Drying-induced changes in the structure of alkali-activated pastes

Drying of cement paste, mortar, or concrete specimens is usually required as a pre-conditioning step prior to the determination of permeability-related properties according to standard testing methods. The reaction process, and consequently the structure, of an alkali-activated slag or slag/fly ash blend geopolymer binder differs from that of Portland cement, and therefore there is little understanding of the effects of conventional drying methods (as applied to Portland cements) on the structure of the geopolymer binders. Here, oven drying (60 °C), acetone treatment, and desiccator/vacuum drying are applied to sodium silicate-activated slag and slag/fly ash geopolymer pastes after 40 days of curing. Structural characterization via X-ray diffraction, infrared spectroscopy, thermogravimetry, and nitrogen sorption shows that the acetone treatment best preserves the microstructure of the samples, while oven drying modifies the structure of the binding gels, especially in alkali-activated slag paste where it notably changes the pore structure of the binder. This suggests that the pre-conditioned drying of alkali activation-based materials strongly affects their microstructural properties, providing potentially misleading permeability and durability parameters for these materials when pre-conditioned specimens are used during standardized testing.     

Dry сoolers and air-condensing units (Review)

The analysis of factors affecting the growth of shortage of freshwater is performed. The state and dynamics of the global market of dry coolers used at electric power plants are investigated. Substantial increase in number and maximum capacity of air-cooled condensers, which have been put into operation in the world in recent years, are noted. The key reasons facilitating the choice of developers of the dry coolers, in particular the independence of the location of thermal power plant from water sources, are enumerated. The main steam turbine heat removal schemes using air cooling are considered, their comparison of thermal efficiency is assessed, and the change of three important parameters, such as surface area of heat transfer, condensate pump flow, and pressure losses in the steam exhaust system, are estimated. It is shown that the most effective is the scheme of direct steam condensation in the heat-exchange tubes, but other schemes also have certain advantages. The air-cooling efficiency may be enhanced much more by using an air-cooling hybrid system: a combination of dry and wet cooling. The basic applied constructive solutions are shown: the arrangement of heat-exchange modules and the types of fans. The optimal mounting design of a fully shopassembled cooling system for heat-exchange modules is represented. Different types of heat-exchange tubes ribbing that take into account the operational features of cooling systems are shown. Heat transfer coefficients of the plants from different manufacturers are compared, and the main reasons for its decline are named. When using evaporative air cooling, it is possible to improve the efficiency of air-cooling units. The factors affecting the faultless performance of dry coolers (DC) and air-condensing units (ACU) and the ways of their elimination are described. A high velocity wind forcing reduces the efficiency of cooling systems and creates preconditions for the development of wind-driven devices. It is noted that global trends have a significant influence on the application of dry coolers in Russia, in view of the fact that some TPP have a surface condensers arrangement. The reasons that these systems are currently less efficient than the direct steam condensation in an air-cooled condenser are explained. It is shown that, in some cases, it is more reasonable to use mixing-type condensers in combination with a dry cooler. Measures for a full import substitution of steam exhaust heat removal systems are mentioned.     

Pharmacological Mechanisms Underlying Gastroprotective Activities of the Fractions Obtained from Polygonum minus in Sprague Dawley Rats

The leaves of Polygonum minus were fractionated using an eluting solvent to evaluate the pharmacological mechanisms underlying the anti-ulcerogenic activity of P. minus. Different P. minus fractions were obtained and evaluated for their ulcer preventing capabilities using the ethanol induction method. In this study, Sprague Dawley rats weighing 150-200 g were used. Different parameters were estimated to identify the active fraction underlying the mechanism of the gastroprotective action of P. minus: the gastric mucus barrier, as well as superoxide dismutase, total hexosamine, and prostaglandin synthesis. Amongst the five fractions from the ethanolic extract of P. minus, the ethyl acetate:methanol 1:1 v/v fraction (F2) significantly (p < 0.005) exhibited better inhibition of ulcer lesions in a dose-dependent manner. In addition, rats pre-treated with F2 showed a significant elevation in superoxide dismutase (SOD), hexosamine and PGE2 levels in the stomach wall mucosa in a dose-dependent matter. Based on these results, the ethyl acetate:methanol 1:1 v/v fraction was considered to be the best fraction for mucous protection in the ethanol induction model. The mechanisms underlying this protection were attributed to the synthesis of antioxidants and PGE2.     

