Structural Evolution Of High Temperature Pure Ga And Ga-Bi Metallic Alloy Melts

Structure of metallic liquid and its evolution are long-standing hot issues in the field of material science and condensed-matter physics,and are both related to appication problems,such as the solidification and performance of metallic material,and fundamental scientific problems,such as glass transition,liquid-liquid transition and liquid anomalies.Therefore,a deeper understanding in the structure of metallic liquid and its evolution are of great importance in both industrial application and fundamental science.In this dissertation,we mainly focus on the metallic liquid with temperature above its melting point,namely the metallic melt.The structure evolution and dynamic behavior of pure Gallium(Ga)melt,eutectic,hypereutectic and hypoeutectic Ga-Bi metallic melts have been studied by nuclear magnetic resonance(NMR)using a home-made in situ high-temperature NMR probe.More details are listed below:(1)NMR is a powerful method in detecting the structure and dynamic change of metallic liquid.Unlike the diffraction pattern(X ray diffraction pattern,neutron diffraction pattern)acquired by using the X ray or neutron interacting with the sample matters,the NMR observables,such Knight shift and relaxation time,themselves are reflactions of the interaction between the central atom and its over all local structure.Our group has a rich experience in high-temperature NMR method,a home-made in situ high temperature NMR probe as high as 1500 K had already been built.However,practical application of the probe reveals that some part can be further improved to get a better performance.Therefore,in chapter 2 of this dissertation,a new in situ high-temperature NMR probe with better performance have been built and several critical drawbacks of the old high-temperture NMR probe have been improved.In addition,as temperature and pressure are the two basic paramaters in the research of structure evolution,based on the diamond anvil cell,an in situ ultra-high pressure NMR probe has been built.The highest pressure 5 GPa,¹H resolution 3ppm can be relized and the probe fits the common superconducting magnet.(2)The structural evolution of pure Ga melt has been studied in chapter 3 of this dissertation.The two quadrupole isotope of Ga makes Ga a superior element for NMR measurement.Using the new home-made high-temperature NMR probe,structure and dynamic evolution of pure Ga melt has been studied in temperature range of 307-1356 K.A continuous structure transformation(structural crossover)around 1000 K has been revealed.The structural evolution of pure Ga melt may associated with the locally favored covalently bonded Ga clusters.Our result demonstrated that complex clusters should exist in the pure Ga melt and certain clusters can even exist at temperature far above the melting point of Ga,and affect the dynamic behavior of metallic liquid.Moreover,a structural crossover rather than a liquid-liquid transition was observed may result from the weak cooperativity J induced by the low constraint of covalent bonds in liquid Ga.(3)Liquid-liquid transition,a first-order phase transition between two different liquid states of the same composition,which can be a probe to get an insight into the structural evolution of metallic liquids and has been extensively studied and debated.In chapter 4 of this dissertation,the eutectic Ga-Bi(Ga₃₉Bi₆₁)metallic melt has been studied using our home-made high temperature probe.Using Ga as the probe element,a liquid-liquid transition has been revealed above the melting point of both Ga and Bi crystal.Based on the study of pure Ga melt,the liquid-liquid transition in eutectic Ga-Bi metallic liquid may associated with the evolution of locally favored covalently bonded Ga-Bi clusters.(4)The origin of the anomalies of liquids is a long standing problem in condensed-matter physics,and the second critical point scenario has attracted the most attention.Based on the study of eutectic Ga-Bi melt,hypereutectic and hypoeutectic Ga-Bi metallic melts have been studied in chapter 5 of this dissertation.Through the analyses of high-temperature NMR result,no obvious structure change is revealed in Ga_(100-x)Bi_x metallic melts with x=30;with Bi concentration increasing,continuous structure transformations and dynamic crossover can be observed in Ga_(100-x)Bi_x metallic liquid for x=40 and 50;and liquid-liquid transitions have been observed in the Ga_(100-x)Bi_x metallic liquid with x=70,80 and 90.According to the two-order parameter model,the different structural evolution behaviors of Ga_(100-x)Bi_x metallic melts may result from the fixed measurement pathway intersect with liquid-liquid coexistence line or the widom line at different place in the T-P plane.The change of the location of liquid-liquid critical point LLCP may be induced by the change of J.Our result provided a new method in studying the second critical point scenario,and from another point of view,the result of Ga₅₀Bi₅₀ have provided an experimental evidence for the second critical point scenario.