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Structure and electrical properties in polymer bonded La0.7Sr0.3MnO3 composites
Available online at www.sciencedirect.com
Physics Procedia 27 (2012) 92 – 95
ISS2011
Structure and electrical properties in polymer bonded La0.7Sr0.3MnO3 composites Liqin Yanga, Xinsheng Yanga , Cuihua Chengb, Li Lva, Yong Zhaoa,b a
Key Lab of Advanced Material Technologies (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu,610031 China b School of Materials Science and Engineering, University of New South Wales, Sydney, 2052, NSW, Australia
Abstract Novel bonded perovskite oxide La0.7Sr0.3MnO3(LSMO) was prepared using epoxy resin as the binder and the microstructure, phase characteristic, density, hardness, electrical and magnetic transport properties were studied. The binder has no reaction with LSMO. With the increase of binder concentration (x), the density and hardness of the sample reach the maximum value when x=2.5%. No insulator to metal phase transition is observed in the whole temperature range. The bonded samples show typical low field magnetoresistance characteristics.
© © 2012 2011 Published Publishedby byElsevier ElsevierB.V. Ltd. Selection Selectionand/or and/orpeer-review peer-reviewunder underresponsibility responsibilityofofISS ISSProgram ProgramCommittee Committee. Keywards: Perovskite oxides; Bonded method; Microstructure
1. Introduction Perovskite manganese oxides of the type RE1-xAExMnO3 (RE = La, Nd, Pr; AE = Ca, Sr, Ba, etc.) have recently attracted considerable research interest because of discovery of the colossal magnetoresistance (CMR) effect [1, 2]. CMR effect is triggered only near the Curie temperature (Tc) at a relative high magnetic field of several Tesla, which limits its practical applications [3]. Recently, another difference magnetoresistance effect has been developed, which associated with the grain boundary in polycrystalline samples of manganese-based perovskite oxides, so called lowfield magnetoresistance (LFMR). LFMR needs only low driving magnetic field (~kOe) and it is non sensitive to temperature, which benefits the practical applications for CMR materials [4]. Ordinarily, enhanced LFMR is obtained by making a composite of the perovskite manganese oxide with a secondary phase such as an insulating oxide, metal and polymer or other CMR oxide [5-9]. This is called the manganese-based two-phase composition. In the process to prepare this kind of manganese oxide composite, however, sintering process is inevitable, which means that diffusion and reaction between two phases cannot be entirely excluded [9]. To obtain manganite-based two-phase composition with clear phase boundary, a newly bonded method has been put forward [10]. Very different from the conventional ceramic, there is no chemical reaction between perovskite manganite oxide and the secondary phase material as no sintering process for bonded perovskite manganite. To prepare bonded sample without sintering process, polymer and metal have been used as the binder. Detailed electrical transport and magnetoresistance properties have been reported elsewhere [10]. The structural characteristics,
Corresponding author. Tel.: +86-28-8760-0787 ; fax: +86-28-8760-0787 . E-mail address: [email protected] . 1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.418
Liqin Yang et al. / Physics Procedia 27 (2012) 92 – 95
93
however, have not been carefully investigated. From the viewpoint of practical application, it is necessary to carry out these studies. In this work, we focus on studying the microstructure, density and hardness properties for polymer (epoxy resin, E-44) bonded La0.7Sr0.3MnO3. 2. Experimental Precursor La0.7Sr0.3MnO3 (LSMO) powders had been synthesized by conventional solid state reaction method. High purity powders of La2O3, SrCO3 and Mn(CH3COO)2·4H2O were mixed in stoichiometric proportion. The mixed powders were ground carefully and preheat at 1200 oC for 10 h. Then 1 wt.% of silane coupling agent (KH-550) was added to the precursor powders and uniform stirred. The coupled powders was mixed uniformly with epoxy resin (E44) and corresponding curing agent with epoxy resin in appropriate ratio (x=0.5-10 wt.%). The resulting mixtures were pelletized at a pressure of 800 MPa, and then aged at room temperature for 10 h. The crystal structures of the samples were characterized by the x-ray diffraction (XRD, X’Pert Panalytical). The surface morphology was observed by scanning electron microscope (SEM, Quanta 200). Density measured by Archimedes method. Hardness was carried out under an external force (500g) and hold pressure 10s by Japanese Akashi (MVKH21) Vickers hardness tester. The electrical and magnetic transport measurements were examined by the PPMS (Physical Property Measurement Systems, Quantum Design). 3. Results and discussion The XRD patters for the boned samples with different amount of binder show the same phase structure, which corresponds to a single perovskite crystalline, shown in Fig. 1. There is no sintering process to prepare the bonded samples, so the phase structure does not change obviously.
