Low-Temperature Deposition of Graphite Coating on La(Fe, Co, Si)₁₃B_0.2 for Room-Temperature Magnetic Refrigeration

Due to the larger magnetic entropy change Δ<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_M</i> and the much lower raw material cost, La(Fe,Co,Si)<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> materials have the potential to replace the room-temperature magnetic refrigeration prototype material Gd. In this work, we have developed a simple chemical vapor deposition (CVD) method for in-situ graphite deposition on the surface of La(Fe,Co,Si)<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub>B<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub>. Facilitated by the decomposition of the solid carbon-source polyethylene glycol (PEG), the CVD process could be carried out at a low temperature of 400 °C and in a short duration of 20 min, which ensures no α-Fe precipitation from and carbon atom diffusion into the La-Fe-Si material. These 400-700 nm thick coatings could significantly enhance the anti-corrosive property with a positive shift of corrosion potential Ecorr from -788 mV to above -600 mV and a reduction of the corrosion current density <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_corr</i> from 1.67×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to 9.14×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> A/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>, accompanied by a concurrent enhancement of the thermal conductivity. More favorably, a large Δ<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_M</i> of ~4 J/kgK at 286 K and a relative cooling power (RCP) value of ~98 J/kg in 0-1.5 T were maintained.