Physics Procedia 27 ( 2012 ) 176 – 179

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committeedoi: 10.1016/j.phpro.2012.03.439

ISS2011

Influences of oxygen content on the carrier concentration and transport properties of Bi-2212 bulks

Shengnan Zhanga, Chengshan Lia*, Qinbin Haoa, Pingxiang Zhanga, Haiming Liua, Shenghui Yangb

aNorthwest Institute of Non-Ferrous Metal Research, Xi’an, 710016, China bNingbo Insititute of Materials Technology & Engineering, Chinese Academy of Science, Ningbo, China

Abstract

Bi Sr Ca0.95Cu O (Bi-2212) bulks have been obtained with Spark Plasma Sintering (SPS) method, with the precursor powders synthesized by co-precipitation process. Due to the vacuum environment involved in SPS, oxygen loss can be expected during sintering. Therefore, extra annealing processes under oxygen atmosphere were performed to enhance the superconducting properties of the bulks. Seebeck coefficient has been measured after different annealing process, in order to calculate the carrier concentration change with annealing process. The carrier concentration underwent a change from 0.178 to 0.198 per Cu ions after the 34 h annealing in oxygen at 773 K. The magnetic field dependence of critical current density, Jc, has been measured at 20 K on the samples with different annealing time. The critical current density increase from 550 to 5100 Acm-2 at zero field after the 34 h annealing, and the optimized Jc value can be obtained at the 34 h annealing sample over the entire magnetic field range. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ISS Program Committee. Keywords: High temperature superconductors; Bi-2212; Spark plasma sintering; Carrier concentration; Critical current density

1. Introduction

Bi2Sr2CaCu2O8 (Bi-2212), as the only materials so far, which can be made into isotropic round wires, has attracted much attentions [1-3] and shows great potential for the practical applications as the high temperature superconducting (HTS) cables [4, 5] and HTS magnets [6, 7]. Especially, due to the limit magnet field of low- temperature superconductor Nb3Sn magnet, Bi-2212 insert coils have been considered to be a necessary part for the fabrication of high field (~30 T) magnet [8, 9]. Therefore, the further enhancement of the superconducting properties and the understanding of transport mechanism of Bi-2212 based materials are the key factors for their practical application.

Spark plasma sintering (SPS) technique [10, 11] is a new method for the densification process. It has been widely used in many research field, such as piezoelectric ceramics [12], and thermoelectric alloys [13], etc.. And today, many studies have also been focused on the sintering of superconducting materials by SPS, like MgB2 [14]. However, there is no report about the SPS sintering of Bi-2212 compounds yet. The spark between grains can partially melt the Bi-2212 at grain boundaries, and combining with the high pressure, higher density and better alignment of grains can be expected [15]. Especially, due to the vacuum environment involved during the sintering process, the obtained samples should be with oxygen content of less then 8. Therefore, it is a chance to tune the oxygen content in Bi-2212 phase by a simple annealing process under oxygen atmosphere and to investigate the relationship among oxygen content, carrier concentration and superconducting properties.

Seebeck coefficient, also known as thermoelectric power, is very sensitive to the electronic band structures, and can be used to calculate the carrier concentrations in superconducting materials [16-18]. Sedky et al [19] has compared with the carrier concentration calculated by Seebeck coefficient and critical temperature. And the validity of this

* Corresponding author. Tel.: +86-29-87931079; fax: +86-29-86224486. E-mail address: csli@c-nin.com.

Available online at www.sciencedirect.com

© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee

Shengnan Zhang et al. / Physics Procedia 27 ( 2012 ) 176 – 179 177

calculation has been proved. In our study, bulk samples sintered by SPS have been annealed under oxygen atmosphere to detect the superconducting properties change with carrier concentration.

2. Experimental

Bi2.1Sr1.96Ca0.95Cu2.0O8 precursor powders were prepared with the modified co-precipitation process [20] with the starting materials of Bi2O3, SrCO3, CaCO3, and CuO (> 99.9%). Single phase Bi-2212 can be obtained after a series of calcination processes in air at 800 oC/ 12 h, 820 oC/ 20 h, and 850 oC/ 20 h with intermediate grinding. The precursor powders were densely packed into a graphite die. During the SPS process, the samples were heated to 800 oC within a very short time and kept at 800 oC for 5 min then cooled down to room temperature. The entire sintering process was performed within 40 min for each sample. The density of samples was measured through Archimedes method. The pellet with the diameter of 12.7 mm and thickness of 1.5 mm has the relative density of 99.5% to the theoretical value.

