Anisotropic Transport Properties Research Articles

Effect of Anisotropic Mass Transport Properties of Porous Layers on Proton Exchange Membrane Fuel Cell Transient Behavior

Effect of Anisotropic Mass Transport Properties of Porous Layers on Proton Exchange Membrane Fuel Cell Transient Behavior

First-principles investigation of in-plane anisotropies in XYTe4 monolayers with X = Hf, Zr, Ti and Y = Si, Ge.

In-plane anisotropic materials can introduce additional degrees of freedom while tuning their physical properties, which expand the range of opportunities for designing novel semiconductor devices and exploring distinct applications. In this work, we investigate the in-plane anisotropic electronic, elastic, transport and piezoelectric properties in a family of isostructural telluride XYTe4 (X = Hf, Zr and Ti, Y = Si and Ge) monolayers based on first-principles calculations. Six types of structures are verified to harbor direct bandgaps at the Γ point ranging between 0.98 and 1.36 eV. The orientation-dependent in-plane elastic stiffness of XYTe4 reveals the anisotropic and ultrasoft nature. Superior dielectric constants and giant switching effects are found in TiGeTe4 monolayers because of giant in-plane anisotropy. Strikingly, the piezoelectric coefficients of XSiTe4 differ by an order of magnitude along the two main directions. The strong in-plane anisotropic elastic properties of XYTe4 monolayers together with outstanding piezoelectric responses show that these structures can compete with that of transition metal dichalcogenides for applications in the field of flexible electronic devices.

Structure and thermoelectric performance of Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds

In this study, we find new Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds in Ag-In-Se system by static diffusion method combined with common X-ray diffraction and backscattering electron analysis. The crystal structure belongs to the trilateral system with the<i> P</i>3<i>m</i>1 space group, which features a two-dimensional layered structure. The unit cell is composed of 9-atom quantum layers arranged in the sequence of Se1-In1-Se2-In2-Se3-Ag/In3-Se4-In4-Se5, and in-between these layers are bonded by the weak van der Waals force. The sintered bulk samples show highly anisotropic transport properties and have an ultra-low lattice thermal conductivity along the direction parallel to sintering pressure about 0.15 W·m<sup>–1</sup>·K<sup>–1</sup> at 873 K. The intrinsically ultra-low lattice thermal conductivity mainly comes from low phonon velocity and the strong coupling between low frequency optical phonon and acoustic phonons. The Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds behave as an n-type conduction. The electrical conductivity is 4×10<sup>4</sup> S·m<sup>–1</sup> and the Seebeck coefficient is –80 μV·K<sup>–1</sup> at room temperature. Therefore, Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds show high electrical transport properties in a wide temperature range, and the power factor is around 5 μW·cm<sup>–1</sup>·K<sup>–2</sup> in a range of 450–800 K. Owing to the ultra-low lattice thermal conductivity along the direction parallel to sintering pressure, Ag<sub>0.407</sub>In<sub>3.198</sub>Se<sub>5</sub> reaches a maximum <i>ZT</i> of 1.01 at 873 K and an average <i>ZT</i> of 0.45 at 300–850 K. The discovery of Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> expands the n-type copper based chalcogenide and lays an important foundation for the application of copper based chalcogenide.

Anisotropic charge transport in 1D and 2D BeN4 and MgN4 nanomaterials: A first-principles study

Low-dimensional Dirac materials have a great potential for practical applications due to their exceptional electronic properties and large surface to volume ratio. Here, we use density functional theory calculations in combination with nonequilibrium Green’s functional formalism to study the ballistic transport properties of recently synthesized single layer BeN4 [Bykov et al., Phys. Rev. Lett. 126 (2021) 175501] and dynamically stable MgN4 [Mortazavi et al., Materials Today Nano 15 (2021) 100125]. We found that these new types of 2D Dirac material show strongly anisotropic electronic transport with current along the armchair direction being significantly larger than in the zig-zag direction. Such in-plane anisotropic electronic transport properties of the materials originate from lattice direction-dependent electron localizations and electrostatic potential variations. Anisotropic charge transport is also obtained for BeN4 nanotube structures. MgN4 sample shows larger current as compared to BeN4 which indicates the possibility of increasing the conductance of BeN4 by replacing Be atoms on the polymeric nitrogen chains with other metal atoms. These findings can be of practical interest in exploring the potential of this novel type of 2D materials for the development of anisotropic multifunctional electronic devices.

