Superconducting Properties of adipic acid doped Bulk MgB2 ... - arXiv

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Arpita Vajpayee1,2, V. P. S. Awana1,*, G. L. Bhalla2, A.K. Nigam3 and H. ... due to its higher transition temperature (Tc), lower raw material cost and good upper.
Superconducting Properties of adipic acid doped Bulk MgB2 Superconductor Arpita Vajpayee1,2, V. P. S. Awana1,*, G. L. Bhalla2, A.K. Nigam3 and H. Kishan1 1

Superconductivity and Cryogenics Division, National Physical Laboratory, Dr K.S. Krishnan Road, New Delhi-110012, India 2

Deparment of Physics and Astrophysics, University of Delhi, New Delhi-110007, India

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Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India

Abstract We report the effect of adipic acid (C6H10O4) doping on lattice parameters, microstructure, critical temperature (Tc), current density (Jc), and irreversibility field (Hirr) for MgB2 superconductor. Actual carbon (C) substitution level for boron (B) is estimated to be from 0.40 at% to 2.95 at% for different doping levels. A reduction in Tc from 38.43 to 34.93 K and in lattice parameter ‘a’ from 3.084(3) Å to 3.075(6) Å is observed for the10 wt% C6H10O4 doped sample in comparison to pristine MgB2. This is an indication of C substitution at boron sites, with the C coming from the decomposition of C6H10O4 at the time of reaction. Interestingly the doped samples have resulted in significant enhancement of Jc and Hirr. All the doped samples exhibit the Jc value of the order of 104 A/cm2 at 5 K and 8 T, which is higher by an order of magnitude as compared to undoped sample. This result indicates that C6H10O4 is a promising material for MgB2 for obtaining the excellent Jc values under higher magnetic fields. Keywords: MgB2 Superconductor, Critical current density, Flux pinning *: Corresponding Author Dr. V.P.S. Awana National Physical Laboratory, New Delhi-110012, India Fax No. 0091-11-25626938: Phone no. 0091-11-25748709 e-mail: [email protected]; www.freewebs.com/vpsawana/

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Introduction Magnesium diboride (MgB2) is a potential competitor to Nb-based superconductor due to its higher transition temperature (Tc), lower raw material cost and good upper critical field Hc2 & critical current density Jc(H) values [1,2]. Further improvements in critical current density (Jc), upper critical field (Hc2) and irreversibility field (Hirr) is required for most of the practical applications because pure MgB2 exhibit rather poor flux pinning behavior. An effective way to improve flux pinning is to introduce foreign pinning centers in MgB2 because MgB2 has large coherence length and small anisotropy. Therefore, chemical doping emerges as a simple and readily scalable technique to pin the fluxoids and thus improving the superconducting performance of pristine MgB2 compound. Number of elements and compounds had been added previously to improve the superconducting performance parameters i.e., Jc, Hc2 and Hirr of MgB2 superconductor [313]. Among the various additives the C-containing compounds, such as SiC, C, B4C or carbon nanotubes (CNT), have been found quite effective dopants [14-17]. It is to be pointed out that the improvement of flux pinning depends on the size (nano) of particle doped in MgB2. However use of nano-particles brings dilemmas such as higher cost and their agglomeration limiting the homogeneity of their mixing with MgB2. For various forms of carbon doping, the substitution of boron cannot be achieved at the same temperatures as that of formation temperatures of MgB2 due to their poor reactivity. In order to overcome these problems, some carbohydrates are proposed to be used as the dopants [18]. In this article, adipic acid (C6H10O4) (aliphatic organic acid) is used as a dopant since it is a good candidate to be as a C source material for doping in MgB2. The melting point of adipic acid is 152oC i.e. it decomposes at temperature that is far below than the formation temperature of MgB2 phase. Hence it produces fresh and highly reactive carbon on atomic scale at the time of the formation of MgB2, which can substitute B easily than the other forms of C. The effect of adipic acid doping on lattice parameters (a and c), critical temperature (Tc), critical current density Jc(H), irreversibility field (Hirr) , flux pinning force (Fp) and microstructures of pristine MgB2 is presented here.

