1. Introduction
It is generally considered that Bi2Sr2CaCu2O8+δ (Bi-2212) round wires are a promising option for use in a range of applications such as round stranding, cable-in-conduit conductors, and Rutherford cables for some high field magnets [1], [2], [3]. Bi-2212 round wires have high upper-critical fields (Hc2 exceeding 100 T at 4.2 K), are isotropic, can be easily twisted, and have multi-filamentary architecture. For example, they have a high engineering critical current density (Je) of 950A/ mm2 at 30 T and 4.2 K, and racetrack coils have been successfully fabricated with these Bi-2212 wires [4].
The powder in tube (PIT) process is used to fabricate Bi-2212 wires. The composition of the precursor powder affects the controlling critical current density (Jc) of Bi-2212/Ag wires [5]. Some efforts have been devoted to controlling the properties of precursor powders. Synthesis methods, such as co-precipitation [6], spray pyrolysis [7], melt cast [8], [9], freeze drying [10], [11], and combustion chemical vapor condensation [12], have been used to improve the quality of precursor powders. In Nexans, the melt casting process has been optimized to tune the composition, particle size and distribution, and carbon content [9], [13], [14]. Several annealing, milling, and pressing steps have been used to obtain high-purity Bi-2212 powders. The powder composition of Bi2.17Sr1.94Ca0.89Cu2Ox (called 521) has been recommended for high-performance Bi-2212 wires [5], [13]. Recently, highly homogeneous Bi-2212 powders have been made using chemical combustion in Engi-mat, and the superconducting performance of wires was greatly improved [4], [15]. For Bi-2212 bulks, the dissolution of the secondary phase or 2201 intergrowths in the Bi-2212 grains was investigated [16], [17]. Investigations on several parameters of powders and preparation methods for Bi-2212 wires have been reported [18]. However, the issue of secondary phase of powders has not been thoroughly considered. It is known that the high-phase purity and homogeneous particles are the basic requirements. The detailed understanding of the effect of secondary phase on the microstructure and final Jc of wires is still far from being achieved. More investigations are still needed regarding the formation mechanism of the secondary phase and the transformation of powders to ultimately achieve good texture and performance of Bi-2212 wires.
In this work, precursor powders were fabricated using ultrasonic spray pyrolysis (USP), and the residual secondary phase was tuned using calcination. A thorough analytical study of the phase and microstructure evolution of precursor powders was used to discuss the origin of the secondary phase in the powders. The role that the secondary phase plays on the Bi-2212 wires is analyzed. Variations in the microstructure, the melting behavior, and the final performance of wires may be ascribed to the residual secondary phase in precursor powders.
2. Experimental methods
Bismuth nitrate, strontium nitrate, calcium nitrate, and copper nitrate were dissolved in pure water to prepare a solution. The stoichiometry of Bi:Sr:Ca:Cu was maintained at 2.17:1.94:0.89:2.00. The solution was directly ultrasonically sprayed into the high temperature pyrolysis chamber to obtain a mixture of oxides [7], [19]. The nitrate solutions [Bi(NO)3, Sr(NO3)2, Ca(NO3)2, and Cu(NO3)2] were directly sprayed into a high-temperature pyrolysis chamber to obtain a mixture of oxides. Then, the corresponding oxides were calcined to form Bi-2212 grains. Each of the following calcination temperatures and dwell times were used: 750 °C/20 h, 790 °C/20 h, 810 °C/20 h, 830 °C/20 h, 830 °C/20 h + 850 °C/20 h, and 850 °C/20 h + 865 °C/20 h. Based on the final calcination temperature, the last three powders were named as P830, P850, and P865, respectively.
The 37 × 18 filaments and monofilament wires were made with P830, P850, and P865 using PIT with a packing tube, drawing, and re-bundling. The corresponding wires were named as WP830, WP850, and WP865, respectively. The final green wires that were obtained had a diameter of Ø1.0 mm. The fully reacted wire was obtained using an overpressure heat treatment process (OPHT) [20], [21]. The pressure was controlled at 5 MPa during the OPHT. The green wire was heated from room temperature to 820 °C at a rate of 160 °C/h, held at 820 °C for 2 h, and heated from 820 °C to 890 °C at a rate of 60 °C/h, and held at 890 °C for 12 min. Then, the wire was cooled from 890 °C to 880 °C at a rate of 10 °C/h, cooled from 880 °C to 830 °C at a rate of 2.5 °C/h, held at 830 °C for 24 h, and cooled from 830 °C to room temperature at a rate of 80 °C/h. For the reference wire, which was prepared with Nexans powder, the parameters of PIT and OPHT were consistent.
