Catalysis wafers

Because the reaction in a CL requires three-phase boundaries (or interfaces) among Nafion (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as reacfanf, an optimized CL structure should balance electrochemical activity, gas transport capability, and effective wafer management. These goals are achieved through modeling simulations and experimental investigations, as well as the interplay between modeling and experimental validation. 

Chemical vapor deposition and heterogeneous catalysis share many kinetic and transport features, but CVD reactor design lags the corresponding catalytic reactor analysis both in level of sophistication and in scope. In the following we review the state of CVD reactor modelling and demonstrate how catalytic reactor design concepts may be applied to CVD processes. This is illustrated with an example where fixed bed reactor concepts are used to describe a commercial "multiple-wafers-in-tube" low pressure CVD reactor. 

It plays the same role as the effectiveness factor in heterogeneous catalysis and is a measure of the film thickness uniformity. It represents the ratio of the total reaction rate on each pair of wafers to that we would obtain if the concentration in the cell formed by the two wafers were equal to the bulk concentration everywhere. Thus, if the surface reaction is the rate controlling step, n = 1, whereas if the diffusion between the wafers controls, n < 1. In the limit of strong diffusion resistance the deposition is confined to a narrow outer band of the wafers and a strongly nonuniform film results. 

Various forms of spectroscopy have been applied to in situ studies of catalysis, and it is appropriate to cite a few examples. FT-IR is frequently employed for in situ investigations. The experimental configurations used can be either transmission studies of free-standing catalyst wafers [2] or diffuse reflectance measurements on samples in catalytic reaction chambers 13]. In situ Raman spectroscopy has also been applied [4]. X-rays have been used to study catalysts in situ, either by powder diffraction methods [5,6] or XAFS [7]. In situ imaging techniques are beginning to be applied to the measurement of spatial distributions and residence times in catalytic reactors. A recent example of this method employed positron-emission tomography [8]. 

AES is sufficiently mature and its application broad. Many general reviews of the technique deal with specific appHcations in general surface and thin film analysis. AES is often used to solve problems in metallurgy, plating, corrosion, and catalysis. Reviews covering these applications are listed in the Further Reading section. Because the primary electron beam can be focused down to a diameter of less than 10 nm, information about local compositions on a specimen s surface can be obtained. This special feature makes AES very attractive for applications in the semiconductor technology where submicrometer features are of interest. To satisfy semiconductor manufacturers, AES systems that are able to handle 300 mm silicon wafers are commercially available now. 

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