Remarkable Anisotropy Research Articles

MgXN2 (X = Hf/Zr) Monolayers: Auxetic Semiconductor with Highly Anisotropic Optical/Mechanical Properties and Carrier Mobility.

Two-dimensional (2D) semiconducting materials with distinct anisotropic physical properties have attracted intense interests. Herein, we show theoretical predictions that MgXN2 (X = Hf/Zr) monolayers are auxetic semiconductors with highly anisotropic electronic, optical, and mechanical properties. The density functional theory calculations coupled with a PSO algorithm (global-minimum search) suggest that both MgHfN2 (MgZrN2) monolayers exhibit orthorhombic symmetry (Pmma) and are direct-gap (indirect-gap) semiconductors with a bandgap of 2.43 eV (2.13 eV). Specifically, the MgHfN2 monolayer exhibits highly anisotropic hole mobility as well as very high electron mobility (∼104 cm2 V-1 s-1). G0W0+BSE calculations indicate that both monolayers bear notable optical anisotropy and relatively large exitonic binding energy (∼0.6 eV). In addition, both monolayers acquire remarkable mechanical anisotropy with a negative in-plane Poisson's ratio (∼-0.2) and high Young's modulus (∼260 N/m). The combination of highly anisotropic electronic, optical, and mechanical properties endows MgXN2 monolayers as potentially useful parts in multifunctional nanoelectronic devices.

Elasticity of Phase H Under the Mantle Temperatures and Pressures: Implications for Discontinuities and Water Transport in the Mid‐Mantle

AbstractPhase H (MgSiO4H2), one of the lower mantle's dense hydrous magnesium silicates (DHMSs), may form and exist in cold slabs and is crucial in carrying water into the deep mantle. Its sound velocities and density are crucial for inferring the mid‐mantle water cycling via seismic approaches. Here we obtain the elastic and thermodynamic properties of phase H under lower‐mantle conditions using first‐principles calculations and discuss the effect of the Mg‐Si disorder on elasticity. The density of phase H is ∼15% and ∼6% lower than that of bridgmanite and periclase, respectively. The dehydration reaction from phase H to bridgmanite, which may occur at the depth of ∼1,300–1,700 km in cold slabs, will cause an increase of 1.0%, 2.7%, and 15% at 1,500 km on VP, VS, and density, respectively. The dehydration of phase H in subduction zones could produce a seismic VS impedance contrast of ∼17% in the mid‐mantle, which can provide an explanation for some seismic discontinuities detected by previous studies. Meanwhile, phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones. Collectively, our results suggest that some seismic observations in mid‐mantle slabs may be related to the presence of phase H formed via the deep water cycle, further constraining the potential water content in local regions of the subducted slabs.

Functional-Unit-Based Material Design: Ultralow Thermal Conductivity in Thermoelectrics with Linear Triatomic Resonant Bonds

We demonstrate the use of functional-unit-based material design for thermoelectrics. This is an efficient approach for identifying high-performance thermoelectric materials, based on the use of combinations of functional fragments relevant to desired properties. Here, we reveal that linear triatomic resonant bonds (LTRBs) found in some Zintl compounds provide strong anisotropy both structurally and electronically, along with strong anharmonic phonon scattering. An LTRB is thus introduced as a functional unit, and compounds are then screened as potential thermoelectric materials. We identify 17 semiconducting candidates from the MatHub-3d database that contain LTRBs. Detailed transport calculations demonstrate that the LTRB-containing compounds not only have considerably lower lattice thermal conductivities than other compounds with similar average atomic masses, but also exhibit remarkable band anisotropy near the valence band maximums due to the LTRB. K5CuSb2 is adopted as an example to elucidate the fundamental correlation between the LTRB and thermoelectric properties. The [Sb-Cu-Sb]5- resonant structures demonstrate the delocalized Sb-Sb interaction within each LTRB, resulting in the softening of TA phonons and leading to large anharmonicity. The low lattice thermal conductivity (0.39 W/m·K at 300 K) combined with the band anisotropy results in a high thermoelectric figure of merit (ZT) for K5CuSb2 of 1.3 at 800 K. This work is a case study of the functional-unit-based material design for the development of novel thermoelectric materials.

