Isotropic Conductivity Research Articles

Joint‐Inspired Liquid and Thermal Conductive Interface for Designing Thermal Interface Materials with High Solid Filling yet Excellent Thixotropy

AbstractFor advanced thermal interface materials (TIMs), massive inorganic addition for high isotropic thermal conductivities conflicts with suitable rheological viscosity for low contact thermal resistance. Traditional strategies rarely resolve such a contradiction, and it remains an academic and industrial challenge. Herein, inspired by the structure and function of the bone joint, a best‐of‐both‐worlds approach is reported that endows a standard polydimethylsiloxane/alumina (PDMS/Al2O3) TIM with simultaneously enhanced rheological mobility and thermal conductivity. It is conducted by employing morphology‐controllable gallium‐based liquid metal (LM) to the surface of Al2O3by a scalable mechanochemical process. At the typical polymer‐LM‐Al2O3interface, LM droplets with low cohesive energy can release the freedom for macromolecular chain relaxation and reduce the viscosity, successfully allowing the high‐loading TIMs (79 vol.%) to keep the thixotropic state and effectively reducing its contact thermal resistance with a copper substrate by 65%. At the same time, adjacent LMs merge to thermally bridge separate Al2O3particles, which facilitates the interfacial thermal conduction and enhances the thermal conductivity from 5.9 to 6.7 W m−1 K−1. Along with additional electrical insulation, this filler modification strategy is believed to inspire others to develop high‐performance polymer‐based TIMs for future advanced electronics.

Thermal analysis of surface micromachined porous silicon membranes using the 3ω method: Implications for thermal sensing

Investigations into porous silicon thermo-resistive type thermal sensors and their advantages in speed-sensitivity trade-off relative to other comparable thermal materials are presented in this work. Porous silicon films were suspended above a silicon substrate using successive patterning and micromachining steps. The 3ω method was used in both supported and suspended configurations, allowing the analysis of both cross-plane and in-plane thermal properties of the micromachined films. By utilising a low noise, broad-frequency measurement of the in-phase and out-of-phase temperature components, thermal conductivity and thermal diffusivity were simultaneously determined. The measurements showed isotropic thermal conductivity of 0.38 ± 0.02 W/m K and 0.12 ± 0.01 W/m K, and thermal diffusivity of 0.39 ± 0.04 mm2/s and 0.23 ± 0.02 mm2/s, for the surface micromachined released porous silicon films at 50 % and 77 % porosity, respectively. The implemented suspended 3ω method leveraged a thermal model that accounted for the finite length of the heater(180 µm) and membranes (400 µm × 400 µm), eliminating the millimetre scale dimensional constraints in previous works. Use of a thermal passivation technique rendered the thermal properties of the films robust against successive photolithography and micromachining. The obtained thermal properties were utilised in finite element modelling of a thermo-resistive thermal sensor with 50 µm × 50 µm × 100 nm dimensions. The modelling results suggest that a thermal time constant of 5 ms could be achieved, comparable to that of amorphous silicon based thermal sensors but with 2–4 times larger temperature sensitivity due to smaller intrinsic thermal conductivity and heat capacity in the porous silicon films, thus achieving a significant improvement in overall speed-sensitivity trade-off.

FEM heat transfer modelling with tomography-based SiCf/SiC unit cell

Modern industry has become increasingly reliant on composite materials for a variety of applications, and the nuclear industry is no exception to this. Among the materials being researched as Enhanced Accident Tolerant Fuels, ceramic matrix composites such as SiC-fiber-reinforced SiC (SiCf/SiC) figure as some prime candidates due to their excellent high temperature performances. SiCf/SiC so far shows adequate nuclear, mechanical and chemical properties; still, the thermal properties need further investigation. The thermal behavior of a material is an important factor for its performance as a nuclear fuel cladding, i.e. the first barrier encapsulating the fuel pellets. Many features determine the resulting properties of composite materials, such as matrix and fiber reinforcement properties and orientation, void fraction, and pore morphology. This study establishes a methodology to study the physical properties of composite materials and applies it to SiCf/SiC. A FEM model is used to characterize the thermal properties of a fundamental SiCf/SiC element, referred to as a �unit cell�, with the objective of accurately predicting the thermal properties of this complex class of materials where experimental data is often difficult to obtain. The unit cell is built based on data acquired with high-resolution tomographic microscopy performed at the TOMCAT beamline of the Swiss Light Source. By using phase-retrieval prior to tomographic reconstruction, the pores, fibers and matrix that compose the material can be distinguished in the data analysis. The separated information is processed to obtain geometrical information about the individual pores and fibers, which is then used to parametrize them as cylindrical objects. This allows constructing a FEM model of a cubic unit cell that is used to extract the effective thermal properties of SiCf/SiC. The analysis scheme includes steady-state and dynamic thermal transport simulations, which yield directional effective thermal conductivity and diffusivity values, respectively. Both modes of analysis show isotropic thermal conductivity values in the range of 71 W/m/K at room temperature, more than three times that of currently employed nuclear cladding materials. Combining these results with the data on the larger structural features of the material will lead to realistic results on the macroscopic thermal properties.

