Physica C 468 (2008) 2283–2287

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Physica C

journal homepage: www.elsevier .com/ locate/physc

On the suppression of superconducting phase formation in YBCO materialsby templated synthesis in the presence of a sulfated biopolymer

Elliott Smith a, Zoe Schnepp a, Stuart C. Wimbush b, Simon R. Hall a,*

a Centre for Organized Matter Chemistry, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UKb Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK

a r t i c l e i n f o

Article history:Received 11 March 2008Received in revised form 31 July 2008Accepted 1 August 2008Available online 14 August 2008

PACS:74.70.�b82.35.Pq81.05�t61.46.Df

Keywords:Superconductor synthesisBiopolymerTemplated synthesisNanoparticle

0921-4534/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.physc.2008.08.002

* Corresponding author.E-mail address: simon.hall@bristol.ac.uk (S.R. Hall

a b s t r a c t

The use of biopolymers as templates to control superconductor crystallization is a recent phenomenonand is generating a lot of interest both from the superconductor community and in materials chemistrycircles. This work represents a critical finding in the use of such biopolymers, in particular the contrain-dicatory nature of sulfur when attempting to affect a morphologically controlled synthesis. Synthesis ofsuperconducting nanoparticles was attempted using carrageenan as a morphological template. Reactivesulfate groups on the biopolymer prevent this, producing instead significant quantities of barium sulfatenanotapes. By substituting the biopolymer for structurally analogous, non-sulfated agar, we show thatsuperconducting nanoparticles could be successfully synthesized.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction ting foams [17]. Control of morphology is achieved by the bio-

Advances in the understanding of materials chemistry have ledto the use of sol–gel methods to produce finer and more homoge-neous superconducting particles [1–3]. By creating morphologi-cally homogeneous superconducting particles, importantfundamental properties such as the critical current density (Jc)can be improved [4,5], owing to the fact that grain boundariesact as weak links in limiting the critical current of high-Tc super-conductors [6–8]. The elimination or minimization of large-anglegrain boundaries by morphological control is therefore a verydesirable target. The field of biomimetic materials chemistry isa powerful one, in that researchers are able not only to producenanoparticles which are morphologically homogeneous andmonodisperse [9,10], but also to concurrently or subsequently or-der them in a very precise manner [11–13], taking as their cuethe procedures and protocols adopted by biomineralizing organ-isms in order to produce a bewildering array of complex andintricate materials [14,15]. We have recently successfully utilizedbiopolymer-mediated syntheses using chitosan and dextran toproduce superconductor nanowires [16] and porous superconduc-

ll rights reserved.

).

polymer acting to chelate and sequester metal cations, therebyproviding both sites of preferred nucleation and growth and ananti-sintering, size-limiting effect on the growing superconductorcrystallites. To date, the range of biopolymers investigated havebeen notable for possessing hydroxide, carboxyl or amino func-tional groups. Whilst primarily hydroxyl mediated, chelation ofmetal ions is improved by complexation with these functionalgroups, leading to improved metal/biopolymer binding. Thisstudy is the first investigation into the suitability of the sulfatefunctional group for controlled superconductor crystal growth,when present as a moiety in the biopolymer carrageenan. Kap-pa-carrageenan consists of repeating disaccharide units of alter-nating (1/3)-a-D-galactose-4-sulfate and (1/4)-b-3,6-anhydro-D-galactose residues joined in a linear chain (Fig. 1a) [18]. Additionof counter cations into kappa-carrageen is known to change itsgelation mechanism. Recent work has shown that in the presenceof Ca2+, cross-bridges are formed between sulfate groups [19] andmultiple aggregations of double helices are seen rather than onlytwo or three units joining together [20]. The result of this is theformation of a much stronger, contracted gel, which is importantin this study as the biopolymer needs to retain the bulk of itsstructure upon calcination long enough for metal nucleationand oxidation to begin.

Fig. 2. TEM images showing (a) nanoparticles from a carrageenan-templated Y124synthesis. (b) Shows a high-resolution image of the BaSO4 nanotape (area outlinedin (b)). Crystallographic axes a and b are marked. Scale bar in (a) is 500 nm and in(b) 5 nm.

Fig. 3. Powder X-ray diffraction pattern of control YBCO material. All peaks areindexed to the superconducting YBa2Cu3O7�d (Y123) phase (JCPDS pattern # 79-1229). Peaks due to CuO are indicated.

Fig. 1. Schematic molecular configuration of (a) kappa-carrageenan and (b) agar.Adapted from Ref. [18].

