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Effects of lowtemperature fastneutron irradiation on ac losses in NbTi B. S. Brown, H. C. Freyhardt, and T. H. Blewitt Citation: Journal of Applied Physics 45, 2724 (1974); doi: 10.1063/1.1663656 View online: http://dx.doi.org/10.1063/1.1663656 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/45/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of low-temperature strain on the structure and the critical-current degradation of the superconducting alloy Nb–Ti Low Temp. Phys. 31, 894 (2005); 10.1063/1.2126947 Features of the low-temperature creep of a Nb–Ti alloy after large plastic deformations at 77 K Low Temp. Phys. 30, 340 (2004); 10.1063/1.1705444 Unexpected time dependence of ac losses in multifilamentary NbTi superconductors Appl. Phys. Lett. 62, 3513 (1993); 10.1063/1.109012 ac lowfrequency magnetic measurement of the proximity effect between fine filaments of superconducting NbTi wires J. Appl. Phys. 73, 1873 (1993); 10.1063/1.353174 Critical current enhancement in Nb3Sn by lowtemperature fastneutron irradiation J. Appl. Phys. 48, 837 (1977); 10.1063/1.323639 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.189.170.231 On: Sat, 20 Dec 2014 16:35:39

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Effects of lowtemperature fastneutron irradiation on ac losses in NbTiB. S. Brown, H. C. Freyhardt, and T. H. Blewitt Citation: Journal of Applied Physics 45, 2724 (1974); doi: 10.1063/1.1663656 View online: http://dx.doi.org/10.1063/1.1663656 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/45/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of low-temperature strain on the structure and the critical-current degradation of the superconductingalloy Nb–Ti Low Temp. Phys. 31, 894 (2005); 10.1063/1.2126947 Features of the low-temperature creep of a Nb–Ti alloy after large plastic deformations at 77 K Low Temp. Phys. 30, 340 (2004); 10.1063/1.1705444 Unexpected time dependence of ac losses in multifilamentary NbTi superconductors Appl. Phys. Lett. 62, 3513 (1993); 10.1063/1.109012 ac lowfrequency magnetic measurement of the proximity effect between fine filaments of superconducting NbTiwires J. Appl. Phys. 73, 1873 (1993); 10.1063/1.353174 Critical current enhancement in Nb3Sn by lowtemperature fastneutron irradiation J. Appl. Phys. 48, 837 (1977); 10.1063/1.323639

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Effects of low-temperature fast-neutron irradiation on ac losses in NbTi*

B. s. Brown, H. C. Freyhardtt , and T. H. Blewitt

Argonne National Laboratory, Argonne, Illinois 60439 (Received 21 January 1974)

The complex magnetic permeability of Nb-44 at. % Ti has been measured using a modified Hartshorn ac bridge and a lock-in amplifier. The out-of-phase signal can be directly related to the energy losses, and critical current can be calculated using the Bean model. Samples were irradiated in a fission neutron spectrum to a dose of 3.2X 1018 neutronlcm2• All measurements were taken in situ at approximately 4.5 OK in an external magnetic field continuously swept to 6.7 kOe while applying ac magnetic fields at 88 Hz with amplitudes between 8 and 500 Oe. Irradiations and measurements were performed on an annealed and a cold-worked sample of the alloy. For the annealed sample, no changes were observed in V L and thus in J c after 8.2X 1017 neutronlcm2 and after an anneal to 320 OK. A decrease of approximately 8% was observed in J c for the cold-worked sample after 3.2X 1018 neutron/cm2• One-half of this decrease was recovered by 270 OK.

INTRODUCTION

The investigation of ac properties in high-field super­conductors is of considerable technological interest. 1-4 If super conducting magnets are to be used for the plas­ma confinement in the fusion reactor,S it is necessary to know the effect of high-energy neutron irradiation on the superconducting properties. The use of pulsed mag­netic fields requires an understanding of the effects of irradiation on the ac losses. The measurement of ac 10sses6 is a useful means of determining in situ critical­current (Je) changes after low-temperature irradiation in bulk samples without attaching electrical contacts. Measurements on high -purity niobium 7 show increases in Je by a factor of 20 after 1. 0 x 1018 n/ cm2. Experi­ments that directly measure the critical current of neu­tron-irradiated NbTi have been made. 8-10 The irradia­tions were performed at both ambient reactor and heli­um temperatures in a reactor spectrum. The effect of different types of irradiation on the critical current has also been investigated in Nb~l, V 3Si, 11,12 and NbZr. 9,13-15 Reviews of radiation effects in superconduc­tors have been given by Cullen16 and Ullmaier. 17

