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Barium carbonate as an agent to improve the electrical properties of neodymium-barium-copper system at high temperature
Journal of Alloys and Compounds 649 (2015) 809e814
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Barium carbonate as an agent to improve the electrical properties of neodymium-barium-copper system at high temperature J.P. Fernandes c, G.W. Duarte c, f, C. Caldart d, C.T. Kniess e, O.R.K. Montedo d, M.R. Rocha d, H.G. Riella c, M.A. Fiori a, b, * Chapeco ria da Regia ~o de Chapeco (UNOCHAPECO), , SC, 89809-000, Brazil Post-Graduate Program in Environmental Science, Universidade Comunita Chapeco ria da Regia ~o de Chapeco (UNOCHAPECO), , SC, Post-Graduate Program in Technology and Management of the Innovation, Universidade Comunita 89809-000, Brazil c polis, SC, 88040-900, Brazil Post-Graduate Program in Chemical Engineering, Universidade Federal de Santa Catarina (UFSC), Floriano d Post-Graduate Program in Science and Materials Engineering, Universidade do Extremo Sul Catarinense (UNESC), Criciúma, SC, 88806-000, Brazil e ~o Paulo, SP, Brazil Post-Graduate Program in Professional Master in Management, Universidade Nove de Julho, Sa f rio Barriga Verde (UNIBAVE), Santa Catarina, SC, Brazil Research Group in Technology and Information, Centro Universita a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2015 Received in revised form 29 May 2015 Accepted 29 June 2015 Available online 6 July 2015
Specialized ceramics are manufactured under special conditions and contain specific elements. They possess unique electrical and thermal properties and are frequently used by the electronics industry. Ceramics containing neodymium-barium-copper (NBC) exhibit high conductivities at low temperatures. NBC-based ceramics are typically combined with oxides, i.e., NBCo produced from neodymium oxide, barium oxide and copper oxide. This study presents NBC ceramics that were produced with barium carbonate, copper oxide and neodymium oxide (NBCa) as starting materials. These ceramics have good electrical conductivities at room temperature. Their conductivities are temperature dependent and related to the starting amount of barium carbonate (w%). © 2015 Elsevier B.V. All rights reserved.
Keywords: NdBaCu alloy Conductive ceramic Conductive oxides alloys Ceramic type NBC
1. Introduction Under most operating conditions, metals and their alloys are widely recognized as good conductors of electricity and heat. However, harsh environments can degrade metals and limit their use. It is therefore necessary to develop new materials capable of overcoming these deficiencies [1]. REBCO-type (RE ¼ rare earth, B ¼ barium, C ¼ copper, O ¼ oxygen) ceramics exhibit great potential for the development of robust conductive elements. These materials primarily consist of metal oxide alloys that are compressed and heated to high temperatures to accelerate atomic diffusion [2e4]. This class of ceramics has been extensively investigated as superconductors [5e8]. These special ceramics are often used in the electronics industry. The introduction of select elements and atypical production conditions can impart peculiar electrical and thermal properties. Some
* Corresponding author. E-mail address: fi[email protected] (M.A. Fiori). http://dx.doi.org/10.1016/j.jallcom.2015.06.261 0925-8388/© 2015 Elsevier B.V. All rights reserved.
ceramics have superconducting properties at low temperatures and are often used in deformation sensors. Neodymiumbarium-copper (NBC) represents the base metallic components for a class of ceramics that exhibits high conductivity at low temperatures. NBC ceramics are typically produced from metal oxides, i.e., neodymium oxide, barium oxide and copper oxide. Many publications have focused on the superconducting properties of these ceramics [9e14], and have studied the effects of Nd and Ba on electrical properties [15e17]. However, the majority of studies have neglected to thoroughly investigate the electrical properties of these ceramics at room and higher temperatures. This class of materials has found commercial use in components of monitoring sensors and their controls. However, variations in temperature can affect the electrical performance of NBC-based ceramics and potentially reduce sensor efficiency. This work describes the synthesis and characterization of NBC ceramics produced with barium carbonate (NBCa) and evaluates their electrical properties. The conductivities of NBCa as a function of temperature and applied potential are described, and a comparison with the properties of NBCo is presented.
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2. Experimental procedure 2.1. NBC samples The NBC samples (neodymiumebariumecopper) were prepared by mixing Nd2O3 (neodymium oxide - Sigma Aldrich), CuO (copper oxide - Vetec), and BaO (barium oxide) or BaCO3 (Barium carbonate - Sigma Aldrich), all with a purity of 99.9%. Two types of NBC mixtures were prepared for electrical measurements: NBCa (Nd2O3 e BaCO3 e CuO) and NBCo (Nd2O3 e BaO e CuO). Powdered samples of different compositions were cylindrically formed with a diameter of 20 mm and a thickness of 3 mm. To eliminate the water content, the compounds were dried in an oven (Fanem Orion 502) with a controlled temperature of 105.0 C for 60 min. Cylindrical samples were compacted with a pressure of 312.5 kgf/cm2, according to the work developed by Fiori et al. and others [18e21]. The pressed NBCa and NBCo samples were sintered at 1020 C (heating rate of 4.00 C/min) for 24 h and then cooled to room temperature. Samples were then polished to eliminate surface irregularities and to improve contact with electrical probes, according to procedures defined by Fiori et al. and other authors [18e21]. To study the effect of the barium concentration (NBCa samples) on the electrical conductivity, samples were produced with three different concentrations of barium carbonate: 50.0 wt%, 45.5 wt% and 40.0 wt%. To study the dependence of electrical conductivity with temperature and electrical polarization, the NBCa samples were prepared with 45.5 wt% BaCO3. Barium concentrations were determined by considering the values adopted by Fiori et al. in their preliminary studies concerning these materials [18]. 2.2. X-ray and SEM characterization The NBCa and NBCo samples were analyzed by x-ray diffraction spectroscopy (XRD) and the results were treated with Rietveld methods. The x-ray analyses were conducted using a Shimadzu LabX XRD-6000 (Cu target with wavelength (l) of 1.5406 Å, 40.0 kV voltage, 30.0 mA current, and scan speed of 2.00 deg/min). Ceramic morphologies were analyzed by scanning electron microscopy (SEM) using a JEOL JSM-6390 LV at 15 kV. 2.3. Electrical characterization To determine the electrical conductivities of the NBCa and NBCo samples, a four-probe method was applied [22]. Electrical measurements of the samples were performed in triplicate as a function of different external biases. The currents of NBC samples were obtained at the following potentials: 0.50 V, 1.00 V, 1.50 V, 2.00 V and 2.50 V. 3. Results and discussion 3.1. X-ray analysis Fig. 1 shows the X-Ray spectra for NBCo and NBCa samples. The results show peaks from oxide phases containing barium, copper and neodymium present in both materials. Additionally, significant differences between the X-Ray diffractograms were observed between 2q values of 20.00 and 50.00 . A comparison between the diffractograms of NBCo and NBCa indicates significant differences in the crystalline structures. While some peaks of the two materials are coincident, other peaks are offset. These displacements are strongly indicative of lattice mismatches, while differences in the magnitudes of the peak signals are related to the degree of crystallinity. These details are presented in Fig. 2a and b.
