Jerzy A. Golczewski

Applications of computational thermodynamics: Group 3: Application of computational thermodynamics to phase transformation nucleation and coarsening

Abstract Computational thermodynamics is applied to two important and closely related phenomena in phase transformations, namely nucleation and coarsening. In both phenomena the surface energy plays a key role. The effect of surface energy on a two-phase equilibrium as well as the activation energy for forming a critical nucleus are both derived for the general multicomponent case. A simplified coarsening equation is derived for the general multicomponent case.Computational thermodynamics is applied to two important and closely related phenomena in phase transformations, namely nucleation and coarsening. In both phenomena the surface energy plays a key role. The effect of surface energy on a two-phase equilibrium as well as the activation energy for forming a critical nucleus are both derived for the general multicomponent case. A simplified coarsening equation is derived for the general multicomponent case.

Phase separation in Si-(B)-C-N polymer-derived ceramcis

Abstract Details of phase separation in the microstructure of amorphous Si–(B)–C–N ceramics derived from polymers have been resolved using the recent results of structural investigations. The formation of an amorphous phase built of atomic compounds SiCi/4N(4–i)/3 and consequently located along the composition line between SiC and Si3N4 in the ternary Si–C–N phase diagram demonstrates a generic feature of phase separation in all these materials. The amorphous carbon phase separates as a counterpart in the microstructure of Si–C–N ceramics, and in the case of Si–B–C–N ceramics such counterpart represents B–N–C domains of the composition (BN)cCy located along the tie line C–BN in the ternary B–C–N phase diagram. The effect of phase separation has been also pondered as a source of exceptional material properties.

A thermodynamic model of amorphous silicates

Abstract A thermodynamic model is proposed to provide a physical picture of the glass transition in amorphous silicates. Below the glass transition temperature Tg the model considers the phase to be a random network of oxide compounds which relaxes to provide local structural fluctuationsas the temperature rises above Tg. Thermally activated development of these fluctuations accounts for gradual transition between the “solid”, as existing below Tg and the liquid state appearing above the melting point T fus . The randomness of the structure and its fluctuation contribute both to the Gibbs energy as estimated near Tg. The model parameters of analytical expressions of Gibbs energy have been adjusted to fit the experimental information. The model has been developed for silicate glasses considered to a first approximation as ideal solution of single amorphous oxides. The relative enthalpy ΔH = H(T) − H(298K) has been calculated for a series of silicate glasses. The computational results agree quite satisfactory with experimental data. The constraints of the model are briefly discussed.

Solar-thermal zinc oxide reduction assisted by a second redox pair

Abstract Thermochemical cycles have been investigated for quite some time as a means to convert heat into storable chemical energy. A particularly attractive reaction is the solar-thermal decomposition of zinc oxide into zinc and oxygen, but the efficiency of this process is impaired by the fast recombination of zinc and oxygen upon cooling of the gas phase. The combination with a second, nickel/nickel oxide redox cycle is presented as a possibility to avoid the simultaneous presence of metal and oxygen in the gas phase.