Thermal Stability and Crystallization Behavior

The thermal stability is a severe limitation if the metallic glass is to be used in as-quenched state for catalysis however, that is not necessarily the case if the glassy alloy is used as catalyst precursor. The thermal stability is mainly influenced by the chemical composition of the metallic glass and the medium to which it is exposed. It has been shown that the crystallization temperature can be significantly lowered in the presence of a hydrogen atmosphere [4.23,24,31,50] or an adsorbed organic compound [4.76]. 

It should also be noted that the crystallization behavior of melt-spun ribbons may be different on both ribbon sides [4.18]. Nucleation for primary crystallization of the transition metals is observed on both sides of the glassy ribbons, while other crystallization reactions have been observed to prefer usually either the free surface or the contact side of the ribbon [4.77]. This phenomenon may lead to different structural and chemical properties of the two ribbon sides, and consequently also to large anisotropy in the catalysts prepared from such ribbons [4.18, 19]. 

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Important properties of glassy metals influencing the structural and chemical properties of the catalyst derived from them are (i) chemical composition (ii) chemical and structural homogeneity (iii) thermal stability and crystallization behavior (iv) oxidation behavior (v) dissolution of gases and (vi) segregation phenomena. These factors together with the conditions used for the chemical transformation of the precursor are crucial to obtain catalysts with the desired properties. 

Morales et al. [323] prepared bionanocomposites of PEA (derived from glycohc acid and 6-aminohexanoic add by in situ polymerization) reinforced with OMMTs. The most dispersed structure was obtained by addition of C25A organoclay. Evaluation of thermal stability and crystallization behavior of these samples showed significant differences between the neat polymer and its nanocomposite with C25A. Isothermal and nonisothermal calorimetric analyses of the polymerization reaction revealed that the kinetics was highly influenced by the presence of the silicate particles. Crystallization of the polymer was observed to occur when the process was isothermally conducted at temperatures lower than 145 °C. In this case, dynamic FTIR spectra and WAXD profiles obtained with synchrotron radiation were essential to study the polymerization kinetics. Clay particles seemed to reduce chain mobility and the Arrhenius preexponential factor. 

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