Surface area roughness factor

Molecular nitrogen will see a significant surface area due to its small size comparable to the dimensions of the surface roughness, while bigger molecules such as pyrene will not be able to see all ridges and valleys and will see a significantly lower effective surface area. These factors have been studied extensively [17] for silica, and authors have found fractal factors to vary between 2 and 3, depending on the silica synthesis, treatment, and so on. 

The break-in process of 3M NSTF films involves voltage cycling to create a smooth polycrystalline Pt surface on the whiskers. The surface area enhancement factors of resulting structures are from 10 to 25, determined from cyclic voltammo-grams in the potential region of hydrogen underpotential deposition. The surface area enhancement factor (or roughness factor or real-to-apparent surface area ratio) of a... 

Roughness factor. Except for the basal plane of HOPG, there is some degree of roughness on the carbon surface. The roughness factor o is defined as the ratio of the microscopic area to the geometric area ... 

Roughness factor calculated for a fractal surface, according to the fractal dimension D and probe area a... 

The scale of the microscopic surface roughness is important to assure good mechanical interlocking and good durability. Although all roughness serves to increase the effective surface area of the adherend and therefore to increase the number of primary and secondary bonds with the adhesive/primer, surfaces with features on the order of tens of nanometers exhibit superior performance to those with features on the order of microns [9,14], Several factors contribute to this difference in performance. The larger-scale features are fewer in number... 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Such an approach revealed objective limitations as it became evident that the equality in the capacitance values for different metals was only a first approximation. The case of Ga is representative. Ga is a liquid metal and the value of capacitance cannot depend on the exact determination of the surface area as for solid metals (i.e., the roughness factor is unambiguously =1). 

In this exercise we shall estimate the influence of transport limitations when testing an ammonia catalyst such as that described in Exercise 5.1 by estimating the effectiveness factor e. We are aware that the radius of the catalyst particles is essential so the fused and reduced catalyst is crushed into small particles. A fraction with a narrow distribution of = 0.2 mm is used for the experiment. We shall assume that the particles are ideally spherical. The effective diffusion constant is not easily accessible but we assume that it is approximately a factor of 100 lower than the free diffusion, which is in the proximity of 0.4 cm s . A test is then made with a stoichiometric mixture of N2/H2 at 4 bar under the assumption that the process is far from equilibrium and first order in nitrogen. The reaction is planned to run at 600 K, and from fundamental studies on a single crystal the TOP is roughly 0.05 per iron atom in the surface. From Exercise 5.1 we utilize that 1 g of reduced catalyst has a volume of 0.2 cm g , that the pore volume constitutes 0.1 cm g and that the total surface area, which we will assume is the pore area, is 29 m g , and that of this is the 18 m g- is the pure iron Fe(lOO) surface. Note that there is some dispute as to which are the active sites on iron (a dispute that we disregard here). 

Such information can be obtained from cyclic voltammetric measme-ments. It is possible to determine the quantity of electricity involved in the adsorption of hydrogen, or for the electrooxidation of previously adsorbed CO, and then to estimate the real surface area and the roughness factor (y) of a R-C electrode. From the real surface area and the R loading, it is possible to estimate the specific surface area, S (in m g ), as follows ... 

GP 8[ [R 7[ The structure of the rhodium catalyst changed during operation. Owing to the microfabrication process (thin-wire pEDM), the surface of the micro channels was rough before catalytic use [3]. After extended operational use, small crystallites are formed, especially in oxygen-rich zones such as the micro channels inlet. Thereby, the surface area is enlarged by a factor of 1-1.5. 

In industrial electrochemical cells (electrolyzers, batteries, fuel cells, and many others), porous metallic or nonmetallic electrodes are often used instead of compact nonporous electrodes. Porous electrodes have large trae areas, S, of the inner surface compared to their external geometric surface area S [i.e., large values of the formal roughness factors y = S /S (parameters yand are related as y = yt()]. Using porous electrodes, one can realize large currents at relatively low values of polarization. 

Many of the same factors which complicate the interpretation of laboratory kinetic studies are among the most important limitations on the application of laboratory dissolution rate data to natural systems. These include uncertainty about 1) the effective surface area in natural systems (56,57) 2) the extent to which surface area and surface roughness change with reaction progress ( 18) and 3) the magnitude of solution composition effects on rates in natural systems. 

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