In this work, ultralight polymer-derived ceramic aerogels (PDCA) were prepared by a facile method of hydrosilylation crosslink and freeze drying. The electromagnetic absorption properties of ultralight PDCA were investigated for the first time. The PDCA pyrolyzed at 1000 °C shows a uniform three-dimensional (3D) framework with a low bulk density of ∼0.19 g/cm3 and high surface area of 134.48 m2/g. The electromagnetic properties of PDCA were characterized by a vector network analyzer. The minimum reflection and absorption bandwidth of PDCA pyrolyzed at 1000 °C, 1200 °C and 1400 °C are −43.37 dB @ 7.6 GHz, −42.01 dB@12.5 GHz, and −31.69 dB@17.3 GHz and 3.8 GHz, 6.6 GHz and 4.2 GHz, respectively, at the frequency range of 2–18 GHz. The strong electromagnetic absorption and wide bandwidth features of PDCA could be attributed to the multiple reflections of microwaves in the 3D framework, as well as the high dielectric loss and proper conductivity.Ultralight polymer-derived ceramic aerogels with wide bandwidth and strong electromagnetic absorption ability.Download high-res image (200KB)Download full-size image

A novel urchin-like ZnS/Ni3S2@Ni composite with a core–shell structure was successfully synthesized by a facile two-stage method. The structure and morphology were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrometry. The influence of reaction temperature on the structure and morphology of the ZnS/Ni3S2@Ni products was investigated by the aid of XRD and SEM techniques. A plausible formation mechanism for the core–shell urchin-like architectures was proposed based on temperature dependent experiments. Electromagnetic absorption measurements show that the urchin-like ZnS/Ni3S2@Ni composite possesses outstanding electromagnetic absorption properties compared with other ZnS/Ni3S2@Ni composites. The optimal reflection loss of −27.6 dB can be observed at 5.2 GHz and the effective absorption (below −10 dB, 90% electromagnetic absorption) bandwidth is 2.5 GHz (12.2–14.7 GHz) with a thickness of only 1.0 mm. The location of the minimum reflection loss can be tuned between 4.6 GHz and 18.0 GHz for the absorber by tuning the thickness between 0.8–2.5 mm. The enhanced electromagnetic absorption properties were attributed to a synergistic effect between dielectric loss and magnetic loss, multiple interfacial polarization resulting from the heterogeneous structure of the core–shell ternary ZnS/Ni3S2@Ni composite, good impedance matching and a unique urchin-like structure.

In this work, the magnetic–dielectric core-shell heterostructure composites with the core of Ni submicron spheres and the shell of SnO2 nanorods were prepared by a facile two-step route. The crystal structure and morphology were investigated by X-ray diffraction analysis, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). FESEM and TEM measurements present that SnO2 nanorods were perpendicularly grown on the surfaces of Ni spheres and the density of the SnO2 nanorods could be tuned by simply varying the addition amount of Sn2+ in this process. The morphology of Ni/SnO2 composites were also determined by the concentration of hydrochloric acid and a plausible formation mechanism of SnO2 nanorods-coated Ni spheres was proposed based on hydrochloric acid concentration dependent experiments. Ni/SnO2 composites exhibit better thermal stability than pristine Ni spheres based on thermalgravimetric analysis (TGA). The measurement on the electromagnetic (EM) parameters indicates that SnO2 nanorods can improve the impedance matching condition, which is beneficial for the improvement of electromagnetic wave absorption. When the coverage density of SnO2 nanorod is in an optimum state (diameter of 10 nm and length of about 40–50 nm), the optimal reflection loss (RL) of electromagnetic wave is −45.0 dB at 13.9 GHz and the effective bandwidth (RL below −10 dB) could reach to 3.8 GHz (12.3–16.1 GHz) with the absorber thickness of only 1.8 mm. By changing the loading density of SnO2 nanorods, the best microwave absorption state could be tuned at 1–18 GHz band. These results pave an efficient way for designing new types of high-performance electromagnetic wave absorbing materials.Keywords: core-shell; dielectric loss; electromagnetic wave absorption; interfacial polarization; Ni/SnO2; SnO2 nanorods

