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Electronic Materials Letters

, Volume 14, Issue 5, pp 646–654 | Cite as

The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures

  • Sungho Lee
  • Woo Il Yang
  • Ho Sang Jung
  • Won-Jae Oh
  • Jiyeong Jang
  • Jae-Hun Lee
  • Kihyeok Kang
  • Seung-Hyun Moon
  • Sang-Im Yoo
  • Sang Young Lee
Article
  • 119 Downloads

Abstract

High-TC GdBa2Cu3O7−δ (GdBCO) superconductor has been popular for making superconductive tapes that have much potential for various fields of large-scale applications. We investigated aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO layer on Hastelloy substrate, so called GdBCO coated conductors (CCs), and Ag-coated GdBCO films on LaAlO3 (LAO) single-crystal substrates at cryogenic temperatures and compared them with each other. Unlike the RS of Ag-coated GdBCO films showing significant degradation in 4 weeks, no significant aging effects were found in our Ag-coated GdBCO CCs aged 85 weeks. The reactive co-evaporation deposition and reaction (RCE-DR) method was used for preparing the Ag-coated GdBCO CCs. Such durability of the Ag-coated GdBCO CCs in terms of the RS could be explained by existence of a protective impurity phase, i.e., Gd-free Ba–Cu–O phase as confirmed by transmission electron microscopy study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on transport properties of other kinds of Ag-coated GdBCO CCs could be realized by means of an artificially-grown protective impurity layer.

Graphical Abstract

Keywords

Aging effect GdBa2Cu3O7−δ Surface resistance Coated conductor Film 

1 Introduction

Among high-TC superconductors with the critical temperature (TC) higher than the liquid nitrogen temperature, GdBa2Cu3O7−δ (GdBCO) has drawn much attention of worldwide researchers having strong interests for large-scale applications. With the critical current density (JC) becoming higher than several MA/cm2 at 77 K [1, 2, 3], GdBCO has been popular for making superconductive tapes, i.e., coated conductors (CCs) that could be used for various fields of large-scale applications including high-field magnets. To date, there are many companies specialized in producing commercial GdBCO CCs, which provides a success story in terms of applications of high-TC materials. For instance, GdBCO CC-based high-field magnets have been known to generate very high magnetic fields of 26 Tesla at 4.2 K [4].

When commercial GdBCO CCs are produced by various factories, the GdBCO layer is usually coated with a Ag layer [5]: The Ag layer acts as a protective layer for the underlying GdBCO layer as well as provides a current path for quench protection. What draws our attention is that there have been recent reports on destructive roles of Ag coating for c-axis oriented GdBCO films on SrTiO3 (100) single crystal substrates [6, 7]. For instance, a reduction in JC at 10 K by about 4 times has been reported by Paturi et al. for a 50 nm-thick Ag-coated GdBCO film aged 5 weeks [7]. They attributed such aging effects to oxygen out-diffusion from the GdBCO, with the Ag layer acting as a catalyst for the oxygen loss. Therefore, assuming that these observations hold for commercial Ag-coated GdBCO CCs, it should be manufacturers’ responsibility to find ways to drastically slow down the aging effects caused by Ag coating. To date, however, aging effects on the transport properties including the JC of Ag-coated GdBCO CCs have been rarely studied.

In this paper, we studied aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO CCs with the GdBCO layer grown on buffered Hastelloy substrate by using the reactive co-evaporation deposition and reaction (RCE-DR) method [4]. We investigated variations in the RS of the Ag-coated GdBCO CCs with aging time, which we compared with those for Ag-coated epitaxially-grown GdBCO films on single crystal substrates. In studying the aging effects for the Ag-coated GdBCO films, we also measured variations in the microwave RS of the films with aging time after the Ag coating had been removed. Striking differences were found between the GdBCO CCs and GdBCO films with no sign of aging effects on the RS of the former after a period of 85 weeks.

