Steady-state Thermal Conductivity Research Articles

Influence of Ag Doping on Thermal Conductivity and Magnetic Levitation of Single Grain YBCO Superconductors for High-Temperature Superconducting Maglev

Ag doping in (RE)Ba2Cu3O7−δ [(RE)BCO] bulk high-temperature superconductors has been recognized as an effective way to improve the mechanical and superconducting properties of REBCO bulk superconductors for practical applications such as high-temperature superconducting (HTS) Maglev. Ag2O nanoparticles doped YBCO samples with different doping amounts (x, x varies from 0∼0.5 wt%) were fabricated by Top-Seeded Melt-Growth method (TSMG). The samples’ steady-state thermal conductivity (λ) and the magnetic levitation force (FL) were measured by a testing platform under a similar condition to the HTS Maglev system. The effects of Ag2O doping on the λ and FL of single domain YBCO bulks have been systematically studied. The λ of the c-axis and the ab-plane were calculated through the steady-state heat flow method. We observed that λ increases distinctly when x ≥0.3 wt%. The doping amount of 0.3 wt% has been evidenced to be the optimal value, of which the λ of the c-axis and ab-plane are approximately 52% and 48% higher than the undoped one respectively. According to the results of the FL measurement, we found FL will increase while the relaxation rate of the FL will decrease by adding trace amounts of Ag2O nanoparticles in comparison to the undoped sample. This observation suggests that Ag-doping is an effective way to enhance thermal conductivity as well as the superconducting properties of YBCO superconductors for practical applications.

Evaluation of Interfacial Thermal Resistance of Al-Si Alloy by Using Image-Based Simulation

Interfacial thermal resistances of Al-Si alloys were evaluated by comparing the measured thermal conductivities and the simulated thermal conductivities. Al-7%Si, Al-18%Si and Al-18%Si held in the solid-liquid coexistence temperature for 90 minutes were fabricated by gravity casting. Thermal conductivities were measured with the steady state thermal conductivity measuring device. Thermal conductivities were also simulated by using optical microscope images. Comparing the measured thermal conductivities and the simulated thermal conductivities, interfacial thermal resistances in Al-Si interfaces were evaluated as about 2.5-4.8×10-9 m2K/W.

Heat transport with a twist.

Despite the desirability of polymers for use in many products due to their flexibility, light weight, and durability, their status as thermal insulators has precluded their use in applications where thermal conductors are required. However, recent results suggest that the thermal conductance of polymers can be enhanced and that their heat transport behaviors may be highly sensitive to nanoscale control. Here we use non-equilibrium molecular dynamics simulations to study the effect of mechanical twist on the steady-state thermal conductance across multi-stranded polyethylene wires. We find that a highly twisted double-helical polyethylene wire can display a thermal conductance up to three times that of its untwisted form, an effect which can be attributed to a structural transition in the strands of the double helix. We also find that in thicker wires composed of many parallel strands, adding just one twist can increase its thermal conductance by over 30%. However, we find that unlike stretching a polymer wire, which causes a monotonic increase in thermal conductance, the effect of twist is highly non-monotonic, and certain amounts of twist can actually decrease the thermal conductance. Finally, we apply the Continuous Chirality Measure (CCM) in an attempt to explore the correlation between heat conductance and chirality. The CCM is found to correlate with twist as expected, but we attribute the observed heat transport behaviors to structural factors other than chirality.

Remarkable Thermal Conductivity Reduction of Silicon Nanowires during the Bending Process.

The one-dimensional geometry of silicon nanowire helps to overcome the rigid and brittle nature of bulk silicon and enables it to withstand substantial bending stresses. This provides exciting opportunities for the development of flexible electronics. The bending strain introduces atomic displacement in the lattice structure, which inherently has a significant impact on the thermal conductivity. The strain-dependent thermal conductivity of silicon nanowire is crucial to the thermal management and performance of flexible electronic devices. However, in situ thermal conductivity measurement of bending silicon nanowires remains challenging and unreported due to the varying thermal contact resistances between the sample and sensor/heat sink. In this study, the Raman spectroscopy-assisted steady state thermal conductivity measurement method is coupled with a micromanipulation system to successively monitor the thermal conductivity variation of silicon nanowires during the bending process. The result shows that the thermal conductivity of silicon nanowires steeply decreases 55-78% owing to the strain-induced structural deformation during bending. Furthermore, the proposed in situ thermal conductivity measurement method can also be extended to other nanomaterials.