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Experimental Procedure Polycrystalline MgB2+(adipic acid)x; (x = 0, 1%, 3%, 5%, 7% & 10%) samples were synthesized by solid-state reaction route. The constituent powders (Magnesium and Boron) were well mixed in stoichiometric ratio through grinding for 1.5 hour. The desired amount of adipic acid powder was dissolved in 10 ml of Acetone. Then we added this solution in ground MgB2 raw powder, which was followed by second grinding in order to form the homogenous mixture. The mixture was pelletized using hydraulic press. The pellets were enclosed in soft iron tube and then annealed at 850oC in argon atmosphere for 2.5 hours. The heating rate was about 425oC per hour. After annealing, the system was allowed to cool down naturally. The optimization of synthesis parameters for the pure MgB2 and its complete physical property characterization are discussed in detail in ref. 19. The X-ray diffraction (XRD) pattern of compounds was taken using CuKα radiation. The resistivity ρ(T) measurements were carried out using four-probe technique. The temperature is measured with an accuracy of ± 0.1K. The scanning electron microscopy (SEM) studies were carried out on prepared samples using a Leo 440 (Oxford Microscopy: UK) instrument. The magnetization measurements were carried out using Quantum Design PPMS, equipped with VSM attachment.

Results and Discussion X-ray diffraction pattern of the doped and undoped samples is shown in Fig. 1. It is found that undoped MgB2 sample has well-developed hexagonal MgB2 phase with a little amount of MgO (marked with * in figure). The presence of small quantity of MgO along with main phase of MgB2 is consistent with earlier reports on similar samples [20,21]. Fitted and observed X-ray diffraction patterns of pure MgB2 are shown in ref.19. The intensity of MgO peak increases monotonically with increasing amount of adipic acid. The peak situated between 2θ = 33o and 2θ = 34o shifts towards the higher 2θ values with increasing x, indicating the contraction in a-axis in crystal lattice. The inset of Fig. 1 shows the shift of (100) peak confirming the decrease in ‘a’ parameter. The lattice parameters

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calculated from the XRD pattern show a large decrease in a-axis parameter but negligible change in c-axis parameter. As shown in Fig. 2(a), lattice parameter ‘a’ decreases from 3.084(3) Å for the undoped sample to 3.075(6) Å for the sample with the highest doping level. On the other hand, no appreciable change in lattice parameter ‘c’ has been observed [shown in Fig. 2(b)]. The decrease in ‘a’ parameter is an indication of the C substitution for B in MgB2 lattice [22,23]. The substituent C atoms are readily available from the C source material i.e. adipic acid. The actual C substitution level for our MgB2 + (C6H10O4)x samples can be estimated indirectly from the change in lattice parameters using formula x = 7.5 ∆(c/a) where ∆(c/a) is the change in c/a compared to pure sample [3,24]. The net C percentage addition is only 49.32% of the adipic acid (C6H10O4). Actual C doping level is calculated to be x ∼ 0.004 to 0.0295 in the composition of Mg(B1-xCx)2 as shown in Fig. 2(d) i.e. for the highest doping level of adipic acid the actual C substitution level is 2.95 at% of Boron. For this substitution level of C the decrease in lattice parameter ‘a’ is in agreement with reported literature [10,25,26]. Fig.2 (c) shows an increase in c/a value as we add the adipic acid in MgB2; which clearly indicates the presence of the lattice strain in doped samples. The lattice strain can be attributed to the C substitution in the structure and unreacted C inside the grains [25]. According to the calculation of single crystal by Lee et al. [27] the actual C substitution level comes out to be 0.0247 for the highest doping level of adipic acid, which is lower than that being calculated from Avdeev et al. [24]. This is discussed in Ref. 3 that according to Avdeev et al the actual carbon content comes out little larger because polycrystalline-carbon-substituted samples may possibly contain some amount of impurity phases with in x-ray resolution limit. Figure 3 shows the SEM images for (a) undoped and (b) 10 wt% C6H10O4 doped MgB2. The microstructure of undoped sample appears inhomogenous, consisting of crystalline grains of size from half of micron to one micron. This figure clearly shows the poor connectivity of the undoped sample in comparison to doped sample. In the 10 wt% C6H10O4 doped MgB2 sample the grains are mixture of platelets and bars and the grain morphology of this sample is refined to smaller and denser compared to undoped sample. The FWHM (full width at half maximum) values derived from the XRD pattern (shown in table I) also support the refinement of grains after doping. The small grains are effective in

