Anisotropic Thermal Transport Research Articles

Morphology Effects on Nonisotropic Thermal Conduction of Aligned Single-Walled and Multi-Walled Carbon Nanotubes in Polymer Nanocomposites

Carbon nanotube (CNT)−CNT contact and CNT distribution effects on anisotropic thermal transport in aligned CNT−polymer nanocomposites (PNCs) are studied using an off-lattice Monte Carlo numerical simulation. Inter-CNT contact and the associated thermal boundary resistance are shown here to significantly affect transport properties of PNCs, including anisotropy ratios. Previous studies have considered the effective thermal conductivities of CNT−PNCs using only a very large CNT−CNT thermal boundary resistance (TBR) compared to that of the CNT−matrix TBR as a limiting case. As CNT−CNT TBR is currently an unquantified parameter for CNT−polymer systems, and because it may be reduced by various techniques, heat transport with CNTs in contact is studied for a wide range of CNT−CNT TBR values, varying from 2 to 25 × 10−8 m2 K/W. The degree of CNT−CNT contact, CNT spatial distribution, and CNT−CNT TBR relative to CNT−matrix TBR are considered for 1−20% volume fraction of aligned single-walled and multi-walled CNTs...

Anisotropic thermal transport in highly ordered TiO2 nanotube arrays

This paper reports on the anisotropic thermal transport in highly ordered amorphous and anatase TiO2 nanotube arrays. Strong anisotropic thermal conductivity is observed: 0.617WK−1m−1 along the tube length direction and 0.077–0.102WK−1m−1 in the cross-tube direction for amorphous TiO2 nanotube arrays. The anatase TiO2 nanotube arrays are found to have a higher and anisotropic thermal conductivity, 1.12WK−1m−1 in the tube-length direction and 0.24WK−1m−1 in the cross-tube direction. The experimental results show that the density of the nanotube arrays is much lower than the value estimated from the geometry of the nanotube array, largely due to the existence of defects and loose contact among nanotubes. The thermal contact resistance between TiO2 nanotubes is characterized to be 15.1 and 20.6Km2W−1 for the two measured amorphous samples, and 5.90Km2W−1 for the anatase TiO2 nanotubes. This quantitatively proves that annealing of amorphous TiO2 nanotubes to transform them to anatase phase also improves the contact between nanotubes.

Anisotropic magnetic and transport properties of orthorhombicAl13Co4

We have investigated anisotropic physical properties (magnetic susceptibility, electrical resistivity, thermoelectric power, Hall coefficient, and thermal conductivity) of the $o{\text{-Al}}_{13}{\text{Co}}_{4}$, an orthorhombic approximant to the decagonal phase. The crystallographic-direction-dependent measurements were performed along the $a$, $b$, and $c$ directions of the orthorhombic unit cell, where $(b,c)$ atomic planes are stacked along the perpendicular $a$ direction. Magnetic susceptibility is predominantly determined by the Pauli-spin paramagnetism of conduction electrons. The in-plane magnetism is stronger than that along the stacking $a$ direction. Anisotropic electrical and thermal conductivities are the highest along the stacking $a$ direction. The anisotropic thermoelectric power changes sign with the crystallographic direction and so does the anisotropic Hall coefficient which changes from negative electronlike to positive holelike for different combinations of the electric current and magnetic-field directions. The investigated anisotropic electrical and thermal transport coefficients were reproduced theoretically by ab initio calculation using Boltzmann transport theory and the calculated anisotropic Fermi surface. The calculations were performed for two structural models of the $o{\text{-Al}}_{13}{\text{Co}}_{4}$ phase, where the more recent model gave better agreement, though still qualitative only, to the experiments.

Anisotropic Thermal Conductivity in Cross-Linked Polybutadienes Subjected to Uniaxial Elongation

Anisotropic thermal transport in cross-linked polybutadienes deformed in simple elongation is investigated experimentally. Using a novel optical technique based on forced Rayleigh scattering, two c...

Anisotropic magnetic, electrical, and thermal transport properties of the Y-Al-Ni-Co decagonal approximant

