Shock Wave Energy Absorption in Metal-Organic Framework.

Recent investigations into the mechanical properties and mechanochemical reactions of metal-organic frameworks (MOFs) have suggested the potential for energy dissipation by multiple mechanisms. Although the possibility of efficient multifunctional shock dissipation by MOFs was suggested by static high pressure studies, there is little known about MOFs under shock compression. Here, we measure the attenuation of shock wave by the MOF denoted zeolitic-imidazolate framework (ZIF-8) in its desolvated, porous state. We find that shock wave dissipation by ZIF-8 occurred by multiple processes: powder compaction, nanopore-collapse, and chemical bond-breakage. The shock energy absorbance in ZIF-8 is proportional to ZIF-8 thickness, allowing the prediction of the thickness of MOF layer needed to attenuate shock waves to a desired lower energy. Compared with PMMA, often used as a standard, ZIF-8 attenuates 7 times more shock energy per unit mass for impacts at a lower velocity of 0.75 km/s and 2.5 times more at a higher velocity of 1.6 km/s. This research illustrates how to improve the ability to attenuate shock waves for personnel and equipment protection by engineering multifunctionality into the shock wave absorbing armor material.

High energy flexible supercapacitors formed via bottom-up infilling of gel electrolytes into thick porous electrodes.

Formation of thick, high energy density, flexible solid supercapacitors is challenging because of difficulties infilling gel electrolytes into porous electrodes. Incomplete infilling results in a low capacitance and poor mechanical properties. Here we report a bottom-up infilling method to overcome these challenges. Electrodes up to 500 µm thick, formed from multi-walled carbon nanotubes and a composite of poly(3,4-ethylenedioxythiophene), polystyrene sulfonate and multi-walled carbon nanotubes are successfully infilled with a polyvinyl alcohol/phosphoric acid gel electrolyte. The exceptional mechanical properties of the multi-walled carbon nanotube-based electrode enable it to be rolled into a radius of curvature as small as 0.5 mm without cracking and retain 95% of its initial capacitance after 5000 bending cycles. The areal capacitance of our 500 µm thick poly(3,4-ethylenedioxythiophene), polystyrene sulfonate, multi-walled carbon nanotube-based flexible solid supercapacitor is 2662 mF cm-2 at 2 mV s-1, at least five times greater than current flexible supercapacitors.

Age-related changes of corneal endothelial cell in healthy Chinese tree shrew measured by non-contact specular microscope.

AIM: To determine the impact of age on the morphology of endothelial cells and central corneal thickness (CCT) in Chinese tree shrew. METHODS: One-hundred and twenty eyes of 60 healthy Chinese tree shrews were studied. Based on age, the tree shrews were divided into four groups. After general anesthesia, the images of endothelium were acquired using non-contact specular microscope Topcon 3000P. Eight parameters of corneal endothelial cells were measured by built-in software, including CCT, endothelial cell density (ECD), percent hexagonality (HG%), coefficient of variability (CV), size of minimal cell (Smin), size of maximal cell (Smax), average cells size (Savg) and size standard deviation (Ssd). Data were analyzed using STATA software. The differences of eight parameters among groups and correlations with age were analyzed. RESULTS: In all studied animals, the average CCT was 249.6±20.29 µm (202-301 µm), ECD was 3080.72± 460.76 cells/mm2 (1239.6-4047.6 cells/mm2) and CV was 29.10±7.60 (13.6-54.6). CV was significantly different among different groups (P<0.001). Strong correlation with age was found in ECD, Smax, Savg, Ssd and CV. CONCLUSION: Cornea of Chinese tree shrews had half CCT of human cornea and similar ECD, CV and size of corneal endothelial cells. Young adult tree shrews had higher ECD, HG% and low CV. ECD, Smax, Savg, Ssd and CV correlated with age significantly.

Energy Storage during Compression of Metal-Organic Frameworks.

Practical applications of metal-organic framework (MOF) materials require an in-depth understanding of their mechanical properties. We have investigated the mechanical properties and energy absorption behavior of single crystals of four isostructural UiO-type MOFs under uniaxial compression. In situ nanocompression experiments were used to measure the mechanical behavior of individual MOF nanocrystals under compression within a transmission electron microscope. The plasticity and endothermicity during deformation of MOFs shows a surprising potential for absorption and dissipation of mechanical shock. At compressive stress below 2 GPa, relatively small amounts of energy (<0.3 kJ/g) are absorbed by the compression of these MOFs. As the stress was increased, however, the energy absorption was significantly enhanced. Above 2 GPa, the energy absorption typically reaches 3-4 kJ/g; for comparison, the energy release in the explosion of TNT is ∼4 kJ/g. Gram for gram, MOFs can absorb as much energy as a high explosive can release.

