IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 5, AUGUST 2017
7400204
Optimization of High-Energy Ball Milling Aided Sintering Process for FeSe Superconductors Shengnan Zhang, Jianqing Feng, Jixing Liu, Botao Shao, Chengshan Li, and Pingxiang Zhang
Abstract—Superconducting FeSe bulks with high superconducting content of PbO-type tetragonal β-FeSe phase were prepared with high-energy ball milling (HEBM) aided sintering process. During the HEBM process, different ball-to-powder ratios of 6:1, 8:1, and 10:1 were adopted. The influences of ball-to-powder ratio on the phase composition of both ball milled powders and sintered bulks, morphology, and superconducting properties of final bulks were systematically investigated. It was noticed that, with the increase of ball-to-powder ratio, the energy input into the ball milling system increased, and the high-energy collision times increased. Therefore, more Fe atoms entered into the Fe-Se matrix, and the ball milling products changed from Fe3 Se4 to Fe7 Se8 . On the other hand, the decreasing total collision times led to the increase of final particle size. After the same sintering process, FeSe bulks with the major phase of β-FeSe were all obtained. However, with the increasing ball-to-powder ratio, the contents of both hexagonal δFeSe phase and residual Fe contents increased. Due to the different Fe/Se ratio in the final β-FeSe phase, the superconducting transition behavior varied obviously. Higher critical temperature of 8.0 K and larger superconducting phase content were achieved with the optimal ball to powder ratio of 6:1. Index Terms—Critical temperature, FeSe, powder metallurgy, superconductors.
I. INTRODUCTION INCE the first discovery in 2008, tetragonal FeSe phase with the superconducting critical temperature Tc of approximately 8.0 K has attracted more and more attentions. It exhibits the simplest lattice structure among all the discovered superconductors, which is composed of only superconducting layers of Fe–Se layers and no blocking layers [1]. With the substitution of Se ions by other chalcogenide elements, FeSe1−x Tex and FeTe1−x Sx are also recognized as “11” system superconductors with both Tc of approximately 15.0 K. The superconductivity
S
Manuscript received April 21, 2017; revised May 22, 2017; accepted June 1, 2017. Date of publication June 16, 2017; date of current version July 7, 2017. This work was supported in part by the National ITER Program of China under Contract 2013GB110001 and in part by the Innovative Research Team of Shaanxi province under Contract 2013KCT-07. The paper was recommended by Associate Editor P. Lee. (Corresponding author: Shengnan Zhang.) S. Zhang, J. Feng, C. Li, and P. Zhang are with the Superconducting Materials Research Center, Northwest Institute for Non-Ferrous Metal Research, Xi’an 710016, China (e-mail:
[email protected];
[email protected]; csli@ c-nin.com;
[email protected]). J. Liu is with the School of Materials Science and Engineering, Northeastern University, Shenyang 110016, China (e-mail:
[email protected]). B. Shao is with the School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2017.2716844
of FeSe is very sensitive to high pressure, the Tc values of FeSe system can be greatly improved up to 37 K under high pressure [2]. Meanwhile, the preparation of unit cell FeSe ultrathin films [3]–[6] has been proved to be effective for the enhancement of Tc to over 65 K. At the same time, the advantages including higher upper critical field of Hc2 approximately 47 T [7] comparing with Nb3 Sn (approximately 35 T), lower cost, and lower toxicity of starting materials compared to the FeAs-based superconductors all suggest their potentials as practical superconductors. Meanwhile, the flux pinning properties in FeSe-based superconductors are also very important. In FeSe superconductors, the multidomain nature of FeSe crystals leads to the combined flux pinning mechanism with both bulk pinning and domain boundary pinning [8]. Thus, nowadays the fabrication of FeSe-based superconductors with optimal superconductivity and strong flux pinning properties has been focused. Aiming at the realization of practical applications in high field magnets, the fabrication of FeSe wires is a necessary way [9]. However, based on previous report, the current carrying density, Jc of FeSe single crystals and films could reach as high as approximately 105 A·cm−2 at 4.2 K, self-field [10]–[12]. But the Jc values of FeSe wires were quite low, which were only in the range of 100 A·cm−2 [13]–[17]. Considering that the Jc of single crystals should be