Detailed Study of the Process of Biomimetic Formation of YBCO

Sep 28, 2012 - and Judith L. MacManus-Driscoll*. ,†. †. Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, ...
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Detailed Study of the Process of Biomimetic Formation of YBCO Platelets from Nitrate Salts in the Presence of the Biopolymer Dextran and a Molten NaCl Flux Zili Zhang,†,‡ Stuart C. Wimbush,† Ahmed Kursumovic,† Hongli Suo,‡ and Judith L. MacManus-Driscoll*,† †

Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Key Laboratory of Advanced Functional Materials, Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, China

‡

S Supporting Information *

ABSTRACT: A novel method of achieving microscopic morphological control during the bulk synthesis of the high temperature superconducting ceramic YBa2Cu3O7 (YBCO) has been studied. By incorporating appropriate amounts of the additives dextran (a biopolymer) and NaCl (a high melting point ionic salt) into the synthesis protocol, it is proven possible to engineer high aspect ratio (platelet) growth of the YBCO crystallites together with localized orientational ordering between adjacent densely packed crystallites. In the optimized protocol, both additives are fully consumed during the synthesis by decomposition (dextran) and vaporization (NaCl), leaving phase-pure YBCO as the final synthesis product. The individual effects of the two additives are separately described and their optimal quantities determined. Routes toward improving the yield and increasing the aspect ratio of the resulting crystallites are outlined. The method is likely applicable to the synthesis of other ceramic materials as an alternative to the conventional solid state synthesis route, where a higher degree of connectivity between crystallites is required than can be achieved through sintering.

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INTRODUCTION

of the challenges inherent to coated conductor production, while also benefiting from much larger current carrying cross sections. In this paper, we further elucidate some aspects of our novel synthesis protocol that were only presented in outline form previously. These include a detailed study of the precise influence of the biopolymer component on the orientational alignment of crystallites critical to this application and of the importance of the fluxing effect of the NaCl addition in achieving the large, high aspect ratio platelet growth. We show that each of these constituents provides a distinct and vital contribution to the final targeted morphology of the synthesis product. While several recent reports9,10 have applied various biomimetic approaches to the formation of YBCO platelets, the focus has been on improved physical properties, and the crystallites obtained have been difficult to isolate in quantity and are limited to submicrometer size. We demonstrate that the combination of liquid flux growth, commonly employed in the production of YBCO single crystals,11,12 with biopolymer-mediated precursor sequestration is the key to achieving the desired morphology.

The fabrication of kilometer length scale conducting cables from the brittle ceramic high temperature superconducting oxides with the added requirement of crystallographic texture to within a few degrees along the entire length of the conductor is an almost inconceivable technical challenge. Nonetheless, immense progress has been made, initially with the first-generation Bi-based superconducting wires formed through the powder-in-tube metallurgical technique1 and, more recently, in the drive toward lower cost and improved in-field performance, with the secondgeneration YBCO-based thin film coated conductor tapes.2−4 This astounding achievement notwithstanding, significant issues remain in regard to the complexity of the resulting process, the reliability of the final product, its ultimate performance, and the total production cost. More seriously, it is no longer apparent how further price/performance gains can be achieved along this route, in order to make the products commercially competitive.5 We recently reported6 on a low-cost method for YBCO conductor fabrication through a bulk processing route that is potentially able to overcome these issues and provide a thick conductor having superior overall performance at a lower price.7 By abandoning the thin film fabrication techniques universally employed for this purpose and instead applying a biomimetic bulk synthesis8 to preform platelets of superconducting material that are subsequently allowed to arrange themselves in a natural preferred orientation on a tapelike substrate, we circumvent many © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Standard (Optimized) Synthetic Protocol. Biomimetic YBCO samples were synthesized by the admixing of 7.50 g dextran (Mr = 70,000)

Received: August 8, 2012 Revised: September 14, 2012 Published: September 28, 2012 5635

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with 2.5 mL of a precursor solution prepared by dissolving Y(NO3)3·6H2O (1.915 g, 0.05 M), Ba(NO3)2 (2.613 g, 0.10 M), and Cu(NO3)2·2.5H2O (3.489 g, 0.15 M) in 100 mL of distilled water. These were mixed in a crucible to form a light blue, viscous paste and left at room temperature for one day to harden. Subsequently, 0.025 g of NaCl (340 mol % relative to YBCO) was sprinkled uniformly across the surface of the hardened mixture before heating in a box furnace to 920 °C at 10 °C/min and sintering for 2 h in air and then cooling to room temperature at 2 °C/min. Variation of Method of Addition of NaCl. NaCl (0.010 g) was added to the reactant mixture in one of three different ways as shown in Figure 1: (A) dissolving in the precursor solution, (B) sprinkling

Figure 2. XRD patterns of biomimetic YBCO samples resulting from the addition of NaCl at different stages of the synthesis: A, B, C. Indexed peaks are YBCO. The scans are offset vertically for clarity.

