Science China Technological Sciences最新文献

This study reports the rare ultralow-frequency (ULF) wave activity associated with the solar wind dynamic pressure enhancement that was successively observed by the GOES-17 (Geostationary Operational Environmental Satellite) in the magnetosphere, the CSES (China Seismo-Electromagnetic Satellite) in the ionosphere, and the THEMIS ground-based observatories (GBO) GAKO and EAGL in the Earth’s polar region during the main phase of an intense storm on 4 November 2021. Along with the enhanced-pressure solar wind moving tailward, the geomagnetic field structure experienced a large-scale change. From dawn/dusk sides to midnight, the GAKO, EAGL, and GOES-17 sequentially observed the ULF waves in a frequency range of 0.04–0.36 Hz at L shells of ∼5.07, 6.29, and 5.67, respectively. CSES also observed the ULF wave event with the same frequency ranges at wide L-shells of 2.52–6.22 in the nightside ionosphere. The analysis results show that the ULF waves at ionospheric altitude were mixed toroidal-poloidal mode waves. Comparing the ULF waves observed in different regions, we infer that the nightside ULF waves were directly or indirectly excited by the solar wind dynamic pressure increase: in the area of L-shells ∼2.52–6.29, the magnetic field line resonances (FLRs) driven by the solar wind dynamic pressure increase is an essential excitation source; on the other hand, around L∼3.29, the ULF waves can also be excited by the outward expansion of the plasmapause owing to the decrease of the magnetospheric convection, and in the region of L-shells ∼5.19–6.29, the ULF waves are also likely excited by the ion cyclotron instabilities driven by the solar wind dynamic pressure increase.

Recently discovered Ising superconductors have garnered considerable interest due to their anomalously large in-plane upper critical fields (B_c2). However, the requisite strong spin-orbital coupling in the Ising pairing mechanism generally renders these superconductors heavy-element dominant with notably low superconducting transition temperatures (T_c). Here, based on the Migdal-Eliashberg theory and the mean-field Bogoliubov-de Gennes Hamiltonian, we demonstrate a significant enhancement of Ising superconductivity in monolayer NbSe₂ through surface fluorination, as evidenced by concomitant improvements in T_c and B_c2. This enhancement arises from three predominant factors. Firstly, fluorine atoms symmetrically and stably adhere to both sides of the monolayer NbSe₂, thereby maintaining the out-of-plane mirror symmetry and locking carrier spins out-of-plane. Secondly, fluorination suppresses the charge density wave in monolayer NbSe₂ and induces a van Hove singularity in the vicinity of the Fermi level, leading to a marked increase in the number of carriers and, consequently, strengthening the electron-phonon coupling (EPC). Lastly, the appearance of fluorine-related, low-frequency phonon modes further augments the EPC. Our findings suggest a promising avenue to elevate T_c in two-dimensional Ising superconductors without compromising their Ising pairing.

The lubrication performance of liquids is severely restricted and is degraded in high-temperature environments. Stable and reliable lubrication in high temperature environments has been a long-standing goal in various industrial fields. In this study, WS₂ and Ti₃C₂T_x MXene nanoflakes were used as oil-based lubricant additives to generate ultra-low friction and even superlubricity (friction coefficient of ∼0.007) at elevated temperatures (400°C), which has hitherto not been achieved by both individual pristine materials, WS₂ and Ti₃C₂T_x MXene. Viscosity and thermogravimetric characterization revealed improvements in the high-temperature rheological properties and thermal stability of the lubricating base oil, indicating improved load-bearing and continuous lubrication capabilities at elevated temperatures. X-ray photoelectron spectroscopy, transmission electron microscopy, and atomic force microscopy demonstrated that the formation of an iron/titanium/tungsten-rich oxide lubricious thin film at the sliding interface reduced the interfacial shear stress, which was responsible for the observed friction and wear reductions at high contact pressures (> 1.1 GPa). Although the titanium/tungsten oxide film was gradually removed after prolonged sliding, a sufficiently thick iron oxide film maintained a low friction coefficient for at least 2 h. The improved surface quality facilitates the achievement of ultra-low friction and reduced wear. The proposed lubrication methodology has a broad utilization potential as a wear-reduction strategy across various industrial fields at elevated temperatures.

