Surface films and chemical reactions

Laboratory studies have clearly established that the onset of waves occurs at higher wind speeds in the presence of organic compounds and that this reduces the gas exchange rate at a given wind 

These experiments open the possibility that some of the scatter in the comparison of natural gas exchange measurements with wind speed may be due to the effect of surfactants. If a way to remotely characterize wave slopes is devised, then it may give insight into second-order effects on the transfer velocity caused by surfactants. Another implication is that pollution of the seas by petroleum may have an inhibitory effect on the global gas exchange rate. 

Calculation of CO2 gas exchange rates in nature from air-water gradients in fco implicitly assumes that CO2 does not react during the transfer across the interface. If reaction did occur it would steepen the CO2 gradient very near the interface (within tens of microns) 

Qoo 5 m d , the rate constant would have to be catalyzed by at least a factor of 10. Redrawn from Emerson (1995). 

This simple calculation suggests that there should be httle CO2 reaction during gas transfer for conditions in the ocean. The residence time of a gas in the diffusive layer is much shorter with respect to diffusion than with respect to reaction. To determine whether this is generally true, CO2 gas exchange rates including the effects of CO2 reaction have been calculated by using a reaction diffusion model for the chemistry of seawater as a function of the gas exchange mass transfer coefficient (Emerson, 1995). Results (Fig. 10.14) indicate that 

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Influence of Surface Films and Chemical Reactions on Air-Water Exchange Advanced Topic)... 

Thin film dielectrics are usually deposited using chemical vapor deposition (CVD). A variation of CVD utilizing a plasma discharge is called plasma-enhanced CVD (PECVD) and is the standard in IC fabrication for the deposition of dielectric films. Plasma-enhanced CVD involves the formation of a solid film in a substrate surface from volatile precursors (vapor or gas) in a plasma discharge. The precursors are chosen to contain the constituent elements of the final film and chemical reactions in the gas phase are encouraged. They are condensed in a substrate that is heated or cooled. It will be shown later that porosity can be introduced in the PECVD films. Spin coating is another preparation technique and a popular choice... 

The objective of this paper is to investigate the role of some potential fillers in modifying the friction and wear behavior of high density polyethylene. It has been studied in terms of the transfer film formation capability and its bonding to the counterface in view of the surface texture and chemical reaction possibilities. ... 

Film-forming chemical reactions and the chemical composition of the film formed on lithium in nonaqueous aprotic liquid electrolytes are reviewed by Dominey [7], SEI formation on carbon and graphite anodes in liquid electrolytes has been reviewed by Dahn et al. [8], In addition to the evolution of new systems, new techniques have recently been adapted to the study of the electrode surface and the chemical and physical properties of the SEI. The most important of these are X-ray photoelectron spectroscopy (XPS), SEM, X-ray diffraction (XRD), Raman spectroscopy, scanning tunneling microscopy (STM), energy-dispersive X-ray spectroscopy (EDS), FTIR, NMR, EPR, calorimetry, DSC, TGA, use of quartz-crystal microbalance (QCMB) and atomic force microscopy (AFM). 

Formation of single-phase ZnTe on zinc substrates at 640 K by using electrochemical ion exchange and chemical reaction/alloying with Te" species, supplied to the substrate as a vapor from TeCU-containing eutectic LiCl-KCl molten salts, was reported recently [118]. ZnTe films with a smooth dense surface and particle diameters less than about 0.8 p,m were obtained, by properly adjusting the TeCU content and the reaction time. 

There are four main processes (i.e., bulk transport chemical reaction film and particle diffusion) which can affect the rate of solid phase chemical reactions and can broadly be classified as transport and chemical reaction processes [10, 31,103 -107]. The slowest of these will limit the rate of a particular reaction. Bulk transport process of a certain pollutant(s), which occurs in the aqueous phase, is very rapid and is normally not rate-limiting. In the laboratory, it can be eliminated by rapid mixing. The actual chemical reaction at the surface of a solid phase (e.g., adsorption) is also rapid and usually not rate limiting. The two remaining transport or mass transfer processes (i.e.,film and particle diffusion processes), either singly or in combination, are normally rate-limiting. Film diffusion invol-... 

