Fast Thermal Processes Research Articles

Thermocouple response time estimation and temperature signal correction for an accurate heat flux calculation in inverse heat conduction problems

Many heat transfer processes do not allow temperature measurements with advanced instruments like IR cameras, and thus require the use of thermocouples – often inserted sheathed thermocouples – in order to measure the temperature in some key-regions. Nevertheless, thermocouples have an intrinsic response time that can dampen substantially temperature measurements and, when applicable, affect heat flux estimations by inverse methods. In this study, a thermocouple measurement correction method is proposed, especially for fast thermal transients like quenching, to avoid underestimation of the boundary heat fluxes due to delayed temperature responses. A simplified energy balance at the hot junction allowed modeling the temperature measurement by a thermocouple, in which heat losses and the response time were taken into account so they can be estimated using the least squares method on experimental data. A series of tests was performed to validate the present method with a heated jet impinging on a copper body instrumented with two thermocouples: one ungrounded and sheathed, then with high response time (”slow”); and another with exposed welded wires, hence with low response time (”fast”). After having estimated the slow thermocouple response time and heat loss parameters, the model allowed a good reconstruction of the fast thermocouple signal using the slow thermocouple measurements. Also, the heat flux estimation by the inverse method using the reconstructed signal resulted in practically the same results obtained using the fast thermocouple data. A parametric sensitivity analysis showed the heat loss parameters can be neglected for heat flux estimations, while simulations showed that temperature noises degrade the response time estimation but data filtering can mitigate this noise effect. Finally, the present method was applied to a large scale jet-cooling of a hot plate – near industrial conditions – and showed a substantially higher dissipated heat flux than the initially estimated. In fact, the results using reconstructed signals demonstrated that the heat flux dissipated by the jet has a higher dependence on the jet Reynolds number than the observed with the original thermocouple measurements, showing how important the temperature measurement correction is to evaluate fast thermal processes.

Asymptotic Description of Fast Thermal Processes in Scalar Harmonic Lattices

Thermal vibrations in a d-dimensional (d = 1, 2) scalar harmonic lattice of a simple structure are under consideration. Redistribution of the averaged kinetic and potential energies of particles after instantaneous thermal excitation (fast process) is described. It is established that the difference between the kinetic and potential energies undergoes power-law damped oscillations at times much longer than the characteristic atomic-vibration period. A typical exponent is –d/2. The oscillation frequencies are determined from the dispersion relation for the lattice. The algorithm of proving allows generalization to scalar and vector lattices with a complex structure and different dimensions. Thermal vibrations in a two-dimensional hexagonal lattice (graphene lattice) are considered as an example of this generalization.

Synthesis and photochromic behaviour of a series of benzopyrans bearing an N-phenyl-carbazole moiety: photochromism control by the steric effect.

Five new N-phenyl-carbazole benzopyrans bearing different substitutions on one of the phenyl rings at the sp3 carbon have been synthesized. Their molecular structures were investigated by X-ray and NMR analyses and through quantum chemical calculations. The photochromic mechanism under UV irradiation in toluene, consisting of the consecutive formation of transoid-cis (TC) and transoid-trans (TT) isomers, was studied by UV-vis spectral and kinetic analyses. These molecules have been specifically designed to ascertain the possibility of favouring the formation of the less thermodynamically stable TT at the photostationary state, upon exploiting steric hindrance effects on the diene part of the molecule. The spectrokinetic study allowed the estimation of most of the spectrokinetic parameters, such as molar extinction coefficients, quantum yields of UV colouration and visible photobleaching, and the rate constants of the fast and slow thermal bleaching processes. Peculiar effects of substituents with different donor strengths on one phenyl ring located at the 3-position were observed on the spectrokinetic properties.

