Metallic contaminant detection system using multi-channel high Tc SQUIDs
Introduction
Finding ultra-small metallic contaminants is an important issue for manufacturers producing commercial products such as lithium-ion batteries. Manufacturers of faulty products suffer significant financial losses when they have to recall their products. Although the industry requirement is to find metallic particles that have a diameter of 50 μm, particles smaller than 100 μm cannot be detected by X-ray imaging, which is commonly used as the inspection method. Therefore, a highly sensitive detection system for small contaminants is required. Some SQUID detection systems for detecting contaminants in food have been developed [1], [2], [3], [4]. One of these systems is commonly available now [5]. However, in most cases, the matrix of industrial products is magnetized, and the magnetic signal from the matrix is sometimes sufficiently large to mask the signals from the contaminants. Thus, we have proposed the use of a planar gradiometer and horizontal magnetization of the sample prior to measurement. This combination reduces the large signal from the matrix and clarifies the signal from the metallic contaminants [6], [7], [8]. For practical use, it is important to increase the detection width of the system by employing multiple sensors with a wide sensing area. We describe a detection system based on eight-channel high-Tc SQUID gradiometers.
Section snippets
Principle
The principle of the detection system is shown in Fig. 1. The magnet horizontally magnetizes the metallic contaminant and the matrix of the test object. The SQUID gradiometer detects these remanent magnetic fluxes. If they are magnetized vertically, since the matrix such as a lithium ion battery electrode is magnetized, a signal from the contaminant is hidden by that from the matrix and cannot be seen [6]. However, it is expected that the magnetic poles appear at both ends of the matrix and the
Conclusion
A multi-channel high-Tc SQUID contaminant detection system for industrial products, with width of more than 70 mm was developed. An iron ball with a diameter of 50 μm was successfully measured by a single-channel SQUID gradiometer when the ball was placed at a distance of 3 mm from the gradiometer. We also demonstrated that the eight-channel system can detect a 95 μm square iron ball in the range of 72 mm with S/N ratio of more than 10. The success of the eight-channel system proposed in this paper
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