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The use of Electron Paramagnetic Resonance (EPR) in the probing of the nanodielectric interface Robert K. MacCrone,1 J. Keith Nelson,1* Linda S. Schadler1 , Robert Smith1 and Robert J. Keefe2 1 Rensselaer Polytechnic Institute Troy, NY 12180-3590, USA 2 EPRI Palo Alto, CA 94304, USA * E-mail : [email protected] Abstract: EPR (sometimes annotated as ESR) has been carried out on functionalized nanoscale silica embedded in crosslinked polyethylene (XLPE) in comparison with the base resin. In situ EPR measurements under an applied electric field have been carried out, and the technique is outlined. The preliminary measurements show that EPR can detect trap sites in the bulk and interfacial polymer. In addition, EPR can detect texture in the polymer due to its processing. At the same time, measurements to determine the dynamics and spatial distribution of the space charge have been carried out under similar conditions using the pulsed electroacoustic analysis technique. The results are discussed in terms of the way in which the nanoparticles affect the local polymer environment. INTRODUCTION AND RATIONALE The finding that enhanced properties can be demonstrated in composite materials formulated using nano particulates has spurred considerable interest in means to optimize performance. It is now becoming clear that the regions associated with the interfaces are pivotal, and, as the nanoparticle size is reduced, these regions dominate in determining the dielectric properties. In this context, Lewis has recently advanced the premise [1] that the interface is characterized by a Stern-Gouy-Chapman layer not unlike the diffuse double layer known to occur in liquid dielectrics. Such layers may affect the local conductivity and contribute to polarization phenomena which give these materials some of their unique properties. Indeed, it is known that the space charges in nanodielectrics behave very differently from the Maxwell-Wagner interfacial process known in conventional dielectric composites [2]. The understanding of these phenomena is undoubtedly the key to being able to tailor the properties of these new dielectrics. For example, the introduction of nanoparticles is shown to have a very significant positive effect on the voltage endurance exhibited. Over two orders of magnitude improvement are seen in widely differing materials – both epoxy thermosets and polyolefins [3]. Often, various chemical treatments (functionalization) are used to modify the interfacial zones seeking to enhance performance. The electrical behavior of such inhomogeneous material depends upon the electrical parameters of the various components (polymer, nanoparticles, interfacial regions) which in turn are determined by their respective resident traps (donors, acceptors). The approach taken here is to identify and characterize the singly occupied traps by electron paramagnetic resonance (EPR), which arises from the unpaired spin. By comparing subsequent measurements of electrical behavior with the corresponding trap content, the goal is to obtain new insights into the mechanisms involved. In addition, knowledge of the macroscopic spatial distribution of the injected or induced charge under an applied electric field is another key consideration. The possibility exists that the time dependencies of the two behaviors may enable a correlation to be made between the corresponding changes, and thus determine where particular processes occur. Such knowledge is important to the ultimate goal to tailor the electrical properties of this new class of materials particularly as particle size is reduced and the interfacial regions start to dominate the properties. THE USE OF EPR AS A PROBE Electron transport in polymer dielectrics takes place by electrons moving between traps or localized states. The term impurity conduction, used here, denotes the tunneling or hopping of electrons from filled donors to empty acceptors without thermal ionization into the conduction band. Despite the recognition that impurity conduction is important in insulator dielectrics, and extensive literature discussion of the trapping of charge in localized traps, the actual donor and acceptor trap sites have eluded direct spectral and structural identification. For example, in the case of thermally stimulated currents, the estimation of the trap site activation energy is deduced from the results, but the actual nature of the trap site is not. One method for probing these electrically active sites is EPR. In this technique, the magnetic field on the sample, placed in an X-band microwave cavity, is increased until the energy difference between the spinup and spin-down orientations, which are also affected by the local environment, match the microwave frequency of the instrument. Strong absorption is then detected. The instrument plots the derivative of the imaginary part of the magnetic susceptibility as a function of field. In the simplest case of spin only, a very sharp peak and trough line would be observed. On the scale of the figures used here, the peak and trough (only a few mT wide) would not be resolved. Fortunately, in the cases studied here, the signal arises from oxygen radicals in a tetragonal orthorhombic environment. The EPR parameters depend on the details of the