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enhancing the flux pinning. In fact the grain boundaries of MgB2 may themselves act as the effective pinning centers [26]. Though the grain size decreases monotonically with an increase in effective C content, the superconducting performance is optimum for 5 wt% C6H10O4 doped MgB2 sample. This we will discuss later after elaborating upon various superconducting parameters of C6H10O4 doped MgB2 samples. Resistivity versus temperature curves for MgB2+(adipic acid)x; (x = 0, 5%, 7% & 10%) samples are shown in Fig. 4. The transition temperature (Tc) of pure sample is 38.43 K. The value of Tc for pristine sample is higher than those reported in ref. 10, 25 & 26. Transition temperature decreases continuously with the addition of adipic acid compared to the pristine sample. The Tc of 10 wt% C6H10O4 doped MgB2 is 34.93 K. The reduction of Tc is due to the increase in C substitution level as we increase the concentration of dopant. The resistivity at 40 K increases from 21 µΩ-cm for the pure MgB2 to 88 µΩ-cm for the 10 wt% C6H10O4 doped MgB2 compound. The resistivity value at 40 K for our pure MgB2 sample is quite lower than those reported in ref. 28, which is 39.7 µΩ-cm. The higher values of residual resistivity for doped samples indicate that the impurity scattering is stronger due to the carbon substitution at boron sites. The residual resistivity ratio (RRR = RT275K/RTonset) values for the pure, 5 wt% and 10 wt% C6H10O4 doped samples are 3.06, 1.89 and 1.69 respectively. The RRR of pure polycrystalline bulk MgB2 in ref. 27 is 3.0, which is close to the present value and in ref.10 the reported RRR value is 2.13, which is lower than our value. After doping in pristine sample the RRR comes down, for example RRR is only 1.5 for MgB2-xCx samples [29]. The RRR values decreases with increase in doping level (shown in table I). C substitution at B site (revealed by contraction in ‘a’ parameter and reduction in Tc) and the inclusion of unreacted C can enhance the electron scattering, which leads to the monotonic decrease in RRR values. Figure 5 depicts the dc susceptibility versus temperature χ(T) plots in an applied field of 100 Oe, in field-cooled (FC) situations. It is evident from the figure that pure MgB2 undergoes a sharp superconducting transition (diamagnetic, Tcdia) at 38.27 K within 1Ktemperature interval. All the samples exhibit one-step transition from normal state to superconducting state, however the transition width increases a bit by increasing the

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amount of dopant. The superconducting critical temperature (Tcdia) being seen from χ(T) measurements is in agreement with the Tc (ρ = 0). The magnetic hysteresis loop for all the doped samples MgB2+(adipic acid)x; (x = 0, 1%, 3%, 5%, 7% & 10%) are shown in Fig. 6(a) at T = 10 K. This figure clearly demonstrates that at T = 10 K the closing of M(H) loop for pure sample is at 7.6 Tesla, while the same is closed at 10.4 Tesla for 5 wt% C6H10O4 doped sample. This indicates that irreversibility field values are improved significantly with addition of adipic acid in parent compound. The variation of irreversibility field with respect to the adipic acid content is shown in Fig. 6(b). The 5 wt% C6H10O4 doped sample has the highest value of Hirr at all the temperatures. The 7 wt% C6H10O4 doped sample has also quite competitive values but slightly lower than as for the 5 wt%. The 10 wt% C6H10O4 doped sample has much lower value of Hirr. In fact it is lower than that of even the pristine sample. It is clear that the value of Hirr is highest for around 5-7 wt% C6H10O4 doping in MgB2. The dependence of critical current density (Jc) on the applied magnetic field at 5 and 15 K is shown in Fig. 7 for undoped and 3, 5 & 7 wt% doped samples. The Jc value for the undoped sample is 2.79 × 103 A/cm2 at 5 K and 8 Tesla whereas for the doped samples the Jc is enhanced to 104 A/cm2 at the same temperature and field. The highest Jc value is achieved for 5 wt% C6H10O4 doped sample i.e. 2.67 × 104 A/cm2; which is almost higher by an order of magnitude compared to undoped sample. If we compare this value with reported literature of organic acid doping then we found that this is better than those [10,26]. For example in ref. 10 highest Jc is found to be 1 × 104 A/cm2 at 5 K in 8 T field; our value is 2.67 times higher than this. Similarly, there is also an increment in Jc value by an order of magnitude at 15 K at higher fields for the 5 wt% C6H10O4 doped sample with respect to the undoped one; for example the Jc value for undoped sample is 1.6 ×103 A/cm2 at 6 T & 5 K and the same is increased to 1.56 × 104 A/cm2 for 5 wt% C6H10O4 doped sample. As far as the optimization of doping content is concerned, the same is found to be 5wt% addition of adipic acid, which amounts empirically to around 1.37% of C at B-site in MgB2. The enhancement of Jc is attributed to the lattice distortion resulting from the incorporation of C atoms into the MgB2 crystal lattice. Worth mentioning is the fact that besides the lattice distortion, the change of band structure and the changed scattering rates

