Physical properties of microwave and solid state synthesized La0.7Na0.3MnO3

We report the structural, magnetic, electrical and broadband microwave absorption in La_0.7Na_0.3MnO₃ sample synthesized by microwave (MW) irradiation (Na_0.3LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na_0.3LMO_SS). Single phase Na_0.3LMO_MW was synthesized at 800 °C in 30 min, whereas, Na_0.3LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at T_C ～324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La_0.7Na_0.3MnO₃.     

Catalytic decomposition of TCE under microwave

The decomposition of trichloroethylene (TCE) by hydrogen over Co/γ-Al₂O₃ and Ni/γ-Al₂O₃ catalysts under microwave heating was studied. The comparison between the catalytic activity in the microwave heating mode and that in the conventional thermal mode demonstrated that the microwave heating could greatly reduce the reaction temperature, accelerate the TCE decomposition speed and improve the TCE decomposition ratio. The results suggest that the microwave heating has a novel effect in the decomposition of TCE.     

Construction of core-shell BaFe12O19@MnO2 composite for effectively enhancing microwave absorption performance

BaFe₁₂O₁₉, a traditional ferrite, has always been extensively investigated as a microwave absorption material because of the application value. Herein, the core-shell BaFe₁₂O₁₉@MnO₂ composite was designed and constructed successfully by a facile hydrothermal method. By introducing nanostructured MnO₂, a typical dielectric loss medium, the electromagnetic wave absorption performance is effectively enhanced. The core-shell structure contributes to high interface polarization, thereby promoting the attenuation of electromagnetic waves. In addition, the optimal temperature of the hydrothermal reaction was explored through the characterization of the morphology and the analysis of microwave absorption performance. The result exhibits that the maximum reflection loss of the prepared BaFe₁₂O₁₉@MnO₂-170 reaches -54.39 dB and the effective absorbing bandwidth reaches 4.64 GHz. The simple preparation method and attractive performance make BaFe₁₂O₁₉@MnO₂ a promising candidate as microwave absorbers.     

Synthesis of hierarchical core-shell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance

A ternary functional composite NiFe₂O₄@MnO₂@graphene was synthesized successfully via a facile method. The phase constitution, microstructures, morphologies and chemical compositions of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). It was observed that the NiFe₂O₄ nanoparticles were coated by hierarchically MnO₂ shells and distributed on the surface of graphene. Investigations of EM wave absorption indicated that NiFe₂O₄@MnO₂@ graphene composite has the strongest reflection loss of −47.4 dB at 7.4 GHz at the matching thickness of 3 mm, compared to NiFe₂O₄ and NiFe₂O₄@MnO₂, and its maximum absorption bandwidth (<−10 dB) is 4.3 GHz (from 5.1 to 9.4 GHz). The enhanced microwave absorption performance can be attributed to the hierarchical structure of MnO₂, void space between MnO₂ and graphene, and better impedance matching of ternary composite. The above results indicate that the novel hierarchical NiFe₂O₄@MnO₂@graphene composite, with intense absorption and wide absorption bandwidth, would be a promising absorber with less EM wave interference.     

Chromium-promoted preparation of forsterite refractory materials from ferronickel slag by microwave sintering

The chromium-promoted preparation of forsterite refractory materials from ferronickel slag was investigated by microwave sintering of the slag with the additions of sintered magnesia and 0–10 wt% chromium oxide (Cr₂O₃). The thermodynamic calculations revealed that the addition of Cr₂O₃ can promote the formations of spinel and liquid phase and maintain high content of forsterite below 1500 °C. The experimental results showed that there existed a stronger promoting effect of Cr₂O₃ additive on the properties of refractory materials in the microwave field than that in conventional sintering. It was attributed to the preferential formation and growth of spinel with stronger microwave absorption than other phases (e.g., enstatite), the existence of more forsterite, and the enhanced densification in association with the presence of more liquid phase at the same temperature. By microwave sintering of the mixture of ferronickel slag, 25 wt% sintered magnesia, and 4 wt% Cr₂O₃ at 1350 °C for 20 min, a superior refractory material with refractoriness of 1801 °C, thermal shock resistance of 6 times, bulk density of 2.97 g/cm³, apparent porosity of 1.4%, and compressive strength of 197 MPa was obtained. Compared with that prepared by conventional sintering at 1350 °C for 2 h, the refractoriness and thermal shock resistance were increased by 175 °C and 100%, respectively. The present study provided a novel method for preparing high-quality refractory materials from ferronickel slag and relevant industrial wastes.     

