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Ball Milling Synthesis and Property of Nd0.7Sr0.3MnO3 Manganites
J. Mater. Sci. Technol., 2010, 26(8), 721-724.
Ball Milling Synthesis and Property of Nd0.7 Sr0.3 MnO3 Manganites Shunsheng Chen1) , Changping Yang1)† , Lingfang Xu1) and Shaolong Tang2) 1) The Provincial Key Laboratory of Piezoelectric Ceramics Materials and Apparatus, Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, China 2) Department of Physics, Nanjing University, Nanjing 210093, China [Manuscript received May 5, 2009, in revised form October 16, 2010]
Strontium doped perovskite-type Nd0.7 Sr0.3 MnO3 ceramics were synthesized completely by high-energy ball milling raw oxides of Nd2 O3 , SrCO3 and MnO2 . The optimal ball milling time and mass ratio of milling balls to raw materials are 4 h and 10:1, respectively. The grain size of as-milled Nd0.7 Sr0.3 MnO3 ceramics ranges from 51 to 93 nm, and the fine particles contain two phases of crystalline phase and amorphous phase. For the Nd0.7 Sr0.3 MnO3 synthesized by ball milling and sequent heat treatment, a remarkable colossal electroresistance (CER) effect is observed and the CER ratio reaches 900% at Curie temperature TC when the load voltage increases from 0.1 to 0.8 V. KEY WORDS: High-energy ball milling; Perovskite structure; Colossal electroresistance (CER); Microstructure defect
1. Introduction Alkali-earth doped perovskite-type manganites R1−x Ax MnO3 (R=rare-earth and A=alkali-earth) have attracted remarkable interest due to their various unique properties[1–4] . Recently, colossal electroresistance (CER) effect, the counterpart of colossal magnetoresistance (CMR), has been greatly focused on by a number of groups[5–9] . Large numbers of works about the CER have been done, however, the view about the physics of the CER effect is still not unanimous. Some groups consider it as the extrinsic properties of materials[10–14] . However, the preparation method and condition are closely associated with the extrinsic properties of materials and can greatly affect the physical properties of materials, especially, the character of electrical transport. Ball milling method is efficient for synthesizing manganite perovskite[15–24] , and it can produce material into metastates, which may contain large numbers of defects such as lattice imperfection, dislocations, bro† Corresponding author. Prof.; Tel.: +86 27 88665447; E-mail address: [email protected] (C.P. Yang).
ken chemical bonds and so on. Accordingly, in this work, we introduced a high-energy ball milling method to synthesize the Nd0.7 Sr0.3 MnO3 perovskite and discussed the optimal milling efficiency, investigated the properties of the as-milled samples as well as the electromagnetic character of the as-milled samples sequently treated by high temperatures. To our surprise, a notable CER effect is observed, which is different from that of stoichiometric NdSrMnO prepared by conventional solid-state reaction method[25] . 2. Experimental MnO2 (purity >99.9%), Nd2 O3 (purity >99.9%) and SrCO3 (purity >99.9%) powders were used as raw materials. Nd2 O3 and SrCO3 were pretreated at 1173 and 673 K for 6 and 4 h, respectively in order to eliminate the moisture. The molar ratio of Nd2 O3 , SrCO3 and MnO2 is 0.35:0.3:1. Raw materials with a total mass of 5 g were put inside a hardened steel pot with a volume of 80 cm3 together with 10 steel balls with diameters of 10 and 6 mm, respectively and milled for different times. The mass ratio of milling balls to raw
S.S. Chen et al.: J. Mater. Sci. Technol., 2010, 26(8), 721–724
722
Table 1 Parameter of milling balls and raw materials Raw materials and milling balls Mol ratio and ball numbers Mass/g Total mass/g
MnO2 0.02 mol 1.7458
SrCO3 0.006 mol 0.8893 5.0
Nd2 O3 0.007 mol 2.3649
Big ball 10 41.4
Small ball 10 8.9 50.3
materials was 10:1. The details are shown in Table 1. A high-energy ball mill with a speed of 1400 r/min was used to mill raw materials under air condition. Crystal structure of the as-milled sample (named as sample A) and the as-milled and post heat-treatment sample (named as sample B) was determined by X-ray diffraction (XRD, DRON-3, CuKα, Japan). Thermomagnetic curves were measured in a vibrating sample magnetometer (VSM, 6N30SE7C, China). Particle shape and size were observed by transmission electron microscopy (TEM, TecnaiG20, 200 kV, USA) and scanning electron microscopy (SEM, JSM-5610LV, Japan). For sample B, resistance measurements were performed using a standard four-wire method. 3. Results and Discussion In order to obtain the optimum efficiency of ball milling, firstly, we investigated the effect of mass ratio of milling balls to raw materials on the synthetic process. Raw materials with different mass ratios of balls to raw materials were respectively milled for the same time then identified by XRD. Just as the result shown in Fig. 1, the Nd0.7 Sr0.3 MnO3 perovskites were well synthesized when the mass ratio of balls to materials was 10:1. For obtaining the optimal milling time, there shares of materials with the same mass ratio of balls to materials (10:1) were prepared to mill for 2, 4 and 9 h, respectively and then examined by XRD. Obviously, for the materials milled for 4 h, the intensity of peak is stronger than the others (Fig. 2), indicating that the Nd0.7 Sr0.3 MnO3 can be formed completely by milling raw materials for 4 h, which is almost the same time with that reported by Bolar´ın et al.