Anisotropic thermal conductivity in epoxy-bonded magnetocaloric composites

Abstract

Thermal management is one of the crucial issues in the development of magnetocaloric refrigeration technology for application. In order to ensure optimal exploitation of the materials “primary” properties, such as entropy change and temperature lift, thermal properties (and other “secondary” properties) play an important role. In magnetocaloric composites, which show an increased cycling stability in comparison to their bulk counterparts, thermal properties are strongly determined by the geometric arrangement of the corresponding components. In the first part of this paper, the inner structure of a polymer-bonded La(Fe, Co, Si)13-composite was studied by X-ray computed tomography. Based on this 3D data, a numerical study along all three spatial directions revealed anisotropic thermal conductivity of the composite: Due to the preparation process, the long-axis of the magnetocaloric particles is aligned along the xy plane which is why the in-plane thermal conductivity is larger than the thermal conductivity along the z-axis. Further, the study is expanded to a second aspect devoted to the influence of particle distribution and alignment within the polymer matrix. Based on an equivalent ellipsoids model to describe the inner structure of the composite, numerical simulation of the thermal conductivity in different particle arrangements and orientation distributions were performed. This paper evaluates the possibilities of microstructural design for inducing and adjusting anisotropic thermal conductivity in magnetocaloric composites.

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

“…The limitation of thermal transport in a certain direction of the regenerator can be used to direct thermal currents towards the heat exchange fluid. It was shown that such an anisotropic thermal conductivity can be achieved by orienting the particles of the composite in a certain direction . The presence of interfacial thermal resistance will presumably enhance the effect of particle orientation on the anisotropy even more because fewer interfaces appear in thermal paths parallel to the preferred particle orientation.…”

“…It was shown that such an anisotropic thermal conductivityc an be achieved by orientingt he particles of the composite in a certain direction. [18] Thep resence of interfacial thermal resistance will presumably enhance the effect of particle orientation on the anisotropy even more because fewer interfaces appear in thermal paths parallel to the preferred particle orientation. Anisotropict hermal conductivity can be very advantageousf or magnetocaloric cooling applications because thermal transport to the heat exchange fluid is improved and simultaneously dissipative losses are reduced.…”

Advanced Magnetocaloric Materials for Energy Conversion: Recent Progress, Opportunities, and Perspective

“…Currently, depending on the configuration of setups, different modalities of techniques like parallel-beam synchrotron X-ray computed tomography, transmission X-ray tomography, scanning transmission X-ray tomography and micro-X-ray fluorescence are available from micrometer to nanometer scale. [359,360] For magnetocaloric research, the lab-based X-ray tomography techniques have been successfully applied for some cases, e.g., (Mn,Fe) 2 (P,Si)-based compounds, [361] La(Fe,Si) 13based compounds, [182,183,[362][363][364][365] 3D printed La 0.6 Ca 0.4 MnO 3 and La(Fe,Si) 13 materials [366,367] to obtain the information like damage/porosity in compacted samples (i.e. bed structure)/composites, density, homogeneity, defects/cracks distribution and interfaces between the layers.…”

Development of a rotary thermomagnetic motor for thermal energy conversion.

“…In order to produce plates of the desired length for the thermomagnetic motor developed, given a maximum length of 2•10 -2 m of the compressed part, as shown in Figure 86.b, the plates were produced in sections that were then connected by tungsten rods and glued using epoxy, the final plates produced are presented in Figure 88. These plates have good mechanical and magnetic properties while maintaining a high ratio of magnetic material to epoxy, only 5% in weight of epoxy is used to produce the plates, as opposed to previous works that have reported percentages of epoxy as high as 45% (WEISE et al, 2016).…”