Transverse Thermal Conductivity Research Articles

Effective thermal conductivity models applied to wood briquettes

Three electrical resistive-circuit models were used to determine the effective transverse and longitudinal thermal conductivity of a wood cell as a constituent element of briquettes chips. The models were applied to the briquettes at 0% moisture content and equilibrium moisture content. With the increase in moisture content above equilibrium moisture content, but below fiber saturation point, both fiber swelling and increase in chips interspaces occurred, so that a change in the cell geometry and model was necessary. Since the transverse series resistive model overestimated the thermal conductivity and the transverse parallel resistive model underestimated the thermal conductivity, the wood cells were combined into serial and parallel circuits until the difference between models became insignificant. General equations were developed in the paper, describing effective transverse thermal conductivity of combined cells. Experimental measurements of thermal conductivity significantly validated the resistive models.

Electronic thermal Hall effect in silicene

We theoretically investigate the electronic thermal Hall effect in silicene via a discrete four-band model. Based on the linear response theory, a formalism to address the transverse thermal conductivity is developed. In the absence of an exchange field, the transverse thermal conductivity vanishes due to the time-reversal symmetry. The transverse conductivity becomes finite in the presence of an exchange field and exhibits several peaks with opposite signs. The peak values increase as the field becomes strong. However, as the temperature becomes high, the peak values begin to decay. The results may be helpful in exploring spin caloritronics based on silicene.

Thermal Transport in the Kitaev Model.

In conventional insulating magnets, heat is carried by magnons and phonons. In contrast, when the magnets harbor a quantum spin liquid state, emergent quasiparticles from the fractionalization of quantum spins can carry heat. Here, we investigate unconventional thermal transport yielded by such exotic carriers, in both longitudinal and transverse components, for the Kitaev model, whose ground state is exactly shown to be a quantum spin liquid with fractional excitations described as itinerant Majorana fermions and localized Z_{2} fluxes. We find that the longitudinal thermal conductivity exhibits a single peak at a high temperature, while the nonzero frequency component has a peak at a low temperature, reflecting the spin fractionalization. On the other hand, we show that the transverse thermal conductivity is induced by the magnetic field in a wide temperature range up to the energy scale of the bare exchange coupling; while increasing temperature, the transverse response divided by temperature decreases from the quantized value expected for the topologically nontrivial ground state and shows nonmonotonic temperature dependence. These characteristic behaviors provide experimentally accessible evidence of fractional excitations in the proximity to the Kitaev quantum spin liquid.

Investigation on the effective thermal conductivity of carbonized high silica/phenolic ablative material

As a phenolic resin-based ablative material, high silica/phenolic composite is widely used in aerospace field. However, the effective thermal conductivity of carbonized ablative material formed during ablation process is rarely reported. In this work, carbonized sample of the high silica/phenolic ablative material was obtained by a carbonization process, and its effective thermal conductivity was measured from 100 to 970°C. In addition, an analysis model of the effective thermal conductivity of the carbonized ablator consisting of fiber yarns and carbonized phenolic was established based on the result of structure analysis. The measured value of the effective thermal conductivity of the carbonized phenolic was used to inverse the transverse effective thermal conductivity of the fiber yarns. When the inversed values were adopted in different empirical models, it was found that the Clayton model is suitable for predicting the effective thermal conductivity of the carbonized high silica/phenolic.

The Thermal Conductivities of Periodic Fibrous Composites as Defined by a Mathematical Model

In this paper, a geometric body-centered model to simulate the periodic structure of unidirectional fibrous composites is presented. To this end, three prescribed configurations are introduced to predict in a deterministic manner the arrangement of internal and neighboring fibers inside the matrix. Thus, three different representative volume elements (RVEs) are established. Furthermore, the concept of the interphase has been taken into account, stating that each individual fiber is encircled by a thin layer of variable thermomechanical properties. Next, these three unit cells are transformed in a unified manner to a coaxial multilayer cylinder model. This advanced model includes the influence of fiber contiguity in parallel with the interphase concept on the thermomechanical properties of the overall material. Then, by the use of this model, the authors propose explicit expressions to evaluate the longitudinal and transverse thermal conductivity of this type of composite. The theoretical predictions were compared with experimental results, as well as with theoretical values yielded by some reliable formulae derived from other workers, and a reasonable agreement was found.

