Comparison of performance data envelopes for thermionic diodes with various tungsten or rhenium emitters and niobium or molybdenum collector

Lancashire, R. B.

Manista, E. J.

Smith, A. L.

English

NASA Technical Reports Server

N A S A  T E C H N I C A L  
M E M O R A N D U M  
N W  -
NASA TM X-67935 
COMPARISON OF COMPUTE!?-ACQUIRED PERFORMANCE DATA FROM 
SEVERAL FIXED SPACED PLANAR DIODES 
by E, J. Manista, A. Lo  Smith, and R. Bo Lancashire 
Lewis Research Center 
Cleveland, Ohio 
TECHNICAL PAPER proposed for  presentation -at 
the Thermionic Conversion Specialists Conference sponsored by 
the Institute of Electrical and Electronics Engineers 
San Diego, California, October 4-6, 1971 
https://ntrs.nasa.gov/search.jsp?R=19710027119 2020-03-23T16:07:45+00:00Z
COMPARISOW OF COMPUTER-ACQUIRED PERFORMANCE DATA 
FROM SEVERAL FIXED SPACED PLANAR DIODES 
E. J. Msnista, A. L. Smith, and R. B. Iancashire 
NASA L e w i s  Research Center 
Cleveland, Ohio 44135 
ABSTRACT 
=I Performance data are  compared for thermionic 
diodes with various tungsten or rhenium emitters 
w and niobium or molybdenum collectors. The planar 
converters have guard-ringed collectors and a 
fixed space of 10 mils (0.254 mm). The data were 
acquired using a computer. The parameters are  the 
temperatures of the emitter "E, collector Tc, and 
cesium reservoir TR. The composite plots have 
constant and varying Tc or TR or both. The 
envelope and composite plots having constant Q 
are  presented. 
(D 
(D 
The diodes were tes ted a t  increments between 
1500 and 2000 K f o r  the  emitters, 750 and 1100 K 
for the  collectors, and 520 and 650 K f o r  the  
reservoirs. 
voltage curves were obtained for  each diode. 
An average of 450 individual current, 
INTRODUCTION 
More power a t  lower temperatures is  the  goal 
f o r  nuclear thermionic diodes. 
improvement means intensive tes t ing  of the best 
exis t ing emitters and collectors, promising new 
electrode materials, and additives. To insure 
success, performance mapping must cover o f f -  
design as  well as  optimum operating conditions -- 
with special  a t tent ion given t o  s t a b i l i t y  problems. 
Providing t h a t  
Moreover, predictions of the output charac- 
t e r i s t i c s  of nuclear thermionic power system 
designs require t h a t  the properties of individual 
converter modules be taken into account. Indeed, 
a l l  converters i n  the network cannot be assumed t o  
be operating a t  nearly ident ical  conditions i f  a 
re l iab le  estimate of the system performance is 
desired. 
Eventually, analytically based correlations 
may reduce the need f o r  the detailed performance 
mapping of thermionic converters. Although tha t  
day may be near, the L e w i s  Research Center has 
accepted the need f o r  massive thermionic converter 
data accumulation and has computerized its per- 
formance mapping t e s t  procedure as  reported by 
Breitwieser, Manista, and Smith i n  reference 1. 
A comparison of the  performance data enve- 
lopes of four different  combinations of electrode 
materials are reported herein. Specifically, the 
planar electrode combinations included i n  the  
comparison are  emitters of physically and chemi- 
cal ly  vapor-deaosited-tungsten against guard- 
ringed niobium collectors and etched-rhenium 
emitters against guard-ringed niobium and molyb- 
denum collectors. More detailed data presentations 
of the performance maus of these electrode combi- 
nations are  available i n  references 2-5. The 
converter structure is  described by Speidel and 
Williams i n  reference 6. 
A l l  converters were tes ted with emitters a t  
g ' s  from 1500 t o  2000 K i n  50 K s t e w ,  collectors 
a t  Tc's fron 750 t o  1100 K i n  50 K steps, and 
cesium reservoirs a t  various TR'S between 520 and 
650 K. Composite d o t s  of the  output current 
density and voltage having constant TIJ, and 
varying TC and TR, are  presented. 
TEST VEHICLF 
A cross section of the planar, guard-ringed 
converter i s  shown i n  figure 1. The interelectrale 
spacing of 10 mils (0.254 mm) was determined from 
indicating surfaces on the emitter and collector 
body. Emitter, collector combinat ions invest i g a M  
were e t  ched-rhenium, niobium; etched-rhenium, 
molybdenum; chemically vapor-deposited-tungsten, 
niobium; and physically vapor-deposited-tungsten, 
niobium. Specific detai ls  of the emitter and 
collector electrodes and t h e i r  preparation are  
