Low-temperature Thermal Conductivity Research Articles

Novel biphasic high-entropy ceramic aerogels and their fiber composites with low thermal conductivity, high thermal stability and significant thermal insulation property

A novel biphasic high-entropy (Y0.2Ho0.2Tm0.2Yb0.2Lu0.2)2SiO5/(Y0.2Ho0.2Tm0.2Yb0.2Lu0.2)2Zr2O7 ceramic aerogel (BP-HEZSA) was prepared using formamide/sol-gel, supercritical CO2 drying and combined with a heat treatment process. The results indicate that BP-HEZSA exhibits a biphasic high-entropy structure at 1200 °C, and it maintained its stability at 1400 °C while demonstrating a homogeneous distribution of elements. BP-HEZSA exhibited nanoscale fine grains (with a size range of 18.82 nm to 27.62 nm), high BET specific surface area (16.24 m2·g−1), and good pore structure, all of which were controlled by the thermal treatment temperature. BP-HEZSA had low room temperature thermal conductivity and high structural and phase thermal stability at high temperatures. Furthermore, the applicability of the aerogel was improved by compounding with aluminosilicate fibers. The composites exhibited low density and low thermal conductivities (0.04–0.14 W·m−1K−1). Butane blowtorch testing confirmed superior thermal insulation in the composites, meeting demands for high-temperature resistance in the advanced nanostructured materials.

The low-temperature thermal conduction and millimeter-wave dielectric properties of β-Si3N4 as a heat-dissipation substrate

Increasing operating frequency is required in wireless communication systems. In use, this generates heat, which results in serious problems for semiconductor devices. Silicon nitride (Si3N4) has high thermal conductivity and fracture strength. Therefore, it is expected to be suitable for heat dissipation in active circuit substrates for wireless communication. In this study, Si3N4 with varying thermal conductivities were prepared by controlling starting material and sintering conditions and their low-temperature thermal conductivities were measured. The mean free path of phonons in β-Si3N4 influences its thermal conductivity. The temperature dependence of the Debye–Waller factor obtained by synchrotron radiation scattering supported the high thermal conductivity of the β-Si3N4. The degradation of thermal conductivity was interpreted in terms of the full width at half maximum of Raman spectra. Further, infrared reflectivity and first-principles calculation for β-Si3N4 confirmed their superior dielectric properties compared with other high thermal conductive materials.

Synthesis, Structure, and Properties of CuBiSeCl2: A Chalcohalide Material with Low Thermal Conductivity.

Mixed anion halide-chalcogenide materials have recently attracted attention for a variety of applications, owing to their desirable optoelectronic properties. We report the synthesis of a previously unreported mixed-metal chalcohalide material, CuBiSeCl2 (Pnma), accessed through a simple, low-temperature solid-state route. The physical structure is characterized through single-crystal X-ray diffraction and reveals significant Cu displacement within the CuSe2Cl4 octahedra. The electronic structure of CuBiSeCl2 is investigated computationally, which indicates highly anisotropic charge carrier effective masses, and by experimental verification using X-ray photoelectron spectroscopy, which reveals a valence band dominated by Cu orbitals. The band gap is measured to be 1.33(2) eV, a suitable value for solar absorption applications. The electronic and thermal properties, including resistivity, Seebeck coefficient, thermal conductivity, and heat capacity, are also measured, and it is found that CuBiSeCl2 exhibits a low room temperature thermal conductivity of 0.27(4) W K-1 m-1, realized through modifications to the phonon landscape through increased bonding anisotropy.

