Metal–organic frameworks (MOFs) are periodic crystalline one-, two-, or three-dimensional structures composed of two major components: a metal ion or cluster of metal ions and an organic molecule called a linker. For this reason, the materials are often referred to as hybrid organic–inorganic materials. They are a subclass of coordination polymers, with the special feature that they are often porous.
MOF are highly porous materials that make it possible to take-up, store, separate, release or protect gases or liquids from their pores. MOFs have the largest internal surface area of any known material that comes from their porous structure means that they can be used in a host of applications, from sensing to gas separation and storage and catalysis. They offer a real-world impact as vital as filtering toxic chemicals through a protective mask. Researchers at North Carolina State University have found that the MOF-treated fabric deactivated the chemical in minutes; this could form the basis for a thin, lightweight shield to degrade some chemical weapons which kill or injure on contact.
The building blocks of the framework – metals and organic linkers – can be combined in almost infinite ways to create novel materials. Therefore, unique structural characteristics can be achieved by tuning the basic materials according to their specified application. As a rule of thumb, MOFs outperform other materials by a factor of 10. There are currently more than 60,000 known MOFs, and they are being investigated as promising materials for gas storage, including CO2 sequestration and hydrogen storage, and can even be used to harvest water in the desert.
Reserachers have also found MOFs important in electromagentic applications. Electromagnetic waves are widely applied in numerous areas and make our daily life convenient through numerous electronics devices, such as mobile phones, WiFi, Near Field Communication (NFC), and wireless charging. With the extensive practical applications of electronic devices and densely packed systems, electromagnetic interference (EMI) becomes a more and more serious problem, which would lead to pernicious impacts on equipment performance, human health, as well as surrounding environment . Furthermore, our individual mini device produces unwanted EM waves, which would influence other nearby devices. Moreover, the global need for some EM waves, such as for military radar stealth, is also boosting, which produces plentiful concerns for human health. Therefore, the protection of electromagnetic radiation has been widely concerned by the whole society.
Carbon-based materials derived from metal–organic frameworks (MOFs) have drawn increasing research attention for high-performance electromagnetic wave absorption due to their unique microstructure. Currently, high efficiency electromagnetic wave absorption plays an important role to keep away from the detection of aircraft by radar and reduce information leakage in various electronic equipment.
Chinese Researchers from School of Materials Science and Engineering, Shandong University, have found the MOF-derived ZrO2/C nanocomposites with light weight, thin thickness, high stability, strong absorption intensity and ultra-wide absorption bandwidth exhibit huge application potential as high-performance electromagnetic wave absorption materials.
“Benefiting from the suitable carbonization treatment, which leads to a strong attenuation capacity and a harmonious impedance matching characteristic, superb electromagnetic wave absorption performances with a minimum reflection loss (RL) value of −58.7 dB (16.8 GHz, 1.5 mm) have been achieved. And the effective absorption bandwidth (EAB; RL < −10 dB) could cover 91.3% (3.4–18.0 GHz) of the measured frequency within the thickness range of 1.0–5.0 mm,” write the authors.
Chinese Researchers find Metal-organic framework absorbs microwaves
Materials that reduce electromagnetic interference between electrical components in advanced electronic circuits and help aircraft, ships, and other military hardware evade radar detection rely on absorbing microwaves. In 2018, researchers have shown, for the first time, that an iron-based metal-organic framework (MOF) has microwave-absorbing properties [Green et al., Materials Today Chemistry 9 (2018) 140].
Many materials from carbon in all its forms to conducting polymers to various metal oxides and composites have been investigated for microwave-absorbing properties, which are thought to arise from dielectric and magnetic losses. But the team from the University of Missouri – Kansas City, Shanghai Institute of Ceramics, China Three Gorges University, Peking University, and Changchun Institute of Optics, Fine Mechanics and Physics think that a novel interference mechanism could be at work in the case of MOFs.
Now we have shown, for the first time, that ferric metal organic frameworks (or MOFs) possess very good microwave absorption properties,” says Xiaobo Chen, who led the research effort. Many materials from carbon in all its forms to conducting polymers to various metal oxides and composites have been investigated for microwave-absorbing properties, which are thought to arise from dielectric and magnetic losses. But the team from the University of Missouri – Kansas City, Shanghai Institute of Ceramics, China Three Gorges University, Peking University, and Changchun Institute of Optics, Fine Mechanics and Physics think that a novel interference mechanism could be at work in the case of MOFs.
The Fe-MOF was synthesized from ferric nitrate [Fe(NO3)3×6H2O], 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid (TMBDC) and 1,4-diazabicyclo[2.2.2]octane (DABCO) mixed in N,N-dimethylformamide (DMF) at room temperature. The resulting brownish-red powder is made up of micrometer-scale amorphous particles within which metal ions are linked together by TMBDC and DABCO pillar ligands.
When bombed with microwaves, the Fe-MOF shows a large reflection loss value (of -54.2 dB), which represents an absorption efficiency of more than 99.999%. The optimum layer thickness appears to be around 2.65 mm, above which a narrower microwave frequency region is shielded from radar detection. Unlike other microwave-absorbing materials, the researchers’ observations indicate that electrical rather than magnetic relaxation within the material explains the high microwave-absorbing properties of Fe-MOF. The researchers suggest that rotation of polar groups or regions within the Fe-MOF are responsible for the remarkable microwave absorption. In effect, as microwaves are reflected from the front to the back surface of the Fe-MOF layer, high levels of interference lead to reflection losses and microwave absorption.
