Thermal Penetration Depth Research Articles

Discrimination of vibrational modes in Ni2MnGa thin films

Vibrational properties in magnetic shape memory alloys are intrinsically connected to the topologies formed due to temperature-dependent breakdown of symmetry. In this sense, herein, we apply Raman spectroscopy in the study of Ni2MnGa films prepared by sputtering on glass and gallium arsenide substrates. Different laser wavelengths probe the Raman active vibrational modes. The non-equilibrium heating mechanisms of the electronic lattice induced by excitation laser power within the thermal penetration depth are used to explain the formation of higher order vibrational modes resulting from phonon-assisted intra-band electronic excitations. The discrimination of the Raman active modes is performed based on calculations carried out within the framework of the density functional theory considering the coexistence of austenite and martensite phases as a function of temperature in films with different crystalline textures and residual stresses. More importantly, this work presents a simple way to discriminate the vibrational modes of the austenite and martensite phases in Ni2MnGa thin films that can be easily applied in the nanodevice development process.

Measurements of Thermal Resistance Across Buried Interfaces with Frequency-Domain Thermoreflectance and Microscale Confinement.

Confined geometries are used to increase measurement sensitivity to thermal boundary resistance at buried SiO2 interfaces with frequency-domain thermoreflectance (FDTR). We show that radial confinement of the transducer film and additional underlying material layers prevents heat from spreading and increases the thermal penetration depth of the thermal wave. Parametric analyses are performed with finite element methods and used to examine the extent to which the thermal penetration depth increases as a function of a material's effective thermal resistance and the degree of material confinement relative to the pump beam diameter. To our surprise, results suggest that the measurement technique is not always the most sensitive to the largest thermal resistor in a multilayer material. We also find that increasing the degree to which a material is confined improves measurement sensitivity to the thermal resistance across material interfaces that are buried 10s of μm to mm below the surface. These results are used to design experimental measurements of etched, 200 nm thick SiO2 films deposited on Al2O3 substrates, and offer an opportunity for thermal scientists and engineers to characterize the thermal resistance across a broader range of material interfaces within electronic device architectures that have historically been difficult to access via experiment.

Anomalously strong size effect on thermal conductivity of diamond microparticles

Diamond has the known highest thermal conductivity of around 2000 W m−1 K−1 and is, therefore, widely used for heat dissipation. In practical applications, synthetic diamond microparticles are usually assumed to have similar thermal conductivity to that of bulk diamond because the particle size is larger than the theoretical phonon mean free path, so that boundary scattering of heat-carrying phonons is absent. In this report, we find that the thermal conductivity of diamond microparticles anomalously depends on their sizes. The thermal conductivity of diamond microparticles increases from 400 to 2000 W m−1 K−1 with the size growing from 20 to 300 μm. We attribute the abnormally strong size effect to the long-range defects during the growth process based on analysis of point defects, dislocations, and thermal penetration depth dependence of thermal conductivity. Our results play a vital role in the design of diamond composites and in the improvement of the thermal conductivity of synthetic diamonds.

Theoretical research on high-efficiency region in cryogenic micro flow heat transfer based on thermal and viscous Knudsen numbers

Cryogenic micro flow heat transfer (CMFHT) has been increasingly interest nowadays due to the urgent needs of superconductive technology, quantum engineering, space cryocooler and other frontier fields. Chasing its high-efficiency region is a key challenge as thermal penetration depth is limited in cryogenic microscale. Therefore, this paper focuses on high-efficiency region in CMFHT, and newly proposes two parameters: thermal and viscous Knudsen numbers (KnT and Knv) based on traditional Knudsen number Kn, which can improve the errors on micro measurement and the inconveniences on high-efficiency region analysis induced by Kn. High-efficiency region is modeled by the three Knudsen numbers and is separated into slip or continuum type bounded by Kn = 0.001. The effects of gas species, temperature and pressure on KnT and Knv are studied and it is found that the reductions of both thermal penetration depth and the range between thermal and viscous penetration depths with temperature dropping deteriorate heat transfer at low temperature. After applicability validations based on opened literatures, it is suggested that in cryogenics ensuring Kn>KnT (L<δT) is put in the first for good heat transfer, and the next is trying the best to ensure Kn<Knv (L>δv) or making Kn and Knv as close as possible.

