Temperature Modulation Frequency Research Articles

Forced Convection during Cooling of Power Supply Box using Pulsation Flow with Piezoelectric Fan

AbstractForced convection flow of air through power supply box with piezoelectric fan of nine identical electrical devices has been numerically investigated. This work simulates a practical situation in which periodic temperature is generated in an electrical device as a result of its periodic input voltage. The finite volume method has been adopted to solve the governing equations. The effects of important parameters such as pulsation flow frequencies and temperature modulation frequency on heat transfer rates were investigated in details for Re = 500 corresponding to velocity of 0.39 m/s. The piezoelectric vibration amplitude is fixed at 0.2 and frequencies are varied in the range of [0–3] corresponding to [0–287] Hz. We have found that the time averaged Nusselt number for each electrical device depends on the pulsation frequency and heat modulation, and it is always larger than that in the steady‐state case. The results show that there exists a short band of frequencies which the enhancement of heat transfer of all electrical devices is higher than 55% compared with that of steady state. In addition, the heat transfer is maximum when heated pulsation frequency in the device is set as St = 3.0 corresponding to frequency of 58.5 Hz of input voltage. The combined pulsation flow by piezoelectric fan and heated pulsation by input voltage show vigorous heat transfer through electrical devices blocks and the heat transfer enhancement was observed in relatively narrow bands of frequencies. The efficiency of piezoelectric fan is expressed by the maximum temperature of the ninth electrical device which not exceed 80°C for time‐averaged velocity of 0.556 m/s and vibration frequency of 34.75 Hz of piezoelectric fan. The pressure drop and power pumping of piezoelectric fan are obtained for different Reynolds numbers. We found that the power loss was 4.6 times higher than for the piezoelectric fan at Re = 500. Good agreement between our numerical simulations of piezoelectric fan and experimental results available from the literature is obtained. © 2023 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Dynamic polarization response of polarization-maintaining fibers by periodic thermal cycling method.

We report a periodic thermal cycling method to investigate the dynamic response of the polarization of a laser propagating through polarization-maintaining (PM) optical fiber, driven by periodic weak temperature modulation. Consequently, temperature modulation on the surface of the coating material of the PM fiber was found to cause a continuous periodic change in the polarization state of the output laser without approaching the steady state of the resulting dynamic polarization response. Additionally, the response was found to depend on the temperature-modulation frequency and amplitude. These experimental results are qualitatively in good agreement with that of the simple theoretical model. Our research would be applied not only to the method of measuring properties of a PM optical fiber by utilizing the continuous modulation of the differential refractive index with a wide modulation-frequency range but also to various applications of the dynamic control of the periodic refractive index in materials.

Realization of dynamic thermal emission control.

Thermal emission in the infrared range is important in various fields of research, including chemistry, medicine and atmospheric science. Recently, the possibility of controlling thermal emission based on wavelength-scale optical structures has been intensively investigated with a view towards a new generation of thermal emission devices. However, all demonstrations so far have involved the 'static' control of thermal emission; high-speed modulation of thermal emission has proved difficult to achieve because the intensity of thermal emission from an object is usually determined by its temperature, and the frequency of temperature modulation is limited to 10-100 Hz even when the thermal mass of the object is small. Here, we experimentally demonstrate the dynamic control of thermal emission via the control of emissivity (absorptivity), at a speed four orders of magnitude faster than is possible using the conventional temperature-modulation method. Our approach is based on the dynamic control of intersubband absorption in n-type quantum wells, which is enhanced by an optical resonant mode in a photonic crystal slab. The extraction of electrical carriers from the quantum wells leads to an immediate change in emissivity from 0.74 to 0.24 at the resonant wavelength while maintaining much lower emissivity at all other wavelengths.

