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Superconducting properties and microstructures of CeO2 doped YBa2Cu3O7−δ films fabricated by pulsed laser deposition
Author’s Accepted Manuscript Superconducting properties and microstructures of CeO2 doped YBa2Cu3O7−δ films fabricated by pulsed laser deposition Wei Wang, Linfei Liu, Tong Zheng, Shunfan Liu, Yijie Li www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32898-0 https://doi.org/10.1016/j.ceramint.2018.10.097 CERI19809
To appear in: Ceramics International Received date: 30 July 2018 Revised date: 11 October 2018 Accepted date: 11 October 2018 Cite this article as: Wei Wang, Linfei Liu, Tong Zheng, Shunfan Liu and Yijie Li, Superconducting properties and microstructures of CeO2 doped YBa2Cu3O7−δ films fabricated by pulsed laser deposition, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.097 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Superconducting properties and microstructures of CeO2 doped YBa2Cu3O7−δ films fabricated by pulsed laser deposition Wei Wang, Linfei Liu*, Tong Zheng, Shunfan Liu, Yijie Li* Key Lab of Artificial Structures & Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, People’s Republic of China [email protected]. [email protected] * Corresponding Author :
Abstract Performance of REBa2Cu3O7−δ superconductors strongly depends on their microstructures and surface morphologies. Using a reel-to-reel pulsed laser deposition (PLD) system, the CeO2-doped YBa2Cu3O7−δ (CeO2-Y123) films were fabricated. Superconducting properties and microstructure of these films were systematically studied for the first time. The results showed that the relationship between the critical current (Ic) values and the deposition temperature depends on the doping content essentially. For all CeO2-doped films, the Ic values increase at first and then decrease with increasing the deposition temperatures. The evolution of surface morphologies should take responsibility for this phenomenon. The optimum deposition temperature of 1 and 2 mol. % CeO2-doped Y123 films is higher than that of 3 mol. % CeO2 doped ones. The substitutions of the Y atoms by Ce atoms are considered as the determinative factor. Furthermore, the in-plane rotation of the crystalline grain orientation on the surface of the CeO2-Y123 films was observed. We think that not only the lattice mismatch, but also the density of dangling bonds contributes to this distinctive phenomenon. Keywords: CeO2-Y123, deposition temperature, Ic value, in-plane rotation.
1. Introduction REBa2Cu3O7−δ (REBCO or RE123) coated conductors, which have great potential for practical applications, have been widely investigated by many groups due to their relatively high critical temperature (Tc) since their discovery [1-4]. The extension of applications of REBCO coated conductors needs the increase of the current carrying capacity especially in high magnetic fields [5-6]. Therefore, finding the effective ways to improve the critical current (Ic) is very important for practical applications under high magnetic fields. There are mainly two methods which can effectively improve the critical current of REBCO films. Firstly, inserting buffer layers between the superconductor layer and the substrate is widely used by scientists [7-9]. The buffer materials, such as LaMnO3 [10], MgO [11-12], Y2O3 [13], Al2O3 [14] are usually utilized in the fabrication of
REBCO coated conductors. Besides providing a smooth textured-surface, the buffer layers can also prevent atoms diffusing from the substrate to REBCO layer. By this way, a high c-axis oriented REBCO films with excellent zero-field superconducting properties can be fabricated successfully. Secondly, doping second-phase particles in the REBCO matrix, generating nano-scale defects as the flux pining centers, has been proved to be a simple and practicable method to increase the critical current density (Jc). Compounds such as BaHfO3 [15-16], BaZrO3 [17-21], BaNb2O6 [22], ZrO2, Ta2O5 [23], metals such as Zr [24-25], Au [26] are all dopants that have been reported to have effects on the Jc improvement of REBCO films. For the latter case, searching proper dopant becomes a matter of significant importance in the fabrication of REBCO films. Cerium oxide (CeO2), due to its high chemical and thermal stability, is usually used as a buffer layer [27]. While in the top-seeded melt-growth (TSMG) process, the CeO2 has been doped into the REBCO bulks to suppress the liquid loss and to improve the Magnetic levitation force at the same time [28-29]. Besides, as one of the rare earth elements, Ce element can occupy the RE site without changing the orthorhombic phase of REBCO structure. Therefore, there is a strong possibility that CeO2 doping can influence the Jc of REBCO films significantly. In this work, we investigated the influence of CeO2 doping on the property of YBCO films. CeO2-Y123 films were grown on the CeO2 buffer layer by pulsed laser deposition. The factors, affecting the critical current of the CeO2-Y123 films, have been discussed. To fully understand the dependence of the property of CeO2-Y123 film on the deposition conditions, our investigation is also focused on the microstructure evolution of CeO2-Y123 films.
