Regulatable Phase Manipulation-Enhanced Polarization and Conductance Loss Enabling Hierarchical 3D Microsphere-like MoS2 with Efficient Microwave Absorption.

Molybdenum disulfide (MoS2) has become a new type of microwave absorption (MA) material due to the abundant functional groups and defects, high polarization effect, and controllable structural design. However, the development of MoS2 has been limited by its inherently low conductance losses and imperfect impedance matching. This study employs ammonium ion (NH4+) intercalation as a phase manipulation strategy to enhance dielectric loss and form heterogeneous structures by incorporating highly conductive 1T phase into the 2H-MoS2 crystal phase. Additionally, the implementation of CTAB as a soft template agent for constructing layered three-dimensional microsphere structures improves impedance matching. The experimental findings demonstrate that the MA performance of MoS2 can be effectively regulated by controlling the 1T phase content and morphological structure design. It is worth noting that A-MoS2-2 possesses excellent multifrequency absorption capability. A-MoS2-2 has a minimum reflection loss (RL) of -53 dB at a coating thickness of 1.99 mm and an effective absorption bandwidth (EAB) of 5.6 GHz at a thinner coating thickness of 1.77 mm. This work improves the MA properties of MoS2 by introducing metallic phases and unique structural design, which opens up new ideas for the development of MA materials.

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 ZZ0.1-600 reaches -60.3 dB at 1.6 mm, the maximum absorption bandwidth of ZZ0.05-600 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.

Silica-Modified Ordered Mesoporous Carbon for Optimized Impedance-Matching Characteristic Enabling Lightweight and Effective Microwave Absorbers.

Ordered mesoporous carbon (OMC) is considered to be a prospective carbon-based material for microwave absorption because of its abundant well-ordered mesoporous structures, high specific surface area, numerous active sites, and facile preparation process. However, its development has been seriously hindered by its poor impedance-matching characteristic. Herein, silica-modified OMC composites with a designable impedance-matching transition layer are successfully fabricated via a self-assembly method and succeeding calcination treatment. In addition, the silica in OMC@SiO2 composites can maintain the mesoporous structure, which facilitates the scattering and reflection of microwaves in the tunnel structure. The as-prepared sample OMC-5@SiO2 exhibits a minimum reflection loss (RL) value of -40.7 dB at 10.8 GHz with 2 mm and an effective absorption bandwidth (RL ≤ -10 dB) of 4.8 GHz with a thinner absorber thickness of 1.5 mm. We believe that the as-prepared OMC@SiO2 composites can be prospective candidates as high-efficiency and lightweight microwave absorbers.

The effect of ZnCl2 activation on microwave absorbing performance in walnut shell-derived nano-porous carbon.

Porous carbon has been expected to be a potential candidate as a lightweight and efficient microwave absorber. Nano-porous carbon carbonized directly from a walnut shell exhibits narrow microwave absorption frequency bandwidth, while the activation process can adjust the pore structure and optimize the microwave absorption performance. Herein, porous carbon materials were successfully prepared using walnut shells as precursors and ZnCl2 as the activating agent. The superior microwave absorption performances of the as-prepared samples could be attributed to the well-developed pore structures and the enhanced dielectric loss capacities of the samples. The interfacial polarization in the walls of the pores and the defects in the samples significantly contributed to the enhancement of the dielectric loss capacities of the samples. In this work, the broadband microwave absorbing porous carbon exhibited an effective absorption bandwidth (reflection loss ≤ -10 dB) of 7.2 GHz (ranging from 10.8 GHz to 18.0 GHz) when the absorber thickness was 2.5 mm. In addition, an effective absorption bandwidth of 6.0 GHz (ranging from 11.4 GHz to 17.4 GHz) could also be achieved when the absorber thickness was only 2.0 mm. The samples exhibited low densities, strong microwave absorption performances and wide effective absorption bandwidths with thin absorber thicknesses, due to which they have a great potential as lightweight and efficient microwave absorbers.

Broadband microwave absorber constructed by reduced graphene oxide/La0.7Sr0.3MnO3 composites.