Ultrahigh Anisotropic Transport Properties of Black Phosphorus Field Effect Transistors Realized by Edge Contact

AbstractHighly anisotropic black phosphorus (BP) has recently attracted significant interest for electronic and optoelectronic devices. To date, in‐plane anisotropic properties of BP field effect transistors (FETs) have been reported only with top contact. However, the 2D top contact geometry is unable to measure the in‐plane electrical conductance precisely, due to the presence of the out‐of‐plane conductance, resulting in underestimation of anisotropy. Here, 1D edge contact method is employed to measure the in‐plane conductance precisely along the armchair and zigzag directions of BP without the contribution of out‐of‐plane conductance. The conductance and mobility anisotropies for BP FETs are measured with edge contact at 300 K to be ≈5.5 and ≈7.5, respectively. The results further show that the mobility of BP FETs with edge contact weakly depends on temperature, indicating that the edge roughness scattering limits the mobility. In contrast, the mobility of BP FETs with top contact strongly depends on temperature, showing that the impurity scattering and phonon scattering limit the mobility at below and above 150 K, respectively. Finally, a scattering phase diagram is demonstrated to understand the role of different scattering mechanisms on the modulation of mobility anisotropy in BP FETs with edge and top contacts.

Performance Investigation of p-FETs Based on Highly Air-Stable Monolayer Pentagonal PdSe2

Pentagonal PdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is a promising candidate for layered electronic devices due to its high air stability and anisotropic transport properties. Here, we investigate the performance of p-type field-effect transistors (p-FETs) based on PdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monolayer using a multiscale simulation framework combining density functional theory and quantum transport. We find that monolayer PdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> devices show excellent switching characteristics (< 65 mV/dec) for the source-drain direction aligned along both [010] and [100] directions. Both directions also show good ON-state current and large transconductance though these are larger along the [010] direction for a 15-nm channel device. The channel length scaling study of these p-FETs indicates that the channel length can be easily scaled down to 7 nm without any significant compromise in the performance. Going below 7 nm, we find that there is a severe degradation in the subthreshold swing for a 4-nm channel length. However, this degradation can be minimized by introducing an underlap structure. The length of underlap is determined by the trade-off between the ON-state current and the switching performance.

KP15: Natural van der Waals material with ultra-low thermal conductivity and excellent thermoelectric performance

Recently, a natural van der Waals material KP15 has been successfully synthesized and reported to possess obvious anisotropy, high carrier mobility, and stable chemical properties. Herein, through combining Boltzmann transport equation and first-principles calculations, we systematically investigate the electronic and phononic transport properties and thermoelectric (TE) performance of KP15. Our calculations show that KP15 exhibits obvious anisotropic lattice thermal conductivity with extremely low intertube thermal conductivity of 1.49 W/mK at room temperature. Such low thermal conductivity is mainly originated from the intrinsic weak intertube interactions and beneficial for the TE performance. Meanwhile, KP15 also hosts evident anisotropic electronic transport properties and relatively high Seebeck coefficients over a wide range of carrier concentrations. Based on the electronic relaxation time obtained from the deformation potential theory, the TE conversion efficiency of KP15 is evaluated, and the thermoelectric figure of merit could approach as high as 1.40 at 700 K along the c-axis under n-type doping. The findings presented in this work indicate that KP15 has extremely low intertube thermal conductivity and excellent TE performance, which will stimulate further experimental exploration of similar alkali metal phosphating systems and more van der Waals materials.