2. Experimental The CeO2-Y123 films were fabricated IBAD-MgO tapes by using a reel-to-reel pulsed laser deposition (PLD) system (PVD Products Inc.). The substrate is 10 mm in width and 50 μm in thickness. Prior to the fabrication of CeO2-Y123 film, a high textured CeO2 buffer layer was prepared by PLD system. The in-plane full width of half maximum (FWHM) value of the CeO2 buffer layer is less than 4 degree. Afterwards, the 200 nm thick CeO2-Y123 films were deposited by PLD in oxygen atmosphere at 800 ℃-840 ℃. After the deposition, a protection layer (silver) was fabricated using the magnetic sputtering method. Then the samples were annealed at 500 °C in flowing O2 gas for 5 h. The architecture of CeO2-Y123 coated conductors is schematically illustrated in Fig. 1. The CeO2-Y123 targets were made by solid state reaction method with mixtures of Y123 and CeO2 powder. The initial material (Y123 powder) was acquired also through a solid state reaction. The mixture of raw powders (Y2O3, BaCO3, and CuO) was calcined at 900 °C for 48 h in air. To improve the purity of the powder, the process was repeated three times. Three pellets, composed of Y123 + 1~3 mol. % CeO2, were prepared with a diameter of 100 mm and a thickness of 10 mm. Next, the pellets were sintered at 800 °C for 24 h and then flipped the pellets and sintered at
800 °C for another 24 h. Finally, in a muffle furnace, the pellets were annealed in flowing oxygen at 400 °C for 12 h. The structure and texture of CeO2-Y123 film were measured by X-ray diffraction system (D8 Discover with GADDS, Bruker Advanced X-ray Solutions, Inc.) with Cu-Kα radiation operated at 40 mA and 40 kV. The surface morphology and surface roughness of the CeO2-Y123 films were observed by Scanning Electron Microscope (SEM, FEI Sirion 200, operated at 5 kV) and atomic force microscopy (AFM, Tapping Mode, BioScope, Veeco Instruments, Inc.).
Figure 1.
Schematic architecture layout of the CeO2-Y123 coated conductor
3. Results and Discussion To evaluate the property of the CeO2-Y123 films, in the liquid nitrogen (77 K), the Ic values of the films were measured by conventional four-probe method in self-field using a criterion of 1 μV/cm. A relatively high Ic (100 A) was obtained in the 1 mol. % CeO2 doped Y123 films. While, the Ic value of the pure Y123 film is merely 75 A. Meanwhile, the relationship between the Ic value of the CeO2-Y123 film and the deposition temperature was shown in Fig. 2. For all the samples, the Ic values firstly increase and then decrease with increasing of the deposition temperature. The optimum temperature of 1 and 2 mol. % CeO2 doped Y123 films is higher than that of 3 mol. % CeO2 doped one. Besides, the Ic values of the CeO2-Y123 films, which grown at their optimum temperatures, decreases along with increasing of the doping content.
Figure 2.
The relationship between the critical current (Ic) of the CeO2-Y123 films and the deposition temperature
To clarify the reasons, the XRD patterns of these films prepared at 820 ℃ were shown in Fig. 3. As can be seen from Fig. 3, only Y123 (00l) peaks were observed, indicating their high c-axis oriented feature. The XRD results proved that the perovskite structure of Y123 films was unchanged within CeO2 doping content. The magnified XRD patterns of the CeO2-Y123 (005) peaks are inserted in Fig. 3. It can be seen that the Y123 (005) peak position shifts to large angle direction with increasing of the doping content.