Fig. 1. X-ray diffractograms 0066or (a) NBCo and (b) NBCa.
The x-ray spectrum for the NBCa show characteristic peaks at 2q equal to 18.40, 20.08, 27.60, 28.90 and 29.90 and the absence of 2q at 19.60, 25.30, 26.08, 30.40, 30.80, 33.60 and 37.00 , which are present in NBCo. The shared peaks correspond to oxide phases that contain all three metals (Ba, Cu, and Nd), but in significantly different arrangements. Fig. 3 shows the relative amount of each phase that contains Ba, Cu and Nd in both NBCo and NBCa ceramics as determined by Rietveld analysis. In NBCa and NBCo the phase Ba(1.55)Cu(3)Nd(1.45)O(7.16) represents more than 50.00% of the total volume. However, a significant difference between the two ceramics is the reduced amount of monometallic oxide phases exhibited by the ceramic sintered with barium carbonate (NBCa) relative to NBCo. In the NBCo ceramic, 34.00% of the phases contain CuO and Nd2O3 (BaO was not observed), while in the NBCa ceramic the combined percentage for these species is 13.50%. As is implied by the above statement, phases containing barium (Ba) occupy a greater volume in NBCa (85.50%) compared with NBCo (65.50%). Of particular note is the increased presence of BaNd2O4 (b) and BaCu3O4 (a) phases in NBCa. The percentages of b and a phases are 19.50% (0.80%) and 18.60% (8.84%) for NBCa (NBCo), respectively. Table 1 summarizes the compositions of NBCa and NBCo as determined by XRD. During the sintering of NBCa precursors at 1020 C, barium carbonate dissociates and carbon monoxide evolves, as shown in equation (1). Carbon monoxide reduces neodymium and copper oxides resulting in the generation and loss of carbon dioxide (equations (2) and (4)). Ultimately, these reactions lead to the
J.P. Fernandes et al. / Journal of Alloys and Compounds 649 (2015) 809e814
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Table 1 Relative percentage of phases in NBCa and NBCo. Sample
BaO (%)
Nd2O3 (%)
CuO (%)
BaNd2O4 (%)
BaCu3O4 (%)
NBCa NBCo difference
0.0 0.0 0.0
2.79 15.76 12.97Y
10.84 18.47 7.63Y
19.50 0.80 þ18.70[
13.60 8.84 þ4.76[
(Nd2O2)þ2 þ (BaO2)2 / Nd2BaO4 [(Phase b]: (1020 C)
(3)
(CO)þ2 þ CuO / (Cu)þ2 þ CO2: (1020 C)
(4)
O2 þ 3(Cu)þ2 þ (BaO2)2 (s) / BaCu3O4 [Phase a]: (1020 C)
(5)
Lattice parameters and volumes of the crystallites were determined by Rietiveld analyses. Table 2 shows the lattice parameters of each phase. The barium phases possess the larger lattice parameters and, consequently, larger crystallite volumes than monometallic-oxide phases. The b phase has the largest unit cell volume, with values of 396.9723 Å (81379-ICSD) and 475.9245 Å (81380-ICSD). The unit cell volume of the a phase is 329.6251 Å (65881-ICSD). The b and a phases are present with high significance only in the NBCa samples and can be attributed to use of barium carbonate. These characteristics favor an increase in the amount of bariumphases and larger volume crystallites than the phases present in NBCo. Therefore, the differences in electrical properties observed between the two types of NBCs can be attributed to the different amounts of b and a phases, which enhance crystallinity and increase electrical conduction. 3.2. SEM analyses Fig. 2. (a) Comparison between x-ray diffractograms of NBCo and NBCa ceramics e 2q from 18 to 27 and (b) between x-ray diffractograms of the NBCo and NBCa ceramics e 2q from 27 38 .
formation of b and a phases, as shown in equations (3) and (5), respectively. BaCO3 / (BaO2)2 þ (CO)þ2: (1020 C)
(1)
(CO)þ2 þ Nd2O3 / (Nd2O2)þ2 þ CO2[: (1020 C)
(2)
Fig. 4 shows SEM images of the NBCo and NBCa samples. The SEM micrographs show a significant difference in surface morphology. There are regular crystal grains on both materials, but the grains are bigger for NBCa. On the surface of NBCa is observed a long range of crystalline regularity. These differences can be attributed to the larger volume of barium-containing phases a and b. Therefore, the greater degree of crystallinity associated with NBCa, in conjunction with the long range crystalline regularity, contribute to the increase in electrical conductivity. 3.3. Electrical measures
Fig. 3. Phases present in NBCo and NBCa ceramics as determined by Rietveld methods.
