Surface crystallinity

Crisp and coworkers found that the development of surface crystallinity was related to the speed of set. The faster the reaction, the shorter was the inhibition period before surface crystallization took place. When the setting time of a cement was between two and three minutes, surface crystallinity developed in a few minutes. When it was seven minutes, surface crystallinity was delayed by three hours. The reaction rate was affected by the chemical composition and physical state of the cement components. Well-ignited zinc oxide, the presence of magnesium in the... 

Gomez R, Orts JM, Feliu JM, Clavilier J, Klein LH. 1997. The role of surface crystalline heterogeneities in the electrooxidation of carbon monoxide adsorbed on Rh(lll) electrodes in sulphuric acid solutions. J Electroanal Chem 432 1 -5. 

Vitus and Davenport showed that upon anodic oxidation of Au(l 11), a monolayer of AuO was formed on the surface, maintaining the surface crystallinity. Subsequent electrochemical reduction of the oxide formed a characteristic wormlike structure that coarsened in a matter of hours to restore the original terrace structure of the substrate surface. This result shows that repeated potential cycling performed in the anodic region may destroy or roughen the surface owing to a lack of coalescence of surface atoms after oxide reduction. 

In contrast, structure-insensitive reactions are those for which turnover frequency under fixed conditions does not depend or depends slightly on the surface crystalline anisotropy of clusters of varying size or of single crystals exposing different faces. For these kinds of reactions, all accessible surface atoms can be considered as equally active sites (Boudart, 1981 and 1995). 

Fig. 7.42. A potentiostatic transient. The current (A-B) ascends almost vertically after being switched on, because all of it goes to charge the double layer. In B-C, the current is increasingly used in the form of electrons crossing the double layer. After C the current should decline slowly as diffusion control sets in. In reality, at solid polycrystalline electrodes, in reactions involving adsorbed intermediates, there is often some further variation of /, owing to, e.g., surface crystalline rearrangements and the effect of impurities from the solution.

Given the complexity that arises from the multitude of interacting variables associated with DPI systems, there are very few excipients that have been incorporated into DPI formulations. Examples of commonly marketed products are listed in Table 2. Lactose has many benefits including a well-established safety profile, low cost, and wide availability. Physicochemical properties of lactose are also relatively desirable from a DPI formulation standpoint smooth surfaces, crystalline, and moderate flow properties. However, lactose may not be suitable for some active... 

Near infra-red 0 to 90 By surface measurement. Highly reflective surfaces, crystalline materials and very dark coloured materials can cause difficulty. 

Finally, it should be realized that, even with a well-defined material in a standard test method, the properties may be dependent on the way in which the test sample has been made the preparation conditions may be different, and, moreover, may strongly deviate from those present in a technical processing operation. This applies, e.g. to shrinkage stresses, nature of the surface, crystalline structure, and, in particular, chain orientations. 

Because X-ray powder diffraction deals with solid samples, the analytical variables are different from those associated with the analysis of liquid or solution samples. Principle among these are particle size effects, uniform sample surface, crystallinity and X-ray absorption. Although particle size and a non-uniform sample surface are serious problems, their... 

The discrepancy in values for the small particle sample (quartz A) as opposed to the large particle sample (quartz B) has been reported (14, 16). This behavior has been explained in terms of the semi-amorphous character of the smallest quartz particles obtained in crushing and grinding operations. While larger particles are believed to be representative of increasing ideality in surface crystalline order, there is, at present, no confirmation for the supposition that the larger (20- to 40-micron) quartz B particles approach the ideal crystal structure. Thermal analysis shows that, relative to the large particles, as much as 48% of the less than 5-micron material may be noncrystalline with respect to the a — j quartz transition (16). 

The highest degree of surface crystallinity is reached if the polymer is moulded on gold foil then the surface crystallinity is about the same as the bulk crystallinity. The smallest degree of crystallinity is reached if the surface of the melt cools down in nitrogen. 

To be examined by ophcal microscopy, a material must, of course, be opaque to visible hght, for its surface to be observed. Contrasts in the produced image are as a result of differences in the reflechvity of the various regions of the microstmcture. Given the penetrahon depths hsted in Table 2.1, it is obvious that not only light, but also electrons, probe just the surfaces (the top-most atomic layers), whereas neutrons and X-rays provide information about the bulk. Hence, light and electron beams are used in microscopes for examining solid surfaces. Because it is well known that the surface crystalline stmc-ture of a sohd may differ from that of the bulk, the surfaces of most samples are usually... 

The HF method has been implemented on periodic systems [15], including bulk and surface crystalline materials. It has proved very useful in the description of magnetic insulators, but it has also successfully been used for describing surface properties of a large number of simple oxide surfaces. 

Typical product description Relatively dense powders formed by irregularly shaped, rough-surface crystalline particles Low-density powders formed by composite porous or hollow amorphous particles Low-density powders formed by loosely packed, regularly shaped crystalline particles... 

For two-layer PS formed on illuminated n-Si the top micro PS may have very different crystal structures varying from amorphous to polycrystalline to distorted single-crystalhne structure. For example, micro PS formed in 5% HF under illumination on n-Si with a doping range from lO to lO Vcm is single crystalline with the same orientation as the substrate. On the other hand, amorphous structure is found in the micro PS formed on illuminated heavily doped n-Si. ° The surface crystalline structure of the PS formed on n-Si under illumination varies from single crystalline to amorphous depending on the silicon resistivity and the condition of anodization as summarized in Fig. 8.55. ... 

The static contact mode is the simplest mode to use for obtaining basic topographic information of solid surfaces. Figure 5.17 is an image of surface crystalline polyethylene obtained with the contact mode. The contact mode was able to detect the nano-scale roughness of crystalline polyethylene, which is believed to be chain folds of polyethylene. The scale of the topography in Figure 5.17 implies that the primary function of an AFM is not the same as a surface profiler... 

Other fillers also produce variable degrees of crystallinity depending on their surface treatment. Carbon black is such an example. Graphitization of carbon black may increase surface crystallinity by 30%." ... 

With different irradiation wavelengths, the photothermal part of the mechanism is changed and the surface crystallinity can be lowered (irradiation at 248 and 308 nm [96]) or increased with shorter wavelengths 193 nm [97]. 

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