Ordered honeycomb-like SnO2 foams were successfully synthesized by means of a template method. The honeycomb SnO2 foams were analyzed by X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), laser Raman spectra, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR). It can be found that the SnO2 foam configurations were determined by the size of polystyrene templates. The electromagnetic properties of ordered SnO2 foams were also investigated by a network analyzer. The results reveal that the microwave absorption properties of SnO2 foams were dependent on their configuration. The microwave absorption capabilities of SnO2 foams were increased by increasing the size of pores in the foam configuration. Furthermore, the electromagnetic wave absorption was also correlated with the pore contents in SnO2 foams. The large and high amounts pores can bring about more interfacial polarization and corresponding relaxation. Thus, the perfect ordered honeycomb-like SnO2 foams obtained in the existence of large amounts of 322 nm polystyrene spheres showed the outstanding electromagnetic wave absorption properties. The minimal reflection loss (RL) is −37.6 dB at 17.1 GHz, and RL less than −10 dB reaches 5.6 GHz (12.4–18.0 GHz) with thin thickness of 2.0 mm. The bandwidth (<−10 dB, 90% microwave dissipation) can be monitored in the frequency regime of 4.0–18.0 GHz with absorber thickness of 2.0–5.0 mm. The results indicate that these ordered honeycomb SnO2 foams show the superiorities of wide-band, high-efficiency absorption, multiple reflection and scatting, high antioxidation, lightweight, and thin thickness.Keywords: electromagnetic properties; honeycomb-like structure; interfacial polarization; multiple reflection; SnO2 foams

In this study, novel porous hollow Ni/SnO2 hybrids were prepared by a facile and flexible two-step approach composed of solution reduction and subsequent reaction-induced acid corrosion. In our protocol, it can be found that the hydrothermal temperature exerts a vital influence on the phase crystal and morphology of Ni/SnO2 hybrids. Notably, the Ni microspheres might be completely corroded in the hydrothermal process at 220 °C. The complex permittivity and permeability of Ni/SnO2 hybrids–paraffin wax composite were measured based on a vector network analyzer in the frequency range of 1–18 GHz. Electromagnetic absorption properties of samples were evaluated by transmission line theory. Ni/SnO2 hybrid composites exhibit superior electromagnetic absorption properties in comparison with pristine Ni microspheres. The outstanding electromagnetic absorption performances can be observed for the hollow porous Ni/SnO2 hybrid prepared at 200 °C. The minimum reflection loss is −36.7 dB at 12.3 GHz, and the effective electromagnetic wave absorption band (RL < −10 dB, 90% microwave attenuation) was in the frequency range of 10.6–14.0 GHz with a thin thickness of 1.7 mm. Excellent electromagnetic absorption properties were assigned to the improved impedance match, more interfacial polarization and unique hollow porous structures, which can result in microwave multi-reflection and scattering. This novel hollow porous hybrid is an attractive candidate for new types of high performance electromagnetic wave-absorbing materials, which satisfies the current requirements of electromagnetic absorbing materials, which include wide-band absorption, high-efficiency absorption capability, thin thickness and light weight.

In this work, amorphous TiO2 and SiO2-coated Ni composite microspheres were successfully prepared by a two-step method. The phase purity, morphology, and structure of composite microspheres are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Due to the presence of the insulator SiO2 shell, the core–shell Ni–SiO2 composite microspheres exhibit better antioxidation capability than that of pure Ni microspheres. The core–shell Ni–SiO2 composite microspheres show the best microwave absorption properties than those of pure Ni microspheres and Ni–TiO2 composites. For Ni–SiO2 composite microspheres, an optimal reflection loss (RL) as low as −40.0 dB (99.99% absorption) was observed at 12.6 GHz with an absorber thickness of only 1.5 mm. The effective absorption (below −10 dB, 90% microwave absorption) bandwidth can be adjusted between 3.1 GHz and 14.4 GHz by tuning the absorber thickness in the range of 1.5–4.5 mm. The excellent microwave absorption abilities of Ni–SiO2 composite microspheres are attributed to a higher attenuation constant, Debye relaxation, interface polarization of the core–shell structure and synergistic effects between high dielectric loss and high magnetic loss.

