. . . . . . .. . Nanocomposite Materials for Dielectric Structures Final Report for EPSRC Grant GR/R 71788/01

Prof. John C. Fothergill and Prof. Leonard A. Dissado working with Professor J. Keith Nelson High Voltage Laboratory, Department of Engineering, University of Leicester, Leicester LE1 7RH

1 Introduction

Nanoparticles are the fundamental building blocks in the design and creation of assembled nano-grained larger scale structures with excellent interfacial and compositional flexibility. However, rather surprisingly, the current push to develop new materials based on nanotechnology has not focused much on the opportunities for dielectric materials; but rather centred on optical and mechanical applications. The few speculative papers that were in the literature (e.g. [1,2]) provided encouragement that this is likely to be very fertile ground. Furthermore, there were good theoretical reasons, cited in the original case for support, why the pursuit of nanomaterials for dielectric applications may have particular promise. From the strictly commercial viewpoint, electrical insulating materials are a huge business, ranging from the large amounts of material used in the high-voltage power industry to thin films for electronics fabrication. It is postulated that even a 10% improvement in an electrical property such as dielectric withstand strength would have enormous commercial impact worldwide.

This project allowed Prof. J. Keith Nelson, as a visiting research fellow, to join forces with the High Voltage Dielectrics Laboratory at the University of Leicester for approximately a 7-month period during the first half of 2002. Nanoparticles made at Rensselaer in the US were used in the programme at Leicester. We used the period to obtain a fundamental understanding of the way in which nanoparticles interacted in a polymer (epoxy resin) matrix to change the dielectric properties. This meshed very well with existing work at Leicester into filled materials and utilized several of the experimental facilities available within the HV Dielectrics Laboratory there. The results of the project reaffirmed our belief that an understanding of the mechanisms controlling the properties of these uniquely structured materials is the key to taking advantage of nanocomposites as advanced dielectrics. A joint proposal under the NSF/EPSRC request for cooperative activities in materials research between US and European Investigators has now been submitted. This will support two PhD students (1 in each country) to carry out more in-depth studies.

2 Experimental Results

This preliminary study was targeted at characterizing resin-based composite thermosets embodying nano-particulates (and contrasting with analogous measurements with conventional micro-filled materials for comparison purposes). A benign base resin was used

(Vantico HY 1300/MY 956 EN). We had experience with filled resin systems and these are easier to make, whilst maintaining high purity levels, than most other insulating systems.

A plurality of characteristics was studied in order to gain some insight into the way in which these composites behave as a preliminary step to the dielectric engineering of new insulating structures. The study involved nano-particulates of alumina (Al2O3), titanium dioxide (TiO2), and zinc oxide (ZnO), chosen to provide a range of permittivities and linearities. One of the salient findings of this study was that, once the size of the particle approaches that of the polymer chain length, then the particles cease to behave like foreign inclusions exhibiting interface phenomena, but start to act cooperatively. This results in a substantial mitigation of the internal charge accumulation within a stressed dielectric. This is clearly seen in Figure 1, which shows the results of a pulsed electro-acoustic (PEA) study of 10% TiO2 composites subjected to an average stress (voltage/gap) of about 4.5 kV/mm. Because the space charge densities are very small in the nano-filled material, the resultant increase in stresses is negligible. However, in the micro-filled material, the space charge increases the internal stresses to almost 10 times higher. The spatial oscillation of charge shown in Figure 1 for the micro-filled material, which is thought to be the result of the effects of field dependent polarization in the high permittivity material, are also characteristically absent in nano-filled epoxy. This is a reproducible result of considerable significance since the high internal fields may prejudice the dielectric integrity of conventional composite materials and provide an explanation of the common finding that the process of filling erodes the electric strength of the matrix material [3].

Figure 1: Space Charge in two samples of 10 % filled epoxy resin after poling for 3 hours at 3 kV. The thinner line shows the large space charge densities found with micro-particles

(1.5µm), the thinner line shows the much-reduced space charge for nanoparticles (38 nm)

Further evidence for this effect may be found in the changes in glass transition temperature that occur in the micro-material which are not evident when nano-sized particulates are used. This is documented in the Differential Scanning Calorimeter (DSC) results shown in Table 1. (A DSC was borrowed from the EPSRC loan for making these measurements.) For post-cured samples, it is evident that the nano-material reduces Tg in contrast to the larger size particles that have the opposite effect. This suggests that particles of nano-metric dimensions

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behave in a similar way to in-filtered plasticisers [4], rather than as “foreign” materials creating a macroscopic interface.