2 Experimental

2.1 Sample Preparation

We used Ag-coated GdBCO CCs prepared by SuNAM for commercial purposes. The GdBCO layer with a thickness of 1.45 μm was prepared on buffered hastelloy substrate by using the RCE-DR method. Oxygen annealing was done after the GdBCO layer is coated with a 600 nm-thick Ag layer grown in situ. The width of the GdBCO CCs is 10 mm. Detailed descriptions on the fabrication procedures have been reported elsewhere [5]. In the RCE-DR method, the deposition ratio among Gd, Ba, and Cu is set to be different from the stoichiometric molar ratio, which is for producing pinning centers such as Gd2O3 and CuO phase inside the GdBCO layer. It has been known that this growth method also results in appearance of impurity phases including Gd-free Ba–Cu–O at the upper part of the GdBCO layer [5]. Figure 1(a) shows X-ray diffraction (XRD) spectra for our GdBCO CCs, where (00l) peaks from stoichiometric GdBCO phase are seen along with those from impurity phases such as Gd2O3 and CuO. We note that the Gd-free Ba–Cu–O phase consists of BaCu2O3 and CuO phases.
Fig. 1

X-ray diffraction spectra for a Ag-coated GdBCO CCs aged 129 weeks (Ag(600)-GdBCO-CC-Top), b pristine Ag-coated GdBCO films (Ag(600)-GdBCO-F2), and c Ag-coated GdBCO film (Ag(600)-GdBCO-F2) aged 212 weeks. In (a), peaks from stoichiometric GdBCO phase are seen along with those from impurity phases such as Gd2O3 and CuO. Only (00 l) peaks from stoichiometric GdBCO are seen in (b), signaling c-axis orientation of GdBCO grains. In (c), All the (00 l) peaks for the aged Ag-coated GdBCO film appears to be much broader than the corresponding ones for the pristine uncoated GdBCO film. All the XRD data were taken after removal of the Ag layer from the specimens

We also prepared Ag-coated GdBCO films for comparative studies on aging effects. The GdBCO films were prepared by using the laser ablation method [8], on which Ag-coating was performed. 500 nm-thick GdBCO films were grown on LaAlO3 (100) single crystal substrates having dimensions of 10 × 10 mm2. We used two different values of 50 nm and 600 nm for the thickness of the Ag coating: The former is to compare our results with earlier reports on 50 nm-thick Ag-coated GdBCO films by other researchers [7], with the latter for comparison between Ag-coated GdBCO films and CCs having the same thickness of 600 nm for the Ag coating. Only (00l) peaks appeared in the XRD spectra for pristine GdBCO films as seen in Fig. 1b. We display XRD spectra for an aged GdBCO film in Fig. 1c. In the figure, the (00l) peaks are mainly seen, which, however, appear to be much broader compared to the corresponding ones for the pristine GdBCO film. Also, we see significant changes in the intensity of the (00l) peaks (I((00l)), on which we discuss later. All the GdBCO specimens, listed in Table 1, have been stored inside an air-filled desiccator at the room temperature.
Table 1

List of GdBCO specimens used in this study

Sample type (Growth method)

Substrate

Sample description

Sample name

Measurement time for the RS

Epitaxially-grown film (Pulsed laser ablation)

LaAlO3(100)

Uncoated GdBCO film

uc-GdBCO-F1

Week 0, week 8

uc-GdBCO-F2

Week 0, week 185

GdBCO film coated with 50 nm-thick Ag layer

Ag(50)-GdBCO-F1

Week 12a, week 16a

Ag(50)-GdBCO-F2

Week 209

GdBCO film coated with 600 nm-thick Ag layer

Ag(600)-GdBCO-F1

Week 9a, week 17a

Ag(600)-GdBCO-F2

Week 208

Coated Conductor (RCE-DR)

Buffered Hastelloy

Top part of the GdBCO layer in GdBCO conductors coated with a 600 nm-thick Ag layer

