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Improvement of critical current density in Sb-doped HgBa2Ca2Cu3O8+δ superconductor prepared by Hg vapour diffusion process
Physica C 325 Ž1999. 109–117
Improvement of critical current density in Sb-doped HgBa 2 Ca 2 Cu 3 O 8qd superconductor prepared by Hg vapour diffusion process J.Q. Li a
a,1
, C.C. Lam a , J.S. Abell
b,)
, G.B. Peacock c , P.P. Edwards c , L.J. Shen
a
Department of Physics and Materials Science, City UniÕersity of Hong Kong, Hong Kong People’s Republic of China b School of Metallurgy and Materials, The UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK c School of Chemistry, The UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 18 June 1999; received in revised form 7 September 1999; accepted 11 September 1999
Abstract Antimony-doped Hg-1223 superconductors have been synthesised from the reaction of a Sb-doped precursor with elemental Hg vapour released from a reactant in sealed silica tubes Žthe Hg vapour diffusion process.. The Hg vapour pressure can be controlled by a small addition of Se in the reactant bars. The phase purity and microstructure of the material have been characterised by XRD and SEM and the superconducing properties measured by electrical resistance and AC susceptibility. High quality superconducting material can be fabricated by this Hg vapour diffusion process with a suitable Se addition in the reactant bars. The microstructure reveals dense, plate-like grains with reduced void formation, when compared with material prepared by a solid state reaction route. Resistance and AC susceptibility measurements give a Tc of 132 K. The diffusion process route significantly enhances the intergrain critical current density Ž Jc s 2.5 = 10 7 Amy2 at 77 K and zero field. and reduces the degradation of Jc in a magnetic field. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Critical current density; Sb-doped Hg-1223 superconductors; Hg vapour
1. Introduction Since the discovery of the high-Tc superconductors in 1986, tremendous efforts have been made to increase their critical current density value, Jc , in order to promote their technical application. Bulk high-Tc superconductors produced by sintering consist of anisotropic superconducting grains, separated )
Corresponding author. Fax: q44-121-414-5232. E-mail address: [email protected] ŽJ.S. Abell. 1 At present on study leave at School of Metallurgy and Materials, University of Birmingham.
by grain boundaries or porosity. Because of the small coherence length of these superconductors, the grain boundaries or porosity act as Josephson weak links. It has been found that the weak links in sintered bulk materials limit the magnitude of Jc and increase the degradation of Jc in a magnetic field. Weak links have also been attributed to grain misorientation w1x and chemical inhomogeneity at interfaces. In order to enhance the intergrain critical current density, it is necessary to improve the coupling between the grains. It is now well established that the Hg-based cuprate superconductor HgBa 2 Ca 2 Cu 3 O 8q d ŽHg1223. has the highest critical temperature ŽTc . among
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 5 0 3 - 1
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all known high-Tc superconductors. High critical current densities were reported in HgBa 2 CaCu 2 O6q d films Ž Jc ) 10 7 Arcm2 at 5 K, and ) 10 5 Arcm2 at 110 K. in the presence of magnetic fields w2x, and even higher Jc is expected for the Hg-1223 phase w3x. Such high critical current densities are at least an order-of-magnitude larger than those of Bi- or Tlbased superconducting films. Unfortunately, the synthesis of Hg-based superconductors is the main impedence to the large-scale production of these materials for applications, due to the complex chemistry and the toxic Hg-containing ingredient. Hg-based cuprate superconductors are usually prepared by sealed silica tube or high pressure techniques. As with other high-Tc superconductors, the weak link problem is present in Hg-based superconductors. Furthermore, these weak links become worse due to the release of O 2 and Hg gas from the sample when HgO and the mercury-cuprate decompose into Hg vapour and O 2 during the synthesis and sintering processes. Doping the Hg-based superconductors with the appropriate metals or oxides can modify the nature of the grain boundaries, as well as promote the formation of the superconducting phase or introduce effective pinning centres. In our previous work w4x, we found that a small addition of Sb in Hg-1223 prepared by a solid state reaction route can improve the intergrain coupling and enhance its intergrain critical current density as well as increase its thermal stability. However, the porosity or voids in the Sbdoped material formed due to the release of O 2 and Hg gas from the sample can still be seen by SEM observation. It is reasonable to believe that these voids could be avoided in material prepared by reacting a Hg-free precursor bar with elemental Hg vapour released from a reactant bar in a sealed silica tube ŽHg vapour diffusion process.. This process is entirely analogous to the ‘‘Controlled Vapour–Solid Reaction ŽCVSR. technique’’ reported by Meng et al. w5x for undoped Hg-1223. Goto w6x reported that the maximum Jc in a F-doped HgBa 2 Ca 2 Cu 3 Re 0.2 O y filament prepared by this diffusion method can reach 1 = 10 4 Arcm2 at 77 K and zero field. Kikuchi et al. w7x also enhanced the critical current density in Tl– Ba–Ca–Cu–O superconductor by the diffusion process with F addition. We have found that the addition of Se can significantly affect the Hg vapour
pressure in the sealed silica tube during synthesis by the ‘‘reactant–precursor technique’’ w7x. Less than 1 wt.% of Se addition in the reactant bars promoted the formation of the Hg-1212 superconducting phase in the precursor bar, while more than 1 wt.% of Se addition promoted the formation of the Hg-1223 superconducting phase in the precursor bars by controlled Hg vapour transport and diffusion. In this work, we have synthesised the Sb-doped Hg-1223 superconductor by this diffusion method and found that the physical properties of the material Žmicrostructure, Jc and Hirr . are significantly improved over ŽHg,Sb.-1223 prepared by a solid–solid reaction.
