Metal Bonding and Interactions in High Temperature Systems

29

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 4, 2016 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch029

Physics and Chemistry of Cesium Thermionic Converters E. J. BRITT Rasor Associates, Inc., Sunnyvale, C A 94086

Thermionic converters are high temperature devices which utilize electron emission and collec tion with two electrodes at different temperatures to convert heat into electric power directly with no moving parts. Most thermionic converters oper ate with a plasma of positive ions in the interelectrode space to neutralize space charge and permit electron current flow. Both the plasma character istics and the surface properties of the electrodes are controlled by the use of cesium vapor in ther mionic diodes. Recently, surfaces with coadsorbed cesium and oxygen have been developed, which have stable work functions of about 1.0 eV. Some of these surfaces operate without loss of oxygen up to 1200 K. Use of these surfaces as advanced collectors in ther mionic converters is currently being investigated. Negative ionic species of cesium and other materials can be formed by interaction with low work function surfaces present in converters. Quadrupole mass spectrographic analyses have confirmed the existence of Cs and other molecular negative ions in ther mionic converter environments. The implications of these negative particles on converter performance is being studied. -

Thermionic energy conversion i s a method o f converting heat d i r e c t l y to e l e c t r i c i t y . A metal e l e c t r o d e , the emitter, i s heated s u f f i c i e n t l y to emit e l e c t r o n s , as shown i n Figure 1. The e l e c t r o n s cross a narrow i n t e r e l e c t r o d e gap and a r e c o l l e c t e d by another metal e l e c t r o d e , the c o l l e c t o r . Heat i s removed from the c o l l e c t o r so that i t s temperature i s lower than the emitter, and the e l e c t r o n s s t r i k i n g the c o l l e c t o r cannot be returned except by

0097-6156/82/0179-0421$06.50/0 ©

1982 American Chemical Society

Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


Figure 1.

Scheme of thermionic diode for directly converting heat into electricity.

ELECTRICAL LOAD

INTERELECTRODE


29.

BRITT

Cesium

Thermionic

423

Converters

flowing through the e x t e r n a l c i r c u i t . The flow of e l e c t r o n s cons t i t u t e s an e l e c t r i c current which d e l i v e r s power to the l o a d . The unique p r o p e r t i e s of cesium play a c r u c i a l r o l e i n the o p e r a t i o n of thermionic converters. Cesium f u n c t i o n s both as adsorbed atomic l a y e r to produce the r e q u i r e d work f u n c t i o n s on the e l e c t r o d e s , and as a plasma medium to form C s ions which n e u t r a l i z e space charge i n the i n t e r e l e c t r o d e r e g i o n . Cesium i s d e s i r able as the plasma medium because of i t s low i o n i z a t i o n p o t e n t i a l and l a r g e atomic mass. Since the s u r f a c e adsorbed l a y e r s are continuously evaporating and being r e p l e n i s h e d by cesium atoms r e f l u x i n g from the vapor, the s u r f a c e p r o p e r t i e s are very s t a b l e . Thermionic converters have operated with no change i n performance f o r more than 5 years. The c y c l e has many s i m i l a r i t i e s to a Rankine c y c l e which uses e l e c t r o n s as a working f l u i d . U n l i k e the normal Rankine c y c l e , however, the working f l u i d ' s "heat of evaporation", approximately the emitter work f u n c t i o n , and i t s "heat of condensation", approximately the c o l l e c t o r work f u n c t i o n , can be v a r i e d i n the thermionic converter. This f e a t u r e provides the converter with great f l e x i b i l i t y i n matching the operating c o n s t r a i n t s of any p a r t i c u l a r system.


