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Effects of ambient and humidity exposure on the MgB2 superconductor
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 516–520
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of ambient and humidity exposure on the MgB2 superconductor Suchitra Rajput , Sujeet Chaudhary 1 Department of Physics, Indian Institute of Technology Delhi, New Delhi 110 016, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 February 2008 Received in revised form 10 April 2008 Accepted 23 June 2008
Exposure effect of ambient (up to a period of 465 days) and humid atmosphere on stability and the physical properties of bulk samples of MgB2 has been carried out based on ac-susceptibility and X-ray diffraction measurements performed at different times during the aging. The motivation of this study came from the device application potential of MgB2 superconductor. It has been observed that on initial exposure to ambient for a period of about 2 months, the MgB2 samples neither show any sign of phase decomposition nor any significant degradation of superconducting parameters. However, on further exposure to ambient for a period greater than 4 months duration, the samples progressively exhibited the partial conversion of MgB2 phase into Mg-deficient phases, viz., MgB6 and MgB12 and also leading to formation of Mg(OH)2 which is accompanied by decrease in the TC and poor connectivity of the superconducting grains. When exposed to humid environment for a period of 30 days, a significant degradation was observed in the superconducting properties of MgB2. In sharp contrast to the case of ambient exposure, the exposure of MgB2 to humid atmosphere (for 30 days) did not result in any noticeable phase decomposition. A comparison of the magnetically inferred critical current density behavior before and after the humidity exposure of MgB2 is also presented. & 2008 Published by Elsevier Ltd.
Keywords: A. Superconductors C. X-ray diffraction
1. Introduction For device application of a superconducting material, the stability of the superconducting phase in the presence of various environments is an issue of critical concern. Thus, the study of the ageing effects on MgB2 is vital in knowing the degree of usefulness of this material. So far, only few attempts have been made to study the effect of ambient atmosphere and the water exposure on MgB2 samples [1,2]. Zhou et al. have investigated MgB2 wires for their ageing effects for 4 months and found the drop in critical current with time. They suggested that the observed drop could be because of oxidation of extra magnesium present in the wire samples [2]. Aswal et al. [1] have also carried out detailed study on the degradation of bulk MgB2 sample after immersing the MgB2 sample in de-ionized water for half an hour and reported the presence of Mg(OH)2, MgCO3 and elemental magnesium on the surface of the degraded sample of MgB2. A detailed study on the correlation of degradation of superconducting properties and/ or phase dissociation of MgB2 samples on exposure to various environments for the wide span of time is therefore desired.
Corresponding author. Tel.: +911 585 244 1066; fax: +9111 26581114.
E-mail addresses: [email protected], [email protected] (S. Rajput), [email protected] (S. Chaudhary). 1 Tel: 9111 26581341; fax: 9111 26581114. 0022-3697/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2008.06.114
In this paper, we discuss the results of the ageing investigations performed under different exposure (ambient and humid) conditions on various polycrystalline MgB2 samples. The resulting changes and evolution of different phases have been investigated in the MgB2 sample exposed to ambient environment over a span of more than a year time so as to find out the stability of the samples and the possible degradation mechanism in the bulk MgB2 samples.
2. Experimental details The polycrystalline MgB2 samples, exhibiting TC in the range of 39–41 K, employed for the degradation study were prepared both by modified solid-state reaction method and sintering under vacuum-sealed conditions [3,4]. Table 1 summarizes the synthesis parameters and exposure conditions of the sample with their increasing age. Sample ‘A’ and ‘B’ have been synthesized by heat treatment at 560 and 820 1C, respectively, for 1.0 h in flowing ‘Ar’ atmosphere whereas sample ‘C’ was made via heat treatment at 700 1C for 1.0 h after placing the pellet sample inside a Ta enclosure followed by its sealing in an evacuated (105 Torr) quartz ampoule. These two sets of MgB2 samples would henceforth be referred to as Set-I (sample A and sample B) and Set-II (sample C), respectively. It may be noted that to compensate for the inevitable loss of the elemental Mg, all these samples were synthesized with excess magnesium composition of the starting
ARTICLE IN PRESS S. Rajput, S. Chaudhary / Journal of Physics and Chemistry of Solids 70 (2009) 516–520
517
Table 1 Details of the MgB2 samples and ageing conditions (environmental condition and exposure time). Set-I
Set-II
Sample A TS ¼ 560 1C; tr ¼ 1 h
Sample B TS ¼ 820 1C; tr ¼ 1 h
Sample C TS ¼ 700 1C; tr ¼ 1 h
Exposure duration Environment condition (days)
Exposure duration Environment condition (days)
Exposure duration Environment condition (days)
0 (As synthesized) In flowing Ar 51 In desiccator (with silica gel) (RH ¼ 20–25%; 25–30 1C) 69 In air (RH ¼ 40–80%; 25–40 1C)
0 (As synthesized) In flowing Ar 103 In desiccator (with silica gel) (RH ¼ 20–25% ; 25–30 1C) 465 In air (RH ¼ 40–80%; 25–40 1C)
0 (As synthesized) In vacuum 30 In humidity cell (RH ¼ 83.4%; 25 1C)
Here, TS is the synthesis temperature, RH is the relative humidity and tr is the time for which sample was kept at TS.
3. Result and discussion
(b) After 69 days
(101) 30
40
50
60
70
80
(112)
(201) MgO
(110) MgO (102)
(002)
Mg B 20
After synthesis
(111)
(a)
(100)
Intensity (a.u.)