[17] . The intensity of peak will decrease gradually (Fig. 2(c)), and it can transform the perovskite phase into amorphous phase if one continues to increase the milling time after synthesizing the single phase[16,23] . Figure 3 shows XRD patterns of samples after milling raw materials for different times. Diffraction peaks of raw materials of Nd2 O3 , SrCO3 and MnO2 can be seen clearly in Fig. 3(a), as marked in the first panel with different symbols. However, it can be seen that the peaks of raw materials dramatically decrease with increasing milling time and almost disappear completely after only 0.5 h (Fig. 3(b)). Meanwhile, the new peaks corresponding to perovskite structure begin to appear with milling time up to 3 h (Fig. 3(c)). It can be seen that the intensities of those new perovskite peaks increase gradually with increasing milling time, and the Nd0.7 Sr0.3 MnO3 single phase is formed completely after milling raw materials for 4 h. No traces of other phase can be detected within the equipment accuracy (Fig. 3(d)). The result shows
Fig. 1 XRD of raw materials with different mass ratio of balls to materials after milling the same time: (a) 20:1, (b) 10:1, (c) 5:1
Fig. 2 XRD of raw materials with the same mass ratio of balls to materials after milling different time: (a) 2 h, (b) 4 h, (c) 9 h
that the ball milling is an another useful method to directly produce the stoichiometric and homogeneous Nd0.7 Sr0.3 MnO3 perovskite in addition to the conventional solid-state reaction method, even though the physical mechanism of ball milling is still unknown to us all. Besides, the intensity of peak of as-milled samples increases evidently if they were treated by high temperatures (Fig. 3(e)), suggesting that high temperatures are of advantage to grain growth. The transmission topography of particles of sample A was examined by TEM and shown in Fig. 4. The obtained particles are globular in shape and the grain size ranges from 51 to 93 nm with an average size of about 91 nm, which is in good coincidence with that estimated from the (200) reflection in Fig. 3(d)
S.S. Chen et al.: J. Mater. Sci. Technol., 2010, 26(8), 721–724
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Fig. 3 XRD patterns of samples after milling raw materials for: (a) 0 h, (b) 0.5 h, (c) 3 h and (d) 4 h, (e) XRD patterns of sample B treated at 1623 K for 1 h
Fig. 5 SEM images of sample A (a) and B (b)
Fig. 4 TEM image of sample A. Inset: the HRTEM image of nano-particle (reprinted with permission from Ref. [22])
using the Scherrer method. The HRTEM (high resolution transmission electron microscopy) results for the locally focused particle show that the particle is composed of crystalline phase (area A of the inset in Fig. 4) and amorphous phase (area B of the inset in Fig. 4). The amorphous phase may not be detected by XRD due to its small quantity relative to that of crystalline phase. It suggests that materials produced by high-energy ball milling are in metastate with lots of defects. In order to obtain further observation about grains, the morphology of Nd0.7 Sr0.3 MnO3 was examined by SEM. Figure 5 (a) and (b) show SEM images of sample A and B, respectively. From Fig. 5(a), we can see that the fine particles of sample A are already agglomerative after milling raw materials for 4 h, and the shape and size of the grains are almost the same with that of TEM. The grain in sample B has an average size of 2.2 μm, which is much bigger than that in sample A (Fig. 5(b)). However, an interesting
thing is that some dislocations-like at the surface of grains are observed, just denoted with arrows. Like the grain boundaries, these dislocations-like divide the grains into many smaller grains, thus, decreasing the grain size and increasing the ratio of the grain boundaries, as a result, influencing the electromagnetic transport. Temperature dependence of magnetization measured in a field cooling at 100 Oe for the sample A and B is shown in Fig. 6 (a) and (b), respectively. The M (T ) curve exhibits a paramagnetic-ferromagnetic (PM-FM) transition with a sharp decrease of the magnetization at Curie temperature, TC =70 K, which is much lower than TC =128 K for sample B. However, TC of sample B is still much lower than that of Nd0.7 Sr0.3 MnO3 (about 230 K) prepared using solidstate reaction method[25] . It suggests that the DE (double exchange) interaction is suppressed strongly by the process of ball milling. Probably, plenty of grain boundaries as well as defects due to ball milling are also responsible for the lower Curie temperature. Temperature dependence of resistance of sample B under different load voltage is shown in Fig. 7. A remarkable CER effect occurs in the whole temperature range measured, especially, the ratio of CER goes up to 900% when load voltage increases from 0.1 to 0.8 V at about 130 K. In our previous study, a remarkable ER was also observed in NdSrMnO polycrystals with slight oxygen-deficiency even under a small applied voltage[25–27] . The CER effect in our present sample may be strongly correlated with the process of ball milling.