Diffusion dominant thermal transport in mixed valent Ba4Sn4Se9

New material developments are of paramount importance from the perspective of developing a fundamental understanding of material properties as well as transitioning from materials to devices. Among the different classes of materials of interest, mixed valence Sn compositions have been known to display technologically important properties and are of interest for the design and synthesis of novel compositions for targeted applications. Herein, we report on electronic, optical and thermal properties of the previously unexplored mixed valence composition Ba4Sn4Se9, a material with a direct optical band gap of 2.36 eV. As revealed by single crystal synchrotron X-ray diffraction, Ba4Sn4Se9 crystallizes in the orthorhombic space group Pnma with a large unit cell comprised of 58 atoms. Heat capacity data reveals a low Debye temperature with many low frequency optic modes. Our low temperature thermal conductivity data and analyses show that thermal diffusion dominates thermal transport above 80 K. Furthermore, high lattice anharmonicity contributes to the very low thermal conductivity. Our results and analyses will motivate further investigations of this as well as other compositions in the Ba-Sn-Se system, as well as other mixed valence chalcogenides for technological applications.

Phonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na2Co2TeO6

The report of a half-quantized thermal Hall effect and oscillatory structures in the magnetothermal conductivity in the Kitaev material α-RuCl3 have sparked a strong debate on whether it is generated by Majorana fermion edge currents, spinon Fermi surface, or whether other more conventional mechanisms are at its origin. Here, we report low temperature thermal conductivity (κ) of another candidate Kitaev material, Na2Co2TeO6. The application of a magnetic field (B) along different principal axes of the crystal reveals a strong directional-dependent B impact on κ, while no evidence for mobile quasiparticles except phonons can be concluded at any field. Instead, severely scattered phonon transport prevails across the B−T phase diagram, revealing cascades of phase transitions for all B directions. Our results thus cast doubt on recent proposals for significant itinerant magnetic excitations in Na2Co2TeO6, and emphasize the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.

Enhancing thermal transport in ABS polymer with graphene oxide: Insights into low-temperature thermal conductivity behavior and correlation with Boson peak anomaly

The thermal conductivity of pure ABS polymer and ABS polymer composite with 0.5 wt% of the thermally reduced graphene oxide (trGO) was measured in a wide temperature range from 2 to 100 K. Adding 0.5 % trGO enhanced the thermal conductivity of ABS polymer by 1.5 times over the entire temperature range. A comparison of the thermal conductivity of ABS and epoxy-resin amorphous polymers and structural glasses shows that it is closely related to the concept of minimal thermal conductivity, which is determined by the intrinsic phonon scattering and the coherence contribution to the thermal conductivity in the material. The temperature dependence of the coherence contribution to the thermal conductivity, related to wave-like tunneling and loss of coherence between different vibrational eigenstates, was approximated by the exponential function of an Arrhenius type with characteristic energy E and a pre-exponential coefficient κ0. A proportional correlation was found between the low-temperature anomaly of the heat capacity, named the calorimetric Boson peak, and the high-temperature behavior of thermal conductivity of amorphous polymers and structural glasses. Thus, this study provides new physical information about the thermal conductivity in disordered materials and indicates a universality of its temperature dependence.

DFT based computational investigations of the physical properties of perovskites-structure SrXO3 (X = Si, Tb, Th) for optoelectronic and thermo-mechanical applications

Density Functional Theory (DFT) method is employed for investigating the structural, elastic, electronic, optical, and thermal properties of Perovskite structure SrXO3 (X = Si, Tb, and Th). The studied lattice parameters show good agreement with the previous theoretical and experimental results. The elastic stiffness constants, (Cij) of SrSiO3, SrTbO3, and SrThO3 satisfied the born stability criteria which confirm the mechanical stability of these phases. It is observed that when Tb or Th is replaced with Si, the brittleness behaviours of SrTbO3 or SrThO3 change to ductility, which improves its machinability for industrial applications. The investigation of anisotropic factors of SrSiO3, SrTbO3, and SrThO3 ensured that SrSiO3 is more anisotropic than SrTbO3 and SrThO3. By evaluating the electronic properties, it can be advantageous for applying the topological insulators (TI), solar cells, or PV (Photovoltaic) cell fabrication. Furthermore, the optical properties are studied and detail discussed with SrXO3 (X = Si, Tb, and Th). The reason of low Debye temperature (ΘD) and low minimum thermal conductivity (Kmin) has been perfectly explicated through the consideration of the mean atomic weight (M/n) of the compounds. Temperature-dependent of heat capacities (Cv, Cp) and linear thermal expansion coefficient (α) are also estimated using the quasi-harmonic Debye model and discussed. The SrTbO3 and SrThO3 compounds can be employed as potential thermal barrier coating (TBC) materials for high-temperature applications, according to the low values of Kmin, ΘD and α.

