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Cryst. Res. Technol. 39, No. 12, 1063 – 1069 (2004) / DOI 10.1002/crat.200410291

Microstructural, thermal, and electrical properties of Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor T. S. Kayed*1, N. Calınlı2, E. Aksu2, H. Koralay2, A. Günen3, İ. Ercan4, S. Aktürk5, and Ş. Çavdar2 1 2 3 4 5

Department of Electrical and Electronics Engineering, Atilim University, 06836 İncek, Ankara, Turkey Ankara Nuclear Research and Training Center, TAEA, 06100 Beşevler, Ankara, Turkey Department of Physics, Faculty of Arts and Science, Gazi University, Teknik Okullar, Ankara, Turkey Abant Izzet Baysal University, Duzce Technical Education Faculty, Bolu, Turkey Department of Physics, Faculty of Arts and Science, Kırıkkale University, Kırıkkale, Turkey

Received 24 February 2004, revised 10 April 2004, accepted 20 April 2004 Published online 1 November 2004 Key words glass-ceramic high temperature superconductors; Hall effect; BSCCO. PACS 74.72.Jt; 74.62.Bf; 74.25.Fy A glass-ceramic Bi1.7V0.3Sr2Ca2Cu3Ox superconductor was prepared by the melt-quenching method. The compound was characterized by scanning electron microscopy, x-ray diffraction, differential thermal analysis, current-voltage characteristics, transport resistance measurements, and Hall effect measurements. Two main phases (BSCCO 2212 and 2223) were observed in the x-ray data and the values of the lattice parameters quite agree with the known values for 2212 and 2223 phases. The glass transition temperature was found to be 426 °C while the activation energy for crystallization of glass has been found to be Ea = 370.5 kJ / mol. This result indicates that the substitution of vanadium increased the activation energy for the BSCCO system. An offset Tc of 80 K was measured and the onset Tc was 100 K. The Hall resistivity ρH was found to be almost fieldindependent at the normal state. A negative Hall coefficient was observed and no sign reversal of ρH or RH could be noticed. The mobility and carrier density at different temperatures in the range 140-300 K under different applied magnetic fields up to 1.4 T were also measured and the results are discussed.

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1

Introduction

There has been intensive research on finding ways to enhance the growth of 2223 phase (because of its high Tc) in order to increase its volume fraction in any BSCCO compound. Several heat treatments together with annealing in various gas atmospheres and applying different pressures were possible ways to obtain pure 2223 phase. Another way to do so is doping or substitution by various elements. For example, the effects of doping BSCCO compounds with Pb and other elements have been previously studied [1-9]. Vanadium substitution with concentrations of 0.1 or above has been found to decrease the Tc of (2223) phase [10-17]. Moreover, because of the smaller size of V compared with Bi, it is able to bring the Cu-O layers close to each other and, consequently, increases the coupling effect. Preparing BSCCO superconducting materials by the melt-quenching method was first achieved by Komatsu [18]; after which, several compositions of BSCCO high-Tc superconductor were prepared by other groups [19-21]. Many advantages may be offered when materials are produced by the glass-ceramic route and in some cases better properties than the properties of samples produced by the usual solid-state reaction method are obtained. For example, superconductors in the shape of bar, pipe, wire, tape, and coil; having high density, homogeneous structure, strong links between the grains, and pore-free can be produced by the glass ceramic method. In this method, a mixture of raw materials is melted at a high temperature and quenched to form glass. ____________________

* Corresponding author: e-mail: [email protected] © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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T. S. Kayed et al.: Properties of Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor

After this, it is reheated for crystallization and the crystallized products are called glass-ceramics. Unfortunately; Yi-, Tl-, and Hg-based high temperature superconductors can’t be synthesized by the glassceramic method and only BSCCO system may be produced because of the existence of glass formers or modifiers like Bi, Ca, and Sr in its composition. In this investigation, vanadium was substituted for 15% of Bi in Bi2Sr2Ca2Cu3Ox to form Bi1.7V0.3Sr2Ca2Cu3Ox compound. Beside the effects of doping on changing the phase, it has also different effects on the physical, electrical, and magnetic properties of the compound. The properties of our composition were studied using Scanning Electron Microscopy (SEM), X Ray Diffraction (XRD), Differential Thermal Analysis (DTA), current-voltage (I-V) characteristics, and Hall effect measurements.

