UC San Diego

Magnetic refrigerator has been receiving a lot of attention in industry because it is environmentally friendly system with higher coefficient of performance (COP). However, magnetocaloric alloys for magnetic refrigerator contain very expensive rare-earth elements or toxic elements so that it is difficult to scale up for widespread use. In this dissertation, development of rare-earth free Mn-based magnetocaloric alloys are described.

In chapter 2 of this dissertation, I describe nanograined Ni-Mn-Sn alloys for room temperature magnetocaloric (MC) material. Ni-Mn-Sn Heusler alloys exhibiting large magnetocaloric effect (MCE) around room temperature have been intensively studied for environmentally-friendly and inexpensive room temperature refrigeration. However, Ni-Mn-Sn alloys have a somewhat narrow MC temperature range and consequently a low refrigerant capacity value around room temperature. In this work, we employed a novel spark erosion process to produce oxide-free nanoparticles of Ni-Mn-Sn alloy and subsequently fabricated nanograined Ni-Mn-Sn material by hot pressing, in order to broaden its MC temperature range. We characterized the crystal structure, magnetic properties, and MC properties of the produced nano-grained Ni48.6Mn37.5Sn13.9 pellets. From the X-ray diffraction and Reitveld refinement, it was determined that the nanograined Ni-Mn-Sn had a modulated 5M (IC) structure, in which a contracted lattice parameter and unit cell volume were observed, compared to as-made bulk Ni-Mn-Sn. For the nanograined Ni-Mn-Sn, we observed an 85% increase in the full-width-halfmaximum (FWHM) of the magnetic field induced entropy change ΔSM(T), leading to an improved refrigerant capacity of 11%, despite a 35% reduction in ΔSM at 3T. Our results suggest that nanostructuring offers a promising approach to enhance the MC performance of Ni-Mn-Sn half-Heusler alloys. Chapter 2, in part, is submitted in Acta Materialia, Dongwon Chun, Chin-Hung Liu, Robin Ihnfeldt, Lizzie Caldwell, Tae Kyung Kim, Yongdeok Kim, Chong Seung Yoon, Ekaterina Novitskaya, Olivia A. Graeve, Sungho Jin and Renkun Chen. The dissertation author was the primary investigator and author of this paper.

In chapter 3 of this dissertation, I describe the structure and properties of Mn-Si alloys for low temperature MC material. In particular, the crystal structure, magnetic properties and MC effect of Mn5-xSi3+x (x = -0.4, -0.3, -0.2, -0.1, 0, 0.3) polycrystalline samples were investigated. Conventional-MCE below 50K was observed in the Mn5- xSi3+x (x = -0.4, -0.3, -0.2, -0.1) samples. As Mn concentration decreased from Mn5-xSi3+x, conventional-MCE was improved, whereas inverse-MCE ~65K was reduced. It was revealed from the X-ray diffraction and Reitveld refinement that lattice parameters (a,b, and c) and unit cell volume contracted, and also, the differences between the measured and calculated results became larger as Mn concentration decreased. These results indicate the MnSi exhibits conventional-MCE below 50K in Mn5-xSi3+x inverse-MCE.