Page 21 - Adsorptionbypowders & poroussolids muyace

CHAPTER 1 INTRODUCTION 7

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Table 1.2. Defirut~ons powders
Tern Defin~tlon
powder Dry matenal composed of discrete parhcles with maxlmum dunens~on less
than about 1 mm
Fme powder Powder with particle slze below about 1 pm
Aggregate Loose, unconsohdated assemblage of particles
Agglomerate bgld, consol~dated assemblage or parhcles
compact Agglomerate formed by compression of powder
Ac~cul~ Needle-shaped
surface area Extent of avalable surface as detemed by a glven method under stated con-
dtlons
Specific surface area Surface area of umt mass of powder, as determmed under stated condibons
External surface Area of external surface of particles, as takmg account of roughness (1 e all
cavrtles whlch are wlder than they are deep). but not porosity
Roughness factor Ratlo of external surface area to area of smoothed envelope around particles
Divided sohd Solid made up of more or less independent pamcles wh~ch may be m the form
of a powder, aggregate or agglomerate

imprecise manner, but it seems reasonable to apply it to a material consisting of par-
ticles less than about 1 p.rn (i.e. particles of colloidal dimensions). The unit mass of a
fine powder contains a large number of small particles and hence exhibits an appre-
ciable surface area. For example, in the simplest case of an assemblage of spherical
particles, all with the same diameter, d, the specific surface area, a, is given by the
relation

where p is the particle absolute density. Thus, a powder composed of smooth spher-
ical particles of d = 1 p.rn and p = 3 g cm-3 would have a specific surface of 2 mZ g-'.
The same calculation would apply to cubic particles, but in this case d would equal
the edge length of the cube. In fact, an area of about 2 mZg-' turns out to be of the
same order of magnitude as the lower limit amenable to investigation by the tech-
niques most often used in routine adsorption measurements.
It is evident that it is more difficult to &fine particle size if the particle shape is not
spherical or cubic. With some other simple geometric forms, a single linear dimen-
sion, d, may be used to calculate the surface area. In particular, when the particle
aspect ratio is sufficiently large, d, is taken as the minimum dimension. Thus, if the
particles are thin or long (i.e. plates or rods), it is the thickness which mainly deter-
mines the magnitude of the specific surface area (Gregg and Sing, 1982).
Perfect spheres are rare, but spheroidal particles are present in some powders pro-
duced at high temperature (e.g. pyrogenic silicas) or by the sol-gel process. The term
sphericity is useful for some purposes. Sphericity has been defined in various ways,
the simplest definition being the ratio of the surface area of a sphere of the same
volume as a given particle to the actual surface area of that particle (Allen, 1990).
The individual particles (pnrnary particles) in a fine powder are usually clustered
together in the form of aggregates or agglomerates. Loosely bonded aggregates
are unconsolidated and non-rigid, but they may be converted into more ng~d,