Nb3Al Wires Research Articles

New-route of flux pinning enhancement in Nb3Al superconductor: density and orientation regulation of stacking faults by nano-oxide particles

Abstract The mechanism of critical current density (Jc) enhancement in rapid heating, quenching and transformation (RHQT) Nb3Al wires with nanoparticles was investigated from microscopic defects. As shown in transmission electron microscopy (TEM) results, stacking faults (SF) density in RHQT Nb3Al could be improved by incorporating nano-oxide particles, because the nanoparticles distorted the Nb3Al crystal lattice and the expansion of the distortion generated stacking faults. The macroscopic measurements based on electron backscatter diffraction (EBSD) showed that many stacking faults were parallel to the Lorentz force (FL//SF) in the pure sample, but orientation of stacking faults distributed randomly in the Nb3Al wires with nano oxide. Stacking faults that were normal to the Lorentz force (FL⊥SF) could hinder the motion of flux lines effectively and thus enhance the total flux pinning force (Fp). As a result, Jc of incorporated nano oxide Nb3Al wires were significantly elevated, where 1 at.% nano-ZrO2 and 1 at.% nano-Dy2O3 sample severally exhibited Jc values of 1.86×105 A/cm2 and 1.95×105 A/cm2@4.2 K,12 T, which improved by about 60% compared to the pure sample. The density and orientation regulation of stacking faults in RHQT Nb3Al revealed the new mechanism of Jc enhancement, which provided a new route to further improve the high-field Jc of Nb3A in future research.

Critical current density improvement of RHQT Nb3Al superconducting wires by incorporating magnetic Dy2O3 nanoparticles as additional flux pinning centers

In this paper, we prepared magnetic Dy2O3 nanoparticles (n-Dy2O3) doped Nb3Al superconducting wires by rapid heating, quenching and transformation (RHQT) method, and investigated the micro-defects, superconducting properties and flux pinning mechanisms. TEM results showed high-density stacking faults (SF) with spacing of 5–25 nm located in Nb3Al (210) crystal planes, which is due to the absence of Nb atoms on the {100} crystal planes. Additionally, many intra-granular and inter-granular Dy2O3 nanoparticles with size of 4–40 nm were also observed in the Nb3Al superconducting phase. Jc was promoted in the doped samples, obtaining the best Jc of 1.95 × 105 A/cm2@ 4.2 K, 12 T in 1 at.% n-Dy2O3 doped Nb3Al wire, which was nearly 70 % higher than pristine Nb3Al wire. By doping n-Dy2O3, the fP(Fp/Fp,max)-h(B/Birr) peaks shift to larger h value (higher field). The flux pinning mechanism by fitting fP-h curves showed dominant surface pinning combined with point pinning, corresponding to the crystal defect of stacking faults and nanoparticles, respectively. The pinning mechanism verified that the nanoparticles acted as additional pinning centers to significantly elevate the Jc performance.

Introducing nano-scale MgO particles into RHQT Nb3Al superconducting wires to significantly improve the critical current density

This research introduced nano-scale particles into the RHQT Nb3Al superconducting wires, which was confirmed by TEM that nanoparticles with size of 5–25 nm distributed in the wires and might act as the extra flux pinning centers. The precursor wires were fabricated by powder in tube process with nano-sized MgO (n-MgO) doped in the raw powder. Uniform microstructure and chemical composition were obtained in the superconducting wires. Besides nanoparticles, high density stacking faults with spacing of 5–20 nm were observed in the Nb3Al superconducting wire, which acted as the dominant flux pinning centers. It had great importance that nanoparticles were introduced into the RHQT Nb3Al wires and combined with the high density stacking faults to significantly improve the critical current density (Jc) of the wires. The magnetic Jc results of 1 wt% n-MgO doped Nb3Al wire were about twice of the non-doped sample, corresponding to Jc values of 3.18 × 105 A/cm2, 2.38 × 105 A/cm2 and 1.72 × 105 A/cm2 at 4.2 K and 8 T, 10 T and 12 T.

