REPLICA AND SHADOWING TECHNIQUES 



replicas is not high and is in the region 50- 

 100 A. A "Formvar" rephca of pearhte struc- 

 ture in steel is shown in Figure 1. 



Oxide Replicas 



Replicas made from thin oxide films (3) 

 are almost entirely confined to aluminum 

 and aluminum alloy specimens. The follow- 

 ing method is widely used. 



An aluminum specimen is anodized in a 

 suitable bath (e.g., Na2HP04— 48 gm, H2SO4 

 (cone.) — 2 ml water — 400 ml at 30 volts for 

 4 mins with a platinum cathode) so that 

 a thin oxide film a few tens of angstrom 

 units in thickness is produced on the surface. 

 This oxide film accurately conforms to the 

 surface topography of the original specimen 

 and it can be separated from the specimen 

 by immersing the whole in mercuric chloride 

 solution. Mercury forms at the interface 

 between the aluminum metal and the oxide, 

 thus releasing the oxide film into the solu- 

 tion. The film is then washed in water to 

 remove mercuric chloride and mounted on an 

 electron microscope support grid. While the 

 method is very simple, careful control of 

 anodizing conditions is rec^uired. Aluminum 

 oxide films have a high electron trans- 

 parency, are non-crystalline, thermally sta- 

 ble, and hence are eminently suitable for the 

 preparation of replicas. It is unfortunate, 

 however, that they possess a self-structure 



o 



of 100-200 A (4) and can only be used in a 

 limited number of cases. Their applications 

 have been widened by means of the so-called 

 aluminum-pressing replica technique (5) 

 in which a piece of aluminum is compressed 

 against a prepared metal surface under very 

 high pressure. The soft aluminum takes up 

 the structure of the metal to a reasonable 

 degree of accuracy. An aluminum oxide 

 replica can then be prepared from the alumi- 

 num pressing. 



The mechanism of image formation in 

 aluminum oxide replicas is c^uite different 

 from that in plastic replicas. The thickness 

 of the oxide film, measured at right angles 



Fig. 1. "Formvar" replica of pearlite struc- 

 ture in carbon steel. X-4500. (After Agar and 

 Re veil) 



to the surface structure, is uniform so that 

 contrast is produced by differences in thick- 

 ness, in the direction of the electron beam, 

 due to the local changes in slope of the 

 replica. Once again it can be seen that this 

 type of replica is almost entirely confined 

 to metal specimens. Figure 2 shows an alumi- 

 num oxide replica of a typical anodizing 

 structure and an etchpit in aluminum. 



Vaeuuni -Deposited Replicas 



A number of evaporated materials can be 

 employed to make electron microscope rep- 

 licas. Two of the most suitable are silicon 

 monoxide (6) and carbon (7). The manipula- 

 tive techniques are basically similar for both 

 materials and depend upon the type of speci- 

 men to be examined. The vacuum evapora- 

 tion techniques are, however, quite different. 

 In the case of silicon monoxide the material 

 is evaporated from a molybdenum foil boat. 



231 



