Ferromagnetic and excellent microwave absorbing properties of CoNi microspheres and heterogeneous Co/Ni nanocrystallines

CoNi microspheres with different diameters and heterogeneous Co/Ni nanocrystallines were synthesized via changing hydrothermal reaction parameters. The heterogeneous Co/Ni nanocrystallines comprised three kinds of particle morphologies, i.e., nanoflakes, nanospheres and needle-like nanowhiskers. The heterogeneous Co/Ni nanocrystalline sample coating (containing 60 wt% powder) exhibited a maximum reflection loss (RL) of −33 dB at 17.6 GHz and a bandwidth of less than −10 dB covering the 15.04–18.00 GHz range with a coating thickness of 1 mm. The CoNi microsphere sample with diameters in the range of 0.4–2.5 μm exhibited excellent microwave absorption abilities in the C-band (4–8 GHz) and X-band (8–11.5 GHz). However, the sample of chain-like assemblies from CoNi microspheres with a diameter above 2 μm presented poor microwave absorption in the 2–18 GHz range. In contrast, the excellent microwave absorption properties of the heterogeneous Co/Ni nanocrystalline sample in the Ku-band (12–18 GHz) could be attributed to the relatively high permeability (1.63–1.10) and optimal impedance matching between permittivity and permeability.

CoNi microspheres with different diameters and heterogeneous Co/Ni nanocrystallines were synthesized via changing hydrothermal reaction parameters.     

Activated carbon fiber/Fe3O4 composite with enhanced electromagnetic wave absorption properties

To obtain a low-density material that is capable of absorbing electromagnetic waves over a wide bandwidth, an activated carbon fiber/Fe₃O₄ composite material (ACF/Fe₃O₄) was prepared using an in situ reduction method. Scanning electron microscopy images show that Fe₃O₄ nanoparticles, approximately 10–40 nm in size, were spread uniformly over the surface of the ACF. The resulting composite exhibited superparamagnetic behavior at room temperature. The ability of the ACF and ACF/Fe₃O₄ composite to absorb electromagnetic waves over a frequency range of 8.2–18 GHz was measured using the arch method. The results showed that the maximum reflectivity of an ACF felt was −12.9 dB at 18 GHz, and the effective microwave-absorbing bandwidth (R < −10 dB) was 1.9 GHz (16.10–18 GHz). The absorption performance of the ACF was greatly enhanced by being loaded with Fe₃O₄ nanoparticles; the maximum reflectivity of the 2 mm layer of the ACF/Fe₃O₄ composite was −30.07 dB at 16.45 GHz, and the effective bandwidth (R < −10 dB) increased to 8.62 GHz (9.38–18 GHz). Coating with nano-Fe₃O₄ magnetic particles can effectively improve the absorption of electromagnetic waves by the ACF, and this technique therefore has great potential for application to the field of electromagnetic shielding.

To obtain a low-density material that is capable of absorbing electromagnetic waves over a wide bandwidth, an activated carbon fiber/Fe₃O₄ composite material (ACF/Fe₃O₄) was prepared using an in situ reduction method.     

Flexible Fe3Si/SiC ultrathin hybrid fiber mats with designable microwave absorption performance

Flexible Fe₃Si/SiC ultrathin fiber mats have been fabricated by electrospinning and high temperature treatment (1400 °C) using polycarbosilane (PCS) and ferric acetylacetonate (Fe(acac)₃) as precursors. The crystallization degree, flexibility, electrical conductivity, dielectric loss and microwave absorption properties of the hybrid fibers have been dramatically enhanced by the introduction of Fe. Fe₃Si nanoparticles with a diameter around 500 nm are embedded in SiC fibers. As the Fe₃Si content increases from 0 to 6.5 wt%, the related saturation magnetization (M_s) values increase from 0 to 8.4 emu g⁻¹, and the electrical conductivity rises from 7.9 × 10⁻⁸ to 3.1 × 10⁻³ S cm⁻¹. Moreover, the flexibility of Fe₃Si/SiC hybrid fiber mats is greatly improved and remains intact after 500 times 180°-bending testing. Compared with pure SiC fibers, the Fe₃Si/SiC hybrid fibers process higher dielectric and magnetic loss, which would be further advanced as more Fe₃Si phase is introduced. At the optimal Fe₃Si content of 3.8 wt%, the Fe₃Si/SiC fibers/silicon resin composite (5 wt%) exhibits minimal reflection loss (RL) of −22.5 dB at 16.5 GHz and 2.5 mm thickness with a wide effective absorption bandwidth (EAB, RL < −10 dB) of 8.5 GHz. The microwave absorption performance can be further promoted by multi component stacking fiber mat composites which contain both low and high Fe₃Si content layers. Furthermore, the position of the microwave absorption bands can also be simply manipulated by designing the stacking components and structure.

