Icosahedron Clusters Research Articles

Simulation study of effects of cooling rate on microstructure of liquid metal Na during solidification processes

A tracing simulation study has been performed for the solidification process of liquid metal Ni with four different cooling rates by means of molecular dynamics method. The pair distribution function g(r) curves, the bond-type index method of Honeycutt-Andersen (HA) and the cluster-type index method have been used to analyze the variations of microstructures during the solidification processes. The results show that the cooling rate plays a critical role in the transitions of microstructures. When the cooling rates are 1.0×1014 K/s and 1.0×1013 K/s, the amorphous structures are formed mainly with the 1551 and 1541 bond-types or polyhedron basic cluster (13 １ 10 2) and icosahedron cluster (12 0 12 0) in the system. When the cooling rates are 1.0×1012 K/s and 1.0×1011 K/s, the crystal structures are formed mainly with 1441 and 1661 bond-types or the bcc basic cluster (14 6 0 8) in the system. At the same time, it has been found that the effects of different cooing rates on the microstructures of metal Na are very small in liquid and supercooled states, however, they are very remarkable in solid (amorphous and crystal) states, and can be fully displayed only near the liquid-solid transition points,i.e., the glass transition temperature Tg and the crystallization temperature Tc, respectively. Accordingly,it possibly provides a new method to determine the Tg and Tc of liquid metals.The cluster-type index method would be more favorable than the bond-type index method for investigating the concrete structural characteristics of the disordering liquid, the amorphous and some crystallized systems.

Hume-Rothery controlled formation of structurally complex alloy phases in the ternary Ga–Mg–Pd system

The Hume-Rothery rule has been used as a guide to search for new approximants of the Mackay icosahedron and Bergman cluster type in the Ga–Mg–Pd system. Ternary alloys with target compositions close to the e/ a ratios 1.67 and 2.09 were synthesized by melting the elements and subsequent annealing in sealed tantalum ampoules. Six new intermetallic phases have been characterized: a cubic face-centered Mackay icosahedron approximant Ga 15− x− y Mg 38+ y Pd 13+ x , a rhombohedral Mackay icosahedron approximant Ga 30− x Mg 32+ x Pd 21, a cubic Bergman cluster approximant Ga 12+ x+ y Mg 11− y Pd 6− x , a cubic phase with interpenetrating I3 and Frank–Kasper networks Ga 32.6Mg 43.4Pd 29, an ordered ternary variant of the NiTi 2 structure type GaMg 3Pd 2 and a ternary Laves phase. The e/ a ratios for the approximants with Mackay icosahedra or Bergman clusters show a strong experimental correlation indicating that the valence electron concentration is an important factor concerning the stability of the phases. The approximant phases exist over a wide range of composition. The structure of the rhombohedral phase is based on the geometry of the canonical cell tiling CB 2C. The results support the proposal by Henley and Mihalkovic on the geometrical structures of Mackay icosahedron-type quasicrystals in ternary systems with one transition and two main group metals.

Structural simulation of clusters in liquid Ni50Al50 alloys

The transition of Ni50Al50 from liquid to amorphous and/or to crystal has been investigated by molecular dynamics simulation based upon tight binding method. The structure parameter such as pair correlation function and pair analysis are adopted to explore the evolution process of clusters in liquid Ni50Al50 alloy at different cooling rate. By visualizing the coordinates of atoms acquired from the simulation, cluster structures can clearly be observed. During the relatively fast cooling, 1551 and 1541 bonded pairs representing the non-crystalline structure and are predominant, while 1431, 1661 and other bonded pairs coexist in the non-crystalline system. Most atoms form into icosahedron clusters and defect icosahedron clusters out of 1551 bonded pairs. The network of icosahedron clusters and defect icosahedron clusters can be seen in the amorphous system. During the relatively slow cooling, the number of 1421 bonded pairs representing fcc-like structure stands out while 1422, 1551 and other bonded pairs coexist in the crystal system. In the latter system, a regular fcc-like structure is formed. It shows the relativity and variation of microstructure between liquid and its solid state.

