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Studies on the Mechanism of Operation of the L Cathode. II
1.Rittner, Ahlert, and Rutledge, J. Appl. Phys. 28, 156 (1957), preceding paper.
2.J. J. Lander, J. Am. Chem. Soc. 73, 5794 (1951).
3.Hughes, Coppola, and Evans, J. Appl. Phys. 23, 635 (1952).
4.E. S. Rittner, Philips Research Repts. 8, 184 (1953).
5.In reference 4 the small (then unknown) heat of formation of from has been neglected. With the use of the barium pressures reported in Sec. V, this quantity has been computed to be and minor corrections to the thermochemical estimates have been based upon this result.
6.J. J. Lander, J. Am. Chem. Soc. 73, 5893 (1954).
7.The heat of formation of from has been assumed identical to the corresponding case for tungsten.
8.O. Kubaschewski and E. L. L. Evans, Metallurgical Thermochemistry (John Wiley and Sons, Inc., New York, 1956), second edition.
9.D. L. Schaeffer and J. E. White [J. Appl. Phys. 23, 669 (1952)] have concluded that the evaporant consists only of barium metal, based upon comparison of the work function of a precleaned tungsten wire covered with a monolayer or a multiatomic layer of evaporant ( 2.02 ev, respectively) with literature values
9.[A. L. Reimann, Thermionic Emission (Chapman and Hall, Ltd., London, 1934)] for a Ba‐W surface (1.58 ev) and for bulk barium (2.11 ev).
9.However, the literature cited is somewhat old and the values differ materially from the more recent ones: 2.15 ev for and 2.51 ev for bulk barium [G. Hermann and S. Wagener, The Oxide‐Coated Cathode (Chapman and Hall, Ltd., London, 1951), Vol. II, p. 98].
10.The oxygen content of the evaporant should strictly be included in M. However, the mass rate of Knudsen flow for pure BaO is higher by only 6%. Since the emerging stream is only about half BaO and since Ba traverses a significant (unknown) fraction of its total path as atoms before acquiring oxygen, the error introduced from the use of is negligible.
11.The plugs were prepared by pressing tungsten powder in a die and sintering at a high temperature in hydrogen. The porosity was varied by altering the pressure, temperature, and firing time. The relationship shown in Fig. 1 is not universal since α is not a single‐valued function of porosity but depends more fundamentally upon the number and size of the pores.
12.The activation energy determined from the slope of line B, 3.0 ev, is slightly lower, which might be explained as follows: If the cathode is incompletely activated, the minimum arrival rate of barium required for complete coverage may be as much as fifteen‐fold higher than curve D.1 Since the slope of D is much lower than that of A or B, the contribution of the adatom evaporation component may not be negligible relative to the Knudsen flow component in relatively fresh, low‐porosity cathodes at lower temperatures, resulting in too low a slope of the single line drawn through the experimental points.
13.Knudsen flow was deemed unimportant by Schaeffer and White [J. Appl. Phys. 23, 669 (1952)]; however, they considered only the flow of BaO vapor, apparently not realizing that the pressure of free Ba could greatly exceed the BaO vapor pressure.
14.Measurements by R. H. Ahlert.
15.A search for the poisoning substance has been based on the hypotheses that it might consist of oxygen from thermal dissociation of or of reaching the emitting surface by volatilization, either of which could account in principle for the increased evaporation rate at the end of life. However, further, experiments have ruled out both possibilities, and the identity and origin of the poisoning agent still stand as major unsolved problems.
16.However, if the cylindrical sides of the tungsten plug are not polished to close the pores, loss of via reaction (3) becomes negligible for a high‐porosity plug. Thus, although polishing reduces the evaporation rate by a factor of about 2, the net improvement in life is only about 1.3 fold.
17.However, the slight disparity of 0.1 to 0.2 mg exceeds experimental error. The origin of this extra Ba has been traced to reduction of by residual impurities in the tungsten. The Ba pressure is too low, however, for these impurities to play a significant role during cathode life.
18.G. E. Moore and H. W. Allison, J. Chem. Phys. 23, 1609 (1955).
19.Moore, Allison, and Morrison, J. Chem. Phys. 18, 1579 (1950).
19.These workers have expressed the composition of the reaction product formed in Eq. (17) as but more recent work by Rooksby and Steward [J. Appl. Phys. 22, 358 (1951)] has shown that it is
20.However, during the first 25 hr of life the instantaneous evaporation rate is substantially higher than the average value and declines rapidly with time, possibly because of incomplete solid solution formation in the oxides immediately after breakdown.
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