High-resolution thermal imaging with a combination of nano-focus X-ray diffraction and ultra-fast chip calorimetry

A microelectromechanical-systems-based calorimeter designed for use on a synchrotron nano-focused X-ray beamline is described. This instrument allows quantitative DC and AC calorimetric measurements over a broad range of heating/cooling rates (≤100000 K s(-1)) and temperature modulation frequencies (≤1 kHz). The calorimeter was used for high-resolution thermal imaging of nanogram-sized samples subjected to X-ray-induced heating. For a 46 ng indium particle, the measured temperature rise reaches ∼0.2 K, and is directly correlated to the X-ray absorption. Thermal imaging can be useful for studies of heterogeneous materials exhibiting physical and/or chemical transformations. Moreover, the technique can be extended to three-dimensional thermal nanotomography.

Phase-controlled superconducting heat-flux quantum modulator

We theoretically put forward the concept of a phase-controlled superconducting heat-flux quantum modulator. Its operation relies on phase-dependent heat current predicted to occur in temperature-biased Josephson tunnel junctions. The device behavior is investigated as a function of temperature bias across the junctions, bath temperature, and junctions asymmetry as well. In a realistic Al-based setup the structure could provide temperature modulation amplitudes up to ∼50 mK with flux-to-temperature transfer coefficients exceeding ∼125 mK/Φ0 below 1 K, and temperature modulation frequency of the order of a few MHz. The proposed structure appears as a promising building-block for the implementation of caloritronic devices operating at cryogenic temperatures.

Multifrequency Interrogation of Nanostructured Gas Sensor Arrays: A Tool for Analyzing Response Kinetics

This paper presents a unique perspective on enhancing the physicochemical mechanisms of two distinct highly sensitive nanostructured metal oxide micro hot plate gas sensors by utilizing an innovative multifrequency interrogation method. The two types of sensors evaluated here employ an identical silicon transducer geometry but with a different morphological structure of the sensitive film. While the first sensing film consists of self-ordered tungsten oxide nanodots, limiting the response kinetics of the sensor-chemical species pair only to the reaction phenomena occurring at the sensitive film surface, the second modality is a three-dimensional array of tungsten oxide nanotubes, which in turn involves both the diffusion and adsorption of the gas during its reaction kinetics with the sensitive film itself. By utilizing the proposed multifrequency interrogation methodology, we demonstrate that the optimal temperature modulation frequencies employed for the nanotubes-based sensors to selectively detect hydrogen, carbon monoxide, ethanol, and dimethyl methyl phosphonate (DMMP) are significantly higher than those utilized for the nanodot-based sensors. This finding helps understand better the amelioration in selectivity that temperature modulation of metal oxides brings about, and, most importantly, it sets the grounds for the nanoengineering of gas-sensitive films to better exploit their practical usage.

Cross-plane thermal diffusivity measurement of an operating vertical cavity surface emitting laser using thermoreflectance

Cross-plane thermal diffusivity of a GaAs/AlGaAs-based vertical cavity surface emitting laser (VCSEL) is measured under operating conditions and active heat sinking using a thermoreflectance technique. Perpendicular thermal diffusivity is determined from the measurements of phase difference between the heating source and the temperature as a function of temperature modulation frequency. The value of (1.22 ± 0.23) × 10-6 m2/s is obtained for thermal diffusivity, which is of the same order as previous values obtained on unbiased VCSEL structures. This is 10−14 times smaller than the corresponding bulk value. The reduction is attributed to the increase in phonon-boundary scattering in the multilayer structure of the VCSEL.

Relaxation dynamics of glass transition in PMMA + SWCNT composites by temperature-modulated DSC

