Science China Technological Sciences最新文献

Many current-carrying contact pairs, such as those found in pantograph-catenary systems, operate in open environments and are susceptible to significant external interference from temperature and humidity variations. This study investigated the evolution of the friction coefficient and contact resistance of C/Cu contact pairs under alternating temperature, humidity, and current conditions. Through experimentation, the wear rate and microtopography of the worn surface were analyzed under various constant parameters. Subsequently, the differences in tribological behavior and current-carrying characteristics of the contact pairs under these three parameters were explored. The results revealed that the decrease in temperature resulted in a significant increase in the friction coefficient of the contact pairs, carbon wear, and copper surface roughness. Additionally, the surface oxidation rate was lower at lower temperatures. Moreover, contact resistance did not consistently increase with decreasing temperature, owing to the combined action of the contact area and the oxide film. Compared with temperature, humidity fluctuations at room temperature exerted less influence on the friction coefficient and contact resistance of the contact pairs. Dry environments rendered carbon materials vulnerable to oxidation and cracking, while excessive humidity fostered abrasive wear and arcing. High-current conditions generally degraded the tribological properties of C/Cu contacts. In the absence of current, the friction coefficient was extremely high, and the copper transfer was high. Under excessive current, copper was susceptible to plowing by carbon micro-bumps and abrasive particles, resulting in a decrease in the friction coefficient. The release of lipids from the carbon surface due to temperature elevation weakened the electrical contact performance and increased the occurrence of arc erosion, thereby exacerbating carbon wear.

Tapered roller bearings (TRBs) can withstand axial loads, radial loads, and overturning moments. The performance, safety, and efficiency of rotating machinery are directly influenced by the friction moments within the TRBs. However, most current research has relied on empirical formulas that focus on axial loads. Additionally, the friction coefficient between the rollers and the inner ring rib has been defined using simple empirical methods. In actual applications, the loads on TRBs are not purely axial or radial, and simple empirical friction coefficients do not adequately account for the varying lubrication conditions. To address this challenge, this study proposes an improved method for calculating the friction moments of TRB under combined axial and radial loads. This study employs a calculation method for sliding friction coefficients that can model dry, boundary, elastohy-drodynamic, and mixed lubrication conditions. To demonstrate the advantages of the proposed method, the friction moments obtained using the existing and proposed methods are compared. Additionally, the influence of TRB structural parameters on the friction moment is discussed. An experimental study is conducted to validate the effectiveness of the proposed method. The findings provide valuable insights for designing TRB structural parameters to minimize friction moments.

Supplier selection is an important business activity in order to realize the purchasing function in supply chain management. The supplier selection process includes four stages, i.e., bidding inviting, bidding, group decision-making, and results disclosure, involving the participation of manufacturing service demanders (MSDs), manufacturing service suppliers (MSSs), and decision-makers. Nowadays, all the participants have raised concerns about the increased transparency in supplier selection. Therefore, this study proposes a transparent supplier selection method by considering the engagement of suppliers. In this method, the Bayesian best-worst method (Bayesian BWM) is used to aggregate decision-makers’ preferences into the overall optimal weights of the alternative MSSs, and the MSS with the largest weight is considered the suitable MSS for MSDs. Furthermore, blockchain is introduced to record the decision-making process information about supplier selection through a customized smart contract, where MSSs act as supervisors to supervise the decision-making process through the distributed consensus mechanism rather than directly participate in the decision-making process. Finally, a case study of supplier selection in purchasing vibration acceleration sensors is presented. The result shows that the proposed method can support MSDs in selecting suitable MSS from alternative MSSs by aggregating decision-makers’ preferences, and blockchain can provide credible information about the supplier selection process for MSSs, MSDs, and decision-makers. In this way, the transparency of supplier selection is enhanced.

As a new concept, mass-entransy is one of the twins in the core of entransy theory. It can describe mass-transfer ability for mass-transfer processes (MTPes), just as thermal-entransy for describing heat-transfer ability. Accordingly, mass-entransy dissipation can be utilized to evaluate the loss of mass-transfer ability. Minimum mass-entransy dissipation (MMED) is utilized to optimize one-way isothermal diffusive MTPes with mass-leakage and mass-transfer law (g ∝ Δ(c), where c means concentration). For a given net amount of mass-transferred key components at the low-concentration side, optimality-condition for the MMED of isothermal diffusive MTPes is obtained by using the averaged-optimization-method. Effects of the amount of mass-transferred and mass-leakage on optimal results are analyzed, and the obtained optimization profiles are compared with those for MTP profiles of constant-concentration-difference (c₁ − c₂ = const) and constant-concentration-ratio (c₁ / c₂ = const). The product of square of key-component-concentration (KCC) difference between high- and low-concentration sides and inert component concentration at high-concentration side for the MMED of the MTP with no mass-leakage is a constant, and the optimal relationship of the KCCs between high- and low-concentration sides with mass-leakage is significantly different from the former. When mass-leakage is relatively small, the MTP with c₁ − c₂ = const strategy is superior to that with c₁ / c₂ = const strategy, and the latter is superior to the former with an increase in mass-leakage. A combination of mass-entransy concept, finite-time thermodynamics, and averaged-optimization-method is a meaningful tool for optimizing MTPes.

Concrete structures undergo integral fragmentation under explosion loads. The fragmentation degree and particle-size distribution of concrete blocks under explosion loads must be considered during mining to ensure safety. In this study, the impulse is calculated based on the relationship between overpressure and time, and the impact energy of the explosion wave is obtained based on blast theory. Subsequently, the Mohr-Coulomb shear strength fracture criterion is introduced to determine the ultimate shear stress of the concrete materials, and an empirical model that can effectively calculate the energy consumption of concrete blocks under explosion loads is established. Furthermore, concrete fragments with different particle sizes under explosion scenarios are quantitatively predicted with the principle of energy conservation. Finally, explosion tests with different top standoff distances are conducted, and the concrete fragments after the explosion tests are recovered, sieved, weighed, and counted to obtain experimental data. The effectiveness of the fragment empirical model is verified by comparing the model calculation results with the experimental data. The proposed model can be used as a reference for civil blasting, protective engineering design, and explosion-damage assessment.

