Josip Horvat

Highly ordered mesoporous cobalt oxide nanostructures: synthesis, characterisation, magnetic properties, and applications for electrochemical energy devices

Highly ordered mesoporous Co(3)O(4) nanostructures were prepared using KIT-6 and SBA-15 silica as hard templates. The structures were confirmed by small angle X-ray diffraction, high resolution transmission electron microscopy, and N(2) adsorption-desorption isotherm analysis. Both KIT-6 cubic and SBA-15 hexagonal mesoporous Co(3)O(4) samples exhibited a low Néel temperature and bulk antiferromagnetic coupling due to geometric confinement of antiferromagnetic order within the nanoparticles. Mesoporous Co(3)O(4) electrode materials have demonstrated the high lithium storage capacity of more than 1200 mAh g(-1) with an excellent cycle life. They also exhibited a high specific capacitance of 370 F g(-1) as electrodes in supercapacitors.

Effect of nano-carbon particle doping on the flux pinning properties of MgB2 superconductor

Abstract Polycrystalline MgB 2− x C x samples with x =0.05, 0.1, 0.2, 0.3 and 0.4 nano-particle carbon powder were prepared using an in situ reaction method under well-controlled conditions to limit the extent of C substitution. The phases, lattice parameters, microstructures, superconductivity and flux pinning were characterized by XRD, TEM, and magnetic measurements. It was found that both the a -axis lattice parameter and the T c decreased monotonically with increasing doping level. For the sample doped with the highest nominal composition of x =0.4 the T c dropped only 2.7 K. The nano-C-doped samples showed an improved field dependence of the J c compared with the undoped sample over a wide temperature range. The enhancement by C doping is similar to that of Si doping but not as strong as for nano-SiC-doped MgB 2 . X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg 2 C 3 and MgB 2 C 2 particles. Nano-particle inclusions and substitution, both observed by transmission electron microscopy, are proposed to be responsible for the enhancement of flux pinning in high fields.

Control of nano carbon substitution for enhancing the critical current density in MgB2

The effects on transition critical temperature, lattice parameters, critical current density, and flux pinning of doping MgB2 with carbon nanoparticles, were studied for bulk, wire and tape under a wide range of processing conditions. Under the optimum conditions, magnetic Jc was enhanced by two orders of magnitude at 5 K for a field of 8 T, and by a factor of 33 at 20 K for a field of 5 T for bulk samples, whereas enhancement by a factor of 5.7 was observed in the transport Ic at 12 T and 4.2 K for a wire sample. Samples sintered at high temperature (900 and 1000 °C) exhibited excellent Jc, approximately 10 000 A cm−2 in fields up to 8 T at 5 K. This result indicates that flux pinning was enhanced by the carbon substitution for B with increasing sintering temperature. Highly dispersed nanoparticles are believed to enhance the flux pinning directly, in addition to the introduction of pinning centres by carbon substitution. Nano-C is proposed to be one of the most promising dopants besides SiC and CNT for the enhancement of flux pinning for MgB2 in high fields.

High transport critical current density and large Hc2 and Hirr in nanoscale SiC doped MgB2 wires sintered at low temperature

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

Improving flux pinning of MgB2 by carbon nanotube doping and ultrasonication

Carbon nanotubes (CNTs) are an excellent candidate for introducing effective pinning centres and at the same time enhancing the upper critical field of MgB2. We report on the use of a low intensity ultrasonication as a method of dispersion of CNTs into precursor magnesium and boron powder. The ultrasonication improved homogenous mixing of CNTs with the MgB2 matrix. Ultrasonication of CNT doped MgB2 resulted in a significant enhancement in the field dependence of critical current density. The density of the sample increased due to the improved adherence between CNTs and MgB2 matrix. CNTs donate carbon that is substituted for boron in MgB2.

Effect of processing temperature on high field critical current density and upper critical field of nanocarbon doped MgB2

Correlation of upper critical field (Hc2) and critical current density (Jc) with processing temperature of nano-C doped MgB2 has been studied in comparison to SiC and pure MgB2. SiC and C doped MgB2 exhibit opposite trends in the dependence of Jc and Hc2 on sintering temperature. This is explained by different reactivities of carbon available upon creation of MgB2 for the two types of doping. Nanocarbon doped MgB2 requires sintering temperatures in excess of 900°C to obtain high boron substitution for carbon, enhancing the vortex pinning and impurity scattering of charge carriers. However, carbon substitution in nano-SiC doped MgB2 occurs at less than 650°C, allowing lower sintering temperature and high degree of carbon substitution. Both pure and SiC doped MgB2 benefit from low sintering temperature, which results in more grain boundary defects. Substantial carbon substitution can compensate for the disadvantage of sintering at high temperature of nano-C doped MgB2, giving the best Jc of 4.8×103A∕cm2 at ...

