涂层导体缓冲层传递织构性质

J Supercond Nov Magn(2012)25:811–816

DOI10.1007/s10948-011-1350-y

O R I G I NA L PA P E R

Texture Transfer Mechanism of Buffer Layer in Coated Conductors

Y.Wang·L.Zhou·C.S.Li·Z.M.Yu·J.S.Li·

L.H.Jin·P.F.Wang·Y.F.Lu

Received:9August2011/Accepted:21October2011/Published online:12November2011

?Springer Science+Business Media,LLC2011

Abstract We have studied the texture transfer process of buffer layers prepared by chemical solution deposition (CSD)methods on YSZ(00l)single crystal substrates and biaxial textured Ni–W substrates.The structure,texture,and surface morphology of the buffer layers were investigated by X-ray diffraction(XRD),atomic force microscopy(AFM), and scanning electron microscopy(SEM).Our results show that the degree of texture and the surface morphology of the buffer layers vary with the changes of the substrate and the lattice mismatch of the top buffer layers with La2Zr2O7 (LZO)after crystallization in argon–hydrogen atmosphere. Moreover,the growth mode of multi-layer?lms and the type of the lattice strain have strong in?uence on the formation and the transfer of the bi-axial texture in multi-layer buffer architecture.It suggests that there exists a possible connec-tion between the strain relaxation and the texture transfer in buffer layer fabricated by CSD https://www.360docs.net/doc/7d13716305.html,rmation on the texture transfer of buffer layer is important for optimizing the buffer layer architecture in coated conductors. Keywords Coated conductors·Buffer layer·Chemical solution deposition method

Y.Wang·L.Zhou·C.S.Li·Z.M.Yu·L.H.Jin·P.F.Wang·

Y.F.Lu( )

Northwest Institute for Nonferrous Metal Research,Xi’an, 710016,P.R.China

e-mail:wyspacestar@https://www.360docs.net/doc/7d13716305.html,

Y.Wang·L.Zhou·J.S.Li

State Key Laboratory of Solidi?cation Processing,

Northwestern Polytechnical University,Xi’an,710072,P.R.China P.F.Wang

Shaanxi Normal University,Xi’an,710062,P.R.China 1Introduction

Coated conductors composed of metal substrate/buffer layer/superconducting layer/protective layer are very attrac-tive for electrical power application and superconducting magnet technology[1–6].Buffer layer,which prevents oxy-gen and metal atoms diffusion and also transfers the biax-ial texture from metal substrate to superconducting layer,is one of the key functional layers in coated conductors[7,8]. Thus the multi-layer architecture is generally adopted to realize the above-mentioned two main functions of buffer layer.However,the thicher buffer layers can lead to a more complicated fabrication procedure and an increase in prepa-ration cost.Therefore,the simpli?cation of the buffer layer architecture is always the desired goal of buffer layer de-velopment.The knowledge of the texture transfer mecha-nism of buffer layer is very useful for the optimization of the buffer layer architecture in coated conductors.However, few studies about texture transfer mechanism of buffer layer, especially fabricated by chemical solution deposition(CSD) methods,have been reported.The study of the texture for-mation and the texture transfer of buffer layer are important for understanding the function of buffer layer and simplify-ing buffer layer architecture in coated conductors.

CSD methods offer the signi?cant potential cost and pro-cessing advantages over physical vapor deposition(PVD) processes[2,9].In addition,the preparation procedure of La2Zr2O7(LZO)thin?lm by CSD is readily controllable and rather mature.Although the LZO with pyrochlore struc-ture presents a relatively high lattice mismatch with YSZ (00l)single crystal substrates and Ni–W substrates,good epitaxial growth of LZO?lm is still possible[10–14].More-over,for the heteroepitaxy system,the lattice mismatch may lead to a series of changes,including the growth mode,mi-crostructure and so on.Therefore,LZO?lms on YSZ(00l)

single crystal substrates and Ni–W substrates were selected as the model to further deposit CeO2,Nd2Zr2O7(NZO),and Nd2Mo2O7(NMO)?lms with different lattice mismatch to LZO by CSD methods,respectively.Then the texture trans-fer mechanism of buffer layer is explored by investigating the texture and surface morphology evolution of buffer lay-ers on YSZ(00l)single crystals and Ni–W tapes.