Diagnostics of Transmission Bearing Units Based on Thermal Load Study

Abstract. The operating temperature of the transmission unit is noted to affect its reliability and can serve as a diagnostic criterion. It is proposed to diagnose the transmission unit technical condition by the temperature in the friction zone in order to take into account the influence of air temperature, solar radiation heating and adjacent heat-producing objects. (Research purpose) To ensure the bearing unit controllability based on the thermal load study. (Materials and methods) The study uses the results of calculating the nominal and operational load of the rear power take-off shaft of the Belarus-82.1 tractor. The study uses the three-dimensional modeling and finite element analysis of the temperature distribution under steady-state thermal conductivity conditions. To establish the functional relationship between the temperature in the friction zone and the diagnostic temperature, the method of finite element analysis is used under steady-state thermal conductivity conditions. (Results and discussion) The maximum load modes and temperatures in the friction zone were determined for the 60310A bearing of the power take-off shaft gearbox during aggregation with different agricultural machines such as 4300 Newtons and 2.4 degrees Celsius for FS-2.0U garden cutter (540 revolutions per minute); 4126 Newtons and 40.7 degrees Celsius for Rovatti T3K80/90/2 (540 revolutions per minute) irrigation pump; 956 Newtons and 13.0 degrees Celsius for ROU-6 (1000) manure spreader; 2615 Newtons and 36.6 degrees Celsius for KPRN-3.0A (1000) mower-conditioner. The maximum temperatures as a diagnostic criterion are established in the friction zone, which equal to 41.7 degrees Celsius at 540 revolutions per minute engine speed and 31 degrees Celsius at 1000 revolutions per minute. (Conclusions) Since the direct measurement of the temperature in the friction zone is hardly possible without changing the bearings design, it is proposed to measure the diagnostic temperature on the unit used for mounting the temperature sensor. The coefficient of proportionality k=0.53 of the finite element model is determined. In order to implement diagnostics in an automatic mode, an algorithm is developed for a digital transmission malfunction recorder. Its design is based on the ATmega328 programmable microcontroller and TMP36 temperature sensors. It is found that the digital transmission malfunction recorder provides automatic control of up to seven different transmission units simultaneously, taking into account the ambient temperature.

Terrestrial heat flow measurement and numerical simulation of lithospheric thermal structure in western Sichuan, China

The western Sichuan in the eastern Qinghai-Tibet plateau has significant geothermal potential. Heat flow is one of the most important parameters in geothermic, its measurement can help obtain a better understanding of the regional lithospheric thermal structure. In this study, we reported 4 heat flow data based on the continuous steady-state temperature logging and rock thermal conductivity test, including two class-A and two class-D data. Furthermore, we calculated the lithospheric thermal structure by a 2D steady-state thermal conduction equation in COMSOL, analyzing the lateral differences of lithospheric thermal structure in the eastern Qinghai-Tibet plateau. Our study has determined that the heat flow values in the western Sichuan range from 94.7 to 116.2 mW/m2, with a corresponding geothermal gradient of 32.1–37.3 °C/km. Moreover, numerical simulation results indicate significant differences in lithospheric thermal structure between western Sichuan and Sichuan Basin, where the Longmen Shan fault zone is a crucial regional tectonic boundary and a significant temperature gradient zone. Specifically, the Moho temperature in the western Sichuan region ranges from 1000 to 1200 °C with an average temperature of 1060 °C, while the Sichuan Basin ranges from 600 to 700 °C. Besides, the thermal lithosphere thickness in the western Sichuan and the Sichuan Basin is 88–97 km and 105–110 km, respectively. These observations suggest that the western Sichuan region is characterized by high thermal anomaly and provide theoretical evidence for the geodynamic and geothermal development in this area. Finally, we suggest that the high thermal anomaly in western Sichuan is the result of a combination of factors, such as thickened crust, high radioactive heat production, and shear frictional heating from deep fault and so on.