Figure 3. XRD patterns of the CeO2-Y123 films with the doping content of 1 mol. %, 2 mol. %, 3 mol. % fabricated at 820℃. The inset shows the magnified XRD patterns of the CeO2-Y123 (005) peaks. The CeO2-Y123 films possess a space group of Pmmm with the c-axis lattice
parameter of 1.16 nm calculated by the Bragg equation, which fit well with the orthorhombic Y123 phase [30].
The SEM images of 1 mol. % CeO2 doped Y123 films fabricated at 800 ℃-840 ℃ were presented in Fig. 4. As shown in Fig. 4(a), a number of large crystalline grains （Y123 crystalline grains） were observed on the surface of the films. With increasing the deposition temperature, the crystalline grains diminished gradually and disappeared at last, as observed in Fig. 4(b)-(d). The diminishing of the large grain boundaries, resulting from the disappearance of the large crystal grains, caused the increase of the Ic values. With further increasing the deposition temperature to 840 ℃, the non-superconducting phase (Y2BaCuO5, Y211) was formed, as observed in Fig 4(e). As is well known, the appearance of the Y211 phase usually results in the decrease of the Ic value. During the film growth process, the nucleation barrier must be overcome in order to form the nucleuses. That is to say, to realize the formation of Y123 nucleus, large potential energy must be achieved. Therefore, at high deposition temperature, many CeO2-Y123 nucleuses were formed. Then all the nucleuses grew in the a, b-direction and converged together, leading to the fast growth of the a, b-plane. On the contrary, at low deposition temperature, the formation of the nucleus was suppressed, resulting in its less nucleuses feature. Further growth of the nucleuses caused the appearance of the large crystalline grains. At the same time, the grain boundary mis-orient angles become large inevitably. Meanwhile, the current carrying capacity reduced essentially, due to these large grain boundaries. Therefore, with increasing the deposition temperature, the Ic values increase at first and then decrease. The AFM photos of the 1 mol. % CeO2 doped Y123 films fabricated at 800 ℃-830 ℃, are presented in Fig. 5. As can be seen from Fig. 5, with increasing the deposition temperature, the surface roughness decreased gradually. Apparently, the decrease of the surface roughness results from the decrease of the large crystalline grains, which fit well with the SEM results.
Figure 4.
The SEM images of the 1 mol. % CeO2 doped Y123 films fabricated at (a) 800 ℃, (b) 810 ℃, (c) 820 ℃, (d) 830 ℃, (e) 840 ℃, respectively.
Figure 5. The AFM photos of the 1 mol. % CeO2 doped Y123 films fabricated at (a) 800 ℃, (b) 810 ℃, (c) 820 ℃, (d) 830 ℃, respectively. The surface roughnesses of the CeO2-Y123 films are 3.41 nm, 3.28 nm, 2.83 nm, 2.16 nm, respectively.