Annealing process under oxygen atmosphere has been performed at 500 oC for 10 h and then another 24 h (total annealing time 34 h). The sample has been weighted before and after each annealing process in order to detect the oxygen absorption values. Seebeck coefficient were measured on ZEM-2 from room temperature (300 K) to 383 K and each value at different temperature has been measured for four times to confirm the accuracy of measurement.

The sintering process was performed on SPSS 5500. Polycrystalline X-ray diffraction (XRD) patterns were taken on a X-ray diffraction (XRD, Philips PW 1710) with Cu-K radiation. Differential scanning calorimery (DSC) and Thermogravimetric analysis (TG) were performed on TGA/DSC1 by Mettler Toledo. The magnetic hysteresis loop was measured suing a DC superconducting quantum interference device (SQUID) at 20 K.

3. Results and Discussion

Co-precipitation technique, combining with a step-by-step calcinations process has been proved to be very efficient for the synthesis of Bi-2212 precursor powders. As shown in Fig. 1(a), after the final calcinations at 850 oC, single phase Bi-2212 has been obtained, which can be indexed into the orthorhombic lattice structure with random orientation. The average powder size is about 10 m according to previous report [20]. After the sintering process, texture structure can be detected. The texture degrees of (00l) peaks are mostly used to judge the quality of intergrain connections, which can be calculated as following,

00 00 2212( ) 100%l l spF I I I (1)

where I00l represents for the intensity of (00l) peak for Bi-2212 phase, I2212 is the total diffraction intensities of Bi-2212, and Isp is the total diffraction intensities of the other secondary phases, including Bi2Sr2CuO6 (Bi-2201), alkali earth cuprates (AEC), etc.. The texture degree of (00l) peak of the SPS bulk is about 60%. After the oxygen annealing for 34 h, the XRD pattern shifted to the higher angle direction, representing to the shrink of lattice. The absorption of oxygen tend to incorporate into Bi-O layers, which can reduce the net positive charge in the Bi-O planes and hence the repulsion between them [21], thus lead to the decrease of c axis. This result is consistent with the previous result [22]. Meanwhile, after the annealing process, extra secondary phase of 1:1 AEC can be detected. The increase of oxygen partial pressure can increase the reacting activity of oxides, thus result in the precipitation of AEC phase.

Fig. 1. (a) X-ray Diffraction patterns of Bi-2212 precursor powders; (b) X-ray Diffraction patterns of Bi-2212 bulk before and after annealing under oxygen atmosphere

In order to observe the dynamic behavior during the annealing process under oxygen atmosphere, DSC and TG has been taken on a piece of bulk sample. As shown in Fig. 2, during the heating process, two oxygen absorption peaks can be detected, corresponding to the absorption of oxygen by Bi-2212, and the decomposition of Bi-2212 into 1:1 AEC and other secondary phase, respectively. Then during the dwell and cooling process, the phase transition from 1:1 AEC back into Bi-2212 take place. Comparing with the weight before and after annealing, a slight weight increase can be detected, corresponding to the net absorption of oxygen into Bi-2212 lattice. With the weight increase value, the value of in Bi2.1Sr1.96Ca0.95Cu2.0Ox+ can be calculated as listed in Table 1. After each annealing cycle, of about 0.05 has been absorbed, which can lead to the increase of carrier concentration.

