Weak Thermal Effects Research Articles

Vector solitonic pulses excitation in microresonators via free carrier effects.

We numerically investigate the excitation of vector solitonic pulse with orthogonally polarized components via free-carrier effects in microresonators with normal group velocity dispersion (GVD). The dynamics of single, dual and oscillated vector pulses are unveiled under turn-key excitation with a single frequency-fixed CW laser source. Parameter spaces associated with detuning, polarization angle, interval between the pumped orthogonal resonances and pump amplitude have been revealed. Different vector pulse states can also be observed exploiting the traditional pump scanning scheme. Simultaneous and independent excitation regimes are identified due to varying interval of the orthogonal pump modes. The nonlinear coupling between two modes contributes to the distortion of the vector pulses' profile. The free-carrier effects and the pump polarization angle provide additional degrees of freedom for efficiently controlling the properties of the vector solitonic microcombs. Moreover, the crucial thermal dynamics in microcavities is discussed and weak thermal effects are found to be favorable for delayed vector pulse formation. These findings reveal complex excitation mechanism of solitonic structures and could provide novel routes for microcomb generation.

Nonlinear Approximation for Mixed Convection Flow From a Vertical Plate With Exponentially Decaying Internal Heat Generation and Nonlinear Thermal Radiation

Abstract This study considered dynamic features of mixed convection flow over a vertical plate influenced by nonlinear thermal radiation and exponentially decaying internal heat generation. The importance of the nonlinear density variation with temperature (NDT) and convective heating is also analyzed. The governing equations are transformed into ordinary differential equations using the similarity variables and solved in MAPLE 2022 by a Runge–Kutta Ferhlberg fourth-fifth order scheme. The results obtained show that, with an increase in the convection process, the internal heat generation convects more fluid away and consequently reduced the rate of heat flowing back into the plate. For a weak internal heat generation λx=0.5, the plate temperature is less than one (1) and the heat is observed to flow from the plate into the fluid on the surface of the right plate. Furthermore, for weak convection and nonlinear thermal radiation effects, the rate at which the heat flows into the plate increases with the convective heat transfer parameter increase. However, for a strong nonlinear thermal radiation effect, the rate at which the heat flows out of the plate increases. The flow feature is not only governed by the rate of internal heat generation but the generation as well reverses the heat flow from the plate since the temperature of the near the plate surface T is much higher than the environmental temperature Tf.

Surface Density Function and Its Evolution in Homogeneous and Inhomogeneous Mixture n-Heptane MILD Combustion

ABSTRACT Moderate or intense low-oxygen dilution (MILD) combustion is a combustion technique with potential to simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the mean behavior of the reactive scalar gradient characterized by the surface density function (SDF = magnitude of the reaction progress variable gradient) and its evolution for exhaust gas recirculation (EGR) type, homogeneous and inhomogeneous mixture n-heptane combustion under MILD conditions using Direct Numerical Simulations (DNS) with reduced chemical mechanism. Two oxygen concentrations have been considered here for the homogeneous mixture case, namely, 3.0% and 4.5% by volume. The characteristics of the SDF and displacement speed were also analyzed in the reaction and propagating-flame-dominated regions of the domain to illustrate the effect of combustion mode on the mean variations of these quantities. The mean values of the SDF in turbulent MILD combustion cases are found to be smaller compared to the corresponding laminar flame cases. Moreover, the effect of mixture inhomogeneity on the SDF variation is found to be marginal for the parameters considered here. It is found that increasing the dilution factor reduces the percentage of flame thickening with respect to the corresponding laminar flame thickness under identical turbulence conditions. The effects of dilatation rate are found to be weak in the studied cases due to the expected low heat release under MILD conditions which leads to weak thermal expansion effects. The effective flame normal and tangential strain rates are found to be dominated by the additional flame normal and tangential strain rates due to flame propagation, respectively. The effective flame normal strain rate has positive values across the flame which promotes flame thickening and could explain the decrease in the SDF values, while the effective flame tangential strain rate is negative throughout the flame and indicates a fractional reduction in flame area that could be attributed to flame surface interactions. The reaction-dominated regions give rise to reduced level of the SDF in the homogeneous mixture cases and increased level of in all cases compared to those calculated in the propagating-flame region.

Morphology of Ionic Micelles as Studied by Numerical Solution of the Poisson Equation.