The electrical resistivity and conductivity values for NBCa and NBCo at different applied potentials are given in Tables 3 and 4. Fig. 5 compares the dependence of electrical conductivity on external bias for both ceramics. The results indicate a higher conductivity for the ceramic manufactured using barium carbonate (NBCa) than for the ceramic produced from barium oxide (NBCo). NBCa has electrical conductivity on the order of 103 S m1, while NBCo has an electrical conductivity on the order of 1 S m1. These results indicate that the variation in electrical conductivity with respect to the evaluated range in bias is not significant for NBCo. However, the electrical conductivity values for NBCa are strongly dependent on the applied potentials studied. An increase in positive bias promotes a significant increase in the electrical conductivity of NBCa until a value of 2.5 V is reached, at which point the electrical conductivity is not significantly modified. The results reveal that conductivities for NBCa were consistently higher than for NBCo, which can be attributed to two main factors:
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Table 2 Lattice parameters and volumes for different phases as determined by Rietveld methodology. Phases
(1) (2) (3) (4) (5) (6) (7)
References ICSO
Ba(1.55)Cu(3)Nd(1.45)O(7.16) BaNd(2)O(4) e 1 BaCu(3)O(4) CuO BaCuNd(2)O(5) Nd(2)O(3) BaNd(2)O(4) e 2
Crystal lattice parameters determined by Rietveld methods
75683 81379 65881 87125 201424 100212 81380
a [Å]
b [Å]
c [Å]
3.894346 10.26672 7.523489 4.691829 6.692198 3.957826 10.70508
3.894346 3.001452 11.78534 3.419484 6.692198 3.957826 3.612342
11.57033 12.88241 3.717567 5.129863 5.866688 6.309534 12.30720
Volume [Å]
175.4748 396.9723 329.6251 82.30165 262.7426 98.83498 475.9245
increase in the percentage of barium-containing phases; as a consequence, there is an increase in crystalline regions and a decrease in the amount of lattice imperfections. As a and b phases are more conductive, their increased presence in NBCa raises the conductivity of the sample as a whole. Fig. 6 shows that conductivity values for the NBCa vary significantly as a function of measurement temperature. These results indicate that increasing the electrical polarization of NBCa leads to an increase in their temperature and, as a consequence, an increase in their electrical conductivity. This suggests the presence of a nonOhmic mechanism as the dominant conducting mechanism; however, with only these results, a definitive conclusion is not possible. Fig. 7 shows the dependence of electrical current on the applied potential that clearly confirms the non-Ohmic behavior for the NBCa. For NBCa, the dependence of the electrical conductivity on temperature and bias is evident. The combined results strongly indicate that increased electrical and thermal energies contribute to the observed increase in conductivity. These contributions can facilitate the promotion of charge carriers to the conduction band and increase the density of available carriers. Both phenomena increase conductivity, however, additional experimentation is required to distinguish between the two mechanisms. Fig. 8 presents the electrical conductivity values for NBCa as a function of the initial amount of barium carbonate (w%). This result shows a significant variation in the electrical conductivity with an increase in barium carbonate and is indicative of an increase in the amount of a and b phases in the NBCa. 4. Conclusions Fig. 4. (a) SEM image of NBCo and (b) SEM image of NBCa.
i) The presence of a and b phases in NBCa that may be more conductive, and ii) the more uniform structure of NBCa due to the increase in the percentage of barium-containing phases. The sintering of the NBC using barium carbonate favors an
NBCa ceramics provided interesting results for conductivity measurements as a function of temperature and applied potential. The electrical conductivity of NBCa increased with temperature and positive bias. In contrast, the electrical conductivity of NBCo did not vary significantly under the same conditions. The results from structural and electrical investigations suggest the presence of new conducting phases in NBCa, denoted as b and a. The presence of barium carbonate in the sintering medium
Table 3 Electrical conductivity determined for NBCa. Polarization (V)
Resistivity (104 Ohm.m) Sample 1
Sample 2
Sample 3
1.00 1.50 2.00 2.50 3.00 3.50 4.00
7.36 3.77 5.73 3.01 2.65 2.84 3.67
7.38 4.32 4.89 2.47 3.18 2.87 2.63
7.65 5.61 5.45 2.40 2.21 2.48 2.49
Average resistivity (104 Ohm.m)
Average conductivity (104 S m1)
7.46 4.57 5.36 2.63 2.68 2.73 2.93
0.13 0.22 0.19 0.38 0.37 0.37 0.34
J.P. Fernandes et al. / Journal of Alloys and Compounds 649 (2015) 809e814
813
Table 4 Electrical conductivity determined for NBCo. Polarization (V)
1.00 1.50 2.00 2.50 3.00 3.50 4.00
Resistivity (Ohm.m) Sample 1
Sample 2
Sample 3
0.31 0.45 0.83 0.34 0.70 0.61 0.52
0.47 0.35 1.09 0.29 0.96 0.58 0.56
0.94 0.22 0.62 0.92 1.16 0.65 0.29
Average resistivity (Ohm.m)
Average conductivity (100 S m1)
0.57 0.34 0.85 0.52 0.94 0.61 0.46
1.75 2.94 1.18 1.92 1.06 1.64 2.17
Fig. 7. Detailed bias-dependent electrical conductivity of NBCa (45.5 wt% BaCO3). Fig. 5. Bias-dependent electrical conductivities of NBCo and NBCa (45.5 wt% BaCO3).
