ELECTRON IVIICKOSCOPY 



these films arc useful only in affording infor- 

 nuitioii about the properties of thin films as 

 such, although they may give indications of 

 changes which might occur in bulk materials. 



l^ajioratcd films of iron, subsequently 

 nitridcd, austcnitized, and quenched to mar- 

 tensite, have been examined by Pitsch (2) 

 by transmission electron microscopy. The 

 martensitic product and the mechanism pro- 

 posed for its formation differ markedly from 

 those in bulk material. This is due to the 

 absence of a constraining lattice on either 

 side of the thin film. 



Takahashi and his co-workers (3-6) have 

 prepared evaporated foils of aluminium- 

 copper alloy by simultaneous evaporation of 

 the two metals. These have been examined 

 by transmission electron microscopy and 

 electron diffraction in the as-deposited con- 

 dition and after heating in the microscope. 

 Again, the structure of these evaporated 

 alloys differs from that of the bulk alloy. 



Deposition from Solution. Large thin 

 single-crystal flakes of gold have been pre- 

 pared by Suito and Uyeda (7) by reduction 

 of a dilute auric chloride solution. The thin 



o 



crystals (100-200 A thick and several mi- 

 crons across) showed trigonal and hexagonal 

 habit and grew parallel to (111). The value 

 of such thin crystals in giving information 

 about bulk properties is limited. 



Electrotleposition. Two methods have 

 been used to produce thin elect rod eposits 

 for examination in the electron microscope. 

 The first, developed by Weisenberger (8), 

 consists in using thin carbon supporting 

 films as conducting electrodes onto which 

 the metal is electrodeposited. This method 

 can be used to in\'estigate the nucleation of 

 electrodeposits, but beyond this the tech- 

 nique is severely Umited. 



The second method, first used by Weil 

 and Read (9), consists in electroplating 

 nickel onto copper or zinc and stripping the 

 thin electroplate from the cathode. This 

 techniciue was appHed by Reimer (10) to the 

 study of the epitaxial growth of electrode- 



posits. Apart from the investigation of the 

 structure of electrodeposits, this technique 

 has little application. 



Thinning Molten Drops by Surface 

 Tension. Takahashi and Kazato (11) have 

 developed a technique for the preparation of 

 thin metal foils from molten material. Foils 

 are made by dipping an elliptical loop (major 

 axis = 1 cm., minor axis = 0.3 cm.) of wire 

 into molten metal and withdrawing it at the 

 rate of 2 cm. /sec. This must be done in an 

 inert atmosphere, when dealing with metals 

 that oxidize readily in air, but otherwise the 

 technique should be applicable to most met- 

 als and alloys. Alloys of tin and lead, alumin- 

 ium and silver, and aluminium with tin or 

 copper have been investigated by Takahashi 

 and his co-workers (6, 11, 12). It was found 

 that the microstructure of the thin parts of 

 the foil was not typical of bulk material, but 

 that the structure of the thicker portions 

 resembled the microstructure of the bulk 

 alloy. This shows that the transition from 

 thin foil to bulk behavior occurs in the 



o 



thickness range of a few thousand Ang- 

 stroms. However, other properties and other 

 alloys may show a different lower limit of 

 thickness for truly bulk behavior, and it is 

 dangerous, therefore, to extend results ob- 

 tained from such foils to bulk material. 



Deformation jVIethods 



Thickness Reduction by Mechanical 

 Work. Some very ductile materials such as 

 gold, silver, and platinum can be beaten into 

 foil '^lOOO A thick. Such foils are so heavily 

 distorted as a result of the deformation that 

 they are virtually useless for pro\'iding data 

 on the properties of the metal. Beaten gold 

 foils were examined in the electron mi- 

 croscope by Hirsch, Kelly, and Menter (13), 

 but the examination yielded only limited in- 

 formation. 



Thin Sections Cut from Bulk Mate- 

 rial with an Ultramicrotome. An ultra- 

 microtome utilizing thermal expansion for 

 advancing the specimen and the magneto- 



182 



