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Phase Evolution and Superconductivity of Bi-2212 Precursor Powder Prepared by Spray Pyrolysis  PDF

  • Cui Lijun 1,2 ✉
  • Yang Fan 2
  • Hou Fanyou 2
  • Sun Xiaguang 2
  • Liu Jianwei 2
  • Yan Guo 2
  • Liu Xianghong 2
  • Feng Yong 2
  • Zhang Pingxiang 1,2 ✉
  • Hao Qingbin 1
  • Zhang Shengnan 1
  • Liu Guoqing 1
1. Northwest Institute for Nonferrous Metal Research, Xi'an 710016, China； 2. National Engineering Laboratory for Superconducting Materials, Western Superconducting Technologies Co., Ltd, Xi'an 710018, China

Updated：2022-12-07

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Abstract

The Bi2Sr2CaCu2Ox (Bi-2212) precursor powder was prepared by spray pyrolysis. The phase evolution during the heat treatment process and the superconductivity of wires were analyzed. Results show that the powder prepared by spray pyrolysis is spherical with the average particle size of 3.03 μm and the dispersion distribution. The phase evolution during the heat treatment process of powder includes four main reaction processes. The decomposition of nitrate and the pre-reaction process between components occur firstly at 527 °C. Then the Bi2Sr2CuOx (Bi-2201) phase is formed at 588 °C due to the high reactivity of spray powder. The Bi-2212 phase is generated at 780 °C, and the powder is completely melted at 834.2 °C. The heat treatment temperature window of the Bi-2212/Ag wire is very narrow. The critical current (Ic) is reduced by 31 A when the maximum heat treatment temperature (Tmax) is changed by ±2 °C. When the optimal Tmax of Bi-2212/Ag wire is 885 °C, the maximum Ic (4.2 K, 0 T) is 486 A. The critical current is increased to 712 A in the oxygen atmosphere during heat treatment.

Science Press

Due to the advantages of high upper critical field and easy processing into isotropic wires, Bi2Sr2CaCu2Ox (Bi-2212) material is a very promising superconductor for high field applications, such as nuclear magnetic resonance, accelerator magnets, and nuclear fusion[

1-4]. The multi-filamentary Bi-2212/Ag wire can be easily fabricated via the powder-in-tube (PIT) method. Besides, the kilometric Bi-2212/Ag wire has already been achieved[5]. The critical current (Ic) is continuously improved by the optimization of powder preparation, wire processing technique, and heat treatment process. The overpressure heat treatment (OP-HT) is a key breakthrough to fabricate Bi-2212/Ag wire with better filament density and grain connectivity[6]. The engineering critical current density (Je) is increased by 8 times when the pressure is increased from 0.1 MPa to 10 MPa.

Generally, the precursor powder can significantly affect the Ic of Bi-2212/Ag wire. Therefore, various techniques have been developed to prepare Bi-2212 precursor powders, including the solid-state reaction[

7,8], sol-gel method[9,10], co-precipitation method[11,12], and spray pyrolysis[13-15]. The properties of the Bi-2212 precursor powder are influenced by the phase composition, particle size, phase content, and powder homogeneity. The precursor powders of the kilometric Bi-2212/Ag wire are usually prepared by the spray pyrolysis and co-precipitation method. The precursor powder prepared by melt casting technique has the optimal Je of 1014 A/mm2 under the conditions of 4.2 K and 5 T[16]. The Bi-2212 precursor powders prepared by co-precipitation has the maximum Je of 1456 A/mm2 under the conditions of 4.2 K and 5 T[17]. The properties of Bi-2212 precursor powder can be further improved by the nanospray method: the Je reaches 1810 A/mm2 under the conditions of 4.2 K and 5 T[15,17]. Therefore, the spray pyrolysis exhibits good industrial application prospect.

The critical current of the Bi-2212/Ag wire is greatly improved by the precursor powder prepared by spray pyrolysis technique. However, the characteristics and the phase evolution of the powders prepared by spray pyrolysis during the heat treatment process are rarely reported. In this research, the characteristics, phase evolution, and super-conductivity of the Bi-2212 precursor powder prepared by the ultrasonic spray pyrolysis technique were investigated.

1 Experiment

The raw material Bi2.17Sr1.94Ca0.89Cu2Ox[

18] was the mixture of four nitrates. All nitrates were prepared as solutions with the ionic content of 1 mol/L. The precursor powder was prepared by the ultrasonic spray pyrolysis at the decomposition temperature of 800 °C. Then the powders were calcined in N2-0.1vol% O2 atmosphere at different temperatures to study the characteristics and phase evolution of the Bi-2212 precursor powders. The Bi-2212/Ag wire with 666 filaments (37×18 configuration) was fabricated by PIT technique, and the heat treatment was conducted by the partial melt process (PMP) at 0.1 MPa[16,17].

