USE OF MELD EMISSION ELECTRON MICROSCOPE 921 



the regions surrounding the 100 plane, such as the 611 or 310 regions, is 

 greatly reduced. Since the mobihty of W atoms on 211 and 110 planes is 

 greater than that on 100 planes, we conclude that the forces required to 

 move a W atom on a 211 or 110 plane from a position of equilibrium to a 

 neighboring position of equilibrium are less than those required to move an 

 atom on a 100 plane. A study of models of these planes leads to the same 

 conclusion. The field effect reduces the displacement work on all these 

 planes, and hence we would expect that the 211 and 110 planes would 

 change their shape faster than the 100 plane. The W atoms involved in such 

 changes pile up in regions near their respective planes and thus increase the 

 local field in such regions. Since the rate of migration increases rapidly 

 with the field, these regions will grow at a still faster rate than before. 

 Hence we would expect that such regions would pile up W atoms at the ex- 

 pense of other regions in which the migration rate started out more slowly. 

 The experimental results, interpreted in this way, lead to the conclusion 

 that high fields can result in movement of W atoms over distances of several 

 thousand Angstroms. 



We have made about five observations in which this effect continued even 

 at room temperature. If by temperature and field treatment one obtains a 

 pattern in which the emission from a few small spots materially exceeds 

 that of other regions, and if the temperature is then reduced to 300°K while 

 the voltage is kept on for hours, it is found that one or two of these spots 

 will grow in size and intensity while other spots and regions get relatively 

 less intense. 



In such cases and in all cases of enhanced local field emission, the pattern 

 can be brought back toward the normal condition by merely glowing the 

 point at temperatures near 1000°K. The more the pattern differed from the 

 normal one, the lower the temperature required to observe changes back 

 toward the normal. One instance of the tendency to approach a normal pat- 

 tern is that of the last photo in Fig. 5. A series of photos showing this tend- 

 ency is given in Fig. 6. 



Effect of Temperature in Changing an Abnormal to a 

 Normal Pattern 



Figure 6 shows first a pattern of normal clean W for 2400°K; then fol- 

 lows a pattern produced by treatment at 1200°K with 8900 volts applied 

 for 90 min. During this time the emission increased from 15 to 31 micro- 

 amps; the next four patterns show the effect of glowing at successively 

 higher temperatures for about an hour. A comparison of photos b and c 

 shows that at 900°K the changes are shght : only a few of the brighter spots 

 near the perimeter of the 110 and 211 planes have decreased in intensity. 