The existing concepts of the ionic micelle structure were specified. It was noted that the composition of dispersed phase particles in a liquid dispersion medium should necessarily include adsorbed counterions rigidly bound to these particles. By numerical solution of the Poisson equation for the two most often used approximations, the Poisson–Boltzmann (PB) model and the Jellium-approximation (JA), the electric potential decay from the Stern potential of dispersed phase particles was defined. A new methodological approach to analyze the reaction of micelle potential decay based on small variability of the CMC value was proposed. It made possible to determine the dimension parameter, which in the presence of weak thermal effects approximately corresponds to the micelle hydrodynamic radius, and to calculate the electrokinetic potential of micelles. The results of theoretical calculations were compared with our previous experimental data on the thickness of the SDS micelle hydrophilic layer obtained by SAXS. A good agreement between the calculated and measured values was obtained, and it was noted that for low concentrations the experimental values are more correctly described by the PB model, but for concentrations greater than 100 mM the JA model is more preferable. It was found that the slipping plane is located near the outer Stern plane and is separated from it only by a few molecular layers of water. The influence stronger than the thermal one can shift the slipping plane closer to the micelle core. Accordingly, the smallest hydrodynamic micelle size is determined by the outer Stern plane. The results of our work allowed us to conclude that the micelle is not something soft and watery, but according to its specified structure, it is a more solid-like particle than was previously assumed. The proposed approach can be extended to investigate other effects of a physicochemical nature, in particular, those observed with the addition of an external electrolyte or nanoparticles.

Anomalies in the temperature evolution of Dirac states in the topological crystalline insulator SnTe

Discovery of topologically protected surface states, believed to be immune to weak disorder and thermal effects, opened up a new avenue to reveal exotic fundamental science and advanced technology. While time-reversal symmetry plays the key role in most such materials, the bulk crystalline symmetries such as mirror symmetry preserve the topological properties of topological crystalline insulators (TCIs). It is apparent that any structural change may alter the topological properties of TCIs. To investigate this relatively unexplored landscape, we study the temperature evolution of the Dirac fermion states in an archetypical mirror-symmetry protected TCI, SnTe employing high-resolution angle-resolved photoemission spectroscopy and density functional theory studies. Experimental results reveal a perplexing scenario; the bulk bands observed at 22 K move nearer to the Fermi level at 60 K and again shift back to higher binding energies at 120 K. The slope of the surface Dirac bands at 22 K becomes smaller at 60 K and changes back to a larger value at 120 K. Our results from the first-principles calculations suggest that these anomalies can be attributed to the evolution of the hybridization physics with complex structural changes induced by temperature. In addition, we discover drastically reduced intensity of the Dirac states at the Fermi level at high temperatures may be due to complex evolution of anharmonicity, strain, etc. These results address robustness of the topologically protected surface states due to thermal effects and emphasize importance of covalency and anharmonicity in the topological properties of such emerging quantum materials.

Vertical Gradient Variations in Radiation Budget and Heat Fluxes in the Urban Boundary Layer: A Comparison Study Between Polluted and Clean Air Episodes in Beijing During Winter

AbstractPrevious studies have reported air pollution‐radiation interactions in the urban boundary layer (UBL), but vertical gradient variations in the radiation budget and heat flux under air pollution conditions are relatively sparse. In this study, based on gradient observations from the Beijing 325 m meteorological tower in December 2015, the characteristics of near‐surface radiation balance and energy budget at three levels under different pollution conditions were comparatively investigated. Relative to clean days, both downward and upward shortwave radiation (DSR and USR) dropped during daytime, while downward and upward longwave radiation (DLR and ULR) enhanced during nighttime on heavily polluted days, showing that with evaluated height, the drop magnitudes of DSR and USR decreased, while the enhancement magnitude of DLR (ULR) decreased (increased). The combined effects of four radiation components significantly induced the reduction in net radiation (Rn) on polluted days, leading to the near‐surface energy budget change. In addition, the monthly averaged anthropogenic heat flux (Qf) was estimated to quantitatively calculate the heat storage (G) term in the surface energy budget. During daytime, compared to the clean episodes, the sensible heat flux (H) was reduced more than Rn in the whole near‐surface UBL during heavy polluted episodes, resulting in smaller H/(Rn + Qf) and larger G/(Rn + Qf). Finally, we revealed that weak thermal forcing effects caused by insufficient availability of net radiation energy at the surface and weak dynamic motion associated with weak winds were both responsible for the larger reduction (increase) in H (G) during pollution episodes in the whole near‐surface UBL.

