TWO NEW METALLIC MATERIALS WITH HIGH SPECIFIC HEAT FOR SUPERCONDUCTOR STABILIZATION

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TWO NEW METALLIC MATERIALS WITH HIGH SPECIFIC HEAT FOR SUPERCONDUCTOR

STABILIZATION

B. Barbisch, K. Kwasnitza

To cite this version:

B. Barbisch, K. Kwasnitza. TWO NEW METALLIC MATERIALS WITH HIGH SPECIFIC HEAT

FOR SUPERCONDUCTOR STABILIZATION. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-

561-C1-565. �10.1051/jphyscol:19841114�. �jpa-00223583�

J O U R N A L DE PHYSIQUE

Colloque C1, supplCment a u n o 1, Tome 45, janvier 1984 page C 1-56 1

TWO NEW M E T A L L I C MATERIALS WITH H I G H S P E C I F I C HEAT FOR SUPERCONDUCTOR S T A B I L I Z A T I O N

B.J. Barbisch and K. ~ w a s n i t z a +

I n s t i t u t e of Solid S t a t e Physics, ETIf-fi6nggerberg, Cff-8093 Ziirich, Slvitzer land

Resume

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Nous avons mesure l a c h a l e u r s p e c i f i q u e de deux composes m e t a l - l i q u e s

a

basse temperature. En r a i s o n de t r a n s i t i o n s de phase de types d i f f e r e n t s , ces composes de t e r r e s r a r e s m o n t r e n t des anomalies t r 6 s im- p o r t a n t e s dans l e u r c a p a c i t e c a l o r i f i q u e , ce q u i l e s r e n d t r e s i n t e r e s - sants pour l a s t a b i l i s a t i o n de supraconducteurs techniques. Les p r e m i e r s e s s a i s d ' a p p l i c a t i o n s o n t p r e s e n t e s .

A b s t r a c t

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We r e p o r t o u r measurement o f t h e low temperature s p e c i f i c h e a t o f two m e t a l l i c r a r e e a r t h compounds i n magnetic f i e l d s up t o 7 Tesla.

Due t o d i f f e r e n t t y p e s o f phase t r a n s i t i o n s , these compounds e x h i b i t v e r y l a r g e peaks i n t h e h e a t c a p a c i t y which makes them a t t r a c t i v e f o r s t a b i l i - z a t i o n o f t e c h n i c a l superconductors. F i r s t attempts f o r a p p l i c a t i o n s a r e presented.

INTRODUCTION

Any d i s t u r b a n c e which occurs i n an a p p l i c a t i o n o f a superconductor wi 11 f i n a l l y l e a d t o an i n c r e a s e o f temperature. Examples f o r such d i s t u r b a n c e s a r e movements o f t h e superconductor due t o t h e magnetic p r e s s u r e on t h e w i n d i n g i n a magnet, c r a c k s i n t h e epoxy r e s i n o f t h e c o i l impregnation due t o thermal s t r e s s , o r i n - duced eddy c u r r e n t s by changing an a p p l i e d magnetic f i e l d o r t h e t r a n s p o r t c u r - r e n t i t s e l f . I f t h e i n c r e a s e i n temperature i s more than t h e c r i t i c a l tempera- t u r e Tc, t h e conductor w i l l l o s e i t s superconducting p r o p e r t i e s and t h e t r a n s - p o r t c u r r e n t i t s e l f w i l l warm up t h e conductor f u r t h e r m o r e by p r o d u c i n g j o u l e h e a t i n g . Among t h e d i f f e r e n t methods t o s t a b i l i z e t h e superconductor, t h e i n - crease o f t h e heat c a p a c i t y may become t h e most i m p o r t a n t one i n c e r t a i n a p p l i - c a t i o n s .

I n 1968 Hancox /1/ proposed t o use l e a d as an e n t h a l p y s t a b i l i z a t i o n m a t e r i a l . This means t h e use o f t h e e n t h a l p y o f substances w i t h h i g h s p e c i f i c heat which keep t h e temperature low i n t h e case o f a d i s t u r b a n c e . Lead has i n t h e v i c i n i t y o f t h e temperature o f l i q u i d helium, where superconductors a r e operated, a heat c a p a c i t y which i s one o r d e r o f magnitude h i g h e r than t h a t o f copper.