Flexible Fe₃Si/SiC ultrathin fiber mats have been fabricated by electrospinning and high temperature treatment (1400 °C) using polycarbosilane (PCS) and ferric acetylacetonate (Fe(acac)₃) as precursors.     

Synthesis of hierarchically structured Fe3C/CNTs composites in a FeNC matrix for use as efficient ORR electrocatalysts

Fe–N–C has a high number of FeN_x active sites and has thus been regarded as a high-performance oxygen reduction reaction (ORR) catalyst, and combining Fe₃C with Fe–N–C typically boosts ORR activity. However, the catalytic mechanism remains unknown, limiting further research and development. In this study, a precipitation-solvothermal process was used in conjunction with pyrolysis to produce a series of Fe–N–C catalysts derived from a zeolitic imidazolate framework (ZIF) that was composited with Fe₃C. The prepared catalysts had a multiscale structure of ZIF-like carbon particles and rod-like structures, as well as bamboo-like carbon nanotubes (CNTs) and carbon layers wrapped with Fe₃C particles while a series of studies revealed the origin of the rod-like structures and Fe₃C phase. The hierarchical structure was beneficial to the enhanced electrocatalytic performance of catalysts for ORR. The optimal sample had the highest half-wave potential of 0.878 V vs. RHE, which was higher than that of commercial Pt/C (0.861 V vs. RHE). The ECSA of the optimal sample was 1.08 cm² μg⁻¹, with an electron transfer number close to 4, and functioning kinetics. The optimal sample exhibited high durability and methanol tolerance for the ORR. Finally, blocking different Fe active sites with coordination ions demonstrated that Fe(ii) was the main active site, indicating that Fe₃C primarily served as a cocatalyst to optimize the electron structure of Fe–N–C, thereby synergistically improving the ORR activity.

The synthesis of an FeNC catalyst with an Fe₃C phase and the important active site was Fe(ii).     

Catalytic graphitization assisted synthesis of Fe3C/Fe/graphitic carbon with advanced pseudocapacitance

We report an environmentally friendly strategy for the synthesis of Fe₃C/Fe/graphitic carbon based on hydrothermal carbonization and graphitization of carbon spheres with potassium ferrate (K₂FeO₄) at 800 °C. The obtained sample consisting of Fe₃C/Fe nanoparticles and graphitic carbon (FC-1-8) delivered an enhanced pseudocapacitance of 428.0 F g⁻¹ at a current density of 1 A g⁻¹. After removal of the Fe₃C/Fe electroactive materials, the graphitic carbon (FC-1-8-HCl) possessed a large specific surface area (SSA) up to 2813.6 m² g⁻¹ with a capacity of 243.3 F g⁻¹ at 1 A g⁻¹, far outweighing the other amorphous carbon electrodes of FC-0-8 (carbon spheres annealed at 800 °C without the treatment of K₂FeO₄). The graphitic material with a porous structure could offer more electroactive sites and improved conductivity of the sample. This method provided guidelines for the synthesis of superior performance supercapacitors with synchronous graphitic carbon and electroactive species.

We report an environmentally friendly strategy for the synthesis of Fe₃C/Fe/graphitic carbon based on hydrothermal carbonization and graphitization of carbon spheres with potassium ferrate (K₂FeO₄) at 800 °C.     