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between the two bands are also responsible for the enhancement of critical fields and currents in MgB2 [7]. The introduction of disorder increases scattering of the charge carriers, which reduces their mean free path and improves the upper critical field. As far as the role of grain boundary pinning is concerned, we would like to mention that though the best values of superconducting parameters are achieved for 5 wt% C6H10O4 doped MgB2 , the grain size is least for 10 wt% C6H10O4 doped MgB2 sample. It seems that besides the grain boundary pinning effects, some more factors like inhomogeneity, agglomeration and lattice strain etc. also contribute in enhancing the flux pinning strength [25]. Seemingly, in present case, the combination of all the favorable factors is optimum for the 5 wt% C6H10O4 doped MgB2 sample. In fact the pinning is also expected to be weaker due to lower transition temperature [7] for higher C6H10O4 doped MgB2 samples. Overall a balanced mechanism is required for getting the best values of superconducting parameters. Grain size alone cannot explain the observed results yet [10,25,26]. In fact besides direct C doping at B-site in MgB2 [14-17], very recently the incorporation of C through various carbohydrates [10,25,26] has attracted a lot of attention. Broadly speaking, it seems that the effect of the most of the used carbohydrates on superconducting performance of MgB2 is more or less the same. These findings can be further supported by flux pinning results. Fig. 8 shows the dependence of reduced flux pinning force (Fp / Fp, max) of all the doped samples along with undoped one at 15 K. The relationship between flux pinning force and critical current density could be described by [30,31] Fp= µ0 Jc(H) H

(1)

Where µ0 is the magnetic permeability in vacuum. Flux pinning curves for the doped samples are shifted to the right as compared to pure MgB2. This indicates towards the significant improvement in flux pinning forces for adipic acid doped samples in comparison of undoped one i.e. all the doped samples have more pinning centers than the undoped one. Due to the substitution of C at B site the formation of nano-domain structure takes place due to the variation of Mg-B spacing. These nano-domains defects having the

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size of 2-3 nm can behave as effective pinning centers. Excess carbon, which can be embedded within the grain of MgB2 as nano-inclusions, can also serve as pinning centers. Conclusions In summary, we used adipic acid (C6H10O4) as the C source material for doping in polycrystalline MgB2. A systematic decrease in lattice parameter ‘a’ and transition temperature (Tc) is observed with increasing amount of dopant. Use of organic acid solution solves the problem of in-homogeneity. Very recently some organic compounds such as sugar [32], benzoic acid [26], mallic acid [25], and tartaric acid [10] have been used as active carbon source for intrinsic pinning in high performance MgB2 superconductor; our results are in confirmation with these reports and on an altogether new additive i.e., adipic acid. It appears to be an inexpensive method instead of using expensive nano-particles [3-6] for improving the superconducting performance of parent MgB2 compound. In addition, the C substitution takes place at the same temperature as the formation of MgB2. The simultaneous activities, substitution of C at B in lattice and the inclusion of excess C within the grains, result in the higher Jc and Hirr values for the doped samples. Further we conclude that the impact of various additive organics on superconducting performance of MgB2 superconductor is more or less the same, may it be sugar [32], benzoic acid [26], mallic acid [25], tartaric acid [10] or the presently studied adipic acid.

Acknowledgement The authors from NPL would like to thank Dr. Vikram Kumar (Director) for his continuous encouragement in present work. Mr. K. N. Sood from SEM Division of NPL is acknowledged for providing us with the SEM micrographs. Arpita Vajpayee would like to thank the CSIR for the award of Junior Research Fellowship to pursue the Ph. D. degree.

