Ball milling surface areas

Fig. 3.Z3 Adsorption isotherm of n-butane at 273 K on a sample of artificial graphite ball-milled for 192 b. The shoulder F appeared at a relative pressure which was the same for all six samples in the first milling run, all six in the second milling run, and also for two of the milled samples which had been compacted. The milling time varied between 0 and 1024 h, and the BET-nilrogen areas of the surfaces between 9 and 610 m g ...

During ball milling the mean particle diameter, d, gas adsorption isotherm specific surface area (BET) and contamination (wt% contaminant) conform to the general law ... 

Grinding was performed using Retsch ball-mill. Particle size distribution was measured using Malvern (Mastersizer) laser particle sizer. BET surface area measurements were carried out using ASAP (Micromeritics) surface area analyzer. 

FIGURE 6.3 (a) Morphology of ball-milled NiO and SDC powders produced separately and (b) NiO-SDC composite powders coproduced by spray pyrolysis, showing a possible mechanism by which the composite powders retain a higher surface area resulting in a higher performance [19]. Reprinted from [19] with permission from Elsevier. 

These difficulties are obviated to some extent by arranging the screens in the form of concentric cylinders, with the coarsest in the centre. The disadvantage of all screens of this type is that only a small fraction of the screening area is in use at any one time. The speed of rotation of the trommel should not be so high that the material is carried completely round in contact with the screening surface. The lowest speed at which this occurs is known as the critical speed and is analogous to the critical speed of the ball mill, discussed in Chapter 2. Speeds of between one-third and a half of the critical speed are usually recommended. In a modified form of the trommel, the screen surfaces are in the form of truncated cones. Such screens are mounted with their axes horizontal and the material always flows away from the apex of the cone. 

VV Bhat, A. Rougier, L. Aymard, G.A. Nazii, J.-M. Tarascon, High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH, J. Alloys Compd. 460 (2008) 507-512. 

Powders of these materials are first prepared and then formed into spherical pellets in a ball mill or into cylindrical pellets by extruding a paste. A catalyticaUy active component such as a noble metal or a multivalent oxide is then frequently added to one of these porous supports by depositing ions from salt solution and heating to dry and decompose the salt. We are then interested not in the total surface area of the pellet but the area of the catalyticaUy active component. 

Hydrolysis of Alkoxides - Although it is simple and economical, the solid state reaction route has the following major drawbacks for the preparation of catalysts (1) very high temperatures are required to complete the formation of the final phase unless ball milling is performed before firing (2) industrial catalysts often exhibit complex chemical composition that is difficult to obtain uniformly through solid state reaction and (3) the low surface areas, that are typically achieved by solid state reaction, may affect the catalytic performances. 

Of the many physical and chemical procedures thus far applied to enhance lignocellulose reactivity (1), fine grinding appears to offer the most direct response to both lignin and crystallinity. When the grinding is done in a vibratory ball mill, particle size can be reduced to micron dimensions, with attendant expansion of external surface area, and crystallinity can be essentially eliminated (2). This combination of events markedly influences carbohydrate accessibility and, hence, the degree of response of a milled lignocellulosic material to chemical, enzymatic, and microbiological attack. 

It is not clear, however, whether electron radiation would have beneficial effects upon the enzymatic hydrolysis of cellulose. It has been shown that the particle size reduction and surface area increase upon ball milling are probably more important than the reduction of crystallinity (19). Electron irradiation may not disrupt the shielding layer provided by the lignin to expose fresh cellulose to the action of large enzyme molecules. 

Ball milling is the most commonly used pretreatment. It reduces crystallinity and particle sizes, while it increases surface area, bulk density, and the water soluble fraction. The major drawbacks of the milling are cost and the fact that noncellulosic substances are not removed. 

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