M.Valentino, C. Bonavolontà Coherentia CNR -INFM, Naples, Italy V. Palmieri

1. Magnetometry and Non Destructive Evaluation M.Valentino, C.Bonavolontà Coherentia CNR -INFM, Naples, Italy V. Palmieri Istituto Nazionale Fisica Nucleare, Legnaro National Laboratories, Legnaro (PD), Italy

2. Outline: • Sensors for magnetometry • Design of high sensitivity magnetic sensors working in unshielded environment • Giant Magneto Resistance (GMR) sensors • GMR devices for magnetic field detectors and Eddy-current measurements • Real time correlation between magnetic signal and corrosion on bulk Niobium • Real time magnetic detection of fatigue stress in metals • 2D high sensitivity magnetic image

3. 10-4 10-5 T 10-6 Hall 10-7 10-8 10-9 nT GMR 10-10 Flux-gate 10-11 HTS pT 10-12 LTS 10-13 10-14 10-15 fT Magnetic sensors & applications Magnetic field (Tesla) Magnetometer field resolution Applications NDE corrosion Fatigue Biomagnetic applications

4. Main magnetic sensors characteristics

5. GMR Sensors Principles Giant Magneto Resistance, (1988) Ferromagnet Co No external Magnetic Field Non magnetic metal Cu Ferromagnet Co J J High Electrical Resistance Ferromagnet Co Non magnetic metal Cu Ferromagnet Co J J Low Electrical Resistance Applied external Magnetic Field Alloy and base transition metal targets (Ni/Fe, Co/Fe, Co/Cr/Pt, Pt/Mn, Ta, Cu, Au)

6. GMR Sensors Principles Electrical resistance variation due to the magnetic field R/R ~ 10%-20%

7. magnetic field noise responses in magnetic unshielded environment Comparison between magnetic field responses of magnetometer, 1st and 2nd order gradiometer

8. 2ndorder transverse gradiometer 1st order off axis gradiometer 1st order transverse gradiometer 1st order Radial gradiometer r r r r F 2 F 3 F 2 b b b F 2 b F 1 b F F F 1 2 1 F 1 z z z z m m m m Useful Gradiometers configuration

9. 5 2 1 3 4 detector reference Niobium Buffered Chemical Polishing BCP Solution : -Hydrofluoric acid -Phosphoric acid Experimental set-up. 1) Niobium disk; 2) Teflon vessel filled with 40ml of Acid solution; 3) Fluxgate magnetometers sensors housed in a Plexiglas holder: detector and reference magnetometers are in a first order gradiometer configuration; 4) Aluminium shields; 5) mu-metal shield.

10. Niobium corrosion BCP 1:1:1 to BCP 1:1:2 increasing the H3PO4 percentage the magnetic signal Intensity increase

11. 0.8 0.7 0.6 nT/Hz1/2 0.5 0.4 0.3 0.2 1 1.5 2 2.5 3 Etching Rate [micron/min] Niobium corrosion The average magnetic field is plotted versus etching rate.

12. cells 50 mm long Corrosion Rate evaluation Corrosion rate According to Faraday law Not explained by Faraday law

13. Cells with different cathode geometries Optimization of the Cu-polishing Flat cathode Curve cathode Magnetic field distribution Line-scan

14. r F 3 b F 2 3th GMR b F 2nd GMR 1 z 1st GMR m GMR 2nd order electronic gradiometer. It detects the in plane magnetic field component reducing the environmental noise 2 d B x 2 dz GMR 2nd order gradiometer 2ndorder Radial gradiometer

15. GMR testing on the electropolishing The solution used: 55% Phosphoric acid 45% n-buthanol Flux gate 1st order gradiometer imaging GMR 2nd order gradiometer imaging

16. 5 mm 4 mm Circular excitation coil to induce eddy current into the specimen GMR sensors for Eddy-current probe

17. Slot 1 Slot 2 Slot 1 Slot 2 GMR Eddy Current probe testing GMR Eddy current probe has been tested using a specimen of AL-Ti alloy with rivets and artificial slots Surface Back side

18. 20 mm 50 mm Detection of sub- millimetric scratch on Niobium surface Test sample GMR imaging 20 mm 50 mm Artificial sub millimetric scratches have been produced on the Nb surface

19. 20 mm 50 mm Comparison with conventional Eddy current induction system Working frequency: 3MHz Test sample Eddy Current induction coil imaging 20 mm 50 mm

20. Physical etching of titanium surface

21. mm mm GMR Eddy current Probe detection

22. Magnetic field scanner to detect fatigue effect in metals • System characteristics • Speed: 3mm/s (0-6/mm/s) • Spatial resolution: 0,1mm • Position Reproducibility : 0,05mm • Magnetic field signal Reproducibility  0.01%

23. …20 mm far from the cut … around the cut Detection of the localized Fatigue process in steel Monitoring the Magnetic field

24. 0,1 mm 0,1 mm Sub-millimetric Magnetic field imaging 0,0 kN 23 kN 9,5 kN 0,0 kN

25. 0,1 mm mm mm Surface plastic deformation imaging, preliminary results GMR responses, spatial resolution: 50m Magnetic field resolution: 1nT/Hz1/2

26. Conclusions • Emerging magnetic sensors, like GMR, allows to detect with high sensitivity the magnetic field produced during the following process: • The ongoing corrosion of the metals electropolished as Cu and Nb. • The Fatigue stress effects in steels. • 2D magnetic field imaging of sub-millimetric defects and plastic deformation in metals with a spatial resolution better than 100 micron. • Development of GMR Eddy current probe • Detection of surface sub-millimetric defects and scratches with depth less than 0.1 mm in Niobium plate. • Detection of corrosion effect with a depth less than 100micron, due to Ion etching on Titanium surface. • Prospective • Development of magnetic sensors array to measure the magnetic field during the electropolishing process of metals on non- planar geometry.