Cryogenic Thermoelectric Cooler With a Passive Branch

7/29/2019 Cryogenic Thermoelectric Cooler With a Passive Branch

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Journal of ElectronicMaterials,Vo 91 6, No. 8, 1997 Reg u l a r I s su e Pa p er

Cryogen ic Therm oe lec t ric Co ole r w i th a Pass ive BranchC . C I P A G A U T A M I N O , t J. W . C O C H R A N E , ~ E . H . V O L C K M A N N , *a n d G . J. R U S S E L L ~* A d v a n ce d E le c t r o n i c M a t e r i a l s G r o u p , S c h o o l of P h y s i c s , T h e U n i v e r s i t y o fN e w S o u t h W a l e s , S y d n e y , N S W 2 0 52 A u s t r a l i a* M a r l ow I n d u s t r i e s , I n c . , 1 04 5 1 V i s t a P a r k R o a d , D a l l a s , T X 7 5 2 3 8 - 1 6 4 5C r y o g e n i c th e r m o e l e c t r i c co o le r s h a v e b e e n f a b r i c a t e d f r o m a n a c t iv e e l e m e n t( p o l y c r y s t a l l i n e B i0.ssSbo.1 2) a n d t h r e e d i f f e r e n t p a s s i v e e l e m e n t s , a h i g h - T cs u p e r c o n d u c t o r ( p o l y c r y s t a l l i n e Y Ba2 Cu3 OT .~ ), a n d l e n g t h s o f h i g h p u r i t y c o p p e ra n d a l u m i n u m w i r e s . T h e r e s u l t s f o r a s i n g le c o u p l e sh o w t h a t f o r l ow , h o tj u n c t i o n ( s in k ) t e m p e r a t u r e s t h e s u p e r c o n d u c t i n g e l e m e n t gi v es r is e t o m a x i -m u m t e m p e r a t u r e d r o p s o f 6 .6 d e g r e e s K a t 7 0 K a n d 7 .3 d e g r e e s K a t 7 5 K i na p p l i e d m a g n e t i c f i el d s o f 0 . 0 a n d 0 . 0 7 T , r e s p e c t i v e l y . T e m p e r a t u r e d r o p s o f ~ 9a n d 1 1 K , r e s p e c t i v e l y , a r e e x p e c t e d f o r s u c h a c o u p l e w h e n f o r m f a c t o r s a r e t a k e ni n to a c c o u n t. T h e c o p p e r c ou p le w i t h a n a p p l i e d m a g n e t i c f i e ld r e s u l t e d i n l a r g ec o o l in g AT v a l u e s a t T ho = 1 5 0 a n d 2 9 3 K , i n d i c a t i n g t h e i m p o r t a n c e o f m e t a l l i cp a s si v e e l e m e n t s f or i n t e r m e d i a t e s i n k t e m p e r a t u r e s . P e r f o r m a n c e c u r v e s f o rt h e s u p e r c o n d u c t o r b a s e d c r y o g e n i c t h e r m o e l e c t r i c co o le r sh o w p r o m i s e w i t hf u r t h e r i m p r o v e m e n t p o s s i b le b y t h e u s e o f s i n g le c r y s t a l l i n e B i0 ssSb0.15.K e y w o r d s : Bi0 .ssSi0 .12, coo lers , c ry og en ic the rm oe lec t r i c , h igh -T cs u p e r c o n d u c t o r s

I N T R O D U C T I O NA t h e r m o e l e c t r i c c o o le r o r P e l t i e r r e f r i g e r a t o r o p e r -a t i n g a t a h o t ju n c t i o n ( s in k ) t e m p e r a t u r e o f l iq u i dn i t r o g e n , w o u l d b e v e r y u s e f u l f o r t h e c o o l i n g o fe l e c t r o n i c a n d o p t o e l e c t r ic d e v i c e s b e l o w 7 7 K t o i m -p r o v e t h e i r e f f ic i e nc y a n d s e n s it i v it y . T h e a d v a n t a g e so f t h e t h e r m o e l e c t r i c c o o l e r o v e r o t h e r a v a i l a b l e c o ol -i n g s y s t e m s , w h i c h a r e m o r e e f f i c i e n t, a r e i t s l o w c o s t,s m a l l s i z e a n d w e i g h t , n o m o v i n g c o m p o n e n t s , l o w

e l e c t r i c a l n o i s e , m a i n t e n a n c e - f r e e o p e r a t i o n , l o n g li fe ,m i n i m a l e n v i r o n m e n t a l i m p a c t , a n d e a s e o f p r o v i d i n gp r e c i s e t e m p e r a t u r e c o n t ro l .T h e p e r f o r m a n c e o f a s i n g l e s t a g e c o o l e r, w h i c h i ss h o w n s c h e m a t i c a l l y i n F ig . 1 , i s c h a r a c t e r i z e d b y i t sf i g u r e o f m e r i t 1,2[ - / \1/2 . \1 /272z - - ( s ,- s o ) 2 / L t o p p ) J ( 1 )

w h e r e S i s t h e S e e b e c k c o e f fi c ie n t, X i s t h e t h e r m a lc o n d u c t i v i t y , p i s t h e e l e c t r i c a l r e s i s t i v i t y , a n d t h e

(Received M arch 3, 1997; accepted Ma rch 28, 1997)

s u b s c r i p t s p a n d n r e f e r t o t h e p o s i t i v e a n d n e g a t i v eb r a n c h m a t e r i a l s . T h e m a x i m u m t e m p e r a t u r e d if fe r-e n c e a c r o s s t h i s c o o l er c a n b e r e a l i z e d i f t h e f o l lo w i n ge q u a t i o n i n v o l v i n g t h e f o r m f a c t o r ( U A ) , w h e r e g ist h e l e n g t h a n d A t h e c r o s s -s e c t io n a l a r e a o f a b r a n c h ,is sa t is f ied , 1,2

A 2 _ _ 2( Pn / ~ n ) ( ~ n / n ) ( p p / t p ) ( ~ p / A p ) ( 2 )T h u s , t h e m a x i m u m t e m p e r a t u r e d i ff e re n c e , f o r ac o l d j u n c t i o n t e m p e r a t u r e TcolV i s g i v e n b y 2