Deciphering the Carrier Transport Properties in Two-Dimensional Perovskites via Surface-Enhanced Raman Scattering.

2D layered organic-inorganic perovskites have attracted substantial attention due to their high stability and promising optoelectronic properties. However, in-depth insights on the anisotropic carrier transport properties of these 2D perovskites are remaining challenging, while they are significant for further designing the high-performance device applications. Here, the carrier transport properties within 2D perovskite single crystals are investigated and a layered-carrier-transport model is developed through the non-invasive and non-destructive surface-enhanced Raman scattering techniques. The carrier transport features of 2D perovskites show clearly the thickness-, applied voltage- and anisotropy-dependent behaviors, which are demonstrated to origin from the quantum confinement effect. The findings elucidate the carrier transport mechanisms within 2D perovskites from their molecular level through Raman spectroscopy, thus providing a promising way for exploring the photo-physical properties in wide-ranged halide perovskites and designing highly efficient perovskite optoelectronic devices.

Rational design of porous GeP2S6 monolayer for photocatalytic water splitting under the irradiation of visible light

Here, we predicted a novel compelling photocatalyst, the porous GeP2S6 monolayer. The stability of the GeP2S6 monolayer was examined via elastic constant calculations, vibratory phonon spectrum and ab initio molecular dynamics simulations. The electronic properties reveal that the GeP2S6 monolayer possesses a direct gap with the value of 2.06 eV. The photocatalytic properties analyses show that the band edges of the GeP2S6 monolayer meets the requirements of the photocatalytic decomposition water. Meanwhile, the GeP2S6 monolayer exhibits better optical absorption ability in the visible region. Additionally, high carrier mobility (electrons: 206.09 cm2/V·s and holes: 261.46 cm2/V·s) and strongly anisotropic transport properties lead to high solar energy conversion efficiency. Interestingly, the photocatalytic decomposition water in GeP2S6 monolayer is robust against compression strain, which will improve the activity of photocatalytic hydrogen evolution reaction. Our findings highlight the porous GeP2S6 monolayer as a good candidate material for electronics and photocatalytic applications.

Mimicking Neuroplasticity via Ion Migration in van der Waals Layered Copper Indium Thiophosphate.

Artificial synaptic devices are the essential components of neuromorphic computing systems, which are capable of parallel information storage and processing with high area and energy efficiencies, showing high promise in future storage systems and in-memory computing. Analogous to the diffusion of neurotransmitter between neurons, ion-migration-based synaptic devices are becoming promising for mimicking synaptic plasticity, though the precise control of ion migration is still challenging. Due to the unique 2D nature and highly anisotropic ionic transport properties, van der Waals layered materials are attractive for synaptic device applications. Here, utilizing the high conductivity from Cu+ -ion migration, a two-terminal artificial synaptic device based on layered copper indium thiophosphate is studied. By controlling the migration of Cu+ ions with an electric field, the device mimics various neuroplasticity functions, such as short-term plasticity, long-term plasticity, and spike-time-dependent plasticity. The Pavlovian conditioning and activity-dependent synaptic plasticity involved neural functions are also successfully emulated. These results show a promising opportunity to modulate ion migration in 2D materials through field-driven ionic processes, making the demonstrated synaptic device an intriguing candidate for future low-power neuromorphic applications.

A multi scale multi domain model for large format lithium-ion batteries

A multi scale multi domain (MSMD) model for large format lithium-ion battery (LIB) cells is presented. In our approach the homogenization is performed on two scales (i) from the particulate electrodes to homogenized electrode materials using an extended Newman model and (ii) from individual cell layer materials to a homogenized battery material with anisotropic electrical and thermal transport properties. Both intertwined homogenizations are necessary for considering electrochemical-thermal details related to microstructural and material features of electrode and electrolyte layers at affordable computational costs. Simulation results validate the MSMD model compared to the homogenized Newman model for isothermal cases. The strength of the MSMD model is demonstrated for non-isothermal conditions, namely for a 120 Ah cell discharged with four different cooling concepts: (i) without cooling (ii) with a base plate cooling (iii) with a tab cooling and (iv) with a side cooling. As one result, temperature gradients cause a local peak discharge up to 2.8 C for a global 2 C discharge rate.