Core–shell microspheres with Ni cores and two phases of TiO2 (anatase, rutile) shells have been successfully synthesized. The crystal structure, morphology and microwave absorption properties of the as-prepared composites were analyzed by X-ray diffraction, field-emission scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and vector network analysis. The core–shell rutile TiO2-coated Ni exhibits better antioxidation ability than that of pure Ni due to the presence of the rutile TiO2 shell, which is confirmed by the thermal gravimetric analysis (TGA). In comparison with bare Ni, these two composites show better microwave absorption properties. The minimum reflection loss (RL) is −38.0 dB at 11.1 GHz with a thickness of only 1.8 mm for the Ni@TiO2 (rutile) composite. The enhanced absorption capability arises from the efficient complementarities between the magnetic loss and dielectric loss, multiple interfacial polarization, high thermal conductivity of rutile TiO2 and microwave attenuation constant. These results show that the thin high-efficiency rutile TiO2-coated Ni composite is a great potential microwave absorbing material for practical applications.

A novel leaf-like NiCu alloy composite was prepared by a simple solution reduction method. The dendritic hierarchical structures were composed of a main stem with several micrometers and plentiful branches. The NiCu paraffin-composite containing 40 wt% NiCu dendrite exhibits excellent microwave absorption properties. The minimal reflection loss (RL) of −21.1 dB can be observed at 14.9 GHz and the bandwidth with RL less than −10 dB reaches 3.4 GHz (13.4–16.8 GHz) with only thickness of 1.4 mm. This novel dendrite-like NiCu alloy could be used as a promising absorbing material with low thickness, high absorption and wide-band as well as easy preparation.

Controllable magnetic–dielectric hybrids with cores of walnut-like Ni and shells of ultra-thin and crumpled ZnS nets have been successfully synthesized by a facile two-step approach. The morphology, microstructure and microwave absorption properties of the as-synthesized core–shell Ni/ZnS composites were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and network analysis. The shapes and microwave absorption properties of Ni/ZnS can be tuned by the hydrothermal temperatures. The core–shell Ni/ZnS composites present significantly enhanced microwave absorption compared with pristine Ni walnuts. When the reaction temperature was 60 °C, the reflection loss (RL) could be as low as −42.4 dB at 12.3 GHz. Moreover, the effective bandwidth (RL < −10 dB) can be recorded in the 11.3–15.6 GHz range with the absorber thickness of only 2.2 mm. The excellent microwave absorption properties were attributed to impedance match, the synergetic effect between the dielectric loss and magnetic loss, interfacial relaxation and conduction loss of unique cross-linked ZnS shells. These results suggest that the as-synthesized crumpled ZnS net-wrapped Ni composites may be an attractive candidate for microwave absorption application.

•The core–shell Ni/Sn6O4(OH)4 composites were prepared by a two-step method.•The microwave absorption properties of Ni/Sn6O4(OH)4 were enhanced compared with raw Ni and Sn6O4(OH)4.•The minimum reflection loss of Ni/Sn6O4(OH)4 was − 32.4 dB at 13.2 GHz.•The excellent microwave absorption properties were attributed to core–shell structure and impedance matching.In this work, core-shell structured microspheres with Ni cores and Sn6O4(OH)4 nanoshells have been successfully synthesized by a solvothermal deposition method. The crystal structure, microstructures and electromagnetic (EM) properties of the samples were investigated by X-ray diffraction, scanning electron microscopy, energy disperse X-ray spectrometry, Fourier transform infrared (FT-IR), transmission electron microscope (TEM) and a network analyser. The Ni/Sn6O4(OH)4 composite microspheres show better microwave absorption properties than those of pure Ni microspheres. An obvious multiple reflection loss phenomenon was observed with the increase of matching thickness. A minimal reflection loss (RL) as low as − 32.4 dB was observed at 13.2 GHz with the absorber thickness of 5.0 mm. The enhanced microwave absorption performances of Ni/Sn6O4(OH)4 composite microspheres were due to good electromagnetic impedance match, geometrical factors, and unique core-shell structure.The electromagnetic wave absorption properties of the Ni microspheres can be significantly enhanced after coating with Sn6O4(OH)4.

