Accepted
, , Received Date: 2025-08-29
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, , Received Date: 2025-09-05
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Relaxor ferroelectric sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) exhibits outstanding ferroelectric characteristics and is widely recognized as a highly promising lead-free ferroelectric material. In order to further promote the application of this environmentally friendly ferroelectric material, it is crucial to gain a comprehensive understanding of its structural evolution and phase transition mechanism under high pressure. This study investigates the structural evolution of NBT under hydrostatic pressure up to 6.8 GPa by integrating in situ high-pressure neutron diffraction experiments with first-principles calculations. Based on high-pressure neutron diffraction experiments conducted at the China Mianyang Research Reactor (CMRR), Rietveld refinement analysis identifies a phase transition from the ambient-pressure R3c phase to the high-pressure Pnma phase in NBT, with a coexistence pressure range of 1.1–4.6 GPa. The bulk modulus of the high-pressure phase Pnma is experimentally determined to be 108.6 GPa for the first time. First-principles calculations further support the thermodynamic tendency for the pressure-induced phase transition from R3c to Pnma and produce a bulk modulus that is in close agreement with the experimental value. By correlating with the experimentally obtained trends of the internal [TiO6] oxygen octahedral structural changes under high pressure in both phases, this study demonstrates that the difference in their macroscopic compressibility originates from the significantly higher pressure sensitivity of the oxygen octahedral distortion degree in the R3c phase than that of the Pnma phase. This relatively softer internal microstructure results in a lower bulk modulus than that of the Pnma phase. By providing a detailed analysis of the pressure-induced phase transition and microstructural evolution, this study clarifies the relationship between the microscopic structural features of the high-pressure and ambient-pressure phases of NBT and their influence on macroscopic mechanical properties, thereby establishing a fundamental connection between microscopic structural responses and bulk physical behavior under high-pressure conditions. These findings provide crucial experimental data and theoretical support for further improving the high-pressure performance and applications of lead-free ferroelectric materials.
, , Received Date: 2025-08-27
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Constructing van der Waals (vdW) heterostructures has emerged as an effective strategy for enriching the physical properties of two-dimensional materials and optimizing their optoelectronic performance. In this work, we systematically investigate the electronic properties and biaxial strain modulation of Janus MoSSe/g-C3N4 heterostructures with two distinct interfacial configurations—SMoSe/g-C3N4 and SeMoS/g-C3N4—by means of first-principles simulations. Binding energy comparisons and AIMD simulations are performed to determine the most stable stacking pattern of each type of the heterostructure. The analyses of the electrostatic potential and work function reveal that the intrinsic dipole of MoSSe layer and the interfacial electric field in the SMoSe/g-C3N4 heterostructure undergo a constructive superposition. This enhances the overall built-in electric field, which points from g-C3N4 layer to MoSSe layer, resulting in a type-I band alignment. In contrast, in the SeMoS/g-C3N4 configuration, the two fields oppose each other, leading to a net electric field directed from MoSSe to g-C3N4 layer. This leads to a type-II band alignment, which facilitates spatial carrier separation and significantly enhances photocatalytic water-splitting activity. Furthermore, this study also demonstrates that biaxial strain can effectively modulate the electronic band structures of both types of heterostructures. In particular, the SeMoS/g-C3N4 system exhibits a reversible transition between type-I and type-II band alignments under specific compressive (–4%) and tensile (+5%) strain states. The underlying mechanism is elucidated by the difference charge density calculations. This study provides theoretical insights into the role of interfacial and intrinsic dipoles combined with strain engineering, offering a viable route for designing efficient MoSSe/g-C3N4-based photocatalysts and optoelectronic devices.
