极限热环境下大功率PCB散热改进研究

通过改进太阳罩结构、改变PCB安装形式、设置隔热等方式,对大功率PCB在大空间极限热环境下的散热方式进行了优化和数值模拟。研究结果表明,根据不同发热电子元件的功率及其在PCB上的安装位置及与PCB的连接形式,采用上述措施,可以显著改善散热效果。     

On some measures and distances for positive random variables

A number of conventional measures of risk as real‐valued functions on the space of positive random variables are considered: the expected shortfall, the mean excess over the threshold, the stop‐loss and some others. Ordering of risks, based on these measures and the distances between corresponding distribution functions, are described. The perturbed measures, describing the effect of changing environment, are discussed. These measures are defined by the accelerated life and proportional hazards models widely used in reliability and survival analysis. The case of a random environment is of a prime interest in the paper. The main result states that if, for instance, the stochastic environment is ‘neutral in expectation’ with respect to the baseline one, the distance between the corresponding distribution functions can be still sufficiently large. Copyright © 2003 John Wiley & Sons, Ltd.     

One-pot tandem Barbier-Prins synthesis of tetrahydropyrans in aqueous media

Tetrahydropyran is an important structural core ofmany carbohydrates and their polymers. Its structure ispresent in numerous natural compounds such as poly-ether antibiotics, marine toxins and pheromones[1,2].Thus many reactions have been developed toward…     

资源区经济与资源环境承载力与可持续发展研究

承载力理论与可持续发展理论密切相关,是可持续发展理论的一个重要组成部分.本文以系统思想为指导,从理论上分析了经济与环境、资源发展的关系;通过实证分析,对资源区进行了定性和定量的研究.     

双无限随机环境中马氏链的暂留性

借鉴确定环境中马氏链暂留性的定义,在随机环境中马氏链的研究领域,引入了各种暂留性的概念,在确定环境中这些暂留性是等价的,而在随机环境中它们不等价.讨论了各类暂留性之间的相互关系及其基本性质,给出了链和状态是暂留的一些判别准则.     

Modeling of chemical tracer transport in the atmospheric environment and its impact on the global climate

Continental regions are experiencing rapid environmental changes due to expansion of industrial activities and land uses in different types of agricultural productions, burning of fossil fuels, etc., which lead to the emanation of huge amount of smog aerosol particulates and chemicals in the atmosphere. Information about these chemical tracers has been found from Indian Ocean Experiment (INDOEX), Intergovernmental Panel for Climate Change (IPCC) assessment reports as well as from other sources. The results of these computations may be interpreted by the chemical tracer transport model. In this paper, we have used a global atmospheric model in which the optical properties and the concentrations of the chemical tracers and aerosols have been incorporated. The aerosols and chemicals are transported in the atmospheric environment by the model cumulus convection and through the model semi-Lagrangian advection process . Thus, they are globally distributed along with the wind flow. The model has been used in studying the impact of the tropospheric chemical perturbations on the global environment.     

自旋相关相对论修正对重夸克偶素性质的研究

自旋相关的相对论修正对有限温度下重夸克偶素的结合和分解的影响进行了讨论,结果表明精细和超精细相互作用特影响对重夸克偶素性质的预言,特别是对ηc和J/ψ起着明显的作用.     

Justifying and Proving in the Cabri Environment

This paper describes a long term teaching experiment carried out with students from the 9th–10th grades. Geometrical constructions in the Cabri environment were selected as a specific field of experience, within which the sense of theory may emerge. The idea of construction constitutes the key to accessing the idea of theorem, moving from a generic idea of justification towards the idea of validating within a geometrical system. The study aims at clarifying the role of the Cabri environment in this teaching-learning processes: analysis of protocols shows the possible evolution of a justification into a proof but at the same time indicates that this evolution is not expected to be simple and spontaneous.This revised version was published online in September 2005 with corrections to the Cover Date.     

Positive solutions of a diffusive prey–predator model in a heterogeneous environment

In this paper, we investigate a diffusive prey–predator model in a spatially degenerate heterogeneous environment. We are concerned with the positive solutions of the model, and obtain some results for the existence and non-existence of positive solutions. Moreover, the multiplicity, stability and asymptotical behaviors of positive solutions with respect to the parameters are also studied.     

METHODS FOR DUSTINESS ESTIMATION OF INDUSTRIAL POWDERS

Handling industrial powders leads to dust emissions. These emissions tend to generate human diseases or cause other environmental effects. A multitude of apparatus has been developed to estimate this dustiness behaviour of powders. Two of these well-known methods are presented and compared with each other. A third recently developed method is also introduced.