Hierarchical SnO2@rGO nanostructures with superhigh surface areas are synthesized via a simple redox reaction between Sn2+ ions and graphene oxide (GO) nanosheets under microwave irradiation. XRD, SEM, TEM, XPS, TG-DTA and N2 adsorption–desorption are used to characterize the compositions and microstructures of the SnO2@rGO samples obtained. The SnO2@rGO nanostructures are used as gas-sensing and electroactive materials to evaluate their property–microstructure relationship. The results show that SnO2 nanoparticles (NPs) with particle sizes of 3–5 nm are uniformly anchored on the surfaces of reduced graphene oxide (rGO) nanosheets through a heteronucleation and growth process. The as-obtained SnO2@rGO sample with a hierarchically sesame cake-like microstructure and a superhigh specific surface area of 2110.9 m2 g−1 consists of 92 mass% SnO2 NPs and ∼8 mass% rGO nanosheets. The sensitivity of the SnO2@rGO sensor upon exposure to 10 ppm H2S is up to 78 at the optimal operating temperature of 100 °C, and its response time is as short as 7 s. Compared with SnO2 nanocrystals (5–10 nm), the hierarchical SnO2@rGO nanostructures have enhanced gas-sensing behaviors (i.e., high sensitivity, rapid response and good selectivity). The SnO2@rGO nanostructures also show excellent electroactivity in detecting sunset yellow (SY) in 0.1 M phosphate buffer solution (pH = 2.0). The enhancement in gas-sensing and electroactive performance is mainly attributed to the unique hierarchical microstructure, high surface areas and the synergistic effect of SnO2 NPs and rGO nanosheets.

Hierarchical In2O3@WO3 nanocomposites, consisting of discrete In2O3 nanoparticles (NPs) on single-crystal WO3 nanoplates, were synthesized via a novel microwave-assisted growth of In2O3 NPs on the surfaces of WO3 nanoplates that were derived through an intercalation and topochemical-conversion route. The techniques of XRD, SEM, TEM and XPS were used to characterize the samples obtained. The gas-sensing properties of In2O3@WO3 nanocomposites, together with WO3 nanoplates and In2O3 nanoparticles, were comparatively investigated using inorganic gases and organic vapors as the target substances, with an emphasis on H2S-sensing performance under low concentrations (0.5–10 ppm) at 100–250 °C. The results show that the In2O3 NPs with a size range of 12–20 nm are uniformly anchored on the surfaces of the WO3 nanoplates. The amounts of the In2O3 NPs can be controlled by changing the In3+ concentrations in their growth precursors. The In2O3@WO3 (In/W = 0.8) sample has highest H2S-sensing performance operating at 150 °C; its response to 10 ppm H2S is as high as 143, 4 times higher than that of WO3 nanoplates and 13 times that of In2O3 nanocrystals. However, the responses of the In2O3@WO3 sensors are less than 13 upon exposure to 100 ppm of CO, SO2, H2, CH4 and organic vapors, operating at 100–150 °C. The improvement in response and selectivity of the In2O3@WO3 sensors upon exposure to H2S molecules can be attributed to the synergistic effect of In2O3 NPs and WO3 nanoplates, hierarchical microstructures and multifunctional interfaces.