Reproducible stable critical current density in powder-in-tube (PIT) Nb3Al superconductors by multi-time rapid heating and quenching process

This paper investigated the microstructure and superconductivity of the powder-in-tube (PIT) Nb3Al short samples with different rapid heating and quenching (RHQ) treatment. Single RHQ processed Nb3Al wires showed inhomogeneous microstructure and the superconductivity were sensitive to rapid heating temperature (Tmax), showing significant fluctuation with a slight change of Tmax. Multi-time RHQ treatment was performed to homogenize the Nb3Al superconductors. The highly uniform microstructure and composition distribution were found in 5-time RHQ processed Nb3Al wires, which exhibited significantly stable and excellent Jc performance. Reproducible stable Jc results of Nb3Al wires were realized in three samples of 5-time RHQ treatment with a difference of Tmax reaching about 200 °C. Jc of the three samples fluctuated less than 20% with results of 1.1 × 105 A/cm2, 9.87 × 104 A/cm2 and 9.51 × 104 A/cm2 at 4.2 K and 12 T. The insensitivity of Jc to Tmax in 5-time RHQ processed Nb3Al samples might attribute to the highly homogeneous Nb3Al A15 phase.

Improvement of critical current density J c in powder-in-tube rapid heating, quenching and transformation Nb3Al wires by doping with nano-SnO2

We prepared Nb3Al superconducting wires doped with nano-size SnO2 (n-SiO2) particles through a multi-time rapid heating and quenching process and investigated their microstructure and superconducting properties. All the samples showed a highly homogeneous A15 Nb3Al phase. Compared with pure Nb3Al, the n-SnO2 doped Nb3Al wires presented a larger ΔT c value and a higher J c value. The best J c at 4.2 K was found in the 1 wt% n-SnO2 doped Nb3Al sample, with 3.37 × 105 A cm−2, 2.55 × 105 A cm−2 and 1.80 × 105 A cm−2 at 8 T, 10 T and 12 T, respectively. These results were an improvement of about 60% compared with pure Nb3Al at the same applied fields. The maximum irreversible field value was obtained in the 1 wt% SnO2-doped Nb3Al wire, with a result of 29.5 T at 4.2 K. The improvement of J c performance in the n-SnO2 doped Nb3Al wires might be attributed to the formation of artificial nanoparticles in the grain, which act as extra effective flux pinning centers.

Correlation between J enhancement and phase homogeneity in Sn-Ge co-doped Nb3Al superconducting wires

In response to the growing demand for critical current density (Jc) in superconducting wires, Sn-Ge co-doped Nb3Al wires were fabricated by the rapid-heating, quenching and transformation (RHQT) method. Physical Property Measurement System-Vibrating Sample Magnetometer (PPMS-VSM) was used to perform magnetic measurements at low-temperature and external field to estimate Jc and superconducting transition temperature (Tc). Backscattered electron (BSE) image acquisition and EDS analysis of the polished surface by FESEM were used to determine phase homogeneity and elemental distribution. A significant positive correlation was found between Jc (up to 7 T at 8 K) and ΔTc. Compared with binary sample, doubled Jc was obtained in the low-dose Sn-Ge co-doped Nb3Al wire, reaching 3.01 × 103 A/mm2, 1.46 × 103 A/mm2 and 0.39 × 103 A/mm2 at 5 T and 8 K, 10 K and 12 K, respectively. The increase in Jc can be attributed to the enhancement of the homogeneity of the superconducting phase, especially the distribution of Al element therein.