As discussed above, the Ic value of 3 mol. % CeO2 doped Y123 is smaller than that of 1 and 2 mol. % CeO2 doped Y123 films. So apart from the reasons discussed above, other factors may play an important role in the high CeO2 doping case. During the fabrication process of CeO2-Y123, there are two factors that affect the Ic value. On one hand, as a negative factor, the Y elements substituted by the Ce elements, resulting in the lattice distortion. In order to realize this substitution the strain energy should be exceeded and thus the extra energy is needed. The Ic value will decrease due to this substitution. On the other hand, as a positive factor, the CeO2 nano-particles were doped into the Y123 matrix as a second phase. These nano-particles can act as the pinning centers [31], leading to the increase of the Ic especially in the high external magnetic field. Apparently, as more CeO2 doped into the Y123 films, the Y sites have more opportunities to be occupied by Ce elements. In order to confirm that the substitution occurred, the magnified XRD patterns of the CeO2-Y123 (005) peaks are inserted in Fig. 3. A shift of the peak position to the right was observed with increasing the doping content, indicating the presence of the substitution. Meanwhile, the substitution becomes easier if the process occurred at high temperature. But for the high CeO2 doped film, a relatively low processing temperature was needed to obtain high current capacity. As a consequence, the optimum temperature for the growth of 3 mol. % CeO2 doped Y123 film is lower than that of 1 and 2 mol. % CeO2 doped films. Meanwhile, on the surface of the CeO2-Y123 films, a distinctive microstructure which is the in-plane rotation of the crystalline grain was observed, as circled by the red line in Fig. 4(a). To clarify the nature of this phenomenon, the lattice mismatch between the Y123 and the CeO2 layer under different in-plane orientations was
calculated, as shown in Table. 1. As can be seen from Table 1, the lattice mismatches of Y123 [110] // CeO2 [100] and Y123 [100] // CeO2 [100] are 0.64 % and 5.67 %, respectively. Obviously, the Y123 films with the in-plane alignment of Y123 [110] // CeO2 [100] is more likely to grow on the CeO2 layer. However, theoretically, the nucleation of these two types can both occur in the fabrication process of the CeO2-Y123 films. The fact is that the in-plane rotation can only be observed on the surface of the CeO2-Y123 film prepared at low temperature (800 ℃). That is to say, except the lattice mismatch between CeO2-Y123 film and the CeO2 layer, other influential factors should also be taken into account. For this reason, the schematic drawings showing the in-plane alignment of c-axis oriented Y123 on the CeO2 layer was presented in Fig. 6. Compared Fig. 6(a) with Fig. 6(b), obviously, a lower density of the dangling bonds was observed in the Y123 [100] // CeO2 [100] case. Table 1. The lattice constant and the lattice matching between the Y123 and the CeO2 layer under different in-plane orientations.
Lattice constant (nm) Y123
CeO2
a
0.3818
0.5411
b
0.3884
0.5411
Figure 6.
lattice mismatch Y123 [110] // CeO2 [100] 1Y123 on 1 CeO2 (unit)
Y123 [100] // CeO2 [100] 3Y123 on 2 CeO2 (unit)
0.64%
5.67 %
Schematic drawings showing the in-plane alignment of c-axis oriented Y123 on the CeO2 layer (a) Y123 [110] // CeO2 [100], (b) Y123 [100] // CeO2 [100].
As is well known, the activity of the atoms on the substrate is essentially affected by the substrate temperature. When high temperature was applied, the crystal nucleus with in-plane alignment of Y123 [100] // CeO2 [100] is unstable due to the existence of the large number of dangling bonds. Therefore, in this condition, even the nucleation process happened, the crystal nucleus with in-plane alignment of Y123 [100] // CeO2 [100] will disappear at last. On the contrary, in the low temperature condition, the crystal nucleuses with in-plane alignment of Y123 [110] // CeO2 [100] and Y123 [100] // CeO2 [100] are both preserved. In this case, the biaxial textured
structure of the CeO2-Y123 film was destroyed by the appearance of the in-plane rotation which resulted in low Ic value.
4. Conclusion In summary, using PLD technique, high c-axis oriented CeO2-Y123 films with a doping content of 1 mol. % - 3 mol. % were fabricated successfully. Our results showed that, with doping CeO2 into the Y123 films, relatively high Ic value can be obtained. For the CeO2-Y123 films with a low doping content (1 mol. % and 2 mol. %) the Ic values increased with the increase of the deposition temperature in the range of 800 ℃ to 830 ℃, due to the disappearance of the large crystal grains and their misorientation angles. Nevertheless, for the high doping content (3 mol. %) case, the Ic value decreased after the deposition temperature reached 820℃, owing to the substitutions of the Y elements by Ce elements. At last, an interesting phenomenon which is the in-plane rotation of the crystalline grain was observed on the surface of the CeO2-Y123 films. Rather than the lattice mismatch, the density of the dangling bonds plays the decisive role in the formation of this microstructure.
