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中文翻译
正电原子在电离过程中碰撞的理论
摘要
我们回顾过去和现在正子原子在电离过程中碰撞理论的发展。从最终状态下合并所有相互作用，在一个同等立足处和保留少量碰撞动力学的一个确切的物体分析开始, 我们进行或重或轻不同的比较, 并且从它们影响电离横剖面的角度进行分析。终于, 我们发现了理论碰撞过程中的连续统一体, 中心点和其它运动学机制。
主题词: 电离; 碰撞动力学; 驱散; 电子光谱; 反物质; 正电子冲击; 中心点电子; 导轨式电子
1. 介绍
正电原子的简单电离碰撞由一个细小的结构微粒冲击, “三体问题”是很多年未解决的一个物理问题。1609 年到1687 年“二体”问题由约翰尼.开普勒和由艾萨克?牛顿共同解决了。三体问题比二体问题更加复杂难懂, 除了一些特殊的现象，它不能被简单的分析解决。1765年, 勒翰得. 依鲁尔发现了原始在线的三大量和依然排列的一种"几何" 解答。不少年后, 拉格朗日发现了五平衡点的存在, 今后大家都称为拉格朗日点。
对三体驱散问题的解答，最早的是三百年前天文学家和数学家用数学工具和相似比的原理解答出来的。例如, 在大量的中心参考系统下, 我们在1836 年描述三体问题由任何空间座标都可能的原因已经由杰库比介绍。所有这些对由线形点标准变革关系, 如所描述[ 1 ] 。在动量空间, 系统由伴生的描述(千吨), (千焦) 和(千牛) 。交换对实验室参考框架, 大量电子最后的动量m, 许多MT (反冲) 目标片段和大量MP 子弹头可能被写根据杰克比冲动Kj 通过伽利略变换[ 1 ]得出
数十年, 电离过程的理论描述承担了三体动力学在最终状态下的简单表示, 根据事实表明
（1）对于离子和原子碰撞, 一个微粒(电子) 比其它二两个原子要轻。
（2）对于电子和正子原子碰撞, 一个微粒(目标中坚力量) 比其它两个原子要重的多。
例如, 根据众所周知的中心论据, 离子和原子电离碰撞的理论描述的决大多数使用冲击参数来设置, 那里子弹头跟随一条未受干扰的直线弹道在碰撞过程过程中, 并且目标中坚力量依然是休息[ 2 ] 。它是
确切, 假设, 子弹头随后而来一条直线弹道没有道理在电子或正子原子碰撞的理论描述。但是, 它通常假设, 目标中坚力量依然是不动。
问题的这些简单化被介绍了在18 世纪。unsolvable t三体问题被简化了, 对所谓的有限的三体问题, 那里一个微粒被承担有一许多足够小不影响其它二个微粒的行动。虽则介绍作为手段提供近似解答对系统譬如太阳行星彗星在古典技工范围内, 它广泛被应用在原子物理在所谓的冲击参量略计对离子原子电离碰撞。
三体问题的其它简单化广泛被使用在19 世纪假设, 一个微粒比其它二巨型的并且依然是在大量的中心镇定自若由其它二。这略计广泛被应用在电子或正子原子电离碰撞。
2. 多个有差别的横剖面
一个三体连续流最后状态的一个运动学上完全描述在任一原子碰撞会要求, 原则上, 九可变物知识, 譬如动量的组分联系了对每个三个微粒在最终状态。但是, 动量和能源节约的情况减少这个数字到五。此外, 每当最初的目标不准备在任何优先方向, 多个有差别的横剖面必须是相称由三体系统的自转在子弹头的行动的最初的方向附近。因而, 搁置一边三个片段的内部结构在最终状态, 只四丧失九可变物是必要完全地描述驱散过程。所以, 电离过程的一个完全描述特性也许被获得以一个四倍有差别的横剖面:
有许多可能的套四可变物使用。为,事例, 我们能选择了电子的方位角角度和其它二个微粒的当中一个, 相对角度在行动之间飞机, 并且一个微粒能量。
这样选择是任意的, 但完成在感觉, 其他套可变物可能与这一个有关。独立可变物一个相似的选择是标准的为原子电离的描述由电子冲击, 理论上和实验性地[ 3,4 ] 。
非常一般四倍有差别的横剖面的图片不是可行的。因而, 它通常是必要减少可变物的数量在横剖面。这可能由修理达到一两他们在某些特殊价值或情况。例如, 我们也许任意地制约自己描述coplanar (i.e. = 0) 或a collinear motion (i.e. = 0 and θ1 = θ2), 以便使问题的依赖性降低到三或二独立可变物, 各自地。
另一选择将集成四倍有差别的横剖面在一个或更多可变物。
前广泛被应用学习电子碰撞, 当后者是主要工具描绘离子原子和正子原子电离碰撞。特别重要对唯一微粒分光学的用途, 那里动量的微粒的当中一个被测量。
3. 单个微粒的动量分布
动量发行为散发的电子和正子礼物几个结构。首先, 我们能观察门限在高电子或正子速度因为有一个极限在任一个微粒可能吸收从系统的动能。第二个结构是土坎被设置沿圈子。它对应于正子的二进制碰撞与散发的电子, 用目标中坚力量充当实际角色。终于, 有尖顶和anticusp 在零速度在电子和正子动量分布, 各自地。第一个对应于电子的励磁于目标的一个低能源连续流状态。秒钟是取尽由于正子的捕获的不可能的事由目标中坚力量。这些动量发行允许我们学习电离碰撞的主要特征。但是, 我们必须记住, 分析只微粒的当中一个在最后状态的任一个实验性技术可能只提供部份洞察入电离过程。四倍有差别的横剖面也许显示由综合化洗涤在这实验的碰撞物产。
4. 理论模型
我们想要讨论在这通信的主要问题是如果有一些重要碰撞物产在正子原子碰撞, 那不是可测的，总共, 单或双有差别的电离横剖面, 并且那因为未被发现。为了了解这些结构的起源, 我们对应的横剖面与那些比较被获得在离子原子碰撞。履行这个宗旨它是必要的有一种充分的量子机械治疗能同时应付电离碰撞由重和轻的子弹头的冲击是因此相等地可适用的- 例如- 对离子原子或正子原子碰撞。一种理论与这特征将允许我们学习倍数任一个指定的特点的变动
有差别的横断面当许多联系在片段之中变化。特别是, 它会允许我们学习变异当改变在二之间制约了运动学情况。
第二重要点将对待所有互作用在最终状态在一个同等立足处。如同我们解释了, 在离子原子碰撞, internuclear 互作用不充当实际在散发的电子的动量发行的角色和因此未被考虑在对应的演算。在这工作, 这假定被避免了。横剖面利益在这范围内是
转折矩阵可能供选择地被写在岗位或预先的形式
那里扰动潜力被定义
为出生类型初始状态
哪些包括子弹头的自由行动和最初的一定的状态Ui 目标, 并且扰动潜力vi 简单地是正子电子和正子中坚力量互作用的总和。转折矩阵也许然后被分解入二个期限依靠是否正子首先与目标中坚力量或电子相处融洽。
为了是一致的与动力学的我们充分的治疗, 它是必要描述最终状态Wf 通过考虑所有互作用在同样立足处的wavefunction 。因而, 我们采取一个被关联的C3 波浪作
那包括畸变D
j 为三活跃互作用。在连续流波浪作用这个选择的最后渠道扰动潜力是[ 5 ]
在纯净的库仑潜力情况下, 畸变被给