2

Experimental details

Amounts of Bi2O3, SrCO3, CaCO3, CuO, and V2O5 were adjusted to obtain the nominal composition Bi1.7V0.3Sr2Ca2Cu3Ox. The powders were mixed for 1 h in an agate mortar to get a homogeneous mixture. This mixture was melted in air for 90 min at 1150 °C. The melted sample was then rapidly quenched between two copper plates in order to sustain the formed phases during melting. Shiny black amorphous glass sheets were obtained at this stage. Part of the glass sheets were then crushed and ground in an agate mortar for thermal analysis. The crystallizations were investigated by DTA technique (using TA SDT-Q600 instrument) from room temperature up to 900 °C with heating rate of 10K / min in flowing nitrogen. The remaining part of the glass sheets was heated from room temperature up to 840 °C at a rate of 2K / min in a tube furnace and kept for 50 h at this temperature in oxygen atmosphere. The surface morphology of the sample was investigated by SEM technique (using JEOL 5600 scanning electron microscope). The formation of superconducting phases was examined by the XRD method. XRD pattern of the sample was obtained with a Rigaku DMAX 2200 diffractometer using monochromatic CuKα radiation in a 2θ range of 5-61°. The search file, PDF2, of the International Centre for Diffraction Data (ICDD) has been used for phase identification. A least square computer program was used to calculate the lattice parameters of each phase. We used the standard DC four-probe method to measure the I-V characteristics and the resistancetemperature (R-T) plot in zero magnetic field. In the R-T measurements, a 1 mA current was applied with a current source (Keithley 228A) between the outer probes and the voltage drop between the inner probes was measured with a nanovoltmeter (Keithley 2182). The I-V characteristics were measured by applying a current that ranges from 1 to 66 mA to the outer probes and measuring the voltage drop between the inner probes. For the Hall effect measurements, we used the van-der-Pauw configuration were four point contacts are fixed on the surface at the corners of the sample. Lake Shore 7507 Hall effect Measurements System (HMS) was used to measure the Hall parameters at different temperatures. Hall effect data were collected using an IEEE interface and a data acquisition software provided by Lake Shore. The sample was cooled in a closed cycle cryostat (Advanced Research Systems) and Lake Shore 340 temperature controller. Measurements were performed under magnetic fields up to 1.4 T, using 7 inch variable gap electromagnet, applied perpendicular to the current direction.

3

Results and discussion

The SEM micrograph of the sample is shown in fig 1. The grains of the sample are randomly oriented and the growth of granular small crystals is observed. This is attributed to the vanadium substitution [22]. Fig 2 shows the x-ray diffraction pattern of the sample. Almost all of the peaks in the pattern were indexed. It is found that the sample contains both BSCCO 2212 (low Tc) and BSCCO 2223 (high Tc) phases. The other phase identified was SrV2O6 in 29.68° peak. The crystal structure was orthorhombic and the lattice parameters were found to be a = 5.390, b = 5.413, c = 30.813 Å for the low Tc phase and a = 5.435, b = 5.412, c = 36.926 Å for the high Tc phase. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 1 SEM micrograph of the surface of Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor with magnification of 2500 times.

Fig. 2 XRD pattern of Bi1.7V0.3Sr2Ca2Cu3Ox glassceramic superconductor showing that the sample contains BSCCO 2212 phase (marked with *) and BSCCO 2223 phase (marked with +), and SrV2O6 (marked with ♦).

The DTA results are shown in fig 3. The sample shows a glass transition temperature (Tg) at 426 °C while the first crystallization temperature Tx1 was found to be 495 °C. The value (Tx1-Tg) is quite wide which is a measure of thermal stability. The other exothermic peaks (Tx2, Tx3, and Tx4) were measured to be Tx2 = 527 °C, Tx3 = 541 °C, and Tx4 = 618 °C respectively. In addition, two endothermic peaks (Ty1 and Ty2) with the partial melting of the sample are found at Ty1 = 776 °C and Ty2 =866 °C. The activation energy of the glass transition was estimated by using the non-isothermal kinetic theory of Kissenger [23]. The basic relation was modified by Matusita and Sakka [24] and is known as an extension of the Johnson-Mehl-Avrami kinetic model [25, 26]. This relation is expressed as; ln ( β Tx2 ) =- ( E a RTx ) +C

where β is the heating rate, R is the gas constant, Tx is the crystallization peak temperature, Ea is the activation

energy for the crystal growth, and C is a constant. The plot of ln ( β Tx2 ) versus 1000/Tx is shown in fig 4 with

heating rates of 5, 10, 15 and 20K/min. The activation energy was found to be Ea = 370.5 kJ/mol for © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Bi1.7V0.3Sr2Ca2Cu3Ox composition. In earlier studies [27-31], the activation energy of BSCCO was found to range from 75 to 340 kJ / mol. Comparing our results with such studies shows that the substitution of vanadium increased the activation energy for crystallization of BSCCO system.

Fig. 3 DTA results for Bi1.7V0.3Sr2Ca2Cu3Ox glassceramic superconductor.

Fig. 5 The temperature dependency of Hall resistivity ρH at different applied magnetic fields up to 1.4 T for Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor. Inset: R-T plot at zero magnetic field.

Fig. 4 Modified Kissenger plot for crystal growth.

Fig. 6 I-V characteristics of Bi1.7V0.3Sr2Ca2Cu3Ox glassceramic superconductor at different temperatures under zero magnetic field. Inset: Change of β with temperature near Tc.

