Niobium antidiffusion barrier reactivity in tin-doped, in situ PbMo6S8-based wires

Journal o f AUoys a n d Compounds, 178 (1992) 447-454 JAL 5045

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Niobium antidiffusion barrier reactivity in tin-doped, i n s i t u PbMo6Ss-based wires P. Rabiller, R. Chevrel a n d M. S e r g e n t Laboratoire de C h i m i e Mindrale B, URA 254 CNRS, Universitd de Rennes I, A v e n u e d u Gdndral Leclerc, F-35042 Rennes Cddex (France)

D. Ansel Laboratoire de MdtaUurgie, INSA de Rennes, URA 254 CNRS, 20 A v e n u e des Buttes de Co,sines, F-35042 Rennes Cddex (France)

M. B o h n Laboratoire Microsonde, Centre de Brest de I'IFREMER, B P 70, F-29253 Plouzand (France)

(Received August 9, 1991; in final form October 14, 1991)

Abstract The reactivity of the niobium antidiffusion barrier at the time of heat treatment is presented in the case of Nb-Cu-sheathed, i n s i t u PbMo6Ss (PMS-) based wires doped with 2.6 at.% Sn. As a result of this reactivity, a lamellar layer grows axially towards the centre of the wire, consuming a significant amount of sulphur at the expense of the formation of the superconducting PMS phase. Investigation of the growth kinetics by scanning electron microscopy under variable experimental conditions indicates that slightly decreasing the heat treatment temperature or increasing the powder densification can considerably minimize the niobium reactivity. In the light of results from electron microprobe analysis together with X-ray powder diffraction data, the possible existence of a new Pbo.l~Nb2Sa compound is discussed.

1. I n t r o d u c t i o n The Chevrel p h a s e PbMo~Sa (PMS) is o n e o f the v e r y p r o m i s i n g sup e r c o n d u c t i n g materials f o r building high field m a g n e t s [1, 2]. Its critical t e m p e r a t u r e T¢ a n d u p p e r critical m a g n e t i c field H¢2, c l o s e t o 15 K and 60 T (at 0 K) respectively, c o u l d allow a p p l i c a t i o n s at liquid helium t e m p e r a t u r e a n d in the 20 T r a n g e p r o v i d e d t h e critical c u r r e n t d e n s i t y J¢ in PMS r e a c h e s a b o u t 5 × 1 0 s A m -2 at t h a t field inside the wires. T h e s e wires are m a d e u s i n g t h e so-called " p o w d e r m e t a l l u r g y t e c h n i q u e " , w h i c h c o n s i s t s o f d r a w i n g ( d o w n to the d e s i r e d c r o s s - s e c t i o n ) a metallic t u b e filled with PMS p o w d e r ( p r e - r e a c t e d r o u t e ) o r a m i x t u r e o f its p r e c u r s o r s ( i n s i t u route). F o r cold p r o c e s s i n g , t h e c a s i n g material is usually c o p p e r . H o w e v e r , d u r i n g h e a t t r e a t m e n t s d o n e after d r a w i n g in o r d e r to either s y n t h e s i z e the s u p e r c o n d u c t i n g PMS p h a s e ( i n s i t u r o u t e ) or r e s t o r e the PMS s u p e r c o n d u c t i n g p r o p e r t i e s ( p r e - r e a c t e d route), n o n - d e s i r e d CuxMosSs

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448

Fig. 1. SEM image showing the growth of a lamellar layer at the innermost niobium barrier surface in an in situ PMS-based wire doped with 2.6 at.% Sn and heat treated at 1000 °C for 3 h. c o m p o u n d s would tend to form in pr e f er e n ce to PMS. This implies that an antidiffusion barrier must be put between the external c o p p e r sheath and the inner powder. For cold-processed wires the antidiffusion barrier is usually either niobium or tantalum. Maximum J¢ values of about 2 × 1 0 s A m -2 (at 4.2 K and 20 T) have been obtained for tin-doped i n s i t u wires using either N b - C u or T a - C u casing [ 3 - 5 ] with typical 1000 °C, 20 min heat t reat m ent to synthesize PMS in the wires. A short annealing time does not allow a com pl et e chemical reaction for the synthesis of PMS, but increasing it causes grain broadening and favours antidiffusion barrier reactivity, which both degrade the J¢ values [3]. In the particular case of Nb-Cu-sheathed wires, concom i t ant significant diffusion occurs during heat t r e a t m e n t in the n e i g h b o u r h o o d of the innermost niobium barrier surface and involves all elements present. As a result a thin lamellar layer grows (see Fig. 1). This layer has received little attention and has sometimes b e e n r e p o r t e d in the literature as being m ade of NbS~ c o m p o u n d s [6-9]. We have intentionally ext ended the range of fabrication conditions (powder stoichiometry, pow de r densification, heat t r e a t m e n t t e m p e r a t u r e and time) in order to study m o r e carefully the trends and c o n s e q u e n c e s of the niobium barrier reactivity. In this p a p e r we report on wires doped with 2.6 at.% Sn (tin is added, not substituted for lead), which have been extensively studied since in our laboratory they gave m a x i m u m Jc values.

