Observation of Curie transition during spark plasma sintering of ferromagnetic materials

• Calibration of temperature during spark plasma sintering (SPS) remains a big challenge.
• Temperature measured by non-contact pyrometers in SPS is not accurate.
• Ferromagnetic materials exhibit abrupt change in permeability at Curie temperature (Tc).
• Iron and Fe–Co alloy showed sharp and reproducible changes in SPS electric current profiles at Tc.
• Ferromagnetic materials can be successfully used to calibrate pyrometers in SPS

The possibility of employing the ferromagnetic–paramagnetic phase transitions of magnetic materials to calibrate temperature during spark plasma sintering (SPS) was investigated using pure Fe and Fe–50Co alloy. A sharp and repeatable change was observed in the electrical current profile at the Curie temperature (Tc) during both sintering and reheating of the sintered samples. Under a pulsed DC current, an abrupt change in the electrical resistance was observed at Tc due to the sudden changes in the permeability and in turn, the skin depth during heating and cooling. These effects can be used to obtain a more accurate in-situ measurement of the sample temperature than the one provided by the pyrometers that are normally used for SPS processing. The temperature measured using a pyrometer was found to be significantly lower (up to 70 °C) than the actual temperature of the specimen.