+

Elementary Converter

Model

A simple a n a l y t i c a l model of thermionic converter performance must be made before the impact of converter performance on system behavior can be s t u d i e d . F o r t u n a t e l y , a very simple model of converter performance has been found to be s u f f i c i e n t l y accurate f o r t h i s purpose. The i d e a l thermionic diode serves as the b a s i s f o r t h i s model. Motive diagrams and converter current v o l t a g e c h a r a c t e r i s t i c s f o r an i d e a l diode are shown i n F i g u r e 2. In the motive diagram (f) and , and the i n t e r e l e c t r o d e spacing d. The s a t u r a t i o n and back-emission current d e n s i t i e s a r e c a l c u l a t e d by the Richardson-Dushman equation. The shooting method i n v o l v e s guessing those values which a r e unknown i n the boundary c o n d i t i o n s a t one s i d e , i n t e g r a t i n g over to the other s i d e using a standard stepping technique, and i t e r a t i n g on the unknown values u n t i l the boundary c o n d i t i o n s at the f a r s i d e are s a t i s f i e d w i t h i n a s p e c i f i e d degree of accuracy. T h i s method i s u s e f u l when the d i f f e r e n t i a l equations to be i n t e g r a t e d are s t r o n g l y nonlinear. In t h i s model, i n t e g r a t i o n i s begun at the c o l l e c t o r edge and proceeds toward the emitter. A flow chart which shows the c a l c u l a t i o n a l l o g i c f o r s o l u t i o n of the IMD-4 computer program i s given i n F i g u r e 6. The c a l c u l a t i o n contains three nested i n t e r a t i o n loops. The inner most loop adjusts the value of the e l e c t r o n temperature a t the c o l l e c t o r edge of the plasma T , by checking the c o n t i n u i t y of heat flux. The second loop, which adjusts the value of the plasma d e n s i t y a t the c o l l e c t o r edge n , i t e r a t e s u n t i l the e l e c t r o n c u r r e n t a t the emitter s i d e has the c o r r e c t value. During each 2

Q

E

c

R

E

c

e C

c


432

METAL BONDING AND INTERACTIONS

ENTER DATA, COMPUTE FIXED PARAMETERS


T

e C

GUESS AND n

c

SOLVE COLLECTOR BOUNDARY CONDITIONS

INTEGRATE EQUATIONS FOR n(x), T (x) V ( x ) , J ( x ) , Q (x)

Figure 6.

Flow chart for the computer program logic in IMD-4 analytical converter model.


29.

BRITT

Cesium

Thermionic

Converters

i t e r a t i o n of the plasma d e n s i t y n , the inner loop on T c must be reconverged. There i s a l s o a small loop on the emitter s i d e boundary c o n d i t i o n which i s executed during every shooting attempt. T h i s loop t e s t s f o r the e x i s t e n c e of a double sheath and then matches the boundary c o n d i t i o n s at the emitter edge. F i n a l l y both values of n and T Q achieve s u f f i c i e n t l y small e r r o r s ; and the r e s u l t s are c a l c u l a t e d , p l o t t e d and p r i n t e d f o r a g i v e n J-V p o i n t . The next p o i n t on the J-V curve i s then c a l c u l a t e d by incrementing the current d e n s i t y . An example of the r e s u l t s obtained with the IMD-4 model i s given i n F i g u r e 7 which shows a computed I-V curve ( s o l i d l i n e ) compared with an experimental curve (dashed l i n e ) . The c a l c u l a t e d motive diagram at v a r i o u s p o i n t s along the curves i s a l s o shown i n the f i g u r e as i n d i c a t e d . For p o i n t s © to © i n the obstructed r e g i o n of the J-V curve, there i s a double sheath b a r r i e r at the emitter edge of the plasma. The emitter sheath i n t h i s r e g i o n of the c h a r a c t e r i s t i c i s approximately constant i n s i z e , equal to about 0.81 to 0.83 v o l t s . The c o l l e c t o r sheath does not vary much e i t h e r i n t h i s r e g i o n . I t remains at about 0.26 to 0.3 v o l t s . Points © to © are i n the q u a s i - s a t u r a t i o n region of the J-V curve, and monotonic sheaths are present on both s i d e s of the plasma. Schottky e f f e c t s on the emitter are i n c l u d e d i n the c a l c u l a t i o n i n t h i s r e g i o n which, along with the i o n c u r r e n t s , give the slope to the c h a r a c t e r i s t i c i n the q u a s i s a t u r a t i o n both the emitter sheath and the c o l l e c t o r sheath i n c r e a s e i n s i z e as the current i s r a i s e d and the plasma becomes more dense. e

