Article pubs.acs.org/JPCC
Magnetic Alignment of Gamma (Core)−Alpha (Shell) Fe2O3 Nanorods in a Solid Polymer Electrolyte for Li-Ion Batteries Dean M. Schaetzl,† Peng Li,‡ Nilima Chaudhari,§ Gary H. Bernstein,‡ and Susan K. Fullerton-Shirey‡,* †
Department of Electrical Engineering Technology, Purdue University, South Bend, Indiana 46634, United States Department of Electrical Engineering, Notre Dame Center for Nano Science and Technology, University of Notre Dame, Notre Dame, Indiana 46556, United States § Center of Excellence in Solar Energy, Physical and Materials Chemistry Division, National Chemical Laboratory (CSIR-NCL), Pune, Maharashtra 411008, India ‡
S Supporting Information *
ABSTRACT: The temperature-dependent ionic conductivity and thermal properties are characterized for a solid polymer electrolyte of poly(ethylene oxide) (PEO) and LiClO4 filled with 1 wt % γ-phase core (maghemite) and α-phase shell (hematite) Fe2O3 nanorods. Samples are solvent-cast in the absence and presence of a 0.5 T magnetic field, dried at room temperature under vacuum for 72 h, and measured under nitrogen. Vibrating sample magnetometry indicates that the magnetic treatment aligns the nanorods to some extent in the desired orientation normal to the electrode surface. For samples with an ether oxygen to lithium ratio (EO/Li) of 10:1, the nanorods induce sample-to-sample variability in the ionic conductivity. The magnetic treatment eliminates this variability, and differential scanning calorimetry data support the observation that the magnetic treatment increases the structural homogeneity of the electrolyte. For samples with an EO/Li of 3:1, the ionic conductivity is 3 orders of magnitude larger for samples containing 5 times more of the crystal structure, (PEO)6/LiClO4. This result is surprising because an inverse relationship between crystallinity and conductivity is normally observed for semicrystalline, solid polymer electrolytes. When the crystal fraction is increased by a factor of 8 via the combination of nanorods and magnetic treatment, the conductivity does not continue to increase, showing that the effect does not persist beyond a critical fraction of (PEO)6/LiClO4. The results demonstrate that field-effect alignment of magnetic nanorods increases the crystal fraction and homogeneity of PEO/LiClO4, but does not affect the ionic conductivity in the range of salt and nanorod concentrations investigated.
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INTRODUCTION Solid polymer electrolytes (SPE) would be a useful replacement for liquid-phase electrolytes in lithium-ion batteries owing to their solid-state properties, mechanical flexibility and electrochemical stability (e.g., resistance to dendrite formation). The room temperature ionic conductivity of poly(ethylene oxide) (PEO)-based SPEs is low (∼10−8−10−6 S/cm).1,2 Efforts to improve conductivity while retaining the solid state property of the electrolyte include polymer alignment via mechanical stretching,3 confinement to nanopores4 and applied electric5,6 and magnetic fields.7,8 For example, Golodnitsky and coworkers increased the conductivity of PEO/LiI by 1 order of magnitude at 65 °C by adding magnetic particles and exposing the sample to a magnetic field.7 They showed that ion transport through both the bulk electrolyte and grain boundaries was improved, but the magnetic treatment had a larger effect on grain-boundary conductivity.9 The addition of peptide nanotubes coated with magnetic nanoparticles improved the conductivity by 2 orders of magnitude and increased the thermal stability.10 This group also achieved comparable improvement by mechanically stretching the sample.3 En© 2014 American Chemical Society