2. E x p e r i m e n t a l details The wires investigated in this study have an out er diameter ranging from 0.3 to 0.8 mm. The starting p o w d e r is a mixture of PbS, tin, MoS2 and m o l y b d e n u m in suitable ratios in order to yield an overall stoichiometxy close to PblSno.4Mo6Ss. Changing the overall stoichiometry modifies the niobium sheath reactivity and its c o n s e q u e n c e s on the superconducting properties of the wire [ 10]. We call the densification of the pow der the ratio

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of the apparent density of the powder in the Cu-Nb tube to the theoretical density of the powder, e x p r e s s e d as a percentage. This densification is about 4 0 % - 6 0 % before drawing and close to 85% in the final p r o c e s s e d wire [ 11 ]. Thus, depending on the initial densification of the powder, the cross-sectional area of the powder core ranges from 6% to 13% of the total cross-section of the wire. Short lengths of wire (about 10 cm) sealed at both extremities have b een heat treated at t e m p e r a t u r e s ranging from 750 to 1050 °C for times from a few minutes to several tens of hours. Hermetically sealed crucibles of niobium filled with variable PMS precursor mixtures have also b e en used, providing a convenient way to tune the powder stoichiometry or densification and to obtain sufficient quantities of the lamellar layer. Cuts of the samples have been e m bedded in epoxy resin under vacuum and then well polished and carbon coated for scanning electron m i croscopy (SEM) investigations (JEOL 35CF) and quantitative electron microprobe analysis (CAMECA SX50, IFREMER, Brest) on transverse cross-sections of the samples. X-ray (Cu Kal) pow der diffraction experiments have been carried out on fragments of the lamellar layer using an INEL-CPS120 apparatus.

3. R e s u l t s

and discussion

3.1. G r o w t h k i n e t i c s

The lamellar layer (Fig. 1) grows axially towards the centre of the wire. Figure 2 shows the square of the mean lamellar layer thickness, e 2, v s . the heat tr eatmen t time t at 1000 °C as a function of the internal radius of the niobium sheath, R0 (different values of Ro can be provided by using wires 4000 [] R o = 67~m O R o = 84~m E

3000

2000

looo

0

10 Heat-treatment

20 time

(h)

Fig. 2. Growth kinetics of the lamellar layer for wires doped o f t h e i n i t i a l i n n e r m o s t d i a m e t e r o f t h e n i o b i u m s h e a t h , Ro.

with 2.6 at.% Sn as a function

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with different outer diameters). An initial linear behaviour followed by a plateau (saturation) can be observed. The saturation thickness es normalized by the initial innermost niobium sheath radius R0 gives a constant ratio (es/ R0) 2 close to 0.25 for each wire diameter. Electron microprobe analysis revealed that saturation of the lamellar layer thickness corresponds to complete consumption of sulphur [3]. With increasing t the inner diameter R of the niobium sheath increases above its initial value R0. The proportional behaviour of e ~ vs. t before saturation has been seen only in the case of wires doped with excess tin. It is observed in the complete temperature range from 800 to 1050 °C. The growth velocity e2/t has been seen not to follow the Arrhenius law but rather follows a power law of the temperature T (K) with an exponent close to 40 as shown in Fig. 3. This suggests that the lamellar layer growth may probably not result from one unique diffusion process. The relative diffusion of tin towards niobium is particularly enhanced by lowering the heat treatment temperature. This leads to the formation of the superconducting NbaSn compound [10]. The changes in the species which are formed, depending on the temperature, make plausible the statement of the existence of several diffusion mechanisms whose prevalences evolve with temperature. This could explain the observed discrepancy with the Arrhenius law. The growth kinetics of the lamellar layer seems to be also influenced by the densification of the powder. This is illustrated on Fig. 4, where the square of the thickness obtained with a 1000 °C, 15 h heat treatment (performed in niobium crucibles) is plotted against the densification of the powder. One can clearly see that increasing the densification of the powder in the wires above its actual value of 85% could be helpful to minimize the niobium reactivity. The influence that the densification of the powder can have on the growth kinetics is not evident. Although we have no direct proof for it, we believe that diffusion processes involving gas phase transport probably occur in the wire and thermodynamic considerations could explain the observed trend.

2

e / t "~ T(K) .~_ E

=. O

m

j

Temperature

(logarithmic

scale)

Fig. :3. I n f l u e n c e o f t h e h e a t t r e a t r a e n t t e m p e r a t u r e o n t h e l a m e n a r l a y e r g r o w t h rate.

451

20OOO

15ooo e-

lO000

-i

5000

0

I

0

60 Densification

I

80 of the

00

powder (%)

Fig. 4. Influence of the densification of t h e p o w d e r o n the lamellar layer t h i c k n e s s for 1000 °C, 15 h h e a t treatment. E x p e r i m e n t s have b e e n carried out in n i o b i u m crucibles. The s a m e h e a t t r e a t m e n t of a wire leads to an estimation of the densification of a b o u t 8596, in good a g r e e m e n t with gravimetric extrapolation.