关于这个模型由佳瑞波帝和马瑞吉拉[ 6 ] 提议为离子原子碰撞, 并且由Brauner 和布里格斯六年后为正子原子和电子碰撞[ 7 ] 。但是, 在所有这些箱子问题的动力学被简化了, 依照被谈论在早先部分, 根据大非对称在介入的片段的大量之间。另外, Garibotti 和Miraglia 忽略了互作用潜力的矩阵元素在接踵而来的子弹头和目标离子之间, 并且做锐化的略计评估转折矩阵元素。这进一步略计被取消了在纸由Berakdar 等。(1992), 虽然他们保留许多制约在他们的离子冲击电离分析。
5. 电子捕获对连续流尖顶
让我们回顾一些结果在立体几何。我们选择作为二个独立参量散发的电子动量组分, 平行和垂线对正子子弹头的行动的最初的方向。子弹头的能量是1 keV 。图2, 我们观察三个不同结构: 二个极小值和土坎。
图2
土坎的起源很好被了解。它对应于电子捕获于连续流(ECC) 尖顶被发现在离子原子碰撞三十年前由Crooks 和Rudd [ 8 ] 。他们测量了电子能量光谱在向前方向和确切地观察了尖顶形状峰顶在子弹头的速度。第一理论解释[ 9 ] 表示, 它分流以与1 相似的方式k 。这个尖顶结构是很多实验性和理论研究焦点。
因为ECC 尖顶是一个推测横跨捕获电离极限入高度激动的一定的状态, 这个同样作用必须是存在在正子原子碰撞。实际上, 这样作用的观察联系了假定物体的形成, 当被预言的二十年前由布朗勒和布里格斯, 依然是一个有争议的问题。
这争执的原因是那, 与离子对比盒, 正子外出的速度与那不是相似冲击, 但主要传播在角度和巨大。因而没有特殊速度在哪里寻找尖顶。并且这一定是如此。如果我们评估双重有差别的横剖面, 我们看见, 尖顶清楚地是可看见的在离子原子碰撞, 但非常温和和被传播的肩膀在正子原子碰撞。因而, 观察这结构它是必要增加横剖面的维度。例如由考虑四倍有差别的横剖面的零的程度裁减在collinear 几何。Kover 和Laricchia 测量了在1998 dr/dEedXkdXK 横剖面在一个collinear 情况在零的程度, 为H2 的电离分子由100 keV 正子冲击[ 10 ] 。结构依照为冲击对重的离子被观察那么尖锐不被定义由于占实验性窗口在正子的卷积
并且电子侦查。从目标反冲不充当在这个实验性情况的重大角色, 当前一般理论给结果相似与那些由Berakdar [ 11 ] 获得, 并且两个跟随严密实验性价值。
这同样实验由Sarkadi 和工友执行了在氩电离由75 keV 氢核冲击。他们第一次测量了四倍有差别的
电离横剖面在collinear 几何为离子原子碰撞, 并且发现ECC 尖顶和在正子冲击在大角度。在这种情况下, 我们必须保留动力学的一个完全帐户为了再生产实验性结果[ 12 ] 。
6. 托马斯机制
现在让我们走回到H2 的电离由1 keV 正子冲击。一个结构在45 可能被观察, 1993 年哪些象由于被预言了和被解释了由Brauner 和布里格斯二个等效双重碰撞机制干涉。每个这些过程包括正子电子二进制碰撞, 被偏折跟随被90 轻的微粒的当中一个被重的中坚力量。这个机制由托马斯[ 13 ] 提议作为扼要负责任电子捕获由快速的重的离子。在这种情况下, 从电子和正子大量是相等的, 这两个过程干涉在45 。如果我们降低能量从1000 年eV 到100 eV, 这个结构在45 消失, 与想法是一致的结果托马斯机制是一个高能作用。但有其它结构, 在大约22.5 。我们在下个部分将考虑这个结构。
7. 备鞍点机制
结构的起源在大约22.5 一定更难辨认。对我们的最佳的知识, 它以前未被预言在正子原子碰撞, 即使机制负责任它的起源几乎已经提议在离子原子碰撞二十年之内以前。想法是, 电子能从离子原子碰撞涌现由在在子弹头和残余的目标离子潜力的备鞍点。1772 年这个机制清楚地与平衡点的当中一个有关由拉格朗日发现, 或对机制由Wannier 提议为低能源电子放射。在离子原子碰撞案件, 查寻这个机制的理论和实验性证据是阴暗由生动的争论[ 14-18 ] 。
在正子原子碰撞情况下, 为电子被困住在正子和残余离子潜力的马鞍, 电子和正子必须首先执行二进制碰撞以便最终获得正确的速度
那里ei 是目标的结合能在初始状态。
能量和动量保护原则的应用表示, 正子偏离在角度
终于, 为电子涌现在方向和正子一样, 它必须遭受随后碰撞以残余中坚力量在a 托马斯象过程。在这第二碰撞, 电子由90和残余目标离子反冲偏转在形成大约135 角度与电子和正子的方向。这个机制被描述在图4.
因而, 检查备鞍点的提案是正确的, 我们看是否我们的演算显示与备鞍点电子生产的这个描述是一致的结构。
图3
图4
极小值被观察在无效性QDCS 。图3 和图4 精确地设置早先条件在任何能量和角度三个微粒符合的那些点。我们做了其它测试在备鞍点机制的有效性和无效性。图5 表示, 结构完全出现从tp 期限。这个结果与提出的机制是一致的, 那里备鞍点结构出现从第一正子电子碰撞之后, 正子和电子被中坚力量驱散。
图 5
8. 结论
总结结果提出了在这通信, 我们由正子的冲击调查了分子氢的电离。被获得的四倍有差别的横断面为电子和正子涌现在同样方向显示三个统治结构。你是知名的电子捕获对连续流峰顶。另外一个是托马斯机制。终于, 有被解释对象由于所谓的"备鞍点" 电离机制的极小值。
虽然主要结论研究的非常充分但也有一些不足。横剖面也许会被很多巨大的困难所阻碍, 但值得高兴的是, 我们一直没有错过对问题许多不同的全方位的观察, 唯一的遗憾就是对总横剖面的研究。
英文原文
Theory of ionization processes in positron–atom collisions
Abstract
We review past and present theoretical developments in the description of ionization processes in positron–atom collisions. Starting from an analysis that incorporates all the interactions in the final state on an equal footing and keeps an exact account of the few-body kinematics, we perform a critical comparison of different approximations, and how they affect the evaluation of the ionization cross section. Finally, we describe the appearance of fingerprints of capture to the continuum, saddle-point and other kinematical mechanisms.