hanced ion transport along the stretching direction was qualitatively11 and quantitatively12 captured by modeling, and the conductivity improvement was attributed to the alignment of PEO helices along the stretching direction. This conclusion has been supported by NMR data.13,14 To understand the relationship between polymer alignment and conductivity, we consider ion transport mechanisms within a PEO-based SPE. Within amorphous domains, ions move via the segmental relaxation of the polymer host.2 Within crystalline domains, ion transport depends on the identity of the crystalline phase, which is a function of temperature, salt identity, and salt concentration. For the most widely studied SPE, PEO/LiClO4, three crystalline phases can form: (1) pure PEO, (2) (PEO)6/LiClO4, and (3) (PEO)3/LiClO4.15 When pure PEO crystallizes, Li+ is expelled to the nearby amorphous domains;16 therefore, this phase prohibits ion transport. (PEO)6/LiX, where X = AsF6,17 PF6,18 and SbF6,18 forms a Received: February 20, 2014 Revised: June 15, 2014 Published: July 18, 2014 18836
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cylinder comprising two chains of PEO with Li+ located in the middle of the cylinder and anions located between cylinders. Remnants of this structure have been detected for PEO/LiClO4 at temperatures above the melting point.19 Peter Bruce’s group showed that the (PEO)6 crystalline phase is 1 order of magnitude more conductive than the amorphous equivalent.20 In a latter publication, they went on to show that fast ion transport is not limited to the (PEO)6 crystalline phase, but has also been observed for (PEO)8 with Na+, K+, and Rb+ cations.21 Although the crystal structure of the (PEO)3 phase has not been reported with LiClO4 as the salt, it has been reported for several chemically similar salts, including NaClO 4 , 22 LiCF3SO3,23 and Li(CF3SO3)2.24 The structure consists of a single, helical PEO chain with cations located within each turn of the helix. Given the similarities between the molecular structures of various PEO-based SPEs, it is reasonable to expect (PEO)3/LiClO4 to have a similar structure. Conductivity data for SPEs containing (PEO)3/LiI suggest that this phase also conducts ions.3 Because the optimal direction for Li+ transport is orthogonal to the electrode surface, the ideal alignment of conductive pathways in SPEs is also orthogonal to the electrode surface. Of the techniques to induce polymer alignment mentioned above, field-induced alignment is attractive because it does not require chemical modification of the polymer, or contact with the electrolyte. However, the low electrical (χe = 2)25 and magnetic (χm = −0.62 × 10−6)26 susceptibilities of PEO suggest that large field strengths would be required to induce an appreciable amount of polymer order. Another technique for improving conductivity is to add metal oxide nanoparticle fillers.27−33 The mechanisms by which nanofillers improve ion transport have been widely debated, but the three most common mechanisms involve (1) nanofiller surface chemistry, (2) long-range nanofiller order, and (3) polymer structure at the nanofiller/SPE interface. First, nanofillers with a higher concentration of hydroxyl termination (i.e., acidic surface chemistry) improve SPE conductivity more than those without hydroxyl termination.34 Therefore, the specific polymorph of the metal oxide nanofiller is important. Second, if the interface between the nanofiller and the SPE favors ion mobility,35 then the long-range order of the nanofillers is also important. Our recent publication showed that Fe2O3 nanofillers with a large aspect ratio (i.e., nanorods) improve the conductivity of PEO/LiClO4 at a loading ten times lower than required for spherical nanoparticles.36 Because the percolation threshold scales inversely with aspect ratio, longrange nanofiller order can be achieved at lower loading in a system that includes nanorods as compared to spherical nanoparticles. Third, there exists some evidence that metal oxide nanofillers may enhance the formation and stabilization of conductive structures, such as (PEO)6/LiClO4.33,36 In combination, the