Magnetic field processing to enhance critical current densities of MgB2 superconductors

A magnetic field of up to 12T was applied during the sintering process of pure MgB2 and carbon nanotube (CNT) doped MgB2 wires. The authors have demonstrated that magnetic field processing results in grain refinement, homogeneity, and enhancement in Jc(H) and Hirr. The extent of improvement in Jc increases with increasing field. The Jc for a 10T field processed CNT doped sample increases by a factor of 3 at 10K and 8T and at 20K and 5T, respectively. Hirr for the 10T field processed CNT doped sample reached 9T at 20K, which exceeded the best value of SiC doped MgB2 at 20K. Magnetic field processing reduces the resistivity in CNT doped MgB2, straightens the entangled CNTs, and improves the adherence between CNTs and the MgB2 matrix.

Effects of enriched biochars containing magnetic iron nanoparticles on mycorrhizal colonisation, plant growth, nutrient uptake and soil quality improvement

Abstract At present, there is little commercial sale of biochar, since farmers find they can not gain a return on their investment in this amendment in the first few years after its application, because of the high cost associated with large application rates. To overcome this constraint, development of artificially aged enriched biochar-mineral complexes (BMCs), having a higher mineral content, surface functionality, exchangeable cations, high concentration of magnetic iron (Fe) nanoparticles, and higher water-extractable organic compounds has been undertaken by a combined team of researchers and a commercial company. Two biochars produced under different pyrolysis conditions were activated with a phosphoric acid treatment. A mixture of clay, chicken litter, and minerals were added to the biochar, and then this composite was torrefied at either 180 or 220 °C. In this study a pot experiment was carried out in glasshouse conditions to determine the effects of four different BMCs, with different formulations applied at rates of 100 and 200 kg ha−1, on the mycorrhizal colonisation, wheat growth and nutrient uptake, and soil quality improvement. It was found that the phosphorus (P) and nitrogen uptake in wheat shoots were significantly greater for a low application rate of BMCs (100 kg ha−1). The present formulation of BMC was effective in enhancing growth of wheat at low application rate (100 kg ha−1). The increase in growth appeared due to an increase in P uptake in the plants that could be partly attributed to an increase in mycorrhizal colonisation and partly due to the properties of the BMC.

Polyhedral magnetite nanocrystals with multiple facets: facile synthesis, structural modelling, magnetic properties and application for high capacity lithium storage.

Polyhedral magnetite nanocrystals with multiple facets were synthesised by a low temperature hydrothermal method. Atomistic simulation and calculations on surface attachment energy successfully predicted the polyhedral structure of magnetite nanocrystals with multiple facets. X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission microscopy confirmed the crystal structure of magnetite, which is consistent with the theoretical modelling. The magnetic property measurements show the superspin glass state of the polyhedral nanocrystals, which could originate from the nanometer size of individual single crystals. When applied as an anode material in lithium ion cells, magnetite nanocrystals demonstrated an outstanding electrochemical performance with a high lithium storage capacity, a satisfactory cyclability, and an excellent high rate capacity.

Effect of sample size on magnetic Jc for MgB2 superconductor

A strong effect of sample size on magnetic Jc(H) was observed for bulk MgB2 when Jc is obtained directly from the critical state model. Thus obtained zero-field Jc (Jc0) decreases strongly with the sample size, attaining a constant value for the samples larger than a few millimeters. On the other hand, the irreversibility field (Hirr) defined at Jc=100 A/cm2 increases with the sample size. The decrease of Jc0 is described in terms of voids in the bulk MgB2 samples and superconducting screening around the cells of superconducting material between these voids (35 μm), because of concentration of the current in the narrow bridges connecting the cells. For samples larger than a few millimeters, the value of magnetic Jc is in agreement with the transport Jc and it is restricted by the voids. The critical state model is not suitable for obtaining Jc for small bulk MgB2. The increase of Hirr with the sample size is an artifact of defining Hirr by the value of Jc at which an additional superconducting screening o...