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

TL;DR

This study investigates the anisotropic mechanical properties of a copper MOF (HKUST-1) using in situ micropillar compression, revealing directional differences in elasticity, plasticity, and fracture toughness at the microscale.

Contribution

It provides the first detailed in situ analysis of the anisotropic mechanical behavior of MOF single crystals along different crystallographic directions.

Findings

Young's modulus varies significantly between (100) and (111) facets.

Yield strength is approximately twice as high in the (111) direction.

Fracture toughness is similar for both facets despite different cracking behaviors.

Abstract

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) ~ 3.6 x E(100), followed by extensive plastic flows. Likewise, the yield strengths are considerably different, in which Y(111) ~ 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 Kc ~ 0.5 MPa m^1/2. This work provides new insights of porous framework ductility at the micron scale and failure by bonds breakage.