178 Shengnan Zhang et al. / Physics Procedia 27 ( 2012 ) 176 – 179

Fig. 2. DSC and TG plots of Bi-2212 bulk during the annealing process under oxygen atmosphere

Table 1. oxygen absorption values of Bi-2212 bulk after the annealing process, the Seebeck coefficient and carrier concentration change during annealing

Before anneal 10 h annealing 24 h annealing

oxygen absorption - +0.05075 +0.05180

Seebeck coefficient (@RT) 12.39 1.626 1.082

Carrier concentration per Cu ion 0.178 0.197 0.198

According to the previous report by Cooper et al, for small hole concentration per Cu ion, P, (P<0.2), the following equation can be used to estimate the hole concentration values by the room temperature Seebeck coefficient:

where, kB and e are the Boltzmann constant and electron charge, S is the Seebeck coefficient with the unit of VK-1, the extra ln2 term comes from the orbital degrees of freedom (assuming two fold orbital degeneracy). This equation is derived using a single-band Hubbard model with the assumption that the bandwidth is much less than kBT. With this equation, P was estimated and listed in Table 1. After the annealing process, carrier concentration increase from 0.178 to 0.198 per Cu ions. Therefore, it can be indicated that the annealing process change the samples from underdoped region to optimal doping region, which may lead to the enhancement of superconducting properties.

Meanwhile, the temperature dependence of Seebeck coefficient can also be used to indicate the transport behavior of samples. As shown in Fig. 3, the Seebeck coefficent of unannealed sample is about 12.3 VK-1 and decrease with the increase of temperature, which representing the semiconductor behavior of carrier transport. The temperature dependence changed from negative to slightly positive, which representing to the change of transport behavior from semiconductor to metal due to the increase of carrier concentration.

Fig. 3. Temperature dependence of Seebeck coefficient from 300 to 383 K for the samples after different annealing time

The magnetic field dependence of critical current density Jc(B) at 20 K have been calculated from the magnetic hysteresis loop from 0 to 20 000 Oe based on the Bean model, Jc=20 M/(va(1-3a/b)), where M is the width of the magnetization loop in emu; v is the volum of the sample in cm3, a and b (a<b) are the sample dimensions perpendicular to the field in cm. As shown in Fig. 4 (a), the superconducting properties is very weak for the sample before annealing due to the low carrier concentration. While after 10 h annealing, the critical current density increased to 3100 Acm-2 at zero field. After another 24 h annealing in oxygen, the Jc value over the entire magnetic field range increase. All these results can be attributed to the increase of carrier concentration. As shown in Fig. 4 (b), the normalized critical current density of 34 h annealing sample decreased rapidly with the magnetic field. The unannealed sample showed the weakest magnetic field dependence at higher magnetic field region. For the unannealed sample, a large number of point defects can be expected due to the nonstoichiomitry of oxygen, which can work as the

1ln ln 22

Bk PS

e P

Shengnan Zhang et al. / Physics Procedia 27 ( 2012 ) 176 – 179 179

pinning centers during the magnetic field transport properties measurement. With the increase of oxygen content, the density of pinning centers decreased therefore, the Jc(B) properties decreased.

Fig. 4. (a) Magnetic field dependence of critical current density at 20 K from 0 to 20 000 Oe; (b) Magnetic field dependence of normalized critical current density at 20 K from 0 to 20 000 Oe

4. Conclusion

Bi-2212 bulks have been prepared with spark plasma sintering technique with proper operation parameters. After sintering, the texture degree of (00l) peak can be calculated from XRD patterns of about 50 %, representing to the slight texture alignment of grains. After annealing under oxygen atmosphere at 500 oC, the lattice parameter decreased due to the increase of oxygen content and small amount of 1:1 AEC can be detected as the secondary phase. The absorption of oxygen lead to the increase of hole concentration, which shift the sample from under doped region to optimal doping region. An obviously increase of critical current density has been observed over the entire magnetic field range at 20 K, attributed to the increase of carrier concentration. The curitical current density we observed here are not quite high comparing with the traditionally prepared bulks of ~105 Acm-2 at 20 K [23], which maybe attributed to the poor texture and the poor oxygen content caused by the SPS method. However, due to the decrease of pinning centers, the normalized critical current density decreased with the applied magnetic field after annealing. Equivalent doping can be expected for the further increase of magnetic field superconducting properties.

Acknowledgements

This research was supported by the National Basic Research Program of China (973 Program) under contract No. 2011CBA00104, the international scientific and technological cooperation projects of China under contract No. S2010GR0518, the National Natural Science Foundation of China under contract No. 51102198.

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