A water-based sol gel technique for chemical solution deposition of (RE)Ba2Cu3O7-y (RE = Nd and Y) superconducting thin films

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A water-based sol gel technique for chemical solution deposition of (RE)Ba2Cu3O7-y (RE = Nd and Y) superconducting thin films
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  I NSTITUTE OF  P HYSICS  P UBLISHING  S UPERCONDUCTOR  S CIENCE AND  T ECHNOLOGY Supercond. Sci. Technol.  19  (2006) 1178–1184 doi:10.1088/0953-2048/19/11/015 A water-based sol–gel technique forchemical solution deposition of  ( RE ) Ba 2 Cu 3 O 7 − y  (RE = Nd and Y)superconducting thin films B Schoofs 1 , V Cloet 1 , P Vermeir 1 , 2 , J Schaubroeck 2 , S Hoste 1 andI Van Driessche 1 1 Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3,9000 Ghent, Belgium 2 Department of Industrial Sciences, Hogeschool Gent, Association Ghent University,Jozef Kluyskensstraat 2, 9000 Ghent, BelgiumE-mail: Bart.Schoofs@UGent.be Received 19 August 2006, in final form 18 September 2006Published 16 October 2006Online at stacks.iop.org/SUST/19/1178 Abstract The achievement of low-cost deposition techniques for high critical current ( RE ) Ba 2 Cu 3 O 7 −  y -coated conductors is one of the major objectives inachieving a widespread use of superconductivity in power applications.Chemical solution deposition techniques are appearing as a very promisingmethodology to achieve highly textured oxide thin films at a low cost, so anintense effort is being carried out to develop routes for all chemically coatedconductor tapes. In this work recent achievements will be presented towardsthe goal of the development of an environmentally friendly, completelywater-based sol–gel technique for the deposition of thin superconductingfilms on SrTiO 3  single-crystal substrates using the dip coat technique. Acomparison is made between aqueous sol–gel synthesis of two ( RE ) Ba 2 Cu 3 O 7 −  y  superconducting systems: YBa 2 Cu 3 O 7 −  y  and itshomologue NdBa 2 Cu 3 O 7 −  y . Our conclusions are that YBa 2 Cu 3 O 7 −  y  stillremains the material of choice for coated conductor development using thissol–gel technique, with a  T  c , onset  of 91 K and  J  c  of 0 . 3 MA cm − 2 .(Some figures in this article are in colour only in the electronic version) 1. Introduction For the development of coated conductors with high  J  c performance,  ( RE ) Ba 2 Cu 3 O 7 −  y  ((RE)BCO) superconductorsstill remain the materials of choice due to their ability tooperate in high magnetic fields. In comparison to the well-known YBa 2 Cu 3 O 7 −  y  (YBCO) system [1], NdBa 2 Cu 3 O 7 −  y (NBCO) is a particularly promising material because of the superior conductive properties. It has the highestsuperconducting transition temperature  T  c  and exhibits anenhanced critical current density  J  c  in intermediate magneticfields [2]. In addition we have noted an increased rateof crystal growth for the Nd-homologue relative to the Y-member of this double perovskite family during in-flamespraying experiments [3]. As this material was only studiedin bulk material, we investigated and compared the thin filmsynthesis andcharacteristicsof both materialsfor future coatedconductor development.Although different vacuum-based techniques yield high-quality thin films for use as coated conductors, their relativelyhigh costs, low throughput and scalability pose impedimentsto industrial implementation [4]. Considering this, alternativelow-cost and non-vacuum processes for the production of (RE)BCO-coated conductors need to be developed in orderto facilitate their introduction into broad fields of application. 0953-2048/06/111178+07 $ 30.00  ©  2006 IOP Publishing Ltd Printed in the UK  1178  The chemical solution deposition of   ( RE ) Ba 2 Cu 3 O 7 −  y  superconducting thin films One of the most promising chemicalsolution depositionroutesfor the production of industrial long-length superconductingtapes by a continuous system is the dip coating technique [5].All the present chemical deposition techniques require thepresence of organic solvents to stabilize the highly reactivemetal organic precursors and thus possess a new restriction ontheir widespread application.Therefore and in conjunction with this dip coattechnique, we have developed an environmentally friendlychemical solution deposition method based on aqueoussol–gel chemistry. This inorganic metal-chelate sol–gelroute possesses considerable environmental advantages incomparison to other presently investigated sol–gel routes(alkoxides, TFA) [6, 7]. In general, the sol–gel process involves the evolutionof inorganic networks through the formation of a colloidalsuspension (sol) and gelation of the sol to form a 3D network in a continuous liquid phase (gel) [8]. From a chemical pointof view, our sol–gel chemistry is based on the hydrolysisand condensation of metal multidentate complexes. In thiswork, the three metal salts are dissolved in water, with