The emission properties of lanthanum hexaboride in an atmosphere of nitrogen were investigated. The emitter was not poisoned by adsorbed nitrogen. This result should have application to magnetohydrodynamic devices in which electron flow from channel walls is required

Carter, Arlen F.

Wood, George P.

English

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NASA MEMO 2-16-59L
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MEMORANDUM
EFFECT OF ADSORBED NITROGEN ON THE THERMIONIC EMISSION
FROM LANTHANUM HEXABORIDE
By Arlen F. Carter and George 1°. Wood
Langley Research Center
Langley Field, Va.
NATIONAL AERONAUTICS AND
SPACE ADMINISTRATION
WASHINGTON
March 1959
https://ntrs.nasa.gov/search.jsp?R=19980230610 2020-06-15T23:16:44+00:00Z

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
M_ORANDUM 2-16-59L
EFFECT OF ADSORBED NITROGEN ON THE THERMIONIC EMISSION
FROM LANTHANUMHEXABORIDE
By Arlen F. Carter and George P. Wood
SUMMARY
The emission properties of lanthanum hexaboride in an atmosphere of
nitrogen were investigated. The emitter was not poisoned by adsorbed
nitrogen. This result should have application to magnetohydrodynamic
devices in which electron flow from channel walls is required.
INTRODUCTION
A study of possible applications of magnetohydrodynamics, with
emphasis on plasma accelerators, has shown that one of the walls of the
channel through which the plasma flows must become an emitter of elec-
trons to close the current circuit through the plasma. Reference l, in
fact, cites the problem of obtaining current flow through a wall as being
of particular significance. A plasma accelerator of one type that has
been proposed requires that there be no discharge and therefore requires
low electric gradients. It follows that field emission or emission by
positive ion bombardment of the surface cannot be used. Thus, thermionlc
emission must be considered. The usual condition of a good vacuum, how-
ever, cannot be realized, but the emission must occur in the presence of
a gas at pressures up to atmospheric. Air would be a desirable choice
for the gas, of course, but at the high temperatures (3,000 ° Kelvin)
required for thermal ionization of seeding material, oxidation is too
much of a problem. Nitrogen would be a good second choice because it is
a major constituent of air. The presence of nitrogen, however, frequently
poisons (greatly reduces) the electron emission from a surface whether the
emission be thermionic, field-induced, or photoelectric. This poisoning
is thought to be caused by the gas molecules adsorbing on the surface and
becoming polarized in such a direction that the work function of the sur-
face is increased. The emission current I is an inverse exponential func-
tion of the work function _ divided by the temperature T (I _ e-_/kT),
and the work functions of good emitters and the temperatures at which
they can be used (which are limited by evaporation and/or melting of the
emitter) have values such that their ratio is rather large. This ratio
islarge enough that the change in its value due to poisoning by adsorbed
gases can change the emitted current by a factor of several powers of 10.
(See ref. 2.) Some typical examples of poisoning of emitters are:
(1) the adsorption of oxygen on thoriated tungsten at lO -14 millimeter of
mercury (ref. 2), (2) the adsorption of a few parts per million of oxygen
on barium-strontium oxides (ref. 2), and (5) the adsorption of nitrogen
on tungsten (refs. 3 and 4). There is evidence that nitrogen in some
cases decreases the work function (ref. 5), in some cases increases it
(refs. 3 and 4), and in some cases has no effect (ref. 6). Available
evidence and prevailing opinion were against the expectation of any
appreciable rate of thermionic emission of electrons in the presence
of nitrogen at pressures of the order of an atmosphere.
The solution to the problem of poissnlng appeared to be so necessary,
however, that considerable attention was given to the problem. In vacuum
technique, surfaces can be outgassed by heating them, in a good vacuum, to
temperatures that approach their melting points. Glass, for example, that
softens at 420 ° C is usually outgassed by heating to 400 ° C. Tungsten,
which melts at 5,350 ° C, can be outgassel by heating to about 2,700 ° C.