Monofilament wires were also subjected to ambient pressure heat treatment of 864 °C/10 h and 890 °C/0h in 0.1 MPa oxygen to study changes in microstructure. The quenched monofilament wire was cooled at a rate of ∼600 °C/min from the high temperature to room temperature.
A Bruker D8 X-ray diffractometer (XRD) was used to investigate the compositions. The microstructure and secondary phase were examined using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS JSM-6700 and 6460). The wires were etched in a mixture of NH4OH and H2O2 for 30 s. The melting behavior was analyzed using thermal gravimetric (TG NETZSCH STA 449F5) analysis at a heating rate of 20 K/min. The critical currents of the wires were measured in liquid helium (4.2 K) using that standard four-probe method.
3. Results
The entire preparation processes of the precursor powders and wires are shown in Fig. 1. First, the corresponding nitrate solution is nebulized using ultrasound to form microdroplets during the ultrasonic spray pyrolysis (USP) process. Then, the microdroplets are carried into a pyrolysis furnace. The dissolved nitrates that within the microdroplets decompose to form intermediate oxides, and hollow polycrystalline spheres are obtained after the water evaporates. Second, the hollow particles break into plate-like grains during calcination. The oxides are transformed into Bi-2212 grains, and small secondary phases form in plate-like Bi-2212 grains. Third, Bi-2212 green wires are prepared using the PIT process with silver sheath. Bi-2212 precursor powders are surrounded by Ag, and wires with 37 × 18 filaments and a diameter of Ø1.0 mm are obtained. Finally, the green wires are heated under the overpressure. Fully reacted Bi-2212 wires are obtained.
Fig. 1. Illustration of the experimental process: (a) Preparation of Bi-2212 precursor powders using the ultrasonic spray pyrolysis process and the origin of the residual secondary phase. (b) Fabrication of Bi-2212 wires. |
3.1. Phase transformation of precursor powders
To analyze the origin of the secondary phase, calcination temperatures in the range of 750 °C-865 °C were used. XRD patterns of powders that were calcined at different temperatures are shown in Fig. 2. The powders from Nexans (lot 82#) are used as a reference; these were synthesized using the melt casting process. The powders that were calcined at 750 °C have the peaks marked as the mixture of Bi2Sr2CuOx (Bi-2201), CaO, CuO, SrO, and SrxBi1-xOy. When the calcination temperature was above 810 °C, there were no obvious peaks of other phases. All of the major peaks can be indexed as the diffraction peaks of (0 0 8), (1 1 3), (1 1 5), (0 0 1 0), (1 1 7), and (2 0 0) for the Bi-2212 phase. The Bi-2201 phase reacts with the secondary phase to form the Bi-2212 phase. This indicates that the Bi-2212 phase, which has relatively high purity, can be obtained in these powders.
Fig. 2. XRD patterns of precursor powders that were calcined using different calcination processes: (a) 750 °C/20 h, (b) 790 °C/20 h, (c) 810 °C/20 h, (d) 830 °C/20 h, (e) 830 °C/20 h + 850 °C/20 h, (f) 850 °C/20 h + 865 °C/20 h, and (g) Nexans powders as a reference. |
Both the relative intensity and full width at half maximum (FWHM) of the (0 0 8) peak vary with respect to the different calcination processes. Fig. 3 shows the changes in the intensity ratio (IR = I(008)/I(0010)) and the FWHM values as a function of temperature. The values for the P865 powders (IR = 0.89, FWHM = 0.14°) are close to those of the reference. The FWHM values of the Bi-2212 (0 0 8) peak decreased with an increase in calcination temperature. This implies that there is an improvement in the degree of crystallization for Bi-2212 grains. Moreover, the IR values increased obviously with an increase in the calcination temperature. The changing trend of IR values is the opposite that of the FWHM values. Rikel et al. observed obvious Bi-2201 peaks that corresponded to the intergrowth of Bi-2201 in Bi-2212 [17], [22]. It is interesting that there are no diffraction peaks of Bi-2201 in our samples, and the IR value may be used to indicate the residue of the secondary phase. The high IR value and low FWHM values suggest that there is a degree of high crystallization of Bi-2212 and less of a secondary phase in the precursor powders.