2. Experimental The samples were synthesised in a sealed silica tube using precursor bars reacted with Hg vapour released from reactant bars. The Hg vapour pressure can be controlled by the amount of Se addition in the reactant bars. For the preparation of precursor and reactant bars, powders of BaO, CaO and CuO were weighed according to the molar ratio of Ba:Ca:Cus 2:2:3, and then they were mixed and ground carefully in an agate mortar. The resulting powder mixture was calcined in an alumina crucible in air at 9008C for 26 h. To prepare the precursor bars, the porous, calcined semi-product was reground and mixed with Sb 2 O 3 powder according to the composition Sb 0.05 Ba 2 Ca 2 Cu 3 O y , pressed into pellets and sintered at 9008C for 26 h in air. These pellets were then reground and hydrostatically pressed into precursor bars Žtypical size 15 = 5 = 2 mm. . The reactant bars were also prepared from the calcined semi-product which was ground, pressed into pellets and sintered at 9008C for 26 h in air. The pellets were reground immediately and mixed with HgO powder according to the composition Hg 1.4 Ba 2 Ca 2 Cu 3 O 8q d . The excess HgO was added to supply sufficient Hg vapour in the sealed capsule to diffuse into the precursor bar. Additionally, 1.5 wt.% of Se was added to this powder mixture, which was then pulverised fully in the agate mortar and hydrostatically pressed into the reactant bars. All the above processes were carried out quickly in air.
J.Q. Li et al.r Physica C 325 (1999) 109–117
The reactant and the precursor bars were sealed in an evacuated quartz tube, with a reactant:precursor mass ratio of ; 3:1. Subsequently, the sealed quartz tubes were put inside a stainless steel tube, closed at both ends, for protection against the possible explosion of the quartz tube. The stainless steel tube was then placed in a tube-furnace which was heated to 9108C in 50 min and kept at 9108C for 3 h. Finally, the samples were furnace-cooled to room temperature. By this treatment, the precursor bars were found to be Sb-doped Hg-1223 superconductors without a trace of the Hg-1212 phase. The samples were oxygenated at 3008C in flowing O 2 for 10 h and 7508C in flowing O 2 for 2 h which was found to remove the small amount of the impurity phase HgCaO 2 from the sample. The phase structure of the samples was identified by means of X-ray diffraction using a Siemens D500 diffractometer. The Tc of samples was measured by using the standard four-point probe electrical resistance method under zero magnetic field. The microstructure observation was performed in a Hitachi S-570 scanning electron microscope. The complex AC susceptibility measurements were performed un-
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der DC field 0 mT F m 0 Hdc F 131 mT at constant AC field amplitude Hac s 153 Amy1 and frequency f s 1 kHz, and without DC magnetic field for AC field 22 Amy1 F Hac F 1045 Amy1 Ž f s 1 kHz.. The samples used for this measurement were cylindrical with a diameter D s 1.28 mm and a height h s 8.80 mm. The critical current densities were estimated based on the Bean model w8x. 3. Results and discussion 3.1. Characterisation of the samples The predominant phase in the precursor bar modified by the Hg vapour diffusion process was found by X-ray diffraction to be HgŽSb.-1223, together with small amounts of impurity phases BaCuO 2 and HgCaO 2 . The impurity phase HgCaO 2 was removed from the sample by annealing at 7508C in flowing O 2 . In Fig. 1, we show the diffraction pattern of the sample after annealing; the pattern consists mainly of the HgŽSb.-1223 phase and a small amount of the precursor phase BaCuO 2 . No trace of Hg-1212 or impurity phase HgCaO 2 exists in these samples.
Fig. 1. X-ray diffraction pattern of the Sb-doped Hg-1223 superconductor prepared by Hg vapour diffusion process after removal of the impurity phase HgCaO 2 . The pattern consists of the HgŽSb.-1223 phase Žwith Miller indices. and the BaCuO 2 phase Žmarked with symbol ‘‘U ’’..
