J. Heimann

Adsorption of divalent metal ions from aqueous solutions using graphene oxide

The adsorptive properties of graphene oxide (GO) towards divalent metal ions (copper, zinc, cadmium and lead) were investigated. GO prepared through the oxidation of graphite using potassium dichromate was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR). The results of batch experiments and measurements by flame atomic absorption spectrometry (F-AAS) indicate that maximum adsorption can be achieved in broad pH ranges: 3-7 for Cu(II), 5-8 for Zn(II), 4-8 for Cd(II), 3-7 for Pb(II). The maximum adsorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) on GO at pH = 5 are 294, 345, 530, 1119 mg g(-1), respectively. The competitive adsorption experiments showed the affinity in the order of Pb(II) > Cu(II) ≫ Cd(II) > Zn(II). Adsorption isotherms and kinetic studies suggest that sorption of metal ions on GO nanosheets is monolayer coverage and adsorption is controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO. Chemisorption was confirmed by XPS (binding energy and shape of O1s and C1s peaks) of GO with adsorbed metal ions. The adsorption experiments show that the dispersibility of GO in water changes remarkably after complexation of metal ions. After adsorption, the tendency to agglomerate and precipitate is observed. Excellent dispersibility of GO and strong tendency of GO-Me(II) to precipitate open the path to removal of heavy metals from water solution. Potential application of GO in analytical chemistry as a solid sorbent for preconcentration of trace elements and in heavy metal ion pollution cleanup results from its maximum adsorption capacities that are much higher than those of any of the currently reported sorbents.

X-ray examination, electrical and magnetic properties of R3Co single crystals (R≡Y, Gd, Dy and Ho)

Single crystals of Y3Co, Gd3Co, Dy3Co and Ho3Co were obtained by the Czochralski method from a levitated melt. A description of the crystal growth technology is given. The crystal quality was examined by X-ray methods. Electrical resistivity and magnetic susceptibility have been measured. Y3Co exhibits Curie-Weiss behaviour with μeff = 1 μB (formula unit)−1. It is the first evidence for the existence of a magnetic moment on cobalt atoms in such a low concentration region. Gd3Co, Dy3Co and Ho3Co show an effective moment that is enhanced by 1 μb relative to the theoretical value for R3Co, assuming that only the R3+ ions contribute a magnetic moment equal to the free ion value. A negative curvature of the temperature-dependent electrical resistivity ϱ(T) is observed in the lower temperature range. For higher temperatures dρdT is small, especially for Gd3Co (5.5 × 10−8Ω m K−1).

Electrical resistivity and magnetic properties of the Re1−xGdxAl2 laves phase (for Re: La, Lu, Y)

Abstract X-ray, magnetic, electrical resistivity and ESR studies of Re1-xGdxAl2 type compounds (Re - La, Lu, Y) were performed. The transition temperatures, effective moments, effective mass, lattice constants, crystallographic parameters, thermal broadening of resonance linewidth and g-factors were obtained. It was found that the simple RKKY model applied to describing the electrical resistivity and magnetic susceptibility in a large range of concentrations x seems to be right.

Electronic structure, magnetic and electrical properties of Fe3−xVxAl compounds

Abstract The electronic band structure, magnetic and transport properties of monocrystalline Fe3−xVxAl compounds with the B2-type structure were investigated. It was found that the magnetic and unusual transport properties are due to a strong hybridization of the 3d states of iron and vanadium and their shift to below the Fermi energy.

Crystal structure, magnetic and electrical properties of YFe4Al8 single crystals

Abstract Single crystals of YFe 4 Al 8 were obtained by the Czochralski method from a levitated melt. Their quality was checked by the Berg-Barrett and Laue methods. The electrical resistivity and the magnetic susceptibility were studied. Besides the antiferromagnetic transition at 195 K, weak ferromagnetism was observed below 100 K.

Investigation of CuxGayCrzSe4 single crystals

From X-ray photoelectron spectroscopy data and X-ray microprobe analysis the formula CuxGayCrzSe4 (x + y + z ≈ 3) for the investigated single crystals was determined. Replacement of Cu by Ga in tetrahedral sites of the spinel-type structure was observed. A negative chemical shift of the Cu 2p levels as well as an absence of Cu shake-up satellite lines was noticed. Tc dependency on the Ga content was found from susceptibility measurements. Effective and saturation magnetic moments at low concentrations of gallium (y < 0.16) were found to be higher than in CuCr2Se4. For gallium-rich samples the values of the moments decrease rapidly. The problem of metal valency in this system is discussed in relation to charge distribution. Covalency and stability of the spinel-type structure is also discussed.

Magnetic susceptibility and luminescence of α-ZnAl2S4 tiospinel doped with chromium

The magnetic susceptibility as well as the optical excitation and luminescence spectra of three samples of α-ZnAl2S4 tiospinel doped with Cr at various concentrations have been measured in the temperature range 77–300 K. The susceptibility has allowed us to estimate the concentration of Cr in the samples studied as lying in the range 0.01–0.06 at.%. Crystal-field theory, including spin–orbit coupling and the trigonal crystal field of the D3d site has been used to fit the optical data. The best-fit parameters of the tiospinel crystal are B20=4800, B40=32500, B43=−28500, F(2)=58800, F(4)=40500 and ξD=300 cm−1. The computational procedure used gives excellent agreement between the experimental and calculated energy levels of the Cr3+ ion in the α-ZnAl2S4 matrix.

Spin glass behaviour and magnetic anisotropy in DyFe4Al8 single crystals

Measurements of electrical resistivity, AC and DC magnetic susceptibilities of a DyFe4Al8 single crystal were performed in the temperature range 4.2–300K. A thermomagnetic effect was found below 30K. The strong anisotropy of the magnetic susceptibility occurs in the whole temperature range and is very pronounced at low temperatures. A T2-dependence of the electrical resistivity in the temperature range 4.2–100K and an antiferromagnetic transition at 185K were found. A cusp in the AC susceptibility was observed below 100K with a maximum at 30K.

Solid solubility of rare earths in aluminium

Abstract Monocrystalline and polycrystalline samples of Al:R (R = Y, Gd, Tb, Dy, Er, Ho) were prepared by arc and levitation melting and the Czochralski method from the levitated melt. It follows from the electrical and magnetic measurements as well as the X-ray examinations that the equilibrium solid solubility is less than 100 ppm. The residual resistivity has a maximum at about 150 ppm and then decreases dramatically with increasing concentration. This is explained by nucleation of the RAl3 phase.

Tc enhancement in Y-Ba-Cu-O by titanium addition

Abstract The critical temperatures of superconducting phases of nominal composition YBa 2 (Cu 1− x Ti x ) 3 O 7−δ were investigated for x in the range from 0 to 0.33. Measurements of the electrical resistivity, the magnetic susceptibility and the powder X-ray diffraction were performed. An enhancement of both T c (χ AC ) and T c (ϱ = 0) was found although the X-ray patterns and χ AC runs indicated that the samples were multiphase.