The generation and measurement of aerosols

Aerosols are of extreme importance to many branches of science and technology although this is seldom realized by non-specialists. The disperse phase of an aerosol may consist of virtually any known material and so includes liquids, pure and unadulterated, as well as solids of simple composition and regular shape in one extreme to complex materials which may be radioactive, electrically charged and of complex shape in the other extreme.The basic characteristics and some of the most important methods of generating aerosols are reviewed. The difficulties that arise in attempting to produce standard aerosols are indicated. In considering measurements on aerosols the difficultues in collecting representative samples are outlined. Diagnostic methods which do not perturb the aerosol such as photographic and holographic techniques as well as light interaction methods are considered in some detail.     

Rapid generation of protein aerosols and nanoparticles via surface acoustic wave atomization

We describe the fabrication of a surface acoustic wave (SAW) atomizer and show its ability to generate monodisperse aerosols and particles for drug delivery applications. In particular, we demonstrate the generation of insulin liquid aerosols for pulmonary delivery and solid protein nanoparticles for transdermal and gastrointestinal delivery routes using 20?MHz SAW devices. Insulin droplets around 3?μm were obtained, matching the optimum range for maximizing absorption in the alveolar region. A new approach is provided to explain these atomized droplet diameters by returning to fundamental physical analysis and considering viscous-capillary and inertial-capillary force balance rather than employing modifications to the Kelvin equation under the assumption of parametric forcing that has been extended to these frequencies in past investigations. In addition, we consider possible mechanisms by which the droplet ejections take place with the aid of high-speed flow visualization. Finally, we show that nanoscale protein particles (50-100?nm in diameter) were obtained through an evaporative process of the initial aerosol, the final size of which could be controlled merely by modifying the initial protein concentration. These results illustrate the feasibility of using SAW as a novel method for rapidly producing particles and droplets with a controlled and narrow size distribution.     

浅析可燃性气体爆炸及爆炸极限

可燃性气体爆炸已经逐渐成为工业生产、生活中主要危害之一,因此研究可燃性气体爆炸机理对预防可燃性气体爆炸具有重要意义,而爆炸极限是研究可燃性气体爆炸的一个重要参数。本文主要以液化石油气、甲烷为例,通过实验及查找文献等方法进行研究。     

Nanofibers: effective generation by electrospinning and their applications

Electrospinning is the most versatile technology in use today, for the generation of polymer nanoscale fibers. The nano materials generated using this technology have a large surface area and are highly porous making it very useful in many applications in diverse fields such as energy storage, healthcare, biotechnology, environmental engineering, defense and security. The production of the fibers and the morphology can be easily controlled by modifications to the processing parameters. The relatively high production rate and simplicity of the setup makes electrospinning highly attractive. This review summarizes the effect of various processing parameters on the effective generation of nanofibers. By simple modifications to the electric field inside the electrospinning chamber the fiber collection can be easily controlled. In addition, the various applications of electrospun fibers in electronic devices, environmental sensors and filters, energy storage, and in biomedicine such as in tissue engineering, drug delivery and enzyme encapsulation are examined and the current research in each field is also explored in this review.     

Harmonic generation in transition metal dichalcogenides and their heterostructures

Few layered transition metal dichalcogenides (TMD), with an absence of crystal inversion symmetry and outstanding optical characteristics, are frequently applied in studies of nonlinear optics (NLO) for harmonic generation. Related materials are regarded as potential candidates for many optoelectronics applications. In order to enhance and manipulate the intrinsically weak NLO responses, TMD’s have been fabricated into heterostructures in recent years. The basic physics of harmonic generation and of TMD optical responses, as well as the interactions in TMD hybrid structures are introduced briefly, and the current state-of-the-art in the performance of TMDs in harmonic generation are reviewed. A particular focus is made on heterostructure studies to enhance and manipulate the response, which represent the core issues for devices and applications.     