S.S. Chen et al.: J. Mater. Sci. Technol., 2010, 26(8), 721–724
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REFERENCES
Fig. 6 M -T curves of sample A (a) and B (b)
Fig. 7 R-T curves of sample B under different load voltages
4. Conclusions (1) Nd0.7 Sr0.3 MnO3 single phase with perovskite structure can be synthesized directly using a highenergy ball milling method. The optimal ball milling time is only 4 h, and the optimal mass ratio of milling ball to material is 10:1. (2) The grain size of Nd0.7 Sr0.3 MnO3 ceramics obtained using the ball milling method ranges from 51 to 93 nm. For sample B, many dislocations can be seen on the surface of grains. (3) For the as-milled and post heat-treatment Nd0.7 Sr0.3 MnO3 ceramics, a notable CER effect is observed in the whole temperature ranges measured. Especially, it gets to 900% at TC when load voltage increases from 0.1 to 0.8 V. Acknowledgements The authors thank the National Natural Science Foundation of China (Grant No. 10774040) and the joint Chinese-Russian Project for their financial supports.
[1 ] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido and Y. Tokura: Phys. Rev. B, 1995, 51, 14103. [2 ] A. Asamitsu, Y. Moritomo, R. Kumai, Y. Tomioka and Y. Tokura: Phys. Rev. B, 1996, 54, 1716. [3 ] M. Rajeswari, C.H. Chen, A. Goyal, C. Kwon, M.C. Robson, R. Ramesh, T. Venkatesan, and S. Lakeou: Appl. Phys. Lett., 1996, 68, 3555. [4 ] K. Chahara, T. Ohno, M. Kasai and Y. Kozono: Appl. Phys. Lett., 1993, 63, 1990. [5 ] R. Gross, L. Alff, B. Buchner, B.H. Freitag, C. Hofener, J. Klein, Y. Lu, W. Mader, J.B. Philipp, M.S.R. Rao, P. Reutler, S. Ritter, S. Thienhaus, S. Uhlenbruck and B. Wiedenhorst: J. Magn. Magn. Mater., 2000, 211, 150. [6 ] X.L. Zhang, Y.G. Zhao, Y.H. Sun, S.N. Gao, P.L. Lang, X.P. Zhang and M.H. Zhu: J. Magn. Magn. Mater., 2006, 306, 55. [7 ] E. Dagotto: Science, 2005, 309, 257. [8 ] Y.Q. Ma, Z.Q. Sun and M.Z. Wu: Solid State Commun., 2007, 143, 245. [9 ] J. Gao and F.X. Hu: Appl. Phys. Lett., 2005, 86, 92504. [10] J. Gao and F.X. Hu: J. Appl. Phys., 2005, 97, 10H706. [11] Y.Q. Ma, Z.Q. Sun and M.Z. Wu: Solid State Commun., 2007, 143, 245. [12] A. Singh, D.K. Aswal, P. Chowdhury, N. Padma, R.M. Kadam, Y. Babu, M.L. Jayanth Kumar, C.S. Viswanadham, S. Kumar, S.K.Gupta and J.V. Yakhmi: Solid State Commun., 2006, 138, 430. [13] H. Yao, F.X. Hu and J. Gao: Thin Solid Films, 2006, 495, 165. [14] Y.W. Xie, J.R. Sun, D.J. Wang, S. Liang and B.G. Shen: J. Appl. Phys., 2006, 100, 033704. [15] M. Muroi, R. Street and P.G. McCormick: J. Appl. Phys., 2000, 87, 3424. [16] Z.Q. Jin, J.R. Zhang, W. Tang and Y.W. Du: Solid State Commun., 1998, 108, 867. [17] A.M. Bolar´ın, F. S´ anchez, S. Palomares, J.A. Aguilar and G. Torres-Villase˜ nor: J. Alloy. Compd., 2007, 436, 335. [18] Q.W. Zhang and F. Saito: J. Alloy. Compd., 2000, 297, 99. [19] D.J. Wang and R.R. Liu: J. ChangChun Univ., 2003, 12, 31. (in Chinese) [20] Z.Q. Jin, H.X. Qin, J.R. Zhang and Y.W. Du: J. Phys.