recognized as intragranular Jc , the low Jc in wires could be attributed to the existence of severe intergrain weak links. Both the existence of pores, which is related to the low superconducting filaments density, and the existence of secondary phases, mainly hexagonal δ-FeSe phase are the major cause of intergrain weak links. According to the Fe–Se binary phase diagram [18], [19], superconducting tetragonal β-FeSe phase can be obtained with the Fe/Se ratio of slightly over 1.05. However, in the same composition region, hexagonal δ-FeSe (or Fe7 Se8 ) can also be obtained, which cannot only decrease the superconducting volume and act as the intergrain weak links, but also decrease the Tc value of FeSe samples by changing the chemical composition of superconducting β-FeSe phase from stoichiometric value [20]. Meanwhile, it has also been reported that the fabrication process can greatly influence the superconducting signal of β-FeSe superconductors [21]. Therefore, the systematically optimization of processing parameters, including the phase composition and average particle size of precursor powders [22], the sintering temperature [23], pressure [20], and chemical compositions [24], is very necessary for the preparation of FeSe with high superconductivity. In our previous study, a novel preparation method called high energy ball milling (HEBM) aided sintering technique has been
1051-8223 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.
7400204
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 5, AUGUST 2017
developed. During this process, Fe and Se powders were ball milled first. Then during sintering, the diffusion distance between Fe and Se atoms became shorter, which was beneficial to the formation of superconducting β-FeSe phase [22]. However, the influences of ball milling parameters on the phase evolution process of Fe-Se system have not been systematically studied. In this study, the ball milling process was optimized by turning the ball to powder ratio from 6:1 to 10:1. The influences of ball milling process on the phase composition of precursor powders and final bulks were explored in details, and optimized ball to powder ratio was determined based on magnetization measurement. Fig. 1. XRD patterns of Fe-Se ball milled powders with different ball to powder ratio.
II. EXPERIMENTAL Fe (99.9%) and Se (99%) powders (from SCR) were weighed as the designed stoichiometric molar ratio of Fe/Se = 1.20 in Ar protected glove box. And the powders were mixed and sealed in a stainless steel (SS) ball milling jar under the atmosphere of Ar. SS balls with the ball-to-powder weight ratio of 6:1, 8:1, and 10:1 were also put into the ball milling jar simultaneously. Highenergy ball milling process was performed at the frequency of 1720 r/min for 6 h with the machine of SPEX-8000M. The ball milled powders were cold pressed into pellets of φ10 mm in diameter and 1.5 mm in thickness with the uniaxial pressure of 10 MPa. Then, the obtained pellets were sealed separately in evacuated quartz tubes. The sintering process was performed at the maximum temperature of 700 °C for 12 h. Phase composition analysis of both the ball milled powders and sintered bulks were performed by X-ray diffractometer of ˚ ReBruker D8 Advance with CuKα radiation (λ = 1.5406 A). itveld refinement method was applied with commercial software Fullprof to determine the phase composition. And the phase content of different phase FM was calculated as FM =
Iβ −FeSe
I M × 100% + Iδ −FeSe + IFe
(1)
where M represents to the target phase as β-FeSe, δ-FeSe, and residual Fe, respectively. IM is the diffraction peaks intensity of M phase, Iβ −FeSe , Iδ −FeSe , and IFe are the diffraction peaks intensities of β-FeSe, δ-FeSe, and Fe, respectively. The microstructures and phase distribution of final bulks were characterized by scanning electron microscopes of JEOL-6700F. The chemical compositional analysis was performed by IncaX-Stream energy-dispersive X-ray spectroscopy (EDX). The superconducting critical temperatures of obtained bulks were obtained by magnetization method, and carried out on the Superconducting Quantum Interference Device (SQUID, MPMSXL-7) from 4.2 K to 30 K under the background field of 10 Oe. The magnetization method is performed by measuring the temperature dependence of magnetization in both zero field cool (ZFC, cooling down with no magnetic field) and field cool (FC, cooling down with a background field of 10 Oe in our measurement) condition. This kind of measurement can be influenced by the residual trapped field inside the superconducting magnet [25], [26].
