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Post heat treatment time induced distinguished superconducting features of JR Nb3Al wires rapidly heated around the Tc peak condition
Intermetallics 110 (2019) 106482
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Post heat treatment time induced distinguished superconducting features of JR Nb3Al wires rapidly heated around the Tc peak condition
T
Changkun Yanga, Xiaguang Suna, Pingyuan Lia, Zhou Yua,∗∗, Yongliang Chena, Yong Zhanga, Xifeng Panc, Guo Yanb, Yong Fengb, Yong Zhaoa,c,∗ a
Key Laboratory of Advanced Technology of Materials (Ministry of Education), Superconductivity and New Energy R&D Center (SNERDC), Southwest Jiaotong University, Chengdu, Sichuan, 610031, China b National Engineering Laboratory for Superconducting Materials (NELSM), Western Superconducting Technologies (WST) Co., Ltd, Xi'an, 710018, China c College of Physics and Energy, Fujian Normal University, Fuzhou, Fujian, 350117, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. intermetallics B. phase transformation C. heat treatment D. grain boundary F. Scanning electron microscopy G. superconducting
This work investigated the effect of post heat treat time on the structure and superconducting properties of Nb3Al wires prepared around the current condition of getting Tc peak. Properties of Nb3Al wires rapidly heated at much lower current of 76 A were also studied for comparison. M-T results show that prolongation of post-heat treatment time can elevate the Tc value of all Nb3Al wires because of the stoichiometry improvement of Nb3Al phase. The best Jc of post heat treated Nb3Al wires rapidly heated at 76 A and 272 A were obtained after post heat treated for 2 h at 800 °C, compared to that of 10 h for the 276 A samples. The different optimism post heat treatment time to get best Jc of the wires might attribute to the disorder degree difference of the Nb(Al)ss phase fabricated under different rapid heating current. The main pinning mechanism of Nb3Al superconducting wires was grain boundary pinning, as deduced from the fitting of pinning curve. Gradually degrade of Jc of the Nb3Al wire performed at longer heat treatment time was attributed to the coarse of Nb3Al grain that attenuated grain boundary pinning.
1. Introduction Compared to Nb3Sn superconducting wires, Nb3Al wires fabricated through rapid heating, quenching and transformation process (RHQT) [1–4] show better strain and stress tolerance at high magnetic fields [5–7], thus Nb3Al was considered as an alternative material to apply in high-energy particle accelerators, nuclear magnetic resonance and magnetic confinement fusion devices [7,8]. Based on Nb-Al binary phase diagram, stoichiometric Nb3Al phase can only stably between 1940 and 2060 °C [9]. Hence, RHQT technique was development to fabricate Nb3Al wires, in which the composite Nb-Al precursor wire was rapidly heated to temperature about 2000 °C and then quickly quenched in a molten Ga bath to get body-centered cubic (bcc) supersaturated solid solution (Nb(Al)ss), and followed by annealing the ductile Nb(Al)ss wires at 800 °C for 10 h to get the Nb3Al A15 phase [10]. After decades of development, RHQT is though as a suitable technique to fabricate long-length Nb3Al wires with excellent superconducting properties, but the practical process for large-scale
commercial manufacture of Nb3Al wires has not been realized owing to some difficulties, such as high heating temperature and complex of processing conditions optimization [11]. In the RHQT process, for a given wounding speed, the phase structure, mechanical properties, and superconducting properties are sensitively dependent on the heating current [12]. With increasing heating current Ih, a peak in the Tc-Ih curve appears before entering into the optimal fabrication condition. It is recently discovered that though the feature of the Tc-Ih curve is also sensitive to the tension during the reel-to-reel RHQ process [11], a peak still exists in the Tc-Ih curve. In a sense, the peak can be regarded as an important mark indicating whether the system is already in the optimal condition. It is interesting to further study the structural and superconducting properties of the Nb3Al wires which were prepared in the condition around the peak, in order to clarify the mechanism behind their unique superconducting properties. In this paper, we compared the superconducting properties of different time post heat treated Nb3Al wires rapidly heated around the “Tc
∗
Corresponding author. Key Laboratory of Advanced Technology of Materials (Ministry of Education), Superconductivity and New Energy R&D Center (SNERDC), Southwest Jiaotong University, Chengdu, Sichuan, 610031, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (Z. Yu), [email protected], [email protected] (Y. Zhao). https://doi.org/10.1016/j.intermet.2019.106482 Received 19 January 2019; Received in revised form 13 April 2019; Accepted 21 April 2019 Available online 28 April 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. XRD patterns for the Nb3Al wires subjected to different post-heat treatment time. (a), (b) and (c) correspond to rapid heating current of 76 A, 272 A and 276 A, respectively.