The experimental technique offered by temperature-modulated differential scanning calorimeter (TMDSC) used to investigate the thermal relaxation dynamics through the glass transition as a function of frequency was studied for pure PMMA and PMMA-single wall carbon nanotubes (SWCNTs) composites. A strong dependence of the temperature dependence peak in the imaginary part of complex heat capacity (Tmax) is found during the transition from the glass-like to the liquid-like region. The frequency dependence of Tmax of the imaginary part of heat capacity (Cp) is described by Arrhenius law. The activation energy obtained from the fitting shows increases while the characteristic relaxation time decreases with increasing mass fraction (ϕm) of SWCNTs. The dynamics of the composites during glass transition, at slow and high scan rates, are also the main focus of this experimental study. The change in enthalpy during heating and cooling is also reported as a function of scan rate and frequency of temperature modulation. The glass transition temperature (Tg) shows increases with increasing frequency of temperature modulation and ϕm of SWCNTs inside the polymer host. Experimental results show that Tg is higher at higher scan rates but as the frequency of temperature modulation increases, the Tg values of different scan rates coincide with each other and alter the scan rate dependence. From the imaginary part of heat capacity, it is obvious that Tmax is not the actual glass transition temperature of pure polymer but Tmax and Tg values can be superimposed when ϕm increases in the polymer host or when the sample undergoes a transition with a certain frequency of temperature modulation.

Characterization of explosives traces by the Nanocalorimetry

Thermal characterization of energetic materials is often a non-trivial task. Experiments on such materials employing differential scanning calorimetry (DSC) have been mainly performed using special high-pressure crucibles, in which the pressure uncontrollably changes during the measurement. Conducting constant pressure experiments would allow addressing the decomposition reaction kinetics in a more quantitative way. In addition, the explosives detection requires sensing much smaller amounts of samples than those used in the DSC measurements. Explosives in the state of traces typically provide the sample sizes in the nano-gram to pico-gram range. The present work has been carried out with the Nanocalorimeter ( www.nanotlab.com), which can safely measure nano-gram-size samples. The Nanocalorimeter is operational in both DC and AC modes. In the DC mode, the device can perform heating ramps at ultra-fast heating rates (10 3–10 6 K/s) that are up to 1 million times faster than the conventional DSC. In the AC mode, which is analogous to the Modulated-DSC, the interval of the temperature modulation frequencies is also unmatched: the upper frequency bound reaches 3.0–10.0 kHz. Similar to the experimental setups described in the literature, the Nanocalorimeter employs a commercial gas sensor with integrated heating elements and thermocouples. In this work, a series of energetic materials of practical interest such as hexogen (cyclotrimethylenetrinitramine) and pentrite are characterized for the first time using ultra-fast heatings. The samples have been deposited on the sensor membrane using either micro-manipulation or spin-coating. Coupling of the Nanocalorimeter to a fast CCD camera was found quite useful to simultaneously visualize the processes occurring on a micro-second time-scale.

Temperature-modulated direct thermoelectric gas sensors: thermal modeling and results for fast hydrocarbon sensors

Direct thermoelectric gas sensors are a promising alternative to conductometric gas sensors. For accurate results, a temperature modulation technique in combination with a regression analysis is advantageous. However, the thermal time constant of screen-printed sensors is quite large. As a result, up to now the temperature modulation frequency (20 mHz) has been too low and the corresponding principle-related response time (50 s) has been too high for many applications. With a special design, respecting the physical properties of thermal waves and the use of signal processing similar to a lock-in-amplifier, it is possible to achieve response times of about 1 s. As a result, direct thermoelectric gas sensors with SnO2 as a gas-sensitive material respond fast and are reproducible to the propane concentration in the ambient atmosphere. Due to the path-independent behavior of the thermovoltage and the temperature, the measured thermopower of two sensors is almost identical.

Relaxation during polymerization on slow heating and the vibrational heat capacity of the polymers

The real and imaginary components of the complex heat capacity, C(p) (') and C(p) ("), and C(p,app) have been measured in real time during the linear chain polymerization on 12 K/h heating of six different (partially) polymerized states of a stoichiometric mixture of cyclohexylamine and diglycidyl ether of bisphenol A. Their C(p,app) shows a sigmoid shape rise with different onset temperatures T(onset), which is followed by a deep exotherm as the viscosity decreases and further polymerization occurs at different rates. The rates of their enthalpy decrease on polymerization determined by subtracting C(p) (') from C(p,app) differ but C(p) (') and C(p,app) of their final states are the same. The relaxation time increases with polymerization and decreases with an increase in T. C(p) (') rises in a sigmoid shape manner, and C(p) (") shows a peak when the relaxation time of the polymerized state is equal to the inverse of the temperature modulation frequency, whether polymerization occurs or not. The unrelaxed or vibrational heat capacity C(p,vib) of the polymers at T>T(onset) is close to C(p) of their glassy state at T<T(onset), showing that C(p) difference between the equilibrium liquid and its glass is mostly configurational. This contradicts a calculation showing that C(p,vib) change of a polymer at T(g) is generally approximately 20% of the total C(p) change.

