Reaction synthesis of spark plasma sintered MoSi2-B4C coatings for oxidation protection of Nb alloy

MoSi₂-B₄C coatings with different B₄C contents were prepared on Nb alloy by spark plasma sintering (SPS) process. Powder mixtures of Mo, Si and B₄C were used as the coating starting materials. Besides MoSi₂ and B₄C phases, small amounts of SiC and MoB are also found in the coatings because of the reactions of Mo, Si and B₄C powders during sintering. Compared with single MoSi₂ coating, the MoSi₂-B₄C coatings show better oxidation resistance at 1450?℃, and dense B₂O₃-SiO₂ oxide scales form after 100?h oxidation. The B₄C or MoB in the MoSi₂-B₄C coatings can serve as the B donor for the formation of B₂O₃. A slight degradation in the microstructure of the MoSi₂-B₄C coatings after oxidation is observed, which can be attributed to the presence of an NbB layer in the inter-diffusion zone of the coatings that retards the inward diffusion of Si from the coating into the substrate alloy. The microstructure development and oxidation behavior of the MoSi₂-B₄C coatings have been discussed.     

基于大数据的五年制高职护生在线学习行为要素研究

在线学习已逐渐成为高职院校一种重要学习形式,该领域也成为当前研究热点之一。对在线学习进行介绍,同时对大数据技术在在线学习平台中的应用进行分析,重点对五年制高职护生在线学习行为要素进行研究分析,为更好地开展在线学习评价提供参考,促进高职院校更好地开展在线学习,提高在线学习质量。     

基于大数据挖掘的用电行为分析

得益于智能电网的发展,电力大数据的价值与作用逐渐被挖掘。通过对电力大数据的深入挖掘与研究,可促进电网企业管理水平的进一步提升,为我国智能电网的发展提供助力。而依托于大数据挖掘的用户用电行为分析,可实现当前企业电网调控与需求侧的充分融合,为电力系统的稳定性运行提供保障。基于此,文章针对大数据挖掘的用电行为分析进行深入探析,以期为智能电网发展提供帮助。     

Innovative focused ultrasound-based sealing method of flexible packaging films—Principles and characteristics

A key process to package goods is the closure procedure. It ensures the good protection against the environment and vice versa. Ultrasonic sealing presents the advantage of converting locally ultrasound into heat, which is optimal for temperature sensitive goods. The disadvantages remain, however, the equipment cost, the noise generation, and the process sensitivity to materials. It is necessary to develop a sealing process to decrease the acquisition cost of the sealing equipment and cover a wide range of polymer films. This paper focuses on a new sealing method based on high-intensity focused ultrasound. The principles and characteristics of this process are discussed and underpinned with experimental results. Both the acoustic and thermal effects of the sealing process are summarized. A study of the process parameter impact on the seam strength in static and dynamic operating is proposed. Low-density polyethylene films with a carrier layer of bi-oriented polypropylene and polyethylene terephthalate are used. A first test bench generates point-shaped seam. The heat generation is recorded with an infrared thermography system. A second test bench, a horizontal form fill and seal machine, produces longitudinal seams. The seam quality is controlled by tensile test and tightness test. A quick heating of the polymer films is found. Point seams and longitudinal seams are successfully generated. The seams behave similarly to the ones generated with ultrasonic sealing process. However, the sealing velocity is limited to 3 m/min. This work provides a summary of the characteristics and principles of the new sealing method in packaging applications.     

Looking at both sides of relationship dynamics in virtual communities: A social exchange theoretical lens

This study draws on social exchange theory to investigate the relationship dynamics of the member–community dyad in virtual community settings. Using a longitudinal design and multiple measurement sources, the findings indicate that social and task communication styles have unique effects on members’ community commitment velocity, which in turn influences member gratitude and entitlement behaviors. A moderated path analysis demonstrates that the effects of social and task communication styles on community commitment velocity are both positively moderated by attachment anxiety and attachment avoidance. It also reveals that members’ level of popularity augments the influence of community commitment velocity on member entitlement behavior.     