Unraveling of the laser drilling of carbon/carbon composites: Ablation mechanisms, shape evolution, and damage evaluation

Laser machining is promising in shaping the brittle carbon/carbon composites (C/Cs) with deep holes, sharp edges, or thin walls. However, there are still many unknowns relating to the laser ablation of carbon materials, and the existing theory and practice is insufficient to guide the industrial machining of C/Cs. Herein the laser drilling of C/Cs was experimentally conducted and numerically modeled to probe into the mechanisms responsible for the material removal, surface formation, and damage evaluation. Firstly, the intrinsic correlations among the anisotropic hole feature, the fiber yarn alignment and the steady-state thermal conduction are revealed. The detailed characterizations of the ablated surface and the recast layer clearly prove that sublimation of the graphitic carbon dominates the material removal process under laser ablation. Furthermore, it is proposed that the greater portion of crystalized graphene layers enables the lower ablation rate of the pyrocarbon matrix than the carbon fibers. Secondly, the combination of the experimental and simulated results unravels that the continuously evolved surface slope and the redeposited recast layer are the decisive factors in the laser-carbon interaction, which affect the efficient absorption coefficient of the laser and result in the nonlinear drilling rate and the self-limiting of the drilling. Finally, the roles of the laser heating and the subsequent rapid cooling in damage initiation and propagation are identified: nanoscale splitting of the pyrocarbon occurs due to the growth and realignment of the graphene layers upon laser heating, and the tensile thermal stress induced by the cooling drives the further growth of high-density but discrete microcracks from these splitting sites. The load bearing capability of the carbon fibers, however, is retained in this severe thermal shock. As a result, the laser drilling induces only a slight degradation of the mechanical strength of the C/Cs.

Comparative assessment of transient- and steady-state soil thermal conductivity using a specially designed consolidometer

Precise estimation of soil thermal conductivity is crucial for reliable heat flow analysis in soil. Research indicates variations in soil thermal conductivity values estimated using transient- and steady-state temperature measurements. However, sources of such reported differences cannot be specified with certainty. This paper presents the development of a soil thermal conductivity measurement device that enables simulation of a specified stress condition on soil specimens and estimation of both transient- and steady-state soil thermal conductivity values from different segments of a single test. A series of thermal conductivity tests were performed on seven different soils using the specially-designed test setup. The difference in soil thermal conductivity values obtained from transient- and steady-state responses are compared to those reported in the literature. In contrast to steady-state soil thermal conductivity, results indicate an increase in transient-state soil thermal conductivity beyond a threshold value of the rate of soil temperature increment during the transient-state measurement. Furthermore, empirical relations are suggested to correlate soil thermal conductivity of fine-grained soils with plasticity index, hydraulic conductivity, and coefficient of consolidation.

Двойственная вариационная модель стационарного процесса теплопроводности, учитывающая пространственную нелокальность

Microcontinuum theories boast a great potential for simulating structurally sensitive materials. There exists a sufficiently large number of works delineating the basics of non-local mechanics using the theory of elasticity as an example. Estimating the investigative capacity of non-local mechanics is at present particularly relevant to simulating nanodevices, nanoelectromechanical systems (NEMS), and media featuring complex internal micro- and nanostructures. Typically, analysing these simulations involves overcoming certain difficulties caused by the necessity to solve integro-differential equations numerically. Variational methods may be successfully applied to analysing mathematical models of continuous media as an additional tool. The paper describes plotting an alternative functional for the problem of steady-state thermal conductivity in a homogeneous body, taking into account non-locality effects and featuring a temperature-independent thermal conductivity coefficient. We show that the stationary conditions for this functional do not differ from those in the absence of non-locality. The alternative functional combined with the fundamental functional presented previously constitute a dual variational model. We quantitatively analyse the problem of an infinite planar plate featuring constantly active internal heat sources. The dual variational formulation of the problem allows us not only to obtain an approximate solution to the problem under consideration, but also to estimate its error, as well as to reduce this error by selecting other approximating functions if necessary

