Periodic open cellular structures (POCS) show an emerging interest as support structures in the research field of heterogeneous catalysis. Regarding the heat management of chemical fixed-bed reactors, metallic POCS reveal promising characteristics compared to conventional packed beds in tubular reactors, e.g. perfused beds of spherical particles. In such a case, due to point-to-point contact, heat conduction contributes only a small amount to the overall heat transport. With strong exothermic reactions, undesired local overheating may occur. By superior heat conduction, POCS enable an improved heat transport within the fixed bed. However, inserted into a tube, a small gap between reactor tube wall and POCS maintains, impairing the heat removal [1].

Designing POCS with an auxetic effect is a promising approach for closing this gap. These structures show a negative Poisson’s ratio, i.e. applying a compression load, their size in one or several of the perpendicular directions decreases. As the main deformation mechanism, the nodes of the auxetic structures rotate and generate a complex stress field and high local strains [2]. Nitinol (NiTi) is able to bear the high strains since it offers 8 % reversible strain due to the shape memory effect (compared to 0.2 % elastic strain of normal metallic materials) [3]. In addition, the degrees of freedom unlocked by electron beam powder bed fusion (EB-PBF) make it possible to fabricate auxetic structures [2].

This study displays the progress of producing NiTi auxetic structures on a freely programmable EB-PBF machine Freemelt One. Moreover, it covers mechanical and thermal simulations computed with the software COMSOL Multiphysics combined with compression tests of EB-PBF built NiTi specimens to determine the reversible strains, maximum stresses and displacements. The aim is to reach an optimization of the geometric parameters of NiTi auxetic re-entrant hexagon structures in terms of improving the heat transfer of tubular reactor systems.

References

[1] G. Littwin, S. Röder, H. Freund, Industrial & Engineering Chemistry Research, 2021, 60, 6753-6766.

[2] C. Körner, Y. Liebold-Ribeiro, Smart Materials and Structures, 2015, 24, 025013.

[3] J.V. Humbeeck, Advanced Engineering Materials, 2001, 11, 837-850.