The effect of filler loading ratios and moisture on DC conductivity and space charge behaviour of SiO2 and hBN filled epoxy nanocomposites (original) (raw)
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Journal of Physics D: Applied Physics, 2018
Nanodielectrics have been expected to improve the electrical performance and considered as dielectrics for the future. It has been recognized that the electrical performance is close related to charge dynamics in the dielectrics material. However, the mechanism of charge dynamics in the interphase of nanodielectrics has not been fully understood, which cause the difficulty in understanding the effect of nanoparticle loading ratios and electric fields applied on the electrical properties. Recently, a model based on the tunneling process with the presence of deep traps has been suggested as one of the conceivable candidates for explaining charge dynamics in nanodielectrics, but the related experiment results are not discussed with tunnelling process. In this paper, the measurements including isothermal surface potential decay and space charge are conducted for the blend polyethylene incorporated with the untreated silica nanocomposites. According to the experimental observation compared with the unfilled blend polyethylene, the electrical properties of nanocomposites with high loading ratios of 2 wt%, 5 wt% and 10 wt% are worsened such as facilitated space charge accumulation and injection, and faster charge carriers transport. On the other hand, regarding the nanocomposites with the low loading ratios of 0.5 wt%, it was observed that slow transport of charge carriers, and suppressed space charge accumulation and injection. The effect of the lower and higher electric field on the electrical properties of the nanocomposites was similar for the low and high loading ratios. The tunnelling process associated with deep traps can effectively explain these observed phenomena of nanocomposites, it is therefore suggested for further explaining the electrical properties and charge dynamics in the nanodielectrics.
Journal of Physics D, 2020
Polymer nanocomposites as dielectrics have attracted a wide range of research interests due to their improved performance. One of the observed characteristics of polymer nanocomposites is the suppression on space charge injection and accumulation and the charge transport mechanism behind is also investigated based on thermally activated hopping (TAH) and quantum mechanical tunnelling (QMT) mechanisms. However, there still lacks research on the effect of moisture on charge transport characteristics and its relationship with experimental results. We herein proposed a method to re-virtualize the distribution of nanoparticles/their aggregates based on the multidimensional scaling (MDS) method in the first step, and a simple numerical method is further following to estimate the contribution of TAH and QMT conductivities to the experimental ones. The results, firstly, indicate the presence of moisture could lead to significant charge injections, and for different relative humidity conditions, due to their diverse water shell thickness, the separation distances of nanoparticles where deep/shallow traps locate show an obvious reduction and consequently vary the contribution of TAH and QMT conductivities in the measured ones. Second, the TAH mechanism plays the main role in charge transport/conduction, especially under lower RH conditions, while the obvious increment of QMT conduction is attributed to the reduced trap distances caused by thicker conductive water shells and support the existence of deep traps. Besides, the proposed model could be potentially extended to other research topics on electrical properties of polymer nanocomposites, such as particle size, dispersion/distribution status and filler loading concentrations which can be reflected and explained via th e variation of nanoparticle surface/trap site distances.
Space Charge Trapping and Conduction in Low-Density Polyethylene/Silica Nanocomposite
Japanese Journal of Applied Physics, 2012
The high field conduction and space charge distribution were investigated in low-density polyethylene (LDPE) and LDPE/silica nanocomposites filled with various concentrations of nanosilica. The results indicate that nanosilica could effectively suppress space charge accumulation at nanofiller concentrations from 0.1 to 5.0 wt %. However, the conduction current at a high field significantly increases at low concentrations from 0.1 to 0.5 wt % and remarkably reduces at high concentrations from 0.5 to 5.0 wt %. It is shown that the trap depth corresponding to the time from 2 to 3600 s significantly decreases at low nanofiller concentrations from 0.1 to 0.5 wt %. However, the depth of deep traps corresponding to the time from 100 to 3600 s increases with the increase in nanofiller concentration from 0.5 to 5.0 wt %. Moreover, the depth of shallow traps corresponding to the time from 2 to 100 s increases at concentrations from 0.5 to 2.0 wt %, and then it decreases at concentrations from 2.0 to 5.0 wt %. In addition, the apparent mobility varies with the modification of trap depth caused by the introduction of nanofiller. The threshold field E {t for remarkable charge injection and E t{c proportional to the total trap density H are significantly lower in the nanocomposite with a low nanosilica concentration, i.e., 0.1 and 0.5 wt %, while both of them increase at concentrations from 0.5 to 5.0 wt %. It is considered that the impurity effect is greater than the nanofiller effect at a low nanofiller concentration. The deep trap is speculated as the chemical trap in the interface of the nanofiller bonding strongly with the polymer chain, while the shallow trap may be related to the chemical trap in the weakly bonded interface. It is clear that the space charge behavior and conduction are significantly affected by modification of the trap depth and density distribution owing to the introduction of nanofiller.