One-step synthesis,characterization and properties of novel hybrid electromagnetic nanomaterials based on polydiphenylamine and Co–Fe particles in the absence and presence of single-walled carbon nanotubes

A one-step preparation method for hybrid electromagnetic nanomaterials based on polydiphenylamine (PDPA) and bimetallic Co–Fe particles in the absence and presence of single-walled carbon nanotubes (SWCNT) was proposed. During IR heating of PDPA in the presence of Co(ii) and Fe(iii) salts in an inert atmosphere at T = 450–600 °C, the polycondensation of diphenylamine (DPA) oligomers and dehydrogenation of phenyleneamine units of the polymer with the formation of C N bonds and reduction of metals by evolved hydrogen with the formation of bimetallic Co–Fe particles dispersed in a polymer matrix occur simultaneously. When carbon nanotubes are introduced into the reaction system, a nanocomposite material is formed, in which bimetallic Co–Fe particles immobilized on SWCNT are distributed in the matrix of the polymer. According to XRD data, reflection peaks of bimetallic Co–Fe particles at diffraction scattering angles 2θ = 69.04° and 106.5° correspond to a solid solution based on the fcc-Co crystal lattice. According to SEM and TEM data, a mixture of particles with sizes of 8–30 nm and 400–800 nm (Co–Fe/PDPA) and 23–50 nm and 400–1100 nm (Co–Fe/SWCNT/PDPA) is formed in the nanocomposites. The obtained multifunctional Co–Fe/PDPA and Co–Fe/SWCNT/PDPA nanomaterials demonstrate good thermal, electrical and magnetic properties. The saturation magnetization of the nanomaterials is M_S = 14.99–31.32 emu g⁻¹ (Co–Fe/PDPA) and M_S = 29.48–48.84 emu g⁻¹ (Co–Fe/SWCNT/PDPA). The electrical conductivity of the nanomaterials reaches 3.5 × 10⁻³ S cm⁻¹ (Co–Fe/PDPA) and 1.3 S cm⁻¹ (Co–Fe/SWCNT/PDPA). In an inert medium, at 1000 °C the residue is 71–77%.

In a self-organizing system within one stage under IR heating conditions, hybrid nanomaterials are formed with a structure that contains bimetallic Co–Fe particles, free or immobilized on the SWCNT surface, dispersed in the polymer PDPA matrix.     

Peculiarities of the microwave properties of hard–soft functional composites SrTb0.01Tm0.01Fe11.98O19–AFe2O4 (A = Co,Ni, Zn,Cu, or Mn)

Herein, we investigated the correlation between the chemical composition, microstructure, and microwave properties of composites based on lightly Tb/Tm-doped Sr-hexaferrites (SrTb_0.01Tm_0.01Fe_11.98O₁₉) and spinel ferrites (AFe₂O₄, A = Co, Ni, Zn, Cu, or Mn), which were fabricated by a one-pot citrate sol–gel method. Powder XRD patterns of products confirmed the presence of pure hexaferrite and spinel phases. Microstructural analysis was performed based on SEM images. The average grain size for each phase in the prepared composites was calculated. Comprehensive investigations of dielectric properties (real (ε′) and imaginary parts (ε′′) of permittivity, dielectric loss tangent (tan(δ)), and AC conductivity) were performed in the 1–3 × 10⁶ Hz frequency range at 20–120 °C. Frequency dependency of microwave properties were investigated using the coaxial method in frequency range of 2–18 GHz. The non-linear behavior of the main microwave properties with a change in composition may be due to the influence of the soft magnetic phase. It was found that Mn- and Ni-spinel ferrites achieved the strongest electromagnetic absorption. This may be due to differences in the structures of the electron shell and the radii of the A-site ions in the spinel phase. It was discovered that the ionic polarization transformed into the dipole polarization.

Paper presents the correlation between the composition, microstructure, and microwave properties of composites based on Tb/Tm-doped Sr-hexaferrites and spinel ferrites (AFe₂O₄), which were fabricated by a one-pot citrate sol–gel method.     