theformation of unstable metal aqua complexes. To prevent fasthydrolysis of these species leading to precipitation of metalhydroxides, the rate of hydrolysis is controlled by addingspecific complexing agents, leading to the formation of metal-chelates and stabilization of the precursor solution. By slowevaporation of the solvent (water), multiple condensations of these complexes will take place, leading to the formation of a homogenous gel network. As this condensation mechanismrequires stable M–OH species to be present, it will only occurafter a judicious selection of several parameters: (i) the valueof pH which is related to the metal source, (ii) the choiceof complexing agent and (iii) the metal to complexing agentratio. For our systems, it should also be mentioned that thefunction of a complexing agent is twofold: (i) stabilizing theprecursor solution through the formation of metal complexesand (ii) allowing cross-linking during gelation through extrafunctional groups in the ligands. As the quality of the finalproduct is highly dependent on the homogeneity and thebonding behaviour of the metal complexes within the sol andthe gel, a systematic study of the chemical properties of thegels derived from varying operational parameters has beenundertaken using potentiometric titrations. These data havebeen described in detail elsewhere [9]. In this paper we describe the chemical synthesis andsuperconducting properties of both YBCO and NBCO thinfilms, with special attention to their microstructure andmorphology. Because of the requirement for high texturein the superconducting layer, the microstructure across thelayer, starting from the interface with the substrate, has alsobeen investigated, resulting in preliminary clues about thenucleation behaviour of the amorphous precursor film and thegrowth of the final crystallinelayer. The determininginfluenceof the thermal decomposition behaviour of the gel network during its conversion to the (RE)BCO phase on the final filmmorphology as well as on the superconductive properties wasalready investigated and published earlier [10]. 2. Experimental work 2.1. Preparation of the precursor solution The YBCO and NBCO precursor solutions were prepared bydissolving stable, cost-effective and easily available inorganicsalts in an aqueous solution of chelating or coordinatingligands. As a result, solvation by water molecules isdiscouraged, and neither hydrolysis nor precipitation is likelyto take place, and hence a very homogeneous solid material isobtained after thermal treatment of the deposited gels.In this study, a stoichiometric mixture of metal acetates ina total concentration of 0.6 M is diluted in a water–acetic acidmixture at60 ◦ C. After refluxingthis mixture during12 h, puretriethanol amine (TEA) is added dropwise as a complexingagentin ametal:TEA proportionof 1:3. The pH of this mixtureat room temperature is equal to 6.75 and no further adjustmentis necessary.Using this synthesis, clear blue precursor solutions witha viscosity close to water are obtained. If stored in a closedand dark container, these solutions are stable for several weekswithout the formation of precipitates. 2.2. Pretreatment of the SrTiO 3  substrate The(RE)BCO filmhastobegrownepitaxiallyonasubstrateorbuffer. Polished (100) SrTiO 3  (STO) single-crystal substratesmeet all the requirements for epitaxial growth of (RE)BCOsuperconductors [11]. Because of the lack of chemical interaction between both materials, STO is particularlyinteresting for the exploration of thin superconducting filmsynthesis prepared by this new sol–gel dip coating method.Afteroptimizationofthesynthesisprocess, this knowledgecanbe extended to buffered metal tapes for long-length productionof applicable coated conductor systems [12]. The preliminary step before dip coating is a thoroughdegreasing and cleaning process of the substrate’s surface. Forwater-based sol–gel systems, this cleaning step is particularlyimportant because dip coating in an aqueous solution requiresa high wettability of the substrate. Poor wettability obviouslyhas a detrimental influence on the final properties of the layer,such as morphology and adhesion, and ultimately  J  c .Cleaning is performed by applying a degreasing sequencein acetone, methanol and trichloroethylene followed by anultrasoniccleaningin acetone. Finallythe degreasedsubstratesare preheated in air at 450 ◦ C for 5 h. After cooling, thesubstrates need to be stored in methanol in order to maintainthis wettabilityfor several days and to reduce the deposition of dust particles on the clean surface.The evolution of wettabilityof the surfacewas determinedby contact angle measurements using the sessile droptechnique [13]. From figure 1, the change of contact angle after every separate cleaning step can be determined. Thecombinationof the degreasing,ultrasoniccleaningand thermalpreheating steps leads to an almost complete wetting with thecontact angle decreasing from 75 ◦ for an untreated surface to7 ◦ after applying the three-step cleaning procedure. 