Full-scale numerical simulations of standing-wave thermoacoustic engines with circular-pore and pin-array stacks

Thermoacoustic engines (TAEs) can convert thermal energy into acoustic energy with no moving parts. Previous numerical studies normally focused on the TAEs with a parallel-plate stack due to their simple structures and used two-dimensional (2-D) computational fluid dynamics (CFD) models to save computational costs. In this study, we conduct full-scale three-dimensional (3-D) CFD simulations on the standing-wave TAEs with more complicated circular-pore and pin-array stacks. Firstly, the dynamic behavior of the standing-wave TAEs in the start-up process is investigated. It is found that the optimal ratios of hydraulic radius rh to thermal penetration depth δk for the TAEs with circular-pore and pin-array stacks are 2 and 3.2, respectively. Secondly, the acoustic, hydrodynamic, and thermodynamic characteristics of the standing-wave TAEs in the steady-state process are explored. We find that when operating at optimal rh/δk, the TAE with a pin-array stack generates much larger acoustic power than that with a circular-pore stack. Examination of the vortex shedding at the stack ends indicates that the pin arrays exhibit less flow resistance than circular pores. At optimal rh/δk, the time-averaged transversal heat flux at the pin array ends is calculated to be around 1.2 × 105 W/m2, which is larger than 7.5 × 104 W/m2 of circular pores, corresponding to a better thermodynamic performance. The 3-D numerical investigations in this work give deeper insights into the performances of TAEs with circular-pore and pin-array stacks which were investigated in experiments, providing useful guidance for the future design and development of more sophisticated thermoacoustic devices for low-grade heat recovery.

Theoretical and experimental investigation on the thermal penetration depth in the surrounding rock of a deeply buried high-temperature tunnel

Deeply buried tunnels that traverse geothermal areas tend to experience elevated temperatures. It is crucial to accurately predict the thermal penetration layer within the rock to address thermal damage in high-temperature tunnels. However, accurately predicting the thermal penetration layer is complicated due to its dependence on ventilation parameters and the thermal properties of the surrounding rock. In this study, the analytical solution of the temperature field of the surrounding rock was derived via the Laplace transformation and then nondimensionalized. Based on this, the correlation equation of the dimensionless radius of the thermal penetration layer (RTPL) was developed for two ranges of Biot numbers: 1 ≤ Bi ≤ 10 and 10 < Bi ≤ 1000. The dimensionless RTPL was found to be a segmented function of Bi and Fo, with Bi = 10 serving as the boundary point. To validate the correlation equation of the dimensionless RTPL, a scaled model experiment was designed and conducted. The obtained results exhibit a remarkable level of agreement. Additionally, the correlation equation is applied to three practical engineering cases. This research provides a valuable theoretical foundation for effectively managing thermal damage in high-temperature tunnels.

Thermal conductivity of SiC and PyC coatings in spherical nuclear fuel particles measured by nanosecond time domain thermoreflectance

TRi-structural ISOtropic (TRISO) nuclear particle fuels are intended for application in several designs of advanced high temperature nuclear reactors. Nanosecond time domain transient thermoreflectance has been applied to the silicon carbide (SiC) and the inner and outer pyrolytic carbon (PyC) coatings of surrogate TRISO particles, measuring mean thermal conductivities of 166 ± 18 W∙m−1K−1, 11.1 ± 1.9 W∙m−1K−1, and 8.6 ± 2.1 W∙m−1K−1, respectively, for each layer. The applicability of the analytical heat diffusion model used to fit the measured traces was examined and it was found that the boundary effect at the SiC/PyC interface becomes perceptible at a lateral distance approximately equal to the thermal penetration depth of SiC (7.1 µm). In addition, steady-state thermal modelling showed that the radial thermal resistance of the particle is dominated by the porous buffer layer. The thermal resistance will likely increase significantly upon circumferential detachment of the porous buffer carbon coating with irradiation. Implications for fuel design and reactor performance are discussed.