Fig. 1. X-ray diffraction patterns of LSMO for different samples.
Fig. 2. SEM images of different samples: (a) sintering ceramic sample, (b) bonded sample, x=2%.
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Liqin Yang et al. / Physics Procedia 27 (2012) 92 – 95
Typical SEM micrographs of bonded samples are shown in Fig. 2. For comparison, SEM for sintering ceramic sample is also given. There are clear grain boundaries in LSMO ceramic, as shown in Fig. 2(a). For the bonded samples, it can be seen that boundaries become ambiguous, as shown in Fig. 2(b). Fig. 3 shows the density curve for the bonded samples with different amount of binder. Sample density increases with the increase of epoxy resin content (x<2.5%), reaching the maximum when x=2.5%. If epoxy resin content exceeds 2.5%, the density shows a decreasing trend with x. For x<2.5%, the binder is mainly dispersed in the sample filling in grain boundary of LSMO, so the density increases. If binder content is higher than 2.5%, excessive binder aggregates itself, decreasing the sample density. The hardness test results of the sample are shown in Fig. 4. The HV-x curve is similar to the Density-x curve with the same mechanism.
Fig. 3. Density curve of LSMO for different samples.
Fig. 4. Hardness curves of LSMO for different samples.
Fig. 5 shows the temperature dependence of resistance in zero ¿eld in the temperature range from 50 to 300 K for all the samples. All samples show purely semiconducting behavior. No insulator-metal transition (I-M) appears in the whole temperature range, which may result from the existence of insulating polymer segregated at the grain boundary. Generally, introduction secondary phase insulation material at the grain boundary will suppress insulator-metal phase transition [3].
Fig. 5. Temperature dependence of zero-field resistance for different samples.
Fig. 6. Zero-field resistivity at 290K for different samples.
Fig. 7. Temperature dependence of MR for different samples under an applied field of 3 kOe.
Liqin Yang et al. / Physics Procedia 27 (2012) 92 – 95
95
Zero-field resistivity at 290 K for the bonded samples with different amount of binder is shown in Fig. 6. Firstly, the resistivity slightly decreases with the increase of binder content; then resistivity reaches the minimum when epoxy resin content x=2.5%; subsequently, resistivity shows an increasing trend. For x=10%, the resistance is about 3 orders of magnitude higher than that for the sample with low doping content. Fig. 7 shows the magnetoresistance (MR) characteristics under an applied field of 3 kOe for the sintering ceramic and the bonded sample (x=1%). MR is de¿ned as (ȡ0íȡH)/ȡ0×100%, where ȡ0 and ȡH are the resistivity under an applied magnetic ¿eld of 0 and 3 kOe, respectively. There is no peak in the MR-T curve, exhibiting obvious LFMR characteristic. Bonded sample and sintering ceramic sample have a similar magnetoresistance characteristic in the temperature range of 50~250K. After temperature higher than 250K, MR value has obvious difference. For the sintering ceramic, MR continues to decrease with increasing of temperature; whereas for the bonded sample, MR remains almost the same value in the temperature range of 250~350 K. 4. Conclusions In the process to prepare bonded perovskite oxide La0.7Sr0.3MnO3, the phase structure does not