Željko Skoko

Some factors influencing forced hydrolysis of FeCl3 solutions

Abstract Phase composition, size and shape of iron oxide particles produced by forced hydrolysis of Fe3+ ions depended on the concentrations of FeCl3 and HCl, and on the presence of 0.005 M quinine hydrogen sulphate (QHS). After 7 days of forced hydrolysis, α-Fe2O3 was produced from the 0.01 M FeCl3 solution, the mixture of β-FeOOH and α-Fe2O3 was produced from 0.02 M FeCl3 solution, whereas β-FeOOH was produced from the 0.1 M FeCl3 solution. Forced hydrolysis of the 0.1 M FeCl3+0.1 M HCl solution yielded α-Fe2O3. In the presence of 0.005 M QHS, the formation of β-FeOOH and α-Fe2O3 was partially or completely suppressed in dependence on the conditions of forced hydrolysis of Fe3+ ions. Specific shapes of β-FeOOH particles (cigar-type, X-, Y-, and star-shapes) were not obtained in the presence of 0.005 M QHS. The bundles of β-FeOOH needles with irregular ends were obtained instead. Also, in the presence of 0.005 M QHS, the formation of α-FeOOH is favoured. The preferential adsorption of sulphate groups and a possible influence of quinine group at the very beginning of FeCl3 hydrolysis were suggested as responsible for the effects observed.

Effects of initial supersaturation on spontaneous precipitation of calcium carbonate in the presence of charged poly-L-amino acids.

Spontaneous precipitation of calcium carbonate was investigated in two precipitation systems: (1) with initial supersaturation lower than that corresponding to the solubility of amorphous calcium carbonate (ACC), at which vaterite precipitated, and (2) with initial supersaturation higher than that of ACC solubility, at which a mixture of calcite and vaterite was formed. After the addition of an acidic polypeptide, poly-L-glutamic acid (pGlu) or poly-L-aspartic acid (pAsp), into (1) a significant inhibition of nucleation, expressed as an increase in induction time, and growth of vaterite, perceived as a dead zone, was observed. Extent of inhibition decreased in the order: Inh(pAps)>Inh(pGlu)>>Inh(pLys). The addition of a polypeptide into (2) caused the inhibition of precipitation and changed the morphology and polymorphic composition of the precipitate; only vaterite appeared at approximately c(pAsp)=3 ppm, c(pGlu)=6 ppm, or c(pLys)=7 ppm. This finding is explained as a consequence of kinetic constraints through the inhibition of calcite nucleation and stronger binding of acidic polypeptide by the calcite surfaces than by the vaterite surfaces. Laboratory precipitation studies using conditions that resemble those in living organism should be run at an initial supersaturation corresponding to the solubility of ACC as a limiting condition.

Long cycle life of CoMn2O4 lithium ion battery anodes with high crystallinity

CoMn2O4 nanomaterials are prepared by a low temperature precipitation route employing metal acetates and NaOH. Structural changes, induced by different annealing temperatures, are comprehensively analyzed by X-ray powder diffraction and Raman spectroscopy. With rising annealing temperature the crystal lattice of CoMn2O4 undergoes changes ; AO4 tetrahedra expand due to thermally induced substitution of Co2+ by larger Mn2+ metal ions on the A-site of the spinel structure, while in contrast, BO6 octahedra shrink since the B-site becomes partially occupied by smaller Co3+ metal ions on account of the migrated Mn ions. CoMn2O4 particle sizes are easily fine-tuned by applying different annealing temperatures ; the particle size increases with increasing annealing temperature. During the battery operation, pulverization and reduction of particle sizes occurs regardless of the initial size of the particles, but the degree of division of the particles during the operation is dependent on the initial particle properties. Thus, contrary to the common assumption that nanostructuring of the anode material improves the battery performance, samples with the largest particle sizes exhibit excellent performance with a capacity retention of 104% after 1000 cycles (compared to the 2nd cycle).