Characterizing the mode transition features of a two-phase thermofluidic oscillator

Two-phase thermofluidic oscillator (TPTO) is a promising technology for harvesting low-grade heat. The multi-physics coupling within TPTO adds to the complex nonlinear dynamic characteristics that affect the operation, while not yet sufficiently analyzed. The mode transition features of a TPTO were experimentally characterized via nonlinear dynamic analysis. The bifurcations were studied with regards to the effect of temperature ratio and relative thermal penetration depth. Mass-spring and acoustic-electric analogy models were developed to predict the incommensurate resonant frequencies, which were then experimentally verified. The experimental results show that the TPTO experiences a series of bifurcations by increasing the temperature ratio, leading to the occurrence of oscillation modes such as the high-frequency limit cycle (HFLC) oscillation, quasiperiodic oscillation and low-frequency limit cycle (LFLC) oscillation. The high frequency in HFLC is approximately 3.1 Hz, which is the natural frequency of the TPTO. The low frequency close to 0.9 Hz in LFLC is the intrinsic frequency of the looped tube. The stability curves of the 2nd, 3rd, and 4th bifurcations show a trend of first growing and then dropping with the increasing relative thermal penetration depth of working fluid along the regenerator. The potential applications of TPTO are then discussed. This study offers insights for comprehending the mechanism of mode transition and provides recommendations for the design and operation of TPTO.

Transient simulations and theoretical modeling of near-junction heat conduction in GaN-on-diamond HEMT

Under high-power working conditions, accurate predictions of the maximum temperature and thermal resistance distribution in devices during transient states are essential for monitoring the health status of the devices. In this work, the heat conduction characteristics of GaN-on-diamond high electron mobility transistors (HEMTs) near the junction region under transient conditions with thermal impedance are systematically investigated with the aid of finite element simulations and thermal impedance analyses. The effects of pulse characteristics, thermal properties of different layers on the maximum temperature and thermal resistance of the device are discussed specifically. The results show that pulse characteristics have a significant effect on the maximum temperature of the device. The shorter the pulse switching time is, the higher the maximum temperature will be. Regarding the effects of the thickness of the GaN layer, the maximum temperature initially decreases and then increases, with the lowest maximum temperature observed at 1.7 μm. As the TBR increases uniformly from 10 to 40 m2K/GW, the temperature increases by approximately 4 % per 10 m2K/GW. The analyses on the thermal resistance show that the GaN layer occupies the largest proportion of thermal resistance, while the diamond substrate occupies the smallest. The optimal substrate thickness is obtained by analyzing the thermal penetration depth in the substrate for working conditions with various pulse widths. For diamond substrate, the optimal thickness at a pulse width of 5 μs is about 25 μm, and increases as the increase in pulse width. At the end, a modified Cauer RC model is proposed for predicting the transient maximum temperature based on heat spreading analysis. The difference between the maximum temperatures predicted by the model and the finite element simulations is within 8 %.

Application of the ESHO-QA guidelines for determining the performance of the LCA superficial hyperthermia heating system

Purpose This study aimed to assess the quality of the lucite cone applicator (LCA), the standard applicator for superficial hyperthermia at the Erasmus MC Cancer Institute, using the most recent quality assurance guidelines, thus verifying their feasibility. Materials and methods The assessment was conducted on each of the six LCAs available for clinical treatments. The temperature distribution was evaluated using an infrared camera across different layers of a fat-muscle mimicking phantom. The maximum temperature increase, thermal effective penetration depth (TEPD), and thermal effective field size (TEFS) were used as quality metrics. The experimental results were validated through comparison with simulated results, using a canonical phantom model and a realistic phantom model segmented from CT imaging. Results A maximum temperature increase above 6 °C at 2 cm depth in the fat-muscle phantom for all the experiments was found. A mean negative difference between simulated and experimental data was of 1.3 °C when using the canonical phantom model. This value decreased to a mean negative difference of 0.4 °C when using the realistic model. Simulated and measured TEPD showed good agreement for both in silico scenarios, while discrepancies were present for TEFS. Conclusions The LCAs passed all QA guidelines requirements for superficial hyperthermia delivery when used singularly or in an array configuration. A further characterization of parameters such as antenna efficiency and heat transfer coefficients would be beneficial for translating experimental results to simulated values. Implementing the QA guidelines was time-consuming and demanding, requiring careful preparation and correct setup of antenna elements.