In this article, 1-(4-ethylphenyl)-nonane-1,3-dione (0206) was prepared by Claisen condensation. By mixing 0206, chelate, and base oil in a ratio of 3.2:4.8:2, a diketone lubricant (PAO=14 (20%)) that can achieve superlubricity was prepared and applied to bearing lubrication experiments. The experimental results show that when the bearing was lubricated by base oil, the friction coefficient (COF) and temperature rise decreased with the decrease of the viscosity of PAO. When PAO=14 (20%) was used as the lubricant, the COF of the bearing was the lowest (0.001), and the wear morphology was comparable to that of the bearing lubricated with commercial lubricant. Compared with the base oil with the same viscosity, it is found that the COF and temperature rise of the bearing lubricated by PAO=14 (20%) were lower under any experimental conditions. And when the amount of lubricant added was 10 µL, the COF of the bearing lubricated by PAO=14 (20%) reached a very low value (0.0004). Bearing ball surface analysis identified the formation of diketone adsorption films. Combined with the previous PAO=14 (20%) superlubricity mechanism, it was considered that the occurrence of tribochemical reaction and the bearing effect of chelates were the main reasons for the existence of ultra-low friction coefficient and low wear. In addition, when there were polar molecules in the lubricant, they were adsorbed on the metal surface through tribochemical reactions, resulting in many irregular pits on the surface.

Low-melting-point alloys have the advantages of good biocompatibility, plasticity, and near-bone mechanical strength, making them suitable as bone defect-filling materials for direct injection into defective bone sites. However, using low-melting-point alloys for orthopedic implants poses the challenge of causing thermal damage to the surrounding bone tissue during injection. In this study, a thermosensitive hydrogel is prepared and synergistically injected into the bone defect site with BiInSn. BiInSn solidifies and releases heat during injection, while the thermosensitive hydrogel absorbs heat and transforms into a gel state, encapsulating BiInSn. Therefore, the surrounding bone tissue is effectively protected from thermal damage. When BiInSn and the thermosensitive hydrogel were injected synergistically, in vitro thermal experiments revealed that the maximum temperature of the surrounding bone tissue reached 42°C. This temperature is below the 47°C threshold, which causes permanent damage to bone tissues. In vivo experiments demonstrated that rats in the BiInSn-thermosensitive hydrogel group exhibited better recovery at the bone defect sites. These results suggest that the synergistic injection of Bi-based alloy and thermosensitive hydrogel is beneficial in reducing thermal damage to bone tissue, guiding bone tissue growth, and effectively facilitating the repair of bone defects.

Flexible heaters with personal thermal management capabilities have great potential in thermal therapy applications due to their excellent flexibility, low power consumption, and portability. However, manufacturing wearable heating devices that are breathable, wear resistant and conformal for long-term use is still challenging. To address these issues, we designed a leather heater using breathable, biocompatible, and tailorable leather as the substrate through a simple in-situ polymerization polypyrrole strategy. This heater exhibits excellent heating and mechanical properties (reaching 64°C at a voltage of 5 V with efficient Joule heat generation of 2286 W/m² and uniform temperature distribution, and functioning properly after 1000 cycles of bendability tests). In addition, this heater displays better wear resistance and water vapor permeability rate (38.04 g/(m² h)). The cuttable and sewable of leather gives the strategy ability to be flexibly designed to mold the heater to the specific requirements of different body parts, providing a new approach to wearable thermal therapy.

Molybdenum disulfide (MoS₂) films are widely deployed in industrial applications owing to their inherent interlayer slip characteristics, offering energy consumption savings and prolonged mechanical part performance. Nevertheless, their practical utility is limited by environmental constraints and the limitations of preparation techniques, which hinder the attainment of robust superlubricity (friction coefficient < 0.01). Herein, through magnetron sputtering technology, we synthesize a core-shell-like nanocomposite composed of MoS₂ nanosheets encapsulating B₄C. The core-shell-like structure enables the resulting films to preferentially grow crystalline MoS₂, providing them with outstanding mechanical properties and efficient lubrication over a wide range of temperatures. Remarkably, such film achieves robust macroscale superlubricity and an ultralow wear rate (1.7 × 10⁻⁸ mm³ N⁻¹ m⁻¹) under high contact stress in a mild vacuum environment. This noteworthy outcome is primarily attributable to the self- segmentation of the macroscale contact interface during the friction process, involving: (1) a large amount of wear debris is embedded into the wear track to create extensive micro-sized asperities; (2) a nanolayer of amorphous carbon enriched with oxide nanoparticles is formed on the uppermost part of these asperities; (3) numerous incommensurate nanocontacts comprising nanoparticles and highly oriented MoS₂ nanosheets are established, culminating in the achievement of robust superlubricity. Our pioneering design, coupled with the elucidation of the underlying superlubricity mechanism, holds significant promise for advancing the development of robust and high performance lubricants.

