38 research outputs found
Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting
The reversible phase transition induced by femtosecond laser excitation of
Gallium has been studied by measuring the dielectric function at 775 nm with ~
200 fs temporal resolution. The real and imaginary parts of the transient
dielectric function were calculated from absolute reflectivity of Gallium layer
measured at two different angles of incidence, using Fresnel formulas. The
time-dependent electron-phonon effective collision frequency, the heat
conduction coefficient and the volume fraction of a new phase were restored
directly from the experimental data, and the time and space dependent electron
and lattice temperatures in the layer undergoing phase transition were
reconstructed without ad hoc assumptions. We converted the temporal dependence
of the electron-phonon collision rate into the temperature dependence, and
demonstrated, for the first time, that the electron-phonon collision rate has a
non-linear character. This temperature dependence converges into the known
equilibrium function during the cooling stage. The maximum fraction of a new
phase in the laser-excited Gallium layer reached only 60% even when the
deposited energy was two times the equilibrium enthalpy of melting. We have
also demonstrated that the phase transition pace and a fraction of the
transformed material depended strongly on the thickness of the laser-excited
Gallium layer, which was of the order of several tens of nanometers for the
whole range of the pump laser fluencies up to the damage threshold. The
kinetics of the phase transformation after the laser excitation can be
understood on the basis of the classical theory of the first-order phase
transition while the duration of non-thermal stage appears to be comparable to
the sub-picosecond pulse length.Comment: 28 pages, including 9 figs. Submitted to Phys. Rev. B 14 March 200
Solving the inverse boundary-value explosion problem using a modified solid-liquid model
Структурні особливості вакуумних конденсатів заліза, легованих вольфрамом
The initial structure and the composition of Fe–W films which were obtained by vacuum deposition were studied by the methods of transmission electron microscopy, x-ray diffraction, energy-dispersive x-ray spectroscopy and x-ray fluorescence analysis. It is shown that microalloying of iron with tungsten results in dispersion iron matrix grain structure. The inhomogeneous distribution of tungsten within each grain was discovered. The observed regularities are explained by the formation of grain boundary segregation of alloying component during binary vapors condensation.Методами просвічуючої електронної мікроскопії, рентгенівської дифрактометрії, енергодисперсійної рентгенівської спектроскопії та спектрального рентгенівського флуоресцентного аналізу вивчена вихідна структура і склад плівок Fe-W, отриманих вакуумним осадженням. Показано, що мікролегування заліза вольфрамом призводить до диспергування зеренної структури залізної матриці. Виявлено неоднорідний розподіл вольфраму в межах кожного зерна. Спостережувані закономірності пояснюються формуванням зерногранічних сегрегацій легуючого компонента при конденсації двокомпонентного пара