. For both phases, there was a strong linear correlation between the quenching rate and the appearance of an absorption band at 685 nm. In addition, quenching was correlated with a loss of CD at approximately 495 nm from Lutein 1 and at 680 nm from chlorophylls a1 and a2, the terminal emitters. The results obtained indicate that quenching of chlorophyll fluorescence in antenna complexes is brought about by perturbation of the lutein 1/chlorophyll a1/chlorophyll a2 locus, forming a poorly fluorescing chlorophyll associate, either a dimer or an excimer.

Investigation of Temperature-Dependent Relaxation Processes in Polyethylene Films by Covalently Attached Anthryl Probe

The dependence of several aspects of the fluorescence from 9-anthryl groups, covalently attached by tethers of differing lengths to interior sites of five polyolefinic films whose crystallinities range from 0 to 74%, has been explored between 40 and 400 K. The data are employed to determine microscopically the onsets of various relaxation processes of the polymers, the sensitivity of the probes to changes in their local morphologies, and the distribution of the groups between amorphous and interfacial sites. The results are compared with those from noncovalently attached (doped) 9-methylanthracene molecules. Specifically, the relationship between the lengths of tethers for the 9-anthryl groups, as well as the chemical method by which each tether is attached to the polymer chains, and the ability of the fluorescence from the lumophores to detect the onsets of host relaxation processes are explored. The two attachment methods lead to different distributions of lumophores in the amorphous and interfacial reg...

Effect of Low Body Temperature During Unilateral Labyrinthectomy on Vestibular Compensation in the Guinea Pig

Objectives - Vestibular compensation, the recovery that follows unilateral vestibular deafferentation (UVD), is a model for central nervous system plasticity. Recovery from the static symptoms of UVD may involve temperature-dependent processes that modulate the immediate effects of UVD and/or the capability of the central nervous system to undergo adaptive plasticity. In this study we investigated changes in oculomotor and postural vestibular symptoms resulting from low body temperature during UVD. Material and methods - To study the effect of low temperatures at the time of UVD on vestibular compensation, we compared the rate of compensation and peak values for postural [roll head tilt (RHT) and yaw head tilt (YHT)] and oculomotor [spontaneous nystagmus (SN)] symptoms in three groups of guinea pigs. Animals in Group 1 (n=6) were maintained at 38°C throughout unilateral labyrinthectomy (UL). Animals in Group 2 (n=6) were not temperature-controlled and animals in Group 3 (n=4) were cooled with ice to 25°C throughout UL. Results - Cooled animals showed significantly higher rates of SN upon recovery from anaesthesia and took a significantly longer time to compensate. Cooled animals were also slower to compensate for postural symptoms (RHT and YHT), with 2 animals showing no compensation for RHT 52 h after UL. Conclusion - Hypothermia (25°C) during UVD surgery exacerbates postural and oculomotor symptoms following UL and significantly slows recovery.

Structural rearrangements in the aqueous phase of cell suspensions and protein solutions induced by a light-oxygen effect

Temperature-dependent transient processes initiated by a direct photogeneration of singlet oxygen in suspensions of human erythrocytes and solutions of serum albumin are studied. The processes appear as anomalous jumps in the temperature dependences of the deformability coefficient of erythrocytes and the refractive index of the extracellular medium and protein solution. In the temperature regions of anomalous jumps, cells and proteins transfer to a metastable state of a lower activity, but they can be isothermally photoreactivated. Simultaneously, a reversible rearrangement of the aqueous phase occurs near the cell and protein surfaces, accompanied by the formation of an extended corona (hydrogel). The transient processes are interpreted as phase transitions in the membrane of erythrocytes and conformation transitions in proteins. The interaction between erythrocytes and albumin via hydrogel is discovered (hydro-conformational interaction). A qualitative physical model of the early stages of the light-oxygen effect is proposed, in which collective magnetic interactions between the electron spins of oxygen molecules and the nuclear magnetic moments of protons in H2O molecules play a dominant role.