Theory confirmed by research

Dwuwymiarowy dyfraktogram rentgenowski krystalicznej warstwy Pd (a) oraz dyfraktogram warstwy całkowicie stopionej impulsem optycznym (b).

Two-dimensional X-ray diffraction pattern of the crystalline Pd layer (a) and the diffraction pattern of the completely melted layer by an optical pulse (b).

It might seem that since the melting of crystalline materials is one of the most common phase transitions, consisting in the transition from solid to liquid, we know everything about this phenomenon. However, this is not so. Scientists from Warsaw University of Technology are currently working on an in-depth study of the mechanisms of transformations between the crystalline and liquid phases.

Lack of definitive experimental evidence for the occurrence of theoretically predicted phenomena, such as heterogeneous melting initiated on defects and liquid nucleation occurring homogeneously in the crystal volume, and kinetic parameters (including the speed of propagation of the melting front of the overheated crystal) known mainly from computer simulations - he faced such challenges research team from the Warsaw University of Technology headed by prof. Jerzy Antonowicz, PhD Eng.

The aim of the project, which is carried out as part of the competition for research grants, Material Technologies-1, was to conduct research on optically induced melting of palladium using the ultra-fast, time-resolved X-ray diffraction method with the use of radiation pulses emitted by an X-ray free electron laser (X -ray free-electron laser - XFEL).

The course of research

The measurements were carried out during the experiment at the European X-Ray Free-Electron Laser Facility (European XFEL), which is part of the extensive research collaboration of prof. Antonowicz with groups of scientists, including from Poland, Germany, Japan and the USA.

Diagram of a system for diffraction measurements using the "pump - probe" method.

Diagram of a system for diffraction measurements using the "pump - probe" method.

- XFEL emits extremely short, bright and high-energy X-ray pulses, which can be used for measurements using the pump-probe method, which allows the study of phase changes occurring in a picosecond time scale inaccessible to conventional measurement methods - says Prof. Jerzy Antonowicz. - During the measurements carried out at the European XFEL, the thin metal layer was excited by a femtosecond optical pulse, which caused it to melt locally. The delayed X-ray sampling pulse was diffracted on the structure of the exposed area of the layer, and the spatial distribution of the scattered radiation was recorded in the form of a two-dimensional diffractogram - he adds.

First conclusions

The preliminary interpretation of the research results showed to a large extent agreement with the theoretical predictions.

Sample fragment view. The white dots in the center of the bright squares are holes created by the interaction of the layer with the X-ray pulse.

Sample fragment view. The white dots in the center of the bright squares are holes created by the interaction of the layer with the X-ray pulse.

- Based on the quantitative analysis of diffraction data, we determined, among others the relative content of the crystalline and liquid phases and the size of the crystalline grains at different stages of the melting process, says Prof. Antonowicz. - We found that the melting process takes place in about 10ps after the femtosecond optical pulse has been absorbed. The analysis of changes in the average crystal grain size showed that melting starts in structurally disordered areas of grain boundaries, and the speed of propagation of the melting front reaches the value of 1000 m / s - he adds.

Determined on the basis of quantitative analysis of X-ray diffraction patterns: content of the crystalline phase 10 ps after irradiation depending on the energy of the laser pulse (a), change in the content of the crystalline phase during partial melting with a pulse of energy of 12 µJ (b) and the accompanying evolution of the mean crystal grain size (b ). The vertical lines in figure (a) correspond to the energy for which the relationships in figures (b) and (c) are plotted, and the vertical lines in figures (b) and (c) represent the delay of the x-ray pulse corresponding to figure (a).

Determined on the basis of quantitative analysis of X-ray diffraction patterns: content of the crystalline phase 10 ps after irradiation depending on the energy of the laser pulse (a), change in the content of the crystalline phase during partial melting with a pulse of energy of 12 µJ (b) and the accompanying evolution of the mean crystal grain size (b ). The vertical lines in figure (a) correspond to the energy for which the relationships in figures (b) and (c) are plotted, and the vertical lines in figures (b) and (c) represent the delay of the x-ray pulse corresponding to figure (a).

Plans for the future

- In the near future, we want to conduct large-scale computer simulations using the molecular dynamics method, and then confront their results with the experimental results - says Prof. Antonowicz. - We also plan to extend our research to systems other than pure metals and time ranges up to nanoseconds and including supercooling and crystallization of the liquid, as well as the transition of supercooled liquid to solid without crystallization, i.e. the process of glass formation.

 

Composition of the research team:

The research team involved in this project is, in addition to applicants from the Warsaw University of Technology (PhD Eng. Jerzy Antonowicz, University Professor; Adam Olczak, M.Sc.), a group of several dozen people from Poland (the group of Prof. Ryszard Sobierajski from the Institute of Physics PAS), and from research centers in Germany, the USA, Great Britain, the Netherlands, Russia and the Czech Republic.

The article was prepared on the basis of materials sent by prof. Jerzy Antonowicz.