High Thermal Diffusivity Research Articles

Manufacturing of Ti-Nb-Cr-V-Ni-Al Refractory High-Entropy Alloys Using Direct Energy Deposition.

High-entropy alloys (HEAs) are composed of 5–35 at% of five or more elements, have high configurational entropy, do not form intermetallic compounds, and have a single-phase face-centered cubic structure or body-centered cubic structure. In particular, refractory HEAs (RHEAs), based on refractory materials with excellent mechanical properties at high temperatures, have high strength and hardness at room temperature and excellent mechanical properties at low and high temperatures. In this study, the Ti-Nb-Cr-V-Ni-Al RHEAs were deposited using direct energy deposition (DED). In the microstructure of Ti-Nb-Cr-V-Ni-Al, the sigma, BCC A2, and Ti2Ni phases appeared to be different from the BCC A2, BCC B2, and Laves phases predicted in the phase diagram. This microstructure was similar to that of the casted Ti-Nb-Cr-V-Ni-Al and had a constructed fine grain size. It was found that the growth of these microstructures was due to the DED process, which has a fast solidification rate. The fine grain size caused high hardness, and the microhardness of the Ti-Nb-Cr-V-Ni-Al was measured to be about 900 HV. In addition, in order to analyze the thermal properties of Ti-Nb-Cr-V-Ni-Al composed of the refractory material, the heat-affected zone (HAZ) was analyzed through a preheating test. The HAZ was decreased, owing to the high thermal diffusivity of Ti-Nb-Cr-V-Ni-Al.

Performance Evaluation of Natural Composites Made from Banyan and Cotton Fibers for Sustainable Thermal Insulation Applications

ABSTRACT Natural composites are gaining more attention due to the light weight and low cost characteristics. In the present investigation, an attempt has been made to fabricate the natural composites using natural fibers like Banyan and Cotton. Composites were made by compression molding technique and five types of composites were prepared, namely, B, BC(3:1), BC(1:1), BC(1:3) and C. Mechanical and thermal properties of the composites were tested. Results showed that Banyan composite exhibited the maximum tensile and flexural strength of 75 MPa and 113 MPa, respectively. This was attributed by the placing of high strength and stiffness Banyan fiber. Further, a maximum thermal resistance of 18.1 × 10−3 m2K/W was observed for Banyan composite. Moreover, the Banyan composite displayed the least coefficient of thermal expansion (CTE) of 1.81 × 10−5/°C which indicated the good dimensional stability of composite. Furthermore, a high thermal diffusivity of 8.79 × 10−7 m2/s was achieved for Banyan composites. Based on the experimental results, it is suggested that the Banyan composite could be a suitable candidate for sustainable thermal insulation applications.

Thermocapillary-enhanced Melting of Different Phase-change Materials in Microgravity

A numerical analysis of the thermocapillary-driven melting of phase change materials (PCMs) in weightlessness is presented. The phase change is explored for different PCMs with moderate melting temperatures, due to their potential for thermal control in space applications. We consider three different alkanes — n-octadecane, n-nonadecane, and n-eicosane — and gallium. Results are discussed in terms of the dimensionless Stefan (Ste) and Marangoni (Ma) numbers, which quantify the importance of the latent heat and the thermocapillary effect during the phase change process, respectively, and the container aspect ratio Gamma. For alkanes, similar results are obtained with melting rate enhancements that depend on Gamma. In short (deep) containers, the thermocapillary effect accelerates melting — with respect to the conduction-driven case — by a factor of as much as 4 depending on Ma, while in large (shallow) containers, this enhancement factor can take values up to 20. The best performance is featured by n-eicosane, followed closely by n-octadecane. For gallium, results differ substantially due to its high thermal diffusivity, leading to a significant reduction of the enhancement up to a value of approximately 1.2 at large Ma and Gamma.

Feasibility of wideband dielectric imaging to guide temperature-controlled atrial fibrillation ablation.

The DiamondTemp (DT) radiofrequency (RF) ablation catheter (Medtronic, Dublin, Ireland) has been introduced for atrial fibrillation (AF) ablation. It features 6 thermocouples at its tip, imbedded in a synthetic diamond material that offers high thermal diffusivity. The technology allows temperature-controlled ablation combining high ablation power levels with a direct temperature feedback consequently resulting in homogeneous atrial lesion formation.1 Recently, a novel wideband dielectric imaging system (KODEX-EPD, EPD Solutions, Philips, Amsterdam, The Netherlands) was introduced to guide catheter ablation of cardiac arrhythmias.