A generation of astronomical telescopes,their users,and publications

Research astronomers and the telescopes they use each have typical life spans of about 40 years. Most of their journals live a good deal longer, though the second most important one today is only 40 years old. This paper looks at numbers for productivity and impact of specific astronomical facilities, changes in equality of opportunities and achievements in observational astronomy, and some aspects of national contributions. The focus is on optical astronomy, though something is also said about radio telescopes and astronomy from space. In summary, nothing stays “best of class” for very long; the fraction of the community with access to the most valuable facilities has increased with time (more equality of opportunity); but the fraction of citations earned by the few super-star papers has also increased (less equality of achievement); and the USA remains the host of the most-cited journals and the most productive telescopes, though Europe (meaning in this context the member nations of the European Southern Observatory, the European Space Agency, and the supporters of the journal Astronomy & Astrophysics) are fast closing the gap, with the UK retaining its own journal and some observing facilities not shared with either the USA or other European countries. Detailed examination of specific facilities indicates that size (of telescope, community, and budget) are all of great importance, but that the most significant “focal plane instrument” is still the astronomer at the virtual eyepiece. The changes have happened against a background of enormous increases in numbers of astronomers, sizes of available facilities (but not total number), numbers of papers (but not of journals), and numbers of citations per paper. A significant subset of the conclusions on turnover of people and facilities accompanying major growth: opportunity versus achievement; Europe versus the USA; and the trade-off between community size and the influence of individual scientists undoubtedly apply in many other fields.     

Synthesis of alpha-alumina nanoribbons: characterization and their morphological evolution

Single crystalline α-alumina nanoribbons were synthesized by reacting aluminum with silicon monoxide at high temperature. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). The intense peaks of XRD pattern indicate that the prepared nanoribbons have a high degree of crystallinity. In the present work, silicon monoxide was used as an oxidant and precursor, which served to control the reaction rate. An interesting morphological evolution that nanoribbons resulted from nanosaws was described, and these novel nanosaws were also carefully characterized.     

表面波电磁声换能器及电声学特性的研究

电磁超声换能器的非接触、信号重复性好等特点使得其在高温自动检测、材料特性测量等领域有着广泛的应用前景。利用电磁超声技术激发表面波更是行之有效的方法,论文着重于理论分析,对表面波电磁声换能器的物理结构、电声学特性作了较为全面和深入的研究,并用等效电路模型探讨了阻抗匹配的方法。结果表明,电磁声换能器为感性负载,不同于压电器件的容性负载特性,因此对换能器的设计和优化也有着独自的特性;随着提离距离的增大,相应的换能效率也将急剧下降;阻抗匹配对于提高信号的输出强度,抑制谐波分量有着重要的作用,而采用电容并联匹配相比串联匹配,能够提供更高的转换效率。     

Morphology evolution of polyaniline microstructures via reverse micelles and their hydrophobicity

Polyaniline morphology evolution in toluene has been observed for the first time. Various structures, including one-dimensional open-ended microtubes, three-dimensional solid microspheres and two-dimensional novel solid microplates were controllably synthesized in the same reaction system. The structures obtained were sensitive to oxidant concentration and molar ratio of monomer to oxidant. Solid microplates were mainly formed on the glass beaker wall and bottom, while tubes and spheres were produced in solution by employing reverse micelles as soft templates. Studies on the effect of reaction temperature, mechanical stirring and the addition of more acid were also carried out. FTIR and UV-Vis results showed that the PANI products were in the emeraldine form. The PANI film prepared from spheres exhibited hydrophobic property due to its rough surface covered with nanoscale dots and the large distribution of sphere diameters.     

泄压与隔爆联用的粉尘爆炸压力特性

作为目前市场上运用最广泛的隔爆产品,隔爆翻板阀一般与泄压板联用,以防止粉尘爆炸传播。为了探究粉尘爆炸时泄压与隔爆联用对容器内压力及隔爆效果的影响,进行了工业规模的粉尘爆炸实验。实验结果表明:由于隔爆翻板阀的影响,容器内部出现了二次峰值压力;随着隔爆翻板阀安装距离的增加,容器内两个峰值压力的时间间隔从28.2 ms增加到62.3 ms,且到达隔爆翻板阀前的峰值压力从0.067 MPa上升至0.101 MPa;泄压面积的增大会导致容器内部和隔爆翻板阀前端峰值压力降低,并可能导致隔爆失败。     

Synthesis of hierarchical barium tungstate corns and their shape evolution process

Barium tungstate nanocorns with lengths of 200-800 nm and with diameters of 20-50 nm in the middle section were synthesized by a facile stepwise solution-phase method. Various comparison experiments showed that several experimental parameters, such as the volume ratio of DMF/H₂O, and the quantity of urea and CTAB, played important roles for the morphological control of BaWO₄ nanostructures. A possible mechanism is offered for the formation of the hierarchical nanostructures. The obtained samples are characterized by means of X-ray diffraction, energy dispersion X-ray spectrometry, scanning electron microscopy, and transmission electron microscopy.     