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11. Berenov A, Serquis A, Liao X Z, Zhu Y T, Peterson D E, Bugoslavsky Y, Yates K A, Blamire M G, Cohen L F and MacManus-Driscoll J L 2004 Supercond. Sci. & Tech. 17 1093 12. Chen S K, Wei M and MacManus-Driscoll J L 2006 Appl. Phys. Lett. 88 192512 13. Lezza P, Senatore C and Flukiger R 2006 Supercond. Sci. & Tech. 19 1030 14. Dou S X, Soltanian S, Horvat J, Wang X L, Zhou S H, Ionescu M, Liu H K, Munroe P and Tomsic M 2002 Appl. Phys. Lett. 81 3419 15. Yamamoto A, Shimoyama J I, Ueda S, Iwayama I, Horii S and Kishio K Supercond. Sci. & Tech. 18 1323 16. Ribeiro R A, Bud’ko S L, Petrovic C and Camfield P C 2003 Physica C 384 227 17. Kim J H, Yeoh W K, Qin M J, Xu X and Dou S X 2006 J. Appl. Phys. 100 013908 18. Kim J H, Zhou S, Hossain M S A, Pan A V and Dou S X 2006 Appl. Phys. Lett. 89 142505 19. Awana V P S, Vajpayee A, Mudgel M, Kishan H, Lalla N P, Ganesan V, Awasthi A M and Bhalla G L 2008 Euro. Phys. J. B. 62 281 20. Yan S C, Yan G, Lu Y F and Zhou L 2007 Supercond. Sci. and Tech. 20 549 21. Dou S X, Pan A V, Zhou S, Ionescu M, Wang X L, Horvat J, Liu H K and Munroe P R 2003 J. Appl. Phys. 94 1850

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22. Kazakov S M, Puzniak R, Rogacki K, Mironov A V, Zhigadlo N D, Jun J, Soltmann C, Batlogg B and Karpinski J 2005 Phy. Rev. B 71 024533 23. Senkowicz B J, Giencke J E, Patnaik S, Eom C B, Hellstrom E E and Larbalestier D C 2005 Appl. Phys. Lett. 86 202502 24. Avdeev M, Jorgensen J D, Ribeiro R A, Budko S L and Canfield P C 2003 Physica C 387 301 25. Kim J H, Dou S X, Hossain M S A, Xu X, Wang J L, Shi D Q, Nakane T and Kumakura H 2007 Supercond. Sci. & Tech. 20 715 26. Li W X, Li Y, Zhu M Y, Chen R H, Xu X, Yeoh W K, Kim J H and Dou S X 2007 IEEE Trans. of Appl. Supercond. 17 2778 27. Lee S, Masui T, Yamamoto A, Uchitama H and Takama S 2003 Physica C 397 7 28. Putti M, Galleani d'Agliano E, Marre D, Napoli F, Tassisto M, Manfrinetti P and Palenzona A 2002 Euro Phys. J. B 25 439 29. Bharathi A, Balaselvi S J, Kalavathi S, Reddy G L N, Sastry V S, Hariharan Y and Radhakrishnan T S 2002 Physica C 370 211 30. Martinez E, Mikheenko P, Martinez-lopez M, Millan A, Bevan A and Abell J S 2007 Phys. Rev. B 75 134515 31. Shen T M, Li G, Cheng C H and Zhao Y 2006 Supercond. Sci. and Tech. 19 1219 32. Shcherbakova O V, Pan A V, Wang J L, Shcherbakov A V, Dou S X, Wexler D, Babic E, Jercinovic M and Husnjak O 2008 Supercond. Sci. and Tech. 21 015005