Microwave-accelerated solvent-free aerobic oxidation of benzyl alcohol over efficient and reusable manganese oxides

A new facile and cost-effective process involving the solvent-free oxidation of benzyl alcohol using molecular oxygen as oxidant under controlled microwave irradiation has been developed for the production of chlorine-free benzaldehyde. Influence of different catalyst parameters (different manganese oxides and other kinds of transition metal oxides) and reaction conditions (reaction period and temperature) on the process performance has been studied. Under optimized reaction conditions, the MnO₂ catalyst showed a superior catalytic performance in the highly selective oxidation of benzyl alcohol as compared to other manganese oxide materials such as MnO, Mn₂O₃ and Mn₃O₄. Moreover, a very stable catalytic activity as a function of cycling test was observed for the MnO₂ catalyst.     

Effects of Different Sintering Methods on the Structural and Electrical Properties of Ca0.9Sr0.1MnO3

Nano-crystalline Ca_0.9Sr_0.1MnO₃ powders were synthesized through citrate sol-gel method. Two different sintering techniques, i.e., conventional sintering and microwave sintering were employed to sinter the prepared samples at 1123 K. The high value of relative density was achieved for both the samples. Phase confirmation, surface morphology, and elemental composition were carried out by powder x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, respectively. DC four-probe measurement results revealed that the microwave sintered sample has higher electrical conductivity values than the conventional sintered sample. The conventional sintered sample exhibited semiconductor nature, whereas the microwave sintered sample indicated semi-metallic properties from room temperature to 673 K.     

Modulation of MoSe2 & MnFe2O4@MnO2 nano-architectures for microwave absorption properties via single- and bilayer method

The electromagnetic pollution problem, particularly at microwave frequencies, poses a threat to not only sensitive technological gadgets but also to the health of humans. Therefore, there is a great need for lightweight and highly effective microwave-absorbing materials (MAMs). Here, we fabricated a hierarchical flower-like MoSe₂ structure and a rod-like MnFe₂O₄@MnO₂ architecture via a solvothermal method. Single-layer and bilayer samples were fabricated to study the microwave absorption feature. In single-layer samples, the flower-like MoSe₂ structure has better microwave absorption properties than the rod-like MnFe₂O₄@MnO₂ architecture. And in bilayer absorbing samples, a sample with a flower-like MoSe₂ structure as the top layer shows high absorption performance. Moreover, in bilayer samples, changes were made to the thickness of both layers to find the best parameters. An optimal bilayer sample has been achieved with a flower-like dielectric MoSe₂ structure as a top layer having a 1 mm thickness and magnetic MnFe₂O₄@MnO₂ as a bottom layer also with a 1 mm thickness; indicating that a strong absorption can only be attained by balancing dielectric loss and magnetic loss. Moreover, the optimal sample shows decent absorption with an effective absorption bandwidth (EAB) of 5.4 GHz (14.7–9.3 GHz) with a 1 mm thickness of each layer. The simulated results of the optimal sample have also been compared with experimental results. These results suggest a different approach for developing MAMs in the future.     

Tailoring La0.8Sr0.2MnO3/La/Sr nanocomposite using a novel complementary method as well as dissecting its microwave,shielding, optical,and magnetic characteristics

In this work, a novel approach was introduced to reduce the oxide nanoparticles and extract the pure metal from them. Accordingly, La_0.8Sr_0.2MnO₃ nanoparticles were prepared through the conventional citrate gel method, and then they were reduced using a solvothermal method by ethylene glycol as a reductive agent. Chemical species, magnetic parameters, crystal structures, and morphological properties of the fabricated structures were deeply studied by Fourier transform infrared (FT-IR), vibrating sample magnetometer (VSM), X-ray powder diffraction (XRD), and field emission scanning electron microscopy (FE-SEM) analyses, respectively. Noticeably, the curves of the diffuse reflection spectroscopy (DRS) suggested a lower energy gap for the La_0.8Sr_0.2MnO₃/La/Sr nanocomposite. Finally, the microwave absorbing characteristics of the specimens were scrupulously investigated using the polystyrene (PS) and polyvinylidene fluoride (PVDF) media. It was found that La_0.8Sr_0.2MnO₃/La/Sr blended in PVDF gained a remarkable reflection loss of 94.68 dB at 15.31 GHz with an only thickness of 1.75 mm, meanwhile displaying an efficient bandwidth as wide as 6.74 GHz (reflection loss (RL) > 10 dB). Noteworthy, La_0.8Sr_0.2MnO₃/PS illustrated a considerable efficient bandwidth of 2.36 GHz (RL > 20 dB). Moreover, La_0.8Sr_0.2MnO₃ composites demonstrated more than 88% electromagnetic interference shielding efficiency (SE) along the X and Ku-band frequency.     

