Preparation of hollow SiC spheres by combustion synthesis

Hollow spherical β-SiC was successfully prepared in argon by combustion synthesis using Si powder and polytetrafluoroethylene (PTFE) powder. The phase composition and morphology of spherical products can be controlled by adjusting the Si/C₂F₄ molar ratio (M_Si/(C2F4)). When M_Si/(C2F4) = 3, the phase content of β-SiC is the highest (up to 85.54%), and hollow spherical products obtained; When M_Si/(C2F4) ≥ 5, the Si/SiC microspheres are solid. The synthesis mechanism of hollow β-SiC microspheres is as follows: Si particles react with PTFE releasing heat. Then unreacted Si absorbs heat to form liquid phase microspheres, which is equivalent to the core template to form β-SiC microspheres by reaction with cracked C. Meanwhile, the silicon diffuses from the core to the shell to form the cavity. This method can synthesize the hollow spherical β-SiC in a simple way without prearranged spherical template and long synthesis cycle.     

碳氢/有机硅/低碳醇三元系泡沫及抑制煤自燃的效果分析

泡沫灭火剂是常用的火灾扑救方法之一,但常规泡沫灭火剂存在半衰期短,析液、聚并迅速而影响灭火效能的问题,基于火灾化学和活性剂技术,提出碳氢表面活性剂SDS、有机硅表面活性剂LS-99和低碳醇的三元系泡沫体系,并系统探究碳氢/有机硅/低碳醇的复配配比。通过表面张力、发泡高度、稳泡系数的大量测试,发现LS-99的临界胶束浓度为0.0083%。LS-99和SDS二元系在降低表面张力、提升发泡高度和稳泡系数方面具有良好的协同增效作用。在此基础上引入适量浓度的能够延缓泡沫析液、聚并的异丁醇,设计出了性能优良的碳氢/有机硅/低碳醇泡沫灭火剂。LS-99、SDS和异丁醇的质量分数为0.1%时,测试结果表明SDS/LS-99/醇三元系泡沫的发泡倍数可达52.5倍,25%析液时间可达210 s,300 s时的稳泡系数高达0.958,半衰期远超常规泡沫。煤自燃的灭火抑制实验表明,SDS/LS-99/醇三元系泡沫作用下,煤自燃各反应阶段的活化能相较于空气氛围均增大,反应难度增强;最大失重速率下降,反应剧烈程度减弱。初期吸热阶段的吸热量为78.3 J/g,大于空气氛围下煤氧复合的吸热量,吸热增幅高达2.16倍。放热阶段的放热量为1765.4 J/g,相较于空气氛围放热降幅达到15.15%,表明SDS/LS-99/醇三元系泡沫对煤自燃具有良好的灭火效果。     

Optimal control and the Pontryagin's principle in chemical engineering: History,theory, and challenges

In the mid-1950s, Pontryagin et al. published a principle that became a fundamental concept in optimal control (OC) theory. The principle provides theoretical and practical methods to find the solution of OC problems, in particular, open-loop control problems. In chemical engineering, the principle has played an important role as a decision making framework for more than 60 years. This study gathers the main contributions on the application of the Pontryagin's principle to the dynamic optimization of chemical processes. A concise overview of the optimality conditions for a wide class of constrained OC problems is provided. Numerical methods to solve the necessary conditions and strategies to address inequality constraints are summarized. The information and illustrative case study presented in this work can be used as a guide to implement the principle in different settings. Opportunities for further application of the principle in relevant chemical engineering problems are also discussed.     

双组分层撞击流反应器浓度场混沌分析

采用平面激光诱导荧光技术测量双组分层式撞击流反应器撞击区浓度分布,利用混沌理论分析不同喷嘴间距、喷嘴直径和射流雷诺数下撞击流反应器浓度场混沌特征参数(关联维、Kolmogorov熵和最大Lyapunov指数)的变化规律。结果表明,撞击流反应器内浓度场混沌特征参数随喷嘴间距的增大呈上下波动的变化趋势,Kolmogorov熵在L=2d时达到最大。浓度场混沌特征参数随喷嘴直径增大呈上下波动的变化趋势,Kolmogorov熵在d=8 mm时整体上最大;射流雷诺数为Re=22 000时整体上混沌特征参数最佳。受二次撞击区的影响,撞击面上靠近二次撞击区各点的混沌特征参数有明显提高。     

主梁绝对最大弯矩的计算方法与特性研究

给出了一组平行荷载直接沿着纵梁移动时,主梁承受结点活载作用下的绝对最大弯矩的计算方法,并分析了主梁绝对最大弯矩的主要特性。填补丁现行结构力学教材以及工程力学手册中关于结点活载作用下,计算主梁绝对最大弯矩的空白,对于桥梁设计与安全评估具有一定的实用价值。     

泥岩地质混凝土顶管施工技术

结合工程实例，根据泥岩特性，介绍了简易工具管的工作原理、适用范围，阐述了泥岩地质混凝土顶管的施工技术和施工管理，指出采用简易工具管取得了良好的经济效益和社会效益。     

城市测绘发展战略的几点思考

城市测绘是我国测绘的重要组成部分,直接服务于城市的规划、建设和管理,在城市国民经济建设和社会发展中承担着基础和先行性的保障作用.本文从城市测绘现状出发,分析了城市发展战略涉及的几个重要问题.     

无砂混凝土灌注桩检测实例分析

近期青岛地区大量使用无砂混凝土灌注桩 ,通过检测发现 .些桩存在嵌固不良的现象 ,其单桩竖向承载力不能满足设计要求 ,选其中一实例对其缺陷的判定进行分析 .     

基于五自由度车辆模型的桥头搭板容许纵坡变化值研究

桥头搭板纵坡变化值是桥头跳车严重与否的决定因素,但迄今为止其容许值多据行车调查确定,本文对其进行理论研究。采用折线形状的路桥过渡段模型和五自由度车辆模型,考虑上桥和下桥两个方向,认为车辆通过路桥过渡段做一定初始条件下的强迫振动,并将其全过程分为首尾相连的四个阶段,而且将前一阶段末车辆的振动状态经修正后作为后一阶段振动的初始条件,用拉普拉斯变换法对人-车-路系统的相互作用进行了动力响应分析,得到了人的加速度及路面对车的作用力随时间的变化曲线、人的加速度最大瞬态振动值和路面对车的最大作用力。最后,考虑汽车行驶的舒适性和安全性,以加速度最大瞬态振动值作为行驶舒适性指标,提出了搭板容许纵坡变化值的确定方法和步骤。     

Dance performance evaluation using hidden Markov models

We present in this paper a hidden Markov model‐based system for real‐time gesture recognition and performance evaluation. The system decodes performed gestures and outputs at the end of a recognized gesture, a likelihood value that is transformed into a score. This score is used to evaluate a performance comparing to a reference one. For the learning procedure, a set of relational features has been extracted from high‐precision motion capture system and used to train hidden Markov models. At runtime, a low‐cost sensor (Microsoft Kinect) is used to capture a learner's movements. An intermediate step of model adaptation was hence requested to allow recognizing gestures captured by this low‐cost sensor. We present one application of this gesture evaluation system in the context of traditional dance basics learning. The estimation of the log‐likelihood allows giving a feedback to the learner as a score related to his performance. Copyright © 2016 John Wiley & Sons, Ltd.