Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments with different facing conditions

This paper presents an experimental study on reduced-scale model tests of geosynthetic reinforced soil (GRS) bridge abutments with modular block facing, full-height panel facing, and geosynthetic wrapped facing to investigate the influence of facing conditions on the load bearing behavior. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. Test results indicate that footing settlements and facing displacements under the same applied vertical stress generally increase from full-height panel facing abutment, to modular block facing abutment, to geosynthetic wrapped facing abutment. Measured incremental vertical and lateral soil stresses for the two GRS abutments with flexible facing are generally similar, while the GRS abutment with rigid facing has larger stresses. For the GRS abutments with flexible facing, maximum reinforcement tensile strain in each layer typically occurs under the footing for the upper reinforcement layers and near the facing connections for the lower layers. For the full-height panel facing abutment, maximum reinforcement tensile strains generally occur near the facing connections.     

Construction and analysis of photovoltaic directly coupled conditions in PEM electrolyzer

The Markov model and the PEM electrolyzer system model for directly coupled photovoltaic are combined to construct an efficient and reliable working condition that fits the fluctuation characteristics of solar energy. The working condition is designed through genetic algorithm so that the average coupling efficiency of the system can reach 98.8%. Then, the durability and recovery test are conducted on the basis of the constructed conditions. It is found that the attenuation rate at the current density of 1A/cm² under the photovoltaic fluctuating condition reached 7.8mV/h, which is twice that under the constant current condition. The charge transfer impedance (R_ct) is the main factor leading to the degradation. It is proved by the recovery experiment that the increase of R_ct is related to the pollution of metal ions. After pickling to remove some metal ions, R_ct can be significantly reduced by 46.8% and 65.2%, respectively. After the durability test, the voltammetric charges under the photovoltaic fluctuating condition and the constant current condition are reduced by 48.3% and 19.1% It indicates that the photovoltaic fluctuation condition will accelerate the attenuation of the effective reaction area of MEA, which is irreversible even after pickling. It can be observed from the SEM images that the catalyst layer of MEA has more obvious peeling under the photovoltaic fluctuation condition, which is not conducive to material transmission and destroys the transmission channel of ions and electrons. This result can provide a reliable reference for the coupling design of PEM electrolyzer and renewable energy in the future.     

温拌LDPE改性沥青混合料性能研究

选择Sasobit和Evotherm 3G两种温拌剂对温拌LDPE沥青混合料制备工艺和性能进行研究。首先,比较变温等体积法和等黏温度法两种方法,并确定两种温拌混合料的拌和及击实温度。在此基础上,分别采用车辙试验、低温弯曲试验、中点加载弯曲试验以及冻融劈裂试验评价70#、LDPE、Sasobit-LDPE以及3G-LDPE四种沥青混合料的高温性能、低温性能、疲劳性能和水稳定性。结果表明,两种温拌技术均是可行的,Sasobit和3G温拌技术可使施工温度降低20℃和15℃。与热拌LDPE沥青混合料相比,Sasobit温拌沥青混合料具有更好的高温性能和水稳定性,在高应力比条件下抗疲劳性能更好,低温性能虽变差,但是优于70#沥青混合料;3G温拌沥青混合料的高温性能,低温性能以及水稳定性则与热拌LDPE沥青混合料各项性能较为接近,抗疲劳性能变差,但是优于70#沥青混合料。     

Effect of different control strategies on rapid cold start-up of a 30-cell proton exchange membrane fuel cell stack

Many places experience extreme temperatures below −30 °C, which is a great challenge for the fuel cell vehicle (FCV). The aim of this study is to optimize the strategy to achieve rapid cold start-up of the 30-cell stack at different temperature conditions. The test shows that the stack rapidly starts within 30 s at an ambient temperature of −20 °C. Turning on the coolant at −25 °C show stability of the cell voltage at both ends due to the end-plate heating, however, voltage of intermediate cells fluctuates sharply, and successful start-up is completed after 60 s. The cold start strategy changes to load-voltage cooperative control mode when the ambient temperature reduced to −30 °C, the voltage of multiple cells in the middle of the stack fluctuate more drastic, and start-up takes 113 s. The performance and consistency of the stack did not decay after 20 cold start-up experiments, which indicates that our control strategies effectively avoided irreversible damage to the stack caused by freeze-thaw process.     

Recent progress of the gas diffusion layer in proton exchange membrane fuel cells: Material and structure designs of microporous layer

Proton exchange membrane fuel cells (PEMFCs) have become the most attractive power supply units for stationary and mobile applications. The operation, design characteristics, as well as performance of PEMFCs, are closely related to the multiphase transport of mass, heat, and electricity in the cell, a critical of which is the gas diffusion layer (GDL). It is very important to guarantee the transmission of water and gasses under high current density, and which is the weakness of PEMFCs at present. Microporous layer (MPL) is considered to be the key variable for mass transfer, so varieties of works focus on modification of MPL materials and its structure design. However, there is still a lack of special review to summarize and prospect the progress of MPL in recent years. This review article therefore focuses on the insights and comprehensive understanding of four critical issues of the MPL, the porosity, pore size distribution, wettability, structural design and the durability of MPL. At last, the conclusion and recommendations section summarized the future prospects and recommendations for possible research opportunities.     