Infrared emissivity measurement device: principle and applications

Infrared emissivity is a necessary parameter for computing models to predict road surface status and pavement temperature irrespective of the weather situation. In this work a new experimental device based on the indirect method was developed for the measurement of surface emissivities. The surface is exposed to modulated isotropic infrared radiation. The intensity reflected by the surface of the sample in a given direction is measured by a detector operating in the spectral range 1–40 µm. This large spectral range allows measurement of the total hemispheric emissivity defined in this case as emissivity. The effect of the temperature modulation frequency, surface composition and surface roughness on the emissivity measurements was investigated. The results show good ability of the device for the determination of emissivities at room temperature.

An unusual behavior in the melting region of isotactic polypropylene crystals revealed by temperature-modulated DSC

We present a new method to analyze irreversible transformation kinetics of melting in polymer crystals with temperature modulated differential scanning calorimetry (TMDSC). In the melting region of several polymers, the apparent heat capacity obtained with TMDSC can be expressed as Cs + (|Fmelt|/β)/(1 + i ω τ (β)), with the true heat capacity,Cs, the endothermic heat flow of melting,Fmelt, the angular frequency of temperature modulation, ω, and the mean time of melting of each crystallite, τ, depending on the underlying linear heating rate, β. In the case of isotactic polypropylene, the frequency dependence cannot be approximated by this formula. The dependence suggests the possibility of the retardation in the melting kinetics to follow temperature modulation.

The development of gas-flow modulation for high-frequency MTDSC measurements

The purpose of this study was to investigate the feasibility of modulating the temperature programme of a conventional DSC by use of an alternating gas-flow system. Modulated temperature differential scanning calorimetry (MTDSC) is an important thermal analysis technique but suffers from a limited applicable frequency range due to the mass of the sample and DSC cell leading to the impingement of thermal conductivity effects. We suggest that the frequency limit can be increased by replacing the cell as the source of temperature modulation with an external gaseous source, directed towards the sample and reference pans. In this evaluation, an alternating gas-flow was passed through a line to a forced gas-flow accessory (FGFA). The FGFA consisted of two matched cylinders containing chambers that allowed pre-temperature-equilibration of the stream of gas before it was passed over the sample and reference pans. The development of this device revealed the essential practical requirements of gas-flow modulation for high-frequency temperature modulation. These include the following: an appropriately sealed tunable gas supply to both sample and reference pans, an effective method for high-frequency cycling of the gas-flow rate, a small aperture to deliver the flowing gas directly over the pan and a temperature equilibration chamber. The results from samples of quenched PET and amorphous Saquinavir indicate that gas-flow modulation is indeed feasible, with the FGFA able to raise the attainable temperature modulation frequency by an order of magnitude compared to conventional MTDSC.

Novel aspects of micro-thermal analysis of polymer blends

In “ac” scanning thermal microscopy, an “active” thermal probe is used also as a heater, so as to inject evanescent temperature waves into a sample and to allow sub-surface imaging of polymers and other materials [1]. The sub-surface detail detected corresponds to variations in heat capacity or thermal conductivity. By suitably choosing the temperature modulation frequency, and hence the penetration depth of the wave, we control the depth of material below the sample surface that is contributing to the image.Micro-Thermal Analysis [2, 3] builds upon this technique, in order to add spatial resolution to two well-established methods of chemical fingerprinting, DTA and DMA. In both cases, a temperature ramp is used to subject the sample to “events” such as a glass transition or melting. The chief advantages of using the active thermal probe to provide the temperature ramp as well as the modulation, without the use of a heating stage, are: (a) the data are obtained from localised regions chosen from a previously-obtained thermal image, (b) apart from these regions, the rest of the sample is preserved in its original unheated state.