Enhancing superconducting properties of MgB2 pellets by addition of amorphous magnetic Ni–Co–B nanoparticles

Amorphous magnetic Ni–Co–B nanoparticles with an average size of 5 nm were added to precursor powders of MgB2 superconductor. The preparation procedure for MgB2 pellets was optimized for obtaining the best critical current density (Jc) at elevated magnetic fields. Addition of Ni–Co–B decreases the Jc for heat treatment of precursor powders at 650 ° C. Heat treatments at 770 ° C and higher improve Jc at 20 and 5 K. This improvement occurs at both temperatures through the increase of the effective connectivity between MgB2 crystals. Vortex pinning was enhanced at 5 K, but not at 20 K. Ni–Co–B nanoparticles reacted with Mg in heat treatments above 730 ° C, forming Mg2Ni and MgCo2 nanoparticles. Ni–Co–B addition was associated with lower oxygen content in MgB2, indicating that reduction of MgO content is the mechanism for improvement of grain connectivity. Decomposition of magnetic Ni–Co–B nanoparticles results mostly in non-magnetic nanoparticles, so magnetic pinning did not occur in our samples.

Effect of magnetic NiCoB nanoparticles on superconductivity in MgB2 wires

A systematic study of the influence of doping MgB2 with single domain magnetic nanoparticles of NiCoB alloy, uncoated and coated with SiO2, has been performed. Electrical resistivity, transport critical current density, Jc(B,T), and magnetization of well characterized undoped and doped with 1.38 and 2.67 wt% of NiCoB particles (both uncoated and coated) MgB2 wires have been investigated in the temperature interval 2–300 K and in magnetic field B ≤ 16T. The superconducting transition temperature, Tc, decreases approximately linearly with the amount of dopand and the intergranular connectivity (the active cross-sectional area fraction, AF) is also reduced upon doping. Reduction of critical fields (irreversibility field, Birr, and upper critical field, Bc2) of doped wires was observed in the whole temperature interval, but an enhancement of Jc of doped wires with respect to the undoped one was observed at low temperature (5 K). Common scaling of Jc(B,T) curves, Birr(T) and volume pinning force, Fp, for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples.

Graphene-oxide-wrapped ZnMn2O4 as a high performance lithium-ion battery anode

Cation distribution between tetrahedral and octahedral sites within the ZnMn2O4 spinel lattice, along with microstructural features, is affected greatly by the temperature of heat treatment. Inversion parameter can easily be tuned, from 5 to 19%, depending on the annealing temperature. The upper limit of inversion is found for T= 400 °C as confirmed by X-ray powder diffraction and Raman spectroscopy. Excellent battery behavior is found for samples annealed at lower temperatures; after 500 cycles the specific capacities for as-prepared ZnMn2O4 is 909 mAh/g, while ZnMn2O4 heat-treated at 300 °C shows 1179 mAh/g which amounts to 101 % of its initial capacity. Despite excellent performance of sample processed at 300 °C at lower charge/discharge rates (100 mAh/g), a drop in the specific capacity is observed with rate increase. This issue is solved by graphene oxide wrapping; specific capacity obtained after 400th cycle for graphene oxide wrapped ZnMn2O4 heat-treated at 300 °C is 799 mAh/g at charge/discharge rate 0.5 A/g, which is higher by factor 6 compared to sample without graphene oxide wrapping.

Enhancement of critical fields and current of MgB2 by co-doping

The electromagnetic properties of well-characterized iron-sheathed MgB2 wires, undoped and doped with dextrin-coated magnetite nanospheres and nanorods, have been studied in the temperature range 5?300?K and a magnetic field up to 16?T. Doping hardly affected the superconducting transition temperature and the active cross-sectional area of the wires, and increased the low temperature upper critical field, Bc2. Wire doped with nanospheres also showed enhanced irreversibility field, Birr, and low temperature (T???15?K) critical current density. Magnetization measurements generally confirm the transport results and indicate an enhancement of flux pinning in nanosphere doped wire for T???20?K. Annealing of wire doped with nanorods at higher temperature (750?? C) enhanced its critical fields, as expected for co-doping.

