Other Methods of Measuring Particle Size

Permeability is one other method for obtaining information about particle diameters. If one packs a tube with a weight of powder exactly equal to its density, and applies a calibrated gas pressure tbrough the tube, the pressure-drop can be equated to an average particle size. The instrument based on this principle is called the Fisher Sub-Sieve Sizer . Only one value can be obtained but the method is fast and reproducible. The instrument itself is not expensive and the method can be applied to quality control problems of powders. Permeametry is usefiil in the particle range of 0.5 to 50 ji. 

Suppose we have a 20.00 gm. sample having a density of 2.0. We measure the surface area as 6.0 m. From the area of a sphere, A = jr, and the volume of a sphere, V = 4/3 tt 1)3, we find the total volume of n spheres to be 10 cc, i.e.- n 4/3 n d3 = 10. The surface area of nfirD spheres is 6 m3. And n, the total number of spheres present is the same in both formulas. Therefore, by substitution, we find D= 10 p. 

If we obtain a particle diameter by some other method and find that it is much smaller than that of the BET method, we infer that the particles are porous. We thus speak of the porosity and need to correct for the pore surface area if we are to make a reasonable estimate of the true diameter by the BET method. Note that the BET method is the only method whereby one can obtain an estimate of the porosity of any given set of particles. 

We are now ready to undertake the art and science of growing a single crystal. A great many methods are available and the choice one makes will 

Measuring particle size and growing single crystals 

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The use of scanning electron microscopy (SEM) is not as satisfactory. Other methods of measuring particle size are somewhat indirect and actually derive their calibration from electron microscopy. In effect, measurements with instruments such as a Coulter Electronics Multisizer or a disc centrifuge average the diameters of a great many particles. In the case of the disc centrifuge, the... 

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