Keywords: Ionization; Collision dynamics; Scattering; Electron spectra; Antimatter; Positron impact; Saddle-point electrons; Wannier; CDW
PACS classification codes: 34.10.+x; 34.50.Fa
1. Introduction
The simple ionization collision of a hydrogenic atom by the impact of a structureless particle, the “three-body problem”, is one of the oldest un solved problems in physics. The two-body problem was analyzed by Johannes Kepler in 1609 and solved by Isaac Newton in 1687. The three-body problem, on the other hand, is much more complicated and cannot be solved analytically, except in some particular cases. In 1765, for instance, Leonhard Euler discovered a “collinear” solution in which three masses start in a line and remain lined-up. Some years later, Lagrange discovered the existence of five equilibrium points, known as the Lagrange points. Even the most recent quests for solutions of the three-body scattering problem use similar mathematical tools and follow similar paths than those travelled by astronomers and mathematicians in the past three centuries. For instance, in the center-of-mass reference system, we describe the three-body problem by any of the three possible sets of the spatial coordinates already introduced by Jacobi in 1836. All these pairs are related by lineal point canonical transformations, as described in [1]. In momentum space, the system is described by the associated pairs (k T,K T), (k P,K P) and (k N,K N). Switching to the Laboratory reference frame, the final momenta of the electron of mass m, the (recoil) target fragment of mass M T and the projectile of mass M P can be written in terms of the Jacobi impulses K j by means of Galilean transformations [1]
For decades, the theoretical description of ionization processes has assumed simplifications of the three-body kinematics in the final state, based on the fact that
? in an ion–atom collision, one particle (the electron) is much lighter than the other two,
? in an electron–atom or positron–atom collision, one particle (the target nucleus) is much heavier than the other two.
For instance, based on what is known as Wick’s argument, the overwhelming majority of the theoretical descriptions of ion–atom ionization collisions uses an impact-parameter approximation, where the projectile follows an undisturbed straight line trajectory throughout the collision process, and the target nucleus remains at rest [2]. It is clear that to assume that the projectile follows a straight line trajectory makes no sense in the theoretical description of electron or positron–atom collisions. However, it is usually assumed that the target nucleus remains motionless.
These simplifications of the problem were introduced in the eighteenth century. The unsolvable three-body problem was simplified, to the so-called restricted three-body problem, where one particle is assumed to have a mass small enough not to influence the motion of the other two particles. Though introduced as a means to provide approximate solutions to systems such as Sun–planet–comet within a Classical Mechanics framework, it has been widely used in atomic physics in the so-called impact-parameter approximation to ion–atom ionization collisions. Another simplification of the three-body problem widely employed in the nineteenth century assumes that one of the particles is much more massive than the other two and remains in the center of mass unperturbed by the other two. This approximation has been widely used in electron–atom or positron–atom ionization collisions.
2. The multiple differential cross section
A kinematically complete description of a three-body continuum final-state in any atomic collision would require, in principle, the knowledge of nine variables, such as the components of the momenta associated to each of the three particles in the final state. However, the condition of momentum and energy conservation reduces this number to five. Furthermore, whenever the initial targets are not prepared in any preferential direction, the multiple differential cross section has to be symmetric by a rotation of the three-body system around the initial direction of motion of the projectile. Thus, leaving aside the internal structure of the three fragments in the final state, only four out of nine variables are necessary to completely describe the scattering process. Therefore, a complete characterization of the ionization process may be obtained with a quadruple differential cross section:
There are many possible sets of four variables to use. For, instance, we can chose azimuthal angles of the electron and of one of the other two particles, the relative angle between the planes of motion, and the energy of one particle.
Such a choice is arbitrary, but complete in the sense that any other set of variables can be related to this one. A similar choice of independent variables has been standard for the description of atomic ionization by electron impact, both theoretically and experimentally [3] and [4].
A picture of the very general quadruple differential cross section is not feasible. Thus, it is usually necessary to reduce the number of variables in the cross section. This can be achieved by fixing one or two of them at certain particular values or conditions. For instance, we might arbitrarily restrict ourselves to describe a coplanar (i.e. = 0) or a collinear motion (i.e. = 0 and θ1 = θ2), so as to reduce the dependence of the problem to three or two independent variables, respectively.
The other option is to integrate the quadruple differential cross section over one or more variables.
The former has been widely used to study electron–atom collisions, while the latter has been the main tool to characterize ion–atom and positron–atom ionization collisions. Particularly important has been the use of single particle spectroscopy, where the momentum of one of the particles is measured.
3. Single particle momentum distributions
In ionization by positron impact it is feasible to study the momentum distribution of any of the involved fragments. As is shown in Fig. 1, the momentum distributions for the emitted electron and the positron present several structures. First, we can observe a threshold at high electron or positron velocities because there is a limit in the kinetic energy that any particle can absorb from the system. The second structure is a ridge set along a circle. It corresponds to a binary collision of the positron with the emitted electron, with the target nucleus playing practically no role. Finally, there is a cusp and an anticusp at zero velocity in the electron and positron momentum distributions, respectively. The first one corresponds to the excitation of the electron to a low-energy continuum state of the target. The second is a depletion due to the
impossibility of capture of the positron by the target nucleus. These momentum distributions allow us to study the main characteristics of ionization collisions. However, we have to keep in mind that any experimental technique that analyzes only one of the particles in the final-state can only provide a partial insight into the ionization processes. The quadruple differential cross sections might display collision properties that are washed out by integration in this kind of experiments.
Fig. 1. Electron and positron momentum distributions for the ionization of
helium by impact of positrons with incident velocity v = 12 a.u.
4. Theoretical model
The main question that we want to address in this communication is if there are some important collision properties in positron–atom collisions, that are not observable in total, single or double differential ionization cross sections, and that therefore have not yet been discovered. In order to understand the origin of these structures, we compare the corresponding cross sections with those obtained in ion–atom collisions. To fulfill this objective it is necessary to have a full quantum-mechanical treatment able to deal simultaneously with ionization collisions by impact of both heavy and light projectiles that is therefore equally applicable – for instance – to
ion–atom or positron–atom collisions. A theory with this characteristics will allow us to study the changes of any given feature of multiple-differential cross-sections when the mass relations among the fragments vary. In particular, it would allow us to study the variation when changing between the two restricted kinematical situations.