Understanding the electrical properties of high-Tc superconductors in the normal (above Tc) and also in the mixed states is a key step for exploring its conduction mechanism. To the authors’ knowledge, Hall effect measurements have not been performed yet in any glass-ceramic superconductor. So we decided to use this powerful tool to characterize our sample. The Hall resistivity ρH was measured at different temperatures in the range from 20 to 300 K under different applied magnetic fields up to 1.4 T. The representative curves are shown in fig 5. ρH was found to be positive for all temperatures and applied magnetic fields (positive or negative). The resistivity drops from about 16.5 µΩ m at 100 K to zero at 80 K showing a wide transition region of about 20 degrees. In the normal state, resistivity is almost the same for all applied magnetic fields. But in the mixed state between 80 and 100 K, some fluctuations were observed in the resistivity values at different magnetic fields. This was not a systematic shift of the curves to lower temperatures with the application of magnetic field as usually observed in magnetoresistive materials. It is believed that the reason for that is the existence of two phases in the sample (BSCCO 2212 and 2223), as confirmed by the XRD analysis and seen in the wide transition width from 100 to © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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80 K. Electrical resistance versus temperature (R-T) at zero magnetic field, measured by the standard fourprobe method at a DC current of 1 mA is shown in the inset of fig 5. Again, a wide transition width is observed in the measurements of transport resistance indicating the existence of more than one phase in the sample. We measured the I-V characteristics in zero magnetic field at different temperatures in the range 60-110 K. These are shown in fig 6. For high-Tc superconductors, the I-V data are found to obey a power law expression V ∝ I β(T) for which the exponential parameter β approaches unity in the normal state [32, 33]. Our I-V data were fitted to the same expression and the values of β at different temperatures and zero magnetic field are shown in the inset of fig 6. For temperatures less than Tc , β has negative values and decreases from –2.48 to – 4.36 as temperature increases from 60 to 74 K. Approaching the critical temperature, β started to increase and suddenly changed from –2.60 to +1.45 at 80 K after which it was saturated to a value of 1 at 110 K.

Fig. 7 The temperature dependency of |RH| at different applied magnetic fields up to 1.4 T for Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor.

Fig. 8 The temperature dependency of the carrier density at different applied magnetic fields up to 1.4 T for Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor.

Fig. 9 The temperature dependency of Hall mobility at different magnetic fields for Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor.

The magnitude of the Hall coefficient |RH| measured at different temperatures in the range 20-300 K under different applied magnetic fields up to 1.4 T is shown in fig 7. RH is negative, indicating that conduction is done by electrons and not by holes; and takes values in the order of magnitude of 10-5 m3 / C. We can notice the sensitivity of Hall coefficient to small magnetic fields where |RH| drops significantly under applied magnetic field of 0.6 T. |RH| increases when the temperature is increased. In high-Tc superconductors, it was found that 1 / RH is linearly dependent on temperature [34, 35], which is a remarkable and puzzling property. In our case, 1 / |RH| was fitted to the linearity relation; © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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1 / |RH| = aT + b, where a and b are constants. 1 / |RH| linearly decreases with increasing temperature especially at temperatures higher than Tc. It is unclear whether the T-dependent RH of our sample is caused by multi-band effects or is a reflection of an unusual transport mechanism. The carrier density curves at different temperatures in the range from 140 to 300 K under different applied magnetic fields up to 1.4 T are shown in fig 8. The carrier density decreases slightly as temperature increases. The behaviour is almost the same for all applied magnetic fields but higher applied fields give higher carrier densities. The measurements of Hall mobility at different magnetic fields in the temperature range from 140 to 300 K are shown in fig 9. Mobility is almost temperature independent and decreases with increasing magnetic field. We could not find any measurements of mobility for high-Tc superconductors in the literature but some reports [36-39] showed the carrier density of La-Sr-Cu-O, Nd-Ce-Cu-O, and Hg-Ba-Ca-Cu-O superconducting systems. According to these reports, it was noticed that the carrier density increases linearly as temperature increases which is in contrast to our case. The carrier density is determined by the Hall effect measurements as nH = 1 / eRH. It is obvious from this relation that increasing RH will decrease nH and consequently the temperature dependency of carrier density will be related to the temperature dependency of RH. We saw that RH of our sample increases with temperature leading to a carrier density decreasing with temperature.

4

Conclusions

We had two main phases (BSCCO 2212 and 2223) in the vanadium doped sample. The glass transition temperature was found to be 426 °C and the activation energy for crystallization of glass has been found to be Ea = 370.5 kJ / mol. This result indicates that the substitution of vanadium increased the activation energy for the BSCCO system. An offset Tc of 80 K was measured and the onset Tc was 100 K. The Hall resistivity ρH was found to be almost field-independent at the normal state. A negative Hall coefficient (indicating an electron-like conduction) was observed without any sign reversal of ρH or RH. The mobility and carrier density results, at different temperatures in the range 140-300 K under different applied magnetic fields up to 1.4 T, showed higher carrier densities for higher applied fields. The sample has an almost temperature independent mobility and the carrier density decreases slightly with temperature. Acknowledgements This work was supported by the Turkish Atomic Energy Authority (TAEA) under project no: DPT 98K120370

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