Abstract
Our society is pursuing chemically recyclable polymers to accelerate the green revolution in plastics. Here, we develop a recyclable polyester library from the alternating copolymerization ...of aldehyde and cyclic anhydride. Although these two monomer sets have little or no thermodynamic driving force for homopolymerization, their copolymerization demonstrates the unexpected alternating characteristics. In addition to readily available monomers, the method is performed under mild conditions, uses common Lewis/Brønsted acids as catalysts, achieves the facile tuning of polyester structure using two distinct monomer sets, and yields 60 polyesters. Interestingly, the copolymerization exhibits the chemical reversibility attributed to its relatively low enthalpy, which makes the resulting polyesters perform closed-loop recycling to monomers at high temperatures. This study provides a modular, efficient, and facile synthesis of recyclable polyesters using sustainable monomers.
Alternating copolymerization of epoxides with cyclic anhydrides is a versatile method to produce functional polyesters. However, this strategy remains challenging to synthesize semicrystalline ...polyesters. Here, we describe a facile and efficient approach to synthesize semicrystalline polyesters via the copolymerization of low-cost isobutylene oxide (IBO) and various cyclic anhydrides. Using a heterogeneous catalyst, zinc–cobalt(III) double metal cyanide complex, inhibits the isomerization of IBO and demonstrates the productivity up to 680 g polyester/g catalyst. The resulting polyesters possess >99% alternating degree, >90% head-to-tail linkages, and molecular weights up to 42.3 kDa. This method also achieves low-molecular weight (3.9–7.1 kDa) hydroxy telechelic polyesters with narrow dispersities (1.3–1.4) upon adding water as the chain transfer agent. These unprecedented polyesters are semicrystalline owing to their high regioregularity and exhibit melting temperatures (T m) ranging from 67 to 141 °C. Interestingly, upon tuning cis/trans isomerization of the CC bond, the unsaturated polyester prepared from IBO and maleic anhydride demonstrates a unique crystalline (T m = 72 °C)-amorphous-crystalline (T m = 153 °C) transformation.
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IJS, KILJ, NUK, PNG, UL, UM
The ablation of thermal protection systems in hypersonic vehicles would generate randomly distributed surface roughness, which prominently influences laminar-turbulent boundary-layer transition. ...While discrete and regular-shaped roughness elements have been extensively studied, the effect of distributed roughness on hypersonic boundary-layer stability and transition is poorly understood. In the present study, experiments were performed on a 7-degree half-angle sharp cone with sandpaper-type roughness in a Mach 6 Ludwieg tube. The transition onset locations were obtained by infrared thermography. Single/two-point focused laser differential interferometer was utilized to characterize the instability waves over distributed roughness along both the streamwise and wall-normal directions within the laminar, transitional and turbulent regions. On distributed roughness, the transition onset location moves forward, and the second mode was still the dominant instability. It exhibits lower frequency, faster growth, and earlier decay compared with the smooth surface. Linear stability calculations were carried out to assist with experiments. Furthermore, bispectral analysis indicates complicated nonlinear phase-locked energy exchanges within the boundary layer. It is shown that the earlier saturation and faster decay of the second mode on distributed roughness is associated with intensified nonlinear interactions.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This work presents a comprehensive analysis of the ballistic response of Q235 steel/6061-T6 aluminum bilayer plates fabricated via explosive welding, with a special focus placed on elucidating the ...mechanisms of ballistic resistance enhancement due to the bonded interface. For comparison purposes, unbonded Q235 steel/6061-T6 aluminum bilayer plates (e.g. being stacked together or spaced with an air gap) were also investigated. Ballistic tests were carried out to compare the residual velocities and failure behaviours of the welded and unwelded bilayer plates. Computational models were built in LS/DYNA and validated with the experiment. Experimental results showed that the ballistic limit velocity of the explosively-welded plate is 13.4% and 9.4% higher than those of the in-contact plate and air-gapped plate. Simulation results suggested that the enhanced ballistic resistance was attributed to its welded interface. On the one hand, the welded interface allowed effective stress attenuation within the steel layer leading to a greater damage tolerance and more energy dissipation during the prolonged response time. On the other hand, when deforming together with the steel layer, the welded interface enforced a uniform indentation on the central region of the aluminum substrate leading to greater energy absorption due to compression and thinning of the material.
•Ballistic resistance of steel/aluminum bilayer armor plates was compared on an equal mass basis.•Explosively-welded plates have superior ballistic resistance over other bilayer plates.•The steel layer in the explosively-welded plate has greater damage tolerance.•The welded interface promotes energy absorption in the substrate due to local compression.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Current deep learning-based change detection approaches mostly produce convincing results by introducing attention mechanisms to traditional convolutional networks. However, given the limitation of ...the receptive field, convolution-based methods fall short of fully modelling global context and capturing long-range dependencies, thus insufficient in discriminating pseudo changes. Transformers have an efficient global spatio-temporal modelling capability, which is beneficial for the feature representation of changes of interest. However, the lack of detailed information may cause the transformer to locate the boundaries of changed regions inaccurately. Therefore, in this article, a hybrid CNN-transformer architecture named CTCANet, combining the strengths of convolutional networks, transformer, and attention mechanisms, is proposed for high-resolution bi-temporal remote sensing image change detection. To obtain high-level feature representations that reveal changes of interest, CTCANet utilizes tokenizer to embed the features of each image extracted by convolutional network into a sequence of tokens, and the transformer module to model global spatio-temporal context in token space. The optimal bi-temporal information fusion approach is explored here. Subsequently, the reconstructed features carrying deep abstract information are fed to the cascaded decoder to aggregate with features containing shallow fine-grained information, through skip connections. Such an aggregation empowers our model to maintain the completeness of changes and accurately locate small targets. Moreover, the integration of the convolutional block attention module enables the smoothing of semantic gaps between heterogeneous features and the accentuation of relevant changes in both the channel and spatial domains, resulting in more impressive outcomes. The performance of the proposed CTCANet surpasses that of recent certain state-of-the-art methods, as evidenced by experimental results on two publicly accessible datasets, LEVIR-CD and SYSU-CD.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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•Friction stir process fabricates a rough and nanosized composite layer on the surface of Tit-6Al-4V alloy.•The composite layer is composed of mainly Ti-6Al-4V and TiO2.•The surface ...microhardness of composites is greatly improved.•The alloy after friction stir process exhibits enhanced cell adhesion and proliferation.•The composite layer shows an increase of corrosion potential.
