NUCLEATION-GROWTH PROCESSES AND ISOTHERMAL KINETICS OF PHASE-TRANSFORMATIONS IN THE METHYLHYDRAZINE MONOHYDRATE

Phase transformations in the methylhydrazine monohydrate (CH3NHNH2 . H 2O) are investigated by differential scanning calorimetry (DSC) over t he temperature range 110-300 K. On cooling a glass is formed by quench ing the liquid. Depending on the heating rate, two solid phases (metas table and/or stable) with slow phase transformation kinetics are obser ved. The optimal temperature for nucleation and growth processes have been determined for the two solid phases. The nucleation rate is maxim um in the glass transition region (T-g congruent to 160 K) and the tem perature growth regions are, respectively, estimated to be 188-202 K f or the metastable solid phase and 212-228 K for the stable solid phase . The metastable and the stable solid phases, respectively, melt at 22 0 and 234 K. The kinetic character of phase transitions is revealed by complementary X-ray measurements at 200 K. A phenomenological model b ased on Avrami's law is proposed for a better understanding of the gro wth mechanisms for the two solid phases. On reheating, the glassy stat e transforms to a metastable liquid. Then, a partial transformation (4 8%) into the metastable solid phase from this liquid begins at about 1 88 K. It is followed by the growth of the stable solid phase from the undercooled liquid and the metastable solid phase. The solid-solid tra nsformation seems to be governed by contacts with the stable solid gra ins.