Development of diamond powder filled carbon fibre pipes

For the High Luminosity (HL) phase of the Large Hadron Collider (LHC) pixel detectors have to meet new and unprecedented challenges for electronics, read-out band widths and also mechanical support and cooling. Traditionally the mechanical support structures (staves) are built from carbon based materials, embedding a metallic cooling pipe through which the cooling liquid, typically CO2, is circulated. In this paper the development and use of a carbon pipe is discussed. Compared to the metallic pipes carbon pipes have potentially advantages like lightness, low radio activation and no CTE mismatch. Their disadvantage of lower transversal thermal conductivity can be mitigated by filling the fabric with diamond powder (DP). The paper reports about the production process of carbon pipes with DP filled epoxy and cyanate ester resin. It discusses that this process leads with reasonable efficiency to pipes that meet the requirements of pressure stability up to 200 bars and leak tightness of 10−6mbarLs-1 for use in HL-LHC staves. The remaining problems and an outlook on potential improvements and further work are presented.

Temperature and Field Dependence of the Quench Propagation Velocity in Industrial REBCO Coated Conductors

A study of the quench propagation velocity in coated conductors produced by various manufacturers is presented. Based on an experimental investigation of the thermal and electrical properties of the coated conductors, the longitudinal normal zone propagation velocity has been calculated as a function of the temperature and magnetic field, and for different orientations of the field with respect to the tape surface. Contrary to what observed in low-temperature superconductors, we have found that the quench propagation velocity in REBa 2 Cu 3 O 7-x coated conductors is weakly affected by the magnetic field and temperature, and is mainly determined by the operating current. The longitudinal normal zone propagation velocity at a given operating current assumes comparable values in the examined coated conductors, regardless of the differences in the structural, thermal, and electrical properties. The normal zone propagation velocity in the direction perpendicular to the tape surface has also been studied, starting from experimental data of the longitudinal and transverse thermal conductivities of the coated conductors.

Thermal Conductivity Measurement of Anisotropic Biological Tissue In Vitro

The accurate determination of the thermal conductivity of biological tissues has implications on the success of cryosurgical/hyperthermia treatments. In light of the evident anisotropy in some biological tissues, a new modified stepwise transient method was proposed to simultaneously measure the transverse and longitudinal thermal conductivities of anisotropic biological tissues. The physical and mathematical models were established, and the analytical solution was derived. Sensitivity analysis and experimental simulation were performed to determine the feasibility and measurement accuracy of simultaneously measuring the transverse and longitudinal thermal conductivities. The experimental system was set up, and its measurement accuracy was verified by measuring the thermal conductivity of a reference standard material. The thermal conductivities of the pork tenderloin and bovine muscles were measured using the traditional 1D and proposed methods, respectively, at different temperatures. Results indicate that the thermal conductivities of the bovine muscle are lower than those of the pork tenderloin muscle, whereas the bovine muscle was determined to exhibit stronger anisotropy than the pork tenderloin muscle. Moreover, the longitudinal thermal conductivity is larger than the transverse thermal conductivity for the two tissues and all thermal conductivities increase with the increase in temperature. Compared with the traditional 1D method, results obtained by the proposed method are slightly higher although the relative deviation is below 5 %.

Thermal Hall Effect in a Phonon-Glass Ba_{3}CuSb_{2}O_{9}.

A distinct thermal Hall signal is observed in a quantum spin liquid candidate Ba_{3}CuSb_{2}O_{9}. The transverse thermal conductivity shows a power-law temperature dependence below 50K, where a spin gap opens. We suggest that because of the very low longitudinal thermal conductivity and the thermal Hall signals, a phonon Hall effect is induced by strong phonon scattering of orphan Cu^{2+} spins formed in the random domains of the Cu^{2+}-Sb^{5+} dumbbells in Ba_{3}CuSb_{2}O_{9}.

Local Postweld Heat Treatment of Heterogeneous Welded Joint

A concept of heterogeneous welded joint is proposed for researching and describing the characteristics of local postweld heat treatment (PWHT) temperature field for asymmetric thermal conductivity welded joint. Its three types have been classified according to thermal conductivity direction around the weld, separately, welded joint with transverse unidirectional thermal conductivity, transverse bidirectional thermal conductivity, transverse and longitudinal thermal conductivity. Compared with the temperature field of symmetric thermal conductivity welded joint, the highest temperature point of heterogeneous welded joint deviates from the heat device center, and uniform temperature area shrinks. In addition, longitudinal temperature difference and dramatic temperature change zone have arisen for the third type heterogeneous welded joint. In order to improve the temperature distribution, two PWHT methods called temperature compensation method and power compensation method have been put forward and developed. Several engineering applications of two methods are illustrated as examples.