given la te r .  !the converters were fabricated and 
f i l l e d  with cesium by Therm0 Electron Engineering 
Corporation. 
Emitter Preparation 
The rhenium emitters (Re) w e r e  fabricated 
from wrought, powder metallurgy material. After 
grinding the surfaces f l a t  (#600 g r i t ) ,  the 
emitters were vacuum annealed a t  2270 K for  about 
one hour. 
f i n a l  converter assembly. 
They were then electro-etched before 
The physically vapor-deposited-tungsten 
emitter (PVD-W) was prepared on an arc-cast 
tungsten substrate. The emitter disc  was ground 
f l a t  and sandblasted before the coating urocess. 
About 1 t o  3 m i l s  of tungsten was then evaporated 
onto the substrate. During the coating process 
the mean substrate temperature was held a t  about 
2600 t o  2800 K while the evaporator was held a t  
3400 K. 
The chemically vapor-deposited-tungsten emitter 
( 0 - W )  was prepared from the  chloride on an arc- 
cast tungsten substrate. The emitter was ground 
f l a t  (jY600 g r i t )  and vacuum annealed a t  2270 K for  
about one hour. It was electropolished a t  10 t o  
12 volts i n  a 3 t o  5 percent solution of NaOH t o  
complete i t s  surface preparation. 
Presented a t  the 1971 Thermionic Conversion Specialists Conference, Oct. 4 through 6, 1971, San Diego,CA 
1 
Collector ?reparation 
The surfaces of the niobium (Nb) collectors 
and guard-rings were machined t o  a f i n a l  f in i sh  of 
about 63 microinches. 
cleaned i n  a solution of trichloroethylene pr ior  
t o  converter assembly. 
The pieces were then 
A molybdenum (Mo) collector and guard-ring 
During t h e  
were fabricated by evaporating about 0.2 mils o f  
molybdenum onto the  niobium pieces. 
vacuum deposition process the  substrates were held 
a t  870 K while the  molybdenum filament was run a t  
2470 K. 
TEST FACILITY 
Test Stations 
Each converter was mounted i n  one of s i x  
vacuum test s ta t ions which are  coupled t o  a 
central  instrumentation control panel. Each 
s ta t ion has its own s e t  o f  emitter, collector, and 
cesium-reservoir heat supplies. Thermal balance 
of  the  collector and reservoir is achieved through 
conduction t o  water l ines .  
~ n s  trumentat ion 
The current developed i n  the converter was 
measured by the  voltage drop across e i ther  a 
0.01- or  0.1-ohm, low-inductance, precision shunt. 
The emitter, collector potent ia l  difference was 
measured at the external shroud of the  converter. 
No corrections were made f o r  the  voltage drop i n  
the emitter support shroud since it is  approxi- 
mately 1.8 millivolts per ampere per square 
centimeter o f  electrode surface. The 1.55-square- 
centimeter collector face determined the current 
density. 
nected t o  the c i rcu i t  on the  opposite s ide O f  the  
shunt from the collector. 
The guard-ring was e lec t r ica l ly  con- 
Collector and- cesium reservoir temperatures 
were sensed by sheathed Chromel, A l u m e l  thermo- 
couples embedded i n  t h e i r  respective converter 
structures. The couples were continuous and were 
brought through the  vacuum wall of  the  test 
s ta t ion  i n t o  a common ambient cold junction zone. 
The temperature of  the ambient zone was sensed by 
a Chromel, A l u m e l  couple tha t  was referenced 
electronically t o  273 K. 'Itro couples were in-  
serted a t  each location. The cesium reservoir 
couples were i n  t h e  copper block surrounding the  
copper tube containing the cesium (see Figure 1). 
Collector couples were within 3.05 millimeters 
(125 m i l s )  of the collector surface. 
A l u m e l  standard calibration for a l l  fou r  couples 
was ver i f ied by an i n  s i t u  comparison against a 
Chromel, A l u m e l  reference couple. 
The Chroxel, 
The emitter temperature was sensed by a 
sheathed high-temperature couple of tungsten, 
5-percent-rhenium versus tungsten, 26-percent- 
rhenium. The couple was inserted t o  a depth of  
6.35 millimeters (250 m i l s )  from the emitter sub- 
s t r a t e  edge and 3.3 millimeters (130 mils) from 
the active face o f  the emitter (see Figure 1). 
Compensating lead w i r e s  were attached t o  the 
couple on the  in te r ior  o f  the  t e s t  chamber and 
were brought out t o  a room temperature junction. 
The high-temperature couple was calibrated i n  s i t u  
against a black-body cavity (length-to-diameter 
r a t i o  of f ive)  i n  the edge of the emitter. "ne 
black-body cavity was observed through a window 