-Condes. Matter, 2000, 12, 3497. [21] Q.W. Zhang, T. Nakagawa and F. Saito: J. Alloy. Compd., 2000, 308, 121. [22] Z.Q. Jin, W. Tang, J.R. Zhang and Y.W. Du: J. Magn. Magn. Mater., 1998, 187, 237. [23] S.S. Chen, C.P. Yang, Z.H. Zhou, D.H. Guo, H. Wang and G.H. Rao: J. Alloy. Compd., 2008, 463, 271. [24] M. Muroi, R. Street and P.G. McCormick: J. Solid State Chem., 2000, 152, 503. [25] C.P. Yang, S.S. Chen, Q. Dai, D.H. Guo and H. Wang: Acta. Phys. Sin., 2007, 56, 4908. (in Chinese) [26] C.P. Yang, V. Morchshakov, Y.L. Huang, I.O. Troyanchuk and K. B¨ arner: Physica B, 2003, 337, 287. [27] C.P. Yang, S.S. Chen, Z.H. Zhou, L.F. Xu, H. Wang, J.F. Hu, V. Morchshakov and K. B¨ arner: J. Appl. Phys., 2007, 101, 063909.
Ball Milling Synthesis and Property of Nd0.7 Sr0.3 MnO3 Manganites Shunsheng Chen1) , Changping Yang1)† , Lingfang Xu1) and Shaolong Tang2) 1) The Provincial Key Laboratory of Piezoelectric Ceramics Materials and Apparatus, Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, China 2) Department of Physics, Nanjing University, Nanjing 210093, China [Manuscript received May 5, 2009, in revised form October 16, 2010]
Strontium doped perovskite-type Nd0.7 Sr0.3 MnO3 ceramics were synthesized completely by high-energy ball milling raw oxides of Nd2 O3 , SrCO3 and MnO2 . The optimal ball milling time and mass ratio of milling balls to raw materials are 4 h and 10:1, respectively. The grain size of as-milled Nd0.7 Sr0.3 MnO3 ceramics ranges from 51 to 93 nm, and the fine particles contain two phases of crystalline phase and amorphous phase. For the Nd0.7 Sr0.3 MnO3 synthesized by ball milling and sequent heat treatment, a remarkable colossal electroresistance (CER) effect is observed and the CER ratio reaches 900% at Curie temperature TC when the load voltage increases from 0.1 to 0.8 V. KEY WORDS: High-energy ball milling; Perovskite structure; Colossal electroresistance (CER); Microstructure defect
1. Introduction Alkali-earth doped perovskite-type manganites R1−x Ax MnO3 (R=rare-earth and A=alkali-earth) have attracted remarkable interest due to their various unique properties[1–4] . Recently, colossal electroresistance (CER) effect, the counterpart of colossal magnetoresistance (CMR), has been greatly focused on by a number of groups[5–9] . Large numbers of works about the CER have been done, however, the view about the physics of the CER effect is still not unanimous. Some groups consider it as the extrinsic properties of materials[10–14] . However, the preparation method and condition are closely associated with the extrinsic properties of materials and can greatly affect the physical properties of materials, especially, the character of electrical transport. Ball milling method is efficient for synthesizing manganite perovskite[15–24] , and it can produce material into metastates, which may contain large numbers of defects such as lattice imperfection, dislocations, bro† Corresponding author. Prof.; Tel.: +86 27 88665447; E-mail address: [email protected] (C.P. Yang).