Micro-particle image velocimetry visualization study of thermal Buoyant-Marangoni flow in microtubes

The coupled momenta induced by thermal effects during evaporation near liquid–vapor interfaces cause complex three-dimensional flow structures, called thermal buoyant-capillary flows. In this study, we investigated the coupled flow mechanisms, observing the internal flow structure of evaporating micro-mini scale menisci in microtubes (680–1560 μm) by the micro-particle image velocimetry (PIV) technique. The horizontal and vertical motions of 1-μm fluorescent particles in volatile ethanol was visualized to obtain the shape of the flow in high-resolution (10–20 μm resolution). The evaporation rate (2–1800 ng/s) and environmental temperature (20–50 °C) were controlled using the Joule heating method. We discuss the effects of varying the thermal conditions and tube size on the position and strength of the vortices. In addition to the experimental work, a simplified numerical simulation was also carried out to estimate the thermal properties that were not measured during experiment. As a result, it was explained that the vortices near the wall of the upper tube are initialized by weak thermal effects from the wall, and higher heating condition cause them to move down the tube and enhance their vorticity by dominant buoyancy effect. In addition, vortices in tubes with large diameters are weaker and their motion is delayed due to the increase in gravitational effects.

Computation of Dynamic Polarizabilities and van der Waals Coefficients from Path-Integral Monte Carlo.

We demonstrate computation of total dynamic multipole polarizabilities using path-integral Monte Carlo method (PIMC). The PIMC approach enables accurate thermal and nonadiabatic mixing of electronic, rotational, and vibrational degrees of freedom. Therefore, we can study the thermal effects, or lack thereof, in the full multipole spectra of the chosen one- and two-electron systems: H, Ps, He, Ps2, H2, and HD+. We first compute multipole-multipole correlation functions up to octupole order in imaginary time. The real-domain spectral function is then obtained by analytical continuation with the maximum entropy method. In general, sharpness of the active spectra is limited, but the obtained off-resonant polarizabilities are in good agreement with the existing literature. Several weak and strong thermal effects are observed. Furthermore, the polarizabilities of Ps2 and some higher multipole and higher frequency data have not been published before. In addition, we compute isotropic dispersion coefficients C6, C8, and C10 between pairs of species using the simplified Casimir-Polder formulas.

Pt-Based Nanostructures for Observing Genuine SERS Spectra of p-Aminothiophenol (PATP) Molecules

By one-pot wet chemical method, Pt-based hollow nanostructures were synthesized at room temperature. Because of the highly damping optical response of the metal, these Pt-based hollow nanostructures exhibited weak thermal effects with a laser focal spot on 4-aminothiophenol (PATP) molecules limiting dimerisation. The isolated surface enhanced Raman scattering (SERS) spectra of PATP, in which the vibrational bands from 4,4′-dimercaptoazobenzene (DMAB) molecules are not observed, were able to be seen, and this was in good agreement with the Raman spectra of PATP powder. In addition, the concentration of PATP molecules was varied, and the illumination time was increased to 2000 s, respectively. It was found that spectra were stable with varied PATP concentrations, and the plasmon-driven chemical conversion of PATP to DMAB was still suppressed, even when the laser illumination time was increased to 2000 s.

Novel organic NLO material bis(N-phenylbiguanidium(1+)) oxalate – A combined X-ray diffraction, DSC and vibrational spectroscopic study of its unique polymorphism

Three polymorphic modifications of bis(N-phenylbiguanidium(1+)) oxalate are reported, and their characterization is discussed in this paper. The non-centrosymmetric bis(N-phenylbiguanidium(1+)) oxalate (I), which was obtained from an aqueous solution at 313K, belongs to the monoclinic space group Cc (a=6.2560(2)Å, b=18.6920(3)Å, c=18.2980(5)Å, β=96.249(1)°, V=2127.0(1)Å3, Z=4, R=0.0314 for 4738 observed reflections). The centrosymmetric bis(N-phenylbiguanidium(1+)) oxalate (II) was obtained from an aqueous solution at 298K and belongs to the monoclinic space group P21/n (a=6.1335(3)Å, b=11.7862(6)Å, c=14.5962(8)Å, β=95.728(2)°, V=1049.90(9)Å3, Z=4, R=0.0420 for 2396 observed reflections). The cooling of the centrosymmetric phase (II) leads to the formation of bis(N-phenylbiguanidium(1+)) oxalate (III) (a=6.1083(2)Å, b=11.3178(5)Å, c=14.9947(5)Å, β=93.151(2)°, V=1035.05(8)Å3, Z=4, R=0.0345 for 2367 observed reflections and a temperature of 110K), which also belongs to the monoclinic space group P21/n. The crystal structures of the three characterized phases are generally based on layers of isolated N-phenylbiguanidium(1+) cations separated by oxalate anions and interconnected with them by several types of N-H...O hydrogen bonds. The observed phases generally differ not only in their crystal packing but also in the lengths and characteristics of their hydrogen bonds. The thermal behaviour of the prepared compounds was studied using the DSC method in the temperature range from 90K up to a temperature near the melting point of each crystal. The bis(N-phenylbiguanidium(1+)) oxalate (II) crystals exhibit weak reversible thermal effects on the DSC curve at 147K (heating run). Further investigation of this effect, which was assigned to the isostructural phase transformation, was performed using FTIR, Raman spectroscopy and X-ray diffraction analysis in a wide temperature range.