Anisotropic highly conductive joints utilizing Cu-solder microcomposite structure for high-temperature electronics packaging

The miniaturization of power conversion systems requires high-power density operation of power modules, causing the heat-density increase. Therefore, it is essential to develop bonding technology to realize highly thermally conductive and reliable high-temperature joints. In this study, we propose a novel anisotropic microcomposite (AMC) joint that integrates a lotus-type porous Cu (LPC) sheet and Sn-based solder for high-temperature electronic applications. The AMC joint was successfully fabricated by infiltrating the molten solder into the unidirectional pores of the LPC sheet during a simple reflow process. Steady-state thermal conductivity measurements for a uniquely designed specimen revealed its equivalent thermal conductivity (142.4 W/m·K), 2.5 times higher than the solder. Finite element simulations supported its excellent thermal performance by investigating the heat flux distribution and thermal conductivity prediction that utilize a three-dimensional image-based constructed model. In addition, the aging test at 200 °C for 1008 h clarified a stable shear strength of over 46 MPa. This indicates a reliable mechanical performance at 200 °C, which is only 20 °C below the melting point of the solder. These experimental and numerical studies proved the potential of the novel joint as a high-temperature electronics joint and offered possible mechanisms for its thermal and mechanical property enhancement.

Origin of high thermal conductivity in disentangled ultra-high molecular weight polyethylene films: ballistic phonons within enlarged crystals

The thermal transport properties of oriented polymers are of fundamental and practical interest. High thermal conductivities ( ≳ 50 Wm−1K−1) have recently been reported in disentangled ultra-high molecular weight polyethylene (UHMWPE) films, considerably exceeding prior reported values for oriented films. However, conflicting explanations have been proposed for the microscopic origin of the high thermal conductivity. Here, we report a characterization of the thermal conductivity and mean free path accumulation function of disentangled UHMWPE films (draw ratio ~200) using cryogenic steady-state thermal conductivity measurements and transient grating spectroscopy. We observe a marked dependence of the thermal conductivity on grating period over temperatures from 30–300 K. Considering this observation, cryogenic bulk thermal conductivity measurements, and analysis using an anisotropic Debye model, we conclude that longitudinal atomic vibrations with mean free paths around 400 nanometers are the primary heat carriers, and that the high thermal conductivity for draw ratio ≳ 150 arises from the enlargement of extended crystals with drawing. The mean free paths appear to remain limited by the extended crystal dimensions, suggesting that the upper limit of thermal conductivity of disentangled UHMWPE films has not yet been realized.

Manipulating thermal fields with inhomogeneous heat spreaders

We design a class of spatially inhomogeneous heat spreaders in the context of steady-state thermal conduction leading to spatially uniform thermal fields across a large convective surface. Each spreader has a funnel-shaped design, either in the form of a trapezoidal prism or truncated cone, and is forced by a thermal source at its base. We employ transformation-based techniques, commonly used to study metamaterials, to determine the required thermal conductivity for the spreaders. The obtained materials, although strongly anisotropic and inhomogeneous, are accurately approximated by assembling isotropic, homogeneous layers, rendering them realisable. An alternative approach is then considered for the conical and trapezoidal spreaders by dividing them into two or three isotropic, homogeneous components respectively. We refer to these simple configurations as neutral layers. All designs are validated numerically both with and without the effects of thermal contact resistance between layers. Such novel designs pave the way for future materials that can manipulate and control the flow of heat, helping to solve traditional heat transfer problems such as controlling the temperature of an object and energy harvesting.

