Thermal Actuation Through Bimaterial Lattices

The goal of this study is to examine the theoretical capability of bimaterial lattices as thermally driven actuators. The lattices are composed of planar non-identical cells. Each cell consists of a skewed hexagon surrounding an irregular triangle; the skew angles of the hexagon and the ratio of the coefficients of thermal expansion (CTEs) of the two component materials determine the overall performance of the actuator. Such a cell has three tailorable CTEs along the lines connecting the points where adjoining cells are connected. Each individual cell and a lattice consisting of such cells can be strongly anisotropic in terms of thermal expansion. While these lattice cells have been used as stress-free connectors for components with differing CTEs, they have not been explored for their actuation capacity. This paper develops models for bimaterial lattices that can be used as mechanical actuators for valves, switches and differential motion. A general procedure for lattice design includes drawing of its skeleton, which identifies the points at which a lattice cell is connected to other cells or substrates; calculation of three CTEs in each cell depending upon the functionality desired; choosing lattice materials; and finding of the skew angles for each cell as solutions of three nonlinear algebraic equations. By changing materials and geometry, we can determine the change of their configuration when the temperature changes. This paper illustrates the concepts with several examples: a two-cell lattice that is connected to a substrate that functions as a lever in a switch; a three-cell lattice that serves as a valve; and a lattice that controls the maximum total deflection of two adjoining parts of a structure.