Ag(600)-GdBCO-CC-Top

Week 0, week 85,

Bottom part of the GdBCO layer in GdBCO conductors coated with a 600 nm-thick Ag layer

Ag(600)-GdBCO-CC-Bottom

Week 2, week 119

aThe GdBCO specimens was stored between week 9 and week 17 after the Ag coating was removed at week 9

2.2 Measurement Procedure

The RS of the Ag-coated GdBCO CCs and films were measured by using the dielectric resonator method, for which a rutile resonator was used. In Fig. 2, we show a diagram of the rutile resonator with a reference YBCO film placed at the bottom and a GdBCO sample at the top. For the resonator, the dimensions of the rutile rod are 3.88 mm in diameter and 2.73 mm in height and the dimensions of the cylindrical cavity are 9.00 mm in diameter and 2.73 mm in height. More details on rutile resonators have been described elsewhere [9, 10, 11].
Fig. 2

(Top) A cross-sectional view of the rutile resonators with a reference YBCO film placed at the bottom and a GdBCO sample at the top of the resonator. (Bottom) A top view of the rutile resonators. Samples with the dimensions of 10 × 10 mm2 are large enough to fully cover the cavity wall

The RS of the GdBCO specimens is obtained using the measured unloaded quality factor (Q0) of the TE011-mode rutile resonator using Eq. (1) [12].
$$\frac{1}{{Q_{O} }} = \frac{{R_{S} (GdBCO)}}{{G_{T} }} + \frac{{R_{S} (YBCO)}}{{G_{B} }} + \frac{{R_{S} (Cu)}}{{G_{SW} }} + k \times \tan \delta$$
(1)

Here GT, GB, and GSW denote the geometrical factors associated with the top plate, the bottom plate, and the inner sidewall of the resonator, respectively, RS(GdBCO) and RS (YBCO) denote the RS of the GdBCO specimens and the reference YBCO film, respectively, and RS (Cu), the RS of the inner sidewall made of oxygen-free high-conductivity copper corresponding to the TE011 mode of the resonator. For reference, the TE011-mode resonant frequency was temperature-dependent with the value of 8.55 GHz at 20 K, where GT = GB = 206 Ω, GSW = 31,920 Ω, and k = 0.9979, respectively. Also, tan δ denotes the loss tangent of the rutile rod used for the resonator, and k, the filling factor for the same TE011 mode. If we just change the top plate of the resonator from one GdBCO specimen to another with the other parts remaining unchanged, the differences in the RS among the GdBCO specimens are directly reflected to the ones in the measured Q0 according to Eq. (1). It is noted that, before measurements, the Ag layer on top of the GdBCO layer should be completely removed. It is because, in our measurement scheme, the measured Q0 is strongly affected by the presence of the Ag layer having significantly higher RS than that of the GdBCO specimens. This makes it impossible to measure the RS of the GdBCO specimens with accuracy. We used a hydrogen peroxide ammonia solution for removal of the Ag layer.

For measuring the RS of the GdBCO specimens using Eq. (1), we determined the RS (YBCO) and the tan δ of rutile in advance using the measured Q0 of the resonator having the same YBCO films placed at the top and at the bottom. For this purpose, we also determined the values for the geometrical factors and the filling factor theoretically using distributions of the electromagnetic fields corresponding to the TE011 mode. More details on this procedure are described elsewhere [12, 13]. Figure 3 shows the measured RS of the reference YBCO films as a function of temperature with the inset showing a tan δ vs. temperature curve for the rutile rod. Both the RS of the YBCO films and the tan δ of rutile appeared to be low enough for us to measure the RS of the GdBCO specimens with accuracy. We note that, for superconductor films having the same intrinsic properties, the measured RS values depend on the film thickness unless the film thickness much greater than the penetration depth [12, 14]. Thus, the measured RS values in this study could mean the effective RS affected by the finite thickness effects.
Fig. 3

