Metamaterials with negative Poisson’s ratio and non-positive thermal expansion

Abstract

Four metallic metamaterials with tailorable mechanical properties are designed using bimaterial star-shaped re-entrant planar lattice structures, which do not involve pins, adhesive, welding or pressure-fit joints and can be fabricated through laser-based additive manufacturing. Three length parameters, one angle parameter and three material combinations are used as adjustable design parameters to explore structure-property relations. For each of the four designed metamaterials, the effects of the design parameters on the Poisson's ratio (PR), coefficient of thermal expansion (CTE), Young's modulus and relative density are systematically investigated using unit cell-based finite element simulations that incorporate periodic boundary conditions. It is found that the bi-material lattice structures can be tailored to obtain 3-D printable metallic metamaterials with positive, near-zero or negative PR and CTE together with an uncompromised Young's modulus. In particular, it is shown that metamaterial # 1 can exhibit both a negative PR and a non-positive CTE simultaneously. These metallic metamaterials can find applications in structures or devices such as antennas and precision instruments to reduce thermomechanical stresses and extend service lives.

Mechanical Metamaterials with Thermoresponsive Switching between Positive and Negative Poisson's Ratios

“…The average CTE is obtained from the relative extension of the composite by the temperature change from 30 to 60 °C resulting in −2.28 × 10 −3 K −1 . We note that this value is the highest CTE among materials and composites, and the actuation is reversible (Figure d).…”

“…However, compared to experimental results, the FEM simulations show the mechanical properties instantaneously responding to temperature changes because the model neglects the time delay of heat conduction. Figure f shows the simulated x ‐ and y ‐direction strains as a function of temperature, indicating that our metamaterial has much larger CTEs ( α x = ±6.44 × 10 −4 K −1 and α y = −4.2 × 10 −3 K −1 ) than other negative thermal expansion materials . Furthermore, the metamaterial exhibits a PPR mode below 45 °C and a NPR mode above the temperature.…”

“…The present work demonstrates a novel strategy that leads to smart mechanical metamaterials with switching between positive and negative Poisson's ratios in response to temperature, where Poisson's ratio is the negative ratio of transverse strain to longitudinal strain. We utilize two‐way shape memory effects to obtain a thermoresponsive rod with an extremely large thermal expansion coefficient, which is the largest value reported thus far . A mechanical model for describing their large‐scale behaviors is further developed to predict their programmable mechanical deformations.…”

Angle‐Dependent Transitions Between Structural Bistability and Multistability

“…In addition, NPR and NTE ceramic aerogels have been fabricated for thermal superinsulation . Recently, four types of 2D and 3D metallic metamaterials with nonpositive PR and CTE were systematically investigated using unit cell‐based finite element simulations that incorporate periodic boundary conditions . 3) For an equivalent CTE and equivalent stiffness , using the shape reconfiguration of metamaterials based on negative stiffness to achieve tunable CTEs of single 1D–3D multistable metastructures has been proposed in our recent study…”

Designing Self Supported SLM Structures via Topology Optimization

“…• shape complexity: it is possible to take advantage of great geometrical freedom in order to reduce several parts in an assembly and/or using design optimization methodologies (TO), which opens a new world of customization possibilities [8][9][10]; • material complexity: multi-material parts can be built [11][12][13]; non-weldable materials can be joined [14]; coatings can be applied [15,16]; and surface finishing can be performed (polishing) [17]; • hierarchical complexity: multi-scale structures can be designed and fabricated using a geometric mesostructure in order to provide information to the part-scale macrostructure (i.e., lattices with negative or zero or positive coefficients of thermal expansion) [9,18,19];…”