High-performance microwave absorbing materials require optimized impedance matching and high attenuation ability. Here we meet the challenge by incorporating electric loss with magnetic loss materials to prepare carbon-based/magnetic hybrids. The reduced graphene oxide (rGO)/La0.7Sr0.3MnO3 (LSMO) composites were prepared by dispersing the LSMO powders into 4.25, 6.25, 8.16, and 10 wt% of the graphene oxide aqueous solution, then the rGO/LSMO composites were formed by hydrothermal method. The pure rGO, LSMO, and rGO/LSMO composites were studied using X-ray diffraction and SEM. Microwave absorption properties were investigated by using coin method. Simulation studies show that 6.25 wt% of rGO/LSMO in a wax matrix exhibits the strongest reflection loss of -47.9 dB @ 10.7 GHz at a thickness of 2.5 mm. Moreover, the effective absorption bandwidth with the reflection loss below -10 dB is up to 14.5 GHz, ranged from 3.5 to 18 GHz for the composites with a thickness of 1.5-5.5 mm, due to a synergism between dielectric loss of rGO and magnetic loss of magnetic LSMO, which is an interesting exploration in the applications of rGO and LSMO. This method can be extended to design and fabricate hybrid absorbers with effective microwave absorption.

Different valence Sn doping - A simple way to detect oxygen concentration variation of ZnO quantum dots synthesized under ultrasonic irradiation.

An ultrasonic method is employed to synthesize the Sn doped Zn0.95Sn0.05O quantum dots with green light emission. Sn2+ and Sn4+ ions are used to create different optical defects inside Zn0.95Sn0.05O quantum dots and the changing trend of oxygen concentration under different ultrasonic irradiation power are investigated. The photoluminescence spectra are employed to characterize the optical defects of Zn0.95Sn0.05O quantum dots. The UV-vis spectra are used to study the band gap of Zn0.95Sn0.05O quantum dots, which is influenced by their sizes. The results indicate that ultrasonic power would influence the size of Zn0.95Sn0.05O quantum dots as well as the type and quantity of defects in ZnO quantum dots. Changing trends in size of Sn2+ and Sn4+ doped Zn0.95Sn0.05O quantum dots are quite similar with each other, while the changing trends in optical defects types and concentration of Sn2+ and Sn4+ doped Zn0.95Sn0.05O quantum dots are different. The difference of the optical defects concentration changing between Sn2+ doped Zn0.95Sn0.05O quantum dots (VO defects) and Sn4+ doped Zn0.95Sn0.05O quantum dots (OZn and Oi defects) shows that the formation process of ZnO under ultrasonic irradiation wiped oxygen out.

Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon.

Lightweight microwave absorbing materials have drawn tremendous attention. Herein, nano-porous biomass carbon materials have been prepared by carbonization with a subsequent potassium hydroxide activation of walnut shells and the microwave absorption properties have also been investigated. The obtained samples have large specific surface areas with numerous micropores and nanopores. The sample activated at 600 °C with a specific surface area of 736.2 m2 g-1 exhibits the most enhanced microwave absorption performance. It has the maximum reflection loss of -42.4 dB at 8.88 GHz and the effective absorption bandwidth (reflection loss below -10 dB) is 1.76 GHz (from 8.08 GHz to 9.84 GHz), corresponding to a thickness of 2 mm. Additionally, the effective absorption bandwidth can reach 2.24 GHz (from 10.48 GHz to 12.72 GHz) when the absorber thickness is 1.5 mm. Three-dimensional porous architecture, interfacial polarization relaxation loss, and the dipolar relaxation loss make a great contribution to the excellent microwave absorption performance. In contrast, the non-activated sample with lower specific surface area (435.3 m2 g-1) has poor microwave absorption performance due to a poor dielectric loss capacity. This comparison highlights the role of micropores and nanopores in improving the dielectric loss property of porous carbon materials. To sum up, porous biomass carbon has great potential to become lightweight microwave absorbers. Moreover, KOH is an efficient activation agent in the fabrication of carbonaceous materials.

High quantum yield ZnO quantum dots synthesizing via an ultrasonication microreactor method.