In-plane anisotropic electronic properties in layered α′-In2Se3

In2Se3 polymorphs have been extensively studied because of their diverse physical properties such as piezoelectricity, photoelectricity, and ferroelectricity, thereby showing plentiful promising applications in integrated electronic devices. These diverse properties are strongly dependent on or affected by their atomic bonding arrangement in the crystal phases. Combining lattice symmetry and local atomic perturbation, we demonstrate a novel layered α′-In2Se3 phase by using the first-principles calculations, which is reconstructed from the inverted tetrahedral bonding configuration by the in-plane displacive middle layer Se atom. The optimized structure of monolayer α′-In2Se3 has triple degenerated atomic configurations with different Se atom orientations. We noted that these degenerated atomic configurations exhibit a moderate switching barrier (about 61 meV/f.u.) between them. To further explore this atom-oriented anisotropic property in α′-In2Se3, the electronic properties were studied with an orthorhombic unit cell. The comparative results for the orthogonal Se atom orientations suggest that the nonbonding orbital coupling of the displacive Se atoms induces large in-plane anisotropic optical absorption and electrical transport properties. This study of the layered α′-In2Se3 phase can extend the realm of switchable anisotropic optoelectronic applications in future electronic devices.

Anisotropic transport properties of graphene-based conductor materials

We analyzed nanographite-based materials in a combined study including experimental analysis via 4-point probe STM and simulation to provide a complete picture of microscopic and macroscopic properties of the material. The two- and three-dimensional transport regimes were determined and evaluated regarding the anisotropy of the conductivity. The experimental results yield the full macroscopic conductivity tensor. Microstructural simulations are used to map those macroscopic properties to the microscopic building blocks of the sample. By combining those two, we present a coherent and comprehensive description of the electrical material parameters across several length scales.

High-performance polarization-sensitive photodetectors on two-dimensional β-InSe.

Two-dimensional (2D) indium selenide (InSe) has been widely studied for application in transistors and photodetectors, which benefit from its excellent optoelectronic properties. Among the three specific polytypes (γ-, ϵ- and β-phase) of InSe, only the crystal lattice of InSe in β-phase (β-InSe) belongs to a non-symmetry point group of n}{}D_{6h}^4, which indicates stronger anisotropic transport behavior and potential in the polarized photodetection of β-InSe-based optoelectronic devices. Therefore, we prepare the stable p-type 2D-layered β-InSe via temperature gradient method. The anisotropic Raman, transport and photoresponse properties of β-InSe have been experimentally and theoretically proven, showing that the β-InSe-based device has a ratio of 3.76 for the maximum to minimum dark current at two orthogonal orientations and a high photocurrent anisotropic ratio of 0.70 at 1 V bias voltage, respectively. The appealing anisotropic properties demonstrated in this work clearly identify β-InSe as a competitive candidate for filter-free polarization-sensitive photodetectors.

Modeling Capillary Transport in Thin Porous Media Using a Composite Continuum-Pore Network Formulation: Effect of Water Blockage on Gas Diffusion and Convection