Microstructure identification of powder in tube Nb3Al wires during rapid heating process and properties of the transformed superconductors

This paper systematically studied the microstructure and phase formation sequence in the powder in tube (PIT) Nb3Al processed by rapid heating and quenching (RHQ), which is a crucial issue to fabricate reliable high performance practical Nb3Al superconductors. Due to significantly inhomogeneous of the Nb/Al diffusion couple distance, an eutectic phase with liquid characteristic happened during the RHQ process, where the phase formation sequence is: Nb + Al → eutectic phase + NbAl3 → Nb3Al + Nb2Al → grid morphology bcc → band feature bcc with the increase of rapid heating temperature. The phase and microstructure during RHQ are mainly determined by the RHQ condition and local Nb/Al distribution that similar feature was shown in the quenched wires with initial Al content of 22–28 at.% in the raw powders. Grid and band microstructure of the bcc phase was related to the Al content of ~32 at.% and ~ 22 at.%, respectively. After transformation process at 800 °C, quenched wires with coexist of gird and band micro-structure show the best superconducting properties, including Tc, ΔTc, Birr, and Jc. The microstructure identification confirms that uniform and thin Al layer in the precursor wire plays an essential role in obtaining a homogeneous bcc solid phase.

Phase formation and enhanced performance of Nb3Al prepared by spark plasma sintering and subsequent annealing

Although the Nb3Al wire prepared by the rapid heating and quenching (RHQ) technique has achieved excellent superconducting critical current density (Jc), various new preparation techniques have still been continuously tried to improve its performance. In this paper, the phase formation and superconducting properties of Nb3Al prepared with spark plasma sintering (SPS) was studied. It is found that the SPS in the temperature range of 1200–1400 °C can quickly produce high-density, nano-structured Nb3Al superconductor, which has a transition temperature around 16 K. Due to its tight and clean grain connection, the sample shows a high-performance current-carrying capacity in self-field. After post-annealing, the flux pinning force and the superconducting performance are significantly improved. The study clarified that the phase formation mechanism of the SPSed Nb3Al is similar to that directly formed from high temperature liquid phase rather than the transformation from bcc phase to A15 phase. The high temperature liquid phase may originate from the local high temperature induced by the plasma heating in the early stage of SPS process. Such a unique phase formation mechanism results in very few structural defects existing inside the sample. The results of this study suggest that chemical doping or other means which can introduce high density structure defects in Nb3Al should be considered to further improve the in-field Jc for the SPSed Nb3Al.

Improved critical current density of Cu-doped Nb3Al wire through optimized Rapid-Heating Quenching and Transformation method

We have prepared 2.5% Cu-doped Nb3Al wire by the Rapid-Heating Quenching and Transformation (RHQT) method with optimized annealing. XRD results show that A15 and supersaturated bcc phase are formed after high current RHQ process, and the supersaturated bcc phase is transformed into A15 phase after conventional annealing (800 °C\\10 h). We observed the microstructures at different annealing time by field-emission scanning electronic microscope (FESEM) and found that σ phase is generated from the decomposition of the metastable A15 phase after prolonged annealing due to its lower free energy at 800 °C, and gradually accumulates Cu to form the Cu-rich σ phase after 20 h. The low-field superconducting properties were also given through magnetic measurement. Both Tc and Bc2 decreased slightly along with the increase of σ phase volume when the annealing is prolonged, while Jc reached its peak after 20 h\\800 °C annealing, which were 1836 A/mm2 at 4.2 K and 5 T and 822 A/mm2 at 8 K and 5 T for 800 °C\\20 h annealing, about 34% and 36% higher than that of the conventional transformed sample, respectively. The calculated Fpmax of the samples with optimized annealing condition also increased accordingly. The fitting analysis of fp using the Dew-Hughes model showed that the pinning mechanism was surface pinning. The additional diffusion channels for Cu provided by newly generated interfaces of σ and Nb(Al) particles enhance the Cu segregation on the A15 boundaries and may be the source of the pinning force improvement.