Sample A
90
2θ (deg) Fig. 1. X-ray diffraction patterns of the MgB2 sample ‘A’ recorded immediately after its synthesis (lower panel) and after the age of 69 days (upper panel).
0 days
Sample A χ'' (a.u.)
2 days 69 days
χ' (a.u.)
powder, i.e., Mg:B ¼ 2:2 [5]. The first set of samples (set I) was exposed to the ambient atmosphere whereas, the second set of samples synthesized via vacuum-sealed process was exposed to the humid conditions for a period of 30 days. Samples ‘A’ and ‘B’ were first kept inside the desiccator having adequate quantity of silica gel which was followed by their exposure to the air for observing the effect of the ambient. For studying the humidity effects on the MgB2, a standard humidity cell was made by saturated solution of KCl possessing 83.4% of humidity at 25 1C. It is well known that when a solid phase of a hygroscopic salt in contact with the saturated solution in water of a same solid specie (viz., KCl), is enclosed in a chamber/container, it maintains the static equilibrium of humidity, provided the temperature remains constant. This method of making humidity cell is advantageous compared to the other static method (viz., sulfuric acid method) as any evaporation of water from the solution is balanced by the precipitation of the solid phase and the amount of the contaminant in the vicinity of the solution is very low [6,7]. The relative humidity maintained above the saturated solution of KCl at different temperatures is found from the literature [8,9]. These MgB2 samples exposed to various environments were subsequently investigated for the variation in superconducting properties with age and the possible phase dissociation. We preferred temperature dependent ac-susceptibility measurements over the resistivity measurements since in the later, the superconducting transition is often dictated by the percolation-driven sharp drop in resistivity. This is in sharp contrast to the change in ac-susceptibility (both in real as well as the imaginary part) at the superconducting transition which is contributed by the whole sample volume through the resistance-less shielding currents. As the degradation mechanism is initiated from the surface, the samples are likely to be inhomogeneous. Therefore, the acsusceptibility technique is expected to provide true picture of the degradation in superconducting behavior of the samples subsequent to their exposure. The critical current density (JC) of the samples has been calculated using modified Bean’s critical state model [10–12].
3.1. Set-I: ambient exposure effects Fig. 1 shows the X-ray diffractograms of sample ‘A’ taken immediately after its synthesis and after 69 days. Sample ‘A’, synthesized at low synthesis temperature and having unreacted magnesium (Fig. 1a) did not show any detectable evidence of formation of new phase. Understandably, no noticeable change was observed in relative fractions of the two primary phases present in the initial sample (i.e., superconducting MgB2 and insulating MgO). In Fig. 2, the variation of real and imaginary parts
38
39
40 T (K)
41
42
Fig. 2. Variation of imaginary and real part of the ac-susceptibility, i.e., w00ac ðTÞ and w0ac ðTÞ, respectively, with temperature for the MgB2 sample ‘A’, recorded at different indicated stages during the ageing under the ambient environment.
ARTICLE IN PRESS
2 days
Sample B χ'' (a.u.)
66 days
χ' (a.u.)
135 days
MgB6 MgB6
Mg(OH)2 MgO
MgO
(101) 30
40
50 60 2θ (deg)
70
(112)
(201)
(200)
(102) (111)
(110)
As-Synthesis (002)
(a)
MgO
Mg
MgO
Age 10 days
(b)
20
MgB6
MgB6
Mg(OH)2
Mg
MgB12 Mg
Age 132 days MgB12 Mg
Mg
(c)
Mg Mg(OH)2
Mg
(d)
Age 465 days
Sample B
(100)
of ac-susceptibility with temperature, as recorded at different indicated durations of exposure of MgB2 sample, is shown. It can be seen that after an exposure for 70 days, while the onset of superconducting transition appeared to be at relatively lower temperature (c.f., the w0ac ðTÞ-plots, see Fig. 2), the temperature at which w00ac ðTÞ exhibits a peak did not show any perceptible shift. Since, this peak temperature is indicative of the average TC of the whole sample, we conclude that the MgB2 sample was more or less remain intact until the studied ambient exposure span of 70 days. This inference is consistent with the X-ray diffraction findings as discussed above. To find the effect of ambient exposure over a wide span of time, an MgB2 pellet, i.e., sample ‘B’, possessing a higher transition temperature (above 40 K) was investigated for more than a year, i.e., for 465 days. Fig. 3 shows the w0ac ðTÞ and w00ac ðTÞ plots recorded for this sample after different span of ambient exposure. Consistent with the result of sample ‘A’, initial degradation after 65–70 days of aging in sample ‘B’ was more or less very negligible. That is, there is no significant change in the transition till the age of 66 days when the sample was kept inside the desiccator with silica-gel (Fig. 3). When the sample was left to the ambient, i.e., in air it exhibited two peaks in w00ac (T)-plots (Fig. 3). This clearly indicates that substantial degradation took place in the sample on prolonged ambient exposure. Such a two loss peak behavior in w00ac ðTÞ is similar to inter-granular and intra-granular shielding contributions, typical of HTSCs such as YBCO and BSCCO. In case of MgB2, this behavior is relatively less observed owing to relatively large coherence lengths. However, their observation in the present case of degradation studies are indicative of appearance of finite weak link effects owing to degradation of the surface of many MgB2 grains inside sample ‘B’. To infer if any change or phase-dissociation could have occurred during the exposure, we showed in Fig. 4 various X-ray diffractograms of sample ‘B’ recorded at the age of 0, 10, 132 and 465 days. It may be recalled here that the sample ‘B’ in the assynthesized form consisted of the prominent MgB2 phase (Fig. 4a) along with the MgO phase. Unlike sample ‘A’, no trace of Mg was present in the pristine sample ‘B’. It should also be noted that in agreement with Eyidi et al. [13] due to weak scattering from ‘B’-rich compounds in X-ray diffraction, we cannot rule out the possibility of the presence of boron and boron oxide in trace amount in sample ‘B’. If we carefully look at the diffractograms
Mg Mg(OH)2
S. Rajput, S. Chaudhary / Journal of Physics and Chemistry of Solids 70 (2009) 516–520
Intensity (a.u.)