III. RESULTS AND DISCUSSIONS Fe-Se mixtures are high- energy ball milled for 6 h with different ball to powder ratio of 6:1, 8:1, and 10:1, respectively. As shown in Fig. 1, the ball milled products are different with different ball to powder ratio. In the powders with ball to powder ratio of 6:1, the major phases can be indexed into both Fe and Fe3 Se4 . The diffraction peak of Fe3 Se4 phase is a hump with the full width at half maximum value of approximately 1.4°. It suggests that the obtained Fe3 Se4 phase is amorphous or weakly crystallized. With the increase of ball to powder ratio to 8:1, small amount of amorphous Fe7 Se8 phase appears as marked by black triangles. Then, with higher ball to powder ratio of 10:1, the major phases changed into Fe and Fe7 Se8 , and most of the Fe7 Se8 phase is crystallized. Based on the powder metallurgy theory, with the increase of ball to powder ratio, the input power into the ball milling system increases, while the energy usage ratio decreases. Thus, it can be deduced that with increasing ball to powder ratio, although the total collision times decrease, the times of high energy collisions increase, which are mainly responsible for the formation of crystallized Fe-Se binary compounds. And more high-energy collision times cause the increase of Fe content in Fe-Se binary compounds, thus product changes from Fe3 Se4 to Fe7 Se8 phase. The phase compositions of sintered Fe-Se bulks are characterized with X-ray diffraction (XRD) patterns as shown in Fig. 2(a). The major phase of all the bulks is tetragonal β-FeSe phase. And small amounts of both δ-FeSe and residual Fe can be observed as secondary phases. Based on the calculation with (1), the contents of δ-FeSe and residual Fe are calculated as plotted in Fig. 2(b). The content of δ-FeSe phase increases monotonously from 0.7% to 1.7% with the increasing ball to powder ratio, and the residual Fe content also increases from 2.1% to 2.6%, simultaneously. The change of phase composition can then lead to the change of chemical composition in β-FeSe phase. Considering that the Fe:Se ratio in hexagonal δ-FeSe phase is lower than that in β-FeSe phase, the increasing δ-FeSe content should be responsible for the larger Fe content in β-FeSe matrix as interstitial Fe. SEM fracture images of FeSe sintered bulks with the ball to powder ratio of 6:1 and 10:1 are observed and shown in Fig. 3(a)
ZHANG et al.: OPTIMIZATION OF HIGH-ENERGY BALL MILLING AIDED SINTERING PROCESS FOR FeSe SUPERCONDUCTORS
7400204
Fig. 2. (a) XRD patterns of sintered FeSe bulks with different ball to powder ratio of 6:1 to 10:1. (b) Phase content of δ-FeSe and residual Fe in sintered bulks with different ball to powder ratio.
Fig. 4. Temperature dependence of magnetization of FeSe sintered bulks fabricated with different ball to powder ratio of (a) 6:1, (b) 8:1, and (c) 10:1.
Fig. 3. SEM fracture images of sintered bulks with different ball to powder ratio of (a) 6:1 and (b) 10:1. Backscattered electron images of (c) 6:1 and (d) 10:1 bulks.
