# The Influence of Surface Morphology of Buffer Layer on the Critical Current Density in YBCO Coated Conductors

Authors: Jie Xiong; Yudong Xia; Fei Zhang; Yan Xue; Kai Hu; Xiaohui Zhao; Bowan Tao
Journal: Advances in Condensed Matter Physics
 (2013)

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## Abstract

1μm-thick YBa2Cu3O7-δ (YBCO) films were grown on the Y2O3/yttria stabilized zirconia (YSZ)/CeO2 buffer layers with different surface morphologies using direct-current sputtering. The critical current density (Jc) value of YBCO was 1.1 MA/cm2 when the root mean square surface roughness (Rrms) of the buffer layer was 2.5 nm. As the Rrms of the buffer layer increased to 15 nm, the Jc decreased to 0.3 MA/cm2. X-ray diffraction and scanning electron microscopy showed the strong relevance of the evolution of the structure and surface morphologies of YBCO films with the buffer layer of different Rrms. A model was proposed to explain the influence of surface morphology on the superconducting properties of YBCO films.


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## Body

## 1. Introduction

Recently, there are more efforts focusing on the fabrication of high performanceYBa2Cu3O7-δ (YBCO) coated conductors on flexible metallic substrates [1, 2]. The application of 2nd generation coated conductors requires higher current carrying capacity. To reach this goal, a high critical current density (Jc) must be maintained in thick (>1 μm) YBCO coatings [3]. Unfortunately, YBCO films deposited on polycrystalline metal substrates showed high angle grain boundaries, leading that the YBCO had a very small critical current [4]. As a result, several methods had been investigated to prepare biaxially aligned templates, such as ion-beam-assisted deposition (IBAD) and rolling assisted biaxially textured substrates (RABiTS) [5, 6]. The RABiTS approach has been proven to be a simple and cost effective manufacturing technology for mass production of coated conductors.In order to block the interdiffusion of the metal and oxygen, and minimize the lattice mismatch of YBCO and flexible metallic substrates, one or several layer thin films are deposited on the metallic substrates to serve as the buffer layer for the subsequent YBCO films. Typical tri-layer stack ofCeO2/yttrium stabilized zirconia (YSZ)/Y2O3 is one of the mostly explored architectures [7]. In this architecture, the Y2O3 acts to transfer the biaxial texture from the RABiTS substrate to the YBCO layer; the YSZ layer is used as a barrier to prevent the interdiffusion between oxides and metal substrate; the CeO2 cap layer on YSZ layer provides good lattice matching and an epitaxial growth template for depositing the YBCO layer. Therefore, buffer layers play a critical role in fabricating high performance coated conductors. So far, enormous efforts had been made toward the manufacture of the biaxial texture of buffer layers on the RABiTS tapes; however, the cap layer had not been paid enough attention [8–10]. In our previous work, we had reported the surface morphology evolvement of the CeO2 films deposited on the YSZ/Y2O3/Ni-alloy substrates [11, 12]. In this study, we investigated in detail the impact of the surface morphologies of buffer layers on the superconducting properties in YBCO films.


## 2. Experimental Details

Biaxially textured{100}〈100〉 Ni-5at.%W alloy (NiW) tapes were used in this study, which were supplied from EVICO GmbH, Germany. The full width at half maximum (FWHM) values of out-of-plane and in-plane were about 5.5° and 6°, respectively. The Y2O3/YSZ/CeO2 multilayers with different surface morphologies were deposited on NiW tapes using reel-to-reel direct-current (DC) reactive magnetron sputtering by modulating the substrate temperature from 500°C to 680°C as reported in [12]. The thickness of all CeO2 cap layers was 40 nm. The subsequent YBCO films were prepared by DC sputtering with the same conditions, which were initially optimized. Detailed conditions were reported elsewhere [3].The film structure was measured with Bede X-ray diffraction (XRD) system includingθ-2θ, ω, and ϕ-scan. Atomic force microscopy (AFM, SEIKO SPA300HA microscopy) and scanning electron microscopy (SEM, JSM-5900) were used to characterize the surface morphology and calculate the root mean square surface roughness (Rrms). Jc-Scan system developed by Germany Leipzig University was used to measure the Jc values.


