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Critical current enhancement in Nb3Sn by low-temperature, fast-neutron induced flux pinning centers
Journal of Nuclear Materials 72 (1978) 142-146 0 North-Holland Publishing Company
CRITICALCURRENTENHANCEMENTIN Nb& BYLOW-TEMPERATURE, FAST-NEUTRONINDUCEDFLUXPINNINGCENTERS* S.L. COLUCCI and H. WEINSTOCK IMnois Institute of Technology, Chicago, Illinois 60616, USA
The short-section critical current densityj, has been measured at 4.2 K for two types of composite Nb$n wires irradiated with fast neutrons at 6 K and then annealed to 77 K. For the first type (A), a fluence of 4.8 X 1017 n/cm2 increased jc at transverse fields above 1.5 T, and the enhancement increased with field to 45% at 10 T. The overall enhancement was a few percent less for a fluence of 1.1 X 1018 n/cm2. In an improved type wire(B) with a larger intrinsic jc, the enhancement for a fluence of 1.8 X 101* n/cm2 was 85% at 10 T. Both types A and B retained partial enhancement after anneals to between 295 K and 540 K. For type B material, T, and Hc2(i”) were measured and H,z(O) calculated using Hc2(0) = 0.693 T, (dHc2/dr)Tc. Consequently the pinning force density Fp = jc X B was plotted as a function of h = H/H,.. The shift in Fp(max) from h = -0.17 to -0.23 following irradiation implies modification in the pinning center microstructure. A 540 K anneal leaves Fp(max) reduced in magnitude, but unchanged in position, suggesting relative stability for the radiationinduced modification. La densite de courant critique de section courtej, a 8tB mesurie P 4.2 K pour deux types de fils de composite Nb$n irradi& par des neutrons rapides i 6 K, puis recuits i 77 K. Pour le premier type (A), une fluence de 4.8 x 1017 n/cm2 augmentait jc pour des champs transversaux au-dessus de 1.5 T et cette am&oration de jc au mentait avec le champ jusqu’8 45% i 10 T. L’am&oration totale itait de quelques % pour une fluence de 1,l X 10’ i n/cm2. Dans un fil de type (B) am&or& avec un jc intrindque plus grand, l’am8lioration pour une fluence de 1,8 X 1Ol8 n/cm2 &tait de 85% B 10 T. A la fois les types A et B retenaient partiellement cette am&oration apr&s des recuits entre 295 et 540 K. Pour le mat&iau B, TC et H,z(T) ont & mesur& et H,..(O) calculee en utilisant la relation H,.. = 0.693 T, (dHc2/dT)T . En conshquence, la densite de force d’kpinglage Fp = jc X B a it6 port&e en fonction de h = H/HC2. Le d&placement $e Fp(max) de la valeur h = - 0.17 jusqu’g -0.23 i la suite de l’irradiation implique une modification de la microstructure des centres d’ipinglage. Un recuit a 540 K provoque une diminution de grandeur mais non de position de Fp(max), ce qui sugg&e une relative stabilite de la modification induite par l’irradiation. Die kritische Stromdichte jc wurde bei 4,2 K an zwei verschiedenen composite-Kurzproben aus Nb$n-DrLhten gemessen, die mit schnellen Neutronen bei 6 K bestrahlt und bis 77 K augeheilt worden waren. Beim ersten Typ (A) wird jc durch eine Dosis von 4,8 X IO l7 n/cm2 bei transversalen Feldern oberhalb 1,s T erhiiht, die Erhijhun steigt mit dem Feld auf 45% bei IO T an. Die gesamte ErhGhung ist wenige Prozent geringer bei einer Dosis von 1,l X 10 1%n/cm2. Bei einem verbesserten Draht vom Typ (B) mit einem grijsseren intrinsischen jc betrSigt die Erhijhung bei einer Dosis von 1.8 X 10 ‘* n/cm2 85% bei 10 T. In beiden Typen A und B bleibt eine partielle ErhShung nach einer Ausheilung bis zu Temperaturen zwischen 295 und 540 K bestehen. Am Material des Typs B wurden T, und H,-.-.(T) gemessen und H&O) mit H,2(0) = 0,693 T, (dHc2/d7)Tc berechnet. Daher wurde die Pinningkraftdichte Fp = jC X B als Funktion von h = H/H,, aufgetragen. Die Verschiebung von Fp(max) von h zO,17 bis -0,23 nach der Bestrahlung schliesst eine Anderung der Mikrostruktur des Pinningzentrums ein. Bei einer Ausheilung bei 540 K wird das Maximum Fp(max) im Betrag erniedrigt, die Lage bleibt ungeHndert; eine relative Stabilitlt fiir die bestrahlungsinduzierte Modifikation wird vermutet.
