Experimental study of AC breakdown strength in ferrofluid during thermal aging

A colloidal ferrofluid (FF) with superparamagnetic iron oxide nanoparticles (SPION) has been investigated for the AC breakdown strength during the accelerated thermal aging test. Three volume concentrations of a transformer oil based FF were subjected to the accelerated thermal tests at the temperature 90 °C. AC breakdown strength (BDS) tests were carried out every 200 h period for up to 600 h. The breakdown probabilities were calculated according Weibull distribution function. Measured BDS populations were compared with a base carrier oil. The BDS median of the clear carrier oil has been observed to fall down 1.54 times, however for particular FF samples it dropped 2.31, 2.90 and 3.63 times, respectively, when comparing properties of the samples before testing with the samples after aging. Final BDS probability distributions show that the dielectrics withstand voltage of FF became lower than that of the carrier oil. The long-term thermal load of the particular FF is critical for its colloidal stability, which is deduced to be the main reason of such a significant BDS reduction. The impact of the thermal aging on the AC magnetic susceptibility is briefly documented, too.