, , Received Date: 2025-08-31
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This study aims to explore two-dimensional semiconductor materials with superior carrier transport properties to meet the growing demands of high-speed electronics and optoelectronic devices, focusing on evaluating the feasibility of monolayer FeGa2S4 as a candidate material through systematic theoretical investigations. First-principles calculations are used to analyze the exfoliation energy of FeGa2S4 bulk crystal, as well as the structural stability, mechanical properties, and strain-dependent optoelectronic behavior of its monolayer counterpart. Strain engineering strategies, including uniaxial and biaxial strain, are used to assess carrier mobility modulation and spectral response. Our calculation results indicate that monolayer FeGa2S4 is an indirect bandgap semiconductor (Eg = 1.65 eV) with low stiffness (Young’s modulus up to 151.6 GPa) and high flexibility (Poisson’s ratio less than 0.25), demonstrating exceptional thermodynamic stability. Under +5% uniaxial tensile strain, its electron mobilities along x and y directions dramatically increases to 5402.4 cm2·V–1·s–1 and 4164.0 cm2·V–1·s–1, fivefold higher than its hole mobility. Biaxial strain outperforms uniaxial strain in bandgap modulation and induces a systematic redshift in optical spectra, significantly enhancing visible-light harvesting efficiency. This work reveals that monolayer FeGa2S4 is a promising high-mobility photoactive material for next-generation solar cells and optoelectronics. The strain-mediated control of electronic and optical properties provides a theoretical framework for optimizing 2D semiconductors and critical guidance for experimental synthesis and device engineering. These findings highlight the potential of materials in advancing energy conversion technology and photonic applications.
, , Received Date: 2025-08-10
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,
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,
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The boundary region between the solar radiation zone and the convection zone ($ T\thicksim180$ eV, $ n_e\thicksim $$ 9\times10^{22}\;{\rm{cm}}^{-3}$) is a critical interface where energy transport in the solar interior transitions from radiation-dominated to convection-dominated regimes. This region also serves as a natural laboratory for studying hot dense plasma. The physical properties of this zone are essential for the reliability of stellar evolution models and the stability of energy transport mechanisms. One of major unresolved issue is how electron collision ionization affects the density of free electrons and radiation properties in this plasma, while accurately describing the impact of hot-dense environments on electron impact ionization (EII) (such as electron screening, ion correlation). To fill this gap, we systematically calculate EII cross sections for C, N, and O ions under realistic solar boundary conditions, with a focus on hot-dense environment impacts. We develop a novel computational framework that merges?hot-dense environment effects into atomic structure calculations: the Flexible Atomic Code (FAC) for atomic structure is combined with the Hyper-netted-Chain (HNC) approximation to capture electron-electron, electron-ion and ion-ion correlations, enabling self-consistent treatment of electron screening and ion correlation. Atomic wave functions are derived by solving the Dirac equation within the ion-sphere model, using a modified central potential that incorporates both free-electron screening and ion–ion interactions. EII cross sections are then computed via the Distorted-Wave (DW) approximation in FAC. The results demonstrate that hot-dense environment effects significantly enhance the electron-impact ionization cross sections of C, N, and O compared to those calculated under the free-atom model. Additionally, a notable reduction in the ionization threshold energy is observed. These effects are attributed to the overlap of atomic potentials due to strong ion coupling and the shift in bound-state energy levels caused by free-electron screening. For instance, under solar boundary conditions, the ionization cross section of C+ increased by up to 50%, with the ionization threshold decreasing from about 24 eV (isolated) to 18 eV (with screening). Similar enhancements were observed for nitrogen and oxygen ions across various charge states. By providing updated ionization cross sections for C, N, and O ions under realistic solar interior conditions, this work offers essential parameters for improving radiation transport models, ionization balance calculations, and equation-of-state models in stellar interiors. The results underscore the necessity of including hot-dense environment effects in atomic process calculations for hot dense plasmas, with implications for astrophysics and inertial confinement fusion research.
,
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In rarefied gas flows, accommodation coefficients (ACs) serve as core parameters for gas-surface interactions and play a crucial role in the accuracy of mesoscopic model simulations. However, there exist significant discrepancies in the ACs obtained by different molecular dynamics simulation methods. To accurately characterize the momentum and energy accommodation properties of rarefied gases with solid surfaces under non-equilibrium conditions, this study systematically investigates the gas-surface interactions between argon molecules and platinum surfaces using molecular dynamics (MD) methods. By employing single scattering (SS) and continual scattering (CS) approaches, the influence of gas-gas collisions on tangential momentum accommodation coefficients (TMAC), normal momentum accommodation coefficients (NMAC), and energy accommodation coefficients (EAC) is comparatively analyzed, along with the operational laws of parameters such as surface morphology, surface temperature, incident velocity, and mean free path (MFP). The results demonstrate that gas density exerts a dual effect on momentum and energy accommodation: at smaller MFP, the high gas density within the interaction region impedes the accommodation of subsequent incident molecules with the surface, resulting in lower ACs; at moderate MFP, gas-gas collisions promote accommodation by increasing the frequency of gas-surface collisions, thereby enhancing ACs. Within the MFP range of 2.0–60.0 nm, the deviation in ACs between the CS and SS methods ranges from –14.88% to 5.21%, validating the dual role of gas density. Furthermore, at larger MFP, the TMAC and NMAC obtained via the CS method exhibit different trends with increasing MFP across surfaces of varying morphologies. In contrast to gas density, increases in both surface temperature and incident velocity shorten the interaction time, leading to reduced ACs. Notably, the effect of temperature varies across surfaces with different morphologies: elevated temperatures on smooth surfaces enhance the thermal fluctuations of surface atoms, thereby increasing NMAC, while elevated temperatures on rough surfaces cause smoothing of rough structures, thus inhibiting accommodation. Under high-speed incident conditions, gas-gas collisions promote NMAC on smooth surfaces, inhibit both TMAC and NMAC on rough surfaces, and suppress EAC across all surfaces. Additionally, the ACs obtained via both the CS and SS methods decrease with increasing incident velocity across surfaces of different morphologies.
