Thermionic Emission from and Surface Structure of Single Crystals of Tungsten

Citation

Smith, George Foster (1952) Thermionic Emission from and Surface Structure of Single Crystals of Tungsten. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7BA0-QT39. https://resolver.caltech.edu/CaltechETD:etd-06292004-102615

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Surface structure of and thermionic emission from single crystals of tungsten grown in 5-mil wires by the Robinson method (2) were investigated. Photomicrographic evidence points to the existence of "plateaus" on the surface normal to the (110), (112), and (001) directions of a crystal heated sufficiently in vacuum to evaporate 3.5 to 4 percent of the wire diameter; these plateaus were about 11A, 6A, and 7A, respectively, in angular width. Superimposed upon the plateaus was to be found a "shingle" structure previously observed by Nichols (7) in the (110) direction. A crystal wire subjected to only mild heat treatment (22 hrs. at 1980[deg]K. and 20 min. at 2280[deg]K.) also showed both types of structure, indicating that evaporation cannot be the only mechanism involved. All wires were heated by a.c. exclusively, to avoid the "d.c. etch." (5,6,7). Polar plots of the thermionic emission vs. angle about the wire show "flats" of emission, confirming the existence of the plateaus. New values of the thermionic constants for different crystal directions were measured on a crystal having a minimum of shingling, in the same tube used by Nichols (7), modified to eliminate an anomalous effect discovered during preliminary tests. The modification did not appreciably alter the emission constants for the (111), (112), (116), and (00l) directions, which constants are in good agreement with Nichols, values (7,8). A second anomalous effect contributed to the currents measured in the low emission (110) direction, rendering normally obtained values erroneous (including those of Nichols'). Although the true (110) constants could not be obtained, an upper limit for the emission is characterized by [...] = 4.72, [...] = 9.7, and a 5.26 volt estimate is deduced for [...]. Contact potentials, determined by comparing retarding potential curves taken in different directions, failed to agree in general with apparent work function differences. The disagreement can be explained by (a) assuming surfaces with temperature dependent work functions, in which case the temperature derivatives obtained also account for departure of the [...] values from 120; or (b) assuming patchy surfaces with temperature independent patch work functions; two-patch surfaces are demonstrated which can account for the observed data. There was no evidence in general for patchiness of the single crystal surfaces investigated; however, the usual tests cannot conclusively deny the existence of patchiness.

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