PARTICLE SIZE EFFECT ON KINETICS AND THERMODYNAMICS OF PHASE TRANSITIONS IN MINERALS

Phase transitions in minerals are transformations naturally occurring when environmental conditions, such as pressure and temperature, change. So, studying their evolution means to understand the modifications of the physical and structural properties of a compound, which can be controlled through industrial processes and exploited in technological applications. On this ground, the present thesis is divided into two big sectors of material science: after a short overview on the basic theory of thermodynamics and kinetics (Part I), the first branch deals with traditional ceramics (Part II), where interest is focused on quartz-cristobalite-tridymite-involving reactions; after that, the field of functional ceramics is discussed in Part III, analysing the behaviour at the nano scale of two widely studied technological materials: the half-doped La and Ca manganite and the tetragonal zirconia stabilized at room temperature by size reduction. All these subjects of study are treated considering the effect of varying the particle size. It is well known, indeed, that reactions are promoted when particles have a greater surface to volume ratio. Therefore, when investigating phase transitions, solid state reactivity and kinetics must reflect the effect of the reduction of grain size. Knowing the mechanisms and the behaviour of materials undergoing high or low temperature processes is essential for industrial procedures to be developed. Thus, phase transitions of the cited compounds are here discussed, in the light of three completely different but complementary approaches: X-Ray Powder Diffraction, performing both reciprocal-space and real-space refinements, and Resonant Ultrasound Spectroscopy, investigating the elastic and anelastic properties of the material of interest.