Acknowledgement This work was supported by the Natural Science Foundation of China (Grant numbers U1832157, 51372150), Shanghai Commission of Science and Technology (Grant numbers 16521108302), and 863 project (Grant number 2014AA032702).
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PII: DOI: Reference:
S0272-8842(18)32898-0 https://doi.org/10.1016/j.ceramint.2018.10.097 CERI19809
To appear in: Ceramics International Received date: 30 July 2018 Revised date: 11 October 2018 Accepted date: 11 October 2018 Cite this article as: Wei Wang, Linfei Liu, Tong Zheng, Shunfan Liu and Yijie Li, Superconducting properties and microstructures of CeO2 doped YBa2Cu3O7−δ films fabricated by pulsed laser deposition, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.097 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Superconducting properties and microstructures of CeO2 doped YBa2Cu3O7−δ films fabricated by pulsed laser deposition Wei Wang, Linfei Liu*, Tong Zheng, Shunfan Liu, Yijie Li* Key Lab of Artificial Structures & Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, People’s Republic of China [email protected]. [email protected] * Corresponding Author :
Abstract Performance of REBa2Cu3O7−δ superconductors strongly depends on their microstructures and surface morphologies. Using a reel-to-reel pulsed laser deposition (PLD) system, the CeO2-doped YBa2Cu3O7−δ (CeO2-Y123) films were fabricated. Superconducting properties and microstructure of these films were systematically studied for the first time. The results showed that the relationship between the critical current (Ic) values and the deposition temperature depends on the doping content essentially. For all CeO2-doped films, the Ic values increase at first and then decrease with increasing the deposition temperatures. The evolution of surface morphologies should take responsibility for this phenomenon. The optimum deposition temperature of 1 and 2 mol. % CeO2-doped Y123 films is higher than that of 3 mol. % CeO2 doped ones. The substitutions of the Y atoms by Ce atoms are considered as the determinative factor. Furthermore, the in-plane rotation of the crystalline grain orientation on the surface of the CeO2-Y123 films was observed. We think that not only the lattice mismatch, but also the density of dangling bonds contributes to this distinctive phenomenon. Keywords: CeO2-Y123, deposition temperature, Ic value, in-plane rotation.
1. Introduction REBa2Cu3O7−δ (REBCO or RE123) coated conductors, which have great potential for practical applications, have been widely investigated by many groups due to their relatively high critical temperature (Tc) since their discovery [1-4]. The extension of applications of REBCO coated conductors needs the increase of the current carrying capacity especially in high magnetic fields [5-6]. Therefore, finding the effective ways to improve the critical current (Ic) is very important for practical applications under high magnetic fields. There are mainly two methods which can effectively improve the critical current of REBCO films. Firstly, inserting buffer layers between the superconductor layer and the substrate is widely used by scientists [7-9]. The buffer materials, such as LaMnO3 [10], MgO [11-12], Y2O3 [13], Al2O3 [14] are usually utilized in the fabrication of
REBCO coated conductors. Besides providing a smooth textured-surface, the buffer layers can also prevent atoms diffusing from the substrate to REBCO layer. By this way, a high c-axis oriented REBCO films with excellent zero-field superconducting properties can be fabricated successfully. Secondly, doping second-phase particles in the REBCO matrix, generating nano-scale defects as the flux pining centers, has been proved to be a simple and practicable method to increase the critical current density (Jc). Compounds such as BaHfO3 [15-16], BaZrO3 [17-21], BaNb2O6 [22], ZrO2, Ta2O5 [23], metals such as Zr [24-25], Au [26] are all dopants that have been reported to have effects on the Jc improvement of REBCO films. For the latter case, searching proper dopant becomes a matter of significant importance in the fabrication of REBCO films. Cerium oxide (CeO2), due to its high chemical and thermal stability, is usually used as a buffer layer [27]. While in the top-seeded melt-growth (TSMG) process, the CeO2 has been doped into the REBCO bulks to suppress the liquid loss and to improve the Magnetic levitation force at the same time [28-29]. Besides, as one of the rare earth elements, Ce element can occupy the RE site without changing the orthorhombic phase of REBCO structure. Therefore, there is a strong possibility that CeO2 doping can influence the Jc of REBCO films significantly. In this work, we investigated the influence of CeO2 doping on the property of YBCO films. CeO2-Y123 films were grown on the CeO2 buffer layer by pulsed laser deposition. The factors, affecting the critical current of the CeO2-Y123 films, have been discussed. To fully understand the dependence of the property of CeO2-Y123 film on the deposition conditions, our investigation is also focused on the microstructure evolution of CeO2-Y123 films.