Термоэлектрические и тепловые свойства суперионных сплавов AgxCu2−xSe (x = 0.01, 0.02, 0.03, 0.04, 0.25)

The results of investigations of the thermoelectric and thermal properties of AgxCu2-xSe (x = 0.01, 0.02, 0.03, 0.04, 0.25) alloys are presented in the paper. The AgxCu2-xSe samples with low silver content were prepared by solid state reaction of the pure elements in argon atmosphere at 773 K temperature. The Ag0.25Cu1.75Se sample was sintered by solid phase reaction of Cu2Se and Ag2Se mixture in argon atmosphere at 673 K. At room temperature X-ray diffraction study revealed the presence of three phases in the samples: the Cu2Se orthorhombic phase, the Cu1.8Se cubic phase and the AgCuSe orthorhombic phase. The heat of the superionic phase transition in Ag0.01Cu1.99Se was measured equal to (3.5 ± 0.3) kJ / mol. For Ag0.25Cu1.75Se sample the heat of the superionic phase transition was found to be (3.1 ± 0.3) kJ / mol. In addition to intense peak of the superionic phase transition occupying the 373-423 K temperature range, the weak thermal effects for Ag0.01Cu1.99Se at 317 K, and ones for Ag0.25Cu1.75Se at 316 K and 437 K were observed too. In the investigated temperature range of 290 - 770 K the electronic conductivity σ decreases, and Zeebeck coefficient α increases with silver content in the compounds. The thermal conductivity of Ag0.03Cu1.97Se compound decreases monotonically from 0.54 to 0.34 W m-1 K-1 in the range 420 - 650 K after the superionic phase transition, resulting in thermoelectric efficiency ZT = σα2T/λ increases monotonically, reaching a value ZT = 1 at 650 K.

Numerical Modeling of Random Magnetization Dynamics

Numerical modeling of random magnetization dynamics driven by a jump-noise process is discussed. In the important case of weak thermal effects, random magnetization dynamics is reduced to stochastic processes on graphs and numerical analysis of these processes are outlined.

Theoretical models of the height of the atmospheric boundary layer and turbulent entrainment at its upper boundary

The planetary boundary layer (PBL), which directly interacts with the underlying surface, differs significantly in its nature from the low-turbulent stably stratified free atmosphere. Fluctuations of the Earth’s surface heat balance immediately affect the PBL and assimilate there owing to the effective mechanism of turbulent heat transfer. In this case the upper boundary of the PBL plays the role of a cover, preventing the direct penetration of thermal effects and contaminants into an overlying atmospheric layer. In view of this, air pollution is especially dangerous when associated with shallow PBL. In addition, local peculiarities of climate change are mainly determined by the PBL height due to the high sensitivity of thin stably stratified PBLs to the thermal effects. Deep convective PBLs are not very sensitive to weak thermal effects, but they significantly affect the formation of convective cloudiness and the climate system as a whole by means of the turbulent entrainment of the thermal energy, humidity, aerosols, and other admixtures through the upper boundary. The PBL height and turbulent entrainment must be calculated when simulating and forecasting air pollution, abnormal frosts and heat, and other hazardous phenomena. In this paper we discuss the state-of-the-art knowledge in the area of PBL height simulation and suggest a new model of turbulent entrainment for convective PBLs.

Thermal effects accompanying stationary binary condensation of vapors into overcritical droplet

A set of equations is derived to calculate the stationary temperature and concentration of a solution in a overcritical droplet with regard to the heat release accompanying the condensation of a binary mixture of vapors in a diffusion or free-molecular regime. In the approximation of an ideal solution, relations are found for the stationary temperature of droplet growing under the conditions of strong and weak thermal effects. For the general case and the cases of strong and weak thermal effects, the temperature and concentration of the droplet and the coefficient of the thermal deceleration of the droplet growth are calculated as functions of the density of a passive gas. The influence of the condensation heat values of the first and second components of the mixture on the stationary temperature and concentration of the solution in the growing droplet is investigated separately.