Figure 1. Schematic diagram of the synthetic protocol showing the three different methods of adding NaCl: (A) dissolving in the precursor solution, (B) sprinkling uniformly across the surface of the precursor/dextran mixture before leaving to harden, (C) sprinkling uniformly across the surface of the hardened precursor/dextran mixture. uniformly across the surface of the precursor/dextran mixture before leaving to harden, (C) sprinkling uniformly across the surface of the hardened precursor/dextran mixture. The method used in the standard synthetic protocol is method C. Variation of Amount of NaCl Added. The amount of NaCl introduced into the synthetic protocol via method C (the standard method) was varied from 0 to 0.100 g. Variation of Amount of Dextran Used. The amount of dextran admixed with the precursor solution was varied from 1.25 to 10.00 g. Variation of Concentration of Precursor Solution. The cation concentration of the precursor solution was varied from 0.025 to 0.125 M with respect to Y. Analysis Techniques. The phase composition of the samples was characterized by Cu Kα X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer equipped with a LynxEye one-dimensional detector. The microstructure of the samples was observed by scanning electron microscopy (SEM, JEOL JSM-5800 LV, 20 kV) with elemental analysis performed by energy dispersive X-ray (EDX) analysis. The samples were screened for superconductivity by measuring their diamagnetic susceptibility in an applied field of 10 mT as a function of temperature in a cryogenic vibrating sample magnetometer (Cryogenic Ltd.).

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RESULTS AND DISCUSSION Figure 2 shows the XRD patterns of the samples resulting from NaCl addition at different stages of the synthesis. All the samples are seen to comprise almost phase pure YBCO, with all polycrystalline peaks above 2% relative intensity evident. There is no significant difference between the XRD scans of the three samples, indicating that the precise method of adding NaCl to the reactant mixture has little effect on the phase formation of the YBCO. Figure 3 shows SEM images of the samples resulting from these different syntheses. Part A shows that dissolving the NaCl in the precursor solution results in a sample having a porous, spongelike morphology, similar to samples prepared without

Figure 3. SEM images of biomimetic YBCO samples resulting from the addition of NaCl at different stages of the synthesis: A, B, C as defined in Figure 1.

NaCl.6,13 Sprinkling the NaCl across the surface of the precursor/dextran mixture before it hardens causes the YBCO crystallites in a few regions of the resulting sample to become ill-defined organic looking platelets as shown in part B, but most parts of the sample remain porous. In contrast, the entirety of the sample prepared by sprinkling the NaCl across 5636

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above 90 K for all the samples up to 0.025 g of NaCl content. The sample with 0.050 g of NaCl, in contrast, had a significantly reduced superconducting onset temperature around 80 K. Its diamagnetic susceptibility was also around half that of the other samples as a result of the high impurity content. Samples with greater amounts of NaCl were not measured due to their low phase fractions of YBCO. Figure 5 shows SEM images of samples synthesized with different amounts of NaCl. As shown in Figure 5a, the sample synthesized without NaCl was porous and spongelike, as is commonly observed.6,13 In the sample with 0.005 g of NaCl, the YBCO formed platelets in very few regions, such as the one highlighted in Figure 5b; however, it was still mainly porous. In the samples with 0.010 and 0.025 g of NaCl, all the YBCO crystallites throughout the sample were platelike, and densely packed to each other so as to comprise clusters with a similar growth direction, as seen in Figure 5c. The morphology of YBCO remains platelike with a correlated growth direction in the sample with 0.050 g of NaCl, as seen in Figure 5d; however, some of the platelets did not aggregate closely. At the same time, some 100 μm of long needle-like grains appear in this sample, which are confirmed by EDX to be CuO. This corresponds with the results of the XRD measurements, which show the emergence of CuO at this level of NaCl addition. When the amount of NaCl is further increased to 0.075 g, a predominance of CuO needles are found, while in the sample with 0.100 g of NaCl, large NaCl particles and CuO needles become the main morphology present, with only a few YBCO platelets found at the surface, as indicated in Figure 5f. It is noted that while this excessive NaCl content acts to disrupt the YBCO phase formation, it nonetheless does not alter the microstructure of the small amount of YBCO that is still formed, which remains platelike. The effect of the NaCl addition, therefore, is to provide a liquid flux environment which encourages the formation of platelike crystallites of YBCO. Balancing the amount of NaCl required to produce YBCO platelets throughout the sample against the phase degradation that occurs if too much NaCl is residually present results in an optimal addition amount between 0.010 and 0.025 g, or around 10−25 wt % relative to the amount of YBCO formed. Figure 6 plots the average platelet size and aspect ratio (length to thickness) of samples synthesized with different amounts of NaCl, as observed in the SEM images. Initially, as the NaCl content is increased, there is a decrease in the platelet thickness while the average platelet length remains roughly constant. The trend in thickness continues to higher NaCl amounts, but the lateral size of the platelets is severely diminished at 0.050 g of NaCl when CuO begins to be form to the detriment of the YBCO. Consequently, only a slight increase in the platelet aspect ratio is observed throughout the range of NaCl addition in this experiment. The potential exists to improve upon this result if the YBCO decomposition at higher NaCl contents can be prevented. This may be possible by extending the dwell time at high temperature to allow the greater NaCl content to evaporate before cooling. In this way, we anticipate being able to achieve platelet aspect ratios of 10:1 or better. Figure 7 shows the XRD patterns of samples synthesized with different amounts of dextran. It is seen that the YBCO phase exists as the predominant phase in all the samples. A variety of impurity phases including CuO and Y2O3 are also seen in the samples with lesser amounts of dextran, up to 5.00 g, and the strength of these impurity peaks becomes weaker as the amount of dextran increases. The YBCO material is virtually