1. Introduction
Nb3Sn wires were examined for an irradiation temperature of about 6 K. Thus the wires were exposed to an environment similar to that for the windings of a superconducting solenoid in some proposed fusion reactors. Two types of Nh3Sn composite wires were measured after a variety of irradiation and annealing histories in order to characterize at least the short-
In the first phase of this work, the fast neutron induced changes in critical current density 0,) of l
Supported by the Energy Research and Development Administration. 142
S.L. Cohcciand H. Weinstock/Current enhancement in Nb3Sn by fast-neutron irradiation
filament of 5.7 X 10e6 cm2 cross-sectional area and jc=1.6X106A/cm2atH=4TandT=4.2Kprior to irradiation. For further information on sample fabrication see ref. [2]. As described previously [2], irradiation was done with essentially a fission neutron spectrum (E > 0.1 MeV) at a temperature of -6 K in the cryogenic facility of the CP-5 reactor at Argonne National Laboratory [3]. Samples about 3 cm long were cut from continuous wires of each type of material, and while contained within an open-topped can, lowered into the reactor port. Following irradiation, the samples were transferred under liquid nitrogen to the measurement facility without ever warming above 77 K (except by design). It has been observed [4,5] that jc changes by less than 20% when low-temperature, neutron-irradiate1 NbsSn is annealed to 77 K. Recovery below 77 K probably results from interstitial migration, with the remaining damage evolving from more stable defect cascades which (it will be argued) produce effective flux-pinning centers as suggested in ref. [4].
section jc variation for these composites over a range of practical interest. In the second phase, T, and Hcz of the wires were measured after similar irradiation and annealing histories (as in the j, work). The aim of this part of the effort was specifically to determine whether the observed jc enhancement was due in part to the introduction of flux pinning centers by the irradiation, or resulted solely from the increase in H,, caused by the increase in resistivity due to the irradiation.
2. Sample preparation The samples were frabricated using the surfacesolid-state reaction of Nb wire codrawn in a Cu-Sn matrix [l] . All wires were reacted at 700°C for 24 h. The first composite (designated type A) contained 19 NbsSn filaments with a total crass-sectional area of 7.4 X 1O-6 cm2 and jc = 1.0 X 106 A/cm2 at H = 4 T and T = 4.2 K before irradiation [ 11.An improved composite (type B) contained one NbsSn
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TEMP
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n-DAMAGED Tlrrad=
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Nb3Sn
-6K
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.
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.
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x 1017 n.crn-’
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K ANNEAL
ANNEAL
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TRANSVERSE MAGNETIC FIELD (T) Fig. 1. Critical current density for type A composites as a function of transverse magnetic field for several neutron doses and annealings.
144
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S.L. Colucci and H. Weinstock/ Current enhancement in Nb#n
fluence exceeded that for producing a maximum j, enhancement. After annealing, the samples still retained a si~~~nt j, e~ancement . It should be emphasized that each ic (H) curve is, of experimental necessity, that of a different sample. However, independent measurements [7] show only about a 5% variation in jC for samples cut from the same wire. In addition, data in fig. 1 for two wires irradiated with 1.l X 1018 n/cm2 are within 5%. The behavior of type A and type B materials is compared in fig. 2. Although intrinsicj,(m values were larger for type B, it is nevertheless observed that the radiation-induced enhancement in j&H) was fractionally greater for type B than for type A. In material B a fluence of 1.8 X 10r8 n/cm2 increased~~(~ for fields above approximately 2 T, with the enhancement increasing with field and reaching 85% at 10 T. Annealing in vacuum at 540 K reduced the enhancement in type B by roughly one-half.
