SCALE EFFECTS IN FILAMENTARY NIOBIUM TIN DIFFUSION COUPLES

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SCALE EFFECTS IN FILAMENTARY NIOBIUM TIN

DIFFUSION COUPLES

I .L. Mcdougall

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 8, Tome 39, aotit 1978, page C6-402

SCALE EFFECTS I N FILAMENTARY N I O B I U M T I N D I F F U S I O N COUPLES

IMI, P.O. Box 216, Witton, Birmingham B3 7BA, England

Ssum6.- On montre que les 6quilibres de phase peuvent avoir une influence sur la temp6rature critique Tc et le champ HF.2 dans les couples bronze-niobium. La force motrice chimique et l'importance du couple de diffusion influent sur le taux de rgaction et le courant critique.

Abstract.- It is shown that phase equilibria can effect critical temperature Tc and field Hc2 in bronze-niobium couples. Chemical driving force and diffusion couple size effect reaction rate and critical current.

CRITICAL PARAMETERS AND COUPLE TIN CONTENT.- The chemical properties of the reaction layer determine the stability of the superconducting state as measured by Tc. The upper critical field Hc2 is similarly determined and, consequently, critical current density J is influenced by chemical properties. It has been shown that in the Cu-Nb-Sn system the composition of the diffusion couple will effect the composition of the niobium tin compound/l/. In a dif diffusion couple the matrix volume is limited and the tin concentration in the bronze decays as reaction proceeds. Early work/2/ on NbsSn has indicated that less than stoichiometric composition gave the highest values of J

.

Values of T decrease as reaction temp is lowered from 700 to 450 OC. Table I shows the effect on Hc2 of different tin contents all starting with 7 at % Sn.

Table I : Chemically dependent critical properties and reaction conditions.

t o 1 R a t i o Bronze:

N b , 700 ' C

2.0 : 1

2.5 : 1

3.0 : 1

CRITICAL CURRENT AND FILAMENT SIZE.- A given composite should not exhibit a size dependence of Jc if equilibrium thermodynamics control compound forma- tion. A set of superconducting to normal transition

measurements were made at Rm detection cove-

ring a range of filament sizes, figure 1 . The width of transition was affected by the filament size and the total tin content the composite, table 11. Data

Fig. 1 : Critical current density for different sizes of filaments. A = 5.0, B = 4.4, C = 3.5, D = 3.2, E = 2.7 microns diameter. 6000 filament conduc- tor 2:l bronze:niobium ratio.

H c p v l a JAB measured 18.8T 21.5T 18.OT 16.5T 600

Table I1 : Width of superconducting transition 700°C 72 hrs reaction. 15.8 16.2 16.8 Reaction temp,2:l 780 OC 700 650 Jc x 1 0 d o v e r a l l A .

m2 a t 10T

lo-14nm l o - l h m

concerning the relationship between grain size and flux pinning efficiency has been published/3/ for large filaments i.e. >IOpm dia. This data on maxi- mum pinning force Fpmax as a function of time can be compared with the results abtained on fine fila-

T, K f o r 1 0 24 lOOhrs 17.7 17.8 17.9 17.3 17.5 17.7 16.9 17.2 17.4 2 : l 8 p m f i l s 4 ~ f l l . s i p f i l s 450 680 480 500 780 580 2.5:l 8 m f i l s _P 600 640 3 : l 5 p f i l s 590 610

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ments down to 2 pm dia. It can be seen in table 111 It has been observed that the growth rate of the that at short reaction times the pinning force does compound can affect superconducting properties/2/. not agree with those calculated. This discrepancy The existance of gradients in physical properties

within the layer may contribute to variation in su-

C a l c u l a t e d Ref 131

Measured 8 p f i l s

Reaction 6 5 0 ' ~ Refb/ 1.5 4.0 5.3 6.0 6.0 5.8 5.3 5.3

perconductivity. Microprobe analysis of composite layers is not very accurate due, mainly, to the effect of beam spread, but gradients in tin content have been observed which result from mass transfer mechanisms and phase boundaries. Recent observations by T.E.M. have demonstrated large gradients in grain Table 111 : Flux pinning force x I O ~ O N ~ - ~ for large

and small filaments. size across reaction layers/6/. Near the niobium,

the grain size is smallest. Diffusion coefficients m s t be due to either changes in pinning mechanisms can be in error by two orders of magnitude when or to modification of intrinsic properties. Compres- calculated without regard to grain boundary effects sion of the compound layer cannot account for the 171. Consequently flux, defect pinning interactions

size degradation of J

.

will not be constant across a layer or with growth.

COMPOUND GROWTH MECHANISM.- It was found that Tc CONCLUSIONS.- (a) The change in composition of the increased with time at constant temperature (Table bronze as diffusion proceeds should alter the equi- I) with a similar variation in Hc2, table IV. librium composition of NbsSn and both T and H

c2' Such changes have been observed starting with tin concentration of 5.5 to 7 at %.

(b) Increasing reaction time gives improved chemical properties but grain growth cau- ses reduction in J

.

Gradients of both composition Table IV : Change in Hc2 with time at 700'~ 2:l Bron- and grain size exist within layers.

ze : Nb ratio 10pm fils. _{(c) The best performance in fila-}

Numerous observations of compound growth rate141 have been fitted by an exponent in the region of 113. This is less than the classic value of 1/2 derived for a reaction at constant driving force controlled by mass transfer across a product layer. The appa- rent diffusion coefficient for tin transfer from bronze to niobium is not constant as filament size is changed to give various degrees of conversion,

mentary niobium tin is obtained when both composition and dislocation networks are optimised. Chan- ging the size of a composite will necessitate a change in reaction regime. For 7 at % tin bronze, 2.5: 1 appears the most flexible bronze to niobium ratio for optimisation of properties by giving ra- pid layer growth i.e. uniform GS while stabilising a niobium rich compound of high H

c2' table V. Standard analytical solutions in cylindri-

- - .- . . . - .-

1

References

[cOnverdbn

a t 24hrsj

Table V : Change in tin diffusion coefficient x ~O-'~cm~s-' with degree of filament conversion.

12 - 2 ~ f i l s m

Semi i n f i n i t e model

~ e + / P67 7 5 0 ' ~

650°C

Limited tin supply ~ e f N P72 7 5 0 ' ~

6 5 0 ' ~ '

/ I / Hopkins,R.H. et al., Met. Trans. AIME

-

8(1977) 9 1 0.1 0.2 0.3 0.4 0.5 0.6- 0.7 0.8 0.9 1.0 0.6 0.5 0.46 0.34 0.12 0.05 0.04 0.02 0.01 0.01 0.12 0.13 0.11 0.02 0.02 0.04 0.03 /2/ McDougall, I .L., Cryogenics

5

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5

(1976) 3792 cal coordinates were used/5/, with input data from