Simple Powder Dispersion Method for Electron Microscopy Saul W. Chaikin, Stanford Research Institute, Menlo Park, Calif.
T" problem in preparing
powders for electron microscope etndy is to achieve a good, representative dispersion so that ultimate particle sizes and shapes may be seen. Where the powders are used in dispersed form in practical materials. the electron microscope dispersion should be a useful comparison to that obtained in the practical material. The electron microscope dispersion technique [Schuster, M. C., Fullam, E. F., IND. ENQ.CEEM., ANAL.ED. 18, 653(1946); Hall, C. E., "Introduction to Electron Microscopy," p. 374, McGraw-Hill, New York, 19531 must not break up particles or permit flocculation of particles once dispersed. Most dispersion techniques depend upon shearing forces to separate agglomerates in viscous liquid suspension. They require several steps to go from the powder to the final dispersion, ready for viewing. Some are tedious, especially when a considerable number of samples are involved. A rapid, effective dispersion technique has been devised and applied to a variety of materials, including ?-iron oxide (Figure l;a), silica (Figure l,b), copper chromite (Egnre lp), cuprous oxide (Figure l,dJ, calcium carbonate, and clay. The method
Figure 1.
consists of rubbing between two 1 X 3 inrh microscope slides for 10 to 20 seconds a mixture consisting of a drop of C.P. ethylene glycol and a few milligrams of the powder. The upper slide is removed by sliding it off the lower one and a collodion-filmed grid is placed promptly (film side down) on the mgture; and immediately withdrawn with a tilting motion. The preparation is then complete and the grid ready for viewing. The operation required 15 minutes to go from the bottled powder to a photogrsphic record. Ethylene glycol has a number of properties which make it especially suitable in the present application. Ita viscosity is high; therefore, high shearing forces are generated during the rubbing process. Because it is a polar material, good wetting of inorganic and other polar particles occurs; its polarity also causes good wetting of thus promoting forthe collodion film, mation of a thin continuous layer of the suspension upon retraction of the filmed grid from themixture. Ethylene glycol has a rather high boiling point (198' CJ, but nevertheless has sufficient vapor pressure a t room temperature (0.1 mm. of Hg) so that in thin films it evapo-
b. C.
d. ANALYTICAL CHEMISTRY
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y-iron oxide Silica Capper chromite Cuprous oxide
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solids. Presumably powders with only slight solubility form representative dispersions because the ratio of suspended to dissolved (and later, precipitated) powder would be high. The medium also furnishes a convenient way to prepare polystyrene spheres for viewing: One drop of a Dow standard latex suspension added to 10 ml. of ethylene glycol may be used to coat a filmed grid, as with the inorganic suspensions. An attempt to use this misture as an internal calibration st,andard,by dispersing an inorganic powder in it, was unsuccessful. The shfaring forces caused the suspended inorganic particles to distort the spheres. ACKNOWLEDGMENT
Thanks are due Donald Ii. Stellman for his assista.ncein this work.
Examples of dispersions made with ethylene glycol. 0.
1808
rates very rapidly (possibly assis*d by the vacuum of the microscope). By the time the preparation is observed in the electron microscope, no liquid ethylene glycol remains. The method is tentatively limited to powders insoluble in ethylene glycol bec$use we have not
