Field Dependence Of Thermal Conductivity Research Articles

Magnetic field dependent thermal conductivity investigation of water based Fe3O4/CNT and Fe3O4/graphene magnetic hybrid nanofluids using a Helmholtz coil system setup

Magnetic hybrid nanofluids are making a name of themselves in mainstream application areas such as heat transfer, solar systems, acoustic applications, etc. These nanofluids are highly favorable as their ability to advance the properties of their constituent particles such as their thermophysical properties. This study aims to investigate the magnetic field dependent thermal conductivity of Fe3O4/CNT – water and Fe3O4/Graphene – water magnetic hybrid nanofluids. The thermal conductivity investigations are carried out with the 3ω method under a uniform magnetic field generated by a 3D Helmholtz coil system. Fe3O4/CNT – water and Fe3O4/Graphene – water magnetic hybrid nanofluids were purchased commercially as 20 wt% colloids. Then, the samples with 1, 2, 3, 4, and 5 wt% were prepared by diluting them with DI water. Thermal conductivity measurements were carried out for the samples under the external uniform magnetic field in the range of 0–250 G in both parallel and perpendicular directions to the temperature gradient generated by the thermal conductivity measurement probe. The results pointed out that the thermal conductivity of the samples increases as the magnetic field and particle concentration increase for both magnetic hybrid nanofluids. Additionally, it is obtained that the thermal conductivity enhancement of Fe3O4/Graphene – water is up to 3 times higher than Fe3O4/CNT – water samples. Moreover, the maximum thermal conductivity enhancement was observed as ∼12 % and ∼9 % for Fe3O4/CNT – water, and ∼51 % and ∼21 % Fe3O4/Graphene – water under external magnetic field application with parallel and perpendicular direction, respectively.

Low-temperature specific heat and heat transport of Tb2Ti2−xZrxO7 single crystals

We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals has an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample.

Magnetic field induced topological transitions and thermal conductivity in a generalized Kitaev model

Recent experiments on Kitaev spin liquid candidate materials reported non-monotonic behavior of thermal conductivity as a function of magnetic field, which lead to conflicting interpretations of its origin. Motivated by this development, we study the magnetic field dependence of thermal conductivity of a generalized Kitaev model, which allows the phase transitions between different flux sectors as a function of the magnetic field. The thermal conductivity due to Majorana fermions shows dip-bump structures as the magnetic field increases, which is caused by either the transitions between different flux sectors of Kitaev spin liquids or the topological transitions that change the Majorana Chern number within the same flux sector. It is shown that the change of Chern number is closely related to the four-Majorana-fermion interaction induced by the magnetic field. The non-monotonic behavior in thermal conductivity emerges at finite temperature, and it becomes weaker when temperature decreases towards zero. Our model provides a generic mechanism for the Kitaev spin liquids to develop non-monotonic magnetic-field dependence of thermal conductivity while the detailed comparison to realistic materials remains an open question for future investigation.

Electric field dependence of thermal conductivity in bulk systems and nanosystems with charged mobile defects

In systems with charged mobile defects, application of an external electric field modifies the spatial distribution of defects. We obtain the corresponding dependence of the thermal conductivity on the electric field due to the redistribution of defects, both in bulk systems (heat transfer described by Fourier’s law) and in nanosystems (heat transfer described by phonon hydrodynamics). We consider the electric field transverse to the heat flux as well as the electric field parallel to the heat flux; in the latter case, we consider a superlattice composed of alternating layers with defects and without defects. For transverse electric fields, the effective thermal conductivity is increased in bulk systems and decreased in nanosystems. This dependence on the field would allow us to control heat transport in an easy and reversible way without modifying the material.

Strongly scattered phonon heat transport of the candidate Kitaev material Na2Co2TeO6

We studied the field dependent thermal conductivity ($\kappa$) of Na$_2$Co$_2$TeO$_6$, a compound considered as the manifestation of the Kitaev model based on the high-spin $d^7$ Co$^{2+}$ ions. We found that in-plane magnetic fields beyond a critical value $B_c \approx$~10 T are able to drastically enhance $\kappa$ at low temperatures, resulting in a double-peak structure of $\kappa(T)$ that closely resembles the behavior of $\alpha$-RuCl$_3$. This result suggests that heat transport in Na$_2$Co$_2$TeO$_6$ is primarily phononic, and it is strongly affected by scattering from magnetic excitations that are highly tunable by external fields. Interestingly, for magnetic fields $B // a$ (i.e., along the zigzag direction of the Co-Co bonds), there is an extended field range which separates the long-range magnetic order for $B\leq B_c\approx10$ T and the partially spin-polarized gapped high-field phase for $B\gtrsim 12$ T. The low-energy phonon scattering is particularly strong in this field range, consistent with the notion that the system becomes a quantum spin liquid with prominent spin fluctuations down to energies of no more than 2 meV.