change. The binder mainly disperses at the grain boundaries. The density and hardness of the sample reach the maximum value when binder concentration is 2.5%. The zero-field resistivity at room temperature firstly decrease with increasing of binder content, reaches the lowest value at x=2.5%, and then shows an increasing trend. All the samples show purely semiconducting behavior. The bonded samples show typical low field magnetoresistance characteristics. The magnetoresistance for the sample containing 1% binder keeps at the same value in the temperature range of 250~350K. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 50872116, 51002125), the Specialized Research Fund for the Doctoral Program of Higher Education (200806130023), and the Fundamental Research Funds for the Central Universities (SWJTU09ZT24, SWJTU11ZT16, SWJTU11ZT29), and Fundamental Research Funds of Sichuan Province (2011JY0031). References [1] Coey JMD, Viret M, Molnar S. von. Mixed-valence manganites. Adv Phys 1999; 48: 167-293. [2] Urushibara A, Maritomo Y, Arima T. Insulator-metal transition and giant magnetoresistance in La1-xSrxMnO3. Phys Rev B 1995; 51: 14103-9. [3] Gaur A, Varma GD, Singh HK. Enhanced low field magnetoresistance in La0.7Sr0.3MnO3/TiO2 composite. J Phys:Appl Phys 2006; 39: 3531–5. [4] Hwang HY, Cheong SW, Ong NP. Batlogg B. Spin-Polarized Intergrain Tunneling in La2/3Sr1/3MnO3. Phys Rev Lett 1996; 77: 2041-4. [5] Yan CH, Huang YH, Chen X, Liao CS, Wang ZM. Improvement of magnetoresistance over a wide temperature range in La2/3Sr1/3MnO3polymer composites. J Phys: Condens Matter 2002; 14: 9607-14. [6] Huang YH, Chen X, Wang ZM, Liao CS, Yan CH. Enhanced magnetoresistance in granular La2/3Ca1/3MnO3/polymer composites. J Appl Phy 2002; 91: 7733-5. [7] Kumar J, Singh RK, Siwach PK, Singh HK, Singh R, Srivastava ON. Low field magneto-transport in LBSMO–PMMA composite. J Magn Magn Mater 2006; 299: 155-60. [8] Yang X, Yang Y, He W, Cheng CH, Zhao Y. Low field magnetoresistance in La0.7Sr0.3MnO3/Ta2O5 composites. J Phys D: Appl Phys 2008; 41:115009. [9] Liu JM, Yuan GL, Sang H. Low-field magnetoresistance in nanosizedLa0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 composites. Appl Phys Lett 2001;78: 11102. [10] Yang L, Yang X, Lv L, Cheng C, Zhao Y. Microstructure and transport properties in tin boned La0.7Sr0.3MnO3 composites. J Supercond Nov Magn 2011; 24:1847–1851.
Physics Procedia 27 (2012) 92 – 95
ISS2011
Structure and electrical properties in polymer bonded La0.7Sr0.3MnO3 composites Liqin Yanga, Xinsheng Yanga , Cuihua Chengb, Li Lva, Yong Zhaoa,b a
Key Lab of Advanced Material Technologies (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu,610031 China b School of Materials Science and Engineering, University of New South Wales, Sydney, 2052, NSW, Australia
Abstract Novel bonded perovskite oxide La0.7Sr0.3MnO3(LSMO) was prepared using epoxy resin as the binder and the microstructure, phase characteristic, density, hardness, electrical and magnetic transport properties were studied. The binder has no reaction with LSMO. With the increase of binder concentration (x), the density and hardness of the sample reach the maximum value when x=2.5%. No insulator to metal phase transition is observed in the whole temperature range. The bonded samples show typical low field magnetoresistance characteristics.