2. Experimental The CeO2-Y123 films were fabricated IBAD-MgO tapes by using a reel-to-reel pulsed laser deposition (PLD) system (PVD Products Inc.). The substrate is 10 mm in width and 50 μm in thickness. Prior to the fabrication of CeO2-Y123 film, a high textured CeO2 buffer layer was prepared by PLD system. The in-plane full width of half maximum (FWHM) value of the CeO2 buffer layer is less than 4 degree. Afterwards, the 200 nm thick CeO2-Y123 films were deposited by PLD in oxygen atmosphere at 800 ℃-840 ℃. After the deposition, a protection layer (silver) was fabricated using the magnetic sputtering method. Then the samples were annealed at 500 °C in flowing O2 gas for 5 h. The architecture of CeO2-Y123 coated conductors is schematically illustrated in Fig. 1. The CeO2-Y123 targets were made by solid state reaction method with mixtures of Y123 and CeO2 powder. The initial material (Y123 powder) was acquired also through a solid state reaction. The mixture of raw powders (Y2O3, BaCO3, and CuO) was calcined at 900 °C for 48 h in air. To improve the purity of the powder, the process was repeated three times. Three pellets, composed of Y123 + 1~3 mol. % CeO2, were prepared with a diameter of 100 mm and a thickness of 10 mm. Next, the pellets were sintered at 800 °C for 24 h and then flipped the pellets and sintered at
800 °C for another 24 h. Finally, in a muffle furnace, the pellets were annealed in flowing oxygen at 400 °C for 12 h. The structure and texture of CeO2-Y123 film were measured by X-ray diffraction system (D8 Discover with GADDS, Bruker Advanced X-ray Solutions, Inc.) with Cu-Kα radiation operated at 40 mA and 40 kV. The surface morphology and surface roughness of the CeO2-Y123 films were observed by Scanning Electron Microscope (SEM, FEI Sirion 200, operated at 5 kV) and atomic force microscopy (AFM, Tapping Mode, BioScope, Veeco Instruments, Inc.).
Figure 1.
Schematic architecture layout of the CeO2-Y123 coated conductor
3. Results and Discussion To evaluate the property of the CeO2-Y123 films, in the liquid nitrogen (77 K), the Ic values of the films were measured by conventional four-probe method in self-field using a criterion of 1 μV/cm. A relatively high Ic (100 A) was obtained in the 1 mol. % CeO2 doped Y123 films. While, the Ic value of the pure Y123 film is merely 75 A. Meanwhile, the relationship between the Ic value of the CeO2-Y123 film and the deposition temperature was shown in Fig. 2. For all the samples, the Ic values firstly increase and then decrease with increasing of the deposition temperature. The optimum temperature of 1 and 2 mol. % CeO2 doped Y123 films is higher than that of 3 mol. % CeO2 doped one. Besides, the Ic values of the CeO2-Y123 films, which grown at their optimum temperatures, decreases along with increasing of the doping content.
Figure 2.
The relationship between the critical current (Ic) of the CeO2-Y123 films and the deposition temperature
To clarify the reasons, the XRD patterns of these films prepared at 820 ℃ were shown in Fig. 3. As can be seen from Fig. 3, only Y123 (00l) peaks were observed, indicating their high c-axis oriented feature. The XRD results proved that the perovskite structure of Y123 films was unchanged within CeO2 doping content. The magnified XRD patterns of the CeO2-Y123 (005) peaks are inserted in Fig. 3. It can be seen that the Y123 (005) peak position shifts to large angle direction with increasing of the doping content.
