Atomic processes

Quantum mechanics tells us that only certain discrete values of E, the total electron energy, and J, the angular momentum of the electrons are allowed. These discrete states have been depicted in the familiar semiclassical picture of the atom (Fig. 1.1) as a tiny nucleus with electrons rotating about it in discrete orbits. In this book, we will examine nuclear structure and will develop a similar semiclassical picture of the nucleus that will allow us to understand and predict a large range of nuclear phenomena. 

The sizes and energy scales of atomic and nuclear processes are very different. These differences allow us to consider them separately. 

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A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

In numerous cases an atomically detailed picture is required to understand function of biological molecules. The wealth of atomic information that is provided by the Molecular Dynamics (MD) method is the prime reason for its popularity and numerous successes. The MD method offers (a) qualitative understanding of atomic processes by detailed analysis of individual trajectories, and (b) comparison of computations to experimental data by averaging over a representative set of sampled trajectories. 

The decay of radioisotopes iavolves both the decay modes of the nucleus and the associated radiations that are emitted from the nucleus. In addition, the resulting excitation of the atomic electrons, the deexcitation of the atom, and the radiations associated with these processes all play a role. Some of the atomic processes, such as the emission of K x-rays, are inherently independent of the nuclear processes that cause them. There are others, such as internal conversion, where the nuclear and atomic processes are closely related. 

In the atomizing process, a stream of molten zinc is broken into tiny droplets by the force of a pressurized fluid impinging on the stream. The fluid can be any convenient material, although air is normally used. The atomized drops cool and soHdify rapidly in a coUection chamber. The powder is screened to specified sizes. Particulate zinc is also produced by other methods such as electrolytic deposition and spinning-cup techniques, but these are not of commercial importance. 

Liquid fuel is injected through a pressure-atomizing or an air-blast nozzle. This spray is sheared by air streams into laminae and droplets that vaporize and bum. Because the atomization process is so important for subsequent mixing and burning, fuel-injector design is as critical as fuel properties. Figure 5 is a schematic of the processes occurring in a typical combustor. 

The electrical characteristics of ceramic materials vary gteady, since the atomic processes ate different for the various conduction modes. The transport of current may be because of the motion of electrons, electron holes, or ions. Electrical ceramics ate commonly used in special situations where reftactoriness or chemical resistance ate needed, or where other environmental effects ate severe (see Refractories). Thus it is also important to understand the effects of temperature, chemical additives, gas-phase equilibration, and interfacial reactions. 

M. Menon and R. E. AUen, New technique for molecular-dynamics computer simulations Hellmann- Feynman theorem and subspace Hamiltonian approach , Phys. Rev. B33 7099 (1986) Simulations of atomic processes at semiconductor surfaces General method and chemisorption on GaAs(llO) , ibid B38 6196 (1988). 

The different growth modes discussed above have been exemplified also from structural studies. Froment and Lincot [247] used structural characterization methods, such as TEM and HRTEM, to determine the formation mechanisms and habits of chemically deposited CdS, ZnS, and CdSe thin film at the atomic level. These authors formulated reaction schemes for the different deposition mechanisms and considered that these should be distinguished to (a) atom-by-atom process, providing autoregulation in normal systems (b) aggregation of colloids (precipitation) ... 

Fig. 3.16 Reaction schemes of different CBD mechanisms for compound semiconductors (a) atom-by-atom process (b) aggregation of colloids and (c) mixed process. (Reprinted from [247], Copyright 2009, with permission from Elsevier)...

The principal intention of the present book is to connect mechanical hardness numbers with the physics of chemical bonds in simple, but definite (quantitative) ways. This has not been done very effectively in the past because the atomic processes involved had not been fully identified. In some cases, where the atomic structures are complex, this is still true, but the author believes that the simpler prototype cases are now understood. However, the mechanisms change from one type of chemical bonding to another. Therefore, metals, covalent crystals, ionic crystals, and molecular crystals must be considered separately. There is no universal chemical mechanism that determines mechanical hardness. 

Finally, it must be remembered that DFT and AIMD can be incorporated into the so-called mixed quantum mechanical/molec-ular mechanical (QM/MM) hybrid schemes [12, 13]. In such methods, only the immediate reactive region of the system under investigation is treated by the quantum mechanical approach -the effects of the surroundings are taken into account by means of a classical mechanical force field description. These DFT/MM calculations enable realistic description of atomic processes (e.g. chemical reactions) that occur in complex heterogeneous envir-... 

There are two other methods in which computers can be used to give information about defects in solids, often setting out from atomistic simulations or quantum mechanical foundations. Statistical methods, which can be applied to the generation of random walks, of relevance to diffusion of defects in solids or over surfaces, are well suited to a small computer. Similarly, the generation of patterns, such as the aggregation of atoms by diffusion, or superlattice arrays of defects, or defects formed by radiation damage, can be depicted visually, which leads to a better understanding of atomic processes. 

Fick s (continuum) laws of diffusion can be related to the discrete atomic processes of the random walk, and the diffusion coefficient defined in terms of Fick s law can be equated to the random-walk displacement of the atoms. Again it is easiest to use a one-dimensional random walk in which an atom is constrained to jump from one... 

Atomic processes, ballistic-like, 14 426, 427 Atomic properties... 

Nuclear/atomic processes, 21 306-309 Nuclear capacities, regional, 17 567t Nuclear chain reaction, modeling, 17 563 Nuclear collisions, energy loss from,... 

Zinc antimonide, 3 44, 53—54 Zinc atomizing process, 26 598 Zinc baths, 9 828-829, 830t Zincblende semiconductors, 22 141 band structure of, 22 142-144 transport properties of, 22 148, 149t Zinc borates, 4 282-283 Zinc brass... 

This section describes the atomization processes and techniques for droplet generation of normal liquids. A comparison of the features of various atomization techniques is summarized in Table... 

Emphasis is placed on the atomization processes used in spray combustion and spray drying from which many atomization processes have evolved. Advantages and limitations of the atomization systems are discussed along with typical ranges of operation conditions, design characteristics, and actual and potential applications. The physical properties of some normal liquids are listed in Table... 

As mentioned in the previous section, a major drawback of the simplex atomizer is the poor atomization quality at the lowest flow rate due to too-low pressure differential if swirl ports are sized to allow the maximum flow rate at the maximum injection pressure. This problem may be resolved by using dual-orifice, duplex, or spill-return atomizers. Alternatively, the atomization processes at low injection pressures can be augmented via forced aerodynamic instabilities by using air or gas stream(s) or jet(s). This is based on the beneficial effect of flowing air in assisting the disintegration of a liquid j et or sheet, as recognized in the application of the shroud air in fan spray and pressure-swirl atomization. 

In air-assist atomization, air is needed usually to augment the atomization process only at low liquid flow rates when the pressure differential is too low to produce satisfactory pressure atomization. In some designs, however, air assistance may be required over the entire range of operating conditions if the atomization quality achieved with a pressure atomizer alone is always poor. In an air-assist atomization process, the impingement of a low-velocity liquid stream by a high-velocity air stream may occur either within or outside the... 

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