mechanisms imply that nanofillers with the proper surface chemistry that are aligned orthogonal to the electrode surface over long distances may enhance ion transport more so than unaligned nanofillers. Therefore, we modify our SPE with nanofillers chosen to meet the following criteria: (1) a high concentration of hydroxyl surface groups, (2) a large aspect ratio, and (3) a large magnetic susceptibility, so that the fillers can be aligned, and (4) their easy axis to coincide with their long axis so that when aligned, they are positioned orthogonal to the electrode surface. We choose magnetic field alignment because it is a noncontact method, and the magnetic susceptibility of the SPE can be increased by
adding magnetic nanofillers, which here are chosen as Fe2O3 nanorods with a magnetic susceptibility, χm, of approximately 2 × 10−3.37 Different polymorphs of Fe2O3 respond to magnetic fields differently: for γ-Fe2O3 nanorods, the shape anisotropy is much stronger than the crystalline anisotropy, so the easy axis is along the long axis of the rods.38 However, the crystalline anisotropy of α-Fe2O3 nanorods is much stronger than the shape anisotropy, causing the magnetic moment to lie in the plane perpendicular to its long axis,39,40 and therefore allowing the rods to point in random directions perpendicular to the field inside the polymer electrolyte. This is similar to Co- and Mn-doped ZnO nanowires that orient perpendicular to the direction of the applied field.41 While this response to the magnetic field is not desirable for our purposes, the surface chemistry of α-Fe2O3 will be more favorable for ion conduction, owing to the presence of surface hydroxyl groups.42,43 Therefore, the ideal magnetic nanofiller is one in which the majority phase is γ-Fe2O3 comprising the core of the nanorod, and the minority phase is α-Fe2O3, comprising the shell. In such a core/shell nanorod, shape anisotropy will dominate the magnetic anisotropy, and the effective easy axis will be the long axis of the rods. This γ-core/α-shell Fe2O3 nanorod can be synthesized by controlled thermal decomposition, and descriptions of the synthesis procedure and core/ shell characterization have been previously published by N. Chaudhari and co-workers.44 Our previous work studying the effect of Fe2O3 nanorods on conductivity indicated that the maximum conductivity is achieved at a filler loading of 1 wt %.36 We therefore add 1 wt % γ-core/α-shell Fe2O3 nanorods to (PEO)10/LiClO4. An ether oxygen to lithium ratio (EO/Li) of 10:1 is chosen because it gives the maximum conductivity improvement with the addition of nanofillers.33 To maximize the extent of alignment, we expose the sample to a 0.5 T field when the sample is in its most disordered state (i.e., dissolved in solvent). The sample is taken from the disordered state to the ordered state by slow solvent evaporation to maximize order and minimize defect formation. We measure the ionic conductivity using impedance spectroscopy, the magnetic response using vibrating sample magnetometry (VSM), and the thermal properties using differential scanning calorimetry (DSC).
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EXPERIMENTAL METHODS Fe2O3 Nanofiller. Fe2O3 nanorods with a γ-phase core (maghemite) and α-phase shell (hematite) were synthesized and characterized according to reference.44 The average diameter and length of the nanorods is 13 and 200 nm, giving an aspect ratio of 15:1. The mass ratio of γ-Fe2O3 (core) to αFe2O3 (shell) is 75:25 as measured by Mossbauer spectroscopy.44 SPE Sample Preparation. Poly(ethylene oxide) (PEO, Mw = 600 000 g/mol) was purchased from Sigma-Aldrich. Because commercial PEO contains micron-sized particles of SiO2, the polymer was purified by dissolving the powder in acetonitrile and then centrifuging at 13 000 rpm for 5 min, producing a white precipitate that was discarded. LiClO4 containing 60 °C where the (PEO)6 phase