简介概要

Microstructure and properties of MOCVD-derived Gd_xY_1-xBa₂Cu₃O_7-δ films with composition fluctuations

来源期刊：Rare Metals2018年第8期

论文作者：Yu-Xi Zhang Fei Zhang Rui-Peng Zhao Yan Xue Hui Wang Qiu-Liang Wang Jie Xiong Bo-Wan Tao

文章页码：675 - 681

摘要：The effects of Gd content on crystalline orientation,microstructure and superconductivity of Gd_xY_1-x-Ba₂ Cu₃ O_7-δ（GdYBCO） films were systematically investigated.By varying the Gd content in the liquid precursor without changing the total amount of rare earth elements,series of GdYBCO films with x values of 0,0.1,0.3,0.5,0.7,0.9 and 1.0 were fabricated by metal organic chemical vapor deposition（MOCVD）.X-ray diffraction（XRD） and scanning electron microscope（SEM） analysis revealed that Gd introducing could restrain the formation of CuYO₂ phase,but induce a-axis growth of GdYBCO film.The increase of x from 0 to 0.5 leads to enhancing critical current density at self-field and 77 K(J_csf) from 1.8 to 2.8 MA cm^-2.which benefits from the decrease in CuYO₂ impurities and improvement of in-plane texture from 5.0° to 4.3°.However,raising × from 0.5 to 1.0 gives rise to abundant a-axis growth of film and degradation of in-plane texture from 4.3°to 5.4°,consequently resulting in the decrease of J_csf from 2.8 to 0.8 MA cm^-2.Even though J_csf has not varied monotonically,the critical transition temperature of GdYBCO films linearly increases from90.75 to 92.25 K and the in-field performance at magnetic field（B） of 0-1.1 T and 77 K as well as B parallel to film normal is also superior with Gd content increasing.

详情信息展示

稀有金属(英文版) 2018,37(08),675-681

Microstructure and properties of MOCVD-derived Gd_xY_1-xBa₂Cu₃O_7-δ films with composition fluctuations

Yu-Xi Zhang Fei Zhang Rui-Peng Zhao Yan Xue Hui Wang Qiu-Liang Wang Jie Xiong Bo-Wan Tao

State Key Laboratory of Electronic Thin Film and Integrated Devices,University of Electronic Science and Technology of China

Applied Research Laboratory of Superconduction and New Material,Institute of Electrical Engineering,Chinese Academy of Sciences

作者简介：*Jie Xiong e-mail:jiexiong@uestc.edu.cn;

收稿日期：15 November 2015

基金：financially supported by the National Science Foundation of China (No.91421110);the National High Technology Research and Development Program of China (No.2014AA032702);the National Basic Research Program of China (No.2015CB358600);the Sichuan Youth Science and Technology Innovation Research Team Funding (No.2011JTD0006);the Sichuan Provincial Fund for Distinguished Young Academic and Technology Leaders (No.2014JQ0011);

Microstructure and properties of MOCVD-derived Gd_xY_1-xBa₂Cu₃O_7-δ films with composition fluctuations

Yu-Xi Zhang Fei Zhang Rui-Peng Zhao Yan Xue Hui Wang Qiu-Liang Wang Jie Xiong Bo-Wan Tao

State Key Laboratory of Electronic Thin Film and Integrated Devices,University of Electronic Science and Technology of China

Applied Research Laboratory of Superconduction and New Material,Institute of Electrical Engineering,Chinese Academy of Sciences

