טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSorkin Viacheslav
SubjectPoint Defects, Lattice Structure and Melting
DepartmentDepartment of Physics
Supervisors Dr. Joan Adler
Professor Emeritus Emil Polturak


Abstract

The melting transition, probably the oldest one known to man, remains a subject of active research. Two scenarios for melting transitions are considered: mechanical melting resulting from lattice instability, and thermodynamic melting which begins at the free surface of the solid.

The purpose of our research was to investigate the melting transition of vanadium (bcc lattice) using molecular dynamics. We found that the Born instability is the trigger for mechanical melting. This instability arises when the solid expands up to a critical volume. We verified that the critical volume at which the crystal melts is independent of the path thru the phase space by which it is reached, i.e. either by heating the perfect crystal or by adding defects at a constant temperature.

Surface melting was studied in detail. We investigated the structural, transport and energetic properties of the low-index surfaces of vanadium at different temperatures. Upon increase of the temperature defects are generated at the surface region and the disorder starts to spread from the topmost layer. A thin quasiliquid film appears in the surface region. The premelting phenomena are most pronounced for the loosest packed Va(111) face.

We applied Born's criterion to the surface region and found a linear relation between the activation energy of surface defects and the premelting temperature. The agreement between the calculated defect activation energy for the Va(111) and  theoretical prediction is reasonable. We concluded that the Born criterion describes both surface and bulk melting, and provides the ``missing link'' which will tie together these two scenarios for melting transitions.