The second important point is to treat all the interactions in the final state on an equal footing. As we have just explained, in ion–atom collisions, the internuclear interaction plays practically no role in the momentum distribution of the emitted electron and has therefore not been considered in the corresponding calculation. In this work, this kind of assumption has been avoided.
The cross section of interest within this framework is
The transition matrix can be alternatively written in post or prior forms as
where the perturbation potentials are defined by (H?E)Ψi = V iΨi and (H?E)Ψf = V fΨf.
For the Born-type initial state
which includes the free motion of the projectile and the initial bound state Φi of the target, and the perturbation potential V i is simply the sum of the positron–electron and positron–nucleus interactions. The transition matrix may then be decomposed into two terms
depending on whether the positron interacts first with the target nucleus or the electron.
In order to be consistent with our full treatment of the kinematics, it is necessary to describe the final state by means of a wavefunction that considers all the interactions on the same footing. Thus, we resort to a correlated C3 wave function
that includes distortions for the three active interactions. The final-channel perturbation potential for this choice of continuum wave function is [5]
(1) In the case of pure coulomb potentials, the distortions are given by
with νj = m j Z j/k j. This model was proposed by Garibotti and Miraglia [6] for ion–atom collisions, and by Brauner and Briggs six years later for positron–atom and electron–atom collisions [7]. However, in all these cases the kinematics of the problem was simplified, as discussed in the previous section, on the basis of the large asymmetry between the masses of the fragments involved. In addition, Garibotti and Miraglia neglected the matrix element of the interaction potential between the incoming projectile and the target ion, and made a peaking approximation to evaluate the transition matrix element. This further approximation was removed in a paper by Berakdar et al. (1992), although they kept the mass restrictions in their ion-impact ionization analysis.
5. The electron capture to the continuum cusp
Let us review some results in a collinear geometry. We choose as the two independent parameters the emitted electron momentum components, parallel and perpendicular to the
initial direction of motion of the positron projectile. The energy of the projectile is 1 keV. In Fig. 2, we observe three different structures: two minima and a ridge.
Fig. 2. QDCS for ionization of H2 by impact of 1 keV positrons for emission of
electrons in the direction of the projectile deflection.
The origin of the ridge is very well understood. It corresponds to the electron capture to the continuum (ECC) cusp discovered in ion–atom collisions three decades ago by Crooks and Rudd [8]. They measured the electron energy spectra in the forward direction and observed a cusp-shape peak at exactly the projectile’s velocity. The first theoretical explanation [9] showed that it diverges in the same way as 1/k. This cusp structure was the focus of a large amount of experimental and theoretical research.
Since the ECC cusp is an extrapolation across the ionization limit of capture into highly excited bound states, this same effect has to be present in positron–atom collisions. In fact, the observation of such an effect associated with positronium formation, while predicted two decades ago by Brauner and Briggs, remained a controversial issue. The reason for this dispute was that, in contrast to the case of ions, the positron outgoing velocity is not similar to that of impact, but is largely spread in angle and magnitude. Thus there is no particular velocity where to look for the cusp. And this is certainly so. If we evaluate the double differential cross section, we see that the cusp is clearly visible in ion–atom collisions, but just a very mild and spread shoulder in positron–atom collisions. Thus, to observe this structure it is necessary to increase the dimension of the cross section. For instance by considering a zero degree cut of the quadruple differential cross section in collinear geometry.
Kover and Laricchia measured in 1998 the dσ/d E e dΩk dΩK cross section in a collinear condition at zero degree, for the ionization of H2 molecules by 100 keV positron impact [10]. The structure is not so sharply defined as for impact observed for heavy ions because of the convolution that accounts for the experimental window in the positron and electron detection. Since the target recoil plays no significant role in this experimental situation, the present
general theory gives results similar to those obtained by Berakdar [11], and both closely follow the experimental values.
The same kind of experiment was performed by Sarkadi and coworkers in Argon ionization by 75 keV proton impact. They measured the quadruple differential ionization cross section in a collinear geometry for ion–atom collisions for the first time, and found the ECC cusp as in positron impact at large angles. In this case, we have to keep a complete account of the kinematics in order to reproduce the experimental results [12].
6. Thomas mechanism
Let us now go back to the ionization of H2 by 1 keV positron impact. A structure at 45° can be observed, which was predicted and explained in 1993 by Brauner and Briggs as due to the interference of two equivalent double-collision mechanisms. Each of these processes consists of a positron–electron binary collision, followed by the deflection by 90° of one of the light particles by the heavy nucleus. This mechanism was proposed by Thomas [13] as the main responsible of electron capture by fast heavy ions. In this case, since the electron and positron masses are equal, these two processes interfere at 45°.
If we lower the energy from 1000 eV to 100 eV, this structure at 45° disappears, a result that is consistent with the idea that the Thomas mechanism is a high energy effect. But there is another structure, at about 22.5°, that persists. We will consider this structure in the next section.
7. Saddle-point mechanism
The origin of the structure at about 22.5° is certainly more difficult to identify. To our best knowledge, it has not been predicted before in positron–atom collisions, even though the mechanism responsible of its origin was already been proposed in ion–atom collisions almost two decades before. The idea was that an electron could emerge from an ion–atom collision by lying in the saddle-point of the projectile and the residual target-ion potentials. This mechanism is clearly related to one of the equilibrium points discovered by Lagrange in 1772, or to the mechanism proposed by Wannier for low-energy electron emission. In the case of ion–atom collisions, the search for theoretical and experimental evidence of this mechanism was overcast by vivid controversy [14], [15], [16], [17] and [18].
In the case of positron–atom collisions, for the electrons to be trapped in the saddle of the positron and residual-ion potentials, the electron and the positron must first perform a binary collision so as to end up with the right velocities
(2)
where εi is the binding energy of the target in the initial state.
Application of energy and momentum conservation principles shows that the positron is deviated in an angle
(3) Finally, for the electron to emerge in the same direction as the positron, it must suffer a subsequent collision with the residual-nucleus in a Thomas-like process. In this second collision, the electron is deflected by 90° and the residual target ion recoils in a direction that forms an angle of about 135° with the electron and the positron. This mechanism is depicted in Fig. 4.
Thus, to check that the proposal of a saddle-point is correct, we look at whether our
calculations show structures that are consistent with this description of saddle-point electron production.
The minimum observed in the QDCS of Fig. 3 and Fig. 4 are located at precisely those points where the previous conditions on the energy and angle of any of the three particles are met.
Fig. 3. QDCS for H2 ionization by 100 eV positrons in the restricted collinear
geometry.
Fig. 4. Mechanism proposed to lead to the observed saddle-like structure.
