Preliminary data indicating thermionic-conversion potentialities for a 111-iridium emitter and collector spaced 0.2 mm apart are presented. These results comprise output densities of current and of power as functions of voltage for three sets of emitter, collector, and reservoir temperatures: 1553, 944, 561 K; 1605, 898, 533 K; and 1656, 1028, 586 K. For the 1605 K evaluation, estimates produced work-function values of 2.22 eV for the emitter and 1.63 eV for the collector with a 2.0-eV barrier index (collector work function plus interelectrode voltage drop) corresponding to the maximum output of 5.5 W/sq cm at 0.24 volt. The current, voltage curve for the 1656 K 111-iridium diminiode yields a 6.2 W/sq cm maximum at 0.25 volt and is comparable with the 1700 K envelope for a diode with an etched-rhenium emitter and a 0.025-mm electrode gap made by TECO and evaluated by NASA

Bair, V. L.

Koeger, E. W.

Morris, J. F.

English

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.R	 NAS/,A% TECHNICAL NASA TMX-73492
R	 MEMORANDUM
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DIMINIODE THERMIONIC CONVERSION 4
WITH 111 — IRIDIUM ELECTRODES S	 e_a
by Erich W Kroeger, Virginia L. Bair, and James F. Morrie
Lewis Research Center
f	 Cleveland, Ohio 44135
September 1976
(NASA-TM- g--',7 3 u 92)	 MiNIODE THERI4I0I1IC N7 °-33	 1
CGNVERSIOld W' :: , 111 --IRIDIUM BLEfCTRODES(NASA)	 11	 p	 $3.50	 CSCZ 201
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G3/75	 057.01 ^.
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1. Report No.
	 ^- 2. Government Accession No, 3, Recipient's Catalog No.
NASA TM X-73492	
—7
4. Title and Subtitle 5. Report Date
DIMINIODE TI-IERMIONIC CONVERSION
6, Performing Organization CodeWITH 111-IRIDIUM ELECTRODES
7, Author(s) 8, Performing Organization Report No,
Erich W, Kroeger, Virginia L. Bair, and James F, Morris E-8888
10, Work Unit No.
9. Performing Organization Name and Address
Lewis Research Center
National Aeronautics and Space Administration
11. Contract or Grant No.
Cleveland, Ohio 44135 13. Type of Report and Period Covered
Technical Memorandum12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration 14, Sponsoring Agency Code
Washington, D. C.	 20546
16. Supplementary Notes
16. Abstract
This report presents preliminary data indicating thermionie-conversion potentialities for a
111-iridium emitter and collector spaced 0, 2 mm apart. 	 These results comprise output den-_
sities of current and of power as functions of voltage for three sets of emitter, collector, and
reservoir temperatures:_ 1553, 944, 561 K; 1605, 898, 533 K; and 1656, 1028; 586 K. 	 For
the 1605 K evaluation, estimates produced work-function values of 2, 22 eV for the emitter
and 1, 63 eV for the collector with a 2 O-eV barrier index (collector work function plus inter-
electrode voltage drop) corresponding to the maximum output of 5. 5 W/cm2 at 0.24 volt.	 The
current,, voltage curve for the 1656 K 111-iridium dlmoniode yields a 6.2 W/cm2 maximum at
0.25 volt and is comparable with the 1700 K envelope for a diode with an etched-rhenium
emitter and a 0.025-mm electrode gap made by TECO and evaluated by NASA.
17, Key Words (Suggested by Authorls)) 18, Distribution Statement
Thermionie Unclassified - unlimited
Diode STAR Category 75
111-Iridium
Diminiode
19, Security Classif, (of this report) 20. Security Classif, fof this page) 21, No; of Pages 22, Price'
Unclassified Unclassified
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t A•
DIMINIODE TI EMMIONIC CONVT 115ION WI`1'11 ill-IRIDIUM 1+'LECTR DES
„-	 by Erich W. hroeger, Virginia L. Bair, and dallies F, Morris
I
Lewis Research Center
SUMMARY
This report presents preliminary data indicating the rm ion ic- conversion
potentialities for a 111- iridium emitter and collector spaced 0.2 mm apart,
r: These results comprise output densities of current and of power as functions
of voltage for three sets of emitter, collector, and reservoir temperatures:
j	 1553, 944, 561 K; 1605, 898, 553 K; and 1656, 1028, 586 K,
	 For the 1605 K
evaluation, estimates produced worli--function values of 2.22 eV for the emitter
,tind 1.63 eV for the collector with 'a 2.O-eV barrier indent (collector work func-
tion plus intereleatrode voltage drop) corresponding to the maximum output of
5.5 NNI/c?m2 at 0.29 volt. 	 The current, voltage curve for the 1656 K 111-1ridium
diminiode yields a 6, 2 W/cm2 maximum at 0.25 volt and is comparable with the
1700 K envelope for a diode with an etched-rhenium emitter and a 0.025-mm
clecstcode gap made by '1ECO and evaluated by NASA.
INTRODUCTION
This report presents preliminary thermionic-conversion results for n
'	
diminiode (ref. 1) with ill.-iridium electrodes separated by 0.2 mm,
	