Comparison among the magnitudes of the terms in Eq. (1) as functions of temperature for the 8.5 GHz TE011 mode rutile resonator. Since RS(GdBCO)/GT is the greatest of all except 1/Q0 throughout the temperatures, the RS(GdBCO) values could be measured with accuracy

3 Results and Discussion

3.1 Aging Effects on the R S of Ag-Coated GdBCO Films

The RS of the our pristine, uncoated GdBCO film (uc-GdBCO-F1) was as good as that of a typical GdBCO single crystal, with the value of 280 μΩ at 8.5 GHz and 20 K being comparable with the corresponding value of ~ 330 μΩ at 10.18 GHz by Ormeno et al. [15]. In Fig. 4a, we also see that the RS of the uc-GdBCO-F1 aged 8 weeks appears to be almost the same as that of the pristine uc-GdBCO-F1 throughout the temperatures, implying that aging process is not fast for uncoated GdBCO films in terms of the RS. We note in Fig. 4a that the RS appears to increase as the temperature gets lower below 20 K. This has been attributed to antiferromagetic Gd3+ ions [15], which enhance the permeability of GdBCO, thus raising the real part of the surface impedance. For reference, the surface impedance ZS (= RS + iXS) is expressed by ZS = (μ/ε)1/2, where μ and ε denote the permeability and the permittivity of the specimens under test, respectively, with XS denoting the surface reactance. The temperature dependence for the ratio of the RS of the uc-GdBCO-F1 aged 8 weeks to that of the pristine uc-GdBCO-F1 is displayed in Fig. 4b, where ratio values of ~ 1 are seen regardless of the temperature.
Fig. 4

a The temperature dependences of the RS of a pristine and an aged uncoated GdBCO films (uc-GdBCO-F1), and aged GdBCO films coated with a 50 nm-thick Ag layer (Ag(50)-GdBCO-F1) and a 600 nm-thick Ag layer (Ag(600)-GdBCO-F1). Here ‘week N/week 0′ denote that the same specimen was measured at week N and week 0 and compared with each other. Also, ‘week N2/week N1′ with N1 > 0 means that the RS was measured at week N1 right after the Ag coating was removed with the RS measured again at week N2. In this case, the specimen was stored without the Ag coating between week N1 and week N2. b The ratios between the RS of the pristine and the aged uc-GdBCO-F1, Ag(50)-GdBCO-F1 and Ag(600)-GdBCO-F1 as functions of temperature. For Ag(50)-GdBCO-F1 and Ag(600)-GdBCO-F1, the Ag layers were removed from the specimens at weeks 12 and 9, respectively, for which the RS data were collected at week 16 for the former and week 17 for the latter

Meanwhile, unlike the uncoated GdBCO film, the RS of Ag(50)-GdBCO-F1 aged 12 weeks showed significant aging effects throughout the temperatures, with the RS enhanced by ~ 90 and 30% at 20 and 70 K, respectively, over the time span. Interestingly, aging process sped up after the Ag layer was removed, with the RS values at 20 and 70 K increased by about 6 times and 3 times, respectively, in 4 weeks after removal of the Ag layer. These results are different from what has been observed for Au-coated GdBCO films by Schlesier et al. [6], where degradation in the JC appeared to proceed fast at first and saturate later.

Aging effects appeared somewhat different on the RS of Ag(600)-GdBCO-F1 from those for Ag(50)-GdBCO-F1, with the RS of the Ag(600)-GdBCO-F1 aged 9 weeks changing a little compared to that of the pristine Ag(600)-GdBCO-F1 throughout the temperatures. It might be due to that that oxygen out-diffusion from the GdBCO layer was slowed down by the thicker Ag layer. However, this does not mean that the 600 nm-thick Ag layer could protect the underlying GdBCO layer from the aging effects for a long period of time. Indeed, a GdBCO layer coated with a 600 nm-thick Ag layer appeared to lose its superconductivity completely after being stored for 184 weeks, on which we discuss later. For Ag(600)-GdBCO-F1, the RS appeared to increase significantly in 7 weeks after removal of the Ag layer throughout the temperatures, with the respective RS values at 20 and 70 K becoming 5 times and 2 times higher than before over this time period. These results also show that aging process continued fast for Ag(600)-GdBCO-F1 after removal of the Ag layer.