c

c


433

Converter

e

Performance

C a l c u l a t e d Power and E f f i c i e n c y . The s i m p l i f i e d a n a l y t i c a l models of thermionic c h a r a c t e r i s t i c s have been used to p r o j e c t the converter e f f i c i e n c y and power d e n s i t y with the b a r r i e r index as a parameter. These p r o j e c t i o n s are shown i n Figures 8 and 9 as f u n c t i o n s of the emitter temperature. The dashed l i n e s i n these two f i g u r e s are f o r a constant current d e n s i t y of 10 A/cm . I f the current d e n s i t y i s adjusted to maximize the e f f i c i e n c y a t each temperature, the c a l c u l a t e d performance i s represented by the s o l i d l i n e s . T y p i c a l present generation thermionic converters operate with V near 2.0. I g n i t e d mode converters i n l a b o r a t o r y experiments have demonstrated p r a c t i c a l o p e r a t i o n with 1.85 < V < 1.90. Other l a b o r a t o r y devices with a u x i l i a r y sources of ions and/or s p e c i a l e l e c t r o d e surfaces have achieved V < 1.5, but u s u a l l y not under p r a c t i c a l operating conditions. 2

B

B

B

Hardware Experience. The e f f i c i e n c y of c y l i n d r i c a l t h e r mionic converters can be determined a c c u r a t e l y because the thermal input to the emitter i s easy to measure. The e f f i c i e n c y of a c t u a l converters i s shown i n F i g u r e 10. The l i n e s f o r


434



Figure 7. Comparison between calculated results from IMD-4 and experimental data from a Planar converter. T , 1700 K; T , 773 K; T , 567 K; 2.654 eV; c, 1.560 eV; and d, 10 ml; Key: , experimental curve. E

c

R

E}



BRUT

Cesium

1000

Thermionic

1200

435

Converters

1400

1600

1800

2000

EMITTER TEMPERATURE (K) Figure 8.

Projected converter efficiency. Key: ,10 amp/cm .

, maximum efficiency; and

2

Figure 9.

Projected converter output power density.


METAL

BONDING AND

INTERACTIONS


436



29.

BRITT

Cesium

Thermionic

437

Converters

1969 and 1972 are converters b u i l t by General Atomic Company f o r the n u c l e a r space power e f f o r t . A group of converters with s p e c i a l l y s t r u c t u r e d (roughened) e l e c t r o d e s u r f a c e s , was b u i l t by Rasor A s s o c i a t e s and produced the r e s u l t s l a b e l e d 1978 i n F i g u r e 10. The performance gains have been made by reducing the emitter temperature while m a i n t a i n i n g the same e f f i c i e n c y . A complementary p l o t showing output power d e n s i t y i s given i n F i g u r e 11. On t h i s f i g u r e , the e a r l i e r converters are a l l grouped i n the shaded r e g i o n l a b e l e d 1965-1978. The 1978 group are the ones b u i l t by Rasor A s s o c i a t e s , which were mentioned i n the previous paragraph. An unusual converter known as the " c o l d s e a l converter", (4) with gas b u f f e r e d heat pipe containment of the cesium vapor, produced the 1979 p o i n t on F i g u r e 11. There are no inherent degradation mechanisms to l i m i t the l i f e of thermionic c o n v e r t e r s . The l i f e t i m e of t e s t devices has u s u a l l y been r e l a t e d to damage from the environment of the heat source. Converters with nuclear f u e l are a f f e c t e d by f i s s o n product s w e l l i n g , which d i s t o r t s the e m i t t e r . Flame heated converter l i f e t i m e s are c o n t r o l l e d by the d u r a b i l i t y of the hot s h e l l , which p r o t e c t s the e m i t t e r from the combustion atmosphere. The r e c o r d holder f o r converter l i f e i s LC-9, a converter b u i l t f o r NASA by General Atomic as p a r t of the i n - c o r e nuclear space r e a c t o r program. LC-9 operated with p e r f e c t l y s t a b l e performance f o r over f i v e years at an e m i t t e r temperature of 1970 K. As shown i n F i g u r e 12 (5), LC-9 had an e l e c t r o d e e f f i c i e n c y of 17%, and generated 8 W/cm of output power (80 KW /m ) . The converter was s t i l l performing s t a b l y when t e s t s were terminated for programmatic reasons. T h i s t e s t i l l u s t r a t e s w e l l the long l i f e c a p a b i l i t y of the thermionic converter process. Recent converter hardware development has concentrated on flame heated devices f o r t e r r e s t r i a l topping c y c l e a p p l i c a t i o n s . S i g n i f i c a n t progress has been made with chemical vapor deposited s i l i c o n c a r b i d e as the o x i d a t i o n p r o t e c t i o n hot s h e l l . Flame heated converters have now been operated by Thermo E l e c t r o n C o r p o r a t i o n with emitter temperatures over 1700 K f o r 7000+ hours (6). 2

