Imaging Damage in Steel Using a Diamond Magnetometer

At a Glance

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	University of Tsukuba, Engineering and Physical Sciences Research Council
Citations	17
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Magnetometry for Non-Destructive Testing (NDT)

This document analyzes the research paper “Imaging damage in steel using a diamond magnetometer” (arXiv:2011.02459v1) to highlight the critical role of high-quality MPCVD diamond and to position 6CCVD as the essential supplier for replicating and advancing this Non-Destructive Testing (NDT) technology.

Executive Summary

This research demonstrates a robust, non-contact method for detecting structural defects in magnetic materials (steel) using ensemble Nitrogen Vacancy (NV) centers in diamond, offering significant advantages over traditional Magnetic Flux Leakage (MFL) techniques.

Novel NDT Method: Utilizes the Optically-Detected Magnetic Resonance (ODMR) of NV centers in diamond to map magnetic flux perturbations caused by structural defects, eliminating the need for magnetic saturation of the target material.
High Spatial Resolution: Achieved high resolution of 1 mm (parallel to the surface, x/y) and 0.1 mm (perpendicular to the surface, z-axis).
Corrosion Under Insulation (CUI) Capability: Successfully imaged defects in 316 stainless steel when covered by thick non-magnetic layers (up to 2 mm fiberglass or 1.5 mm brass), demonstrating suitability for CUI detection.
High Lift-Off Distance: The fiber-coupled sensor design maintained functionality at a lift-off distance up to 3 mm from the steel surface.
Robust Sensor Platform: Diamond’s inherent properties enable sensor operation in harsh environments, including high radiation and temperatures up to 300 °C.
Material Requirement: The core technology relies on high-quality Single Crystal Diamond (SCD) optimized for ensemble NV center creation and integration into a compact, fiber-coupled sensor head.

Technical Specifications

The following hard data points were extracted from the experimental results, detailing the performance and parameters of the diamond-based magnetometer system.

Parameter	Value	Unit	Context
Spatial Resolution (Parallel, x/y)	1	mm	Limited by the size of the 1 mm bias magnet.
Spatial Resolution (Perpendicular, z)	0.1	mm	Limited by the scanning stage resolution.
Maximum Lift-Off Distance	3	mm	Distance from sensor head to 316 stainless steel surface.
Non-Magnetic Cover Thickness (Fiberglass)	2	mm	Maximum thickness tested for CUI simulation.
Non-Magnetic Cover Thickness (Brass)	1.5	mm	Thickness tested for CUI simulation.
MW Power (Optimum)	10	W	Used for structural defect quantification measurements.
Frequency Modulation Amplitude	4.5	MHz	Used for structural defect quantification measurements.
Modulation Frequency	3.0307	kHz	Used for structural defect quantification measurements.
ODMR Reference Voltage	0.6	V	Chosen to allow for higher lift-off distance.
Sensor Operating Temperature (Expected)	Up to 300	°C	Based on diamond NV center stability.

Key Methodologies

The experiment utilized a compact, fiber-coupled diamond sensor head operating via ODMR in an intentionally inhomogeneous magnetic field setup.

Diamond Material: An ensemble of NV centers in diamond was used. While isotopically-purified ¹²C diamond was previously used for high sensitivity, the current setup prioritized spatial resolution, making natural abundance diamond sufficient.
Bias Field Application: An inhomogeneous magnetic field was deliberately applied using two permanent magnets. A 1 mm permanent magnet was placed 2 mm from the diamond to induce Zeeman splitting of the NV centers.
Microwave (MW) Delivery: MW excitation was delivered via an integrated antenna loop to perform ODMR. The sensor head design included a small Faraday shield to prevent MW leakage.
Detection Mechanism: The sensor detects magnetic flux profile perturbations arising from structural defects in the steel. These perturbations cause shifts in the NVC Zeeman splitting, which are quantified by monitoring the voltage output of a Lock-In Amplifier (LIA).
Scanning and Calibration: Two-dimensional (x/y) scans were performed using a scanning stage. Calibration was achieved by monitoring LIA voltage changes as the lift-off distance (z-axis) was varied over a damage-free area of the sample.
Defect Quantification: Defects were quantified by analyzing the change in the LIA shift voltage and the Full Width at Half Maximum (FWHM) of the cross-sectional profiles, correlating these metrics to defect depth and width.

6CCVD Solutions & Capabilities

The successful replication and advancement of this diamond-based NDT technology rely on high-precision, custom-engineered MPCVD diamond components. 6CCVD is uniquely positioned to supply the necessary materials and fabrication services to enhance spatial resolution and sensor integration.

Applicable Materials

Research Requirement	6CCVD Material Recommendation	Technical Rationale
NV Center Host Material	Optical Grade Single Crystal Diamond (SCD)	Provides the highest purity and lowest defect density necessary for creating stable, high-density NV ensembles required for robust ODMR signals.
Future Sensitivity Enhancement	Isotopically Purified ¹²C SCD	For future designs requiring sub-nanotesla sensitivity (pT/√Hz range) or homogeneous field operation, 6CCVD supplies ¹²C SCD to minimize decoherence caused by ¹³C impurities.
Sensor Substrate/Support	High-Purity Polycrystalline Diamond (PCD)	Available in large formats (up to 125 mm) and thicknesses (up to 10 mm) for robust substrate mounting or large-area sensor arrays.

Customization Potential

The paper explicitly notes that reducing the size of the 1 mm bias magnet is necessary to improve spatial resolution (< 1 mm). This requires a corresponding reduction and precise integration of the diamond sensor element.

Research Need	6CCVD Customization Service	Specification Match
Miniaturization for Resolution	Precision Laser Cutting and Dicing	We provide custom dimensions for SCD plates, enabling the fabrication of sensor elements significantly smaller than the 1 mm cube used in the current setup, crucial for achieving sub-millimeter resolution.
MW Antenna Integration	Custom Metalization Services	6CCVD offers in-house deposition of metals (Ti, Pt, Au, Cu, Pd, W) for creating the integrated microwave delivery loops and contact pads directly onto the diamond surface, ensuring low-loss signal transmission.
Optical Coupling Optimization	Ultra-Low Roughness Polishing	Our SCD material is polished to an industry-leading surface roughness of Ra < 1 nm, minimizing scattering losses and optimizing the efficiency of the fiber-coupled optical detection system.
Thickness Control	Custom Thickness Control	We supply SCD and PCD materials with precise thickness control from 0.1 µm up to 500 µm, allowing engineers to optimize the active NV volume for sensitivity versus spatial resolution trade-offs.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth parameters and material optimization for quantum sensing applications. We can assist researchers in selecting the optimal diamond grade (e.g., specific nitrogen concentration for ensemble NV creation) and geometry for similar Magnetic NDT and Structural Defect Imaging projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.15.024015