( A T ) ~ = ZTi2ola 2 (3)A c c o r d i n g t o t h e a n a l y s i s o f G o l d s m i d e t a l ., a t h e b e s tm a t e r i a l f o r t h e n e g a t i v e b r a n c h o f t h e t h e r m o e l e -m e n t a t l iq u i d n i t r o g e n t e m p e r a t u r e s a r e t h e a l l oy s o fb i s m u t h w i t h a n t i m o n y , e s p e c i a ll y i n t h e p r e s e n c e o fa m a g n e t i c f i el d . ~T h e c h o ic e o f m a t e r i a l f o r t h e p o s i ti v e b r a n c h w o u l dn o r m a l l y b e a b i s m u t h - t e l l u r i d e a l l o y b u t , a s i n -d i c a t e d b y G o l d s m i d e t a l. , t h e f i g u r e o f m e r i t o f t h et h e r m o e l e m e n t w o u l d b e s i g n i f i c a n t l y i m p r o v e d a tl iq u i d n i tr o g e n t e m p e r a t u r e s b y t h e u s e o f a n o r m a l

91 5


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916 Mino, Cochrane, Volckmann, and Russell

Lo w temperature ~derCcld ~'~r cop per C.31romet-ol~l

difler entiol' ~ t e r o c c + J p ~ e

Y 1 2 3 !C u r r e n tl e o dO~ode

Copper -s

J lC o p p e r o rI L

d o , ,

a t u m i n i u m w i r e

Jc o p p e rbFig. 1. Schemat ic diagra m of the thermoelectric coole r with (a) asuperconductor as passive elem ent, and (b) a metal wire as passivee lement .metal as a passive branch. However, ira superconduc-tor is used to replace the p-type branch in Eq. (1), Zreduces to:

Z = z = S: /~nPn (4)as S and p are both zero for a superconductor belowthe t ransi tion t empera ture , T c.With the appropriate form factor, a superconductorwould provide a perfect passive branch for the ther-moelectric cooler. This improvem ent arises from thefact that the superconductor branch does not contrib-ute to the Joule heating which limits the maximumtemp erat ure difference for a junction fabricated witha normal metal branch.High -T superconducting mater ials having T c >90Kare now available for liquid nitrogen temperatureoperation. According to Goldsmid et al., the cross-sectional area of the superconducti ng passive ele-ment should have a th erma l conductance < 10% of the

active element and support a curren t density, J = 240A cm -2 at 77K in a mag net ic field of -0.2T . In 1988,bulk high temper atur e supercondu cting (HTSC) ma-terial with these characteristic properties was notavailable a nd th e predictio ns of Goldsmid et al. couldnot be verified.In 1992, Dashevskii et alY fabricated a single-element Bil_xSbx-(YBa2Cu3OT~ + Ag) thermocouplewith th e superconducting passive element havi ng a T c= 92K and J > 103 A cm-2 at 77K. The effect of anexternal magnetic field on Jc was not reported andconsequently the increased cooling of the thermo-couple due to a magnetic field was not given. Theresults reported indicated a maximum temperaturedifference, with the sink temp erat ure at 77K, in therang e 5.8 to 9 degrees K depend ing on the crystall in-ity of the active branch, t hat is, wheth er an extrudedpolycrystal or single crystal, respectively. This workconfirms the ideas o fGoldsmid et al. 3 and shows tha tthe m any problems relat ed to the fabrication of sucha thermocouple, for example, very low resistanceconnections between the branches and different ther-mal expansion coefficients of the materials , can beovercome.A second single-elemen t thermo electri c cooler basedon a passive superconducting branch was fabri catedby Nak an o and Ha sh im ot o ~ us in g a BiosgSb011-(Bi,Pb)2Sr2Ca2Cu3Oy thermocouple. The (Bi-2223)mater ial ha d a T c= ll 0K , a J = 580 A cm ~ at 78K, butagain no magnetic field data were reported. Themaximu m temper ature difference obtained was 7.1Kwith the temperatu re of the sink temperatu re being78K. Using the thermocouple Bio85Sbl~-(Bi-2223), itwas also reported t hat cooling could be obtained attemperatu res above the superconductivity transitiontemperature.Other work on a thermoelectric cooler using a su-perconducting passive branch has also been reportedby Fee. 9 Using an active b ismuth -anti monide alloybranc h and a pas sive YBa2Cu3OT~ branch, the maxi-mum cooling obtained was 5.35 degrees K below asink temperature of 79K. It was further indicatedth at imp rovements could produce temp erat ure differ-ences up to 16 degrees K. Fur the r, Troda hl and Fee 1~have reported that a Cu vs Bi-Sb cooler achieved atemp erat ure difference of -5 degrees K in a magneticfield at liquid nitrogen temperature. An optimizedsingle stage ele men t sh ould pr oduce a AT >_ 14K.In this study, we report on the characteristics of aBi0 ssSbo.t2-YBa2CthOT_~ hermoelectri c cooler as a func-tion of sink tempe ratur es, above and below T of thecsuperconductor, with and without an external ap-plied magnetic field. Comparison is also made withthe results from a similar couple using either purecopper or alumi num wire as the passive element.

E X P E R I M E N T A LA single stage thermoelectric cooler was fabricatedfrom a polycryst all ine Bio.ssSbo.~2 ma te ri al {negative(active) branch} and ei ther a polycryst alline high-Tc

YBa2Cu3OT~ mat eri al, a pur e copper wire or a pur e


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Cryogen ic Thermo elec t r ic Coo ler wi th a Pass ive Branch 917

