Amorphous internal defects

BET studies of both the commercial and laboratory scale particles discussed above indicate that there is little internal area accessible to BET adsorbate molecules. This holds for both amorphous and polycrystall ine particles. If the individual particles are composed of multiple crystalline substructures, internal defects capable of adsorption would be expected. However, the BET measurements. show that internal pore.s, if they are present, are not accessible to adsorbate gases. A possible explanation is that annealing by solid-state diffusion occuin sufftcienily rapidly al the temperatures of formation to block access of the external gas to dislocations and grain boundaries. However, the origins of the crystallites within the particles and the mechanisms of crystallization tire not understood al present. 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

The discussion so far assumed that the superimposed uniaxial anisotropy is perfectly uniform. This is rather the exception than the rule in reality, mostly due to internal stresses and/or surface defects. The typical fluctuation wavelengths are much larger than Zex and range from a few to about 100 pm. Such Ku fluctuations ultimately provide the limiting factor for the soft magnetic properties in amorphous and optimized crystalline (conventional or nanocrystalline) alloys. 

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