Fig. 2. (a) Enlarged XRD patterns of (210) peak for the three typical RHQ Nb3Al wires subjected to different post-heat treatment time. (b) Variation of full width at half maximum (FWHM) of the (210) peak with post-heat treatment time for the three typical RHQ Nb3Al superconducting wires.
treat time is 2 h, compared to that of 10 h for the 276 A wires rapidly heated on the right side of the Tc peak.
peak” condition. The M-T results show that prolongation of post-heat treatment time can improve the Tc and stoichiometry of the Nb3Al wires. The disorder degree of the Nb(Al)ss phase is different for Nb3Al wires under different RHQ heating current, resulting in the difference of optimal post heat treat time to obtain best Jc of the wires. For the wires rapidly heated on the left side of the Tc peak [11], the optimal post heat
2. Experimental The single core Nb3Al precursor wire composed of multiple Nb/Al 2
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Fig. 3. (a), (b) and (c) are the M-T curves for the samples with RHQ heating current of 76 A, 272 A and 276 A, respectively. (d) The Tc as a function of post-heat treatment time for the samples.
temperature and magnetic field by using the SQUID magnetometer (MPMS-7T, Quantum Design). The length of all the samples were 4 mm to maintain roughly the same demagnetization factor during the measurement. The temperature range of M-T curves was set from 5 K to 20 K. During M-T measurement process, the samples were cool to 5 K in a zero field and then the magnetic moment was measured at intervals of 0.1 K until the temperature reached 20 K in a field of 20 Oe. Jc was calculated by using the Bean model [18]: (Jc = 15ΔM/R), in which R was the diameter of the samples and the △M was the width of the magnetization loops.
layers was fabricated by the jerry-roll (JR) process [13–16]. Nb foil and Al foil with size of 0.15 × 300 × 2100 mm and 0.05 × 300 × 2100 mm was stacked and wrapped on a Nb rod by hand to an outer diameter of 42 mm. Then the Nb/Al composite was inserted into a Cu can, sealed in high vacuum and extruded into round Cu/Nb-Al rod by hydrostatic extrusion. Subsequently, the wire was carefully cold drawn down to diameter of 0.8 mm by conical dies. Specific process of manufacturing the precursor wires have reported elsewhere [17]. Nb3Al superconducting wires were fabricated RHQT process. In the RHQ process, the mono-filamentary wires without the Cu sheath were reel to reel rapidly heated to high temperature and then were quenched in a molten Ga bath to form bcc Nb(Al)ss phase. Where after, low temperature transformation process was performed to convert the resulting Nb(Al)ss phase into Nb3Al A15 phase with stoichiometric composition and fine grain size. In this work, using a home-made RHQ equipment [17], we have carried out the RHQ process at different joule heating currents (76 A, 272 A and 276 A) with constant wire feeding rate of 0.3 m/s. After finishing the RHQ process, the wires were sealed in quartz tube at vacuum pressure of 10−3 Pa, then annealed at 800 °C with the same ramping rate (200 °C/h) and different holding time (2 h, 10 h, 20 h, 50 h and 100 h) to convert Nb(Al)ss phase to Nb3Al A15 superconducting phase. X-ray diffraction (XRD) with scanning range from 10 to 90° was performed at room temperature to investigate the crystal structure of RHQT-processed Nb3Al wires. The cross-section microstructure and composition were obtained by field emission scanning electron microscopy (FESEM) equipped with an energy-dispersive X-ray spectroscopy (EDX). The magnetic moments were measured as a function of the
3. Results and discussion Fig. 1 displays the XRD patterns for three typical RHQT Nb3Al superconducting wires post heat treated at different time. Fig. 1(a), (b) and (c) show that the main phase of the wires is Nb3Al A15 phase, which can be indexed as (210), (211) and (310) planes of Nb3Al. Besides Nb3Al phase, a small amount of Nb2Al phase was detected in the wires with the rapidly heating current of Ih = 76 A, which could attribute to insufficient reaction of Nb and Al layers at such low heating current. Diffraction peaks of Nb was also found in all annealed wires since the wires are actually a composite material composed of Nb filament in the center of wires. In addition, with prolonging the post-heat treatment time, the intensity of the (210), (211) and (310) peaks gradually increases, especially for the wires rapidly heated below the “Tc peak” current condition, indicating crystallinity improvement of the formed Nb3Al phase. The enlarged patterns of the (210) peak shown in Fig. 2(a) reveal that 3
Intermetallics 110 (2019) 106482
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Fig. 4. (a), (b) and (c) are Jc-H curves for the Nb3Al superconducting wires. (d) Jc measured at 6 T for the Nb3Al samples heat treated at 800 °C for various time.