Absence of long-range order in an XY pyrochlore antiferromagnet Er2AlSbO7

Rare-earth pyrochlores are known to exhibit exotic magnetic phenomena. We report a study of crystal growth and characterizations of a new rare-earth compound Er$_2$AlSbO$_7$, in which Al$^{3+}$ and Sb$^{5+}$ ions share the same positions with a random distribution. The magnetism are studied by magnetic susceptibility, specific heat and thermal conductivity measurements at low temperatures down to several tens of milli-kelvin. Different from the other reported Er-based pyrochlores exhibiting distinct magnetically ordered states, a spin-freezing transition is detected in Er$_2$AlSbO$_7$ below 0.37 K, which is primarily ascribed to the inherent structural disorder. A cluster spin-glass state is proposed in view of the frequency dependence of the peak position in the ac susceptibility. In addition, the temperature and field dependence of thermal conductivity indicates rather strong spin fluctuations which is probably due to the phase competition.

Effect of magnetic field-dependent thermal conductivity on natural convection of magnetic nanofluid inside a square enclosure

Purpose The purpose of this paper is to investigate the convective heat transfer of magnetic nanofluid (MNF) inside a square enclosure under uniform magnetic fields considering nonlinearity of magnetic field-dependent thermal conductivity. Design/methodology/approach The properties of the MNF (Fe3O4+kerosene) were described by polynomial functions of magnetic field-dependent thermal conductivity. The effect of the transverse magnetic field (0 < H < 105), Hartmann Number (0 < Ha < 60), Rayleigh number (10 <Ra <105) and the solid volume fraction (0 < φ < 4.7%) on the heat transfer performance inside the enclosed space was examined. Continuity, momentum and energy equations were solved using the finite element method. Findings The results show that the Nusselt number increases when the Rayleigh number increases. In contrast, the convective heat transfer rate decreases when the Hartmann number increases due to the strong magnetic field which suppresses the buoyancy force. Also, a significant improvement in the heat transfer rate is observed when the magnetic field is applied and φ = 4.7% (I = 11.90%, I = 16.73%, I = 10.07% and I = 12.70%). Research limitations/implications The present numerical study was carried out for a steady, laminar and two-dimensional flow inside the square enclosure. Also, properties of the MNF are assumed to be constant (except thermal conductivity) under magnetic field. Practical implications The results can be used in thermal storage and cooling of electronic devices such as lithium-ion batteries during charging and discharging processes. Originality/value The accuracy of results and heat transfer enhancement having magnetic field-field-dependent thermal conductivity are noticeable. The results can be used for different applications to improve the heat transfer rate and enhance the efficiency of a system.

Magnetic field dependent thermal conductivity measurements of magnetic nanofluids by 3ω method

Abstract This study aims to investigate the thermal conductivity of the Fe3O4-water magnetic nanofluid under the influence of external magnetic field. The Fe3O4-water magnetic nanofluid sample with 4.8% vol. concentration was purchased. Then, it is diluted to four different vol. concentrations of 1, 2, 3, 4%. The external magnetic field in the range of 0–400 G was applied in parallel and perpendicular directions to the temperature gradient generated by the thermal conductivity measurement probe. Before thermal conductivity measurements, the external magnetic field generated in the air gap between permanent magnets has been analyzed both numerically and experimentally. Then a non-uniformity parameter was defined to relate magnetic field in measurement region to chain aggregation of the nanoparticles. Thermal conductivity measurements were conducted with 3ω method. The results showed that the thermal conductivity enhances with the increasing of the magnetic field strength up to a specific point, then the enhancement decreases. It is also observed that this specific point varies according to the particle concentration. Unlike the most of available study in the literature, in this study the lower thermal conductivity enhancements were obtained. The maximum enhancements with the highest concentration were 10% and 5.6% in parallel and perpendicular orientations, respectively.