has melted. When left at room temperature under vacuum for several days after the second heating scan, the samples do not regain the high conductivity observed on the first heating scan. This suggests that the high conductivity values are related to the slow solvent anneal, which provides the maximum amount of polymer mobility and the best opportunity for the electrolyte to achieve the most energetically favorable state. This result contradicts the conventional understanding that crystallinity must be decreased in favor of polymer mobility to increase ion transport in an entangled, polymer electrolyte melt. Although higher fractions of (PEO)6 correspond to enhanced conductivity, there is a limit to the effectiveness: when the (PEO)6 fraction is increased further by adding Fe2O3 nanorods
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CONCLUSIONS We conclude that neither the presence of 1 wt % γ-core/α-shell Fe2O3 nanorods, or a 0.5 T magnetic treatment affect the conductivity of SPEs comprising (PEO)3/LiClO4 or (PEO)10/ LiClO4. In samples with an EO/Li of 10:1, the nanorods increase sample-to-sample variation in the conductivity, and this variability is eliminated when the nanorod-filled samples are magnetically treated. Both the conductivity and DSC data suggest that nanorods promote structural disorder in the samples, while the magnetic treatment promotes structural order. For samples with an EO/Li ratio of 3:1, the conductivity increases by 3 orders of magnitude at room temperature when the crystal fraction of (PEO)6 increases by a factor of 4 to 6. When this crystal structures is melted (i.e., T > 60 °C), the conductivity decreases. This is a surprising observation because ionic conductivity is strongly correlated to polymer mobility. However, treatments that further increase the fraction of (PEO)6 (i.e., the addition of nanorods and/or magnetic treatment) do not further increase the conductivity, demonstrating a limit to the effectiveness of (PEO)6 under the sample conditions investigated. It was first reported in 2001 that the ionic conductivity of crystalline (PEO)6 was more conductive than the amorphous equivalent. Gadjourova and co-workers demonstrated this for a fully crystalline, powder sample of pure (PEO)6, created with low molecular weight PEO.20 Here, we show a conductivity improvement of 3 orders of magnitude at room temperature for a semicrystalline, fully entangled, polymer electrolyte when the (PEO)6 fraction increases by 4 to 6 times. These results motivate reconsidering the general approach of improving 18843
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Electrolytes for Lithium Batteries. Electrochem. Solid State Lett. 2004, 7, A412−A415. (8) Majewski, P. W.; Gopinadhan, M.; Osuji, C. O. Magnetic Field Alignment of Block Copolymers and Polymer Nanocomposites: Scalable Microstructure Control in Functional Soft Materials. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 2−8. (9) Kovarsky, R.; Golodnitsky, D.; Peled, E.; Khatun, S.; Stallworth, P. E.; Greenbaum, S.; Greenbaum, A. Conductivity Enhancement Induced by Casting of Polymer Electrolytes under a Magnetic Field. Electrochim. Acta 2011, 57, 27−35. (10) Goldshtein, K.; Golodnitsky, D.; Peled, E.; Adler-Abramovich, L.; Gazit, E.; Khatun, S.; Stallworth, P.; Greenbaum, S. Effect of Peptide Nanotube Filler on Structural and Ion-Transport Properties of Solid Polymer Electrolytes. Solid State Ionics 2012, 220, 39−46. (11) Durr, O.; Dieterich, W.; Maass, P.; Nitzan, A. Effective Medium Theory of Conduction in Stretched Polymer Electrolytes. J. Phys. Chem. B 2002, 106, 6149−6155. (12) Gitelman, L.; Averbuch, A.; Nathan, M.; Schuss, Z.; Golodnitsky, D. Stochastic Model of Lithium Ion Conduction in Poly(Ethylene Oxide). J. Appl. Phys. 2010, 107. (13) Chung, S. H.; Wang, Y.; Greenbaum, S. G.; Golodnitsky, D.; Peled, E. Uniaxial Stress Effects in Poly(Ethylene Oxide)-LiI Polymer Electrolyte FilmA