Shape-Configurable MXene-Based Thermoacoustic Loudspeakers with Tunable Sound Directivity.

Film-type shape-configurable speakers with tunable sound directivity are in high demand for wearable electronics. Flexible, thin thermoacoustic (TA) loudspeakers-which are free from bulky vibrating diaphragms-show promise in this regard. However, configuring thin TA loudspeakers into arbitrary shapes is challenging because of their low sound pressure level (SPL) under mechanical deformations and low conformability to other surfaces. By carefully controlling the heat-capacity-per-unit-area and thermal effusivity of an MXene conductor and substrates, respectively, we fabricated an ultrathin MXene-based TA loudspeaker exhibiting high SPL output (74.5dB at 15kHz) and stable sound performance for 14 days. Loudspeakers with the parylene substrate, whose thickness was less than the thermal penetration depth, generated bidirectional and deformation-independent sound in bent, twisted, cylindrical, and stretched-kirigami configurations. Furthermore, we constructed parabolic and spherical versions of ultrathin, large-area (20cm × 20cm) MXene-based TA loudspeakers, which displayed sound-focusing and 3D omnidirectional-sound-generating attributes, respectively. This article is protected by copyright. All rights reserved.

Effect of channel size on the performance of a PCHE-based supercritical [formula omitted] thermoacoustic engine

Thermoacoustic engine is a promising energy conversion device. Recently, supercritical CO2 has been successfully employed as the working fluid in a standing-wave thermoacoustic engine operating under transcritical temperature conditions. The core components of the device are integrated as a printed circuit heat exchanger (PCHE), where the CO2 channel size plays a vital role in the performance of the device. In the present work, guided by approaching two to four times thermal penetration depth, a PCHE module with improved channel size is designed, fabricated, and tested. Results suggest improvements are achieved in pressure amplitude Δp (the highest record increases from 0.419 to 0.455 MPa) and onset temperature difference (the lowest record drops from 14.8 to 11.2 °C), which are attributed to the enhanced thermally-driven expansion and contraction during one oscillation period. This paper shows that the average thermal penetration depth is a notable variable generally positively correlated to the applied temperature difference. By studying the transition and saturation in operation mode observed in the experiments, it is revealed that three to four times the average thermal penetration depth is the best interval for the CO2 channel size. Beyond this interval, the power law relation between dimensionless pressure amplitude and the temperature difference holds. Below this interval, the saturation becomes so strong that the growth of pressure amplitude with temperature difference decelerates significantly.

Physics-based modelling and data-driven optimisation of a latent heat thermal energy storage system with corrugated fins

Solid-liquid phase transformation of a phase change material in a rectangular enclosure with corrugated fins is studied. Employing a physics-based model, the influence of fin length, thickness, and wave amplitude on the thermal and fluid flow fields is explored. Incorporating fins into thermal energy storage systems enhances the heat transfer surface area and thermal penetration depth, accelerating the melting process. Corrugated fins generate more flow perturbations than straight fins, improving the melting performance. Longer and thicker fins increase the melting rate, average temperature, and thermal energy storage capacity. However, the effect of fin thickness on the thermal characteristics seems insignificant. Larger fin wave amplitudes increase the heat transfer surface area but disrupt natural convection currents, slowing the melting front progress. A surrogate model based on an artificial neural network in conjunction with the particle swarm optimisation is developed to optimise the fin geometry. The optimised geometry demonstrates a 43% enhancement in thermal energy storage per unit mass compared to the case with planar fins. The data-driven model predicts the liquid fraction with less than 1% difference from the physics-based model. The proposed approach provides a comprehensive understanding of the system behaviour and facilitates the design of thermal energy storage systems.

