AS-06: New measurement method and instrument for direct measurement of the adiabatic temperature change to enable magnetic refrigeration

Magnetic refrigeration based on the magnetocaloric effect has the potential to significantly improve energy efficiency of existing vapor-compression-based temperature control systems, while being environmentally benign. However, the existing measurement methods, direct (adiabatic temperature changes under magnetization/demagnetization) and indirect (heat capacity or isothermal magnetization measurements), have large uncertainties (30% or more) and a large discrepancy in values. For example, the adiabatic temperature change for gadolinium varies between 3.9 K to 5.8 K using a field change of about 2 T. To progress, this technology requires a clear connection between the properties of the magneto-caloric materials (MCMs) and their performance in the magnetic refrigeration cycle. Therefore, we are proposing a new instrument and measurement method to directly measure the refrigeration capacity of MCMs with acceptable uncertainties: a pressure-energy rig that uses the incompressibility of a fluid as an amplifier to increase the sensitivity of the measurement. In this method, a chamber is moved into/out of an applied magnetic field (Fig. 1). The sample chamber is made of pure diamagnetic glass to eliminate eddy current heating and minimize changes to the magnetic field seen by the sample, and is surrounded by a (high) vacuum jacket to make the chamber quasi-adiabatic. A glass “fence” is in the center of the sample chamber to hold the sample stationary while the actuator is operating, but maintains good thermal contact with the surrounding fluid. The sample surface, sample center, and chamber walls have their temperatures monitored to measure heat in/from the sample. A non-magnetic pressure sensor measures the pressure change in the fluid around the sample, and it can also be used to verify the maximum sample temperature change. The measurement technique is calibrated by comparison with the sample temperature rise from the injection of a known amount of heat, provided by a known applied current to a heater of known resistance located in the center of the sample. Preliminary results for Gd at room temperature are 2.5 K for a 1 T field change.