NUMERICAL SOLUTION OF HEAT TRANSFER BY CONDUCTION INSIDE ORTHOTROPIC SOLID MATERIALS IN CYLINDRICAL GEOMETRY

This study presents a numerical solution of heat transfer within two orthotropic mediums in cylindrical geometry, subjected to an imposed flux density on its lateral surface and to a temperature over the two cylinders sections. The heat equation for the two orthotropic cylinders with the corresponding limit conditions, the continuities of the flux, and the temperatures at the interface of both cylinders have been resolved by using the method of alternating direction implicit (ADI). The established numerical code has been successfully validated with the results exist in the literature for a particular case. The purpose of this study is to investigate the thermal behavior of the two orthotropic cylinders according to mediums mains thermal conductivities ratio, radial thermal conductivity ratio of the two mediums in addition to the time necessary to reach steady-state thermal conduction. The results show that isotherms, axial and radial flux depend on the data problem values. Furthermore, the steady-state time is rather affected by the mains thermal conductivities ratio. The lower value of this time corresponds to higher values of mediums axial thermal conductivities.

The comparison of a discrete-continuous approach and a method of finite differences in solving the problem of unsteady-state heat and moisture transfer in the building envelope

This article is devoted to the development of methods for calculating heat and humidity regime in the building envelope. The equation of steady-state thermal conductivity with boundary conditions of the third kind and the formula for calculating heat losses of a building based on the heat transfer equation have been considered. The equation of unsteady-state thermal conductivity as well as its solution using the discrete-continual approach has also been studied. The solution of the unsteady-state heat conductivity problem with invariable over time boundary conditions using the discrete-continuous approach was proposed by A.B. Zolotov and P.A. Akimov. The subsequent modernization of the solution was conducted by V.N. Sidorov and S.M. Matskevich. The unsteady-state equation of moisture transfer based on Fick’s second law using the theory of moisture potential is derived. The solution of the unsteady-state moisture transfer equation using the finite difference method according to an explicit difference scheme as well as the solution of the unsteady-state moisture transfer equation using the discrete-continuous approach is demonstrated. To prove the effectiveness of using the discrete-continuous approach in the area of the unsteady-state humidity conditions we compared the calculation results of the distribution of moisture in a single-layer enclosing structure made of aerated concrete using two methods of moisture potential theory. It was found that the difference in the results of calculation by the discrete-continual formula and by the method of finite differences does not exceed 3.2%.

Prevention of Frost Heave in the Surrounding Rocks of Tunnel for Ski Resorts

Taking the four-season ski resort in Jilin Beishan as an example, this paper analyses the temperature field of surrounding rocks by applying the theory of transient thermal conductivity of semi-infinite large objects, and explains the necessity of insulation isolation layer and heating system setting. The structure and insulation performance of the insulation isolation layer are specified, and the flat wall steady-state thermal conductivity theory is used to calculate that the heat transfer on both sides of the insulation layer is about 29.66kW. This heat is not only a part of the cooling load of the snow resort refrigeration system, but also a part of the heat load of the wall heating system. For the refrigeration system, the COP value of the chiller is 2.5kW/kW. Even excluding the energy consumption of the transmission and distribution process, the annual power consumption of the cooling load alone is about 104,000 KWH, which explains the importance of the temperature control system and insulation layer. The heat source of the heating system of this project innovatively adopts the waste heat of cooling generated in the refrigeration process. This measure makes the heating source of this project no longer consume fossil energy, resulting in significant energy saving and emission reduction benefits. As described, a tunnel ski resort similar to a new or rebuild one can be used as a reference. Some of the results can be applied to prevention of frost heave of traffic tunnels in high-cold and high-latitude areas.

Thermo-elastic analysis of functionally graded graphene nanoplatelets (GPLs) reinforced closed cylindrical shells

This paper investigates, for the first time, the thermo-elastic responses of a functionally graded (FG) graphene nanoplatelets (GPLs) reinforced composite (FG-GPLRC) closed cylindrical shell under a temperature field due to steady-state thermal conduction along thickness direction. A comprehensive thermo-elastic model covering various thermal boundary conditions (TBCs) is established. Analytical solutions for both radial stress and hoop stress of the shell are derived based on the plane hypothesis and displacement continuity conditions between the cylindrical shell and hemispherical ends. A parametric study is conducted to investigate the effects of GPL distribution pattern, weight fraction and geometry as well as TBCs on thermal stresses and deformation of the shell. It is found that GPL distributions V and O are preferred patterns for both FG-GPLRC cylindrical shell and hemispherical ends, respectively. The closed FG-X-GPLRC cylindrical shell with higher GPL concentration has higher thermal conductivity consequently higher thermal stresses. The research findings are of practical importance for the applications and design of closed FG-GPLRC shells in a variety of engineering sectors.