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Fig. 2. The r vs. T curve for the Sb-doped Hg-1223 superconductors obtained by the diffusion process.
Fig. 2 shows the temperature dependence of the electrical resistivity Ž r ; T curve. for the sample after removal of the impurity phase HgCaO 2 . The zero resistance Tc of the oxygenated samples was 132 K, which is slightly higher than that measured on material prepared by the previously reported reaction method w4x. Fig. 3 shows an SEM micrograph of a representative region of the surface of the annealed sample. Similar to the other Hg-based superconductors, the microstructure of the superconducting phase HgŽSb.1223 comprises thick flake-shaped lamina stacked one on the other. These spiral, flake-like crystal grains were confirmed to be HgŽSb.-1223 by EDX analysis, as shown in Fig. 4. No Sb is evident from the EDX analysis, although the expected Sb peak at 3.6048 keV may be masked for the small amount of Sb used, by the large Ca-peak. From Fig. 3, it can be seen that the flake-like superconducting grains grow perpendicular to the sample surface and pack densely, indicating that the grains probably grow from the surface to the centre of the sample during the Hg vapour diffusion process. There are some impurity phases, mainly BaCuO 2 existing in the grain boundaries. Small pores or voids, formed probably by the decomposition of impurity HgCaO 2 during annealing can also be seen in the grain boundaries. However,
they are much smaller than those in the material prepared by the previous reaction method w4x, which
Fig. 3. Microstructure of the sample surface obtained by the diffusion process after removal of impurity phase HgCaO 2 .
J.Q. Li et al.r Physica C 325 (1999) 109–117
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Fig. 4. The EDX pattern for the HgŽSb.-1223 grains shown in Fig. 3.
formed due to the release of O 2 and Hg gas from the material. This implies that the Hg vapour diffusion process may enhance the intergrain critical current density of the superconductor. 3.2. Intergrain critical current density and irreÕersibility line from AC susceptibility measurements Fig. 5Ža. shows the real part x X and the imaginary part x Y of the magnetic susceptibility vs. temperature for a sample measured under a frequency f s 1 kHz and AC fields Hac s 22, 55, 153, 210, 418, 623, 787 and 1045 Amy1 in zero DC field. Fig. 5Žb. is an enlarged plot of x Y , which has been used for determining the peak of the imaginary part of the magnetic susceptibility. The real and imaginary parts of the magnetic susceptibility vs. temperature, measured under frequency f s 1 kHz and Hac s 153 Amy1 with various DC fields m 0 Hdc s 0, 4.63, 9.00, 13.3, 22.1, 44.7, 87.5, 131.1 mT are shown in Fig. 6Ža. and Žb.. The characteristic drops of the real part of the susceptibility x X in the low AC or low DC field curves of these figures correspond to diamagnetic shielding at 132 K consistent with the Tc Ž R s 0. in the corresponding r ; T measurements. For convenience of comparison, the AC susceptibilities in Fig. 5 are normalised to their maximum value.
Only one large peak appears in the x Y ; T curves for the sample in all AC or DC fields ŽFigs. 5Žb. and 6Žb... In our previous work w4x, we found that a small peak appeared together with the large peak at a certain DC field in the x Y ; T curves for an Sb-doped sample prepared by the solid-state reaction method ŽFig. 7Žb. in Ref. w4x.. Clearly, the large x Y-peak and the small x Y-peak correspond to the inter- and intragrain current system, respectively. For a Sb-doped sample prepared by the solid-state reaction method, the two peaks merge together in x Y ; T curves at low DC field and separate as the DC field gets larger. For the Sb-doped sample prepared by the present diffusion process, the small peak and large peak merge together in the x Y ; T curves in all AC or DC fields. Similarly, we believe the peaks in x Y ; T curves in Figs. 5 and 6 are mainly attributable to the intergrain current system. Furthermore, this large peak is much sharper than the corresponding peak for the Sb-doped or pure Hg-1223 samples prepared by the solid-state reaction method w4x and shifts to low temperature much more slowly with increasing AC or DC field. This means that the diffusion process can further improve the intergrain coupling and modify the characteristic nature of the superconducting grain boundaries in Sb-doped Hgbased superconductors. In other words, by using the
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X Y Y Fig. 5. Ža. The real part x and the imaginary part x of magnetic susceptibility and Žb. the corresponding enlarged imaginary part x vs. temperature of the Sb-doped Hg-1223 superconductors obtained by the diffusion process measured under a frequency 1 kHz and AC fields Žwith peak positions left to right. Hac s 22, 55, 153, 210, 418, 623, 787 and 1045 Amy1 , with zero DC field.
diffusion process we are able to further enhance the intergrain critical current density, Jc .
