EFFECTS OF SINTERING TEMPERATURE ON SUPERCONDUCTIVITY IN Ti-SHEATHED MgB 2 WIRES. G. Liang 1, H. Fang 1, S. Guchhait 2, C. Hoyt 1, J. T.

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1 EFFECTS OF SINTERING TEMPERATURE ON SUPERCONDUCTIVITY IN Ti-SHEATHED MgB 2 WIRES G. Liang 1, H. Fang 1, S. Guchhait 2, C. Hoyt 1, J. T. Markert 2 1 Department of Physics, Sam Houston State University, Huntsville, Texas 77341, USA 2 Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA ABSTRACT The effects of sintering temperature on the superconducting properties of Ti-sheathed MgB 2 wires have been studied. The wires were fabricated by in situ powder-in-tube (PIT) method and characterized by x-ray diffraction, magnetization, scanning electron microscopy and transport measurements. Samples were sintered for 30 minutes at the following temperatures: 650 C, 700 C, 750 C, 800 C, and 850 C. It is found that the cores of these wires are almost in pure MgB 2 superconducting phase, indicating that the Tisheath does not react with Mg or B. The superconducting transition temperature T c decreases from 36 K to 34.2 K with the decrease of the sintering temperature from 850 C to 650 C. At 5 K and 20 K, critical current density J c peaks up for samples sintered at 800 C, which is in sharp contrast with previously reported result that for Fe-sheathed MgB 2 wires, J c peaks up at sintering temperature of 650 C. KEYWORDS: MgB 2 superconductors, critical current density, magnetization, x-ray diffraction, SEM graph INTRODUCTION Lightweight superconducting magnets are preferred for applications such as electric space propulsion and they require the use of light metal as the sheath material in the powder in tube (PIT) process for making MgB 2 wires [1, 2]. For such applications, 281

2 titanium (Ti) appears to be the most favorable sheath metal due to its low mass density, favorable chemical compatibility (i.e., non-reactive with MgB 2, B, or Mg during sintering process), non-magnetic property, and high mechanical strength (50% harder than Fe) [3, 4]. Recently, we have successfully fabricated Ti-sheathed MgB 2 wires with sintering temperature at 800 ºC [5, 6] and it was found that the values of the magnetic critical current density (J c ) are in general comparable to the J c of the Fe-sheathed MgB 2 wires fabricated with similar conditions. Currently, much effort has been made by our group to improve J c for Ti-sheathed MgB 2 wires. Recently, it was reported by some groups that J c can have strong dependence on sintering temperature for Fe-sheathed MgB 2 wires [7-9]. For example, Kim et al. [7] found that J c peaks up for sample sintered at 650 ºC and decreases significantly with the increase of sintering temperature from 650 ºC to 900 ºC. Very recently, we carried out the first study on the dependence of J c and T c on sintering temperature for Ti-sheathed MgB 2 wires to see how J c is affected by sintering temperature. In this paper, we present the result on magnetic J c for these newly fabricated Ti-sheathed MgB 2 wires, together with the results from x-ray diffraction (XRD), energy dispersive spectrometry (EDS), scanning electron microscopy (SEM), electrical resistivity, and temperature dependent magnetization measurements. EXPERIMENTAL The Ti-sheathed monocore MgB 2 wires were fabricated by an in situ powder-in-tube (PIT) method, which has been described in detail elsewhere [5,6]. The milled Mg + 2B powder particles are very uniform in size and smaller than 2 m, as checked by SEM [6]. The cross-sectional areas for the wires and cores are about 1mm 2 (1 mm 1mm) and mm 2, respectively. Wires of about six inches were then cut from the as-drawn wires and sintered in tube furnaces in flowing high purity argon for 30 minutes at the following five different temperatures: 850 ºC, 800 ºC, 750 ºC, 700 ºC, and 650 ºC. The ramping up rate for reaching these temperatures was 300 C/hour and the cooling down rate after sintering was 100 C/hour. The XRD patterns were obtained using a Rigaku x-ray diffractometer with Cu K radiation. The SEM images and EDS spectra were taken using a JEOL JSM- 6330F Field Emission Scanning Electron Microscope. The temperature (T) dependent electrical resistivity was measured by a standard four-probe dc technique. The magnetization (M) was measured using both an Oxford Instrument s Maglab Vibrating Sample Magnetometer (VSM) and a Magnetic Properties Measuring System (MPMS) magnetometer from Quantum Design. RESULTS AND DISCUSSIONS X-ray Diffraction Figure 1 shows the powder XRD patterns for the superconducting core material of the Ti-sheathed MgB 2 wires, together with the patterns of two reference compounds: Ti and MgB 2 (from Alfa Aesar). All of the major peaks in the patterns of the MgB 2 wires can be indexed with the MgB 2 phase. Compared with the patterns of the references, the two very weak impurity peaks located at 2 = 38.4 and 40.1 can be attributed to the phase of Ti. We believe that the Ti impurity contamination was produced during the preparation of the XRD sample slides when some Ti particles were scraped from the inner wall of the Ti- 282