Fig. 6. Bias-dependent electrical conductivity and corresponding temperature for NBCa (45.5 BaCO3 wt%).
promotes the formation of these phases, which are present at very low levels in NBCo. Therefore, the NBCa ceramics produced with barium carbonate in the sintering medium showed a larger percentage of the bariumcontaining phases and higher electrical conductivity at room temperature when compared with NBCo, whose source of barium was monometallic-oxide. Further experimentation is required to
Fig. 8. Electrical conductivities of NBCa manufactured with different initial amounts of BaCO3.
further elucidate the dominant mechanism behind the enhanced conductivity observed in NBCa ceramics. Acknowledgments The Authors thank the National Council for Scientific and Technological Development e CNPq by the financial support of the work.
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References [1] S. Bhadrakumari, P. Predeep, YBa2Cu3O7x/Thermoplastic polymer composite thermistors, Eur. Polym. J. 45 (2009) 226e229. [2] H.A. Blackstead, J.D. Dow, M. Lehmannh, Th-doped Nd2CuO4 homologues, Solid State Commun. 107 (7) (1998) 323e327. [3] Yuan Li, Shumin Han, Jinhua Li, Xilin Zhu, Lin Hu, The effect of Nd content on the electrochemical properties of low-Co LaeMgeNi-based hydrogen storage alloys, J. Alloys Compd. 458 (2008) 357e362. [4] J. Valo, M. Leskela, B.C. Hauback, H. Fjellva, S.-M. Koo, K.V. Rao, Nd substitution effects in YBa2Cu4O8 prepared using a conversion reaction from Y1xNdxBa2Cu3O7d and CuO, Int. J. Inorg. Mater. 2 (2000) 269. [5] T. Kikuchi, A. Wongsatanawarid, Y. Homma, K. Suzuki, N. Koshizuka, M. Murakami, Processing conditions for (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors, Phys. Proc. 27 (2012) 132e135. [6] L. Cheng, L.S. Guo, Y.S. Wu, X. Yao, D.A. Cardwell, Multi-seeded growth of melt processed GdeBaeCueO bulk superconductors using different arrangements of thin film seeds, J. Cryst. Growth 366 (2013) 1e7. [7] Tomoyuki Naito, Daiki Furuta, Takahiro Arayashiki, Hiroyuki Fujishiro, Yousuke Yanagi, Yoshitaka Itoh, Position dependence of irreversibility line on thin plate Gd-Ba-Cu-O bulk superconductor, Phys. Proc. 27 (2012) 168e171. [8] K. Iida, K. Nenkov, G. Fuchs, G. Krabbes, B. Holzapfel, B. Büchner, L. Schultz, Effect of addition of planetary milled Gd-211 on the microstructures and superconducting properties of air-processed single grain GdeBaeCueO/Ag bulk superconductors, Phys. C. Supercond. 470 (2010) 1153e1157. [9] N. Nancheva, N. Feschiev, D. Tzaneva, M. Misheva, N. Djurelov, Positron trapping at defects in Y-Ba-Cu-O, La-Nd-Ba-Cu-O and La-Nd-Pb-Cu-O superconductors, J. Mater. Process. Technol. 68 (1997) 8e12. [10] M. Matsui, N. Sakai, M. Murakami, Effect of cooling rate on the trapped-fields of NdeBaeCueO bulk superconductors, Phys. C. Supercond. 378e381 (2002) 651e656.
[11] H. Kojo, S.I. Yoo, M. Murakami, Melt processing of high-Tc Nd-Ba-Cu-O superconductors in air, Phys. C. Supercond. 289 (1997) 85e88. [12] M. Matsui, T. Miyamoto, S. Nariki, N. Sakai, M. Murakami, Fabrication of largegrain NdeBaeCueO superconductor, Phys. C. Supercond. 357e360 (2001) 694e696. [13] K. Iida, J. Yoshioka, M. Murakami, Superconducting properties of NdeBaeCueO fabricated in air, Phys. C. Supercond. 372e376 (2002) 1152e1154. [14] M. Matsui, M. Murakami, Enhancement of the trapped-fields and mechanical properties of NdeBaeCueO bulk superconductors, Ceram. Int. 29 (2003) 117e137. [15] H. Fujimoto, H. Ozaku, Superconducting and mechanical properties of SmeBaeCueO bulk superconductors, Phys. C 412e414 (2004) 657e661. [16] W.H. Tang, J. Gao, Influence of Nd at Ba-sites on superconductivity of YBa2xNdxCu3Oy, Phys. C 298 (1998) 66e72. [17] H.H. Dong, Effects of the Ba-site dopants on the superconductivity of the RBCO system, Phys. C 302 (1998) 299e303. [18] C. Caldart, et al., Influence of High Energy Milling on the Airflow Sensor property of the NBCa Ceramic, Mater. Sci. Forum 727e728 (2012) 499, 10.4028/www.scientific.net/MSF.727-728.499. [19] L. CIONTEA, et al., Synthesis and properties of YBa2Cu307d made by a modified hydroxycarbonate method, Phys. C 257 (1996) 304e312. [20] L. ZHAO, et al., Characterization and evaluation of NdBaCo2O5þd cathode for proton-conducting solid oxide fuel cells, Int. J. Hydrog. Energy 35 (2010) 753e756, http://dx.doi.org/10.1016/j.ijhydene.2009.10.102. [21] T. KIKUCHIA, et al., Processing conditions for (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors, Phys. Proc. 27 (2012) 132e135, http://dx.doi.org/ 10.1016/j.phpro.2012.03.428. trica DC em So lidos: [22] E.M. Girotto, I.A. Santos, Medidas de Resistividade Ele -las Corretamente, Quim. Nova 25 (2002) 639. como Efetua
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Barium carbonate as an agent to improve the electrical properties of neodymium-barium-copper system at high temperature J.P. Fernandes c, G.W. Duarte c, f, C. Caldart d, C.T. Kniess e, O.R.K. Montedo d, M.R. Rocha d, H.G. Riella c, M.A. Fiori a, b, * Chapeco ria da Regia ~o de Chapeco (UNOCHAPECO), , SC, 89809-000, Brazil Post-Graduate Program in Environmental Science, Universidade Comunita Chapeco ria da Regia ~o de Chapeco (UNOCHAPECO), , SC, Post-Graduate Program in Technology and Management of the Innovation, Universidade Comunita 89809-000, Brazil c polis, SC, 88040-900, Brazil Post-Graduate Program in Chemical Engineering, Universidade Federal de Santa Catarina (UFSC), Floriano d Post-Graduate Program in Science and Materials Engineering, Universidade do Extremo Sul Catarinense (UNESC), Criciúma, SC, 88806-000, Brazil e ~o Paulo, SP, Brazil Post-Graduate Program in Professional Master in Management, Universidade Nove de Julho, Sa f rio Barriga Verde (UNIBAVE), Santa Catarina, SC, Brazil Research Group in Technology and Information, Centro Universita a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2015 Received in revised form 29 May 2015 Accepted 29 June 2015 Available online 6 July 2015
