Thermal Relaxation Processes Research Articles

Memory effects in a gas of viscoelastic particles

We study a granular gas of viscoelastic particles (kinetic energy loss upon collision is a function of the particles' relative velocities at impact) subject to a stochastic thermostat. We show that the system displays anomalous cooling and heating rates during thermal relaxation processes, this causing the emergence of thermal memory. In particular, a significant Mpemba effect is present, i.e., an initially hotter/cooler granular gas can cool down/heat up faster than an in comparison cooler/hotter granular gas. Moreover, a Kovacs effect is also observed, i.e., a nonmonotonic relaxation of the granular temperature—if the gas undergoes certain sudden temperature changes before fixing its value. Our results show that both memory effects have distinct features, very different and eventually opposed to those reported in theory for granular fluids under simpler collisional models. We study our system via three independent methods: approximate solution of the kinetic equation time evolution and computer simulations (both molecular dynamics simulations and direct simulation Monte Carlo method), finding good agreement between them.

Mpemba effect in inertial suspensions.

The Mpemba effect (a counterintuitive thermal relaxation process where an initially hotter system may cool down to the steady state sooner than an initially colder system) is studied in terms of a model of inertial suspensions under shear. The relaxation to a common steady state of a suspension initially prepared in a quasiequilibrium state is compared with that of a suspension initially prepared in a nonequilibrium sheared state. Two classes of Mpemba effect are identified, the normal and the anomalous one. The former is generic, in the sense that the kinetic temperature starting from a cold nonequilibrium sheared state is overtaken by the one starting from a hot quasiequilibrium state, due to the absence of initial viscous heating in the latter, resulting in a faster initial cooling. The anomalous Mpemba effect is opposite to the normal one since, despite the initial slower cooling of the nonequilibrium sheared state, it can eventually overtake an initially colder quasiequilibrium state. The theoretical results based on kinetic theory agree with those obtained from event-driven simulations for inelastic hard spheres. It is also confirmed the existence of the inverse Mpemba effect, which is a peculiar heating process, in these suspensions. More particularly, we find the existence of a mixed process in which both heating and cooling can be observed during relaxation.

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

A thermodynamic description of the hysteresis in specific-heat curves in glass transitions

By refining the definition of thermodynamic equilibrium and state variables (thermodynamic coordinate, TC) for solids, it is determined that the state of a glass substance transforms into an equilibrium state after it is solidified. In contrast, the state of a glass substance during the glass transition is a nonequilibrium state. The specific-heat (C) versus temperature (T) curve exhibits hysteresis, which is traditionally believed to invalidate thermodynamic methods. However, the glass transition slowly occurs in a manner such that structural change is decoupled with the fast process of thermal relaxation of phonons, which enables us to describe the hysteresis by thermodynamic methods. The hysteresis is caused by the structural relaxation and the time of relaxation is determined by the energy barrier, which depends solely on the current value of TCs. Therefore, the state in hysteresis can be described by the information of the current structure alone: history-dependent response functions are unnecessary. On the basis of these conclusions, the behavior of the C-T curve with changing heating/cooling rate γ is simulated. The main features of the hysteresis, the shift of C to higher temperatures with increasing γ, the hump structure, and the memory effect are well reproduced from a structure-dependent energy barrier. In view of the structural dependence of the energy barrier, it is not surprising to observe deviations from the Arrhenius law. However, only the terms that are higher than linear in T are observed in Arrhenius plot as the deviation. An important finding of this study is that the apparent energy barrier obtained using the Arrhenius plot significantly overestimates the real value. The extraordinarily large values of the pre-exponential factor of the relaxation time can also be understood on this basis.

Revisiting the thermal relaxation of neutron stars

In this work, we revisit the thermal relaxation process for neutron stars. Such a process is associated with the thermal coupling between the core and the crust of neutron stars. The thermal relaxation, which takes place at around 10–100 years, is manifested as a sudden drop in the star’s surface temperature. Such a drop is smooth for slowly cooling objects and very sharp for fast-cooling ones. In our study, we focused particularly on the cooling of neutron stars whose mass is slightly greater than the value above which the direct Urca (DU) process sets in. Considering different mechanisms for neutrino production in each region of the star, and working with equations of state with different properties, we solved the thermal evolution equation and calculated the thermal relaxation time for an ample range of neutron star masses. By performing a comprehensive study of neutron stars just above the onset of the DU process, we show that stars under these conditions exhibit a peculiar thermal relaxation behavior. We demonstrate that such stars exhibit an abnormally late relaxation time, characterized by a second drop in its surface temperature taking place a later age. We qualified such behavior by showing that it is associated with limited spatial distribution of the DU process in such stars. We show that as the star’s mass increases, the DU region also grows, and the star exhibits the expected behavior of fast-cooling stars. Finally, we show that one can expect high relaxation times for stars in which the DU process takes place in a radius no larger than 3 km.