Material + Filler Size (nm) Loading % weight

Tg °C

CY1300 Resin N/A N/A 63.8

CY1300 + TiO2 Micro (1500) 1 76.1

CY1300 + TiO2 Micro (1500) 10 73.9

CY1300 + TiO2 Micro (1500) 50 79.9

CY1300 + TiO2 Nano (38) 1 62.9

CY1300 + TiO2 Nano (38) 10 52.4

CY1300 + TiO2 Nano (38) 50 62.1

Table 1: Glass-transition temperatures of resin filled with nano and microTiO2 particles

A parallel pattern of behaviour is evident from Thermally Stimulated Current (TSC) characteristics. Continuing to use TiO2 as an example, Figure 2 portrays TSC spectra under the same conditions for the base resin, together with micro-and nano-filled composites. While the base resin and the micro-filled material exhibit structure related to the α-transition, the characteristics for the nano-material are benign. Although the full mechanistic explanation of this is not yet understood, it is very important since it would appear that carrier de-trapping is hindered in the nano-material. However, this is brought about without affecting the relaxation time appreciably, as might have been supposed. This can be seen from the conductivity (current/stress) data presented in Figure 3 which shows that the conductivity of the nano-material is, if anything, a little lower than the base resin for a 10% (by weight) loading. This is, of course, not true at very high loadings when the percolation limit is exceeded and these materials start to show appreciable conductivity.

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Figure 2: Thermally stimulated current in TiO2

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resin

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Fig 4. Permittivity & loss tangent for micro-filled epoxy. Bottom: to top: 293, 318, 343, 368, 393 K

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Figure 5. As Figure 1, but for nano-filled

material

Some insight into the way that the incorporation of materials on nano-metric dimensions affects the dielectric properties may be obtained by examining the variation of the real and imaginary components of relative permittivity as a function of temperature and frequency, Figures 4 and 5. At a nominal 10% (weight percent) particulate loading, the dielectric response of the resin when filled with particles of micron size (1.5 µm) is virtually indistinguishable from the base resin. This suggests that the low frequency process is probably associated with charges at the electrodes and not due to particulates in the bulk. With the filler replaced with 10% of nano-metric size TiO2 (38 nm average diameter measured by TEM), the main differences observed relate to a marked modification of the process seen in the base resin at low frequencies and high temperatures. For the nano-metric material, the process exhibits a flat tan δ response at low frequencies in marked contrast to the micron-sized filler. This suggests that a percolation conduction process is operative. In the presence of the nano-filler, the mid frequency dispersion is noticeably reduced. The nano materials are clearly inhibiting motion (see PEA and TSC results above). The mid-frequency process shows a small change in estimated activation energy from 1.7 eV to 1.4 eV. Reduction of the particulate loading from 10 to 1% (by weight) did not have any very obvious fundamental 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 changes engineered by the nano-materials do require loadings greater than a few percent.

3 Appraisal

Very marked differences in charge accumulation are seen in filled materials depending on whether the filler has micro- or nano-metric dimensions. Furthermore, the characteristics suggest that, for the micron-sized filler, carriers are blocked at the anode yielding a heterocharge situation, and giving rise to the large anomalous field distortions. This behaviour clearly has substantial implications for the subsequent migration of charges and probably accounts for the fact that temporal studies (not given here) show that the image

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charge in the cathode at first decreases and then recovers. Again in contrast to the micro-filled material, the decay of charge in the nano-filled TiO2 is very rapid; with insignificant homocharge remaining after just 2 minutes. Although there is some injection of negative charge at the cathode, the nano-filled material is characterized by much less transport perhaps brought about by the larger density of shallower traps.

The PEA results taken in conjunction with the Dielectric Spectroscopy and DSC studies suggest that significant interfacial polarization is implied for conventional fillers which is mitigated in the case of particulates of nanometric size, where a short-range highly immobilized layer develops near the surface of the nano-filler (1-2 nm). This bound layer, however, influences a much larger region surrounding the particle in which conformational behaviour and chain kinetics are significantly altered. This interaction zone is responsible for the material property modifications especially as the curvature of the particles approaches the chain conformation length of the polymer. Evidence suggests that the local chain conformation and configuration play major roles in determining the interactions of a polymer with nano-fillers [5], as is evidenced here by the DSC results of Table 1. The polymer binding to the nanoparticles replaces some of the cross-linking and thus loosens the structure. In contrast, the micron scale case produces significant Maxwell-Wagner polarization giving rise to the characteristics of Fig.4.

