Effects Of Carbon Substitution Research Articles

Stoichiometry dependence of the critical current density in pure and nano-SiC doped MgB 2/Fe tapes

The stoichiometry dependence of the microstructure and superconducting properties of pure and nano-SiC doped MgB 2/Fe tapes was systematically investigated. The tapes prepared with the composition of a slight deficiency of Mg (Mg 0.9B 2 and Mg x B 2(SiC) 0.1 ( x = 0.9 and 1.0)) showed the best transport J c. Adding a slight excess of Mg like 5%, as being done by many researchers, was not beneficial for the J c improvement as expected. The onset T c was not significantly changed in both doped and undoped tapes by adding excess of boron or magnesium, but the transition widths were broadened due to the induced impurities. The slightly-Mg-deficient pure samples show smaller grain sizes, which corresponds to a better J c– B performance at high magnetic field due to the enhanced grain boundary pinning. The field dependence of J c in nano-SiC doped tapes was almost not influenced by varying the starting Mg content although microstructural difference can still be seen, suggesting that the flux pinning ability was mainly controlled by the carbon substitution effect for boron.

ＭｇＢ２超伝導体の性質とその可能性 ＭｇＢ２超伝導体の臨界電流特性における炭素置換効果

The relationship between the reactivity, doping effects of metal carbide and superconducting properties of MgB2 were studied to investigate the intrinsic carbon substitution effects on the critical current properties of MgB2. Decreases in Tc and a-axis length of MgB2 due to the carbon substitution were observed. We found that only a part of the carbon in nominal composition was actually substituted into the MgB2 lattice and that not nominal but actual carbon content in MgB2 lattice played an essential role for determining the lattice properties and superconductivity of MgB2. Improved field dependence of Jc was observed in carbide doped MgB2 bulk samples especially at low-temperatures under high magnetic fields. On the other hand, surplus doping of carbide was found to decrease Jc due to serious suppression of Tc and a reduction in the effective current path. These findings suggested that optimizing the actual carbon content by selecting suitable carbon source dopants and heating conditions is important for improving the critical current properties of superconducting MgB2 materials.

Effect of SiC nanoparticle addition on the critical current density ofMgB2 tapesfabricated from MgH2, B and MgB2 powder mixtures

The effects of the addition of SiC nanoparticles were compared forMgB2 tapesprepared from two different starting powders with respect to the improvement in the critical current, Jc, at 4.2 K and 10 T. The starting powder was prepared by blending premixedMgH2 andB(MgH2+2B) andpremixed (MgH2+2B) and MgB2 (30% MgB2) in a ratio of 7:3. All tapes were fabricated by the in situ powder-in-tube method. The use of30% MgB2 as standard powderyields higher Jc valuesthan MgH2+2B in the caseof the pure MgB2 tape;however, the highest Jc (1.8 × 104 A cm−2) was obtainedfor the tape from MgH2+2B with 10 mol% SiC additive. The tape from30% MgB2 contained regionswith Si-free MgB2 grains, while Si compounds were distributed homogeneously across the tape prepared fromMgH2+2B. Furthermore,the use of 30% MgB2 instead of MgH2+2B was found to reduce the effect of carbon substitution of the SiCadditive. These data indicate that the technique, which enhancesJc–B characteristicsof the pure MgB2 tape, does not promise further improvement through the addition of nanosized impurityparticles. It is concluded that we should improve the fabrication technique of theMgB2 tape by keeping (or improving) the overall reactivity between the standard powder andnanosized impurity particles.

Carbon-substitution effect on vortex order-disorder transition inMgB2single crystals

We have investigated the carbon substitution effect on the peak effect of $\mathrm{Mg}{\mathrm{B}}_{2}$ single crystals. The peak effect originates from the order-disorder transition of vortices, so that disorders introduced by carbon substitution are expected to have strong influence on the vortex phase diagram. In fact, a systematic study of the rf penetration depth ${\ensuremath{\lambda}}_{\mathrm{rf}}$ has revealed that the order-disorder transition line monotonously shifts to the low-field region with carbon content in $H\ensuremath{\Vert}ab$ and $c$. The results are discussed in terms of the effect of weak point disorder on the vortex order-disorder transition.

Carbon-substitution effect on superconducting properties in MgB 2 single crystals

The superconducting properties of carbon-substituted MgB 2 single crystals are studied. We found that C-substitution modifies superconducting properties such as anisotropy and irreversibility field. The highest H c2 can be achieved by the C-substitution around 3–5% per B-atom. In the superconductivity transition, the resistivity is almost ohmic; this is remarkable contrast to the case of C-free crystals.