Preparation and mechanical properties of SiCw-reinforced WC-10Ni3Al cemented carbide by microwave sintering

SiC_w-reinforced WC-10Ni₃Al cemented carbide was prepared by microwave sintering method, and the effects of the sintering temperature and SiC_w content on the microstructure and mechanical properties of WC-10Ni₃Al cemented carbide were investigated; the promotion effect and strengthening mechanism of SiC_w were then analysed. The experimental results showed that the relative density, hardness, flexural strength and fracture toughness of WC-10Ni₃Al cemented carbide increased and then decreased with increasing SiC_w addition and sintering temperature. When the sintering temperature was 1500 °C and the content of SiC_w was 0.3 wt%, the sample reached the highest mechanical properties and had a relative density of 96.5%, hardness of 1570 HV, flexural strength of 1275 MPa and fracture toughness of 13.1 MPa mm^1/2, which were 4.0%, 23.1%, 12.5% and 8.1% higher than those of the sample without SiC_w, respectively. During microwave sintering of WC-Ni₃Al, the addition of an appropriate SiC_w content can increase the microwave absorption of the sample, and produce many micro-high-temperature regions within the sample, which can accelerate the generation of the Ni₃Al liquid phase. This promotes liquid phase flow to fill pores and rearrange the WC grains, thereby improving density and mechanical properties of the sample. The strengthening mechanisms of SiC_w on microwave sintered WC-Ni₃Al consist of promoting densification enhancement, fine-grained strengthening, and solid solution strengthening of Ni₃Al by Si atoms.     

Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field

Micron-sized lanthanum oxide powders are prepared by the calcination of lanthanum carbonate hydrate in microwave field. The decomposition process of lanthanum carbonate hydrate was analyzed by TG-DSC and indicates the reaction undergoes three stages, resulting in the generation of lanthanum oxide at 770 °C. For microwave assisted calcination, XRD patterns demonstrate that hexagonal La₂O₃ structure is initially formed after calcination at 650 °C for 2 h, and FT-IR analyses confirm the decomposition of precursor is complete after calcination at 750 °C for 2 h. SEM investigations reveal that 800 °C is the optimal calcination temperature to generate La₂O₃ powders with uniform morphologies. In comparison, conventionally calcination experiments are carried out in electrical furnace. Both XRD and FT-IR analyses are in consistence with TG-DSC, which indicate the temperature required for fully decomposition of lanthanum carbonate hydrate by conventional heating is higher than that of microwave heating. SEM images present irregular morphologies and wide particle size distribution of conventionally prepared samples. All the techniques are utilized to prove the feasibility of decomposing La₂(CO₃)₃ to generate La₂O₃ in microwave field and highlight the advantages of microwave heating.     

Enhanced high temperature microwave absorption of La0.9Sr0.1MnO3/MgAl2O4 composite ceramics based on controllable electrical conductivity

The high temperature microwave absorbing efficiency (HTMAE) of xLa_0.9Sr_0.1MnO₃/(1 − x)MgAl₂O₄ composite ceramics was investigated by studying the crystal structure, electrical conductivity, and permittivity. The crystal structure of La_0.9Sr_0.1MnO₃ and MgAl₂O₄ were maintained, but the Mn³⁺ and Al³⁺ ions were exchanged with each other through doping. The conductivity and permittivity of the composite ceramics increased with the increase of La_0.9Sr_0.1MnO₃ content and test temperature. When x = 0.36, the electrical conductivity in La_0.9Sr_0.1MnO₃ significantly enhanced the microwave polarization of the composite ceramics at high temperature. According to transmission/reflection modelling, the composite ceramics with x = 0.24 showed excellent HTMAE near the optimal thickness of 1.8 mm. Although the optimal thickness of the composite with x = 0.36 was reduced to 1.1 mm, the HTMAE was seriously lessened due to an impedance mismatch. xLa_0.9Sr_0.1MnO₃/(1 − x)MgAl₂O₄ are promising as thin and efficient microwave absorbing materials at high temperatures and the microwave permittivity can be further enhanced by adjusting the conductivity of La_0.9Sr_0.1MnO₃.     