Abstract：

The effects of Gd content on crystalline orientation,microstructure and superconductivity of Gd_xY_1-x-Ba₂ Cu₃ O_7-δ(GdYBCO) films were systematically investigated.By varying the Gd content in the liquid precursor without changing the total amount of rare earth elements,series of GdYBCO films with x values of 0,0.1,0.3,0.5,0.7,0.9 and 1.0 were fabricated by metal organic chemical vapor deposition(MOCVD).X-ray diffraction(XRD) and scanning electron microscope(SEM) analysis revealed that Gd introducing could restrain the formation of CuYO₂ phase,but induce a-axis growth of GdYBCO film.The increase of x from 0 to 0.5 leads to enhancing critical current density at self-field and 77 K(J_csf) from 1.8 to 2.8 MA cm^-2.which benefits from the decrease in CuYO₂ impurities and improvement of in-plane texture from 5.0° to 4.3°.However,raising × from 0.5 to 1.0 gives rise to abundant a-axis growth of film and degradation of in-plane texture from 4.3°to 5.4°,consequently resulting in the decrease of J_csf from 2.8 to 0.8 MA cm^-2.Even though J_csf has not varied monotonically,the critical transition temperature of GdYBCO films linearly increases from90.75 to 92.25 K and the in-field performance at magnetic field(B) of 0-1.1 T and 77 K as well as B parallel to film normal is also superior with Gd content increasing.

Keyword：

Gd content; GdYBCO; MOCVD; Microstructure; Flux pinning;

Received： 15 November 2015

1 Introduction

Critical current density (J_c) of YBa₂Cu₃O_7-δ(YBCO)superconducting film decays severely with applied magnetic field due to the motion of vortices [ 1] .How to immobilize the vortices to improve the in-field performance of YBCO film has captured the researchers'attention.Although naturally grown defects [ 1] ,such as boundaries,in-plane or out-of-plane misorientation,dislocations and voids,can somewhat pin flux vortices,yet reports [ 2, 3, 4, 5, 6] have revealed that the density or effectiveness of these naturally grown defects was not high enough to retain high-field J_c at a necessary level.

Substantial efforts on introducing artificial pinning centers (APCs) have been made to improve the flux pinning effects.For instance,nano-sized Y₂O₃ particles resulted from the excess of yttrium in YBCO films could form point,line and/or plane defects,thus enhanced the flux pinning effects [ 7, 8, 9, 10] .Alternatively,perovskites such as BaZrO₃ [ 10, 11, 12, 13, 14, 15] ,BaSnO₃ [ 16] ,BaHfO₃ [ 17] and BalrO₃ [ 18] could also form randomly oriented particles or c-axis oriented columnar defects in YBCO matrix to improve the flux pinning effects.Additionally,inclusion of nanoparticles of Gd₃TaO₇ [ 19] ,Ba₂GdTaO₆ [ 20] ,Ba₂YNbO₆ [ 21] ,YBa₂CuO₅ [ 22] or Y₂BaCuO₅ [ 23] has also been demonstrated to be an effective way to raise the in-field performance of YBCO films.In addition to introduction of the above-mentioned extra particulates,the partial or complete substitution of lanthanides for yttrium to form REBCO(RE=rare earth) has also manifested the enhanced flux pinning effects.As an example,GdBa₂Cu₃O_7-δ(GdBCO)was reported being of superior in-field performance to YBCO [ 24] .However,systematic research about the effects of Gd content on the properties of Gd_xY_1-xBa_2-Cu₃O_7-δ(GdYBCO) superconducting films has been barely reported [ 25] .

Hence,the authors have applied metal organic chemical vapor deposition (MOCVD) to fabricate GdYBCO films with various Gd contents on the metallic substrates and systematically investigated the effects of Gd content on the crystalline orientation,in-plane and out-of-plane textures,surface morphology,self-field and in-field superconductivity at 77 K.

2 Experimental

The MOCVD system applying single mixed liquid precursor of metal organics was used to deposit GdYBCO films.And the home-made buffer stack of LaMnO₃/homoepi MgO/ion beam-as sis ted deposition (IBAD)-MgO/Y₂O₃/Al₂O₃/Hastelloy tape [ 26, 27] ,collectively termed as IB AD template,was used as substrate,which was preheated to 810℃during the deposition of GdYBCO film.The precursor was prepared by dissolving solid-state metal organics of Y(tmhd)₃,Gd(tmhd)₃,Ba(tmhd)₂ and Cu(tmhd)2 into tetrahydrofuran.To fabricate GdYBCO films with various Gd contents,the mole ratio of Gd to the constant total amount of Gd and Y in precursor was set as0%,10%,30%,50%,70%,90%and 100%,corresponding to x values of 0,0.1,0.3,0.5,0.7,0.9 and 1.0 in GdYBCO film,respectively.More experimental details could be found elsewhere [ 28] .