In Fig. 5, we show variations in the ratio of the intensities of two XRD peaks of (005) and (007) (henceforth called I(005)/I(007)) with aging time for different GdBCO specimens. Correlation between I(005)/I(007) and oxygen deficiency has been reported by Ye et al. [16], who observed continuous decrease in I(005)/I(007) with increase in δ for YBaCuO7−δ films. In Fig. 5, I(005)/I(007) decreases below 2.9 for Ag(600)-GdBCO-F1, Ag(50)-GdBCO-F2, Ag(600)-GdBCO-F2, and Ag(600)-GdBCO-CC-Bottom with increasing aging time, reflecting significant aging effects observed for the specimens. Meanwhile, the values remain higher than 2.9 for Ag-GdBCO-CC-Top and uc-GdBCO-F2, which could reflect lack of the aging effects in the GdBCO layer. However, it is not clear if the criterion based on the I(005)/I(007) value could be used for correlating the aging effects in the Ag-coated GdBCO films with the observed I(005)/I(007) values. For instance, we see in Fig. 5 decrease of I(005)/I(007) below 2.9 over a time span of 184 weeks for the other uncoated GdBCO, i.e., uc-GdBCO-F1, despite that its RS values show no signs of the aging effects. Furthermore, despite that significant aging effects were observed for Ag(50)-GdBCO-F1, its I(005)/I(007) values remained higher than 2.9. Similar observations were also made by Paturi et al. [7], who attributed such mixed tendency with regard to the I(005)/I(007) values to substitution of Gd ion to the site of Ba ions in aged GdBCO.
Fig. 5

Variations of I(005)/I(007) with the aging time for different GdBCO specimens. The values appear to decrease below 2.9 for both the aged Ag(50)-coated GdBCO and Ag(600)-GdBCO films. It is noted that the I(005)/I(007) value remains almost the same at above 2.9 for an uncoated GdBCO film (Gd-F2) showing no aging effects. The straight line at I(005)/I(007) = 2.9 and the dotted lines are guides to eyes

3.2 Aging Effects on the R S of Ag-Coated GdBCO Coated Conductors

For most commercial GdBCO CCs, the GdBCO layer is covered with an Ag layer. Therefore, it is essential to compare aging effects in Ag-coated-GdBCO films with those in commercial Ag-coated GdBCO CCs. In Fig. 6, we show the ratio of the RS of the Ag-coated GdBCO CCs aged 85 weeks with that of the pristine one, where the values are close to 1 throughout the temperatures due to little difference between the two sets of the measured RS as seen in its inset. We note that these results for our Ag-coated GdBCO CCs, i.e., Ag(600)-GdBCO-CC-Top in Table 1, are in striking contrast with those for the Ag-coated-GdBCO films experiencing significant aging effects due to the presence of the Ag layer (see Sect. 3.1). For instance, the respective ratios between the RS values of Ag(600)-GdBCO-CC-Top aged 85 weeks and the pristine one turned out to be 1.04 and 0.99 at 20 and 70 K, which are virtually the same with the measurement uncertainty of 2% in Q0 taken into consideration.
Fig. 6

The ratio between the RS of the pristine Ag-coated GdBCO CC and the Ag-coated GdBCO CC aged 85 weeks. Inset: The temperature dependences of the RS of the specimens above

Based on what we observed for the uc-GdBCO-F1 and Ag(600)-GdBCO-F1, it is clear that durability of our Ag-coated GdBCO CCs with regard to the RS is not attributable to the relatively large thickness of 600 nm of the Ag layer. It is because, for Ag(600)-GdBCO-F1, degradation in the RS still occurred in 7 weeks after removal of the Ag coating. We note that there was no sign of the aging effects in uc-GdBCO-F1 aged 8 weeks (see Fig. 4). For addressing proper reasons for durability of the Ag-coated GdBCO CCs, we compared compositional and structural differences between GdBCO CCs grown by using the RCE-DR method and epitaxially grown GdBCO films as follows.