272 A and 276 A. Fig. 3(d) shows the Tc as a function of post-heat treatment time for the Nb3Al samples. Tc of 272 A and 276 A samples are much higher than that of 76 A. With the prolongation of post-heat treatment time, all Nb3Al wires show upward trend of Tc value, indicating improvement of stoichiometry of the Nb3Al phase. More accurately, the Tc of Nb3Al wires rapidly increased as prolonging heat treatment time from 2 h to 10 h, and then the Tc elevates much slower as longer heat treatment time. Fig. 4 shows the Jc performance of Nb3Al wires measured at 4.2 K and 10 K. Jc of the 272 A and 276 A rapidly heated Nb3Al wires reach around 106 A/cm2 at 4.2 K, 0 T, which are closed to the result of RHQT Nb3Al wires [21,22]. Jc value of the 76 A Nb3Al samples at 4.2 K, 0 T are much lower than that of 272 A and 276 A samples because of formation of off-stoichiometry Nb3Al phase. As prolonging the post heat treatment time from 2 h to 100 h, Jc of the 76 A and 272 A samples monotonously decreases, as shown in Fig. 4(a) and (b); however, Jc of the 276 A samples (Fig. 4(c)) increases first and then decreases where the best Jc of the wires was obtained after heat treatment for 10 h. Fig. 4(d) summaries Jc values at 4.2 K, 6 T of the Nb3Al samples after post heat treated at various time. Jc values of 272 A and 276 A samples are nearly two orders of magnitude higher than that of 76 A samples. Both 76 A and 272A samples obtain the best Jc of the wires after post heat treated for 2 h. However, for the 276 A samples, best Jc of the wires was got at post-heat treatment time of 10 h. Tc peak with the increase of RHQ heating current has been reported [11], and 272A and 276A correspond to the left and right side of the current getting the Tc peak. Therefore, the disorder degree difference of the Nb(Al)ss phase in 272 A and 276 A samples leads to different optimal post heat treatment time to obtained the best Jc of the wires. For the 276 A wires possesses more
not only the reflection intensity changed but also the diffraction peak shape modified by different post-heating process. Fig. 2(b) shows that the full width at half maximum (FWHM) of the reflection peak (210) decreases with post-heating time. According to Scherrer formula (D = λ K/β cosθ), where D, λ, K, β and θ is the grain size, the incident X-ray wavelength, the Scherrer constant, FWHM and the Bragg diffraction angle, the grain size gradually grows with the prolongation of post-heat treatment time, confirming that the crystallinity of Nb3Al A15 phase is significantly affected by the post-heat treatment time. Fig. 3(a), (b) and (c) show the temperature dependence of magnetization for the Nb3Al superconducting wires. These figures reveal that the superconducting transition temperature (Tc) of the wires show a slight upward trend with the prolongation of post-heat treatment time. Jorda et al. [9] found that the Tc of Nb3Al increases with the improvement of Al content in Nb3Al. Hence, the gradually increase of Tc might attribute to the stoichiometry improvement of the Nb3Al phase. However, Tc of Nb3Al wires rapidly heated at 76 A is relative low only about 12 K, indicating the composition of Nb3Al phase deviates from stoichiometry. This Tc result is compared to the Nb3Al superconductors formed by conventional low temperature diffusion progress with Tc value of 12–15 K [19,20], corresponding to 22.5 at.% of the solubility of Al in Nb lattice according to Nb-Al phase diagram [9]. Therefore, rapid heating current of 76 A is not enough form Nb(Al)ss phase because of insufficient heating temperature, subsequently, post heat treatment at 800 °C just occurs element diffusion of Nb and Al in the wires rather than the phase transformation form Nb(Al)ss to A15. The Tc values of Nb3Al wires rapidly heated at 272 A and 276 A reach around 17.5 K, which is much higher than that of 76 A. The high Tc indicates the formation of nearly stoichiometry Nb3Al A15 phase which is transformed from Nb(Al)ss phase obtained after rapidly heating at 4
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Fig. 5. The typical SEM images for Nb3Al superconducting wires subjected to RHQ heating current of 276 A post heat-treated for different times.
Fig. 6. EDX spectra of the samples after transformation process of 20 h.
