The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures
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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.
KeywordsAging effect GdBa2Cu3O7−δ Surface resistance Coated conductor Film
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 .
When commercial GdBCO CCs are produced by various factories, the GdBCO layer is usually coated with a Ag layer : 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 . 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 . 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.
2.1 Sample Preparation
List of GdBCO specimens used in this study
Sample type (Growth method)
Measurement time for the RS
Epitaxially-grown film (Pulsed laser ablation)
Uncoated GdBCO film
Week 0, week 8
Week 0, week 185
GdBCO film coated with 50 nm-thick Ag layer
Week 12a, week 16a
GdBCO film coated with 600 nm-thick Ag layer
Week 9a, week 17a
Coated Conductor (RCE-DR)
Top part of the GdBCO layer in GdBCO conductors coated with a 600 nm-thick Ag layer
Week 0, week 85,
Bottom part of the GdBCO layer in GdBCO conductors coated with a 600 nm-thick Ag layer
Week 2, week 119
2.2 Measurement Procedure
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.
3 Results and Discussion
3.1 Aging Effects on the R S of Ag-Coated GdBCO Films
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. , 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.
3.2 Aging Effects on the R S of Ag-Coated GdBCO Coated Conductors
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.
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.
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.
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.
This work was supported by Konkuk University in 2014.
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