In situ micropillar compression of an anisotropic metal-organic framework single crystal

Understanding of the complex mechanical behavior of metal-organic frameworks (MOF) beyond their elastic limit will allow the design of real-world applications in chemical engineering, optoelectronics, energy conversion apparatus, and sensing devices. Through in situ compression of micropillars, the uniaxial stress-strain curves of a copper paddlewheel MOF (HKUST-1) were determined along two unique crystallographic directions, namely the (100) and (111) facets. We show strongly anisotropic elastic response where the ratio of the Young's moduli are E-(111) approximate to 3.6 x E-(100), followed by extensive plastic flows. Likewise, the yield strengths are considerably different, in which Y-(111) approximate to 2 x Y-(100) because of the underlying framework anisotropy. We measure the fracture toughness using micropillar splitting. While in situ tests revealed differential cracking behavior, the resultant toughness values of the two facets are comparable, yielding K-c similar to 0.5 MPa root m. This work provides insights of porous framework ductility at the micron scale under compression and failure by bonds breakage.

Automated summary

Understanding the mechanical behavior of MOF beyond their elastic limit is crucial for their practical applications. The study determined the stress-strain curves of a copper paddlewheel MOF (HKUST-1) along different crystallographic directions and revealed anisotropic elastic response and extensive plastic flows. The fracture toughness values of two facets were comparable, indicating the ductility of porous framework under compression and bonds breakage.