We made another test on the validity of the saddle-point mechanism. Fig. 5 shows that the structure arises exclusively from the t P term. This result is consistent with the proposed mechanism, where the saddle-point structure arises from a first positron–electron collision. Afterwards, both positron and electron are scattered by the nucleus.
Fig. 5. QDCS for ionization of H2 by impact of positrons at 100 eV and electron
energy E e = 19 eV.
8. Conclusions
Summarizing the results presented in this communication, we have investigated the ionization of molecular hydrogen by the impact of positrons. The obtained quadruple differential
cross-sections for the electron and the positron emerging in the same direction show three dominant structures. One is the well-known electron capture to the continuum peak. Another one is the Thomas mechanism. Finally, there is a minimum that might be interpreted as due to the so-called “saddle-point” ionization mechanism.
But the main conclusion is that the study of the fully differential cross section might be hindered by a great number of difficulties, but the reward is that many different structures can be observed that otherwise are missed in double, single differential or total cross sections.
外文出处： 《Exploiting Software How to Break Code》By Greg Hoglund, Gary McGraw Publisher : Addison Wesley Pub Date : February 17, 2004 ISBN : 0-201-78695-8 译文标题： JDBC接口技术 译文： JDBC是一种可用于执行SQL语句的JavaAPI（ApplicationProgrammingInterface应用程序设计接口）。它由一些Java语言编写的类和界面组成。JDBC为数据库应用开发人员、数据库前台工具开发人员提供了一种标准的应用程序设计接口，使开发人员可以用纯Java语言编写完整的数据库应用程序。 一、ODBC到JDBC的发展历程 说到JDBC，很容易让人联想到另一个十分熟悉的字眼“ODBC”。它们之间有没有联系呢？如果有，那么它们之间又是怎样的关系呢？ ODBC是OpenDatabaseConnectivity的英文简写。它是一种用来在相关或不相关的数据库管理系统（DBMS）中存取数据的，用C语言实现的，标准应用程序数据接口。通过ODBCAPI，应用程序可以存取保存在多种不同数据库管理系统（DBMS）中的数据，而不论每个DBMS使用了何种数据存储格式和编程接口。 1．ODBC的结构模型 ODBC的结构包括四个主要部分：应用程序接口、驱动器管理器、数据库驱动器和数据源。应用程序接口：屏蔽不同的ODBC数据库驱动器之间函数调用的差别，为用户提供统一的SQL编程接口。 驱动器管理器：为应用程序装载数据库驱动器。 数据库驱动器：实现ODBC的函数调用，提供对特定数据源的SQL请求。如果需要，数据库驱动器将修改应用程序的请求，使得请求符合相关的DBMS所支持的文法。 数据源：由用户想要存取的数据以及与它相关的操作系统、DBMS和用于访问DBMS的网络平台组成。 虽然ODBC驱动器管理器的主要目的是加载数据库驱动器，以便ODBC函数调用，但是数据库驱动器本身也执行ODBC函数调用，并与数据库相互配合。因此当应用系统发出调用与数据源进行连接时，数据库驱动器能管理通信协议。当建立起与数据源的连接时，数据库驱动器便能处理应用系统向DBMS发出的请求，对分析或发自数据源的设计进行必要的翻译，并将结果返回给应用系统。 2．JDBC的诞生 自从Java语言于1995年5月正式公布以来，Java风靡全球。出现大量的用java语言编写的程序，其中也包括数据库应用程序。由于没有一个Java语言的API，编程人员不得不在Java程序中加入C语言的ODBC函数调用。这就使很多Java的优秀特性无法充分发挥，比如平台无关性、面向对象特性等。随着越来越多的编程人员对Java语言的日益喜爱，越来越多的公司在Java程序开发上投入的精力日益增加，对java语言接口的访问数据库的API 的要求越来越强烈。也由于ODBC的有其不足之处，比如它并不容易使用，没有面向对象的特性等等，SUN公司决定开发一Java语言为接口的数据库应用程序开发接口。在JDK1．x 版本中，JDBC只是一个可选部件，到了JDK1．1公布时，SQL类包（也就是JDBCAPI）
附录 科技译文： Numerical Control Numerical Control(NC) is a method of controlling the movements of machineComponents by directly inserting coded instructions in the form of numerical data(numbers and data ) into the system.The system automatically interprets these data and converts to output signals. These signals ,in turn control various machine components ,such as turning spindles on and off ,changing tools,moving the work piece or the tools along specific paths,and turning cutting fluits on and off. In order to appreciate the importer of numerical control of machines ,let’s briefly review how a process such as machining has been carried out traditionally .After studying the working drawing of a part, the operator sets up the appropriate process parameters(such as cutting speed ,feed,depth of cut,cutting fluid ,and so on),determines the sequence of operations to be performed,clamps the work piece in a workholding device such as chuck or collet ,and proceeds to make the part .Depending on part shape and the dimensional accuracy specified ,this approach usually requires skilled
附录一英文科技文献翻译 英文原文： Experimental investigation of laser surface textured parallel thrust bearings Performance enhancements by laser surface texturing (LST) of parallel-thrust bearings is experimentally investigated. Test results are compared with a theoretical model and good correlation is found over the relevant operating conditions. A compari- son of the performance of unidirectional and bi-directional partial-LST bearings with that of a baseline, untextured bearing is presented showing the bene?ts of LST in terms of increased clearance and reduced friction. KEY WORDS: ?uid ?lm bearings, slider bearings, surface texturing 1. Introduction The classical theory of hydrodynamic lubrication yields linear (Couette) velocity distribution with zero pressure gradients between smooth parallel surfaces under steady-state sliding. This results in an unstable hydrodynamic ?lm that would collapse under any external force acting normal to the surfaces. However, experience shows that stable lubricating ?lms can develop between parallel sliding surfaces, generally because of some mechanism that relaxes one or more of the assumptions of the classical theory. A stable ?uid ?lm with su?cient load-carrying capacity in parallel sliding surfaces can be