Modulated dilatometry as a tool for simultaneous study of vacancy formation and migration

Abstract A model is presented to derive both vacancy formation and migration characteristics from length change measurements upon modulated time-linear heating. The length variation with linear heating yields access to the equilibrium concentration of thermal vacancies. The modulation amplitude and the phase shift between modulated temperature and length change is determined by the ratio of equilibration rate and modulation frequency which yields access to the vacancy migration characteristics. The contribution from thermal lattice expansion is obtained from a reference measurement at high modulation frequencies. Compared to static isothermal equilibration measurements after temperature jumps, the processes are monitored under quasi-equilibrium conditions avoiding obstacles associated with fast temperature changes. Furthermore, in contrast to the static isothermal case where the equilibration rate is obtained from the time-exponential decay, its determination from the amplitude and phase shift of modulation offers higher precision. The method is suitable for materials with high thermal vacancy concentrations and low vacancy diffusivities, among which is the important class of intermetallic compounds with B2-structure.

A new strategy to obtain thin ZrO2–Al2O3 composite aerogel coating with prominent high–temperature resistance and rapid heat dissipation

High–temperature resistant aerogel coating usually prevents damage to walls and equipment in a harsh high–temperature environment, but these coating still has some disadvantages in practical application, such as short time of high–temperature resistance or low stability. In this work, ZrO2–Al2O3 composite aerogel coating with prominent high–temperature resistance and rapid heat dissipation was successfully fabricated by employing (Al(H2PO4)3, AHP) as high–temperature adhesive to form stable phosphates products. The thermal conductivity and thermal diffusivity of the obtained ZrO2–Al2O3 composite aerogel coating are 0.1602–0.1623 ​W/mK and 0.3488–0.4702 ​mm2/s, respectively. The resultant aerogel coating with a thickness of 1.4 ​cm showed good performance in heat resistance with tolerance of flame up to ∼1300oC for 15 ​min, and rapid heat dissipation that can be cooled to 32.7–38.4oC within 60–70 ​s. Phosphate products, such as AlPO4 and ZrP2O7, are mainly formed at high temperature with the introduction of AHP, which fundamentally improves the high–temperature resistance and flame retardant of aerogel coating. Different microstructure and pore structure are responsible for the heat dissipation performance of the aerogel coating, which shows high thermal diffusivity of 0.3488–0.4702 ​mm2/s. Overall, this work provides a new strategy for preparing aerogel coating with high–temperature resistance, flame retardant, thermal stability and rapid heat dissipation suitable for harsh conditions.

Numerical Study of the Thermal Performance of a Mems Pressure Sensor with Self-Calibration Capabilities.

Recent industry trends toward more complex and interconnected systems have increased the demand for more reliable pressure sensors. By integrating a microactuator with a pressure sensor, the sensor can self-calibrate, eliminating the complexities and costs associated with traditional sensor calibration methods to ensure reliability. The present work is focused on furthering understanding and improving the thermal performance of a thermopneumatic actuated self-calibrating pressure sensor. A transient numerical model was developed in ANSYS and was calibrated using experimental testing data. The numerical model provided insights into the sensor’s performance not previously observed in experimental testing. Furthermore, the model was utilized for two design studies. First, it was found that a substrate with low thermal conductivity and high thermal diffusivity is ideal for both the sensor’s efficiency and a faster transient response time. The second design study showed that decreasing the size of the sealed reference cavity lowers power consumption and transient response time. The study also showed that reducing the cavity base dimension has a greater effect on lowering power consumption and response time. Overall, the present work increases understanding of the self-calibrating pressure sensor and provides insight into potential design improvements, moving closer to optimized self-calibrating pressure sensors.

Bridgman growth and characterization of Dy:YCa4O(BO3)3 crystal

A 1-inch Dy3+:YCa4O(BO3)3 single crystal has been grown by the Bridgman method. The segregation coefficient of Dy3+ is 0.784. The structures, thermal and optical properties of Dy3+:YCOB crystal was studied. The results show that the Dy3+:YCOB crystal has high transmittance, specific heat and thermal diffusion coefficient. The Y-axis-oriented Dy3+:YCOB crystal has the largest absorption cross-section σabs(0.85×10−21cm2@444nm) and emission cross-section σem(2.54×10–21cm2@585nm). In addition, the J-O intensity parameters Ωt(t=2,4,6) of the Dy3+:YCOB crystal were calculated, where Ω2, Ω4, Ω6 are 5.01×10−20cm2, 0.33×10−20cm2, 0.52×10−20cm2 respectively. The fluorescence branching ratio β is 78.7%, and the radiative lifetime τrad is 1.588 ms. The fluorescence lifetime τf is 384.47 μs at 585 nm. The above data confirms that the Y-axis-oriented Dy3+:YCOB crystal is a good yellow laser gain material, which has potential application value in the field of yellow laser operation.