Li-7 Nuclear Magnetic Resonance Study. Electrochem. Solid State Lett. 1999, 2, 553−555. (14) Golodnitsky, D.; Livshits, E.; Ulus, A.; Barkay, Z.; Lapides, I.; Peled, E.; Chung, S. H.; Greenbaum, S. Fast Ion Transport Phenomena in Oriented Semicrystalline LiI-P(EO)n-Based Polymer Electrolytes. J. Phys. Chem. A 2001, 105, 10098−10106. (15) Robitaille, C. D.; Fauteux, D. Phase-Diagrams and Conductivity Characterization of Some PEO-LiX Electrolytes. J. Electrochem. Soc. 1986, 133, 315−325. (16) Fullerton-Shirey, S. K.; Maranas, J. K. Effect of LiClO4 on the Structure and Mobility of PEO-Based Solid Polymer Electrolytes. Macromolecules 2009, 42, 2142−2156. (17) MacGlashan, G. S.; Andreev, Y. G.; Bruce, P. G. Structure of the Polymer Electrolyte Poly(Ethylene Oxide)6: LiAsF6. Nature 1999, 398, 792−794. (18) Gadjourova, Z.; Marero, D. M.; Andersen, K. H.; Andreev, Y. G.; Bruce, P. G. Structures of the Polymer Electrolyte Complexes PeO6: LiXF6 (X = P, Sb), Determined from Neutron Powder Diffraction Data. Chem. Mater. 2001, 13, 1282−1285. (19) Mao, G.; Saboungi, M. L.; Price, D. L.; Badyal, Y. S.; Fischer, H. E. Lithium Environment in PEO-LiClO4 Polymer Electrolyte. Europhys. Lett. 2001, 54, 347−353. (20) Gadjourova, Z.; Andreev, Y. G.; Tunstall, D. P.; Bruce, P. G. Ionic Conductivity in Crystalline Polymer Electrolytes. Nature 2001, 412, 520−523. (21) Zhang, C. H.; Gamble, S.; Ainsworth, D.; Slawin, A. M. Z.; Andreev, Y. G.; Bruce, P. G. Alkali Metal Crystalline Polymer Electrolytes. Nat. Mater. 2009, 8, 580−584. (22) Lightfoot, P.; Mehta, M. A.; Bruce, P. G. Structure of the Poly(Ethylene Oxide) Sodium-Perchlorate Complex PEO3-NaClO4 from Powder X-Ray-Diffraction Data. J. Mater. Chem. 1992, 2, 379− 381. (23) Lightfoot, P.; Mehta, M. A.; Bruce, P. G. Crystal-Structure of the Polymer Electrolyte Poly(Ethylene Oxide)3LiCF3SO3. Science 1993, 262, 883−885. (24) Andreev, Y. G.; Lightfoot, P.; Bruce, P. G. Structure of the Polymer Electrolyte Poly(Ethylene Oxide)3: Lin(SO2CF3)2 Determined by Powder Diffraction Using a Powerful Monte Carlo Approach. Chem. Commun. 1996, 2169−2170. (25) Wong, T.; Brodwin, M.; Papke, B. L.; Shriver, D. F. Dielectric and Conductivity Spectra of Polyethylene Oxide Complexes of Sodium-Salts. Solid State Ionics 1981, 5, 689−692. (26) Privalko, V. P.; Sobolev, V. B.; Rekhteta, N. A.; Sichko, V. N. Structure-Dependent Magnetic Susceptibility of Sharp Poly(Ethylene Oxide) Fractions. J. Macromol. Sci.-Phys. 1998, B37, 765−771. (27) Reddy, M. J.; Chu, P. P.; Kumar, J. S.; Rao, U. V. S. Inhibited Crystallization and Its Effect on Conductivity in a Nano-Sized Fe
conductivity in SPEs by increasing polymer mobility and eliminating crystallinity.
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ASSOCIATED CONTENT
* Supporting Information S
Conductivity data for (PEO)10/LiClO4 + 2.5 wt % core/shell Fe2O3 nanorods, the magnetic field strength profile of the neodymium magnets in the iron yoke, Nyquist plots of the unfilled sample, and the method used to extract the conductivity values from the impedance data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +1 (574) 631-1367; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the Notre Dame Center for Nano Science and Technology (NDnano), and the U.S. Army TARDEC under Contract No. W56HZV08-C-0236, through a subcontract with Mississippi State University. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. Army TARDEC. The National Science Foundation supported the VSM measurements under Grant No. ECCS-0923243. We thank N. S. Do for her assistance on the DSC measurements, and E. Kinder for depositing the top Al contacts. Disclaimer: References herein to any specific commercial company, product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the Department of the Army (DoA). The opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or the DoA, and shall not be used for advertising or product endorsement purposes.