Reactivity of H 2/air mixtures over hot metal surfaces

This paper reports on a systematic study of hydrogen oxidation in the presence of different materials, ranging from quartz to carbon steel. The reactivity tests conducted at low hydrogen concentration revealed significant promotion of the oxidation reaction by carbon steel in a temperature range (300–400 °C) where gas phase reactions occur to a limited extent. This effect is strongly enhanced by surface modifications induced by acid corrosion. Stainless steel surfaces behaved quite differently under similar reaction conditions, showing rather an inhibiting role in H 2 oxidation. The physicochemical characterization of carbon steel samples before and after acid treatment revealed that the surface area undergoes a strong increase due to corrosion, and also the structure is modified with surface enrichment of trace metal components, which can exert a catalytic role. The potential risk represented by the oxidation properties of these widely used materials is mitigated by the low value of the heat of combustion of hydrogen per unit volume; with a composition below the lower flammability limit, weak thermal effects were in fact observed by infrared measurements of the surface temperature. However, using a simplified kinetic expression, it can be estimated that when hydrogen-rich mixtures come into contact with an overheated metal surface, the exothermic reaction can generate dangerous hot spots.

Stationary fronts due to weak thermal effects in models of catalytic oxidation

We analyze the possible existence of an infinite number of stationary front solutions in a microkinetic model of a catalytic reaction coupled with weak enthalpy effects in the domain of kinetics bistability. The kinetic model incorporates three steps: dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. The infinitude of stationary front solutions emerges due to the lack of intercrystallites communication of surface species in supported catalysts; thermal conductions and gas-phase diffusion are the only means of interaction. Incorporation of surface species diffusion leads to a very slow front motion. We complement this analysis with simulations of stationary states on one- (wire and ring) and two-dimensional (disk) systems which may be subject to control or to fluid flow. These results account for certain experimental results and may have implications for various technological problems.

Moving waves and spatiotemporal patterns due to weak thermal effects in models of catalytic oxidation

We analyze the behavior of a microkinetic model of a catalytic reaction coupled with weak enthalpy effects to show that under fixed gas-phase concentrations it can produce moving waves with an intrinsic length scale, when the underlying kinetics is oscillatory. The kinetic model incorporates dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. Three typical patterns may emerge in a one-dimensional system (a long wire or a ring): homogeneous oscillations, a family of moving waves propagating with constant velocities, and patterns with multiple source/sink points. Pattern selection depends on the ratio of the system length to the intrinsic wave length and the governing parameters. We complement these analysis with simulations that revealed a plethora of patterned states on one- and two-dimensional systems (a disk or a cylinder). This work shows that weak long-range coupling due to high feed rates maintains such patterns, while low feed rates or strong long-range interaction can gradually suppress the emerging patterns.

Microwave power handling capability of HTS superconducting thin films: weak links and thermal effects induced limitation

Microwave power-handling capabilities of YBCO thin superconducting films, up to 150 W of the input power, have been investigated using a 14 GHz shielded dielectric cavity. The YBCO films were deposited by the thermal reactive co-evaporation and the dc sputtering method on LaAlO/sub 3/, NdGaO/sub 3/, and Al/sub 2/O/sub 3/ substrates. A simple nonlinear R(P)-L(P)-equivalent circuit model was used to simulate a nonlinear cavity response. The correlation of nonlinearity with global and local heating is discussed. The analysis of experimental data confirms, that for all films, the heating of weak links switched to normal state is one of the rf power handling capability major limitations.

Renormalization group analysis of weak collective pinning

The effect of impurity disorder on vortex lattices in type-II superconductors is studied within the framework of the weak collective pinning theory. Using a renormalization group approach, we calculate the sizeR c of collectively pinned vortex bundles in the regimea o <R c <λ (small bundles) for isotropic superconductors. We assume weak disorder and neglect thermal effects. Our findings quantitatively refine the lowest-order perturbation result,R c ∞\(\sqrt B \), obtained by Larkin and Ovchinnikov in 1979 (B is the magnetic induction). In particular, we find a different numerical constant in the exponential as well as a new algebraic prefactor. These improvements provide us with a more accurate functional dependence of the critical current on the magnetic field.

Nonlinear electrostatic wave equations for magnetized plasmas. II

For pt.I see ibid., vol.26, p.443-7 (1984). The problem of extending the high frequency part of the Zakharov equations for nonlinear electrostatic waves to magnetized plasmas, is considered. Weak electromagnetic and thermal effects are retained on an equal footing. Direction dependent (electrostatic) cut-off implies that various cases must be considered separately, leading to equations with rather different properties. Various equations encountered previously in the literature are recovered as limiting cases.