THE ROLE OF HETEROATOMS IN JET FUEL INSTABILITY

ABSTRACT

The test fuel used in this investigation was a petroleum derived jet A fuel from Rastanura refinery, Saudi Arabia. The physical and chemical tests of fuel, and one month sunlight storage experiment exhibited the stability of fuel. Accelerated and ambient storage stability tests, conducted using model nitrogen compounds in jet-A fuel and n-decane, indicated that the rate of sediment formation was dependent on the structural features of nitrogen compounds and storage conditions. The pyrrole and indole derivatives did not produce any appreciable sediment except 2,6-dimethylpyrrole. Air, temperature, dissolved oxygen and light all strongly accelerate the process of sediment formation. On the other hand removal of dissolved oxygen significantly reduces it. Acids and phenols act respectively as accelerator and inhibitor towards sedimentation. The reaction has a low apparent activation energy and appears to involve a free radical oxidative self condensation of nitrogen compounds. The tower O/C ratios of sediment also supported the view point that oxidation is a key aspect of jet fuel thermal instability. Approximately 25 different types of possible structures were identified from average properties of sediments as analysed by elemental analysis, infrared and mass spectrometric methods.     

改性木质素磺酸盐分散剂对陶瓷料浆性能的影响

常用的陶瓷分散剂为无机盐类,其分散稳定效果不佳,而高分子分散剂价格高,有必要开发性价比优良的分散剂。本论文系统研究了改性木质素磺酸盐陶瓷分散剂(WAL)的应用性能。掺WAL的陶瓷料浆颗粒表面的Zeta总体高于掺对比样的无机盐分散剂,尤其在掺量0.35%时,掺WAL的Zeta电位绝对值为38.89mv,比无机盐分散剂的高6.0mv。掺WAL的陶瓷料浆球磨后颗粒的平均粒径由空白的19.47μm减小至12.61μm,小于掺无机盐分散剂的13.45μm。掺WAL的浆体流出时间为49.53s,比掺无机盐分散剂的缩短7.08s;通过显微镜放大1000倍也可以看出WAL的分散效果优于无机盐分散剂,浆体中絮体小且均匀。掺WAL浆体的分散稳定性指数120min时为2.5,小于无机盐分散剂的3.0;颗粒粒径沉降60min后为15.37μm,小于无机盐分散剂的16.29μm。综合表明,WAL是一种性能优良的陶瓷分散剂,有良好的应用前景。     

一种含Re单晶镍基合金的组织结构与蠕变性能

通过组织形貌观察及蠕变曲线测定,研究了一种含Re镍基单晶合金在高温的蠕变行为。结果表明,合金经完全热处理后的组织结构是立方γ′相以共格方式嵌镶在γ基体中。在试验的温度和应力范围内,与无Re合金相比较,含Re合金有较好的蠕变抗力及较长的蠕变寿命,并测算出合金在稳态期间的蠕变激活能与应力指数。通过分析位错攀移越过筏状γ′相及影响合金应变速率的因素,研究了合金在稳态蠕变期间的变形机制,其中,元素Re溶入γ基体后可有效阻碍位错运动,降低合金的应变速率,是使合金具有较低应变速率和较长蠕变寿命的主要原因。     

面向复杂大部件装配的多机器人制造系统运动规划研究进展

移动机器人单元精确、稳定的运动规划是其自主完成既定任务的基础和前提,也是当前多机器人制造系统应用研究所需要解决的技术难题.通过对目前已有多机器人制造系统的研究综述,介绍了运动规划算法的研究现状及其趋势,分析了各种算法优劣势,为多机器人制造系统的工业应用提供理论基础.讨论了实现多机器人制造系统工业应用所需解决的问题,并对...     

在纯镁上激光熔覆镁铝合金层提高表面的耐蚀性

研究了在纯镁基底上用激光熔覆的镁铝合金A的显微组织,物相结构及腐蚀行为,表明该合金的组成相为α-Al和β-Mg_2Al_3,在界面上则生成了共晶层;合金内生成了多晶结构;合金的腐蚀速率比纯镁低两个数量级。     

基于深度学习的三维模型检索算法综述

近年来,深度学习被广泛应用于各个领域并取得了显著的进展,如何利用深度学习高效管理呈爆炸式增长的三维模型一直是一个研究热点.本文介绍了发展至今主流的基于深度学习的三维模型检索算法,并根据实验得出的算法性能评估分析了其优缺点.根据检索任务的不同,可将主要的三维模型检索算法分为两类:(1)基于模型的三维模型检索方法,即检索对...     

Improving the accuracy of multisided travelling wave determination of location of a fault in a power line by means of the difference–distance-measuring technique

The double-ended travelling wave method of determination of location of a fault in electrical networks has significant errors due to changes in the propagation velocity of electromagnetic waves. We have developed a travelling wave method of improving the accuracy of location of a fault determination in power transmission lines that is based on navigation algorithms. The method is applicable to lines with branches. The distance to fault determination for the developed method is up to twice as accurate as the double-ended travelling wave location of a fault method. The accuracy of the developed method is less influenced by external factors (change of sag, soil resistance, the instantaneous value of the current at the fault time) than is the accuracy of the double-ended travelling wave location of a fault method. The proposed method allows reducing the errors in determining distance to the location of a fault, and its accuracy is less affected by external factors. The developed method can be incorporated into existing and prospective devices based on travelling wave methods of location of a fault determination.