5
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significantly improved for the wires heated to high temperature of about 2000 °C, therefore the Nb layer and Al layer react to form Nb (Al)ss with high Al content in the Nb lattice. However, in some inhomogeneous area with thicker Al layer and thinner Nb layer where the atom ratio of Al to Nb larger than 1:3, besides the formation of Nb(Al)ss near the Nb layer, amorphous layer of Nb-Al with smaller Al content in Nb lattice may form surrounding the Nb(Al)ss after RHQ process. This is consistent with the results of recent work of preparing Nb3Al superconductor by mechanical alloy [23], where Nb-Al amorphous layer formed after short-time ball milling and prolonging milling time results the Nb-Al amorphous layer to crystallize into Nb(Al)ss solid solution with higher solubility of Al in lattice of Nb when its thickness increases beyond a critical thickness. After post heat treatment at 800 °C, Nb(Al)ss phase and amorphous Nb-Al layer transform to A15 Nb3Al phase with different Al content. With prolonging the post heat treat time, the diffusion distance of Al increases and atom ordering of A15 Nb3Al Nb becomes better, leading to shrunken of the dark interspace areas with higher Al content and improvement of the stoichiometry of Nb3Al phase. Pinning behaviors of the Nb3Al superconducting wires were studied. The magnetic flux pinning force (Fp) at 10 K was calculated by Fp = μ0H × Jc. The field dependence of pinning force Fp shown in Fig. 7(a) and Fig. 7(b) exhibit that the maximum value of Fp is about 1.8 × 109 N/m3 and 2.7 × 109 N/m3 for the 272 A and 276 A samples at applied magnetic field of 2 T (μ0H = 2 T). The field dependence of pinning force Fp in the mixed state of type-II superconductors can be complied with the relationship of fp ∝ hm(1-h)n [24,25], where fP = Fp/ Fpmax and h = H/Hirr. Fig. 5 (a1) and (b1) show fP vs h (H/Hirr) curves
Table 1 The average composition of 276 A subjected to post heat-treated for different time. Element
2 h (at.%)
10 h (at.%)
20 h (at.%)
50 h (at.%)
100 h (at.%)
Al Nb
23.32 76.68
24.30 75.70
24.68 75.32
24.75 75.25
24.70 75.30
disordered Nb(Al)ss phase, longer post heat treat time of 10 h is required to get best Jc of the wires. Fig. 5 shows the typical SEM images of 276 A Nb3Al wires post heattreated at various time. Small holes and dark interspace areas can be observed in the Nb3Al superconducting wires, which might attributed to irregular deformation and inhomogeneous distribution of the Nb and Al layers during wire drawing process because of significant hardness difference of Nb and Al laminate. After RHQT, these inhomogeneous Nb and Al layers cause excessive or incomplete reaction, producing holes and interspace areas [17]. Fig. 5 shows that longer post heat treatment time can reduce down the area of dark interspace and improve the homogenization of the wires, which is consistent with the increase of Tc value. Fig. 6 shows the typical EDX spectra of the samples after transformation at 20 h, in which the content of Al is higher in dark interspace areas. Table 1 summaries the average composition of 276 A samples post heat-treated for different time. Improvement of stoichiometry can be observed with the prolongation of post heat-treat time. During the rapid heating and quenching (RHQ) process, the diffusion coefficient of Al and the solubility of Al in the lattice of Nb was
Fig. 7. (a) and (b) The field dependence of magnetic flux pinning force Fp at 10 K for the Nb3Al wires with RHQ heating current of 272 A and 276 A. (a1) and (b1) Normalized pinning force fP (Fp/Fpmax) vs h (H/Hirr) curves and fitting pinning model curves of fp ∝ hm(1-h)n for the samples. 6
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and a fitting curve of fp ∝ hm(1-h)n for a typical sample. From the fitting curves, m is in the range of 0.56–0.65, and n is in the range of 1.96–2.31 for the 272 A RHQT Nb3Al wires; and m is in the range of 0.57–0.67, and n is in the range of 2.06–2.23 for the 276 A RHQT Nb3Al. In both cases, parameters of m and n are close to the results of m = 0.5, n = 2 corresponding to the grain boundary pinning mechanism in D.DewHughes pinning theory [26]. Excessive post-heat treatment time will coarsen the grain size of Nb3Al, which attenuates the grain boundary pinning, resulting in decrease in the Jc performance.
[8]
[9] [10]
[11]
4. Conclusion In this work, we systemically studied the effect of post-heat treatment time on the superconducting properties of Nb3Al wires rapidly heated around the current condition getting Tc peak. With the prolongation of post-heat treatment time, Tc of all RHQT Nb3Al wires show an increasing trend, indicating improvement of the stoichiometry of Nb3Al phase. For the samples rapidly heated at 76 A and 272 A, the best Jc of Nb3Al wires were obtained at post heat treatment time of 2 h, but best Jc of 276 A wires was obtained at 10 h. The disorder degree of the Nb(Al)ss phase was different for 272 A and 276 A samples, resulting in difference of optimal post heat treatment time to obtain the best Jc of the wires. 272 A and 276 A locates on the left and right side of current getting the Tc peak, therefore the Nb(Al)ss phase in 276 A samples is more disordered and longer post heat treatment time is required to obtain the best Jc of the wires. Grain boundary pinning is the main flux pinning mechanism in the Nb3Al superconducting wires.
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements [18]
The authors are grateful to the financial support of the National Key R&D Program of China (No. 2017YFE0301401), the Program of International S&T Cooperation of China (Grant No. 2013DFA51050), and the National Natural Science Foundation of China (Grant Nos. 61404109, 51377138, and 51702266).