Carbon-substitution effect on superconducting properties in MgB2 single crystals

The superconducting properties of carbon-substituted MgB2 single crystals are studied. We found that C-substitution modifies superconducting properties such as anisotropy and irreversibility field. The highest Hc2 can be achieved by the C-substitution around 3–5% per B-atom. In the superconductivity transition, the resistivity is almost ohmic; this is remarkable contrast to the case of C-free crystals.

Sm(Co, Fe, Cu, Zr, C)8.2 ribbons for high-temperature magnets

Abstract The effect of carbon substitution for cobalt on the structure and magnetic properties at room and high temperature of melt spun ribbons with composition Sm(Co0.86−xFe0.1Zr0.04Cx)8.2 and Sm(Co0.74−xFe0.1Cu0.12Zr0.04Cx)8.2 (x=0−0.015) has been studied. Arc-melted bulk samples have Th2Ni17-type structure while ribbons have TbCu7-type structure. Average grain size for Cu-free sample with x=0.005 is 23 nm while for Cu-doped samples it varies from 53 to 22 nm when x varies from 0.005 to 0.015, respectively. Coercive as-spun samples have Hc∼2.2–7.8 kOe and reduced remanence mr=Mr/Ms up to 0.74. Ribbons of the Cu-free composition with x=0 and 0.005 and annealed at 750°C for 1 h have TbCu7-type structure with coercivity values 4.9 and 3.0 kOe respectively, while for x=0.01 and 0.015, FCC-Co is the dominant phase. Cu-doped ribbons with x=0.005, after annealing at 750°C for 2 h, are characterized at room temperature by square hysteresis loop with Hc=8.3 kOe, reduced remanence mr=0.75 and (BH)max=6.3 MGOe. Loops at higher temperatures are also square with high mr; for x=0.01 the magnetization is 89 emu/g and the coercive field is 1.8 kOe at 400°C.

Effect of carbon substitution on the magnetic properties of Nd–Fe–(B,C) nanocomposite magnets

The purpose of the present work is to study the effect of carbon substitution for boron on the magnetic and structural properties in nanocomposite Nd 10Fe 82B 8− x C x , ( x=0, 2, 4, 5, 6, 7) ribbons in correlation with their crystallization behavior. Four different structural states were observed in as-spun ribbons depending on the carbon content x and the wheel speed: (i) Nd 2Fe 14(B,C)+α-Fe, (ii) Nd 2Fe 14(B,C)+Nd 2Fe 17C x +α-Fe, (iii) Nd 2Fe 17C x +α-Fe and (iv) amorphous. In annealed ribbons, the 2:17:C x phase transforms into the 2:14:1, whereas the amount of α-Fe phase increases with x. The coercivity has a maximum of 8.7 kOe for x=2, followed by a gradual decrease with further increase of carbon concentration. The deterioration of the magnetic properties with x can be associated with the change in the crystallization behavior and the coarsening of the microstructure for x>4.

The effects of carbon substitution on the behavior of hydrogen in Nd–Fe–B-type magnetic alloys

In the present study, the hydrogen absorption and desorption behavior of Nd–Fe–B-type alloys with the substitution of carbon for boron was investigated focusing on the disproportionation and recombination reaction of the hydrogenation, disproportionation, desorption and recombination (HDDR) process. Furthermore, the HDDR treatment of carbon-substituted Nd–Fe–B alloy was carried out. It was found that the most distinct effect of carbon substitution for boron in Nd–Fe–B-type alloy is the lowering of the starting temperature of disproportionation reaction in the HDDR process. The HDDR-treated Nd 12.6Fe bal.Co 11.6Ga 1.0Zr 0.1B 4.0C 2.0 powder shows better magnetic properties than the Nd 12.6Fe bal.Co 11.6Ga 1.0Zr 0.1B 6.0 powder at short disproportionation reaction time. This may be due to the rapid disproportionation of Nd 2Fe 14(B,C) phase at the disproportionation reaction temperature.