•Au/SnO2@WO3 composites were formed by reducing HAuCl4 with SnCl2 on WO3 nanoplates.•Au/SnO2@plate-WO3 is high sensitive to H2S detection at low temperature of ∼50 °C.•Au/SnO2@plate-WO3 is highly selective to H2S detection in various gases or vapors.•Synergistic effect of Au/SnO2 and WO3 results in enhancement in H2S-sensing property.In order to improve the gas-sensing performance at low temperature, binary Au/SnO2 species were used to modify WO3 nanoplates, i.e., Au/SnO2@plate-WO3 composites, which were synthesized by in-situ reducing HAuCl4 with SnCl2 adsorbed on the surfaces of WO3 nanoplates derived via an intercalation and topochemical conversion route. XRD, XPS, SEM, TEM and UV–vis DR spectra were used to characterize the samples. The gas-sensing properties of the samples were evaluated using H2S as target gas. The Au/SnO2 nanoparticles with small sizes (several nanometers) are uniformly anchored on the surfaces of WO3 nanoplates. The response of the 0.5%Au/SnO2@plate-WO3 sensor to 10 ppm H2S is up to 220 at 50 °C, 28 times higher than that of the plate-WO3 sensor. The optimal operation temperature of the plate-WO3 and Au/SnO2@plate-WO3 sensor for H2S detection is about 150 °C. The responses of the Au/SnO2@plate-WO3 sensor to 100 ppm of CO, SO2, H2, CH4 and organic vapors are negligibly low (1.2–8.0) at low temperatures. The possible explanation for the high selectivity and response in H2S detection at low temperatures can be the synergistic effect of the binary Au/SnO2 nanoparticles and ultra-thin WO3 nanoplates in adsorption, reaction and diffusion of the gas molecules.

•Au@plate-WO3 nanocomposites were synthesized by a chemical process.•The Au@plate-WO3 sensors were highly selective to NO gases with low concentrations.•The Au@plate-WO3 sensors had the highest sensitivity operating at about 170 °C.•The optimum amount of Au nanoparticles was about 1 wt.%.•Au nanoparticles and the loose aggregates enhanced the NO-sensing performance.Au-modified WO3 nanoplates (Au@plate-WO3) were synthesized by chemically reducing HAuCl4 on the surfaces of two-dimensional WO3 nanoplates, which were derived from an intercalation–topochemical process. XRD, SEM, TEM, XPS and UV–vis DR spectra were used to characterize the WO3 nanoplates and Au@plate-WO3 nanocomposites. The gas-sensing properties of the WO3 nanoplates and Au@plate-WO3 nanocomposites were comparatively investigated using inorganic gases and organic vapors as the target gases, with an emphasis on exploring the response and selectivity of NO gases with low concentrations (0.5–10 ppm) at low operating temperature (130−250 °C). The results indicated that Au nanoparticles (Au NPs) enhance the low-temperature sensitivity and selectivity of the Au@plate-WO3 sensors for NO detection when compared with the performance of the WO3 sensors. The Au@plate-WO3 nanocomposite with 1 wt.% Au NPs has the best NO-sensing performance at the optimum operating temperature of ∼170 °C. In addition, the Au@plate-WO3 sensors show highly selective to NO gas among various inorganic gases (i.e., H2, SO2 and CO) and organic vapors (i.e., alcohol, acetone, methanal and benzene). The enhancement in sensitivity and selectivity for NO detection is probably due to the synergistic effect of Au NPs and the house-of-card structure of WO3 nanoplates.

SiC particulate-reinforced Al composites were prepared by powder metallurgy (PM) method and conventional atmospheric sintering. Scanning electron microscope (SEM), X-ray diffraction (XRD) techniques were used to characterize the sintered composites. The effect of temperature on the density, hardness, strength, and microstructure of composites was investigated. Detailed failure behavior was analyzed. It was found that the segregation of SiC appeared at higher temperature. The highest micro-hardness of 80 MPa occurred at 700 °C. The strength tended to increase with the increasing temperature due to the formation of Al2Cu. Both ductile and brittle fracture features were observed.