Figure 3. XRD patterns of the CeO2-Y123 films with the doping content of 1 mol. %, 2 mol. %, 3 mol. % fabricated at 820℃. The inset shows the magnified XRD patterns of the CeO2-Y123 (005) peaks. The CeO2-Y123 films possess a space group of Pmmm with the c-axis lattice
parameter of 1.16 nm calculated by the Bragg equation, which fit well with the orthorhombic Y123 phase [30].
The SEM images of 1 mol. % CeO2 doped Y123 films fabricated at 800 ℃-840 ℃ were presented in Fig. 4. As shown in Fig. 4(a), a number of large crystalline grains （Y123 crystalline grains） were observed on the surface of the films. With increasing the deposition temperature, the crystalline grains diminished gradually and disappeared at last, as observed in Fig. 4(b)-(d). The diminishing of the large grain boundaries, resulting from the disappearance of the large crystal grains, caused the increase of the Ic values. With further increasing the deposition temperature to 840 ℃, the non-superconducting phase (Y2BaCuO5, Y211) was formed, as observed in Fig 4(e). As is well known, the appearance of the Y211 phase usually results in the decrease of the Ic value. During the film growth process, the nucleation barrier must be overcome in order to form the nucleuses. That is to say, to realize the formation of Y123 nucleus, large potential energy must be achieved. Therefore, at high deposition temperature, many CeO2-Y123 nucleuses were formed. Then all the nucleuses grew in the a, b-direction and converged together, leading to the fast growth of the a, b-plane. On the contrary, at low deposition temperature, the formation of the nucleus was suppressed, resulting in its less nucleuses feature. Further growth of the nucleuses caused the appearance of the large crystalline grains. At the same time, the grain boundary mis-orient angles become large inevitably. Meanwhile, the current carrying capacity reduced essentially, due to these large grain boundaries. Therefore, with increasing the deposition temperature, the Ic values increase at first and then decrease. The AFM photos of the 1 mol. % CeO2 doped Y123 films fabricated at 800 ℃-830 ℃, are presented in Fig. 5. As can be seen from Fig. 5, with increasing the deposition temperature, the surface roughness decreased gradually. Apparently, the decrease of the surface roughness results from the decrease of the large crystalline grains, which fit well with the SEM results.
Figure 4.
The SEM images of the 1 mol. % CeO2 doped Y123 films fabricated at (a) 800 ℃, (b) 810 ℃, (c) 820 ℃, (d) 830 ℃, (e) 840 ℃, respectively.
Figure 5. The AFM photos of the 1 mol. % CeO2 doped Y123 films fabricated at (a) 800 ℃, (b) 810 ℃, (c) 820 ℃, (d) 830 ℃, respectively. The surface roughnesses of the CeO2-Y123 films are 3.41 nm, 3.28 nm, 2.83 nm, 2.16 nm, respectively.