jc measurements
The critical current for each sample was measured in a transverse magnetic field of up to 10 T. Details of the j, measurement procedure have been previously described [6J. The critical current at a given field value was defined as the net current passing through the sample when the potential developed across a l-cm central segment equalled 27 E.IV.This voltage is well below the linear flux flow region of the I-V characteristic. The behavior ofj, for type A composites is summarized in fig. 1 and has been previously discussed in detail [6]. Basically, a fluence of 4.8 X 1017 r&m2 depressed je(JZ) for H-- 1.5 T, but at higher fields j,.(H) was enhanced, with the increase becoming larger with field, up to 45% at the highest field measured (10 T). Increasing the fluence for a second group of type A wires to 1.l X 101a n/cm’, resulted in a slightly smaller jC enhancement, suggesting this
II
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by fast-neutron irradiation
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TEMP
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n- DAMAGED Firad=
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-6K
TmMs= 4.2K
l.8.6 4 -
MATERIAL 0 o A
A
unirradi ated 4.8 x 10” n.cr# 1.1 x lOI n.cti*
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= .
B
unit-radiated 1.8 x 10” n.cti’ 1.8x lo’@ n.crrT* + 540
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Fig. 2. Comparison of j&l) for type A and type B composites.
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S.L. Colucci and H. Weinstock / Current enhancement in Nb$‘n by fast-neutron irradiation
4. &2(T)
measurement
For the determination of HC2 as a function of temperature, the transition temperature TC was measured as a function of applied field. A coaxial stainless steel dewar for thermal isolation was installed in the bore of the superconducting solenoid used in the j,(H) measurements. A four-electrode spring clamp inserted into the dewar held the sample wire parallel to the external field. A resistance heater attached to the sample holder could elevate the temperature of the sample above the ambient temperature of the liquid helium bath, maintaining a uniform temperature over the sample length. The voltage produced by a 5 mA current through the sample was measured by a nanovoltmeter. Again, the samples were mounted and measured without warming above 77 K, except when intentionally annealed. For a given external field, He2 (9, TC was defined as the temperature for which the sample voltage was half that for a completely normal sample. Fig. 3 shows H,,(T) for type B wires unirradiated, irradiated with 1.8 X 10’8n/cm2 and irradiated and annealed. Flagged data are for other type B wires with the same 11,
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. UNIRRADIATED .
1.8
.x 10’8
n cm’2
4 1 8 x 1018 n cm’2
+ 540
K ANNEAL
history, indicating the acceptable consistency of comparing measurements made on different wires of the same type. The zero-field T, is listed in table 1. Limitations in the magnitude of the applied field prevented a direct determination of H&O). But for data up to a field of only 10 T, it can be seen that H&O) is increased by irradiation. However, both H,.(O) and the normal state resistivity p of the NbaSn are related to the value of the slope evaluated at T,. Assuming that the Ginsburg-Landau theory in the dirty limit can be applied, and since NbsSn is not parama~etica~y limited 1891, H&O) = 0.693 Tc
The calculated values of I&a(O) are given in table 1. Although TC is lowered by irradiation, apparently the increase in p is sufficient to give a net gain in HC2. As an ~dication of the app~cabi~ty of the above equation He2 (T = 4.2 K) for similar wires has been measured [lo], and found to be 16 T to 17 T. Thus, the value of He2 (T = 0) = 18 T for the unirradiated sample is very reasonable. Knowing Hc2(0) for these samples, the volume pinning force density, Fp = jc X B, is plotted in fig. 4 as a function of reduced magnetic field, h = HI%I,.. Here it is assumed H&4.2) 5 He2 (0). The maximum in Fr, for the unirradiated samhle occurs at h = 0.17. For the irradiated sample, the maximum is shifted to h = 0.23. When interpreted in the flux pinning model developed largely by Kramer [ 111, this significant shift in location of maxima (hn) is sufficient to demonstrate a modification of the pinning center ~crostructure. Upon annealing, the magnitude of Fp is reduced, but its m~mum remains at h, C=0.23. Table 1 Values computed from data in fig. 3 for type B composites. Sample
Fig. 3. Upper critical field for type B composite wires as a function of temperature; Flagged points indicate data taken for a different wire length.