2.3. Dip coating and thermal process In order to avoid dust contamination during thin filmdeposition, all manipulation of cleaned and dip-coated films1179  B Schoofs  et al Figure 1.  Sessile drop contact angle measurement on STO substrates(A: degreased STO; B: ultrasonic cleaned STO; C: preheated at450 ◦ C; D: untreated STO; E: combination of three cleaning steps). are performed in a dedicated clean room at a class 10000 andcomputer-controlledprecision dip coater in a laminar flow boxat class 10. Dust particles are removed from all solutions byfiltering the solutions with 0 . 45  µ m hydrophilicpolypropylene(GHP) membrane filters.The (RE)BCO precursorfilms on STO substrates were dipcoated at a maximum withdrawal speed of 170 mm min − 1 .One single dip of the cleaned substrate at this speed leads toa final layer thickness of approximately 500 nm, as was laterdetermined from AFM measurements. This thickness shouldbe sufficient for technological applications and thicker layerscan be obtained either by multiple dip coating sequences or byincreasing the viscosity of the precursor solution [14].Immediatelyafterdip coating,theliquidlayeris convertedinto a gel by heating the sample for several hours at 60 ◦ Cin a dust-free furnace. Subsequently, this amorphous gel istransformed to the desired crystalline superconducting phaseby a heat treatment in a tube furnace dependent on thetype of superconductor. In order to obtain highly textured,superconducting YBCO films, the samples were sinteredat 940 ◦ C for 5 h in pure oxygen. After this sinteringprocess the layers were annealed at 450 ◦ C for 5 h, alsounder a 100% oxygen atmosphere. The preparation of NBCO material is much more sensitive to the proper choiceof synthetic parameters. As is well known, the NBCOsystem exhibits a strong tendency for Nd/Ba substitutionwith the formation of Nd 1 +  x  Ba 2 −  x  Cu 3 O 7 −  y  solid solutionphase, which has detrimental effects on the superconductingproperties of the final layer [15, 16]. Our earlier investigations showed that the Nd/Ba substitution in spray-dried and sol–gelprepared bulk material could be depressed by the use of highsintering temperatures in combination with a reduced oxygenatmosphere [17]. Therefore the dip-coated and dried layerswere heated at 10 ◦ C min − 1 to 940 ◦ C for 7 h under a reducedoxygen atmosphere containing 1% O 2  /Ar. After this sinteringprocess the layers were annealed at 450 ◦ C for 5 h under a100% oxygen atmosphere. 2.4. Characterization of the deposited thin layers The thermal decomposition behaviour of the precursor gelnetwork was investigated previously by applying TGA–DTAanalysis (Stanton-Redcroft STA 1500) on bulk samples withthe same composition in metal ions and complexants asthe thin films. IR spectra (4000–400 cm − 1 ) were taken atroom temperature(KBr-pellets)using a Mattson UnicamFTIRspectrometer. The microstructure of the deposited layers wascharacterized by x-ray diffraction (XRD; Siemens D5000,Cu K α ) using  θ  –2 θ   geometry in combination with pole figuresfor determination of the degree of biaxial texture of the layer.Local defects in the layer and at the substrate–layer interfacewereidentifiedusinghigh-resolutionTEM measurements(JeolJEM 3010). Combining these results, it was also possibleto observe the initial nucleation during conversion of theamorphous layer to the oriented crystalline phase. The overallmorphology of the thin films was characterized by opticalmicroscopy (Leitz) and SEM (Philips 501), while surfaceroughness and thickness of the layer were determined using aninterferometric profilometer (Wyko). The critical temperatureof the superconductive layers was determined by resistivitymeasurementsusingacustom-madefour-pointtestdevice. Thecritical current density measurements were performed using aTheva cryoscan. 3. Results and discussion 3.1. Thermal decomposition precursor 3.1.1. NBCO precursor.  In order to establish the bestheat-treatment profile of both films, the decompositionbehaviour of both precursors was studied experimentallyby thermogravimetry by applying the same atmosphere asduring thin film synthesis. The TGA–DTA spectrum of the precursor gel is shown in figure 2(a). (heating rate5 ◦ C min − 1 , 1% O 2  /Ar atmosphere). The broad endothermbeneath 200 ◦ C corresponds to the evaporation and release of gel network water. Above 200 ◦ C, three exothermic peaks areobserved, corresponding to the stepwise decomposition of thegel network. Intermediary products are formed at 220 and350 ◦ C, while after the last decomposition step at 400 ◦ C nofurther weight loss is noticed. IR spectroscopy showed that theremaining species are formally metal oxides of Nd, Cu and Ba. 3.1.2. YBCO precursor.  