3 FIGURE 1. XRD pattern for the core materials of the Ti-sheathed MgB 2 wires sintered at five different temperatures from 650 o C to 850 o C for 30 minutes. For comparison, the XRD patterns for the commercial MgB 2 and Ti powders are also shown. sheath. The broad peak located at 2 = 62.3 can be attributed to MgO impurities. In the presence of oxygen, MgO impurities could be formed by the reaction 2Mg + O 2 2MgO during the sintering process. The main source of the oxygen could be from the air trapped in the cores during the short-time crimping sealing of the end of the Ti tube (with Mg+2B mixture packed in) in air. SEM and EDS Results The analysis and conclusion made in last section are consistent with our EDS result. Shown in Figure 2 is a typical EDS spectrum taken for the entire cross-sectional area of the cores for these MgB 2 wires. In the spectrum, only a Mg peak and a weak O peak are observed and there is no Ti peak seen. This clearly indicates that the Ti sheath did not react with either Mg or B to form Ti compounds in the core of the wire. The O peak in the EDS spectrum is also consistent with the XRD result that some MgO impurities are formed in the core material. In Fig. 3, we show the SEM images for the cores of four wire samples sintered at different temperatures. The surfaces of the cores exposed at the ends of the 283

4 FIGURE 2. The EDS spectrum taken on the entire cross-section of the MgB 2 core. FIGURE 3. SEM images of the core region of the Ti-sheathed MgB 2 wires which are (a) sintered at 650 ºC, (b) sintered at 750 ºC, (c) sintered at 800 ºC, and (d) ) sintered at 850 ºC. wires were polished. The existence of a large number of holes or voids can be directly observed from the SEM images. Most of these holes/voids have diameters of 1-3 m, similar to the size of the Mg powder particles in the milled Mg + 2B powder precursor. This observation suggests that the holes/voids could be due to the evaporation of the Mg 284

5 particles during the sintering process when some of the evaporated Mg escaped from the ends of the wire. Temperature Dependent Magnetization and Electrical Resistivity Shown in Fig. 4 are the dc magnetization M(T) curves for the five samples sintered at different temperatures from 650 ºC to 850 ºC. All of these curves are normalized to M = -1 at 5 K. The M(T) data were measured at 20 Oe in zero-field-cooled (ZFC) modes. It is observed from Fig. 4 that the transition temperature, T c,on, defined as the temperature at which the M(T) starts to drop, decreases steadily from 36 K to 34.2 K with the decrease of sintering temperature from 850 ºC to 650 ºC. The width of the transition, T, defined by the difference between the temperatures at 10% and 90% of the full drop of M(T), appears almost unaffected by the variation of the sintering temperature. The decrease of T c with decreasing sintering temperature is consistent with the electrical resistivity data shown in Fig. 5. Such a decrease of T c with decreasing sintering temperature has been observed previously in Fe-sheathed MgB 2 wires [7-9] and can be attributed to the better crystallinity at higher sintering temperatures. Magnetization Hysteresis Loops and Magnetic J c The magnetization hysteresis loops, M (H), were measured at two temperatures, 5K and 20 K, for all of the five samples. In the measurements, the axes of the wires were always kept parallel to the direction of the applied magnetic field. In Fig. 6, we show such FIGURE 4. Temperature dependent dc magnetization, M (T), measured in ZFC mode in a field of 20 Oe for the Ti-sheathed MgB 2 wires sintered at five different temperatures. The data are normalized to M = -1 at T = 5 K. 285

6 FIGURE 5. Electrical resistivity curves for Ti-sheathed MgB2 wires sintered at 800 ºC, 750 ºC, and 650 ºC. The data are normalized to the values at 55K. FIGURE 6. Magnetization hysteresis loops measured at 5 K and 20 K for the Ti-sheathed MgB 2 wire sample sintered at 700 ºC for 30 minutes hysteresis loops for the sample sintered at 700 ºC as an example. The fluctuation of the M (H) curve observed for the hysteresis loop measured at 5 K can be attributed to the flux jumps [5]. Shown in Fig. 7 are the field dependent magnetic J c (H) curves for the five samples. The magnetic J c was calculated by J c = 20 M/[a(1-a/3b)] [5, 10, 11] from the Bean critical 286