Studies on the structure and the magnetic properties of high-entropy spinel oxide (MgMnFeCoNi)Al2O4

The study of high-entropy materials has attracted enormous interest since they could show new functional properties that are not observed in their related parent phases. Here, we report single crystal growth, structure, thermal transport, and magnetic property studies on a novel high-entropy oxide with the spinel structure (MgMnFeCoNi)Al2O4. We have successfully grown high-quality single crystals of this high-entropy oxide using the optical floating zone growth technique for the first time. The sample was confirmed to be a phase pure high-entropy oxide using x-ray diffraction and energy-dispersive spectroscopy. Through magnetization measurements, we found (MgMnFeCoNi)Al2O4 exhibits a cluster spin glass state, though the parent phases show either antiferromagnetic ordering or spin glass states. Furthermore, we also found that (MgMnFeCoNi)Al2O4 has much greater thermal expansion than its CoAl2O4 parent compound using high resolution neutron Larmor diffraction. We further investigated the structure of this high-entropy material via Raman spectroscopy and extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. From Raman spectroscopy measurements, we observed (MgMnFeCoNi)Al2O4 to display a combination of the active Raman modes in its parent compounds with the modes shifted and significantly broadened. This result, together with the varying bond lengths probed by EXAFS, reveals severe local lattice distortions in this high-entropy phase. Additionally, we found a substantial decrease in thermal conductivity and suppression of the low temperature thermal conductivity peak in (MgMnFeCoNi)Al2O4, consistent with the increased lattice defects and strain. These findings advance the understanding of the dependence of thermal expansion and transport on the lattice distortions in high-entropy materials.

Natural Convection in the Melting of Phase Change Materials in a Cylindrical Thermal Energy Storage System: Effects of Flow Arrangements of Heat Transfer Fluid and Associated Thermal Boundary Conditions

Abstract Latent thermal energy storage systems (LTESS) have received widespread attention due to their high energy density to store a significant amount of thermal energy in the form of latent heat into phase change materials (PCM) at a nearly constant melting temperature. The thermal efficiency of LTESS is usually limited by poor heat conduction in PCM but enhanced by the natural convection of molten PCM. The natural convection increases the uniformity of temperature by mixing in a PCM enclosure and therefore increases the heat transfer rates and accelerates the melting. While there is negligible natural convection, periodic reciprocation of heat transfer fluid (HTF) through the PCM enclosure has been demonstrated to increase the heat transfer rates to PCM by increasing the melt interface area and reducing temperature gradients across PCM compared to fixed-directional flow arrangements. The current study examines the effects of HTF flow direction on the strength and duration of natural convection in PCM in a vertical cylindrical shell-and-tube container. Gallium is used as the PCM because of its low melting temperature and high thermal conductivity, and water is used as the HTF. A 2-D axisymmetric numerical model developed in ANSYS Fluent was used for the study of the cylindrical LTESS. The aspect ratio of the cylindrical container is nearly 1, which allows the generation of natural convection currents of strong magnitude in molten PCM in the vertical orientation. The irregular melting front in the PCM is caused by both natural convection in molten PCM and thermal boundary conditions for different HTF flow arrangements. The temperature and melting front profiles of PCM with the reciprocating flow arrangement are compared to unidirectional flows in upward and downward directions. The influence of HTF operating parameters such as temperature, velocity, and reciprocation period on PCM melting is studied. Scale analysis is also applied to characterize the different melting regimes of PCM under different flow arrangements.

