The Study Of Dynamic And Structural Evolutions In Glassy Systems

Amorphous structural materials?also known as glasses in scientific term?are important building blocks of the world.Compared with the corresponding stable crystalline state,they occupy many unique properties at the aspects of mechanics,optics,thermology,electrics,and magnetics.According to the thermodynamic rules,disordered matters have higher free energy.Therefore,they stay at non-equilibrium state and evolve with time,which is closely related to surrounding environment and the thermal history.The study of evolution of amorphous materials is of great value in understanding the instinct characteristics and commercial applications of glassy materials.However,due to the complexity and instability of the disordered structure,the researches always face great challenges.In this paper,taking the respective advantages of different glassy systems,the temporal dynamic and structural evolution of amorphous materials is studied at the single-particle level,to establish the correlation between macro-properties and micro-configurations.The main work of this thesis is summarized as follow:Owing to its excellent oxidation resistance,Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass is used to study the crystallization behavior of amorphous surface near the glass transition temperature?T_g?.Near the T_g point,the dynamics of glass surface is decoupled form that of the bulk.By adjusting the annealing temperature?around T_g-20 K?,a periodic superlattice-like modulated nanostructure is grown on the disordered free surface.Upon increasing temperature,the heterogeneous surface crystallization transforms into homogeneous nucleation of the entire glassy system and traditional polycrystal is created.The growth mechanism of superlattice is proposed by observing the cross-sectional atomic rearrangements,and its generality is verified in other several metallic glassy systems.By analyzing the diffusing behavior of constituent elements,the correlation between the liquid-like behavior of the amorphous surface and the crystallization characteristics is revealed,which also provides a new method and sheds light on the synthesis of sophisticated superlattice nanostructures.By synthesizing PNIPAM colloidal particles with different degree of temperature sensitivity,their size difference could change within a considerable scope.Starting from binary crystal at high temperature,the system experiences order-to-disorder transition during cooling.Via in-situ observation of the microscopic process,it reveals that the phase transition accompanies with the percolation of disordered defects.Moreover,this novel experimental method pays a way for future systematic researches on solid-state phase transformation mechanism.Taking advantage of the temperature-sensitivity of PNIPAM colloids and flexible control of volume fraction,the dynamics and structure of the glassy system can be continuously tracked at singe-particle level.Through standard two-step temperature-jump protocol,the Kovacs memory effect is observed in two-dimensional colloidal glasses.The influence of thermal history on non-monotonic Kovacs effect is studied.Via calculating single particles'Debye-Waller factor and local structural entropy,it is found that the defected regions,which have not been completely frozen in the physical aging process,are sensitive to the temperature rise.They form local liquid-like islands,and else regions are activated subsequently.The statistical analysis reveals that all the particles contribute to the recovery of macroscopic quantities.These results uncover a coherent physical picture connecting the aging process with Kovacs effect.Comparing the existing theoretical interpretations,our results exclude the multi-relaxation mode theory and support the hierarchical scenario of glass.Similar to the temperature cycle conducted in spin glass,we carry out three-step temperature protocol on PNIPAM colloidal glass.Like spin glass,the aging,rejuvenation and memory effect appear at corresponding step.It indicates the common instinct properties in both structural and functional glass,and proves the existence of complex hierarchical structure in potential energy landscape of glasses.The above researches take advantage of metallic glass and colloidal glass,via observing the dynamics and structural evolution in real space to study their phase transitions and relaxation behaviors,which are of great significance to understand the non-equilibrium nature of amorphous materials.