Tailor Made Conductivities of Polymer Matrix for Thermal Management: Design and Development of Three-Dimensional Carbonaceous Nanostructures

The thermal conductivity of graphene is limited in the transverse direction due to the exfoliation of the interplanar distance. In this study, the multiwalled carbon nanotubes (MWCNTs) with high thermal conductivities were immobilized into the exfoliated few layers graphene with an objective to enhance the transverse thermal conductivity of MWCNT embedded graphene three-dimensional architectures. Reconstructing the topographically controlled synthetic hybrid nanostructure assemblies in a polymer matrix provides the opportunity to gain insight into the electrical and thermal conduction mechanism. The thickness of the resin layer between the nanohybrid fillers was found to be critical for the phonon transport through the coating whereas this epoxy insulating layer prevents tunneling of the electron. By controlled functionalization and site specific orientations of hybrid nanoscale morphologies in the fiber reinforced composite, the higher mechanical strength could be achieved with increased electrical but r...

Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS–BEC Crossover

There is growing evidence that the superconducting semimetal FeSe ($T_c\sim8$ K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities, $\kappa_{xx}$ and $\kappa_{xy}$, respectively, in magnetic fields up to 20 T. The field dependences of $\kappa_{xx}$ and $\kappa_{xy}$ imply that a highly anisotropic small superconducting gap forms at the electron Fermi-surface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below $\sim1.0$ K, both $\kappa_{xx}$ and $\kappa_{xy}$ exhibit distinct anomalies (kinks) at the upper critical field $H_{c2}$ and at a field $H^*$ slightly below $H_{c2}$. The analysis of the thermal Hall angle ($\kappa_{xy}/\kappa_{xx}$) indicates a change of the quasiparticle scattering rate at $H^*$. These results provide strong support to the previous suggestion that above $H^*$ a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.

A Heuristic Approach to the Determination of the Effective Thermal Conductivity Coefficients of Biperiodic Composite Media

A heuristic approach to determining the effective thermal conductivity coefficients of unidirectionally reinforced biperiodic composite media is suggested allowing one to substantially refine the calculated values of the effective coefficients of transverse thermal conductivity of the indicated compositions by using the simplest partitioning of a periodicity cell into thin layers (subelements) with subsequent application of the simplest formulas of averaging by simple and inverse mixture rules. A comparison of the calculated values of these coefficients with familiar experimental data is made. The good agreement of the prediction with experiment allows an assumption that the method developed can be used for practical calculations of the thermophysical characteristics of fibrous media with biperiodic structure. At the present time the accuracy of more complex structural models of the thermal conductivity of unidirectionally reinforced composites lacks a strict experimental justification.

Numerical predictions of the effective thermal conductivity for needled C/C-SiC composite materials

ABSTRACTIn the present paper, the complicated structures of needled C/C-SiC composite materials with random distributions of fibers and pores are reconstructed. A multiple-relaxation-time (MRT) lattice Boltzmann model with off-diagonal elements in the relaxation time matrix is adopted to predict longitudinal and transverse thermal conductivities of needled C/C-SiC composite materials whose constituents are anisotropic. The accuracy of the proposed method is verified by the good agreements between the numerical results and experimental data obtained by the Hot Disk thermal constants analyzer. After validations, the factors that influence the effective thermal conductivities of the composite materials are investigated.

Thermographic and tomographic methods for tridimensional characterization of thermal transfer in silica/phenolic composites

Abstract Silica/phenolic composite materials are often used in thermal protection systems (TPS) for atmospheric re-entry. The present work aims to compare two different approaches to assess heat transfer properties of these materials: i) using standard and specific experimental methods, and ii) with the development of 3D thermal transfer multiscale model using 2D (microscopy) and 3D (tomography) images. The latter procedure, based on computations on images, is a two-step change of scale from microscopic scale to mesoscopic scale and then to the macroscopic one. Two silica/phenolic composites with different spatial organizations are studied and their thermal properties are compared. Several experimental methods have been used, including space-resolved diffusivity determinations. Numerical results are compared to experimental ones in terms of transverse and longitudinal thermal conductivities of the composites, and were found to be in good agreement. A discussion is made on the different possible sources of uncertainty for both methods.

Effects of in-plane extension on transverse thermal conductivity of a carbon–carbon 8-harness satin weave composite

A finite element-based technique for coupled thermo-mechanical analysis of woven Ceramic Matrix Composites sheets is presented for the prediction of the degradation of transverse thermal transport behaviour with in-plane extension. The thermal conductivity–strain characteristics have been determined, at the tow level, from the properties of the constituent elements, and then extended to tows and composite. The non-linear thermal conductivity-extension behaviour of the tow has been discretised by multi-linear curves, and implemented in a user-defined subroutine in ABAQUS to model the behaviour of the homogenised orthotropic unidirectional tow and its matrix. By using this approach, an 8-Harness Satin weave HITCO C/C composite unit cell has then been analysed. The variation of through-thickness thermal conductivity degradation with in-plane extension has been predicted and compared with the results of experiments. Very good agreement has been achieved. Two classes of behaviour have been experimentally observed: one that exhibits a brittle response, and another that shows a quasi-ductile behaviour. Both classes of behaviour have been predicted and shown to relate, respectively, to strain localisation and instantaneous pull-out deactivation, without localisation being invoked. These responses are reflected directly in the predicted and experimental rates of decay of transverse thermal conductivity with axial extension. It is advocated that the reduction in transverse thermal conductivity with extension and damage can be used as a Structural Integrity Monitor for CMC operational components.