Based on the Bean critical-state model w8x, the critical current density Jc at the peak temperature Tp
J.Q. Li et al.r Physica C 325 (1999) 109–117
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X Y Y Fig. 6. Ža. The real part x and the imaginary part x of susceptibility and Žb. the enlarged imaginary part x vs. temperature for Sb-doped Hg-1223 superconductors obtained by the diffusion process measured under a frequency 1 kHz and the DC fields Žwith peak positions left to right. m 0 Hdc s 0, 4.633, 9.00, 13.3, 22.1, 44.7, 87.5, 131.1 mT, with a constant AC field Hac s 153 Amy1 .
of the imaginary part of the AC susceptibility curve Ž x Y . for a cylindrical sample can be determined by the formula Jc Ž Hac , Tp . s HacrR, where Hac is the amplitude of the AC field and R is the radius of the
cylindrical sample w4,9x. Fig. 7 shows the critical current density Ž Jc . vs. temperature for Sb-doped Hg-1223 sample prepared by the diffusion process calculated in this way; values for pure and Sb-doped
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Fig. 7. The critical current density Ž Jc . vs. temperature for the Sb-doped Hg-1223 superconductors obtained by the diffusion process ŽU ., compared with Sb-doped ŽHg 0.95 Sb 0.05 .-1223 Ž`. and pure Hg-1223 Žv . superconductors prepared by a solid-state reaction method w5x. Jc is evaluated based on the Bean model.
Hg-1223 samples obtained in our previous work w4x Žsolid-state reaction method. are also plotted in this figure for comparison. From Fig. 7, it can be seen that the intergrain critical current density Jc for the Sb-doped sample prepared by the diffusion process increases much more significantly than those in the
other two samples. By linear extrapolation of the Jc ; T curve for individual samples, the Jc values at 77 K for the Sb-doped Hg-1223 superconductor prepared by the diffusion process, Sb-doped Hg-1223 and pure Hg-1223 superconductors obtained by reaction method are 2.5 = 10 7, 1.1 = 10 7 and 7.0 = 10 6
Fig. 8. The irreversibility lines ŽIL. for the Sb-doped Hg-1223 obtained by the diffusion process ŽU ., compared with Sb-doped ŽHg 0.95 Sb 0.05 .-1223 Ž`. and pure Hg-1223 Žv . superconductors prepared by a solid-state reaction method w5x.
J.Q. Li et al.r Physica C 325 (1999) 109–117
A.my2 , respectively. Thus, a significant improvement in the critical current density is obtained by the Hg-diffusion process. By defining the temperature where the maximum x Y occurs at a certain DC field as the irreversibility temperature ŽTir ., the irreversibility line for the sample obtained by the diffusion process can be plotted in Fig. 8 from the data of Fig. 6. The irreversibility lines from the data of our previous work w4x for the pure and Sb-doped Hg-1223 superconductors prepared by the reaction method are also plotted in this figure for comparison. The irreversibility line of the Hg-1223 superconductor is shifted significantly to higher temperature by doping with Sb w4x; however, the shift to higher temperature is even more dramatic in the sample prepared by using the Hg vapour diffusion process. This shift reflects the reduction of degradation in Jc under magnetic field. Therefore, the Hg vapour diffusion process method can reduce the degradation in Jc on increasing the magnetic field, when compared to an equivalent sample preparaed by a more conventional solid-state reaction method.
4. Conclusion We have successfully synthesized Sb-doped Hg1223 superconductor by a Hg vapour diffusion process and studied its superconductivity. The experi-
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mental results show that this diffusion process avoids the voids formed due to the release of O 2 and Hg gas during the previously employed solid state reaction route, and further modifies the nature of superconducting grain boundaries. It further enhances significantly the intergrain critical current density Jc and reduces the degradation in Jc under magnetic field. Acknowledgements The authors would like to express their gratitude to the Research Committee of the City University of Hong Kong for their financial support to this research . References w1x D. Dimos, P. Chaudhari, F.K. LeGoues, Phy. Rev. Lett. 61 Ž1988. 219. w2x L.K. Elbaum, C.C. Tsuei, A. Gupta, Nature 373 Ž1995. 679. w3x R.L. Meng, B.R. Hickey, Y.Y. Sun, Y. Cao, C. Kinalidis, J. Meen, Y.Y. Xue, C.W. Chu, Physica C 260 Ž1996. 1. w4x J.Q. Li, C.C. Lam, K.C. Hung, L.J. Shen, Physica C 304 Ž1998. 133. w5x R.L. Meng, L. Beauvais, X.N. Zhang, Z.J. Huang, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Physica C 216 Ž1993. 21. w6x T. Goto, Physica C 282–287 Ž1997. 891. w7x A. Kikuchi, T. Kinoshita, N. Nishikawa, S. Komiya, K. Tachikawa, Jpn. J. Appl. Phys. 34 Ž1995. L167. w8x C.P. Bean, Rev. Mod. Phys 36 Ž1964. 31. w9x F. Gomory, P. Lobatka, Solid State Communication 66 Ž6. Ž1988. 645.