考虑行走弹性影响下的车辆换挡过程分析

为研究行走弹性对工程车辆行走换挡过程的影响，建立了考虑驱动轮轮胎变形引起的车辆行走弹性影响的整车行走动力学模型，该模型适用于换挡过程分析。计算与分析了典型装载机动力换挡过程，将考虑或不考虑行走弹性时计算得到的整车加速度和瞬时冲击度的变化过程分别进行了对比，利用实车试验对单一换挡过程进行了验证；并将考虑行走弹性时升挡过程中变速箱输入输出转速变化过程、离合器滑摩功率特性的计算结果与不考虑行走弹性时换挡过程的计算结果进行了对比。研究结果表明：与不考虑行走弹性的情况相比，行走弹性变形使得换挡过程中整车加速度的变化更加平缓；换入挡离合器有两次滑摩过程，冲击度更小，换挡时间更长；离合器接合时的冲击度与其接入时间有关；滑摩功更小。     

Review of hydrogen crossover through the polymer electrolyte membrane

Proton exchange membrane fuel cell (PEMFC) is considered to be a promising, clean, and efficient energy conversion device. At present, the main challenges faced by the application of PEMFC in the automotive are cost and durability. Hydrogen from anode to the cathode through polymer electrolyte membrane (i.e. crossover hydrogen) affects the durability of fuel cells. In this paper, the existing literature on hydrogen crossover is reviewed and summarized from consequences, causes, mitigation measures, and detection methods. The influences of hydrogen crossover on the components and performance are discussed. The causes are analyzed from structural permeation and membrane degradation. The methods of alleviating the degradation of the membrane are summarized. The electrochemical and non-electrochemical monitoring methods are described, and the segmented current method is explained separately. The existing problems and research prospects are put forward, which lays a foundation for further research on hydrogen crossover and improvement fuel cell durability.     

The quasi ternary system TiB2-CrB2-WB2 between 1900 and 2300°C

The quasi-ternary TiB₂-CrB₂-WB₂ system has been studied up to 2300 °C by means of hot-pressing, SPS and pressureless dilatometry. Liquid phase is generated by two binary eutectics at the TiB₂-WB₂ edge at 2230 ± 30 °C (e₁) and at the CrB₂-WB₂ edge at 1965 °C (e₂). A large homogeneity range of a (Ti,W,Cr)B₂ solid solution has been noticed. A new ternary γ-phase emerges from a critical tie line between (W,Cr,Ti)₂B₄ and α-(Ti,W,Cr)B₂ solid solution at 1950 °C, spreads slightly into both the WB₂ and CrB₂ direction and decomposes by the peritectic reaction P: γ ⇆ α-(Ti,Cr,W)B₂ + (W,Cr,Ti)₂B₄ + Liquid at 2170 ± 30 °C. γ reacts with the CrB₂-WB₂-rich liquid according to the transition equilibrium T: δ-(Ti,Cr,W)B₂ + (W,Cr,Ti)₂B₄ ⇆ γ + Liquid at 2050 °C. The γ-phase is characterized by broad 00 l x-ray reflections which implies a layerwise accumulation and differentiation of Cr and W atoms in the basal plane of the AlB₂-type structure.     

Phase-transition induced optimization of electrostrain,electrocaloric refrigeration and energy storage of LiNbO3 doped BNT-BT ceramics

((Bi_0.5Na_0.5TiO₃)_0.88-(BaTiO₃)_0.12)_(1-x)-(LiNbO₃)_x (x = 0.0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, and 0.07; abbreviated as LiNbO₃-doped BNT-BT) ceramics possessing many excellent performances (large electrostrain, negative electrocaloric effect and energy storage density with high efficiency) was fabricated by the conventional solid-state reaction method. A large electrostrain (maximum ~ 0.34% at 100 kV/cm and room temperature) with high thermal stability over a broad temperature range (~80 K) is obtained at x = 0.03. A large energy storage density (maximum W_energy ~ 0.665 J/cm³ at 100 kV/cm and room temperature) with a high efficiency (η ~ 49.3%) is achieved at x = 0.06. Moreover, a large negative electrocaloric (EC) effect (maximum ΔT ~ 1.71 K with ΔS ~ - 0.22 J/(K kg) at 70 kV/cm)) is also obtained at x = 0.04. Phase transition (from ferroelectric to antiferroelectric and then to relaxor) induced by increasing the doping amount of LiNbO₃ plays a very key role on the optimization of these performances. These findings and breakthroughs make the LiNbO₃-doped BNT-BT ceramics very promising candidates as multifunctional materials.     

Experimental investigation on the voltage uniformity for a PEMFC stack with different dynamic loading strategies

The operating life of the proton exchange membrane fuel cell stack is mainly decided by performances of its weakest single cell because of the “Buckets effect”, thus high voltage uniformity during a dynamic loading process is key to the stack durability. In this work, a 3-kW stack is examined experimentally on its voltage uniformity (voltage coefficient variation (Cv)) under conditions of loading from open-circuit state (0 A) to nominal current (165 A) and stack temperatures of 30 °C, 45 °C and 65 °C. Different dynamic loading strategies, namely constant loading rate strategy, decreasing loading rate strategy, and increasing loading rate (square/cube increasing loading rate) strategy, are examined and compared. Results display that during the loading process, (a) the voltage uniformity rises abruptly and goes down quickly when the loading current is small (e.g. from 0 A to 22 A), (b) the voltage uniformity under a small loading current is better than that under the open-circuit state, and (c) voltage uniformity decreases as the loading current increases from a small value to the nominal current. Comparisons of different current loading strategies show that as the stack temperature rises from 30 °C to 65 °C, the stack Cv value under the open-circuit state increases from 1.12 to 1.84 and decreases from 3.85 to 2.45 in the nominal current state. The maximum Cv for the decreasing loading rate strategy decreases from 16.25 to 9.49 and that of the constant loading rate strategy also decreases from 5.85 to 4.96. Cv values of the square current increasing loading rate strategy keep below 3.85 under conditions of the three stack temperatures and display a slight fluctuation during the whole current loading process, which indicates that the strategy can effectively make the stack being of an excellent voltage uniformity during the instantaneous response process.