The paper presents results of dielectric spectroscopy and space charge (PEA) measurements on epoxy resin filled with 10% w/w micro- and nano- sized particles of TiO2, Al3O2 and ZnO. The results appear to show that the material from which the nano-particle is made is not highly significant in influencing these results. The results support the proposition that the dielectric properties of such nano-filled composites are controlled by Stern-Gouy-Chapman layers (“interaction zones”) around the particles

Fothergill, J.

Fu, M.

Nelson, J. K.

English

Crossref

Dielectric properties of epoxy nanocomposites containing TiO/sub 2/, Al/sub 2/O/sub 3/ and ZnO fillers

City Research Online

Fothergill, J., Nelson, J. K. & Fu, M. (2004). Dielectric properties of epoxy nanocomposites containing TiO2, Al2O3 and ZnO fillers. Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, 406 - 409. doi: 10.1109/CEIDP.2004.1364273 <http://dx.doi.org/10.1109/CEIDP.2004.1364273>City Research OnlineOriginal citation: Fothergill, J., Nelson, J. K. & Fu, M. (2004). Dielectric properties of epoxy nanocomposites containing TiO2, Al2O3 and ZnO fillers. Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, 406 - 409. doi: 10.1109/CEIDP.2004.1364273 <http://dx.doi.org/10.1109/CEIDP.2004.1364273>Permanent City Research Online URL: http://openaccess.city.ac.uk/1340/ Copyright & reuseCity  University  London has developed City  Research Online  so that  its  users  may access the research outputs of City University London's staff. Copyright © and Moral Rights for this paper are retained by the individual author(s) and/ or other copyright holders. Users may download and/ or print one  copy  of  any  article(s)  in  City  Research  Online  to  facilitate  their  private  study  or  for  non-commercial research. Users may not engage in further distribution of the material or use it for any profit-making activities or any commercial gain. All material in City Research Online is checked for eligibility for copyright before being made available in the live archive. URLs from City Research Online may be freely distributed and linked to from other web pages. Versions of researchThe version in City Research Online may differ from the final published version. Users are advised to check the Permanent City Research Online URL above for the status of the paper.EnquiriesIf you have any enquiries about any aspect of City Research Online, or if you wish to make contact  with the author(s) of this paper, please email the team at publications@city.ac.uk.   Dielectric Properties of Epoxy Nanocomposites containing TiO2, Al2O3 and ZnO fillers   J. C. Fothergill1, J. K. Nelson2, M. Fu1   1University of Leicester, United Kingdom 2Rensselaer Polytechnic Institute, United States   Abstract: The paper presents results of dielectric spec-troscopy and space charge (PEA) measurements on epoxy resin filled with 10% w/w micro- and nano- sized particles of TiO2, Al3O2 and ZnO.  The results appear to show that the material from which the nano-particle is made is not highly significant in influencing these re-sults.  The results support the proposition that the dielectric properties of such nano-filled composites are controlled by Stern-Gouy-Chapman layers (“interaction zones”) around the particles. Introduction In an earlier paper [1], we reported on studies aimed at providing an understanding of the charge storage and transport of an epoxy resin containing TiO2 nanoparti-cles. It was shown that nanometric fillers mitigate the interfacial polarization characteristic of conventional materials with a reduction in the internal field accumulations. In a more recent paper [2] we have shown that this has beneficial effects both for the dielectric withstand and voltage endurance.  In this paper, we present further, similar results for two other types of nanocomposites; these are based on the same base resin containing nano-particulates of alumina (Al2O3), and zinc oxide (ZnO), chosen to pro-vide a range of permittivities and linearities. These results are compared with the earlier results obtained for TiO2.  Specimen Preparation The material used in this investigation was a Bisphenol-A epoxy (Vantico CY1300 + HY956) containing parti-cles of TiO2 (average diameter 23 nm and 1.5 µm), Al2O3 (17 nm and 1.5 µm), or ZnO (24 nm and 0.3 µm) to form nano- and micro-composite materials respec-tively. The resin was selected because it was benign (i.e. without other fillers or dilutents), had a low initial vis-cosity, and a glass transition below 100 ºC.   