Rosenblum e t a1 /2/ proposed i n 1977 Gd2O3 and GdA103 as new m a t e r i a l s t o en- hance a d i a b a t i c s t a b i 1 it y . These two compounds a r e showing s p e c i f i c h e a t anoma- l i e s ; i n GdA103 due t o a n t i f e r r o m a g n e t i c o r d e r i n g a t 3.7 K and i n Gd2O3 one i s s u g g e s t i n g a complex combination o f c r y s t a l l i n e f i e l d s p l i t t i n g e f f e c t s and long range o r d e r . The main advantage o f t h i s substances i s t h a t t h e i r heat c a p a c i t y i s e x t r e m e l y h i g h due t o t h e phase t r a n s i t i o n s ; f o r a copper composite o f 7.2 wt% Gd2O3 t h e y r e p o r t e d an averaged v a l u e f o r t h e s p e c i f i c heat o f 4.2 mJ/gK between 3 K and 4 K, which i s about 60 times t h a t o f p u r e copper.

+Present address: Swiss I n s t i t u t e o f Nuclear Research (SIN), CH-5234 V i l l i g e n , S w i t z e r l a n d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841114

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PHYSIQUE

The m a t e r i a l s mentioned above a r e e i t h e r metal 1 i c o r have an e x c e p t i o n a l l y h i g h s p e c i f i c heat, b u t t h e y do n o t have t h e two f a v o r a b l e p r o p e r t i e s t h e same time.

Therefore, we p u t o u r i n t e r e s t i n t o t h e search o f m e t a l l i c m a t e r i a l s w i t h phase t r a n s i t i o n s because t h e m e t a l l i c c h a r a c t e r l e t s us expect a good t h e r m a l conduc- t i v i t y .

MATERIALS

The i n t e r m e t a l l i c compound PrCu2 c r y s t a l l i z e s c o n g r u e n t l y i n t h e o r t h o r o m b i c CeCu2 s t r u c t u r e . Our r e s u l t s f o r t h e h e a t c a p a c i t y p e r volume u n i t between 4 K and 10 K i n a p p l i e d magnetic f i e l d s up t o 7 T a r e shown i n f i g . 1.

1 The measured s p e c i f i c e a t p e r u n i t volume o f

+

PrCu2 i n a p p l i e d magnetic f i e l d s . For comparison t h e s p e c i f i c h e a t p e r u n i t volume o f NbTi and copper i s d i s p l a y e d a l s o .

The d e t a i l s o f sample p r e p a r a t i o n and c a l o r i c measurement w i l l be d e s c r i b e d elsewhere /3/. From t h e l i t e r a t u r e /4,5/ i t i s known t h a t t h i s substance under- goes a t 7 K a s t r u c t u r a l phase t r a n s i t i o n , an induced Jahn T e l l e r e f f e c t . The s p e c i f i c heat o f t h i s anomaly i s superposed b y a S c h o t t k y - e f f e c t w i t h a peak a t 12 K t o 13 K. The h i g h e s t v a l u e o f t h e s p e c i f i c h e a t a t B = OT i s 215 m ~ / c m ~ K a t 6.4 K. T h i s i s a f a c t o r 85 compared t o t h e s p e c i f i c h e a t o f copper p e r volume u n i t . For temperatures o f 4 K t o 6 K t h e s p e c i f i c h e a t l i e s about two o r d e r s o f magnitude h i g h e r t h a n t h a t o f copper.

The praseodymium hexaboride PrB6 i s another m e t a l l i c compound which e x h i b i t s a l o w temperature s p e c i f i c h e a t anomaly. FIg. 2 shows our s p e c i f i c h e a t measure- ments. The k i n d o f t h e phase t r a n s i t i o n i s n o t f u l l y understood. Lee e t a1 /6/

c l a i m e d t h a t PrB6 i s a n t i f e r r o m a g n e t i c a l l y o r d e r i n g and shows a tendency towards a f i r s t o r d e r t r a n s i t . i o n . Mc C a r t h y e t a1 /7/ r e p o r t e d t h e e x i s t e n c e o f a second phase t r a n s i t i o n a t a t e m p e r a t u r e o f 4.2 K which has o n l y a s m a l l i n f l u e n c e on t h e s p e c i f i c heat. By measuring d i f f e r e n t batches, we found a c e r t a i n dependence o f t h e s p e c i f i c h e a t on sample p r e p a r a t i o n , b u t t h e l a r g e amount i n h e a t capa- c i t y was always found. The v a l u e s d i f f e r e d b y about 15%. Between a t e m p e r a t u r e o f 4.5 K and 7 K, t h e s p e c i f i c h e a t i s l a r g e r t h a n 100 m ~ / c m ~ K, which i s h i g h e r a t t h e s e temperatures compared t o copper b y a f a c t o r o f 100 and 30, r e s p e c t i - v e l y . We found t h a t t h e PrB6 compound i s s t a b l e a t room temperatures, and no s i g n i f i c a n t o x y d a t i o n was observed i n a i r .