Specialized ceramics are manufactured under special conditions and contain specific elements. They possess unique electrical and thermal properties and are frequently used by the electronics industry. Ceramics containing neodymium-barium-copper (NBC) exhibit high conductivities at low temperatures. NBC-based ceramics are typically combined with oxides, i.e., NBCo produced from neodymium oxide, barium oxide and copper oxide. This study presents NBC ceramics that were produced with barium carbonate, copper oxide and neodymium oxide (NBCa) as starting materials. These ceramics have good electrical conductivities at room temperature. Their conductivities are temperature dependent and related to the starting amount of barium carbonate (w%). © 2015 Elsevier B.V. All rights reserved.
Keywords: NdBaCu alloy Conductive ceramic Conductive oxides alloys Ceramic type NBC
1. Introduction Under most operating conditions, metals and their alloys are widely recognized as good conductors of electricity and heat. However, harsh environments can degrade metals and limit their use. It is therefore necessary to develop new materials capable of overcoming these deficiencies [1]. REBCO-type (RE ¼ rare earth, B ¼ barium, C ¼ copper, O ¼ oxygen) ceramics exhibit great potential for the development of robust conductive elements. These materials primarily consist of metal oxide alloys that are compressed and heated to high temperatures to accelerate atomic diffusion [2e4]. This class of ceramics has been extensively investigated as superconductors [5e8]. These special ceramics are often used in the electronics industry. The introduction of select elements and atypical production conditions can impart peculiar electrical and thermal properties. Some
* Corresponding author. E-mail address: fi[email protected] (M.A. Fiori). http://dx.doi.org/10.1016/j.jallcom.2015.06.261 0925-8388/© 2015 Elsevier B.V. All rights reserved.
ceramics have superconducting properties at low temperatures and are often used in deformation sensors. Neodymiumbarium-copper (NBC) represents the base metallic components for a class of ceramics that exhibits high conductivity at low temperatures. NBC ceramics are typically produced from metal oxides, i.e., neodymium oxide, barium oxide and copper oxide. Many publications have focused on the superconducting properties of these ceramics [9e14], and have studied the effects of Nd and Ba on electrical properties [15e17]. However, the majority of studies have neglected to thoroughly investigate the electrical properties of these ceramics at room and higher temperatures. This class of materials has found commercial use in components of monitoring sensors and their controls. However, variations in temperature can affect the electrical performance of NBC-based ceramics and potentially reduce sensor efficiency. This work describes the synthesis and characterization of NBC ceramics produced with barium carbonate (NBCa) and evaluates their electrical properties. The conductivities of NBCa as a function of temperature and applied potential are described, and a comparison with the properties of NBCo is presented.
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J.P. Fernandes et al. / Journal of Alloys and Compounds 649 (2015) 809e814
2. Experimental procedure 2.1. NBC samples The NBC samples (neodymiumebariumecopper) were prepared by mixing Nd2O3 (neodymium oxide - Sigma Aldrich), CuO (copper oxide - Vetec), and BaO (barium oxide) or BaCO3 (Barium carbonate - Sigma Aldrich), all with a purity of 99.9%. Two types of NBC mixtures were prepared for electrical measurements: NBCa (Nd2O3 e BaCO3 e CuO) and NBCo (Nd2O3 e BaO e CuO). Powdered samples of different compositions were cylindrically formed with a diameter of 20 mm and a thickness of 3 mm. To eliminate the water content, the compounds were dried in an oven (Fanem Orion 502) with a controlled temperature of 105.0 C for 60 min. Cylindrical samples were compacted with a pressure of 312.5 kgf/cm2, according to the work developed by Fiori et al. and others [18e21]. The pressed NBCa and NBCo samples were sintered at 1020 C (heating rate of 4.00 C/min) for 24 h and then cooled to room temperature. Samples were then polished to eliminate surface irregularities and to improve contact with electrical probes, according to procedures defined by Fiori et al. and other authors [18e21]. To study the effect of the barium concentration (NBCa samples) on the electrical conductivity, samples were produced with three different concentrations of barium carbonate: 50.0 wt%, 45.5 wt% and 40.0 wt%. To study the dependence of electrical conductivity with temperature and electrical polarization, the NBCa samples were prepared with 45.5 wt% BaCO3. Barium concentrations were determined by considering the values adopted by Fiori et al. in their preliminary studies concerning these materials [18]. 2.2. X-ray and SEM characterization The NBCa and NBCo samples were analyzed by x-ray diffraction spectroscopy (XRD) and the results were treated with Rietveld methods. The x-ray analyses were conducted using a Shimadzu LabX XRD-6000 (Cu target with wavelength (l) of 1.5406 Å, 40.0 kV voltage, 30.0 mA current, and scan speed of 2.00 deg/min). Ceramic morphologies were analyzed by scanning electron microscopy (SEM) using a JEOL JSM-6390 LV at 15 kV. 2.3. Electrical characterization To determine the electrical conductivities of the NBCa and NBCo samples, a four-probe method was applied [22]. Electrical measurements of the samples were performed in triplicate as a function of different external biases. The currents of NBC samples were obtained at the following potentials: 0.50 V, 1.00 V, 1.50 V, 2.00 V and 2.50 V. 3. Results and discussion 3.1. X-ray analysis Fig. 1 shows the X-Ray spectra for NBCo and NBCa samples. The results show peaks from oxide phases containing barium, copper and neodymium present in both materials. Additionally, significant differences between the X-Ray diffractograms were observed between 2q values of 20.00 and 50.00 . A comparison between the diffractograms of NBCo and NBCa indicates significant differences in the crystalline structures. While some peaks of the two materials are coincident, other peaks are offset. These displacements are strongly indicative of lattice mismatches, while differences in the magnitudes of the peak signals are related to the degree of crystallinity. These details are presented in Fig. 2a and b.