518
80
90
Fig. 4. X-ray diffractograms of MgB2 sample ‘B’ recorded immediately after synthesis and at the age of 10, 132 and 465 days during their exposure to ambient air environment.
(Fig. 4) taken at the age of 0, 10, 132 and 465 days, it is clear that as the age of the sample increases, it resulted in the formation of elemental magnesium. The other important observation is that strength of the main MgO peak relative to the main MgB2 peak also decreased. Moreover, other magnesium-deficient compound, viz., MgB6 and MgB12 also appeared along with the elemental magnesium. Apart from all these phases, diffraction peak corresponding to the Mg(OH)2 can also be seen from the age of 132 days onwards. The observation of all these phases is indicative of the dissociation of MgB2 phase to the magnesium-deficient phases. Here, it should be mentioned that the earlier studies on the degradation of MgB2 did not report the observation of magnesium-deficient phases [1,2], although Aswal et al. [1] have found some amount of Mg(OH)2 in the degraded samples. They have suggested it to be due to the reaction between MgB2 and H2O leading to the formation of Mg(OH)2, B and H2, which on further reaction with CO2 formed MgCO3 along with the oxide of boron. However, in the present case the absence of MgCO3 and oxide of boron indicate that different degradation mechanism has taken place in these MgB2 samples. The presence of the magnesiumdeficient phases such as MgB6, MgB12 suggest the following dissociation mechanism for the degraded MgB2 sample in the ambient atmosphere: ambient
MgB2 ! MgB6 þ MgB12 H2 O
þ Mg!MgB6 þ MgB12 þ MgðOHÞ2 þ Mg þ H2 m
(1)
The reduction in the intensity of MgO peak (Fig. 4) can be possibly accounted for in terms of its reaction with the water content present in the atmosphere:
38
39
40 T (K)
41
42
Fig. 3. Variation in the imaginary and real part of the ac-susceptibility with temperature showing the shift in the transition temperature in sample ‘B’ during the ageing under ambient.
H2 O
MgO!MgðOHÞ2 þ H2 m
(2)
Thus, the exposure of MgB2 samples for longer period of time results in the formation of Mg-deficient Mg–B-phases, hydroxide of magnesium and elemental magnesium. The observed decrease
ARTICLE IN PRESS S. Rajput, S. Chaudhary / Journal of Physics and Chemistry of Solids 70 (2009) 516–520
107
Jc (A/m2)
106 Before After 105
38
χ'' (a.u.) χ' (a.u.)
Before After
36
37
38
39
40
41
T (K) Fig. 5. Variation in the imaginary and real part of the ac-susceptibility with temperature showing the shift in the transition temperature for MgB2 sample ‘C’ before and after keeping it in the humidity cell for a period of 30 days.
40
41
T (K)
20
30
40
(112)
(201) MgO
(102)
(110) MgO
(002)
After humidity effect
50 60 2θ (deg)
70
80
(112)
(201) MgO
(102)
(110) MgO
After synthesis (002)
(100)
Intensity (a.u.)
(100)
(101)
Fig. 6. Temperature dependence of the critical current density of the MgB2 sample ‘C’ before and after keeping it in the humidity cell for a period of 30 days.
3.2. Set II: humid exposure effects Sample ‘C’ was kept in the humidity cell for a period of 30 days to study the possible degradation effects in response to the humid environment. Fig. 5 shows the variation of ac-susceptibility with temperature for sample ‘C’ recorded immediately after its synthesis and after keeping it in the 83.7% humid environment for 30 days. It is clearly visible from Fig. 5 that after the humidity attack on the sample ‘C’, its transition temperature lowered significantly. However, we did not observe any two-peak feature in its w00ac ðTÞ behavior subsequent to humid exposure. To have more insight on the effect of the possible appearance of additional phases, we recorded w00ac (T)-plots at various applied ac fields for this sample ‘C’, and obtained the critical current density JC(T) behavior of the samples before and after the humid exposure. Fig. 6 presents the critical current density JC(T) behavior of sample ‘C’ before and after keeping it in the humidity cell. From the JC(T) behavior in the two cases (Fig. 6), one can see that no considerable change in JC and hence pinning effects in the vortices is observed at low temperatures. The shift of JC(T) drop occurring at the
39
(101)
in TC (the 2nd peak in w00ac ðTÞ and the presence of another peak at lower T in w00ac ðTÞ having higher magnitude) can be, respectively, accounted for compositional fluctuation within the grain and some of the phase dissociation of MgB2 at the inter-granular regions. Thus the ageing in these granular MgB2 samples results in some enrichment of the magnesium and magnesium-deficient phases predominantly near the grain surface in the degraded sample. Here it should also be mentioned that the microstructure and porosity in sample synthesized with magnesium-rich composition and in flowing Ar has already been presented in the earlier studies [14]. The observed changes in TC and phases suggest that MgB2 superconductor is also sensitive to ambient conditions and bear similarity with HTSC’s (BSCCO and YBCO), though the degradation is sluggish as compared to YBCO, in particular. Earlier studies on YBCO and Bi-2223 also predicted the ageing effects in these superconductors. There is formation of non-superconducting Y2BaCuO5 phase along with CuO and Ba(OH)2 on exposing YBCO to the humid atmosphere [15]. Similarly, a sample having mixed phase of Bi-2223 and 2212 was reported to decompose into the mixture of Bi2CuO4, CuO and SrCO3 with the small fraction of CaCO3 whereas, single-phasic Bi2223 was found to be stable with respect to the exposure to the humid conditions [16].
