Electroless plating Ni-P coatings on La(Fe,Si)_(13) hydride bulks for room-temperature magnetic-refrigeration application

摘要

La(Fe,Si)_(13) based magnetocaloric alloys have been implemented as regenerators in room-temperature cooling systems in search for environmental-friendly alternatives to conventional vapour-compression refrigeration. Unfortunately,their corrosion resistance in water related transfer fluids has as yet been insufficient,although various attempts were made,including optimizing elemental compositions,customizing heat transfer fluids,and surface coating. This work presents a simple electroless plating strategy to deposit continuous Ni-P layers on the La_(0.7)Ce_(0.3)Fe_(11.45_Mn_(0.2)Si_(1.35) hydride thin-plates sintered under a high pressure,affording super corrosion protection and simultaneous improvement of heat transfer ability. Upon deposition of 2-6 μm Ni-P layers possessing robust interfacial coupling with the substrate,a large magnetic entropy change of about 12 J/kg K under 0-2 T was retained at room temperature. More significantly,the charge transfer resistance in 3.5% NaCl solution was increased to 6-17 times and the corrosion current density was reduced to 5-7 times compared to the high-pressure sintered hydride bulk,accompanied by a simultaneous increase of thermal conductivity to about 2 times. The Ni-P layers consisting of columnar microstructure may facilitate relief of the local stress from the field-induced lattice expansion,thus improving cycling stability of the magnetic refrigerants.