Thermal expansion coefficient nanocomposites

For most metal-reinforced nanocomposites the thermal expansion coefficient of the metal phase will be larger than that of the matrix, reversing the expected stress fields compared to SiC-reinforced alumina. Thus while the tensile radial stresses surrounding occluded particles may induce transgranular cracking, the compressive hoop stresses may inhibit crack propagation if the particles are located at grain boundaries. Macrostresses in sub-micron Ni... 

Because nanocomposites are made from different phases with different thermal expansion coefficients and elastic moduli, they inevitably develop residual thermal stress during cooling after sintering. Assuming the dispersion phase is spherical particulate in the matrix material, residual stresses can be developed due to differences in the thermal expansion and elastic constants between the matrix and the particles [23] ... 

Utracki, L. A., Compressibility and thermal expansion coefficients of nanocomposites with amorphous and crystalline polymer matrix. Ear. Polym. J., 45, accepted (2009e). 

Hitachi has developed a heat-resistant material for printed wiring boards, consisting of an epoxy polymer containing nanosized silica, evenly dispersed by means of a sol-gel reaction. At 260 °C, the nanocomposite s elasticity was said to be about ten times higher than that of epoxy, and its thermal expansion coefficient was about one-third. 

Montmorillonite platelets are particularly effective for reducing thermal expansion coefficients of plastics, as shown in Figure 4.2 for used exfoliated polyamide 6 nanocomposites. 

FIGURE 4.2 Coefficient of thermal expansion coefficient of montmorillonite-filled polyamide nanocomposites. 

Organic/inorganic nanocomposites prepared by in situ polymerization methods have received extensive attention in recent years. Unlike microscale fillers, nanoscale fillers can offer excellent properties to a polymer matrix. Nanosized filler, with a few weight percent in the reinforced polymer nanocomposites, strongly influences the macroscopic properties of the polymer. The resultant polymer nanocomposites can significantly improve some of their properties, such as higher heat distortion temperatures, enhanced flame resistance, increased modulus, better barrier properties, reduced thermal expansion coefficient, and altered electronic and optical properties. 

The structure and properties of biofibers, mainly of cellulose, were described in this chapter. First, the hierarchy microstructure of natural plant fiber and then a variety of crystal modifications of cellulose were mentioned. The ultimate mechanical properties (modulus of 138 GPa and strength of 17.8 GPa) and thermal properties (thermal expansion coefficient of 10 order) were emphasized as quite excellent for cellulosic fiber, enough for use as reinforcement in the composites. With the manifestation of these intrinsic properties in macroscopic material, the oH-cellulose composite was shown to possess excellent mechanical properties, thermal resistance, and optical transparency, besides being composed of fully sustainable resources and hence, biodegradable. Nowadays, the interest in cellulosic nanocomposites has increased considerably [60, 61] and they are expected to be used in many fields such as electronic devices, vehicles, and windmills to replace glass and/or carbon fibers. 

The intercomponent adhesion level can be estimated quantitatively with the aid of the parameter b (Figure 9.9), which is determined by an independent method using the thermal expansion coefficient of epoxy polymers. In Figure 9.27 the curves 1-3 represent three basic types of the dependences of the linear thermal expansion coefficient of the considered epoxy polymers on the relative contents of nanoclusters. The straight line 1 illustrates the case when adhesion is absent between two structural components of natural nanocomposite and at the thermal expansion coefficients of the loosely packed matrix and nanoclusters, respectively) the loosely packed matrix will expand on heating independently from... 

The thermal expansion is one of the criteria of choice of engineering polymers for their application in either quality [35]. As a rule, polymers have high thermal expansion coefficient, which makes their application in contact with other materials difficult. In paper [62] the structural analysis of the thermal expansion of crosslinked polymers considered as natural nanocomposites was carried out. 

Nanocomposites are a new class of particle-filled composites in which at least one dimension of the dispersed particles is within 100 nm. Because of the dispersion of nanosize clay particles, polymer-clay nanocomposites exhibit improved moduli and strength, decreased thermal expansion coefficient, decreased gas permeability, increased swelling resistance, better thermal stability and enhanced ionic conductivity when compared to the pristine polymers or microscale composites [149-151], They find increased applications in various fields such as automobile, packaging, electronic, coating and aerospace industries [152,153],... 

Two active species such as a free radical and a hydrated electron generated during y-irradiation were used in preparing polymer nanocomposites. Various polymer-clay nanocomposites using y-ray polymerization of the desired monomers can be prepared in a one-step process at room temperature and ambient pressure. The polymer-clay nanocomposites have enhanced moduK, decreased thermal expansion coefficients, reduced gas permeabiUty and increased ionic conductivity. Precious metals have been studied most extensively among polymer-metal nanocomposites and used as catalysts in sensors, photochromic and electrochromic devices and recording materials. Various functional groups can be introduced on the CNT surface by y-irradiation polymerization as a one-step process. The polymer-CNT nanocomposites can be used as supports to immobilize biomolecules in biosensors. 

The thermal properties of UP-CNF nanocomposites were also improved. As shown in Table 1, the glass transition temperature (Tg) of the UP resin rose and the thermal expansion coefficients declined with the increase of the CNF content in the composites. Well-dispersed CNFs have a large surface area and strong interactions with polymer molecules, which may restrict the movement of polymer chains and lead to higher transition temperatures and lower thermal expansion coefficients. 

PI nanocomposites have been prepared by various methods with different fillers. The nanocomposites might have many applications starting from barrier and thermal resistance to a compound with low coefficient of thermal expansion (CTE) [154-167]. These hybrid materials show very high thermal and flame retardation as well as barrier resistance and adhesion. Tyan et al. [158] have shown that depending on the structure of the polyimide the properties vary. Chang et al. [159] have also investigated the dependency of the properties on the clay modifiers. 

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