We have investigated anisotropic physical properties (magnetic susceptibility, electrical resistivity, thermoelectric power, Hall coefficient, and thermal conductivity) of Y-Al-Ni-Co decagonal approximant with composition ${\text{Al}}_{76}{\text{Co}}_{22}{\text{Ni}}_{2}$. The crystalline-direction-dependent measurements were performed along three orthogonal directions ${a}^{\ensuremath{\ast}}$, $b$, and $c$ of the Y-Al-Ni-Co unit cell, where $(a,c)$ monoclinic atomic planes are stacked along the perpendicular $b$ direction. Anisotropic magnetic susceptibility of conduction electrons is paramagnetic for the field lying within the $(a,c)$ atomic planes and diamagnetic for the field along the $b$ direction. Anisotropic electrical resistivity is low in all crystalline directions, appearing in the order ${\ensuremath{\rho}}_{{a}^{\ensuremath{\ast}}}g{\ensuremath{\rho}}_{c}⪢{\ensuremath{\rho}}_{b}$. Thermopower shows electron-phonon enhancement effect. Anisotropic bare thermopower (in the absence of electron-phonon interactions) was extracted, appearing in the same order as the resistivity, $|{S}_{{a}^{\ensuremath{\ast}}}^{\text{bare}}/T|g|{S}_{c}^{\text{bare}}/T|g|{S}_{b}^{\text{bare}}/T|$. Anisotropic thermal conductivity appears in the order ${\ensuremath{\kappa}}^{b}g{\ensuremath{\kappa}}^{c}g{\ensuremath{\kappa}}^{{a}^{\ensuremath{\ast}}}$, so that $b$ is the most conducting direction for both electricity and heat. Hall coefficient ${R}_{H}$ exhibits pronounced anisotropy, where the magnetic field in a given crystalline direction yields the same ${R}_{H}$ for the current along the other two crystalline directions in the perpendicular plane. These anisotropies are analyzed in terms of the anisotropic structure of the Y-Al-Ni-Co phase and the ab initio calculated anisotropic Fermi surface. The results are compared with the literature-reported anisotropy of the physical properties of the $d$-Al-Ni-Co decagonal quasicrystal.

Anisotropic electrical, magnetic and thermal transport properties of the Al80Cr15Fe5 decagonal approximant

The anisotropic magnetic, electrical and thermal transport properties were studied of a single-crystalline Al80Cr15Fe5 approximant to the decagonal quasicrystal. The temperature-dependent electrical resistivity along the b and c crystalline directions shows non-metallic behaviour with a broad maximum, whereas it exhibits a metallic positive temperature coefficient along the a direction perpendicular to the (b, c) atomic planes. The resistivity can be treated with the model of slow charge carriers, where the increased electron–phonon scattering upon raising the temperature induces a transition from dominant Boltzmann (metallic) to dominant non-Boltzmann (insulating-like) regime. The magnetic susceptibility and thermal conductivity also reveal a small anisotropy.

Fast and highly anisotropic thermal transport through vertically aligned carbon nanotube arrays

This letter reports on fast and highly anisotropic thermal transport through millimeter-tall, vertically aligned carbon nanotube arrays (VANTAs) synthesized by chemical vapor deposition on Si substrates. Thermal diffusivity measurements were performed for both longitudinal and transverse to the nanotube alignment direction, with longitudinal values as large as 2.1±0.2cm2∕s and anisotropy ratios as large as 72. Longitudinal thermal conductivities of 15.3±1.8W∕(mK) for porous 8±1vol% VANTAs in air and 5.5±0.7W∕(mK) for epoxy-infiltrated VANTAs already exceed those of phase-changing thermal interface materials used in microelectronics. Data suggest that further improvements are possible through optimization of density and defects in the arrays.

Enhanced spin fluctuations inCa3Co4−xTixO9single crystals

The effect of Ti doping on anisotropic magnetic, electronic, and thermal transport properties of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{9}$ single crystals has been investigated. The Ti substitution drives the system from a ferrimagnetic state for $x=0$ to spin-glass state for $x\ensuremath{\geqslant}0.2$ at low temperatures. Two anomalies in the $d{\ensuremath{\chi}}_{c}∕dT$ versus $T$ curve are observed for all samples, which are suggested to be related to the appearance of short-range spin-density-waves (SDW) and ferrimagnetic state. The in-plane resistivity of all studied samples shows a metal-insulator transition at ${T}_{\ensuremath{\rho}\phantom{\rule{0.2em}{0ex}}\mathrm{min}}$. ${T}_{\ensuremath{\rho}\phantom{\rule{0.2em}{0ex}}\mathrm{min}}$ is essentially constant at about $85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x\ensuremath{\leqslant}0.6$ and reaches $142\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as $x=0.8$. However, the out-of-plane resistivity shows the semiconducting behavior in the whole temperature and doping range studied. The in-plane thermopower increases monotonically with the increase of temperatures in the studied temperature range. Strikingly, for Ti-doping samples, the out-of-plane thermopower exhibits a steeper upturn at low temperatures and is obviously suppressed by an applied magnetic field. Combined with the magnetic measurements, the low-temperature upturn of out-of-plane thermopower is suggested to originate from the spin fluctuation induced by Ti doping. In addition, both the resistivity and thermopower increase monotonically with increasing the Ti content, which is considered to be related to the variation of carrier concentration due to Ti doping.

