Benzoic acid doping to enhance electromagnetic properties of MgB2 superconductors

Transcription

1 University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences 2007 Benzoic acid doping to enhance electromagnetic properties of 2 superconductors W. X. Li Shanghai University, wenxian@uow.edu.au Y. Li Shanghai University M. Y. Zhu Shanghai University R. H. Chen Shanghai University X. Xu University of Wollongong, xun@uow.edu.au See next page for additional authors Publication Details Li, W., Li, Y., Zhu, M., Chen, R. H., Xu, X., Yeoh, W., Kim, J. & Dou, S. X. (2007). Benzoic acid doping to enhance electromagnetic properties of 2 superconductors. IEEE Transactions on Applied Superconductivity, 17 (2), Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au

2 Authors W. X. Li, Y. Li, M. Y. Zhu, R. H. Chen, X. Xu, W. K. Yeoh, J. H. Kim, and S. X. Dou This journal article is available at Research Online:

3 2778 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007 Benzoic Acid Doping to Enhance Electromagnetic Properties of 2 Superconductors W. X. Li, Y. Li, M. Y. Zhu, R. H. Chen, X. Xu, W. K. Yeoh, J. H. Kim, and S. X. Dou Abstract The effect of benzoic acid doping on lattice parameters, microstructure, critical temperature (T c ), critical current density (J c ) and flux pinning force of 2 has been studied. In this work we used benzoic acid as an example of aromatic acids as an additive to 2. For different sintering process, actual carbon (C) substitution for boron (B) was estimated to be from 3.5 at% to 5.0 at% of B while T c dropped about 2 4 K. The advantages of aromatic acid doping include homogeneous mixing of precursor powders, avoidance of expansive nano-additives, production of highly reactive C and significant enhancement in J c of 2, compared to un samples. High level C substitution of B can induce the 2 crystals grow into bar shape microstructure. The J c value of Acm 2 at 5 K and 8 T for preheated 10 wt% C 7 H 6 O 2 2 sample is higher than that of the un 2 by a factor of 13. As there are numerous aromatic acids readily available this finding has significant ramifications not only for the fabrication of 2 but also for many C based compounds and composites. Index Terms Aromatic acids, benzoic acid, 2, superconductivity. I. INTRODUCTION FOR the relatively high critical temperature of about 39 K [1], the high critical current density of more than in moderate fields, and the lack of weak links, magnesium diboride superconductors is believed to be a promising candidate for engineering applications. The temperature range of choice is from 20 K to 30 K, for which the traditional low temperature superconductors (LTS) cannot play any role but a cryogen-free cooling system is readily available. Many works are devoted to the enhancement of the critical current density in applied magnetic field, invoking techniques like the chemical doping [2] [14], irradiation [15], and various thermomechanical processing techniques [16] [19]. Since has a relatively large coherence length and small anisotropy, the fluxoids to be pinned are string-like and amenable to pinning by inclusions and precipitates in the grains. This opens a window to the success of chemical doping in this material as chemical doping is a simple and readily scalable technique. Various forms of carbon (C) dopants have remarkably enhanced the electromagnetic properties of [2] [13]. Many novel techniques have been directed towards the fabrication Manuscript received August 29, This work was supported in part by the Natural Science Foundation of China (No ). W. X. Li, Y. Li, M. Y. Zhu, and R. H. Chen are with the School of Materials Science and Engineering, Shanghai University, Shanghai , P.R. China ( shliwx@yahoo.com). X. Xu, W. K. Yeoh, J. H. Kim, and S. X. Dou are with the Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia ( shi@uow.edu.au). Digital Object Identifier /TASC of technically usable high- wires [2], [8], [10], [11]. Among the numerous forms of C-containing dopants, SiC doping has achieved a record high in-field,, and irreversibility in [2], [9], [14]. These record properties have been confirmed and reproduced by other groups [8] [11]. However, the best high field values achieved in the SiC wires were compromised by the reduction in self-field and low-field. Although nanosize precursor particles were chosen for the doping process, it is difficult to achieve homogenous distribution of a small amount of nano-dopants within the matrix materials through solid state mixing. There are always agglomerates of nano-additives in the precursors. For various forms of C doping, the substitution of C for boron (B) can not be achieved at the same temperatures as that of the formation reaction due to their poor reactivity. Hydrocarbons, such as benzene, have been used for C substitution [20]. However, the C substitution level is limited by the high volatility of these compounds. In order to overcome these problems, some certain carbohydrates are proposed to be used as the dopants [21]. In this study, benzoic acid is used as a representative of aromatic acid dopant. The lattice parameters (a and c), critical temperature, critical current density, flux pinning force, and microstructures are presented in comparison with that of the un reference. II. EXPERIMENTAL superconductor bulks were prepared by an in-situ reaction process with the addition of benzoic acid. The selected amount of (99%), 10 wt% of total, was dissolved in toluene (, 99.5%). The solution was mixed with an appropriate amount of amorphous B powder (99%, Tangshan, China). This slurry was dried in vacuum so that the B powder particles were coated by the. Some of this uniform composite was then mixed with an appropriate amount of Mg (99%) powder. The other composite was preheated at 380 in argon gas for 30 min to decompose the into benzene and carbon dioxide, and then mixed with an appropriate amount of Mg (99%) powder. These mixed powders were ground in glove box, and pressed in closed tubes, and then sintered at 900 for 30 min under high purity argon gas. The heating rate was 5. All samples were characterized by X-ray diffraction (XRD) and field emission gun scanning electron microscopy (FEG- SEM). The crystal structure was refined with the aid of the program FullProf. was defined as the onset temperature at which diamagnetic properties were observed. The magnetization was /$ IEEE