In this work, a composite continuum-pore network formulation is presented to model two-phase transport in thin fibrous porous media [1,2]. The composite model incorporates a control volume (CV) mesh at the layer scale, which embeds a structured cubic pore network. Capillary transport is simulated using the discrete pore network, considering the Purcell toroid model to determine the local entry capillary pressures in the fibrous material [3]. In addition, the pore-network model is used to determine analytically local anisotropic effective transport properties (local effective diffusivity and permeability), which are mapped onto the CV mesh to simulate transport within the porous layer using a continuum formulation. The relative effect of local water blockage on gas diffusion and convection is taken into account using a power-law function of the form (1-s)n, where s is the local liquid saturation. Good agreement is found between the predicted global effective transport properties (global effective diffusivity and permeability) under dry and wet conditions and previous experimental data reported in the literature for Toray TGP-H series carbon paper. Moreover, water saturation distributions are compared with results obtained using X-ray computed tomography [4]. As shown by García-Salaberri et al. [4,5], local water accumulation is found to play an important role on gas diffusion and convection. As a result, the exponent n increases from nearly n=2 at the local, pore scale to nearly n=3 at the global, layer scale. Currently, work is underway to implement the composite model in multiphysics models of a polymer electrolyte fuel cell and a redox flow battery to examine the impact of material heterogeinities on performance and durability.[1] P.A. García-Salaberri, I.V. Zenyuk, J.T. Gostick, A.Z. Weber, Modeling Gas Diffusion Layers in Polymer Electrolyte Fuel Cells Using a Continuum-based Pore-network Formulation, ECS Trans. 97 (7) 615.[2] P.A. García-Salaberri, Modeling diffusion and convection in thin porous transport layers using a composite continuum-network model: Application to gas diffusion layers in polymer electrolyte fuel cells, Int. J. Heat Mass Trans. (2020), accepted.[3] J.T. Gostick, Random Pore Network Modeling of Fibrous PEMFC Gas Diffusion Media Using Voronoi and Delaunay Tessellations, J. Electrochem. Soc 160 (2013) F731–F743.[4] P.A. García-Salaberri, G. Hwang, M. Vera, A.Z. Weber, J.T. Gostick, Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of through-plane saturation distribution, Int. J. Heat Mass Trans. 86 (2015) 319–333.[5] P.A. García-Salaberri, J.T. Gostick, G. Hwang, A.Z. Weber, M. Vera, Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of local saturation and application to macroscopic continuum models, J. Power Sources 296 (2015) 440–453.. Figure 1. Global relative effective diffusivity in the through-plane direction, gTP=Deff TP,wet/Deff TP,dry, as a function of average saturation, savg, computed with the composite continuum-pore network model. The gas species concentration distributions corresponding to different average saturations are shown on the top.. Figure 1

Microstructure reconstruction of the gas diffusion layer and analyses of the anisotropic transport properties

The gas diffusion layer (GDL) is a key component in a proton exchange membrane fuel cell and a comprehensive understanding of its transport properties is imperative for improving the performance and durability of a fuel cell. In this study, two microscopic reconstruction methods, stochastic numerical and X-ray computed tomography (XCT) reconstruction, are employed to generate 3D microstructure of two different types of GDL. The stochastic numerical reconstruction method simulates all available phases, including the pores, carbon fibers, binders, and PTFE, to generate the 3D microstructure of the GDL in comparison with experimentally obtained GDL using the XCT method. The porosity and pore size distribution of the reconstructed GDL are compared and analyzed. Pore scale model is employed to obtain the effective and anisotropic transport properties including gas diffusivity, electrical and thermal conductivity as a function of porosity caused by different compression strain. Furthermore, Lattice-Boltzmann method is used to determine the anisotropic liquid water permeability and saturation as a function of capillary pressure. The results show significant anisotropic transport properties of Toray GDL due to its disc-shape binder existed at the intersection of the fibers. On the other hand, Freudenberg GDL shows mostly isotropic transport properties due to its uniformly distributed carbon fiber without binder. The combined results not only provide a reliable framework for investigating GDL microstructure, but also provide useful property correlations for fuel cell simulations and operations.

