Metamaterials with a controllable thermal-mechanical stability: Mechanical designs, theoretical predictions and experimental demonstrations

Mechanical metamaterials with tunable thermal expansion are of increasing interest due to their capability for matching the thermal expansion of surrounding components to maintain high-level thermal-mechanical stability upon a temperature change and show their promising applications in the fields of flexible electronics, aerospace, MEMS, and precision instruments. Though numerous of studies on metamaterials with zero thermal expansion of coefficient (CTE) has been developed to access these demands, the experimentally demonstrated designs with a remarkable zero CTE value (e.g. <0.5 ppm/K) still remain a challenge. In this paper, a design concept of metamaterial capable of offering a tailorable CTE, especially the excellent thermal-mechanical stability, is systematically studied, whose underlying mechanism is the conversion of thermal-induced deflection of bilayer beams (in Al and Ti) into unusual, effective thermal expansion or shrinkage of metamaterials. Notably, the experimentally demonstrated CTE of this metamaterial is 0.42 ppm/K, which is the unprecedented zero CTE result as compared to the previously reported studies. The theoretical model for CTE and elastic modulus of this design, which is verified by FEA and experiments, provides a clear understanding of relationships between configuration parameters and effective thermal and mechanical performances. Combined theoretical predictions, FEA, and experimental studies demonstrate the capability of this metamaterial to serve as a design platform for devices or components that require excellent thermal-mechanical stability.