Fe/Co/N–C/graphene derived from Fe/ZIF-67/graphene oxide three dimensional frameworks as a remarkably efficient and stable catalyst for the oxygen reduction reaction

The development of non-noble metal catalysts with high-performance, long stability and low-cost is of great importance for fuel cells, to promote the oxygen reduction reaction (ORR). Herein, Fe/Co/N–C/graphene composites were easily prepared by using Fe/ZIF-67 loaded on graphene oxide (GO). The Fe/Co/porous carbon nanoparticles were uniformly dispersed on graphene with high specific surface area and large porosity, which endow high nitrogen doping and many more active sites with better ORR performance than the commercial 20 wt% Pt/C. Therefore, Fe/Co/N–C/graphene composites exhibited excellent ORR activity in alkaline media, with higher initial potential (0.91 V) and four electron process. They also showed remarkable long-term catalytic stability with 96.5% current retention after 12 000 s, and outstanding methanol resistance, compared with that of 20 wt% Pt/C catalysts. This work provides an effective strategy for the preparation of non-noble metal-based catalysts, which could have significant potential applications, such as in lithium–air batteries and water-splitting devices.

Fe/Co/N–C/graphene was facilely and successfully prepared by a calcination process, which has remarkable electrocatalytic ORR activity in alkali solutions and also displays an exceptional stability for the ORR and methanol tolerance.     

Hofmann-like metal–organic-framework-derived PtxFe/C/N-GC composites as efficient electrocatalysts for methanol oxidation

Pt_xFe/C/N-GC electrocatalysts were prepared using a composite of Hofmann-like Pt/Fe-based metal–organic frameworks and two-dimensional oxidized graphene. The Pt_xFe/C/N-GC-700 composite (annealed at 700 °C) exhibited an enhanced mass activity in the electrocatalytic methanol oxidation reaction, which was ten times higher than that of commercial Pt/C (20%) catalysts.

Pt-based bimetallic catalysts derived from Pt/Fe-based metal–organic frameworks were prepared for electrocatalytic methanol oxidation, for which the mass activity is ten times higher than that of commercial Pt/C (20%) catalysts.     

Synthesis of a hollow-structured flower-like Fe3O4@MoS2 composite and its microwave-absorption properties

In order to realize the characteristics of new types of wave-absorbing materials, such as strong absorption, broad bandwidth, low weight and small thickness, a hollow-structured flower-like Fe₃O₄@MoS₂ composite was successfully prepared by simple solvothermal and hydrothermal methods in this paper. The structural properties were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Besides, the microwave properties and magnetic properties were measured using a vector network analyzer and via a hysteresis loop. SEM and TEM images revealed that MoS₂ nanosheets grew on the surface of hollow nanospheres. The results showed that the composite exhibited excellent absorbing property. When the molar ratio of Fe₃O₄ and MoS₂ was 1 : 18, the minimum reflection loss value reached −49.6 dB at 13.2 GHz with a thickness of 2.0 mm and the effective absorption bandwidth was 4.24 GHz (11.68–15.92 GHz). Meanwhile, the effective absorption in the entire X-band (8–12 GHz) and part of the C-band (4–8 GHz) and Ku-band (12–18 GHz) could be achieved by designing the sample thickness. In addition, the hollow structure effectively reduced the density of the material, which was in line with the current development trend of absorption materials. It could be predicted that the hollow core–shell structure composite has a potential application prospect in the field of microwave absorption.

A hollow-structured flower-like Fe₃O₄@MoS₂ composite was synthesized. The minimum reflection loss value reached −49.6 dB at 13.2 GHz with a thickness of 2.0 mm and the effective absorbing bandwidth was 4.24 GHz (11.68–15.92 GHz).     

Facile synthesis of porous Fe3O4@C core/shell nanorod/graphene for improving microwave absorption properties

Porous Fe₃O₄@C core/shell nanorods decorated with reduced graphene oxide (RGO) were fabricated through a facile one-pot method. The microwave absorption properties of the samples can be adjusted by the weight ratio of RGO. The addition of RGO not only effectively reduces the agglomeration of Fe₃O₄@C, but also has a great effect on impedance matching and dielectric loss, resulting in enhanced microwave absorption abilities. The thickness corresponding to optimum reflection loss (R_L) decreases as the weight ratio of RGO increases. For the Fe₃O₄@C/RGO composite, a maximum R_L value of −48.6 dB can be obtained at 13.9 GHz with a thickness of 3.0 mm, and the absorption bandwidth with R_L below −10 dB is 7.1 GHz from 10.9 GHz to 18 GHz. These results demonstrate a facile method to prepare a highly efficient microwave absorption material with special microstructure.

