Jingyu Qin

Study on the structural relationship between the liquid and amorphous Fe78Si9B13 alloys by ab initio molecular dynamics simulation

The structure of the liquid and amorphous Fe78Si9B13 alloys is investigated by ab initio molecular dynamics simulation. The amorphous structure bears a strong resemblance to the liquid structure on both the atomic and electronic levels. Chemical short-range order is evidenced by higher Fe coordinating proportion than the nominal one around both Si and B atoms in both the liquid and amorphous states. The atomic distances and the local densities of states show that Fe–Si bonding is stronger than Fe–B bonding and Si and B tend to repulse each other.

Breaking mechanism of single molecular junctions formed by octanedithiol molecules and Au electrodes.

We present a theoretical study of the elongation process of molecular junctions formed by octanedithiol molecule and Au electrodes. Five types of junctions that have different molecule-electrode coupling geometries are considered. It is found that the behavior of the H atom in the -SH group plays a crucial role in the system structure variation. The variation of the total energy and the average force needed to break the molecular junction are calculated, and each type of molecular junctions is found to have a characteristic breaking force. Comparing our theoretical results with those from experiment shows that the most probable coupling geometry was neglected in almost all the previous work. A dynamic analysis of the electronic structure of the molecular junctions is used to understand the variation of the system configuration.

Indication of liquid-liquid phase transition in CuZr-based melts

We study the dynamic behavior of CuZr-based melts well above the liquidus temperature. The results show a discontinuous change in viscosity during cooling, which is attributed to an underlying liquid-liquid phase transition (LLPT) in these melts. The LLPT is further verified by thermodynamic response in the same temperature region. We find that the LLPT in the Cu46Zr46Al8 melt is reversible above 1350 K upon repeated heating and cooling. Based on the concept of fluid cluster in metallic melts, the reversible LLPT is attributed to the structural transition from the strongly ordered high-density liquids to the weak-local low-density liquids upon cooling.

Heredity of medium-range order structure from melts to amorphous solids

A medium-range order (MRO) structure characterized by the pre-peaks in the structure factor S(Q) curves of typical glass-forming Au55Cu25Si20 melt has been detected using the high temperature x-ray diffractometer. Combining the ab initio molecular dynamics simulations and the experiments, we explore and discuss the structure of the MRO cluster. During the rapid solidification, some structural information carried by the MRO structure is inherited from the melt to the amorphous solid, which promotes the glass formation. Through the comparison of microstructures between amorphous and crystal phases, we also discuss the heredity mechanism.

Metal-based magnetic functional fluids with amorphous particles

Two metal-based magnetic functional fluids, Ga–Sn and Ga–In magnetic liquids, were fabricated by doping with Fe73.5Nb3Cu1Si13.5B9 metallic glass particles and nanoscale Fe3O4 particles, respectively. The saturation magnetization of the metallic glass particles, Fe73.5Nb3Cu1Si13.5B9, is about 125 emu g−1, nearly 50% larger than that of Fe3O4 crystalline particles which are usually used in water-based magnetic fluids. It is discovered that functional fluids, Ga85.8In14.2 and Ga91.6Sn8.4 alloy liquids, with Fe73.5Nb3Cu1Si13.5B9 particles exhibit high saturation magnetization as well as low coercivity and remanence. Furthermore, the magnetic hysteresis curves confirm that the liquid metal-based magnetic functional fluids with Fe73.5Nb3Cu1Si13.5B9 particles have higher magnetization than the metal-based Fe3O4 fluids. Owing to the high alloy boiling point more than 2000 K, the metal-based functional fluids should be useful materials for engineering applications when the surrounding temperature is relatively high. Interestingly, these functional fluids offer the properties of superior thermal or electrical conductivity over conventional water-based fluids.