i n  the t e s t  s ta t ion with a disappearing filament 
opt ical  pyroxeter. 
were calibrated against a National Bureau o f  
Standards (NBS) tungsten s t r i p  lamp. The maximum 
uncertainty associated with the observed tempera- 
tu re  is  approximately f10 Kelvin degrees. This 
estimate includes the accuracy o f  the NBS cal i -  
bration, the reversal capabi l i t ies  of the  opt ical  
pyrometer and observer, and the effect  o f  the 
approximate black-body cavity i n  the emitter. 
The temperature difference between the black-body 
cavity and the active face of the emitter i s  
considered negligible for  tungsten and is a maxi- 
mum o f  20 K f o r  rhenium. These estimates are  
based on a one-dimensional heat balance o f  the 
radiation across the interelectrode gap and the 
heat conducted through the emitter. This model 
neglects any heat flow through the emitter- 
support shroud since the electron bombardment 
filament was designed t o  nul l i fy  t h i s  heat path. 
Electron cooling and heating e f fec ts  on the 
surface temperatures a re  a l so  negligible since 
the time interval  over which the load is  applied 
is  very short and t h e  converter is held a t  a low- 
current, retarded-voltage condition between 
tests.l 
i q  negligible. 
The opt ical  path and pyrometer 
The contribution o f  gaseous conduction 
E33C PROCEDURE 
The computer-controlled data acquisition 
system is programed t o  t r igger  the variable load 
a t  up t o  s i x  different emitter temperatures 
during a given t e s t  interval  o f  about 20 seconds. 
(The computer program was developed by E. Wnista 
and C. Kadm of  NASA-kwis.)  The system performs 
t h i s  task by monitoring the emitter temperature 
(the high-temperature couple output). When TE 
reaches a predetermined value, t h e  load is 
triggered. Temperature levels a t  which the load 
is  t o  b.e triggered are introduced into th8 pro- 
gram by the operator as  independent input data. 
The data recording program, synchronized with the 
application o f  the variable load, samples the J,V 
charaqteristics o f  the  converter a t  9 points 
during the load variation of approximately 10 
milliseconds duration. Sample and hold amuli- 
f i e r s  coordinate i n  time the collector current 
and collector, emitter uotent ia l  difference. The 
data recording program then waits u n t i l  the  f u l l  
load variation i s  over (by monitoring the load 
driving source) before recording the temperature 
data associated with the par t icular  sweep. 
Between sweeps, these analog temperatures are 
converted by the computer t o  t h e i r  values i n  
degrees Kelvin and s re  printeil out t o  a id  the 
operator in set t ing aDproDriate t e s t  conditions. 
The converters were mapped by fixing the 
2 
temperatures of the  cesium reservoir and the 
collector and by heating or cooling the emitter 
t o  t h e  predetermined values. 
trigger levels, emitter temperatures between 
1500 and 2000 K were observed i n  50 K increments. 
The temperature of the  collector or of the  cesium 
reservoir was then changed and the preceding pro- 
cedure repeated. Table I give the nominal 
temperature conditions a t  which J , V  data were 
obtained for  each of the converters tested. 
Through programmed 
COI’NERTER 
Cesium Reservoir 
% 
Temperature, 
K 
RAW PFZSEN!CATION 
- 
ETCHED-RHENIUM, F’VD-TUNGSTEN, CVD-TUNGSTEN, ETCHED-RHENIUM, 
NIOBIUM NIOBIUM NIOBIUM MOLYBDENJM 
525, 555, 575, 550, 575, 600, 550, 575, 600, 540, 560, 580, 
590, 600, 610, 600, 625, 650 625, 650 625, 650 I C20, 640 
Since the loca l  computer can s tore  and 
r e c a l l  only a limited number of successive 
sweeps, the data are transmitted t o  the Lewis 
Central Computing Center for  storage on magnetic 
tape and for some engineering calculations. The 
data from each converter are sorted into groups 
of common emitter temperatures and are displayed 
i n  order of ascending TE on microfilm output. 
Both J , V  and P,V (power density) curves are  
displayed on t h i s  output, with the  J and P 
scales being determined by the maximum of each 
sweep. 
Central Computer: The data are grouped by 
comon emitter and collector temperatures and by 
common emitter and reservoir temperatures. The 
computer plots a l l  the  sorted J , V  data on com- 
posite plots and displays them on the microfilm 
output. The current density scales on these 
plots  are  a l l  common and limited t o  a maxiumum J 
of 30 amperes per square centimeter. 
