Abstract: The use of nanocomposites as dielectric materials is expected to lead to improved electrical performance. However, recent research has shown that moisture absorption can cause a deterioration in the electrical performance of nanocomposites. Although it is generally accepted that hydroxyl groups attached to nanoparticle surfaces are the main cause of moisture absorption, the impact of this absorption on the electrical properties of nanocomposites is still not fully understood. In this paper, a series of measurements, including thermogravimetric analysis, DC breakdown, surface potential decay and space charge, are conducted with the aim of determining the impact of moisture absorption on the electrical properties of polyethylene/silica nanocomposites. The results show that the loading ratio of nanosilica and the humidity of the conditioning environment determine the amount of absorbed moisture. According to the Zhuravlev model, the main contribution to the deterioration in electrical properties of nanocomposites comes from the large amount of moisture absorbed in multilayer form. It is found that the loading ratio of nanosilica is the most significant factor in reducing DC breakdown strength.
Abstract: Crosslinked polyethylene (XLPE) nanocomposites and blends are extensively utilized in different industries owing to their superior properties and characteristics compared to uncrosslinked and pristine polyethylene (PE). These excellent properties enable XLPE nanocomposites and blends to be employed in specialized applications and conditions. Despite their numerous favorable features, they face several risks, challenges, and problems that bring limitations and restrictions for their usage in some cases. Cable industry is one of the leading industries employing XLPE nanocomposites and blends. While these materials can grant outstanding characteristics to the cable insulation (such as dielectric properties), they can also encounter operative challenges in the meantime. This chapter attempts to define these risks and limitations via a comprehensive survey in recent case studies and publications. The described topics are categorized into several parts. They include risks, challenges, and constraints associated with electrical issues, embedded nanoparticles, crosslinking agents, recyclability, surface characteristics, and aging behaviors of XLPE nanocomposites and blends. The definitions of the mentioned problems are accompanied by the most recent and updated proposed solutions to resolve them. For providing inclusive insights in each of the categories, they are additionally subcategorized based on the suggested addressing approaches.
Abstract: Agglomeration is a major challenge in the research of nanodielectrics. Recognition of agglomerates in scanning electron microscopy (SEM) images can effectively support tackle this issue. Motivated by the fast development of image recognition, we propose a new approach for agglomerates recognition in SEM images of nanodielectrics by semantic segmentation algorithm. On the basis of convolutional neural network, pixel blocks classification network and full convolutional segmentation network employed with data augmentation are investigated in this work. Both networks can preliminarily recognize the agglomerates of spherical silica-based blend polyethylene nanocomposites. The average intersection over union (mIoU) of the pixel blocks classification network is 0.837 and it takes 48 seconds to process an image, while the mIoU of the full convolutional segmentation network is 0.777 and it takes 0.059 seconds to process an image.
Abstract: 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 the variation of nanoparticle surface/trap site distances.
Abstract: A review article is presented of the research results obtained by the author on the properties of amorphous silica surface. It has been shown that in any description of the surface silica the hydroxylation of the surface is of critical importance. An analysis was made of the processes of dehydration (the removal of physically adsorbed water), dehydroxylation (the removal of silanol groups from the silica surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of experimental and theoretical studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chemistry of amorphous silica. The main advantage of this physico-chemical model lies in the possibility to determine the concentration and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the silica surface as a function of the pretreatment temperature of SiO2 samples. The model makes it possible to determine the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO2 skeleton. The magnitude of the silanol number, that is, the number of OH groups per unit surface area, αOH, when the surface is hydroxylated to the maximum degree, is considered to be a physico-chemical constant. This constant has a numerical value: αOH,AVER=4.6 (least-squares method) and αOH,AVER=4.9 OH nm−2 (arithmetical mean) and is known in literature as the Kiselev–Zhuravlev constant. It has been established that adsorption and other surface properties per unit surface area of silica are identical (except for very fine pores). On the basis of data published in the literature, this model has been found to be useful in solving various applied and theoretical problems in the field of adsorption, catalysis, chromatography, chemical modification, etc. It has been shown that the Brunauer–Emmett–Teller (BET) method is the correct method and gives the opportunity to measure the real physical magnitude of the specific surface area, SKr (by using low temperature adsorption of krypton), for silicas and other oxide dispersed solids.