First-principles study on phonon transport properties of MoTe2 and WTe2 monolayers in different phases

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDC) monolayer is a key method for manipulating the lattice thermal conductivity. In this paper, we investigate the phonon thermal transport properties of MoTe2 and WTe2 monolayers with H and T′ phases (H-TMDC and T′-TMDC) via first-principles calculations. H-MoTe2 and H-WTe2 monolayers have isotropic lattice thermal conductivities, whereas the lattice thermal conductivities of T′-MoTe2 and T′-WTe2 monolayers are highly anisotropic. Meanwhile, in comparison with the H phase, the maximal lattice thermal conductivity in T′ phase has a reduction of 83.85% and 77.92% at 300 K for MoTe2 and WTe2 monolayers, respectively. Furthermore, we observe that phonon heat capacities and phonon lifetimes of T′-TMDC monolayers are lower than those of H-TMDC monolayers, while the phonon group velocities of T′-TMDC monolayers are larger. Hence, the competitive relationship among these parameters leads to a lower lattice thermal conductivity of the T′-TMDC monolayer. In addition, lower critical phonon MFPs (mean free path) of T′-TMDC monolayers prove that the lattice thermal conductivity has a weaker size dependence. These findings demonstrate that MoTe2 and WTe2 monolayers with different phases have a great potential in 2D nanoelectronic devices.

A chiral SrSi2 (srs) superstructure constructed by a dual interaction system showing isotropic electrical conductivity

Most porous conductive frameworks are highly anisotropic in their structures thus leading to anisotropic charge transport. Here we report a supramolecular self-assembly which is constructed by intermolecular hydrogen bonding and π···π interactions. This material features a chiral, porous, cubic framework structure with π-stacked helical columns along all of the three Cartesian coordinates. As a result, isotropic charge transport with an electrical conductivity (σ) of 2.1 × 10–7 S/cm is achieved. By achieving isotropic charge transport in a π-stacked supramolecular assembly, these results provide a new type of isotropic conductive framework materials alternative to conductive metal-organic frameworks (MOFs).

Bounds for the Electrical Resistance for Non-homogeneous Two-dimensional Conducting Body

A mathematical model is developed to determine the steady-state current flow through in non-homogeneous isotropic conductor whose shape is a two-dimensional hollow domain. The corresponding electric boundary value problem is formulated using Maxwell's theory of electricity. The determination of the two-dimensional motion of charges is based on the concept of the electrical conductance. The Cauchy-Schwarz inequality is used to get the lower and upper bounds for the electrical conductance. The derived upper and lower bound formulae are illustrated by numerical examples.

Triggering Fast Lithium Ion Conduction in Li4PS4I

All-solid-state batteries are receiving keen interest as emerging alternatives to conventional liquid electrolyte batteries owing to potential benefits that include the possibility of higher energy densities, enablement of anode-less designs, a wider range of temperature operation, and improvement in battery safety. The discovery and development of new solid-state electrolytes is a critical factor. One sought-after material is Li4PS4I that has long been predicted to exhibit high ionic conductivity, while experiment has disappointingly proven the contrary. Here, we address this long-standing issue and show that inducing Li sublattice disorder is key. We demonstrate the remarkable effects of aliovalent substitution in Li4+xP1–xSixS4I (x = 0.12 and 0.30) using a combination of single-crystal X-ray and powder neutron diffraction; Raman and impedance spectroscopy; ab initio molecular dynamics simulations; and the bond valence site energy approach. With increasing Si4+ and thus Li+ content in Li4+xP1–xSixS4I, configurational disorder of the Li sublattice is induced, leading to isotropic 3D-fast Li ion conductivity of 1.46 mS·cm–1 at room temperature for the Li4.3P0.7Si0.3S4 composition and a low activation energy of 0.32 eV. The unit cell volume is half that of the parent phase, Li4PS4I, which exhibits a fully ordered Li substructure that explains the latter’s poor conductivity and high activation energy (0.046 mS·cm–1 and 0.44 eV, respectively). The deliberate creation of Li sublattice disorder (via Li ion “stuffing”) is a pivotal strategy toward the development of new, fast ion conductors.