Thermal contact conductance at melting and crystallization of metal micro-droplets

To measure interfacial thermal contact conductance in fast thermal processes, ultrafast scanning calorimetry combined with high-resolution high-speed infrared thermography is applied. The dynamics of temperature distribution on the sample surface is measured by thermography during melting and crystallization of a tin particle of about 580 ng and 70 μm in diameter. The temperature difference on the sample/sensor interface is measured and used to determine the interfacial thermal contact conductance with acceptable accuracy on a millisecond time scale. It is shown that the temperature difference can be of the order of 100 K at melting and crystallization. This is very essential for applications with fast temperature changes like additive manufacturing and for calorimeter calibration. The method can be applied to different materials in fast thermal processes on a micro-scale. During crystallization, the effect of reheating (about 100 K) is observed.

Optothermally induced mechanical oscillation in a silk fibroin coated high-Q microsphere

Protein-based optical devices with biocompatibility and biodegradability have distinct advantages for applications in biomedical sensing. Silk fibroin with unique optical, thermal, and mechanical properties renders great flexibility in designing functional photonic platforms. Here, we report the experimental observation of optothermally induced mechanical oscillation in a silk-fibroin coated microcavity. Theoretical analysis reveals that the observed oscillation results from the interplay of several nonlinear effects in the silk-coated-microsphere as well as the coexistence of fast and slow thermal dynamic processes. The physics in our study breaks ground for the study of nonlinear dynamics of structural protein optical material that can be used for functional optical devices.

Thermal inactivation of Aspergillus flavus in peanut kernels as influenced by temperature, water activity and heating rate

Infection of Aspergillus flavus, which can produce aflatoxin, is a major problem for peanut safe storage. Thermal inactivation kinetics of Aspergillus flavus is essential to design an effective heat treatment process. In this study, thermal inactivation kinetics of Aspergillus flavus in peanut kernel flour at four water activity (aw) levels (0.720, 0.783, 0.846, and 0.921) with three temperatures for each aw was studied using a thermal-death-time heating block system and fitted with first-order kinetic and Weibull models. The influence of heating rates on thermotolerance of Aspergillus flavus was also investigated. The results showed that the Weibull distribution model had better coefficient of determination from 0.954 to 0.996, as compared to that (from 0.866 to 0.980) of the first-order kinetic model. An upward concavity was found with the inactivation curve, indicating a tailing effect. Model parameters (D, δ, and p) were estimated with the modified Bigelow equations to predict survival curves of Aspergillus flavus at any temperature and aw. The reduced heat resistance of Aspergillus flavus at high heating rates above 1 °C/min suggests that developing fast thermal processes is preferred for pasteurizing peanuts in food industry. A case study was presented for applying the cumulated lethal time model to design the industrial heating process based on the thermal kinetics of Aspergillus flavus.

Fast and slow thermal processes in harmonic scalar lattices

An approach for analytical description of thermal processes in harmonic lattices is presented. We cover longitudinal and transverse vibrations of chains and out-of-plane vibrations of two-dimensional lattices with interactions of an arbitrary number of neighbors. The motion of each particle is governed by a single scalar equation and therefore the notion ‘scalar lattice’ is used. The evolution of initial temperature field in an infinite lattice is investigated. An exact equation describing the evolution is derived. Continualization of this equation with respect to spatial coordinates is carried out. The resulting continuum equation is solved analytically. The solution shows that the kinetic temperature is represented as the sum of two terms, one describing short time behavior, the other large time behavior. At short times, the temperature performs high-frequency oscillations caused by redistribution of energy among kinetic and potential forms (fast process). Characteristic time of this process is of the order of ten periods of atomic vibrations. At large times, changes of the temperature are caused by ballistic heat transfer (slow process). The temperature field is represented as a superposition of waves having the shape of initial temperature distribution and propagating with group velocities dependent on the wave vector. Expressions describing fast and slow processes are invariant with respect to substitution t by . However, examples considered in the paper demonstrate that these processes are irreversible. Numerical simulations show that presented theory describes the evolution of temperature field at short and large time scales with high accuracy.

Thermal oscillatory behavior analysis and dynamic modulation of refractive index in microspherical resonator.

