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Thermo Electron

CORPORATION

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DISTRIBUTION JF THIb 0( LJ ' / t f . ' ! iS UHV. ,*

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Thermo

[ ' U i P n R A M O KJ

K Electron

Report No. TE4233/4237-60-78

NASA/DOE

ADVANCED THERMIONIC

TECHNOLOGY PROGRAM

PROGRESS REPORT NO. 28

October 1977

NASA Contract NAS3-20959 ERDA Contract EY-76-G-02-3056

P r e p a r e d By

The rmo Elec t ron Corporat ion 101 F i r s t Avenue

Waltham, Massachuse t t s 02154

t^ UiSTPiBJTiON : T,-SS i v ^^ .

W/M^ Thermo f/c. Electron f ( ) n I •( i n AT I CIN

I . SURFACE STUDIES

A, SURFACE CHARACTERIZATION CHAMBER

Samples from surface charac te r i za t ion and conver ter exper iments

were analyzed by Auger spec t roscopy . A p o s t - m o r t e m Auger analysis

of Converter No. 166 indicated that lanthantim had t r ans fe r r ed from

the lanthanum hexaboride col lector to the tungsten e m i t t e r . In addi ­

t ion, carbon, oxygen, sodiusxi, cesium^ tungsten, and rhenium were

detected on the e m i t t e r . The e33aitter surface had become a dull g ray

that could be peeled eas i ly with a r a z o r blade. The exposed a r e a s con­

tained no rhenitim but did contain a g r e a t e r amount of lanthan\im. The

source of rheniura mus t be the molybdenum-rhenium braze that bonds

the era i t ter to the emi t t e r s l eeve . No molybdentxm peaks were readi ly

apparent ; however , if any weak peaks were p resen t , they could have

been dominated by the tungsten spec t rum. Rhenium and molybdenum

oxides a r e both known to be vola t i le , rhenitim being considerably

m o r e s o .

Suprisingly, no rhenium was evident on the col lector although

carbon, oxygen, ni t rogen, sodium, nickel , ces ium, lanthanum, and

tungsten were detected. The lanthanxim and tungsten signals were of

comparable s t r eng ths . Except for one out of the seven spots analyzed,

carbon concentrat ions were s m a l l . Most l ikely, nickel and sodium

w e r e introduced during handling subsequent to the opening of the

c o n v e r t e r .

The Se len ium-Reservo i r Conver te r , No, 173, was dissected and

i t s contents were analyzed. The coEector had a dark deposit along the

edge of the opening to the se lenium r e s e r v o i r . Auger analysis indicated

1

ZJZ Thermo f/c Electron t I ) t KM I n A n ci N

this deposit to be e lementa l seleniijxn.. The emi t te r surface was shiny

(like e lectropol ished tungsten) except for a number of smal l da rk

specks about 1/2 inm in d i a m e t e r . The highest concentration of these

specks appeared in the "shadow" of the selenium r e s e r v o i r opening.

Auger analys is showed the specks to have a high concentration of

selenixira. A few of the selenium Auger spec t ra obtained from var ious

spots on the emi t t e r surface were d is tor ted - indicative of a selenium

compound.

The emi t t e r of Converter No. 173 was subjected subsequently to

an e lec t ron mic roprobe ana lys i s . An analysis taken over an a r ea

about 1 m m at the center of the emi t t e r surface showed the presence

of se lenium, tungsten, ces ium, and a slight amount of nickel . F igure

I - l shows a port ion of this analyzed a r e a magnified 1000 t i m e s . The

l ighter rec tangular portion of the is land-l ike s t ruc tu re was found to

be ces ium, a s were the nximerous globules; the smoother , open a r e a s

were tungsten. The c rus ty region contained cesitim, xnost likely

ces ium hydroxide , which dr ied and cracked under the vacuum environ­

ment of the e lec t ron m i c r o p r o b e . Thus , the emi t te r surface is s e g r e ­

gated into regions of tungsten, selenitim, and ces ium (hydroxide). The

observed d is tor ted selenitoin Auger spec t ra a r e mos t l ikely caused by

the complicated surface topography of the sample . These ana lyses ,

coupled with the independence of the conver te r cha rac te r i s t i c s as a

function of se lenium r e s e r v o i r t e m p e r a t u r e , suggest that a selenium

cold spot existed in the diode. A second conver ter is being constructed

in o rder to continue these invest igat ions .