Liquid metal droplets bouncing higher on thicker water layer

Liquid metal (LM) has gained increasing attention for a wide range of applications, such as flexible electronics, soft robots, and chip cooling devices, owing to its low melting temperature, good flexibility, and high electrical and thermal conductivity. In ambient conditions, LM is susceptible to the coverage of a thin oxide layer, resulting in unwanted adhesion with underlying substrates that undercuts its originally high mobility. Here, we discover an unusual phenomenon characterized by the complete rebound of LM droplets from the water layer with negligible adhesion. More counterintuitively, the restitution coefficient, defined as the ratio between the droplet velocities after and before impact, increases with water layer thickness. We reveal that the complete rebound of LM droplets originates from the trapping of a thinly low-viscosity water lubrication film that prevents droplet-solid contact with low viscous dissipation, and the restitution coefficient is modulated by the negative capillary pressure in the lubrication film as a result of the spontaneous spreading of water on the LM droplet. Our findings advance the fundamental understanding of complex fluids’ droplet dynamics and provide insights for fluid control.

Numerical analysis for permafrost temperature field in the short term of permafrost subgrade filling

Purpose It is of great significance to study the influence of subgrade filling on permafrost temperature field in permafrost area for the smooth construction and safe operation of railway.Design/methodology/approach The paper builds up the model for the hydrothermal coupling calculation of permafrost using finite element software COMSOL to study how permafrost temperature field changes in the short term after subgrade filling, on which basis it proposes the method of calculation for the concave distortion of freezing front in the subgrade-covered area.Findings The results show that the freezing front below the subgrade center sinks due to the thermal effect of subgrade filling, which will trigger hydrothermal erosion in case of sufficient moisture inflows, leading to the thawing settlement or the cracking of the subgrade, etc. The heat output of soil will be hindered the most in case of July filling, in which case the sinking and the distortion of the freezing front is found to be the most severe, which the recovery of the permafrost temperature field, the slowest, constituting the most unfavorable working condition. The concave distortion of the freezing front in the subgrade area increases with the increase in temperature difference between the filler and ground surface, the subgrade height, the subgrade width and the volumetric thermal capacity of filler, while decreases with the increase of the thermal conductivity of filler. Therefore, the filler chose for engineering project shall be of small volumetric thermal capacity, low initial temperature and high thermal conductivity whenever possible.Originality/value The concave distortion of the freezing front under different working conditions at different times after filling can be calculated using the method proposed.

Dual template strategy to prepare ultralight and high-temperature resistant ceramic nanorod aerogels for efficient thermal insulation

Ceramic aerogels are considered as promising thermal insulation materials used in extreme environments. However, the combination of ultralight, high heat tolerance and ultra-low high-temperature thermal conductivity is still a great challenge for both brittle granular aerogels and micron-porous fibrous aerogels. Here, a dual-template strategy is proposed for the simple fabrication of high-performance nanoporous ceramic aerogels (SARAs) by adopting one-dimensional high-crystallinity and high-aspect-ratio Al2O3 nanorods as the basic unit. After delicate design of the composition and pore structures, SARAs exhibit excellent comprehensive properties including ultra-low density (8.12–28.43 mg cm−3), low room temperature thermal conductivity (down to 0.0246 W m−1 K−1) and extremely high temperature resistance (up to 1400 °C). In addition, the unique nanoporous structure lapped by nanorods also confer SARAs superior high-temperatures thermal insulation performance (0.0949 W m−1 K−1 at 1000 °C). The successful preparation of nanorod aerogels with fascinating properties not only provides a new vision on the pore structure regulation of aerogels for thermal insulation, but also offers an opportunity to develop new ultralight functional ceramic aerogels.

Structural and thermal properties of Eu2Ga11Sn35

Clathrates have been reported to form in a variety of different structure types; however, inorganic clathrate-I materials with a low-cation concentration have yet to be investigated. Furthermore, tin-based compositions have been much less investigated as compared to silicon or germanium analogs. We report the temperature-dependent structural and thermal properties of single-crystal Eu2Ga11Sn35 revealing the effect of structure and composition on the thermal properties of this low-cation clathrate-I material. Specifically, low-temperature heat capacity, thermal conductivity, and synchrotron single-crystal x-ray diffraction reveal a departure from Debye-like behavior, a glass-like phonon mean-free path for this crystalline material, and a relatively large Grüneisen parameter due to the dominance of low-frequency Einstein modes. Our analyses indicate thermal properties that are a direct result of the structure and composition of this clathrate-I material.

