use of field emission electron microscope 917 



Effect of Temperature in Determining the Size of 110, 

 211 & 100 Planes 



Figure 4 shows the effect of glowing the point at temperatures from 2600 

 to 1200°K. In each case the temperature was held at a constant value until 

 an approximate steady state was reached. When the photographs were 

 taken, the applied voltages were adjusted slightly so as to maintain a con- 

 stant average screen intensity. After the 1200°K glowing, the 2400°K was 

 repeated. All the films came from the same pack and were developed to- 

 gether. Other tests showed that the result for a given temperature did not 

 depend upon temperature treatments that preceded it. 



From this series of tests we conclude: (1) A single crystal of tungsten 

 which is approximately a paraboloid with a radius of curvature of 3 X 10~^ 

 cm and which has been heated for approximately an hour at 2400°K, as- 

 sumes a surface configuration in which 110, 211, and 100 planes develop. 

 The remainder of the surface consists of small facets or crystal planes of the 

 order of 100 A across. (2) The 110 planes are rectangular in shape with 

 rounded corners; the 211 planes are slightly elliptical; the 100 planes are 

 circular. (3) As the temperature of glowing decreases from 2600 to 1200°K 

 the diameter of the 100 plane increases from 260 A to 500 A; the major 

 axis of the 211 plane increases from 400 A to 750 A; the 110 plane changes 

 only sHghtly in size. (4) At 1200°K small 310 and 111 planes develop and 

 step-like structures develop in the 111 zone between the 110 and 211 planes. 

 (5) The rate at which the surface changes from one steady state configura- 

 tion to another decreases with temperature. Below 1050°K this rate be- 

 comes too slow for convenient observation. (6) At 1050°K in one hour the 

 211 planes enlarge and the W atoms migrate in the 111 direction on this 

 plane; the excess W is deposited on the adjoining paraboloidal surface. 



Effect of High Fields and Temperature in Determining 

 Surface Configuration 



Figure 5 shows a series of photographs in which normally clean W was 

 kept at 1400°K while approximately 8000 volts was appHed to the anode 

 for one minute to 39 minutes. The last photograph was taken after the W 

 was heated to 1430°K for three minutes without an applied voltage. All 

 photographs were taken with the W at room T. The voltage was adjusted 

 until the total field emission was 30 microamps. As the treatment progresses: 

 the 211 and 100 planes enlarge, while the 110 planes change their shape; 

 the emission density from the 100 and surrounding regions decreases while 

 that near the HI and regions surrounding the 110-211 planes increases; 



