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Physica C 441 (2006) 126–129

Grain boundary flux penetration and resistivity in large grainniobium sheet q

P.J. Lee a,*, A.A. Polyanskii a, A. Gurevich a, A.A. Squitieri a, D.C. Larbalestier a,P.C. Bauer b, C. Boffo b, H.T. Edwards b

a Applied Superconductivity Center, University of Wisconsin-Madison, Madison, WI 53706, USAb Fermilab, Batavia, IL, USA

Abstract

Kneisel, Ciovati, Myneni and co-workers at TJNAF have recently fabricated two superconducting cavities from the center of a largegrain Nb billet manufactured by CBMM. Both cavities had excellent properties with one attaining an accelerating gradient of 45 MV/m(2 K) after a 48 h and 120 �C bake [P. Bauer et al., An investigation of the properties of BCP niobium for superconducting RF cavities,in: K.-J. Kim, C., Eyberger (Eds.), Proceedings of the Pushing the Limits of RF Superconductivity workshop, Argonne National Lab-oratory Report ANL-05/10, March 2005, pp. 84–93]. An investigation is underway to use magneto-optical (MO) imaging to observe theflux penetration behavior of a sheet sliced from this billet. The large grain size (some larger than 50 mm) allowed us to isolate multiple bi-crystals and tri-crystals. In the first stage of the present study we have taken the as-received sheet (RRR �280), which has been etched toreveal the grain structure. By magneto-optical examination we observed preferential flux penetration at some grain boundaries of a bi-crystal where the grain boundary was almost perpendicular to the sample surface and there was <1 lm surface step across the boundary.At other grain boundaries, with large steps or where the grain boundaries were not normal to the surface, we observed no preferentialflux penetration. Preliminary transport measurements on a bi-crystal showed greater normal state resistance and lower superconductingcritical current at the grain boundary.� 2006 Elsevier B.V. All rights reserved.

Keywords: Magneto-optical effects; Grain boundary; Niobium; Resistivity; Superconducting RF; Superconductivity

1. Introduction

An understanding of what degrades the performance ofhigh purity Nb used for SRF cavity fabrication is vital tothe success of the ILC. Using magneto-optical (MO) imag-ing we have recently shown that some Nb grain boundariesshow preferential flux penetration below that seen in thegrains [1]. This result was the first direct evidence ofdepressed superconductivity at a Nb grain boundary(GB), however there was also extensive surface topology

0921-4534/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2006.03.027

q The work at UW-Madison was funded by Fermilab contract #559390and DOE-HEP under grant DE-FG02-91ER40643.

* Corresponding author.E-mail address: lee@engr.wisc.edu (P.J. Lee).

associated with many grain boundaries in that study andthe BCP sample had undergone an extensive heat treatmentin order to enlarge the grain size. This heat treatment mayalso have contributed to the poisoning of the GB. In thisstudy we examine grain boundaries in a slice of one ofthe CBMM extremely large-grain billets. In this first reportwe examine the behavior of samples from this as-receivedingot slice after minimal BCP polishing and etching toreveal the grain locations.

In this first series of experiments we examine bi-crystaland tri-crystal regions of the sample in the as-cut state thatis close to the as-received state of the slice. Only light BCPand GB etching was applied. Cutting has left some residualcold work in the samples. In subsequent studies we willmake the same measurements at each step of a typicalBCP and heat treatment procedure.

P.J. Lee et al. / Physica C 441 (2006) 126–129 127

The advantages of using single crystal or very large grainsheet for cavities and recent results on such cavities manu-factured by TJNF are reported elsewhere in this volume [2].

2. Experimental technique

The billet slice, in the as received condition, is shown inFig. 1. The side shown was given sufficient BCP treatmentat TJNAF to reveal the grain boundary locations but someresidual marks from cutting can still be seen in the top lefthand corner. The reverse side was in the rough-cut condi-tion which necessitated further mechanical polishing fol-lowed by etching so that we could determine thelocations of the boundaries after traversing the thicknessof the slice. The three-dimensional geometry was thendetermined using the ‘‘A 3D’’ [3] plugin for the ImageJ[4] image analysis package as well as the standalone pro-gram Reconstruct [5]. Height maps were determined usingthe Extended Depth of Field plugin [6] for ImageJ.

2.1. Magneto optical imaging

The magneto-optical (MO) technique is described indetail in [7]. It uses the strong Faraday effect in Y–Fe gar-

Fig. 1. Overview of as-received ingot slice showing bi- and tri-crystalregions used for this series of experiments.

net to measure the vertical magnetic field component abovea sample. The spatial resolution attained is �5 lm whenthe garnet is placed directly on the face of the sampleand fields of �1 mT can be resolved. The sample is typi-cally a 5 · 5 mm2 rectangle (�2 mm thick). The continuousflow cryostat permitted sample temperatures down to�6 K. An external solenoid can apply vertical fields up to�120 mT to the sample. The field enhancement in the dia-magnetic state is considerable, rising to a maximum of 2.7at the mid-point of each edge as calculated using finite ele-ment analysis for a 5 · 5 · 2 mm3 sample. The fieldenhancement pattern produces the characteristic ‘‘rooftop’’field penetration pattern typified later in this paper by thebehavior of the tri-crystal samples.

2.2. Inter- and intra-grain resistivity

For inter- and intra-grain resistivity measurements anadditional bi-crystal bar was cut from bi-crystal #1. Threesides of the bar were cut with a diamond saw and the thirdwas left with a heavily cold-worked band saw cut. GoldVoltage contacts were sputtered onto the surface oppositethe band-saw cut after the surface had been Ar-ion etched.Regions between contacts were masked using Ag paint,which was subsequently removed (Fig. 2).