Heat transport of the kagome Heisenberg quantum spin liquid candidate YCu3(OH)6.5Br2.5 : Localized magnetic excitations and a putative spin gap

The spin-1/2 kagom\'{e} Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, which unfortunately suffers from the notorious orphan spins problem caused by magnetic disorders. Here we turn to YCu$_3$(OH)$_{6.5}$Br$_{2.5}$, recently recognized as another host of a globally undistorted kagom\'{e} Cu$^{2+}$ lattice free from the orphan spins, thus a more feasible system for studying the intrinsic kagom\'{e} quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishing small residual linear term of $\kappa/T$ ($T\rightarrow 0$), and thus clearly rule out itinerant gapless fermionic excitations. Unusual scattering of phonons grows exponentially with temperature, suggesting thermally activated phonon-spin scattering and hence a gapped magnetic excitation, consistent with a $\mathbb{Z}_2$ quantum spin liquid ground state. Additionally, the analysis of magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized.

Heat capacity and thermal conductivity of CdCr2Se4 ferromagnet: Magnetic field dependence, experiment and calculations

The low-temperature specific heat capacity and thermal conductivity under magnetic field 0–13 T is investigated on polycrystalline sample of the ferromagnetic insulator CdCr2Se4, possessing spinel structure with magnetic Cr3+ ions in the octahedrally coordinated sites. The phonon (lattice) and magnon contributions are separated by their characteristic temperature dependencies and by quenching of the magnon component under high magnetic fields in the isothermal measurements. The results are analyzed with respect to formulas for model systems and confronted with ab-initio calculations of phonon and magnon spectra. The theoretical formula for the field-induced quenching of magnon heat capacity is well reproduced, but there are two kinds of deviations from expected behavior deserving attention: (1) Heat capacity at low fields is by several percent lower than expected and approaches the theoretical curve only above ∼0.5 T. We propose that the zero-field magnon heat capacity is diminished by a presence of magnetic domain walls and the expected field dependence is restored when magnetic field converts the sample into a single domain state. (2) The suppression of magnon heat capacity under high magnetic field is incomplete. While the data follow well the theoretical line for given temperature as regards the curvature, the final saturation seems to tend several percent above the expected value for lattice heat capacity. We propose that this behavior is related to the phonon-magnon coupling arising due to Zeeman gap in the magnon spectra and subsequent formation of intersection points with the phonon spectra. The thermal conductivity has shown substantial deviations with respect to model predictions as concerns temperature dependence of the phonon contribution and suppression of the magnon contribution in markedly lower magnetic fields than predicted by the model. We ascribe this discrepancy to a complex character of the intergrain heat transfer.

Vat photopolymerization 3D printing of thermal insulating mullite fiber-based porous ceramics

To satisfy the eagerness for mullite fiber-based porous ceramics (Mu f -based porous ceramics) with complex geometries in high-temperature thermal insulation fields, vat photopolymerization three-dimensional (3D) printing was first employed to prepare the highly complex Mu f -based porous ceramics. Herein, a photosensitive hydroxysiloxane HPMS-KH570 was employed as the resin matrix, which not only prevented the fiber agglomeration and improved the rheological properties of the slurries but also acted as a high-temperature binder to fix the fiber cross points after sintering and consequently improved the structural stability of the 3D-Mu f -based porous ceramics. The effects of mullite fiber (Mu f ) aspect ratio and content on the performance of Mu f slurries, microstructure and physical properties of the 3D-Mu f -based porous ceramics were investigated. When the fiber aspect ratio was 45 and the fiber content was 6.67 vol%, the Mu f slurry exhibited the most suitable properties for vat photopolymerization 3D printing, and the corresponding 3D-Mu f -based porous ceramics exhibited a well-defined 3D printed lattice structure with struts consisting of randomly crisscrossing Mu f . This unique 3D skeleton structure endowed the 3D-Mu f -based porous ceramics with low density (0.47 g/cm 3 ) and low room temperature thermal conductivity (0.11 W/(m·K)), enabling it a promising candidate for high-temperature (1000-1400 ℃) insulation materials. Furthermore, this printing strategy provided references for vat photopolymerization 3D printing of other fiber-based materials. • A strategy for DLP printing of 3D-Mu f -based porous ceramics was first presented. • The addition of HPMS-KH570 can prevent fiber agglomeration. • HPMS-KH570 can transform into SiO 2 binders and fix the fiber cross points. • 3D-Mu f -based porous ceramics exhibited an excellent thermal insulation property. • This work provides references for the DLP printing of other fiber-based materials.

