EI.KCTRON Mir.HOSCOPY 





Fig. 2. Aluminum oxide replica of aluminum 

 showing structure produced by the anodization 

 process. X20,000. (After Welsh) 



In the case of carbon the material is evapo- 

 rated by passmg a high electric current 

 through tAVO pointed carbon rods lightly 

 sprung together in vacuo. Resistance heating 

 at the points causes the carbon to evaporate. 

 Carbon can also be deposited by a "gas dis- 

 charge" method (8), but this is not in gen- 

 eral use. 



Replica techniques employing evaporated 

 materials can be divided into two main 

 groups: single-stage methods and multi- 

 stage methods. The former are most suitable 

 for particulate specimens and the latter for 

 specimens such as metal surfaces. The two 

 basic procedures using evaporated carbon 

 are shown in Figure 3. In a typical single- 

 stage technique (9) (Figure 3) a particulate 

 suspension is dried onto a glass microscope 

 slide, which is then placed in a vacuum 

 chamber and coated with the replicating 

 material. The replica film, with the particles 

 still embedded in it, is floated onto a water 

 surface. It is next transferred to the surface 

 of a bath containing a solvent for the parti- 

 cles. When the particles have dissolved, the 

 remaining replica film can be washed, by 

 floating on water, and mounted on an elec- 

 tron microscope support grid. This replica 

 is then ready for examination in the electron 



microscope. If it is made from silicon monox- 

 ide or carbon it will follow the surface to a 

 very high degree of accuracy. Figure 4a 

 shows photographic halide crystals repli- 

 cated in this way. 



In two-stage methods (10), the specimen 

 is coated with a layer of plastic (generally 

 "Formvar") from a solution in a suitable sol- 

 vent, and then backed with a further thick 

 layer of another plastic, for example, "Bedac- 

 rjd." The double plastic film is dry-stripped 

 by means of adhesive tape, and then placed 

 in a A^acuum unit. The structure surface of 

 the "Formvar" is coated with silicon monox- 

 ide or carbon, and after removal from the 

 plant, the tape is placed in a solvent for the 

 backing plastic; after this has been dissolved, 

 the "Formvar"/carbon or "Formvar"/sili- 

 con monoxide film is separated from the tape 

 and mounted on grids. Finally, the "Form- 

 var" is washed away, leaving a replica film 

 on the grid ready for examination. 



There are a number of variants to these 

 techniques, for example, thick layers of 

 evaporated metals can be used instead of 

 "Formvar" in the first stage of a two-stage 

 process. Sometimes it maj^ be more advisable 

 to use a thermoplastic such as polystyrene 

 for the first stage (11). Of course, a piece of 

 metal can be coated with carbon and the 

 carbon separated by dissolving the metal 

 (12). This constitutes a single-stage process 

 for a metal specimen but has the disadvan- 

 tage that the surface structure is destroyed. 



Extraction Replicas 



A replica technique specially designed for 

 use with metallurgical specimens has been 

 developed by Smith and Nutting (13) and 

 is known as the carbon extraction replica 

 technique. The aim is to replicate the surface 

 of the matrix and to extract anj^ precipitates. 

 Thus the arrangement and size distribution 

 of the precipitates can be found and their 

 constitution determined by means of electron 

 diffraction. 



To prepare such a replica the metal surface 



232 