Condensation of cadmium aerosols

Cadmium aerosols were prepared by the heat-pulse method and fall-out collected at temperatures T from ambient to just above the melting point T _f. Deposits were examined by electron microscopy after coating with Au-Pd. Representative particles were removed for electron and X-ray diffraction studies.The particles are crystalline. The types, and their proportions and size distributions vary with temperature. At low temperatures particles are monocrystals of high specific surface, chiefly prisms with deep fissures and cavities but also rough spheres and stellate dendrites. At higher temperatures the crystal forms are perfected, the proportion of prisms falls, and polycrystalline as well as monocrystalline spheres are found. Spheres can exceed 50m in diameter but the largest prisms are 2 to 3 m and disappear at 0.8 to 0.9 T _f. Up to this point spheres have one or more circular {0001} depressions, or dishes, depending on the number of crystallites they contain. At higher temperatures they are quite smooth.It is concluded that smooth spheres are droplets which have supercooled and frozen on the collector, and as such are not aerosol particles. Droplets that freeze in the cloud become dished spheres, and their subsequent growth involves condensation on areas between the dishes. All particles are nucleated from the vapour close to the source, the prisms apparently at T<T _f.     

Condensation of zinc aerosols

Condensation of zinc aerosols in the presence of argon was studied in the heat-pulse cloud chamber; as with cadmium, two distinct series of particles are produced as the wall temperature is increased, first prisms and then spheres. The prismatic particles are monocrystalline and include complicated dendrites and a species not previously seen, the capped prism. Whiskers grow from the basal surfaces of the simple prisms. The spherical particles include mono-, bi-, and poly-crystals.The production of simple prisms and dendrites is attributed to condensation of solid in the presence of particularly steep temperature and vapour-pressure gradients. Crystallographic evidence supports the view that the spheres condense as liquid droplets. It is concluded that the droplets solidfy from nuclei that grow rapidly into circular {00.1} rafts on the liquid surface, in the manner observed with large sessile drops by Mutaftschiev and Zell. The formation of hillocks on the basal raft or its conversion into a concave dish is evidence of growth in the vapour.The numbern of basal rafts on a solidified sphere may be identified with the number of nuclei. When unimpeded by other nuclei, a raft grows to a maximum radiusr0.55R, whereR is the droplet radius. The ratior/R is used to deduce the orientation of the pyramidal planes which develop at the edge of the raft as it begins to thicken. At low wall temperatures some of these planes tend to be very coarse, while when the growth front reaches the opposite side of the particle, the solid surface adopts a relatively smooth spherical outline.As the wall temperature is raised, the highest value observed forn increases from 1 to a maximum of 20 to 30 and then falls abruptly as the melting point of the metal is approached. The temporary increase inn may be an effect of drop size but the final fall is ascribed to the failure of supercooled droplets to nucleate until withdrawn from the apparatus. Spheres withn>1 deform on cooling as the result of thermal expansion anisotropy. Forn>2 cracking and slip are observed.     

Case Study of an Aerosol Explosion and a Method to Determine Explosion Temperatures

A failure analysis case study is presented for a two-piece aerosol containing tetrafluoroethane, commonly referred to as Refrigerant 134a. A gentleman was preparing to recharge the air conditioning system of an automobile when the bottom exploded off the aerosol container, propelling the body of the aerosol container like a rocket, which hit the man in the eye and blinded him in that eye. The aerosol was never connected to the air conditioner, therefore backpressure from the air conditioner (AC) compressor was ruled out as a cause for the explosion. The objective of the study was to determine why the aerosol exploded. Several recently developed test methods were used, including two types of heat-to-burst tests and a puncture chamber to measure the pressure-versus-temperature behavior of aerosols. More common test methods were also used, such as water bath pressure tests, hydro pressure burst tests, pneumatic pressure burst tests, hardness measurements, weight measurements, metallography, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and an accident scenario recreation. A semi-empirical correlation between the hardness and weights of the container bottoms was used to determine the explosion temperature and/or pressure. This semi-empirical correlation agrees in principle with an analysis of the explosion pressures using finite-element analysis (FEA). The root cause for the explosion was determined to be a lack of strength of the bottom of the two-piece aerosol coupled with heating the aerosol to temperatures significantly above room temperature.     