Anomalous Radial Anisotropy and Its Implications for Upper Mantle Dynamics Beneath South China From Multimode Surface Wave Tomography

AbstractThe strength of radial anisotropy can effectively reveal vertical or horizontal material flow, which may provide significant dynamic implications. The South China Block (SCB) is a classic laboratory to study strong deformation under compression from the surroundings. We construct a radially anisotropic 3‐D shear‐wave model of the entire SCB using multimode surface wave tomography with a lateral resolution of 2.0°. Distinctive structural contrasts are revealed in the S‐wave velocity anomalies in the Cathaysia Block and the Yangtze Craton. In the Cathaysia Block, a low‐velocity anomaly becomes evident with increasing depth, particularly in the VSH model. The Yangtze Craton is, however, characterized by a high‐velocity anomaly, especially in the Sichuan Basin, where the high‐velocity feature can extend down to a depth of 300 km. Interestingly, an abrupt change in radial anisotropy is observed beneath the Sichuan Basin, with positive radial anisotropy (faster VSH) in the uppermost mantle and negative radial anisotropy (faster VSV) at depths from 80 to 250 km. The lower negative anisotropic layer may reflect the directional arrangement of constituent materials caused by eastward compression and erosion of mantle flow under the Tibetan Plateau. In addition, remarkable negative radial anisotropy is found in the asthenosphere beneath Hainan Island and its surroundings. The features beneath Hainan Island and its surroundings may be related to the Hainan mantle plume in the Cenozoic.

Anisotropic tensile mechanics of vertically aligned carbon nanotube reinforced silicon carbide ceramic nanocomposites

Vertically aligned carbon nanotubes (VACNTs)/silicon carbide (SiC) nanocomposites (VSNs) present remarkably anisotropic mechanical properties originated from their intrinsic heterogeneous microstructures. However, the studies related to tensile mechanical behaviors and failure mechanisms of VSNs are still suspended so far. In this work, the tensile mechanical properties of VSNs were investigated by an in-situ transmission electron microscopy (TEM) tensile test combined with classic molecular dynamics (MD) simulation. The testing results show that the fracture strength and Young's modulus of VSNs along CNT-axis (∥ direction) are higher than those perpendicular to CNT-axis (⊥ direction). In addition, the fracture surface along ∥ direction is uneven whereas it is flat along ⊥ direction. The MD simulation reveals that the stronger behavior along ∥ direction is mainly benefited by the excellent tensile mechanical properties of CNTs, however, the relatively weak van der Waals interaction between CNTs and SiC matrix dominates the tensile performances of VSNs along ⊥ direction. Finally, the remarkable mechanical anisotropy of VSNs is explained by pulling-out and peeling-off failures of CNTs based on MD simulation. This work provides experimental and theoretical insights into the dominant mechanisms of mechanical anisotropy in typical VACNT/ceramic nanocomposites.

Molecular Dynamics Simulations of Polymer Nanocomposites Welding: Interfacial Structure, Dynamics and Strength.

Polymer welding has received numerous scientific attention, however, the welding of polymer nanocomposites (PNCs) has not been studied yet. In this work, via coarse-grained molecular dynamics simulation, the attention on investigating the welding interfacial structure, dynamics, and strength by constructing the upper and lower layers of PNCs, by varying the polymer-nanoparticle (NP) interaction strength εNP-p is focused. Remarkably, at low εNP-p , the NPs gradually migrate into the top and bottom surface layer perpendicular to the z direction during the adhesion process, while they are distributed in the middle region at high εNP-p . Meanwhile, the dimension of polymer chains is found to exhibit a remarkable anisotropy evidenced by the root-mean-square radius of gyration in the xy- (Rg,xy ) and z- (Rg,z ) component. The welding interdiffusion depth increases the fastest at low εNP-p , attributed to the high mobility of polymer chains and NPs. Lastly, although the mechanical properties of PNCs at high εNP-p is the strongest because of the presence of the NPs in the bulk region, the welding efficiency is the greatest at low εNP-p . Generally, this work provides a fundamental understanding of the interfacial welding of PNCs, in hopes of guiding to design and fabricate excellent self-healable PNCs.