a l u m i n u m w i r e { p o s i ti v e ( p a s s i v e ) b r a n c h } , s e e F i g . 1 .The po lycrys ta l l ine Bio .s sSbo .~2m a t e r i a l w a s s u p p l ie db y M a r l o w I n d u s t r i e s a n d i ts t e m p e r a t u r e d e p e n d e n tf i g u r e o f m e r i t i s s i m i l a r t o t h o s e r e p o r t e d b y Y i m a n dA m i t h f o r s i n g le c r y s t a l l i n e m a t e r i a l s , 6 s e e F i g . 2 .O b v i o u s l y , s i n g l e c r y s t a l l i n e m a t e r i a l w o u l d b e t h ei d e a l n e g a t i v e b r a n c h e l e m e n t , b u t d u e t o i t s h a n d l i n gd i f fi c u lt i es a n d t e n d e n c y t o c l e a v e w h e n s t r e s s e d ,w h i c h m a y o c c u r d u r i n g r e p e a t e d t e m p e r a t u r e c y-c l i ng o f p r o t o t y p e d e v i c e s , it w a s n o t u s e d i n t h i ss t u d y . F u r t h e r , w e h a v e i n c l u d e d t h e c a l c u l a t e d ( u s-i n g a p p r o x i m a t i o n g i v e n i n R ef . 2 ) t e m p e r a t u r e d e -p e n d e n t f i g u r e o f m e r i t f o r a c o u p le u s i n g t h e p o l y-cr ys ta l l i ne Bio~ssSbo.12a s t h e n e g a ti v e b r a n c h a n d t h eb e s t a v a i l a b l e p - t y p e s e m i c o n d u c t i n g m a t e r i a l - e x c e s sT e- do pe d (Sb2 Te3)72 (B i 2 T%)25 (Sb2S%)31~- as th e pos i-t i v e b r a n c h .I t i s c l e a r t h a t t h e f i g u r e o f m e r i t o f t h e n - p s e m i c o n -d u c t o r co u p l e d e c r e a s e s a s t h e t e m p e r a t u r e d e c r e a s e si n t h e v i c i n i t y o f 10 0 K a n d t h e r e s u l t i s ty p i c a l f o r a l ls u c h c o u p l e s . I f a m a g n e t i c f i e l d o f 0 . 7 5 T is u s e d w i t ht h e c o u p le a t th e s e l o w t e m p e r a t u r e s , t h e n t h e b e s ta v a i l a b l e p - t y p e m a t e r i a l i s B io .s sS bo .1 2d o p e d w i t h 3 0 0p p m S n . 6 H o w e v e r , t h e r e s u l t a n t Z f or th i s c o u p l e a ts i n k t e m p e r a t u r e - 8 0 K i s st il l s i g n i f ic a n t l y b e lo wt h a t f o r a s u p e r c o n d u c t i n g p a s s i v e b r a n c h a n d a v e r ys m a l l a p p l i e d m a g n e t i c f i e ld , a s w e s h o w b e l o w . T h epo lyc ry s ta l l in e Bio .s sSbo .~2a c t i v e e l e m e n t u s e d i n t h i sw o r k h a d a n a r e a o f 4 . 25 4 . 2 5 m m 2 a n d a l e n g t h o f4 . 50 m m .T h e p o ly c r y s t a l l in e s u p e r c o n d u c t i n g m a t e r i a lYBa2Cu3OT_~w a s p r e p a r e d f r o m a s t o ic h i o m e t r i c m i x -t u r e o f Y 2 03 , C u O , a n d B a C O 3 , c a l c i n e d a t 8 5 0 ~ i nn i t r o g e n f o r 2 4 h , g r o u n d a n d s i n t e r e d a t 9 3 0 ~ i n a irf o r 2 4 h, f o l l o w e d b y t w o m o r e s i n t e r i n g c y c le s . T h em a t e r i a l w a s f i n a l ly o x y g e n a t e d a t 5 0 0 ~ i n I a t m o -s p h e r e o f p u r e o x y g e n f o r 1 5 0 h, f o l l o w e d b y a v e r ys l o w c o ol t o r o o m t e m p e r a t u r e . T h e Y B C O h a d a T =9 1 .5 K , a J c > 1 5 0 0 A c m -2 i n z e ro f i e l d a n d a J ~ = 3 4 0A c m -2 i n a f i e l d o f 0 . 5 T a t 7 7 K . T h e Y B C O e l e m e n tu s e d h a d a n a r e a o f 2 . 6 8 x 2 . 6 8 m m 2 a n d a l e n g t h o f4 . 5 0 m m . T h i s i s n o t a n o p t i m u m s i z e d c o m p o n e n t a si t s c r o s s - s e c t i o n a l a r e a i s - 4 t i m e s t h a t r e q u i r e d a t8 0 K ; b u t d u e t o t h e p r o t o t y p e f a b r i c a t i o n p r o c e s s e s , i ta l l o w e d f or e a s e o f h a n d l i n g , i n c l u d i n g t h e p r e p a r a -t i o n o f u l t r a - f la t , p a r a l l e l s i d e d ( t o p a n d b o t t o m )s u r f a c e s o f t h e m a t e r i a l s u s e d i n e a c h b r a n c h . F u r -t h e r w o r k is c o n t i n u i n g i n th i s a r e a .T h e h i g h -p u r i t y c o p p e r a n d a l u m i n u m w i r e s u s e da s p a s s i v e m e t a l e l e m e n t s h a d a r e a s o f 2 . 38 a n d 0 . 7 9m m 2, r e s p e c t i v e l y . U s i n g p u b l i s h e d r e s i s t i v i t y a n dt h e r m a l c o n d u c t i v i t y d a t a a s a f u n c t i o n o f t e m p e r a -t u r e f o r t h e s e e l e m e n t s , ~ l e n g t h s o f 6 7 , 2 0 5 , a n d 4 1 0m m f o r c o p p e r a n d 4 5 , 9 0 , 2 0 0 , a n d 4 0 0 m m f o ra l u m i n u m w e r e c h o se n . T h e f ir s t l e n g t h s h o u l d c o r re -s p o n d t o a s u i t a b l e f o r m f a c t o r a t r o o m t e m p e r a t u r ew h i l e t h e 4 1 0 m m C u a n d 2 0 0 m m A 1 t o f o r m f a c t o r sa t ~ 8 0 K . W i r e l e n g t h > 4 0 0 m m c o u l d n o t b e f i tt e d i n t ot h e c r y o c o o l e r c h a m b e r w i t h o u t s i g n i f i c a n t s t r a i nb e i n g p r o d u c e d i n t h e w i r e . I n fa c t , t o d e t e r m i n e a na c c u r a t e t h e o r e t i c a l f o r m f a c t o r f o r t h e w i r e s o v e r a