The nucleation and growth of Helium hubbles at grain boundaries of bcc tungsten: a molecular dynamics simulation

Tungsten (W) is a potential candidate for plasma facing materials (PFMs) of fusion reactor. The helium (He) produced in fusion reaction is insoluble and easy to gather and form to He bubbles in W, resulting in embrittlement and degradation of the performance of the W matrix. In this paper, based on molecular dynamics, the nucleation and growth of helium bubbles in the bulk and at ∑3[211](110) and ∑9[110](411) grain boundaries of W was studied. As a result, the growth mechanism of Helium bubbles at grain boundary of W was different from in bulk. Helium bubbles in bulk W grow up by extruding dislocation rings. The growth mechanism of helium bubbles at ∑3[211](110) grain boundary was as follows: Firstly, a small amount of W interstitial atoms were extruded and emitted. And then the 1/2<inline-formula><tex-math id="Z-20200121175109">\begin{document}$\left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175109.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175109.png"/></alternatives></inline-formula> dislocation line was extruded. Finally, the 1/2<inline-formula><tex-math id="Z-20200121175126">\begin{document}$\left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175126.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175126.png"/></alternatives></inline-formula> dislocation line would migrate along the direction of [111] of the grain boundary interface. Moreover, the emission of W interstitial atoms and dislocation extrusion of the helium bubble were not observed in our simulated time scale at the ∑9[110](411) grain boundary. Then we used the NEB method to calculate the diffusion barrier of self-gap atoms in the bulk and at ∑3[211](110) and ∑9[110](411) grain boundaries of W, which explained the simulation results. The migration energy barrier of W self-gap atoms in the bulk and at ∑3[211](110) grain boundary was only a few to a few millielectron volts. So as long as W self-gap atoms dissociated from the surface of the He bubble in the thermal relaxation process, they can be easily migrated out. However, The migration energy of the W self-gap atom at the ∑9[110](411) grain boundary can be from a few tenths to a few electron volts. Even during the thermal relaxation process, the W self-gap atoms dissociated from the surface of the He bubble. It was difficult for the W self-gap atoms migrated out. Finally, the correlation between He bubble size and stress released was given. Either in bulk or at ∑3[211](110) and ∑9[110](411) grain boundaries of W, after the pressure of the helium bubble becomes stable with time, the radius of the helium bubble would increase rapidly whenever the pressure dropped sharply. So there was a small step on the curve of the evolution of the radius of the helium bubble with time. Thus, helium bubbles in W could promote growth by releasing pressure intermittently.

Superconductivity of Y5Rh6Sn18; Coexistence of the high temperature thermal lattice relaxation process and superconductivity

The heat capacity, magnetic, and electrical transport properties of skutterudite-related Y5Rh6Sn18 compound indicate intriguing coexistence of the high temperature thermal lattice relaxation process at temperatures higher than 350 K and superconductivity below the superconducting transition temperature Tc=3.08 K. Y5Rh6Sn18, having a cage-like structure crystallises in tetragonal structure with the space group I41/acd. The gap structure of this compound is found not to be characteristic of conventional BCS type superconductors. Anomalous behaviour observed in the electronic specific heat C(T)/T and the upper critical field Hc2(T) data may suggest the presence of two superconducting gaps in this compound. The ab initio band structure calculations showed a significant contribution of s and p Sn states as well as Rh and Y d-electron states to the total density of states near the Fermi level, which supports the multi-band superconductivity of this compound. Several electronic quantities and parameters characterising the superconducting and normal state of Y5Rh6Sn18 are calculated within the Ginzburg-Landau-Abrikosov-Gorkov theory. The data reveals dirty limit superconductivity in Y5Rh6Sn18 superconductor of type-II.

