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.

No abstract

We evaluated the additive effects of malic acid (C 4 H 6 O 5 ), from 0 to 30 wt% of the total MgB 2 , on the lattice parameters, lattice strain, amount of carbon (C) substitution, microstructures, weight fraction of MgO, critical temperature (T c ), critical current density (J c ), and irreversibility field (H irr ) of a MgB 2 superconductor. The calculated lattice parameters show a large decrease in the a-axis lattice parameter for MgB 2 + C 4 H 6 O 5 samples from 3.0861(6) to 3.0736(1) Å, with even a 10 wt% addition. This is an indication of C substitution into boron sites, with the C coming from C 4 H 6 O 5 , resulting in enhancement of J c and H irr . Specifically, the H irr of the MgB 2 + C 4 H 6 O 5 samples prepared by the chemical solution route reached around 7 T at 20 K, with a T c reduction of only 1.5 K. In addition, the self-field J c of the MgB 2 + C 4 H 6 O 5 samples was only slightly reduced at an additive level as high as 30 wt%. However, residual oxygen after evaporation processing contributed to a large amount of MgO in our MgB 2 + 30 wt% C 4 H 6 O 5 samples. These problems can be further controlled by the amount of C 4 H 6 O 5 additive or different evaporation temperatures.

We succeeded in simple and rapid synthesis of surface-modified monoclinic ZrO 2 nanoparticles using a supercritical hydrothermal method. The precursor Zr(OH) 4 was treated in the presence of various surface modifiers with carboxyl group (-COOH) in a batch-type reactor at 400 C for 10 min. Oleic, sebacic, dodecanedioic, and 12-aminododecanoic acids were used as surface modifiers. Addition of surface modifiers resulted in smaller particle (crystallite) sizes than the unmodified nanoparticles, suggesting that there is an interaction between the surface modifiers and the nanoparticles. The reduced particle size in the presence of surface modifiers was attributed to the inhibition of the growth of the crystalline surface due to the surface modification. The FT-IR spectra revealed that the surface modifiers were attached to the surface of the nanoparticles through coordination bonds between the carboxylate group (-COO À ) and the Zr ion. The FT-IR spectra also confirmed the presence of functional groups, such as methyl (-CH 3 ), carboxyl (-COOH), and amine (-NH 2 ), at the surface. The surface modification was also verified by the thermogravimetric analysis. The number of the surface modifiers attached to the surface of the products was about 2 molecules per nm 2 . The nanoparticles with carboxyl and amine surface functional groups were water dispersible; the isoelectric point shifted to low pH ranges because of the nature of the groups.

We report on comprehensive measurements of the magnetic, transport, and thermal properties of the Heusler type compound Fe 2+x V 1−x Al at −0.05 x 020. We show that while stoichiometric Fe 2 VAl is a nonmagnetic semimetal (or narrow-gap semiconductor), a substitution on the nominal V-sites with the Fe atom leads to a ferromagnetic ground state above x = x c (∼0.05) with a rising Curie temperature T C and an ordered moment M s . At x = 0.1 and 0.2, the reduced value of the ratio M s /P eff 1, where P eff is the effective Curie-Weiss moment, together with the analysis of the magnetization data M(H ,T ), shows magnetism is itinerant. At a higher temperature T ∼ 60 K, a Schottky anomaly in specific heat C is indicated prominently at x > 0, while the anomaly is observable in the whole experimental range of x. At a lower temperature, an electronic component C/T in specific heat shows a divergence that arises at both x < 0 and x > 0. The resistivity-temperature curve ρ(T ) in the vicinity of the ferromagnetic quantum critical point, x ∼ 0.05, shows a non-Fermi liquid behavior, ρ ∼ T n (n ∼ 1), above H > 20 kOe.

No abstract

A comprehensive study of the effects of carbohydrate doping on the superconductivity of MgB 2 has been conducted. In accordance with the dual reaction model, more carbon substitution is achieved at lower sintering temperature. As the sintering temperature is lowered, lattice disorder is increased. Disorder is an important factor determining the transition temperature for the samples studied in this work, as evidenced from the correlations among the lattice strain, the resistivity, and the transition temperature. It is further shown that the increased critical current density in the high field region can be understood by a recentlyproposed percolation model [M. Eisterer et al., Phys. Rev. Lett. 90, 247002 (2003)]. For the critical current density analysis, the upper critical field is estimated from a correlation that was reported in a recent review article [M. Eisterer, Supercond. Sci. Technol. 20, R47 (2007)], where a sharp increase in the upper critical field by doping is mainly due to an increase in lattice disorder or impurity scattering. On the other hand, it is shown that the observed reduction in self-field critical current density is related to the reduction in the pinning force density by carbohydrate doping. KeywordsCorrelation, between, doping, induced, disorder, superconducting, properties, carbohydrate, doped, MgB2 Disciplines Engineering | Physical Sciences and Mathematics Publication DetailsKim, J, Dou, SX, Oh, S, Jercinovic, M, Babic, E, Nakane, T & Kumakura, H (2008), Correlation between doping induced disorder and superconducting properties in carbohydrate doped MgB2, Journal of Applied Physics, 104(6), 063911-1-063911-5. AuthorsJung Ho Kim, S X. Dou, Sangjun Oh, M Jercinovic, E Babic, T Nakane, and Hiroaki Kumakura A comprehensive study of the effects of carbohydrate doping on the superconductivity of MgB 2 has been conducted. In accordance with the dual reaction model, more carbon substitution is achieved at lower sintering temperature. As the sintering temperature is lowered, lattice disorder is increased. Disorder is an important factor determining the transition temperature for the samples studied in this work, as evidenced from the correlations among the lattice strain, the resistivity, and the transition temperature. It is further shown that the increased critical current density in the high field region can be understood by a recently-proposed percolation model ͓M. Eisterer et al., Phys. Rev. Lett. 90, 247002 ͑2003͔͒. For the critical current density analysis, the upper critical field is estimated from a correlation that was reported in a recent review article ͓M. Eisterer, Supercond. Sci. Technol. 20, R47 ͑2007͔͒, where a sharp increase in the upper critical field by doping is mainly due to an increase in lattice disorder or impurity scattering. On the other hand, it is shown that the observed reduction in self-field critical current density is related to the reduction in the pinning force density by carbohydrate doping.

No abstract