4 LI et al.: BENZOIC ACID DOPING TO ENHANCE ELECTROMAGNETIC PROPERTIES OF SUPERCONDUCTORS 2779 TABLE I MEASURED DATA FOR UN-DOPED AND +C7H O SAMPLES WITH DIFFERENT PROCESSES measured at 5 K and 20 K using a Physical Property Measurement System (PPMS, Quantum Design) in a time-varying magnetic field with sweep rate 50 Oe/s and amplitude 8.5 T. Since there is a large sample size effect on the magnetic for [22], [23], all the samples for superconductivities measurement were made to the same size for comparison. The magnetic was derived from the width of the magnetization loop using Bean s model. versus magnetic field was measured up to 8.5 T. III. RESULTS AND DISCUSSION Table I shows the measured data for the un and samples with different sintering process. The lattice parameters calculated from XRD (shown in Fig. 1) show a large decrease in the a-axis parameter with 10 wt% inclusions, especially in the preheated sample, but slight change in the c-axis parameter. This is an indication of the C substitution for B in the lattices. The actual C substitution level can be estimated from the a-axis change [24]. It should be noted that the net C percentage addition is wt% of the addition. The actual C substitution levels of 3.5 at% to 5.0 at% of B at different process are clearly higher than those with other forms of C dopants, which is attributable to the high reactivity of fresh C released from the decomposition of at low temperature. For the preheating can decompose into and, more C atoms substitute for B in the closed sintering space. Accordingly, of preheated sample decreases about 4 K than the un for the intrinsic superconductivity has been damaged due to the C substitution. Table I also lists results of the AC susceptibility measurements as function of the temperature. With the increasing of C substitution from 3.5 at% to 5.0 at%, decreases from 35.5 K to 33.5 K. Fig. 2 shows the magnetic field dependence of in all samples at 5 K and 20 K. The high-field of the samples were much higher than that of the un. It should be noted that values in high field were increased by more than an order of magnitude. For example, the Jc value of at 5 K and 8 T for preheated 10 wt% sample is higher than that of the un by a factor of 13. Even though at 20 K and 5 T, the value of preheated 10 wt% sample is higher than that of the un by a factor of 6.5. In addition, there was no degradation in self-field for the preheated 10 wt% sample. Fig. 1. XRD curves of 10 wt% benzoic acid and pre-heated 10 wt% benzoic acid. Fig. 2. Magnetic field dependence of J of un and benzoic acid samples with different sintering process at 20 K and 5 K. Moreover, of preheated sample is slightly higher than that of the non-preheated one in the same applied measurement magnetic field. These findings can be further supported by the flux pinning results. Fig. 3 plots the field dependence of the volume pinning force,, of all samples at 20 K. The is normalized by the maximum volume pinning force,. The flux pinning for the samples was significantly higher than that of the un one at. This result indicates that the of samples was improved by the C substitution effect and nano-c inclusions within the grains. The reaction producing free carbon is more effective than the other kinds of mechanically mixed carbon dopants. Fig. 4 shows the SEM images for (a) un, (b) 10 wt%, and (c) preheated 10 wt%. The microstructure of un sample appears inhomogeneous, consisting of crystalline grains from several tens of nm in size to 500 nm [Fig. 4(a)]. Although

5 2780 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007 = 2 Fig. 3. Field dependence of the volume pinning force, F J B, of un and 10 wt% benzoic acid at 20 K. The F is normalized. by the maximum volume pinning force, F grains are mixture of plates and bars, the morphology of the sample was refined to smaller, 10 wt% denser, and more homogeneous grains compared to the unone. As to the preheated sample, however, grains appear to have a bars and larger plates shape, with their width up to 250 nm and length up to 600 nm, in a well connected grain network [Fig. 4(c)]. The SEM images suggest that the growing into a higher C substitution level induces the bar-shape grains microstructure to remedy the distortion of lattices. Although the distorted lattices result in the decrease, they can afford many defects in the superconductor as strong flux pinning centers to drag the flux movement. The grain doping is supported refinement effect by 10 wt% by the FWHM results derived from XRD patterns. The small grains are effective in enhancing flux pinning because act as the effective pinning the grain boundaries of centers, as in the case of A15 metallic superconductors. The fine microstructures improve the crystals linkage enhancing effective current path. Using organic solution, such as toluene, to dissolve the aromatic acid, such as benzoic acid, for manufacturing superconductors shows significant advantages in the raw materials mixing process. Aromatic acids can be dissolved in a suited volatilizable solvent so that the solution can form slurry with B powder. After evaporating the solvent the aromatic acid forms a coating on the B powder surfaces, giving a highly uniform mixture. The aromatic acids in the mixture melt at lower temperatures and decompose at temperatures below the formation temperature of, hence producing highly reactive and fresh C on the atomic scale, which can substitute B easily than the other kinds of C. For the freshly formed C elements are high reactivity, the C substitution for B can take place at the same. The simultemperature as the formation temperature of taneous dual reactions promote C substitution for B in the lattice and the inclusion of excess C within the grains, resulting in the enhancement of, and inevitably, in the depression of. Fig. 4. The microstructure photographs from field emission gun-scanning elec, (b) 10 wt% tron microscopy (FEG-SEM): (a) un, and (c) preheated 10 wt%. CHO CHO IV. CONCLUSION In summary, benzoic acid doping results in a depression in but significantly increases the C substitution level, reduces the performance agglomerate impurities, and hence improves at all the operating temperatures and especially in the high field range. Manufacturing nano- materials using the organic solution route solves the agglomeration problem, avoids the use of expensive nano-additives, and achieves improved performance properties for aromatic acids are soluble, cheap, abundant, and readily available. As aromatic acids consist of a large range of materials, this work will have significant implications for further improvement of the performance properties, as well as many other C-based compounds and of composites.

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