Physica C: Superconductivity and its Applications
Effects of TiC doping on the upper critical field of MgB2 superconductors
Introduction
The enhancement of upper critical field (Hc2) for MgB2 superconductors is crucial for magnet applications. The better results to improve the Hc2 of MgB2 are achieved in the carbon [1], [2], [3], [4], [5] or carbides [6], [7], [8] doped MgB2. Since carbon has one more electron than boron, it is expected that electrons are doped into the system by carbon substitution. The increase of Hc2 for the carbon doped MgB2 is attributed to its two-gap nature. An earlier work has shown that in C-doped MgB2 thin films the Hc2 (0 K) can exceed 70 T [5]. However, for C or SiC doped wires or bulks the Hc2 (0 K) is close to 35–40 T [3], [8]. The carbon solubility in MgB2 varies considerably depending on the synthesis route and starting materials. Mickelson et al. [8] achieved better mixing of B and C atoms using boron carbides B4C as a source of carbon. The samples had an estimated composition Mg(B0.9C0.1)2. Ribeiro et al. [9] reported that the carbon substitution level is about 10% in the nearly single-phase Mg(B1−xCx)2 sample, it is most likely solid solubility of carbon in polycrystalline MgB2 obtained under ambient pressure. Recently, Kazakov et al. [3] found that the carbon solubility can reach to 15% in the carbon doped single crystal MgB2 sample obtained under high pressure. However, the highest Hc2 in carbon doped MgB2 sample can be achieved by the carbon substitution around 3–5% per boron atom [3], [8]. In general, the MgB2 samples were prepared by sintering the mixed powders of Mg, B and dopants in previous experiments. The so-obtained polycrystalline MgB2 samples have a high porosity, which is a limit for promoting the increase of current density of MgB2 superconductors due to decreasing the effective carried current area. In addition, the nano-scale dopants were widely used in order to improve the reactivity of dopants. This causes the material costs to increase substantially due to the higher price for the nano-scale doping powders. To solve above problems we have developed the two-step reaction method. This method is used to fabricate the lower cost micro-scale C [10] and SiC [11] doped MgB2 bulks. The experimental results clearly demonstrate that this method could effectively increase both the sample density and reactivity of micro-scale dopants, and therefore improve the superconducting performance of MgB2.
In this work, we have investigated the doping effect of TiC on the superconducting properties of MgB2. The nano-TiC and the micro-TiC doped MgB2 bulks were obtained by single step reaction and two-step reaction methods, respectively. It is found that the Hc2 of the TiC doped MgB2 obtained by two-step reaction are improved significantly.
Section snippets
Experiments
Using the powders of magnesium (99%, −325mesh), amorphous boron (99.9%, 1–2 μm), and TiC (99.9%, <5 μm) as raw materials, the bulk Mg(B0.95(TiC)0.05)2 samples were prepared by two-step reaction method. In the first step, in order to decrease the Mg volatilization these powders were mixed with the stoichiometry of Mg(B0.95(TiC)0.05)4 (sample A) by mechanical grinding, and then pressed into pellet with the dimension of Ф2 × 0.5 cm under a pressure of 10 MPa. The pellet was put into a niobium crucible,
Results and discussion
Fig. 1 shows the XRD patterns for the sample A, B and C. It can be seen that for the sample A the MgB2 is a major phase, and small amount of MgB4, TiB2 and Mg2C3 are formed. This indicates that the reaction of 3Mg + 3TiC + 8B = MgB2 + 3TiB2 + Mg2C3 take places in the first step sintering reaction. Indeed, as reported by Yamamoto et al. [12] that the reaction between micro-TiC and Mg could occur when the sintering temperature is higher than 950 °C. The MgB4 are resulted from the decomposition of MgB2[13].
Summary
In summary we have investigated the doping effects of TiC on the superconducting properties of MgB2 superconductors. The nano-TiC and micro-TiC doped MgB2 bulks were prepared by single step reaction and two-step reaction, respectively. The SEM results indicated that the grain size of MgB2 in nano-TiC doped sample is smaller than that of micro-TiC doped sample. Improved Jc properties were observed for the nano-TiC doped MgB2 due to increase of grain boundaries. The XRD results indicated that the
Acknowledgments
This work is supported by the National Natural Science Foundation of China under the Contract No. 50472099, the National Basic Research Program of China under the Contract No. 2006CB601004 and the Doctorate Foundation of Northwestern Polytechnical University of China.
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