519
90
Fig. 7. X-ray diffractograms of the MgB2 sample ‘C’ recorded before and after exposing it to 83.7% humid environment for 30 days. Apart from broadening of peaks corresponding to MgO an unidentified peak was noticed after the humidity attack on sample ‘C’.
superconducting transition to lower temperature is consistent with the change in wac ðT Þ behavior (Fig. 5) after ageing, possibly related with the composition fluctuation effects within the superconducting grains. Here it should also be mentioned that Zhou et al. [2] studied the ageing of MgB2 wires and they reported the 20% decrease in the critical current in the MgB2 wires after an ageing for 4 months. They associated it to be due to the oxidation of the extra magnesium present in the wire sample. In Fig. 7, we show the diffractograms of the sample ‘C’ recorded before and after keeping it in the humidity cell. The broadening of the peak corresponding to MgO phase when the sample was attacked by the humid environment along with the appearance of one unknown impurity peak was noticed. These two observations might have resulted due to the composition fluctuation in the sample as an evidence by the degradation in TC on humidity exposure (Fig. 5). It should also be noticed that, in contrast to ambient exposure effects shown by sample ‘B’, no sign of phase dissociation of MgB2 phase was observed on exposing the sample ‘C’ to humid environment for a period of 30 days. For knowing the surface degradation effects, we recorded SEM micrographs (at 6000 magnification, Fig. 8(a) and (b)) for sample ‘C’ before and after the humidity attack. The SEM images of the unfractured surface of sample ‘C’ clearly indicates the degradation of the
ARTICLE IN PRESS 520
S. Rajput, S. Chaudhary / Journal of Physics and Chemistry of Solids 70 (2009) 516–520
Fig. 8. The SEM images of surface of MgB2 sample ‘C’ showing the degradation of the surface of the sample due to humid environment: (a) before and (b) after exposing the sample to the 83.7% humid atmosphere. The bar represents the 5 mm length.
sample surfaces after the humidity attack. The effect of humidity exposure is clearly evident from the absence of the uniform/ homogenous distribution of granular structure seen prior to humidity exposure. The porosity of sample ‘C’ has been discussed in a separate study [17]. This could possibly result from the amorphous nature of the degraded product, which understandably gives inferior contrast in SEM micrograph.
4. Conclusions On the basis of degradation investigations presented here for the MgB2 superconductor exposed to ambient as well as humid atmosphere, it can be inferred that the MgB2 system is sensitive to environmental conditions. In ambient conditions up to 70 days, the degradation in the MgB2 superconductor with regards to the phase-dissociation and/or lowering of TC was found to be negligible. However, exposure for prolonged duration reveled that MgB2 samples are more prone to degradation when exposed to ambient for longer durations. A model has been proposed wherein the MgB2 partially transform to MgB6 and MgB12 and also leading to the formation of Mg(OH)2 and Mg when exposed in ambient condition for more than 70 days. When exposed to humid atmosphere, MgB2 samples showed severe degradation of the surface and also of the superconducting parameters, however, without leading to magnesium-deficient phases as observed on ambient exposure. The extent of degradation can be definitely reduced if samples are sheathed or kept in vacuum environment.
Acknowledgment The authors would like to thank Mr. D. C. Sharma for his help in SEM studies.