The formulation and processing of the planar specimens (having thicknesses between 500 and 750 µm) have been described previously [1]. These are cast under carefully controlled conditions to ensure good mixing, dispersion and the minimization of voids. Large shear forces were needed in the mixing process to obviate unwanted clustering of the particles. For some of the measurements, 100 nm aluminium electrodes were evaporated onto the cast films. In all cases, careful optical microscopic examination of the specimens was undertaken to reject those with agglomerations, cavities, and other defects. Dielectric Spectroscopy Insight into the way that the incorporation of materials of nanometric dimensions affected the dielectric proper-ties was obtained by examining the variation of the real and imaginary components of relative permittivity as a function of temperature and frequency.  Figure 1: Real relative permittivity 10% TiO2 nano-filled epoxy 1E+01E+11E+21E+31E+41E+51E+61E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6frequency/HzReal relative permittivity293K318K343K368K393K Figure 2: Imaginary relative permittivity 10% TiO2 nano-filled epoxy 1E-21E-11E+01E+11E+21E+31E+41E+51E+61E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6frequency/HzImaginary relative permittivity 293K318K343K368K393K A Solartron H.F. frequency response analyser (type 1255) in combination with a Solatron Dielectric Inter-face, Type 1296 was used for this purpose with typically five temperatures spanning 100 K and a frequency range   of 10-3-106 Hz. An example of the relative permittivity Bode plots for the material filled with 10% w/w TiO2 nano-particles is shown in Figure 1 and Figure 2. An Arrhenius plot, Figure 3, shows that the low frequency processes (judged by the frequency at which εr"=100) are temperature activated with an activation energy of 1.64 eV.  Figure 3: Arrhenius plot representing low frequency processes pre-sented in Figure 1 and Figure 2 -16-14-12-10-8-6-4-20240.0024 0.0026 0.0028 0.003 0.0032 0.0034 0.0036(1/Temperature)/[1/K]log e{frequency( εr"=100)/[Hz]}slope = -1.90x104 K-1activation energy = 1.64 eV Dielectric spectroscopy measurements were also made on the base resin (i.e. without any filler) and with 10% w/w filled epoxy containing micro-particles of TiO2 and Al2O3 and 10% nano-particles TiO2 and ZnO. Represen-tative results from measurements at 393 K are shown here in Figure 4 and Figure 5. Figure 4: Real relative permittivity of unfilled and 10% micro- and nano- filled materials at 393 K 1E-11E+01E+11E+21E+31E+41E+51E+61E+71E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6Frequency /HzReal Relative Permittivity base resinmicro 10% TiO2nano 10% TiO2micro 10% Al2O3nano 10% ZnO Figure 5: Imaginary relative permittivity of unfilled and 10% micro- and nano- filled materials at 393 K 1E-11E+01E+11E+21E+31E+41E+51E+61E+71E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6Frequency /HzImaginary Rel. Permittivity base resinmicro 10% TiO2nano 10% TiO2micro 10% Al2O3nano 10% ZnO Figure 6 shows the loss tangent (ε"/ε') calculated from this data. It can be seen that the dielectric properties of the systems are similar, irrespective of the chemical nature of the filler that was used. Figure 6: Loss tangent of unfilled and 10% micro- and nano- filled materials at 393 K 1E-21E-11E+01E+11E+21E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6Frequency /HzLoss tangent (tan δ)base resinmicro 10% TiO2nano 10% TiO2micro 10% Al2O3nano 10% ZnO At higher frequencies, the micron-filled material has a higher real relative permittivity due to the high relative permittivity of the filler, e.g. εr(TiO2)≈99. Simple calcu-lations show that the increase of permittivity from the base resin value of 9.99 to 13.8 when micro-filled is reasonable using the Lichtenecker-Rother logarithmic law of mixing [2]. However, the nanocomposite, under the same conditions, exhibits a measured value of 8.49, which is significantly less than that of the base resin matrix. This is true at all the temperatures measured. This surprising result, which has been confirmed using an independent second test utilizing a different spec-trometer, appears to indicate that the interaction zone surrounding the nanoparticles has a profound effect on the dielectric behaviour of the nanocomposite; a conclu-sion that is consistent with the space-charge measure-ments described below.  