Fig. 3. Intensity ratio of Bi-2212 peaks and FWHM value of the Bi-2212 (008) peak as a function of final calcination temperature (▲ IR = I(008)/I(0010), ● FWHM of (008), ■ and □ FWHM and IR values of Nexans powders as the reference). |
SEM images of the precursor powders are given in Fig. 4. A black dot is highlighted in the inset of the figure. During the USP process, spherical and hollow noncrystalline agglomerations formed. After calcination at 790 °C, thin plates, needle-like grains (light grey), and many dark dots are observed (as marked by arrows) on hollow particles (Fig. 4b). Very thin plate-like particles are confirmed to be Bi-2201 grains. At calcination temperatures in the range of 810 °C∼830 °C, spherical particles decomposed into plate-like Bi-2212 grains (Fig. 4c-4d), and the number of dark dots decreased. With a further increase in the calcination temperature to the range of 850 °C∼865 °C, lamellar Bi-2212 grains grew bigger and thicker (Fig. 4e-4f). The average size of particles increased from ∼1.5 µm (P830) to ∼2.6 µm (P865). It is noteworthy that the number of dark dots decreased with an increase in calcination temperature, and the dark dots disappeared completely in the P865 sample. The dark dots are attributed to the secondary phase. The secondary phase, which has different contrast, indicates a different composition. The changes in morphology agree with XRD results.
Fig. 4. SEM images of precursor powders that were calcined using different calcination processes: (a) 750 °C/20 h, (b) 790 °C/20 h, (c) 810 °C/20 h, (d) 830 °C/20 h, (e) 830 °C/20 h + 850 °C/20 h, and (f) 850 °C/20 h + 865 °C/20 h. The residual dark dots are labeled by red arrows, and the inset figure shows the details of the residual secondary particles. |
Typical EDS patterns of plate-like grains and dark dots in P850 powders are shown in Fig. 5. The atomic ratios of Ca and Cu are different in the black dots. The compositions of the Bi-2212 grains and the secondary phase are determined as listed in Table 1. The normalized compositions of dark dots for P830 and P850 powders are Bi2Sr1.95Ca0.96Cu7.38Ox and Bi2Sr1.69Ca0.99Cu6.41Ox, respectively. Compared to the normalized composition of Bi-2212 grains, the secondary phase is determined to be the copper-rich phase or (Sr,Ca)CuOx on the surface of lamellar Bi-2212 grains. This also suggests that there is no sufficient transformation of the Bi-2212 plate. When the amount of residual black dots increased, the degree of crystallization of Bi-2212 decreased (Fig. 4). Furthermore, a clean and uniform surface without dark dots can be obtained in the P865 sample only when there is adequate crystallization. From the above analysis, the amount of the secondary phase in the lamellar Bi-2212 grains can be tuned using the calcination process.
Fig. 5. (a) Typical energy dispersive X-ray spectrometry results of P850 powders and (b) EDS pattern acquired at dot 2 in panel a, and (c) EDS pattern acquired at dot 3 in panel a. |
Table 1. Phase compositions of powders and the secondary phase. |
| Powder Number | Phase | Compositions measured using EDS | Normalized EDS compositions |
|---|---|---|---|
| P830 | Bi-2212 | Bi12.54Sr12.16Ca4.62Cu13.55Ox | Bi2Sr1.94Ca0.74Cu2.16Ox |
| Black dot | Bi8.09Sr7.89Ca3.87Cu29.86Ox | Bi2Sr1.95Ca0.96Cu7.38Ox | |
| P850 | Bi-2212 | Bi16.93Sr15.91Ca7.56Cu18.42Ox | Bi2Sr1.88Ca0.89Cu2.17Ox |
| Black dot | Bi10.73Sr9.07Ca5.31Cu34.41Ox | Bi2Sr1.69Ca0.99Cu6.41Ox |
3.2. Superconducting performance of wires
The final diameter of the OPHT-wire was about Ø 0.95 mm. Fig. 6 shows the critical current (Ic) and critical current density (Jc) of wires that were prepared with different powders. The values of the critical current (Ic) of the WP830, WP850, WP865, and reference wires were 520A, 839A, 1105A, and 966A, respectively, under 4.2 K and a self-field. The Jc (4.2 K, 0 T) values of the wires were 3186 A/mm2, 5143 A/mm2, 6773 A/mm2, 6140 A/mm2, respectively, and the Je (4.2 K, 0 T) values of the wires were 733 A/mm2, 1183 A/mm2, and 1558 A/mm2, and 1362 A/mm2, respectively. Higher Jc values are obtained with a decrease in the content of the secondary phase in P865, which is higher than that of the reference wires that were prepared with Nexans powders. The maximum value of Jc for WP865 is double of that for WP830. The good performance of the WP865 wires is close to the results that were obtained at the Fermi Lab [5]. This implies that the disappearance of the secondary phase in the lamellar grain is beneficial for the transport properties.