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2. Experimental

Yttrium barium copper oxide (YBCO) sols and composite mate-rials were prepared and analysed using adapted previously pub-lished protocols. Yttrium oxide nanoparticles (31.25 mmol) weredissolved in 0.2 M acetic acid (100 ml) and stirred for 10 hours at55 �C in a covered beaker. 0.5 M copper acetate (50 ml) was addedto the now clear solution and stirred for a further 2 h at the sametemperature. Finally 0.5 M barium acetate (25 ml) was added andthe solution was stirred for 2 h at room temperature. Smallamounts of dissolved tartaric acid were then dropped into the ace-tate solution to bring its pH value down to 5.6. Solutions of carra-geenan were prepared by dissolving kappa-carrageenan (0.15 g) indistilled water (10 ml) under stirring and heating at 60 �C forapproximately 1 h. Composite superconductor precursor/carra-geenan thin films were prepared by the addition of variousamounts of the YBCO precursor solution from 0.1 ml to 3.0 ml in10 ml hot carrageenan solution, followed by vigorous mixing to en-sure a homogeneous fluid. The liquid was then cast into Petridishes and gels were formed upon cooling to room temperature.The gels were dried by being left at 35 �C for 72 h. Once dry, thecomposite films were broken into small pieces and placed in cruci-bles in the furnace. The initial heating was at 1 �C/min up to 500 �Cwhere they remained for 2 h. Once cooled to room temperature,the films were crushed into a fine powder and placed back in thefurnace. The final calcination stage was at 1 �C/min up to 920 �Cfor a further 2 h. The microstructures of the resulting nanoparticleswere characterized by 200 kV JEOL 2010F field-emission transmis-sion electron microscopy (FETEM), equipped with an annular darkfield detector and an Oxford energy-dispersive X-ray (EDX) detec-tor. High-resolution TEM images and EDX analysis were used todetermine the phase composition of the nanoparticles. PowderXRD was carried out using a Bruker D8 Advance powder diffrac-tometer (CuKa radiation, 1.54056 Å; 2h values 15–55�, step inter-val 0.02�). SQUID magnetometry was performed using a QuantumDesign Magnetic Property Measurement System equipped with a5 T superconducting magnet. The measured data was not correctedfor demagnetisation effects. Field cooled (FC) and zero field cooled(ZFC) dc susceptibility was measured as a function of temperatureunder an applied field of 1 mT.

3. Results and discussion

Control samples of YBCO were produced via the sol–gel methodin the absence of carrageenan. YBCO typically has large irregularlyshaped crystals even when synthesised by chimie-douce tech-niques such as the sol–gel method; using transmission electronmicroscopy (TEM), the structure of the control sample was indeedshown to consist of irregularly shaped crystallites on average 5 lm

Fig. 4. SQUID magnetometry data showing critical current density (Jc) and criticaltemperature (Tc) of a control Y123 samples. The material has a Tc of 91 K and acritical current density of 3.5 MA cm�2 at 10 K, 1 T.

Fig. 5. Electron diffraction pattern of a representative nanotape present in thecarrageenan-templated YBCO syntheses. Spots indicated are indexed to BaSO4

(JCPDS pattern # 24-1035). Crystallographic axes a and b are shown.

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in length (data not shown). This compared markedly to the samplesynthesized in the presence of carrageenan, which showed a con-trolled crystallization, producing nanoparticles of between 100nm and 200 nm in dimension (Fig. 2a).

Powder X-ray diffraction confirmed that the control materialwas primarily Y123 (Fig. 3). SQUID magnetometry of this materialshowed that the control was superconducting, with a critical tem-perature Tc of 91 K and a Jc of 3.5 MA cm�2 (10 K, 1 T) (Fig. 4). The

Fig. 6. (a) X-ray diffraction pattern of carrageenan-templated YBCO. Peak legend, j – BaSmagnetometry of non-superconducting carrageenan-templated material and (c) X-ray difindexed, peaks marked j correspond to BaCuO2. (d) SQUID magnetometry of agar-temp

upturn in susceptibility at the lowest temperatures measured isthe contribution due to the paramagnetic (non-superconducting)

O4;4– Y2BaCuO5; d – Y247; – CuYO2; X – CuO; s – BaO2; – Y2(SO4)3; (b) SQUIDfraction pattern of superconducting agar-templated material. Peaks due to Y124 arelated Y124; curves for both Tc and Jc are shown.

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fraction of the sample. After the calcination process, the controlsamples were black, whereas samples produced with carrageenanas a template were dark grey. This was the first indication that anappreciable amount of an associate phase was present. When sol–gel synthesis was carried out in the presence of carrageenan, theformation of morphologically distinct nanotapes was observed, inaddition to nanoparticulate growth. The nanotape dimensions var-ied considerably. Their length ranged from 100 to 1500+ nm, andtheir widths from 40 to 80 nm. Fig. 2b shows a high resolutionTEM image of the edge of one such nanotape, which reveals thatthe nanotapes are single crystals of barium sulfate, BaSO4. Thenanotapes possessed pronounced lattice fringes, which ran parallelto the crystallographic axis of elongation. The lattice spacing per-pendicular to the length of the nanotapes was determined to be0.7 nm, consistent with the crystallographic a-axis of BaSO4 (ortho-rhombic lattice, space group Pbnm, JCPDS pattern #24-1035). Elec-tron diffraction of the nanotapes produced intense spot patternswhich were able to be indexed to BaSO4 (Fig. 5). Highly anisotropiccrystals are a common morphology in the barium sulfate systemand have been observed previously in the case of simple organic/aqueous biphase interfacial crystallization [21], or in reverse mi-celles and microemulsions [22,23]. Under the synthesis conditionspresented here, BaSO4 is produced upon admixing the supercon-ductor precursor solution with the carrageenan and on calcination