This procedure results in rather rapid oatgassing. Much slower rates,
of the order of hours, result when the t_mperature is much less than the
melting point. It was thought that possibly a thermlonic emitter could
be depoisoned (outgassed) by heating it _lmost to its melting point, even
in the presence of gas, rather than in a good vacuum. In the experiments
reported herein, the emitter was not heated to a high enough temperature
to drive off the adsorbed nitrogen rapidly, but the attempt led to the
discovery that nitrogen not only did not poison the emitter that was used
(lanthanum hexaboride) but actually enhaaced the emission rate.
APPARATU B
Lanthanum hexaboride was chosen as the emitter. It has been studied
and developed by Lafferty (ref. 7), who nade a thorough investigation of
its properties, of which there are a numoer of desirable ones. It is
commercially available and ready for use in powder form, but, when sin-
tered, it can be machined. It is therefore physically rugged. It is
chemically stable, unaffected by oxygen _r moisture at room temperature,
easy to activate, and has an emission rate per unit surface area that,
at a given temperature, exceeds that of _ny other known emitter except
those of the barium-strontium oxide type. It has a reasonably high
melting point (2,175 ° C) and the lanthanlm evaporated from its surface
is completely replenished by diffusion f_om within at temperatures up
to 2,000 ° C.
The emitter was mixed with amyl acetate to form a paste which was
packed into a graphite cup. The inside diameter of the cup was
30.207 centimeter and the area of the emitter was thus 0.034 square cen-
timeter. The cup was held in a small hollow molybdenum cylinder. (See
fig. i.) The cathode-to-anode gap was set at 0.08 centimeter. Ample
heating power was provided by two lO0-watt tungsten-filament heaters
mounted adjacent to the molybdenum cylinder. Three molybdenum cylin-
ders surrounded the assembly to serve as radiation shields. The anode
was made of tantalum, and all leads were of tantalum wire. Cleaning
of the components and the assembly was effected by washing with trichlo-
roethylene (reagent grade). The complete assembly was mounted on a heat-
resistant glass base for ease in handling and was set under a glass bell
J ar.
The vacuum system uses a mechanical pump backing an oil diffusion
pump. A liquid-oxygen cold trap keeps any oil vapor from reaching the
test section. The vacuum system was so designed, cleaned, and assembled
as to avoid leaving volatile contaminants inside. Brass was used through-
out except for the stainless-steel base plate and the glass bell Jar.
Before assembly, each brass component was cleaned thoroughly by etching
in a solution of sulfuric acid and potassium dichromate and washing in
distilled water and in methyl alcohol (chemically pure) to remove any
traces of acid. During assembly, all soft-soldered Joints except the
final ones were flushed with distilled water a_ cleaned with alcohol to
remove any acid flux. On silver-soldered joints, dilute sulfuric acid
was used to remove the borax flux and the Joints were washed with dis-
tilled water and then with alcohol.
Three gages were used to cover the desired pressure range. A
Knudsen gage covered the range from 1 × lO -6 millimeter of mercury to
1 × 10-3 millimeter of mercury, a thermopile gage from 1 × 10-3 milli-
meter of mercury to 1 millimeter of mercury and a diaphragm-type dial
gage from 1 millimeter of mercury to atmospheric pressure. A needle-
type leak valve with a Teflon seat was designed and constructed to admit
nitrogen in any desired quantity. The temperature was measured by an
optical pyrometer that was directed at the graphite cup through holes
in the radiation shields. The radiation from the graphite is not black-
body and therefore a temperature correction was made. The value of
emissivity was taken as 0.8 which, with a 4-percent loss by reflection
from each of the glass bell-Jar surfaces, gives an effective emissivity
e of 0.74. This value is used in Wien's law to obtain the true temper-
ature T from the apparent temperature Tapparent by the equation
(ref. 8)
1 h logee 1
-- = +
T c2 Tapparent
4where h = 0.65 _ and c2 _ hc _ 1.438 :< 104 _-°K (h is Planck'sk
constant, k is Boltzmann's constant, _id c the velocity of light).
The probable error in true temperature i3 +_5° C becaRseof the uncer-
tainty in emissivity (e = 0.74 +-0.05) _id about +-25_ C because of the
uncertainties in reading the pyrometer.