2

Prospects f o r Performance Improvement. Performance improvement i n thermionic converters i s being pursued by suppressing arc v o l t a g e drop i n the plasma and by reducing the s i z e of the c o l l e c t o r work f u n c t i o n . The v o l t a g e l o s s i n a spontaneous i g n i t e d mode plasma i s about 0.5 v o l t . T y p i c a l cesium converters behave as though the c o l l e c t o r work f u n c t i o n i s about 1.5 ev, yet l a b o r a t o r y surfaces can be produced with (J>c - 1.0 ev. Reduction of e i t h e r V or (f)c i n p r a c t i c a l converters would l e a d to a s u b s t a n t i a l improvement i n converter performance, s i n c e the output v o l t a g e of a t y p i c a l i g n i t e d cesium diode i s only about 0.5 v o l t . d


METAL

BONDING AND

INTERACTIONS


438


BRITT

29.

Cesium

Thermionic

Converters

439


20

15

>• o

z

10 -

LJ O u.

X AT ELECTRODE

5 -

+ AT LEAD

0 10 Z

CM

*

a: £ LJ

w

AT ELECTRODE

•

o

AT LEAD i

Q.

10,000

20,000

30,000

I

i

40,000

50,000

OPERATING TIME (HOURS) Figure 12. Operating history of converter test LC-9 at General Atomic Company. Stable performance was monitored for > 5 years with no failure.


