Porous structure to improve microwave absorption properties of lamellar ZnO

Comparative evaluation of α-Bi2O3/CoFe2O4 and ZnO/CoFe2O4 heterojunction nanocomposites for microwave induced catalytic degradation of tetracycline

Two microwave (MW) responsive heterojunction nanocomposite catalysts, i.e., α-Bi2O3/CoFe2O4 (BO/CFO) and ZnO/CoFe2O4 (ZO/CFO), with weight% ratio of 70/30, 50/50, 30/70 were synthesized by sequential thermal decomposition and co-precipitation methods, and used for the degradation of tetracycline (TC) under MW irradiation. The formation of desired catalysts was confirmed through the characterization results of XRD, FT-IR, SEM, VSM, UV-DRS, XPS, BET, etc. Using batch MW experiments, the catalyst dose, pH, initial TC concentration, reaction temperature, and MW power were optimized for TC removal. Under the following reaction conditions: catalyst dose ∼1 g/L, initial TC concentration ∼1 mg/L, temperature ∼90 °C, MW ∼450 W, BO/CFO, and ZO/CFO showed ∼97.55% and 88.23% TC degradation, respectively, after 5 min. The difference in the catalytic response against TC degradation indicated the difference in reflective loss (RL) between these two catalysts. The presence of other competitive anions has affected the removal efficiency of TC due to the scavenging effect. The radical trapping study revealed the significant contribution of TC degradation by hydroxyl radicals in the case of ZO/CFO, whereas for BO/CFO, superoxide (●O2-) and hydroxyl radicals (●OH) both played influential roles. The Z-scheme heterojunction of BO/CFO allowed the formation of ●O2- but the same was inhibited in type-II heterojunction of ZO/CFO due to the valance band position. The dielectric loss, magnetic loss, interfacial polarization, and high electrical conductivity, 'hotspots' were produced over the catalyst surface alongside electron-hole separation at heterojunctions, which were responsible for the generation of reactive oxygen species. In addition, Co3+/Co2+ and Fe3+/Fe2+ redox cycles have promoted ●O2- and sulfate radical production during persulfate application. Among the two MW responsive catalysts, BO/CFO could be a potential material for rapidly destroying emerging organic pollutants from wastewater without applying other oxidative chemicals under MW irradiation.

MnFe2O4/polyaniline/diatomite composite with multiple loss mechanisms towards broadband absorption

The research and development of absorbing materials with high absorbing capacity, wide effective absorption bandwidth, and lightweight has always been interesting. In this research, a facile hydrothermal method was used to prepare MnFe2O4, and the grain size of MnFe2O4 decreased with increasing hydrothermal temperature. When the size of MnFe2O4 nanoparticles is less than 10 nm, its quantum size effect and surface effect make its electromagnetic microwave absorption performance greatly optimized. When the thickness of MnFe2O4-110 °C is 2.57 mm, the minimum reflection loss (RLmin) is -35.28 dB. Based on this, light porous diatomite and a three-dimensional polyaniline network are introduced. Diatomite is used as the base material to effectively reduce the agglomeration of MnFe2O4 quantum dots. The relatively high surface area introduced by a three-dimensional network of polyaniline promotes the orientation, interfacial polarization, multiple relaxation, and impedance matching, thereby generating further dielectric loss. Additionally, the magnetic properties of manganese ferrite and the strong electrical conductivity of polyaniline play an appropriate complementary role in electromagnetic wave absorption. The RLmin of MnFe2O4/PANI/diatomite is -56.70 dB at 11.12 GHz with an absorber layer thickness of 2.57 mm. The effective frequency bandwidth (RL < -10 dB) ranges from 9.21 to 18.00 GHz. The absorption mechanism indicates that the high absorption intensity is the result of the synergistic effect of impedance matching, conduction losses, polarization losses, and magnetic losses.

Autogenous and Tunable CNTs for Enhanced Polarization and Conduction Loss Enabling Sea Urchin-Like Co3ZnC/Co/C Composites with Excellent Microwave Absorption Performance

ZIF-67-derived magnetic metal/carbon composites are considered prospective candidates for use as microwave absorption (MA) materials owing to their magnetoelectric synergy. However, the structure of ZIF-67-derived MA materials mainly depends on the morphology and composition of pristine metal-organic frameworks (MOFs), and their microstructures lack a rational design. Herein, a multidimensional sea urchin-like carbon nanotubes (CNTs)-grafted carbon polyhedra-encapsulated Co₃ZnC/Co nanoparticle composite was prepared by one-step catalytic pyrolysis of ZIF-67/ZnO using a rational structural design. The autogenous and tunable CNTs obtained with the assistance of zinc evaporation not only overcome the limitation of homogeneous dispersion but also endow the Co₃ZnC/Co/C composite with outstanding MA properties owing to the conduction loss provided by CNTs, polarization loss caused by multiple components, and electromagnetic wave trap composed of a special sea urchin-like structure. Consequently, the minimum reflection loss of ZZ_0.1-600 reaches -60.3 dB at 1.6 mm, the maximum absorption bandwidth of ZZ_0.05-600 is 6.24 GHz (covering nearly the entire Ku band) at 1.9 mm, and the structure has a low weight ratio (30 wt %). Compared with Z-600 and pure ZnO, the MA performance of the sea urchin-like Co₃ZnC/Co/C composite obtained by rational structural design has been greatly improved; this strategy offers a new approach for optimizing the MA performance of materials according to their structural design.

Pyrolysis, morphology and microwave absorption properties of tobacco stem materials

The recent development of microwave radiation technology has increased the application possibilities of waste tobacco stems (WTSs). In this study, the morphology and microwave absorption properties of tobacco stem materials as well as the pyrolysis of the resultant biomass (BMTS) were studied via thermogravimetry-differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and a vector network analysis (VNA). The results show that the BMTS pyrolysis involves four stages in air: dehydration, heat transfer, pyrolysis, and carbonisation, and it involves three stages in N2: moisture evaporation, de-volatilization, and charring. The microwave-assisted expansion of WTSs can improve the pore diameter and total porosity of the expanded tobacco stems (ETSs) and BMTS. The latter is a macroporous material with a total porosity of 78.2% and a probable pore size of 29.5 μm. Its pore size distribution ranges from 10.7 nm to 227 μm. The microwave absorption properties of the WTSs are affected by the moisture content, bulk density, and grain size; the properties can be enhanced by decreasing the grain size and increasing the moisture content and bulk density within the experimental range. The 3 dB bandwidth and amplitude vary by 0.45 MHz and - 0.406 dB per 1% increase in the moisture content of the materials, respectively. Our results demonstrate that tobacco stem materials with different moisture contents and grain sizes should be classified before the expansion or re-drying steps to ensure heating uniformity and product quality during the microwave radiation treatment.