Theory of the (3×2) reconstruction of the GaAs(001) surface

We discuss an ab initio density functional theory investigation of a (3×2) reconstruction of the Ga-rich GaAs(001) surface. Recent experiments identified this new reconstruction as a stable surface of epitaxially grown GaAs doped with indium or carbon (L. Li, et al., Appl. Phys. A 66 (1998) S501; L. Li, et al., Ultramicroscopy 73 (1998) 229). Using our recently developed real-space pseudopotential density functional code, we investigate this (3×2) reconstruction and evaluate stabilization mechanisms for the model proposed by the experimentalists, which does not satisfy the electron counting rule. In order to study the effect of In substitution, we consider the dependence of the surface energies of competing reconstructions on surface strain and relative chemical potentials. The effects of carbon substitution are modeled using a charged (3×2) reconstruction. These studies support and clarify the proposed structure and stabilization mechanisms. We provide a simple physical explanation for the behavior of the surface energy of the (3×2) reconstruction.

Magnetic properties of Nd/sub 2/Fe/sub 13.8/Mo/sub 0.1/Nb/sub 0.1/B/sub 1-x/C/sub x/ alloys

The effect of carbon substitution on the magnetic properties of Nd/sub 2/Fe/sub 13.8/Mo/sub 0.1/Nb/sub 0.1/B/sub 1-x/C/sub x/ (with x=0, 0.2, 0.4, 0.6, 0.8) was studied. The anisotropy field was measured from 77 K to 379 K using the singular point detection technique. It was found that both the anisotropy field and the critical field increase with increasing carbon content. A slight decrease of both Curie temperature and saturation magnetization were observed. The increase of anisotropy field may be due to a variation of the crystal field behavior due to Nd ions when boron is substituted by carbon. The increase of critical field with increasing carbon content may be due to the variation of high-order crystalline electric field terms. >

Bipolarons in boron-rich icosahedra: Effects of carbon substitution.

Analysis of electronic-transport and magnetic measurements on boron carbides has indicated that charge transport occurs via bipolaron hopping between the twelve-atom icosahedral structural units of these solids. Two types of isoelectronic icosahedral units are neutral icosahedral boron clusters (${\mathrm{B}}_{12}$ icosahedra) and positively charged clusters of eleven boron atoms and a carbon atom [(${\mathrm{B}}_{11}$C${)}^{+}$ icosahedra]. In particular, the transport analysis assumes that it is energetically favorable for an electron pair to occupy a (${\mathrm{B}}_{11}$C${)}^{+}$ icosahedron rather than a ${\mathrm{B}}_{12}$ icosahedron. Here, we study the energy changes and atomic displacements associated with adding two electrons to a (${\mathrm{B}}_{11}$C${)}^{+}$ icosahedron in neutral surroundings and compare our results with our previous study of two electrons added to an analogous ${\mathrm{B}}_{12}$ icosahedron. We find that the addition of the two electrons reduces the (${\mathrm{B}}_{11}$C${)}^{+}$ system's energy by about 18 eV.Most of this energy reduction, \ensuremath{\simeq}16--17 eV, is associated with the two electrons filling bonding orbitals associated with all of the atoms of the positively charged icosahedron. The remainder of the energy reduction is associated with the polaronic displacements, the contraction of the icosahedron induced by the placing of the two electrons in icosahedral bonding orbitals. Indeed, the energy reduction associated with adding two electrons to form a (${\mathrm{B}}_{11}$C${)}^{\mathrm{\ensuremath{-}}}$ icosahedron is greater than that associated with adding two electrons to form a ${\mathrm{B}}_{12}$${\mathrm{}}^{2\mathrm{\ensuremath{-}}}$ icosahedron by about 14 eV. This is because the increased Coulombic attraction of the two electrons arising from the extra positive charge provided by the carbon nucleus exceeds the increases in electronic kinetic energy and electron-electron repulsion energy associated with the icosahedron's reduced symmetry. Thus, it is energetically preferable for an electron pair to occupy a (${\mathrm{B}}_{11}$C${)}^{+}$ icosahedron rather than a ${\mathrm{B}}_{12}$ icosahedron. This supports the notion, arising from analysis of electronic transport experiments, that bipolaron hopping occurs between ${\mathrm{B}}_{11}$C icosahedra.

Effect of carbon substitution on the specific heat of Fe 80B 20− xC x (0≤ x≤8)

The specific heat of ferromagnetic metallic glasses Fe 80B 20− x C x (0≤ x≤8) has been measured in the temperature range between 1.5 and 10 K. It is found that the electronic specific heat coefficient is independent of the carbon concentration x. The Debye temperature has a broad peak around x = 4. Both these results are in sharp contrast to the case of FeB binary metallic glasses and could be qualitatively understood in terms of magnetic and structural properties.