Fig. 6. Ic and Jc values of wires that were prepared with different precursor powders. The inset figure is the voltage-current curves of the wires. |
The cross-sectional morphologies of fully OPHT-wires are shown in Fig. 7. All of the filaments of the three wires were dense without large voids (Fig. 7a, 7d, and 7g). EDS results confirmed that the large grey areas and light grey areas were Bi-2212 and Bi-2201 grains with EDS; this is a typical morphology of reacted Bi-2212 multifilament wires. Some bridges between filaments can be observed in the samples; this includes the merging filaments and the thin outgrowths of filaments [23], [24]. However, there are some large filament-merging bridges in different wires, and three or more filaments merged into a large cluster in the sub-bundle (Fig. 7b, 7e, 7h). Microcracks are also noted in the large cluster of bonded filaments (Fig. 7c and 7f). In contrast, there are few and small clusters in the WP865 wires (Fig. 7h and 7i). The microstructure of the filaments appears to be more uniform, and this leads to a high value of Jc [25]. The filament configurations of different wires are the same, and thus, variations in the precursor powders may be responsible for the different microstructures. When the secondary phase in the precursor powders is prohibited, the large cluster of bonded filaments can be reduced; this may be useful for high Jc values in wires. Therefore, it is shown that the secondary phase has a great influence on the performance and morphology of wires.
Fig. 7. (a) SEM images of transverse cross-sections for fully heat treated WP830 wires (37 × 18), (b) a 37 filaments sub-bundle of WP830 wires, (c) filament bridges of WP830 wires, (d-f) SEM images of WP850 wires with different magnifications, (g-i) SEM images of WP865 wires with different magnifications. |
3.3. Melting growth behaviors of wires
Thermal analysis was performed to investigate the melting behaviors of the wires. Fig. 8 shows DSC curves of unsintered wires in the temperature range of 800 °C-920 °C. Also, the entre curves are shown in the inset. The strong endothermic peak is confirmed to be the melting point of Bi-2212. The endothermic peak for each WP850 and WP865 is at ∼883 °C. However, the endothermic peak of WP830 is at ∼880 °C. The starting melting point shifts slightly to a lower temperature. This may be ascribed to the effect of the secondary phase.
Fig. 8. DSC curves of Bi-2212 green wires that were prepared with different precursor powders. The inset figure is DSC curves over the whole temperature range. |
To discuss the morphology of the filaments before the melting process, two monofilament wires (WP830 and WP865) were heated at 864 °C/10 h and at 890 °C/0h. The microstructures of the monofilament wires were compared to analyze the evolution of Bi-2212 grains during different stages of the partial melt processing. Fig. 9 shows the longitudinal section of a filament after it was heated at 864 °C with a dwell time of 10 h. The green wires were used as a reference (Fig. 9a and 9c). After the heat treatment and before the melting stage, there are clear and large Bi-2212 grains in WP830 (Fig. 9b). However in WP865, the grains are smaller and the microstructure is more uniform (Fig. 9d). This indicates that there is a transformation of the secondary phase to the Bi-2212 phase during a low temperature step. The change in the microstructure before melting suggests the recrystallization of Bi-2212, and this leads to different melting behaviors (Fig. 8). The microstructures of two monofilament wires heated at 890 °C with a dwell time of 0 h are shown in Fig. 10. At the onset of the melting stage, Bi-2212 (grey) and Bi-2201 (light grey) and alkaline-earth cuprates (dark dots) formed. There are more Bi-2201 grains in WP830 (Fig. 10a and 10b). In contrast, there are fewer Bi-2201 grains in the WP865 filament, and the Bi-2201 grains are small (Fig. 10c and 10d). At the onset of the melting stage, a residual secondary phase may result in the appearance of more and larger Bi-2201 grains in the WP830 filament.