Natural Convection in an Annular Enclosure: Influence of Magnetic Field-dependent Thermal Conductivity on Heat Transfer

Natural Convection in an Annular Enclosure: Influence of Magnetic Field-dependent Thermal Conductivity on Heat Transfer

Thermal conductivity and stability of commercial MgB2 conductors

This paper presents a study of the thermal transport properties of MgB2 tapes differing in architecture, stabilization and constituent materials. The temperature and field dependence of thermal conductivity, was investigated both along the conductor and in the direction perpendicular to the tape. These data provide fundamental input parameters to describe the 3D heat diffusion process in a winding. Thermal transport properties—even in field—are typically deduced using semi-empirical formulas based on the residual resistivity ratio of the stabilizer measured in absence of magnetic field. The accuracy of these procedures was evaluated comparing the calculated κ values with the measured ones. Based on the experimental thermal conduction properties and critical current surface we determined the dependence of minimum quench energy and normal zone propagation velocity on the operating parameters of the conductor. The correlation between thermal properties and tape layout allowed us to provide information on how to optimize the thermal stability of MgB2 conductors.

Maneuvering thermal conductivity of magnetic nanofluids by tunable magnetic fields

We report an experimental investigation of magnetic field dependent thermal conductivity of a transformer oil base magnetic fluid as a function of volume fractions. In the absence of magnetic field, thermal conductivity increases linearly with an increase in volume fraction, and magnitude of thermal conductivity thus obtained is lower than that predicted by Maxwell's theory. This reveals the presence of clusters/oligomers in the system. On application of magnetic field, it exhibits a non-monotonous increase in thermal conductivity. The results are interpreted using the concept of a two-step homogenization method (which is based on differential effective medium theory). The results show a transformation of particle cluster configuration from long chain like prolate shape to the aggregated drop-like structure with increasing concentration as well as a magnetic field. The aggregated drop-like structure for concentrated system is supported by optical microscopic images. This shape change of clusters reduces thermal conductivity enhancement. Moreover, this structure formation is observed as a dynamic phenomenon, and at 226 mT field, the length of the structure extends with time, becomes maximum, and then reduces. This change results in the increase or decrease of thermal conductivity.

Magneto thermal conductivity of superconducting Nb with intermediate level of impurity

Niobium materials with intermediate purity level are used for fabrication of superconducting radio frequency cavities (SCRF), and thermal conductivity is an important parameter influencing the performance of such SCRF cavities. We report here the temperature and magnetic field dependence of thermal conductivity κ for superconducting niobium (Nb) samples, for which the electron mean free path le, the phonon mean free path lg, and the vortex core diameter 2rC are of the same order of magnitude. The measured thermal conductivity is analyzed using the effective gap model (developed for le ≫ 2rC (Dubeck et al 1963 Phys. Rev. Lett. 10 98)) and the normal core model (developed for le ≪ 2rC (Ward and Dew-Hughes 1970 J. Phys. C: Solid St. Phys. 3 2245)). However, it is found that the effective gap model is not suitable for low temperatures when le ∼ 2rC. The normal core model, on the other hand, is able to describe κ(T,H) over the entire temperature range except in the field regime between HC1 and HC2 i.e. in the mixed state. It is shown that to understand the complete behavior of κ in the mixed state, the scattering of quasi-particles from the vortex cores and the intervortex quasi-particle tunneling are to be invoked. The quasi-particle scattering from vortices for the present system is understood in terms of the framework of Sergeenkov and Ausloos (1995 Phys. Rev. B 52 3614) extending their approach to the case of Nb. The intervortex tunneling is understood within the framework of Schmidbauer et al (1970 Z. Phys. 240 30). Analysis of the field dependence of thermal conductivity shows that while the quasi-particle scattering from vortices dominates in the low fields, the intervortex quasi-particle tunneling dominates in high fields. Analysis of the temperature dependence of thermal conductivity shows that while the quasi-particle scattering is dominant at low temperatures, the intervortex quasi-particle tunneling is dominant at high temperatures.

