PARTICLE SIZE AND SHAPE MEASUREMENTS AND STATISTICS 



The proper preparation of slides to pro- 

 duce representative, well-dispersed samples 

 is especially important. It is, of course, 

 necessary to start with a small representa- 

 tive sample of the powder, a few milligrams 

 of which are usually dispersed in a drop of 

 liquid on a clean slide. A wooden toothpick 

 is a convenient dispersing tool which has the 

 advantage over glass or steel that it is not 

 apt to crush or shatter the particles. The 

 suspension is spread over the glass slide as 

 a thin film and covered with a clean cover 

 glass to complete the preparation. The fol- 

 lowing are a few of the many liquids which 

 have been used for dispersing powders. 



Water 



Water and ammonia 



Water and potassium 



citrate (2%) 

 Water and Calgon 



(0.1%) 

 Glycol 

 Glycerol 

 Ethanol 

 Acetone 

 Cyclehexanol 



Butanol 



Dammar 



Turpentine 



Duco cement 



Glucose syrup 

 Xylene 



Polymethylmethacrylate 

 Polystyrene 



Solutions of surface-ac- 

 tive agents 



Considerable experimentation with differ- 

 ent liquids is sometimes required before one 

 is found which is satisfactory for a given 

 powder. 



A sufficiently large number of particles 

 must be measured so that the size distribu- 

 tion obtained is representative of the original 

 powder. Usually at least 200 particles are 

 measured. If the size distribution is very 

 wide there may be hundreds or thousands 

 of small particles for each of the large ones. 

 In this case it is often helpful to measure all 

 the larger particles in a relatively large area 

 with a low-power objective and all the small 

 particles in a much smaller area with a high- 

 power objective; the results are combined 

 arithmetically for any arbitrary area. 



The number of fields which should be ex- 

 amined depends largely on the number of 

 particles per field. The choice of fields can 

 be random or according to a pattern, but 



should be made prior to observing the fields 

 through the microscope in order to avoid 

 unconscious preferential selection. 



When measuring the sizes of particles 

 whose diameters are close to the theoretical 

 limit of resolution of the microscope, proper 

 alignment and illumination are particularly 

 important. Thus, when illuminating with 

 transmitted light ("brightfield" illumina- 

 tion) critical or Kohler illumination should 

 be used to obtain the full benefit of the capa- 

 bilities of the microscope. 



Images of particles observed with the mi- 

 croscope are larger than the particles by an 

 amount approximately equal to the limit of 

 resolution of the microscope. The amount of 

 this enlargement varies somewhat with the 

 illumination, the type of material, and the 

 ability of the observer to distinguish various 

 degrees of contrast. Some microscopists sub- 

 tract the limit of resolution from the meas- 

 ured diameters when determining particle 

 sizes which are close to the limit of resolu- 

 tion. 



The thickness of particles can be measured 

 with the fine-focusing adjustment. The grad- 

 uations on this adjustment must be cali- 

 brated and this is readily accomplished with 

 small beads or cylindrical fibers. The diame- 

 ter of the bead or fiber is measured with an 

 ocular micrometer and the microscope is 

 focused on the glass slide and then on the 

 top of the fiber or bead, noting the readings 

 on the fine adjustment. The difference in 

 readings corresponds to the diameter. Ob- 

 viously the thickness of an irregular object 

 can then be obtained by focusing on the 

 slide and on the top of the object and noting 

 the difference in readings. 



Many special techniques are available for 

 determining particle sizes and size distribu- 

 tions. Electronic methods have been devel- 

 oped for scanning the image formed by a 

 microscope. For example, a television rastor 

 can be placed at the ocular of the microscope 

 so that a minute spot of light scans the parti- 

 cle field on the microscope shde, while the 



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