Study About Polyamorphic Phase Transition Of Cerium-based Metallic Glasses And Structure And Phase Transition In Amorphous Sulfur

The problem of amorphous structure and its phase transition is a frontier topic in condensed matter physics,and the transition between two different amorphous phases is called amorphous polymorphic phase transition.In this dissertation,amorphous polymorphic phase transitions have been studied for two different amorphous systems,namely metallic glasses and amorphous sulfur.Pressure and temperature are the basic physical parameters of changing the state of matter.Pressure is applied to change the interatomic distance within the material and even modulating the atomic structure,the overlap of electron orbits of adjacent atoms,etc.It has an important reference value to study the amorphous polymorphic phase transition of metallic glass under the high pressure for understanding the disordered structure of metallic glass.Thermodynamic behaviors of amorphous materials such as glass transition,supercooled liquid-to-liquid phase transition and the like are temperature-induced amorphous polymorphic phase transitions.A single component of amorphous sulfur is an ideal target for the study of disordered systems.Compared to the melt quenching and the crystalline sulfur pressure-induced amorphization methods,the bulk amorphous sulfur prepared by the rapid pressure coagulation method in this group exhibits high thermal stability and is not easily crystallized,which provide a unique condition to study the structure and phase transition of amorphous sulfur above the room temperature.This paper is divided into the following sections:?1?In this work,the polymorphic phase transitions of Ce₇₀Al₁₀Cu₂₀0 and Nd₆₀Fe₃₀Al₁₀metallic glasses are investigated by synchrotron radiation X-ray diffraction combined with resistance measurement under high pressure.The results of high energy X-ray diffraction experiments show that both of the two metallic glasses have a phase transition from low-density amorphous state to high-density amorphous state under pressure.With the pressure-induced volume collapse,it is found that Nd₆₀Fe₃₀Al₁₀0 has a volume collapse at21.09 GPa and the contraction is 8.4%.Identically,Ce₇₀Al₁₀Cu₂₀0 collapses to 2.7%at 8.39GPa.The resistivity of the two kinds of metallic glasses is measured by strip anvil under high pressure.It indicates that there appears a discontinuous change of the slope of the resistivity of Ce₇₀Al₁₀Cu₂₀0 at 1.73 GPa,which is the approximately same with initial pressure of polymorphic phase transition through observing X-ray diffraction dates.?2?The synchrotron radiation X-ray diffraction and the radial distribution function are performed to study the polymorphic phase transitions of Ce₆₀Al₂₀Cu₂₀0 and Ce₅₅Al₄₅5 metallic glasses from low density to high density induced by the pressure.The experiment shows that for Ce₆₀Al₂₀Cu₂₀0 and Ce₅₅Al₄₅5 metallic glasses,the volume varies discontinuously with the pressure in the range of 3.50 to 6.32 GPa and 2.20 to 6.89 GPa,respectively.Based on the radial distribution function,there appears a change of the distance between the nearest neighboring atoms and the relative distance between different shells under different pressures for Ce₆₀Al₂₀Cu₂₀0 and Ce₅₅Al₄.Meanwhile,it concludes that the volume reduction is related to the pressure-induced the shortening of bond during the phase transition process,and the polymorphic phase transition is caused by the pressure-induced the delocalization of4f electrons combined with local atomic rearrangement.?3?Abnormal exothermic melting process of amorphous sulfur is studied by high energy X-ray diffraction,radial distribution function and Raman spectroscopy.Using experimental dates and the simple chain model,it proves that the amorphous sulfur consists of a polymer chain with an average chain length of 16 and an S₈ ring,the weight percentage of which is about 7:3.The coordination number of the first nearest neighbor atom of amorphous sulfur decreases from 1.91 to 1.81,meanwhile,the intensity of the Raman vibration peak at 461 cm^-1of the polymer chain decreases significantly during the exothermic melting process,indicating that the polymer chain breaks during melting.In addition,a re-entry of?-transition is also proposed,namely,the parent liquid structure?above the?-transition temperature?of amorphous sulfur is retained to a certain extent during the preparation process,then,the melt structure of amorphous sulfur obtained above the?-transition temperature changes into the liquid structure below the?-transition temperature,and last,the liquid structure is again transformed into the melt structure above the?-transition temperature.?4?Differential scanning calorimetry,UV-Vis absorption spectroscopy,and tensile testing are utilized to investigate the glass transition temperature,absorption spectra,and stress-strain curves of two kinds of amorphous sulfur which are solidified under rapid compression before and behind the?-transition temperatures.The experimental consequences show that the amorphous sulfur prepared at the same temperature and higher pressure has a higher glass transition temperature.It is found that there is more polymer chains under higher pressurizing rate and larger pressurizing amplitude conditions.The optical band gap of the two kinds of amorphous sulfur prepared at the same pressure and different parent liquid temperatures are 2.24 eV and 2.49 eV,respectively.Tensile test results demonstrate that the fracture strength of the two kinds of amorphous sulfur are 718.6 and949.1 KPa,and the elongation are 3007.3%and 2560%at a stretch rate of 25?m/s,respectively.In summary,this paper has carried out a series of work about amorphous polymorphic phase transitions,including the pressure-induced polymorphic phase transition of metallic glass at room temperature and the temperature-induced supercooled liquid-to-liquid polymorphic phase transition of amorphous sulphur at atmospheric pressure.Through studying the difference between the two kinds of fast-pressing amorphous sulfur,the polymorphic phase transition of liquid sulfur,namely,the?-transition mechanism,is derived.These research results have important reference value for understanding the microstructure and phase transition of amorphous materials,and the correlation between the amorphous materials and the parent liquid structure.