Porous Fe₃O₄@C core/shell nanorods decorated with reduced graphene oxide were synthesized by a facile one-pot method, and exhibit high microwave absorption performance: maximum reflection loss reaches −48.6 dB.     

Microwave absorption enhancement by adjusting reactant ratios and filler contents based on 1D K–MnO2@PDA and poly(vinylidene fluoride) matrix

One-dimensional K–MnO₂ nanorods were prepared by a wet chemical process. Dopamine hydrochloride (PDA) layers with various thicknesses were coated and finally, the composites were filled in a poly(vinylidene fluoride) (PVDF) matrix using the hot-molding procedure. The complex permittivity and permeability of the K–MnO₂@PDA/PVDF composites could be adjusted by reactant amount ratios and filler contents. The minimum reflection loss could reach −49.4 dB and an effective absorption bandwidth (<−10 dB) covering 11.12 GHz was achieved with 20% filler content when the reactant amount ratio between K–MnO₂ and PDA was 4 : 0.375, which was derived from effective internal polarization processes. It is expected that these novel composites can be used as high-performance microwave absorbers.

The microwave absorption properties of K–MnO₂@PDA/PVDF composites are greatly enhanced due to appropriate reactant amount ratios and filler contents, which result in an effective internal polarization process.     

Preparation and properties of CF–Fe3O4–BN composite electromagnetic wave-absorbing materials

A three-layered electromagnetic (EM) wave-absorbing material was prepared by depositing a Fe₃O₄ and boron nitride (BN) coating onto the surface of a carbon fiber (CF) by in situ hybridization. The structure, chemical composition, morphology, high-temperature resistance, EM characteristics and EM wave absorption of the composite materials were analyzed. The composite materials contained CFs, and Fe₃O₄ was distributed along the axial direction of the fiber, whereas BN was found in the outermost coating layer. The proposed preparation method improved the oxidation resistance and EM wave absorption of CF. When the solubility of the metal salt was 20 g/100 ml, the decomposition temperature of the prepared CF/Fe₃O₄(3)/BN increased by more than 200 °C compared with that of CF/Fe₃O₄(3). The EM wave loss of less than −5 dB ranged within 8.8–18 GHz, and the effective EM wave-absorbing bandwidth (R < −10 dB) was 4.2 GHz (11.2–15.4 GHz). The prepared CF-based composite material had a lightweight structure, wide absorption band, and strong oxidation resistance. All these findings can serve as a reference for the study of other EM wave-absorbing materials.

A three-layered electromagnetic (EM) wave-absorbing material was prepared by depositing a Fe₃O₄ and boron nitride (BN) coating onto the surface of a carbon fiber (CF) by in situ hybridization.     

Selective hydrodeoxygenation of p-cresol as a model for coal tar distillate on Ni–M/SiO2 (M = Ce,Co, Sn,Fe) bimetallic catalysts

Ni–M/SiO₂ with different binary metals M (M = Ce, Co, Sn, Fe) prepared by an incipient impregnation method was used in the hydrodeoxygenation (HDO) of low-temperature coal tar distillate, which is rich in phenolic compounds. p-Cresol, as a model compound of the distillate, was used to evaluate the activity and selectivity of BTX products on the series of reduced Ni–M/SiO₂ catalysts in a fixed bed reactor. The properties of the catalysts were characterized by N₂ adsorption–desorption, ICP-AES, XRD, H₂-TPR, and XPS. Benzene and toluene as the direct deoxygenation (DDO) products and cyclohexane and methylcyclohexane as the hydrogenolysis (HYD) products were detected to evaluate the selectivity of the path in the deoxygenation process. In this series of catalysts, the order of reactivity was Ni–Ce > Ni–Sn > Ni–Co > Ni–Fe > monometallic Ni. Meanwhile, the addition of Ce and Co loaded in the Lewis acid sites of the catalyst affected the electron distribution of nickel atom and its atomic arrangement on the surface of the carrier. Compared to monometallic Ni, the DDO path become dominant on Ni–Ce and Ni–Co and the selectivity for BTX products increased from 58.8% to 77.4% and 71.1%, respectively. The binary metal Sn, unlike the former two metals, formed a Ni₃Sn crystal form with Ni, which resulted in significant enhancement of the HYD path while obviously increasing the reactivity.