Making ^ 1
both the emitter and collector of the same bulk material counteracts converter t
vaporization, deposition effects (ref, 2),	 The choice of 111-iridium derives
from "the fact that higher work function (metallic) surfaces are known to pro-
vide a lower work function value in the presence of a specified Cs pressure
Intl substrate temperature" (refs- 3 to 5): 	 The bare work function of ill-
iridium is 5.76 eV; of 0001 rhenium, 5. 34 eV; of 110 tungsten, 5.25 eV; and
0f 1.10- molybdenum, X1.92 eV (ref. ?,)	 So of the _potential metallic electrodes, }
lli i ridium promises the lowest cesiated work functions:, which means greater
z ari ,ent densities from the emitter and greater output voltages from the collec-
to (",	 In addition the vapor pressure of iridium is simular to that of molybdenum
'	 losing- less> than 10 `1 mm a year at 1700'X in vacuum (fig . . 1) .	 Thus, lll­ iridiuzxa
1, an excellent prespect among possible metallic electrodes for Cesium Cher- tJ
nionic converters,
ategory 75
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This preliminary report is the product of initial testing of a variable-gap
diminiode with 111-iridium electrodes: A malfunction terminated the evalua
tion, And subsequent examination revealed irreparable damage as well as a
capillary opening covered by liquid cesium in the end of the reservoir. No 	 r
ether indication of a leak had been apparent. While a replacement diminiode
is in process, the present results can serve to indicate the thermionic-
conversion potentialities of 111-iridium electrodes. To that end these data
represent observed maxiin-im. performances corresponding to three sets of
emitter, collector, and reservoir temperatures, 1553, 994, 561 If; 1605,
898, 553 ll and 1656, 1028, and 586 K.
As a background reference 6 cites references 7 to 10 on cesium diodes
with iridium emitters. References 7 and 8 report 5. 5 W/em 2 for a 1703 K
emitter and a 0,76 mm electrode gap, Reference 9 reveals that a 1643 x
polycrystalline:-iridium emitter 0.23 mm from a: stainless-steel collector
produced 5.8 W/cm2 at 0.3 volt and a maximum of 7 2 W/cm2 at 0,16 volt
with about 2 torr of cesium vapor. Adding 60 torr of xenon increased those
performances to 7.4 W/cm2 at 0.3 volt and a maximum of 8.4 W/cm 2 at 0.2
volt. "As before, high cesium pressure with close spacing yields better high-
current output at lower voltages, and low cesium, pressure with wider spacing
yields higher output at higher voltages" (ref, 9) . Incidentally reference 10
discusses effects of pulsed discharges of the reference 9 diode.
Enlightened consensus indicates that the usually higher performances
found in early thermionic-converter studies probably involved oxygenation...
effects unrecognized at that time. The same sophisticated opinions propose
that additives used in those early studies, particularly inert-gas additions,
generally contained oxygen as Gimpurity. And "the presence of minute
quantities of oxygen (10 to 10 torr) in a cesium thermionic converter re-
sults in improved performance characteristics" (ref, 11). In fact, such obser-
vations received support en masse as more effective quality control in diode
processing led to lower and lower thermionic- conversion outputs. These find-
ings contributed to the initiation of a program that presently explores oxygen
ation of cesiated electrodes (ref. 12), And today a-practical demonstration of
this effect is the increased performance of the TRCO cesium, oxygen, tungsten
diode over that of the standard nonoxygenated version.
Thus the High power densities of the reference 9 diode undoubtedly re-
dounded at least partially from undefined prductive oxygen effects. The re-
sults from the 1.11-iridium, diminiode may also involve some unspecified
w
'1141 film il
13
3
oxygenation. But the absence of leakage symptoms and the presence of per-
formance levels substantially below those of reference 9, indicate probably
Y
negligible additive influences in this diminiode study..
W. E, Frey and R. D. Schaal developed and performed special proce-
dures and conducted research tests necessary to fabricate this diminiode.
1
EQUIPMENT AND PROCEDURES
Reference 1 provides detailed descriptions of all equipment and proce
dares used in the experiments treated in this report.
RESULTS
I^
Figures 2 to 4 show selected data representing maximum performances
observed in limited initial testing of a diminiode with 1114ridium electrodes
spaced 0.2 mm apart. These results comprise output densities of current
and power as functions of voltage for three sets of emitter, collector, , and
reservoir temperatures: 1553, 944, 561 K; 1605, 898, 553 K; and 1656,
1028, 586 K.
Because insufficient data exist to form constant-emitter-temperature
"
	