3.2. Electron microprobe analysis The following points derive from the elementary concentration profiles obtained from the quantitative X-ray analysis and apply to both niobium crucible samples and wires [3] heat treated at 1000 °C. For short annealing times (which usually yield maximum Jc values), sulphur diffusion towards the niobium sheath is clearly observable but no chemical composition of the lamellar layer can be determined precisely. It is therefore not surprising that not much information is available yet in the literature about the lamellar layer composition. When a long enough annealing time is used, trends can be clearly identified and different regions resulting from the niobium sheath reactivity can be summarized as shown in Fig. 5. The lamellar layer with homogeneous concentrations of elementary lead, niobium and sulphur (3, 38 and 57 at.% respectively) extends over a thickness of about e. The tin distribution, in contrast to that of lead, is not homogeneous [3] and its mean elementary concentration does not exceed 2 at.%. Thus we think that the chemical composition in the lamellar layer is close to Pbo.15Nb2S3. From the standpoint of elementary concentrations the Nb:S ratio of 2:3 does not correspond to any reported Nb-S binary compound. Right inside the lamellar layer, before the mean elementary contents correspond to those of PMS of actual radius R I after annealing time t, there is a region where molybdenum accumulates which is also a " g a p " in both sulphur and lead contents. At the saturation thickness all available sulphur is absorbed by the lameUar layer and no PMS phase remains, so R~--0. In

452

100

Mo ,

so

S 60

Nb c~ 40

R

R-e Distance

111 (a.u.)

Fig. 5. S c h e m a t i c illustration o f the diffusion o f e l e m e n t s c a u s e d by n i o b i u m b a r r i e r reactivity.

The peak in tin concentration has been included arbitrarily to illustrate its inhomogeneous distribution. that case, equating the amount of sulphur per unit length of wire absorbed in the lamellar layer to that available in the powder core before heat treatment leads to an estimation of (es/Ro) 2 close to 0.3 (taking into account a densification of the powder of about 85% which lowers the amount of sulphur compared to that available in a 100% dense powder). This is in good agreement with the value deduced from SEM. The estimated available PMS cross-section is roughly proportional to RI 2. After a typical heat treatment of 10-20 min at 1000 °C it can be 15%-30% lower than that defined by R02. Thus minimizing the lamellar layer growth is of great importance in order to optimize the superconducting properties of the PMS-based wires. This is also true since during the lamellar layer growth there will occur a degradation of the intrinsic superconducting properties of the PMS phase. [10].

3.3. X - r a y diffraction Intentionally long heat treatments performed on niobium crucibles enabled us to take off sizable amounts of the lamellar layers to perform X-ray (Cu Kal) powder diffraction experiments. The best recorded diffraction pattern is shown in Fig. 6. Traces of metallic tin, lead or molybdenum are not unlikely to be present among the collected samples and so it is not excluded that they could contribute to the observed diffraction pattern (the positions of the possible coinciding peaks axe indicated in Fig. 6). No reported binary Nb-S or ternary P b - N b - S (or Sn-Nb-S) compounds fit the experimental diffraction data. However, we have noticed some similarity with the theoretical monoclinic powder diffraction pattern of the Nb2Se3 compound [12]. This latter c o m p o u n d has also been reported as being thought to possess a layerlike structure [ 13 ]. The possibility that a similar and new Nb2S3 compound

453 A

lO

20

30

40

50

60

70

Diffraction angle (2 Theta)

Fig. 6. Experimental X-ray (Cu Kal) powder diffraction pattern of the lamellar layer fragments. Peaks indicated by A may be partly due to the presence of metallic traces of tin, lead or molybdenum.

could exist is thus s uppor t e d by the elementary concentration analysis together with the diffraction data. In that event, as in the Nb3S4 c o m p o u n d [14], one has to consider the possibility of lead filling partically insterstitial sites in the lattice.

4. Conclusions We have seen that the reaction towards the niobium barrier in tin-doped, i n s i t u PMS-based wires at the time of heat t r e a t m e n t p r o c e e d s at the expense of optimum superconductivity in the wires, since the am ount of sulphur a b s o r b ed by the lamellar layer growth is not available for formation of the s u per co n d u ctin g PMS phase. The new routes of producing wires we are currently exploring favour b o th decreasing the heat t r eat m e nt t e m p e r a t u r e and increasing the powder densification, thus minimizing the niobium barrier reactivity. Electron m i c r o p r o b e analysis and X-ray pow der diffraction make plausible the existence of a new Pb0.~sNb2S3 compound. An on-going effort is being m a d e to p r o d u c e larger crystal plates of this c o m p o u n d to provide accurate X-ray diffraction data from which the structure could be established without ambiguity.

Acknowledgments The authors wish to thank L. Le Lay (Applied Superconductivity Center, University of Wisconsin, Madison, USA.) for helpful discussions. This work received financial s u p p o r t from the Agence Fran~aise p o u r la Makrise de l'Energie and Minist~re de la Recherche et de la Technologie within a E u r o p e a n p r o g r a m m e Eu r e ka 96 coordinated in France by Alsthom.

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