基于静电作用制备硅基气凝胶及其性能研究

针对第二类复合材料(基体与增强相间通过化学键连接)增强导致二氧化硅气凝胶密度及导热率升高等不足,利用带负电荷的二氧化硅气凝胶与带正电荷的聚合物间静电吸引作用制备二氧化硅气凝胶/聚合物杂化复合材料,分析基于静电作用的二氧化硅气凝胶的增强、透光与传热性能.研究表明,通过静电吸引作用在二氧化硅气凝胶骨架表面引入聚合物层,可以有效提高气凝胶材料的强度,聚合物的引入使气凝胶内部分微孔转变为中孔,同时由于静电吸引相界面的高透光和高热阻性质,使气凝胶复合材料基本保持原有的透光性能和隔热性能.     

离子液体/乙烯-醋酸乙烯共聚物复合材料的制备与性能

分别将离子液体三正丁基甲铵(双三氟甲磺酰)亚胺盐([TBMA][NTf₂])和1-乙基-3-甲基咪唑四氟硼酸盐([EMIM][BF₄])与乙烯-醋酸乙烯共聚物弹性体(EVM)熔融共混制备复合材料,研究了离子液体与EVM的相容性及抗静电性能。 样品宏观透明性和SEM测试结果显示,[TBMA][NTf₂]与EVM具有较好的相容性,相应的EVM/[TBMA][NTf₂]复合材料的抗静电效果更好;热重分析(TGA)结果表明,[TBMA][NTf₂]促进EVM热解,而[EMIM][BF₄]对EVM的热稳定影响较小。 室温下在EVM中填加[TBMA][NTf₂]的共混物表现出较好的柔性,当[TBMA][NTf₂]质量分数为20%时,EVM/[TBMA][NTf₂]复合材料的模量为0.58 MPa,断裂伸长率可达到2997%。     

Radiation of electric arc burning in argon

Using real experimentally obtained integral values, the paper deals with modelling of electric arc stabilised by flowing gas. Attention is focused namely on approximate estimation of radiation coefficient of argon. A designed model of electric arc burning in argon of atmospheric pressure inside arc heater’s anode channel is described. The model makes it possible to compute axial and/or radial dependencies of some quantities of interest (temperature, velocity, electric field intensity, arc radius, etc.), and subsequently to judge energy exchange between the arc and its surroundings. Sets of model’s input data, including arc voltage, arc current, argon flow-rate, and flow-rates and temperatures of water cooling individual parts of the arc heater, have been measured during numerous experiments. In a studied case with relatively high argon flow-rate, radiation has been found to be prevailing mechanism of energy transfer from arc to anode channel walls. Based on this finding, techniques have been designed for simple approximate estimation of radiation coefficient of argon in a limited extent of temperatures. As an example, they have been tested on a particular set of measured and computed data. Argon radiation coefficient estimated in this way has been compared with the results of theoretical computations carried out by other authors. Considering simplifications used and differences between a real situation and an ideal theoretical model, agreement of the results is within satisfactory limits.     

Ignition of a cloud of dry powder using super brush discharges

Ignition of a cloud of dry powder is a major concern in the field of industrial process safety. The different types of discharges are already defined (spark discharges, brush discharges, propagating discharges, cone discharges, corona discharges) such as their ignition properties in a gas or a dust atmosphere. For example, it is known that a classic brush discharge cannot ignite a cloud of dry flammable dust [6,13]. Glor and Schwenzfeuer performed direct ignition tests using brush discharges and defined that even if the energy released by this kind of discharge equaled the one of a spark, the power released by the brush discharge is too low to trigger an ignition.However, some doubts remained for super brush discharges. A brush discharge as a super brush discharge occurs between a charged insulating object and a conductive electrode. The main difference lies in the surface charge density reached on the insulator that is much higher for a super brush discharge than for a brush discharge. A high charge density can be reached for example using pipes of polyethylene individually charged by tribo-charging piled one above another. Such a configuration was evocated by Lüttgens [12] and tested by Larsen [11] who performed direct ignition tests in oxygen enriched atmospheres.This study is relevant with the actual safety problems since pharmaceutical and chemical powders are well known to generate electrostatic charges during their transport or handling and since the same configuration of independent polyethylene fibers can be found in flexible bulk containers that are one of the most common solutions to package this kind of powder.This paper presents the experimental set-up and the results of direct ignition tests performed with a polyethylene wax whose MIE is lower than 1  mJ at ambient conditions. The electric field reached at 1 m and the charge transfer were also registered and are described. Finally, numerical simulations are carried out to define the original surface charge density in order to help to understand the phenomenology of this discharge and its frequency of occurrence in industry.     