Effect of Process Parameters on Laser Powder Bed Fusion of Al-Sn Miscibility Gap Alloy

Al-Sn binary system is a miscibility gap alloy consisting of an Al-rich phase and a Sn-rich phase. This system is traditionally applied in bearings and more recently found application as form-stable phase change material (PCM) exploiting solid-liquid phase transition of Sn. A careful choice of production process is required to avoid macro-segregation of the two phases, which have different densities and melting temperatures. In the present study, the additive manufacturing process known as laser powder bed fusion (LPBF) was applied to an Al-Sn alloy with 20% volume of Sn, as a rapid solidification process. The effect of process parameters on microstructure and hardness was evaluated. Moreover, feasibility and stability with thermal cycles of a lattice structure of the same alloy were experimentally investigated. An Al-Sn lattice structure could be used as container for a lower melting organic PCM (e.g., a paraffin or a fatty acid), providing high thermal diffusivity thanks to the metallic network and a “safety system” reducing thermal diffusivity if the system temperature overcomes Sn melting temperature. Even if focused on Al-Sn to be applied in thermal management systems, the study offers a contribution in view of the optimization of manufacturing processes locally involving high solidification rates and reheat cycles in other miscibility gap alloys (e.g., Fe-Cu) with similar thermal or structural applications.

Climate and energy balance of the ground in University Valley, Antarctica

AbstractWe report 3 years of data from one meteorological and three smaller stations in University Valley, a high-elevation (1677 m) site in the Dry Valleys of Antarctica with extensive dry permafrost. Mean air temperature was -23.4°C. Summer air temperatures were virtually always < 0°C and were consistent with the altitude lapse rate and empirical relationships between summer temperature, distance from the coast and elevation. The measured frost point (-22.5°C) at the 42 cm deep ice table is equal to the surface frost point and above the atmospheric frost point (-29.6°C), providing direct evidence that surface conditions control ground ice depth. Observed peak surface soil temperatures reach 6°C for ice-cemented ground > 15 cm deep but stay < 0°C when it is shallower. We develop an energy balance model tuned to this rocky and dry environment. We find that differences in peak soil surface temperatures are primarily due to the higher thermal diffusivity of ice-cemented ground compared to dry soil. Sensitivity studies show that expected natural variability is insufficient for melt to form and significant excursions from current conditions are required. The site's ice table meets the criteria for a Special Region on Mars, with 30% of the year > -18°C and water activity > 0.6.

Petrophysical characterization of the Los Humeros geothermal field (Mexico): from outcrop to parametrization of a 3D geological model

The Los Humeros Volcanic Complex has been characterized as a suitable target for developing a super-hot geothermal system (> 350 °C). For the interpretation of geophysical data, the development and parametrization of numerical geological models, an extensive outcrop analogue study was performed to characterize all relevant key units from the basement to the cap rock regarding their petrophysical properties, mineralogy, and geochemistry. In total, 226 samples were collected and analyzed for petrophysical and thermophysical properties as well as sonic wave velocities and magnetic susceptibility. An extensive rock property database was created and more than 20 lithostratigraphic units and subunits with distinct properties were defined. Thereby, the basement rocks feature low matrix porosities (< 5%) and permeabilities (< 10–17 m2), but high thermal conductivities (2–5 W m−1 K−1) and diffusivities (≤ 4·10–6 m2s−1) as well as high sonic wave velocities (≥ 5800 m s−1). Basaltic to dacitic lavas feature matrix porosities and permeabilities in the range of < 2–30% and 10–18–10–14 m2, respectively, as well as intermediate to low thermal properties and sonic wave velocities. The pyroclastic rocks show the highest variability with respect to bulk density, matrix porosity (~ 4– > 60%) and permeability (10–18–10–13 m2), but feature overall very low thermal conductivities (< 0.5 W m−1 K−1) and sonic wave velocities (~ 1500–2400 m s−1). Specific heat capacity shows comparatively small variations throughout the dataset (~ 700–880 J kg−1 K−1), while magnetic susceptibility varies over more than four orders of magnitude showing formation-related trends (10–6–10–1 SI). By applying empirical correction functions, this study provides a full physiochemical characterization of the Los Humeros geothermal field and improves the understanding of the hydraulic and thermomechanical behavior of target formations in super-hot geothermal systems related to volcanic settings, the relationships between different rock properties, and their probability, whose understanding is crucial for the parametrization of 3D geological models.