[19]
[20]
[21]
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Post heat treatment time induced distinguished superconducting features of JR Nb3Al wires rapidly heated around the Tc peak condition
T
Changkun Yanga, Xiaguang Suna, Pingyuan Lia, Zhou Yua,∗∗, Yongliang Chena, Yong Zhanga, Xifeng Panc, Guo Yanb, Yong Fengb, Yong Zhaoa,c,∗ a
Key Laboratory of Advanced Technology of Materials (Ministry of Education), Superconductivity and New Energy R&D Center (SNERDC), Southwest Jiaotong University, Chengdu, Sichuan, 610031, China b National Engineering Laboratory for Superconducting Materials (NELSM), Western Superconducting Technologies (WST) Co., Ltd, Xi'an, 710018, China c College of Physics and Energy, Fujian Normal University, Fuzhou, Fujian, 350117, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. intermetallics B. phase transformation C. heat treatment D. grain boundary F. Scanning electron microscopy G. superconducting
This work investigated the effect of post heat treat time on the structure and superconducting properties of Nb3Al wires prepared around the current condition of getting Tc peak. Properties of Nb3Al wires rapidly heated at much lower current of 76 A were also studied for comparison. M-T results show that prolongation of post-heat treatment time can elevate the Tc value of all Nb3Al wires because of the stoichiometry improvement of Nb3Al phase. The best Jc of post heat treated Nb3Al wires rapidly heated at 76 A and 272 A were obtained after post heat treated for 2 h at 800 °C, compared to that of 10 h for the 276 A samples. The different optimism post heat treatment time to get best Jc of the wires might attribute to the disorder degree difference of the Nb(Al)ss phase fabricated under different rapid heating current. The main pinning mechanism of Nb3Al superconducting wires was grain boundary pinning, as deduced from the fitting of pinning curve. Gradually degrade of Jc of the Nb3Al wire performed at longer heat treatment time was attributed to the coarse of Nb3Al grain that attenuated grain boundary pinning.
1. Introduction Compared to Nb3Sn superconducting wires, Nb3Al wires fabricated through rapid heating, quenching and transformation process (RHQT) [1–4] show better strain and stress tolerance at high magnetic fields [5–7], thus Nb3Al was considered as an alternative material to apply in high-energy particle accelerators, nuclear magnetic resonance and magnetic confinement fusion devices [7,8]. Based on Nb-Al binary phase diagram, stoichiometric Nb3Al phase can only stably between 1940 and 2060 °C [9]. Hence, RHQT technique was development to fabricate Nb3Al wires, in which the composite Nb-Al precursor wire was rapidly heated to temperature about 2000 °C and then quickly quenched in a molten Ga bath to get body-centered cubic (bcc) supersaturated solid solution (Nb(Al)ss), and followed by annealing the ductile Nb(Al)ss wires at 800 °C for 10 h to get the Nb3Al A15 phase [10]. After decades of development, RHQT is though as a suitable technique to fabricate long-length Nb3Al wires with excellent superconducting properties, but the practical process for large-scale
commercial manufacture of Nb3Al wires has not been realized owing to some difficulties, such as high heating temperature and complex of processing conditions optimization [11]. In the RHQT process, for a given wounding speed, the phase structure, mechanical properties, and superconducting properties are sensitively dependent on the heating current [12]. With increasing heating current Ih, a peak in the Tc-Ih curve appears before entering into the optimal fabrication condition. It is recently discovered that though the feature of the Tc-Ih curve is also sensitive to the tension during the reel-to-reel RHQ process [11], a peak still exists in the Tc-Ih curve. In a sense, the peak can be regarded as an important mark indicating whether the system is already in the optimal condition. It is interesting to further study the structural and superconducting properties of the Nb3Al wires which were prepared in the condition around the peak, in order to clarify the mechanism behind their unique superconducting properties. In this paper, we compared the superconducting properties of different time post heat treated Nb3Al wires rapidly heated around the “Tc
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Corresponding author. Key Laboratory of Advanced Technology of Materials (Ministry of Education), Superconductivity and New Energy R&D Center (SNERDC), Southwest Jiaotong University, Chengdu, Sichuan, 610031, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (Z. Yu), [email protected], [email protected] (Y. Zhao). https://doi.org/10.1016/j.intermet.2019.106482 Received 19 January 2019; Received in revised form 13 April 2019; Accepted 21 April 2019 Available online 28 April 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. XRD patterns for the Nb3Al wires subjected to different post-heat treatment time. (a), (b) and (c) correspond to rapid heating current of 76 A, 272 A and 276 A, respectively.