EXPERIMENTAL

The ellipsoidal specimens were 25 mm long and had a 2. 9-mm maximum diameter. The Nb-44 at. % Ti was cold worked to r.od form and hand machined to the final shape. One specimen (NbTi 1) was annealed at 1200°C for 1 h in a vacuum better than 9 x 10-9 Torr, the other specimen (NbTi 2) remained in the cold-worked state. The samples were investigated in situ in a mutual-induc­tance bridge using a lock-in amplifier at the bridge out­put, which is similar to the technique used by Sekula. 6

The temperature at which the experiment was conducted was approximately 4.5 OK, which is greater than the boil­ing point of liquid helium because of the residual gamma heating with the reactor down. The ac magnetic perme­ability is measured as a steady magnetic field is swept (70 Oe/sec) up to 6.7 kOe in a superimposed 88-Hz sin­usoidal field of amplitude ho. ho was varied between 8 and 500 Oe. Consistent data were obtained for a bias­field sweep rate of 10 Oe/sec. As described by Sekula, the bridge was balanced with the pick-up coils both emp­ty and containing the sample, and the phase adjustment was made for all ac field amplitudes by assuming that

2724 Journal of Applied Physics, Vol. 45, No.6, June 1974

no losses exist in the Meissner state for a sample cooled in zero magnetic field from temperatures above Te. The fundamental component of the nonlinear response of the sample to the sinusoidal field excitation can be resolved to obtain a signal V L (loss voltage), which is proportion­al to the imaginary component iJ." of the complex perme­ability. This is simply related to the power dissipation in the sample. 18

The irradiation was performed in the fast-neutron cryogenic facility of the Argonne CP-5 reactor, 19 which consists of a uranium converter with an inner concen­tric boron-carbide shield that yields essentially a fast­neutron spectrum of 1 xl012 fast neutrons/cm2:", sec (E> O. 1 MeV). The gamma heating resulted in an irradi­ation temperature of 6 ± 10K. Short-term temperature oscillations of ± 0.1 OK made measurements quite diffi­cult with the reactor on; therefore, measurements were taken at approximately 4. 5 OK with the reactor down. The sample was raised 60 cm from the irradiation posi­tion into the superconducting magnet and pick-up coils. Before each measurement the samples were annealed

-3~--~----~2~--~3~---4~--~5~--~6~--~7

H (kOe)

FIG. 1. de magnetization curve for NbTi 1 at 4. 5 OK.

Copyright © 1974 American Institute of Physics 2724

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2725 Brown, Freyhardt, and Blewitt: Effects of neutron irradiation on ac losses in NbTi 2725

0.9

0.8

0.7

0.6

> 0.5 E. ...J

> 0.4

0.3

0.2

0.1

7

H. kOe

FIG. 2. Loss voltage VL vs applied field H for ac amplitudes of 65 and 130 Oe for NbTi 1 (annealed).

to 16 oK to exclude the trapped flux from the previous measurement. Previous measurements showed little ef­fect of a 16 oK anneal on the defect state of NbTi. 20

RESULTS AND DISCUSSION

Figure 1 shows a dc magnetization curve of NbTi 1 measured by a conventional electronic integration meth­od21 with no superimposed ac field. The loss voltage V L

is plotted as a function of the applied field H in Fig. 2 for NbTi 1 at ac field amplitudes, 110, of 65 and 130 Oe. Although the phase setting was determined for the sam­ple cooled in zero magnetic field, all subsequent mea­surements were taken after the field was cycled to 6.7 kOe and back to zero, and the sample therefore contain­ed trapped flux. For fields greater than 0.5 kOe, the curves for the unirradiated samples with and without trapped flux are identical. Sekula6 has shown that the energy diSSipation per cycle and per unit sample surface area, W., is given (in MKS units) by

(1)

where N is the number of turns in the pick-up coil en­closing the sample of radius R. The loss, W;, per unit area per second is given by W. times frequency;

W~= W.f=hoVL/2Y21TNRo

Thus, for the data in Fig. 2, the sample losses at 6 kOe are 8 and 54 f..L W / cm2, respectively, for 110 = 65 and 130 Oe.