440

METAL

BONDING

AND

INTERACTIONS

Recently, two types of surfaces with coadsorbed cesium and oxygen have shown promise f o r low work f u n c t i o n o p e r a t i o n i n thermionic converters. The f i r s t type of surface i s made by cod e p o s i t i o n of a " t h i c k l a y e r " (~ 30 A) of cesium. As shown i n Figure 13 (7), i f the p r o p o r t i o n s of cesium and oxygen are p r o p e r l y c o n t r o l l e d , a work f u n c t i o n as low as 1.0 ev can be obtained. The s u b s t r a t e m a t e r i a l does not a f f e c t the low (J) obtained with a t h i c k C - 0 l a y e r . This type of s u r f a c e could p o t e n t i a l l y be maintained i n a thermionic converter by an e q u i l i b r i u m mixture of cesium, oxygen, and cesium oxide. Experiments to demonstrate t h i s i n operating diodes are underway ( 8 ) . The second type of s u r f a c e i s an atomic l a y e r of oxygen s t r o n g l y bound to a tungsten 110 s u r f a c e i n the form of a 2-D oxide. This surface w i l l not l o s e oxygen up to a temperature of 1200 K; and when cesium i s adsorbed on i t with proper coverage, a work f u n c t i o n near 1.0 ev can be obtained. An example ( 9 ) i s shown i n F i g u r e 14. In previous attempts to use low (f>c surfaces i n cesium t h e r mionic diodes, the s u r f a c e performs with an e f f e c t i v e work funct i o n of > 1.5 ev. I t has been p o s t u l a t e d that a space charge l a y e r of negative ions near the c o l l e c t o r may be preventing the low work f u n c t i o n surfaces from being u t i l i z e d . Negative Cs ions and other negative ions with smaller masses have been detected by a quadrupole mass spectrometer attached to a thermionic converter (10). Atomic cesium and cesium dimers have too small a v a l u e of e l e c t r o n a f f i n i t y (11) to produce a s e r i o u s space charge l a y e r . But near the s u r f a c e there may be a l a r g e c o n c e n t r a t i o n of trimers and other atomic c l u s t e r s . I t i s very p o s s i b l e that these l a r g e r p a r t i c l e s could become negative ions i n s u f f i c i e n t q u a n t i t i e s to a f f e c t converter performance. Further research and understanding i s needed to determine whether negative ions are a problem i n thermionic c o n v e r t e r s . Another route to performance improvement i s through operat i o n with plasma c o n d i t i o n s d i f f e r e n t from the cesium i g n i t e d mode. Considerable research has been done on converters with plasmas produced by a t h i r d e l e c t r o d e discharge or pulsed diodes. Operation with gases other than cesium may y i e l d a b e t t e r plasma medium. Inert gases are i n t e r e s t i n g i n t h i s regard because the Ramsauer e f f e c t r e s u l t s i n very small e l e c t r o n - n e u t r a l s c a t t e r i n g at thermionic energies. Converters with other types of a l k a l i metal vapors are a l s o of i n t e r e s t . The a s s o c i a t i v e i o n i z a t i o n of e x c i t e d sodium atoms to produce dimer i o n s , (12) i s p a r t i c u l a r l y promising f o r a thermionic converter plasma. Thermionic conversion i s a technology that needs, and can immediately use, research on high temperature p r o p e r t i e s of a l k a l i metals. E l e c t r o n t r a n s p o r t p r o p e r t i e s of a l k a l i vapors and c h a r a c t e r i s t i c s of atomic c l u s t e r s are p a r t i c u l a r l y important. Improved understanding i n these areas could lead to performance improvements that would more than double the output power d e n s i t y and e f f i c i e n c y of cesium i g n i t e d mode thermionic c o n v e r t e r s . c


s



29.

BRITT

Figure 13.

Cesium

Thermionic

Converters

Low work junction of a thick layer of CsO obtained by codeposition of Cs and O.


441



442

Figure 14. Low work junction obtained by Cs adsorbed on an atomic layer of annealed 2-D oxide on a W(110) surface (-A-) at 295 K. Work function of Cs on a clean W(110) (-O-) surface is shown for comparison.


29.

BRITT

Cesium

Thermionic

443

Converters


NOMENCLATURE Symbol

Units

d

= i n t e r e l e c t r o d e gap width

cm

e

= e l e c t r o n charge

coulombs

E

= electric

volts/cm

eE

field

= f u n c t i o n used i n e v a l u a t i n g the i o n drag term

-dimensionless-

= f u n c t i o n used i n e v a l u a t i n g the drag term

-dimensionless-

= net forward e l e c t r o n current d e n s i t y

A/cm^

= net forward e l e c t r o n current d e n s i t y at the emitter sheath-plasma i n t e r f a c e (computed from boundary c o n d i t i o n s )

A/ cm^

= net forward i o n current

density

A/

2

A/cm J

= net current d e n s i t y through the diode

k

= Boltzmann s

k

A/ cm^ 1

T

1

constant

= e l e c t r o n thermal d i f f u s i v i t y

erg/°K -dimensionless-

= i o n thermal d i f f u s i v i t y

-dimensionless-

- e l e c t r o n thermal c o n d u c t i v i t y

watt/cm

= atom d e n s i t y

^eE

R.

