Schematic illustration of the interference microwave reflection loss (RL) mechanism within the Fe-MOF.
Schematic illustration of the interference microwave reflection loss (RL) mechanism within the Fe-MOF.

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. Now 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].

MOFs are highly porous composite materials constructed from organic ligands or linking molecules and metal ions or clusters. The resulting coordinated network forms a three-dimensional structure with record-breaking surface areas and pore volumes, which has attracted attention for catalysis, sensing, drug delivery and gas storage.

“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.