DH4512型霍尔效应实验仪的参数标定

针对实验教学中存在的问题,在给定霍尔灵敏度的情况下,标定了霍尔电流(Ιs)和励磁电流(IM),并测量了不同情况下的磁场误差。研究表明:磁场稳定位置x=0 mm左右且KH=169 m V/m A时,其Is和IM选取1 m A和100 m A时,误差最小,较为准确,该结论可用于大学物理实验教学。     

Solar Electricity and Solar Fuels: Status and Perspectives in the Context of the Energy Transition

The energy transition from fossil fuels to renewables is already ongoing, but it will be a long and difficult process because the energy system is a gigantic and complex machine. Key renewable energy production data show the remarkable growth of solar electricity technologies and indicate that crystalline silicon photovoltaics (PV) and wind turbines are the workhorses of the first wave of renewable energy deployment on the TW scale around the globe. The other PV alternatives (e.g., copper/indium/gallium/selenide (CIGS) or CdTe), along with other less mature options, are critically analyzed. As far as fuels are concerned, the situation is significantly more complex because making chemicals with sunshine is far more complicated than generating electric current. The prime solar artificial fuel is molecular hydrogen, which is characterized by an excellent combination of chemical and physical properties. The routes to make it from solar energy (photoelectrochemical cells (PEC), dye‐sensitized photoelectrochemical cells (DSPEC), PV electrolyzers) and then synthetic liquid fuels are presented, with discussion on economic aspects. The interconversion between electricity and hydrogen, two energy carriers directly produced by sunlight, will be a key tool to distribute renewable energies with the highest flexibility. The discussion takes into account two concepts that are often overlooked: the energy return on investment (EROI) and the limited availability of natural resources—particularly minerals—which are needed to manufacture energy converters and storage devices on a multi‐TW scale.     

The Physical Tourist¶Physics in “Lake Wobegon”: A Tour of Three Minnesota Museums of Science and Technology

I provide a tour of three museums of science and technology in the Twin Cities of Minneapolis and St. Paul, Minnesota: the Science Museum of Minnesota, the Pavek Museum of Broadcasting, and The Bakken Library and Museum.     

三相供电线路的漏电流分布规律

面对安全用电的严峻形式 ,经国内外的实践证明 ,安装使用漏电保护开关对于防止触电身亡 ,避免设备接触地故障损坏和漏电引起火灾事故 ,及减少漏电引起的电能损耗 ,具有明显效果。然而对于漏电保护开关的跳闸临界值具有很大歧义 ,因此我们首先对漏电流进行分布拟合 ,将得出的参数值进行随机模拟 ,以此来制定漏电保护开关的跳闸临界值     

菲涅耳公式的几何图示与应用

电磁波通过两种透明介质的分界面时,反射波、折射波和入射波的电矢量分量的大小和方向之间的关系可由菲涅耳公式说明,作通过分析和推导,给出了菲涅耳公式的几何图示,这使得在分析光学问题时,谱得简单和直观。     

Numerical studies on velocity, temperature history and heat transfer to the particles injected into the argon plasma

A new approach to study the particle velocity in a thermal plasma in relation to input parameters (power, gas flow rate, injection velocity of the particle and particle size) and nozzle dimensions (nozzle length and diameter) has been made. Injected particle's temperature and thermal history were calculated for particles of three different materials (alumina, tungsten and graphite) in argon plasma. Allowable powder feed rate was calculated for the particles. Heat transfer per particle injected in to the plasma is reported. Liquid fraction of the particle after it reached the melting point is also reported. Particle velocity is found to increase with increase in power, gas flow rate and injection velocity and decrease with increase in particle size, nozzle length and nozzle diameter. Thermal histories of the particles in relation to the plasma temperature and particle diameter are presented. Particle's residence time is found to increase with increase in diameter of the particle. Allowable powder feed rate for complete melting of the particle is higher at higher percentage utilisation of the plasma power. Powder feed rate is seen to decrease with increase in particle size and it is higher for tungsten and lower for graphite particle. Heat transfer rate from plasma to particle is seen to decrease with increase in time and the same is higher for plasmas of higher temperature and smaller sized particle. Received 4 May 2000 and Received in final form 15 March 2001