Interfacial Thermal Resistance in Magnetocaloric Epoxy‐Bonded La‐Fe‐Co‐Si Composites

Abstract

Magnetocaloric composites made from La−Fe−Co−Si particles and an epoxy binder matrix exhibit mechanical stability and good magnetocaloric properties, but also a large characteristic time for thermal transport. Here, the origin of this large time constant is examined by comparing two measurement techniques, direct and contactless, to finite‐element simulations based on a tomographic dataset of the sample. The combination of the low thermal conductivity of the epoxy matrix and a thermal resistance at the interface between epoxy and La−Fe−Co−Si is shown to be in good agreement in simulations and experiments. The findings help to disentangle the role of the thermal conductivity and the interfacial thermal resistance for the heat flow in magnetocaloric composites. It is shown that the low thermal conductivity of the epoxy alone cannot explain the large time constant and possibilities for using the interfacial thermal resistance to tailor anisotropic thermal conductivity for directional heat transfer in magnetocaloric composites are presented.

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“…The constant delay with respect to the field variation, observable for all samples, is due to the characteristic time of the temperature sensor, as described in detail in literature. [ 18,37,38 ] In contrast, the adiabatic temperature change for the composite without TEGO is characterized by a response time to the magnetic field about two times larger ( τ = 0.21 ± 0.03 s) as compared with the composite with TEGO. The same delay has been reported also for epoxy‐bonded composites of LaFeSi ( τ = 0.22 s [ 38 ] ) and MnfePSi ( τ = 0.20 s [ 18 ] ) compounds.…”

“…Indeed, it has already been demonstrated that the main cause of the worsening of heat transport in a composite made of metallic powder and an epoxy binder matrix is the interfacial thermal resistance at the powder–matrix interface, which is not considered in the simulated model. [ 38 ]…”

“…[ 18,37,38 ] In contrast, the adiabatic temperature change for the composite without TEGO is characterized by a response time to the magnetic field about two times larger ( τ = 0.21 ± 0.03 s) as compared with the composite with TEGO. The same delay has been reported also for epoxy‐bonded composites of LaFeSi ( τ = 0.22 s [ 38 ] ) and MnfePSi ( τ = 0.20 s [ 18 ] ) compounds. These results clearly demonstrate that TEGO improves the heat transport in the composite, thus recovering the in‐time MC response of the bulk metallic compound and preventing the effect of low thermal conductivity of the polymeric resin.…”

“…Indeed, it has already been demonstrated that the main cause of the worsening of heat transport in a composite made of metallic powder and an epoxy binder matrix is the interfacial thermal resistance at the powder-matrix interface, which is not considered in the simulated model. [38] The relevant effect of TEGO addition on the transport of heat induced by the MCE can be explained by considering an effective decrease in the thermal resistance at the powder-matrix interface and the creation of a percolation network that provides a low thermal resistance path for phonon transport. These beneficial effects have been already observed in heat-conductive composite materials enriched with carbon nanostructures.…”

“…The time dependence of the MC response of composites has been investigated by directly measuring the adiabatic temperature change induced in the samples by using different magnetic field variation rates. [18,37,38] Figure 6 shows the time evolution of the adiabatic temperature change of the bulk NiMnIn compound and the composites. The two panels report the dynamic MC response of samples in two different time scales.…”