The mid-range frequency behaviour has been discussed in detail elsewhere (e.g. [1,2]), where it is concluded that nanoparticles restrict chain movement.  The base resin exhibits classic Maxwell-Wagner interfacial polarisation below a frequency of 0.1 Hz with slopes of –2 and –1 on the real and imagi-nary permittivity Bode plots [3] and a peak in the loss tangent at 0.1 Hz (Figure 6). The micro-filled epoxies also display this peak in loss tangent, which is therefore also attributed to interfacial polarisation at the elec-trodes. At very low frequencies, the dielectric behaviour of the base resin and microcomposite become very similar as electrode effects dominate.  The thickness of the samples (~750 µm) was approximately 500 times the inter-microparticle distance. It is interesting that the deviation between real permittivities of the microcom-posite and the base-resin starts to occur at approxi-mately 500 times the frequency of 0.1 Hz, giving some support for the theory that the microcomposite was   exhibiting Maxwell-Wagner polarisation due to the microparticle interfaces. The nano-composites both show quite different behaviours from the base resin and micro-composites below 0.1 Hz.  Instead of the real part of the relative permittivity having a slope of –2, it is parallel to the imaginary part with a slope of –1.  This, together with the flattening out of the tan-δ response at lower fre-quencies (Figure 6), is indicative of “low-frequency dispersion (LFD)” [3] or “quasi-DC” (QDC) behaviour [4]. We consider that this is due to partially overlapping charge layers (Stern and Gouy-Chapman layers [5]), along the lines that Lewis has considered (e.g. [6, 7]), which form fractal-like quasi-conducting paths. This would allow a distribution of path lengths over which charges could move with greater or lesser ease of movement. This has been considered more quantita-tively in [2] and is also discussed in [8]. Reduction of the particulate loading from 10% to 1% (by weight) did not have any very obvious fun-damental changes, but the nano-filled material then does start to exhibit a low frequency response more typical of the base resin and micro-filled material, suggesting that loadings greater than a few percent are required to en-sure sufficient overlap of the interaction zones. Space Charge Measurement Space charge measurements were made using a pulsed electro-acoustic (PEA) system. A voltage of 3 kV was applied to disks approximately 0.75 mm thick (i.e. an average field of 4 kV.mm-1) for 10,800 s (3 hours) at room temperature (293±3 K). The sample was then short-circuited for at least 3600 s (1 hour). The space charge was measured periodically during the charging and discharging periods.  We report selected representa-tive results here for 10% w/w filled samples showing the charge accumulation at 3 hours and the subsequent decay.  As the charge accumulates, the electric field is distorted and so we also report the observed electrical field after the 3-hour charging period. In each graph, the vertical dotted lines indicate the cathode (left) and an-ode (right), the grey line indicates the charge distribu-tion at the end of the charging period (i.e. before short circuiting) and arrows show the charge decay with time.  Figure 7 and Figure 8 recap the comparison made in [1] between the epoxy composites micro- and nano- filled with TiO2. In the nano-filled material, there is homo-charge near the electrodes whereas in the mi-cro-filled there is hetero-charge.  Since hetero-charge gives rise to increased, and homo-charge to decreased, electric fields at the electrodes, where breakdown is most likely to initiate, homo-charge is generally felt to be preferable to hetero-charge under DC conditions. The nano-filled material is therefore behaving preferentially under these conditions. Figure 7: 10% TiO2 micro-filled charge decay. Measurements made at 0 (volts on – grey curve), 30, 600, 1800, 3600, 7200 and 10,800 s. -8-6-4-2024680 0.2 0.4 0.6 0.8 1Position/(mm)Charge density /(C.m-3 ) Figure 8: 10% TiO2 nano-filled charge decay. Measurements made at 0 (volts on – grey curve), 10,30,60,120,300,600,900 and 1800 s. -4-3-2-1012340 0.2 0.4 0.6 0.8 1Position /(mm)Charge density /(C.m-3 ) Figure 9: The maximum field intensity with the micro- and nano- filled TiO2 systems as a function of time 0246810121E+2 1E+3 1E+4 1E+5time of voltage application/(s)Max. field intensity/(kv.mm-1)`MicroNanoApplied Figure 9 shows how the space charge distorts the field in the two systems and it is clear that the maximum field in the micro-filled system builds up to over twice the average (“applied”) field whereas the nano-filled system stabilises at approximately 5.5 kV.mm-1.  