The TGA–DTA spectrum isshown in figure 2(b) (heating rate 5 ◦ C min − 1 , 100% oxygenatmosphere). Again, the broad endotherm beneath 200 ◦ Ccorresponds to the evaporation and release of gel network water. Above 200 ◦ C, three exothermic peaks are observed,corresponding to the stepwise decomposition of the gelnetwork at 200 and 280 ◦ C with formation of intermediates,whileafterthelastdecompositionstepat420 ◦ C, theremainingspecies are determined by IR spectra as being metal oxides of Y, Cu and Ba.The decomposition behaviour of the multi-metal gelnetwork is completely different to the thermal decomposition1180  The chemical solution deposition of   ( RE ) Ba 2 Cu 3 O 7 −  y  superconducting thin films (a)(b)(c) Figure 2.  (a) TGA–DTA measurement of an acetate–TEA-basedNBCO sol–gel solution under a 1% O 2  /Ar atmosphere.(b) TGA–DTA YBCO precursor in 100% oxygen atmosphere.(c) TGA–DTA measurement of the separate metal acetates. behaviour of the separate metal acetates given in figure 2(c).Most remarkable is the marked lowering of the decompositionfrom the metal complexes to the respective oxides. The lastdecomposition step in the mixture occurs at 400 ◦ C, whereasfor the separate metal salts the formation of the oxides takesplace at 300 ◦ C for Cu acetate, 500 ◦ C for Ba acetate, 650 ◦ Cfor Nd acetate and 800 ◦ C for the Y acetate. This highreactivity of the amorphous precursor is probably caused bythe homogenous distribution of the metal ions inside the gelnetwork. 3.2. Morphology Figure 3(a) shows the SEM micrograph of the NBCO layer deposited from the acetate–TEA precursor, while figure 3(b)shows the SEM micrograph of the YBCO film. Thick homogeneous and crack-free layers are obtained for bothmaterials.Figure 4(a) shows the average roughness AFM measure- ments of the acetate–TEA-based NBCO layer with values of average roughness equal to  R a , acetate - TEA NBCO layer  =  318 nm.The average roughness measurement for the YBCO film, de-rived under the same circumstances, is lower in comparison to (a)(b) Figure 3.  (a) SEM image of NBCO film (640 × ). (b) SEM image of YBCO film (640 × ). NBCO, with an  R a , acetate - TEA YBCO layer  =  177 . 91 nm. Theseobservations are in contrast to the results obtained by vacuumdeposition techniques, where NBCO films show a lower  R a value in comparison to YBCO films [18]. 3.3. Microstructure Figure 5 shows the  θ  –2 θ   XRD spectrum of the sintered andannealed NBCO (blue curve) and YBCO (red curve) layer dipcoatedon a ( l 00) polishedSTO substrate. From the intensityof the different (00 l ) peak reflections, a strong  c -axis orientationof the layer can be observed. No peaks to be attributed to otherorientations of the (RE)BCO phase or impurity phases wereobserved.Figures 6(a) and (b) show the (103) pole figures for the NBCO and YBCO layer and exemplifies a high in-planetexture of the (RE)BCO phase. This is confirmed by the verylow degreeof misorientationangles(averagevalueof 3 . 05 ◦ forNBCO, 2 . 66 ◦ for YBCO) calculatedfromtheFWHM valuesinthe phi-scan.It is well known that the good lattice match with theSTO single-crystal substrate induces the textured growth of the (RE)BCO phase. For vacuum techniques, this is thoughtto occur in an atomic layer-by-layer growth. However, using asol–gel system, one starts from a bulky amorphous layerwhichis already present in large amounts on top of the substrateand which has to be converted to the final crystalline phaseafterwards. To induce texture in this layer this means that theinitialnucleationof the (RE)BCO phasehas to takeplaceattheinterface between the substrate and the amorphous layer.1181  B Schoofs  et al (a)(b) Figure 4.  (a) AFM measurement of the acetate–TEA-based NBCOlayer (  R a  = 318 . 45 nm). (b) AFM measurement of the acetate–TEAbased YBCO layer (  R a  = 177 . 91 nm). Figure 5.  XRD of (RE)BCO thin film. This assumption was corroborated by HR-TEM measure-ments at the interface (see figure 7(a)) where the orientation of  the HTSC layer starts immediately at the interface. In fact, thisstudy demonstrates that the quality of the biaxial textures ob-tained in films deposited by an aqueous sol–gel method are atleastcomparableto thoseobtainedinvacuum-baseddepositiontechnologies.The diffraction pattern given in figure 7(b) correspondingto this interface proves the correct growth of both the cubicsingle-crystal substrate and the orthorhombic NBCO layer ontop.This nucleation mechanism at the interface is alsoconfirmed by the TEM picture given in figure 8 proving thatthegrowthof theNBCO layerfollowsthedefectspresentatthesubstrate surface during thousands of packed unit cell layers.It can therefore be safely concluded that the texturedgrowth of the superconductor coating probably starts withnucleation of the material at the interface followed by the (b)(a) Figure 6.  (a) Pole figure of acetate–TEA-based NBCO layer.(b) Pole figure of acetate–TEA-based YBCO layer. (a)(b) Figure 7.  (a) HR-TEM picture of NBCO/STO interface.(b) Diffraction pattern of interface. development of a crystallization front proceeding through theamorphous gel.1182
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