Fig. 2. (a) Schematic description of resistivity sample with (b) images ofeach side of the actual sample. In (c) we show a Laser Scanning ConfocalMicroscope image of the 1 lm thick Au contact layer.

Fig. 5. Bi-crystal from region #6 (a) with 3D model showing �30� tilt toboundary (b) and a classic rooftop magneto optical image for ZFCT = 5.5 K and our maximum field, H = 120 mT.

128 P.J. Lee et al. / Physica C 441 (2006) 126–129

3. Results

3.1. Bi-crystals

The grain boundary in region #1 runs almost perpendic-ular to the surface of the slice (Fig. 3(b)), and for the sam-ple cut from this region there is clearly premature fluxpenetration along the grain boundary at fields much lowerthan Hc1. The sequence of magneto optical images in Fig. 4(ZFC, T = 7.6 K) shows an initial penetration at 12 mTthat is localized to the grain boundary (note the curvaturematching the model surface in Fig. 3(b)). There is virtuallyno topography at the grain boundary (Fig. 3(c)).

This behavior contrasts with the second bi-crystalregion, Fig. 5, where the boundary is at a �30� tilt to thesurface and a classic rooftop MO image, Fig. 5(c), isobserved up to our maximum field of 120 mT.

3.2. Tri-crystals #4 and #5

In tri-crystal #4 (Fig. 6) there is a 13 lm step at the topleft grain boundary, marked ‘‘A’’, which produces a cur-rent inflection indicated by the dark lines in MO images(d) and (e). A small notch is observed in the 60 mT MOimage where the ‘‘A’’ boundary meets the top edge of thesample as well as a larger notch in location ‘‘B’’. Neitherfeature results in bulk penetration of the sample and thenotches are not observed at our maximum field of120 mT (Fig. 6(e)).

Fig. 3. (a) Bi-crystal #1 (a) with (b) a height map detail from the areaindicated, showing sub 5 lm topography at the grain boundary and (c) a3D model of the bi-crystal. A series of MO images, (d)–(g), at increasingfield (T = 5.6 K ZFC), shows premature flux penetration starting at a fieldof only 8.1 mT, �Hc1.

Fig. 4. The reverse side of bi-crystal #1 shown in Fig. 3: magneto optical imaboundary with increasing field.

For tri-crystal region #5, Fig. 7, we again see no prema-ture flux penetration into the sample associated with thegrain boundaries or topography.

ge sequence at 7.5 K showing flux penetration increasing along the grain

Fig. 6. Tri-crystal #4 (a) with 3D model (b) and a height map of the topleft hand grain boundary showing a 13 lm step. A typical ‘‘rooftop’’magneto-optical image (d) is obtained at T = 5.5 K, H = 60 mT and (e) atT = 5.7, H = 120 mT. In (d) and (e) a faint line, marked ‘‘A’’ indicating acurrent inflection is observed and in (d) an edge notch, marked ‘‘B’’, whichdoes not extend into the sample with increasing field (e).

Fig. 7. (a) Light microscope image of tri-crystal #5 compared with (b) amagneto optical image after zero field cooling and then applying ourmaximum field of 120 mT at 5.3 K showing the classic rooftop pattern.

P.J. Lee et al. / Physica C 441 (2006) 126–129 129

3.3. Resistivity measurements

The Au deposition technique was successful and we wereable to measure a RRR across the grain boundary of 187,11% less than the intra-grain boundary value of 211. Super-conducting state measurements showed lower criticalcurrent across the grain boundary as compared to theintra-grain region.

4. Summary

(1) MO Imaging shows premature flux penetration at aperpendicular grain boundary in an as-received sliceof Nb with residual cold work on surface. Thisbehavior occurs at 8.1–20 mT (7.5 K), much lowerthan Hc1.

(2) The premature flux penetration behavior observed inperpendicular boundary sample does not appear tobe topologically related as much larger topologicalfeatures do not to cause this behavior for the othersamples.

(3) Initial resistivity measurements indicate grain bound-ary weakness.

(4) This work is continuing with the samples receivingfurther processing typical of cavity processing.

Acknowledgements

We would like to thank Peter Kneisel for providing theNb slice, Bill Starch for metallographic preparation andMatt Feldman for sputtering the Au contacts.

References

[1] P. Bauer et al., An investigation of the properties of BCP niobium forsuperconducting RF cavities, in: K-J. Kim, C, Eyberger (Eds.),Proceedings of the Pushing the Limits of RF Superconductivityworkshop, Argonne National Laboratory Report ANL-05/10, March2005, 84–93.

[2] P. Kneisel et al., Performance of Large Grain and Single CrystalNiobium Cavities, Presentation MoP09 at the 12th workshop on RFsuperconductivity, Ithaca, NY, July 2005.

[3] A. Cardona, A 3D editing plugin. Available from: <http://www.pensament.net/java/>.

[4] W.S. Rasband, ImageJ, National Institutes of Health Bethesda, MD,USA. Available from: <http://rsb.info.nih.gov/ij/>, 1997–2005.

[5] J.C. Fiala, Reconstruct: a free editor for serial section microscopy, J.Microsc. 218 (2005) 52.

[6] D. Sage, Extended Depth of Field plugin by Biomedical ImagingGroup, University of Lausanne, Available from: <http://bigwww.epfl.ch/demo/edf/>.

[7] A.A. Polyanskii, D.M. Feldmann, D.C. Larbalestier, Magneto-opticalcharacterization techniques, in: D. Cardwell, D. Ginley (Eds.), TheHandbook on Superconducting Materials, Institute of Physics, UK,2003, p. 1551.