Mn5-xGe3Nix refrigerant for active magnetic refrigeration

Mn5-xNixGe3 (x=0, 0.025, 0.075, 0.1, 0.15, 0.2, 0.25, and 0.4) alloys were synthesized to demonstrate that the magnetocaloric (MCE) properties [Curie temperature, magnetic entropy change (Delta S-M), and adiabatic temperature change (Delta T-ad)] can be fine-tuned by changing the Ni fraction. An active magnetic regenerative (AMR) cycle testbed was built in-house to investigate the suitability of Mn5-xNixGe3 as a solid magnetic refrigerant operating at room temperature and as a composite refrigerant for an AMR cycle. The maximum temperature difference between the hot and cold ends, Delta T-span, measured from the AMR testbed was 7.5K and 5.6K for Gd and Mn5Ge3, respectively. The Delta T-span values were reasonable considering the inferior Delta S-M and Delta T-ad for Mn5Ge3 compared to those of Gd. A two-layer composite refrigerant consisting of Mn5Ge3 and Mn4.975Ni0.025Ge3 produced Delta T-span of 6.3K, which is 13% higher than that of the single Mn5Ge3 refrigerant, indicating the effectiveness of a multi-layer refrigerant selected to closely match the temperature gradient in an AMR cycle. This work highlights the importance of tailoring the MCE properties as required to build a composite refrigerant.