Paul G Munroe

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2−x(SiC)x/2 with x=0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4×105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because o...

Shifting paradigms: development of high-efficiency biochar fertilizers based on nano-structures and soluble components

Many biochars have a complex carbon lattice structure with aromatic and aliphatic domains, acidic and basic groups, vacancies, metallic and non-metallic elements, and free radicals. Biochars also have separate mineral oxide, silicate and salt phases, and small and large organic molecules. In the rhizosphere, such constituents can be involved in chemical and biological processes along a soil–microbe–plant continuum, including nutrient cycling, metal chelation and stabilization, redox reactions, and free radical scavenging. It is hypothesized that the greater the amount of these nanoparticles and dissolved components, the greater will be plant and microbial responses. We provide suggestions for developing low-dose, high-efficiency biochar–nanoparticle composites, as well as initial field trial results and detailed characterization of such a biochar–fertilizer composite, to highlight the potential of such biochars.

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.

Improved irreversibility behavior and critical current density in MgB2-diamond nanocomposites

MgB2-diamond nanocomposite superconductors have been synthesized by addition of nanodiamond powder. Microstructural analysis shows that the nanocomposite superconductor consists of tightly packed MgB2 nanograins (∼50–100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (∼10–20 nm) inside the grains. The Jc–H and Hiir–T characteristics have been significantly improved in this MgB2-diamond nanocomposite, compared to MgB2 bulk materials prepared by other techniques. Also, the Jc value of 1×104 A/cm2 at 20 K and 4 T and the Hirr value of 6.4 T at 20 K have been achieved.

Doping effect of nano-diamond on superconductivity and flux pinning in MgB2

The doping effect of diamond nanoparticles on the superconducting properties of MgB2 bulk material has been studied. It is found that the superconducting transition temperature Tc of MgB2 is suppressed by the diamond doping, however, the irreversibility field Hirr and the critical current density Jc are systematically enhanced. Microstructural analysis shows that the diamond-doped MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50–100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (~10–20 nm) inside the grains. The high density of dislocations and diamond nanoparticles may be responsible for the enhanced flux pinning in the diamond-doped MgB2.

Transport critical current density in Fe-sheathed nano-SiC doped MgB/sub 2/ wires

The nano-SiC doped MgB/sub 2//Fe wires were fabricated using a powder-in-tube method and an in-situ reaction process. The depression of T/sub c/ with increasing SiC doping level remained rather small due to the counterbalanced effect of Si and C co-doping. The high level SiC co-doping allowed creation of the intra-grain defects and nano-inclusions, which act as effective pinning centers, resulting in a substantial enhancement in the J/sub c/(H) performance. The transport J/sub c/ for all the wires is comparable to the magnetic J/sub c/ at higher fields despite the low density of the samples and percolative nature of current. The transport I/sub c/ for the 10wt% SiC doped MgB/sub 2//Fe reached 660A at 5K and 4.5T (J/sub c/=133000A/cm/sup 2/) and 540A at 20K and 2T (J/sub c/=108000A/cm/sup 2/). The transport J/sub c/ for the 10wt% SiC doped MgB/sub 2/ wire is more than an order of magnitude higher than for the state-the-art Fe-sheathed MgB/sub 2/ wire reported to date at 5K and 10T and 20K and 5T respectively. There is a plenty of room for further improvement in J/sub c/ as the density of the current samples is only 50%.

Comparison between nano-diamond and carbon nanotube doping effects on critical current density and flux pinning in MgB2

Doping effects of nano-diamond and carbon nanotubes (CNTs) on critical current density of bulk MgB2 have been studied. CNTs are found prone to be doped into the MgB2 lattice whereas nano-diamond tends to form second-phase inclusions in the MgB2 matrix, leading to a more significant improvement of Jc(H) by doping by nano-diamond than by CNTs in MgB2. TEM reveals tightly packed MgB2 nanograins (50–100 nm) with a dense distribution of diamond nanoparticles (10–20 nm) inside MgB2 grains in nano-diamond-doped samples. Such a unique microstructure leads to a flux pinning behaviour different from that in CNTs-doped MgB2.