The melting behavior and phase evolution were analyzed based on the thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. The phase composition was determined by X-ray diffraction (XRD). The powder morphologies were observed by scanning electron microscope (SEM). The critical currents of the Bi-2212/Ag wires were measured by the standard four-point probe with a criterion of 1 μV/cm.

2 Results and Discussion

During the ultrasonic spray pyrolysis, the nitrate solution is ultrasonically atomized into the micron-sized droplets which pass rapidly through the high temperature zone and are dehydrated to form the powder particles in the flowing gas. The decomposition and reactions occur in this process. Fig.1 shows the particle size distribution and morphology of the as-prepared powders. The powders have the average particle size of only 3.03 μm, and the concentrated particle size distribution can also be observed. The powders are spherical with mutual diffusion distribution and no agglomeration can be observed. The reaction mainly occurs inside the particles, resulting in the uniformity of elements and phases.

Fig.1  Particle size distribution (a) and morphology (b) of precursor powder prepared by ultrasonic spay pyrolysis

Fig.2 shows TGA and DSC curves of the precursor powders in N2-0.1vol% O2 atmosphere at the heating rate of 10 °C/min. Three significant endothermic peaks at 527.0, 587.9, and 834.2 °C can be observed. The mass loss is 21.1% when temperature is increased from 400 °C to 600 °C. The Bi-2212 phase is generally formed above 700 °C. Therefore, the main reactions at 400~600 °C are the decomposition of nitrates and the reaction between the components.

Fig.2  TGA and DSC curves of precursor powders in N2-0.1vol% O2 atmosphere at the heating rate of 10 °C/min

The mass barely decreases above 600 °C. However, at the endothermic peak of 834.2 °C, a significant mass loss of about 1% is observed, which indicates the melting of Bi-2212 phase. Thus, the formation of Bi-2212 phase occurs at 587.9~834.2 °C.

The heat treatments were conducted in the N2-0.1vol% O2 atmosphere at 527, 588, 780, and 834 °C for 12 h for phase evolution analysis of the precursor powders. SEM morphologies of the powders after heat treatments are shown in Fig.3. The powders after heat treatments at 527 and 588 °C maintain the spherical morphology, but their surface is coarser than that before heat treatment due to the decomposition of nitrates. The morphology changes when the heat treatment temperature is increased to 780 °C, as shown in Fig.3c. The powders consist of flaky grains, which correspond to the Bi2Sr2CuOx (Bi-2201) or Bi-2212 phases. The average particle size of the powder is less than 1 μm. When the temperature is further increased to 834 °C, the powder melts completely, as shown in Fig.3d.

Fig.3  SEM morphologies of precursor powders after heat treatment at different temperatures for 12 h: (a) 527 °C, (b) 588 °C, (c) 780 °C, and (d) 834 °C

Fig.4 shows the XRD patterns of the precursor powders after heat treatments at 527, 588, and 780 °C for 12 h in N2-0.1vol% O2 atmosphere. The precursor powder after heat treatment at 527 °C consists of nitrates and several oxides, including Sr(NO3)2, Bi-2201, CaO, CuO, Sr-Bi-oxide, and Sr-Ca-oxide. The Bi-2201 formation occurs at 588 °C due to the high reactivity of powders. When the temperature is further increased to 780 °C, the main phase of the precursor powder is completely transformed into the Bi-2212 phase, accom-panied by a small amount of Bi-2201 phase and CuO. Therefore, the phase evolution can be divided into four stages. Stage Ⅰ: Sr(NO3)2+Bi-2201+CaO+Sr0.9Bi1.1O2.55+SrxCayOz+CuO; Stage Ⅱ: Bi-2201 +(Sr,Ca)xCuyOz+CaO+CuO; Stage Ⅲ: Bi2212+Bi-2201+CuO; Stage Ⅳ: complete melting reaction.

Fig.4  XRD patterns of precursor powders after heat treatments at 527, 588, and 780 °C for 12 h

The Bi-2212/Ag wires were fabricated by the precursor powder after heat treatment at 780 °C for 12 h in N2-0.1vol% O2 atmosphere. The heat treatment process of the Bi-2212/Ag wire is according to the process in Ref.[

17]. The short wires with the diameter of 1 mm suffered heat treatments with different maximum heat treatment temperatures (Tmax). Fig.5 shows the relationship between Ic (4.2 K, 0 T) and Tmax of the Bi-2212/Ag wire. The peak Ic of 486 A is achieved at Tmax of 885 °C, and the maximum Ic decrement is 31 A when the Tmax changes by ±2 °C. It can be seen that the heat treatment temper- ature window of Bi-2212/Ag wire is very narrow.