Hierarchical manganese sillenite Bi12MnO20 microparticles assembled by nanocubes with microwave absorption enhancement

Pure manganese sillenite (Bi₁₂MnO₂₀) microparticles with average sizes of 11.5–16.8 μm were prepared via an oxidation-precipitation method assisted by microwave-heating. Their microstructures are characterized by the shapes of single or multiple intersecting tetrahedrons and the massive nanocubes in ordered arrays on each tetrahedron surface. Such hierarchical Bi₁₂MnO₂₀ microparticles have not been reported so far, while their growth mechanisms may include the formation of active centers, the selective nucleation on active center surfaces and the growth of nanocubes along certain orientations. In electromagnetic determination, noticeable resonances both in permittivity and permeability were revealed beyond relaxations. A cone-like reflection loss peak was obtained with the minimum value of −39.1 dB and a broad effective microwave absorption bandwidth covering 12.7–14.6 GHz. Moreover, the reflection loss peak stagnates at the same frequency regardless of the absorbent thickness variations, suggesting a high performance in microwave attenuation.     

Comparison between microwave and conventional thermal reactivations of spent activated carbon generated from vinyl acetate synthesis

Microwave and traditional thermal reactivations of activated carbon (AC) used as catalyst support in vinyl acetate synthesis have been investigated. Experiments have been carried out by using a single mode microwave device (MW) operating at 2450 MHz and a conventional electric furnace (CF) under steam and CO₂ atmosphere, respectively. The surface properties of the spent AC and the reactivated samples were characterized by means of N₂ adsorption and SEM, and compared the effects of different heating mechanisms and activating agents on the adsorption capacities and pore structures of the reactivated AC. These results indicated that the AC obtained by microwave irradiation showed higher adsorption capacities for iodine, methylene blue (MB) and acetate acid, higher BET surface areas and mesoporosity than those obtained by conventional thermal heating. The reactivated samples activated by steam had a narrower and more extensive microporosity as well as higher BET than those activated by carbon dioxide under the same heating equipment. From the results, it was concluded that microwave heating combined with steam as an activating agent could remarkably increase the reactivating efficiency compared to the traditional thermal heating.     

Influence of heating approach (microwave vs. muffle furnace) and fuel in auto-combustion design of nanostructured Ca2Mn3O8 as support for efficient and reusable catalyst used in green fuel production

In this article, MgO/Ca₂Mn₃O₈ nanocatalyst was used for the first time in the transesterification process for the production of green fuel from sunflower oil. Besides, the synthesis of layered nanostructured Ca₂Mn₃O₈ was done for first applied by combustion route as well. The effects of two heating sources including muffle furnace and microwave irradiation, and also various fuels including urea, glycine, and ethylene glycol were investigated on the characteristics of calcium-manganese oxide (Ca₂Mn₃O₈) as the support of the nanocatalysts loaded by MgO as active phase. The synthesized samples were characterized by different analyzes such as AFM, HRTEM, FESEM, FTIR, EDX, XRD, and BET-BJH. The nanocatalyst produced by the microwave irradiation combustion method and urea fuel scored the best performance. This nanocatalyst had the highest specific surface area (46.4 m²/g), and better pore size distribution, too. The roughness of this sample was reported at 5.1 nm in the range of 2 μm. It was found that the optimum sample has converted 96.7% of the oil extracted from sunflower seed to fatty acid esters that are gained by transesterification of fats with methanol. This sample exhibited better reusability in the same operating conditions in proportion to the sample synthesized with muffle furnace in twelve runs, which justified the positive influence of microwave irradiation on the catalytic properties.     

Design of double-layer ceramic absorbers for microwave heating

We propose a guide for designing double-layer ceramic absorbers in microwave heating by optimizing the thickness based on the analysis of reflection loss (RL) of a double-layer absorber consisting of a high-loss SiC layer and a low-loss Al₂O₃ layer. The calculated reflection losses for individual layers of SiC and Al₂O₃ show that the former with a thickness of 0.0054 m has the maximum microwave absorption while the latter in the thickness range up to 0.1 m is identified as a poor microwave absorbing material with RL larger than −0.4 dB. By using a 0.0054-m-thick SiC layer as the susceptor, the absorption in the Al₂O₃ layer and of the entire double-layer absorber increases significantly. The results demonstrate that high microwave absorption throughout the heating process can only be achieved in a sample with a small thickness in which a slight absorption peak shift during heating (less than one eighth-wavelength in the medium) occurs.     