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Figure & Table captions Table I. Critical temperature (Tc), residual resistivity ratio (RRR), actual carbon substitution level and FWHM for the undoped and adipic acid doped MgB2 samples Figure 1. X-ray diffraction pattern of pure and 1,3,5,7 & 10 wt% adipic acid doped samples Figure 2. (a) a-axis lattice parameter, (b) c-axis lattice parameter, (c) c/a values and (d) actual amount of C substitution of MgB2+(adipic acid)x (x = 0, 1%, 3%, 5%, 7% & 10%) samples Figure 3. SEM pictures of (a) Pure MgB2 & (b)10 wt% adipic acid added sample Figure 4. Variation of resistivity with temperature ρ(T) plots for Pure, 5, 7 and 10 wt% adipic acid added samples Figure 5. Magnetization versus temperature plot of MgB2+(adipic acid)x; (x = 0, 1%, 3%, 5%, 7% & 10%) samples Figure 6. (a) Magnetization loop M(H) for MgB2+(adipic acid)x; (x = 0, 1%, 3%, 5%, 7% & 10%) samples up to 13 Tesla field at 10 K and (b) Variation of irreversibility field Hirr with respect to adipic acid concentration at 5 K, 10 K & 20 K Figure 7. Jc(H) plots for adipic acid doped samples along with pristine MgB2 at (a) 15 K & (b) 5 K Figure 8. Variation of reduced flux pinning force (Fp/Fp,max) with magnetic field for MgB2+(adipic acid)x; (x = 0, 1%, 3%, 5%, 7% & 10%) at 15 K

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Table I.

C6H10O4 (wt%)

Tc (K)

ρ40K

(ρ→0) (µΩ-cm)

RRR values

Actual C substitution (x) in MgB2-xCx

FWHM of

FWHM of

(101) (deg)

(100) (deg)

0

38.43

21.22

3.06

0

0.385

0.308

3

36.95

107.98

2.06

0.0108

0.492

0.372

5

36.62

51.69

1.89

0.0137

0.510

0.384

7

35.88

38.57

1.76

0.0176

0.457

0.342

10

34.93

88.09

1.69

0.0295

0.514

0.373

13

# Mg * MgO

1 - 10 wt% adipic acid 2 - 7 wt% adipic acid 3 - 5 wt% adipic acid 4 - 3 wt% adipic acid 5 - 1 wt% adipic acid 6 - Pure MgB2

(100)

(101)

Figure 1.

*

#

(201)

(200)

(110)

(002)

2 (102) (111)

1

(001)

Intensity (a.u.)

(100)

1

3

2

4 3

5

4 5

6

6

20

30

40

50

60

70

33.0 33.2 33.4 33.6 33.8 34.0 2θ ( deg.) 80

2θ (deg.)

14

Figure 2.

3.085 o

a-axis (A )

(a) 3.080

3.075

o

c-axis (A )

3.527

(b)

3.526 3.525 3.524

c/a

1.1460

(c)

1.1445

x in Mg(B1-xCx)2

1.1430 0.03

(d )

0.02 0.01 0.00

0

2

4

6

8

10

Amount of adipic acid (wt%))

15

Figure 3.

(a)

(b)

16

Figure 4.

150µ

Pure MgB2 5 wt% adipic acid 7 wt% adipic acid 10 wt% adipic acid

ρ (Ω-cm)

100µ

50µ

0 50

100

150

200

250

300

T (K)

17

Figure 5.

0.000 Pure MgB2 1 wt% adipic acid 3 wt% adipic acid 5 wt% adipic acid 7 wt% adipic acid 10 wt% adipic acid

M (emu/g)

-0.002

H = 100 Oe

-0.004

-0.006

-0.008

-0.010

10

20

30

40

50

T (K)

18

Figure 6 (a).

50 Pure MgB 2 1 w t% adipic acid 3 w t% adipic acid 5 w t% adipic acid 7 w t% adipic acid 10 wt% adipic acid

M (emu/g)

25

0 T = 10 K

-25

-50

4

6

8

10

12

H (T)

19

Figure 6 (b).

12

11

Hirr (Tesla)

10

9

8

5K 10K 15K

7

6 0

2

4

6

8

10

wt % of adipic acid

20

Figure 7

6

10

15K

5K Pure MgB2 3 wt% adpic acid 5 wt% adpic acid 7 wt% adpic acid

5

2

Jc (A/cm )

10

4

10

3

10

4

6

8

10

H (T)

21

Figure 8.

Pure MgB 2 1 w t% adipic acid 3 w t% adipic acid 5 w t% adipic acid 7 w t% adipic acid 10 w t% adipic acid

1.0

0.8

Fp / Fp,max

T = 15 K 0.6

0.4

0.2

0.0

0

2

4

6

8

H (Tesla)

22