2007
The dielectric properties of epoxy resin were studied as a function of hydration by dielectric spectroscopy. The dielectric spectroscopy measurements show different conduction and quasi-DC behaviors at very low frequencies (<10-2 Hz) with activation energies dependent on the hydration. These observations lead to the development of a model in which a "water shell" is formed around the nanoparticles. The multiple shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 10 monolayers as the first layer. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 5 nm thick at 100% RH. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. PEA results show that the wetted epoxy specimens have a higher threshold field of space charge accumulation than such dry specimens since water enhances charge decay.
The effect of water absorption on the dielectric properties of epoxy nanocomposites
IEEE Transactions on Dielectrics and Electrical Insulation, 2008
In this research, the influence of water absorption on the dielectric properties of epoxy resin and epoxy micro-composites and nano-composites filled with silica has been studied. Nanocomposites were found to absorb significantly more water than unfilled epoxy. However, the microcomposite absorbed less water than unfilled epoxy: corresponding to reduced proportion of the epoxy in this composite. The glass transition temperatures of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The T g decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20K as the humidity was increased from 0% to 100%. This implied that for all the samples, the amount of water in the resin component of the composites was almost identical. It was concluded that the extra water found in the nanocomposites was located around the surface of the nanoparticles. This was confirmed by measuring the water uptake, and the swelling and density change, as a function of humidity as water was absorbed. The water shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. This has been discussed in terms of a percolation model. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 5 monolayers. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive; this may extend for approximately 25 nm. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water surrounding a given size of nanoparticle appeared to be independent of the concentration of nanoparticles but approximately proportional to water uptake. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. Comments are made on the possible effect on electrical aging.
Interfacial charge behavior in nanodielectrics
2009 IEEE Conference on Electrical Insulation and Dielectric Phenomena, 2009
In recent years, the availability and low cost of nanometric-sized filler particles have generated great interest in polymer nanocomposites for a host of applications, including electrical insulation with enhanced breakdown and voltage endurance properties. This work combines the results of several experiments to add insight to the processes taking place in the crucial polymer transition region near the particle surfaces. The relative tendency to accumulate space charge under a high DC field is investigated through pulsed electroacoustic (PEA) apparatus. DC transient (absorption) currents reveal a quasi-DC conductivity that is surprisingly high in the nanocomposite, a result that is reinforced by dielectric spectroscopy, which also indicates a reduction in the nanocomposite's real permittivity. Thermally-stimulated currents reveal the presence of shallow traps that accompany the nanoparticle inclusions. Taken together, the results of the study indicate that the transition region is responsible for the desirable nanocomposite bulk properties which are today of interest, and help explain the difference in performance between these new materials and both unfilled resin and conventional composites composed of micronsized fillers.
Internal charge behaviour of nanocomposites
Nanotechnology, 2004
The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO 2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current, photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics.
Solution of the tunneling-percolation problem in the nanocomposite regime
Physical Review B, 2010
We noted that the tunneling-percolation framework is quite well understood at the extreme cases of percolation-like and hopping-like behaviors but that the intermediate regime has not been previously discussed, in spite of its relevance to the intensively studied electrical properties of nanocomposites. Following that we study here the conductivity of dispersions of particle fillers inside an insulating matrix by taking into account explicitly the filler particle shapes and the inter-particle electron tunneling process. We show that the main features of the filler dependencies of the nanocomposite conductivity can be reproduced without introducing any a priori imposed cutoff in the inter-particle conductances, as usually done in the percolation-like interpretation of these systems. Furthermore, we demonstrate that our numerical results are fully reproduced by the critical path method, which is generalized here in order to include the particle filler shapes. By exploiting this method, we provide simple analytical formulas for the composite conductivity valid for many regimes of interest. The validity of our formulation is assessed by reinterpreting existing experimental results on nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the characteristic tunneling decay length, which is found to be within the expected range of its values. These results are concluded then to be not only useful for the understanding of the intermediate regime but also for tailoring the electrical properties of nanocomposites.