Physical and thermal properties of unidirectional banana–jute hybrid fiber-reinforced epoxy composites

During the last few years, the hybrid composite materials are replacing the conventional composite materials because of their superior properties. In the present communication, unidirectional banana–jute hybrid fiber-reinforced epoxy composites were prepared by varying the fiber content from 0 to 40 wt% with different weight ratios. The physical and thermal properties of the hybrid composites were tested as per ASTM standards. The influence of fiber content on density, thermal conductivity, specific heat, thermal diffusivity, thermal stability, and water absorption of hybrid composites was investigated. A new micromechanical model for the transverse thermal conductivity of hybrid fiber-reinforced polymer composites is developed using the law of minimal thermal resistance and equal law of specific equivalent thermal conductivity. The results are validated with the results obtained by experimental, numerical simulation, and analytical methods existing in the literature. In numerical, steady state heat transfer simulations were performed to calculate thermal conductivity by using ANSYS software. It is encouraging to notice that the experimental and numerical results are in close approximation with the values predicted by the micromechanical model suggested in this work. It is found that the measured properties of the hybrid composites are suitable for building components and automobiles in order to decrease energy consumption.

Wiedemann-Franz law in iron-based superconductor Fe1+dTe1−xSex

We present results of measurements of the transport phenomena in the series of Fe1+dTe1−xSex single crystals. Namely, we investigate the longitudinal and transverse electrical and thermal conductivities in the parent compound Fe1+dTe as well as superconducting Fe1+dTe0.85Se0.15 and Fe1+dTe0.59Se0.41. This set of data allows us to verify validity of the Wiedemann–Franz law in the systems studied and consequently it is stated that the electronic system in the paramagnetic phase of Fe1+dTe behaves like the Fermi liquid being predominantly elastically scattered. In contrast, low temperature properties of Fe1+dTe0.85Se0.15 and Fe1+dTe0.59Se0.41 are affected likely by the emergence of antiferromagnetic fluctuations, which are expected to be (π,0) and (π,π), respectively. These seem to be coupled with the hole-like band in case of Fe1+dTe0.85Se0.15, whereas with the electron-like band in case of Fe1+dTe0.59Se0.41.

Thermal properties of wood-based panels: thermal conductivity identification with inverse modeling

Accuracy and effectiveness of predicting the heat transfer in wood-based panels is increasingly important for describing their behavior, especially for varying environmental conditions. To model the heat transfer in wood-based panels it is essential to input credible data on their thermal properties. Therefore, proper estimation of the specific heat and thermal conductivity is fundamental. A finite element inverse analysis procedure was developed. The procedure was designed in such a way that anisotropy of the thermal conductivity was accounted for. For all analyzed wood-based panels, in-plane thermal conductivity was significantly higher compared to the transverse one, and it was recommended to consider the anisotropy, and to use both in-plane and transverse thermal conductivity for modeling heat transfer. The effect of temperature on thermal conductivity was not clearly manifested. The thermal conductivity values were decreasing or increasing with temperature. In some cases this influence was practically insignificant (i.e. OSB), while for low density fiberboard the effect of temperature on thermal conductivity was the highest. The identification procedure was validated and its credibility was assessed. It was shown that data on thermal properties available in the literature should not be recommended to model the heat transfer.

2D hierarchical heat transfer computational model of natural fiber bundle reinforced composite

In this paper, a two-dimensional (2D) hierarchical computational model was developed for analysis of heat transfer in unidirectional composites filled with doubly periodic natural fiber bundle. The reinforcement in the composite encloses large number of small lumens, which hints the composite consisting of matrix and natural fiber bundles involves structures at several level scales. In the model, the unit representative volume element (RVE) of composite with fibers arranged periodically was taken into consideration and equivalent models were converted from differently scaled RVEs by a two-step homogenized procedure. Subsequently, numerical simulation of heat–transfer process in each model was performed by finite element analysis and the overall transverse thermal conductivity of each model was obtained numerically. To verify the developed composite models, an optional interrelationship between the overall thermal conductivity of the equivalent natural fiber bundle and the solid region phase in it was obtained for the first-step homogenization and then was compared with analytical results or numerical results from other methods. Finally, a sensitivity analysis was conducted with the models to investigate how changes in the values of important variables such as thermal conductivity and volume fraction of constituent can affect the effective thermal properties of the composite.