Fig. 2. (a) Enlarged XRD patterns of (210) peak for the three typical RHQ Nb3Al wires subjected to different post-heat treatment time. (b) Variation of full width at half maximum (FWHM) of the (210) peak with post-heat treatment time for the three typical RHQ Nb3Al superconducting wires.
treat time is 2 h, compared to that of 10 h for the 276 A wires rapidly heated on the right side of the Tc peak.
peak” condition. The M-T results show that prolongation of post-heat treatment time can improve the Tc and stoichiometry of the Nb3Al wires. The disorder degree of the Nb(Al)ss phase is different for Nb3Al wires under different RHQ heating current, resulting in the difference of optimal post heat treat time to obtain best Jc of the wires. For the wires rapidly heated on the left side of the Tc peak [11], the optimal post heat
2. Experimental The single core Nb3Al precursor wire composed of multiple Nb/Al 2
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Fig. 3. (a), (b) and (c) are the M-T curves for the samples with RHQ heating current of 76 A, 272 A and 276 A, respectively. (d) The Tc as a function of post-heat treatment time for the samples.
temperature and magnetic field by using the SQUID magnetometer (MPMS-7T, Quantum Design). The length of all the samples were 4 mm to maintain roughly the same demagnetization factor during the measurement. The temperature range of M-T curves was set from 5 K to 20 K. During M-T measurement process, the samples were cool to 5 K in a zero field and then the magnetic moment was measured at intervals of 0.1 K until the temperature reached 20 K in a field of 20 Oe. Jc was calculated by using the Bean model [18]: (Jc = 15ΔM/R), in which R was the diameter of the samples and the △M was the width of the magnetization loops.
layers was fabricated by the jerry-roll (JR) process [13–16]. Nb foil and Al foil with size of 0.15 × 300 × 2100 mm and 0.05 × 300 × 2100 mm was stacked and wrapped on a Nb rod by hand to an outer diameter of 42 mm. Then the Nb/Al composite was inserted into a Cu can, sealed in high vacuum and extruded into round Cu/Nb-Al rod by hydrostatic extrusion. Subsequently, the wire was carefully cold drawn down to diameter of 0.8 mm by conical dies. Specific process of manufacturing the precursor wires have reported elsewhere [17]. Nb3Al superconducting wires were fabricated RHQT process. In the RHQ process, the mono-filamentary wires without the Cu sheath were reel to reel rapidly heated to high temperature and then were quenched in a molten Ga bath to form bcc Nb(Al)ss phase. Where after, low temperature transformation process was performed to convert the resulting Nb(Al)ss phase into Nb3Al A15 phase with stoichiometric composition and fine grain size. In this work, using a home-made RHQ equipment [17], we have carried out the RHQ process at different joule heating currents (76 A, 272 A and 276 A) with constant wire feeding rate of 0.3 m/s. After finishing the RHQ process, the wires were sealed in quartz tube at vacuum pressure of 10−3 Pa, then annealed at 800 °C with the same ramping rate (200 °C/h) and different holding time (2 h, 10 h, 20 h, 50 h and 100 h) to convert Nb(Al)ss phase to Nb3Al A15 superconducting phase. X-ray diffraction (XRD) with scanning range from 10 to 90° was performed at room temperature to investigate the crystal structure of RHQT-processed Nb3Al wires. The cross-section microstructure and composition were obtained by field emission scanning electron microscopy (FESEM) equipped with an energy-dispersive X-ray spectroscopy (EDX). The magnetic moments were measured as a function of the
3. Results and discussion Fig. 1 displays the XRD patterns for three typical RHQT Nb3Al superconducting wires post heat treated at different time. Fig. 1(a), (b) and (c) show that the main phase of the wires is Nb3Al A15 phase, which can be indexed as (210), (211) and (310) planes of Nb3Al. Besides Nb3Al phase, a small amount of Nb2Al phase was detected in the wires with the rapidly heating current of Ih = 76 A, which could attribute to insufficient reaction of Nb and Al layers at such low heating current. Diffraction peaks of Nb was also found in all annealed wires since the wires are actually a composite material composed of Nb filament in the center of wires. In addition, with prolonging the post-heat treatment time, the intensity of the (210), (211) and (310) peaks gradually increases, especially for the wires rapidly heated below the “Tc peak” current condition, indicating crystallinity improvement of the formed Nb3Al phase. The enlarged patterns of the (210) peak shown in Fig. 2(a) reveal that 3
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Fig. 4. (a), (b) and (c) are Jc-H curves for the Nb3Al superconducting wires. (d) Jc measured at 6 T for the Nb3Al samples heat treated at 800 °C for various time.