The thermal nonlinear effects in whispering-gallery-mode resonators are characterized by oscillatory behavior in the transmission spectrum. Although the thermal linewidth broadening is proven to be practical in mode-locking and dynamic control of the optical path, the oscillatory behavior always leads to instability of mode-locking and influences the control accuracy. We theoretically and experimentally illustrate the thermal oscillatory behavior using a model that combines slow and fast thermal relaxation processes of the microsphere and fluctuations of the pump wavelength. We also report dynamic modulation of the refractive index based on the fast thermal relaxation process.

Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J cm−2 single-shot irradiation under UHV condition and 0.25–2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm−2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J cm−2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.

Photoemission measurements of temperature in pulsed laser heating of various materials

The possibilities for the photoemission method of measuring the temperature of various materials heated by millisecond laser pulses have been investigated. The temperature of graphite, tungsten, tantalum, silicon plates, and silicon dioxide films was determined experimentally with a time resolution of 1 μs within the range 1200-2900 K. Introduction. Measurement of the temperature of fast processes is a complex problem in itself, and it is fur- ther aggravated if the temperature of an object changes from room to 3000 K at a rate of 10 8 K ⁄ s and there is no possibility of taking account of the accompanying change in the emissivity. In this case, to measure the temperature of the object, use can be made of the photoemission method in which modulation with a high-frequency electronic flux rather than a low one is produced, and one can compare the energy distribution of photoelectrons in two broad spec- tral intervals, one of which being part of the other, with the difference between the measured temperature and the ther- modynamic one turning out to be insignificant. The procedural error of such measurements does not exceed 0.3% at a surface emissivity approximately equal to 0.4. This property was revealed experimentally (1) and later confirmed theo- retically (2, 3). Measurement of the temperature of an object by the photoemission method from the spectra of photoelec- trons released by thermal radiation is based on the dependence of the energy distribution of photoelectrons of the ex- ternal photoeffect on the energy distribution in the object's radiation spectrum. In this case the gas of photoelectrons near the emitting body surface is the thermometric substance, and its temperature can be determined from the cut-off voltage of the volt-ampere characteristics of a photoelectronic device, for example, a photomultiplier for light fluxes normalized by the photocurrent (1) or by the change in the energy distribution of photoelectrons as a function of temperature (2, 3). As the body temperature increases, the maximum of the spectral energy distribution of its radiation is dis- placed to the side of short wavelengths (the Wien displacement), with the maximum of energy distribution of photo- electrons being displaced to the side of high energies. In this case, the body temperature is determined by the parameter k, which is the ratio of the photocurrent I 0 measured in the absence of the retarding field to the partially blocked photocurrent I bl measured in the presence of the retarding field in the cathode region of the photomultiplier. Since the relative content of electrons of different energies in this region is independent of the magnitude of the light flux arriving at the device, within the boundaries of linearity of its light characteristic k = f(T) (3). The process of measuring the body temperature is reduced to the recording of a two-level oscillogram of the amplitude of the voltage pulse at the photomultiplier anode U(t) in the process of electron flux modulation in the cath- ode region of the photomultiplier by rectangular pulses that are negative relative to the photocathode. Using these lev- els of the photomultiplier signal, the function k(T) is calculated in each period of the modulated signal, and using the calibrating function T(k) obtainable with the use of the standard thermal radiation, one determines the dynamics of the temperature of the investigated body T(t). The simplicity of the electron flow modulation in the cathode region of the photomultiplier with a frequency of 1 MHz allows one to make such measurement with a time resolution of 1 μs. The possibilities of the indicated method in measuring the temperature of fast thermal processes is illustrated below using several examples. Using an FMK-01 pulse photoemission pyrometer, we measured the temperature of sev-

Surface analysis correlated with the Raman measurements of a femtosecond laser irradiated Ca F2