BIOHEAT EQUATION WITH FOURIER AND NON-FOURIER HEAT TRANSPORT LAWS: APPLICABILITY TO HEAT TRANSFER IN HUMAN TISSUES

The paper is dedicated to mathematical problem formulations for the heat propagation in biological tissues based on the Fourier and non-Fourier laws at different boundary conditions. The heating of the tissues is provided by external heat sources like low intensity lasers or light-emitting diodes which are widely used in contemporary medical care. Numerical computations on the standard Pennes bioheat equation with Fourier heat conduction give the temperature curves for both heating and thermal relaxation processes that do not correspond to the in vivo measurement data on human skin tissue. It is shown the modified bioheat equation based on the Guyer-Krumhansl heat conduction with correct formulation of the boundary conditions produces realistic temperature curves when the distributed heat sources and sinks in the tissue are accounted for. The former corresponds to the metabolic heat and temperature dependent chemical reactions, while the latter is provided by the heat convection with blood microcirculation system. The proposed model gives realistic two time temperature curves. The perspective applications of the novel mathematical formulation are discussed.

Information-Theoretical Bound of the Irreversibility in Thermal Relaxation Processes.

We establish that entropy production, which is crucial to the characterization of thermodynamic irreversibility, is obtained through a variational principle involving the Kulback-Leibler divergence. A simple application of this representation leads to an information-theoretical bound on entropy production in thermal relaxation processes; this is a stronger inequality than the conventional second law of thermodynamics. This bound is also interpreted as a constraint on the possible path of a thermal relaxation process in terms of information geometry. Our results reveal a hidden universal law inherent to general thermal relaxation processes.

Electric field assisted ion exchange of silver in soda-lime glass: A study of ion depletion layers and interactions with potassium

The introduction of silver ions into glass by diffusion from an external source is of interest in modifying the optical and electrical properties of glass for device applications. Silver is introduced to fabricate in-glass waveguides, while potassium ions are introduced to pattern the silver diffusion by locally impeding the diffusion. Electric fields assist the silver diffusion, leading to faster diffusion rates of ions and allowing greater concentrations of silver without a metallic cluster formation. However, ion depletion layers are formed in the glass as a result of the application of electric field. Here, we study the nature of these depletion layers using depth profiles of composition after electric field diffusion, using cross-sectional energy dispersive x-ray spectroscopy analysis and infrared spectroscopy (Fourier transform infrared attenuated total reflection spectroscopy). We confirm the accelerated diffusion of silver by an electric field and show that potassium impedes the diffusion of silver even when a field is present. We find that an ion depletion layer is formed below the glass surface by the applied electric field which leads to a thermal relaxation and depolarization process when the samples are reheated. Observation of this process enables activation energies and threshold temperatures for the diffusion of Ag, K, and Na to be evaluated and compared with values obtained by composition profiles. Potassium was found to increase the initially low temperature threshold for silver diffusion, increase the activation energy, and also impede sodium diffusion.

Investigation of recombination front region in detached plasmas in a linear divertor plasma simulator

Abstract We have investigated detailed plasma structure around the recombination front region (RFR), where volume electron-ion recombination (EIR) strongly occurs in detached plasmas, by using two-dimensional (2D) movable probe system as well as laser Thomson scattering diagnostics. The 2D distribution of electron density, electron temperature, and plasma potential around the RFR, has been revealed for the first time. The spatial distribution of the particle loss rate due to EIR estimated from electron temperature and electron density profiles indicated that the three-dimensional structure of the RFR adopts a cone structure. Furthermore, in the peripheral region of the RFR, the radial profile of the electron density was widened locally with increasing cross-field transport caused by plasma fluctuation. From the heat transport analysis of electrons, it was found that the thermal relaxation process between electrons and ions is important for the cooling of electrons, and that the convection of electrons regulates electron heat transport near the RFR.

A novel multiphotochromic system with orthogonal light excitations

Multiphotochromic systems are of immense importance for promising applications nowadays, while it remains a formidable challenge to develop a compound containing various photoswitches with spectral complementarity. Herein, we describe a novel multiphotochromic system based on a first generation donor-acceptor Stenhouse adduct (DASA) and an aurone, in which all physicochemical properties of two photoswitches are preserved and four isomers are selectively reachable with three different wavelengths of light sources (365 nm, 430 nm, 585 nm) and a thermal relaxation process in an orthogonal manner. Multi-addressable behavior of the system could be beneficial for potential applications such as logic devices and photo-printing.