Ultra-Fine Nb3Al Mono-Core Wires and Cables

Jelly-roll (JR) processed wires using metal thin foils as starting materials have excellent cold workability and drawability. We have successfully fabricated Nb/Al composite mono-core wires having 50 μm in outer diameter and over 400 meters in length. Furthermore, some of the wires were additionally drawn to 30 μm in outer diameter. It is known that the critical current density (Jc) of Nb3Al synthesized by low-temperature diffusion reactions increases with decreasing the diffusion distance between Nb and Al. In this study, we have confirmed that Jc would increase with decreasing the wire diameter, and the maximum Jc has been obtained on Nb3Al wires with 30 μm in diameter. In addition, a 7-wire round cable has also been demonstrated using 80 μm diameter wire, with an Ic 7 times the Ic of a single wire. A transport current test was performed for the cable sample simulating W&R and R&W processes using a 50 mm diameter G10 holder. There were no differences between the Ic and the n-values in the W&R and R&W processes. Moreover, as a feasibility study, a 3rd bundle Rutherford cable has been successfully fabricated using 3,108 pure copper wires 50 μm in diameter.

Microstructure and Superconducting Properties of Rapid Heating, Quenching, and Transformation (RHQT) Powder‐in‐Tube Nb3Al Wires Doped with Sn Element

Properties of rapid heating, quenching, and transformation (RHQT)‐treated powder‐in‐tube (PIT) Nb3Al1−xSnx wires are studied herein. After the RHQ process, the body‐centered cubic (bcc) phase is formed in the Sn‐doped Nb3Al wires. The transformed superconducting A15 phase shows a uniform microstructure and homogenous component distribution feature. With the increase of the Sn doping level, the critical transition temperature (Tc), critical current density (Jc), and irreversibility field (Birr) of the superconducting wires increase first and then reduce, achieving the highest Tc value of 17.6 K and the smallest ΔTc of 0.6 K in the 4% Sn doped Nb3Al wire. The 4% Sn addition helps to improve the layer Jc of the sample to 2.1 × 105 A cm−2 at 8 K and 5 T, which is about 2.5 times that of the pure Nb3Al sample. Furthermore, the Nb3Al1−xSnx (x = 0.04) wire gives the highest Birr value of 17.22, 13.23, and 9.11 T at 8, 10, and 12 K, respectively. The addition of Sn helps to achieve stoichiometric Nb3Al A15 phase and formation of precipitations with the size of 10 nm, which might act as the flux‐pinning center.

Effect of Cu doping on properties of RHQT processed powder in tube Nb3Al wires

Powder-in-tube (PIT) Nb3(Al1-xCux)(x = 0–0.04) superconducting wires were fabricated by rapid heating, quenching and transformation (RHQT) process. With the addition of Cu, disordered Nb3Al A15 phase with stoichiometric composition was formed in the quenched wires and atomic long range ordering occurred during the low temperature annealing process, resulting in improvement of Tc, Jc and Birr performance compared to the pure Nb3Al sample. The highest Tc of 18.0 K, narrowest ΔTc of 0.6 K and best layer Jc of about 30481 A/cm2 @12 K, 4T was found in the Nb3(Al0·97Cu0.03) wire, which is nearly four times of pure Nb3Al sample. The Nb3(Al0·97Cu0.03) wire also shows the highest Birr value of 25.7T at 8 K and 12.9T at 12 K, respectively. The formed A15 phase exhibit uniform microstructure and composition. Flux pinning of the Nb3(Al1-xCux) wires follows the surface pinning mechanism, where grain boundary and stacking faults were considered as pinning centers.