Although Ti-6Al-4V has been widely used in biomaterial field. Compared with other classes of materials, it still encounters some problems such as low surface hardness and relative low biocompatibility. To solve these problems friction stir processing (FSP) was applied to fabricate a nanosized composite layer of TiO2 and Ti-6Al-4V. Uniform distribution of TiO2 particles with some clusters on the surface of alloy can be observed. Due to severe plastic deformation and stirring heat, nanocrystallines and amorphous TiO2 can be observed in stir zone. FSPed samples show significant improvement in surface microhardness and biocompatibility due to its modified structure compared with original sample. In addition, through corrosion behaviors of the samples in simulated body fluid, it is found that FSP can enhance whilst TiO2 reduces the possibility and corrosion rate of material in environment of human body.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•Frontal coating on steel plates enhances ballistic performance.•The frontal coating has a cushioning effect to attenuate and disperse contact force.•The frontal polyurea-coating ...promotes energy dissipation of the steel substrate.
This study investigates the effect of polyurea coating on the ballistic resistance of polyurea-steel laminate plates. Ballistic tests are carried out to compare the residual velocities and failure modes of the monolithic steel plate and polyurea-steel laminate plates on an equal mass basis. Experimental results show that polyurea coating could lead to an enhanced ballistic resistance of the laminate plate (with 5.2% greater ballistic limit velocity), but only when applied at the impact-receiving face. Finite element simulations are performed to provide insights into the penetration process and the energy partitioning to reveal the enhancing mechanisms of the frontal polyurea coating. Excellent agreement was obtained between the numerical simulations and the experimental results. Simulation results suggest that the frontal polyurea-coating has a cushioning effect to alleviate and redistribute the total contact force acting on the substrate steel layer, which helps to spread the local impact to a far-field response. The polyurea-coating on the impact-receiving face provides additional ballistic resistance by promoting greater energy dissipation of the steel layer through global deformation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Polyesters with both cyclic topology and chemical recyclability are attractive. Here, the alternating copolymerization of cyclic anhydride and o‐phthalaldehyde to synthesize a series of cyclic and ...recyclable polyesters are reported for the first time. Besides readily available monomers, the copolymerization is carried out at 25 °C, uses common Lewis/Brønsted acids as catalysts, and achieves high yields within 1 h. The resulting polyesters possess well‐defined alternating sequences, high‐purity cyclic topology, and tunable structures using distinct two monomer sets. Of interest, the copolymerization manifests obvious chemical reversibility as revealed by kinetic and thermodynamic studies, making the unprecedented polyesters easy to recycle to their distinct two monomers in a closed loop at high temperatures. This work furnishes a facile and efficient method to synthesize cyclic polyesters with closed‐loop recyclability.
The cationic copolymerization of cyclic anhydride and o‐phthalaldehyde demonstrates alternating and chemical reversible characteristics, yielding novel polyesters with alternating sequence, high‐fidelity cyclic topology, closed‐loop chemical recyclability, and easy‐to‐tune structure.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This paper focused on ice processing and snow melting on urban pavement during the winter time in South China under the emergent situation. A kind of electrothermal cushion with carbon fiber was ...developed to improve resistance to skidding of pavement in winter and help melt ices and snows on the surface of pavement to ensure driving safety. This special cushion is composed of multilayer materials and structures. Laboratory tests for the cushion were conducted to analyze its electrothermal performance, heating uniformity, effect of ice melting, skidding resistance and frictions. At the same time, the durability of the ice melting cushion was evaluated. The results indicate that the developed anti-skidding electrothermal ice melting cushion with carbon fiber is stable in heating ability, high in heating speed, and the surface and bottom temperature of the ice melting mat during operation is not more than 30 ℃ without burning pavements and wheel tires, and the melting rate can reach 78.7% for one hour. The cushion can achieve excellent skid resistance and interlayer friction performance. The cushion can completely meet the requirements such as repeated rolling compaction under vehicle load, abrasion, bursting, etc., and have good durability. Therefore, the cushion is recommended to directly be placed on the pavement surface without interrupting traffics to improve the anti-skidding performance and short-term ice and snow melting in winter. The cushion has lower heating and deicing cost and has a better deicing effect and better economic benefits. Moreover, the cushion only needs to be used in winter, which extends its service life and can be reused without adverse impact on road performance.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