## 3. Results and Discussion

Figure1 shows the surface morphologies of the different CeO2 layers for this series experiments. The Rrms values of the CeO2 films from (a) to (e) are shown in Table 1. The XRD θ-2θ scan, ω-scan of CeO2 (200) peak, and ϕ-scan of CeO2 (111) plane are shown in Figures 2(a), 2(b), and 2(c), respectively; all the Y2O3/YSZ/CeO2 films are completely c-axis oriented with representative in-plane FWHM value of 5.2° and out-of-plane FWHM value of 3.2°.Table 1
TheRrms of the buffer layers and the values of out-of-plane and in-plane textures of the corresponding YBCO films.
Sample
a
b
c
d
e
R
rms of CeO2
2.5 nm
4.6 nm
5.8 nm
9.5 nm
15.2 nm
Δ
ω of (002) YBCO
2.37°
2.98°
2.92°
2.97°
3.2°
Δ
ϕ of (103) YBCO
4.91°
4.77°
5.07°
4.72°
5.48°AFM images of differentCeO2 cap layers.
(a)
(b)
(c)
(d)
(e)Typical XRD patterns ofCeO2/YSZ/Y2O3 buffer layer (a) θ-2θ scan, (b) ω-scan of (002) CeO2, and (c) ϕ-scan of (111) CeO2.
(a)
(b)
(c)The XRD patterns of YBCO films deposited on the buffer samples from (a) to (e) are shown in Figure3. The deposition time was 20 hours and the typical thickness of YBCO film was 1 μm. The 2θ peaks at 33.02°, 33.8°, and 34.9° were indexed as the (00l) reflections of CeO2, Y2O3, and YSZ, respectively. Weak NiO and NiWO4 reflections were observed at the 2θ angles of 19.28°, 37.26°, and 43.29°. All of the patterns indicated pure c-axis orientation except for the sample (e), which had a small amount of a-axis grains. The out-of-plane and in-plane textures of YBCO films deposited on samples from (a) to (e) are documented in Table 1. With the exception of the YBCO deposited on sample (e), all the out-of-plane FWHM values of YBCO films were smaller than 3°, demonstrating good out-of-plane alignments. The FWHM values of sample (e) were slightly bigger than others, ascribed to the a-axis grains, and leaded to the formation of high angle grain boundaries in the YBCO film. The in-plane FWHM values of YBCO films were between 4.7° and 5.5°, indicating no obvious tendency on the in-plane texture when the YBCO films deposited on different buffer layers.Figure 3
XRDθ-2θ patterns of YBCO films deposited on the buffers of samples from (a) to (e).The SEM images of YBCO films deposited on different surface morphologies ofCeO2 cap layers are shown in Figure 4, which corresponded with (a) to (e) of Figure 1, respectively. Figure 4(a) showed the relative flattest surface morphology with typical small holes attributed to the rich yttrium in YBCO films [13]. As the Rrms of the buffer layers increased, the holes on the surface of the YBCO films became bigger, as observed in Figures 4(b) and 4(c). As the Rrms of the buffer layers increased more than 10 nm, the needle-like a-axis particles of YBCO films were distributed on the surface of YBCO film shown in Figures 4(d) and 4(e).SEM images of YBCO films deposited on different surface morphologies of buffer layers.
(a)
(b)
(c)
(d)
(e)The relationship between theJc of corresponding YBCO films is sketched in Figure 5. The Jc of YBCO deposited on the buffer of sample (a) was 1.1 MA/cm2. The YBCO deposited on samples (b)–(d) showed Jc values of 0.9 MA/cm2, 0.8 MA/cm2, and 0.5 MA/cm2, respectively. As the Rrms of the buffer layers increased to 15 nm, the Jc value of YBCO films continuously decreased to 0.3 MA/cm2, revealing the strong relevance between the superconducting properties and the surface morphology.Figure 5
The relationship between theJc of YBCO films and surface morphologies of CeO2 layers.In order to study the mechanism of the effect of the buffers’ surface on the performance of YBCO films, the corresponding line profiles ofCeO2 films of samples (a) and (e) were plotted in Figures 6(a) and 6(b), respectively. As shown in Figure 6, the grain height was in the range of −50 nm~50 nm, and the gap between in grain boundaries of sample (a) was much smaller than that of sample (e). In the sputter process, the sputter atoms diffused and nucleated on the substrate surface. However, YBCO is a multielement oxide with different atom radii of Y, Ba, and Cu. For different surface morphologies of CeO2 films such as sample (e), the gap between grain boundaries was deeper and the grain boundaries area was larger than that of sample (a). If metal atoms diffused from one grain of CeO2 surface to another, it should overcome the superior potential barrier, while the potential barrier of a Ba atom is bigger than that of Cu and Y atoms. Therefore, if the grain boundary area was large, Ba atoms would be enriched on the grain boundaries, which led to the deviation of YBCO chemical contents, and as observed white particles formed on the surface in Figure 4(e), which was identified as the CuO nonconductive particles. Further demonstrations are necessary for a better understanding by transmission electron microscopy which is ongoing.The corresponding line profiles ofCeO2 films (a) of sample (a) and (b) of sample (e).
(a)
(b)


## 4. Conclusions

The influence of surface morphologies ofCeO2 cap layers on the microstructure and critical current density of YBCO films was systematically investigated. YBCO films on the flat surface revealed high quality epitaxial c-axis oriented growth and had much better superconducting properties. As the Rrms of the buffer layer increased, the nonconductive particles and a-axis grains on the surface of YBCO were observed, leading to deteriorating the Jc of YBCO films. A model was proposed to understand the phenomena, demonstrating that the surface morphology of buffer layers had a strong effect on the current carrying capacity of YBCO coated conductors.



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*Source: 673948-2013-12-04.xml*