Effect of Polymer Coating on Magnetocaloric Properties of Garnet

Magnetocaloric materials can be applicable in magnetic cooling technologies, thermomagnetic motors, or medical treatments. For any practical applications, magnetocaloric materials are formed into various geometries, e.g., cylinders, plates, spheres, etc. Oxide-based MCE materials can be formed using high-temperature sintering or via mixing with polymers to increase the material’s formability. Further, coating these particles with polymers is a practical and effective method for inhibiting their agglomeration. However, the effect of polymer coating on MCE oxide material is not well explored. Given the above, this study focuses on understanding the influence of polyvinylpyrrolidone (PVP) on Gd₃Fe₅O₁₂ garnet’s structural, magnetic, and magnetocaloric properties.<br/><br/>The garnet Gd₃Fe₅O₁₂ samples were synthesized using the sol-gel auto combustion method. PVP coating of powder proceeds by adding 5% PVP (M_w-31000-50000) to a solution of 150 mg of Gd₃Fe₅O₁₂ nanoparticles suspended in distilled water. The presence of PVP was recorded via FTIR and TGA. The XRD studies showed that the PVP coating did not affect the crystal structure of Gd₃Fe₅O₁₂. However, the XRD peaks became slightly broadened and decreased in intensity after coating. The BET-specific surface area of nanostructured uncoated and PVP-coated Gd₃Fe₅O₁₂ oxides was determined to be 54.41 (m²/g) and 109.1 (m²/g), respectively. A marked increase in BET surface area is observed for the PVP-Gd₃Fe₅O₁₂ sample.<br/><br/>Magnetic measurements revealed that both coated and uncoated particles exhibited superparamagnetic behavior at 300K and 5K, respectively. The PVP-coated particles showed lower magnetization than the uncoated particles, attributed to reduced effective mass and degradation of particle dipole-dipole interactions caused by the non-magnetic PVP coating. Further pinning of surface spins via bonding with PVP molecules may extend deeper into the particle, reducing the effective magnetization of the compound. Magnetic entropy (-ΔSm) and the samples’ relative cooling power (RCP) were determined from the isothermal magnetization M vs. H data at different temperatures. The results showed a considerable increase in the peak temperature (57.5K) for PVP-coated Gd₃Fe₅O₁₂ compared to the uncoated sample. The peak temperature of 57.5K corresponds to a significant alteration in the material's magnetic properties. The maximum value of magnetic entropy change (-ΔSm) for uncoated Gd₃Fe₅O₁₂ was determined to be 3.80 Jkg⁻¹K⁻¹ at 37.5K with a 5T applied field, accompanied by a relative cooling power (RCP) of 380 Jkg⁻¹. On the other hand, for PVP-coated Gd₃Fe₅O₁₂, the maximum -ΔSm was found to be 3.38 Jkg⁻¹K⁻¹ at 47.5K with a 5T applied field, and the RCP was 308 Jkg⁻¹. Indeed, the observed maximum magnetic entropy changes at higher temperatures for the PVP-coated Gd₃Fe₅O₁₂ sample are noteworthy. Further, there is an unnoticeable change in -ΔSm value with PVP coating. This characteristic indicates that the PVP-coated garnet may have an advantage in terms of usability over a wider temperature range compared to the uncoated counterpart, which can potentially be a promising material for applications in cryogenic temperature magnetic refrigeration.