Cooling rate dependence of structural properties of aluminium duringrapid solidification

Constant-pressure molecular dynamics simulations and a systematic analysis ofthe local atomic structures have been performed to study the structuralevolution of aluminium on different cooling runs. The regular and defectiveicosahedral atomic configurations depend strongly on the cooling rate, whichis responsible for the cooling rate dependence of the enthalpy. In thesimulated amorphous aluminium there exist three kinds of microstructure unitthat do not depend on the cooling rate. Two of them are similar to those inthe fcc crystal containing interstitialcy and hardly change with decreasingtemperature after the glass formation. The third can be considered as crystalgerms. Our results also suggest that there exists a critical cooling ratebelow which the icosahedra form a percolating cluster and the glass exhibitshigh stability. At a certain quenching rate that is slower than this criticalcooling rate, the strength of the icosahedron clusters infinite network mayhave a saturation value, i.e. a maximum. This may be the origin of theexistence of an ideal quenching rate at which the glass exhibits the higheststructural stability.

Stoichiometry and phase behavior of carbon tetrachloride solvates of C 60

Two solvated crystals of C 60 were formed from CCl 4 solutions. It was found that the cubic solvated crystal is composed of a C 60(CCl 4) 13 formula unit while the hexagonal crystal has a C 60(CCl 4) 2 unit. These solvates have phase behavior comparable with that of cyclohexane solvates of C 60, being closely related to binary sphere mixtures with a characteristic diameter ratio of 0 .58. The present calorimetric study strongly suggests that the C 60(CCl 4) 13 crystal has an AB 13-type structure, in which icosahedron clusters of CCl 4 like those in the cyclohexane analogue are formed.

The structure of deposited metal clusters generated by laser evaporation

Metal clusters have been produced using a laser evaporation source. A Nd-YAG laser beam focused onto a solid silver rod was used to evaporate the material, which was then cooled to form clusters with the help of a pulsed high pressure He beam. TOF mass spectra of these clusters reveal a strong occurrence of small and medium sized clusters (n<100). Clusters were also deposited onto grid supported thin layers of carbon-films which were investigated by transmission electron microscopy. Very high resolution pictures of these grids were used to analyze the size distribution and the structure of the deposited clusters. The diffraction pattern caused by crystalline structure of the clusters reveals 3-and 5-fold symmetries as well asfcc bulk structure. This can be explained in terms of icosahedron and cuboctahedron type clusters deposited on the surface of the carbon layer. There is strong evidence that part of these cluster geometries had already been formed before the depostion process. The non-linear dependence of the cluster size and the cluster density on the generating conditions is discussed. Therefore the samples were observed in HREM in the stable DEEKO 100 microscope of the Fritz-Haber-Institut operating at 100 KV with the spherical aberrationcS=0.5 mm. The quality of the pictures was improved by using the conditions of minimum phase contrast hollow cone illumination. This procedure led to a minimum of phase contrast artefacts. Among the well-crystallized particles were a great amount of five- and three-fold symmetries, icosahedra and cuboctahedra respectively. The largest clusters with five- and three-fold symmetries have been found with diameters of 7 nm; the smallest particles displaying the same undistorted symmetries were of about 2 mm. Even smaller ones with strong distortions could be observed although their classification is difficult. The quality of the images was improved by applying Fourier filtering techniques.

The Properties of Deposited Metal Clusters generated by Laser Evaporation

Metal clusters have been produced using a laser evaporation source. A Nd-YAG laser beam focused onto a solid silver rod was used to evaporate the material, which was then cooled to form clusters with the help of a pulsed high pressure He beam. TOF mass spectra of these clusters reveal a strong occurrence of small and medium sized clusters (n less than 100). Clusters were also deposited onto grid supported thin layers of carbon-films which were investigated by transmission electron microscopy. Very high resolution pictures of these grids were used to analyze the size distribution and the structure of the deposited clusters. The diffraction pattern caused by crystalline structure of the clusters reveal 3- and 5-fold symmetries besides the pattern reflecting the fcc bulk structure. This can be explained in terms of icosahedron and cuboctahedron type clusters deposited on the surface of the carbon layer. There is strong evidence that part of these cluster geometries had already been formed before the deposition process. The non-linear dependence of the cluster size and the cluster density on the generating conditions is discussed.