2672 IEEE TRANSACTIONS ON APPLIED …

1 2672 ieee TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011. Effects of the Size of the Doped SiC Nanoparticles on the Critical Current Density of the Ti-Sheathed MgB2. Superconducting Wires G. Liang, H. Fang, Z. P. Luo, S. Keith, and C. Hoyt Abstract The effects of the grain size of the doped SiC nanopar- structures could react differently with the Mg and B powder ticles on the critical current density ( c ) of the Ti-sheathed MgB2 in the precursor to form different impurities [5], the compli- superconducting wires were studied. The concentration of the SiC cating factors for the formation of the impurities (some of them dopant was 10% and the sizes of the SiC particles were 20 nm, 45 nm, and 123 nm.)

2 Contrary to the c results reported on the could act as pinning centers) due to different phases of the three SiC-doped Fe-sheathed MgB2 wires, we found that the c for the SiC powders could not be excluded in the study of Soltanian Ti-sheathed MgB2 wires decreased with the size of the SiC par- et al. [3] Thus, it appears to be necessary to carry out a study ticles. We also found that the c is greater than that of the un- on the grain size effect on Jc using SiC nanoparticles with dif- doped MgB2 wires only for the wires with 123 nm SiC size. This ferent sizes but the same crystal structures or forms ( , amor- unusual dependence of c on the size of the SiC dopant is discussed in association with the results from the magnetization, electrical re- phous or crystalline form).

3 Recently, we have successfully fab- sistivity, x-ray diffraction, and transmission electron microscopy ricated Ti-sheathed, un-doped MgB wires with high for the measurements. application of MgB in lightweight superconducting magnets. Index Terms Critical current density, magnesium diboride, Nano-SiC appears to be an effective way to further increase the magnetization, SiC nanoparticles. in this wires. In this study, we examine the effect of the grain size on for Ti-sheathed, SiC-doped MgB wires. To rule out the complicating factors on due to the difference in phases, I.

4 INTRODUCTION only SiC powders with the same crystal structure were selected. Surprisingly, we found that the effects of the SiC grain size on F OLLOWING the first report of Dou et al. [1] on the dra- matic enhancement of in SiC-doped MgB supercon- ductors, extensive studies have been carried out on nano-SiC. the are different from what Soltanian et al. [3] reported. This paper reports the detailed analysis of these effects. doped MgB wires/tapes/pellets [1] [4]. However, among all of these studies, except for the one carried out by Soltanian et al. II. EXPERIMENTAL. [3], it seems that only a specific size of SiC nanoparticles had Ti-sheathed MgB wires with SiC doping were fabricated been used.

5 Soltanian et al. [3] used three particle sizes for the using the standard powder-in-tube (PIT) method as described SiC dopants: 20 nm, 300 nm, and 35 m. They observed that previously [6] [8] in the fabrication of the undoped MgB. the magnetic for the SiC-doped samples was much higher wires. 10 wt. % SiC powder were added to the milled Mg B. than that of undoped samples in the higher field regions, and it mixture [8], [9] for the fabrication of the SiC-doped MgB. increased with the decrease of the grain size of SiC. They ex- wires. The SiC powders of different sizes, Mg powder ( plained their observations by proposing that the doped nano-SiC.)

6 Purity, 325 mesh), and amorphous B powder ( purity, and the Mg Si inclusions be embedded inside the MgB grains 325 mesh) were purchased from Alfa Aesar. The scanning functioned as the effective pinning centers. The study by Solta- electron microscopy (SEM) images showed that the Mg B. nian et al. [3] seems like a systematic one, however, the three powder particles after milling were very uniform in size with SiC powders used in their study had different crystal structures: an average diameter of about 1 m, [6]. The cross-sectional the 20 nm SiC powder was amorphous, the 300 nm SiC powder areas of the finished wires were about 1 mm 1 mm and the was crystallized in the -SiC phase, and the 35 m SiC powder MgB cores had a nearly square cross-sectional area of about was crystallized in an unknown (non -SiC) phase.

7 Since the nanoparticles of an element or compound with different crystal mm . The sizes of the commercial SiC powders and the microstruc- ture of the cores of the SiC doped MgB wires were measured Manuscript received August 03, 2010. Date of publication December 20, by both transmission electron microscopy (TEM) and SEM. 2010; date of current version May 27, 2011. This work was supported in part The TEM work was carried out using a JEOL 2010 microscope by the National Science Foundation under Grant CHE-0718482, an Award from at a working voltage of 200 kV, and the SEM work was per- the Research Corporation for Advancement, and an EGR Grant of Sam Houston State University.

8 Formed using a JEOL JSM-6330F Field Emission Scanning G. Liang, H. Fang, S. Keith, and C. Hoyt are with the Department of Physics, Electron Microscope. The x-ray diffraction (XRD) pattern was Sam Houston State University, Huntsville, TX 77341-2267 USA (e-mail: obtained using a Rigaku x-ray 2005 diffractometer with Cu K. radiation. The temperature dependent resistivity was mea- Z. P. Luo is with the Microscopy and Imaging Center, Texas A&M University, College Station, TX 77843-2257 USA. sured by a standard four-probe dc technique. The hysteresis Digital Object Identifier loops of the magnetization were measured using an Oxford 1051-8223/$ 2010 ieee .)

9 LIANG et al.: EFFECTS OF SIZE OF DOPED SiC NANOPARTICLES ON OF Ti-SHEATHED MgB WIRES 2673. Fig. 1. TEM and SEM images of the SiC particles with average size of (a) 20. nm, (b) 43 nm, (c) 123 nm, and (d) m. Instrument's Maglab 9-Tesla Vibrating Sample Magnetometer (VSM) with a field ramping step of Oe. The undoped and SiC-doped MgB wire samples have roughly the same dimension of mm mm mm. In the Fig. 2. XRD patterns for the core materials of Ti-sheathed, SiC-doped MgB. hysteresis loop measurements, the wire samples were wires and some reference materials. oriented along the direction of the APPLIED magnetic field.

10 The temperature dependent magnetization, , was measured in both zero-field-cooled (ZFC) and field cooled (FC) modes such as those observed in the nano-carbon doping of MgB [5]. using a Magnetic Properties Measurement System (MPMS) Thus, only the powders 1, 2, and 3 were selected as SiC dopants magnetometer from Quantum Design. The experimental details in this study. on these measurements have been described previously [8]. B. XRD Results III. RESULTS AND DISCUSSION Fig. 2 shows the XRD patterns for the core material of the undoped and SiC-doped MgB wires. The patterns for the 20.