In the case of nano-fillers, there is evidence that a grafted layer is formed by the absorption of end-functionalised polymers onto the surface especially when the functional groups are distributed uniformly along the polymer backbone. Hence the local chain conformation is critical to determining the way in which bonding takes place (and thus the cohesive energy density). The defective nature of nano-scale particles can be expected to enhance the bonding if chemical coupling agents (CVD coatings on nanoparticles or tri-block copolymers) are employed.

The large interaction zone in nano-filled polymers with reduced mobility (free volume) should be accompanied by a significant change in electrical properties. Studies of electrical behavior thus provide an opportunity both for a fundamental study of this interaction zone, and also an opportunity for optimizing performance for specific and critical applications.

The finding that conventional fillers are accompanied by substantial bulk charge accumulation is clearly a factor in the common experience of the lower electric strengths exhibited for filled materials. The mitigating effects of nanoparticles provide encouragement that nanocomposites can be engineered with strengths that are commensurate with the base polymer. This is the basis for the further studies proposed.

4 Dissemination of Results

The results presented above were the subject of an oral presentation at the IEEE’s principle conference on Dielectrics and Electrical Insulation [6]. This is possibly the first paper to present results on the dielectric properties of nano-filled dielectrics. Since then, the authors have been invited to write a paper for the Nanotechnology journal. This will incorporate other results found during the project. These include work with the other fillers and also results from SAXS and Raman spectroscopy.

5 Objectives and Future Work

The objectives of this work were:

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1. To characterise the physical and dielectric properties of materials with nanoscale filler formulations.

2. To gain an understanding of the underlying physics, in particular the way in which nanoparticles interact in a polymer matrix and how they affect dielectric ageing and breakdown.

3. To lay the foundation for future collaborative work in this area with Prof. Nelson at the Rensselaer Polytechnic Institute, USA.

It is clear that an enormous amount of work has been completed in the 7 months allowed; objective 1 has certainly been met. There have been considerable gains in the underlying physics; the space charge measurements in particular show that nano-filled materials are likely to behave much better in terms of electrical ageing and breakdown that micro-filled matierals. Objective 2 has therefore been met. Full breakdown and ageing studies will take approximately 3 person-years and are part of the objectives of the follow-on proposal. This has been submitted following the NSF/EPSRC request for cooperative activities in materials research between US and European Investigators. Objective 3 has therefore been met.

6 International Competitiveness and Industrial Interest

We believe this was the first work to be published in this area. Since this work was started, an EC Framework 6 network (“NanoPETS”) has been proposed and we have been invited to join this. This includes industrial and well as academic partners. The work has therefore been at the forefront of international competition and we hope to continue to lead. The follow-on NSF/EPSRC proposal will assist this and also has industrial support.

7 Management and Use of Resources

We believe that project, costing less than £50,000, was an excellent use of resources.

8 References

1. Lewis T.J., “Nanometric Dielectrics”, IEEE Trans on Diel. And Elect. Ins., Vol.1, pp 812-25, 1994

2. Frechette, M. F.; Trudeau, M.; Alamdari, H. D.; Boily, S., “Introductory remarks on nanoDielectrics” CEIDP Annual Report, (2001), p 92-99

3. Khalil M.S., “The role of BaTiO3 in modifying the dc breakdown strength of LDPE”, IEEE Trans., Vol. DEI-7, 2000, pp261-68

4. Sabuni M.H. and Nelson J.K., "The effects of plasticizer on the electric strength of polystyrene", J. Materials. Sci., Vol. 14, 1979, pp 2791-96

5. Schmidt-Rohr K. and Spiess H.W., “Nature of non-exponential loss of correlation above the glass transition investigated by multidimensional NMR”, Phys. Rev Lett., Vol. 66, p 3020, 1991

6. J.K. Nelson, J.C. Fothergill, L.A. Dissado and W. Peasgood, “Towards an understanding of nanometric dielectrics”, IEEE Conf. Elec. Insulation & Dielectric Phenomena, Mexico, Oct. 2002

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