SPECIMEN PREPARATION 





' V 



Fig. 4. UO2 aerosol after transfer. 



box, and cutting a small opening in the bot- 

 tom to admit a microscope slide with 

 "Formvar"-coated specimen screens. 



Procedure for sample preparation : 



A small amount of the sample is placed in 

 an atomizer, the hand bulb remo\'ed and 

 the atomizer attached to a compressed air 

 hne. The nozzle of the atomizer is inserted 

 into the box opening and a short blast of air 

 applied. 



Immediately after, the coated screens are 

 placed on the floor of the "cloud chamber" 

 and the door is sealed. Settling times from 

 20 minutes up to several hours, according to 

 particle size range desired, proved to be suffi- 

 cient. 



With this techniciue, although the agglom- 

 erates are not completely broken up, par- 

 ticles appear in preferential chain-type ag- 

 glomeration with a good percentage of single 

 layers evident; statistical counting, individ- 

 ual particle shape determinations as well as 

 elect ronmicrography are thus facilitated. 



Example: Figure 5 carbon black particles 

 74,000 X. 



J. H. Bender and J. D. Steely 



Sorting and Fractography of Particles 



Particles in the size range of 400 to 325 

 mesh (37-44 m/i), of interest to powder metal- 

 lurgists, are sometimes coated with a dissimi- 

 lar material to enhance or change the basic 

 physical properties and/or form alloys in 

 the sintering process. 



Since statistical counting of coated vs. un- 

 coated particles and determination of the 

 coating thickness were not feasible by stand- 

 ard E.M. techniques, the following method 

 was developed : 



Statistical counting may be accomplished 

 optically by color difference at ^-^1000 X on 

 as-received powders using reflected, polar- 

 ized, full-wave compensated light. Standard 

 samples of the coating and base material are 

 used for reference. Coating thickness may be 

 determined in the following way: 



1. Mix proportional amounts of particles 

 and epoxy resin. 



2. Cast this mixture in a "pencil-shaped" 

 mold. 



3. Cut thin sections from the cured cast- 

 ing with a biological microtome. 



4. Direct stripping plastic rephcas can 

 then be taken from the cut end of the cast- 

 ing. 





0.1 -" 



Fig. 5. Carbon black particles. 4O,00OX. (Los 

 Alamos Scientific Laboratory GMX-1 GT-SITE) 



273 