Tunable thermal properties of the biphenylene and the lateral heterostructure formed with graphene: A molecular dynamics investigation

Recently, biphenylene was successfully synthesized as a novel allotrope of carbon. In this investigation, non-equilibrium molecular dynamics calculations are conducted to explore the intrinsic thermal properties of biphenylene. The isotropic thermal conductivity of biphenylene is obtained, which is also sensitive to size and temperature. Furthermore, the graphene/biphenylene lateral heterostructure is constructed to possess an interfacial thermal conductance of about 2.84 × 109 W K−1 m−2. The external tensile strain can induce a redshift of the vibrational density of states of pristine graphene and biphenylene, and the improved overlap also results in an enhanced heat flux in the biphenylene/graphene heterostructure. Our approach can provide theoretical guidance to design a thermal management device based on graphene and biphenylene.

High thermal conductivity in wafer-scale cubic silicon carbide crystals

High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as both active functional materials and thermal management materials. We report an isotropic high thermal conductivity exceeding 500 W m−1K−1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-standing puzzle that the literature values of thermal conductivity for 3C-SiC are lower than the structurally more complex 6H-SiC. We show that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which avoids the exceptionally strong defect-phonon scatterings. Moreover, 3C-SiC is a SiC polytype which can be epitaxially grown on Si. We show that the measured 3C-SiC-Si thermal boundary conductance is among the highest for semiconductor interfaces. These findings provide insights for fundamental phonon transport mechanisms, and suggest that 3C-SiC is an excellent wide-bandgap semiconductor for applications of next-generation power electronics as both active components and substrates.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen-Bonded Phosphate Network.

Utilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10-4 S cm-1 ) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three-dimensional (3D) hydrogen-bonded (H-bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H-bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10-3 S cm-1 at ≈351 K, which is the first example for organic molecular crystal. Variable-temperature X-ray structural analysis and solid-state 2 H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H-bonded phosphate network.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen‐Bonded Phosphate Network

AbstractUtilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10−4 S cm−1) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three‐dimensional (3D) hydrogen‐bonded (H‐bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H‐bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10−3 S cm−1 at ≈351 K, which is the first example for organic molecular crystal. Variable‐temperature X‐ray structural analysis and solid‐state 2H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H‐bonded phosphate network.

Growth-microstructure-thermal property relations in AlN thin films

AlN thin films are enabling significant progress in modern optoelectronics, power electronics, and microelectromechanical systems. The various AlN growth methods and conditions lead to different film microstructures. In this report, phonon scattering mechanisms that impact the cross-plane (κz; along the c-axis) and in-plane (κr; parallel to the c-plane) thermal conductivities of AlN thin films prepared by various synthesis techniques are investigated. In contrast to bulk single crystal AlN with an isotropic thermal conductivity of ∼330 W/m K, a strong anisotropy in the thermal conductivity is observed in the thin films. The κz shows a strong film thickness dependence due to phonon-boundary scattering. Electron microscopy reveals the presence of grain boundaries and dislocations that limit the κr. For instance, oriented films prepared by reactive sputtering possess lateral crystalline grain sizes ranging from 20 to 40 nm that significantly lower the κr to ∼30 W/m K. Simulation results suggest that the self-heating in AlN film bulk acoustic resonators can significantly impact the power handling capability of RF filters. A device employing an oriented film as the active piezoelectric layer shows an ∼2.5× higher device peak temperature as compared to a device based on an epitaxial film.

Bounds for the electrical resistance for non-homogeneous conducting body

Abstract A mathematical model is developed to determine the steady-state electric current flow through in non-homogeneous isotropic conductor whose shape has a three-dimensional hollow body. The equations of the Maxwell's theory of electric current flow in a non-homogeneous isotropic solid conductor body are used to formulate the corresponding electric boundary value problem. The determination of the steady motion of charges is based on the concept of the electrical conductance. The derivation of the upper and lower bound formulae for the electrical conductance is based on Cauchy-Schwarz inequality. Two numerical examples illustrate the applications of the derived upper and lower bound formulae.