The crystalline orientation and texture of as-deposited GdYBCO films were examined by X-ray diffractometer(XRD,Bede D1).The microstructure was observed by scanning electron microscope (SEM,JEOL JSM-7001F).The critical transition temperature (T_c) was measured by four-probe method using the current of 1 mA,and the GdYBCO samples were cooled by cold head of Sumitomo(RDK-101D).The J_c at 77 K and 0 T (J_csf) was obtained by induction method using LeipzigJ_c-scan system.The critical current in the magnetic field (B) from 0 to 1.1 T was measured on the sample bridge of 1 mm×5 mm(width×length) with magnetic field direction parallel to the film normal,and the criteria to determine the critical current was 1μV·cm^-1.Additionally,the film thickness was tested by step profiler (Veeco,Dektak 150).

3 Results and discussion

500-nm-thick GdYBCO films with x values of 0,0.1,0.3,0.5,0.7,0.9 and 1.0 were fabricated on IBAD-MgO templates.Figure 1 shows the detailedθ-2θpatterns,which exhibit clear and sharp peaks of (00l) GdYBCO and a few unexpected peaks of CuYO₂ phases and (h00)GdYBCO.As x<0.5,only (00l) GdYBCO peaks can be clearly observed,indicating that the corresponding films crystallize well and align their c-axis perpendicular to the substrate surface.However,the films mix with a few CuYO₂ particles,demonstrated by the presence of CuYO₂peaks in Fig.1.As x≥0.5,the peaks of CuYO₂ disappear,revealing that the increase in Gd content prevents the formation of CuYO₂ precipitates and the composition ratio of the film body is closer to the stoichiometric value.However,the presence of (h00) GdYBCO peaks suggests that the film transfers from purely c-axis orientation to mixed orientation of c-axis and a-axis.It should also be noted that the intensity of (002) peak of the film becomes weaken with Gd content increasing.To give quantitative understanding,I₍₀₀₂₎ and I₍₀₀₃₎ are used to represent the intensities of (002) peak and (003) peak of the film,respectively,and theI₍₀₀₃₎/I₍₀₀₂₎ values are summarized in Fig.2.It is obvious that I₍₀₀₃₎/I₍₀₀₂₎ value is enlarged as Gd content increases.As reported in Ref. [ 29] ,it is the intrinsic character of GdBCO that the (002) peak is much weaker than (003) peak.Hence,it is easy to understand that such increase in I₍₀₀₃₎/I₍₀₀₂₎ ratio is ascribed to the change of film body from YBCO to GdBCO with Gd content increasing from 0 to 1.Conversely,the curve in Fig.2 can also be used to semi-quantitatively determine the Gd content of GdYBCO film when the Gd content is uncertain.

Figure 3 showsχ-scans [ 30] on (102) planes of all GdYBCO films.As x<0.5,there is only one peak around57°corresponding to (102) plane of c-axis oriented GdYBCO grains,whereas there arises one more peak around 33°corresponding to (102) planes of a-axis oriented GdYBCO grains when x further increases to 1.0.The above-obtained results are in agreement with those yielded from 2θscans,further demonstrating that the orientation of GdYBCO films transfer from c-axis orientation to a-axis orientation.Such conversion of orientation suggests that the MOCVD conditions optimized for YBCO film deposition are not appropriate any more,since the film body has transferred from YBCO to GdBCO as x>0.5.For example,with experiments it has been found that the deposition temperature of GdBCO film was about 15℃higher than that of YBCO film.