Unhradiated
-1.52
17.2
18.1
1.8 X 10t8n/cm2 + 71 K anneal
-1.93
15.3
20.7
1.8 X 101*n/cm2 + 540 K anneal
-1.76
16.3
19.9
S.L. Coiucci and H. Weinstock j Current a~~unce~ent in Nb#
146
Nb,Sn . . *
1 UNIRRADIATED 1 $3 x 10’8 1.8
x 10” -540
“.cm‘2 n‘cmq” K ANNEAL
FP= J,x B
‘0
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’
.I
111’ .2 .3
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.4
.5
.6
.7
.8
.9
h Waco)
Fig. 4. Volume pinning force density for type B composites plotted as a function of reduced magnetic field. The locations of maxima in F,, are indicated by hp.
From this evidence one might propose the following scenario. Flux pinning centers in the unirradiated state are, for the most part, NbsSn grain boundaries [ 121. Irradiation at low temperature introduces defect cascades, which are randomly distributed and act as new pinning centers. At the same time, irradiation increases the nodal-state resistivity, p and thus f&. , as has been observed [ 131. The combined effect is to increase Fp. Upon annealing, some point defects are removed, reducing p and H,, , but producing little change in the number or location of the larger defect systems which act as pinning centers. Thus, annealing as described reduces the magnitude of Fp, but does not change h,. The correspondence between increased F,, and increased h,, which is characteristic in this experiment, is generally not observed in other material treatments. This correspondence has been seen, however, in Nb-Ti alloy systems [ 141 that have been severely cold-worked and then annealed to several temperatures. The reduction of Fp and h, with annealing was explained as resulting from an ordering of the dislocations into dis-
b_yfast-neutron irrad~tion
location walls. The present work suggests that the increase in Fp and h, with irradiation results from a change in the flux pinning microstructure: the effect of randomly located defect cascades is added to that of the grain bo~da~es. In conclusion, jC can be increased by fast-neutron irradiation at low temperatures. The increase is apparently greater in material with higher pre-irradiation jC. The reason for this behavior is currently under investigation. There is strong evidence that the mechanism for jC enhancement can not be accounted for exclusively by increases in H,.. . A complete explanation must include modification of the flux-pinning microstructure. The authors wish to thank Drs. T.H. Blewitt and B.S. Brown of Argonne National Laboratory and Dr. CL. Snead of Brookhaven National Laboratory for valuable discussions. They are also grateful to Dr. M. Suenaga of Brookhaven National Laboratory for supplying the Nb s&r material and for his continued interest in this study. References [I] M. Suenaga, T.S. Luhman and W.B. Sampson, J. Appl. Qhys. 45 (1974) 4049. k&I S.L. Colucci, H. Weinstock and M. Suenaga, J. Appl. Qhys. 48 (1977) 837. [3] AC. Klank, T.H. Blewitt, J.J. Minarik and T.L. Scott, Bull. Inst. Froid. Suppl. 5 (1966) 373. [4] B.S. Brown, T.H. Blewitt and D.G. Wozniak, J. Appl. Qhys. 46 (1975) 5163. [S] M. Soeil, H. Bauer, K. Boening and R. Bett, Qhys. Lett. A51 (1975) 83. [6] S.L. Colucci, H. Weinstock and B.S. Brown, Appl. Phys. Lett. 28 (1976) 667. [ 71 M. Suenaga, private communications. [8] K. Maki, Physics 1 (1964) 127. [9] R.R. Hake, Phys. Rev. 158 (1967) 356. [lo] CL. Snead, private communications. [ 111 E.J. Kramer, J. Elect. Mat. 4 (1975) 839. [ 121 T. Luhman, C.S. Pande and D. Dew-Hughes, J. Appl. Qhys. 47 (1976) 1459. [13] CL. Snead and D. Qarkin, Nucl. Tech. 29 (1976) 264. [14] W.W. Webb, J. Appl. Qhys. 42 (1971) 102.