Highly Anisotropic Tip-Induced Domain Growth in Polydomain Triglycine Sulfate

Ferroelectrics are widely used as modern functional materials for nonlinear optics, optoelectronics, nonvolatile data storage, and nanoelectronics. Most of these applications require creation of precise micron-scale domain patterns. We have studied the current limited wall motion regime driven by anisotropic bulk screening processes during local switching in polydomain triglycine sulfate single crystals. The field application using a conductive tip of a scanning probe microscope (SPM) in a dry atmosphere resulted in formation of a partially switched area, strongly elongated in the c crystallographic direction. At the same time, switching at high humidity led to a circular shape of the area. The calculation of the spatial distribution of the external field demonstrated the possibility of domain wall shift at distances up to hundreds of microns from the SPM tip. However, after external field switch-off the depolarization field in nonscreened areas led to reverse domain wall motion. Thus, the domain wall stabilization is possible only in the regions with sufficient screening of the depolarization field. We have shown that the growth of the area size with time obeys the same current-limited wall motion expressions that were derived previously for domain growth during local switching by a highly resistive electrode. The change in the area shape with humidity was attributed to the change in the governing screening mechanism from anisotropic bulk conductivity to isotropic surface conductivity through an adsorbed water layer. The obtained results demonstrate the essential role of the screening processes during periodical poling and pave the way for further improvement of the domain engineering methods.

Isotropic porous graphite foam/epoxy composites with outstanding heat dissipation and excellent electromagnetic interference shielding performances

In the field of radio frequency chips package, design of isotropic material was a significant challenge to efficiently resolve undesirable electromagnetic radiation and excessive heat accumulation in confined tiny spaces. Here, a brand-new material design concept was proposed and successfully verified through controllable graphitization degree of carbon materials and curvature of twisted graphite sheets during heating and foaming process of asphalt. Concretely, aromatic compounds of the asphalt condensed to form sheet-like macromolecules, which were twisted and oriented along the pore walls. Eventually, these macromolecules were highly graphitized, which expectedly realized the consummate integration of local orientation and macroscopic isotropy. Particularly, a special interlocking epoxy was introduced to prepare epoxy/graphite foam composites (EP-GFs). The EP-GFs (average pore size 0.09 mm) demonstrated an isotropic thermal conductivity up to 118.5 ± 5.8 W·m−1·K−1. Astoundingly, EP-GFs (average pore size 0.244 mm) showed an isotropic electromagnetic interference shielding efficiency (EMI SE), reaching up to 113 dB in 8–12 GHz. Therefore, the EP-GFs completely integrated outstanding thermal conductivity, excellent EMI SE and isotropy macroscopically. Interestingly, the EP-GFs can be cured by Joule heat of low-voltage direct current, and the primary heating region was the contact interface between copper sheets instead of the whole bonded product. This work offered a facile feasible strategy for fabricating isotropic materials to address heat dissipation and electromagnetic pollution of advanced electronic devices in an economical and environmentally friendly manner.

Quasi-isotropic thermal conductivity of polymer films enhanced by binder-free boron nitride spheres

Heat dissipation has become increasingly crucial in miniaturized modern electronics. Thanks to the high in-plane thermal conductivity yet excellent electrical insulation, hexagonal boron nitride (h-BN) has been considered as an ideal filler material to enhance the thermal conductivity of polymers for efficient heat dissipation. However, the low out-of-plane thermal conductivity significantly limits their practical applications and a heat spreader that propagates heat along multiple directions is favorable. In this work, micro-sized, binder-free boron nitride spheres (BNSs) have been successfully synthesized using a two-step process of spray drying and high-temperature sintering. Consisting of randomly dispersed boron nitride nanosheets (BNNSs), the BNSs show an isotropic thermal conductivity of 34.8 W/mK. Using the BNSs as fillers, the out-of-plane thermal conductivity of poly(vinyl alcohol) (PVA) film is significantly enhanced to 8.1 W/mK, the second highest value compared with previously reported BN based PVA composite films. In the meantime, the in-plane thermal conductivity, up to 10.6 W/mK, is not sacrificed, indicating the quasi-isotropy in thermal conductivity. The significant thermal conductivity enhancement (∼3700%) of PVA is attributed to the formation of isotropic thermally conductive networks within the polymer matrix and strong interactions between BNNSs inside BNSs. This study provides a practical route to fabricate BN-enhanced composite films with isotropic thermal conductivity and promising materials that are valuable for heat dissipation in new-era advanced electronics and related applications.

Effect of Anisotropic Electrical Conductivity Induced by Fiber Orientation on Ablation Characteristics of Pulsed Field Ablation in Atrial Fibrillation Treatment: A Computational Study.