Fig.5  Relationship between Ic (4.2 K,0 T) and Tmax of Bi-2212/Ag wire

The properties of the precursor powders are greatly influenced by the heat treatment temperature of Bi-2212/Ag wire. The optimal Tmax is about 890.5 °C in Ref.[

19], and the peak Je appears at about 894 °C in Ref.[20]. However, the optimal Tmax in this research is only 885 °C, indicating the significantly high reactivity of spray pyrolysis powders.

The Bi-2212 phase partially melts at Tmax and nucleates through the slow cooling process to obtain the high-texture grains, thereby increasing the Ic of Bi-2212/Ag wires. Due to the low density of the wire and the carbon residue in the precursor powder, the voids are formed during the partial melting process. The relative content of the void is defined as the ratio of the void area to the whole wire area.

Fig.6 shows the relationship between the relative content of the void and Tmax. The content of the liquid phase is increased gradually with increasing the Tmax. These liquid phases gather together through the cracks between grains and finally form the voids during the cooling process. Therefore, the relative content of the void is monotonically increased with increasing the Tmax. Because the maximum Ic occurs at Tmax of 885 °C (Fig.5), the optimal liquid phase content can be corre-spondingly obtained. The liquid phase content is continuously increased when Tmax is higher than 885 °C, resulting in the excess formation of voids, which seriously blocks the current path. The liquid phase content is decreased when the temperature is lower than 885 °C, resulting in the poor grain texture, which is not conducive to the Ic enhancement of the Bi-2212/Ag wire.

Fig.6  Relationship between relative content of void and Tmax of Bi-2212/Ag wire

Fig.7 shows the optical microscope (OM) images of the Bi-2212/Ag wires after heat treatment at different Tmax. The void area is increased obviously with increasing the Tmax. The voids appear firstly in the inner filaments and gradually in the outer filaments with increasing the Tmax. The Bi-2212/Ag wire was heat-treated at Tmax for only 0.2 h and then cooled slowly at the cooling rate of 10 °C/h. Due to the heat conduction, the cooling rate in the central area is slower than that in the outer layer of the wires. Thus, the central area contains more liquid phase, which leads to the difference of void area in the inner and outer filaments.

Fig.7  OM images of Bi-2212/Ag wires after heat treatments at different Tmax: (a) 883 °C, (b) 887 °C, (c) 891 °C, and (d) 895 °C

The Bi-2212/Ag wires fabricated by the precursor powders after heat treatments at 770 °C for 12 h in N2-0.1vol% O2 atmosphere and at 860 °C for 12 h in O2 atmosphere were then heat-treated by PMP with the optimal Tmax of 885 °C. Fig.8 shows the voltage-current (V-I) curves of the two Bi-2212/Ag wires. Ic (4.2 K, 0 T) of the Bi-2212/Ag wire prepared by the precursor powder after heat treatment at 770 °C for 12 h in N2-0.1vol% O2 atmosphere is about 486 A, and the corresponding Je is 619 A/mm2. However, when the heat treatment atmosphere changes to oxygen, Ic is about 712 A, and the corresponding Je is 907 A/mm2, which is comparable to the Je (4.2 K, 5 T) of 1100 A/mm2 in Ref.[

21]. The oxygen content in the heat treatment atmosphere of the precursor powder preparation has an important influence on the superconductivity of the Bi-2212/Ag wire.

Fig.8  V-I curves of Bi-2212/Ag wires prepared by precursor powders after heat treatment at 770 °C for 12 h in N2-0.1vol% O2 atmosphere and at 860 °C for 12 h in O2 atmosphere

3 Conclusions

1) The spray pyrolysis powders exhibit a concentrated particle size distribution and their average particle size is 3.03 μm. The powders are spherical and diffusively distributed without agglomeration.

2) The phase evolution of the spray pyrolysis powder consists of four stages, and the powder morphology changes. The Bi-2212 phase is formed during the heat treatment at 780 °C for 12 h in N2-0.1vol% O2 atmosphere.

3) The heat treatment temperature window of Bi-2212/Ag wire is very narrow. The maximum critical current (Ic) decrement is 31 A with the variation of ±2 °C in maximum heat treatment temperature (Tmax). The optimal Tmax is 885 °C, and the maximum Ic (4.2 K, 0 T) is about 486 A.

4) The oxygen content in the heat treatment atmosphere of the precursor powder has an important influence on the superconductivity of the Bi-2212/Ag wire. Ic (4.2 K, 0 T) of Bi-2212/Ag wire is increased to 712 A when oxygen is used as the heat treatment atmosphere.

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