2Experimental Procedures

All of the MOD precursor solutions were prepared in am-bient atmosphere.The stable yellow-colored LZO starting solution was produced by mixing a stoichiometric amount of reagents lanthanum(III)2,4-pentanedionate(La(acac)3) and zirconium(IV)2,4-pentanedionate(Zr(acac)4)(re-ceived from Strem)with propionic acid along with con-tinuous stirring at75?C for25min.Similarly,cerium (III)2,4-pentanedionate(Ce(acac)3)(received from Strem) was dissolved in propionic acid by heating with continu-ously stirring to synthesize a stable brown-colored CeO2 precursor solution.However,the NZO and NMO precur-sor solutions were prepared from neodymium(III)acetyl-acetonate(Nd(acac)3),Zr(acac)4and molybdenum(IV) dioxo acetylacetonate(MoO2(acac)2)in methanol and pro-pionic acid.Details of these solution preparation processes can be found elsewhere[15–20].Spin coating was used to deposit LZO on YSZ(00l)single crystal substrates and textured Ni–W substrates at a spin rate of2500rpm for30s.The spin coated?lms were then heat-treated at 900?C for40min in a?owing mixture of Ar-4%H2gas. Then the CeO2precursor solution was spin-coated onto the above-obtained crystallized LZO?lms,followed by heat treatment at1000?C for15min in Ar-4%H2to fab-ricate CeO2/LZO/YSZ and CeO2/LZO/NiW.On the other hand,the NZO and NMO precursor solutions were re-spectively spin coated onto the as-grown LZO/YSZ and LZO/Ni–W templates.Subsequently,these coatings were heated at550?C for1h and then annealed at1000?C for2h in a reducing forming gas atmosphere of Ar-4%H2.At the end of the heat-treatment cycles,the samples were quenched to room temperature in the same atmosphere to fabricate NZO/LZO/YSZ,NMO/LZO/YSZ,NZO/LZO/Ni–W,and NMO/LZO/Ni–W.

All the buffer?lms were characterized by X-ray diffrac-tion(XRD),which was performed to carry outθ–2θscan using CuKαradiation at40kV and50mA,for phase purity and texture.In addition,the out-of-plane and in-plane scans were also con?rmed by XRD for the degree of texture of buffer layer.The?lm thicknesses of all buffer layers were calibrated and determined byα-step apparatus.The surface homogeneity and microstructure analysis of these samples were performed by JSM-6700?eld emission scanning elec-tron microscopy(SEM)and a digital instruments nanoscope SPI3800-SPA-400atomic force microscopy(AFM)in con-tact mode.

3Results and Discussion

The typicalθ–2θscans for LZO/YSZ,CeO2/LZO/YSZ, NZO/LZO/YSZ,and NMO/LZO/YSZ samples are shown in Fig.1.All of the LZO,CeO2,NZO,and NMO?lms prepared in Ar-4%H2have been completely crystallized to form the c-axis orientation because only highly(00l) diffraction peaks of buffer layers and YSZ(00l)single crys-tal substrates are observed,and neither(111)nor(222)peak is present in the patterns.

In addition,Fig.1also shows the development of the FWHM values of the out-of-plane and in-plane texture of the buffer layers as a function of the lattice mismatch of the top buffer layers with LZO.The variation of the FWHM values of the top buffer layers represents mainly a texture transfer effect of the bottom buffer layer.Therefore,the relative FWHM from the omega and phi scans(derived from the speci?c value FWHM buffer(00l)/FWHM LZO(004), and FWHM buffer(111)/FWHM LZO(222)or FWHMLZO(111)/ FWHM LZO(222))are plotted in Fig.1,which reveals the tex-ture transfer of the buffer layer with the lattice mismatch of the top buffer layer to LZO.The relative FWHM value from omega scans in0.3%of lattice mismatch between

the Fig.1XRD patterns for the as-grown LZO/YSZ,CeO2/LZO/YSZ, NZO/LZO/YSZ,and NMO/LZO/YSZ samples in Ar-4%H2.The de-velopment of the relative FWHM from omega scans and phi scans of buffer layer as a function of the lattice mismatch of the top buffer layer with LZO