Semi-Analytical Modeling of Pyrolysis Front and Ignition Temperatures of Thermally Reactive Single Solid Particles

A semi-analytical approach is developed for predicting pyrolysis front temperature in a charring solid undergoing thermal decomposition. The pre-reaction heating stage is described using an analytical formulation and invoking the concept of thermal penetration depth. The solution for the solid conversion stage accounts for decomposition enthalpy, the convective flow of volatiles, and a reaction front characterized by a uniform temperature that progresses toward the inner layers. This method incorporates empirical relations into the analytical model. Two scenarios are considered. First, the solution of the pyrolysis model combined with the data of conversion time versus external heat flux leads to an algebraic expression that reveals the existence of a maximum pyrolysis-front temperature. Explicit relations are derived for both the extremum pyrolysis temperature and optimum applied heat flux. In the second case, an expression is derived for the ignition temperature of a solid fuel (e.g., wood) by incorporating ignition delay time measurements into the heating stage model. The newly derived expression allows the ignition temperature to be described as a function of the Biot number and external heat flux. The relation obtained for the ignition temperature explains the experimental trends reported in some previous studies where two local extremums were observed for the ignition temperature in the absence of volatile reactions.

Noninvasive and Continuous Monitoring of the Core Body Temperature through the Quantitative Measurement of Blood Perfusion Rate.

Core body temperature (CBT) is one of the four vital signs that must be monitored continuously. The continuous recording of CBT is possible through invasive methods by inserting a temperature probe into specific body sites. We report a novel method to monitor CBT through the quantitative measurement of skin blood perfusion rate (ωb,skin). By monitoring the skin temperature, heat flux, and ωb,skin, the arterial blood temperature, equivalent to CBT, can be extracted. ωb,skin is quantitatively evaluated thermally via sinusoidal heating with regulated thermal penetration depth so that the blood perfusion rate is acquired only in the skin. Its quantification is significant because it indicates various physiological events including hyper- or hypothermia, tissue death, and delineation of tumors. A subject showed promising results with steady values of ωb,skin and CBT of 5.2 ± 1.05 × 10-4 s-1 and 36.51 ± 0.23 °C, respectively. For periods where the subject's actual CBT (axillary temperature) did not fall within the estimated range, the average deviation from the actual CBT was only 0.07 °C. This study aims to develop a competent methodology capable of continuously monitoring the CBT and blood perfusion rate at a distant location from the core body region for the diagnosis of a patient's health condition with wearable devices.

Near-field thermophotovoltaic energy conversion analysis based on enhanced radiative absorption distribution

Near-field radiation has been widely shown to greatly boost the electrical power of thermophotovoltaic (TPV) cells. However, there is a lack of theoretical analysis exploring the important influences of near-field effects on radiative absorption distributions as well as TPV energy conversion performances. This work investigates the electrical performances of near-field TPV cells made of InGaSb coupled with different practical emitters such as plain tungsten (W), indium tin oxide (ITO) film, and alternate W and alumina multilayer in detail. A comprehensive analysis is conducted to systematically compare the impacts of evanescent wave tunneling, surface plasmon resonance, and hyperbolic modes on spatial distributions of radiative absorption and the profiles of local carrier concentrations. The detailed and accurate analysis reveals the crucial role of near-field radiation emitted by various emitters in charge collection efficiency, thermal photon flux penetration depth, and photocurrent generation. Thus, the results certify that the electric power could be enhanced by utilizing ITO and multilayer emitters instead of a plain W emitter. The efficiency for an ITO emitter increases with decreasing vacuum gap owing to the suppressed bulk recombination but decreases when the vacuum gap falls below 18 nm due to increased surface recombination. While the efficiency for a multilayer emitter is comparatively lower due to the larger sub-bandgap photons and inefficient n-region. Furthermore, we verify the strategies for performance improvement via decreasing the surface recombination and optimizing the p-region thickness. The underlying mechanism is interpreted based on the spatial distribution and the collection efficiency of minority carriers.

