Time temperature stability of magnetic properties of ceramic magnetic temperature transducers

Three ceramic magnetic compositions within the CuZnTi ferrite system, havin g the Curie temperatures centered around 60, 80 and 100 degrees C, respecti vely, were investigated in order to determine the time-temperature stabilit y of their main magnetic properties. Permeability and the slope of permeabi lity around their Curie points were determined on ring shaped samples, befo re and after being subjected to long term (over 6000 h) ageing at their Cur ie temperature and for a shorter time of about 50h at higher temperatures u p to 700 degrees C. Rates of changes of 33, 66 and 100 ppm h-l of the maxim um slopes of permeability were found for the samples aged at 60, 80 and 100 degrees C, respectively. The samples aged at temperatures higher than 200 degrees C show a rather sudden decrease of maximum slope of permeability, b ut no shift of the Curie temperature and the working point, corresponding t o the temperature where the slope has the maximum value. This is the most i nteresting result as concerns the use of such magnetic temperature sensors for the construction of highly sensitive temperature controllers, for examp le ultrathermostats. The results are discussed in terms of the migration pr ocesses of the cations, especially Cu2+,from metastable positions on which they were frozen during the rapid cooling of the sample from the sintering temperature, to the more stable ones, namely the octahedral sites into the spinel lattice. (C) 1999 Elsevier Science Limited. All rights reserved.