As discussed above, the Ic value of 3 mol. % CeO2 doped Y123 is smaller than that of 1 and 2 mol. % CeO2 doped Y123 films. So apart from the reasons discussed above, other factors may play an important role in the high CeO2 doping case. During the fabrication process of CeO2-Y123, there are two factors that affect the Ic value. On one hand, as a negative factor, the Y elements substituted by the Ce elements, resulting in the lattice distortion. In order to realize this substitution the strain energy should be exceeded and thus the extra energy is needed. The Ic value will decrease due to this substitution. On the other hand, as a positive factor, the CeO2 nano-particles were doped into the Y123 matrix as a second phase. These nano-particles can act as the pinning centers [31], leading to the increase of the Ic especially in the high external magnetic field. Apparently, as more CeO2 doped into the Y123 films, the Y sites have more opportunities to be occupied by Ce elements. In order to confirm that the substitution occurred, the magnified XRD patterns of the CeO2-Y123 (005) peaks are inserted in Fig. 3. A shift of the peak position to the right was observed with increasing the doping content, indicating the presence of the substitution. Meanwhile, the substitution becomes easier if the process occurred at high temperature. But for the high CeO2 doped film, a relatively low processing temperature was needed to obtain high current capacity. As a consequence, the optimum temperature for the growth of 3 mol. % CeO2 doped Y123 film is lower than that of 1 and 2 mol. % CeO2 doped films. Meanwhile, on the surface of the CeO2-Y123 films, a distinctive microstructure which is the in-plane rotation of the crystalline grain was observed, as circled by the red line in Fig. 4(a). To clarify the nature of this phenomenon, the lattice mismatch between the Y123 and the CeO2 layer under different in-plane orientations was
calculated, as shown in Table. 1. As can be seen from Table 1, the lattice mismatches of Y123 [110] // CeO2 [100] and Y123 [100] // CeO2 [100] are 0.64 % and 5.67 %, respectively. Obviously, the Y123 films with the in-plane alignment of Y123 [110] // CeO2 [100] is more likely to grow on the CeO2 layer. However, theoretically, the nucleation of these two types can both occur in the fabrication process of the CeO2-Y123 films. The fact is that the in-plane rotation can only be observed on the surface of the CeO2-Y123 film prepared at low temperature (800 ℃). That is to say, except the lattice mismatch between CeO2-Y123 film and the CeO2 layer, other influential factors should also be taken into account. For this reason, the schematic drawings showing the in-plane alignment of c-axis oriented Y123 on the CeO2 layer was presented in Fig. 6. Compared Fig. 6(a) with Fig. 6(b), obviously, a lower density of the dangling bonds was observed in the Y123 [100] // CeO2 [100] case. Table 1. The lattice constant and the lattice matching between the Y123 and the CeO2 layer under different in-plane orientations.
Lattice constant (nm) Y123
CeO2
a
0.3818
0.5411
b
0.3884
0.5411
Figure 6.
lattice mismatch Y123 [110] // CeO2 [100] 1Y123 on 1 CeO2 (unit)
Y123 [100] // CeO2 [100] 3Y123 on 2 CeO2 (unit)
0.64%
5.67 %
Schematic drawings showing the in-plane alignment of c-axis oriented Y123 on the CeO2 layer (a) Y123 [110] // CeO2 [100], (b) Y123 [100] // CeO2 [100].
As is well known, the activity of the atoms on the substrate is essentially affected by the substrate temperature. When high temperature was applied, the crystal nucleus with in-plane alignment of Y123 [100] // CeO2 [100] is unstable due to the existence of the large number of dangling bonds. Therefore, in this condition, even the nucleation process happened, the crystal nucleus with in-plane alignment of Y123 [100] // CeO2 [100] will disappear at last. On the contrary, in the low temperature condition, the crystal nucleuses with in-plane alignment of Y123 [110] // CeO2 [100] and Y123 [100] // CeO2 [100] are both preserved. In this case, the biaxial textured
structure of the CeO2-Y123 film was destroyed by the appearance of the in-plane rotation which resulted in low Ic value.
4. Conclusion In summary, using PLD technique, high c-axis oriented CeO2-Y123 films with a doping content of 1 mol. % - 3 mol. % were fabricated successfully. Our results showed that, with doping CeO2 into the Y123 films, relatively high Ic value can be obtained. For the CeO2-Y123 films with a low doping content (1 mol. % and 2 mol. %) the Ic values increased with the increase of the deposition temperature in the range of 800 ℃ to 830 ℃, due to the disappearance of the large crystal grains and their misorientation angles. Nevertheless, for the high doping content (3 mol. %) case, the Ic value decreased after the deposition temperature reached 820℃, owing to the substitutions of the Y elements by Ce elements. At last, an interesting phenomenon which is the in-plane rotation of the crystalline grain was observed on the surface of the CeO2-Y123 films. Rather than the lattice mismatch, the density of the dangling bonds plays the decisive role in the formation of this microstructure.
Acknowledgement This work was supported by the Natural Science Foundation of China (Grant numbers U1832157, 51372150), Shanghai Commission of Science and Technology (Grant numbers 16521108302), and 863 project (Grant number 2014AA032702).
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