Ni–M/SiO₂ with different binary metals M (M = Ce, Co, Sn, Fe) prepared by an incipient impregnation method was used in the hydrodeoxygenation (HDO) of p-cresol as model of phenols in coal tar to produce BTX.     

Biomass carbon derived from pine nut shells decorated with NiO nanoflakes for enhanced microwave absorption properties

Electromagnetic absorption materials have gained increasing attention. In this study, we report NiO decorated biomass porous carbon derived from pine nut shells as a promising microwave absorbing material by a facile strategy. The NiO/biomass porous carbon (BPC) is thermally converted from Ni(OH)₂/BPC with BPC as the base for precipitation. All products were characterized by XRD, Raman, and SEM techniques, which reveals that the NiO nanoflakes were uniformly self-assembled on the surface of the activated carbon. Compared with counterparts of pure Ni(OH)₂ and Ni(OH)₂/BPC, a large reflection loss peak of −33.8 dB at 16.4 GHz is achieved for the NiO/BPC composites, and the absorption bandwidth less than −10 dB can reach up to about 6.7 GHz (from 11.3 to 18.0 GHz) with a thickness of 8 mm. The enhanced microwave absorption properties originate from the electric/dielectric polarization and the unique NiO decorated BPC structures. The expanded interfaces, such as NiO–NiO, Ni–BPC and NiO–paraffin interfaces in the complicated porous composites, could boost the interfacial polarization as well as related relaxation which results in enhanced dielectric loss and electromagnetic absorbing properties. In addition, NiO/BPC nanocomposites exhibit comparatively better matching of permittivity and permeability. It is expected that our present investigation could provide a new possibility for biomass based fabrication of potential microwave absorbing materials.

A facile strategy was applied to synthesize NiO/biomass carbon composites with excellent microwave absorption properties.     

Facile preparation of rGO/MFe2O4 (M = Cu,Co, Ni) nanohybrids and its catalytic performance during the thermal decomposition of ammonium perchlorate

Reduced graphene oxide/metal ferrite (rGO/MFe₂O₄, M = Cu, Co, Ni) nanohybrids are successfully prepared through a simple, one-step hydrothermal method. The rGO/MFe₂O₄ nanohybrids are characterized by XRD, TEM, FT-IR, XPS, Raman and BET surface area measurements. The rGO/MFe₂O₄ nanohybrids demonstrate amazing catalytic activities on the thermal decomposition of ammonium perchlorate (AP). DSC results indicate that rGO/MFe₂O₄ nanohybrids (3 wt%), could decrease the decomposition temperature of pure AP from 424.7 °C to 329.1 °C, 338.3 °C, and 364.8 °C, respectively. This enhanced catalytic performance is mainly attributed to the synergistic effect of NPs and rGO. The activation energy (E_a) of AP mixed with nanohybrids is investigated by two isoconversion methods, Flynne–Walle–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS), on a conversion degree (α) range from 0.05 to 0.95. The values of E_a calculated from the above two methods matched with each other. A strong dependence of E_a on α is observed, indicating a complex decomposition process.

The rGO/MFe₂O₄ (M = Cu, Co, Ni) nanohybrids show amazing catalytic activity in the thermal decomposition process of AP.     

Dopamine-derived cavities/Fe3O4 nanoparticles-encapsulated carbonaceous composites with self-generated three-dimensional network structure as an excellent microwave absorber

Dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with self-generating three-dimensional (3D) network structure were successfully fabricated by a facile synthetic method, in which sodium alginate provided carbon matrix pores and excellent microwave absorption performance was established. The hollow cavities derived from the core–shell-like CaCO₃@polydopamine were creatively introduced into the 3D absorber to significantly improve the absorption performance. The sample calcined at 700 °C exhibited the most outstanding microwave absorption performance, with minimal reflection loss up to −50.80 dB at 17.52 GHz with a rare thickness of only 1.5 mm when filler loading was 35% in paraffin matrix. The effective absorption bandwidth of reflection loss < −10 dB reached 3.52 GHz from 14.48 GHz to 18 GHz, corresponding to the same thickness of 1.5 mm. In contrast, the sample without hollow dopamine-derived cavities showed poor performance due to poor impedance matching, and this highlights the role of hollow cavities brought into the 3D structure, which led to a difference in interfacial polarization, multiple reflections and scattering. The novel dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with 3D network structure can be regarded as a promising candidate for application as a microwave absorber with strong absorption.

