2, Nagoya University, Nagoya, Aichi, Japan
To realize negative thermal expansion (NTE), porous composites made of two materials with different coefficients of thermal expansion are being actively researched. NTE can be realized by taking advantage of the thermal deformation mechanisms of a composite's internal geometry. However, the ability to tune thermal expansion is desirable not only to make it negative. Designing a material with arbitrary thermal expansion, including positive thermal expansion, using these phenomena extends the material's applications. Also, additive manufacturing (AM) could provide a new way to fabricate composites by layering multiple materials on arbitrary points in three-dimensional space. In this study, we developed a porous composite with arbitrary thermal expansion, including negative and positive thermal expansion as well as isotropic and anisotropic thermal expansion, by using multi-material photopolymer AM. The internal geometries producing such characteristics were designed by topology optimization, which is the most effective structural optimization method both in realizing macroscopic inward deformation and in maintaining stiffness. The designed structures were converted to three-dimensional models and fabricated by multi-material photopolymer AM. Using laser scanning dilatometry, we measured the thermal expansion of these specimens, revealing well-ordered thermal expansion, from anisotropic positive thermal expansion to anisotropic negative thermal expansion, over a wide range of about -3x10-4K-1 to 1x10-3K-1.