Influence of loading direction on tensile deformation behavior of a lean duplex stainless steel sheet: The role of martensitic transformation

In this work, the tensile deformation behavior of a hot-rolled lean duplex stainless steel with metastable austenite (named as present TLDX) was studied at 20 °C along the loading directions of 0°, 45° and 90° to the rolling direction (corresponding to the L-oriented, D-oriented and T-oriented specimens, respectively). The initial microstructure, martensitic transformation characteristics, damage and fracture behavior were clarified through microstructure observations. The intrinsic mechanism of anisotropy in tensile deformation behavior was comprehensively discussed referring to the microstructure characteristics. The present TLDX exhibited remarkable mechanical anisotropy at 20 °C, characterized by the highest values of ultimate tensile strength (σu = 853 MPa) and plasticity (et = 70.6%, eu = 62.3%) for the L-oriented specimen and the lowest values (i.e., σu = 790 MPa, et = 53.3%, eu = 50.4%) for the T-oriented specimen, although the yield strength is comparable (σs∼524 MPa) for different loading directions. Strain-induced martensitic transformation (SIMT) with a sequence of γ→ε→α′ was detected for all specimens. The L-oriented specimen is most favorable for SIMT, followed by the D-oriented specimen, and finally the T-oriented specimen, which, caused by the different strain partitioning response, is the main factor contributing to the mechanical anisotropy at 20 °C. Different damage and fracture characteristics for different loading directions were also identified, which are closely associated with the initial microstructure.

Influence of Wallach’s Rule on Chiral AIE Systems and Its Application in Cryptographic Information Storage

Influence of Wallach’s Rule on Chiral AIE Systems and Its Application in Cryptographic Information Storage

Effects of SiC content on the mechanical and thermophysical properties of 3D Cf/SiC–Al composites

3D Cf/SiC–Al composites were achieved through the pressure infiltration of liquid Al–Si alloy into porous 3D Cf/SiC preform, which was produced by different cycles of precursor infiltration and pyrolysis. The effect of silicon carbide volume fraction on the microstructure, anisotropic mechanical response, and thermophysical characteristics of the 3D Cf/SiC–Al composites was investigated. The results demonstrated that the initial microstructure of 3D Cf/SiC can be retained, and the obtained Cf/SiC–Al composites presented remarkable anisotropy characteristics. As the silicon carbide ceramics content increased from 12.5 vol% to 41.8 vol%, the thermal conductivity and thermal expansion coefficient of 3D Cf/SiC–Al composites decreased, whereas the bending strength initially increased and then decreased in the Z-direction. The bending strength perpendicular (Z) to the carbon cloth layer of 3D Cf/SiC–Al composites was higher than that of parallel (X–Y) to the carbon cloth layer. However, a significant anisotropy in the thermal conductivity values was the opposite. The 3D Cf/SiC–Al composite with 1ow ceramic content (17 vol%) had a higher thermal conductivity in the X–Y direction (64 W m−1 K−1) than in the Z-direction (34 W m−1 K−1). The thermal expansion coefficient of all the 3D Cf/SiC–Al composites along the X–Y direction also decreased initially and then increased in the range of 100–450 °C, which presents low expansion characteristics.

Mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy compressed at elevated temperature

The mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy was investigated via experiments and molecular dynamics simulations. According to the experimental results, laths occurring in γ-phase grains were identified as γ-phase twins, and increasing strain promoted the γ → α2 (fct → hcp) phase transformation when the alloy was deformed at 1150 °C. A large number of ultrafine γ lamellae precipitated when deformed at 1200 °C, which was attributed to the thermal-mechanical coupling. The atomic mechanisms of the α2(α) → γlamella phase transformations were investigated using molecular dynamics simulations. The results showed that the generalised stacking fault energy along different directions (<1‾010>) exhibited remarkable anisotropy on the basal plane (0001) of the hcp crystal structure, regardless of the ordering or disordering phase. The disordering of the hcp crystal structure significantly affected the formation of the γ lamellae. Regardless of the ideal stoichiometric or off-stoichiometric composition, the stress-induced hcp → fcc crystal structure transformation occurred more easily in the disordered phases. Grain-boundary nucleation dominated the precipitation of the ultrafine γ lamellae. In addition, intragranular nucleation of γ lamellae was observed, which was mainly attributed to the activation and movement of the Shockley partial dislocation loop with a Burgers vector of 1/6<1‾010> whose formation was not directly related to the dissociation of the superpartial dislocation 1/6<112‾0>.