Thermal modeling of a dual‐purposed snap biopsy acquisition procedure in a suspected cancerous lesion

AbstractThe needle‐based biopsy procedure is common in cancer detection and patient‐specific targeted therapy, wherein a tissue sample from the potential diseased site is acquired and frozen instantly with the help of a coolant medium. While liquid nitrogen (LN2) is the most widely used coolant for preserving the acquired sample and performing biopsy tests on the same at a later time, cold ischemia leads to inevitable cell degradation beyond a threshold time. In an effort to circumvent this challenge, here we aim to put forward the concept of an integrated biopsy sample acquisition and cryotherapy procedure, by incorporating an exclusively designed cooling circuit in a conventional biopsy‐needle for freezing the sample in vivo as soon as it is acquired, while causing cryoablation in the surrounding tissues simultaneously. An enthalpy‐based approach is employed to develop a bioheat transfer model for the cryotherapy design, with illustrative simulation data presented for breast cancer. Our model is demonstrated by considering a constant LN2 cooling temperature of 77.15 K, and cooling powers ranging from 2 to 10 W. The model results elucidate procedure‐specific insights such as the thermal penetration depth and the cooling time on being subjected to the cryoablation. The cooling rates thus obtained are further assessed from the simultaneous considerations of cryopreservation and cryosurgery, deriving critical insights on tissue survival and damage for acting as a precursor to patient‐specific treatment planning.

In-plane thermal diffusivity determination using beam-offset frequency-domain thermoreflectance with a one-dimensional optical heat source

We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experiments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically <100 kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a linear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer. We achieve this by an original approach based on patterning the transducer using focused ion beam, with the key purpose of limiting in-plane heat transport through the thin metallic transducer.

Evolution of the microstructure of superfine grain graphites under thermal oxidation

Nuclear graphite is used as a moderator and structural material in a number of Gen-IV reactor designs, such as the (Very) High Temperature Reactors. Chronic or acute thermal oxidation affects the mechanical properties of the graphite components and the impact on the structural integrity of the core depends on the evolution of the microstructure during oxidation.This study qualitatively examined the microstructure of four superfine grain graphites using polarised and fluorescent optical microscopy. These graphites had similar physical properties but notable differences in the oxidation rate at 700 °C. Using fluorescent microscopy, it was possible to distinguish between open and closed porosity in graphite, which is useful in understanding the progression of thermal oxidation and penetration depth. Based on the samples in this work, fine open and closed porosity may be linked to increased surface oxidation and relatively high oxidation rate, whereas high penetration depth may be linked to relatively low open porosity consisting mainly of large pores. Fairly large domains of coherent crystallite orientation, perhaps due to the presence of agglomerates, may also indicate lower oxidation rates at 700 °C.This paper demonstrated that fluorescent microscopy combined with image analysis, is a useful tool in understanding the progression of thermal oxidation and the corresponding effect on the microstructure.

Experimental study of the thermal probing depth of a skin calorimeter

In this work we perform an experimental study of the thermal probing depth of a skin calorimeter developed to measure in vivo the heat capacity and the thermal resistance of a 4 cm2 skin region. To determine these properties, a small thermal excitation is applied to the skin and then, the static and dynamic response of the calorimetric signal is studied. This excitation consists of a periodic change of the calorimeter thermostat temperature while the device is applied on the skin. We have found that the thermal penetration depth depends mainly on the period of this periodic thermal excitation. This time dependence is exponential for non-semi-infinite domains. Using a time–variant model, we performed measurements on the dorsal and volar areas of the left wrist of a healthy 64-year-old male subject. For a 4 cm2 region of skin, the results show that the heat capacity in both zones are of the same order of magnitude. Its initial value is 4 J/K and increases exponentially up to 7 J/K with a time constant of 6.5 min. Thermal resistance also increases exponentially with time: from 27 to 36 K/W for the volar zone, and from 32 to 45 K/W for the dorsal zone.