F i g . 2 The s p e c i f i c h e a t o f PrB, measured as a f u n c t i o n o f tempe- r a t u r e i n t h e range 3 K t o 10 K f o r d i f f e r e n t values o f magnetic f i e l d .

ATTEMPTS FOR TECHNICAL APPLICATION

The m a t e r i a l s d e s c r i b e d above are hard, t h e y cannot be pressed o r drawn i n t o w i r e s . F o r t h e use as s t a b i l i z a t i o n m a t e r i a l s i n s u p e r c o n d u c t i n g magnets o r cables, one has t o b r i n g them i n a f a v o r a b l e form.

With PrCu2 we f i r s t t r i e d t o i n c o r p o r a t e i t i n t o s o l d e r . PrCu2 powder was cove- r e d w i t h s i l v e r by e v a p o r i z a t i o n and t h e n mixed w i t h l i q u i d s o l d e r , i n o u r case l i q u i d indium. The c o v e r i n g w i t h s i l v e r was necessary because o t h e r w i s e t h e r e was no b i n d i n g between powder and s o l d e r . Then we checked t h e s p e c i f i c h e a t o f t h i s f i l l e d s o l d e r , b u t no peak was d i s c o v e r e d . We suppose t h a t t h e PrCu2 powder o x y d i z e d i n t h e a i r b e f o r e i t was covered and p r o t e c t e d b y s i l v e r , b u t t h e pos- s i b i l i t y t h a t PrCu2 r e a c t s w i t h i n d i u m can n o t be excluded.

The n e x t experiment we made was t o b r i n g i t i n t o v a r n i s h . F i g . 3 shows t h e mea- s u r e d s p e c i f i c heat o f a m i x t u r e o f GE 7031 v a r n i s h and PrCu2 powder.

h e a t measurements o f GE 7031 v a r n i s h . Sol i d

30 l i n e s r e p r e s e n t t h e expected q u a n t i t a t i v e c o n t r i b u t i o n s o f PrCu2 and v a r n i s h t o t h e

0 t o t a l s p e c i f i c heat, which should be t h e

sum o f t h e s e two curves.

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The s p e c i f i c h e a t i s n o t r a i s e d as much as one c o u l d expect f r o m t h e h e a t capa- c i t i e s o f t h e two p a r t s . The c h a r a c t e r i s t i c peak o f PrCuz i n t h e t e m p e r a t u r e range between 4 K t o 8 K i s r a t h e r suppressed, b u t n o t t h e enhancement over 8 K.

It seems t h a t t h e Jahn T e l l e r e f f e c t d o e s n ' t t a k e p l a c e and t h a t t h e l e v e l scheme o f t h e c r y s t a l f i e l d p r o d u c i n g t h e S c h o t t k y anomaly remains more o r l e s s unchanged. But t h e s p e c i f i c h e a t values a r e s t i 11 h i g h enough, and we be1 i e v e t h a t PrCu2 powder would be advantageous as a f i l l e r i n t h e epoxy used f o r c o i l i m p r e g n a t i o n .

I n c o l l a b o r a t i o n w i t h t h e company M e t a l l w e r k e Plansee ( A u s t r i a ) we produced a s i n t e r w i r e c o n t a i n i n g PrB6. PrB6 was g r i n d e d and mixed w i t h copper powder, f i l - l e d i n a copper tube, h e a t t r e a t e d under pressure, and t h e n drawn t o a w i r e . The l e n g t h o f t h e w i r e was about 40 m and t h e diameter was 0.65 mn. For t h e f i r s t a t t e m p t we used n o t more t h a n 5.2 wt% PrB6 (about 10% i n volume). F i g . 4 shows a microphotograph and i n F i g . 5 t h e s p e c i f i c h e a t o f t h i s w i r e i s r e p o r t e d .