The r e su l t s of other Auger ana lyses a r e given in subsequent

sect ions of this r e p o r t .

2

w 7711-15

F igure I - l , Scanning Elec t ron Photomicrograph of Central Region of the E |n i t t e r (Converter No. 173) (Magnified lOOOx)

m/gm Thermo f/c, Electron f ( ) t I P ( ) R A I if TM

I I . PLASMA STUDIES

A. CONVERTER THEORY

Schottky and Goiilorab sca t te r ing effects were added to the analyt ical

raodel of the ignited in te re lec t rode p l a s m a . The computer p rog ram was

improved to accommodate these two effects, with prel inainary resu l t s

plotted in F igure I I - 1 . These calculations indicate that Schottky effects

can produce output cu r r en t s significantly l a rge r than the corresponding

sa tura t ion c u r r e n t s . Coulomb sca t te r ing was found to have liie poten­

t ia l of substant ial ly increas ing the a r c d rop .

The requ i red emi t t e r sheath height for the formation of the double

sheath can be calculated f rom a s imple sheath model by matching solu­

tions der ived f rom independent analyses of the p lasma and eini t ter

sheath^ as shown in F igure I I -2 . The double sheath is seen to be

formed at a cu r r en t r a t io of approximately 0 . 5 5 . This value is con­

s is tent with exper imenta l observations^ assuming such formation is

assoc ia ted with the knee of the I-V cha rac t e r i s t i c s of diodes, and

appears at the c u r r e n t ra t ios between 0 .4 and 0 . 6 ,

B , EXPERIMENTAL PLASMA ANALYSES

Spectroscopic analyses of conver te r p lasmas under operating

conditions were ini t ia ted. Spectra a r e being taken throughout the

4000 to 9000 A region. Line width, and re la t ive line and continuum

intensi ty m e a s u r e m e n t s allow determinat ion of e lec t ron densi t ies and

t e m p e r a t u r e s , as well a s excited state population densi t ies through­

out the in te re lec t rode reg ion . Sensitive detection may enable t r ace

consti tuents ( e . g . , oxides) to be moni tored . Changes in p lasma con­

ditions due to var ia t ions in T , T j T , e lect rode spacing, and L C R

4

7710^4

1.1

1.0

0.9

(0 - 3

0.8

0.7

0.6

0.5

0.4

1 ! 1 1 ! 1 ! 1 r WITH COULOMB SCATTERING ^

WITH SCHOTTKY EFFECT

/

/

/

/

WITHOUT SCHOTTKY' EFFECT

// ' '^^wiTH couLoye / / / SCATTERING

J I I L J i™__„_L

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 OB 0.9

V^ (VOLTS)

Figure 11-1. Schottky Effect on Converter Per formance

7710-13

>

5.0

4.0

3.0

2.0

LO

0

1 r I 1 1 —

/ -—PLASMA / SOLUTION

/

/

/ SHEATH'S. / SOLUTION >y y^

- ^/L^""""^ 1 ^ 1 1 1 1

-

-

-

0.5 0.6 0.7

J /J ,

0.8 0.9 1.0

Figure I I -2 . Format ion of Double Valve Sheath

migm Thermo i c : Electron f ( ) M n ( i n A r I O N

current were observed. Temporal decay of interelectrode conditions

in piilsed converters are also being investigated. Preliminary mea­

surements of a converter with a tungsten oxide collector (T = 1425 K, E

T = 751 K, 0,25 tor r cesium, and l-3xim electrode spacing) have ^ 14 -3

shown maximtim electron densities of 2,8 x 10 cm and electron

temperatures below 3000 K. Relative line intensity measurements

indicate progressively less equilibrated excited state poptxlation den­

sities with decreasing energy levels. C. ENHANCED MODE CONVERSION EXPERIMENTS

Refined measurements were performed to determine surface

thermal deformation of a molybdenum lase r -mi r ror electrode during

heating. The apparatus (see Figure 11-3) allowed the splitting of a

l - m m beam from a He-Ne laser into two components and reflecting

one from the center of the electrode and the other from its edge.