obtained, for example, with macro or micro surface structure of di?erent types. These include waviness [1] and protruding microasperities [2–4]. A good literature review on the subject can be found in Ref. [5]. More recently, laser surface texturing (LST) [6–8], as well as inlet roughening by longitudinal or transverse grooves [9] were suggested to provide load capacity in parallel sliding. The inlet roughness concept of Tonder [9] is based on ??e?ective clearance‘‘ reduction in the sliding direction and in this respect it is identical to the par- tial-LST concept described in ref. [10] for generating hydrostatic e?ect in high-pressure mechanical seals. Very recently Wang et al. [11] demonstrated experimentally a doubling of the load-carrying capacity for the surface- texture design by reactive ion etching of SiC
机械设计 摘要：机器是由机械装置和其它组件组成的。它是一种用来转换或传递能量的装置，例如：发动机、涡轮机、车辆、起重机、印刷机、洗衣机、照相机和摄影机等。许多原则和设计方法不但适用于机器的设计，也适用于非机器的设计。术语中的“机械装置设计”的含义要比“机械设计”的含义更为广泛一些，机械装置设计包括机械设计。在分析运动及设计结构时，要把产品外型以及以后的保养也要考虑在机械设计中。在机械工程领域中，以及其它工程领域中，所有这些都需要机械设备，比如：开关、凸轮、阀门、船舶以及搅拌机等。 关键词：设计流程设计规则机械设计 设计流程 设计开始之前就要想到机器的实际性，现存的机器需要在耐用性、效率、重量、速度，或者成本上得到改善。新的机器必需具有以前机器所能执行的功能。 在设计的初始阶段，应该允许设计人员充分发挥创造性，不要受到任何约束。即使产生了许多不切实际的想法，也会在设计的早期，即在绘制图纸之前被改正掉。只有这样，才不致于阻断创新的思路。通常,还要提出几套设计方案，然后加以比较。很有可能在这个计划最后决定中,使用了某些不在计划之内的一些设想。 一般的当外型特点和组件部分的尺寸特点分析得透彻时，就可以全面的设计和分析。接着还要客观的分析机器性能的优越性，以及它的安全、重量、耐用性，并且竞争力的成本也要考虑在分析结果之内。每一个至关重要的部分要优化它的比例和尺寸，同时也要保持与其它组成部分相协调。 也要选择原材料和处理原材料的方法。通过力学原理来分析和实现这些重要的特性，如那些静态反应的能量和摩擦力的最佳利用，像动力惯性、加速动力和能量；包括弹性材料的强度、应力和刚度等材料的物理特性，以及流体润滑和驱动器的流体力学。设计的过程是重复和合作的过程，无论是正式或非正式的进行，对设计者来说每个阶段都很重要。 最后，以图样为设计的标准，并建立将来的模型。如果它的测试是符合事先要
附录1：外文资料翻译 A1.1外文资料题目 26.22 接地故障电路开关 我们目前为止报道的接地方法通常是充分的, 但更加进一步的安全措施在某些情况下是必要的。假设例如, 有人将他的手指伸进灯口（如Fig.26.45示）。虽然金属封入物安全地接地, 但那人仍将受到痛苦的震动。或假设1个120V 的电炉掉入游泳池。发热设备和联络装置将导致电流流入在水池中的危害，即使电路的外壳被安全地接地，现在已经发展为当这样的事件发生时，设备的电源将被切断。如果接地电流超过5mA ，接地开关将在5 ms 内跳掉，这些装置怎么运行的？ 如Fig.26.46所示，一台小变流器缠绕上导线 ，第二步是要连接到可能触发开合120 V 线的一台敏感电子探测器。 在正常情况下流过导体的电流W I 与中性点上的电流N I 准切的相等，因此流经核心的净潮流(N W I I -)是零。 结果，在核心没有产生电流，导致的电压F E 为零，并且开关CB 没有动作。 假设如果某人接触了一个终端(图Fig.26.45示)，故障电流F I 将直接地从载电线漏到地面，这是可能发生的。如果绝缘材料在马达和它的地面封入物之间断开，故障电流也会被产生。在以下任何情况下，流经CT 的孔的净潮流等于F I 或L I ，不再是零。电流被产生，并且产生了可以控制CB 开关的电压F E 。 由于5 mA 不平衡状态只必须被检测出，变压器的核心一定是非常有渗透性的在低通量密度。 Supermalloy 是最为常用的，因为它有相对渗透性典型地70000在通量密度仅4mT 。 26.23 t I 2是导体迅速发热的因素 它有时发生于导体短期内电流远大于正常值的情况下，R I 2损失非常大并且导体的温度可以在数秒内上升几百度。例如，当发生严重短路时，在保险丝或开关作用之前，会有很大的电流流过导体和电缆。 此外，热量没有时间被消散到周围，因此导体的温度非常迅速地增加。 在这些情况下什么是温度上升？ 假设导体有大量m ，电阻R 和热量热容量c 。 而且，假设电流是I ，并且那它流动在t 少于15秒期间。 在导体上引起的热 Rt I Q 2= 从Eq.3.17，在功率一定的情况下我们可以计算导体上升的温度差：
毕业设计（论文）外文文献翻译 文献、资料中文题目：软件开发概念和设计方法文献、资料英文题目： 文献、资料来源： 文献、资料发表（出版）日期： 院（部）： 专业： 班级： 姓名： 学号： 指导教师： 翻译日期： 2017.02.14
外文资料原文 Software Development Concepts and Design Methodologies During the 1960s, ma inframes and higher level programming languages were applied to man y problems including human resource s yste ms,reservation s yste ms, and manufacturing s yste ms. Computers and software were seen as the cure all for man y bu siness issues were some times applied blindly. S yste ms sometimes failed to solve the problem for which the y were designed for man y reasons including: ?Inability to sufficiently understand complex problems ?Not sufficiently taking into account end-u ser needs, the organizational environ ment, and performance tradeoffs ?Inability to accurately estimate development time and operational costs ?Lack of framework for consistent and regular customer communications At this time, the concept of structured programming, top-down design, stepwise refinement，and modularity e merged. Structured programming is still the most dominant approach to software engineering and is still evo lving. These failures led to the concept of "software engineering" based upon the idea that an engineering-like discipl ine could be applied to software design and develop ment. Software design is a process where the software designer applies techniques and principles to produce a conceptual model that de scribes and defines a solution to a problem. In the beginning, this des ign process has not been well structured and the model does not alwa ys accurately represent the problem of software development. However,design methodologies have been evolving to accommo date changes in technolog y coupled with our increased understanding of development processes. Whereas early desig n methods addressed specific aspects of the