Two additional sor t s  are  done by the 
Figure 2 i l l u s t r a t e s  the density of data 
points used t o  define the observed optimum J , V  
envelope from a converter at a given emitter 
temperature. 
accumulated a t  that  emitter temperature f o r  the  
converter and thus folds i n  the e f fec t  of both 
collector and cesium reservoir temperature 
variations. 
?he presentation uses a l l  the  data 
Constant emitter temperature envelopes are  
given i n  Figures 3 t o  6 f o r  each of the  converters 
t o  i l l u s t r a t e  the performance obtained as a func- 
t ion  of emitter temperature. Comparisons of the  
performance as a function of electrode combination 
a r e  given i n  Figures 7 t o  10 f o r  emitter tempera- 
tures  of 1700, 1800, 1900, and 2000 K. 
Over most of the  J , V  operating points the 
etched-rhenium, niobium diode was superior. The 
exception being the region of lorr current density, 
near end below 6 amwres per square centimeter. 
A t  these conditions, a l l  the  diodes give nearly 
ident ical  output. For current densities greater 
than 6 amperes per squere centimeter the diodes 
can be ordered i n  terms of decreasing oerformance 
as etched-rhenium, niobiwl; chemically vapor- 
deposited-tungsten, niobium; and a toss-up between 
the physically vapor-deoos i ted-  t ungs t en, niobium 
and the etched-rhenium, molybdenum. 
An anomaly and poor performance is  shown by 
the etched-rhenium, molybdenum diode. One would 
expect its performance t o  have been equal t o  or  a 
l i t t l e  be t te r  than tha t  obtained from the etched- 
rhenium, niobium diode. The rhenium, molybdenum 
and the rhenium, niobium current, voltage curves 
are  being examined i n  an attempt t o  uncover the 
primary reasons f o r  t h i s  discrepancy. 
CONCLUDING FtEMARIzS 
Detailed thermionic converter performance 
data from four different electrode combinations 
were compared a t  constant emitter temperatures of 
1700, 1800, 1900, and 2000 K. The etched-rhenium, 
niobium electrode combination was found t o  give 
superior performance. Considerable lower perform- 
ance was found for  the etched-rhenium, molybdenum 
converter. Detailed examinations of the individ- 
ual  J , V  curves are  being made i n  order t o  ascer- 
t a i n  the nature of t h i s  anomaly. 
Table I 
Nominal temperature conditions a t  which data were. acquired for  the fixed 
spacing converters. All converters were tes ted a t  emitter temperatures of: 
1500, 1550, 1600, 1650, 1700, 1750, 1800, 1850, 1900, 1950, 2000 K. An 
exception was the etched-rhenium, niobium converter which was not tes ted 
a t  l5OO K. 
3 
References 
1. R. Breitwieser, E. J. Manista, and A. L.Smith, 
"Computerized Performance Mapping of a Thermi- 
onic Converter With Oriented Tungsten 
Electrodes," Proc. IEEE Thermionic Conversion 
Specialists &nf., Carmel, Calif., Oct. 1969, 
pp 90-99, and NASA X-52714, 1969. 
2. R. B. Lancashire, "Computer-Acquired Wrform- 
ance of an Etched-Rhenium, Niobium Planar 
Diode," Proc. IEEE Thermionic Conversion 
Specialists Conf., Miami, Fla., Oct. 1970, 
pp 487-491, and NASA TM X-2262, 1371. 
3. R. B. Lancashire, "Computer-Acquired Perform- 
ance Data from a Physically Vapor-Deposited- 
!Cungsten, Niobium Planar Diode," HASA 
IM X-2330, 1971 
4. A. L. Suith, "Computer-Acquired Wrformance 
Data from a Chemically Vapor-Deposited- 
Tungsten, Niobium Planar Diode," NASA 
TM X-2373, 1971. 
5. E. J. Mnista ,  "Computer-Acquired Performance 
Data from an Etched-Wenium, klybdenum 
Planar Diode,'' ( t o  be published as NASA TI4 X),  
1971. 
6. T. 0. Speidel and R. M. Williams, "Fixed- 
Space Planar Thermionic Diode With Collector 
Guard Ring, " 
Specialists Conf., 1968, pp 113-117. 
Roc: IEEE Thermionic Conversion 
4 
9 
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CONVERTER CONFIG U RAT1 ON 
Figure 1. - Converter conf igurat ion.  
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ELECTRODE POTENTIAL, V 
Figure 2. - Computer processed plot of cur ren t  density, voltage data at con- 
stant emitter temperature of 1750 K for the etched-rhenium molybdenum 
converter. Collector temperature, 750 to 1100 K; cesium reservoir tem- 
perature, 540 to 640 K; interelectrode space, 0.254 mill imeters (10 mils). 
4 
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NASA-Lewis-Com'l 


Comparison of computer-acquired performance data from several fixed spaced planar diodes

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