A magnetocaloric composite based on molecular coolers and carbon nanotubes with enhanced thermal conductivity

Abstract

In spite of a remarkably large magnetocaloric effect at temperatures below ca. 10 K, molecular coolers are yet to become excellent magnetic refrigerants. Their main limitation is the heat transport across molecules, which is expected to decrease too drastically at cryogenic temperatures. Here, this prediction is corroborated for the [Gd₂(OAc)₆(H₂O)₄]·4H₂O molecular cooler by thermal conductivity experiments and direct measurements of the magnetocaloric effect, together with numerical simulations. As a way out, a hybrid composite material is formed by attaching carboxylate-bridged Gd(III) molecules onto oxidized multi-walled carbon nanotubes. Notably, the molecular component of this composite maintains a large magnetocaloric effect, while the thermal conductivity of oriented composite buckypapers surpasses that of bulk [Gd₂(OAc)₆(H₂O)₄]·4H₂O by a factor of 2–3 below 20 K. Eventually, direct measurements of the magnetocaloric effect on the composite evidence the benefits arising from its higher thermal conductivity, in the form of relatively lower temperatures reached in magnetic refrigeration experiments, for example 1.2 K vs. 1.5 K, respectively, for the oriented composite buckypapers and bulk molecular cooler, starting from 1.96 K and 1 T applied field.