Thermal Modeling of a Multilayered Film via Taylor Series Expansion- and Least Squares-Based-Lattice Boltzmann Method

An enhanced lattice Boltzmann scheme, Taylor series expansion- and least squares-based-lattice Boltzmann method (TLLBM), is adopted to simulate transient thermal behavior in a thin multilayer. The meshless formulation of TLLBM not only handles the changes in material properties from one layer to another, but also facilitates the general geometry handling capabilities. The energy carrier interaction at the interface is governed via diffusive mismatch model (DMM). We simulated transient thermal behavior of a hot-spot in the dual-layer by incorporating a heat source in one of the layers. The sub-continuum effect of anisotropic thermal transport is presented where the heat preferentially flows laterally in the layer with higher conductivity as its thickness is reduced. The insight into the nanoscale thermal behavior acquired via a relatively simple model will be critical for the design and operation of complex data storage and electronic systems, where thermal transport plays an active and critical role

A novel heat transfer model and its application to information storage systems

Lattice Boltzmann method (LBM) based on Boltzmann transport equation is developed to simulate the nanoscale heat transport in solids. The LBM can simulate both the metals and semiconductors by properly incorporating the energy carriers. We found that boundary scattering of phonons results in an anisotropic thermal transport in nanoscale solids. The electron-phonon coupling is introduced to accurately describe the thermal behavior of nanoscale confined solids. Our numerical tool will be suitable for simulating complex multiscale systems involving multiple energy carriers with different length and time scales, and is useful in magnetic recording technology when the thermal response plays a crucial role such as for reliability of the head-disk interface and the heat assisted magnetic recording systems.

Cooling behaviour of particle filled polypropylene during injection moulding process

The effects of thermal properties of various fillers (magnetite, barite, copper, talc, glass fibres and strontium ferrite) in various proportions on the cooling behaviour of polypropylene matrix composites are investigated in an injection moulding process. A thermocouple in the cavity of the mould records the temperatures at the surface of the composite during injection moulding. From the slope of the cooling curves the thermal diffusivities of the composites are estimated and compared with thermal diffusivities at room temperature and elevated temperatures measured with a transient technique. The cooling curves show different merging sections affected by the after pressure, the diffusivity of the composite and the diffusivity of polypropylene matrix. The cooling behaviour depends on the anisotropic thermal diffusivity of the used composite, which is caused by the alignment of filler material due to the injection moulding process and the interconnectivity of the filler particles. The thermal diffusivity shows the highest value for 30 vol% talc filled polypropylene, whereas the shortest cooling time was found for 35 vol% copper filled polypropylene. The knowledge of the systematic variation of thermal transport properties of composites due to different filler material and filler proportions allows to optimize the mould process and to customize the heat flow properties. Furthermore, the strongly anisotropic thermal transport properties of talc filled polypropylene allow the design of composites with a predefined maximum heat flow capability to transport heat in a preferred direction.

Anisotropic thermal transport of 2D quasicrystals of decagonal Al–Ni–Co system

The anisotropic thermal conductivity of 2D quasicrystals of the decagonal Al–Ni–Co system has been measured over a wide range of temperature. The thermal conductivity value along the periodic axis shows the nearly same behavior as the dirty alloy system. The quasiperiodic thermal conductivity exhibits a typical plateau dependence similar to that of the disordered system. The anisotropic behavior of the measured thermal conductivity is discussed on the basis of phonon relaxation time approximation. The main contributions of phonon scattering processes in the periodic and quasiperiodic directions are phonon–electron interaction and point-defect-like scattering effect arising from Penrose tiling, respectively. A small increase above the plateau region in the quasiperiodic transport indicates hopping conduction of localized vibration modes assisted by phonons such as fracton excitations in the disordered system.

Holographic interferograms from laser fusion code simulations

The simulation of interferograms for laser fusion-type targets is described. Three-dimensional ray-tracing postprocessors to the two-dimensional hydrodynamic codes lasnex and sage are used for this purpose. Simulated interferograms can then be directly compared with experimentally obtained interferograms, thus providing insight into various aspects of target physics, such as anisotropic thermal transport and details of the hydrodynamic flow. The method is illustrated for two specific KMS microdisk target shots diagnosed with double-exposure holographic interferometry.

Anisotropic electronic and thermal transport properties of lightly oxygen-doped La 2CuO 4+ δ

We track the systematic variation in magnetic susceptibility (χ), thermal conductivity (κ), anisotropic resistivity (ϱ), and thermoelectric power ( S) of crystalline La 2CuO 4+ δ as a function of light hole doping. The level of hole doping (δ) was carefully varied by anneals under controlled oxygen-partial-pressure performed in such a way that the antiferromagnetic ordering temperature T N ranged from 250 to 310 K. χ data indicate that increased oxygen hole doping acts to reduce T N below the δ = 0 value of 320 K. Anomalies are present in κ at T n that result from Cu spin fluctuations about the magnetically ordered ground state. Anomalies are also present in ϱ and S that occur at T N for air-annealed samples wherein T N = 250 K; when more lightly hole-doped so that T N >250 K, the anomalies in ϱ and S do not occur at T N, but instead occur at T<250 K. In analogy with heavily oxygen-loaded La 2CuO 4+ δ , these transport anomalies may result from phase separation involving the diffusion of non-stoichiometric, excess oxygen.

Origin of Thermal Conductivity Anisotropy in Liquid Crystalline Phases

Thermal conductivity measurements have been performed in nematic and smectic liquid crystalline phases by the forced Rayleigh light-scattering technique. Results show clearly that the thermal transport anisotropy is governed by the molecular shape anisotropy and by the molecular orientation but is independent of the smectic layer ordering.