Cryogenic thermal conductivity of 7050 aluminum alloy subjected to different heat treatments

The cryogenic thermal conductivity of high-strength 7050 aluminum alloy subjected to the processes with different combinations of solution, aging and deep cryogenic treatment, was measured by a one-dimensional steady-state thermal conduction method. The microstructure evolution of this alloy after different treatments was also characterized to investigate the changes in thermal conductivity. The results showed that the thermal conductivity of aluminum alloy was approximately linearly reduced with the decrease of temperature in general. Under the effects of different treatments, it was found that the thermal conductivity at different temperatures was sharply decreased by 30% after solution. Deep cryogenic treatment further decreased the thermal conductivity when it was conducted after solution. However, the conduction of aging increased the thermal conductivity after solution or solution-deep cryogenic treatment. Intensive lattice distortion and abundant of point defects that enhanced the electro scattering were the main reasons for decreasing the thermal conductivity after solution. The relatively large size of Al-Cu precipitates and increased dislocation density contributed to the further decrease of thermal conductivity after deep cryogenic treatment. The precipitation of a large number of ultra-fine secondary phase particles after aging increased the thermal conductivity by sufficiently releasing the lattice distortion. This work also provided an efficient method for investigating the mechanisms of deep cryogenic treatment on aluminum alloy by studying the thermal conductivity at low temperatures.

Evaluation of Interfacial Thermal Resistance of Al-AlN Composites by Using Image-Based Calculation

Interfacial thermal resistance of Al-AlN composites was evaluated by comparing the measured thermal conductivity and the simulated thermal conductivity. Al-10vol.%AlN and Al-20vol.%AlN composites were fabricated by spark plasma sintering. Effective thermal conductivity was measured with the steady state thermal conductivity measuring device. Effective thermal conductivity was also simulated by using FE-SEM image and the measured relative density. Comparing the measured thermal conductivity and the simulated thermal conductivity, interfacial thermal resistance in Al-AlN composites was evaluated as about 1.27-6.2510-9 m2K/W.

A steady-state heat conduction problem of a nonhomogeneous conical body

Numerous studies and textbooks deal with the steady-state thermal conduction of radially nonhomogeneous circular cylinder. In contrast, there are relatively few studies on the thermal conduction problems of conical solid bodies. This study is intended as a modest contribution to the solution of thermal conductance problems of nonhomogeneous conical bodies. A one-dimensional steady-state heat conduction in nonhomogeneous conical body is considered. The thermal conductivity of the hollow conical body in a suitable chosen spherical coordinate system depends on the polar angle but is independent of the radial coordinate and azimuthal angle coordinate. A functionally graded type of material inhomogeneity is considered. All results of the paper are based on Fourier’s theory of heat conduction in solid bodies.

Условия теплового взрыва в пластине при конвективно-радиационном теплообмене

The processes of obtaining and storing energy-saturated substances are characterized by energy release in their volume. The intensity of this energy release increases with increasing temperature. The stability of the stationary temperature state of a solid with a temperature-dependent intensity of volumetric energy release is directly related to the conditions of heat transfer of this body with the environment. If the heat energy released in the volume of the body can no longer be diverted into the environment, the steady temperature state of the body becomes impossible. The paper studies the conditions for a thermal explosion in a solid in the form of a plate with a temperature-dependent coefficient of thermal conductivity and convective-radiation heat transfer on its surfaces. The statement of the nonlinear problem of steady-state thermal conductivity in the plate is represented by a system of integral relations. The limits of integration of the integrals included in these relations are the desired functions and parameters which determine the temperature state of the plate. A quantitative analysis of these relationships makes it possible to establish the influence of the parameters which determine the intensity of heat transfer and the dependence of the thermal conductivity of the plate material on the conditions for a thermal explosion with an arbitrary law of variation with temperature of the volumetric power of the energy release in the plate. The results of such an analysis are presented in the framework of a one-parameter model of the stationary theory of thermal explosion