and (b). As discussed previously, the increase of ball to powder ratio can lead to the decrease of total collision times. Thus, the final powder size is obviously larger with the ball to powder ratio of 10:1. Meanwhile, the backscattered electron images of two bulks are shown in (c) and (d), respectively. It is known that in the backscattered electron images, the contrast represents to
the different phase distribution. Therefore, combined with EDX characterization, the distribution of residual Fe particles, which are observed as the dark gray areas, can be observed as marked by arrows. Due to the effectively reduced particle size in the powders with 6:1 ball to powder ratio, the particle size of residual Fe also has been greatly decreased from approximately 10 to 2 μm. Moreover, the pore average size of the black areas is also bigger in the bulk with 10:1 ball to powder ratio, which causes the decrease of density and weak intergrain connections. Therefore, the uniformly distributed chemical composition, decreasing size of secondary phase, and the decreasing porosity in the bulks with ball to powder ratio of 6:1 all can contribute to the optimization of superconducting properties of β-FeSe. As shown in Fig. 4(a)–(c), the temperature dependence of magnetization of all the sintered bulks with different ball to powder ratios are measured. And the related critical temperature values, Tc (onset), are estimated by taking the onset of the ZFC curve in the M(T) measurement as shown in each inset
7400204
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 5, AUGUST 2017
of (a)–(c). Although the XRD patterns of all the three sintered bulks are similar with only slight change of secondary phase and lattice parameters, great differences are observed in the superconducting properties. The critical temperature, Tc (onset) values of 8.0 K, 7.8 K, and 7.4 K are obtained on the FeSe bulks with the ball to powder ratio of 6:1, 8:1, and 10:1, respectively. The decrease of Tc values with larger ball to powder ratio should be attributed to the variation of Fe/Se ratio in β-FeSe phase from optimal value due to the existence of larger content of δ-FeSe and residual Fe. On the other hand, the residual magnetization values on the normal state, which decreases with increasing ball to powder ratio consistently, should be attributed to the residual Fe. And with the ball to powder ratio of 6:1, larger magnetization difference at 4.2 K, ΔM = MFC − MZFC , implies a larger superconducting phase content. The obtained magnetization difference ΔM of approximately 0.08 emu/g is comparable with that obtained value by Williams et al. [27] with the sample prepared through a higher temperature, longer time sintering process. Therefore, it can be concluded that the ball to powder ratio is an important parameter for the HEBM aided sintering of FeSe bulks, and the optimal value should be 6:1 under this synthesis circumstance. IV. CONCLUSION In this study, ball to powder ratio was optimized during the HEBM aided sintering process of FeSe superconducting bulks. The increase of ball to powder ratio from 6:1 to 10:1 led to the decrease of total collision times, therefore, the final average particle size increased. However, the high-energy collision times increased, which contributed to the change of Fe–Se binary compounds from Fe3 Se4 to Fe7 Se8 in the ball milled powders. After the same sintering process, the FeSe bulks with high chemical composition uniformity, small particle size of secondary phase, high density, and larger superconducting phase content were achieved with the ball to powder ratio of 6:1. And the critical temperature of 8.0 K was obtained. Thus, the optimal ball to powder ratio was determined as 6:1 and will be adopted in the future study. REFERENCES [1] F. C. Hsu et al., “Superconductivity in the PbO-type structure α-FeSe,” Proc. Nat. Acad. Sci. USA, vol. 105, pp. 14262–14264, 2008. [2] M. Bendele, E. Pomjakushina, K. Conder, R. Khasanov, and H. Keller, “Pressure effects in the iron chalcogenides,” J. Supercond. Novel Magn., vol. 27, pp. 965–968, 2014. [3] S. L. He et al., “Phase diagram and electronic indication of hightemperature superconductivity at 65 K in single-layer FeSe films,” Nature Mater., vol. 12, pp. 605–610, 2013. [4] W. Li et al., “Superconductivity in a single-layer alkali-doped FeSe: A weakly coupled two-leg ladder system,” Phys. Rev. B, vol. 88, 2013, Art. no. 140506. [5] W. H. Zhang et al., “Interface charge doping effects on superconductivity of single-unit-cell FeSe films on SrTiO3 substrates,” Phys. Rev. B, vol. 89, 2014, Art. no. 060506(R).