Improvement of critical current density in Sb-doped HgBa 2 Ca 2 Cu 3 O 8qd superconductor prepared by Hg vapour diffusion process J.Q. Li a
a,1
, C.C. Lam a , J.S. Abell
b,)
, G.B. Peacock c , P.P. Edwards c , L.J. Shen
a
Department of Physics and Materials Science, City UniÕersity of Hong Kong, Hong Kong People’s Republic of China b School of Metallurgy and Materials, The UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK c School of Chemistry, The UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 18 June 1999; received in revised form 7 September 1999; accepted 11 September 1999
Abstract Antimony-doped Hg-1223 superconductors have been synthesised from the reaction of a Sb-doped precursor with elemental Hg vapour released from a reactant in sealed silica tubes Žthe Hg vapour diffusion process.. The Hg vapour pressure can be controlled by a small addition of Se in the reactant bars. The phase purity and microstructure of the material have been characterised by XRD and SEM and the superconducing properties measured by electrical resistance and AC susceptibility. High quality superconducting material can be fabricated by this Hg vapour diffusion process with a suitable Se addition in the reactant bars. The microstructure reveals dense, plate-like grains with reduced void formation, when compared with material prepared by a solid state reaction route. Resistance and AC susceptibility measurements give a Tc of 132 K. The diffusion process route significantly enhances the intergrain critical current density Ž Jc s 2.5 = 10 7 Amy2 at 77 K and zero field. and reduces the degradation of Jc in a magnetic field. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Critical current density; Sb-doped Hg-1223 superconductors; Hg vapour
1. Introduction Since the discovery of the high-Tc superconductors in 1986, tremendous efforts have been made to increase their critical current density value, Jc , in order to promote their technical application. Bulk high-Tc superconductors produced by sintering consist of anisotropic superconducting grains, separated )
Corresponding author. Fax: q44-121-414-5232. E-mail address: [email protected] ŽJ.S. Abell. 1 At present on study leave at School of Metallurgy and Materials, University of Birmingham.
by grain boundaries or porosity. Because of the small coherence length of these superconductors, the grain boundaries or porosity act as Josephson weak links. It has been found that the weak links in sintered bulk materials limit the magnitude of Jc and increase the degradation of Jc in a magnetic field. Weak links have also been attributed to grain misorientation w1x and chemical inhomogeneity at interfaces. In order to enhance the intergrain critical current density, it is necessary to improve the coupling between the grains. It is now well established that the Hg-based cuprate superconductor HgBa 2 Ca 2 Cu 3 O 8q d ŽHg1223. has the highest critical temperature ŽTc . among
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 5 0 3 - 1
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all known high-Tc superconductors. High critical current densities were reported in HgBa 2 CaCu 2 O6q d films Ž Jc ) 10 7 Arcm2 at 5 K, and ) 10 5 Arcm2 at 110 K. in the presence of magnetic fields w2x, and even higher Jc is expected for the Hg-1223 phase w3x. Such high critical current densities are at least an order-of-magnitude larger than those of Bi- or Tlbased superconducting films. Unfortunately, the synthesis of Hg-based superconductors is the main impedence to the large-scale production of these materials for applications, due to the complex chemistry and the toxic Hg-containing ingredient. Hg-based cuprate superconductors are usually prepared by sealed silica tube or high pressure techniques. As with other high-Tc superconductors, the weak link problem is present in Hg-based superconductors. Furthermore, these weak links become worse due to the release of O 2 and Hg gas from the sample when HgO and the mercury-cuprate decompose into Hg vapour and O 2 during the synthesis and sintering processes. Doping the Hg-based superconductors with the appropriate metals or oxides can modify the nature of the grain boundaries, as well as promote the formation of the superconducting phase or introduce effective pinning centres. In our previous work w4x, we found that a small addition of Sb in Hg-1223 prepared by a solid state reaction route can improve the intergrain coupling and enhance its intergrain critical current density as well as increase its thermal stability. However, the porosity or voids in the Sbdoped material formed due to the release of O 2 and Hg gas from the sample can still be seen by SEM observation. It is reasonable to believe that these voids could be avoided in material prepared by reacting a Hg-free precursor bar with elemental Hg vapour released from a reactant bar in a sealed silica tube ŽHg vapour diffusion process.. This process is entirely analogous to the ‘‘Controlled Vapour–Solid Reaction ŽCVSR. technique’’ reported by Meng et al. w5x for undoped Hg-1223. Goto w6x reported that the maximum Jc in a F-doped HgBa 2 Ca 2 Cu 3 Re 0.2 O y filament prepared by this diffusion method can reach 1 = 10 4 Arcm2 at 77 K and zero field. Kikuchi et al. w7x also enhanced the critical current density in Tl– Ba–Ca–Cu–O superconductor by the diffusion process with F addition. We have found that the addition of Se can significantly affect the Hg vapour
pressure in the sealed silica tube during synthesis by the ‘‘reactant–precursor technique’’ w7x. Less than 1 wt.% of Se addition in the reactant bars promoted the formation of the Hg-1212 superconducting phase in the precursor bar, while more than 1 wt.% of Se addition promoted the formation of the Hg-1223 superconducting phase in the precursor bars by controlled Hg vapour transport and diffusion. In this work, we have synthesised the Sb-doped Hg-1223 superconductor by this diffusion method and found that the physical properties of the material Žmicrostructure, Jc and Hirr . are significantly improved over ŽHg,Sb.-1223 prepared by a solid–solid reaction.