CRITICALCURRENTENHANCEMENTIN Nb& BYLOW-TEMPERATURE, FAST-NEUTRONINDUCEDFLUXPINNINGCENTERS* S.L. COLUCCI and H. WEINSTOCK IMnois Institute of Technology, Chicago, Illinois 60616, USA
The short-section critical current densityj, has been measured at 4.2 K for two types of composite Nb$n wires irradiated with fast neutrons at 6 K and then annealed to 77 K. For the first type (A), a fluence of 4.8 X 1017 n/cm2 increased jc at transverse fields above 1.5 T, and the enhancement increased with field to 45% at 10 T. The overall enhancement was a few percent less for a fluence of 1.1 X 1018 n/cm2. In an improved type wire(B) with a larger intrinsic jc, the enhancement for a fluence of 1.8 X 101* n/cm2 was 85% at 10 T. Both types A and B retained partial enhancement after anneals to between 295 K and 540 K. For type B material, T, and Hc2(i”) were measured and H,z(O) calculated using Hc2(0) = 0.693 T, (dHc2/dr)Tc. Consequently the pinning force density Fp = jc X B was plotted as a function of h = H/H,.. The shift in Fp(max) from h = -0.17 to -0.23 following irradiation implies modification in the pinning center microstructure. A 540 K anneal leaves Fp(max) reduced in magnitude, but unchanged in position, suggesting relative stability for the radiationinduced modification. La densite de courant critique de section courtej, a 8tB mesurie P 4.2 K pour deux types de fils de composite Nb$n irradi& par des neutrons rapides i 6 K, puis recuits i 77 K. Pour le premier type (A), une fluence de 4.8 x 1017 n/cm2 augmentait jc pour des champs transversaux au-dessus de 1.5 T et cette am&oration de jc au mentait avec le champ jusqu’8 45% i 10 T. L’am&oration totale itait de quelques % pour une fluence de 1,l X 10’ i n/cm2. Dans un fil de type (B) am&or& avec un jc intrindque plus grand, l’am8lioration pour une fluence de 1,8 X 1Ol8 n/cm2 &tait de 85% B 10 T. A la fois les types A et B retenaient partiellement cette am&oration apr&s des recuits entre 295 et 540 K. Pour le mat&iau B, TC et H,z(T) ont & mesur& et H,..(O) calculee en utilisant la relation H,.. = 0.693 T, (dHc2/dT)T . En conshquence, la densite de force d’kpinglage Fp = jc X B a it6 port&e en fonction de h = H/HC2. Le d&placement $e Fp(max) de la valeur h = - 0.17 jusqu’g -0.23 i la suite de l’irradiation implique une modification de la microstructure des centres d’ipinglage. Un recuit a 540 K provoque une diminution de grandeur mais non de position de Fp(max), ce qui sugg&e une relative stabilite de la modification induite par l’irradiation. Die kritische Stromdichte jc wurde bei 4,2 K an zwei verschiedenen composite-Kurzproben aus Nb$n-DrLhten gemessen, die mit schnellen Neutronen bei 6 K bestrahlt und bis 77 K augeheilt worden waren. Beim ersten Typ (A) wird jc durch eine Dosis von 4,8 X IO l7 n/cm2 bei transversalen Feldern oberhalb 1,s T erhiiht, die Erhijhun steigt mit dem Feld auf 45% bei IO T an. Die gesamte ErhGhung ist wenige Prozent geringer bei einer Dosis von 1,l X 10 1%n/cm2. Bei einem verbesserten Draht vom Typ (B) mit einem grijsseren intrinsischen jc betrSigt die Erhijhung bei einer Dosis von 1.8 X 10 ‘* n/cm2 85% bei 10 T. In beiden Typen A und B bleibt eine partielle ErhShung nach einer Ausheilung bis zu Temperaturen zwischen 295 und 540 K bestehen. Am Material des Typs B wurden T, und H,-.-.(T) gemessen und H&O) mit H,2(0) = 0,693 T, (dHc2/d7)Tc berechnet. Daher wurde die Pinningkraftdichte Fp = jC X B als Funktion von h = H/H,, aufgetragen. Die Verschiebung von Fp(max) von h zO,17 bis -0,23 nach der Bestrahlung schliesst eine Anderung der Mikrostruktur des Pinningzentrums ein. Bei einer Ausheilung bei 540 K wird das Maximum Fp(max) im Betrag erniedrigt, die Lage bleibt ungeHndert; eine relative Stabilitlt fiir die bestrahlungsinduzierte Modifikation wird vermutet.