[6] D. F. Liu et al., “Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor,” Nature Commun., vol. 3, p. 931, 2012. [7] M. H. Fang et al., “Weak anisotropy of the superconducting upper critical field in Fe1 . 1 1 Te0 . 6 Se0 . 4 single crystals,” Phys. Rev. B, vol. 81, 2010, Art. no. 020509. [8] A. Leo et al., “Vortex pinning properties in Fe-chalcogenides,” Supercond. Sci. Technol., vol. 28, 2015, Art. no. 125001. [9] Y. W. Ma, “Progress in wire fabrication of iron-based superconductors,” Supercond. Sci. Technol., vol. 25, 2012, Art. no. 113001. [10] C. S. Yadav and P. L. Paulose, “The flux pinning force and vortex phase diagram of single crystal FeTe0 . 6 Se0 . 4 ,” Solid State Commun., vol. 151, pp. 216–218, 2011. [11] W. H. Zhang et al., “Direct observation of high temperature superconductivity in one-unit-cell FeSe films,” Chin. Phys. Lett., vol. 31, 2014, Art. no. 017401. [12] W. D. Si et al., “High current superconductivity in FeSe0 . 5 Te0 . 5 -coated conductors at 30 Tesla,” Nature Commun., vol. 4, p. 1347, 2013. [13] Y. Mizuguchi, H. Izawa, T. Ozaki, Y. Takano, and O. Miura, “Transport properties of the single- and 3-core Fe-Se wires fabricated by a novel chemical-transformation PIT process,” Supercond. Sci. Technol., vol. 24, 2011, Art. no. 125003. [14] Z. S. Gao et al., “Superconducting properties of FeSe wires and tapes prepared by a gas diffusion technique,” Supercond. Sci. Technol., vol. 24, 2011, Art. no. 065022. [15] Y. Mizuguchi et al., “Fabrication of the iron-based superconducting wire using Fe(Se,Te),” Appl. Phys. Exp., vol. 2, 2009, Art. no. 083004. [16] T. Ozaki et al., “Transport properties and microstructure of mono- and seven-core wires of FeSe1 −x Tex superconductor produced by the Fediffusion powder-in-tube method,” Supercond. Sci. Technol., vol. 24, 2011, Art. no. 105002. [17] A. Tsukada et al., “Pulsed laser deposition conditions and superconductivity of FeSe thin films,” App. Phys. A, vol. 104, pp. 311–318, 2011. [18] H. Okamoto, “The Fe–Se (Iron-Selenium) system,” J. Phase Equilib., vol. 12, pp. 383–389, 1991. [19] S. R. Svendsen, “Decomposition pressures and standard enthalpy of formation for the iron selenides FeSe, Fe7 Se8 , Fe3 Se4 and FeSe2 ,” Acta Chemica Scandinavica, vol. 26, pp. 3757–3774, 1972. [20] G. Rahman, I. G. Kim, and A. J. Freeman, “Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe,” J. Phys.: Condens. Matter, vol. 24, 2012, Art. no. 095502. [21] C. F. Zignani et al., “Fabrication and characterization of sintered IronChalcogenide superconductors,” IEEE Trans. Appl. Supercond., vol. 26, no. 3, Apr. 2016, Art. no. 7400105. [22] S. N. Zhang et al., “Effective method for the fabrication of FeSe Superconductors: High-energy ball milling aided sintering process,” Mater. Chem. Phys., vol. 163, pp. 587–593, 2015. [23] H. Izawa, Y. Mizuguchi, T. Ozaki, Y. Takano, and O. Miura, “Evolution of tetragonal phase in the FeSe wire fabricated by a novel chemicaltransformation PIT process,” Jpn. J. Appl. Phys., vol. 51, 2012, Art. no. 010101. [24] M. K. Wu et al., “The development of the superconducting PbO-type β-FeSe and related compounds,” Physica C, vol. 469, pp. 340–349, 2009. [25] A. Galluzzi et al., “Critical current and flux danamics in Ag-doped FeSe supercondutor,” Supercond. Sci. Technol., vol. 30, 2016, Art. no. 025013. [26] D. Zola, M. Polichetti, C. Senatore, and S. Pace, “Magnetic relaxation of type-II superconductors in a mixed state of entrapped and shielded fulx,” Phys. Rev. B, vol. 70, 2004, Art. no. 224504. [27] A. J. Williams, T. M. McQueen, and R. J. Cava, “The stoichiometry of FeSe,” Solid State Commun., vol. 149, pp. 1507–1509, 2009.
Authors’ biographies not available at the time of publication.