the surface of the hardened precursor/dextran mixture comprises well-defined YBCO platelets with angular edges, connected to each other to form clusters having a similar growth direction, as shown in part C. It is found that only by adding the NaCl after the mixture of precursor solution and dextran hardens (C) is it possible to obtain homogeneous YBCO platelets throughout the sample. If added earlier, either to the liquid precursor solution (A) or while the precursor/ dextran mixture is still in unhardened liquid form (B), the NaCl dissolves into the liquid and disperses throughout the solution. Consequently, it is incorporated into the YBCO matrix, rather than acting as a molten salt flux at high temperatures (above its melting point), assisting with the anisotropic crystal growth of the YBCO. As a result, the final morphology of the YBCO formed in these cases is predominantly the same as that obtained without NaCl. Where the NaCl is added to the surface of the unhardened mixture, the high viscosity of the paste may result in some fraction of the NaCl failing to dissolve before the mixture hardens. This is likely to be the cause of formation of the few, poorly formed platelets that are observed within this sample. Keeping the NaCl solid until an elevated temperature is reached is necessary to produce platelike YBCO crystallites. Here, this is accomplished by the use of dextran to sequester the precursor solution away from the NaCl; however, it is possible to conceive of the use of precursors in which NaCl is not soluble or indeed solid precursor material for a more conventional sintering approach. Figure 4 shows the XRD patterns of samples prepared with different amounts of NaCl sprinkled over the surface of the

Figure 4. XRD patterns of biomimetic YBCO samples synthesized with different amounts of NaCl. Indexed peaks are YBCO. Peaks originating from residual NaCl and a CuO impurity phase are marked. The scans are offset vertically for clarity.

hardened precursor/dextran mixture (standard method C). It is seen that YBCO is the dominant phase for amounts of NaCl up to 0.025 g but that additions of NaCl beyond this amount cause rapid suppression of the YBCO phase and its replacement by CuO and NaCl as the primary phases present. This is most clearly seen in the gradual replacement of the (005)/(014)/(104) YBCO peak by an adjacent CuO peak and in the emergence of the most intense NaCl peaks. Since the total yield of YBCO of a standard synthesis is about 0.1 g, these results are broadly as expected. It is important to keep the amount of NaCl added to no more than 0.025 g to ensure YBCO phase purity. In this case, it is suggested that the total holding time at temperatures above the melting point of the NaCl (801 °C) is sufficient for the small amount of liquid flux present to fully evaporate during processing. These results are also reflected in susceptibility measurements (Supporting Information Figure S1) which reveal the onset of superconductivity 5637

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Figure 5. SEM images of biomimetic YBCO samples synthesized with different amounts of NaCl: (a) none, (b) 0.005 g, (c) 0.025 g, (d) 0.050 g, (e) 0.075 g, and (f) 0.100 g.

Figure 6. Average platelet size and aspect ratio of biomimetic YBCO samples synthesized with different amounts of NaCl.