近似模拟地下核爆炸冲击震动效应方法的探讨

在对地下核爆炸冲击震动效应分析的基础上,对地下核爆炸的冲击震动效应的模拟方法进行了探讨,对封闭化学爆炸的冲击震动效应进行了计算.结果表明:地下核爆炸冲击震动的应力波峰值、质点的速度、质点的加速度和质点位移,都可以用封闭化学爆炸来进行模拟,而且封闭化学爆炸与地下核爆炸之间的有效当量系数在6％～30％之间,这与已知的有效当量系数基本吻合.结论:地下核爆炸的冲击震动效应可以用封闭化学爆炸近似模拟,将会给地下核爆炸的研究带来极大方便.     

Condensation and crystal morphology of bismuth aerosols

Condensation aerosols of bismuth were prepared in the heat-pulse cloud chamber at various wall temperatures by flash evaporation of metal into argon at atmospheric pressure. The particles nucleate from the vapour in the liquid state only. When prepared at low wall temperatures the droplets solidify to tapered twins with an asymmetrically sited protuberance. At wall temperatures approaching the melting point of the metal the morphology of the smaller particles is affected by thermal ageing. Complex multiple twins occur, as well as twins of simpler, rhombohedral shape. A solidification mechanism is proposed for the tapered particles, by which the freshly nucleated crystal, growing at the surface of the droplet with an emergent corner, experiences a twinning shear under the influence of fluctuating stresses imposed by the particle motion. The crystal growth rate is thereby enhanced unidirectionally, and the particle becomes elongated as a result of the volumetric expansion.     

Modelling preventive measures and their effect on generation times in emerging epidemics

We present a stochastic epidemic model to study the effect of various preventive measures, such as uniform reduction of contacts and transmission, vaccination, isolation, screening and contact tracing, on a disease outbreak in a homogeneously mixing community. The model is based on an infectivity process, which we define through stochastic contact and infectiousness processes, so that each individual has an independent infectivity profile. In particular, we monitor variations of the reproduction number and of the distribution of generation times. We show that some interventions, i.e. uniform reduction and vaccination, affect the former while leaving the latter unchanged, whereas other interventions, i.e. isolation, screening and contact tracing, affect both quantities. We provide a theoretical analysis of the variation of these quantities, and we show that, in practice, the variation of the generation time distribution can be significant and that it can cause biases in the estimation of reproduction numbers. The framework, because of its general nature, captures the properties of many infectious diseases, but particular emphasis is on COVID-19, for which numerical results are provided.     

Models of driving behavior: A review of their evolution

This paper reviews models that emphasize the cognitive components of driving behavior. Studies of individual differences have sought predictors of accident histories. Typically low correlations and reliance on post hoc explanations reflect theoretical deficiencies and problems with the use of accident measures. Motivational models emphasize transient, situation-specific factors rather than stable, individual predictors. However, neither testable hypotheses nor suitable methods have been developed to study situational factors and motives that influence driving. More recent models have incorporated a hierarchical control structure, which assumes concurrent activity at strategic, maneuvering, and operational levels of control. At the same time, automaticity has emerged as a central construct in cognitive psychology. All activities are assumed to combine fast, automatic components with slower, more deliberate, controlled processing. It is argued that identifying the situational factors that increase drivers' uncertainty and thus trigger a shift in attention from automatic to controlled processing will help integrate concepts of automaticity and motivational models. Finally, recent theorizing has suggested that errors associated with the inherent variability of human behavior may be more important to roadway crash causation than systematic errors, which are attributable to the known limits of the human information-processing system. Drivers' abilities to recover from errors may also be important to crash causation. It is concluded that the hierarchical control structure and theories of automaticity and errors provide the potential tools for defining alternative criterion measures, such as safety margins, and developing testable theories of driving behavior and crash causation. Two examples of models that integrate information-processing mechanisms within a motivational framework are described.