These reflected light beams were observed as two spots on a screen

placed approximately 20 m away, Deforxnation of the electrode sur­

face is measured by the relative motion between these two spots. To

first order, this relative motion is independent of the thermal distor­

tion of the sample support s tructure. In a set of experiments with

electrode temperatures ranging up to 1675 K, maximum deformations -5 never exceeded 8 x 1 0 cm. This value is sufficiently low to allow

efficient operation of close-spaced diodes.

7

7710-15

•SCREEN

20 METERS

MIRRORS BEAM SPLITTER

He~Nt LASER

\«8i-METAL BELL JAR

TO VACUUM SYSTEM

Figure I I - 3 . Exper imenta l Ar rangemen t for Surface Therm.al Deform.ation Measurement

8

wi^ Thermo ^ Electron f I 1M p n f 1A I I f 1 fsj

III. CONVERTER DEVELOPMENT

A . LOW-TEMPERATURE CONVERSION EXPERIMENTS

1. Tungsten E m i t t e r , Sprayed Lanthanum Hexaboride Collector

(Converter No, 180)

This var iab le -spac ing device employs a specially gxiarded col­

lec tor s t ruc tu re (see F igure I I I - l ) in o rder to de termine m o r e a c c u r ­

a te ly the work function of i ts sprayed lanthanum hexaboride surface

a t low cu r r en t l eve l s . Plots of col lector work function v e r s u s T /T^^

const ructed f rom re tard ing inode data show good cor re la t ion with dc

back-emiss ion v a l u e s . Work functions a s low as 1.45 eV were ob­

tained at T ^ / T values between 1.4 and 1.5 from re tarding plots

C R

(see F igure 111-2). At tempts to introduce l ow-p re s su re oxygen past

the cesixrm r e s e r v o i r , in o rde r to reduce the col lector work function,

have been unsuccessful . Work is cur ren t ly under way with higher

p r e s s u r e s of oxygen.

B . HIGH-EFFICIENCY CONVERSION EXPERIMENTS

1, Tungsten E m i t t e r , Sintered Lanthanum Hexaboride Collector

("Showerhead" Conver ter No. 166)

Measuremen t s of the effects of oxygen diffusion through the porous

LaB/ col lec tor were continued. Oxygen penetrat ion was observed to

init ial ly deignite the conve r t e r . Pumping on the back of the porous

co l lec tor would subsequently re igni te i t . The saturat ion cu r r en t of

the conver te r was always a factor of 2 to 3 g rea t e r than p r io r to the

oxygen dose . However, this inc reased performance would not l as t

m o r e than 20 to 30 m i n u t e s . Eventual ly the conver ter could not be

9

7510-IOA

COLLECTOR FEEDTHROUGH

COLLECTOR

COLLECTOR INSULATION CERAMIC

SEAL

FLEXIBLE BELLOWS

EMITTER FLANGE

EMITTER SLEEVE

OUTGASSING TUBE

/ EMITTER ^WITH PYROMETER AND

THERMOCOUPLE HOLES TO CESIUM RESERVOIR

Figure I I I - l . Schematic of Guarded Conver ter

2.2

2.0

:0N

(eV

) CD

o 1.6 z

cr il.4 QC O 1 -

i i i . 2 o o

1.0

—

__

+ \ s +

\

'BACK EMISSION M~800K ® - 7 5 0 K X - 7 0 0 K + - 6 5 0 K • - 600K

X X X

1

i

J^ ^

t r CONVERTER NO. 180 SPRAYED La Be COLLECTOR

1

® ®

7710-2

®

M

—

'

1.4 L8 2.0 22

T/Tr

Figure 111-2. Work Function of Sprayed Lanthanum Hexaboride Collector Ve r sus T / T R

r p Thermo f/C Electron ( I 1 w R n R A r I C) N

ignited under any condit ions, and it was assumed that a l l the ces ium

had become oxidized. In fact, when the cesiuin r e s e r v o i r was slowly

heated up to 1000 K, no ignition was possible; the conver te r was taken

off t es t and was Auger analyzed.