ken chemical bonds and so on. Accordingly, in this work, we introduced a high-energy ball milling method to synthesize the Nd0.7 Sr0.3 MnO3 perovskite and discussed the optimal milling efficiency, investigated the properties of the as-milled samples as well as the electromagnetic character of the as-milled samples sequently treated by high temperatures. To our surprise, a notable CER effect is observed, which is different from that of stoichiometric NdSrMnO prepared by conventional solid-state reaction method[25] . 2. Experimental MnO2 (purity >99.9%), Nd2 O3 (purity >99.9%) and SrCO3 (purity >99.9%) powders were used as raw materials. Nd2 O3 and SrCO3 were pretreated at 1173 and 673 K for 6 and 4 h, respectively in order to eliminate the moisture. The molar ratio of Nd2 O3 , SrCO3 and MnO2 is 0.35:0.3:1. Raw materials with a total mass of 5 g were put inside a hardened steel pot with a volume of 80 cm3 together with 10 steel balls with diameters of 10 and 6 mm, respectively and milled for different times. The mass ratio of milling balls to raw
S.S. Chen et al.: J. Mater. Sci. Technol., 2010, 26(8), 721–724
722
Table 1 Parameter of milling balls and raw materials Raw materials and milling balls Mol ratio and ball numbers Mass/g Total mass/g
MnO2 0.02 mol 1.7458
SrCO3 0.006 mol 0.8893 5.0
Nd2 O3 0.007 mol 2.3649
Big ball 10 41.4
Small ball 10 8.9 50.3
materials was 10:1. The details are shown in Table 1. A high-energy ball mill with a speed of 1400 r/min was used to mill raw materials under air condition. Crystal structure of the as-milled sample (named as sample A) and the as-milled and post heat-treatment sample (named as sample B) was determined by X-ray diffraction (XRD, DRON-3, CuKα, Japan). Thermomagnetic curves were measured in a vibrating sample magnetometer (VSM, 6N30SE7C, China). Particle shape and size were observed by transmission electron microscopy (TEM, TecnaiG20, 200 kV, USA) and scanning electron microscopy (SEM, JSM-5610LV, Japan). For sample B, resistance measurements were performed using a standard four-wire method. 3. Results and Discussion In order to obtain the optimum efficiency of ball milling, firstly, we investigated the effect of mass ratio of milling balls to raw materials on the synthetic process. Raw materials with different mass ratios of balls to raw materials were respectively milled for the same time then identified by XRD. Just as the result shown in Fig. 1, the Nd0.7 Sr0.3 MnO3 perovskites were well synthesized when the mass ratio of balls to materials was 10:1. For obtaining the optimal milling time, there shares of materials with the same mass ratio of balls to materials (10:1) were prepared to mill for 2, 4 and 9 h, respectively and then examined by XRD. Obviously, for the materials milled for 4 h, the intensity of peak is stronger than the others (Fig. 2), indicating that the Nd0.7 Sr0.3 MnO3 can be formed completely by milling raw materials for 4 h, which is almost the same time with that reported by Bolar´ın et al.[17] . The intensity of peak will decrease gradually (Fig. 2(c)), and it can transform the perovskite phase into amorphous phase if one continues to increase the milling time after synthesizing the single phase[16,23] . Figure 3 shows XRD patterns of samples after milling raw materials for different times. Diffraction peaks of raw materials of Nd2 O3 , SrCO3 and MnO2 can be seen clearly in Fig. 3(a), as marked in the first panel with different symbols. However, it can be seen that the peaks of raw materials dramatically decrease with increasing milling time and almost disappear completely after only 0.5 h (Fig. 3(b)). Meanwhile, the new peaks corresponding to perovskite structure begin to appear with milling time up to 3 h (Fig. 3(c)). It can be seen that the intensities of those new perovskite peaks increase gradually with increasing milling time, and the Nd0.7 Sr0.3 MnO3 single phase is formed completely after milling raw materials for 4 h. No traces of other phase can be detected within the equipment accuracy (Fig. 3(d)). The result shows
Fig. 1 XRD of raw materials with different mass ratio of balls to materials after milling the same time: (a) 20:1, (b) 10:1, (c) 5:1
Fig. 2 XRD of raw materials with the same mass ratio of balls to materials after milling different time: (a) 2 h, (b) 4 h, (c) 9 h