Fig. 1. X-ray diffractograms 0066or (a) NBCo and (b) NBCa.
The x-ray spectrum for the NBCa show characteristic peaks at 2q equal to 18.40, 20.08, 27.60, 28.90 and 29.90 and the absence of 2q at 19.60, 25.30, 26.08, 30.40, 30.80, 33.60 and 37.00 , which are present in NBCo. The shared peaks correspond to oxide phases that contain all three metals (Ba, Cu, and Nd), but in significantly different arrangements. Fig. 3 shows the relative amount of each phase that contains Ba, Cu and Nd in both NBCo and NBCa ceramics as determined by Rietveld analysis. In NBCa and NBCo the phase Ba(1.55)Cu(3)Nd(1.45)O(7.16) represents more than 50.00% of the total volume. However, a significant difference between the two ceramics is the reduced amount of monometallic oxide phases exhibited by the ceramic sintered with barium carbonate (NBCa) relative to NBCo. In the NBCo ceramic, 34.00% of the phases contain CuO and Nd2O3 (BaO was not observed), while in the NBCa ceramic the combined percentage for these species is 13.50%. As is implied by the above statement, phases containing barium (Ba) occupy a greater volume in NBCa (85.50%) compared with NBCo (65.50%). Of particular note is the increased presence of BaNd2O4 (b) and BaCu3O4 (a) phases in NBCa. The percentages of b and a phases are 19.50% (0.80%) and 18.60% (8.84%) for NBCa (NBCo), respectively. Table 1 summarizes the compositions of NBCa and NBCo as determined by XRD. During the sintering of NBCa precursors at 1020 C, barium carbonate dissociates and carbon monoxide evolves, as shown in equation (1). Carbon monoxide reduces neodymium and copper oxides resulting in the generation and loss of carbon dioxide (equations (2) and (4)). Ultimately, these reactions lead to the
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Table 1 Relative percentage of phases in NBCa and NBCo. Sample
BaO (%)
Nd2O3 (%)
CuO (%)
BaNd2O4 (%)
BaCu3O4 (%)
NBCa NBCo difference
0.0 0.0 0.0
2.79 15.76 12.97Y
10.84 18.47 7.63Y
19.50 0.80 þ18.70[
13.60 8.84 þ4.76[
(Nd2O2)þ2 þ (BaO2)2 / Nd2BaO4 [(Phase b]: (1020 C)
(3)
(CO)þ2 þ CuO / (Cu)þ2 þ CO2: (1020 C)
(4)
O2 þ 3(Cu)þ2 þ (BaO2)2 (s) / BaCu3O4 [Phase a]: (1020 C)
(5)
Lattice parameters and volumes of the crystallites were determined by Rietiveld analyses. Table 2 shows the lattice parameters of each phase. The barium phases possess the larger lattice parameters and, consequently, larger crystallite volumes than monometallic-oxide phases. The b phase has the largest unit cell volume, with values of 396.9723 Å (81379-ICSD) and 475.9245 Å (81380-ICSD). The unit cell volume of the a phase is 329.6251 Å (65881-ICSD). The b and a phases are present with high significance only in the NBCa samples and can be attributed to use of barium carbonate. These characteristics favor an increase in the amount of bariumphases and larger volume crystallites than the phases present in NBCo. Therefore, the differences in electrical properties observed between the two types of NBCs can be attributed to the different amounts of b and a phases, which enhance crystallinity and increase electrical conduction. 3.2. SEM analyses Fig. 2. (a) Comparison between x-ray diffractograms of NBCo and NBCa ceramics e 2q from 18 to 27 and (b) between x-ray diffractograms of the NBCo and NBCa ceramics e 2q from 27 38 .
formation of b and a phases, as shown in equations (3) and (5), respectively. BaCO3 / (BaO2)2 þ (CO)þ2: (1020 C)
(1)
(CO)þ2 þ Nd2O3 / (Nd2O2)þ2 þ CO2[: (1020 C)
(2)
Fig. 4 shows SEM images of the NBCo and NBCa samples. The SEM micrographs show a significant difference in surface morphology. There are regular crystal grains on both materials, but the grains are bigger for NBCa. On the surface of NBCa is observed a long range of crystalline regularity. These differences can be attributed to the larger volume of barium-containing phases a and b. Therefore, the greater degree of crystallinity associated with NBCa, in conjunction with the long range crystalline regularity, contribute to the increase in electrical conductivity. 3.3. Electrical measures
Fig. 3. Phases present in NBCo and NBCa ceramics as determined by Rietveld methods.