A new LaB/ "showerhead" conver te r is being designed with a

window to pe rmi t spec t roscopic ana lyses of the oxygen diffusion

p r o c e s s e s ,

2, Lanthanum Hexaboride E m i t t e r , Guarded Nickel Collector

This conver te r is essen t ia l ly a repea t of No. 168 with a gxiarded

nickel col lector (see F igure i n - 3 ) , The problem with Converter

No, 168 was that the cu r r en t from the tungsten cup could not be

separa ted f rom the cu r r en t frora the LaB , emi t te r s lug. The guarded

col lec tor should al leviate this p rob lem in that the cu r r en t from the

tungsten shoxild flow to the gxiard, whereas the cu r r en t from the LaB . o

slug shoxild flow to the col lec tor button. The construct ion of this

conver t e r is near ly completed.

12

7711-6

GUARD

COLLECTOR

COLLECTOR INSULATION

KOVAR SLEEVE

SAPPHIRE WINDOW

EMITTER HOHLRAUM

SPACING PAD

COLLECTOR FEEDTHROUGH

0.004 SPACING LaBg EMITTER ELECTRODE

RHENIUM WIRE SUPPORT TUNGSTEN EMITTER BODY

Figure 111-3, LaB, Emitter, Guarded Nickel Collector Converter

m» Thermo f/c. Electron ( ( ] n p) n n A I I n f^j

IV, COMPONENT HARDWARE PROGRAM

A, CVD COMPONENT DEVELOPMENT

The f i r s t f ree-s tanding si l icon carbide-graphi te- tungs ten com­

posite hot shel l has been fabr ica ted. This shel l , shown in F igure

I V - 1 , m e a s u r e s 9. 53 cm in length by 2. 54 cm in d i ame te r . The

tungsten and graphite l aye r s have uniform th icknesses of 0.02 cm

and 0,1 cm, respec t ive ly . The si l icon carbide outer layer is 0 ,28 cm

thick at the he in i spher ica l end and 0.05 cm throughout the cyl indrical

por t ion . Design studies a r e in p r o g r e s s to determine a method to

grow g r e a t e r th icknesses of s i l icon carbide at the round end, and to

es tab l i sh a hydrogen purge to the inside of the shell during CVD of

the si l icon ca rb ide . The hydrogen purge is n e c e s s a r y to maintain the

in te rna l tungsten deposit free f rom t r a c e amounts of silicon ca rb ide .

The t e m p e r a t u r e and flow r a t e set t ings of the tungsten CVD

appara tus have , for the t ime being, becoine es tabl ished. Tungsten

she l l s , which a r e 0,05 cm thick at the open end and 0 ,04 cm thick

at the round end, a r e now routinely formed after an 85-minute expo­

s u r e to a 150-SCCM flow of tungsten hexafluoride.

A capabili ty for producing composite hot shells has been demon­

s t r a t ed . The CVD hot shel l effort of the component ha rdware p r o ­

g r a m wil l now be directed along the l ines of establishing routinely

reproducib le fabrication p r o c e s s e s .

B . ALLOY HOT SHELL DEVELOPMENT

Simulated furnace test ing continued at a furnace gas t empera tu re -7

of 1573 K. The inside of each shel l is evacuated to 10 t o r r . Table

IV-1 p r e s e n t s the status of these t e s t s as of November 2, 1977, A

14

w

• ; • • • » • • • ' .

.-. -ilrWtel

7710-17

F igure I V - 1 . Composite CVD Silicon Carbide and Tungsten Emi t t e r Subassembly

15

•

TABLE IV-1

SIMULATED FURNACE TESTS (November 2, 1977)

TEST P O R T NO,

1

2

3

4

5

6

HOT S H E L L

REACTION BONDED SILICON CARBIDE

KANTHAL A !

KANTHAL A l

INCONEL 671

CHROMIZED STAINLESS S T E E L 446

KANTHAL A l

T E S T HOURS

14 ,213

12.626

12,492

5 , 6 6 3

5, 172

2, 810

COMMENT

LEAKTIGHT THIS HOT S H E L L WAS BRAZED TO A M O L Y B ­DENUM S L E E V E WITH N I C K E L - C O P P E R

LEAKTIGHT

LEAK OBSERVED REMOVED 3 ! Oct . 77

LEAKTIGHT

LEAK OBSERVED

LEAKTIGHT

DATE OF TEST INITIATION

9 A p r i l 75

25 Aug. 75

9 Sept . 75

17 Nov. 76

17 Nov. 76

7 May 77

»y^ Thermo • ' . - • • f/^ Electron { ( ) H P D F ^ A T I O N

leak was observed on the Kanthal A-1 sample on October 31 , 1977,

This shell has been removed from the furnace for failure ana lys i s .