The critical current calculated for 110 = 130 Oe, plotted in Fig. 3, can be found from the Bean22 relation.

W. = t[f..Loli5/ Jc(H)], (2)

which together with Eq. (1) yields

(3)

where C is independent of the biaSing field. From Fig.

J. Appl. Phys., Vol. 45, No.6, June 1974

2, the data are seen to be nearly linear with the biasing field H

VL=A+BH.

Combining Eqs, (3) and (4) yields the Kim23 expression for Jc(H)

Jc(H) = (l/(Ho +H).

At H = 6 kOe, for 110 = 65 Oe, calculations yield (l = 7. 9 x104 kOeA/cm2

, Ho=0.18 kOe, and Jc=1.3x104 A/cm2, and for 110= 130 Oe, (l =10.2 x104 kOeA/cm2, Ho =0. 51 kOe, and Jc=1.6x104 A/cm2.

The dependence of the loss voltage V L for NbTi 1 on the amplitude 110 of the ac field at H = 1 and 6 kOe is shown in Fig. 4. The line of slope 2 indicates that V L is proportional to h~, as predicted by Eq. (3). The effect of the neutron irradiation on the losses for NbTi 1 can be seen in Fig. 4 and for NbTi 2 in Fig. 5. From Fig. 4 it is apparent that the neutron irradiation produced lit­tle change in the loss signal in NbTi 1 (less than the mea­suring error of 4%) up to a dose of 8.2 x 1017 fast neu­tron/cm2 for all ac field amplitudes and, consequently, little change in Jc(H). In Fig. 4, a line of slope 2, as de­termined by a least-squares fit, has been drawn through the unirradiated data. Similar fits for NbTi 2 show an increase in VL of 8±4% after 3.2x101B neutron/cm2 and a subsequent decrease in V L of 5% after the 270 OK an­neal. This indicates an 8% decrease in J c after irradia­tion, of which about one-half recovers upon annealing to 270oK. NbTi 1 has a V L approximately five times larger than NbTi 2, and thus NbTi 2 has a Jc five times that of NbTi 1. A large number of pinning centers introduced by cold working were removed by the 1200°C vacuum an­neal in NbTi 1, leading to a smaller J c' Resistivity mea­surements at 14.5 OK after low-temperature fast-neutron

I04~ __ +-__ ~ __ ~ __ -+ __ ~ __ ~~~~~ o 2345678

H(kOe)

FIG. 3. Critical current Jc as a function of applied field H cal­culated for ho= 130 Oe for NbTi 1 (annealed).

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2726 Brown, Freyhardt, and Blewitt: Effects of neutron irradiation on ac losses in NbTi 2726

> E

10.0

1.0

....I

> 0.1

0.01

NbTil

v PREIRRADIATED

DAFTER 2.36 x 1017FAST NEUTRONS/cm2

A 5.76 x " o 8.23 x " ° 60

0

C ANNEAL

0.001L.--+--* ....... +-~:-±:--±,--~~,.....,--L_---I 30 50 100 200 300 500

ho(Oe)

FIG. 4. Loss voltage VL vs ac amplitude ho at applied fields of 1 and 6 kOe for NbTi 1 (annealed).

irradiation on Nb-64 at. % Ti20 have shown large in­creases in the normal-state resistivity, and thus large defect productions, as expected for defects produced at a temperature below their mobility point. This differ­ence in the effects of the defects on the behavior of the normal-state resistivity and the flux pinning has been seen in both pure and alloy superconductors. 16,17