References [1] D.K. Aswal, A.K. Muthe, A. Singh, S. Sem, K. Shah, L.C. Gupta, S.K. Gupta, V.C. Sahani, Degradation behaviour of MgB2 superconsuctor, Physica C 363 (2001) 208–214. [2] S. Zhou, A.V. Pan, H. Liu, S. Dou, Single- and multi-filamentary Fe-sheathed MgB2 wires, Physica C 382 (2002) 349–354. [3] S. Rajput, S. Chaudhary, S.C. Kashyap, P. Srivastava, On the study of phase formation and critical current density in superconducting MgB2, Bull. Mater. Sci. 29 (2006) 207–211. [4] S. Rajput, S. Chaudhary, S.C. Kashyap, On the synthesis of MgB2 from magnesium rich powders, Indian J. Pure Appl. Phys. 44 (2006) 461–467. [5] S. Rajput, Ph.D. Thesis, Synthesis and characterization of polycrystalline bulk and tapes of MgB2 superconductors, IIT Delhi, India, 2007 (Chapters 3–4). [6] R.L. Opila, C.J. Weschler, R. Schubert, Acidic vapors above saturated salt solutions commonly used for control of humidity, IEEE Trans. Components Hybrid Manuf. Technol. 12 (1989) 114–120. [7] R. Schubert, H.G. Tompkin, Effects of exposure to vapors of saturated salt aqueous solutions, IEEE Trans. Components Hybrid Manuf. Technol. 5 (1982) 425–430. [8] R.C. West, M.J. Astle, W.H. Beyer, CRC Handbook of Chemistry and Physics, 98th ed, CRC Press Inc., Boca Raton, FL, USA, 1984 (p. E-42). [9] J.A. Dean, N.A. Lange (Eds.), Lange’s Handbook of Chemistry, 13th ed, McGrawHill Book Company, New York, NY, 1985. [10] C.P. Beans, Magnetization of high-field superconductor, Rev. Mod. Phys. 36 (1964) 31–39. [11] D.-X. Chen, J. Nogues, K.V. Rao, ac-Susceptibility and intergranular critical current density of high-Tc superconductors, Cryogenics 29 (1989) 800. [12] J.R. Clem, Hysteretic ac losses and susceptibility of thin superconducting disks, Phys. Rev. B 50 (1994) 9355. [13] D. Eyidi, O. Eibl, T. Wenzel, K.G. Nickel, M. Giovannini, A. Saccone, Phase analysis of superconducting polycrystalline MgB2, Micron 34 (2003) 85. [14] S. Rajput, S. Chaudhary, D.K. Pandya, S.C. Kashyap, Fabrication & characterization of polycrystalline samples tape and of superconducting MgB2— a future prospect for an electromagnet, in: Lim Hock, Serguei Matitsine, Gan Yeow Beng (Eds.), Proceedings of the ICMAT-2005 Conference on Electromagnetic Materials, World Scientific Publishing Co. Pt. Ltd., Singpore, 2005, p. 253. [15] B. Gogia, R.D. Tarey, D.K. Pandya, S.C. Kashyap, K.L. Chopra, Degradation mechanism of YBa2Cu3O7k superconductors under humid conditions, Rev. Sol. Stat. Sci. 2 (1988) 367–371. [16] B. Gogia, S.C. Kashyap, D.K. Pandya, K.L. Chopra, Effect of humid atmosphere on the stability of superconducting phases of BSCCO system, Physica C 185 (1991) 521–522. [17] S. Rajput, S. Chaudhary, On the Mg out-diffusion and superconductivity in vacuum synthesized MgB2 samples, J. Phys. Chem. Sol., under review.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of ambient and humidity exposure on the MgB2 superconductor Suchitra Rajput , Sujeet Chaudhary 1 Department of Physics, Indian Institute of Technology Delhi, New Delhi 110 016, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 February 2008 Received in revised form 10 April 2008 Accepted 23 June 2008
Exposure effect of ambient (up to a period of 465 days) and humid atmosphere on stability and the physical properties of bulk samples of MgB2 has been carried out based on ac-susceptibility and X-ray diffraction measurements performed at different times during the aging. The motivation of this study came from the device application potential of MgB2 superconductor. It has been observed that on initial exposure to ambient for a period of about 2 months, the MgB2 samples neither show any sign of phase decomposition nor any significant degradation of superconducting parameters. However, on further exposure to ambient for a period greater than 4 months duration, the samples progressively exhibited the partial conversion of MgB2 phase into Mg-deficient phases, viz., MgB6 and MgB12 and also leading to formation of Mg(OH)2 which is accompanied by decrease in the TC and poor connectivity of the superconducting grains. When exposed to humid environment for a period of 30 days, a significant degradation was observed in the superconducting properties of MgB2. In sharp contrast to the case of ambient exposure, the exposure of MgB2 to humid atmosphere (for 30 days) did not result in any noticeable phase decomposition. A comparison of the magnetically inferred critical current density behavior before and after the humidity exposure of MgB2 is also presented. & 2008 Published by Elsevier Ltd.
Keywords: A. Superconductors C. X-ray diffraction
1. Introduction For device application of a superconducting material, the stability of the superconducting phase in the presence of various environments is an issue of critical concern. Thus, the study of the ageing effects on MgB2 is vital in knowing the degree of usefulness of this material. So far, only few attempts have been made to study the effect of ambient atmosphere and the water exposure on MgB2 samples [1,2]. Zhou et al. have investigated MgB2 wires for their ageing effects for 4 months and found the drop in critical current with time. They suggested that the observed drop could be because of oxidation of extra magnesium present in the wire samples [2]. Aswal et al. [1] have also carried out detailed study on the degradation of bulk MgB2 sample after immersing the MgB2 sample in de-ionized water for half an hour and reported the presence of Mg(OH)2, MgCO3 and elemental magnesium on the surface of the degraded sample of MgB2. A detailed study on the correlation of degradation of superconducting properties and/ or phase dissociation of MgB2 samples on exposure to various environments for the wide span of time is therefore desired.
Corresponding author. Tel.: +911 585 244 1066; fax: +9111 26581114.
E-mail addresses: [email protected], [email protected] (S. Rajput), [email protected] (S. Chaudhary). 1 Tel: 9111 26581341; fax: 9111 26581114. 0022-3697/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2008.06.114
In this paper, we discuss the results of the ageing investigations performed under different exposure (ambient and humid) conditions on various polycrystalline MgB2 samples. The resulting changes and evolution of different phases have been investigated in the MgB2 sample exposed to ambient environment over a span of more than a year time so as to find out the stability of the samples and the possible degradation mechanism in the bulk MgB2 samples.
