A 2D auxetikos system based on interconnected shurikens
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A 2D material that expands biaxially upon thermal change, regardless of whether the temperature increases or decreases, is introduced herein. To do so, it must possess an overall conventional or positive thermal expansion under the influence of heating, but the coefficient of thermal expansion must switch to a negative value upon cooling. A novel microstructure is proposed herein in the form of interconnected shuriken network, whereby each rigid shuriken is connected to four connecting rods via four pairs of rotating rods. All rods contract during cooling but the swiveling of the rotating rod produces a net increase of the unit cell expansion in spite of the connecting rod contraction, thereby leading to an overall negative thermal expansion. During heating, the rotating rods are made redundant through the action of an interlocking mechanism while the connecting rods expand, thereby resulting in an overall positive thermal expansion. The capability for the material system to flip the sign of its thermal expansivity paves a way for engineers to design material systems that possess opposing properties in order to respond in a consistent manner in spite of opposing stimuli.
Keywords2D materials Auxetikos Network Shurikens
List of symbols
Half-length of connecting rod
Length of rotating rod
Half-length between sharp edges of shuriken
Half-length of unit cell dimension measured along the x-axis
Original half-length of unit cell dimension measured along the x-axis
Coefficient of thermal expansion (CTE) of connecting rod
Coefficient of thermal expansion (CTE) of rotating rod
Coefficient of thermal expansion (CTE) of unit cell in x-direction
Half-angle of shuriken corner
Note that the thermal strain for auxetikos system is always positive to indicate it having the tendency to increase in dimension. This characteristic translates into two distinct slopes of the plot of thermal strain versus temperature, and the corresponding abrupt change in CTE at the original state. There has been no precedence whereby any material system expands based on temperature change magnitude, and therefore the auxetikos system considered herein is being presented for the first time.
3 Results and discussion
It is therefore unsurprising that Fig. 5 exhibits two sets of CTE—one for increasing temperature and one for decreasing temperature—although the CTE is non-existent for no change in temperature.
4 Conclusions and recommendation
An auxetikos material, i.e. one that tends to increase, has been defined herein as a material system that exhibits positive strain whether the stimulus is of positive or negative value. By adopting the interconnected shuriken network, it has been shown that the material system expands based on the magnitude of the thermal change. This has been made possible by the presence of two sets of effective microstructure. One set of microstructure, whereby the rotating rod is redundant, takes effect upon heating wherein only the connecting rods expand. The other set of microstructure, in which the rotating and connecting rods are permitted to contract simultaneously, takes effect upon cooling. The capability of demonstrating two effective microstructures from just a single microstructure has been made possible through the implementation of a jamming or locking mechanism to render the rotating rods redundant under heating but functional under cooling. Results reveal that the effective CTE can be made more negative (under cooling) or less positive (under heating) by decreasing the αc/αr or lc/lr ratios, or by increasing the θ angle. It is herein proposed that a refined model be developed to take into consideration the finite change in temperature and more realistic geometrical details be incorporated, such as rod width, so as to produce a more accurate predictive model. The capability for materials to exhibit similar response under opposing conditions—due to their ability to manifest opposing properties under opposing conditions—paves a way for engineers to design materials that are able to change their behavior to suit the environment.
Compliance with ethical standards
Conflict of interest
The corresponding author states that there is no conflict of interest.
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