Two additional hot shel ls were placed on tes t on October 31, 1977.

Both of these shel ls were del ivered to Thermo Elec t ron on October

6, 1977 in connection with Subcontract 7070-4190-411-Mod #3 , The

samples were mounted a s follows;

Tes t P o r t No, 3: 446 CRES P l a s m a a r c sprayed with n ichrome, 0. 01 to 0. 02 cm thick and difftised for 8 hours at 1367 K in a hydrogen a tmosphe re .

Tes t P o r t No. 5; 446 CRES P l a s m a a r c sprayed with Cr O , 0.01 to 0.02 cm thick.

Both samples had 2. 54 c m inside d i a m e t e r s , 0, 38 cm wall th icknesses

and were 20, 3 cm long. The end caps were TIG welded. The asseanbly

was s t r e s s re l ieved for one hour at 1256 K and a i r cooled.

17

m» Thermo W^ Electron r I J M p n ^̂ A ! I c) NI

V. COMBUSTION-HEATED THERMIONIC DEVICE

The objective of this t ask is to design, construct , and tes t a

f lame-heated thermionic device for eventual use in combustion-heated

pane l s . The f i r s t step in this development will be the construct ion and

f larae-heated t e s t of a "miniature device" employing the mos t promis ing

m a t e r i a l s in s izes that a r e readi ly avai lable . The next step wil l be the

fabricat ion of a l a r g e r "workhorse device^ " which will be evaluated in

a t e s t furnace that provides coinbustion p a r a m e t e r s s imi la r to tih.ose in

fossi l - fuel powerplants . The data and experience with the minia ture

and workhorse devices wil l then be factored into a l a rge r "prototype

d e v i c e . "

The design of the min ia ture device (see Figxire V-1) in tegra tes

the r e su l t s of the al loy hot shel l development at TRW with the va r i ab le -

spacing diode t e s t s a t T h e r m o E lec t ron . To date , the mos t promising-

m a t e r i a l for alloy hot shel ls is INCONEL 671. This high chromium

nickel aUoy has shown good stabil i ty in s h o r t - t e r m t e s t s a t TRW and

a l so in a l ong - t e rm t e s t in the Simulated Furnace Faci l i ty a t The rmo

E lec t ron .

The hot shel l d imensions a r e determined by readi ly available

INCONEL 671 tubing (nominally 2. 54 c m in d i ame te r ) . The effective a r ea 2

of the conver te r is 2, 0 cm and is de termined by the maximum d iam­e te r of the col lector that can be inse r ted into the emi t te r hot shell without shor t - c i r cu i t ing .

The L605 emi t t e r m a t e r i a l was chosen on the basis of the favor­

able data f rom the var iable spacing Converter No. 143. These data

were repor ted in the P r o g r e s s Repor t for August 1976.

18

MOUNTING BRACKET

CESIUM RESERVOIR-

7710-4

MOLYBDENUM REINFORCEMENT RING'

NICKEL COLLECTOR

INCONEL WICK / EMITTER

:LLOWS WICK SUPPORT

INCONEL 671 HOT SHELL

Figure V - 1 . Miniature F lame-Hea ted Device

mf^ Thermo ^ Electron

In o rde r for the t empe ra tu r e drop through the collector s t ruc ture

to be min imized , a potassixim filled heat pipe is used in the collector

stejXL. A cooling fin, a n d / o r an axxxiliary hea te r , may be clamped to

the outboard end of the col lec tor for diagnostic tes t ing .

The hea t source for heating the emi t t e r of the device will be a

gas t o r c h . A p r o p a n e / a i r , or propane/oxygen, torch will be used

in the init ial t e s t s .

Construction of the min ia ture device is in p r o g r e s s . Test resxilts

should be available next month .

20