2. Experimental details The polycrystalline MgB2 samples, exhibiting TC in the range of 39–41 K, employed for the degradation study were prepared both by modified solid-state reaction method and sintering under vacuum-sealed conditions [3,4]. Table 1 summarizes the synthesis parameters and exposure conditions of the sample with their increasing age. Sample ‘A’ and ‘B’ have been synthesized by heat treatment at 560 and 820 1C, respectively, for 1.0 h in flowing ‘Ar’ atmosphere whereas sample ‘C’ was made via heat treatment at 700 1C for 1.0 h after placing the pellet sample inside a Ta enclosure followed by its sealing in an evacuated (105 Torr) quartz ampoule. These two sets of MgB2 samples would henceforth be referred to as Set-I (sample A and sample B) and Set-II (sample C), respectively. It may be noted that to compensate for the inevitable loss of the elemental Mg, all these samples were synthesized with excess magnesium composition of the starting
ARTICLE IN PRESS S. Rajput, S. Chaudhary / Journal of Physics and Chemistry of Solids 70 (2009) 516–520
517
Table 1 Details of the MgB2 samples and ageing conditions (environmental condition and exposure time). Set-I
Set-II
Sample A TS ¼ 560 1C; tr ¼ 1 h
Sample B TS ¼ 820 1C; tr ¼ 1 h
Sample C TS ¼ 700 1C; tr ¼ 1 h
Exposure duration Environment condition (days)
Exposure duration Environment condition (days)
Exposure duration Environment condition (days)
0 (As synthesized) In flowing Ar 51 In desiccator (with silica gel) (RH ¼ 20–25%; 25–30 1C) 69 In air (RH ¼ 40–80%; 25–40 1C)
0 (As synthesized) In flowing Ar 103 In desiccator (with silica gel) (RH ¼ 20–25% ; 25–30 1C) 465 In air (RH ¼ 40–80%; 25–40 1C)
0 (As synthesized) In vacuum 30 In humidity cell (RH ¼ 83.4%; 25 1C)
Here, TS is the synthesis temperature, RH is the relative humidity and tr is the time for which sample was kept at TS.
3. Result and discussion
(b) After 69 days
(101) 30
40
50
60
70
80
(112)
(201) MgO
(110) MgO (102)
(002)
Mg B 20
After synthesis
(111)
(a)
(100)
Intensity (a.u.)
Sample A
90
2θ (deg) Fig. 1. X-ray diffraction patterns of the MgB2 sample ‘A’ recorded immediately after its synthesis (lower panel) and after the age of 69 days (upper panel).
0 days
Sample A χ'' (a.u.)
2 days 69 days
χ' (a.u.)
powder, i.e., Mg:B ¼ 2:2 [5]. The first set of samples (set I) was exposed to the ambient atmosphere whereas, the second set of samples synthesized via vacuum-sealed process was exposed to the humid conditions for a period of 30 days. Samples ‘A’ and ‘B’ were first kept inside the desiccator having adequate quantity of silica gel which was followed by their exposure to the air for observing the effect of the ambient. For studying the humidity effects on the MgB2, a standard humidity cell was made by saturated solution of KCl possessing 83.4% of humidity at 25 1C. It is well known that when a solid phase of a hygroscopic salt in contact with the saturated solution in water of a same solid specie (viz., KCl), is enclosed in a chamber/container, it maintains the static equilibrium of humidity, provided the temperature remains constant. This method of making humidity cell is advantageous compared to the other static method (viz., sulfuric acid method) as any evaporation of water from the solution is balanced by the precipitation of the solid phase and the amount of the contaminant in the vicinity of the solution is very low [6,7]. The relative humidity maintained above the saturated solution of KCl at different temperatures is found from the literature [8,9]. These MgB2 samples exposed to various environments were subsequently investigated for the variation in superconducting properties with age and the possible phase dissociation. We preferred temperature dependent ac-susceptibility measurements over the resistivity measurements since in the later, the superconducting transition is often dictated by the percolation-driven sharp drop in resistivity. This is in sharp contrast to the change in ac-susceptibility (both in real as well as the imaginary part) at the superconducting transition which is contributed by the whole sample volume through the resistance-less shielding currents. As the degradation mechanism is initiated from the surface, the samples are likely to be inhomogeneous. Therefore, the acsusceptibility technique is expected to provide true picture of the degradation in superconducting behavior of the samples subsequent to their exposure. The critical current density (JC) of the samples has been calculated using modified Bean’s critical state model [10–12].
3.1. Set-I: ambient exposure effects Fig. 1 shows the X-ray diffractograms of sample ‘A’ taken immediately after its synthesis and after 69 days. Sample ‘A’, synthesized at low synthesis temperature and having unreacted magnesium (Fig. 1a) did not show any detectable evidence of formation of new phase. Understandably, no noticeable change was observed in relative fractions of the two primary phases present in the initial sample (i.e., superconducting MgB2 and insulating MgO). In Fig. 2, the variation of real and imaginary parts
38
39
40 T (K)
41
42
Fig. 2. Variation of imaginary and real part of the ac-susceptibility, i.e., w00ac ðTÞ and w0ac ðTÞ, respectively, with temperature for the MgB2 sample ‘A’, recorded at different indicated stages during the ageing under the ambient environment.
ARTICLE IN PRESS
2 days
Sample B χ'' (a.u.)
66 days
χ' (a.u.)
