Novel graphite-coated La(Fe, Si)13Hy plates for magnetocaloric cooling

Abstract

La(Fe, Si)₁₃ hydrides suffer from intrinsically poor mechanical, anti-corrosive and thermally conductive properties, which impede their implementation as efficient regenerators. The current improvement strategies are faced with difficult trade-offs among these properties. In this work, we developed a facile chemical vapor deposition (CVD) method for low-temperature in-situ deposition of a thin graphitic layer of ∼ 200 nm on La_0.8Ce_0.2Fe_xMn_0.215Si_1.3 plates of 0.5–1 mm thickness by evaporating polyethylene glycol (PEG) polymer as a solid carbon source. These plates could be machined directly from their casting ingot due to the mechanical property enhancement from ∼ 3–6 wt% α-Fe. Favorably, these graphite-coated plates could be successfully fully hydrogenated at atmospheric pressure for room-temperature application. The ∼ 0.03 wt% graphite coating could endow the composites with striking anti-corrosive properties with the sum of the film resistance and the charge transfer resistance increasing from 105 (for the graphite-free hydride) to 11298 Ω·cm² by more than two orders of magnitude. Moreover, these graphite composites exhibit large room-temperature magnetic entropy change ΔS_M of 8.7 J/kg•K in 0–1.5 T and high thermal conductivity λ of 7.9–8.6 W/Km compared with the corresponding values of 3.5 J/kg•K and 9.5 W/Km for the prototype room-temperature magnetocaloric material Gd. The intriguing functionality of the graphite coating may advance the practical application of La(Fe, Si)₁₃-based materials.