Fig. 7. TEM images showing (a) the control of crystal morphology by calcination inthe presence of agar, producing a fine, reticulated crystalline structure comprisingindividual crystallites (b) of the order of 500 nm in length.

forms a white crystalline material which is stable with regards toformation of BaO up to 1100 �C.

The persistence of BaSO4 as an impurity in the reaction mixturehas an adverse effect on the stoichiometry of the superconductingphase. Powder X-ray diffraction (Fig. 6a) confirmed the initialhypothesis that a mixed phase was present. By abstracting a largeamount of barium, the sulfate groups on the carrageenan renderthe attainment of a superconducting phase unlikely, instead leadingto the formation of a large number of phases normally regarded asimpurity phases in YBCO superconductor synthesis. Predominantpeaks were assigned to BaSO4 with reflections at d = 3.95 (111),3.77 (201), 3.58 (002), 3.39 (210), 3.29 (102), 3.11 (211), 2.20(221) and 1.59 Å (132). Other phases present were CuYO2

(d = 5.71 (003), 2.82 Å (006)); Y211 (d = 2.92 (211), 1.74 Å(322)); CuO (d = 2.53 (002), 2.35 (111), 1.83 Å (�202)); BaO2

(d = 2.16 (112), 1.91 (200), 1.64 Å (211)), Y2(SO4)3 at d = 4.71 Å(104) and pronounced peaks due to Y247 (d = 4.22 (0012), 2.68 Å(115). SQUID magnetometry (Fig. 6b) showed that unlike the con-trol, the samples made with carrageenan were non-superconduc-ting down to 10 K. The magnetic susceptibility showed a typicalparamagnetic response. In order to confirm that it is the presenceand reactivity of the sulfate groups and not biopolymer control itselfwhich is the cause of the non-superconductivity of the samples, abiopolymer structurally analogous to carrageenan was chosen to ef-fect morphological control over crystal growth in a similar fashion.

Agar (Fig. 1b) is from the same family of Rhodophycae as thecarrageenans, but possesses L-3,6-anhydro-a-galactopyranoserather than the D-3,6-anhydro-a-galactopyranose units seen in car-rageenan and more importantly, a lack of sulfate groups. The struc-tural similarities can been seen by comparing Fig. 1a and b. TEMimaging confirms that control of crystal morphology is effectedby calcination in the presence of agar, with a fine, reticulated crys-talline structure comprising individual crystallites of the order of500 nm in length (Fig. 7).

XRD shows that the agar-templated sample is Y124, with smallimpurity peaks due to BaCuO2 and Y211 (Fig. 6c), with SQUID mag-netometry confirming that the material is superconducting with anonset Tc of �55 K and a Jc at 10 K and 1 T field of 0.05 MA cm�2

(Fig. 6d). Although there is a suppression of the Tc in the Y124superconductor, (previously observed in Y124 biotemplatedgrowth with chitosan16) these data indicate that for a biopolymerwhich is structurally analogous to carrageenan, templated crystal-lization of superconducting YBCO is possible. This immediatelysuggests that it is the presence of the sulfate groups in carrageenanwhich are detrimental to the successful synthesis of an YBCOphase. We postulate that this is due to the formation of BaSO4

through the reaction of barium in the precursor solution with sul-fate groups of the carrageenan. This is evidenced by the presence oflarge quantities of BaSO4 nanowires in the carrageenan-templatedYBCO syntheses.

4. Conclusion

We have demonstrated that by using the biopolymer carra-geenan, template control of mineralization in order to achieveYBCO nanoparticles has been compromised by the reactivity of sul-fate groups on the biopolymer. Previous biopolymer-mediatedgrowth of YBCO nanoparticles has been successful, and this canbe inferred to be due to the biopolymer in each case acting as aphysical sequestering agent, cross-linking and cocooning the cat-ionic species rather than acting to chemically passivate the reac-tant species. By changing the biopolymer to one which isstructurally similar but containing none of the reactive functionalgroups, control of crystal morphology is achieved with none ofthe abstraction of barium leading to the creation of the intended

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superconducting YBCO phase. In this way, we have shown that notall biopolymers are suitable for template control of YBCO crystalli-zation; the potential for chemical interaction between the biopoly-mer and elements in the superconductor precursor sol must betaken into account.

Acknowledgements

The authors thank the Royal Society and EPSRC for financialsupport. Correspondence and requests for materials should be ad-dressed to SRH (simon.hall@bristol.ac.uk).

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