Quantum heat distribution in thermal relaxation processes.

We analyze the heat exchange distribution of open quantum systems undergoing a thermal relaxation process with a time-dependent effective temperature. We show that such processes arise, for example, if the dynamics maximizes the entropy production. Using a two-point measurement scheme, we find an expression for the heat moment generating function that depends solely on the system's partition function and on the thermalization function (i.e., the law of cooling) describing the effective temperature. Applications include the relaxation of free bosonic and fermionic modes, for which closed-form expressions for the time-dependent heat distribution function are derived. Multiple free modes with arbitrary dispersion relations are also briefly discussed. In the semiclassical limit our formula agrees with previous results of the literature for the heat distribution of an optically trapped nanoscopic particle far from equilibrium.

Molecular organization and optical switching of liquid-crystalline azobenzenes in monomolecular films

The photoisomerization and thermal relaxation processes of three amphiphilic azobenzene derivatives (LCAn) showing liquid-crystalline properties were studied in solutions and in monomolecular films formed using Langmuir and Langmuir-Blodgett techniques. The well-controlled isomeric state of LCAn in chloroform during the UV irradiation allowed to prepare spreading solutions containing trans isomers, cis isomers or their mixture and to investigate molecular organization in the Langmuir films made of them. Surface pressure-mean molecular area isotherms and compression modulus-surface pressure dependences delivered information on a thermodynamic state, phase transitions and intermolecular interaction in the monolayers. LCAn in the trans state create densely-packed films, in which the photoisomerization do not occur due to the lack of free volume for the change of the molecules conformation and their strong tendency for the H-type aggregation. However, the reversible switching of the LCAn conformation during cyclic photoisomerization and thermal relaxation processes was observed in the LB films initially deposited onto quartz substrates in the cis state, where molecules are loosely-packed and exist in the monomeric form. Kinetics and rate constants of the photoisomerization and thermal relaxation were determined based on the recorded absorption spectra in order to obtain information on their efficiency and on the impact of the replacement of the alkyl chain attached in the LCAn para position with the alkyloxy chain. Our research show that LCAn are promising materials for application in optical memories, logic gates and holographic gratings.

Phonon branch-resolved electron-phonon coupling and the multitemperature model

Electron-phonon $(e\text{\ensuremath{-}}p)$ interaction and transport are important for laser-matter interactions, hot-electron relaxation, and metal-nonmetal interfacial thermal transport. A widely used approach is the two-temperature model (TTM), where $e\text{\ensuremath{-}}p$ coupling is treated with a gray approach with a lumped coupling factor ${G}_{ep}$ and the assumption that all phonons are in local thermal equilibrium. However, in many applications, different phonon branches can be driven into strong nonequilibrium due to selective $e\text{\ensuremath{-}}p$ coupling, and a TTM analysis can lead to misleading or wrong results. Here, we extend the original TTM into a general multitemperature model (MTM), by using phonon branch-resolved $e\text{\ensuremath{-}}p$ coupling factors and assigning a separate temperature for each phonon branch. The steady-state thermal transport and transient hot electron relaxation processes in constant and pulse laser-irradiated single-layer graphene (SLG) are investigated using our MTM respectively. Results show that different phonon branches are in strong nonequilibrium, with the largest temperature rise being more than six times larger than the smallest one. A comparison with TTM reveals that under steady state, MTM predicts 50% and 80% higher temperature rises for electrons and phonons respectively, due to the ``hot phonon bottleneck'' effect. Further analysis shows that MTM will increase the predicted thermal conductivity of SLG by 67% and its hot electron relaxation time by 60 times. We expect that our MTM will prove advantageous over TTM and gain use among experimentalists and engineers to predict or explain a wide ranges of processes involving laser-matter interactions.

Tracking coherent population transfer and thermal population relaxation in condensed system by broad-band transient grating spectroscopy

Broad-band transient grating (BB-TG) spectroscopy was proposed to track both coherent population transfer (CPT) and thermal population relaxation processes in a condensed system of solvated molecules in solution (Rhodamine101 in methanol). A broad band around 1500 cm−1 and a relative narrow band near 2900 cm−1 emerge in TG and transient absorption contour plots when pump and probe pulses overlap in the sample. The experimental results matched well with the vibrational modes of Rhodamine101 that were obtained by theoretical calculation. In addition, it was found that the population of CPT particles can be evaluated quantitatively through the intensity of the TG signal.