Similar results, shown in Figure 10 and Figure 11 are obtained for the ZnO micro- and nano- filled systems. Here the micro-filled system does show homo-charge accumulation but also a large change from posi-tive to negative charge accumulation in the central part.    The nano-filled material shows a small negative charge in the centre after poling for three hours, but relatively little charge elsewhere. Figure 10: 10% ZnO micro-filled charge decay. Measurements made at 0 (volts on – grey curve), 10, 30, 60, 300, 600, 900, and 1800 s. -6-4-202460 0.2 0.4 0.6 0.8Position /(mm)Charge density /(C.m-3 ) Figure 11: 10% ZnO nano-filled charge decay. Measurements made at 0 (volts on – grey curve), 30, 60, 300, 3600, and 7200 s. -6-4-202460 0.2 0.4 0.6 0.8Position /(mm)Charge density /(C.m-3 ) The results on the Al2O3 nano-filled samples appear to show a large heterocharge accumulation near the anode, but we would like to repeat these results before we have sufficient confidence in them to present here. A summary of the results after poling for three hours is shown in Figure 12 and a summary presented in Table 1.  Figure 12: Electric field throughout all samples. The very thick line is the base resin (unfilled).  The grey lines are the micro-filled and the black lines are the nano-filled samples. -15-10-50510PositionElectrical field /(kV.mm-1 )ZnO Al2O3Base resinTiO2TiO2 Al2O3ZnO Table 1: Comparison of systems Filler  Anode Cathode Central Max Field (kV/mm) None  Hetero Negative 6, cathode Al2O3 micro Homo (small)   >4, cathode Al2O3 nano Hetero (big) Hetero (small)  12, anode ZnO micro Homo Homo pos/neg 13, centre ZnO nano   Small neg 5, cathode TiO2 micro Hetero Hetero  8, anode TiO2 nano Homo Homo Positive 6, centre Conclusions The dielectric spectroscopy results show very little dependence upon the particle material; only the size appears to be important.  The effect of space charge build up on the internal electric fields is similar for the TiO2 and ZnO particles but there are some anomalies that require further work for the Al3O2 particles.  The results appear to support our earlier conclusions regard-ing the formation of quasi-conductive regions around the particles that partially overlap in the case of the nano-particles.  These allow charge dissipation and have been shown to greatly improve the dielectric withstand and voltage endurance characteristics (e.g. [2]).  Author address: Prof. John. C. Fothergill, University of Leicester, University Road, Leicester, LE1 7RH, UK Email: fothers@ieee.org References  1  J.K. Nelson, J.C. Fothergill, L.A. Dissado and W.  Peas-good, “Towards an understanding of nanometric dielectrics”, IEEE Conf. Elec. Insulation & Dielectric Phenomena, Mexico, Oct. 2002, pp 295-298 2  Nelson, J.K. Fothergill, J.C.; “Internal charge behaviour of nanocomposites” Nanotechnology, volume 15, issue 5, (2004) p 586 – 595 3  Jonscher A.K, “Dielectric relaxation in solids”, Chelsea Dielectric Press 1983, ISBN:0950871109 4  Dissado L.A. and R.M., “Anomalous low frequency dis-persion. A near DC conductivity in disordered low dimensional materials”, J. Chem. Soc. Faraday Trans. 2, 80, pp 291-319, 1984 5  Chapman D.L., “A contribution to the theory of electro-capillarity”, Phil. Mag., Vol 25, 1913, pp 475-481 6  Lewis T.J., “Nanometric Dielectrics”, IEEE Trans on Diel. And Elect. Ins., Vol.1, pp 812-25, 1994 7  Lewis T.J., “Interfaces and Nanodielectrics are Synony-mous, Proc. IEEE 2004 International Conference on Solid Dielectrics, Toulouse, France, July 5-9, 2004, IEEE Pub. 04CH37539, Vol. 2, pp792-795 8  J. K. Nelson, L. A. Utracki, R.K. MacCrone and C. W. Reed, “Role of the Interface in Determining the Dielectric Properties of Nanocomposites”, this conference 

Dielectric properties of epoxy nanocomposites containing TiO2, Al2O3 and ZnO fillers

Abstract

Similar works

Full text

Available Versions

Crossref

City Research Online