2. Experimental The samples were synthesised in a sealed silica tube using precursor bars reacted with Hg vapour released from reactant bars. The Hg vapour pressure can be controlled by the amount of Se addition in the reactant bars. For the preparation of precursor and reactant bars, powders of BaO, CaO and CuO were weighed according to the molar ratio of Ba:Ca:Cus 2:2:3, and then they were mixed and ground carefully in an agate mortar. The resulting powder mixture was calcined in an alumina crucible in air at 9008C for 26 h. To prepare the precursor bars, the porous, calcined semi-product was reground and mixed with Sb 2 O 3 powder according to the composition Sb 0.05 Ba 2 Ca 2 Cu 3 O y , pressed into pellets and sintered at 9008C for 26 h in air. These pellets were then reground and hydrostatically pressed into precursor bars Žtypical size 15 = 5 = 2 mm. . The reactant bars were also prepared from the calcined semi-product which was ground, pressed into pellets and sintered at 9008C for 26 h in air. The pellets were reground immediately and mixed with HgO powder according to the composition Hg 1.4 Ba 2 Ca 2 Cu 3 O 8q d . The excess HgO was added to supply sufficient Hg vapour in the sealed capsule to diffuse into the precursor bar. Additionally, 1.5 wt.% of Se was added to this powder mixture, which was then pulverised fully in the agate mortar and hydrostatically pressed into the reactant bars. All the above processes were carried out quickly in air.
J.Q. Li et al.r Physica C 325 (1999) 109–117
The reactant and the precursor bars were sealed in an evacuated quartz tube, with a reactant:precursor mass ratio of ; 3:1. Subsequently, the sealed quartz tubes were put inside a stainless steel tube, closed at both ends, for protection against the possible explosion of the quartz tube. The stainless steel tube was then placed in a tube-furnace which was heated to 9108C in 50 min and kept at 9108C for 3 h. Finally, the samples were furnace-cooled to room temperature. By this treatment, the precursor bars were found to be Sb-doped Hg-1223 superconductors without a trace of the Hg-1212 phase. The samples were oxygenated at 3008C in flowing O 2 for 10 h and 7508C in flowing O 2 for 2 h which was found to remove the small amount of the impurity phase HgCaO 2 from the sample. The phase structure of the samples was identified by means of X-ray diffraction using a Siemens D500 diffractometer. The Tc of samples was measured by using the standard four-point probe electrical resistance method under zero magnetic field. The microstructure observation was performed in a Hitachi S-570 scanning electron microscope. The complex AC susceptibility measurements were performed un-
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der DC field 0 mT F m 0 Hdc F 131 mT at constant AC field amplitude Hac s 153 Amy1 and frequency f s 1 kHz, and without DC magnetic field for AC field 22 Amy1 F Hac F 1045 Amy1 Ž f s 1 kHz.. The samples used for this measurement were cylindrical with a diameter D s 1.28 mm and a height h s 8.80 mm. The critical current densities were estimated based on the Bean model w8x. 3. Results and discussion 3.1. Characterisation of the samples The predominant phase in the precursor bar modified by the Hg vapour diffusion process was found by X-ray diffraction to be HgŽSb.-1223, together with small amounts of impurity phases BaCuO 2 and HgCaO 2 . The impurity phase HgCaO 2 was removed from the sample by annealing at 7508C in flowing O 2 . In Fig. 1, we show the diffraction pattern of the sample after annealing; the pattern consists mainly of the HgŽSb.-1223 phase and a small amount of the precursor phase BaCuO 2 . No trace of Hg-1212 or impurity phase HgCaO 2 exists in these samples.
Fig. 1. X-ray diffraction pattern of the Sb-doped Hg-1223 superconductor prepared by Hg vapour diffusion process after removal of the impurity phase HgCaO 2 . The pattern consists of the HgŽSb.-1223 phase Žwith Miller indices. and the BaCuO 2 phase Žmarked with symbol ‘‘U ’’..