Tunable anisotropic thermal transport in super-aligned carbon nanotube films

Super-aligned carbon nanotube (CNT) films have intriguing anisotropic thermal transport properties due to the anisotropic nature of individual nanotubes and the important role of nanotube alignment. The exact relation between the alignment and the anisotropic thermal conductivities, however, was not well understood due to the challenges in both the preparation of high-quality super-aligned CNT samples and the thermal characterization of highly anisotropic and porous thin films. Here, super-aligned CNT films with different alignment configurations are fabricated and their anisotropic thermal conductivities are measured using an elliptical-beam approach of time-domain thermoreflectance (TDTR). The results suggest that the alignment configuration can tune the cross-plane thermal conductivity kz from 6.4 to 1.5 W/(mK), and the in-plane anisotropic ratio from 1.2 to 13.5. This work confirms the important role of CNT alignment in tuning the thermal transport properties of super-aligned CNT films and provides an efficient way to design thermally anisotropic films for thermal management.

Tuning the electronic structures and anisotropic transport behaviors of GeSe monolayer by substitutional doping

The electronic structures and anisotropic transport properties of doped-GeSe monolayers are systematically explored by performing the density functional theory combined with non-equilibrium Green’s function method. Numerical results of the cohesive energy and formation energy show that it is possible to substitute Ge/Se with isovalent atoms and group-V atoms at room temperature. Isovalent substitutions of Ge/Se atoms can maintain the semiconducting features of monolayer GeSe. While the doping of group-V atoms modifies drastically the electronic structures of doped-GeSe and makes metallic properties appear. In addition, the anisotropy of electronic transport of monolayer GeSe also can be effectively manipulated by substitutional doping. In particular, the isotropic electronic transport properties are shown in the devices based on GeSe with P substituting Ge atom. An obvious negative differential resistance behavior with a large current peak-to-valley ratio up to the radio of 103 is gained in N-dope device. These results are useful for the future potential applications of GeSe materials in next-generation high-efficiency electronic devices.

Highly In-Plane Anisotropic Two-Dimensional Ternary Ta2NiSe5 for Polarization-Sensitive Photodetectors.

Intriguing anisotropic electrical and optoelectrical properties in two-dimensional (2D) materials are currently gaining increasing interest both for fundamental research and emerging optoelectronic devices. Identifying promising new 2D materials with low-symmetry structures will be rewarding in the development of polarization-integrated nanodevices. In this work, the anisotropic electron transport and optoelectrical properties of multilayer 2D ternary Ta2NiSe5 were systematically researched. The polarization-sensitive Ta2NiSe5 photodetector shows a linearly anisotropy ratio of ≈3.24 with 1064 nm illumination. The multilayer Ta2NiSe5-based field-effective transistors exhibit an excellent field-effective mobility of 161.25 cm2·V-1·s-1 along the a axis (armchair direction) as well as a great current saturation characteristic at room temperature. These results will promote a better understanding of the optoelectrical properties and applications in new categories of the in-plane anisotropic 2D materials.

Zintl Phase BaAgSb: Low Thermal Conductivity and High Performance Thermoelectric Material in Ab Initio CalculationSupported by the National Key R&D Program of China (Grant Nos. 2016YFA0401503 and 2017YFA0403700), the National Natural Science Foundation of China (Grant Nos. 11675255, U1932220, 11675195, and U1932220), the Key Research Program of Frontier Sciences, CAS (Grant No. 292016YQYKXJ00135), and the Program of State Key Laboratory

Thermoelectric materials are critical parts in thermal electric devices. Here, Zintl phase BaAgSb in space group of P63/mmc is reported as a promising thermoelectric material in density function theory. The anisotropic lattice thermal conductivity and phonon transport properties are investigated in theory. The strong phonon-phonon scattering in BaAgSb exhibits ultra-low lattice thermal conductivity of 0.59 W⋅m−1⋅K−1 along c-axis at 800 K, and high thermoelectric performance ZT = 0.94 at 400 K. The mix of covalent and ionic bond supports high carrier mobility and low thermal conductivity. The unusual features make BaAgSb a potential thermoelectric material.