“This work opens up a new application field for MOF materials, while providing a promising material candidate (and likely many MOF candidates in the future) for microwave absorption,” says Chen. Fe-MOF is easy to fabricate in large quantities from widely available, cost-effective reagents under mild conditions, he points out, and can be used to coat any objects that need to be shielded from radar detection or electromagnetic interference via simple brush-on or roll-to-roll approaches.
New kind of supercapacitor made without carbon
Supercapacitors use technology that is significantly different from the reversible chemical reactions used in rechargeable batteries. They store electrical energy by building up a separation of positive and electric charge and this ability enables them to supply quick bursts of energy needed, for example, to power the acceleration of electric cars, or open emergency doors in aircraft. They have a weakness, however, in the relatively low quantity of energy that they can store, a property known as their energy density.
In 2016, Mircea Dincă, an MIT associate professor of chemistry; Yang Shao-Horn, the W.M. Keck Professor of Energy have found an entirely new class of materials for supercapacitors. Dincă and his team have been exploring for years a class of materials called metal-organic frameworks, or MOFs, which are extremely porous, sponge-like structures. These materials have an extraordinarily large surface area for their size, much greater than the carbon materials do. That is an essential characteristic for supercapacitors, whose performance depends on their surface area. But MOFs have a major drawback for such applications: They are not very electrically conductive, which is also an essential property for a material used in a capacitor.
Dinca et al. had designed a supercapacitor using MOF which exceeds the performance of carbon based due to with high surface area and high electrical conductivity, This discovery is “very significant, from both a scientific and applications point of view,” says Alexandru Vlad, a professor of chemistry at the Catholic University of Louvain in Belgium “the supercapacitor field was (but will not be anymore) dominated by activated carbons.”
One advantage of the material used in these experiments, technically known as Ni3(hexaiminotriphenylene)2, is that it can be made under much less harsh conditions than those needed for the carbon-based materials, which require very high temperatures above 800 degrees Celsius and strong reagent chemicals for pretreatment. “One of our long-term goals was to make these materials electrically conductive,” Dincă says, even though doing so “was thought to be extremely difficult, if not impossible.” But the material did exhibit another needed characteristic for such electrodes, which is that it conducts ions (atoms or molecules that carry a net electric charge) very well.
While there has been much research on MOFs, most of it has been directed at uses that take advantage of the materials’ record porosity, such as for storage of gases. “Our lab’s discovery of highly electrically conductive MOFs opened up a whole new category of applications,” Dincă says. Besides the new supercapacitor uses, the conductive MOFs could be useful for making electrochromic windows, which can be darkened with the flip of a switch, and chemoresistive sensors, which could be useful for detecting trace amounts of chemicals for medical or security applications.
And a key advantage of that, he explains, is that “this work shows only the tip of the iceberg. With carbons we know pretty much everything, and the developments over the past years were modest and slow. But the MOF used by Dinca is one of the lowest-surface-area MOFs known, and some of these materials can reach up to three times more [surface area] than carbons. The capacity would then be astonishingly high, probably close to that of batteries, but with the power performance [the ability to deliver high power output] of supercapacitors.”
But that’s likely just the beginning, Dincă says. MOFs are a large class of materials whose characteristics can be tuned to a great extent by varying their chemical structure. Work on optimizing their molecular configurations to provide the most desirable attributes for this specific application is likely to lead to variations that could outperform any existing materials. “We have a new material to work with, and we haven’t optimized it at all,” he says. “It’s completely tunable, and that’s what’s exciting.”
In 2020, the KAUST research team found a way to increase the energy density using materials known as covalent organic frameworks (COFs). These are crystalline porous polymers formed from organic building blocks held together by strong “covalent” bonds—the type that holds atoms together within molecules. The reason for the previously low performance of COFs, the team found, is related to their low conductivity. They were able to overcome this limitation by exploring modified structures, which allowed electrons to become “delocalized,” meaning that they were able to move widely throughout the molecules. Furthermore, including carefully selected molecular functional groups also assisted the chemical changes required for increased energy storage performance. The researchers designed layered two-dimensional COFs to effectively exploit multiple charge storage mechanisms in a single material. In so doing, they were able to significantly increase the charge storage capacity of the COF.
Principal investigator Dr. Joseph Hunt’s work – “Synthesis and Characterization of Carbon Nanotube-Metal Organic Framework Composites” – could be used to develop new electromagnetic materials with enhanced, tunable properties with applications in electromagnetic offense and defense, and electric weapons in addition to chemical, biological and radiological protection.
“The Metal Organic Framework nanotube composites could be transitioned to a variety of operational areas in which thin layers of material with high electromagnetic lossiness is desired,” said Hunt. “The other permittivity and potential electronic properties could be used in electric weapons or directed energy projects.”
Hunt’s project – performed to produce composites with enhanced properties by combining carbon nanotube and reticular chemistry – advance the state of the art by exploring how the material properties of Metal Organic Framework are affected by the incorporation of increasing amounts of carbon nanotubes.
“This work enables future weapon systems by providing control over the electromagnetic properties of the material as well as providing the improved conductivity necessary for sensors and other electronic systems utilized by the Navy and Department of Defense.”
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