chemical bonding. A random orientation of such a set of moieties gives rise rather to a very broad (25 – 50 mT) asymmetrical peak. In this work, such a set is identified by the location of the low field maximum on the magnetic field axis. The SiO2/XLPE polymer nanocomposite used in this work has previously [4] been shown to be rich in oxygen radicals. It is important to note that: • • • oriented vertically in the web due to flow during molding. Thus when the magnetic field is parallel to the electric field, it will be across the chain direction and when the magnetic field is perpendicular to the electric E there can be several different structural types of oxygen radical species present, each type of oxygen radical species is associated with a unique donor or acceptor energy, and there were few organic radicals, or defects, that could be detected that are associated with the carbon atoms of the organic polymer itself. The work on identifying qualitatively and quantitatively the acceptor and donor traps around the nano-particles strongly suggests that the Gouy-Chapman layer so often invoked in discussion, may, in fact, involve these oxygen and organic radicals. More importantly, they may be involved not only as ions, but also as acceptor and donor states responsible for impurity conduction as defined above. To prove this conclusively requires that the filled donor and empty acceptor concentration changes induced by the electric field be monitored. This implies in situ measurements. Consider a simple model shown below of 5 oxygen radicals. O(.) – O(.) – O(.) – O(.) – O(.) Here the (.) represents an unpaired (EPR active) electron spin on the oxygen. The EPR spectrum of this structure would have a relative amplitude of 5. On polarization the electron arrangement might be O( ) – O(.) – O(.) – O(.) – O(..) now having a relative amplitude of 3. This predicted decrease in EPR intensity is considerably more general than the very specific model used above might imply. EXPERIMENTAL ARRANGEMENTS The principle of the EPR technique has been previously described by Wolter et al. [5] and the configuration used here is depicted in Fig.1. The innovation involves the application of electric fields to the polymer specimen while being scanned in the microwave cavity. A special ‘I’-shaped sample is molded to achieve large creep surfaces to accommodate the high-voltage (See insert of Fig.1). Note that the polymer chains will be Figure 1: A schematic of the EPR arrangement – sample configuration shown in the insert field, it will be along the chain direction. RESULTS AND INTERPRETATION To investigate the nature of the trap, donors, unpaired electrons and holes (hereafter called EPR active moieties) in the polymer/matrix materials, EPR was measured on thin slabs of XLPE, and a 12½ wt % functionalized XLPE/SiO2 polymer. The formulation of the cross-linked polyethylene (XLPE) materials and the functionalized nanocomposite have been described earlier [4]. EPR spectra for a XLPE sample is shown in Fig.2 without any applied electric field on the specimen. The two plots represent two orthogonal orientations of the sample, the directions are designated B⊥ and B║. Tests conducted at intermediate angles (not shown here) showed a systematical variation from the two extremes shown in Fig. 2, and, presumably, also for Fig 3 (see later). The immediate salient features of note are: ● The spectra extend over a wide magnetic field, more than 0.2T as evidenced by the significant signal on either side of the peak. ● For both specimen types, the B║ spectra are very different from the B⊥ spectra. This is due to the preferred orientation (texture) of the polymer chains. ● The B║ spectrum of the XLPE shows only one low field maximum at ~ 0.30 T, while that for the 12½% nanocomposite material shows two low field maxima at ~ 0.30 and ~0.26 T respectively (see arrows in Fig.3). These values will be used to label the different sets of oxygen radicals [4] which in turn reflect the different respective polymer structures. An assumption is being made here that these unpaired spins are associated with oxygen radicals, even though the pure polymer contains no extrinsic silica. It is well known that, during normal polymer processing, oxygen radical formation occurs and in this case peroxides are added to aid in crosslinking which certainly introduce oxygen to the system. Interestingly, in-house characterization of the starting functionalized SiO2 powders used in the preparation of the nanocomposite also showed no detectable oxygen radical species. Differential of Imaginary Mag. Susceptibility 2.15 2.1 2.05 2 1.95 1.9 1.85 1.8 From these features several important observations can be made: 1.75 1.7 0.24 0.26 0.28 0.3 0.32 0.34 0.36 0.38 0.4 Applied Flux Density (T) Differential of Imaginary Mag. Susceptibility 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 0.24 0.26 0.28 0.3 0.32 0.34 0.36 0.38 0.4 Applied Flux Density (T) Figure 2. EPR Spectra of XLPE. Magnetic field (a) perpendicular, B⊥, (b) parallel. B║. Differential of Imaginary Mag. Susceptibility 2.65 2.55 (a) 2.45 2.35 2.25 2.15 2.05 0.22 0.27 0.32 0.37 0.42 Applied Flux Density (T) Differential of Imaginary Mag. Susceptibility 2.3 2.2 (b) 2 1.9 1.8 1.7 1.6 0.27 0.32 0.37 To investigate the motion of the carriers along the chains, selected