Imaging Damage in Steel Using a Diamond Magnetometer
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-02-05 |
| Journal | Physical Review Applied |
| Authors | L Q Zhou, R.L. Patel, A. C. Frangeskou, A. Nikitin, B.L. Green |
| Institutions | National Institutes for Quantum and Radiological Science and Technology, Engineering and Physical Sciences Research Council |
| Citations | 17 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates a novel, robust, and contactless method for Non-Destructive Testing (NDT) of magnetic materials, such as steel, utilizing Nitrogen-Vacancy (NVC) centers in diamond.
- Core Innovation: The sensor deliberately uses an inhomogeneous bias magnetic field, which is distorted by structural defects in the steel. Detecting these distortions allows for damage profile reconstruction.
- High Spatial Resolution: The system achieved a spatial resolution of 1 mm in the plane parallel to the surface (x/y axes) and 0.1 mm in the plane perpendicular to the surface (z-axis).
- Contactless Operation: The sensor maintains functionality at a large lift-off distance of up to 3 mm from the steel surface, enabling inspection through thick, non-magnetic coatings (e.g., insulation).
- Operational Robustness: The fiber-coupled design operates effectively in an unshielded environment and requires only low bias magnetic fields supplied by small permanent magnets.
- MFL Alternative: This technique avoids the primary limitation of standard Magnetic Flux Leakage (MFL) measurements by not requiring the magnetic saturation of the target material.
- Extreme Environment Potential: Due to the diamond sensor head, the device is expected to be suitable for operation in high-radiation environments and temperatures up to 300 °C.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Spatial Resolution (Parallel, x/y) | 1 | mm | Limited by the size of the small bias magnet. |
| Spatial Resolution (Perpendicular, z) | 0.1 | mm | Limited by the nominal resolution of the scanning stage. |
| Maximum Lift-off Distance | 3 | mm | Distance from sensor head to 316 stainless steel surface. |
| MW Excitation Power | 10 | W | Used for optimal structural defect measurements. |
| Frequency Modulation Amplitude | 4.5 | MHz | Used for optimal structural defect measurements. |
| Modulation Frequency | 3.0307 | kHz | Used for optimal structural defect measurements. |
| Bias Field Source | Permanent Magnets | N/A | Two magnets used; one 1 mm and one 2 mm from the diamond. |
| Reference Sensitivity (Previous Work) | 310 ± 20 pT/âHz | N/A | Achieved in the 10 to 150 Hz range using a similar 12C diamond configuration. |
| Operating Temperature (Potential) | Up to 300 | °C | Based on NVC properties cited in literature [4]. |
| Non-Magnetic Coating Thickness Tested | 1.5 (Brass) / 2 (Fiberglass) | mm | Demonstrated successful defect mapping through these layers. |
Key Methodologies
Section titled âKey MethodologiesâThe experimental methodology focuses on using the NVC Zeeman splitting shift to map magnetic flux distortions caused by defects in the steel.
- Sensor Setup: A compact, fiber-coupled sensor head containing an ensemble of NVCs in diamond was utilized. The setup deliberately operated without magnetic shielding or compensation coils.
- Inhomogeneous Bias Field: Two small permanent magnets were positioned near the diamond (1 mm and 2 mm away) to induce Zeeman splitting and create a specific, inhomogeneous magnetic flux profile incident upon the steel sample.
- Microwave (MW) Excitation: A 10 W MW source was used, delivered via a loop antenna, to excite the NVCs and enable Optically-Detected Magnetic Resonance (ODMR).
- Signal Detection and Modulation: NVC fluorescence was detected using a balanced photodiode setup. A lock-in amplifier (LIA) digitized the output, with the MW frequency modulated at 3.0307 kHz (4.5 MHz amplitude).
- Scanning and Data Acquisition: The sensor head was affixed to a scanning stage and moved across the steel samples (316 stainless steel with defined slot defects) in the x and y axes. A dwell time of 1 second was implemented before data acquisition at each point.
- Defect Mapping: Structural defects (e.g., slots) in the steel altered the magnetic permeability, causing localized distortions in the applied inhomogeneous bias field. These distortions were measured as shifts in the NVC Zeeman splitting, quantified by changes in the LIA voltage output.
- Defect Quantification: The resulting 2D maps were analyzed using Lorentzian fits to cross-sectional profiles. Changes in the magnitude of the shift voltage indicated defect depth, while changes in the Full Width at Half Maximum (FWHM) indicated defect width.
Commercial Applications
Section titled âCommercial ApplicationsâThis NVC-based magnetometry technique is highly relevant for industries requiring robust, contactless inspection of metallic infrastructure.
- Oil and Gas Industry:
- Pipeline Integrity Management: Used for detecting and characterizing corrosion, pitting, and cracking in pipelines.
- Corrosion Under Insulation (CUI): The large 3 mm lift-off distance is critical for inspecting pipelines and vessels where corrosion occurs beneath thick, non-magnetic insulation layers, avoiding costly insulation removal.
- Infrastructure and Manufacturing:
- Structural Health Monitoring: NDT of common steel structures (bridges, storage tanks) where magnetic saturation required by MFL is difficult or impossible.
- Quality Control: High-resolution mapping of defects in magnetic components during manufacturing or maintenance.
- Extreme Environment Applications:
- Nuclear and Energy Facilities: The diamond sensor head is inherently radiation-hard [5] and capable of operating at elevated temperatures (up to 300 °C), making it suitable for monitoring critical components in harsh environments.
- Quantum Sensing Technology:
- Advancement of compact, fiber-coupled quantum sensors for industrial deployment, demonstrating high spatial resolution without the need for complex magnetic shielding.
View Original Abstract
We demonstrate a simple, robust, and contactless method for nondestructive testing of magnetic materials such as steel. This uses a fiber-coupled magnetic sensor based on nitrogen-vacancy centers (NVCs) in diamond without magnetic shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic field on the diamond to improve the sensitivity. In contrast, here we show that the spatial resolution for imaging is improved by applying an inhomogeneous magnetic field to the steel even though this leads to an inhomogeneous magnetic field on the diamond. Structural damage in the steel distorts the inhomogeneous magnetic field and by detecting this distortion we reconstruct the damage profile through quantifying the shifts in the NVC Zeeman splitting. With a 1-mm magnet as the source of our inhomogeneous magnetic field, we achieve a high spatial resolution of 1 mm in the plane parallel and 0.1 mm in the direction perpendicular to the surface of the steel. This works even when the steel is covered by a nonmagnetic material. The lift-off distance of our sensor head from the surface of 316 stainless steel is up to 3 mm.