Measurement of He neutral temperature in detached plasmas using laser absorption spectroscopy

The reduction of the heat load onto plasma-facing components by plasma detachment is an inevitable scheme in future nuclear fusion reactors. Since the control of the plasma and neutral temperatures is a key issue to the detached plasma generation, we have developed a laser absorption spectroscopy system for the metastable helium temperature measurements and used together with a previously developed laser Thomson scattering system for the electron temperature and density measurements. The thermal relaxation process between the neutral and the electron in the detached plasma generated in the linear plasma device, NAGDIS-II was studied. It is shown that the electron temperature gets close to the neutral temperature by increasing the electron density. On the other hand, the pressure dependence of electron and neutral temperatures shows the cooling effect by the neutrals. The possibility of the plasma fluctuation measurement using the fluctuation in the absorption signal is also shown.

PCH1 and PCHL promote photomorphogenesis in plants by controlling phytochrome B dark reversion

Phytochrome B (phyB) is the primary red light photoreceptor in plants, and regulates both growth and development. The relative levels of phyB in the active state are determined by the light conditions, such as direct sunlight or shade, but are also affected by light-independent dark reversion. Dark reversion is a temperature-dependent thermal relaxation process, by which phyB reverts from the active to the inactive state. Here, we show that the homologous phyB-binding proteins PCH1 and PCHL suppress phyB dark reversion, resulting in plants with dramatically enhanced light sensitivity. Moreover, far-red and blue light upregulate the expression of PCH1 and PCHL in a phyB independent manner, thereby increasing the response to red light perceived by phyB. PCH1 and PCHL therefore provide a node for the molecular integration of different light qualities by regulation of phyB dark reversion, allowing plants to adapt growth and development to the ambient environment.

Depolarization current relaxation process of insulating dielectrics after corona poling under different charging conditions

As an insulating dielectric, polyimide is favorable for the application of optoelectronics, electrical insulation system in electric power industry, insulating, and packaging materials in space aircraft, due to its excellent thermal, mechanical and electrical insulating stability. The charge storage profile of such insulating dielectric is utmost important to its application, when it is exposed to electron irradiation, high voltage corona discharge or other treatments. These treatments could induce changes in physical and chemical properties of treated samples. To investigate the charge storage mechanism of the insulating dielectrics after high-voltage corona discharge, the relaxation processes responsible for corona charged polyimide films under different poling conditions were analyzed by the Thermally Stimulated Discharge Currents method (TSDC). In the results of thermal relaxation process, the appearance of various peaks in TSDC spectra provided a deep insight into the molecular status in the dielectric material and reflected stored space charge relaxation process in the insulating polymers after corona discharge treatments. Furthermore, the different space charge distribution status under various poling temperature and different discharge voltage level were also investigated, which could partly reflect the influence of the ambiance condition on the functional dielectrics after corona poling.

Manipulation of Nonlinear Optical Properties of Graphene Bonded Fiber Devices by Thermally Engineering Fermi–Dirac Distribution

AbstractGraphene's atomic thickness, gapless Dirac–Fermionic band structure, and large thermal conductivity make it a promising element for applications in photonic integrated devices. Importantly, actively tunable graphene‐waveguide‐integrated optoelectronic devices can potentially be utilized for the realization of reconfigurable photonic systems. Since electrical device control is always preferred, researchers have previously demonstrated gate‐variable graphene devices; however, electrical control remains challenging, particularly in all‐fiber systems because of the required complicated configuration and fabrication techniques along with additional signal loss. Here, a graphene‐fiber‐integrated platform is proposed and the manipulation of its nonlinear optical properties is demonstrated by engineering the Fermi–Dirac distribution of graphene based on a convenient electric heating method. For the first time, it is experimentally shown that the nonlinear optical absorption of graphene is correlated to temperature via the thermal relaxation process. In the experiments, the modulation depth variation exceeds 60%. The configuration is simple, cost‐effective, and can be readily extended to other 2D materials. It is believed that this work can contribute to building miniature and compact graphene‐fiber‐integrated devices for actively tunable multifunctional applications.