Thermal Design Method for Optimization of Dry Heat Transfer through Hemp-Based Knitted Fabrics

ABSTRACT In the present research, folding of yarns was applied as a method for designing thermal transport characteristics of hemp-based knits. The rib-knitted fabrics were made by combining one of two cotton two-folded yarns (differing in folding twist) with one of the two complex hemp yarns. The parameters describing the thermal properties in the steady-state and transient regimes of heat transfer were determined. The results indicated that the packing density of the fibers in the yarn plays a crucial role in the heat conduction through the fabric. The two-assembled hemp component appeared to be the main heat conductor in the knitted fabrics containing it. Between the knits composed of the two-folded hemp and two-folded cotton yarns, that having two-folded more twisted cotton yarn as a component was characterized by higher thermal conductivity. The higher thermal conductivity resulted in higher thermal absorptivity, but not necessarily in higher thermal diffusivity, as the increased volumetric heat capacity may limit the speed of heat spreading through the fabric. Therefore, the knits containing two-assembled hemp yarn differed in thermal diffusivity despite their similar thermal conductivity. Both knits with the two-folded more twisted cotton yarn as a component had similar thermal diffusivity despite their different thermal conductivity.

Nanoparticle-enhanced absorptivity of copper during laser powder bed fusion

Laser powder bed fusion (LPBF) of pure copper for thermal and electrical applications is hampered by its low near-infrared absorptivity and high thermal diffusivity. These material properties make it very difficult to localize the thermal energy needed to produce high density 3D printed parts. Modification of metal powders via nanoparticle additives is a promising approach to increasing absorptivity, but the effect of nanoparticles on absorptivity and melting behavior during LPBF is not well understood. In this study, we developed an in situ calorimetry system to measure effective absorptivity during LPBF on copper substrates. We decorated copper substrates using three nanoparticle systems (CuS, TiB2, multilayer graphene flakes) and demonstrated an enhanced absorptivity of the decorated substrates relative to pure copper. Graphene nanoflakes resulted in the highest improved absorption relative to pure copper from 0.09 to 0.48, due to their stability at high laser scanning powers. A thermomechanical model with convective heat transfer provided confidence in the measurements by reproducing the experimental melt pool traces. Full 3D cylindrical prints demonstrated an improvement in relative density of the copper-graphene powder prints (in the range of 0.930–0.992), relative to that of as-purchased copper powder prints (in the range of 0.854–0.972). This work provides a fundamental study of nanoparticle-enabled LPBF of highly reflective metals and demonstrates a viable route for expanding the library of reliably printable metals.

Photothermal Optimization of Quantum Dot Converters for High‐Power Solid‐State Light Sources

AbstractQuantum dots (QDs) exhibit remarkable photoelectric performance. However, the conversion efficiency and thermal performance of QD converters so far is not satisfactory for high‐power solid‐state light (SSL) applications owing to the serious reabsorption loss of QDs and the poor thermal diffusion of polymer materials. Here, an efficient QD converter based on an anodic aluminum membrane (AAM), with high light enhancement and thermal diffusion, is introduced for high‐power SSL applications. Nanoporous AAMs are utilized as templates for confining the QD deposition into the nanowire (NW) shape by incorporating inkjet printing and vacuum‐deposition methods, forming vertical QD NW arrays. Compared with conventional converters, QD‐AAM yields nearly twofold photoluminescence intensity. Three‐dimensional finite‐difference (FDTD) simulations attribute the fluorescence‐enhancement mechanism to the enhancement of QD conversions and boosting of the waveguide effect in QD‐AAM. Furthermore, QD‐AAM exhibits excellent thermal diffusion performance under high‐power excitation, owing to the inner heat conduction path in the AAM framework. This study provides a practical and efficient strategy to comprehensively enhance the optical and thermal performance of QD converters for high‐power SSL applications.

Geopolymer Concrete Performance Study for High-Temperature Thermal Energy Storage (TES) Applications

Solar energy is an energy intermittent source that faces a substantial challenge for its power dispatchability. Hence, concentrating solar power (CSP) plants and solar process heat (SPH) applications employ thermal energy storage (TES) technologies as a link between power generation and optimal load distribution. Ordinary Portland cement (OPC)-based materials are widely used in sensible TES, but their use is limited to operation temperatures below 400 to 500 °C because of thermal degradation processes. This work proposes a geopolymer (GEO)-based concrete as a suitable alternative to OPC concrete for TES that withstands high running temperatures, higher than 500 °C. To this end, thermophysical properties of a geopolymer-based concrete sample were initially measured experimentally; later, energy storage capacity and thermal behavior of the GEO sample were modeled numerically. In fact, different thermal scenarios were modeled, revealing that GEO-based concrete can be a sound choice due to its thermal energy storage capacity, high thermal diffusivity and capability to work at high temperature regimes.