Shock Wave Chemistry in a Metal-Organic Framework.

Metal-organic frameworks (MOFs) have potential applications as energy absorbing materials for shock wave energy mitigation due to their nanoporosity. Here we have examined km/s laser-driven flyer plate impacts on a prototypical MOF, ZIF-8. We observed particle fragmentation and morphological changes in microcrystals of ZIF-8 at lower shock pressures (≈2.5 GPa), and amorphization and structural collapse at higher pressures (≈8 GPa). High-speed emission spectroscopy revealed that 50 ns after flyer plate impacts, an emission pulse was generated by ZIF-8 resulting from chemical bonds that were broken and subsequently reformed. MOFs may prove useful in the dissipation of shock wave energy through large structural changes (free volume collapse and endothermic bond breakage).

Impact of Shape Persistence on the Porosity of Molecular Cages.

Porous materials provide a plethora of technologically important applications that encompass molecular separations, catalysis, and adsorption. The majority of research in this field involves network solids constructed from multitopic constituents that, when assembled either covalently or ionically, afford macromolecular arrangements with micro- or meso-porous apertures. Recently, porous solids fabricated from discrete organic cages have garnered much interest due to their ease of handling and solution processability. Although this class of materials is a promising alternative to network solids, fundamental studies are still required to elucidate critical structure-function relationships that govern microporosity. Here, we report a systematic investigation of the effects of building block shape-persistence on the porosity of molecular cages. Alkyne metathesis and edge-specific postsynthetic modifications afforded three organic cages with alkynyl, alkenyl, and alkyl edges, respectively. Nitrogen adsorption experiments conducted on rapidly crystallized and slowly crystallized solids illustrated a general trend in porosity: alkynyl > alkenyl > alkyl. To understand the molecular-scale origin of this trend, we investigated the short and long time scale molecular motions of the molecular cages using ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations. Our combined experimental and computational results demonstrate that the microporosity of molecular cages directly correlates with shape persistence. These findings discern fundamental molecular requirements for rationally designing porous molecular solids.

Bond breakage under pressure in a metal organic framework.

The internal free volume of porous materials diminishes upon mechanical compression, and such volume collapse can have chemical consequences. We report here the endothermic bond breakage in a metal-organic framework (MOF) during compression-induced collapse. Upon bulk compression at 1.9 GPa, the effective number for Zr-O bonds between Zr(iv) ions and carboxylate groups in UiO-66 decreased from 4.0 to 1.9, as determined by EXAFS, and the internal free volume was synchronously collapsed. Consistent with the EXAFS data, IR spectra confirmed conversion of syn-syn bridging carboxylates to monodentate ligation, thus establishing mechanochemical reactions induced by external compression of MOFs. Substantial mechanical energy (∼4 kJ g-1) was absorbed by UiO-66 nanocrystals during compression, as demonstrated from nanocompression of single crystals (600 nm) in situ during scanning electron microscopy, which establishes the potential application of MOFs as mechanical energy absorbers for hydrostatic and shock compression.

Compression-induced deformation of individual metal-organic framework microcrystals.

The deformation and mechanical behavior of individual zeolitic-imidazolate framework (ZIF-8) micro- and sub-microcrystals were observed under compression. Young's modulus and volume changes as a function of applied pressure were determined on individual single crystals, offering insights in the relationship among structure, morphology, and mechanical properties. Dramatic volume decreases and amorphization were detected during compression over a pressure range of 0-4 GPa for individual 1.2 µm ZIF-8 microcrystals, and the deformed microcrystals partially recovered after pressure release. The orientation and size effects on the mechanical behavior of ZIF-8 nano- and microcrystals were also investigated. The presence of solvates within the pores of the ZIF-8 has a dramatic effect on the mechanical properties of the single crystals. Methanol-solvated ZIF-8 microcrystals are much less deformable than the desolvated microcrystals and shatter completely at very low applied force.