Hollow dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with self-generating 3D network structure were fabricated for potential application as excellent microwave absorbers.     

Boosting the hydrogen evolution activity of a Co–N–C electrocatalyst by codoping with Al

Co, Al and N tri-doped graphene (CANG) was successfully fabricated via annealing N-doped graphene with Co and Al precursors. The material was characterized by scaning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, physical adsorption, and X-ray photoelectron spectroscopy (XPS). It was found that the as-prepared CANG features a robust three-dimensional hierarchically porous structure. The contents of Co and Al can achieve the maximum value of 2.18 at% and 0.51 at% at the annealing temperature of 950 °C. Upon using the electrocatalyst for the hydrogen evolution reaction (HER), the CANG exhibited remarkable electrocatalytic performance in both acidic (η₁₀ = 105 mV) and alkaline media (η₁₀ = 270 mV), and outperforms Co,N-codoped graphene and Al,N-codoped graphene, respectively. In combination with the density functional theory (DFT) calculations, it was revealed that the introduction of the Al heteroatom can decrease the absolute value of hydrogen adsorption free energy (ΔG(H*)) of Co–N–C catalysts, thus greatly enhancing the HER activity. This discovery will provide new guidance to the design of advanced and inexpensive carbon materials for fuel cell, water-splitting and other electrochemical devices.

The electrocatalytic activity of Co–N–C catalyst toward hydrogen evolution reaction can be significantly boosted by codoping with Al.     

Aqueous phase hydrogenolysis of glycerol with in situ generated hydrogen over Ni/Al3Fe1 catalyst: effect of the calcination temperature

The present work studied the influence of the calcination temperature on the aqueous phase hydrogenolysis of glycerol with in situ generated hydrogen over a Ni/Al₃Fe₁ catalyst. The Ni/Al₃Fe₁ catalyst was synthesized by the co-precipitation method at 28 mol% of Ni (Ni/(Ni + Al + Fe)) and a molar ratio of Al/Fe of 3/1. The prepared catalyst was calcined at different temperatures (500–750 °C). The obtained samples were tested for the aqueous phase hydrogenolysis (APH) of glycerol and characterized by several analytical techniques (ICP-OES, H₂-TPR, XRD, N₂-physisorption, NH₃-TPD, STEM, FESEM, and TGA). The catalyst calcined at 625 °C was selected as the best sample due to its high acidity, metal dispersion, and catalytic activity; 1,2-propanediol was the highest carbon selectivity product. In addition, it experienced lower metal leaching than the catalyst calcined at 500 °C.

Calcination temperatures of 500, 625 and 750 °C were studied over a Ni/Al₃Fe₁ catalyst on glycerol valorization. The catalyst calcined at 625 °C showed the best performance due to its high activity and selectivity to 1,2-propanediol.     

Bain and Nishiyama–Wassermann transition path separation in the martensitic transitions of Fe

The importance of martensitic transformations has led to tremendous efforts to explore the microscopic martensitic transition paths. There are five possible transformation paths (for γ → α transition) known for Fe at present, and at an arbitrary activation energy, any of the five paths might be followed. It then becomes considerably difficult to monitor the microscopic phase transition mechanism in experiments. Therefore, it is helpful to realize only one of the paths in a physical process. Based on first-principles calculations, we show that at suitable activation energies the Nishiyama–Wassermann (N–W) transformation path can be realized without the involvement of the Bain path, since the condition E_NW(θ) < E < E_Bain can be satisfied by pure Fe. E is the activation energy of the system, and E_NW(θ) and E_Bain are the energy barriers for the N–W and Bain transformations, respectively. In particular, the potential energy surface (PES) for the N–W transformation has been calculated as being four-dimensional, i.e., E = E(a,b,c,θ), where (a, b, c) are the lattice constants and θ is the shear angle involved in the shear distortion of the N–W path.

The importance of martensitic transformations has led to tremendous efforts to explore the microscopic martensitic transition paths.