Sensor detects cracks in magnetic matter
doi:10.1038/nindia.2012.21 Published online 15 February 2012
Researchers have developed a new kind of optical sensor from an emulsion of magnetic nanofluid containing droplets of iron oxide nanoparticles. The sensor could be used to image defects such as cracks, corrosion and erosion buried in rail tracks, pipelines and tubes.
External magnetic fields generate magnetic flux leakage signals near defects in magnetic materials. This signal can be used in a device such as a hall probe sensor to provide information on the defect. However, scientists have yet to develop a visual and non-contact sensor for this purpose.
To design such a sensor, the researchers first employed a simple emulsification process to prepare a magnetic nanofluid from oil-based ferric oxide in the presence of water and sodium dodecyl sulphate, a surface-active molecule. They then made a thin-film sensor by placing the nanofluid between two microscopic glass plates. The nanofluid droplets contained ferric oxide nanoparticles of around 6.5 nm in size. The researchers placed the head of the sensor near the rear surface of mild steel plates containing artificial defects.
They magnetized the steel plate and sensor using a horseshoe-sized bar magnet. Applying an external magnetic field generated magnetic flux lines around the defective region of steel plate, which led to the formation of one-dimensional nanodroplet arrays in the direction of the field. The droplets generated bright colours when irradiated with white light, thus providing a signature of the defects buried in the material.
Violet light was produced where the magnetic flux leakage signal was at a maximum. "The most important aspect of this technique is the reusability of the flux sensor due to perfectly reversible one-dimensional arrays of droplets formed in the fluid," says lead researcher John Philip.
Mahendran, V. et al. Nanofluid based optical sensor for rapid visual inspection of defects in ferromagnetic materials. Appl. Phys. Lett. 100, 073104 (2012) | Article |