Fig. 9. Longitudinal sectional SEM images of Bi-2212 monofilament wires: (a) WP830 green wires, (b) WP830 wires heated at 864 °C/10 h, (c) WP865 green wires, and (d) WP865 wires heated at 864 °C/10 h. |
Fig. 10. Longitudinal sectional SEM images of Bi-2212 monofilament wires heated at 890 °C/0h. (a) WP830 wires, (b) high-magnification view of panel a, (c) WP865 wires, and (d) a high-magnification view of panel c. |
4. Discussion
The phase transformation is influenced by calcination, depending on the precursor powders used in the USP fabrication process (Fig. 1). The intermediate phases (SrxBi1-xOy, CuO, SrO and CaO etc.) convert gradually into Bi-2201 and Bi-2212 grains (Fig. 2). Dark dots are clearly observed on the plate surface (Fig. 4). The normalized composition of the secondary phase dots are determined to be Bi2Sr1.69Ca0.99Cu6.41Ox in P850 powders (Fig. 5). The residual secondary phase is estimated to be a small unreacted copper-rich phase or (Sr,Ca)CuOx. With further heat treatment, the dark dots disappeared completely from the plate surface, and this implies that pristine Bi-2212 grains can be achieved (Fig. 4).
Furthermore, a dark secondary phase in the precursor powders has an important influence on the superconducting property and the microstructure of OPHT wire (Fig. 6, Fig. 7). The melting temperature of Bi-2212 decreased with the appearance of the secondary phase (Fig. 8), and thus, the melting growth behavior changes accordingly. More large clusters of bonded filaments are formed during the melting process (WP830, Fig. 7). After the heat treatment and before melting, clear and large Bi-2212 grains were observed in the WP830 single filament (Fig. 9). The residual secondary phases converted into Bi-2212 grains in this stage. At the onset of the melting stage, more and larger Bi-2201 grains were found in the WP830 filament compared to those of WP865 (Fig. 10). This suggests that the secondary phases lead to the subsequent formation of more Bi-2201 in WP830. Thus, uniform filaments and optimal merging bridges are only obtained in the WP865 wire with pristine Bi-2212 powders. Based on variations in the microstructures, the maximum Jc value (6773 A/mm2 at 4.2 K, 0 T) of WP865 is double than that of WP830. From the above analysis, different melting growth behaviors may result in changes in morphology and superconducting properties.
5. Conclusions
Bi-2212 precursor powders with secondary phases have been tuned using ultrasonic spray pyrolysis and calcination processes. The origin of the secondary phase is analyzed, and the influences of the secondary phase on the microstructure and Jc of wires were investigated. The dark dots are derived from the intermediate phases, which were determined to be Bi2Sr1.69Ca0.99Cu6.41Ox. This can be ascribed to the unreacted copper-rich phase or (Sr,Ca)CuOx phase. The presence of the secondary phase can cause a low melting temperature, recrystallization of Bi-2212 grains before the melting stage, and formation of larger Bi-2201 at the onset of melting stage. This results in the formation of more large clusters of bonded filaments and a low Jc value of the Bi-2212 wires. With a higher calcination temperature, the secondary phase can be eliminated completely from the surface Bi-2212 grains. A high Je value of 1558 A/mm2 and Jc value of 6773 A/mm2 are obtained in Bi-2212 wires (WP865). These results indicate that the residual secondary phase plays a critical role in the melting behavior and the morphological and superconducting properties of round wires. Controlling the secondary phase is an effective and significant way to achieve a high current capacity for bismuth-based superconducting round wires.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Key R&D Program of China (2021YFB3800201), the National Science Fund Program of China (No. 51777172, 51902267), and the Natural Science Basic Research Plan in Shaanxi Province (No. 2022GY-392, 2021JQ-884).