F i g . 4 The c r o s s s e c t i o n o f t h e copper s i n t e r w i r e w i t h 5.2% PrB6. The d i a m e t e r o f t h e w i r e i s 0.65 mm.

2.5 CP

[$I

2.0

I I I I I I I I I I

- -

o Cu+ 5.2 wt% PrBs

- 7 1

1.5 - OoOCo OoOO 0 0

0 F i g . 5 The measured s p e c i f i c

0 h e a t o f t h e s i n t e r w i r e . The

o s p e c i f i c h e a t o f p u r e copper

1.0 ^- i s d i s p l a y e d by t h e s o l i d l i n e .

-

l l 1 I I l I I

2 3 4 5 6 7 8 9 10 1 1 1 2 T [ ~ ]

The h e a t c a p a c i t y o f t h e w i r e shows a c e r t a i n i n c r e a s e compared t o t h a t o f copper, a l t h o u g h one d o e s n ' t see t h e t y p i c a l sharp peak o f t h e PrB6 compound. I t seems t h a t , due t o t h e f i n e s i z e o f t h e PrBs g r a i n s , t h e peak i s smeared o u t i n temperatures o v e r s e v e r a l degrees. The s p e c i f i c h e a t i s r a i s e d b y a f a c t o r of 5. One has t o t a k e i n t o account t h a t t h e r e i s o n l y 5.2 wt% s t a b i l i z i n g m a t e r i a l i n t h e copper; we expect t h a t t h i s r a t e can be increased t o t h e double.

Furthermore one has t o n o t e t h e v e r y good e l e c t r i c a l and thermal c o n d u c t i v i t y of t h i s w i r e . We measured a f t e r a heat t r e a t m e n t o f 3 h and 6 0 6 C 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 t P C , and t h e r e s i d u a l r e s i s t a n c e r a t i o between P C and l i q u i d h e l i u m temperature. The r e s u l t s a r e 2.4 pQcm f o r t h e e l e c t r i c a l r e s i s t i v i t y , and 200 f o r t h e RRR. Using t h e Wiedemann-Franz law w i t h a L o r e n t z number L = 2.1

W Q / K ~ we c a l c u l a t e d f o r t h e w i r e a thermal c o n d u c t i v i t y k = 7,35 W/cm

K

a t 4.2 K. The d u c t i l i t y i s a l s o good, t h e minimum bending r a d i u s was 3 mn ( h e a t t r e a t e d as mentioned above).

We t h i n k t h a t such a w i r e s h a l l be u s e f u l i n t h e c o n s t r u c t i o n o f s u p e r c o n d u c t i n g c a b l e s . Soldered t o t h e superconducting w i r e s , i t i n c r e a s e s b o t h t h e copper c r o s s s e c t i o n and t h e s p e c i f i c h e a t .

CONCLUSION

We have p r e s e n t e d s t u d i e s o f t h e p o s s i b i l i t i e s t o use h i g h s p e c i f i c h e a t mate- r i a l s f o r s t a b i l i z i n g s u p e r c o n d u c t i n g a p p l i c a t i o n s l i k e c a b l e s o r magnets. We f o u n d t h a t one can i n c o r p o r a t e h i g h h e a t c a p a c i t y m a t e r i a l s as f i l l e r i n epoxy r e s i n s . F u r t h e r we have shown t h e p o s s i b i l i t y t o f a b r i c a t e a h i g h conducting, h i g h s p e c i f i c h e a t copper w i r e .

F u r t h e r r e s e a r c h t o i n c r e a s e t h e volume p r o p o r t i o n o f t h e s t a b i l i z i n g m a t e r i a l s seems necessary. Also some p r o o f i s needed t h a t these m a t e r i a l s work w e l l as s t a b i l i z e r s i n t h e case o f s h o r t t i m e d i s t u r b a n c e s .

ACKNOWLEDGEMENTS

We wish t o express our thanks t o P r o f . J.L. Olsen f o r h i s c o n t i n u o u s i n t e r e s t i n t h i s p r o j e c t . We acknowledge t h e sample p r e p a r a t i o n b y F. H u l l i g e r and t h e h e l p - f u l d i s c u s s s i o n s w i t h W. G l a t z l e (Plansee). For p r o d u c i n g t h e s i n t e r w i r e we a r e v e r y i n d e b t e d t o t h e Plansee company.

T h i s work was supported by t h e f i n a n c i a l a i d o f t h e Swiss Energy Foundation NEFF.

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