PETKOGRAPHIC THIX SECTIONS 



the grains and was not distributed uniformly 

 over the full carrier surface (Figure 1). 



Structure of Urea Prills. In one case, 

 the reason for the abnormal, weakness of 

 urea prills was sought. Thin sections revealed 

 that the crystal bundles of which the indi- 

 vidual prills were composed did not, in the 

 case of the weak prills, meet along uniformly 

 narrow contact zones in the normal manner 

 but were irregularly separated by relatively 

 wide void spaces (Figure 2). This observa- 

 tion could be correlated with operating con- 

 ditions. In another case, the prill structures 

 of a number of competitive brands of urea 

 were examined by means of thin sections. In 

 three brands, round cross-sections and com- 

 pact structures characteristic of prills cooled 

 directly from the melt were observed. In the 

 fourth brand, the cross-section was irregular 

 and the rather loosely bonded urea crystals 

 showed a strong peripheral alignment ; it was 

 deduced that these prills were probably 

 formed by mechanical means. 



Examination of Heat Exchanger De- 

 posit. A thick scale removed from a heat 

 exchanger tube was found through thin sec- 

 tion microscopy to consist of a single, readily 

 identifiable species in definite, even bands. 

 The interpretation of the observed structure 

 was that the scale was deposited during dis- 

 crete intervals between which neither erosion 

 nor deposition of scale occurred. These ob- 

 servations could be correlated with plant 

 operating conditions. 



Internal Structure of Crystals. In an 

 investigation of the effects of environmental 

 conditions on the crystallization of am- 

 monium sulfate, internal twinning and other 

 imperfections were studied by means of thin 

 sections. 



Strain Patterns in Resin Test Pieces. 

 Thin sections were helpful in the study of 

 strain patterns in deformed resin test pieces, 

 since (1) strain patterns which were too in- 

 tense for resolution in the full thickness of 

 the specimen were easily resolvable at re- 

 duced thickness and (2) patterns could be 



Fig. 1. Silica gel plugging. Coarse catalyst car- 

 rier pores. (XlOO). 





^t^v^c;^ 





Fig. 2. Mechanically weak urea prills. (X40). 



Fig. 3. Strained Izod specimen — end of notch. 

 Ordinar}^ transmitted light. (X14). 



studied in any desired orientation of the 

 specimen. Figure 3 shows, in ordinary trans- 

 mitted light, a portion of a section approxi- 

 mately .25 mm in thickness of the notch 

 area of a notched Izod impact test specimen 



475 