Fabrication and Electromagnetic Properties of 24-Filament Jelly-Roll Nb3Al Superconducting Long Wire with Reel-to-Reel Rapid Heating and Quenching Heat Treatment

In this paper, a simple 24-filament structure Nb3Al superconducting wire has been firstly prepared by using the jelly-roll Nb/Al composite wire, the reel-to-reel rapid heating and quenching (RHQ), and phase transformation heat treatment. In order to obtain the Nb3Al long wire with good superconducting properties, various heating current (HC) and wire moving speed (WMS) were used to produce high-quality Nb(Al)ss supersaturated solid solution during RHQ process; thereafter, the Nb(Al)ss wire was put into the Cu tube and drawn for preparing the Cu-cladded Nb3Al wire. The results suggest that for the Nb3Al wire with the HC 262 A and WMS of 300 mm/s, its onset Tc and layer Jc at 4.2 K and 12 T measured by the transport way reach 17.7 K and 1.58 × 105 A/cm2, respectively. The magnetization measurement (M-H) of Nb3Al wires indicate that the diameter reduction of Nb(Al)ss solid solution wires after RHQ will significantly improve their grain connectivity and thus Jc properties at the optimal heat treatment conditions. The work suggests the simple-structured Nb3Al superconducting wire is promising to the practical high-field magnet application.

Superconducting property improvement of RHQT Nb3Al wires through doping of Ti

This work reports the improvement of superconducting properties of powder in tube RHQT Nb3Al wires by Ti doping. The crystal structure, superconducting properties and microstructure of RHQT (Nb1-xTix)3Al (x = 0–0.03) wires were systematically studied. Compared to pure Nb3Al sample, the RHQT (Nb1-xTix)3Al wires with Ti doping content beyond 2 at% show improvement of critical transition temperature (Tc), critical current density (Jc) and irreversible field (Birr). Among all RHQT (Nb1-xTix)3Al wires, (Nb1-xTix)3Al (x = 0.025) sample exhibits the highest Tc value of 16.7 K and the smallest ΔTc of 0.5 K, indicating better phase uniformity of the wire. Layer Jc of (Nb1-xTix)3Al (x = 0.025) sample is about 2.63 × 105 A/cm2 at 8 K, 5 T, which is about twice of the pure Nb3Al sample. This sample also shows the highest Birr value of 12 T, 9.6 T and 7 T at 8 K, 10 K and 12 K, respectively. Flux pinning mechanism of the (Nb1-xTix)3Al wires is consistent of surface pinning model. Surface morphology of the RHQT (Nb1-xTix)3Al superconductors is compact with excellent grain connectivity.

Comparison of phase transformation and superconducting properties of RHQT Nb3Al wires fabricated at Tc peak and valley condition

This work compared the phase transformation feature and superconducting properties of Nb3Al wires rapidly heated at 272 A and 315 A, closing to the Tc peak and valley current condition, respectively. The fabricated rapidly heating, quenching and transformation (RHQT) Nb3Al wires are mainly consisted of A15 phase with trace Nb and Nb2Al impurity phase. With the increase of annealing temperature from 700 °C to 1100 °C, the Tc value and Jc performance of Nb3Al wires improve first and then gradually decrease. M-T and Jc-H curves reveal that the best Tc and Jc performance of 272 A samples was obtained at the annealing temperature of 800 °C, compared to that of 900 °C for the 315 A samples. In both 272 A and 315 A samples, lower annealing temperature cannot promote the phase transform process from bcc to A15 phase. Excessive heating beyond 1000 °C caused the already transformed Nb3Al phase to decompose into Nb2Al and Al-poor Nb3Al phase, resulting in degraded Tc and Jc performance of the Nb3Al wires. The main pinning mechanism of Nb3Al wires was grain boundary pinning, as deduced from the fitting of flux pinning force versus applied field curves.

Fabrication of Nb3Al Superconducting Wires Through Rapid Heating/Quenching in Open Air

Nb 3 Al wire requires a simplified fabrication process as well as improved critical current density. We found that the reel-to-reel Rapid Heating/Quenching (RHQ) treatment could be safely operated in open air without an expensive dynamic vacuum system or a large metal vacuum chamber. It should provide a significant cost reduction. The electrode distance between the Cu pulley and the liquid Ga bath surface is set for 125 mm. The precursor wire is moving at speeds of 0.3, 0.6, or 1.0 m/s. The wire surface material (Nb) reacted with oxygen and made a homogeneous Ga-Nb oxide layer of several microns in thickness. Its surface morphology was very smooth, and it acted as a lubricant for the metal drawing dies of which would be considered a similar effect as an anode oxidation drawing technique. The Nb-Al supersaturated bcc solid solution alloy could be synthesized by RHQ in open air, and its synthesis condition might be shifted to slightly lower input heating energy. The heating energy in open air is probably adding an additional heating energy from the surface oxidation reaction.