	 envelopes, the present exemplary curves serve only to imply potentialities
for thermionic converters with both electrodes made of 111-iridium. For
example, outputs reach at least 2.9 W/cm2 at 0.14 volt for a 1553 K
emitter, 5.5 W/cm2
 at 0024 volt for 1605 K, and 6.2 W/em2 at 0.25 volt
for 1656 K.
Work function estimates for the 1605 K test are 2.22 eV for the emitter
and 1.63 eV for the collector. These values compare with 2,38 and 1.;54 eV
for cesiated rhenium at appropriate electrode-to--reservoir temperature
ratios of 2. 9 and 1.6 (ref. 13) . The work functions for this 1605 K 111
iridium diminiode indicate a barrier index (collector work function plus
interelectrode voltage drop) of 2.0 eV corresponding to the 5.5 -.W/cm
maximum.
Figure 5 reveals that the 1656 K current, voltage curve is similar to the
better 1700 K envelope for diodes with etched-rhenium emitters and 0.25-mm
	
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electrode gaps made by TECO and evaluated by NASA (ref. 14),
Such results merit further investigation: Another ill-iridium diminiode'
is now in the fabrication process,
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REFERENCES
1. Morris, J. F„ Energy-Conversion Research and Development with
Diiminic^des.	 First International Conference of Plasma Science.	 IEEE,
} Knoxville, Tennessee, May 1974 (NASA TM X-71533).
2. Morris, J. F.? and Lundholm, J. G.: NASA Thermionic-Conversion Pro-
gram.
	
Eleventh Intersceiety Energy Conversion Engineering Conference,
State ,Line, Nevada, September 1976 (NASA TM X-73430).
3. Swanson, L W. p NASA ERDA TEC-ART (Thezmionic-•Energy- Conversion
Applied-Research-and, Technology) Program Review. Naval Research
1 Laboratory, Washington, D. C< , January 1975.
4. Swanson, L. W.; and Strayer, R. W. : Field Electron-Microscopy Studies
of Cesium Layers on Various Refractory Metals; Work Function Change.
J. Chem, Phys, 4^, 24i:^1 J968).
5. Bell;, A. E.; Bennette, C. J.; Strayer, R. W.; and Swanson, L. W.
= Single Crystal Work Finact ons and Behavior of Various Adsorbates on
;Metal Surfaces.
	
Final Report, NAS3-11820 (1970).
6 . Morris, J, F.: Cesium-Diode Performances from the 1963-to,­1971 Ther-
mionic Conversion Specialist Conferences. 	 NASA TM X- 2589.
7. Talaat, M. E.	 Iridium Thermionic Converter Studies. 	 PIC No. 734,
i
March 1963.
8, Martini, W. R.	 Internal Flame-Heated Thermionic Converters.	 Deport
on the Thermionic Gcz',;vt!^rsion. Specialist Conference (pp. 356-361).
z
1
IEEE, Gatlinburg, Tprx.--ssee, October 1963.
:a
`	 9, Kaplan, C. • and Mer rz(:,aich J. B.	 Xenon Addition Experiment in a
Thermionic Co v( rtr- r.	 Report on the Thermionic ` Conversion Specialist
I
Meeting (pp, 333 ,3 344) .	 IEEE, Cleveland, Ohio, October 1964.
10, Kaplan, C.: Pulsed Discharge Experiments in a Thermionic Converter.
Report on the Thermionic Conversion Specialist Meeting (pp. 326- 332) .
IEEE, Cleveland, Ohio, October 1964,
i
11, Rufeh, F.; Lieb, D.
	
and Fraim, F.. Recent Experimental Results on
Electronegative Additives=
	
Report on the Thermionic Converseion
Specialist Meeting (pp. 25 to 28).
	 IEEE, Palo Alto, California,
1 October 1967.
	
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1v
7
12, NASA Contracts with Thermo Electron Corporation (TECO); NAS7-508,
NAS3-14413, and NAS3-19866
1$. 110sopoulos, G. N..; and E, P. Gyftopoulos, Thermionic Energy
Conversion Volume 1: Processes and Devices. MIT Press, 1973,
14, Upnista, E. J. Smith, A. L.; and Lancashire, R. B., Comparison of
Computer-Acquired Performance Data from Several Fixed Spaced Planar
Diodes, Report on the Thermionic Conversion Specialist Meeting, San
Diego, California, October 1971. (NASA TM X-67935,)
15, Storms, E. K,: NASA ERDA TEC-ART Program Review, University of
Texas, Austin, May 1976,
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Diminiode thermionic conversion with 111-iridium electrodes

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