p a r t i c u l a r t e m p e r a t u r e r a n g e , p r e c i se v a l u e s o f t h et h e r m a l c o n d u c t i v i t y a n d e l e c t r i c a l r e s i s t i v i t y m u s tb e k n o w n o v e r t h e t e m p e r a t u r e r a n g e u n d e r c o n s i de r -a t i o n .I n f a b r i c a t i n g t h e c o o l e r, a c o p p e r b r i d g e w a s u s e da s t h e c o l d u n c t i o n ; b u t a s f o r a l l s u c h d e v i c e s n o t o n l ym u s t t h e c o n t a c t re s i s t a n c e b e t w e e n t h e d i f f e r e n tm a t e r i a l s b e a s s m a l l a s p o s si b l e b u t t h e c o n t a c t sm u s t b e s u f f i c i e n tl y f le x i bl e to a c c o m m o d a t e t h e d if -f e r e n t t h e r m a l e x p a n s i o n c o e f f ic i e n ts o f t h e c o u p l em a t e r i a l s . T h e s e t w o c o m p e t i n g g o a l s c r e a t e t h e p r i -m a r y o b s t a c l e in t h e f a b r i c a t i o n p r o c e s s . A f t e r ex p e r i -m e n t i n g w i t h a l ar g e n u m b e r o f m a t e r i a l s a n d t e c h -n i q u e s t o p r o v i d e t h e r e q u i s i t e c o n t a c t s , w e f i n a l lyu s e d t h e f o l l o w i n g p r o c e s s. T h e t w o o p p o s i t e e n d s o ft h e B io ss Sb 0. n e l e m e n t w e r e n i c k e l p l a t e d a n d s o l-d e r e d t o t h e c o p p e r h e a t s i n k a n d b r i d g e u s i n g a lo wt e m p e r a t u r e ( m e l t i n g p o i n t 5 8 ~ s o l d e r o f c o m p o s i -t i o n 2 3 % I n : 4 7 % B i : 1 8 % P b : 1 2 % S n . T h e c o n t a c t s f o rt h e Y B C O e l e m e n t w e r e f a b r i c a t e d b y e v a p o r a t i n g a7 p m l a y e r o f s i l v e r o n t o t h e e n d s a n d t h e n a n n e a l i n gi n o x y g e n f o r t w o d a y s a t 5 0 0 ~ B o t h t h e Y B C O a n dc o p p e r e l e m e n t s w e r e s o l d e r e d i n t o p o s i ti o n u s i n g t h el o w t e m p e r a t u r e s o ld e r.T h e f a b r i c a t e d c o o l e r w a s a t t a c h e d t o a c ol d f i n g e ro f a c r y o c o o le r b y n y l o n s c r e w s w i t h a m i c a s p a c e r f o re l e c t ri c a l i so l a t i o n a n d t h e r m a l p a s t e f o r g o o d t h e r -m a l c o n t a c t ( F ig . l a ) . I n t h e c a s e o f t h e a l u m i n u mw i r e , a p u r e z i nc s o l d e r a n d a t e m p e r a t u r e o f 4 0 0 ~w a s u s e d t o f o r m a v e r y lo w r e s i s t a n c e c o n t a c t b e -t w e e n t h e a l u m i n u m w i r e a n d e a c h o f t h e c o p p e r h e a ts i n k a n d b r i d g e . T h e s e c o p p e r e l e m e n t s w e r e t h e ns o l d e r e d i nt o p o s i ti o n u s i n g t h e l o w t e m p e r a t u r es o l d e r (F i g. l b ) . I n a l l c a s e s, t h e f i n a l t h e r m o e l e m e n th a d a to t a l r e s i s t a n c e < 0 .1 o h m .T h e t e m p e r a t u r e o f t h e h o t j u n c t i o n w a s m e a s u r e dw i t h b o t h a d i o d e a n d a c o p p e r - c o n s t a n t a n t h e r m o -

7 .0 J ~....~Bi85Sb15 single crystal along the tr igonol ax is (6)

"~ S i8 8 S b 12 v ia powder metaLLurgy rom Mar'tow ndust ries% 5 0 [ _ / \ ' f ~ - ~ - 8S 'b121M l )/ lSb 2 e3 72 |B i Te3 ) zS(Sb2Se3) 3r e f ' [ 1 Z ] }

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T / KF ig . 2 . T e m p e r a t u r e v a r i a t i o n o f t h e f i g u r e s o f m e r i t f o r th r e e i n d i v i d u a lac t i v e e le m en t s : s in g le c ry s t a l l i ne B io .s s Sbo. ls , s i ng le crysta l l ineBio.~Sbo.12,po l y c ry s t a l l i ne B io .= Sbo12 r o m M a r l o w I n d u s t r i e s a n d t h r e et h e r m o e l e c t r i c c o o l e r s , p o t y c r y s ta l l in e B i o . ~ S b o . J C u w i t h a n appl iedm a g n e t i c f i e ld a n d p o l y c r y s t a ll i n e B i o . ~ S b o . ~ ( 1 2 3 w i t h n o m a g n e t i cf ie l d , a s o b t a i n e d i n t h i s w o r k a n d a c a l c u l a t e d2 Z f o r p o l y c r y s t a l li n eBio.~Sbo.,21(Sb2Te3)z2 Bi2 Te3)2s (Sb2Se3)3. 2


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F ig . 4 . M ax imum coef f i c i en t o f per formance as a func t i on o f thet em pe r a t u r e d r op f o r t he B i ~ . ~ S bo . jY 123 c oup l e ope r a t i ng a t d i f f e ren ts ink temperatures.c o u p l e to a n a c c u r a c y o f 0 .1 d e g r e e K w h i l e t h et e m p e r a t u r e d i ff e r en c e w a s m e a s u r e d w i t h a v e r yf i n e g a u g e c h r o m e l - a l u m e l t h e r m o c o u p l e t o a n a c c u-r a c y o f + 0 .1 d e g r e e K . T h e t h e r m o e l e c t r i c c o o l e r c u r -r e n t w a s l i m i t e d to 1 0 a m p e r e , t h e a p p l i e d m a g n e t i cf i e ld t o 1 .0 T , a n d t h e s i n k t e m p e r a t u r e s s t u d i e d w e r em a i n l y i n t h e r a n g e 7 0 - 2 9 3 K w i t h a f o c u s o n te m p e r a -t u r e s a b o u t 7 7 K , a s l i q u id n i t r o g e n w o u l d b e t h e i d e a lc o o l a n t f or c o m m e r c i a l o p e r a t i o n .