top buffer layer and LZO reaches a maximum value and then decreases as the lattice mismatch increases;?nally,it slightly increases when the lattice mismatch between the top buffer layer and LZO increases from 1.3%to 3.3%.How-ever,the relative FWHM value from phi scans drops moder-ately as the lattice mismatch increases and reaches a min-imum value in 1.3%of the lattice mismatch between the top buffer layer and LZO,and then increases markedly as the lattice mismatch increases to 3.3%.Compared with the LZO ?lm on YSZ substrate,the degraded out-of-plane tex-ture of CeO 2layer on the LZO/YSZ template may be related to its especial surface morphology presented below.How-ever,the larger lattice mismatch between NZO and LZO may increase the in?uence of surface energy on the ori-entation of buffer layer,which leads to a smaller FWHM value determined by rocking curve measurement [21].For NMO/LZO/YSZ sample,the worse out-of-plane and in-plane texture could be explained by the strain relaxation in NMO ?lm with increasing the thickness of the buffer layer [17].It should be pointed out that the thickness of the top and bottom buffer layers in these different architectures is almost the same value of around 50nm.

Figure 2shows the XRD patterns obtained from the LZO/Ni–W,CeO 2/LZO/Ni–W,NZO/LZO/Ni–W,and NMO/LZO/Ni–W multilayer architectures.Besides the ma-jor (002)diffraction peaks of the Ni–W substrates,only the (00l)re?ections of the buffer layers appear,which indicates the good c-axis texture of the buffer

?lms.

Fig.2Typical θ–2θscans of the as-prepared LZO/Ni–W,CeO 2/LZO/Ni–W,NZO/LZO/Ni–W,and NMO/LZO/Ni–W samples in Ar-4%H 2.Dependence of the relative FWHM from omega scans and phi scans of buffer layer on the lattice mismatch of the top buffer layer with LZO

Figure 2also presents the relationship between the rela-tive FWHM value of the omega scans as well as phi scans and the lattice mismatch of the top buffer layer with LZO.The relative FWHM value from omega scans slightly de-creases when the lattice mismatch between the top buffer layer and LZO increases to 0.31%.Furthermore,the rela-tive FWHM suddenly increases as the lattice mismatch in-creases.It should be pointed out that the variation trend in the relative FWHM value from phi scans is very similar to that of omega scans as the lattice mismatch between the top buffer layer and LZO increases but only the ?uctuation range of relative FWHM value is wider in omega scans than that in phi scans.We found that the FWHM values of out-of-plane and in-plane alignments of CeO 2?lm has improved compared with that of the bottom LZO buffer layer.This tex-ture improvement was generally observed in many samples,which was already reported by other researchers [22–24].This result is related to the lattice mismatch of the top buffer layer with LZO and the growth mode of the top buffer layer on LZO/Ni–W templates.

For LZO/YSZ,the round-shape grains have an average grain size of about 40nm and the root means square rough-ness value (R rms )of 0.63nm is calculated over an area of 5μm ×5μm,as seen in Fig.3(a).It indicates that this very smooth LZO surface is especially bene?cial for the subse-quent growth of the top buffer layers.Figure 3(b)–(d)shows the surface morphology of CeO 2,NZO,and NMO ?lms on LZO/YSZ template,and their R rms values are 2.10,0.51,and 1.42nm with a scan area of 5μm ×5μm,respectively.The CeO 2/LZO/YSZ grown in Ar-4%H 2indicates a volcanic-vent shaped surface structure,which results in a larger R rms value and is related to the large FWHM values from its omega and phi scans.In addition,NZO ?lm with a ?at crystalline plain has been obtained on LZO/YSZ template.In contrast to the surface morphology of NZO/LZO/YSZ,NMO layer deposited on LZO/YSZ presents a relatively rough topography along with large crystalline growing out of the coating volume.Thus,it can be considered that the surface and interface roughness have little effect on the epi-taxial growth of the buffer layers on LZO/YSZ templates due to the very smooth surface of the bottom buffer layer.This suggests that the surface morphology evolution is re-lated to the change of the epitaxial growth and texture trans-formation behavior of buffer layers.