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Fig. 2. The r vs. T curve for the Sb-doped Hg-1223 superconductors obtained by the diffusion process.
Fig. 2 shows the temperature dependence of the electrical resistivity Ž r ; T curve. for the sample after removal of the impurity phase HgCaO 2 . The zero resistance Tc of the oxygenated samples was 132 K, which is slightly higher than that measured on material prepared by the previously reported reaction method w4x. Fig. 3 shows an SEM micrograph of a representative region of the surface of the annealed sample. Similar to the other Hg-based superconductors, the microstructure of the superconducting phase HgŽSb.1223 comprises thick flake-shaped lamina stacked one on the other. These spiral, flake-like crystal grains were confirmed to be HgŽSb.-1223 by EDX analysis, as shown in Fig. 4. No Sb is evident from the EDX analysis, although the expected Sb peak at 3.6048 keV may be masked for the small amount of Sb used, by the large Ca-peak. From Fig. 3, it can be seen that the flake-like superconducting grains grow perpendicular to the sample surface and pack densely, indicating that the grains probably grow from the surface to the centre of the sample during the Hg vapour diffusion process. There are some impurity phases, mainly BaCuO 2 existing in the grain boundaries. Small pores or voids, formed probably by the decomposition of impurity HgCaO 2 during annealing can also be seen in the grain boundaries. However,
they are much smaller than those in the material prepared by the previous reaction method w4x, which
Fig. 3. Microstructure of the sample surface obtained by the diffusion process after removal of impurity phase HgCaO 2 .
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Fig. 4. The EDX pattern for the HgŽSb.-1223 grains shown in Fig. 3.
formed due to the release of O 2 and Hg gas from the material. This implies that the Hg vapour diffusion process may enhance the intergrain critical current density of the superconductor. 3.2. Intergrain critical current density and irreÕersibility line from AC susceptibility measurements Fig. 5Ža. shows the real part x X and the imaginary part x Y of the magnetic susceptibility vs. temperature for a sample measured under a frequency f s 1 kHz and AC fields Hac s 22, 55, 153, 210, 418, 623, 787 and 1045 Amy1 in zero DC field. Fig. 5Žb. is an enlarged plot of x Y , which has been used for determining the peak of the imaginary part of the magnetic susceptibility. The real and imaginary parts of the magnetic susceptibility vs. temperature, measured under frequency f s 1 kHz and Hac s 153 Amy1 with various DC fields m 0 Hdc s 0, 4.63, 9.00, 13.3, 22.1, 44.7, 87.5, 131.1 mT are shown in Fig. 6Ža. and Žb.. The characteristic drops of the real part of the susceptibility x X in the low AC or low DC field curves of these figures correspond to diamagnetic shielding at 132 K consistent with the Tc Ž R s 0. in the corresponding r ; T measurements. For convenience of comparison, the AC susceptibilities in Fig. 5 are normalised to their maximum value.
Only one large peak appears in the x Y ; T curves for the sample in all AC or DC fields ŽFigs. 5Žb. and 6Žb... In our previous work w4x, we found that a small peak appeared together with the large peak at a certain DC field in the x Y ; T curves for an Sb-doped sample prepared by the solid-state reaction method ŽFig. 7Žb. in Ref. w4x.. Clearly, the large x Y-peak and the small x Y-peak correspond to the inter- and intragrain current system, respectively. For a Sb-doped sample prepared by the solid-state reaction method, the two peaks merge together in x Y ; T curves at low DC field and separate as the DC field gets larger. For the Sb-doped sample prepared by the present diffusion process, the small peak and large peak merge together in the x Y ; T curves in all AC or DC fields. Similarly, we believe the peaks in x Y ; T curves in Figs. 5 and 6 are mainly attributable to the intergrain current system. Furthermore, this large peak is much sharper than the corresponding peak for the Sb-doped or pure Hg-1223 samples prepared by the solid-state reaction method w4x and shifts to low temperature much more slowly with increasing AC or DC field. This means that the diffusion process can further improve the intergrain coupling and modify the characteristic nature of the superconducting grain boundaries in Sb-doped Hgbased superconductors. In other words, by using the
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X Y Y Fig. 5. Ža. The real part x and the imaginary part x of magnetic susceptibility and Žb. the corresponding enlarged imaginary part x vs. temperature of the Sb-doped Hg-1223 superconductors obtained by the diffusion process measured under a frequency 1 kHz and AC fields Žwith peak positions left to right. Hac s 22, 55, 153, 210, 418, 623, 787 and 1045 Amy1 , with zero DC field.
diffusion process we are able to further enhance the intergrain critical current density, Jc .