1. Introduction
Nb3Sn wires were examined for an irradiation temperature of about 6 K. Thus the wires were exposed to an environment similar to that for the windings of a superconducting solenoid in some proposed fusion reactors. Two types of Nh3Sn composite wires were measured after a variety of irradiation and annealing histories in order to characterize at least the short-
In the first phase of this work, the fast neutron induced changes in critical current density 0,) of l
Supported by the Energy Research and Development Administration. 142
S.L. Cohcciand H. Weinstock/Current enhancement in Nb3Sn by fast-neutron irradiation
filament of 5.7 X 10e6 cm2 cross-sectional area and jc=1.6X106A/cm2atH=4TandT=4.2Kprior to irradiation. For further information on sample fabrication see ref. [2]. As described previously [2], irradiation was done with essentially a fission neutron spectrum (E > 0.1 MeV) at a temperature of -6 K in the cryogenic facility of the CP-5 reactor at Argonne National Laboratory [3]. Samples about 3 cm long were cut from continuous wires of each type of material, and while contained within an open-topped can, lowered into the reactor port. Following irradiation, the samples were transferred under liquid nitrogen to the measurement facility without ever warming above 77 K (except by design). It has been observed [4,5] that jc changes by less than 20% when low-temperature, neutron-irradiate1 NbsSn is annealed to 77 K. Recovery below 77 K probably results from interstitial migration, with the remaining damage evolving from more stable defect cascades which (it will be argued) produce effective flux-pinning centers as suggested in ref. [4].
section jc variation for these composites over a range of practical interest. In the second phase, T, and Hcz of the wires were measured after similar irradiation and annealing histories (as in the j, work). The aim of this part of the effort was specifically to determine whether the observed jc enhancement was due in part to the introduction of flux pinning centers by the irradiation, or resulted solely from the increase in H,, caused by the increase in resistivity due to the irradiation.
2. Sample preparation The samples were frabricated using the surfacesolid-state reaction of Nb wire codrawn in a Cu-Sn matrix [l] . All wires were reacted at 700°C for 24 h. The first composite (designated type A) contained 19 NbsSn filaments with a total crass-sectional area of 7.4 X 1O-6 cm2 and jc = 1.0 X 106 A/cm2 at H = 4 T and T = 4.2 K before irradiation [ 11.An improved composite (type B) contained one NbsSn
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n-DAMAGED Tlrrad=
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Nb3Sn
-6K
l-
.a.6.4 -
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o
UNI RRADIATED
.
4.8
d
1 1 x 10’8
n .crn-‘,
WIRE
1
.
1.1
x 10”
n.cni*.
WIRE
2
.
1.1
x 1d8
n.cni*
+ 415K
x 1017 n.crn-’
+ 295
K ANNEAL
ANNEAL
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TRANSVERSE MAGNETIC FIELD (T) Fig. 1. Critical current density for type A composites as a function of transverse magnetic field for several neutron doses and annealings.
144
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S.L. Colucci and H. Weinstock/ Current enhancement in Nb#n
fluence exceeded that for producing a maximum j, enhancement. After annealing, the samples still retained a si~~~nt j, e~ancement . It should be emphasized that each ic (H) curve is, of experimental necessity, that of a different sample. However, independent measurements [7] show only about a 5% variation in jC for samples cut from the same wire. In addition, data in fig. 1 for two wires irradiated with 1.l X 1018 n/cm2 are within 5%. The behavior of type A and type B materials is compared in fig. 2. Although intrinsicj,(m values were larger for type B, it is nevertheless observed that the radiation-induced enhancement in j&H) was fractionally greater for type B than for type A. In material B a fluence of 1.8 X 10r8 n/cm2 increased~~(~ for fields above approximately 2 T, with the enhancement increasing with field and reaching 85% at 10 T. Annealing in vacuum at 540 K reduced the enhancement in type B by roughly one-half.