© © 2012 2011 Published Publishedby byElsevier ElsevierB.V. Ltd. Selection Selectionand/or and/orpeer-review peer-reviewunder underresponsibility responsibilityofofISS ISSProgram ProgramCommittee Committee. Keywards: Perovskite oxides; Bonded method; Microstructure
1. Introduction Perovskite manganese oxides of the type RE1-xAExMnO3 (RE = La, Nd, Pr; AE = Ca, Sr, Ba, etc.) have recently attracted considerable research interest because of discovery of the colossal magnetoresistance (CMR) effect [1, 2]. CMR effect is triggered only near the Curie temperature (Tc) at a relative high magnetic field of several Tesla, which limits its practical applications [3]. Recently, another difference magnetoresistance effect has been developed, which associated with the grain boundary in polycrystalline samples of manganese-based perovskite oxides, so called lowfield magnetoresistance (LFMR). LFMR needs only low driving magnetic field (~kOe) and it is non sensitive to temperature, which benefits the practical applications for CMR materials [4]. Ordinarily, enhanced LFMR is obtained by making a composite of the perovskite manganese oxide with a secondary phase such as an insulating oxide, metal and polymer or other CMR oxide [5-9]. This is called the manganese-based two-phase composition. In the process to prepare this kind of manganese oxide composite, however, sintering process is inevitable, which means that diffusion and reaction between two phases cannot be entirely excluded [9]. To obtain manganite-based two-phase composition with clear phase boundary, a newly bonded method has been put forward [10]. Very different from the conventional ceramic, there is no chemical reaction between perovskite manganite oxide and the secondary phase material as no sintering process for bonded perovskite manganite. To prepare bonded sample without sintering process, polymer and metal have been used as the binder. Detailed electrical transport and magnetoresistance properties have been reported elsewhere [10]. The structural characteristics,
Corresponding author. Tel.: +86-28-8760-0787 ; fax: +86-28-8760-0787 . E-mail address: [email protected] . 1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.418
Liqin Yang et al. / Physics Procedia 27 (2012) 92 – 95
93
however, have not been carefully investigated. From the viewpoint of practical application, it is necessary to carry out these studies. In this work, we focus on studying the microstructure, density and hardness properties for polymer (epoxy resin, E-44) bonded La0.7Sr0.3MnO3. 2. Experimental Precursor La0.7Sr0.3MnO3 (LSMO) powders had been synthesized by conventional solid state reaction method. High purity powders of La2O3, SrCO3 and Mn(CH3COO)2·4H2O were mixed in stoichiometric proportion. The mixed powders were ground carefully and preheat at 1200 oC for 10 h. Then 1 wt.% of silane coupling agent (KH-550) was added to the precursor powders and uniform stirred. The coupled powders was mixed uniformly with epoxy resin (E44) and corresponding curing agent with epoxy resin in appropriate ratio (x=0.5-10 wt.%). The resulting mixtures were pelletized at a pressure of 800 MPa, and then aged at room temperature for 10 h. The crystal structures of the samples were characterized by the x-ray diffraction (XRD, X’Pert Panalytical). The surface morphology was observed by scanning electron microscope (SEM, Quanta 200). Density measured by Archimedes method. Hardness was carried out under an external force (500g) and hold pressure 10s by Japanese Akashi (MVKH21) Vickers hardness tester. The electrical and magnetic transport measurements were examined by the PPMS (Physical Property Measurement Systems, Quantum Design). 3. Results and discussion The XRD patters for the boned samples with different amount of binder show the same phase structure, which corresponds to a single perovskite crystalline, shown in Fig. 1. There is no sintering process to prepare the bonded samples, so the phase structure does not change obviously.
Fig. 1. X-ray diffraction patterns of LSMO for different samples.
Fig. 2. SEM images of different samples: (a) sintering ceramic sample, (b) bonded sample, x=2%.
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Typical SEM micrographs of bonded samples are shown in Fig. 2. For comparison, SEM for sintering ceramic sample is also given. There are clear grain boundaries in LSMO ceramic, as shown in Fig. 2(a). For the bonded samples, it can be seen that boundaries become ambiguous, as shown in Fig. 2(b). Fig. 3 shows the density curve for the bonded samples with different amount of binder. Sample density increases with the increase of epoxy resin content (x<2.5%), reaching the maximum when x=2.5%. If epoxy resin content exceeds 2.5%, the density shows a decreasing trend with x. For x<2.5%, the binder is mainly dispersed in the sample filling in grain boundary of LSMO, so the density increases. If binder content is higher than 2.5%, excessive binder aggregates itself, decreasing the sample density. The hardness test results of the sample are shown in Fig. 4. The HV-x curve is similar to the Density-x curve with the same mechanism.
Fig. 3. Density curve of LSMO for different samples.
Fig. 4. Hardness curves of LSMO for different samples.
Fig. 5 shows the temperature dependence of resistance in zero ¿eld in the temperature range from 50 to 300 K for all the samples. All samples show purely semiconducting behavior. No insulator-metal transition (I-M) appears in the whole temperature range, which may result from the existence of insulating polymer segregated at the grain boundary. Generally, introduction secondary phase insulation material at the grain boundary will suppress insulator-metal phase transition [3].
Fig. 5. Temperature dependence of zero-field resistance for different samples.
Fig. 6. Zero-field resistivity at 290K for different samples.