135 days
MgB6 MgB6
Mg(OH)2 MgO
MgO
(101) 30
40
50 60 2θ (deg)
70
(112)
(201)
(200)
(102) (111)
(110)
As-Synthesis (002)
(a)
MgO
Mg
MgO
Age 10 days
(b)
20
MgB6
MgB6
Mg(OH)2
Mg
MgB12 Mg
Age 132 days MgB12 Mg
Mg
(c)
Mg Mg(OH)2
Mg
(d)
Age 465 days
Sample B
(100)
of ac-susceptibility with temperature, as recorded at different indicated durations of exposure of MgB2 sample, is shown. It can be seen that after an exposure for 70 days, while the onset of superconducting transition appeared to be at relatively lower temperature (c.f., the w0ac ðTÞ-plots, see Fig. 2), the temperature at which w00ac ðTÞ exhibits a peak did not show any perceptible shift. Since, this peak temperature is indicative of the average TC of the whole sample, we conclude that the MgB2 sample was more or less remain intact until the studied ambient exposure span of 70 days. This inference is consistent with the X-ray diffraction findings as discussed above. To find the effect of ambient exposure over a wide span of time, an MgB2 pellet, i.e., sample ‘B’, possessing a higher transition temperature (above 40 K) was investigated for more than a year, i.e., for 465 days. Fig. 3 shows the w0ac ðTÞ and w00ac ðTÞ plots recorded for this sample after different span of ambient exposure. Consistent with the result of sample ‘A’, initial degradation after 65–70 days of aging in sample ‘B’ was more or less very negligible. That is, there is no significant change in the transition till the age of 66 days when the sample was kept inside the desiccator with silica-gel (Fig. 3). When the sample was left to the ambient, i.e., in air it exhibited two peaks in w00ac (T)-plots (Fig. 3). This clearly indicates that substantial degradation took place in the sample on prolonged ambient exposure. Such a two loss peak behavior in w00ac ðTÞ is similar to inter-granular and intra-granular shielding contributions, typical of HTSCs such as YBCO and BSCCO. In case of MgB2, this behavior is relatively less observed owing to relatively large coherence lengths. However, their observation in the present case of degradation studies are indicative of appearance of finite weak link effects owing to degradation of the surface of many MgB2 grains inside sample ‘B’. To infer if any change or phase-dissociation could have occurred during the exposure, we showed in Fig. 4 various X-ray diffractograms of sample ‘B’ recorded at the age of 0, 10, 132 and 465 days. It may be recalled here that the sample ‘B’ in the assynthesized form consisted of the prominent MgB2 phase (Fig. 4a) along with the MgO phase. Unlike sample ‘A’, no trace of Mg was present in the pristine sample ‘B’. It should also be noted that in agreement with Eyidi et al. [13] due to weak scattering from ‘B’-rich compounds in X-ray diffraction, we cannot rule out the possibility of the presence of boron and boron oxide in trace amount in sample ‘B’. If we carefully look at the diffractograms
Mg Mg(OH)2
S. Rajput, S. Chaudhary / Journal of Physics and Chemistry of Solids 70 (2009) 516–520
Intensity (a.u.)
518
80
90
Fig. 4. X-ray diffractograms of MgB2 sample ‘B’ recorded immediately after synthesis and at the age of 10, 132 and 465 days during their exposure to ambient air environment.
(Fig. 4) taken at the age of 0, 10, 132 and 465 days, it is clear that as the age of the sample increases, it resulted in the formation of elemental magnesium. The other important observation is that strength of the main MgO peak relative to the main MgB2 peak also decreased. Moreover, other magnesium-deficient compound, viz., MgB6 and MgB12 also appeared along with the elemental magnesium. Apart from all these phases, diffraction peak corresponding to the Mg(OH)2 can also be seen from the age of 132 days onwards. The observation of all these phases is indicative of the dissociation of MgB2 phase to the magnesium-deficient phases. Here, it should be mentioned that the earlier studies on the degradation of MgB2 did not report the observation of magnesium-deficient phases [1,2], although Aswal et al. [1] have found some amount of Mg(OH)2 in the degraded samples. They have suggested it to be due to the reaction between MgB2 and H2O leading to the formation of Mg(OH)2, B and H2, which on further reaction with CO2 formed MgCO3 along with the oxide of boron. However, in the present case the absence of MgCO3 and oxide of boron indicate that different degradation mechanism has taken place in these MgB2 samples. The presence of the magnesiumdeficient phases such as MgB6, MgB12 suggest the following dissociation mechanism for the degraded MgB2 sample in the ambient atmosphere: ambient
MgB2 ! MgB6 þ MgB12 H2 O
þ Mg!MgB6 þ MgB12 þ MgðOHÞ2 þ Mg þ H2 m
(1)
The reduction in the intensity of MgO peak (Fig. 4) can be possibly accounted for in terms of its reaction with the water content present in the atmosphere:
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39
40 T (K)
41
42
Fig. 3. Variation in the imaginary and real part of the ac-susceptibility with temperature showing the shift in the transition temperature in sample ‘B’ during the ageing under ambient.
H2 O
MgO!MgðOHÞ2 þ H2 m
(2)
Thus, the exposure of MgB2 samples for longer period of time results in the formation of Mg-deficient Mg–B-phases, hydroxide of magnesium and elemental magnesium. The observed decrease
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107
Jc (A/m2)
106 Before After 105
38
χ'' (a.u.) χ' (a.u.)
Before After
36
37
38
39
40
41
T (K) Fig. 5. Variation in the imaginary and real part of the ac-susceptibility with temperature showing the shift in the transition temperature for MgB2 sample ‘C’ before and after keeping it in the humidity cell for a period of 30 days.
40
41
T (K)
20
30
40
(112)
(201) MgO
(102)
(110) MgO
(002)
After humidity effect
50 60 2θ (deg)
70
80
(112)
(201) MgO
(102)
(110) MgO
After synthesis (002)
(100)
Intensity (a.u.)
(100)
(101)
Fig. 6. Temperature dependence of the critical current density of the MgB2 sample ‘C’ before and after keeping it in the humidity cell for a period of 30 days.