Low Temperature Specific Heat and Thermal Conductivity in Superconducting UTe2

The measurements [Phys. Rev. B 100, 220504(R) (2019)] do not detect noticeable thermal conductivity in superconducting UTe2 in the T = 0 limit. At the same time the same crystals exhibit a large residual density of states comparable with its normal state value. The improvement of samples quality leads to the augmentation of critical temperature of transition to the superconducting state accompanied by the decreasing of residual specific heat ratio C(T)/T at T → 0. There is presented an analytic derivation of this inverse correlation property in concrete cases of triplet superconducting states with node-less and point-nodes order parameters allowed by symmetry in UTe2. The obtained explicit formulas for the residual thermal conductivity are in reasonable correspondence with observations.

Resistivity and thermal conductivity of an organic insulator β′–EtMe3Sb[Pd(dmit)2

A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator β′–EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, while the low-temperature thermal conductivity significantly depends on the cooling rate, the high-temperature resistivity is almost perfectly independent of the cooling rate. These results indicate that in the samples with the finite residual term, the mean free path of the quasiparticles that carry the heat at low temperatures is governed by disorders, whose characteristic length scale of the distribution is much longer than the electron mean free path that determines the high-temperature resistivity. This explains why recent X-ray diffraction and nuclear magnetic resonance measurements show no cooling rate dependence. Naturally, these measurements are unsuitable for detecting disorders of the length scale relevant for the thermal conductivity, just as they cannot determine the residual resistivity of metals. Present results indicate that very careful experiments are needed when discussing itinerant spin excitations in β′–EtMe3Sb[Pd(dmit)2]2.

Carbothermal reduction synthesis of high porosity and low thermal conductivity ZrC-SiC ceramics via an one-step sintering technique

In this work, porous ZrC-SiC ceramics with high porosity and low thermal conductivity were successfully prepared using zircon (ZrSiO4) and carbon black as material precursors via a facile one-step sintering approach combining in-situ carbothermal reduction reaction (at 1600 °C for 2 h) and partial hot-pressing sintering technique (at 1900 °C for 1 h). Carbon black not only served as a reducing agent, but also performed as a pore-foaming agent for synthesizing porous ZrC-SiC ceramics. The prepared porous ZrC-SiC ceramics with homogeneous microstructure (with grain size in the 50–1000 nm range and pore size in the 0.2–4 µm range) possessed high porosity of 61.37–70.78%, relatively high compressive strength of 1.31–7.48 MPa, and low room temperature thermal conductivity of 1.48–4.90 W·m−1K−1. The fabricated porous ZrC-SiC ceramics with higher strength and lower thermal conductivity can be used as a promising light-weight thermal insulation material.

Dielectric AlN/epoxy and SiC/epoxy composites with enhanced thermal and dynamic mechanical properties at low temperatures

In a superconducting magnet, the thermal properties of the insulating material at low temperatures is an important technical index to ensure the normal operation of a superconducting coil. Herein, a low-temperature thermal conductivity testing device with a tiny uncertainty was developed to measure the composites. Furthermore, SiC/epoxy and AlN/epoxy composites were prepared and their properties were tested at low temperatures. The results showed that the composites have a better thermal conductivity at low temperatures than pure epoxy. In addition, the AlN/epoxy and SiC/epoxy composites have excellent dynamic mechanical properties and low coefficient of thermal expansion. Due to the fair thermal and dynamic mechanical properties, the composites will have potential application in the superconducting magnet.