•The novel porous lamellar ZnO was prepared by a facile two-step method.•The novel porous lamellar ZnO exhibits enhanced microwave absorption properties.•The minimum reflection loss of porous lamellar ZnO is −34.5 dB at 10.7 GHz.•The good properties were mainly attributed to multi-reflection of porous structure.The novel porous ZnO nanoflakes were fabricated by a facile two-step method containing preparation of precursor ZnCO3 and subsequently calcination of ZnCO3. The as-prepared products were analyzed by X-ray diffraction, scanning electron microscopy, and thermalgravimetric analysis. The results reveal that the porous ZnO nanoflakes were in the diameter and thickness of several to tens micrometers and 100–500 nm, respectively. The microwave absorption properties of porous ZnO nanoflakes were investigated by the network analyzer, which exhibit the minimal reflection loss of −34.5 dB at 10.7 GHz with only thickness of 1.5 mm. The effective absorption (below −10 dB) bandwidth can be tuned between 7.0 GHz and 17.1 GHz by tuning absorber thickness of 1.0–2.2 mm. Thus, the porous lamellar ZnO could be used as a promising absorbing material with the features of high efficiency absorption, wide-band and light weight.The porous lamellar ZnO nanostructures present the enhanced microwave absorption properties.

In this study, novel porous hollow Ni/SnO2 hybrids were prepared by a facile and flexible two-step approach composed of solution reduction and subsequent reaction-induced acid corrosion. In our protocol, it can be found that the hydrothermal temperature exerts a vital influence on the phase crystal and morphology of Ni/SnO2 hybrids. Notably, the Ni microspheres might be completely corroded in the hydrothermal process at 220 °C. The complex permittivity and permeability of Ni/SnO2 hybrids–paraffin wax composite were measured based on a vector network analyzer in the frequency range of 1–18 GHz. Electromagnetic absorption properties of samples were evaluated by transmission line theory. Ni/SnO2 hybrid composites exhibit superior electromagnetic absorption properties in comparison with pristine Ni microspheres. The outstanding electromagnetic absorption performances can be observed for the hollow porous Ni/SnO2 hybrid prepared at 200 °C. The minimum reflection loss is −36.7 dB at 12.3 GHz, and the effective electromagnetic wave absorption band (RL < −10 dB, 90% microwave attenuation) was in the frequency range of 10.6–14.0 GHz with a thin thickness of 1.7 mm. Excellent electromagnetic absorption properties were assigned to the improved impedance match, more interfacial polarization and unique hollow porous structures, which can result in microwave multi-reflection and scattering. This novel hollow porous hybrid is an attractive candidate for new types of high performance electromagnetic wave-absorbing materials, which satisfies the current requirements of electromagnetic absorbing materials, which include wide-band absorption, high-efficiency absorption capability, thin thickness and light weight.

A novel urchin-like ZnS/Ni3S2@Ni composite with a core–shell structure was successfully synthesized by a facile two-stage method. The structure and morphology were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrometry. The influence of reaction temperature on the structure and morphology of the ZnS/Ni3S2@Ni products was investigated by the aid of XRD and SEM techniques. A plausible formation mechanism for the core–shell urchin-like architectures was proposed based on temperature dependent experiments. Electromagnetic absorption measurements show that the urchin-like ZnS/Ni3S2@Ni composite possesses outstanding electromagnetic absorption properties compared with other ZnS/Ni3S2@Ni composites. The optimal reflection loss of −27.6 dB can be observed at 5.2 GHz and the effective absorption (below −10 dB, 90% electromagnetic absorption) bandwidth is 2.5 GHz (12.2–14.7 GHz) with a thickness of only 1.0 mm. The location of the minimum reflection loss can be tuned between 4.6 GHz and 18.0 GHz for the absorber by tuning the thickness between 0.8–2.5 mm. The enhanced electromagnetic absorption properties were attributed to a synergistic effect between dielectric loss and magnetic loss, multiple interfacial polarization resulting from the heterogeneous structure of the core–shell ternary ZnS/Ni3S2@Ni composite, good impedance matching and a unique urchin-like structure.