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F i g u r e 3 s h o w s t h e t e m p e r a t u r e d r o p , AT ( K ), a c r o s st h e c o o l e r, a s a f u n c t i o n o f t h e r m o c o u p l e c u r r e n t f o r as e r i e s o f s i n k t e m p e r a t u r e s , a n d n o a p p l i e d m a g n e t i cf ie l d. A t a s i n k t e m p e r a t u r e o f 8 5 K , th e Y 1 2 3 s u p e r -c o n d u c t i n g e l e m e n t s t a r t s t o b e c o m e n o r m a l a t ac u r r e n t > 6 A . F o r t h e l o w e r s i n k t e m p e r a t u r e s s h o w n ,t h e s u p e r c o n d u c t o r d o e s n o t g o n o r m a l a t 1 0 A , t h em a x i m u m c u r r e n t a l lo w e d b y t h e s y s t e m w i r in g . I ts h o u l d b e n o t e d t h a t t h e s e s m o o t h c u r v e s a r e " b e s tf it s" to a v e r a g e d e x p e r i m e n t a l d a t a t h a t s h o w e d at i m e v a r i a t i o n o f 0 . 2K , w h i l e a c t u a l p e a k v a l u e s a r et a k e n f r o m t h e o r i g i n al d a t a . T o s h o w t h e a c t u a le x p e r i m e n t a l d a t a i n s u c h f ig u r e s w o u l d c o m p l e t e l yo b s c u r e t h e r e s u l t s .I t is c l e a r t h a t a t e m p e r a t u r e d r o p d o e s o cc u r f o r a l ls i n k t e m p e r a t u r e s f r om r o o m t e m p e r a t u r e t o 7 0K ,w h e t h e r t h e s u p e r c o n d u c t i n g e l e m e n t i s in t h e n o r-m a l o r s u p e r c o n d u c t i n g s t a t e . F r o m t h i s s e t o f c u r v e s ,t h e m a x i m u m t e m p e r a t u r e d r op a t ta i n e d u n d e r t h ee x p e r i m e n t a l c o n d it io n s f or t h e t e m p e r a t u r e r a n g e7 0 - 1 0 0 K w a s 6 . 6 K w i t h a t h e r m o c o u p l e c u r r e n t o f 9 .5A a n d a s in k t e m p e r a t u r e o f 7 0K . F o r s i n k t e m p e r a -t u r e s b e l o w 7 0 K , ( AT )m ~ w a s f o u n d t o d e c r e a s e v e r ys i g n i f ic a n t l y a n d t h e c o u p l e c u r r e n t s w e r e < 1 0 A .A l s o , c oo l i n g o f t h e c o l d j u n c t i o n d e c r e a s e s s i g n if i -c a n t l y as th e s i n k t e m p e r a t u r e a p p r o a c h e s a n d e x -c e e d s T c o f t h e s u p e r c o n d u c t i n g e l e m e n t .U n d e r i d e al co n d it io n s th e m a x i m u m t e m p e r a t u r ed r o p s h o u l d b e - 9 d e g r e e s K a t 8 0 K w h i c h i n d i c a t e st h a t a f u r t h e r 3 0 % i n c r e a s e i n ( A T ) ~ x c a n b e o b t a i n e dw i t h i m p r o v e d d e s i g n . I n f a c t , t h e c r o s s - s e c t i o n a l a r e ao f t h e s u p e r c o n d u c t i n g p a s s i v e e l e m e n t w a s a f a c to ro f f o u r l a r g e r t h a n t h a t r e q u i r e d f o r o p t i m u m p e r f o r-m a n c e , 3 a n d t h e r e s u l t s p r e s e n t e d h e r e c a n b e i m m e -d i a t e l y i m p r o v e d b u t r e q u i r e s e x p e r t i s e i n t h e f a b ri -c a t i o n o f r a t h e r s m a l l d e v i c e s .U s i n g E q . ( 3) f o r d e r i v a t i o n o f t h e f i g u r e o f m e r i t , Z ,o f t h e B i 0 . s s S b 0 1 ~ 1 2 3 c o u p l e , F i g . 2 s h o w s t h e v a r i a -t i o n o f Z w i t h t h e s i n k t e m p e r a t u r e f o r o u r P e l t i e rr e f r ig e r a to r . A s ex p e c te d , th e m e a s u r e d m a x i m u mf i g u r e o f m e r i t i s 2 .0 x 1 0 -3 K -1 a t 8 0 K w h i c h i s - 6 0 %o f t h e v a l u e g i v e n f o r t h e n e g a t i v e b r a n c h e l e m e n t o f3 . 5 x 1 0 -3 K -1 a t 8 0 K . T h i s l a t t e r v a l u e a p p e a r s t o b er e l a t i v e c o n s t a n t a r o u n d 7 5 K w h i l e f o r o u r r e f r i g e ra -t o r o p e r a t in g a t 7 0 K t h e m e a s u r e d f i gu r e o f m e r i ti n c r e a s e s t o 3 .3 x 1 0 3 K . T h i s f u r t h e r i n c r e a s e o f Z f ors i n k t e m p e r a t u r e s i n th e r a n g e 7 0 t o 8 0 K is r a t h e ru n e x p e c t e d a s z i s a l m o s t c o n s t a n t f o r t h e a c t i v ee l e m e n t o v e r t h e t e m p e r a t u r e r a n g e 7 0 - 1 0 0 K , w h i l et h e t h e r m a l c o n d u c t i v i t y of t h e s u p e r c o n d u c t i n g e l e -m e n t i n c r e a s e s s i g n i fi c a n t l y i m m e d i a t e l y b e l o w t h et r a n s i t i o n t e m p e r a t u r e . t 3 T h u s , Z f o r t h e c o u p l e s h o u l dd e c r e a s e c o n t i n u o u s ly f o r s i n k t e m p e r a t u r e < 8 0K .F u r t h e r w o r k i s b e i n g u n d e r t a k e n t o e l u c i d a te t h isp o i n t .T h e m o s t i m p o r t a n t p a r a m e t e r t h a t d e t e r m i n e s t h ee f f e c t i v e n e s s o f t h e P e l t i e r r e f r i g e r a t o r i s t h e c o ef fi -