Effects of Modulated Differential Scanning Calorimetry (MDSC) Variables on Thermodynamic and Kinetic Characteristics During Gelatinization of Waxy Rice Starch

ABSTRACTThe total, reversing, and nonreversing thermal properties during gelatinization of waxy rice starch (starch‐to‐water ratio = 1:2, w/w) were examined by modulated differential scanning calorimetry (MDSC). The effect of MDSC operating variables (i.e., the amplitude and frequency of temperature modulation and the underlying heating rate) on these thermal properties was determined by response surface methodology (RSM) and statistical analysis. The frequency of temperature modulation and the underlying heating rate significantly influenced the gelatinization temperatures and enthalpy changes in total and nonreversing endotherms. In addition, the combination of 0.025Hz and 4–8°C/min with a properly low degree of oscillation was suitable for characterization of starch gelatinization by MDSC. The enthalpy changes in the reversing (thermodynamic) endotherms increased, but those in the nonreversing (kinetic) endotherms decreased with increasing periods (i.e., decreasing frequencies) and underlying heating rates. However, the total enthalpy changes were only slightly influenced by the MDSC variables studied. In addition, the activation energies for the total and nonreversing events were 281.8–417.3 and 386.3–739.7 kJ/mol, respectively, depending on the MDSC conditions. From the compensation relationship between the activation energy and frequency factor, we concluded that the total and nonreversing endotherms were linked to the same single transition.

Frequency domain calorimetry to study phase transitions and relaxation processes in condensed matter

This paper describes a modulated scanning calorimeter for studying, both in the isothermal and in the temperature scanning mode, liquids and solids that are chemically stable or undergoing structural transformations. The calorimeter concurrently measures, at a temperature modulation frequency, the complex heat capacity C ∗ p and the enthalpy change during chemical reactions or phase transitions. The time and temperature variation of the real and the imaginary parts of C ∗ p provide information on the molecular relaxation processes and the mutual influence between structure and dynamics in highly viscous samples as well as the glass transition. The principles and main features of the instrument and a set of experimental tests performed are described and accuracy, sensitivity, temporal stability and reproducibility of the data are given.

Enthalpy relaxation and glass transition behaviour of sucrose by static and dynamic DSC

The relaxation process of the sucrose in the glassy state and its glass transition are studied by differential scanning calorimetry in both the conventional and the dynamic mode. The data on the kinetics of glassy relaxation are experimentally evaluated by measuring the enthalpy relaxation peaks as a function of the ageing times at three different ageing temperatures, namely 45, 55 and 60°C. It is shown that the ageing process is a complex phenomenon with a rather large distribution of relaxation times. The value of the breath of the distribution of relaxation processes, β, is 0.32 and it is in the lower range of the distributions commonly observed. The dynamic components of the heat capacity ( C′ p and C″ p) have been obtained from the experiments carried out at different frequency of temperature modulation ( ν=8.3−83 mHz). These components are unaffected by the ageing process and therefore are more reliable for the correct T g determination. The activation energies for the relaxation process and for the glass transition process are also evaluated.

Heat capacity measurement by modulated DSC at constant temperature

The mathematical equations for step-wise measurement of heat capacity (C p ) by modulated differential scanning calorimetry (MDSC) are discussed for the conditions of negligible temperature gradients within sample and reference. Using a commercial MDSC, applications are evaluated and the limits explored. This new technique permits the determination ofC p by keeping the sample continually close to equilibrium, a condition conventional DSC is unable to meet. Heat capacity is measured at ‘practically isothermal condition’ (often changing not more than ±1 K). The method provides data with good precision. The effects of sample mass, amplitude and frequency of temperature modulation were studied and methods for optimizing the instrument are proposed. The correction for the differences in sample and reference heating rates, needed for high-precision data by standard DSC, do not apply for this method.