7 state model [12], where M (in emu/cm 3 ) is the difference between the upper and lower branches of the M(H) curve. There is no contribution to M from the paramagnetic Tisheath [3, 4]. Fig. 7 shows that J c peaks up at for the sample sintered at 800 ºC and decreases when the sintering temperature is away from this temperature. Due to the intensive flux jumps at low fields, the low-field portion of the J c curves for the sample sintered at 800 ºC is not shown. For this sample, the magnetic J c at 5 K and fields of 1.7 T, 5 T, and 7 T are about A/cm 2, A/cm 2, and A/cm 2, respectively. At 20 K and fields of 0.6 T and 4 T, the J c values for this sample are about A/cm 2 and 10 4 A/cm 2, respectively. It is observed from Fig. 7 that with the decrease of sintering temperature from 800 ºC to 650 ºC, the magnetic J c decreases steadily. This result is in sharp contrast with previously reported result that for Fe-sheathed MgB 2 wires, J c peaks up at 650 ºC and decreases with increasing sintering temperature. [7] FIGURE 7. The field dependence of the magnetic J c, estimated by the Bean s model [12], for the Tisheathed MgB 2 wire samples sintered at five different sintering temperatures. 287

8 CONCLUSIONS The effects of sintering temperature on the structural and superconducting properties of Ti-sheathed MgB 2 wires were studied for the first time. All of the wires were fabricated by an in situ PIT method and characterized by x-ray diffraction, EDS, SEM, transport, and magnetization measurements. Samples were sintered for 30 minutes at five different temperatures from 650 C to 850 C. For all samples, it was found that the cores of these wires are almost in pure MgB 2 phase, indicating that the Ti-sheath does not react with Mg or B. The superconducting transition temperature T c decreases steadily from 36 K to 34.2 K with the decrease of the sintering temperature from 850 C to 650 C. At 5 K and 20 K, critical current density J c peaks up for samples sintered at 800 C This variation of J c with sintering temperature is in sharp contrast with previously reported results by other group that for Fe-sheathed MgB 2 wires, J c peaks up roughly for samples sintered at 650 C. ACKNOWLEDGEMENTS This work is supported by the National Science Foundation under Grants No. CHE and No. DMR Welch Foundation under Grant No. F-1191, an award from Research Corporation for Science Advancement, and EGR grants of Sam Houston State University. REFERENCES 1. Nagamatsu J.; Nakagawa N.; Muranaka T.; Zenitani Y. and Akimitsu J., Nature 410, pp. 63 (2001). 2. Flükiger R.; H. Suo L.; Musolino N.; Beneduce C.; Toulemonde P. and Lezza P. Physica C 385, pp. 286 (2003). 3. Collins E. W. and Gehlen P. C., J. Phys. F: Metal Phys. 1, pp. 908 (1971). 4. Wasilewski R. J., J. Appl. Phys. 40, pp (1969). 5. Liang G.; Fang H.; Hanna M.; Yen F.; Lv B.; Alessandrini M.; Keith S.; Hoyt C.; Tang Z.; and Salama K., Supercond. Sci. Technol. 19, pp (2006). 6. Liang, G.;Alessandrini M.;Yen F.; Hanna M.; Fang H.; Hoyt C.; Lv B.; Zeng J. and Salama K. Physica C 457, pp (2007). 7. Kim J. H.; Dou S. X.; Wang J. L.; Shi D. Q.; Xu X.; Hossain M. S. A.; Yeoh W. K.; Choi S. And Kiyoshi T., Supercond. Sci. Technol. 20, pp (2007). 9. Chen S. K.; Tan K. S.; Glowacki B. A.; Yeoh W. K.; Soltanian S.; Horvat J. And Dou S. X., Appl. Phys. Lett. 87, pp (2005). 10. Soltanian S.; Qin M. J.; Keshavarzi S.; Wang X. L. And Dou S. X., Phys. Rev. B 68, pp (2003). 11. Gyorgy E. M.; van Dover R. B.; Jackson K. A.; Schneemeyer L. F. and, and Waszczak J. V., Appl. Phys. Lett. 55, pp. 283 (1989). 12. Bean C. P., Phys. Rev. 8, pp. 250 (1962). 288