The electrical resistivity and conductivity values for NBCa and NBCo at different applied potentials are given in Tables 3 and 4. Fig. 5 compares the dependence of electrical conductivity on external bias for both ceramics. The results indicate a higher conductivity for the ceramic manufactured using barium carbonate (NBCa) than for the ceramic produced from barium oxide (NBCo). NBCa has electrical conductivity on the order of 103 S m1, while NBCo has an electrical conductivity on the order of 1 S m1. These results indicate that the variation in electrical conductivity with respect to the evaluated range in bias is not significant for NBCo. However, the electrical conductivity values for NBCa are strongly dependent on the applied potentials studied. An increase in positive bias promotes a significant increase in the electrical conductivity of NBCa until a value of 2.5 V is reached, at which point the electrical conductivity is not significantly modified. The results reveal that conductivities for NBCa were consistently higher than for NBCo, which can be attributed to two main factors:
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Table 2 Lattice parameters and volumes for different phases as determined by Rietveld methodology. Phases
(1) (2) (3) (4) (5) (6) (7)
References ICSO
Ba(1.55)Cu(3)Nd(1.45)O(7.16) BaNd(2)O(4) e 1 BaCu(3)O(4) CuO BaCuNd(2)O(5) Nd(2)O(3) BaNd(2)O(4) e 2
Crystal lattice parameters determined by Rietveld methods
75683 81379 65881 87125 201424 100212 81380
a [Å]
b [Å]
c [Å]
3.894346 10.26672 7.523489 4.691829 6.692198 3.957826 10.70508
3.894346 3.001452 11.78534 3.419484 6.692198 3.957826 3.612342
11.57033 12.88241 3.717567 5.129863 5.866688 6.309534 12.30720
Volume [Å]
175.4748 396.9723 329.6251 82.30165 262.7426 98.83498 475.9245
increase in the percentage of barium-containing phases; as a consequence, there is an increase in crystalline regions and a decrease in the amount of lattice imperfections. As a and b phases are more conductive, their increased presence in NBCa raises the conductivity of the sample as a whole. Fig. 6 shows that conductivity values for the NBCa vary significantly as a function of measurement temperature. These results indicate that increasing the electrical polarization of NBCa leads to an increase in their temperature and, as a consequence, an increase in their electrical conductivity. This suggests the presence of a nonOhmic mechanism as the dominant conducting mechanism; however, with only these results, a definitive conclusion is not possible. Fig. 7 shows the dependence of electrical current on the applied potential that clearly confirms the non-Ohmic behavior for the NBCa. For NBCa, the dependence of the electrical conductivity on temperature and bias is evident. The combined results strongly indicate that increased electrical and thermal energies contribute to the observed increase in conductivity. These contributions can facilitate the promotion of charge carriers to the conduction band and increase the density of available carriers. Both phenomena increase conductivity, however, additional experimentation is required to distinguish between the two mechanisms. Fig. 8 presents the electrical conductivity values for NBCa as a function of the initial amount of barium carbonate (w%). This result shows a significant variation in the electrical conductivity with an increase in barium carbonate and is indicative of an increase in the amount of a and b phases in the NBCa. 4. Conclusions Fig. 4. (a) SEM image of NBCo and (b) SEM image of NBCa.
i) The presence of a and b phases in NBCa that may be more conductive, and ii) the more uniform structure of NBCa due to the increase in the percentage of barium-containing phases. The sintering of the NBC using barium carbonate favors an
NBCa ceramics provided interesting results for conductivity measurements as a function of temperature and applied potential. The electrical conductivity of NBCa increased with temperature and positive bias. In contrast, the electrical conductivity of NBCo did not vary significantly under the same conditions. The results from structural and electrical investigations suggest the presence of new conducting phases in NBCa, denoted as b and a. The presence of barium carbonate in the sintering medium
Table 3 Electrical conductivity determined for NBCa. Polarization (V)
Resistivity (104 Ohm.m) Sample 1
Sample 2
Sample 3
1.00 1.50 2.00 2.50 3.00 3.50 4.00
7.36 3.77 5.73 3.01 2.65 2.84 3.67
7.38 4.32 4.89 2.47 3.18 2.87 2.63
7.65 5.61 5.45 2.40 2.21 2.48 2.49
Average resistivity (104 Ohm.m)
Average conductivity (104 S m1)
7.46 4.57 5.36 2.63 2.68 2.73 2.93
0.13 0.22 0.19 0.38 0.37 0.37 0.34
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Table 4 Electrical conductivity determined for NBCo. Polarization (V)
1.00 1.50 2.00 2.50 3.00 3.50 4.00
Resistivity (Ohm.m) Sample 1
Sample 2
Sample 3
0.31 0.45 0.83 0.34 0.70 0.61 0.52
0.47 0.35 1.09 0.29 0.96 0.58 0.56
0.94 0.22 0.62 0.92 1.16 0.65 0.29
Average resistivity (Ohm.m)
Average conductivity (100 S m1)
0.57 0.34 0.85 0.52 0.94 0.61 0.46
1.75 2.94 1.18 1.92 1.06 1.64 2.17
Fig. 7. Detailed bias-dependent electrical conductivity of NBCa (45.5 wt% BaCO3). Fig. 5. Bias-dependent electrical conductivities of NBCo and NBCa (45.5 wt% BaCO3).