that the ball milling is an another useful method to directly produce the stoichiometric and homogeneous Nd0.7 Sr0.3 MnO3 perovskite in addition to the conventional solid-state reaction method, even though the physical mechanism of ball milling is still unknown to us all. Besides, the intensity of peak of as-milled samples increases evidently if they were treated by high temperatures (Fig. 3(e)), suggesting that high temperatures are of advantage to grain growth. The transmission topography of particles of sample A was examined by TEM and shown in Fig. 4. The obtained particles are globular in shape and the grain size ranges from 51 to 93 nm with an average size of about 91 nm, which is in good coincidence with that estimated from the (200) reflection in Fig. 3(d)
S.S. Chen et al.: J. Mater. Sci. Technol., 2010, 26(8), 721–724
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Fig. 3 XRD patterns of samples after milling raw materials for: (a) 0 h, (b) 0.5 h, (c) 3 h and (d) 4 h, (e) XRD patterns of sample B treated at 1623 K for 1 h
Fig. 5 SEM images of sample A (a) and B (b)
Fig. 4 TEM image of sample A. Inset: the HRTEM image of nano-particle (reprinted with permission from Ref. [22])
using the Scherrer method. The HRTEM (high resolution transmission electron microscopy) results for the locally focused particle show that the particle is composed of crystalline phase (area A of the inset in Fig. 4) and amorphous phase (area B of the inset in Fig. 4). The amorphous phase may not be detected by XRD due to its small quantity relative to that of crystalline phase. It suggests that materials produced by high-energy ball milling are in metastate with lots of defects. In order to obtain further observation about grains, the morphology of Nd0.7 Sr0.3 MnO3 was examined by SEM. Figure 5 (a) and (b) show SEM images of sample A and B, respectively. From Fig. 5(a), we can see that the fine particles of sample A are already agglomerative after milling raw materials for 4 h, and the shape and size of the grains are almost the same with that of TEM. The grain in sample B has an average size of 2.2 μm, which is much bigger than that in sample A (Fig. 5(b)). However, an interesting
thing is that some dislocations-like at the surface of grains are observed, just denoted with arrows. Like the grain boundaries, these dislocations-like divide the grains into many smaller grains, thus, decreasing the grain size and increasing the ratio of the grain boundaries, as a result, influencing the electromagnetic transport. Temperature dependence of magnetization measured in a field cooling at 100 Oe for the sample A and B is shown in Fig. 6 (a) and (b), respectively. The M (T ) curve exhibits a paramagnetic-ferromagnetic (PM-FM) transition with a sharp decrease of the magnetization at Curie temperature, TC =70 K, which is much lower than TC =128 K for sample B. However, TC of sample B is still much lower than that of Nd0.7 Sr0.3 MnO3 (about 230 K) prepared using solidstate reaction method[25] . It suggests that the DE (double exchange) interaction is suppressed strongly by the process of ball milling. Probably, plenty of grain boundaries as well as defects due to ball milling are also responsible for the lower Curie temperature. Temperature dependence of resistance of sample B under different load voltage is shown in Fig. 7. A remarkable CER effect occurs in the whole temperature range measured, especially, the ratio of CER goes up to 900% when load voltage increases from 0.1 to 0.8 V at about 130 K. In our previous study, a remarkable ER was also observed in NdSrMnO polycrystals with slight oxygen-deficiency even under a small applied voltage[25–27] . The CER effect in our present sample may be strongly correlated with the process of ball milling.
S.S. Chen et al.: J. Mater. Sci. Technol., 2010, 26(8), 721–724
724
REFERENCES
Fig. 6 M -T curves of sample A (a) and B (b)
Fig. 7 R-T curves of sample B under different load voltages
4. Conclusions (1) Nd0.7 Sr0.3 MnO3 single phase with perovskite structure can be synthesized directly using a highenergy ball milling method. The optimal ball milling time is only 4 h, and the optimal mass ratio of milling ball to material is 10:1. (2) The grain size of Nd0.7 Sr0.3 MnO3 ceramics obtained using the ball milling method ranges from 51 to 93 nm. For sample B, many dislocations can be seen on the surface of grains. (3) For the as-milled and post heat-treatment Nd0.7 Sr0.3 MnO3 ceramics, a notable CER effect is observed in the whole temperature ranges measured. Especially, it gets to 900% at TC when load voltage increases from 0.1 to 0.8 V. Acknowledgements The authors thank the National Natural Science Foundation of China (Grant No. 10774040) and the joint Chinese-Russian Project for their financial supports.
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