Selective Oxidation due to the Heating of the 

 Evaporated Film of a-Brass 



N. Takahashi and K. Mihama 



Yainanashi University, Kofii, and Japan Electron Optics Laboratory Co., Ltd., Tokyo 



It is well known that, when an alloy is heated in 

 the air, one of the constituent metals is oxidized 

 selectively [6]: these conditions are determined by 

 the temperature and the pressure [3]. On a-brass, it 

 has been known that CuoO is formed at a low 

 temperature and ZnO is developed at a high tempe- 

 rature [6]. 



We have prepared the single crystal film of a-brass 

 by evaporation /// vacuo and we have performed the 

 experiment on the selective oxidation by heating in 

 the electron microscope when observing the electron 

 microscopic and the electron diffraction images 

 continuously. (At the place of the specimen chamber 

 of the JEM-5 electron microscope with a four-lens 

 system, the specimen heating or cooling adaptors 

 can be introduced. These adaptors make it possible 

 to study continuously the thermal change of the 

 specimen in the ranges from room temperature to 

 about 1000 C or from about r200 C to the liquid 

 nitrogen temperature respectively.) 



A single crystal of a-brass has been prepared by 

 successive evaporation of copper and zinc /"// vacuo onto 

 the cleavage surface of rock salt which was kept at about 

 400''C. To obtain a single crystal of a-brass, copper must 

 be evaporated prior to zinc without breaking the 

 vacuum. 



«-ii£3i! J-. 



Fig. 1. Electron diffraction pattern of a-brass. 



Fig. 2. Corresponding electron micrograph of fig. 1. 



(3) (2) 



Fig. 3. Electron diffraction pattern of a-brass with CujO. 

 Fig. 4. Three relative orientations between a-brass and Cu.^O. 



Single crystal of oi-brass. — According to the equi- 

 librium diagram of Cu-Zn alloy, the a-phase, which is 

 the solid solution in substitution type, exists between 

 and 40 °o in weights of zinc. 



Electron diflfraction image of the above a-brass 

 film at room temperature is shown in fig. 1 : the 

 plane (100) is always parallel to the cleavage surface 

 of rock salt. Appearances of irrational spots have 

 been observed and discussed by Briick [1], Menzer 

 [5], Laue [4], Gottsche [2], etc. and have been rec- 

 ognized in the present case also. The corresponding 

 electron micrograph is shown in fig. 2. The regions 

 of black and white are due to the interferences of 

 equal inclination of the illuminated electron beam 

 for the specimen and the directions seem to corres- 

 pond to [100] direction or a-brass. 



Formation of Cii^O. — CujO can frequently be seen 

 even at room temperature. The composite film of 

 copper and zinc may be oxidized on account of the 

 water action at the time of dissolution of rock salt. 



Heating the specimen at a temperature below 

 300°C, CuaO appears as shown in fig. 3. Three rela- 

 tive orientations between Cu.>0 and a-brass are 

 observed, that is, 



(00r)cu,o//(001)a.brass and [010]cu,o//[010],_brass, (D 

 (111 )cu.O /(OOl)^.brass and [1 T0]cu,o//[lT0]a-brass (2) 



and 



(111 )cu.O;'/(001 )a-brass and [lT0]cu,o//[l 10]a-brass- (3) 



These three relative orientations are shown sche- 

 matically in fig. 4. The effect of secondary scattering 

 can be observed and it results in the appearances of 

 irrational electron diffraction spots. The correspond- 

 ing electron micrograph is shown in fig. 5. The inter- 

 ference fringes disappear only due to the heating. 







A. 



Fig. 5. Corresponding electron micrograph of Fig. 3. 



Fig. 6. Electron micrograph of CU2O formation in lower 



vacuum. 



