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.
materials can be employed in electrical apparatus, such as cables,
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.
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).
2: Comparison of
space charge patterns relevant to EVA pure, (A), and filled (6%), (B),
poling field 60kV/mm.
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.
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
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.
 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.