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Due to their special appearance, spherical α‐Al2O3 nanoparticles play an important role for obtaining high‐performance structural and functional ceramics. However, there are still problems such as easily agglomerates to form worm‐like structures at high temperatures and difficult availability of spherical nanoparticles. In this study, spherical α‐Al2O3 nanoparticles with high dispersion were prepared by a combination of a microwave hydrothermal method and an addition of nano‐Al particles as seeds. First, spherical amorphous alumina precursors were synthesized by the microwave hydrothermal method at 100°C for 30 min using Al2(SO4)3·18H2O, Al(NO3)3·9H2O, and urea, as raw materials, and then spherical α‐Al2O3 nanoparticles with a diameter of about 66 nm were acquired after calcined the precursor at 1050°C for 90 min by adding nano‐Al seeds, which reduced the calcination temperature by 50°C and holding time by 30 min compared to that without seeds. Kinetic analysis shows that 5 wt.% nano‐Al seeds can reduce the activation energy of crystalline transition of γ‐Al2O3 to α‐Al2O3 from 516.51 to 474.37 kJ/mol. Moreover, the microscopic mechanism of nano‐Al particles as seeds was investigated. The characterizations of sintering properties show that spherical α‐Al2O3 nanoparticles facilitate the acquisition of uniform microstructure for resulting ceramic and the fracture modes include both intergranular and transgranular fractures. [ABSTRACT FROM AUTHOR]

In this work, the hierarchical NiCo2O4 self-assembled by lamellar flakes were successfully synthesized by a simple solvothermal route and subsequent thermal treatment. The composition and microstructure of the hierarchical NiCo2O4 were characterized by XRD and SEM, respectively. The microwave absorption properties were also evaluated. An optimum reflection loss (RL) of − 45.5 dB at 4.6 GHz with a composite thickness of 4.2 mm and effective bandwidth (RL < − 10 dB) of 7.7 GHz were obtained. The excellent electromagnetic wave absorption property of the hierarchical NiCo2O4 is due to the conduction loss and dielectric loss of NiCo2O4, the hierarchical structure could also enhance the microwave absorbing properties by the strong multi-reflections and scattering of microwave between stacking layers. [ABSTRACT FROM AUTHOR]

Ti 3 C 2 T X /Ni-spheres hybrids were successfully prepared through in situ hydrothermal assembly of Ni spheres on Ti 3 C 2 T x layer, and then the electromagnetic wave absorbing materials were investigated in the range of 2–18 GHz. The as-synthesized Ti 3 C 2 T X /Ni-spheres hybrids contain uniformly distributed Ni spheres on the edge of Ti 3 C 2 T x layer and some of them enter the interspace between individual lamella. The Ti 3 C 2 T X /Ni-spheres hybrids show an optimal reflection loss of −47.06 dB with the thickness of 1.5 mm at 12.4 GHz. The effective absorption bandwidth (EAB) reaches 3.6 GHz. The excellent electromagnetic wave absorbing properties can be explained in two ways. First is the conduction loss and dielectric loss of Ti 3 C 2 T x and the second is the magnetic loss of Ni, in addition, the laminated structure could also enhance the microwave absorbing properties by the strong multi-reflections and scattering between MXene layers. The work of this article not only provides a new method for preparing novel electromagnetic wave absorbers, but also fine balance among small thickness, broad range band, and lightweight of the Ti 3 C 2 T X /Ni-spheres hybrids, which makes Ti 3 C 2 T X /Ni-spheres hybrids a promising materials for electromagnetic wave absorption application. [ABSTRACT FROM AUTHOR]

In the present work, polymer-derived SiCN ceramic aerogels (PDCA-SiCN) were fabricated via a combined sol-gel/freeze drying/polymer precursor conversion method. The microstructure of PDCA-SiCN was studied by regulating the synthesis temperature and time during the sol-gel process. PDCA-SiCN showed a unique three-dimensional network structure, and the specific surface area and pore size of PDCA-SiCN prepared at 150 °C for 20 h were 134 g/m2 and 18 nm, respectively. To assess the electromagnetic wave absorption (EMA) properties of PDCA-SiCN, the materials were uniformly blended with common paraffin, and the influence of PDCA-SiCN/paraffin ratio on the EMA properties was also investigated. The sample with a PDCA-SiCN/paraffin ratio of 20:80 exhibited the best EMA performance, with a minimum reflection loss (RL) of −43.37 dB at 7.6 GHz and electromagnetic absorption bandwidth of 3.8 GHz, which correspond to an absorption of 99.99 % of the electromagnetic waves. The excellent EMA properties of PDCA-SiCN could be attributed to a synergistic effects of good impedance matching, multiple reflections and high dielectric loss. [ABSTRACT FROM AUTHOR]