272 A and 276 A. Fig. 3(d) shows the Tc as a function of post-heat treatment time for the Nb3Al samples. Tc of 272 A and 276 A samples are much higher than that of 76 A. With the prolongation of post-heat treatment time, all Nb3Al wires show upward trend of Tc value, indicating improvement of stoichiometry of the Nb3Al phase. More accurately, the Tc of Nb3Al wires rapidly increased as prolonging heat treatment time from 2 h to 10 h, and then the Tc elevates much slower as longer heat treatment time. Fig. 4 shows the Jc performance of Nb3Al wires measured at 4.2 K and 10 K. Jc of the 272 A and 276 A rapidly heated Nb3Al wires reach around 106 A/cm2 at 4.2 K, 0 T, which are closed to the result of RHQT Nb3Al wires [21,22]. Jc value of the 76 A Nb3Al samples at 4.2 K, 0 T are much lower than that of 272 A and 276 A samples because of formation of off-stoichiometry Nb3Al phase. As prolonging the post heat treatment time from 2 h to 100 h, Jc of the 76 A and 272 A samples monotonously decreases, as shown in Fig. 4(a) and (b); however, Jc of the 276 A samples (Fig. 4(c)) increases first and then decreases where the best Jc of the wires was obtained after heat treatment for 10 h. Fig. 4(d) summaries Jc values at 4.2 K, 6 T of the Nb3Al samples after post heat treated at various time. Jc values of 272 A and 276 A samples are nearly two orders of magnitude higher than that of 76 A samples. Both 76 A and 272A samples obtain the best Jc of the wires after post heat treated for 2 h. However, for the 276 A samples, best Jc of the wires was got at post-heat treatment time of 10 h. Tc peak with the increase of RHQ heating current has been reported [11], and 272A and 276A correspond to the left and right side of the current getting the Tc peak. Therefore, the disorder degree difference of the Nb(Al)ss phase in 272 A and 276 A samples leads to different optimal post heat treatment time to obtained the best Jc of the wires. For the 276 A wires possesses more
not only the reflection intensity changed but also the diffraction peak shape modified by different post-heating process. Fig. 2(b) shows that the full width at half maximum (FWHM) of the reflection peak (210) decreases with post-heating time. According to Scherrer formula (D = λ K/β cosθ), where D, λ, K, β and θ is the grain size, the incident X-ray wavelength, the Scherrer constant, FWHM and the Bragg diffraction angle, the grain size gradually grows with the prolongation of post-heat treatment time, confirming that the crystallinity of Nb3Al A15 phase is significantly affected by the post-heat treatment time. Fig. 3(a), (b) and (c) show the temperature dependence of magnetization for the Nb3Al superconducting wires. These figures reveal that the superconducting transition temperature (Tc) of the wires show a slight upward trend with the prolongation of post-heat treatment time. Jorda et al. [9] found that the Tc of Nb3Al increases with the improvement of Al content in Nb3Al. Hence, the gradually increase of Tc might attribute to the stoichiometry improvement of the Nb3Al phase. However, Tc of Nb3Al wires rapidly heated at 76 A is relative low only about 12 K, indicating the composition of Nb3Al phase deviates from stoichiometry. This Tc result is compared to the Nb3Al superconductors formed by conventional low temperature diffusion progress with Tc value of 12–15 K [19,20], corresponding to 22.5 at.% of the solubility of Al in Nb lattice according to Nb-Al phase diagram [9]. Therefore, rapid heating current of 76 A is not enough form Nb(Al)ss phase because of insufficient heating temperature, subsequently, post heat treatment at 800 °C just occurs element diffusion of Nb and Al in the wires rather than the phase transformation form Nb(Al)ss to A15. The Tc values of Nb3Al wires rapidly heated at 272 A and 276 A reach around 17.5 K, which is much higher than that of 76 A. The high Tc indicates the formation of nearly stoichiometry Nb3Al A15 phase which is transformed from Nb(Al)ss phase obtained after rapidly heating at 4
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Fig. 5. The typical SEM images for Nb3Al superconducting wires subjected to RHQ heating current of 276 A post heat-treated for different times.
Fig. 6. EDX spectra of the samples after transformation process of 20 h.