K

-3

cm -3

= charged p a r t i c l e d e n s i t y a t the c o l l e c t o r plasma-sheath i n t e r f a c e

cm

= charged p a r t i c l e d e n s i t y i n the plasma

cm

= t o t a l e l e c t r o n .energy f l u x

watts/cm

= e l e c t r o n energy f l u x a t the emitter sheath-plasma i n t e r f a c e ( c a l c u l a t e d from the boundary c o n d i t i o n s )

watts/cm

= i o n drag term

volt/cm*

-3 ,

2

,

2

4

le




i o n source c o e f f i c e n t

cm /sec

e l e c t r o n temperature

°K

i o n temperature

°K

cesium r e s e r v o i r temperature

°K

e l e c t r o n temperature at c o l l e c t o r sheath-plasma i n t e r f a c e

°K

emitter temperature

°K

c o l l e c t o r temperature

°K

electron potential

volts

arc drop

volts

b a r r i e r index

volts

output v o l t a g e

volts

cesium i o n i z a t i o n p o t e n t i a l

volts

recombination c o e f f i c i e n t

cm^/sec

electron mobility

2 cm / v o l t - s e c 2

ion mobility

cm / v o l t - s e c

f i e l d - f r e e emitter work f u n c t i o n

volts

c o l l e c t o r work f u n c t i o n

volts


29.

BRITT

Cesium

Thermionic

Converters

4 4 5

Literature Cited


1. 2.

Rasor, N. S. and Warner, C., JAP, 1964, 35, 2589-2600. B r i t t , E. J . and McVey, J . , "Advanced Thermionic Energy Conversion, J o i n t H i g h l i g h t s and Status Report"; Rasor A s s o c i a t e s . , C00-2263-16, NSR 2-16, 1979. 3. W i l k i n s , D. R. and Gyftopoulos, E. P., J . Appl. Phys., 1966, 37, 3533-3540. 4. Smith, M. D., Manda, M. L., and Britt, E. J . , "Utilization of Low Temperature I n s u l a t o r s and Seals i n Thermionic Con v e r t e r s " , 15th I n t e r s o c i e t y Energy Conversion Engineering Conference, Washington, 1980. 5. "Thermionic Converter and Fuel Element T e s t i n g Summaries at Gulf General Atomic Company, " GULF-GA-C12345, C a l i f o r n i a , 1972. 6. Goodale, D. B., Reagan, P. Miskolczy, G., L i e b , D. and Huffman, F. N.; " C h a r a c t e r i s t i c s of CVD S i l i c o n Carbide Thermionic Converters", 16th I n t e r s o c i e t y Energy Conversion Engineering Conference, A t l a n t a , GA, 1981. 7. Papageorgopoulos, C. A. and Desplat, J.-L.; Surface Science, 1980, 92, 119. 8. Hansen, L. K. and Woo, H.; "Thermionic Converters with a T h i c k Cesium Oxide C o l l e c t o r " , IEEE I n t e r n a t i o n a l Conference on Plasma Science, Madison, Wisconsin, 1980, 75. 9. Desplat, J.-L. and Papageorgopoulos, C. A. Surface Science, 1980, 92, 97. 10. Laskowski, B. "Lowest Autodetaching States of Cs-", IEEE I n t e r n a t i o n a l Conference on Plasma Science, Madison, Wisconsin, 1980, 44. 11. Hansen, L. K. and Woo, H. " P o s i t i v e and Negative Ions from a Plasma Diode", IEEE I n t e r n a t i o n a l Conference on Plasma Science, Madison, Wisconsin, 1980, 45. 12. Koch, M. E., Verma, K. K. and Stwalley, W. C., J . Opt. Soc. Am., 1980, 90, 627. RECEIVED August

26,

1981.


Metal Bonding and Interactions in High Temperature Systems

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