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Title: Quantitative studies of the nanoscale mechanical properties of metal organic framework materials
Author: Zeng, Zhixin
ISNI:       0000 0004 7654 3554
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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The nanoscale mechanical properties of metal-organic framework (MOF) materials including the elasticity (Young's modulus), plasticity (hardness, yield stress, ultimate strength, and work hardening coefficient), adhesion property (adhesive force and energy), and failure mechanisms (failure modes, corresponding threshold forces and failure strengths) have been studied in the range of sub-nm to ~100s nm in depth. MOF materials are a topical class of porous crystalline solids that are constructed from the self-assembly of metal clusters and organic linkers, to yield tuneable framework structures with a vast range of physical and chemical properties. Herein, three model MOF systems are of particular interest: the nanocrystals and micron-sized crystals of ZIF-8, two-dimensional nanosheets of CuBDC, and submillimetre-sized crystals of HKUST-1. The three MOF materials were prudently selected because they represent three distinctive structures in terms of the response to stress. In this work, depth-sensing nanoindentation techniques, particularly the 'conventional' instrumented indentation technique (force sensitivity ~ 50 nN) and the atomic force microscopy (AFM)-based nanoindentation (force sensitivity < 0.05 nN), have been intensively employed and adapted to enable the accurate characterisation of the fine-scale mechanical behaviour of MOF materials. By leveraging the force and displacement sensitivities of the AFM instrument, the AFM-based nanoindentation method has been rationally established and improved by extending its application to study MOF mechanics at the nanoscale. This is a challenging field that has not yet been rigorously addressed by means of detailed experiments. Amongst the depth-sensing techniques, the AFM-based nanoindentation measurements are usually criticised for their instability and low reproducibility. Herein, the major factors that could interfere with the measurement precision have been systematically investigated. The research has led to the development of comprehensive approaches for AFM instrumental controls to achieve reliable force-displacement data and improved contact models for nanoindentation data analysis. Quantification of the nanoscale mechanical properties sheds light on the complicated deformation behaviour of the porous MOF structures. AFM nanoindentation data obtained at the nanometre length scale reveal new structure-property information about the surface energy, incipient plasticity, and fracture mechanisms underpinning the inorganic-organic building blocks of MOF systems. Accurate materials data about mechanical performance and durability will be fundamental towards the development of functional devices and practical applications.
Supervisor: Tan, Jin-Chong Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Quantitative Study ; Engineering Science ; Metal-Organic Framework (MOF) ; Mechanical Property Characterisation ; Nanomaterial