In this chapter we discuss how solid surfaces can be modified. Surface modification is essential for many applications, for example, to reduce friction and wear, to make implants biocompatible, or to coat sensors [405,406], Solid surfaces can be changed by various means such as adsorption, thin film deposition, chemical reactions, or removal of material. Some of these topics have already been discussed, for example in the chapter on adsorption. Therefore, we focus on the remaining methods. Even then we can only give examples because there are so many different techniques reflecting diverse applications in different communities. 

Once the thermodynamic parameters of stable structures and TSs are determined from quantum-chemical calculations, the next step is to find theoretically the rate constants of all elementary reactions or elementary physical processes (say, diffusion) relevant to a particular overall process (film growth, deposition, etc.). Processes that proceed at a surface active site are most important for modeling various epitaxial processes. Quantum-chemical calculations show that many gas-surface reactions proceed via a surface complex (precursor) between an incident gas-phase molecule and a surface active site. Such precursors mostly have a substantial adsorption energy and play an important role in the processes of dielectric film growth. They give rise to competition among subsequent processes of desorption, stabilization, surface diffusion, and chemical transformations of the surface complex. 

The selection of diffusion equation solutions included here are diffusion from films or sheets (hollow bodies) into liquids and solids as well as diffusion in the reverse direction, diffusion controlled evaporation from a surface, influence of barrier layers and diffusion through laminates, influence of swelling and heterogeneity of packaging materials, coupling of diffusion and chemical reactions in filled products as well as permeation through packaging. 

The surface films react chemically with solution species, thus leading to their dissolution as reaction products [17]. Surface species such as oxides, hydroxides, and nitrides may be highly nucleophilic, while many polar aprotic solvents are highly electrophilic. Hence, chemical dissolution of pristine surface films on active metals in solutions is a very probable route [18]. 

CFD [Chemical Fluid Deposition] A process for depositing thin films on solid surfaces by a chemical reaction in a liquid such as supercritical carbon dioxide. Superior to CVD in being capable of operation at almost ambient temperatures. Demonstrated for depositing platinum metal on silicon wafers, polymer substrates, and porous solids by hydrogen reduction of an organo-platinum compound at 80°C. 

Chemical vapor methods for thin-film synthesis bring to the substrate surface the chemical reagents, in the gas phase, needed to synthesize the material to be prepared as a thin film. The chemical reaction is allowed to occur at or near the substrate surface, and the resulting material, having a vapor pressure considerably less than the reagent gases, deposits onto the surface. A nice example of this is the synthesis of the classic... 

Explicitly developed are models of several theoretical multiphase distributions, with corresponding depth-profile results on thin-film plasma polymers, phase-separated block copolymers, and chemical reactions on fiber surfaces. Ion impact is treated from three points of view as an analytical fingerprint tool for polymer surface analysis via secondary ion mass spectroscopy, by forming unique thin films by introducing monomers into the plasma, and as a technique to modify polymer surface chemistry. 

During CMP, the temperature between the polishing pad and the wafer is elevated due to friction and chemical reactions [43,44]. The elevated temperature may have an impact on the surface adsorption behavior of the passivating agent. The passivating film may become thinner due to an... 

There is a variety of fundamental physical and chemical principles lhat can control the deposition rate and quality of a film resulting from a CVD process. We briefly introduce them here, but refer the reader to Chapter 2 and other books on CVD for more detailed discussions. The basic processes underlying CVD can be subdivided into mass transport effects and chemical effects, each of which can occur in both the gas and solid phases. Chemical effects can be further subdivided into thermodynamic effects and kinetic effects. In some cases, a particular effect can be separated out as rate limiting, and a CVD process can be said to be mass-transport controlled or surface-kinetics controlled. In reality, transport and chemical reactions are closely coupled, with their relative importance varying with the details of the operating conditions. 

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