Ultraslippery/hydrophilic patterned surfaces for efficient fog harvest

The rapid and controllable sliding of droplets on anisotropic slippery liquid infused porous surfaces (SLIPS) has promising application prospects in the fields of energy, lab-on-a-chip, etc. In this work, the fabrication of hydrophilic patterned SLIPS on copper substrates that can achieve efficient droplet transport and fog harvest was reported. A single superhydrophilic stripe and array of superhydrophilic stripes were fabricated on SLIPS background, respectively. Contact angles, sliding angles and contact angle hysteresis of the surfaces were investigated and analyzed. Results showed that hydrophilic patterned SLIPS with superhydrophilic stripe array had remarkable anisotropy in contact angle hysteresis in the parallel and perpendicular directions due to the large energy barrier induced by liquid surface tension in the direction perpendicular to the stripes, which can be used to transport droplets rapidly and precisely. In addition, the fog harvest performance, as a typical example of droplet transport, was significantly improved on hydrophilic patterned SLIPS due to high nucleation density on the hydrophilic SLIPS, high condensate mobility, efficient condensate transferring from SLIPS to superhydrophilic stripes and liquid shedding along the stripes for departing.

Theoretical study of anisotropy and ultra-low thermal conductance of porous graphene nanoribbons

The thermal transport properties of porous graphene nanoribbons are studied by the non-equilibrium Green's function method. The results show that owing to the existence of nano-pores, the thermal conductance of porous graphene nanoribbons is much lower than that of graphene nanoribbons. At room temperature, the thermal conductance of zigzag porous graphene nanoribbons is only 12% of that of zigzag graphene nanoribbons of the same size. This is due to the phonon localization caused by the nano-pores in the porous graphene nanoribbons. In addition, the thermal conductance of porous graphene nanoribbons has remarkable anisotropy. With the same size, the thermal conductance of armchair porous graphene nanoribbons is about twice higher than that of zigzag porous graphene nanoribbons. This is because the phonon locality in the zigzag direction is stronger than that in the armchair direction, and even part of the frequency phonons are completely localized.

Remarkable anisotropy in rhombohedral Ge2Sb2Te5 compound: a promising thermoelectric material with multiple conduction bands and acoustic-optical branches coupling

First-principles calculations and Boltzmann transport theory are used to evaluate the electronic structure and thermoelectric properties of the high-temperature rhombohedral Ge2Sb2Te5 compound. In this work, we have investigated the phonon spectrum, band structure and density of states with K-H configuration, respectively. In the phonon spectrum, the acoustic and optical branches are coupled, and their combined effect significantly reduces the thermal lattice conductivity. The conduction band contributed by the Ge, Sb, and Te has the characteristics of more multiple valleys and degenerate features. When the electron-doping level is 4.3 × 1019 cm−3, a large number of conduction bands are activated, thereby enhancing the conductivity and seebeck coefficient. The layered Ge2Sb2Te5 has significant anisotropy, and its thermoelectric performance along the z-axis is excellent. The thermoelectric transport coefficients are calculated using Boltzmann transport theory combined with relaxation time approximation. For p type, the largest ZT value of 1.509 could be obtained with the carrier density of 1.3 × 1020 cm−3 at 900 K, while for n type, the considerable ZT value of 3.124 could be obtained with the carrier density of 4.5 × 1019 cm−3 at 700 K.

Antiferroelectric Anisotropy of Epitaxial PbHfO3 Films for Flexible Energy Storage

AbstractDielectric capacitors are widely studied for power supply systems because they can quickly store and release electrical energy. Among various kinds of dielectric materials, antiferroelectrics show promising features of high energy‐storage density and efficiency. In this study, epitaxial antiferroelectric PbHfO3 films with different orientations are fabricated, in which remarkable anisotropies of polarization and energy storage properties are discovered. With the optimization of film orientation, much‐improved energy density and excellent high‐temperature efficiency are achieved in the PbHfO3 films. Moreover, the PbHfO3 films are fabricated onto flexible mica substrates, which exhibit excellent property robustness against mechanical bending. This study provides a fundamental understanding of the anisotropic antiferroelectric behaviors of epitaxial PbHfO3 films and provides a generalizable pathway for flexible energy‐storage dielectric capacitors.