Based on the Bean critical-state model w8x, the critical current density Jc at the peak temperature Tp
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X Y Y Fig. 6. Ža. The real part x and the imaginary part x of susceptibility and Žb. the enlarged imaginary part x vs. temperature for Sb-doped Hg-1223 superconductors obtained by the diffusion process measured under a frequency 1 kHz and the DC fields Žwith peak positions left to right. m 0 Hdc s 0, 4.633, 9.00, 13.3, 22.1, 44.7, 87.5, 131.1 mT, with a constant AC field Hac s 153 Amy1 .
of the imaginary part of the AC susceptibility curve Ž x Y . for a cylindrical sample can be determined by the formula Jc Ž Hac , Tp . s HacrR, where Hac is the amplitude of the AC field and R is the radius of the
cylindrical sample w4,9x. Fig. 7 shows the critical current density Ž Jc . vs. temperature for Sb-doped Hg-1223 sample prepared by the diffusion process calculated in this way; values for pure and Sb-doped
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Fig. 7. The critical current density Ž Jc . vs. temperature for the Sb-doped Hg-1223 superconductors obtained by the diffusion process ŽU ., compared with Sb-doped ŽHg 0.95 Sb 0.05 .-1223 Ž`. and pure Hg-1223 Žv . superconductors prepared by a solid-state reaction method w5x. Jc is evaluated based on the Bean model.
Hg-1223 samples obtained in our previous work w4x Žsolid-state reaction method. are also plotted in this figure for comparison. From Fig. 7, it can be seen that the intergrain critical current density Jc for the Sb-doped sample prepared by the diffusion process increases much more significantly than those in the
other two samples. By linear extrapolation of the Jc ; T curve for individual samples, the Jc values at 77 K for the Sb-doped Hg-1223 superconductor prepared by the diffusion process, Sb-doped Hg-1223 and pure Hg-1223 superconductors obtained by reaction method are 2.5 = 10 7, 1.1 = 10 7 and 7.0 = 10 6
Fig. 8. The irreversibility lines ŽIL. for the Sb-doped Hg-1223 obtained by the diffusion process ŽU ., compared with Sb-doped ŽHg 0.95 Sb 0.05 .-1223 Ž`. and pure Hg-1223 Žv . superconductors prepared by a solid-state reaction method w5x.
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A.my2 , respectively. Thus, a significant improvement in the critical current density is obtained by the Hg-diffusion process. By defining the temperature where the maximum x Y occurs at a certain DC field as the irreversibility temperature ŽTir ., the irreversibility line for the sample obtained by the diffusion process can be plotted in Fig. 8 from the data of Fig. 6. The irreversibility lines from the data of our previous work w4x for the pure and Sb-doped Hg-1223 superconductors prepared by the reaction method are also plotted in this figure for comparison. The irreversibility line of the Hg-1223 superconductor is shifted significantly to higher temperature by doping with Sb w4x; however, the shift to higher temperature is even more dramatic in the sample prepared by using the Hg vapour diffusion process. This shift reflects the reduction of degradation in Jc under magnetic field. Therefore, the Hg vapour diffusion process method can reduce the degradation in Jc on increasing the magnetic field, when compared to an equivalent sample preparaed by a more conventional solid-state reaction method.
4. Conclusion We have successfully synthesized Sb-doped Hg1223 superconductor by a Hg vapour diffusion process and studied its superconductivity. The experi-
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mental results show that this diffusion process avoids the voids formed due to the release of O 2 and Hg gas during the previously employed solid state reaction route, and further modifies the nature of superconducting grain boundaries. It further enhances significantly the intergrain critical current density Jc and reduces the degradation in Jc under magnetic field. Acknowledgements The authors would like to express their gratitude to the Research Committee of the City University of Hong Kong for their financial support to this research . References w1x D. Dimos, P. Chaudhari, F.K. LeGoues, Phy. Rev. Lett. 61 Ž1988. 219. w2x L.K. Elbaum, C.C. Tsuei, A. Gupta, Nature 373 Ž1995. 679. w3x R.L. Meng, B.R. Hickey, Y.Y. Sun, Y. Cao, C. Kinalidis, J. Meen, Y.Y. Xue, C.W. Chu, Physica C 260 Ž1996. 1. w4x J.Q. Li, C.C. Lam, K.C. Hung, L.J. Shen, Physica C 304 Ž1998. 133. w5x R.L. Meng, L. Beauvais, X.N. Zhang, Z.J. Huang, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Physica C 216 Ž1993. 21. w6x T. Goto, Physica C 282–287 Ž1997. 891. w7x A. Kikuchi, T. Kinoshita, N. Nishikawa, S. Komiya, K. Tachikawa, Jpn. J. Appl. Phys. 34 Ž1995. L167. w8x C.P. Bean, Rev. Mod. Phys 36 Ž1964. 31. w9x F. Gomory, P. Lobatka, Solid State Communication 66 Ž6. Ž1988. 645.