Structure And Properties Of Liquid And Amorphous Binary Alloys Simulated By Ab Initio Molecular Dynamics

Amorphous alloys are widely applied in advanced materials and catalysis field because of their special electronic and structural properties of short-range order and long-range disorder. For example, Ni-B, Ni-P, FeB and rapidly quenched skeletal Ni have superior catalytic performance to the industrilized Raney Ni catalyst in the hydrogenation of furfural, benzene and cyclopentadiene and have a promising potential application. In recent years, more and more literatures have focused on metal glass and amorphous solid. The alloys can be prepared by chemical reduction or rapid quenching methods, and experimentally characterized by neutron diffraction, XPS, XRD, and XAFS and so on. Theoretically molecular dynamics (MD) can be used to simulate rapid quenching process and analysis the structural and electronic properties of amorphous alloys. Our laboratory has performed a lot of research on the preparation, characterization and catalysis mechanism for a long time and studied the microcosmic structure by ab intio method and cluster model.MD can reproduce the macrocosmic properties of the systems exactly and store much microcosmic information, becoming a bridge between microcosmic and macrocosmic properties. Classical MD, which simplifies the interactions among atoms by potential function, is applied most often in simulation of a supercell containing tens of thousands of atoms and shows a high speed in calculation. However, new parameters must be reseted for a new system because the parameters in potential functions can not be used in any systems commonly. Moreover, classical MD can not handle the electronic structure so that new methods are required to conquer its shortcoming.Recently, ab intio molecular dynamics (AIMD) has been introduced along with the development of computer hardware and software. The simulation process of AIMD can be described as follows: firstly, total potential and total electronic energy of the systems simulated are calculated iteratively according to Kohn-Sham local density functional; secondly, the forces acting on every ions (or atoms) are obtained by Hellman-Feynman theorem; then a appropriate time step is chosen integrate numerically the position coordinate and obtain a new configuration for ions by Newton s equation of motion; the same processes as above are repeated for many and many times until the system arrive at dynamic equilibrium. Until now a NTV process for the simulated system is finished. Because of the emulate and fluctuation in the process we can calculate many thermodynamic and dynamic functions for liquid systems statistically. After that we can simulate the rapid quenching and let the system reach room temperature. Obviously AIMD is far more timeconsuming and is limited to simulate the system containing about 100 particles. However, AIMD is more reliable than classical MD because it takes electronic structure into account. Perfect agreement between experimental results and simulation results by AIMD are found in a lot of literatures. In the paper we employ AIMD and supercell model to simulate the rapid quenching process of liquid alloys and study their structure and properties. Reliability of results depends on the size of supercell and length of simulation time. In the paper we chose two small systems, namely Ni₂₅ Al₇₅ (64 particles) and Ni₆₄ B₃₆ (100particles), to test the reliability of supercell model and simulation by AIMD. Results indicate that our results by AIMD simulation are in good agreement with experimental results although there are only 64 (or 100) particles in the supercell in our simulation. It means AIMD is a reliable method to simulate liquid and amorphous systems. All the supercells employed in our paper contain more than 100 particles and the simulation time is about several picoseconds.At first we studied three Ni-Al systems (Ni₂₅ Al₇₅ , Ni₅₀ Al₅₀ and Ni₇₅ Al₂₅ ) by AIMD. Results indicate that Ni-Al amorphous alloys can be prepared by rapid quenching and their structure is similar to that of liquid alloys. In Ni-Al alloys the first peak position of Ni-Al pair correlation function is smaller than the sum of Ni and Al atomic radii, meaning a strong interaction between Ni and Al atoms and a chemical bonding with weak covalence. The point is clarified by electronic structure analysis which shows a strong interaction between d electrons of Ni and p electrons of Al. Reversely, interaction between the same kind of atoms is relatively weak. Specially, we obtain a small Ni-Ni coordination number in Ni₂₅ Al₇₅ alloy, meaning Ni atoms well dispersed in Al matrix. After Al removal many pores are formed around Ni. It explains the experimental fact that skeletal Ni has a large surface area and a high catalytic activity. Bond pair analysis indicates that in both liquid and amorphous Ni-Al alloys the tetrahedral local order is dominant. The fivefold symmetry local order is also present, but is obscured by the tetrahedral packing. The Fermi level of the liquid and amorphous Ni-Al alloys mainly consists of the d-electrons of Ni and the p-electrons of Al. The contribution of Ni d-electrons increases as the content of Ni increase in the Ni-Al alloys. We find a good agreement between our results and former ones in literatures, showing the reliability of our simulation method.Secondly, we studied liquid and amorphous Ni₈₁ B₁₉ and Ni₈₀ P₂₀ and made a comparison between their structural properties. AIMD simulation has revealed some common structural properties of Ni₈₁ B₁₉ and Ni₈₀ P₂₀ alloys: Amorphous Ni₈₁ B₁₉ and Ni₈₀ P₂₀ alloys can be prepared by rapid quenching method. They can preserve some structural properties of liquid alloy and have a more ordered structure. There is a strong interaction between the d-electrons of Ni and the p-electrons of B (or P) to form the valence band of the system, which leads to the strong covalent interactions between Ni and B (or P) atoms. There are also some important differences in their structural and electronic properties. Calculations indicate that Ni-Ni interaction in Ni₈₁ B₁₉ alloy is stronger than metallic bond in pure Ni. However, the Ni-Ni interaction in pure Ni is stronger than that in Ni₈₀ P₂₀ alloy. Direct B-B contact in Ni₈₁ B₁₉ alloys appears in our simulation, however no direct P-P contact in Ni₈₀ P₂₀ alloys is found. Bond pair analysis indicates that the local environment of an atom in liquid and amorphous Ni₈₁ B₁₉ alloys is dominated by icosahedral and distorted icosahedral inherent structures. However, in liquid and amorphous Ni₈₀ P₂₀ alloys tetrahedral local order is preponderant. We also discussed the reason that lead to the difference of microcosmic atomic arrangements between Ni₈₁ B₁₉ and Ni₈₀ P₂₀ amorphous alloy. Then we simulated Fe₈₀ B₂₀ and Fe₇₅ B₂₅ systems. Results indicate that Fe-B alloys have a similar structure with Ni-B alloys. For example, amorphous Fe-B can be prepared by rapid quenching method. They can preserve some structural properties of liquid alloy and have a more ordered structure. There is a strong interaction between the d-electrons of Fe and the p-electrons of B to form the valence band of the system. Amorphous Fe₈₀ B₂₀ alloy is dominated by icosahedral and distorted icosahedral inherent structures since 1551 and 1541 bond pairs are preponderant, whose sum amounts to more than 43% of the total number of all bond types. Interestingly, in liquid Fe₇₅ B₂₅ the situation is reversed. About 35% of 1441, 1431, 1421 and 1422 pairs are found, indicating that tetrahedral local order is preponderant. The proportion of 1551 and 1541 pairs decreases to 10%. This is because more B atoms destroy the fivefold symmetry structure in alloys with the increase of B contents.Bulk simulation is a starting point for simulating the surface structure and properties. In the last part of this paper we construct a supercell containing vacuum layer to simulate the rapid quenching process in the surface of Ni-B and Ni-P. Results indicate that metalloid B or P has a tendency of surface accumulation. We have just started an innovative research since there are few reports in this aspect.