INORGANIC REPLICATION IN ELECTRON MICROSCOPY 813 



The granularity barely discernible in the enlargement, which again shows 

 a slight imperfection in focusing, is attributable to plastic. Resolution is 

 perhaps 100 A. 



Metallic film replicas could easily be used to replicate surfaces in sealed- 

 off tubes. For example, chromium can be plated on a tungsten wire which is 

 suitably mounted in the tube, and thoroughly outgassed during pumping, 

 the surface to be replicated being shielded from the Cr source during the 

 process. Later it is evaporated to form the repHca. As an illustration, Fig. 

 12 is from a Cr repHca of the activated surface of an oxide-coated cathode. 

 It was actually prepared by evaporation at a pressure of 2 X 10~^ mm and 

 not in a sealed-off tube, but the suggested technique is certainly practicable. 

 No films were observed in shadows in this replica, which was less than 



'•4 



Fig. 12 — Chromium replica (te = lOOA, da = 30°) of surface of an oxide-coated cathode. 

 One-step repUca. Whitish areas are due to impurity in the oxide (probably silica) re- 

 deposited on the replica. 



100 A thick with da = 30°. However, the film was very flimsy and exhibited 

 a large number of cracks, usually originating in shadows (e.g., the elongated 

 black area). The film was freed from the surface by dissolving the oxide in 

 dilute acid; since no plastic mold was involved, the fine scale features are 

 characteristic of the oxide surface. The whitish areas (near bottom) are 

 due to some impurity in the oxide redeposited on the rephca, probably silica 

 from the ball-milling process to which the original barium carbonate powder 

 had been subjected. RepHca resolution is perhaps 100 A, although suitable 

 features to test higher resolving power are not present. Shadow edges indi- 

 cate instrumental resolution less than 50 A. 



In Figs. 13 and 14 the use of germanium as a repHca ting material is il- 

 lustrated. Germanium is easily evaporated from a conical carbon crucible 

 supported and heated by a conical helix of tungsten. As in the case of silica 

 and silicon monoxide, the thickness of the resulting film is only roughly 

 known. For the repHca of Fig. 13, 2 mg of Ge at 8 cm distance, 6a = 30°, 



