Thermal expansion oxygen atom

Around 400 K there is a phase transition to a disordered but still cubic structure associated with the terminal oxygen atom in a WO4 tetrahedron which can migrate to another tetrahedron, thereby reversing the direction in which a pair of tetrahedra point. Nevertheless the same general atomic mechanisms are responsible for the negative thermal expansion up to the decomposition temperature. 

Oxynitride glasses are silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms. As nitrogen increases, glass transition temperature, elastic modulus, viscosity and hardness increase while thermal expansion coefficient decreases. 

Figure 30 Rocking of the ReOa stmcture indicating possible correlation of the transverse thermal motions of the oxygen atoms. The octahedra do not change volume on rocking, but the distance between the octahedra decreases as they rock away from the strucmre in the middle producing negative thermal expansion. (Reprinted from Tao and SleighE Elsevier 2003)...

Figure 12.12 Variation in the number ofbridging oxygen atoms (O ), the coefficient of thermal expansion (a) and the softening temperature Sp) as a function of alkali-oxide concentration in a B2O3 - containing binary glass (After Balta and Balta, 1976).

The nanodispersion of Si and O throughout the polyimide polymer matrix leads to the formation of a protective silica layer on the polyimide surface when the material reacts with AO [15]. Our data indicates that upon AO exposure, the organic material in the polymer surface erodes, while the atomic oxygen reacts with the nanodispersed POSS to form a silica layer. Therefore, when POSS is copolymerized to form POSS-PI, it imparts remarkable AO resistance, and does so with minor effects in the storage modulus, glass transition temperature, and coefficient of thermal expansion [15]. 

Single crystal measurements at different temperatures have also been made and analysed to explain in detail the remarkable negative thermal expansivity observed for many microporous solids. Simplistically, enhanced transverse thermal vibration of oxygen atoms in T-O-T bonds can result in the reduction of T-T distances in the frameworks and the shrinkage of cell dimensions that depend on these T T distances. ... 

In the case of SrLaAlO, the bonds between A1 and the apical oxygen 02 are longer than those between A1 and the equatorial 01 atoms. Bond valence sums indicate that the apical bonds are weaker than the equatorial bonds, and this is reflected in the resultant axial relative thermal expansion values (as defined earlier) and the axial thermal expansion, which is found to be greater along the c-axis than along the a- (or b-)axes (Figure 9.1b) ... 

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