Experiments have been performed that directily mea­sure the critical transport current of neutron -irradiated NbTi. SoelI et al. 8 irradiated a single filament of 45 fila­ment Nb-66 at. % Ti having a total diameter of 11 and 21 /lm at 5°K inthe Munich research reactor. Their di­rect measurement of J c showed decreases in J c after rel­atively large doses. The l1-/lm wire showed a 6 and 37% decrease in J c at 6 kOe after 3.2 and 7.5 X 1018 neutron/ cm2, (E > 0.1 MeV), respectively. This decrease was 73% recovered after a 270 0 K anneal. The 21-/lm wire showed a 13% decrease after 4.5 x 1018 neutron/cm2 at 7. 8 kOe, an additional 7% decrease after annealing to 60 oK, and 60% recovery after a 200 0 K anneal. The ir­radiations by So elI et al. were performed in a reactor spectrum with relatively large thermal neutron flux, whereas ours were conducted in a fast-neutron spectrum. This may be a possible source of differences when com­paring experiments. Horak and Blewitt24 have pointed out the importance of thermal neutrons in observing low-

J. Appl. Phys., Vol. 45, No.6, June 1974

temperature radiation-induced resistivity changes and annealing behavior. Despite this, both experiments indi­cate modest decreases in J c after doses of > 3 x 1018 neu­tron/cm2 at super conducting temperatures. Soell et aL 8 attribute the flux pinning to the dislocation cell walls and explain the decrease in pinning to an increase in the ef­fective defect density for pinning within the cell cores. If the relative change in the defect structure in the cell walls is small, the large increase in defect density with­in the cell cores decreases the difference in effective pinning density between the cell walls and the cell cores. This increased homogeneity in the strain field and resi­dual reSistivity would lead to a decrease in pinning. On the basis of this model, the cold-worked sample (NbTi 2) should have a higher J c because of higher cell-wall con­centration,25-27 as it does. It can be argued that the an­nealed specimen (NbTi 1) could show an increase in Jc upon irradiation. If the dislocation concentration is suf­fiCiently low, the enhanced pinning due to the radiation­induced defects could lead to increased rather than de­creased pinning. The small change in NbTi 1 indicates that neither of these effects is large at low doses.

Two measurements have been made to investigate the effect of neutron irradiation at ambient reactor temper­ature (- 60 °e) on the critical transport current of NbTi. Parkin and SchweitzerlO irradiated multifilament wires in a reactor spectrum. Their critical current decrease was less than 4% at 10 kOe after 3xl017 neutron/cm2 and 21 % after 1. 6 x 1019 neutron/ cm2. Sugisaki et al. 9 irra­diated 0.15-mm-diam Nb-50 at. % Ti wire to a dose of

> S

..J >

1.0 NbTi 2

" UNIRRADIATED

• AFTER 3.16 x Id8 n/cm2

° AFTER 270K ANNEAL

o

0.001 30 50 100 200 500

ho(Oe)

FIG. 5. Loss voltage VL vs ac amplitude ho at an applied field of 6 kOe for NbTi 2 (cold worked).

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2727 Brown, Freyhardt, and Blewitt: Effects of neutron irradiation on ac losses in NbTi 2727

3.5 X 1018 fast neutron/cm2 and 7.4 X 1018 thermal neu­tron/ cm2. They observed a 125% increase in the critical transport current at 6 kOe. Wohlleben28 has recently ob­served a 19% decrease in J e after irradiation with 3.1-MeV protons to a dose of lxlQ17 photon/cm2 at 25°K on Nb-66 at. % Ti. After annealing for 1 h at 285 oK only 40% of the J e degredation remained.

Metallurgical preparation, i. e., alloy composition and defect state, appears to be an important consideration in predicting the effect of irradiation on Je in the niobium­titanium system. The Nb-44 at. % Ti is {3 single phase with no a-titanium precipitates. 26-27 Higher-percentage titanium alloys can result in pinning by a-titanium pre­cipitates and possibly w-phase precipitates. 29-30 It has been shown that the super conducting parameters Te. He2• and Je are u:1.iquely related to the form and distribution of the metallurgical phases. 26,27,29 The role of the dif­ferent phases in neutron-irradiation experiments is not fully determined but must be considered when compar­ing experiments with different percentage titanium al­loys. The change in Je after neutron irradiation can also depend on the initial Je as determined by the cold-work­ing and heat-treatment schedule before irradiation.