Comparative study of microwave absorption properties of Ni–Zn ferrites obtained from different synthesis technologies

Spinel ferrites are widely used for electromagnetic wave (EMW) absorption applications. In this study, spinel Ni–Zn ferrites with excellent microwave absorption properties were synthesized. Their EMW absorption characteristics and interaction mechanisms were studied to lay the foundation for the study of the role of Ni–Zn ferrite as a magnetic substrate for composites. Herein, Ni_0·5Zn_0·5Fe₂O₄ was prepared by the hydrothermal method (H-NZFO) and the sol–gel auto-combustion method (S-NZFO); both samples exhibited distinct microwave absorption properties. The S-NZFO absorber (thickness = 3.72 mm) demonstrated the best dual-zone microwave absorption with two strong reflection loss peaks at 5.1 and 10.5 GHz. The corresponding effective absorption bandwidth (EAB) reached 9.0 GHz, which covered part of the S-band and all of the C- and X-bands. These results were attributed to the high saturation magnetization, outstanding complex permeability, and multiple magnetic loss channels of S-NZFO. The H-NZFO sample exhibited excellent absorption capability and matching thickness. At a thickness as low as 1.71 mm, the minimum reflection loss (RL_min) of the H-NZFO absorber reached -60.2 dB at 13.1 GHz. The maximum bandwidth corresponding to RL below -10 dB was 4.6 GHz. These results can be attributed to small particle size, high complex permittivity, and multiple dielectric loss channels of H-NZFO. The observed wide effective absorption bandwidth of S-NZFO and strong microwave absorption capability of H-NZFO suggest the potential of both materials as substrates for efficient microwave absorbers in military as well as civilian absorption applications.     

α-MnO2 nanorods-polyaniline (PANI) nanocomposites synthesized by polymer coating and grafting approaches for screening EMI pollution

In this work, α-MnO₂ nanorods-polyaniline nanocomposites were synthesized using polymer coating and grafting approaches. The synthesized nanocomposites were characterized by XRD, FESEM, EDAX, TEM, TG-DTA and FT-IR techniques. The Electromagnetic properties of prepared samples were measured using vector network analyzer in the 8–18 GHz (X and Ku-Band) frequency region. The α-MnO₂-NH₂-PANI nanocomposite synthesized by grafting approach showed better electrical conductivity, excellent dielectric loss with superior microwave absorption ability. In comparison with pure MnO₂, the microwave absorption characteristics of α-MnO₂-NH₂-PANI nanocomposite display considerable improvements, with an effective absorption band at 10.8 GHz and 14.5 GHz under ?10 dB and minimum reflection loss (RL) of ?30.79 dB at 14.5 GHz. The α-MnO₂-NH₂-PANI sample also showed considerable shielding effectiveness (SE) i.e. ?20.85 dB in the 8–18 GHz frequency region. The observed value of RL and SE surpasses the required value for being utilized at a commercial level. These results are surely helpful to explore the microwave absorption study of different combinations of organic/inorganic nanocomposite materials particularly for shielding and microwave absorption applications.     

Self-cleaning functionalized FeNi/NiFe2O4/NiO/C nanofibers with enhanced microwave absorption performance

Microwave absorbing materials with enhanced microwave absorption performance and self-cleaning function are of great interest for military applications and human health caused by electromagnetic radiation pollution. Herein we report the synthesis of FeNi/NiFe₂O₄/NiO/C nanofibers (NFs) via electrospinning technique using nickel acetylacetonate (Ni(acac)₂), ferric acetylacetonate (Fe(acac)₃), and N, N-dimethylformamide (DMF)/polyacrylonitrile (PAN) solution as precursor. We also show the abilities of the materials to attenuate electromagnetic microwave and their ease of self-cleaning performance. X-ray diffraction patterns and HRTEM images reveal that the materials possess FeNi, NiFe₂O₄, NiO and graphite. HAADF-STEM images show that the magnetic nanoparticles distribute uniformly along the fibers. Contrast experiments had been conducted on different calcination temperatures to elucidate the impedance matching and loss mechanism. Based on the results of the experiment, excellent microwave absorption was exhibited by blending the NiFe₂O₄/NiO/CNFs and FeNi/NiFe₂O₄/NiO/CNFs into paraffin at 50 wt% and 5 wt%, respectively. Moreover, the contact angles (CA) of the as-prepared fiber films calcined at 650, 750 and 950 °C were 143°, 141° and 144°, respectively, indicating that fiber films exhibited excellent hydrophobicity and self-cleaning function. It suggested that the as-obtained NFs had an excellent application prospect in self-cleaning microwave absorbing materials.     

Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering

Structural, dielectric and ferroelectric properties of polycrystalline BaTiO₃ (BTO) ceramics prepared with hybrid sintering i.e., microwave assisted radiant heating (MARH) are reported. It is observed that the permittivity (ε) and true switched ferroelectric charge density (Q_SW) of BTO ceramics can be enhanced by employing MARH. An enhancement of 58% in ε and 17% in Q_SW is observed for the BTO sample prepared with 30% microwave power applied during MARH as compared to the conventional radiant heating. The results are explained in terms of microstructure resulting from the microwave assisted sintering.