铌锆管表面开裂原因分析及热处理行为的研究

铌锆管表面SEM(EDS)表明:成品铌锆管表面存在大量的Si、O元素,在临界加工率下表面剧烈变形,最终导致后续加工严重开裂,通过对成品铌锆管的两因子-五变量温度-时间正交实验设计,结果表明:成品铌锆管在40%~62.5%加工率下1350×60 min退火下,晶粒组织较好,再结晶充分,力学性能稳定,铌锆管的表面开裂倾向较小。     

Scaling behavior of dynamic hysteresis of PMN-PT relaxor ferroelectric ceramics near the morphotropic phase boundary

Due to the feature of domains, ferroelectric materials display hysteresis behavior with respect to the change of the applied electric field. Every ferroelectric material has its own unique hysteresis loop reflecting the information of domain reversal under an electric field. In this work, the scaling behaviors of dynamic hysteresis in relaxor (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT) ceramics with different compositions were studied systemically. Our results showed that the evolution of scaling behavior in PMN-PT ceramics can be divided into three stages, which is independent of the phase structure of the ceramics and the testing electric field frequency. The relationship between hysteresis area <A> and field amplitude E₀ obeys the power law <A>∝ in the low and high E₀ regions, where the reorientations of 180° and non-180° domain are dominant, respectively. However, the dynamic hysteresis area <A> does not follow the power law in the intermediate E₀ regions, which is attributed to the interaction of different domain reversal mechanisms. Furthermore, the hysteresis area <A> decreases gradually with increasing frequency at a certain E₀ and the time-dependent domain reversal process was also discussed.     

Layer-by-layer self-assembly of multifunctional enzymatic UF membranes

In order to eliminate membrane fouling and to ensure enzymatic hydrolysis of urea, multifunctional biocatalytic membranes were prepared by using urease (URE) and trypsin (TRY) enzymes on the sulfonated polysulfone (SPSf) ultrafiltration membrane via layer-by-layer (LbL) self-assembly method. The membrane architecture consisted of multilayer assembly with TRY and URE enzymes as the outer layer and inner sandwiched layer, respectively. Polyethyleneimine (PEI) and alginate (ALG) were used as cationic and anionic polyelectrolytes. Sulfonation and PEI deposition were successfully accomplished as confirmed by attenuated total reflectance Fourier transform infrared spectroscopy, differential scanning calorimetry (DSC) and scanning electron microscopy analysis, contact angle measurements, staining with toluidine blue and Congo red dyes and dead-end filtration experiments. A characteristic value of SPSf membrane with a high water permeability (1000 L/m².h.bar) and 95% bovine serum albumin (BSA) rejection was observed. In static conditions, URE activities of SPSf2-PEI-URE membrane were not affected by BSA fouling, while TRY immobilizations with increased concentrations (SPSf2-PEI-URE-PEI-ALG-TRY) significantly lowered the activity of URE. In dynamic conditions, each deposited layer exhibited individual resistance to flow that can be considered as irreversible fouling and caused 90% of flux decline for the SPSf2-PEI-URE-PEI-ALG-TRY membrane assembly. The recovery of the initial flux for the multilayered membrane at the end of six fouling and washing cycles was observed 85%. Moreover, at the end of 5 cycles, 78% of the URE initial activity of the multilayered membrane was preserved. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48750.     

基于网络学习行为聚类分析的在线课堂优化策略研究

利用聚类分析法对网络学习行为分类，从而挖掘不同类型学习者在学习行为和学习效果间的差异。以大学计算机公共课《数据库技术及应用》为例，综合学习者学习行为类别差异、在线课程内容组织以及课程特点等要素为在线课堂进一步发展提供优化策略。     

The explosion thermal behavior of H2/CH4/air mixtures in a closed 20 L vessel

In this work, the explosion thermal behavior of H₂/CH₄/air mixtures, at different equivalence ratios (0.6–1.6) and hydrogen volume fractions (0%–100), was investigated in a confined 20-L chamber. The parameters of explosion time and pressure, as well as the explosion heat loss were quantitatively studied and analyzed. Moreover, the dominant chain reactions of the explosion process and heat release were identified via the detailed mechanism of the Foundational Fuel Chemistry Model (FFCM1). The results indicated that an increased H₂ volume fraction in the mixtures increased the peak explosion pressure, maximum pressure rise rate and deflagration index. In addition, the explosion duration and fast-burning period were greatly shortened. Both the adiabatic flame temperature and thermal diffusivity monotonically increased with increasing H₂ volume ratio. Moreover, the enhancement effect of the H₂ ratio on the thermal diffusivity of H₂/CH₄ mixtures was more prominent for fuel-rich mixtures than for fuel-lean mixtures. The obtained quantitative results are helpful for developing measures to prevent the potential explosion accidents.