Pulsed field ablation (PFA) is a promising new ablation modality for the treatment of atrial fibrillation (AF); however, the effect of fiber orientation on the ablation characteristics of PFA in AF treatment is still unclear, which is likely an essential factor in influencing the ablation characteristics. This study constructed an anatomy-based left atrium (LA) model incorporating fiber orientation and selected various electrical conductivity and ablation targets to investigate the effect of anisotropic electrical conductivity (AC), compared with isotropic electrical conductivity (IC), on the ablation characteristics of PFA in AF treatment. The results show that the percentage differences in the size of the surface ablation area between AC and IC are greater than 73.71%; the maximum difference in the size of the ablation isosurface between AC and IC at different locations in the atrial wall is 3.65 mm (X-axis), 3.65 mm (Z-axis), and 4.03 mm (X-axis), respectively; and the percentage differences in the size of the ablation volume are greater than 6.9%. Under the condition of the pulse, the amplitude is 1000 V, the total PFA duration is 1 s, and the pulse train interval is 198.4 ms; the differences in the temperature increase between AC and IC in LA are less than 2.46 °C. Hence, this study suggests that in further exploration of the computational study of PFA in AF treatment using the same or similar conditions as those used here (myocardial electrical conductivity, pulse parameters, and electric field intensity damage threshold), to obtain more accurate computational results, it is necessary to adopt AC rather than IC to investigate the size of the surface ablation area, the size of the ablation isosurface, or the size of the ablation volume generated by PFA in LA. Moreover, if only investigating the temperature increase generated by PFA in LA, adopting IC instead of AC for simplifying the model construction process is reasonable.

Ultralow electron-surface scattering in nanoscale metals leveraging Fermi-surface anisotropy

Increasing resistivity of metal wires with reducing nanoscale dimensions is a major performance bottleneck of semiconductor computing technologies. We show that metals with suitably anisotropic Fermi velocity distributions can strongly suppress electron scattering by surfaces and outperform isotropic conductors such as copper in nanoscale wires. We derive a corresponding descriptor for the resistivity scaling of anisotropic conductors, screen thousands of metals using first-principles calculations of this descriptor and identify the most promising materials for nanoscale interconnects. Previously-proposed layered conductors such as MAX phases and delafossites show promise in thin films, but not in narrow wires due to increased scattering from side walls. We find that certain intermetallics (notably CoSn) and borides (such as YCo$_3$B$_2$) with one-dimensionally anisotropic Fermi velocities are most promising for narrow wires. Combined with first-principles electron-phonon scattering predictions, we show that the proposed materials exhibit 2-3x lower resistivity than copper at 5 nm wire dimensions.

Spherical boron nitride/silicone rubber composite with high isotropic thermal conductivity via pre‐constructing thermally conductive networks

AbstractThermal interface material (TIM) is crucial for electronic devices to dissipate heat, but the high interface thermal resistance between polymer matrix and filler is a major problem affecting its thermal conductivity (TC). In this study, we prebuilt excellent thermally conductive pathway of isotropic spherical boron nitride (sBN) using a facile method, that is, pressing fillers, to decrease the interface thermal resistance before filling matrix silicon rubber (SR), and prepared a TIM with high isotropic TC. At 50 wt% filler content, the through‐plane and in‐plane TCs of sBN/SR composite reached 9.36 and 7.82 W/(m·K), respectively, which are higher than the highest value of previous research on bulk BN/SR composites (not including film), 4.13 and 6.56 W/(m·K), respectively. Meanwhile, the thermal decomposition temperature of TIM is 47.2°C higher than that of pure SR. In addition, the TIM has a low hardness (shore A hardness <70) and can be bent and folded, so can be used to substitute traditional thermally conductive silicone pad.

Electric current and magnetization distributions’ self-organization features in a magnetoresistive film nanoelement under the influence of an external magnetic field

The new generalized Laplace’s equation for an electric potential is presented, which in the general case, is nonlinear for media with resistance anisotropy caused by the anisotropic magnetoresistance (AMR) effect and determined by the orientation of the magnetization vector relative to the current density vector at an arbitrary point of the sample. The solution to this equation is obtained for an anisotropic medium with a non-uniform distribution of current density and magnetization in the case of an oblique-shaped plate of nanoscale thickness. In addition, the analytical solution was obtained for the current density distribution in such a medium using conformal mapping, which confirms the numerical solution in the case of an isotropic conductor. An analysis of the results showed that the presence of the AMR effect leads to a significant change in the current density distribution in the sample compared to the isotropic case, which is expressed in the deviation of the current density vector in perpendicular to the magnetization vector direction in the plate. It has been established that the presence of an external magnetic field in the film plane, which leads to the emergence of an inhomogeneous magnetization distribution, entails self-organization of the electric current distribution. The graphs of the plate resistance as a function of applied magnetic field demonstrate asymmetry for the cases of the direct and reverse directions of the external magnetic field, which cannot be obtained within the framework of simplified models of current flow in media with the AMR effect.