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REFERENCES
(1) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nanocomposite Polymer Electrolytes for Lithium Batteries. Nature 1998, 394, 456−458. (2) Gray, F. M. Solid Polymer Electrolytes Fundamentals and Technological Applications; VHC Publishers, Inc.: New York, NY, 1991. (3) Golodnitsky, D.; Livshits, E.; Rosenberg, Y.; Peled, E.; Chung, S. H.; Wang, Y.; Bajue, S.; Greenbaum, S. G. A New Approach to the Understanding of Ion Transport in Semicrystalline Polymer Electrolytes. J. Electroanal. Chem. 2000, 491, 203−210. (4) Bishop, C.; Teeters, D. Crystallinity and Order of Poly(Ethylene Oxide)/Lithium Triflate Complex Confined in Nanoporous Membranes. Electrochim. Acta 2009, 54, 4084−4088. (5) Huang, Y. P.; Lee, M. J.; Yang, M. K.; Chen, C. W. Montmorillonite Particle Alignment and Crystallization and IonConducting Behavior of Montmorillonite/Poly(Ethylene Oxide) Nanocomposites. Appl. Clay Sci. 2010, 49, 163−169. (6) Sodeye, A. I. I.; Huang, T. Z.; Gido, S. P.; Mays, J. W. Polymer Electrolyte Membranes from Fluorinated Polyisoprene-Block-Sulfonated Polystyrene: Microdomain Orientation by External Field. Polymer 2011, 52, 5393−5396. (7) Golodnitsky, D.; Livshits, E.; Kovarsky, R.; Peled, E.; Chung, S. H.; Suarez, S.; Greenbaum, S. G. New Generation of Ordered Polymer 18844
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Article
Oxide Composite PEO Solid Electrolyte. J. Power Sources 2006, 161, 535−540. (28) Krawiec, W.; Scanlon, L. G.; Fellner, J. P.; Vaia, R. A.; Giannelis, E. P. Polymer NanocompositesA New Strategy for Synthesizing Solid Electrolytes for Rechargeable Lithium Batteries. J. Power Sources 1995, 54, 310−315. (29) Dissanayake, M.; Jayathilaka, P.; Bokalawala, R. S. P.; Albinsson, I.; Mellander, B. E. Effect of Concentration and Grain Size of Alumina Filler on the Ionic Conductivity Enhancement of the (PEO)9LiCF3SO3:Al2O3 Composite Polymer Electrolyte. J. Power Sources 2003, 119, 409−414. (30) Wang, L. S.; Yang, W. S.; Wang, J.; Evans, D. G. New Nanocomposite Polymer Electrolyte Comprising Nanosized ZnAl2O4 with a Mesopore Network and PEO-LiClO4. Solid State Ionics 2009, 180, 392−397. (31) Dey, A.; Karan, S.; De, S. K. Molecular Interaction and Ionic Conductivity of Polyethylene Oxide-LiClO4 Nanocomposites. J. Phys. Chem. Solids 2010, 71, 329−335. (32) Lin, C. W.; Hung, C. L.; Venkateswarlu, M.; Hwang, B. J. Influence of TiO2 Nano-Particles on the Transport Properties of Composite Polymer Electrolyte for Lithium-Ion Batteries. J. Power Sources 2005, 146, 397−401. (33) Fullerton-Shirey, S. K.; Maranas, J. K. Structure and Mobility of PEO/LiClO4 Solid Polymer Electrolytes Filled with Al2O3 Nanoparticles. J. Phys. Chem. C 2010, 114, 9196−9206. (34) Croce, F.; Persi, L.; Scrosati, B.; Serraino-Fiory, F.; Plichta, E.; Hendrickson, M. A. Role of the Ceramic Fillers in Enhancing the Transport Properties of Composite Polymer Electrolytes. Electrochim. Acta 2001, 46, 2457−2461. (35) Adebahr, J.; Best, A. S.; Byrne, N.; Jacobsson, P.; MacFarlane, D. R.; Forsyth, M. Ion Transport in Polymer Electrolytes Containing Nanoparticulate TiO2: The Influence of Polymer Morphology. Phys. Chem. Chem. Phys. 2003, 5, 720−725. (36) Do, N. S. T.; Schaetzl, D. M.; Dey, B.; Seabaugh, A. C.; Fullerton-Shirey, S. K. Influence of Fe2O3 Nanofiller Shape on the Conductivity and Thermal Properties of Solid Polymer Electrolytes: Nanorods