Measurements of the self-sustained enhancement of field emission by carbon fiber microemitters

Two types of self-sustained enhancement in field emission by carbon fibers are described. In the first, the field is increased until the emission current switches from zero to between 1 and 10 μA. Next the field is reduced, but not so far that the current would drop. Then the current remains for several hours to several days, with transient increases from the 10 μA to between 14 and 22 μA. It is believed that the transients are caused by the activation of new microtips on the fiber surface. These effects were noted when the carbon fiber tip was mounted in a closed glass vacuum bulb pumped by barium getters, and also in a vacuum system using the combination of a molecular drag pump and ion pumps. The second type of enhancement occurs under ultrahigh vacuum conditions, during in situ thermal treatment of the carbon fiber tip while the emission current is about 2.5 μA. A specially built cathode assembly enables heating the tip to ∼725°C. After continuous heating at 570°C for 20 to 35 h, the current suddenly increases to between 13 and 25 μA. This enhancement is reversible if the emitted current is kept at the newly increased value for at least 30 min. The current–voltage characteristics at several temperatures were recorded and analyzed. Similar field-forming phenomena were previously observed with Molybdenum and ZnO–W tips.