Ultra fast electronic and thermal processes for energy deposition mechanism during femtosecond laser ablation of CaF2 have been identified by means of Atomic Force Microscopy (AFM) and Raman scattering technique. For this purpose, a single crystal CaF2 (111) was exposed with 800nm, 25fs Ti: Sapphire laser pulses at different laser fluences both in air and under UHV condition. Various nano and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Around the ablation threshold fluence, most significant features identified at the CaF2 surface are nanohillock like structures. These nanohillocks are typical features related to the fast electronic processes and are explainable on the basis of Coulomb explosion. At moderate irradiation fluence, bump formation is considered to be due to ultrafast melting. Further increase in the laser fluence resulted into strong enhancement of the thermal process with the appearance of larger humps and craters. These fast electronic and thermal processes are well correlated with the structural and crystallographic alterations inferred from Raman spectroscopy analysis. The nanohillocks appearing at a lower fluence are due to calcium colloid formation (aggregates of metal clusters). At higher fluences and dozes, the compressive as well as tensile stresses along with the presence of calcium carbonate are associated to diffusion, transformation and aggregation of defects which are typical features of thermal processes leading to the growth of larger humps.

The Photoreactions of Recombinant Phytochrome CphA from the Cyanobacterium Calothrix PCC7601: A Low‐Temperature UV–Vis and FTIR Study

The photoreactions of recombinant phytochrome CphA from cyanobacterium Calothrix sp. PCC7601 reconstituted with phycocyanobilin were investigated using UV-Vis and Fourier transform infrared (FTIR) difference spectroscopy, stabilizing intermediates at low temperature. The yield of the forward reaction strongly depends on temperature, unlike the backward reaction. Because of the very fast thermal relaxation processes in the Pr to Pfr pathway, no pure difference spectra of the Pr photoconversion products could be directly measured. Thus, the contribution of the Pfr:Pr pathway was taken into account by applying an appropriate correction procedure both in the UV-Vis and FTIR experiments. Three intermediates have been trapped at -25, -45 and -120 degrees C, which show the characteristic vibrational band pattern of the plant phytochrome phyA intermediates meta-Rc, meta-Ra and lumi-R, respectively. In the backward reaction, two intermediates corresponding to meta-F and lumi-F were trapped at -70 and -140 degrees C, respectively. FTIR spectra of all intermediates, as well as of the Pfr state, show remarkable similarities with the corresponding spectra of Cph1 phytochrome from cyanobacterium Synechocystis and the 59 kDa N-terminal fragment of Cph1, and, albeit not so pronounced, also with plant phyA. The spectral similarities and differences between the various phytochromes are discussed in terms of structural changes of the chromophore and the chromophore-protein interactions.

Fast electronic and thermal processes in femtosecond laser ablation of Au

Velocity distribution, pulse width dependence studies, and two-pulse correlation measurements have been used to study the possibility of the occurrence of ultrafast electronic and thermal ablation processes in Au exposed to ultrashort laser pulses in the femtosecond to picosecond time domain. Three distinct different velocity groups (5.5, 1.5, and 0.25eV) have been observed and can be attributed to two ultrafast electronic processes (Coulomb explosion and rapid plasma formation) and a thermal process. The buildup of a rapid plasma favors the laser energy absorption around 400fs after the beginning of the laser-matter interaction.

The Scaling Effect on the Thermal Processes at Mini/Microscale

The present work discusses the scaling effect on steady and unsteady thermal processes at mini/microscale that behave differently from the bulk case. Fast transient thermal processes could not be realized at bulk scale because of large thermal inertia and seldom have applications for practical use. However, due to the development of state-of-the-art technology, the miniaturization of device size makes it possible to utilize thermal responses due to the significantly reduced thermal inertia. For the steady case, the thermal effect varies with the boundary conditions and flow parameters because the relative importance of different forces changes with size at mini/microscale. Thus, the thermal phenomena are different from the conventional cases and vary with size.

