High-surface materials

It was found essential to maintain cleanliness around the Coulter Counter at all times. Dust and moisture must be avoided or high background counts and instability will result. The Coulter Counter analysis was found to be more rapid than the Micromerograph, requiring approximately 15 minutes per analysis as compared to 90 minutes for high surface material on the Micromerograph. The use of a multichannel Coulter Counter would reduce the time per analysis to less than five minutes... 

Lattice constants and variations of these constants have oec.asionallj been discussed as being related to the specific activities of certain catalysts. The poor x-ray diagrams usually obtained with high surface materials do not lend themselves to such determinations, particularly if it is considered that the lattice constants in the uppermost surface layers of the catalyst woidd have to be measured in order to give information on the catalytic properties. The dimensions of a lattice of a given crystal are usually a function of the temperature at which the crystal is held. However, if other elements or compounds can go into solid solution in this crystal, changes in the lattice may occur. 

Cyclopentadiene reacts with alkaline oxides and hydroxides forming the alkaline cyclopentadienyl salts. Alkaline hydroxides supported on high surface materials such as kiesclguhr and alumina are good absorbents for trace amounts of CPD contaminant in other hydrocarbons (27). 

Solid-state absorbers (such as hydrides or high-surface materials)... 

With this testing method an evaluation is possible within shortest time, i.e. directly after the heat impulse. The high temperature difference between a delamination and sound material is affected - among other parameters - by the thickness of the layer. Other parameters are size and stage of the delamination Generally, a high surface temperature refers to a small wall thickness and/or layer separation [4],... 

None of the above methods is sufiBcient for neutrons, however. Neutrons penetrate mader so easily that the only effective approach is to use materials with a very high surface-to-vohuue ratio. This can be accomplished with small particles and exfoliated graphite, for instance, but the teclmique has essentially been abandoned in surface sdidies [7, 8]. 

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

The central idea underlying measurements of the area of powders with high surface areas is relatively simple. Adsorb a close-packed monolayer on the surface and measure the number A of these molecules adsorbed per unit mass of the material (usually per gram). If the specific area occupied by each molecule is A then the... 

If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

Furthermore, it must be remembered that highly disperse materials are, from their very nature, difficult to prepare with exactly reproducible surface properties, in respect of either the extent of the surface or the nature of the surface itself. Consequently, highly precise values of the absolute area of individual samples, even if attainable by some method as yet undeveloped, would be of little more value in practice than the BET specific surface, calculated from carefully measured isotherms. 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

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