Green emission ZnO quantum dots were synthesized by an ultrasonic microreactor. Ultrasonic radiation brought bubbles through ultrasonic cavitation. These bubbles built microreactor inside the microreactor. The photoluminescence properties of ZnO quantum dots synthesized with different flow rate, ultrasonic power and temperature were discussed. Flow rate, ultrasonic power and temperature would influence the type and quantity of defects in ZnO quantum dots. The sizes of ZnO quantum dots would be controlled by those conditions as well. Flow rate affected the reaction time. With the increasing of flow rate, the sizes of ZnO quantum dots decreased and the quantum yields first increased then decreased. Ultrasonic power changed the ultrasonic cavitation intensity, which affected the reaction energy and the separation of the solution. With the increasing of ultrasonic power, sizes of ZnO quantum dots first decreased then increased, while the quantum yields kept increasing. The effect of ultrasonic temperature on the photoluminescence properties of ZnO quantum dots was influenced by the flow rate. Different flow rate related to opposite changing trend. Moreover, the quantum yields of ZnO QDs synthesized by ultrasonic microreactor could reach 64.7%, which is higher than those synthesized only under ultrasonic radiation or only by microreactor.

Fast synthesize ZnO quantum dots via ultrasonic method.

Green emission ZnO quantum dots were synthesized by an ultrasonic sol-gel method. The ZnO quantum dots were synthesized in various ultrasonic temperature and time. Photoluminescence properties of these ZnO quantum dots were measured. Time-resolved photoluminescence decay spectra were also taken to discover the change of defects amount during the reaction. Both ultrasonic temperature and time could affect the type and amount of defects in ZnO quantum dots. Total defects of ZnO quantum dots decreased with the increasing of ultrasonic temperature and time. The dangling bonds defects disappeared faster than the optical defects. Types of optical defects first changed from oxygen interstitial defects to oxygen vacancy and zinc interstitial defects. Then transformed back to oxygen interstitial defects again. The sizes of ZnO quantum dots would be controlled by both ultrasonic temperature and time as well. That is, with the increasing of ultrasonic temperature and time, the sizes of ZnO quantum dots first decreased then increased. Moreover, concentrated raw materials solution brought larger sizes and more optical defects of ZnO quantum dots.

[Luminescent properties of Eu3+ doped layered perovskite structure M2TiO4 (M = Ca, Sr, Ba) red-emitting phosphors].

Series of Eu3+ doped layered perovskite structure M2TiO4: Eu3+ (M = Ca, Sr, Ba) red phosphors were prepared by the high-temperature solid state reaction method. Their phase compositions and photoluminescence properties were investigated by XRD, UV-Vis DRS and fluorescence spectra The results indicated that pure Sr2 TiO4 and Ba2 TiO4 powers could be prepared under 1 100 degrees C for 2 hours, but Ca2 TiO4 powers could not be synthesized even raising the calcination temperature and lengthening the calcination time. Ba2TiO4: Eu3+ phosphor emitted 594 nm (5D0 --> 7F1) and 615 nm (5D0 --> 7F2) orange-red light under the excitation of 395 nm. Sr2TiO4 : Eu3+ phosphor gave a unusual and strong orange-red emission of 578 nm (5D0 --> 7F0) and 626 nm (5 D0 --> 7F2) under the excitation of near ultraviolet or blue light, resulting in the better color purity and higher luminescent intensity. In addition, this phosphor had the highest luminous efficiency when excited by the charge migration excitation at 363 nm and it had the great potential to be a red phosphor for N-UV LED and blue light chip.

[Up-conversion luminescent materials of Y2O3: RE(RE=Er or Er/Yb) prepared by sol-gel combustion synthesis].

Y2O3 powders doped with rare-earth ions were synthesized by sol-gel combustion synthesis. Effects of different calcinating temperatures, Er+ doping concentration and Yb3+ doping concentration were investigated. It was shown that the single well crystallized Y2O3 powders could be obtained at 800 degrees C; as the calcinating temperature increased, the crystallinity and upconversion luminescence intensity were higher; the particle size was uniform around 1 microm at 900 degrees C; when Er3+ doping concentration was 1 mol%, the green upconversion luminescence intensity reached the maximum, but for red upconversion luminescence, when Er3+ doping concentration was 4 mol%, its luminescence intensity reached the maximum; as the ratio of Yb3+ to Er3+ was 4:1, the green emission intensity reached the maximum, while the red emission intensity was always increasing as Yb3+ doping concentration increased.