During growth of the GdBCO layer, impurity phases such as Gd2O3 and Gd-free Ba–Cu–O are also formed with the Gd-free Ba–Cu–O phase appearing at the top of the GdBCO layer [17]. This results in the Gd-free Ba–Cu–O phase being placed between the Ag layer and the GdBCO layer in the Ag-coated GdBCO CCs. Therefore, if the Gd-free Ba–Cu–O phase could block oxygen out-diffusion from the underlying GdBCO layer, the RS of Ag-coated GdBCO CCs would remain intact over a long period of time despite existence of the Ag layer at the top. Furthermore, the Gd-free Ba–Cu–O layer might also prevent the Ag layer from acting as a catalyst for the oxygen out-diffusion. In Fig. 7, we show a TEM picture for a typical GdBCO CC grown by using the RCE-DR method, where a 50 nm-thick layer of the Gd-free Ba–Cu–O phase is seen at the top of the GdBCO layer.
Fig. 7

A TEM picture for a typical GdBCO CC grown by using the RCE-DR method. A Gd-free Ba–Cu–O layer with a thickness of ~ 50 nm is seen on top of the GdBCO layer

We also investigated the aging effects at the bottom part of the GdBCO layer being in direct contact with the buffered Hastelloy substrate. Although there is no Gd-free Ba–Cu–O layer at the bottom part of the GdBCO layer, oxygen out-diffusion from the bottom part is prevented by the buffered Hastelloy substrate. Thus, we expected that the RS of the bottom part of the GdBCO layer would remain intact over a long period of time.

In the inset of Fig. 8, we show the temperature-dependent RS of the bottom part prepared from pristine Ag-coated GdBCO CCs and that of the corresponding bottom part prepared from Ag-coated GdBCO CCs aged 119 weeks. In Fig. 8, the ratio between the latter and the former is seen as a function of temperature, where there is little difference between the two sets of the RS values over this period of time: The respective ratios are 1.07 and 0.97 at 20 and 70 K. However, when the bottom part of the GdBCO layer was coated with a 600 nm-thick Ag layer, the Ag coating appeared to be fully reacted with the underlying GdBCO layer after 184 weeks, making the Ag-coated bottom part completely lose its superconductivity.
Fig. 8

The ratio between the RS of the bottom part of the GdBCO layer in Ag-coated GdBCO CCs aged 119 weeks (RS(aged)) and that in Ag-coated GdBCO CCs aged 2 weeks (RS(pristine)). Inset: The temperature dependences of the RS of the GdBCO specimens above

In Fig. 9, we display microscope pictures for the Ag-coated GdBCO CCs aged 184 weeks, the pristine bottom part of the GdBCO layer prepared from pristine GdBCO CCs, and the 600 nm-thick Ag-coated bottom part of the GdBCO layer aged 184 weeks. We note in the figure that the Ag-coated bottom part aged 184 weeks appears to be in stark contrast with the clean surface of the pristine bottom part. We also see in Fig. 9 that the Ag coating on top of the Gd-free Ba–Cu–O/GdBCO layer remains intact. These results provide a direct evidence for the fact that existence of the Gd-free Ba–Cu–O layer provides the reason for the observed durability of the Ag-coated GdBCO CCs in terms of the RS.
Fig. 9

Microscope pictures for (Left) a Ag-coated GdBCO CC aged 325 weeks, (Center) the bottom part of a pristine GdBCO layer, and (Right) the bottom part of a GdBCO layer aged 184 weeks after an 600 nm-thick Ag layer was deposited on top of the bottom part. All the pictures are magnified by 20 times