Fig. 6. Bias-dependent electrical conductivity and corresponding temperature for NBCa (45.5 BaCO3 wt%).
promotes the formation of these phases, which are present at very low levels in NBCo. Therefore, the NBCa ceramics produced with barium carbonate in the sintering medium showed a larger percentage of the bariumcontaining phases and higher electrical conductivity at room temperature when compared with NBCo, whose source of barium was monometallic-oxide. Further experimentation is required to
Fig. 8. Electrical conductivities of NBCa manufactured with different initial amounts of BaCO3.
further elucidate the dominant mechanism behind the enhanced conductivity observed in NBCa ceramics. Acknowledgments The Authors thank the National Council for Scientific and Technological Development e CNPq by the financial support of the work.
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References [1] S. Bhadrakumari, P. Predeep, YBa2Cu3O7x/Thermoplastic polymer composite thermistors, Eur. Polym. J. 45 (2009) 226e229. [2] H.A. Blackstead, J.D. Dow, M. Lehmannh, Th-doped Nd2CuO4 homologues, Solid State Commun. 107 (7) (1998) 323e327. [3] Yuan Li, Shumin Han, Jinhua Li, Xilin Zhu, Lin Hu, The effect of Nd content on the electrochemical properties of low-Co LaeMgeNi-based hydrogen storage alloys, J. Alloys Compd. 458 (2008) 357e362. [4] J. Valo, M. Leskela, B.C. Hauback, H. Fjellva, S.-M. Koo, K.V. Rao, Nd substitution effects in YBa2Cu4O8 prepared using a conversion reaction from Y1xNdxBa2Cu3O7d and CuO, Int. J. Inorg. Mater. 2 (2000) 269. [5] T. Kikuchi, A. Wongsatanawarid, Y. Homma, K. Suzuki, N. Koshizuka, M. Murakami, Processing conditions for (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors, Phys. Proc. 27 (2012) 132e135. [6] L. Cheng, L.S. Guo, Y.S. Wu, X. Yao, D.A. Cardwell, Multi-seeded growth of melt processed GdeBaeCueO bulk superconductors using different arrangements of thin film seeds, J. Cryst. Growth 366 (2013) 1e7. [7] Tomoyuki Naito, Daiki Furuta, Takahiro Arayashiki, Hiroyuki Fujishiro, Yousuke Yanagi, Yoshitaka Itoh, Position dependence of irreversibility line on thin plate Gd-Ba-Cu-O bulk superconductor, Phys. Proc. 27 (2012) 168e171. [8] K. Iida, K. Nenkov, G. Fuchs, G. Krabbes, B. Holzapfel, B. Büchner, L. Schultz, Effect of addition of planetary milled Gd-211 on the microstructures and superconducting properties of air-processed single grain GdeBaeCueO/Ag bulk superconductors, Phys. C. Supercond. 470 (2010) 1153e1157. [9] N. Nancheva, N. Feschiev, D. Tzaneva, M. Misheva, N. Djurelov, Positron trapping at defects in Y-Ba-Cu-O, La-Nd-Ba-Cu-O and La-Nd-Pb-Cu-O superconductors, J. Mater. Process. Technol. 68 (1997) 8e12. [10] M. Matsui, N. Sakai, M. Murakami, Effect of cooling rate on the trapped-fields of NdeBaeCueO bulk superconductors, Phys. C. Supercond. 378e381 (2002) 651e656.
[11] H. Kojo, S.I. Yoo, M. Murakami, Melt processing of high-Tc Nd-Ba-Cu-O superconductors in air, Phys. C. Supercond. 289 (1997) 85e88. [12] M. Matsui, T. Miyamoto, S. Nariki, N. Sakai, M. Murakami, Fabrication of largegrain NdeBaeCueO superconductor, Phys. C. Supercond. 357e360 (2001) 694e696. [13] K. Iida, J. Yoshioka, M. Murakami, Superconducting properties of NdeBaeCueO fabricated in air, Phys. C. Supercond. 372e376 (2002) 1152e1154. [14] M. Matsui, M. Murakami, Enhancement of the trapped-fields and mechanical properties of NdeBaeCueO bulk superconductors, Ceram. Int. 29 (2003) 117e137. [15] H. Fujimoto, H. Ozaku, Superconducting and mechanical properties of SmeBaeCueO bulk superconductors, Phys. C 412e414 (2004) 657e661. [16] W.H. Tang, J. Gao, Influence of Nd at Ba-sites on superconductivity of YBa2xNdxCu3Oy, Phys. C 298 (1998) 66e72. [17] H.H. Dong, Effects of the Ba-site dopants on the superconductivity of the RBCO system, Phys. C 302 (1998) 299e303. [18] C. Caldart, et al., Influence of High Energy Milling on the Airflow Sensor property of the NBCa Ceramic, Mater. Sci. Forum 727e728 (2012) 499, 10.4028/www.scientific.net/MSF.727-728.499. [19] L. CIONTEA, et al., Synthesis and properties of YBa2Cu307d made by a modified hydroxycarbonate method, Phys. C 257 (1996) 304e312. [20] L. ZHAO, et al., Characterization and evaluation of NdBaCo2O5þd cathode for proton-conducting solid oxide fuel cells, Int. J. Hydrog. Energy 35 (2010) 753e756, http://dx.doi.org/10.1016/j.ijhydene.2009.10.102. [21] T. KIKUCHIA, et al., Processing conditions for (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors, Phys. Proc. 27 (2012) 132e135, http://dx.doi.org/ 10.1016/j.phpro.2012.03.428. trica DC em So lidos: [22] E.M. Girotto, I.A. Santos, Medidas de Resistividade Ele -las Corretamente, Quim. Nova 25 (2002) 639. como Efetua