jc measurements
The critical current for each sample was measured in a transverse magnetic field of up to 10 T. Details of the j, measurement procedure have been previously described [6J. The critical current at a given field value was defined as the net current passing through the sample when the potential developed across a l-cm central segment equalled 27 E.IV.This voltage is well below the linear flux flow region of the I-V characteristic. The behavior ofj, for type A composites is summarized in fig. 1 and has been previously discussed in detail [6]. Basically, a fluence of 4.8 X 1017 r&m2 depressed je(JZ) for H-- 1.5 T, but at higher fields j,.(H) was enhanced, with the increase becoming larger with field, up to 45% at the highest field measured (10 T). Increasing the fluence for a second group of type A wires to 1.l X 101a n/cm’, resulted in a slightly smaller jC enhancement, suggesting this
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by fast-neutron irradiation
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n- DAMAGED Firad=
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-6K
TmMs= 4.2K
l.8.6 4 -
MATERIAL 0 o A
A
unirradi ated 4.8 x 10” n.cr# 1.1 x lOI n.cti*
MATERIAL l
= .
B
unit-radiated 1.8 x 10” n.cti’ 1.8x lo’@ n.crrT* + 540
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TRANSVERSE
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MAGNETIC
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Fig. 2. Comparison of j&l) for type A and type B composites.
% I 10
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S.L. Colucci and H. Weinstock / Current enhancement in Nb$‘n by fast-neutron irradiation
4. &2(T)
measurement
For the determination of HC2 as a function of temperature, the transition temperature TC was measured as a function of applied field. A coaxial stainless steel dewar for thermal isolation was installed in the bore of the superconducting solenoid used in the j,(H) measurements. A four-electrode spring clamp inserted into the dewar held the sample wire parallel to the external field. A resistance heater attached to the sample holder could elevate the temperature of the sample above the ambient temperature of the liquid helium bath, maintaining a uniform temperature over the sample length. The voltage produced by a 5 mA current through the sample was measured by a nanovoltmeter. Again, the samples were mounted and measured without warming above 77 K, except when intentionally annealed. For a given external field, He2 (9, TC was defined as the temperature for which the sample voltage was half that for a completely normal sample. Fig. 3 shows H,,(T) for type B wires unirradiated, irradiated with 1.8 X 10’8n/cm2 and irradiated and annealed. Flagged data are for other type B wires with the same 11,
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1.8
.x 10’8
n cm’2
4 1 8 x 1018 n cm’2
+ 540
K ANNEAL
history, indicating the acceptable consistency of comparing measurements made on different wires of the same type. The zero-field T, is listed in table 1. Limitations in the magnitude of the applied field prevented a direct determination of H&O). But for data up to a field of only 10 T, it can be seen that H&O) is increased by irradiation. However, both H,.(O) and the normal state resistivity p of the NbaSn are related to the value of the slope evaluated at T,. Assuming that the Ginsburg-Landau theory in the dirty limit can be applied, and since NbsSn is not parama~etica~y limited 1891, H&O) = 0.693 Tc
The calculated values of I&a(O) are given in table 1. Although TC is lowered by irradiation, apparently the increase in p is sufficient to give a net gain in HC2. As an ~dication of the app~cabi~ty of the above equation He2 (T = 4.2 K) for similar wires has been measured [lo], and found to be 16 T to 17 T. Thus, the value of He2 (T = 0) = 18 T for the unirradiated sample is very reasonable. Knowing Hc2(0) for these samples, the volume pinning force density, Fp = jc X B, is plotted in fig. 4 as a function of reduced magnetic field, h = HI%I,.. Here it is assumed H&4.2) 5 He2 (0). The maximum in Fr, for the unirradiated samhle occurs at h = 0.17. For the irradiated sample, the maximum is shifted to h = 0.23. When interpreted in the flux pinning model developed largely by Kramer [ 111, this significant shift in location of maxima (hn) is sufficient to demonstrate a modification of the pinning center ~crostructure. Upon annealing, the magnitude of Fp is reduced, but its m~mum remains at h, C=0.23. Table 1 Values computed from data in fig. 3 for type B composites. Sample
Fig. 3. Upper critical field for type B composite wires as a function of temperature; Flagged points indicate data taken for a different wire length.