Figure 7. XRD patterns of biomimetic YBCO samples synthesized with different amounts of dextran. Indexed peaks are YBCO. Peaks originating from CuO and Y2O3 impurity phases are marked. The scans are offset vertically for clarity.

phase-pure when the amount of dextran exceeds 5.00 g. The strong presence of YBCO in spite of the existence of minor impurity phases in some samples means that the susceptibility curves of all these samples (Supporting Information Figure S2) evidence superconductivity with an onset temperature above 90 K and little significant difference between them.

Figure 8 shows SEM images of samples synthesized with different amounts of dextran. The morphology of the samples synthesized with 1.25 and 2.50 g of dextran mainly comprises clustered, locally oriented YBCO platelets, as shown in Figure 8a, c. CuO is also seen in these two samples. The CuO appears as needles in the sample synthesized with 1.25 g of dextran 5638

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Figure 8. SEM images of biomimetic YBCO samples synthesized with different amount of dextran: (a, b) 1.25 g; (c, d) 2.50 g; (e) 7.50 g; (f, g, h) 10.0 g.

The lesser amount of CuO cannot form large needle-like grains throughout the sample, and instead, it segregates to the surface. The samples synthesized with 5.00 and 7.50 g of dextran exhibit dense clusters of YBCO platelets with localized orientation throughout each cluster, as shown in Figure 8e. As the amount of dextran used reaches 10.00 g, three distinct YBCO microstructures are evident: clusters of platelets as seen in the samples with 5.00 and 7.50 g of dextran, shown in Figure 8f; continuous regions of connected platelets, not formed into clusters, shown in Figure 8g; and a porous structure with only a few residual YBCO platelets embedded in it, as indicated by the arrows in Figure 8h. The cause of the changes in microstructure of the samples synthesized with different amounts of dextran can be deduced

Figure 9. Photographic images of the hardened precursor solution/ dextran mixture containing different amounts of dextran ranging from insufficient (left) to excessive (right).

(Figure 8b) and as smaller isotropic grains at the surface in the sample synthesized with 2.50 g of dextran (Figure 8d). 5639

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does the well-formed mixture, forming dense clusters of well aligned platelets. Intermediate material is exposed to the NaCl, and thus forms platelets, but is forced to crystallize much faster as the NaCl evaporates, resulting in random orientation. In summary, an incorrect amount of dextran, either too much or too little, causes an inhomogeneous distribution of the hardened precursor solution in relation to the subsequently added NaCl, which ultimately leads to an inhomogeneous morphology of the resulting YBCO in accordance with the local amount of NaCl seen by the precursor. Figure 10 plots the average platelet size and aspect ratio (length to thickness) of samples synthesized with different amounts of dextran, as observed in the SEM images. There is a very slight increase in both the thickness and the lateral size of the platelets as the amount of dextran used increases; however, the two increase in proportion, with the consequence that the platelet aspect ratio remains constant at ∼6. This is a significant increase over the aspect ratio of ∼3 typically observed for the growth of YBCO crystals from flux,12,15 as determined by the crystal lattice parameter ratio via Bravais’ Law. The sample synthesized with 7.50 g of dextran has anomalously large platelets but again retains the same aspect ratio. There is consequently no morphological benefit to the YBCO in increasing the amount of dextran used in the synthesis. The natural question arises of whether we can increase the yield of YBCO from our synthesis given that, under the present condition, a yield of just 0.1 g of YBCO results from the use of 10 g of dextran. Since the amount of dextran used in relation to the amount of precursor solution has been shown to be a critical factor, the option that remains open to us is to increase the cation concentration of the precursor solution. To this end, a range of different concentration solutions were investigated under the standard synthesis protocol. Figure 11 shows the XRD patterns of samples synthesized from different concentrations of precursor solution. It is seen that the YBCO phase exists as the predominant phase in all the samples. In the sample prepared from the lowest concentration precursor, 0.025 M, a significant CuO impurity content is observed; the samples covering the concentration range from 0.050 to 0.100 M are broadly similar, and the sample prepared from the highest concentration solution, 0.125 M, exhibits somewhat stronger YBCO peaks. Figure 12 shows SEM images of samples synthesized from different concentrations of precursor solution. Aggregated YBCO platelets of a similar size and aspect ratio were observed in all samples, as shown in Figure 12a, c, d. However, in the sample prepared from the 0.025 M precursor, there are additionally a lot of rounded grains surrounding the YBCO clusters as shown in Figure 12b. These grains were confirmed by EDX to be CuO, corresponding with the XRD results. The CuO vanishes on increasing the precursor concentration to 0.050 M (the standard concentration), again in agreement with the XRD results, but on increasing the precursor concentration further to 0.125 M, the porous, spongelike microstructure reappears across much of the sample, accompanied by a worsening in the localized orientational alignment of the platelets. This systematic variation from CuO impurity content through phase-pure YBCO platelets to spongelike YBCO as the precursor concentration is increased mirrors the effect of decreasing the amount of NaCl in the synthesis (relative to the total amount of YBCO present). It therefore suggests that a route to achieving a higher yield of YBCO is to simultaneously

Figure 10. Average platelet size and aspect ratio of biomimetic YBCO samples synthesized with different amounts of dextran.