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c i e n t o f p e r f o r m a n c e w h i c h s h o w s t h e r e l a t i o n s h i pb e t w e e n t h e r a t e o f c o ol in g a n d t h e e n e r g y e x p e n d e df o r t h i s c o o l i n g .F r o m G o l d s m i d, 2 t h e m a x i m u m c o e ff i ci e n t o f p e r-f o r m a n c e m a y b e w r i t t e n a s

w h e r e T M is t h e m e a n t e m p e r a t u r e ( T i . k + T o~d)/2.F i g u r e 4 s h o w s Om ~p l o t t e d a s a f u n c t i o n o f ( T ~ k - T o~a)= A T f o r d if f e r e n t s i n k t e m p e r a t u r e s . I t i s c l e a r ri n c r e a se s w i t h d e c r e a s in g T ~ k r e a c h i n g a m a x i m u mv a l u e a t 7 0 K . I n f a c t , t h i s s i n g l e e l e m e n t d e v ic e f i r s tb e c o m e s e f f i c i e n t , t h a t i s , r > 1 w i t h a c o o l i n g o f 2 . 3d e g r e e s K a t 7 0K , a n d w o u l d b e c o m m e r c i a l l y v i a b lei n t h e f o r m o f a m u l t i - e l e m e n t d e v ic e o p e r a t i n g w i t ha s in k t e m p e r a t u r e a t 7 7K .T o f u r t h e r c h a r a c t e r i z e o u r s i n g l e e l e m e n t c o u p le ,w e h a v e c a l c u l a t e d t h e c o o li n g p o w e r o f t h e c o u pl e ,t h a t i s , t h e P e l t i e r a b s o r p t i o n p o w e r a t t h e c o l dj u n c t io n m i n u s h a l f o f h e J o u l e h e a t g e n e r a t e d i n t h ec o u p l e a t a p p l i e d c u r r e n t s o f 3 , 6 , a n d 9 A . H e r e , w eh a v e a s s u m e d t h a t t h e o n l y h e a t c o n d u c ti o n i n t h ec o u p le i s d u e to P e l t i e r a d s o r p t i o n a n d J o u l e h e a t i n g ,h a l f o f w h i c h i s t r a n s m i t t e d t o t h e c o ld j u n c t i o n , w i t ht h e o t h e r h a l f t o t h e h o t j u n c t io n . F i g u r e 5 s h o w s t h er e s u l ts f or t h e r a n g e o f h o t ju n c t i o n t e m p e r a t u r e s 7 0 -1 50 K . I n p r o d u c i n g t h e s e c u r v e s , e x p e r i m e n t a l v a l-u e s w e r e u s e d f o r th e S e e b e c k c o e f f ic i e n t o f t h e t w ob r a n c h e s a n d t h e t o t a l e l e c t r ic a l r e s i s t a n c e o f t h ec o u pl e . N o t e t h e s u d d e n d e c r e a s e o f J o u l e h e a t i n g a tt h e s u p e r c o n d u c t in g t ra n s i t io n t e m p e r a t u r e a n d t h e

s i g n i fi c a n t c o o li n g p o w e r o f t h e s i n g le e l e m e n t w h e nt h e p o s i t iv e b r a n c h b e c o m e s s u p e r c o n d u c t i n g . M a x i -m u m c o o li n g o c c u r s f o r h o t j u n c t i o n t e m p e r a t u r e sb e t w e e n 7 5 - 8 0 K w i t h t h e P e l t i e r a b s o r p ti o n d e c r e a s -i n g w i t h d e c r e a s i n g h o t j u n c t i o n t e m p e r a t u r e s a s a l s of o u n d by N a k a n o a n d H a s h i m o t o 2 T h i s d e c re a s ee x p l a i n s t h e d i m i n u t i o n o f (A T)m = a t l ow t e m p e r a -t u r e s .

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0-2 0-1, 0.6 0 8 1.0Applied m a g n e t i c f i e l d { T )

F i g . 7 . ( a ), ( b ) , a n d ( c ) a r e : v a r i a t io n o f t h e t e m p e r a t u r e d r o p w i t h a p p l i e d c o u p l e c u r r e n t f o r d i f f e r e n t l e n g t h s o f c o p p e r w i r e i n t h e p a s s i v e b r a n c ha n d d i f f e r e n t s i n k t e m p e r a t u r e s . ( d ) T o t a l t e m p e r a t u r e d r o p a s a f u n c t i o n o f a p p l i e d m a g n e t i c f ie l d f o r a fi x e d l e n g t h o f c o p p e r w i r e , a c o u p l e c u r r e n ts e t t o t h e v a l u e t o o b t a i n ( A T )m a t t h e d i f fe r e n t T ,~ ,k e m p e r a t u r e s w i t h n o a p p l i e d m a g n e t i c fi e ld .P a s s i v e S u p e r c o n d u c t i n g B r a n c h a n d a nA p p l i e d M a g n e t i c F i e ld