RESULTSANDDIS_ISSION
First, the lanthanum hexaboride was activated and partially sin-
tered by heating to 1,375° C for about 25 minutes at a pressure of about
5 × l0 -4 millimeter of mercury. Then it_ emission rate was measuredas
function of anode potential at 1,340° C. The results are shownin the
lower curve of figure 2. The saturation value shownin reference 7 for
the sametemperature and area is 13 milliamperes. There are a numberof
possible reasons that the emission rate per unit surface area was less
than that obtained by Lafferty. The lan-_hanumhexaboride was not very
well sintered and it could easily be flaked off the surface with a sharp-
pointed instrument. Also, the anodevol-_age could not be raised high
enough to obtain saturated emission because of anode overheating.
Pulsing equipment for the anodevoltage would solve this problem but it
was not readily available.
After the data shownin the lower c1_ve of figure 2 were obtained,
prepurified nitrogen (99.996 percent pur,_) was admitted over a period
of 22 minutes until a pressure of 735 millimeters of mercury was reached.
The temperature was held at 1,340 ° C; thc_ anode voltage, at lO volts;
and the current was measured as function of pressure. (The results are
shown in figure 5 and are discussed subsc_quently.) Then the system was
exhausted to its initial pressure of about 5 × 10 -4 millimeter of mercury.
Current-voltage data were then obtained _nd the results are shown by the
upper curve of figure 2. Comparison of _,he two curves indicates that
exposure to nitrogen increased the therm_.onic current by 2 or 3 times.
At a later time, after the system had been exposed to atmosphere,
it was evacuated and the emitter was rea_tivated by heating for 25 min-
utes at 1,315 ° C. Then the heaters were turned off. After an hour and
a half, nitrogen was admitted until the pressure reached 740 millimeters
of mercury and the system was allowed to sit for a half hour. Then
l0 volts were applied to the anode and h_ater power sufficient to raise
the temperature to 1,522 ° C was suddenly applied and current-time data
were taken. These data are shown in fi_e 5. The current rises quickly
to about 50 microamperes and then decrea:_es. In 7 minutes it falls to
13 microamperes. The decrease in currenl, as time increases is believed
to be due to the loss of adsorbed nitrogen from the emitter.
5In what chemical form the nitrogen may have been adsorbed was not
ascertained in the present investigation. Dr. J. M. Lafferty of the
General Electric Research Laboratory, Schenectady, N. Y., has pointed
out to the authors that the adsorbed nitrogen probably combines with
lanthanum on the surface to form there a surface layer of LaN.
Dr. Lafferty has found that bulk LaN has, at a given temperature, a
greater emission rate than LaB 6. This result would explain the increase
in emission current that results from adsorption of nitrogen and would
also explain the decrease in emission current with increasing time as
probably being due to evaporation of LaN from the surface.
Further information on the effect of nitrogen is shown in figure 4.
After the emitter had been activated, nitrogen was admitted to a pressure
of 1 millimeter of mercury and this pressure was held for 20 minutes.
Temperature was kept at 1,250 ° C during this time. Then the system was
evacuated to 3 x lO -4 millimeter of mercury. With an anode potential of
lO volts the variation of current with time at different temperatures
was determined. At 1,257 ° C, the current was about 210 microamperes.
When the temperature was suddenly raised to 1,476 ° C, the current rose
to 1,050 microamperes and then dropped linearly in 5 minutes to 800 micro-
amperes. This drop is believed to be due to the loss of adsorbed nitro-
gen. When the temperature was returned to 1,257 ° C, the current was
little more than half its original value at that temperature. This result
seems to indicate that driving off the adsorbed nitrogen decreased the
emission rate, probably by increasing the effective work function. Then
as time increased, the current increased_ this condition probabd_y indi-
cated that at the rather low temperature of 1,25T ° C nitrogen _as again
adsorbed on the surface and the work function was therefore decreased.
Further results are shown in figure 5, which shows current as a
function of pressure. The maximum of 390 microamperes for an anode
potential of i0 volts (which, with an anode-cathode distance of
0.08 centimeter gives a gradient of 125 volts per centimeter) is set
by the simple version of Child's law for space-charge-limited emission
in a vacuum. The experimental results are seen to approach this value
at low pressure and to fall considerably below this value as the pres-
sure increases. The electron mobility, of course, decreases as the
mean free path decreases and becomes small compared with the anode-
cathode distance.
