Covalent adaptable networks (CANs; also known as dynamic covalent networks or vitrimers) are appealing for developing simple and efficient techniques for recycling thermosetting polymers. In this paper, ethylene glycol (EG) is used as a solvent to enable pressure-free surface welding, surface repair, and recycling of a malleable epoxy where the transesterification-type bond exchange reaction (BER) imparts a dynamic nature to the covalent network. At a high temperature, the EG molecules participate in the BER, leading to dissolution of the epoxy network. If the EG is allowed to evaporate, the dissolved epoxy can re-form into a solid. The effects of EG content, temperature, and catalyst concentration on EG-assisted BERs are investigated. It is found that the amount of EG can be adjusted to tune the solution/solid transformation: An excessive amount of EG is required to dissolve the epoxy; on the other hand, a shortage of EG can shift the reaction back to re-form the polymer. Furthermore, the catalyst concentration defines the point at which dissolution initiates, while the degradation rate depends on temperature. This new EG-assisted method is further used for surface welding, surface damage repair, and powder-based reprocessing. The EG-assisted method does not require pressure and can achieve the properties of a fresh sample. It also provides potential opportunities to develop facile recycling techniques for thermosetting polymers.
Abstract Linear chains in thermoplastics make them relatively weak in performance but inherently weldable and recyclable. By contrast, thermosets with permanently crosslinked networks possess outstanding mechanical performance, thermal, and chemical stability, but are unweldable and unrecyclable. In the last decade, a kind of thermoplastics‐like thermosets termed as vitrimer has been developed with extensive applications, in which welding of vitrimers plays a central and fundamental role. Herein, we present the current state of the art of the welding of vitrimers and discuss the welding of vitrimers from a broad picture of chemistry, physics, and mechanics: i) chemistry and mechanics of the welding of vitrimers; ii) applicability of the mechanical assessment methods for the welding of vitrimers; iii) design principles and implement strategies to the welding of vitrimers; iv) effects of welding conditions on the welding strength and toughness; and v) applications to the adhesion of chemically inert materials. Finally, advantages, challenges, and open questions to the welding of vitrimers are highlighted, and future opportunities in chemistry, mechanics, design of tough welding, artificial intelligence aided programming of welding technology, mechanical assessment standard, and so on are discussed. The development of vitrimer welding would fuse disciplines and make transformative impact in polymer industry.
Tungsten oxide (WO3) is widely used as a functional material for “smart windows” due to its excellent electrochromic properties, however it is difficult to overcome the conflict between its optical modulation and cyclic stability. In this work, WO3 thin films with different crystal structures were prepared by DC reactive magnetron sputtering method. The effects of substrate temperature on the structure, composition, and electrochromic properties of WO3 films were investigated. The results show that the crystallinity of the WO3 film increases with increasing deposition temperature, indicating that temperature plays an important role in controlling the structure of the WO3 film. For WO3 thin films formed at a substrate temperature of 573 K, the film is in an amorphous state to a crystalline transition state. From X-ray diffraction (XRD) analysis, the thin film showed a weak WO3 crystallization peak, which was in the composite structure of amorphous and nanocrystalline. Which has the best electrochromic properties, with modulation amplitude of 73.1% and bleached state with a coloration efficiency of 42.9 cm2/C at a wavelength of 550 nm. Even after 1500 cycles, the optical modulation still contains 65.4%, delivering the best cyclic stability.
This paper describes a simulation model for non-point source pollution assessment at a landscape scale based on the interactions between landscape units during pollutant transport along the flow path.By differentiating the interactions into pull and drag effects,the loss of a pollutant to the receiving water body can be estimated.Thereafter,the spatial distribution of water quality conditions along the river channel is obtained based on flow accumulation values.We applied this model to four watersheds in Hailun of Heilongjiang in Northeast China.The modeling results were compared with observed data for ten storm events within two years,for which the simulated nitrogen and phosphorus concentrations correlated significantly with measured total nitrogen and particulate phosphorus concentrations.These results indicated that the method is able to describe the spatial and temporal distribution of nitrogen and phosphorus source pollution and water quality characteristics in a watershed.However,the simulated nitrogen and phosphorus concentrations were higher than the observed.Further improvement of the method with respect to qualitative water quality assessment is suggested.
By the dataset of the salinity in the Yellow Sea every month from May in 1977 to November in1981,the mechanism and spatial-temporal model on the seasonal cycle of salinity in five layers in Yellow Sea was studied with the analysis methods of Rotated Empirical Orthogonal Function(REOF),harmonic analysis and the delay correlation analysis and so on.The results showed that the seasonal field in salinity field in Yellow Sea was divided into two kinds of the spatial-temporal salinity models,the first one of them was the response to the river runoff,the Subei coastal waters,the Bohai sea southern coastal waters and the seasonal change of evaporation minus precipitation;the second one was the response to the seasonal changes of the transport of the Yellow sea warm currents with high salinity.The spatial variable component in the first model in vertical direction represented a two-layer structure,while it in the second one a one-layer structure.The variable phase of the two spatial-temporal salinity models spreading from the surface to the bottom needed the average two months.And in the layer with the same salinity the seasonal variable phase of the second model lagged behind the first model in two-three months.The temporal variable components in two kinds of the spatial-temporal salinity models formed the seasonal cycle time of the normal symmetry.Moreover,the temperature model and the salinity one in surface were the notable minus correlation at the same time,and they in every layer under the surface had the notable minus correlation in one-two month lagged.The seasonal change of the salinity files in Yellow Sea,easily affected by its inter-annual change,become abnormal.
Magnetic tunnel junction nanopillar with interfacial perpendicular magnetic anisotropy (PMA-MTJ) becomes a promising candidate to build up spin transfer torque magnetic random access memory (STT-MRAM) for the next generation of non-volatile memory as it features low spin transfer switching current, fast speed, high scalability, and easy integration into conventional complementary metal oxide semiconductor (CMOS) circuits. However, this device suffers from a number of failure issues, such as large process variation and tunneling barrier breakdown. The large process variation is an intrinsic issue for PMA-MTJ as it is based on the interfacial effects between ultra-thin films with few layers of atoms; the tunneling barrier breakdown is due to the requirement of an ultra-thin tunneling barrier (e.g., <1 nm) to reduce the resistance area for the spin transfer torque switching in the nanopillar. These failure issues limit the research and development of STT-MRAM to widely achieve commercial products. In this paper, we give a full analysis of failure mechanisms for PMA-MTJ and present some eventual solutions from device fabrication to system level integration to optimize the failure issues.
The effects of particle size, temperature, time, and pressure on the mechanical properties of regenerated epoxy-acid vitrimers were investigated, which helped to refine the vitrimer reprocessing condition parameter toolbox.
According to the characteristics of short circuit fault current in DC system,a rapid identification device about DC short circuit current was designed based on DSP,judging with the current rate of rise and the amplitude of the current. The device adopts the PCB Rogowski coil and the Hall sensor to collect the current signal.After being amplified and filtered,the signal then inputted to the DSP control circuit to analyze,a corresponding operation signal was outputted. The software was designed. The testing results show that the device can identify short circuit current accurately,quickly and reliably.
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