Versus Nanospheres. J. Phys. Chem. C 2012, 116, 21216− 21223. (37) Schenck, J. F. The Role of Magnetic Susceptibility in Magnetic Resonance Imaging: MRI Magnetic Compatibility of the First and Second Kinds. Med. Phys. 1996, 23, 815−850. (38) Ngo, A. T.; Pileni, M. P. Cigar-Shaped Ferrite Nanocrystals: Orientation of the Easy Magnetic Axes. J. Appl. Phys. 2002, 92, 4649− 4652. (39) Reufer, M.; Dietsch, H.; Gasser, U.; Hirt, A.; Menzel, A.; Schurtenberger, P. Morphology and Orientational Behavior of SilicaCoated Spindle-Type Hematite Particles in a Magnetic Field Probed by Small-Angle X-Ray Scattering. J. Phys. Chem. B 2010, 114, 4763− 4769. (40) Hammond, M. R.; Dietsch, H.; Pravaz, O.; Schurtenberger, P. Mutual Alignment of Block Copolymer-Magnetic Nanoparticle Composites in a Magnetic Field. Macromolecules 2010, 43, 8340− 8343. (41) Zhang, S. J.; Pelligra, C. I.; Keskar, G.; Majewski, P. W.; Ren, F.; Pfefferle, L. D.; Osuji, C. O. Liquid Crystalline Order and Magnetocrystalline Anisotropy in Magnetically Doped Semiconducting Zno Nanowires. ACS Nano 2011, 5, 8357−8364. (42) Bergermayer, W.; Schweiger, H.; Wimmer, E. Ab Initio Thermodynamics of Oxide Surfaces: O2 on Fe2O3 (0001). Phys. Rev. B 2004, 69. (43) Jones, F.; Rohl, A. L.; Farrow, J. B.; van Bronswijk, W. Molecular Modeling of Water Adsorption on Hematite. Phys. Chem. Chem. Phys. 2000, 2, 3209−3216. (44) Chaudhari, N. S.; Warule, S. S.; Muduli, S.; Kale, B. B.; Jouen, S.; Lefez, B.; Hannoyer, B.; Ogale, S. B. Maghemite (Hematite) Core (Shell) Nanorods Via Thermolysis of a Molecular Solid of FeComplex. Dalton Trans. 2011, 40, 8003−8011. (45) Fullerton-Shirey, S. K.; Ganapatibhotla, L.; Shi, W. J.; Maranas, J. K. Influence of Thermal History and Humidity on the Ionic
Conductivity of Nanoparticle-Filled Solid Polymer Electrolytes. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 1496−1505. (46) Li, P.; Csaba, G.; Sankar, V. K.; Ju, X. M.; Lugli, P.; Hu, X. S. R.; Niemier, M.; Porod, W.; Bernstein, G. H., Switching Behavior of Lithographically Fabricated Nanomagnets for Logic Applications. J. Appl. Phys. 2012, 111. (47) Chatterjee, J.; Haik, Y.; Chen, C.-J. Size Dependent Magnetic Properties of Iron Oxide Nanoparticles. J. Magn. Magn. Mater. 2003, 257, 113−118. (48) Tang, B.; Wang, G. L.; Zhuo, L. H.; Ge, J. C.; Cui, L. J. Facile Route to Alpha-FeOOH and Alpha-Fe2O3 Nanorods and Magnetic Property of Alpha-Fe2O3 Nanorods. Inorg. Chem. 2006, 45, 5196− 5200. (49) Song, Y. Y.; Sun, Y. Y.; Lu, L.; Bevivino, J.; Wu, M. Z. SelfBiased Planar Millimeter Wave Notch Filters Based on Magnetostatic Wave Excitation in Barium Hexagonal Ferrite Thin Films. Appl. Phys. Lett. 2010, 97, 3. (50) McCulloch, B.; Portale, G.; Bras, W.; Pople, J. A.; Hexemer, A.; Segalman, R. A. Dynamics of Magnetic Alignment in Rod-Coil Block Copolymers. Macromolecules 2013, 46, 4462−4471. (51) Golodnitsky, D.; Ardel, G.; Peled, E. Effect of Plasticizers on the CPE Conductivity and on the Li-CPE Interface. Solid State Ionics 1996, 85, 231−238. (52) Michael, M. S.; Jacob, M. M. E.; Prabaharan, S. R. S.; Radhakrishna, S. Enhanced Lithium Ion Transport in PEO-Based Solid Polymer Electrolytes Employing a Novel Class of Plasticizers. Solid State Ionics 1997, 98, 167−174. (53) Song, J. Y.; Wang, Y. Y.; Wan, C. C. Review of Gel-Type Polymer Electrolytes for Lithium-Ion Batteries. J. Power Sources 1999, 77, 183−197.
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