Hierarchical In2O3@WO3 nanocomposites, consisting of discrete In2O3 nanoparticles (NPs) on single-crystal WO3 nanoplates, were synthesized via a novel microwave-assisted growth of In2O3 NPs on the surfaces of WO3 nanoplates that were derived through an intercalation and topochemical-conversion route. The techniques of XRD, SEM, TEM and XPS were used to characterize the samples obtained. The gas-sensing properties of In2O3@WO3 nanocomposites, together with WO3 nanoplates and In2O3 nanoparticles, were comparatively investigated using inorganic gases and organic vapors as the target substances, with an emphasis on H2S-sensing performance under low concentrations (0.5–10 ppm) at 100–250 °C. The results show that the In2O3 NPs with a size range of 12–20 nm are uniformly anchored on the surfaces of the WO3 nanoplates. The amounts of the In2O3 NPs can be controlled by changing the In3+ concentrations in their growth precursors. The In2O3@WO3 (In/W = 0.8) sample has highest H2S-sensing performance operating at 150 °C; its response to 10 ppm H2S is as high as 143, 4 times higher than that of WO3 nanoplates and 13 times that of In2O3 nanocrystals. However, the responses of the In2O3@WO3 sensors are less than 13 upon exposure to 100 ppm of CO, SO2, H2, CH4 and organic vapors, operating at 100–150 °C. The improvement in response and selectivity of the In2O3@WO3 sensors upon exposure to H2S molecules can be attributed to the synergistic effect of In2O3 NPs and WO3 nanoplates, hierarchical microstructures and multifunctional interfaces.

Core–shell microspheres with Ni cores and two phases of TiO2 (anatase, rutile) shells have been successfully synthesized. The crystal structure, morphology and microwave absorption properties of the as-prepared composites were analyzed by X-ray diffraction, field-emission scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and vector network analysis. The core–shell rutile TiO2-coated Ni exhibits better antioxidation ability than that of pure Ni due to the presence of the rutile TiO2 shell, which is confirmed by the thermal gravimetric analysis (TGA). In comparison with bare Ni, these two composites show better microwave absorption properties. The minimum reflection loss (RL) is −38.0 dB at 11.1 GHz with a thickness of only 1.8 mm for the Ni@TiO2 (rutile) composite. The enhanced absorption capability arises from the efficient complementarities between the magnetic loss and dielectric loss, multiple interfacial polarization, high thermal conductivity of rutile TiO2 and microwave attenuation constant. These results show that the thin high-efficiency rutile TiO2-coated Ni composite is a great potential microwave absorbing material for practical applications.

In this work, amorphous TiO2 and SiO2-coated Ni composite microspheres were successfully prepared by a two-step method. The phase purity, morphology, and structure of composite microspheres are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Due to the presence of the insulator SiO2 shell, the core–shell Ni–SiO2 composite microspheres exhibit better antioxidation capability than that of pure Ni microspheres. The core–shell Ni–SiO2 composite microspheres show the best microwave absorption properties than those of pure Ni microspheres and Ni–TiO2 composites. For Ni–SiO2 composite microspheres, an optimal reflection loss (RL) as low as −40.0 dB (99.99% absorption) was observed at 12.6 GHz with an absorber thickness of only 1.5 mm. The effective absorption (below −10 dB, 90% microwave absorption) bandwidth can be adjusted between 3.1 GHz and 14.4 GHz by tuning the absorber thickness in the range of 1.5–4.5 mm. The excellent microwave absorption abilities of Ni–SiO2 composite microspheres are attributed to a higher attenuation constant, Debye relaxation, interface polarization of the core–shell structure and synergistic effects between high dielectric loss and high magnetic loss.