A t e a c h si n k t e m p e r a t u r e s t u di e d , t h e th e r m o -c o u p l e c u r r e n t c o r r e s p o n d i n g t o (A T)m a w a s a p p l i e d t ot h e r e f r i g e ra t o r a n d m a i n t a i n e d c o n s t a n t, w h i l e t h ee f f e ct o f a n a p p l i e d m a g n e t i c f ie l d (r a n g e 0 - 1 T ) w a st h e n r e c o r d ed . T h e i n c r e m e n t a l c h a n g e i n te m p e r a -t u r e w i t h f i e ld is s h o w n i n F i g . 6 a w h i l e F i g. 6 b s h o w st h e t o t a l t e m p e r a t u r e d r o p ( AT ) w i t h a p p l i e d f ie l d f o ra t h e r m o c o u p l e c u r r e n t O f I a x. I t s h o u l d b e n o t e d t h a ta t 7 0 K n o i n c r e a s e i n ( AT )m ~ w a s f o u n d w i t h a p p l i e df ie l d, w h i l e t h e f i g u r e o f m e r i t f o r t h e c o u p l e r e a c h e di t s m a x i m u m v a l u e a t 7 5 K , w h i c h i s b e l o w t h e s u p e r -c o n d u c t in g t r a n s i ti o n t e m p e r a t u r e o f t h e s u p e r c o n -d u c t i n g m a t e r i a l .T h e m a x i m u m t e m p e r a t u r e d r o p o f 7 .3 d e gr e e s Kw a s o b t a i n e d a t a s i nk t e m p e r a t u r e o f 7 5 K in a na p p l i e d m a g n e t i c f i el d o f 0 . 07 T . T h i s r e s u l t i s c on -s i s t e n t w i t h t h e w o r k o f Y im a n d A m i t h 8 w h o s h o wt h a t fo r s i n g l e c ry s t a l s o f t h e a l l o y B i0 .8 8S bo.1 2 t h et h e r m o e l e c t r i c f i g u r e o f m e r i t ( a l o ng t h e t r i g o n a l a x i s)i s a m a x i m u m a t a t e m p e r a t u r e o f 8 0 K fo r a m a g n e t i cf i e l d - 0 . 0 3 T a p p l i e d a l o n g t h e b i s e c t r i x a x i s; w h i l e f o rt h e h i g h e r z a l l o y B i o . s s S b 0 .1 ~ t h e e q u i v a l e n t f i e l d i s- 0 .1 3 T . F u r t h e r , t h e m a x i m u m f i g u r e o f m e r i to b t a i n e d w a s 3 . 3 x 1 0 -3 K -1 a t 7 5 K w h i c h i s c l o s e t ot h a t e x p e c t e d fo r t h e p o l y c ry s t a l l i n e B i o . s s S b o .1 2 e l e -m e n t ( 4 .2 x 1 0 4 K -1 a t 7 5 K ) a s s i n g l e c r y s t a l l i n eBio.88Sbo.12 on ly in cr ea se s fr om 6 x 10 -3 to 7 10 -s K -1( Y i m a n d A m i t h ) 6 w i t h a n o p t i m u m m a g n e t i c f ie ld a ta t e m p e r a t u r e o f 8 0 K . T h u s , a 1 7 % i n c r e a s e i n z i se x p e c t e d f o r t h e p o l y c r y s t a l l in e a c t i v e e l e m e n t i n a no p t i m u m f i e ld , w i t h t h e e x p e r i m e n t a l i n c r e a s e o f0 .7 x 10 -3 K -~ r e p r e s e n t i n g a 2 7 % i n c r e a s e . T h i s m o r et h a n e x p e c t e d i n c r e a s e , a s w e l l a s th e w e l l d e f i n e dp e a k i n ( A T)m ~ a t 7 5 K , m a y b e a s s o c i a t e d w i t h ad e c r e a s e i n t h e t h e r m a l c o n d u c t i v i t y ~ o f t h e p o l y -c r y s t a l l i n e Y B C O w h e n p l a c e d i n a m a g n e t i c f ie l d.S u c h a d e c r e a s e i n ~ h a s b e e n f o u n d f o r si n g l e c r y s t a l so f Y B C O 4 a n d a g a i n f u r t h e r w o r k i s r e q u i r e d i n t h i sa r e a u s i n g b u l k a n d m e l t - pr o c e s s e d s u p e r c o n d u c t in gm a t e r i a l s .F r o m t h e c u r v e s o f F i g . 6, it i s s e e n t h a t t h e p a s s i v eb r a n c h w a s c o m p l e t e ly s u p e r c o n d u c t in g a t t e m p e r a -t u r e s b e l o w 8 5 K . H o w e v e r , e v e n w h e n t h e s u p e r c o n -

d u c t o r i s i n t h e n o r m a l s t a t e , t h a t i s , i t i s a m e t a l , i ts t il l m a i n t a i n s a s i g n i fi c a n t t e m p e r a t u r e d r o p f o rs i n k te m p e r a t u r e s i n t h e r a n g e 9 0 - 2 9 3 K . F u r t h e r ,t h e a d v a n t a g e o f u s i n g a h i g h e r T c s u p e r c o n d u c t i n gm a t e r i a l w i t h t h e r e l e v a n t c u r r e n t a n d m a g n e t i cp r o p e r t i e s w o u l d l e a d t o le s s c r i ti c a l it y w h e n o p e r a t -i n g a t a l i q u i d n i t r o g e n s i n k t e m p e r a t u r e .T h e u s e o f s i n g l e c ry s t a l l i n e B io .85 Sb o.1 5 a l l o y w i t ha p p l i e d m a g n e t i c f i e ld s h o u l d s h o w a s i g n i fi c a n t i n -c r e a s e i n t h e f i n a l f i g u r e o f m e r i t f o r t h i s s u p e r c o n -d u c t i n g p a s s i v e b r a n c h r e f r i g e r a t o r 2 S u p e r c o n d u c t -i n g m a t e r i a l w i t h th e r e l e v a n t c h a r a c t e r i s t i c s ( m e l tp r o c e s s e d m a t e r i a l ) i s n o w r e a d i l y a v a i l a b l e a n d w i t hf u r t h e r w o r k o n s in g l e e l e m e n t c o o l e r s i t s h o u l d n o t b ea b i g s t e p t o t h e d e v e l o p m e n t o f a p r a c t i c a l c o o l in gu n i t o p e r a t i n g a t a s i n k t e m p e r a t u r e o f 7 7 K .P a s s i v e M e t a l B r a n c h