SEM micrographs for LZO ?lm on Ni–W substrate,CeO 2,NZO,and NMO ?lms on LZO/Ni–W templates are shown in Fig.4.The grain boundary grooves of Ni–W sub-strates were found to be completely covered.Figure 4shows that all of the ?lm surfaces are homogenous as well as crack free.For CeO 2/LZO/Ni–W and NZO/LZO/Ni–W,there is no signi?cant difference with respect to the surface mor-phology compared with that of the LZO/Ni–W sample ex-cept that the average grain size increases to a certain ex-

Fig.3AFM images for as-grown(a)LZO/YSZ,(b)CeO2/LZO/YSZ,(c)NZO/LZO/YSZ,and(d)NMO/LZO/YSZ samples in Ar-4%H2,re-spectively

tent as the layer amount of the buffer layer increases.In ad-dition,we have noticed that the average grain size for the CeO2?lm is larger than that for NZO?lm on LZO/Ni–W. It suggests that the growth of NZO grains under the given conditions is slower than that of CeO2,which leads to a higher grain boundary concentration in NZO?lm than that in CeO2?lm.However,the larger grain size may result in the lager residual stress in?lms.It indicates that the part of strain relaxation takes place in NZO/LZO/Ni–W sam-ple because of the larger lattice mismatch of NZO with LZO than that of CeO2.This result is assigned to the larger FWHM values from omega and phi scans of NZO?lm than that of CeO2?lm on LZO/Ni–W.It is expected that a larger strain relaxation in the NZO?lm is not in fa-vor of the texture transfer in NZO/LZO/Ni–W architecture. However,some large grains bulge on the surface of the NMO/LZO/Ni–W,which is very similar to the surface mor-phology of NMO/LZO/YSZ,can be related to its especial epitaxial growth mode on LZO?lm with a large lattice mis-match of3.3%.The strain energy in strained?lm increases as the lattice mismatch of epitaxial?lm with substrate in-creases.The result reveals that the growth of CSD-NMO ?lms on LZO/Ni–W and LZO/YSZ templates does not take a two-dimensional layer-by-layer mode.It may result in the decrease of texture degree of NMO layers,which was concluded from the largest FWHM values in out-of-plane scans and in-plane scans of NMO?lms on LZO/Ni–W and LZO/YSZ.Therefore,we believe that their surface morphol-ogy evolution is associated with the texture transfer of the buffer layers.

For the heteroepitaxy system,the lattice distortion gener-ates the tensile stress or compressive stress in the?lm by the short range attractive interactions.The lattices distor-tion may be enhanced as the lattice mismatch of buffer layer

Fig.4SEM micrographs of as-prepared(a)LZO/Ni–W,

(b)CeO2/LZO/Ni–W,

(c)NZO/LZO/Ni–W,and

(d)NMO/LZO/Ni–W samples in Ar-4%H2,

respectively

with LZO increases,which may be in favor of the coher-ent epitaxial growth of buffer layer during the high tempera-ture heat-treatment process.However,the strain relaxation can occur when the stress in the?lms overstep a limita-tion value,which may disfavor the epitaxial growth of buffer layer and the texture transfer from the substrate to the buffer layer.The larger lattice mismatch of buffer layer with sub-strate leads to the smaller the critical thickness of epitaxial ?lm,at which the lattice relaxation takes place.After the buffer layer deposited on LZO?lm,the lattice distortion of LZO layer may be relaxed because of an opposite effect in the mis?t strain between LZO and the underlying substrate, which results in extending the strain to the top buffer layer during the subsequent epitaxial growth of buffer layer[25]. In addition,surface energy of precursor?lm,which may be related to the growth mode of buffer layer,could in?u-ence the orientation growth of buffer layer on template layer. Therefore,the texture transfer in multi-layer architecture of buffer layer is not only in?uenced by the lattice mismatch between the buffer layer and LZO,but also affected by the growth mode of buffer layer,especially for the buffer layer on LZO/Ni–W template.In other words,compared with the epitaxial growth of buffer layer on LZO/YSZ,the mismatch between the buffer layer and LZO is a minor contribution to the texture transfer from Ni–W substrate to the top buffer layer,which is mainly determined by growth mode of the top buffer layer.4Conclusions

We have obtained LZO,CeO2,NZO,and NMO?lms with strong(00l)orientation for further fabrication of coated con-ductors by the CSD route and studied the texture transfer mechanism of buffer layer in the CSD process.The XRD patterns,AFM images,and SEM micrographs have shown that the texture and surface morphology are dependent on the lattice mismatch of the top buffer layers with LZO and their growth mode.The lattice mismatch of the top buffer layer with LZO in?uences the strain relaxation process, which could be related to the texture formation and texture transfer of buffer layer in coated conductors. Acknowledgements This work was?nancially supported by the Na-tional Science Fund Program and the National High Technology Re-search and Development Program China(Grant No.50872115and 2009AA03Z203).

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