Fig.1 XRDθ-2θpatterns of GdYBCO films fabricated at various Gd contents (x value).Symbols asterisk and number sign indicating diffraction peaks of MgO and Hastelloy tape,respectively

Fig.2 Relationship between ratio value ofI₍₀₀₃₎/I₍₀₀₂₎ and Gd content.I₍₀₀₃₎ andI₍₀₀₂₎ representing peak intensities of(003)GdYBCO and (002)GdYBCO,respectively

Fig.3 XRDχ-scans of (102)GdYBCO of as-fabricated films

To determine the out-of-plane and in-plane textures of the films,the co-scans andΦ-scans were performed,respectively,on (005) GdYBCO and (103) GdYBCO,and the corresponding full width at half maximum (FWHM)values (Δω,ΔΦ) are summarized in Fig.4.As Gd content rises up to 0.5,Δωhardly changes,whileΔΦdecreases from 5.0°to 4.3°,indicating that Gd substitution for Y deeply improves the in-plane texture rather than out-ofplane texture.Such improvement on in-plane texture would be owed to the disappearance of CuYO₂ impurities.However,ΔΦis enlarged as Gd content further increases,suggesting that the in-plane texture deteriorates.And this deterioration is attributed to the appearance of a-axis oriented growth,which results from the above-mentioned improper deposition conditions of GdYBCO films.

Fig.4 Relationship of FWHMs ofω-scans on (005) planes andΦ-scans on (103) planes of GdYBCO films with various Gd contents

SEM images of GdYBCO films are shown in Fig.5,exhibiting dense and crack-free microstructure as well as some surface particles.As x=0,there is a large number of particles identified as Y-Cu-O phases by energy dispersive spectroscopy (EDS) analysis (not shown here),which is consistent with the XRD examinations in Fig.1.As x is enlarged,the number of Y-Cu-O particle decreases,which is in agreement with the weakening of diffraction peak of CuYO₂ phase in Fig.1.However,the a-axis oriented GdYBCO grains (rectangular shape particles in Fig.5) also arise with Gd content increasing,especially for x>0.5.When x>0.5,the size and density of a-axis grains increase with Gd content increasing,consistent with the higher (h00) GdYBCO peaks at larger x in Fig.1.Such microstructure with large number of a-axis grains for x>0.5 further demonstrates that the deposition conditions are not appropriate anymore and the substrate temperature is somewhat low,especially for those of higher Gd content.

The superconductivity of GdYBCO film is characterized by the measurements of critical transition temperature (T_c)and J_csf,of which the results are depicted in Fig.6.In Fig.6a,the T_c almost linearly increases from 90.75 to92.25 K,whereas the transition width (ΔT) firstly increases and then decreases with the increase in Gd content.AndΔT is 0.65 and 0.93 K for YBCO and GdBCO,respectively,and reaches its maximum of～2 K at x=0.5.Owing to the higher T_c of GdBCO than YBCO,such variation ofΔT results from the co-existence of YBCO phase and GdBCO phase at 0<x<1.0.In Fig.6b,J_csfincreases from 1.8 to 2.8 MA·cm^-2 as 0≤x≤0.5,which benefits from the rising of T_c and the above-mentioned improvement of in-plane texture yielded from the decrease in CuYO₂ impurities.When Gd content is above 0.5,the film body transfers to GdBCO and the improper deposition conditions leads to the degradation of in-plane and out-ofplane textures as well as the presence of misoriented grains including (103) GdYBCO and (h00) GdYBCO.The texture degradation and grain misorientation are harmful to the transmission of superconducting current,consequently resulting in the drop of J_csf from 2.8 to 0.8 MA·cm^-2.