The hydrogenolysis/hydrocracking of waste polyethylene (PE) has recently been intensively studied, with the general pursuit of low-temperature reaction conditions, increased oil-phase yield, and narrower carbon chain distribution. Before this, we utilized a ball-milled ZSM-22 catalyst loaded with Ru nanoparticles (NPs), which exhibited excellent hydroconversion performance. It deconstructed PE into >80 wt.% oil products under low temperatures and short reaction times. Herein, we investigated the influence of varying temperature/pressure parameters on the degree of specific hydrocracking/internal hydrogenolysis/terminal hydrogenolysis reactions. From the comprehensive energy efficiency perspective, including stirring, reaction, and product separation, as well as taking into account the degree of product isomerization and catalyst lifespan, we analyzed the optimization of parameters. This research abandons the notion that lower temperatures are better and proposes a more comprehensive evaluation framework for low-consumption hydroconversion of PE to produce high-value products.

Torque sensors are essential components of robotic joints. In the past, structure optimization of force-sensing elements has been the common approach to improve the performance of the torque sensors. In this work, we demonstrate a torque sensor with bulk metallic glasses as a force-sensing element. Compared with the sensors made of stainless steel and aluminum alloy, the use of bulk metallic glass as a force-sensing element significantly improves sensor sensitivity, linearity, repeatability, hysteresis, and measuring range. Our work not only opens up a new avenue for the application of bulk metallic glasses, but also provides opportunities for enhancing the performance of force/torque sensors through materials optimization.

In addition to the plasmon-mediated resonant coupling mechanism, the excitation of hot electron induced by plasmon presents a promising path for developing high-performance optoelectronic devices tailored for various applications. This study introduces a sophisticated design for a solar-blind ultraviolet (UV) detector array using linear In-doped Ga₂O₃ (InGaO) modulated by platinum (Pt) nanoparticles (PtNPs). The construction of this array involves depositing a thin film of Ga₂O₃ through the plasmonenhanced chemical vapor deposition (PECVD) technique. Subsequently, PtNPs were synthesized via radio-frequency magnetron sputtering and annealing process. The performance of these highly uniform arrays is significantly enhanced owing to the generation of high-energy hot electrons. This process is facilitated by non-radiative decay processes induced by PtNPs. Notably, the array achieves maximum responsivity (R) of 353 mA/W, external quantum efficiency (EQE) of 173%, detectivity (D*) of approximately 10¹³ Jones, and photoconductive gain of 1.58. In addition, the standard deviation for photocurrent stays below 17% for more than 80% of the array units within the array. Subsequently, the application of this array extends to photon detection in the deep-UV (DUV) range. This includes critical areas such as imaging sensing and water quality monitoring. By leveraging surface plasmon coupling, the array achieves high-performance DUV photon detection. This approach enables a broad spectrum of practical applications, underscoring the significant potential of this technology for the advancement of DUV detectors.

Aging treatments are the key process to obtain satisfactory strength for 7xxxAl alloys and their composites. However, traditional single-stage (SS) aging is time-consuming to reach a peak strength condition. In this study, an efficient 120°C + 160°C two-stage (TS) aging treatment was proposed on a B₄C/7A04Al composite fabricated via powder metallurgy (PM) technology, which could acquire similar peak-aging strength but only took about 15% of the time compared to traditional 120°C SS aging. The evolution of precipitation during the TS aging was investigated, as well as those of the 7A04Al alloys for comparison. In the second stage aging process, the higher aging temperature accelerated the nucleation of η′ phases inside the grains and thus increased the density of precipitates. Moreover, the short aging time limited the coarsening of precipitates and the broadening of precipitate-free zones. The above factors were beneficial for quickly obtaining satisfactory precipitation strengthening effects. The B₄C/7A04Al composite exhibited slower aging kinetics than the 7A04Al alloy in the TS aging. Mg elements consumption by the chemical reaction between B impurities introduced by B₄C particles and the Al matrix was considered to potentially retard the aging kinetics of the B₄C/7A04Al composite. Nevertheless, the precipitation sequence was not affected.

Concerning the high demand for lightweight and multifunctional properties of engineering structures, the coral skeleton-inspired sheet-based (CSS) structure, which was a novel bio-mimicking coral skeleton wall-septa architecture with a unique ability to resist wave shocks was fabricated using NiTi alloy by laser powder bed fusion (LPBF) technology. The effects of laser energy density (LED) on surface morphologies, microstructures, phase transformation behavior, and mechanical properties of LPBF-fabricated CSS structures were systematically investigated. The results indicated that the size deviation was predominantly governed by powder adhesion and step effect. NiTi CSS structures with LED of 71 J·mm⁻³ possessed superior compressive modulus (∼100 MPa), ultimate strength (∼13 MPa), and energy absorption efficiency (∼69%). The compression fracture mechanism of the LPBF-fabricated NiTi CSS structures was revealed to be predominantly brittle fracture accompanied by ductile fracture. Furthermore, the Ni₄Ti₃ nanoprecipitates induced the precipitation strengthening effect, enabling better shape memory response at LED of 71 J·mm⁻³, with a recoverable strain of 3.63% and recovery ratio of 90.8%, after heating under a pre-strain of 4%. This study highlights the importance of a bionic design strategy for enhancing the mechanical properties of NiTi components and offers the possibility to tailor its functional properties.