Unhradiated
-1.52
17.2
18.1
1.8 X 10t8n/cm2 + 71 K anneal
-1.93
15.3
20.7
1.8 X 101*n/cm2 + 540 K anneal
-1.76
16.3
19.9
S.L. Coiucci and H. Weinstock j Current a~~unce~ent in Nb#
146
Nb,Sn . . *
1 UNIRRADIATED 1 $3 x 10’8 1.8
x 10” -540
“.cm‘2 n‘cmq” K ANNEAL
FP= J,x B
‘0
Y
’
.I
111’ .2 .3
I
I
5
I
I
I
.4
.5
.6
.7
.8
.9
h Waco)
Fig. 4. Volume pinning force density for type B composites plotted as a function of reduced magnetic field. The locations of maxima in F,, are indicated by hp.
From this evidence one might propose the following scenario. Flux pinning centers in the unirradiated state are, for the most part, NbsSn grain boundaries [ 121. Irradiation at low temperature introduces defect cascades, which are randomly distributed and act as new pinning centers. At the same time, irradiation increases the nodal-state resistivity, p and thus f&. , as has been observed [ 131. The combined effect is to increase Fp. Upon annealing, some point defects are removed, reducing p and H,, , but producing little change in the number or location of the larger defect systems which act as pinning centers. Thus, annealing as described reduces the magnitude of Fp, but does not change h,. The correspondence between increased F,, and increased h,, which is characteristic in this experiment, is generally not observed in other material treatments. This correspondence has been seen, however, in Nb-Ti alloy systems [ 141 that have been severely cold-worked and then annealed to several temperatures. The reduction of Fp and h, with annealing was explained as resulting from an ordering of the dislocations into dis-
b_yfast-neutron irrad~tion
location walls. The present work suggests that the increase in Fp and h, with irradiation results from a change in the flux pinning microstructure: the effect of randomly located defect cascades is added to that of the grain bo~da~es. In conclusion, jC can be increased by fast-neutron irradiation at low temperatures. The increase is apparently greater in material with higher pre-irradiation jC. The reason for this behavior is currently under investigation. There is strong evidence that the mechanism for jC enhancement can not be accounted for exclusively by increases in H,.. . A complete explanation must include modification of the flux-pinning microstructure. The authors wish to thank Drs. T.H. Blewitt and B.S. Brown of Argonne National Laboratory and Dr. CL. Snead of Brookhaven National Laboratory for valuable discussions. They are also grateful to Dr. M. Suenaga of Brookhaven National Laboratory for supplying the Nb s&r material and for his continued interest in this study. References [I] M. Suenaga, T.S. Luhman and W.B. Sampson, J. Appl. Qhys. 45 (1974) 4049. k&I S.L. Colucci, H. Weinstock and M. Suenaga, J. Appl. Qhys. 48 (1977) 837. [3] AC. Klank, T.H. Blewitt, J.J. Minarik and T.L. Scott, Bull. Inst. Froid. Suppl. 5 (1966) 373. [4] B.S. Brown, T.H. Blewitt and D.G. Wozniak, J. Appl. Qhys. 46 (1975) 5163. [S] M. Soeil, H. Bauer, K. Boening and R. Bett, Qhys. Lett. A51 (1975) 83. [6] S.L. Colucci, H. Weinstock and B.S. Brown, Appl. Phys. Lett. 28 (1976) 667. [ 71 M. Suenaga, private communications. [8] K. Maki, Physics 1 (1964) 127. [9] R.R. Hake, Phys. Rev. 158 (1967) 356. [lo] CL. Snead, private communications. [ 111 E.J. Kramer, J. Elect. Mat. 4 (1975) 839. [ 121 T. Luhman, C.S. Pande and D. Dew-Hughes, J. Appl. Qhys. 47 (1976) 1459. [13] CL. Snead and D. Qarkin, Nucl. Tech. 29 (1976) 264. [14] W.W. Webb, J. Appl. Qhys. 42 (1971) 102.