Flux pinning and inhomogeneity in magnetic nanoparticle doped MgB2/Fe wires

The effects of magnetic nanoparticle doping on superconductivity of MgB2/Fe wires have been investigated. Fe2B and SiO2-coated Fe2B particles with average diameters 80 and 150 nm, respectively, were used as dopands. MgB2 wires with different nanoparticle contents (0, 3, 7.5, 12 wt.%) were sintered at temperature 750°C. The magnetoresistivity and critical current density Jc of wires were measured in the temperature range 2–40 K in magnetic field B ≤ 16 T. Both transport and magnetic Jc were determined. Superconducting transition temperature Tc of doped wires decreases quite rapidly with doping level (~ 0.5 K per wt.%). This results in the reduction of the irreversibility fields Birr(T) and critical current densities Jc(B,T) in doped samples (both at low (5 K) and high temperatures (20 K)). Common scaling of Jc(B,T) curves for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples. Rather curved Kramers plots for Jc of doped wires imply considerable inhomogeneity.

Electronic structure and glass forming ability in early and late transition metal alloys

Abstract A correlation between the change in magnetic susceptibility (Δχexp) upon crystallisation of Cu–Zr and Hf metallic glasses (MG) with their glass forming ability (GFA) observed recently, is found to apply to Cu–Ti and Zr–Ni alloys, too. In particular, small Δχexp, which reflects similar electronic structures, ES, of glassy and corresponding crystalline alloys, corresponds to high GFA. Here, we studied Δχexp for five Cu–Ti and four Cu–Zr and Ni–Zr MGs. The fully crystalline final state of all alloys was verified from X-ray diffraction patterns. The variation of GFA with composition in Cu–Ti, Cu–Zr and Cu–Hf MGs was established from the variation of the corresponding critical casting thickness, dc. Due to the absence of data for dc in Ni–Zr MGs their GFA was described using empirical criteria, such as the reduced glass transition temperature. A very good correlation between Δχexp and dc (and/or other criteria for GFA) was observed for all alloys studied. The correlation between the ES and GFA showed up best for Cu–Zr and NiZr2 alloys where direct data for the change in ES (ΔES) upon crystallisation are available. The applicability of the Δχexp (ΔES) criterion for high GFA (which provides a simple way to select the compositions with high GFA) to other metal-metal MGs (including ternary and multicomponent bulk MGs) is briefly discussed.

Thermal behavior of novel catanionic cholates XRPD technique in solving problems

In this article, we bring new insight into room temperature structure of catanionic cholates and complement their thermal behavior given by the conventional thermal techniques with the XRPD technique. The comparative study of the addition of each dodecyl chain and ammonium group is made bearing in mind the complete architecture of synthesized cholates. The examined samples are crystal smectic phases at room temperature, with proposed sandwich-type structure, promoted by cholates architecture. For most of the studied compounds, thermal behavior is characterized as formation of structural varieties and/or polymorphs as low-temperature phases and formation of high-temperature mesomorphic, lamellae-like phases. The exception is dimeric dicholate, which only forms SmA phase before its decomposition. The dependence of the isotropization temperatures, enthalpy and entropy changes, on the increasing ammonium headgroup number, points to the fact that thermal stability of these catanionics depends on the structure of cationic component that is its constituent, where cholate anion shows minor effect. The chemistry of amphiphiles, their supramolecular behavior and thermotropic affinity is at the frontier of the contemporary research and design of the new functional materials, because this is simple but effective way to control the nature and location of reactions. From that point of view, the systematic analysis of physico-chemical properties of various catanionic amphiphiles brings new findings of their chemical structure–properties relationship, therefore enabling simpler and reliable way of new materials synthesis with desired properties.