The calculated curve shown in figure 5 results from Child's law
with consideration of electron mobility. Cobine's derivation (ref. 9)
gives
J
9.95 x IO -14 I¢_2
x3
where J is current density, K is the mobility, V is the voltage,
and x the gap width, all in practical cgs units. Theoretical values
of the mobility K, derived either on the basis of kinetic theory or of
Ramsauer cross sections, are shown in reference i0 to disagree with
experimental values. The experimental values, taken from figure 1.24
of reference i0, were therefore used in obtaining the calculated curve
shown in figure 5.
The experimental values are slightly great._r than the calculated
values and differ by a factor of about 2 at the most. The two curves
are very nearly parallel, however, and again show that nitrogen does
not poison the emission. When the mobility is calculated, the tempera-
ture of the gas must be taken into consideration. The temperature was
not actually known but was assumed to be the s_e as the temperature of
the cathode.
It should be mentioned that the lanthanum i_exaboride and the graph-
ite cup were removed and a clean graphite cup ilserted in the molybdenum
cylinder after the data were taken. The temperature was raised to 1,335 ° C
to determine whether there was any emission current from the components.
With an anode potential of I0 volts, a reading of 37 microamperes was
noted. This value is about i0 percent of the r,_ading obtained under the
same conditions with lanthanum hexaboride in th,_ cup. Inasmuch as the
purpose of the investigation did not require theft absolute measurements
be made, this discrepancy was not taken into ac_:ount in the data.
CONCLUDING REMARKS
The thermionic electron emission from lant_mum hexaboride is not
poisoned by the presence of nitrogen at pressur,_s up to atmospheric.
This finding should be useful in practical appl_[cations of magnetohydro-
dynamics in which electron flow from channel wai_ls is required.
Langley ReSearch Center,
National Aeronautics and Space Administrat_lon_
Langley Field, Va., November 25, 1958.
REFERENCES
I. Resler, E. L., Jr., and Sears, W. R. : The Prospects for Magneto-
Aerodynamics. Jour. Aero. Sci., vol. 25, no. 4, Apr. 1958,
pp. 259-245, 258.
2. Nottingham, Wayne B.: Thermlonic Emission. Handbuch d. Physik,
Bd. XXI, Sprlnger-Verlag (Berlin), 1956, pp. 1-175.
3. M_ller, Erwin W.: Field Emission Microscopy. Physical Methods in
Chemical Analysis, vol. III, Walter G. Berl, ed., Academic Press,
Inc. (New York), 1956, pp. 135-182.
4. Eisinger, Joseph: Electrical Properties of Nitrogen Adsorbed on
Tungsten. The Jour. Chemical Physics, vol. 28, no. i, Jan. 1958,
pp. 165-166.
5- Hughes, Arthur Llewelyn, and DuBridge, Lee 01vin: Photoelectric
Phenomena. McGraw-Hill Book Co., Inc., 1932.
6. Weissler, G. L.: Photoionization in Gases and Photoelectric Emission
From Solids. Eandbuch d. Physik, Bd. XXI, Sprlnger-Verlag (Berlin),
1996, pp. 504-382.
7- Lafferty, J. M.: Boride Cathodes. Jour. Appl. Physics, vol. 22,
no. 5, Mar. 1951, pp. 299-309.
8. Dike, Paul H.: Pyrometry by Radiation Methods. High-Temperature
Technology, I. E. Campbell, ed., John Wiley & Sons, Inc., c.1956,
pp. 335-357.
9- Coblne, James Dillon: Gaseous Conductors. Theory and Engineering
Applications. Dover Publications, Inc., c.1958.
I0. Meek, J. M., and Craggs, J. D.: Electrical Breakdown of Gases. The
Clarendon Press (Oxford), 1953.
8GRAPHITE CUP
/ i _-T.&NTALUM ANODE
MOLYBDENUM i /
CYLINDER_ _ /
_- TUNGSTEN HEATERS
_-- MOLYBDENUM
RADIATION SHIELDS
Figure i.- Details of emission dioce construction.
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Figure 2.- Variation of anode current with anode potential for LaB 6
before and after exposure to N2. Temperature, 1,340 ° C; pressure,
5 × 10 -4 millimeter of mercury.
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Effect of Adsorbed Nitrogen on the Thermionic Emission from Lanthanum Hexaboride

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