The MnO 2 /paraffin composites (50 wt% loading) present extraordinary microwave absorption performance, and the minimum reflection loss (RL) values is −51.3 dB at 4.9 GHz with the thickness of 3.5 mm. The excellent electromagnetic absorption properties can be ascribed to the unique hollow structure, perfect impedance matching behavior and the multiple interface polarization effect. • Hollow γ-MnO 2 sphere was obtained by annealing the precursor at 400 °C for 2 h. • The minimum RL values of hollow γ-MnO 2 spheres is −51.3 dB at 4.9 GHz (3.5 mm). • The hollow structure promotes the EM waves multiple reflections and scattering. • Abundant interface and defects in γ-MnO 2 spheres increase the Maxwell–Wagner effect. Hollow γ-MnO 2 sphere was obtained by annealing the precursor at 400 °C with different holding time. The influences of different holding time on the morphology and crystalline structure of final products have been discussed in detail, and the microwave absorption properties of the as-products were also investigated. The results exhibited that finer crystalline feature of the γ-MnO 2 and larger pore size in the hollow γ-MnO 2 sphere could be obtained with the extended holding time. The MnO 2 /paraffin composites (50 wt% loading) present extraordinary microwave absorption performance, and the minimum reflection loss (RL) values is −51.3 dB at 4.9 GHz with the thickness of 3.5 mm. The excellent electromagnetic absorption properties can be ascribed to the hollow structure, perfect impedance matching behavior and the multiple interface polarization effect. [ABSTRACT FROM AUTHOR]

Single transition metal oxide photocatalysts face the problems about low utilization of sunlight and easy recombination of photogenerated carries. In this paper, TiO 2 @Ti 3 C 2 /Cd x Zn 1−x S (MT-xC) composite photocatalysts were prepared in one step with the aid of microwave hydrothermal method by synergistic in situ compounding and construction of semiconductor heterojunction. The results showed that when the microwave hydrothermal temperature was 140 °C, the reaction time was 0.5 h and the CdZnS loading was 30 wt%, MT-3 C photocatalyst demonstrated good photocatalytic performance, with a degradation efficiency of 93.66% for 20 mg/L rhodamine B (RhB) within 90 min. The main reason for this significant improvement is that TiO 2 and Cd x Zn 1−x S have matched band structure, the spatial separation of photogenerated carries in photocatalytic reactions can be accelerated by constructing a Z-type heterojunction. Moreover, the efficient and homogeneous heating of microwaves can provide a large amount of energy for the oxidation of Ti 3 C 2 within a short time. The synergistic effect of semiconductor heterojunction composed of Ti 3 C 2 , TiO 2 and Cd x Zn 1−x S and microwave can further improve the catalytic activity of the composite photocatalyst. • TiO 2 @Ti 3 C 2 /Cd x Zn 1−x S were rapidly synthesized by one step with microwave hydrothermal method. • This method reduces the reaction temperature while shortening the preparation time. • In situ oxidation of Ti 3 C 2 and introduction of Cd x Zn 1−x S has obtained an effective photocatalyst. [ABSTRACT FROM AUTHOR]

[Display omitted] • Spherical Al 2 O 3 precursors were rapidly synthesized at low temperature by M-H. • Effects of process factors in M-H on the properties of particles were studied. • The mechanism of M-H synthesis of spherical Al 2 O 3 was investigated. Spherical α-Al 2 O 3 particles with the size of 1–2 μm were obtained by microwave hydrothermal(M-H) method and subsequent microwave calcination. In the synthesis, Al 2 (SO 4) 3 ·18H 2 O was used as the aluminum sources, and CH 4 N 2 O was used as the precipitant. The effects of M-H temperature, heating preservation time and pressure in the reactor on the morphology, phase composition, particle size and surface area of the particles were studied. The results show that amorphous precursors with high sphericity and good dispersion can be rapidly (30 min) produced by M-H method at low temperature (100 °C). Increasing the temperature or extending the holding time will destroy the spherical morphology of the precursor, while transforming its phase from amorphous to γ-AlOOH. Pressure in the reactor does not change the phase composition of the precursor, but affects its dispersion and homogeneity. The precursor retains its spherical morphology after microwave calcination at 1100 °C for 90 min. The resulting powders were hot pressed and sintered at 1650 °C for 30 min to obtain a high-performance ceramic plate with a thermal conductivity of 27.1 W/(m·K) and a Vickers hardness of 21.4 GPa. [ABSTRACT FROM AUTHOR]

Ultra-wide absorption bandwidth is one of the goals of ideal absorbing materials. In this article, three-dimensional porous microstructure Co/SiCN ceramic aerogel composites (CACs) were synthesized through a reverse microemulsion method, which is aimed to avoid the aggregation of cobalt nanoparticles, and makes better impedance matching between CACs surface and air to obtain an outstanding microwave absorption (MA) performance. The complex permittivity of the CACs could be tuned via the concentration of cobalt acetate solution. The MA results indicate that CACs with cobalt acetate solution concentration of 0.5 mol/L shows an excellent MA properties with ultrabroad bandwidth 8.5 GHz covering the whole K band (18–26.5 GHz). The excellent MA performance can be attributed to good impedance matching, multiple reflections, polarization processes, and the synergistic effects in the CACs. Therefore, our study paves a novel way to fabricate the promising lightweight MA materials with ultrabroad absorption bandwidth at higher frequency. Co/SiCN ceramic aerogel composites shows the ultrabroad effective absorption bandwidth of 8.5 GHz covering the whole K band (18–26.5 GHz), which was attributed to good impedance matching, multiple reflections and polarization processes. Image 1 • Co/SiCN CACs were fabricated by combination of reverse microemulsion and calcination. • Co/SiCN CACs were used as high-efficiency microwave absorber. • Co/SiCN CACs possesses ultrabroad effective absorption bandwidth of 8.5 GHz. • Good impedance matching, multiple reflections and polarization processes contributed to absorption. [ABSTRACT FROM AUTHOR]