Coherent-interface-induced second hardening deformation of Al-Mg-Al nanolayers by molecular dynamics simulations.

Thermal diffusion plays an important role in the determination of the structures and properties of interfaces and nanolayers. Here we report results from molecular dynamics simulations of the tensile behavior of Al-Mg-Al nanolayers with their Al/Mg interfaces being joined by the thermal diffusion of atoms. We find that a different deformation mechanism applies in each case: low thermal diffusion temperatures (300 ≤ T1 < 664 K) and high thermal diffusion temperatures (664 ≤ T1 ≤ 846 K). The formation of coherent Al/Mg interfaces in the case of high T1 induces the second hardening deformation of Al-Mg-Al nanolayers before the stress reaching the tensile strength, significantly enhancing the tensile properties of Al-Mg-Al nanolayers in comparison to the case of low T1. This difference would provide guidance on the improvement of the mechanical properties of Al-Mg layered systems.

Thermal Properties of AM Series Magnesium Alloys

This paper presents experimental investigation on thermal properties (thermal diffusivity and thermal conductivity) of AM series cast magnesium alloys. The effects of the changing in Al content (from 2 to 9 Al and constantly 0.5 Mn, wt. %) in the alloys on thermal properties, density and hardness were comparatively analyzed. It was observed that intermetallic phases (Mg17Al12 ve Al8Mn5) found in the microstructure of the alloys have an effect on thermal diffusivity, thermal conductivity, density and also hardness. The thermal properties of the alloys were decrease with increasing Al content in the alloy. The thermal properties of the alloys were increase with increasing temperature (the temperature range from 25°C to 400°C). The highest thermal diffusivity was measured on AM20 alloy and the lowest thermal diffusivity were observed on AM90 alloy.

Long-term performance evaluation for deep borehole heat exchanger array under different soil thermal properties and system layouts

To meet the prospect of carbon neutrality, Deep borehole heat exchanger (DBHE) shows good potentiality in extracting deep geothermal energy for building heating, especially in densely populated urban areas of northern China. To investigate the influence on different soil thermal properties and system layouts of the DBHE array, a comprehensive numerical model has been established by OpenGeoSys software coupled TESPy toolkit and a series of scenarios are simulated. Results show that thermal conductivity lay a more important influence on heat extraction performance for DBHE array, rather than volumetric heat capacity. The thermal plume of DBHE array will grow larger along with higher thermal diffusivity. For typical geological parameters in Xi'an, the inter-borehole spacing should not be set below 15 m or it will bring a risk of freeze in circulation. The heat extraction performance and long-term sustainability of single-line layout are obviously better than other layout patterns, also with a smaller ground area needed to deploy the boreholes. This study implies that soil thermal conductivity is the core factor in determining the heat extraction performance of DBHE array and also gives suggestions for the system design of DBHE array in the aspect of borehole spacing and system arrangement.

Numerical study on regenerative effectiveness of parallel-plate regenerator for Stirling engine

ABSTRACT Due to the excellent heat transfer performance and low pressure drop, the parallel-plate regenerator (PPR) is very suitable for use in Stirling engines. At present, researches on the application of PPR in Stirling engines are rare. The axial heat conduction loss in PPR is a key factor that affects the heat transfer performance. Therefore, in this paper, the theoretical model of PPR operated under the Stirling engine working conditions is studied numerically, the regenerative effectiveness is taken as the target parameter, and the influence of axial heat conduction and other important parameters are explored. The results show that the axial heat conduction is detrimental to regenerative effectiveness, the regenerative effectiveness does not vary monotonically with the increase in the thermal conductivity, the copper regenerator with higher thermal conductivity has a lower regenerative effectiveness because of its higher axial thermal conductivity. The working frequency and the thermal properties of the working fluids have a comprehensive effect on the regenerative effectiveness, for high thermal diffusivity working fluids, such as helium and hydrogen, the regenerative effectiveness is high, while for low thermal diffusivity working fluid, such as carbon dioxide, the regenerative effectiveness is relatively low, and does not vary monotonously with frequency. The regenerative effectiveness increases with the reduction of the plate spacing, the plate spacing should be determined by considering the regenerative effectiveness and the pressure drop comprehensively. It is recommended that the stainless steel be a suitable material for the plates, and helium be the suitable working fluid.

Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design

The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas’s high thermal conductivity, heat capacity, and thermal diffusivity.