EPR spectra were measured under the action of an electric field in the E direction (see Fig.1). First, in principle, without charge injection, it would be expected that many of the distances between the carriers would be changed due to the charge motion and subsequent pile up at internal barriers. Unfortunately, with the electric fields up to 15 kVmm-1 and limited signal averaging, the expected changes were not seen. Second, with charge injection, it would be expected that the Fermi Energy would increase to accommodate the injected charge, and the number of unpaired spins would decrease. This being the case, the magnitude of the absorption would also be expected to decrease. This was also not observed. IMPACT FOR INTERNAL CHARGE 2.1 1.5 0.22 ● The orientation of the different oxygen radical species configurations is not isotropic as evidenced by the difference between the B║ spectra with the B⊥ spectra. Since the polymer segments in such polymers are known to be textured, the EPR active oxygen moieties almost certainly lie along the polymer segments and are also ultimately responsible for the texturing. ● The 12½ wt % specimen not only contains the oxygen radical structure present in the XLPE only polymer, namely the 0.3 T structure, but also contains another different radical structure as well, namely, the 0.26 T structure. It is reasonable to associate this latter structure (0.26 T) with the interfacial polymer. Interestingly, the texturing process has not discriminated between the two material samples. 0.42 Applied Flux Density (T) Figure 3. EPR Spectra of functionalized SiO2/XLPE nanocomposite. Magnetic field (a) perpendicular, B⊥, (b) parallel. B║. Figure 4 provides the time-resolved spatial distribution of the internal charge in the two materials with the polarization charge subtracted out. It is evident that homocharge is injected both from the anode (A) and cathode (C) in both cases. However, although the applied nominal field (30 kVmm-1) is the same in both instances, the base XLPE is characterized by the injection of copious charge (see arrows) not seen to the same extent in Fig. 4(b). This is consistent with Fig. 5 which shows that the onset of charge injection (where the charge magnitudes depart from linearity) for the nanocomposite is about 31 kVmm-1 in contrast with a stress of about 20 kVmm-1 for the XLPE alone. Fig. 5 relates to an applied field which has been applied for 2 hr. Calculations of the associated field show that the behavior results in a 32% increase in the peak field over the nominal value for the XLPE which is reduced to only 7% for the nanocomposite under the same conditions. This may, in part, account for the improved Charge Density (C/m 3 ) 3 2 (a) Material Anode Cathode XLPE 54.5 ks 16.6 ks SiO2/XLPE nanocomposite 0.8 ks 11.1 ks Maxwell-Wagner polarization which has been seen to be characteristic of nanocomposites [2]. 1 0 -1 C CONCLUSION A -2 0.00 0.20 0.40 0.60 0.80 1.00 Normalized Position 1 min 30 min 1 hr 2 hr Charge Density (C/m 3 ) 3 2 (b) 1 0 -1 C ACKNOWLEGEMENTS 0.20 0.40 0.60 0.80 1.00 Normalized Position 1 min 30 min 1 hr 2 hr Figure 4. Distribution and dynamics of internal calibrated charge for (a) XLPE and (b) SiO2/XLPE nanocomposites. breakdown properties seen in nanocomposites. However, perhaps of greater importance in the context of this study is the charge decay characteristic (curves not shown here). The decay time constants are estimated from the charge dynamics when the samples are short circuited and depicted in Table 1. This suggests that the environment changes associated with the particles reflected in the EPR results involves mobile charges which would act to mitigate the 60 Charge Density (C/m ) The authors are grateful for funding received from EPRI during the conduct of this work. Pulsed electroacoustic tests were conducted by Mrs. C. Liang whose efforts are also much appreciated. REFERENCES [1] T.J. Lewis, “Interfaces are the dominant feature of dielectrics at the nanometric level”, IEEE Trans EI. Vol. 11, 2004, pp 739-53. [2] J.K. Nelson and J.C. Fothergill, “Internal charge behaviour in nanocomposites”, Nanotechnology, Vol. 15, 2004, pp 586-9 [3] J.K. Nelson, “The promise of dielectric nanocomposites”, IEEE Int. Symposium on Elect. Ins.., Toronto, Canada, June 2006, pp 452-57 [4] Roy M., Nelson J.K., MacCrone R.K., Schadler L.S., Reed C.W., Keefe R. and Zenger W., “Polymer nanocomposite dielectrics – the role of the interface”, Trans. IEEE, Vol. DEI-12, 2005, pp 629-43 and p 1273 80 40 20 0 -20 0 The EPR results show that the oxygen radicals associated with the interfacial polymer have lower ligand field splittings compared to their counter-part oxygen radicals in the pure polymer: specifically 0.26T:0.33T. These energy differences, resulting from structural changes which are induced by proximity to nanoparticles may play a crucial role in determining the conductivities of the various regions. These, in turn, are reflected in the behavior of the measured internal charge. A -2 0.00 3 Table 1. Charge decay time constants in XLPE and SiO2/XLPE nanocomposite 10 20 30 40 -40 -60 -80 -100 Electric Field (kV/mm) Figure 5. Determination of injection threshold. 12.5% SiO2/XLPE nanocomposite. 2 hr stressing. [5] K.D. Wolter, J.F. Johnson and J. Tanaka in “Engineering Dielectrics”, Vol 2B, ASTM, 19