Anisotropic optical properties of Cu2ZnSn(SxSe1−x)4 solid solutions: First-principles calculations with TB-mBJ+U

The main aim of this work is to determine and explain the relationships between optoelectronic properties and the sulfur anion content in Cu2ZnSn(SxSe1−x)4 solid solution. Band gap and absorption coefficient are of primary interest to the engineers and scientists researcher worked in optoelectronic field. Herein, the electronic and optical properties are calculated based on 128 conventional atoms within lattice parameters obtained at 300 K by using the FP-LAPW method combined with quasi-harmonic Debye model. The composition dependent band gaps of CZTSSe solid solutions are investigated by TB-mBJ+U. As results, all materials are semiconductors with a direct band gap ranging from 0.614 to 0.99 eV. The band gap variation increases as a function of sulfur anion content and showed a positive deviation from Vegard's law with a very small downward bowing parameter of + 0.079 eV. The density of state (DOS) calculations indicate that the energy bands of VBM involve Cu_d/anion(S/Se)_p hybridized antibonding-like states. Based on band alignment, the Ec offset between CZTS and CZTSe is larger than the Ev offset. These results are reported previously in other work and are confirmed in this study. Our work included a systematic comparison of the influence of S/(S+Se) atomic ratios on optical quantities. The dielectric function tensors show remarkable anisotropy. In addition, the static dielectric constants are found to decrease with sulfur anion content. The CZTSSe is proved to be suitable for good solar cells with high absorption coefficient (> 104 cm−1). Such deep optical studies would be helpful for future optoelectronic applications of these compounds with different S/(S+Se) atomic ratios.

Simultaneous topology and deposition direction optimization for Wire and Arc Additive Manufacturing

A remarkable elastic anisotropy in plates of austenitic stainless steel produced by the Wire and Arc Additive Manufacturing process is recently reported. The Young’s modulus depends on the angle of orientation with respect to the material deposition direction. Here, for the first time, this anisotropy is exploited to maximize structural stiffness by simultaneously optimizing the structural design layout and the local deposition path direction for WAAM. The results obtained indicate deposition that is commonly preferred along the load-path directions for WAAM is sub-optimal and stiffness can be increased at least 53% upon optimizing the deposition directions.

Effective masses of the tin sulfide influenced by intrinsic defects: A theoretical prediction

We performed a theoretical study within the Density Functional Perturbation Theory framework to analyze the influence of intrinsic defects in the effective mass of the charge carriers of tin sulfide. We modeled the orthorhombic phase Aem2(39) space-group symmetry and evaluated defects such as residual stress and 1.6%,3.1%, and 4.7% tin vacancy concentration. Our results show that the effective masses exhibit remarkable anisotropy for both holes and electrons; they are larger parallel to the longest axis of the unit cell; besides, the effective masses of the holes present a slight increase when the concentration defect is higher. These results demonstrate that the vacancy defect modifies the effective masses and allow us to infer that large values of effective mass contribute to low mobility between parallel planes with Van der Waals forces. Consequently, it would be reasonable to propose this material like a prominent rectifier diode.

Preparation and thermoelectric properties of ZnTe-doped Bi0.5Sb1.5Te3 single crystal

In this work, Bi0.5Sb1.5Te3 single crystal doped with 6 mol% ZnTe was fabricated using a vertical Bridgman method. A region about 20 × 70 mm2 in dimensions along (001) cleavage plane is obtained. The as-grown crystal exhibits remarkable anisotropy of TE properties that parallel and perpendicular to (001) in 300–480 K temperature range. It is found the highest power factor PF∥(001)= ~50 μWcm-1K−2 and PF⊥(001)= ~18 μWcm-1K−2 are obtained, and the total thermal conductivities for both directions are κ∥(001) = 1.8 Wm-1K−1 and κ⊥(001) = 0.8 Wm-1K−1. Thus, the peak figure of merit ZT∥(001) = 1.05 and ZT⊥(001) = 0.9 are realized, which might expand its application scope compared with the commercial Bi2Te3-based TE materials.

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy has been recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This issue inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Waals interaction. To do this, we made correlative far- and near-field characterizations validated by first-principle calculations that reveal a huge birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this remarkable anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

Tunable Poisson’s ratio and tension-compression asymmetry of graphene-copper nanolayered composites

The Poisson’s ratios of graphene-copper nanolayered (GrCuNL) composites under tension and compression are investigated by molecular dynamics and theoretical analysis. The Poisson’s ratio of a GrCuNL composite can be tuned by tailoring its repeat layer spacing without changing the topological structures. The effect of constituent nanocrystalline Cu grain size on the Poisson’s ratio is negligible. There are remarkable in-plane anisotropy and tension-compression asymmetry in the Poisson’s ratio due to the chiral difference in compressive stress in graphene layers. A mechanical model considering the chirality and repeat layer spacing is proposed, which can accurately predict the Poisson’s ratio of a GrCuNL composite. For stable GrCuNL composites, the repeat layer spacing should be larger than 2 nm, and their tunable range of Poisson’s ratio is 0.1–0.35.