, , Received Date: 2025-08-29
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, , Received Date: 2025-08-15
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,
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Nickel-based superalloys are extensively used in aero-engin due to their combined high strength, toughness, corrosion and creep resistance at elevated temperatures. Strain rate, temperature, and strain are important factors influencing the microstructural evolution of nickel-based superalloys. In this work, a typical nickel-based superalloy, GH4738 alloy, is selected to study the dynamic compressive deformation behavior of this material. Split Hopkinson pressure bar (SHPB) compression test was performed on GH4738 superalloy at strain rates of 1000~7500 s-1 with temperature ranges from RT to 500 °C. The yield strength of GH4738 superalloy decreases with increasing temperature and increases with increasing strain rate; however, at the temperature of 500 °C and the strain rate of 7500 s-1, it drops sharply. In order to understand the microscopic deformation behavior of GH4738 superalloy, parallel specimens were prepared with SHPB at frozen strains of -0.02, -0.05, -0.10, -0.20 and -0.25 at a strain rate of 3000 s-1 for the cases of RT, 400 °C and 500 °C, respectively. Neutron diffraction technique was employed to characterize the evolution of lattice constants and elastic lattice strains. We define the horizontal lattice mismatch as the lattice misfit at the γ/γ' interface that is perpendicular to the SHPB compressed direction, and the vertical lattice mismatch as the lattice misfit parallel to the SHPB compression direction. As the frozen strain increases, the horizontal lattice mismatch exhibits positive values and an increasing trend, while the vertical lattice mismatch changes from positive to negative values; the elastic lattice strain of the γ' phase consistently increases, while that of the γ phase remains almost unchanged. The lattice strains of the {111} and {220} planes are negative at 400 °C and 500 °C but positive at RT; the lattice strain of the {200} plane alternates between positive and negative values from RT to 500 °C, while that of the {311} plane remains negative throughout this temperature range. However, at a frozen strain of -0.25, the lattice strain of the {311} plane exhibits a significant rebound at both RT and 500 °C, indicating generation of significant intergranular stresses in the material. Dislocation configurations are characterized using transmission electron microscopy (TEM) to interpret the underlying mechanism. At RT, plastic deformation is dominated by γ-γ' co-deformation, with defects manifesting as parallel slip bands and stacking faults. Lattice misfit is effectively relaxed due to the formation of dislocation networks at γ/γ' interfaces, resulting in minimal residual lattice strain at RT. At 500 °C, dislocation density increases substantially because both γ and γ' phases readily undergo plastic deformation under thermal activation. Under such conditions, dislocation networks fail to compensate for lattice distortions induced by defect multiplication, resulting in high lattice misfit and residual lattice strain. At 400 °C, the alternating dominance of dislocation climb and slip induces fluctuations in both lattice misfit and residual lattice strain. Due to slow dislocation density accumulation, {hkl} lattice strains continuously increase. This contrasts with the RT and 500 °C scenarios, where rising dislocation density partially recovers elastic lattice distortion and even induces {hkl} lattice strain rebound at high strains (ε = -0.20~-0.25).
,
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In the field of solar cell technology, the conversion efficiency of silicon heterojunction (SHJ) solar cells has reached 27.08%. Meanwhile, perovskite/SHJ tandem solar cells based on this structure have achieved an efficiency of 34.85%, surpassing the 33.7% theoretical limit for single-junction devices. As the industry shifts from single-junction to tandem configurations, SHJ cells—benefiting from their distinctive structure and low-temperature fabrication process—offer superior compatibility with perovskite layers. This positions SHJ technology to play a critical role in the development of perovskite/tandem solar cells.
