Consideration of Magnetic Dipole Orientation in Liquid Detected by Metallic Contaminants Detection System Using High-Tc SQUID

The market for high-performance lithium-ion (Li-ion) batteries is growing rapidly as automobiles become electrified. The presence of small metallic particles of the order of 10 μm in a battery is likely to cause failure; therefore, it is important to eliminate them. We have developed a prototype system for detecting metallic foreign matter in liquid components for Li-ion batteries using a high-temperature superconducting radio-frequency superconducting quantum interference device, which is a highly sensitive magnetic sensor. Signal waveforms of a magnetized metallic piece passing through a trench located below the superconducting quantum interference device (SQUID) were measured. We found that the waveform depended on the direction of the magnetic dipole at the time of detection and could be roughly divided into six categories to explain all the experimental results. The maximum magnitude obtained for each sample was plotted against the equivalent spherical diameter of the sample. Our results indicate that the signal intensity was proportional to the cube of the sample diameter and that particles larger than φ20 × 30 μm (equivalent spherical diameter 23 μm) were detected with signal-to-noise ratio ≥3.