  Additive manufacturing (AM) could be a novel method of fabricating composite
and porous materials having various effective performances based on mechanisms
of their internal geometries. Materials fabricated by AM could rapidly be used
in industrial application since they could easily be embedded in the target
part employing the same AM process used for the bulk material. Furthermore,
multi-material AM has greater potential than usual single-material AM in
producing materials with effective properties. Negative thermal expansion is a
representative effective material property realized by designing a composite
made of two materials with different coefficients of thermal expansion. In this
study, we developed a porous composite having planar negative thermal expansion
by employing multi-material photopolymer AM. After measurement of the physical
properties of bulk photopolymers, the internal geometry was designed by
topology optimization, which is the most effective structural optimization in
terms of both minimizing thermal stress and maximizing stiffness. The designed
structure was converted to a three-dimensional STL model, which is a native
digital format of AM, and assembled as a test piece. The thermal expansions of
the specimens were measured using a laser scanning dilatometer. The test pieces
clearly showed negative thermal expansion around room temperature.