Abstract: Owing to their unique mechanical properties, carbon nanotubes are considered to be ideal candidates for polymer reinforcement. However, a large amount of work must be done in order to realize their full potential. Effective processing of nanotubes and polymers to fabricate new ultra-strong composite materials is still a great challenge. This Review explores the progress that has already been made in the area of mechanical reinforcement of polymers using carbon nanotubes. First, the mechanical properties of carbon nanotubes and the system requirements to maximize reinforcement are discussed. Then, main methods described in the literature to produce and process polymer–nanotube composites are considered and analyzed. After that, mechanical properties of various nanotube–polymer composites prepared by different techniques are critically analyzed and compared. Finally, remaining problems, the achievements so far, and the research that needs to be done in the future are discussed.
Abstract: Keywords: Droplets Bubbles Microfluidics Encapsulation Emulsion a b s t r a c t In this paper, we emphasize our long series of experiments proving that the physical processes along fluid interfaces can be exploited for creating unusual fluidic objects. We report for the first time a couple of new fluidic objects so-called “liquid onions” and “mayonnaise” droplets. The study starts from the observation of antibubbles, exhibiting unstable liquid‐air‐liquid interfaces. We show that the lifetime of such a system has the same origin as floating/coalescing droplets on liquid surfaces. By analyzing such behaviours, we created droplets bouncing on a liquid bath. The methods and physical phenomena collected in this paper provide a basis for the development of a discrete microfluidics. Open questions are underlined, experimental challenges and future applications are proposed.
"Influence of moisture absorption on..." refers background or methods in this paper
...According to the Zhuravlev model, moisture attached to the surface of nanosilica can be divided into two phases: multilayer moisture, which can be removed at 25 °C in vacuum, and monolayer moisture, which requires a temperature of 190 °C in vacuum for its removal ....
...To obtain spherical nanosilica, both flame hydrolysis and polymerisation processes will hydroxylate the surface of the silica, generating Si–OH (silanol) groups on this surface [11,14]....
...The surface chemical groups of nanosilica and their influence on absorbed moisture are discussed in terms of the Zhuravlev model ....
...The OH groups are covalently bonded to the nanosilica surface, whereas the H2O molecules are physically bonded to the OH groups primarily via hydrogen bonds but also via van der Waals bonds ....
...It is known that the presence of OH groups on the surface of nanosilica particles can change the surface properties, depending on the concentration of these groups ....
Abstract: It is suggested that a major field of study in the future development of dielectrics will concern their properties when relatively few molecules are involved. Such smallness arises naturally at interfaces of nanometric thickness and will occur also when dielectrics are employed in the nano-technical devices of the future. It already occurs in living systems where the dielectric and conductive properties of biomaterials are vital in sustaining activity. The transverse and lateral properties of interfaces, including the effects of molecular ordering, are considered and it is suggested that the advent of scanning tunneling and atomic force microscopies provides a significant opportunity for nanometric dielectric studies. An important feature, suggested for future exploitation, is the cross-coupling in interfaces of force fields arising from electrical, mechanical, chemical and entropic potential gradients. Application of these concepts to biology and to the behavior of polymer gels which may lead to development of muscle-like actuators and transducers are considered. Finally, attention is drawn to the likely role of nanometric interfacial processes in the initiation of electrical breakdown in insulating materials. >
Abstract: 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 the reduced proportion of the epoxy in this composite. The glass transition temperatures (Tg) of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The Tg decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20 K 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 with a thickness of approximately 5 monolayers. A second layer of water is proposed that is dispersed but 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.