Silicon, thermomechanical properties

In recent years silicon-based polymers were investigated as precursors for SiC and Si3N4 ceramics, as well as for crystalline or amorphous Si/C/N and SiC/Si3N4 composite materials [1, 2]. This is due to the very interesting chemical and thermomechanical properties of silicon carbonitrides, such as high hardness, toughness and corrosion resistance. In most of these studies polycarbosilanes, polysilazanes and polycarbosilazanes were applied [3]. 

Thermal and thermomechanical properties. Silicones, as a class, are rated among the highest temperature stable polymers. They can withstand temperatures of200°C, almost continuously, without degradation of physical or electrical properties and have been used at temperatures as high as 300°C. Because of their high thermal stabilities, they are used as adhesives and encapsulants for electronic modules that are expected to perform in extreme temperature environments, such as near automotive engines and in deep-well sensors. Because of their low moduli of elasticity, silicones also fare well at very low temperatures. They are rated for continuous use at -80°C, but may be used at even lower temperatures. 

Nafion -Si02 nanocomposite membranes are used to increase the proton conductivity and water retention at higher temperatures as well as improve the thermomechanical properties. Mauritz [53] investigated the preparation of Nafion/SiO composites by infiltration of Nafion membranes with silicon alkoxides. They exploited the fact that the pendant group clusters catalyze the sol-gel reac-... 

Polymer derived ceramics have been known for the last four decades and are prepared via solid-state thermolysis of preceramic polymers. They exhibit a unique combination of remarkable properties due to their covalent bonding and amorphous nature. Thus, silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) based ternary PDCs have been shown to possess outstanding high-temperature properties such as stability with respect to crystallization and decomposition, oxidation and corrosion resistance as well as excellent thermomechanical properties (e.g., near zero steady state creep resistance up to temperatures far beyond 1000 °C). Their properties are directly influenced by the chemistry and the architecture of the preceramic precursors, thus there is an enormous potential in tuning the microstructure and properties of the PDCs by using tailored polymers. Furthermore, suitable chemical modification of the preceramic precursors leads to quaternary and multinary ceramics, as it has been shown for instance for silicon boron carbonitride ceramics in the last 25 years, which in some cases exhibit improved properties as compared to those of the ternary materials. 

Three carbon fibre-reinforced polyimides were exposed to UV radiation at 177C, at three different intensities for three different times, so that the product of intensity and time was a constant. Intensities of 1,2 and 3 suns, where one sun is the power in space at one earth-sun distance, were used, for a time periods of 500, 250 and 167 h. The samples were characterised by X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, thermomechanical analysis and dynamic mechanical analysis. Measurement of bulk properties showed no difference between samples exposed to heat and UV radiation, and control samples. Surface analysis by XPS showed an apparent decrease in carbonyl concentration on the surface of some exposed samples. This was correlated to surface contamination by a silicone-containing material. 3 refs. 

The properties of aluminum alloys (mechanical, physical, and chemical) depend on alloy composition and microstructure as determined by casting conditions and thermomechanical processing. While certain metals alloy with Al rather readily [9], comparatively few have sufficient solubility to serve as major alloying elements. Of the commonly used alloying elements, magnesium, zinc, copper, and silicon have significant solubility, while a number of additional elements (with less that 1% total solubility) are also used to confer important improvements to alloy properties. Such elements include manganese, chromium, zirconium, titanium, and scandium [2,10]. 

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