5
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significantly improved for the wires heated to high temperature of about 2000 °C, therefore the Nb layer and Al layer react to form Nb (Al)ss with high Al content in the Nb lattice. However, in some inhomogeneous area with thicker Al layer and thinner Nb layer where the atom ratio of Al to Nb larger than 1:3, besides the formation of Nb(Al)ss near the Nb layer, amorphous layer of Nb-Al with smaller Al content in Nb lattice may form surrounding the Nb(Al)ss after RHQ process. This is consistent with the results of recent work of preparing Nb3Al superconductor by mechanical alloy [23], where Nb-Al amorphous layer formed after short-time ball milling and prolonging milling time results the Nb-Al amorphous layer to crystallize into Nb(Al)ss solid solution with higher solubility of Al in lattice of Nb when its thickness increases beyond a critical thickness. After post heat treatment at 800 °C, Nb(Al)ss phase and amorphous Nb-Al layer transform to A15 Nb3Al phase with different Al content. With prolonging the post heat treat time, the diffusion distance of Al increases and atom ordering of A15 Nb3Al Nb becomes better, leading to shrunken of the dark interspace areas with higher Al content and improvement of the stoichiometry of Nb3Al phase. Pinning behaviors of the Nb3Al superconducting wires were studied. The magnetic flux pinning force (Fp) at 10 K was calculated by Fp = μ0H × Jc. The field dependence of pinning force Fp shown in Fig. 7(a) and Fig. 7(b) exhibit that the maximum value of Fp is about 1.8 × 109 N/m3 and 2.7 × 109 N/m3 for the 272 A and 276 A samples at applied magnetic field of 2 T (μ0H = 2 T). The field dependence of pinning force Fp in the mixed state of type-II superconductors can be complied with the relationship of fp ∝ hm(1-h)n [24,25], where fP = Fp/ Fpmax and h = H/Hirr. Fig. 5 (a1) and (b1) show fP vs h (H/Hirr) curves
Table 1 The average composition of 276 A subjected to post heat-treated for different time. Element
2 h (at.%)
10 h (at.%)
20 h (at.%)
50 h (at.%)
100 h (at.%)
Al Nb
23.32 76.68
24.30 75.70
24.68 75.32
24.75 75.25
24.70 75.30
disordered Nb(Al)ss phase, longer post heat treat time of 10 h is required to get best Jc of the wires. Fig. 5 shows the typical SEM images of 276 A Nb3Al wires post heattreated at various time. Small holes and dark interspace areas can be observed in the Nb3Al superconducting wires, which might attributed to irregular deformation and inhomogeneous distribution of the Nb and Al layers during wire drawing process because of significant hardness difference of Nb and Al laminate. After RHQT, these inhomogeneous Nb and Al layers cause excessive or incomplete reaction, producing holes and interspace areas [17]. Fig. 5 shows that longer post heat treatment time can reduce down the area of dark interspace and improve the homogenization of the wires, which is consistent with the increase of Tc value. Fig. 6 shows the typical EDX spectra of the samples after transformation at 20 h, in which the content of Al is higher in dark interspace areas. Table 1 summaries the average composition of 276 A samples post heat-treated for different time. Improvement of stoichiometry can be observed with the prolongation of post heat-treat time. During the rapid heating and quenching (RHQ) process, the diffusion coefficient of Al and the solubility of Al in the lattice of Nb was
Fig. 7. (a) and (b) The field dependence of magnetic flux pinning force Fp at 10 K for the Nb3Al wires with RHQ heating current of 272 A and 276 A. (a1) and (b1) Normalized pinning force fP (Fp/Fpmax) vs h (H/Hirr) curves and fitting pinning model curves of fp ∝ hm(1-h)n for the samples. 6
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and a fitting curve of fp ∝ hm(1-h)n for a typical sample. From the fitting curves, m is in the range of 0.56–0.65, and n is in the range of 1.96–2.31 for the 272 A RHQT Nb3Al wires; and m is in the range of 0.57–0.67, and n is in the range of 2.06–2.23 for the 276 A RHQT Nb3Al. In both cases, parameters of m and n are close to the results of m = 0.5, n = 2 corresponding to the grain boundary pinning mechanism in D.DewHughes pinning theory [26]. Excessive post-heat treatment time will coarsen the grain size of Nb3Al, which attenuates the grain boundary pinning, resulting in decrease in the Jc performance.
[8]
[9] [10]
[11]
4. Conclusion In this work, we systemically studied the effect of post-heat treatment time on the superconducting properties of Nb3Al wires rapidly heated around the current condition getting Tc peak. With the prolongation of post-heat treatment time, Tc of all RHQT Nb3Al wires show an increasing trend, indicating improvement of the stoichiometry of Nb3Al phase. For the samples rapidly heated at 76 A and 272 A, the best Jc of Nb3Al wires were obtained at post heat treatment time of 2 h, but best Jc of 276 A wires was obtained at 10 h. The disorder degree of the Nb(Al)ss phase was different for 272 A and 276 A samples, resulting in difference of optimal post heat treatment time to obtain the best Jc of the wires. 272 A and 276 A locates on the left and right side of current getting the Tc peak, therefore the Nb(Al)ss phase in 276 A samples is more disordered and longer post heat treatment time is required to obtain the best Jc of the wires. Grain boundary pinning is the main flux pinning mechanism in the Nb3Al superconducting wires.
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements [18]
The authors are grateful to the financial support of the National Key R&D Program of China (No. 2017YFE0301401), the Program of International S&T Cooperation of China (Grant No. 2013DFA51050), and the National Natural Science Foundation of China (Grant Nos. 61404109, 51377138, and 51702266).
[19]
[20]
[21]
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