Post heat treatment time induced distinguished superconducting features of JR Nb3Al wires rapidly heated around the Tc peak condition

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.

Improvement of Superconducting Properties by Optimized Post-Heat Treatment for Nb3Al Wires Fabricated by RHQ

With extremely high critical current density (Jc) and excellent strain tolerance, Nb 3 Al superconductor is considered as an alternative to Nb 3 Sn for application of high-filed magnets. However, complexity in the phase formation of Nb 3 Al hinders the Nb 3 Al superconducting wires to satisfy the requirement of engineering applications at present. Here, we have reported the improved performance of simple-structured 18-filamnet jelly-roll Nb 3 Al precursor long wires fabricated with rapid heating and quenching (RHQ) process. After processed with RHQ heat-treatment under various heating conditions, the Nb 3 Al wire were post-heat treated at a temperature between 700 °C to 1100 °C. The phase formation and the superconducting properties of the wires were investigated. It has been found that, even if the RHQ process deviates from the optimum processing state, the superconducting properties of the wire can be improved by appropriately selecting post-heat treatment conditions.

Reel-to-Reel Heat Treatment, Microstructure, and Superconducting Properties of Jelly-Roll Nb3Al Superconducting Wire

In this work, we utilized the jelly-roll method to make a single-filament Nb3Al precursor long wire, combining with a homemade reel-to-reel rapid heating and quenching (RHQ) equipment, and consequently the high-performance Nb3Al superconducting wires had been prepared. By investigating the heating current and wire moving speed, the optimized RHQ heat treatment conditions of this Nb3Al wire were obtained. The results suggest that the Tc for the Nb3Al superconducting wire is up to 17.4 K and the non-Cu critical current density (non-Cu Jc) reaches 1238 A/mm2 at 4.2 K and 12 T, which indicates the bcc-phase Nb–Al supersaturated solid solution has already been generated. Considering the Nb3Al precursor wire with a large thickness of Al layer, about 690 nm, such high Jc is quite unexpected. The SEM analysis presents that there is still some unreacted Nb and Nb2Al phase in the superconducting wire, which may result from the inhomogeneity of Nb and Al foils. This work implies the Nb3Al superconducting wire is promising to the application of high-field superconducting magnets.

Superconducting Properties and Crystalline Structure of High-Performance Nb3Al Wires Fabricated by RHQ and Mechanical Alloying Methods

In this paper, we compare the superconducting properties of Nb3Al wires prepared by two different techniques. The first is the in situ powder-in-tube method: first, the powder of the Nb–Al saturated solid solution, Nb(Al)ss, prepared by mechanical alloying is filled into a metal niobium tube, and then subjected to cold working such as swaging and drawing to form a wire. Heat treatment at 800–1000 °C converts the Nb(Al)ss phase to the Nb3Al phase. Another method is to use the jelly-roll technology to make the Nb–Al precursor wire, then use the rapid heating and quenching (RHQ) process to form the Nb(Al)ss phase, and finally, to transform the Nb(Al)ss phase into Nb3Al by heat-treatment at 800–900 °C. We found that although both the mechanical alloying method and the RHQ technique can make the Nb(Al)ss phase, the properties of the finally obtained Nb3Al wire have a big gap: the Nb3Al wire prepared by the RHQ process has much higher Tc and Jc . Phase structure analyses show that the RHQ energy density in the RHQ method plays a similar role as the milling time does in the mechanical alloying method, both of these parameters play important roles in controlling the crystal structure and microstructure of the Nb3 Al wires.