附录 INTEGRATION OF MACHINERY （From ELECTRICAL AND MACHINERY INDUSTRY）ABSTRACT Machinery was the modern science and technology development inevitable result, this article has summarized the integration of machinery technology basic outline and the development background .Summarized the domestic and foreign integration of machinery technology present situation, has analyzed the integration of machinery technology trend of development. Key word：integration of machinery ，technology，present situation ，product t，echnique of manufacture ，trend of development 0. Introduction modern science and technology unceasing development, impelled different discipline intersecting enormously with the seepage, has caused the project domain technological revolution and the transformation .In mechanical engineering domain, because the microelectronic technology and the computer technology rapid development and forms to the mechanical industry seepage the integration of machinery, caused the mechanical industry the technical structure, the product organization, the function and the constitution, the production method and the management system has had the huge change, caused the industrial production to enter into “the integration of machinery” by “the machinery electrification” for the characteristic development phase. 1. Integration of machinery outline integration of machinery is refers in the organization new owner function, the power function, in the information processing function and the control function introduces the electronic technology, unifies the system the mechanism and the computerization design and the software which constitutes always to call. The integration of machinery development also has become one to have until now own system new discipline, not only develops along with the science and technology, but also entrusts with the new content .But its basic characteristic may summarize is: The integration of machinery is embarks from the system viewpoint, synthesis community technologies and so on utilization mechanical technology, microelectronic technology, automatic control technology,
本科毕业论文(设计) 外文翻译 学院：机电工程学院 专业：机械工程及自动化 姓名：高峰 指导教师：李延胜 2011年05 月10日 教育部办公厅 Failure Analysis，Dimensional Determination And
Analysis，Applications Of Cams INTRODUCTION It is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designed．Sometimes a failure can be serious，such as when a tire blows out on an automobile traveling at high speed．On the other hand，a failure may be no more than a nuisance．An example is the loosening of the radiator hose in an automobile cooling system．The consequence of this latter failure is usually the loss of some radiator coolant，a condition that is readily detected and corrected．The type of load a part absorbs is just as significant as the magnitude．Generally speaking，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be considered．Another concern is whether the material is ductile or brittle．For example，brittle materials are considered to be unacceptable where fatigue is involved． Many people mistakingly interpret the word failure to mean the actual breakage of a part．However，a design engineer must consider a broader understanding of what appreciable deformation occurs．A ductile material，however will deform a large amount prior to rupture．Excessive deformation，without fracture，may cause a machine to fail because the deformed part interferes with a moving second part．Therefore，a part fails(even if it has not physically broken)whenever it no longer fulfills its required function．Sometimes failure may be due to abnormal friction or vibration between two mating parts．Failure also may be due to a phenomenon called creep，which is the plastic flow of a material under load at elevated temperatures．In addition，the actual shape of a part may be responsible for failure．For example，stress concentrations due to sudden changes in contour must be taken into account．Evaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductile． In general，the design engineer must consider all possible modes of failure，which include the following． ——Stress ——Deformation ——Wear ——Corrosion ——Vibration ——Environmental damage ——Loosening of fastening devices
中英文资料对照外文翻译 机械设计 摘要： 机器由机械和其他元件组成的用来转换和传输能量的装置。比如：发动机、涡轮机、车、起重机、印刷机、洗衣机和摄影机。许多机械方面设计的原则和方法也同样适用于非机械方面。术语中的“构造设计”的含义比“机械设计”更加广泛，构造设计包括机械设计。在进行运动分析和结构设计时要把产品的维护和外形也考虑在机械设计中。在机械工程领域中，以及其它工程领域，都需要机械设备，比如：开关、凸轮、阀门、船舶以及搅拌机等。 关键词：设计流程设计规则机械设计 设计流程 设计开始之前就要想到机器的实用性，现有的机器需要在耐用性、效率、重量、速度，或者成本上得到改善。新的机器必需能够完全或部分代替以前人的功能，比如计算、装配、维修。 在设计的初级阶段，应该充分发挥设计人员的创意，不要受到任何约束。即使有一些不切实际的想法，也可以在设计的早期，即在绘制图纸之前被改正掉。只有这样，才不致于阻断创新的思路。通常,必须提出几套设计方案,然后进行比较。很有可能在这个计划最后指定使用某些不在计划方案内的一些想法的计划。 一般当产品的外型和组件的尺寸特点已经显现出来的时候，就可以进行全面的设计和分析。接着还要客观的分析机器性能、安全、重量、耐用性，并且成本也要考虑在内。每一个至关重要的部分要优化它的比例和尺寸，同时也要保持与其它组成部分的平衡。 选择原材料和工艺的方法。通过力学原理来分析和实现这些重要的特性，如稳定和反应的能量和摩擦力的利用，动力惯性、加速度、能量；包括材料的弹性强度、应力和刚度等物理特性，以及流体的润滑和驱动器的流体力学。设计的过程是一个反复与合作的过程，无论是正式的还是非正式的，对设计者来说每个阶段都很重要。。产品设计需要大量的研究和提升。许多的想法,必须通过努力去研究成为一种理念,然后去使用或放弃。
中文翻译 STC89C52处理芯片 电气工程的研究和解决方案中心（ceers） 艾哈迈德为吉.波特 首要性能： 与MCS-51单片机产物兼容、8K字节在系统可编程视频存储器、1000次擦拭周期，全静态操作：0Hz~33Hz、三级加密程序存储器，32个可编程I/O接口线、三个16位定时器（计数器），八个中断源、低功能耗空闲和掉电模式、掉电后间断可唤醒，看门狗定时器、双数值指针，掉电标示符。 关键词：单片机，UART串行通道，掉电标示符等 前言 可以说，二十世纪跨越了三个“点”的时代，即电气时代，电子时代和现已进入的电脑时代。不过，这种电脑，通常指的是个人计算机，简称PC机。还有就是把智能赋予各种机械的单片机（亦称微控制器）。顾名思义，这种计算机的最小系统只用了一片集成电路，即可进行简单的运算可控制。因为它体积小，通常都是藏在被控机械的内部里面。它在整个装置中，起着有如人类头脑的作用，他出了毛病，整个装置就会瘫痪。现在，单片机的种类和适用领域已经十分广泛，如智能仪表、实施工控、通讯设备、导航系统、家用电器等。各种产品一旦用上了单片机，就你能起到产品升级换代的功效，常在产品名称前冠以形容词——“智能型”，如智能洗衣机等。接下来就是关于国产STC89C52单片机的一些基本参数。 功能特性描述: STC89C52单片机是一种低功耗、高性能CMOS8位微控制器，具有8K在系统可编程视频播放存贮器使用高密度非易失性存储器技术制造，与工业80C51 产物指令和引脚完全兼容。片上反射速度允许程序存储器在系统可编程，也适用于常规的程序编写器。在其单芯片上，拥有灵敏小巧的八位中央处理器和在线系统可编程反射，这些使用上STC89C52微控制器为众多嵌入式的控制应用系统提供高度矫捷的、更加有用的解决方案。STC89C52微控制器具有以下的标准功效：8K字节的反射速度，256字节的随机存取储存器，32位I/O串口线，看门狗定时器，2个数值指针，三个16 为定时器、计数器，一个6向量2级间断结构，片内晶振及钟表电路。另外，STC89C52可降至0HZ静态逻辑操作，支持两种软件可选择节电模式、间断继续工作。空闲模式下，CPU停止工作，允许RAM、定时器/计数器、串口、间断继续工作。掉电保护体式格局下，RAM内容被生成，振动器被冻结，单片机一切的工作停止，直到下一个间断或者硬件复位为止。8位微型控制器8K字节在系统中可编程FlashSTC89C52.。