Figure 11. XRD patterns of biomimetic YBCO samples synthesized from different concentrations of precursor solution. Indexed peaks are YBCO. Peaks originating from a CuO impurity phase are marked. The scans are offset vertically for clarity.

as follows. As outlined in ref 14, the dextran forms a cage structure which locks cations from the precursor solution within the cage after mixing and hardening. In the case of insufficient dextran, after mixing, not all the precursor solution can be sequestered within the dextran cage and some remains free. However, the flow of this extra precursor solution is blocked by the dextran, so it is not homogeneously distributed throughout the sample but confined to some areas, which causes the color variations across the material seen in the left image of Figure 9. When NaCl is added homogeneously across the surface of the hardened mixture, the areas without extra precursor solution will have a proportionately higher amount of NaCl relative to the amount of YBCO present. As described earlier, a high concentration of NaCl is not conducive to the synthesis of YBCO but rather promotes CuO formation. Consequently, during the subsequent sintering, such regions will predominantly form CuO, as observed in Figure 8b and d. As the amount of dextran is increased to become sufficient to sequester all of the precursor solution, the local NaCl concentration variations disappear, as shown in the center image of Figure 9, and all the YBCO synthesized under such conditions forms platelets. Finally, as the amount of dextran becomes excessive, as shown on the right image of Figure 9, the precursor solution loses its fluidity and hardens into an undulate structure. As a result, different parts of the mixture have differing degrees of exposure to the NaCl applied to the surface. Material far from the surface does not see the NaCl and so becomes porous, while material at the surface behaves as 5640

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Figure 12. SEM images of biomimetic YBCO samples synthesized from different concentrations of precursor solution: (a, b) 0.025 M; (c) 0.075 M; (d, e) 0.125 M.

reaction. When these optimal quantities are used, the high temperatures of the synthesis result in complete decomposition of the dextran and gradual vaporization of the NaCl, leaving behind a phase-pure synthesis product. The potential exists to modify the method to produce a higher yield of superconductor and higher aspect ratio platelets. By developing and optimizing a morphologically controlled bulk synthesis route for YBCO having the essential characteristics required for conductor fabrication (crystallographic alignment of adjacent crystallites and high aspect ratio platelet growth), we provide the first steps toward supplanting existing complex thin film approaches with a straightforward method offering the potential for dramatically increased performance and reduced production cost.

increase both the precursor concentration and the amount of NaCl used.

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CONCLUSION The individual influence of the additives dextran and NaCl on the biomimetic synthesis of YBCO from nitrate biopolymer dextran acts as an encapsulating medium, creating nanoreactors of the precursor materials in the early stages of reaction that promote fine nucleation and a constrained crystallite growth mode resulting in a dense local orientational alignment of the platelets. The high melting point salt NaCl remains inert throughout these early stages of reaction so long as it can effectively be prevented from dissolving in the precursor solution, in this case through addition at a stage in the process where the precursor has been sequestered by the dextran, only participating as the temperature rises and it melts to form a flux, encouraging highly anisotropic crystallite growth. The relative amount of each of these additions is an essential parameter to achieving good phase formation. A sufficient quantity of dextran (around 10.0 g per 2.5 mL of precursor solution) is required to promote YBCO phase formation in preference to simpler binary phases, while the quantity of NaCl must be just sufficient (around 0.010 g per 0.1 g of hardened precursor) to influence the crystallite growth without interfering with the

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ASSOCIATED CONTENT

S Supporting Information *

Susceptibility curves indicating the superconducting transition temperatures of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +44 (0)1223 334468. Fax: +44 (0)1223 334567. E-mail: [email protected]. 5641

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.Z. and H.S. were supported by the National Natural Science Foundation of China (51171002), the program of New Century Excellent Talents in University of China, and 211 Program of Beijing University of Technology. S.C.W. was supported in this work by The Leverhulme Trust with supplementary funding from The Isaac Newton Trust. J.L.M.-D. acknowledges the European Research Council for the Advanced Investigator Grant, Novox, ERC-2009-adG 247276.

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REFERENCES

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