Structural disorder in metallic glass-forming liquids

We investigated structural disorder by a new structural parameter, quasi-nearest atom (QNA), in atomistic configurations of eight metallic glass-forming systems generated through molecular dynamics simulations at various temperatures. Structural analysis reveals that the scaled distribution of the number of QNA appears to be an universal property of metallic liquids and the spatial distribution of the number of QNA displays to be clearly heterogeneous. Furthermore, the new parameter can be directly correlated with potential energy and structural relaxation at the atomic level. Some straightforward relationships between QNA and other properties (per-atom potential energy and α-relaxation time) are introduced to reflect structure-property relationship in metallic liquids. We believe that the new structural parameter can well reflect structure disorder in metallic liquids and play an important role in understanding various properties in metallic liquids.

Unusual coordination structure in undercooled eutectic Ga-In alloy melt

The local structures in the normal and undercooled liquid eutectic Ga-In alloy are investigated by using X-ray absorption spectroscope (XAS), X-ray diffractometer (XRD) and the ab initio molecular dynamics (AIMD) simulations. The results from these three techniques all indicate that the nearest-neighbor coordination number decreases with decreasing temperature in the liquid eutectic Ga-In alloy. Moreover, the nearest-neighbor interatomic distance increases and the local atomic packing density decreases with decreasing temperature. The X-ray absorption near edge structure (XANES) measurement shows that the atomic rearrangement at the undercooled temperature results in the formation of low-coordinated polyhedron clusters with high symmetries. This finding provides a new perspective for understanding the liquid-solid transition in undercooled conditions.

Abnormal resistivity and viscosity behavior in Sb-rich Pb–Sb melts

Abstract Electrical resistivity and viscosity of Pb–Sb alloys are measured to investigate Peierls distortion behavior in the melts. In Pb 30 Sb 70 , Pb 20 Sb 80 , and Pb 10 Sb 90 melts, temperature dependence of resistivity deviates from linear dependence during cooling. At 663 °C, different trends in isothermal behavior between experimental and theoretical resistivities, are interpreted as the existence of Peierls distortion in Sb-rich melts. In Pb 30 Sb 70 and Pb 20 Sb 80 melts, abnormal viscosity results verify the existence of abnormal structure transition, which is attributed to the formation of large Sb clusters with Peierls distortion. In undercooled liquid Pb 20 Sb 80 , minute resistivity coefficient and quadratic resistivity behavior are interpreted as the rapid increase of cluster size of Sb clusters with Peierls distortion, which provides preferential nucleation sites for higher structure similarity to the crystalline and lower liquid-solid interfacial energy.

Structure of liquid Cu–Sb alloys by ab initio molecular dynamics simulations, high temperature X-ray diffraction, and resistivity

Structure of Cu–Sb melts has been studied by ab initio molecular dynamics simulations, high-temperature X-ray diffraction and resistivity measurements. Over the whole concentration range, heterogeneous coordination numbers are larger than that of homogeneous atoms. This indicates preferential Cu–Sb coordination in Cu–Sb melts. A drop is observed in maximum position of simulated Sb–Sb partial distribution functions around Cu75Sb25, which reveals the rapid increase of Sb–Sb coordination. Around eutectic melts, main peak splitting is observed in both structure factor and simulated total pair distribution functions, which reveals the co-existence of Cu–Sb heterogeneous and Sb–Sb clusters. Abnormal changes in temperature coefficient of resistivity are observed around pure Sb and in compound-forming range, which are well interpreted as reinforcement of Peierls distortion and Cu3Sb compound clusters, respectively. Structural inhomogeneity that results from atomic size effect also has been discussed by analyzing concentration dependence of Warren–Cowley parameters and concentration correlation functions.

Growth of single crystalline boron nanotubes in a Cu alloy

Herein, we report the successful synthesis of single crystalline boron nanotubes in a Cu alloy via a novel and simple direct melt reaction process. The boron nanotubes are perfect single crystals with a β-rhombohedral structure, whose size can be controlled and changed. The combined experimental and dynamic simulation results lead to the conclusion that the boron nanotubes in the Cu alloy can be considered as a crystal skeleton, closely related to the liquid structure and solidification characteristics of the Cu–B alloy, which is significantly affected by the cooling rate and boron concentration. In addition, the stable morphology of the boron nanotubes, which minimizes the energy content, is not a round tube but a parallelogram. Thus, boron tubes with large size range scale can be prepared in Cu alloys.