Method of rapid (100 000 K s−1) controlled cooling and heating of thin samples

A method of rapid controlled cooling and heating of thin samples is developed. The apparatus comprises of a thin-film resistor embedded in a membrane and a sample placed on it, a narrow gap between the membrane and a heat sink, and a control circuit. The resistor acts both as a heater and as a temperature sensor to reduce time constant of the control circuit. The gap between the membrane and the heat sink is filled with gas (e.g., N2 or He) acting as cooling medium with low thermal inertia. The control circuit applies given rate of temperature changes and measures the power, released on the heater, to determine sample thermal properties such as heat capacity, exothermal/endothermal heat flow, etc. The method allows realizing fast (100000Ks−1) controlled (e.g., linear) cooling and heating rates regardless of thermal events in a sample, and fast (∼10μs) switching between ramps and isotherms. Beside calorimetry, the method can be used in a variety of applications, requiring rapid controlled temperature changes of a sample, and in studying of fast thermal processes.

Fast electromagnetic balance

Instead of reading the equilibrium value, the deflection of the balance as a function time can be measured and the equation of motion \(T = J\ddot \alpha + k\dot \alpha + C\alpha \) can be used to calculate the unknown torque T and to relate the other quantities in the equation to the actual instrument-constants. In this way, balance reading could be much faster and weighing errors due to faulty instrument and environmental influences can be smaller than those in equilibrium position. This enables the use of microbalances for the observation of fast chemical or thermal processes and to use it as fast checkweigher for control of sorting machines. In the present paper we present results from calculations of a simulation procedure.

An Infrared Diagnosis Channel for an Electronuclear Installation

A data-acquisition system is described for use with fast thermal processes in an electronuclear installation. The diagnostic channel consists of an optical system, coordinate-sensitive detector, and electronic units for amplifying, processing, and managing the data and interfacing with the computer. The data readout works under the real-time disk operating system.

Multichannel system for fast measurement of temperatures at 1-4.2K

The report presents a set of electronics designed for investigation of fast thermal processes taking place in He I and He II. Its main features are as follows. First, temperature field is measured at several distanced points in short time. This is useful, for example, while dealing with second-sound shock waves, superfluid turbulence dynamics, etc. Second, minimal wiring is sufficient for multipoint temperature measurement what is principal, especially, in the case of forced flow cryostats. Third, the measurement procedure allows to achieve rather high precision with the use of rather low-sensitive Ge thermometers. Warm and cold tests have shown the system efficiency and reliability.

The use of pyrometry in the study of fast thermal processes involving initially solid samples

The theoretical background relevant to the use of pyrometry in the study of fast processes involving rapid temperature changes initially solid samples is outlined. Pyrometers of various kinds are described briefly and their applications in the study of pyrotechnic reactions, schock-wave research, thermal imaging and the measurement of thermophysical properties are reviewed.

Chemical reactions at solid-solid interfaces induced by pulsed laser annealing

Al thin films (thickness < 1000 Å) evaporated on the (111) face of an Sb single crystal, and Al-Sb multilayer thin films (thickness < 1000 Å) evaporated on glass and quartz substrates were irradiated in UHV by the pulsed beam of an excimer laser. The samples were characterized in-situ during the growth of the films and after the irradiations by Auger and photoelectron spectroscopies without breaking the vacuum, and later, by scanning electron microscopy. The formation of the AlSb compound is observed above the fluence threshold previously reported. Lateral transport of matter and inhomogeneous reaction are observed after irradiation of the Al films deposited on Sb crystals, leading to the formation of an inhomogeneous surface with holes and particles. A strong chemical reaction between the substrate and the films also leads to a partial oxidation of the film evaporated on quartz or glass. We observed the same behaviour under the IR laser beam from a Q-switched Nd:YAG laser. One has taken advantage of fast thermal processes of thin films induced by pulsed or scanned CW laser beams for applications like synthesis of compounds by laser alloying, oxidation of thin films, planarization, and phase changes in optical data storage media. The relevant parameters needed to predict the evolution of the film/substrate interface under strong laser irradiation using the thermal model are not only: (i) the thermal properties of the substrate, and (ii) the optical characteristics of the film-substrate system, but also (iii) the chemical stability of the interface, and (iv) the free surface energies of the substrate and of the film, at least above the melting fluence threshold or if thermal cycling is expected over large periods of time.