Narrow Results

The point-like defects known as E' centers are the most abundant natural defects in silicon dioxide (SiO₂) and have been identified as unpaired sp³ dangling bonds. Their importance stems from the fact that they deeply affect the quality of electronic and optical devices. For this reason, particular attention has been paid in their characterization since the early 1960s. In this work, we review theoretical and experimental results concerning these kinds of defects, focusing on the related charge and spin states. In particular, the defect known as E' in crystalline quartz and its analogous E'_γ in amorphous SiO₂, detected by electron spin resonance, are shown to be due to the Si dangling bonds that arise either upon removal or displacement of an oxygen atom in a SiO₂ network, accompanied by an asymmetric relaxation of the network.

In this work, SiO₂-encapsulated copper particles/PA12 (Cu-SiO₂/PA12) composite powders were prepared by electroless composite plating, and the laser sintering behavior was investigated. Results showed that Cu, Cu₂O, CuO, and SiO₂ (Cu-SiO₂) composite particles were plated on the surface of KH550-modified PA12 powders. The Cu-SiO₂ particles existed independently on PA12 surface, and the size was around 200 nm. The melting temperature and crystallization temperature of Cu-SiO₂/PA12 composite powders were 183^∘C and 150^∘C. The results indicate that the selective laser sintering (SLS) process involved the contact of Cu-SiO₂/PA12 powders, the formation of sintering neck, the growth of sintering neck, and the formation of fused solid. The Cu-SiO₂ composite particles uniformly dispersed in the part due to surface tension, and the contact interface was good due to their similar polarity. The Cu-SiO₂/PA12 SLS parts had excellent dimensional precision. The tensile strength of the 15W-sintered Cu-SiO₂/PA12 specimen was 48MPa.

Composite polyurethane (PU)-SiO₂ hollow fiber membranes were successfully prepared via optimizing the technique of dry-jet wet spinning, and their pressure-responsibilities were confirmed by the relationships of pure water flux-transmembrane pressure (PWF-TP) for the first time. The origin for this phenomenon was analyzed on the basis of membrane structure and material characteristics. The effects of SiO₂ content on the structure and properties of membrane were investigated. The experimental results indicated that SiO₂ in membrane created a great many interfacial micro-voids and played an important role in pressure-responsibility, PWF and rejection of membrane: with the increase of SiO₂ content, the ability of membrane recovery weakened, PWF increased, and rejection decreased slightly.

Good breakdown strength is an important feature for the selection of dielectric materials, especially in high-voltage engineering. Although nanocomposites have been shown to possess many promising dielectric properties, the breakdown strength of nanocomposites is often found to be negatively affected. Recently, imposing nonisothermal crystallization processes on polyethylene blends has been demonstrated to be favorable for breakdown strength improvements of dielectric materials. In an attempt to increase nanocomposites’ voltage rating, this work reports on the effects of nonisothermal crystallization (fast, moderate and slow crystallizations) on the structure and dielectric properties of a polyethylene blend (PE) composed of 80% low density polyethylene and 20% high density polyethylene, added with silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) nanofillers. Through breakdown testing, the breakdown performance of Si₃N₄-based nanocomposites was better than SiO₂-based nanocomposites. Since nanofiller dispersion within both nanocomposite systems was comparable, the enhanced breakdown performance of Si₃N₄-based nanocomposites is attributed to the surface chemistry of Si₃N₄ containing less hydroxyl groups than SiO₂. Furthermore, the breakdown strength of SiO₂-based nanocomposites and Si₃N₄-based nanocomposites improved, with the DC breakdown strength increasing by at least 12% when both the nanocomposites were subjected to moderate crystallization rather than fast and slow crystallizations. This is attributed to changes in the underlying molecular conformation of PE in addition to water-related effects. These results suggest that apart from changes in the nanofiller surface chemistry, changes in the underlying molecular conformation of polymers are also important to improve the breakdown performance of nanocomposites.