Low-temperature synthesis of β-FeSe superconductor from soluble precursors is a great challenge in the chemical solution approaches. Here, we develop a new and facile solution-based synthetic route to first fabricate narrow-phased β-FeSe superconductor with soluble iron and selenium sources as starting materials. The growth mechanism of β-FeSe superconductors is discussed by kinetically controllable syntheses in various reaction conditions. Chemically engineering the stoichiometry of β-FeSe products by selenium-diffusion process gives rise to a transition of antiferromagnetic-superconducting-antiferromagnetic (AFM-SC-AFM) order. Once the AFM order is suppressed, SC β-FeSe nanosheets show a tunable initial superconducting transition temperature (TC) from 3.2 to 10 K in the superconducting regime. Electrical measurements on superconducting β-FeSe exhibit an upper critical magnetic field higher than 14 T, showing potential application of β-FeSe nanosheet for superconducting device. This method provides guidance for future applications in such chemical solutions for diffusion-controlled synthesis of narrow-phased functional materials, which are enriched of abundant fundamental physics and potentials for future applications.

•Ni/Ni3C/Ni3N nanocomposites were synthesized by a one-pot solution chemistry method at lower temperature of 200  °C.•Ni3N nanomaterials were generally obtained by two steps, here we use one-step for preparation of Ni3N nanomaterials.•The formation mechanism, magnetic properties and textural structures of Ni/Ni3C/Ni3N nanocomposite were investigated.A one-pot solvothermal synthesis of Ni/Ni3C/Ni3N nanocomposite, using hydrazine hydrate (N2H4) as reducing agent and tetraethylene glycol (TEG) as solvent, is reported. The contents of Ni3C and Ni3N decrease with increasing the amount of N2H4. The magnetic hysteresis loops at room temperature of the Ni/Ni3C/Ni3N display ferromagnetic characteristics. Saturation magnetization increases gradually with increasing Ni content of the Ni/Ni3C/Ni3N. Growth mechanism is discussed based on the characterization results of X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TG), nitrogen adsorption desorption, and elemental analysis. Furthermore, Ni3N was fabricated by such a solution chemistry method at low-temperature of 200 °C.Ni/Ni3C/Ni3N nanocomposites were synthesized by a one-pot solution chemistry method at lower temperature of 200 °C. The formation mechanism, magnetic properties and textural structures of Ni/Ni3C/Ni3N nanocomposite were investigated.Download high-res image (191KB)Download full-size image

Monoclinic Fe3−xCrxSe4 nanostructures (0 ≤ x ≤ 2.5) were synthesized using a high-temperature solution chemical method. With increasing the Cr doping, the peak positions in the X-ray diffraction (XRD) patterns of Fe3−xCrxSe4 nanostructures slightly shifted to lower 2θ values due to the changes in lattice parameters. Expansions in the unit cell volumes of Fe3−xCrxSe4 nanostructures (x > 0.3) may have been responsible for enhancing the ferromagnetic (FM) interaction between magnetic ions, which resulted in a significant increase in the Curie temperature (TC) from 331 K for Fe3Se4 to 429 K for FeCr2Se4, distinctly differing from the magnetic properties of the corresponding bulk materials. A room-temperature coercivity (HC) analysis showed an obvious increase from 3.2 kOe for Fe3Se4 to 12 kOe for Fe2.3Cr0.7Se4 nanostructure, but gradually decreased upon further increasing the Cr content.

Electromagnetic (EM) wave absorption properties of metallic perovskite lanthanum nickel oxide (LNO) powder and dielectric-modulated LNO–FeCo/C composites were investigated. Reflection loss (RL) of the LNO–paraffin composites depends on the mass ratio of LNO powder dispersion in the paraffin matrix, in which the optimal RL of −24.7 dB at 17.6 GHz with the absorbent layer thickness of 1.6 mm is obtained at the 10 wt% LNO–paraffin composite, just below the percolation threshold (PC) of the LNO dispersion in the paraffin matrix. Three absorption peaks in the frequency dependence of RL of the 10 wt% LNO–paraffin composite are ascribed to the dielectric relaxations which occurred at frequencies of about 3.3, 9.2 and 15.1 GHz. Dielectric-modulation by metal-conductive LNO powder significantly increases the relative complex permittivity of the (x)LNO–(y)FeCo/C–paraffin composites and a good impedance match coming from the dielectric-modulation by the LNO powder would be obtained just below the PC of the LNO powder dispersion in the paraffin matrix. The (8 wt%)LNO–(32 wt%)FeCo/C–paraffin composite exhibited an enhanced EM wave absorption performance, in which RL values less than −20 dB can be obtained in the 2–18 GHz range by choosing an appropriate thickness from 1.3 mm to 8 mm and the minimum RL is −50.6 dB at 9 GHz with the absorbent layer thickness of 2.4 mm.

Microwave absorption properties of core double-shell FeCo/C/(x)BaTiO3 nanocomposites were investigated in the 1–18 GHz frequency range. High resolution transmission electron microscopy studies reveal the core double-shell type nanocomposite with FeCo nanoparticles as the center, while carbon and BaTiO3 are the inside and the outside shells, respectively. Enhanced relative permittivity made the core double-shell FeCo/C/(x)BaTiO3 nanocomposites with better electromagnetic impedance matching than that of a FeCo/C and BaTiO3 mixture. Reflection loss (RL) values of FeCo/C/(20 wt%)BaTiO3–paraffin composite are almost double those of the FeCo/C–paraffin composite at the absorbent thickness from 2 to 7.5 mm due to enhanced interfacial effects. The RL value of the FeCo/C/(20 wt%)BaTiO3–paraffin composite is −41.7 dB at 11.3 GHz at the absorbent thickness of 2 mm and a broad absorption bandwidth of 5.1 GHz (RL values exceeding −10 dB) covers the 9.4–14.5 GHz frequency range.

The M–Cl (M=Fe, Co, Ni) boracites are prepared by a procedure consisting of solution mixing, evaporating to dryness, grinding and heating reaction in N2 atmosphere. The lattice parameter and the phase transition temperatures of the boracites Co3B7O13Cl and Ni3B7O13Cl are investigated by means of X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. There are three reversible phase transitions in the Co–Cl boracite and one phase transition in the Ni–Cl boracite above room temperature. Temperature dependences of dielectric loss of M–Cl (M=Fe, Co, Ni) boracites are studied by the rectangular cavity perturbation method at a microwave frequency of 2.45 GHz. The ferroelectric property at room temperature of the Co3B7O13Cl is demonstrated.