On the theory of magnetic field dependence of thermal conductivity in dielectrics in terms of the isotropic model

Polarization of phonons in a magnetic field has been analyzed in terms of the isotropic model. It has been demonstrated that, in the presence of spin-phonon interaction, phonons possess circular polarization, which is responsible for the appearance of the heat flux component perpendicular to the temperature gradient and the magnetic field.

Ni 2X 2 (X = pnictide, chalcogenide, or B) based superconductors

We review the properties of Ni-based superconductors which contain Ni 2X 2 (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi 2As 2. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in T c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.

Fully gapped superconductivity and strong electron correlations in the [formula omitted]-pyrochlore [formula omitted

To elucidate the superconducting gap structure and the influence of rattling motion on quasiparticle dynamics in the superconducting state of KOs 2 O 6 , thermal conductivity and microwave surface impedance were measured at low temperatures. The magnetic field dependence of thermal conductivity at the lowest temperature and the temperature dependence of penetration depth strongly indicate a fully gapped superconductivity in KOs 2 O 6 . The quasiparticle scattering time is strongly enhanced in the superconducting state, indicating a strong electron–electron inelastic scattering in the normal state. These results highlight that KOs 2 O 6 is unique among superconductors, which is characterized by the coexistence of fully gapped superconductivity and strong electron correlations.

Full-gap superconductivity with strong electron correlations in the [formula omitted]-pyrochlore KOs 2O 6

To elucidate the superconducting gap structure and the influence of rattling motion on quasiparticle dynamics in the superconducting state of KOs 2O 6, the thermal conductivity and microwave surface impedance were measured at low temperatures. The magnetic field dependence of thermal conductivity and temperature dependence of penetration depth demonstrate full-gap superconductivity in KOs 2O 6. The quasiparticle scattering time is strongly enhanced in the superconducting state, indicating a strong electron inelastic scattering in the normal state. These results highlight that KOs 2O 6 is unique among superconductors with strong electron correlations.

Exotic Superconducting Properties in the Electron-Hole-Compensated Heavy-Fermion “Semimetal”URu2Si2

We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including the first-order transition at Hc2 with a reduction of entropy flow. This is a consequence of multiband superconductivity with compensated electronic structure in the hidden order state of this system. We provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology.

Field-dependent thermal and electrical transports in Cu∕CoFe multilayer

This paper reports field-dependent thermal and electrical conductivity measurements of a 144 nm thick Cu∕CoFe giant magnetoresistive multilayer made of extremely thin periodic layers (12 and 21 Å for CoFe and Cu layers, respectively), using steady-state Joule heating and electrical resistance thermometry in suspended bridges between 300 and 380 K. Large decreases in the electrical and thermal resistivities from antiparallel to parallel alignment of the magnetization in the film, referred to as the giant magnetoresistance (GMR) and giant magnetothermal resistance (GMTR), are observed. GMR ratios of 17% and 12% and large GMTR ratios of 25% and 58% are measured at 300 and 380 K, respectively. It is concluded that different electron scattering rates for charge and heat transports in the ferromagnetic CoFe layer are responsible for the difference between the GMR and GMTR ratios. While the previous works only reported the relative change in thermal conductance due to applied magnetic field, the present manuscript reports the absolute value of the field-dependent thermal conductivity of Cu∕CoFe multilayer.

Avoiding spurious velocity overshoots in hydrodynamic simulations of deep submicron devices by a physical modelling of heat transport

We show that spurious peaks in velocity profiles emerging from hydrodynamic simulators of submicron n+nn+ Si structures are washed out by using a field-dependent thermal conductivity calculated within a correlation function formalism.

Thermal conductivity of organic superconductors in oriented magnetic field

We report study of thermal conductivity of organic superconductors in magnetic field of varying orientation with respect to the conducting plane, paying special attention to the parallel case. At low temperatures the field dependence of thermal conductivity in this configuration shows notable difference from usual behavior, both for electronic and phononic contributions. We show that the behavior of the electronic thermal conductivity in organic superconductor λ-(BETS) 2 GaCl 4 is consistent with the formation of an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov superconducting state. In a closely related antiferromagnetic superconductor κ-(BETS) 2 FeBr 4 interaction with magnetic sublattice leads to a first order transition at H c2 in parallel magnetic field and reentrant superconductivity at high fields. The phonon thermal conductivity is shown to be a good tool for picking up a dimensional crossover and lock-in transition in the vortex system.