I / 11 本科毕业设计外文翻译 <2018届） 论文题目基于WEB 的J2EE 的信息系统的方法研究 作者姓名[单击此处输入姓名] 指导教师[单击此处输入姓名] 学科(专业 > 所在学院计算机科学与技术学院 提交日期[时间 ]
基于WEB的J2EE的信息系统的方法研究 摘要：本文介绍基于工程的Java开发框架背后的概念，并介绍它如何用于IT 工程开发。因为有许多相同设计和开发工作在不同的方式下重复，而且并不总是符合最佳实践，所以许多开发框架建立了。我们已经定义了共同关注的问题和应用模式，代表有效解决办法的工具。开发框架提供：<1）从用户界面到数据集成的应用程序开发堆栈；<2）一个架构，基本环境及他们的相关技术，这些技术用来使用其他一些框架。架构定义了一个开发方法，其目的是协助客户开发工程。 关键词：J2EE 框架WEB开发 一、引言 软件工具包用来进行复杂的空间动态系统的非线性分析越来越多地使用基于Web的网络平台，以实现他们的用户界面，科学分析，分布仿真结果和科学家之间的信息交流。对于许多应用系统基于Web访问的非线性分析模拟软件成为一个重要组成部分。网络硬件和软件方面的密集技术变革[1]提供了比过去更多的自由选择机会[2]。因此，WEB平台的合理选择和发展对整个地区的非线性分析及其众多的应用程序具有越来越重要的意义。现阶段的WEB发展的特点是出现了大量的开源框架。框架将Web开发提到一个更高的水平，使基本功能的重复使用成为可能和从而提高了开发的生产力。 在某些情况下，开源框架没有提供常见问题的一个解决方案。出于这个原因，开发在开源框架的基础上建立自己的工程发展框架。本文旨在描述是一个基于Java的框架，该框架利用了开源框架并有助于开发基于Web的应用。通过分析现有的开源框架，本文提出了新的架构，基本环境及他们用来提高和利用其他一些框架的相关技术。架构定义了自己开发方法，其目的是协助客户开发和事例工程。 应用程序设计应该关注在工程中的重复利用。即使有独特的功能要求，也
沈阳工业大学工程学院 毕业设计（论文）外文翻译 毕业设计（论文）题目：工具盒盖注塑模具设计 外文题目：Friction , Lubrication of Bearing 译文题目：轴承的摩擦与润滑 系（部）：机械系 专业班级：机械设计制造及其自动化0801 学生姓名：王宝帅 指导教师：魏晓波 2010年10 月15 日
外文文献原文： Friction , Lubrication of Bearing In many of the problem thus far , the student has been asked to disregard or neglect friction . Actually , friction is present to some degree whenever two parts are in contact and move on each other. The term friction refers to the resistance of two or more parts to movement. Friction is harmful or valuable depending upon where it occurs. friction is necessary for fastening devices such as screws and rivets which depend upon friction to hold the fastener and the parts together. Belt drivers, brakes, and tires are additional applications where friction is necessary. The friction of moving parts in a machine is harmful because it reduces the mechanical advantage of the device. The heat produced by friction is lost energy because no work takes place. Also , greater power is required to overcome the increased friction. Heat is destructive in that it causes expansion. Expansion may cause a bearing or sliding surface to fit tighter. If a great enough pressure builds up because made from low temperature materials may melt. There are three types of friction which must be overcome in moving parts: (1)starting, (2)sliding, and(3)rolling. Starting friction is the friction between two solids that tend to resist movement. When two parts are at a state of rest, the surface irregularities of both parts tend to interlock and form a wedging action. To produce motion in these parts, the wedge-shaped peaks and valleys of the stationary surfaces must be made to slide out and over each other. The rougher the two surfaces, the greater is starting friction resulting from their movement . Since there is usually no fixed pattern between the peaks and valleys of two mating parts, the irregularities do not interlock once the parts are in motion but slide over each other. The friction of the two surfaces is known as sliding friction. As shown in figure ,starting friction is always greater than sliding friction . Rolling friction occurs when roller devces are subjected to tremendous stress which cause the parts to change shape or deform. Under these conditions, the material in front of a roller tends to pile up and forces the object to roll slightly uphill. This changing of shape , known as deformation, causes a movement of molecules. As a result ,heat is produced from the added energy required to keep the parts turning and overcome friction. The friction caused by the wedging action of surface irregularities can be overcome
机械设计理论 机械设计是一门通过设计新产品或者改进老产品来满足人类需求的应用技术科学。它涉及工程技术的各个领域，主要研究产品的尺寸、形状和详细结构的基本构思，还要研究产品在制造、销售和使用等方面的问题。 进行各种机械设计工作的人员通常被称为设计人员或者机械设计工程师。机械设计是一项创造性的工作。设计工程师不仅在工作上要有创造性，还必须在机械制图、运动学、工程材料、材料力学和机械制造工艺学等方面具有深厚的基础知识。如前所诉，机械设计的目的是生产能够满足人类需求的产品。发明、发现和科技知识本身并不一定能给人类带来好处，只有当它们被应用在产品上才能产生效益。因而，应该认识到在一个特定的产品进行设计之前，必须先确定人们是否需要这种产品。 应当把机械设计看成是机械设计人员运用创造性的才能进行产品设计、系统分析和制定产品的制造工艺学的一个良机。掌握工程基础知识要比熟记一些数据和公式更为重要。仅仅使用数据和公式是不足以在一个好的设计中做出所需的全部决定的。另一方面，应该认真精确的进行所有运算。例如，即使将一个小数点的位置放错，也会使正确的设计变成错误的。 一个好的设计人员应该勇于提出新的想法，而且愿意承担一定的风险，当新的方法不适用时，就使用原来的方法。因此，设计人员必须要有耐心，因为所花费的时间和努力并不能保证带来成功。一个全新的设计，要求屏弃许多陈旧的，为人们所熟知的方法。由于许多人墨守成规，这样做并不是一件容易的事。一位机械设计师应该不断地探索改进现有的产品的方法，在此过程中应该认真选择原有的、经过验证的设计原理，将其与未经过验证的新观念结合起来。 新设计本身会有许多缺陷和未能预料的问题发生，只有当这些缺陷和问题被解决之后，才能体现出新产品的优越性。因此，一个性能优越的产品诞生的同时，也伴随着较高的风险。应该强调的是，如果设计本身不要求采用全新的方法，就没有必要仅仅为了变革的目的而采用新方法。 在设计的初始阶段，应该允许设计人员充分发挥创造性，不受各种约束。即使产生了许多不切实际的想法，也会在设计的早期，即绘制图纸之前被改正掉。只有这样，才不致于堵塞创新的思路。通常，要提出几套设计方案，然后加以比较。很有可能在最后选定的方案中，采用了某些未被接受的方案中的一些想法。