Fig. 7. Temperature dependence of MR for different samples under an applied field of 3 kOe.
Liqin Yang et al. / Physics Procedia 27 (2012) 92 – 95
95
Zero-field resistivity at 290 K for the bonded samples with different amount of binder is shown in Fig. 6. Firstly, the resistivity slightly decreases with the increase of binder content; then resistivity reaches the minimum when epoxy resin content x=2.5%; subsequently, resistivity shows an increasing trend. For x=10%, the resistance is about 3 orders of magnitude higher than that for the sample with low doping content. Fig. 7 shows the magnetoresistance (MR) characteristics under an applied field of 3 kOe for the sintering ceramic and the bonded sample (x=1%). MR is de¿ned as (ȡ0íȡH)/ȡ0×100%, where ȡ0 and ȡH are the resistivity under an applied magnetic ¿eld of 0 and 3 kOe, respectively. There is no peak in the MR-T curve, exhibiting obvious LFMR characteristic. Bonded sample and sintering ceramic sample have a similar magnetoresistance characteristic in the temperature range of 50~250K. After temperature higher than 250K, MR value has obvious difference. For the sintering ceramic, MR continues to decrease with increasing of temperature; whereas for the bonded sample, MR remains almost the same value in the temperature range of 250~350 K. 4. Conclusions In the process to prepare bonded perovskite oxide La0.7Sr0.3MnO3, the phase structure does not change. The binder mainly disperses at the grain boundaries. The density and hardness of the sample reach the maximum value when binder concentration is 2.5%. The zero-field resistivity at room temperature firstly decrease with increasing of binder content, reaches the lowest value at x=2.5%, and then shows an increasing trend. All the samples show purely semiconducting behavior. The bonded samples show typical low field magnetoresistance characteristics. The magnetoresistance for the sample containing 1% binder keeps at the same value in the temperature range of 250~350K. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 50872116, 51002125), the Specialized Research Fund for the Doctoral Program of Higher Education (200806130023), and the Fundamental Research Funds for the Central Universities (SWJTU09ZT24, SWJTU11ZT16, SWJTU11ZT29), and Fundamental Research Funds of Sichuan Province (2011JY0031). References [1] Coey JMD, Viret M, Molnar S. von. Mixed-valence manganites. Adv Phys 1999; 48: 167-293. [2] Urushibara A, Maritomo Y, Arima T. Insulator-metal transition and giant magnetoresistance in La1-xSrxMnO3. Phys Rev B 1995; 51: 14103-9. [3] Gaur A, Varma GD, Singh HK. Enhanced low field magnetoresistance in La0.7Sr0.3MnO3/TiO2 composite. J Phys:Appl Phys 2006; 39: 3531–5. [4] Hwang HY, Cheong SW, Ong NP. Batlogg B. Spin-Polarized Intergrain Tunneling in La2/3Sr1/3MnO3. Phys Rev Lett 1996; 77: 2041-4. [5] Yan CH, Huang YH, Chen X, Liao CS, Wang ZM. Improvement of magnetoresistance over a wide temperature range in La2/3Sr1/3MnO3polymer composites. J Phys: Condens Matter 2002; 14: 9607-14. [6] Huang YH, Chen X, Wang ZM, Liao CS, Yan CH. Enhanced magnetoresistance in granular La2/3Ca1/3MnO3/polymer composites. J Appl Phy 2002; 91: 7733-5. [7] Kumar J, Singh RK, Siwach PK, Singh HK, Singh R, Srivastava ON. Low field magneto-transport in LBSMO–PMMA composite. J Magn Magn Mater 2006; 299: 155-60. [8] Yang X, Yang Y, He W, Cheng CH, Zhao Y. Low field magnetoresistance in La0.7Sr0.3MnO3/Ta2O5 composites. J Phys D: Appl Phys 2008; 41:115009. [9] Liu JM, Yuan GL, Sang H. Low-field magnetoresistance in nanosizedLa0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 composites. Appl Phys Lett 2001;78: 11102. [10] Yang L, Yang X, Lv L, Cheng C, Zhao Y. Microstructure and transport properties in tin boned La0.7Sr0.3MnO3 composites. J Supercond Nov Magn 2011; 24:1847–1851.