We believe that our results for the RS of Ag-coated GdBCO CCs could be applied to the JC of the specimens due to the following reasons. At first, both the RS and the JC reflect the intrinsic transport properties of superconductor specimens. Secondly, the relation of RS(T) = C0/JC(T) has been reported by Ohshima et al., for YBCO films, with C0 = 2.0 × 107 Ω (A cm− 2) being temperature-independent and T denoting the temperature [18]. It is clear that the relation of between RS and JC cannot be directly applied to GdBCO films due to the existence of antiferromagnetic Gd3+ ions in GdBCO. However, the relation of RS ∝  1/JC might still hold if proper corrections are made with the existence of antiferromagnetic Gd3 + ions taken into consideration. In this context, we checked if our results are consistent with Paturi et al.’s with regard to the speed of the aging process. In Fig. 10, we show the ratio between the RS values of the pristine and the aged GdBCO specimens, where, for Ag(50)-GdBCO-F1, the RS appears to increase by 4 times over a time span of 4 weeks, Interestingly, this corresponds to what Paturi et al. obtained from the 1/JC of their GdBCO films having 50 nm-thick Ag coating over the same time span.
Fig. 10

Variations in the ratio between the RS values of uc-GdBCO-F1, Ag(50)-GdBCO-F1, and Ag(600)-GdBCO-F1 at 8.5 GHz and 10 K with the aging time. Paturi et al.’s results with regard to the aging effects on the JC at 10 K of Ag-coated GdBCO films similar to Ag(50)-GdBCO-F1 are compared [7]. The RS of Ag(600)-GdBCO-F1 are the respective values measured for Ag(600)-GdBCO-F1 aged 9 weeks and aged 17 weeks, during which the Ag(600)-GdBCO-F1 was aged without the Ag layer

Our results show that, unlike Ag-coated GdBCO films, aging effects could be insignificant for Ag-coated GdBCO CCs: The RS of our Ag-coated GdBCO CCs aged 85 weeks showed no sign of the aging effects. Such durability of the Ag-coated GdBCO CCs with regard to the RS could be explained by existence of impurity phases, mainly Gd-free Ba–Cu–O phase that covers the top part of the GdBCO layer in our Ag-coated GdBCO CCs. The mechanisms on how the Gd-free Ba–Cu–O phase prevent both oxygen diffusion from the GdBCO layer and the roles of Ag as a catalyst cannot be addressed at the moment. Here we note that presence of such Gd-free Ba–Cu–O phase depends on the growth techniques and our results on Ag-coated GdBCO CCs should be strictly applied to the ones prepared by using the RCE-DR method. Further studies are needed to address the aging effects for other Ag-coated GdBCO CCs prepared by different growth techniques.

4 Conclusion

We compared aging effects on the microwave surface resistance (RS) of Ag-coated GdBa2Cu3O7−δ (GdBCO) coated conductors with those for Ag-coated GdBCO films. The Ag-coated GdBCO CCs were prepared by using the RCE-DR method and the GdBCO films were prepared by using the laser ablation method. Unlike the RS of Ag-coated GdBCO films, no significant aging effects were found for our Ag-coated GdBCO CCs, with the RS of the Ag-coated GdBCO CCs showing no sign of the aging effects after a period of 85 weeks. Such durability of the Ag-coated GdBCO CCs in terms of the RS seems due to existence of Gd-free Ba–Cu–O phase on top of the GdBCO layer as confirmed from TEM study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on the transport properties such as RS and JC of Ag-coated GdBCO CCs prepared by other growth techniques could be realized by means of an artificially-grown protective impurity layer.

Notes

Acknowledgements

This work was supported by Konkuk University in 2014.

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Copyright information

© The Korean Institute of Metals and Materials 2018

Authors and Affiliations

  1. 1.Department of PhysicsKonkuk UniversitySeoulKorea
  2. 2.Department of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM)Seoul National UniversitySeoulKorea
  3. 3.SuNAM Co., Ltd.AnseongKorea

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