The hot deformation behavior and microstructure evolution of GH3536-TiB₂ composites fabricated by powder metallurgy (PM) were examined in the temperature range of 950–1150°C and strain rate range of 0.001–1 s⁻¹. The hot compression stress-strain curves and the constitutive equation were obtained. In addition, the hot processing map was drawn, which indicated that the appropriate hot working window was 950–1050°C/0.001–0.1 s⁻¹ and 1050–1100°C/0.001–0.01 s⁻¹. The microstructure analysis showed that the splitting and spheroidization of M₃B₂ led to a decrease in size and volume fraction at 950–1100°C. At 1150°C, the eutectic microstructure of M₃B₂ + γ was formed due to the dissolution of M₃B₂, which caused macroscopic cracking of the deformed sample. Additionally, the deformation temperature and the strain rate had little effect on the size and volume fraction of M₃B₂. Besides, discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) were found in the deformed microstructure, while the former was dominant. Within the test range of this work, the dynamic recrystallization (DRX) fraction of the deformed composites was high due to the bulging nucleation of numerous interfaces. The DRX grain size increased with increasing deformation temperature or decreasing strain rate. Texture analysis showed that the deformation texture of <101>//compression direction RD existed in the matrix when the deformation temperature was below 1100°C, and the texture type became <001>//RD at 1100°C. Additionally, it was also found that the <001>//RD texture was formed in M₃B₂ under the strain rates of 0.1 and 0.01 s⁻¹.

Accurate prediction of junction temperature is crucial for the efficient thermal design of silicon nano-devices. In nano-scale semiconductor devices, significant ballistic effects occur due to the mean free path of phonons comparable to the heat source size and device scale. We employ a three-dimensional non-gray Monte Carlo simulation to investigate the transient heat conduction of silicon nanofilms with both single and multiple heat sources. The accuracy of the present method is first verified in the ballistic and diffusion limits. When a single local heat source is present, the width of the heat source has a significant impact on heat conduction in the domain. Notably, there is a substantial temperature jump at the boundary when the heat source width is 10 nm. With increasing heat source width, the boundary temperature jump weakens. Furthermore, we observe that the temperature excitation rate is independent of the heat source width, while the temperature influence range expands simultaneously with the increase in heat source width. Around 500 ps, the temperature and heat flux distribution in the domain stabilize. In the case of dual heat sources, the hot zone is broader than that of a single heat source, and the temperature of the hot spot decreases as the heat source spacing increases. However, the mean heat flux remains unaffected. Upon reaching a spacing of 200 nm between the heat sources, the peak temperature in the domain remains unchanged once a steady state is reached. These findings hold significant implications for the thermal design of silicon nano-devices with local heat sources.

Microstructural rafting of Ni-based single-crystal (SC) superalloys is inevitable at elevated temperatures during long-term service with mechanical loading, which significantly affects the mechanical behaviour of the material. In this study, the effects of rafting on the mesodeformation and fracture behaviour of a Ni-based SC superalloy under cyclic and tensile loads were investigated using in situ scanning electron microscopy (SEM), digital image correlation (DIC), and crystal plasticity finite element method (CP-FEM) simulations. The results indicated that the tensile strength decreased significantly in the rafted specimens. In the cyclic tests, both the virgin and rafted specimens showed an increase in the maximum shear strain with cycle number. The interaction of cross-slip bands was captured by in situ SEM-DIC around the micro-notch of the virgin specimens during the tensile test, while a more homogeneous local deformation field was observed in the rafting specimens. In addition, the fracture behaviour was strongly influenced by the rafting morphology. The crack exhibited instantaneous and long-range fracture features along the octahedral plane as it propagated in the rafting specimen, whereas it deflected over a short distance between the crystallographic planes at an early stage in the virgin specimen, which is consistent with the CP-FEM results. Furthermore, the CP-FEM results for the crack initiation direction on ((1{bar1}1)) dominant slip plane were consistent with the in situ SEM observations.