W i t h t h e a c t i v e e l e m e n t b e i n g i d e n ti c a l t o t h a t u s e df o r t h e p r e v i o u s c o u p l e , F i g. 7 s h o w s t h e t e m p e r a t u r ed r o p , A T , a s a f u n c t i o n o f a p p l i e d c u r r e n t f o r d i f f e r e n tl e n g t h s o f C u w i r e . A n e x t e r n a l m a g n e t i c f i e l d w a so n l y a p p l i e d f o r t h e r e s u l t s s h o w n i n F i g . 7 c. I t i s c l e a rf r o m t h e r e s u l t s t h a t a l e n g t h o f c o p p e r w i r e > 4 1 0 m mo r a n a p p l i e d c u r r e n t > 1 0 A i s r e q u i r e d i f w e a r e t od e t e r m i n e e x p e r i m e n t a l l y t h e o p t i m u m l e n g t h o f w i r ea t b o t h l ow te m p e r a t u r e s a n d r o o m t e m p e r a t u r e .H o w e v e r , t h e c o p p e r r e s u l t s s h o w t h a t a t t e m p e r a -t u r e s - 8 0 K , t h e m a x i m u m t e m p e r a t u r e d r o p a t 10 Ai s - S K w h i c h is o n ly - 1 K l e s s t h a n t h a t f o r t h es u p e r c o n d u c t in g e l e m e n t . O p t i m i z a t i o n o f b o t h t y p e so f p a s s i v e e l e m e n t s w o u l d i n d i c at e a m a x i m u m t e m -p e r a t u r e d r o p - 7 K w h i c h w o u l d be c o n s is t e n t w i tht h e f i g u r e o f m e r i t o f t h e a c t i v e e l e m e n t ( - 9 d e g r e e sK ).I n t h e c a s e o f t h e a l u m i n u m p a s s i v e e le m e n t , t h e 4 5a n d 9 0 m m l e n g t h s g a v e ri s e to t h e l a r g e s t t e m p e r a -t u r e d r o p f o r a l l s i n k t e m p e r a t u r e s , w h i l e t h e l o n g e rl e n g th s s h o w e d th e e x p e c t e d t e m p e r a t u r e s e q u e n c ef o r (A T)= ~. H o w e v e r , fo r s i n k t e m p e r a t u r e s - 8 0 K , t h em a x i m u m t e m p e r a t u r e d r op o b ta i n ed w a s - 4 d e g r e e sK , w h i c h s u g g e s t s f u r t h e r w o r k i s r e q u i r e d f o r t h i sp a s s i v e e l e m e n t . A c c o r d i n g t o T r o d a h l a n d F e e , 1~ h ea l u m i n u m p a s s i v e e l e m e n t s h o u l d g iv e a l a r g e r t e m -p e r a t u r e d r o p t h a n t h a t o f c o p p e r . T h i s h a s n o t b e e nt h e c a s e a n d i t m a y a r i s e f r o m t h e d i f f ic u l t y o f o b t a i n -


7/7

Cryogenic Thermoelectric Cooler with a Passive Branch 921ing a very low contact resistance between the alu-min um wire and copper sinks.Since the results for the long copper length appearsclose to the expected optimum results, a magneticfield was applied to the couple whe n a cur ren t flowedthrough the junction corresponding to (AT)m~. Figure7d shows the t emp era ture drop as a function of ap-plied magnetic field. The low temper atu re results arevery similar to those obtained with the superconduct-ing passive ele ment, the significant differences being(AT)m~ - 1.5K smaller for sink temperatures -80K,while (AT)m~ - 6 degrees K large r for sink tem-peratures of 150 and 293K. Also, the curves corre-sponding to these latter temperatures are signifi-cantly different to those of Fig. 6b for the super-conducting passive el ement.

CONCLUSIONThe results clearly confirm the original work ofGoldsmid et al2 concerning the effectiveness of a

passive ele ment for a Peltier ref rigerat or operating atlow tempera tures . Even without optimization of theform factors for the passive elements, it is clear tha tat low, hot junction (sink), tempera tures , the super-conductor/active couple provides cooling performance,with and without an applied magnetic field, that isnot totally consistent with th at expected theoreticallyfor the active element ofpolycrystallineBi0 ssSb0.12. Asan a lternat ive passive element, lengths of pure Cuand A1 wire have been used. Optimization of thelengths was not achieved for eithe r wire type, but inthe case of Cu the results were only 1.5 degrees Klessfor the temperature drop at Ts~ -80K than thatobtained for the case of a superconducting passiveelement. At Ts~ = 150 and 293K, (AT), with an appliedmagnetic field, was very large and significantly dif-ferent to the equivalent result for the metallicsuperconducting mater ials branch.Finally, these results show clearly that using asuperconducting passive element, a magnetic fieldand a single crystall ine Bi0.s~Sb0.1~ act ive element,

thermoelec tric coolers should be able to be fabricatedtha t are commercially efficient when opera ted at asink temp era ture of 77K, have a substa ntial perfor-mance when fabricated in cascade form and shouldproduce an ideal single element temp era ture drop aslarge as 16 degrees K in zero field or 27K in a 0.12 Tfield.3,6,9 In fact, by opt imizing th e form factors for aparallel combination of a met al a nd superc onductoras the passive branch, it should be possible to takeadvantage of both materials, above and below thesuperconducting transition temperature .

A C K N O W L E D G M E N TThe authors would like thank Professor H.J.Goldsmid for stimula ting discussions and suggestionsduring this work.

R E F E R E N C E S1. A.F. Ioffe, S e m i c o n d u c t o r T h e r m o e l e m e n t s a n d T h e r m o el e c-tr ic Cooling, (London: Infosearch, 1957).2. H.J. Goldsmid, Thermoe lec t r ic Re f r igera t ion , (New York:

Plen um Press, 1964).3. H.J. Goldsmid, K.K. Gopinathan, D.N. Matthews, K.N.R.Taylor and C.A. Baird, J . Phys . D:Appl . Phys . 21,344 (1988).4. G.E. Smith and R. Wolfe, J . App l . Phys . 33, 841 (1962).5. R. Wolfe and G.E. Smith, Appl . Phys . Le t t . 1, 5 (1962).6. W.M. Yim and A. Amith, Sol id -S ta te E lec t ron . 15, 1141(1972).7. Z.M. Dashevskii, N.A. Sidorenko, N.A. Tsvetkova, C. YaSkipidarov and A.B. Mosolov,Supercond . Sc i . Techno l . 5,690(1992).8. T. Nakano and J. Hashimoto, Jpn . J . App l . Phys . 33, L1728(1994).9. M.G. Fee, Appl . Phys . Le t t . 62, 1161 (1993).10. H.J. Trodah l and M.G. Fee, A d v a n c e s i n S u p e r c o n d u c t i vi t yIV: Proc. 6 th In t l . Symp. on Superconduc t iv i t y ( ISS '93) ,October, 1993, Hiroshima (Springer-Verlag, 1994), p. 1215.11. G.K. White, E x p e r i m e n t a l T e c h n i q u es i n L o w - T e m p e r a t u r ePhys ics , (Oxford: Clarendon Press, 1959).12. W.M. Yim and F.D. Rosi, Sol id -S ta te E lec t ron . 15, 1121(1972).13. B.M. Suleiman, I. U1-Haq, E. Karawacki, A. Maqsood andS.E. Gustafsson, Phys . Rev . B 48, 4095 (1993).14. R.A.Richardson, S.D. Peacor, F. Nori and C. Uher, Phys . Rev .Le t t . 67, 3856 (1991).

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Cryogenic Thermoelectric Cooler With a Passive Branch