The application of high-performance silver-coated copper (Ag@Cu) paste for electrode metallization provides a viable approach to reduce the cost and improve the performance of SHJ cells. However, the micron-scale particle size of Ag@Cu powder (typically several micrometers) limits the packing density of the electrode layer. To address this, nano-silver powder (~100 nm) is commonly introduced as an additive, enhancing both the packed density of the powder and the electrical conductivity through nano-effects. Although many studies focus on isolated aspects such as paste conductivity, a systematic evaluation covering contact resistivity, printed and cured electrode morphology, overall cell performance, and long-term stability remains scarce. Potential adverse effects of nano-silver addition have also been overlooked. Therefore, a thorough investigation into the role of nano-silver in low-temperature Ag@Cu pastes is necessary.
Highly conductive low-temperature curing pastes typically employ binary or ternary composite powders with well-separated particle sizes to achieve high packing density according to the dense packing theory. In this work, we systematically adjusted the proportions of three conductive powders: micro-sized Ag@Cu (3-5 μm), sub-micron silver (500 nm), and nano-silver (100 nm), to study the effect of nano-silver on key properties of Ag@Cu paste. These include: curing temperature and sintering behavior, microstructure of cured electrodes, interface structure between electrodes and the silicon wafer, electrical resistivity, and the overall conversion efficiency of SHJ solar cells. The aim is to clarify the underlying mechanisms and optimize the nano-silver content.
This research reveals several significant impacts of nano-silver addition on Ag@Cu paste properties: (1) It markedly reduces the resistivity of the cured electrode. Compared to sub-micron silver, nano-silver facilitates improved lateral conductivity at lower sintering temperatures. (2) It introduces additional pores at the contact interface with the silicon wafer, increasing contact resistivity. A thickened organic layer at the interface also forms, which reduces the open-circuit voltage of the cell. (3) It enhances paste thixotropy, leading to narrower printed electrode lines that reduce shading loss and increase short-circuit current density. Concurrently, it raises electrode height and cross-sectional area, which helps improve the fill factor. (4) With nano-silver content controlled at 15%, the efficiency of SHJ cells matches or approaches that of reference cells with pure silver electrodes, mainly due to enhanced fill factor and short-circuit current density.
In summary, an optimized amount of nano-silver powder (e.g., 15%) enables simultaneous improvement in electrode conductivity, printability, and opto-electrical performance, yielding SHJ cells with efficiency comparable to those using pure silver electrodes. This demonstrates the potential of Ag@Cu pastes as a cost-effective alternative without compromising performance. Future studies should focus on the long-term reliability of such paste systems and their scalability, supporting the mass adoption of this technology in perovskite/SHJ tandem solar cells.
The application of high-performance silver-coated copper (Ag@Cu) paste for electrode metallization provides a viable approach to reduce the cost and improve the performance of SHJ cells. However, the micron-scale particle size of Ag@Cu powder (typically several micrometers) limits the packing density of the electrode layer. To address this, nano-silver powder (~100 nm) is commonly introduced as an additive, enhancing both the packed density of the powder and the electrical conductivity through nano-effects. Although many studies focus on isolated aspects such as paste conductivity, a systematic evaluation covering contact resistivity, printed and cured electrode morphology, overall cell performance, and long-term stability remains scarce. Potential adverse effects of nano-silver addition have also been overlooked. Therefore, a thorough investigation into the role of nano-silver in low-temperature Ag@Cu pastes is necessary.
Highly conductive low-temperature curing pastes typically employ binary or ternary composite powders with well-separated particle sizes to achieve high packing density according to the dense packing theory. In this work, we systematically adjusted the proportions of three conductive powders: micro-sized Ag@Cu (3-5 μm), sub-micron silver (500 nm), and nano-silver (100 nm), to study the effect of nano-silver on key properties of Ag@Cu paste. These include: curing temperature and sintering behavior, microstructure of cured electrodes, interface structure between electrodes and the silicon wafer, electrical resistivity, and the overall conversion efficiency of SHJ solar cells. The aim is to clarify the underlying mechanisms and optimize the nano-silver content.