Fig.5 SEM images of GdYBCO films with various Gd contents:a x=0,b x=0.1,c x=0.3,dx=0.5,e x=0.7,fx=0.9 and g x=1.0

Fig.6 Dependence of superconductivity of as-deposited GdYBCO films on Gd content:a critical transition temperature (T_c) and transition width (ΔT) and b J_c at 77 K and 0 T

The dependence of critical current (I_c) on B was measured on the samples with Gd content of 0,0.3,0.5,0.7 and1.0 at 77 K and plotted in Fig.7a,which shows that I_c of all samples declines with B.To intuitively exhibit the effects of Gd content,the curves in Fig.7a are normalized by I_c(B=0) and replotted in Fig.7b.It is clearly illustrated that the increase in Gd content slows down the declining rate of normalizedI_c/I_c(B=0) with B increasing,which benefits from the flux pinning effects induced by substituting Gd for Y [ 24, 25] .At B=1 T and 77 K,the normalized I_c/I_c(B=0) corresponding to Gd content of 1.0retains about 27%,twice more than that of Gd content of 0,demonstrating that GdBCO film has in-field performance quite superior to that of YBCO film at 77 K.As reported,I_c(B) is proportional to B^-αin the field range of 0.1-1.0 T(αvalue is calculated from the I_c-B curve in Fig.7) and the smallerαvalue means the better in-field performance [ 1] .By fitting the data points (shown in inset of Fig.7b),theαvalues corresponding to Gd content of 0,0.3,0.5,0.7 and1.0 are 0.400,0.397,0.394,0.375 and 0.332,respectively,among which the smallest value of 0.332 is comparable to those reportedαvalues [ 12, 13, 15, 31] .Figure 8 shows the magnetic field dependence of flux pinning force (F_p),where F_p=J_c(B)×B [ 31] .AndJ_c(B) is deduced from the data in Fig.7a through the following equation:

where w and t represent the width and thickness of the micro-bridge of GdYBCO films used in I_c measurements,being 1 and 500 nm,respectively.

In view of the various J_csf shown in Fig.6b,the comparative analysis of Gd content of 0 and 1.0 as well as 0.3and 0.7 is more illustrative.With regard to Gd content of 0and 1.0,F_p of the former is higher than that of the latter as B<0.2 T,which is attributed to the much higherJ_csf of1.8 MA·cm^-2 of the former than 0.8 MA·cm^-2 of the latter.As 0.2 T<B<1.0 T,F_p of Gd content of 1.0 is almost same with that of Gd content of 0,indicating that the flux pinning effect of GdBCO is stronger than that of YBCO at this moment.As for Gd content of 0.3 and 0.7,they have the sameJ_csf and their F_p is same too as B<10 mT,whereas F_p of the latter is increasingly higher than that of the former as B increases to 1.0 T,suggesting that the latter is of superior flux pinning effect.All the above discussion suggests that substitution of Gd for Y is beneficial to enhance the flux pinning effect of the film.

Fig.8 Magnetic field (B) dependence of flux pinning force (F_p),where F_p=J_c×B

4 Conclusion

Gd content deeply affects the in-plane texture,morphology and superconducting performance of GdYBCO films.The increase in Gd content from 0 to 0.5 could suppress the formation of CuYO₂,thus improves the in-plane texture and micro structure of the films and finally enlargesJ_csf from 1.8to 2.8 MA·cm^-2.As Gd content further increases from 0.5 to1.0,the growth of a-axis GdYBCO grains arises abundantly and becomes much severer at Gd content of 1.0,which consequently deteriorates the texture and thus dropsJ_csf from2.8 to 0.8 MA·cm^-2.However,the increasing Gd content brings the linear increase of T_c from 90.75 to 92.25 K and the slower decay of normalized I_c/I_c(B=0) as well as the stronger flux pinning effect.Even though the flux pinning force of GdBCO is smallest due to the lowest J_c,the authors believe that further modification on MOCVD process would raise J_c and then enhance the flux pinning force.

Fig.7 Dependence of a critical current (I_c) and b normalized I_c/I_c(B=0) on magnetic field (B) with B perpendicular to normal of sample surface

Acknowledgements This study was financially supported by the National Science Foundation of China (No.91421110),the National High Technology Research and Development Program of China (No.2014AA032702),the National Basic Research Program of China (No.2015CB358600),the Sichuan Youth Science and Technology Innovation Research Team Funding (No.2011JTD0006) and the Sichuan Provincial Fund for Distinguished Young Academic and Technology Leaders (No.2014JQ0011).