Rutile TiO2 Nanoparticles Encapsulated in a Zeolitic Imidazolate Framework-Derived Hierarchical Carbon Framework with Engineered Dielectricity as an Excellent Microwave Absorber.

Aiming to solve the poor response of titanium dioxide (TiO2) in the microwave frequency, versatile series of N-doped carbon (NC) components are employed to improve the conductivity and polarization strength of TiO2-based composites. The bimetallic zeolitic imidazolate framework-derived TiO2@NC complex (TNC-3) exhibits hierarchical microstructures and large-scale hetero-interfaces, whereas the pyrolysis composite of metal-polydopamine-coated TiO2 (TNC-4) possesses the vesicle-like NC shell and bulk TiO2 core. Thus, the optimal reflection loss and efficient absorption bandwidth of TNC-3 realize -44.0 dB at 3.0 mm and 5.4 GHz at only 2.0 mm of coating thickness, respectively. Nevertheless, the corresponding attenuation ability of TNC-4 is separately -24.3 dB and 4.8 GHz with a thickness of 5.0 and 2.0 mm, respectively. Importantly, the conduction and polarization loss can be enhanced by the large-scale interfacial contacts between nanoscale rutile nanoparticles and hierarchical graphitized carbon. Meanwhile, the superior performance of TNC-3 stems from the large proportion of pyridinic N and pyrrolic N, which provides asymmetric lone pairs to strengthen the dipole rotation. These results are of great value in constructing semiconductor-based complexes by carbon-coating engineering as functional materials.

Enhanced polarization from flexible hierarchical MnO2 arrays on cotton cloth with excellent microwave absorption.

To develop flexible microwave absorbers with strong attenuation capability has become a formidable challenge for applications of camouflage, stealth, and anti-electromagnetic pollution. Herein, a series of highly uniform cotton cloth@MnO2 (CC@MnO2) hierarchical structures with superior absorption performances were fabricated by simultaneously changing their intrinsic (α/δ phase) and extrinsic (2D/1D geometry) characteristics. The distinct absorption capability was dominantly contributed by the vertically grown dielectric MnO2 1D nanotube and conductive CC substrate, which could serve as a highly oriented backbone to ensure rapid electron transportation. Therefore, a well-designed CC@MnO2 sample (α phase instead of the δ phase) exhibits the best absorption performance. The maximum reflection loss (RL) is -53.2 dB at 5.4 GHz and the effective bandwidth is 5.84 GHz for a thickness of only 2 mm. This unique structure exhibits polarization, conduction loss, and strong dissipation capability, which can be attributed to the high density of accumulated charges trapped at the interface, as confirmed by the electron holography analysis. Meanwhile, the MnO2 coating does not affect the original flexibility of the CC and yields a massive interface and electronic conduction path. It is expected that CC@MnO2 might shed a new light on the design of microwave absorbers.

Boosted Interfacial Polarization from Multishell TiO2 @Fe3 O4 @PPy Heterojunction for Enhanced Microwave Absorption.

Core@shell structures have been attracting extensive attention to boost microwave absorption (MA) performance due to the unique interfacial polarization. However, it still remains a challenge to synthesize sophisticated 1D semiconductor-based materials with excellent MA competence. Herein, a hierarchical cable-like TiO2 @Fe3 O4 @PPy is fabricated by a sequential process of solvothermal treatment and polymerization. The complex permittivity of ternary composites can be optimized by tunable PPy coating thickness to improve the loss ability. The maximum reflection loss can reach -61.8 dB with a thickness of 3.2 mm while the efficient absorption bandwidth can achieve over 6.0 GHz, which involves the X and Ku band at only a 2.2 mm thickness. Importantly, the heterojunction contacts constructed by PPy-Fe3 O4 and Fe3 O4 -TiO2 contribute to the enhanced polarization loss. Besides, the configuration of magnetic Fe3 O4 sandwiched between dielectric TiO2 and PPy facilitates the magnetic stray field to radiate into the TiO2 core and out of the PPy shell, which significantly promotes magnetic-dielectric synergy. Electron holography validates the distinct charge distribution and magnetic coupling. The new findings might shed light on novel structures for functional core@shell composites and the design of semiconductor-based materials for microwave absorption.

Morphology-controlled synthesis and excellent microwave absorption performance of ZnCo2O4 nanostructures via a self-assembly process of flake units.

Pure dielectric microwave absorbers with strong attenuation capability and wide-band response become a challenge for efficient electromagnetic wave energy absorption. Herein, a series of ZnCo2O4 hierarchical structures with superior absorption performance have been achieved by tuning their surface architectures from ball-, hydrangea- to cabbage-, and pineapple-like morphologies. A facile one-step synthesis strategy using a self-assembly process with ZnCo2O4 crystalline flakes as structural units was proposed. The deionized water solution and urea addition were found to critically determine the formation of our unique cabbage-like ZnCo2O4 self-assembled morphology. The wide band and distinct absorption was dominantly contributed from dielectric ZnCo2O4 flakes, which could be furthermore adjusted by the above-mentioned morphologies. Due to its abundant void volume stacked by flakes, the cabbage-like ZnCo2O4 demonstrated the best absorption performance where the RLmax reached -36.33 dB at 9.5 GHz with an efficient bandwidth of 5.11 GHz (RL < -10 dB, 11.17-16.28 GHz). Adjusting the simulating thickness from 1 to 5 mm, the bandwidths range from 5.8 to 18 GHz. This unique structure has the polarization, conduction loss and strong dissipation capability resulting from the high density of accumulated charges trapped by the flake gap, confirmed by the analysis of electromagnetic parameters and electronic holography. It is expected that the self-assembled ZnCo2O4 microsphere might shed new light on the design of novel microwave absorption materials.