Selective Oxidation of y:- Brass 



323 



Fig. 7. Electron ditVraction pattern ofa-brass withZnO. 

 Fig. 8. Two relative orientations between a-brass and ZnO. 



Fig. 9. Corresponding electron micrograph of Fig. 7. 



Fig. 10. Electron micrograph for fmal stage of oxydalion for 

 a-brass. 



Fig. 1 1. Secondary diffraction effect for Cu.,0. 



and the small black points are distributed uniformly 

 on the surface of a-brass. 



Furthermore, when a-brass is heated in some bad 

 vacuum, one may see the agglomerations with the 

 definite external shape as shown in fig. 6, which 

 can be considered as CuaO. 



Formation of ZnO. — When heating the a-brass 

 over 450 C in the electron microscope, ZnO appears 

 in the state of single crystals. Fig. 7 shows 

 the electron diffraction image of ZnO which was 

 obtained by heating a-brass at 450 C during 30 

 min. The larger spots correspond to a-brass and the 

 other spots are due to ZnO. The arrangement 

 shows that ZnO develops as a hexagonal prism 

 which has the situation of perpendicularity to the 

 cubic face of a-brass. The diagonal of (0001) of 

 ZnO is parallel to the direction of [100] of a-brass. 

 Fig. 8 shows schematically the two relative orienta- 

 tions between ZnO and a-brass, that is, 



(000 1 )z„o// (00 1 ),.brass and [ 1! 20]znO, \ [ 1 TOJa-brass (4) 



and 



(0001)zno//(00I),.b,.assand [1 I 20]z„o//[l lOUrass- (5) 



The diffraction spots around the direct electron beam 

 in hg. 7 show the above two orientations. 



Discussion. — ( 1 ) Relative orientations between t/ie 

 oxides and y.-lvass. The appearance of the orienta- 

 tion (I) can be understood easily by the fact that 

 oxygen atoms enter in the lattice of a-brass to form 

 CU2O. The orientations (2) and (3) seem to devel- 

 op after the formation of the orientation (I): copper 

 atoms traverse into the layer of oxide already pro- 

 duced and at the surface of the oxide layer copper 

 atoms combine with oxygen atoms. 



One of the authors has earlier shown that the 

 oxidation of brass starts in the direction of the 

 most dense atomic plane, that is (100) plane for 

 a-brass and (III) plane of /^-brass [7]. 



As in the case of Cu.O, the direction [1120] 

 of ZnO appears parallel to the direction of the 

 most dense atomic plane (110) of a-brass, and the 

 plane (0001) of ZnO appears for the same reason 

 parallel to the plane of (III) for CuaO as the 

 result of the replacement of copper atoms by zinc 

 atoms at the superficial layer. 



The development of the orientations (4) and (5) 

 can be explained in the same manner for the case of 

 CuoO. It is also explained by the hypothesis of 

 "rotational slip" which was proposed by Wilman 

 [8]. 



The important role of the direction [110] of a-brass 

 is also explained and its evidence can be observed 

 in the electron microscope as shown in fig. 10, in 

 which ZnO appears with some unknown substance 

 at the final stage of the oxidation of a-brass at 



Fig. 12. Schematic representation of secondary diffraction 

 effect due to ZnO. The small black and white circles corres- 

 pond to the orientations (4) and (5) respectively. (Cf. fig. 7.) 



