Nanocomposite materials

Nanotechnologies represent a very attractive research field that can have a powerful impact on the development of advanced electrical and electronic products. LIMAT has being involved recently in reserach programmes on nanocomposite materials for electrical application with other national and international partners.

A few percent of nanofillers are often enough to modify significantly polymer behavior, as regards mechanical, chemical, environmental and electrical properties. Polymeric nanocomposite materials can be employed in electrical apparatus, such as cables, capacitors, machines.

Investigation regards, at present, new families of ethylene/vinylacetate, EVA, and isotactic polypropylene, PP, nanocomposites, which are filled by organophilic layered silicates.

Electrical properties are investigated at LIMAT lab, trying to single out the influence of the in-situ nanofiller formation, nanofiller self-assembly, silicate modification and polymer processing, through space charge and charging-discharging current measurements, comparing the behavior of unfilled and filled materials and considering various filler concentrations and poling electrical field values.

Electric strength measurements, life tests and partial discharge measurements, are also performed, in order to single out the effect of nanofilling on material performance as electrical insulation.

The nanocomposite morphology can be evaluated trough transmission electron microscopy (TEM). Fig. 1 shows the TEM micrograph of the EVA nanocomposite containing 6 wt% of the exfoliated organohectorite (Fig. 1A) and  PP nanocomposite (Fig 1B) containing the 20 wt% of nano-additive.

                

Figure 1: TEM micrographs of a) EVA and b) PP compounds containing 6 wt%  and 20 wt% of nano-additive.


Figure 2 shows a comparison of space charge patterns relevant to EVA pure and filled (poling field 60 kV/mm). As can be seen, significant amount of charge (mainly positive) affects base EVA, which diminishes considerably for the nanocomposite having 6% of filler.

From space charge threshold characteristic a behavior common to both materials can be extracted, that is, introduction of nanofiller reduces the amount of accumulated charge at medium-high fields (fields possibly used for the design of HVDC insulation systems, such as energy cables), while an increase of charge at low fields is observable (see Fig. 3).


Figure 2: Comparison of space charge patterns relevant to EVA pure, (A), and filled (6%), (B), poling field 60kV/mm.

 

Figure 3: Space charge threshold characteristics for EVA base and filled (6%) (Fig. 3A) and for PP base and filled (6%) (Fig. 3B). Results obtained at 40kV/mm with filler content 2 and 4% are also reported. The threshold fields for space charge accumulation are indicated by arrows.

 DC voltage breakdown tests indicate that the presence of nanofiller affects electric strength. While in the case of EVA electric strength slightly decreases, for PP an increase of electric strength is detected (this is shown by the scale paramenter, α, of Weibull probability distribution relevant to electric strength values, see Fig. 4). It is noteworthy that the shape parameter, β, of Weibull distribution increases for nanocomposite, which means that material becomes more homogeneous (smaller content of weak points, generally consisting of macrovoids). This can impact positively on insulation system design, where the so-called dimensional effect reduces significantly the design field in the presence of low β values. Figure 4 summarizes the values of the scale parameter, α, for both PP and EVA (with confidence intervals at probability 95%).

Figure 4: Scale parameter values for both PP and EVA, base and filled (6%), confidence intervals at probability 95%.

Life estimation with DC voltage polarity inversion, that are able to emphasize the effect of space charge accumulation on insulation performance, are being carried out to strengthen breakdown voltage and space charge observations as regards expected long term performance of the investigated materials applied to DC insulation.

 

References

[1] C. Zilg, D. Kampfer, R. Thomann, R. Mulhaupt, G.C: Montanari, "Electrical properties of polymer nanocomposites based upon organophilic silicates", IEEE CEIDP, pp. , Albuquerque, USA, October 2003.