3.2. Set II: humid exposure effects Sample ‘C’ was kept in the humidity cell for a period of 30 days to study the possible degradation effects in response to the humid environment. Fig. 5 shows the variation of ac-susceptibility with temperature for sample ‘C’ recorded immediately after its synthesis and after keeping it in the 83.7% humid environment for 30 days. It is clearly visible from Fig. 5 that after the humidity attack on the sample ‘C’, its transition temperature lowered significantly. However, we did not observe any two-peak feature in its w00ac ðTÞ behavior subsequent to humid exposure. To have more insight on the effect of the possible appearance of additional phases, we recorded w00ac (T)-plots at various applied ac fields for this sample ‘C’, and obtained the critical current density JC(T) behavior of the samples before and after the humid exposure. Fig. 6 presents the critical current density JC(T) behavior of sample ‘C’ before and after keeping it in the humidity cell. From the JC(T) behavior in the two cases (Fig. 6), one can see that no considerable change in JC and hence pinning effects in the vortices is observed at low temperatures. The shift of JC(T) drop occurring at the
39
(101)
in TC (the 2nd peak in w00ac ðTÞ and the presence of another peak at lower T in w00ac ðTÞ having higher magnitude) can be, respectively, accounted for compositional fluctuation within the grain and some of the phase dissociation of MgB2 at the inter-granular regions. Thus the ageing in these granular MgB2 samples results in some enrichment of the magnesium and magnesium-deficient phases predominantly near the grain surface in the degraded sample. Here it should also be mentioned that the microstructure and porosity in sample synthesized with magnesium-rich composition and in flowing Ar has already been presented in the earlier studies [14]. The observed changes in TC and phases suggest that MgB2 superconductor is also sensitive to ambient conditions and bear similarity with HTSC’s (BSCCO and YBCO), though the degradation is sluggish as compared to YBCO, in particular. Earlier studies on YBCO and Bi-2223 also predicted the ageing effects in these superconductors. There is formation of non-superconducting Y2BaCuO5 phase along with CuO and Ba(OH)2 on exposing YBCO to the humid atmosphere [15]. Similarly, a sample having mixed phase of Bi-2223 and 2212 was reported to decompose into the mixture of Bi2CuO4, CuO and SrCO3 with the small fraction of CaCO3 whereas, single-phasic Bi2223 was found to be stable with respect to the exposure to the humid conditions [16].
519
90
Fig. 7. X-ray diffractograms of the MgB2 sample ‘C’ recorded before and after exposing it to 83.7% humid environment for 30 days. Apart from broadening of peaks corresponding to MgO an unidentified peak was noticed after the humidity attack on sample ‘C’.
superconducting transition to lower temperature is consistent with the change in wac ðT Þ behavior (Fig. 5) after ageing, possibly related with the composition fluctuation effects within the superconducting grains. Here it should also be mentioned that Zhou et al. [2] studied the ageing of MgB2 wires and they reported the 20% decrease in the critical current in the MgB2 wires after an ageing for 4 months. They associated it to be due to the oxidation of the extra magnesium present in the wire sample. In Fig. 7, we show the diffractograms of the sample ‘C’ recorded before and after keeping it in the humidity cell. The broadening of the peak corresponding to MgO phase when the sample was attacked by the humid environment along with the appearance of one unknown impurity peak was noticed. These two observations might have resulted due to the composition fluctuation in the sample as an evidence by the degradation in TC on humidity exposure (Fig. 5). It should also be noticed that, in contrast to ambient exposure effects shown by sample ‘B’, no sign of phase dissociation of MgB2 phase was observed on exposing the sample ‘C’ to humid environment for a period of 30 days. For knowing the surface degradation effects, we recorded SEM micrographs (at 6000 magnification, Fig. 8(a) and (b)) for sample ‘C’ before and after the humidity attack. The SEM images of the unfractured surface of sample ‘C’ clearly indicates the degradation of the
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Fig. 8. The SEM images of surface of MgB2 sample ‘C’ showing the degradation of the surface of the sample due to humid environment: (a) before and (b) after exposing the sample to the 83.7% humid atmosphere. The bar represents the 5 mm length.
sample surfaces after the humidity attack. The effect of humidity exposure is clearly evident from the absence of the uniform/ homogenous distribution of granular structure seen prior to humidity exposure. The porosity of sample ‘C’ has been discussed in a separate study [17]. This could possibly result from the amorphous nature of the degraded product, which understandably gives inferior contrast in SEM micrograph.
4. Conclusions On the basis of degradation investigations presented here for the MgB2 superconductor exposed to ambient as well as humid atmosphere, it can be inferred that the MgB2 system is sensitive to environmental conditions. In ambient conditions up to 70 days, the degradation in the MgB2 superconductor with regards to the phase-dissociation and/or lowering of TC was found to be negligible. However, exposure for prolonged duration reveled that MgB2 samples are more prone to degradation when exposed to ambient for longer durations. A model has been proposed wherein the MgB2 partially transform to MgB6 and MgB12 and also leading to the formation of Mg(OH)2 and Mg when exposed in ambient condition for more than 70 days. When exposed to humid atmosphere, MgB2 samples showed severe degradation of the surface and also of the superconducting parameters, however, without leading to magnesium-deficient phases as observed on ambient exposure. The extent of degradation can be definitely reduced if samples are sheathed or kept in vacuum environment.
Acknowledgment The authors would like to thank Mr. D. C. Sharma for his help in SEM studies.
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