This research reveals several significant impacts of nano-silver addition on Ag@Cu paste properties: (1) It markedly reduces the resistivity of the cured electrode. Compared to sub-micron silver, nano-silver facilitates improved lateral conductivity at lower sintering temperatures. (2) It introduces additional pores at the contact interface with the silicon wafer, increasing contact resistivity. A thickened organic layer at the interface also forms, which reduces the open-circuit voltage of the cell. (3) It enhances paste thixotropy, leading to narrower printed electrode lines that reduce shading loss and increase short-circuit current density. Concurrently, it raises electrode height and cross-sectional area, which helps improve the fill factor. (4) With nano-silver content controlled at 15%, the efficiency of SHJ cells matches or approaches that of reference cells with pure silver electrodes, mainly due to enhanced fill factor and short-circuit current density.
In summary, an optimized amount of nano-silver powder (e.g., 15%) enables simultaneous improvement in electrode conductivity, printability, and opto-electrical performance, yielding SHJ cells with efficiency comparable to those using pure silver electrodes. This demonstrates the potential of Ag@Cu pastes as a cost-effective alternative without compromising performance. Future studies should focus on the long-term reliability of such paste systems and their scalability, supporting the mass adoption of this technology in perovskite/SHJ tandem solar cells.
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Optical Stokes vector skyrmions, as novel fully Poincaré spherical vector beams, hold broad application prospects in optical communication, optical computing, multiplexing, and super-resolution imaging. However, existing research primarily focuses on the controllable generation of single optical skyrmions, with limited exploration of continuous modulation of different skyrmion configurations and insufficient investigation into generation in the terahertz frequency band. This paper proposes a multilayer metasurface that generates higher-order topological configurations of Stokes vector skyrmions through rotation. For instance, a two-layer structure enables rotational control of two skyrmion types, while a three-layer design achieves control over four skyrmion types. A twist-tunable double-layer Moiré metasurface design is simultaneously developed, where the two metasurface layers are designed with complementary Moiré phases to achieve continuous modulation of the radial skyrmion order. By synergistically modulating the geometric and dynamic phases of the metasurface, the topological invariance of free-space propagating skyrmions is preserved while maintaining beam intensity. The paper presents detailed theoretical analysis and numerical results, validated through full-wave simulation studies. This multilayer metasurface design enables dynamic control of Stokes vector and skyrmion configurations solely by adjusting the relative rotation angles between layers, eliminating the need to alter incident light or external conditions. This approach breaks through the limitations of traditional phase modulation methods reliant on phase-change materials. Furthermore, the dual-layer Moiré metasurface design significantly enhances device integration, offering a highly integrated and flexible technical pathway for realizing multidimensional light field manipulation and long-distance terahertz optical communication systems.
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This study employs a phase-field–lattice Boltzmann coupled model to investigate the effect of forced convection on the lamellar eutectic growth of Al-Cu alloys. Results indicate that forced convection tilts lamellar structures toward the flow direction, enhances solute diffusion, and causes solute concentration to deviate asymmetrically from the solid phase centerline. Greater convection intensity leads to more pronounced interface asymmetry. Increased undercooling weakens convective effects and reduces tilt angles, while larger lamellar widths diminish convective influence and yield smaller tilt angles. The study reveals a synergistic regulatory mechanism between these factors. Simulations indicate that without convection, layers grow vertically with solute symmetrically distributed along the solid centerline. Interlayer lateral diffusion promotes synergistic α-β phase growth. Forced convection (along the x+ direction) enhances solute transport in the flow direction while weakening counter-current transport. This shifts the triple point and creates asymmetric solute distribution—e.g., higher α phase concentration to the left of the centerline—causing layer tilting. Increasing convection intensity (expressed as A/A0, where A0 corresponds to the coefficient for a 0.5° tilted layer) exacerbates asymmetry at the solid-liquid interface and reduces the distance between the interface peak and the triple point. Higher undercooling (0.8-1.4 K) enhances growth driving force and reduces solute trapping capacity, weakening convective effects and decreasing the tilt angle. When undercooling is minimal and convection is strong, the tilt angle significantly increases. As the interlayer spacing (6.4-19.2 μm) increases, solute exchange at the interface becomes more frequent, convective interference weakens, and the tilt angle decreases; under conditions of small spacing and strong convection, solutes are easily washed away, inhibiting lamellar growth. In summary, forced convection directly alters the morphology of the solute transport control layer. Supercooling and interlayer width indirectly modulate convective effects by influencing growth driving forces and interfacial solute exchange. These three factors synergistically regulate eutectic growth, providing a theoretical basis for controlling eutectic microstructure.
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