The experiments, with the exception of Sugisaki' s, show a consistent behavior of critical-current changes,

. with which our results agree, although ours involves the indirect measurement of the Bean critical-current den­sity, whereas the former investigations used a direct critical transport current measurement. The presence of a high thermal-neutron flux may be important in the ambient-temperature irradiations. Thermal-neutron capture results in a recoiling ion that produces many low-energy events which result in a more uniform defect density than the high-defect-density cascades resulting from fast-neutron damage. The thermal-neutron defect distribution favors radiation-enhanced diffusion31 with the result that the different species in an alloy can re­distribute themselves. This effect is smaller at helium temperature as a result of the immobility of the atoms. To test this hypothesis, a fast-neutron ambient-temper­ature irradiation is being performed with and without a thermal flux.

ACKNOWLEDGMENTS

The author would like to thank S. T. Sekula for supply­ing the samples and for many helpful discussions. They are grateful to T. L. Scott, A. C. Klank, and the reactor operators of the Argonne CP-5 reactor for experimental assistance.

J. Appl. Phys., Vol. 45, No.6, June 1974

*Work performed under the auspices of the U. S. Atomic Ener­gy Commission.

tPermanent address: Institut fUr Metallphysik, der Univer­sitat Gottingen, 34 Gottingen, Hospitalstrabe 12, West Germany.

IAn extensive list of references may be found in S. L. Wipf, Brookhaven National Laboratory Report No. 50155, p. 511 (unpublished) •

2W.T. Beall, Jr. andT.W. Meyerhoff, J. Appl. Phys. 40, 2052 (1969).

3R.C. Bentley and B. Graham, Cryogenics 11, 55 (1971). 4S. T. Sekula and R. H. Kernohan, Proceedings of the 1972 Applied SUperconductivity Conference, Annapolis, Maryland, p. 468 (unpublished).

SR. Carruthers, P.A. Davenport, andJ.T.D. Mitchell, 5th Symposium on Fusion Technology, Oxford, 1968 (unpublished).

6S. T. Sekula, J. Appl. Phys. 42, 16 (1971). 7B. S. Brown (unpublished). 8M. Soell, S. L. Wipf, and G. Vogl, Proceedings of the 1972 Applied SUperconductivity Conference, Annapolis, Maryland, p. 434 (unpublished).

9T. Sugisaki, T. Okada, and T. Suita, Technol. Rep. Osaka Univ. 21, 385 (1971).

IOD. Parkin and D. Schweitzer, Trans. Am. Nucl. Soc. 17, 147 (1973).

IIp.S. Swartz, H.R. Hart, Jr., andR.L. Fleischer, Appl. Phys. Lett. 4, 71 (1964).

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13R. Benaroya, T.H. Blewitt, J.M. Brooks., and C. Laverick, IEEE Trans. Nuc. Sci. NS-14, 383 (1967).

14H. T. Coffey, E. L. Keller, A. Patterson, and S. H. AutIer, Phys. Rev. 156, 355 (1967).

ISS. T. Sekula (private communication). 16C. W. Cullen, Brookhaven National Laboratory Report No.

50155, p. 437 (unpublished). HH. Ullmaier, Proceedings of the 1973 International Confer­

ence on Defects and Defect Clusters in B. C. C. Metals and Their Alloys, Gaithersburg, Maryland, 1973, p. 363 (unpubli shed).

18D. deKlerk and C. A. M. van der Klein, J. Low Temp. Phys. 6, 1 (1972).

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22C.p. Bean, Rev. Mod. Phys. 36,31 (1964). 23Y.B. Kim, C.F. Hempstead, andA.R. Strnad, Phys. Rev.

129, 528 (1963). . 24J. A. Horak and T. H. Blewitt, Phys. Status Solidi A 9, 721

(1972). 25R. G. Hampshire and M. T. Taylor, J. Phys. F 2, 89 (1972). 26C. Baker, J. Mater. Sci. 5,40 (1970). 27D. F. Neal, A. C. Barber, A. Woolcock, and J. A. F. Gidley,

Acta Metall. 19, 143 (1971). 28K. Wohlleben, J. Low Temp. Phys. 13, 269 (1973). 29C. Baker and J. SUtton, Philos. Mag. 19, 1223 (1969). 30D. Kramer and C. G. Rhodes, Trans. Metall. Soc. AIME

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