Junhai Xia

From clusters to phase diagrams: composition rules of quasicrystals and bulk metallic glasses

Metallic elements having negative enthalpies of mixing tend to form characteristic local atomic clusters. In this review, we use the structural information in the first nearest neighbour shell level, or first-shell atomic cluster, to derive the composition rules of two types of complex alloy phases, quasicrystals and bulk metallic glasses, both being composed of elements with negative enthalpies of mixing. We first show the composition phenomena in quasicrystal-forming systems, where major composition rules such as cluster line, electron concentration and atomic size criteria are derived. Then we analyse the composition rules of bulk metallic glasses using the very same approaches. Finally, we summarize their common composition rules into more general rules and basic theories.

Ternary bulk metallic glasses formed by minor alloying of Cu8Zr5 icosahedron

Icosahedron Cu8Zr5, derived from the Cu8Zr3 phase structure, corresponds to the deepest eutectic Cu0.618Zr0.382. Near this composition is the best glass-forming Cu–Zr composition, Cu0.64Zr0.36. (Cu0.64Zr0.36)1−xMx and (Cu0.618Zr0.382)1−xMx were examined for glass formation, where M represents Nb, Sn, Mo, Si, V, Ag, or Ta. A series of Cu–Zr based bulk metallic glasses are discovered with minor Nb, Sn, Mo, Ag, and Ta additions (lower than 3at.%). The minor alloying mechanism was discussed in the light of atomic size and electron concentration factors.

Design of Cu8Zr5-based bulk metallic glasses

Basic polyhedral clusters have been derived from intermetallic compounds at near-eutectic composition by considering a dense packing and random arrangement of atoms at shell sites. Using such building units, bulk metallic glasses can be formed. This strategy was verified in the Cu–Zr binary system, where we have demonstrated the existence of Cu8Zr5 icosahedral clusters in Cu61.8Zr38.2, Cu64Zr36, and Cu64.5Zr35.5 amorphous alloys. Furthermore, ternary bulk metallic glasses can be developed by doping the basic Cu–Zr alloy with a minority element. This hypothesis was confirmed in systems (Cu0.618Zr0.382)100−xNbx, where x=1.5 and 2.5at.%, and (Cu0.618Zr0.382)98Sn2. The present results may open a route to prepare amorphous alloys with improved glass forming ability.

Investigation of correlation between the microstructure and electrical properties of sol-gel derived ZnO based thin films

Pure ZnO and aluminum doped ZnO films (ZAO) were prepared by sol-gel method and the effect of Al doping on the microstructure and electrical properties of the films was investigated. The results showed that the transformation from granular to columnar structure could be observed in pure ZnO films with the increase in heating time while in aluminum doped films little structural changes occurred even after a prolonged heating time. Additionally, measurements of electrical properties showed that both microstructural evolution and doping could significantly improve the conductivity of the films, which could be assigned to an increase both in Hall mobility and carrier concentration. The relationship between microstructure and the electrical properties of the films was discussed, and various scattering mechanisms were proposed for sol-gel derived ZnO and ZAO films as a function of the carrier concentration.

Formation and Optimization of Cu-Based Cu-Zr-Al Bulk Metallic Glasses

The present paper covers in a comprehensive manner the formation of the Cu-based Cu-Zr-Al bulk metallic glasses (BMGs). Composition optimization for BMG formation is realized by using an “e/a-variant criterion”. This criterion is incorporated into the ternary phase diagram by a straight composition line, which is defined by linking a specific binary composition to the third element. There e/a-variant composition lines are constructed: (Cu9Zr4)1-xAl, (Cu61.8Zr38.2)1-xAlx and (Cu56Zr44)1-xAlx, where Cu9Zr4, Cu61.8Zr38.2 and Cu56Zr44 are specific cluster compositions in the Cu-rich Cu-Zr binary system. No Cu-based BMGs are obtained in the composition line (Cu9Zr4)1-xAlx using our suction casting techniques, while BMGs are obtained within an e/a span from 1.24 to 1.3 and from 1.28 to 1.36 respectively along the other two lines (Cu61.8Zr38.2)1-xAlx and (Cu56Zr44)1-xAlx. Thermal analysis results indicate that the BMGs on every composition line manifest increased thermal stability and glass forming ability (GFA) with increasing e/a ratios. The maximum appears in Cu58.1Zr35.9Al6 with the e/a value of 1.3, which belongs to the (Cu61.8Zr38.2)1-xAlx series. The characteristic thermal parameters of this BMG are Tg = 760K, Tg/Tm = 0.659 and Tg/Tl = 0.648, which are all superior to those reported for the known Cu55Zr40Al5 BMG.

Magnetism of amorphous carbon nanofibers

Amorphous carbon nanofibers (ACNFs) have been synthesized by a thermal chemical vapor deposition technique. The ACNFs grow as two branches perpendicular to {111} facets of a catalytic copper nanoparticle. The carbon nanofibers are composed of disordered localized nanofragments which in turn consist of several graphene layers. The ACNFs show a paramagnetic characteristics at 2, 5, and 10 K. The magnetic moments are suggested to originate from a large amount of defects in the graphene layers of the nanofragments.

Hexahedral nanocementites catalyzing the growth of carbon nanohelices

The formation mechanism of carbon nanohelices grown on iron needles by microwave plasma assisted chemical vapor deposition is studied by means of transmission electron microscopy and scanning electron microscopy. The catalyst assisting the helix formation is identified to be the single crystalline cementite (Fe3C) particle at the tip of each carbon nanohelix. The Fe3C particles show the general morphology of a hexahedron with six (different) crystallographic planes as the surface planes. The different catalytic effect of different crystallographic surface planes produces an anisotropic growth on the front surface of the carbon nanostructure, which results in a rotation of the cementite particles. The rotating particles catalyze the growth of the carbon nanostructure in a helical way.

Confined Au-Pd Ensembles in Mesoporous TiO2 Spheres for the Photocatalytic Oxidation of Acetaldehyde

Titanium dioxide (TiO2) is the most widely used photocatalyst for environmental remediation and solar-fuel production, owing to its low cost, nontoxicity, and abundance. In TiO2based photocatalysis, the photogenerated electrons and holes separate and migrate to the surface to be involved in the surface redox reactions. However, a high recombination rate of the photogenerated charge carriers leads to low photocatalytic efficiency. Various noble metals have been introduced to trap photogenerated electrons across the interface and, thus, improve the charge separation within the TiO2 semiconductor. [2]

The size effect of catalyst on the growth of helical carbon nanofibers

Cu-catalyzed carbon nanofibers are investigated by means of transmission electron microscopy. Straight and helical carbon nanofibers are observed to connect to the catalyst particles of octahedron or triangular prism in the samples prepared using the same processing conditions. Statistic analysis of the results leads to evidence that the morphology of the nanofibers depends on the size of the catalyst particles. Small size of catalyst particles favors formation of the helical fibers, while large size of catalysts results in the straight fibers. Based on the observed results, growth, and morphology formation of the carbon nanofibers are discussed. The growth model in which the rotating catalysts catalyze the growth of the carbon nanostructure in a helical way is proposed.

Effects of ion bombardment on the morphology and microstructure of carbon nanomaterials grown by microwave plasma chemical vapor deposition

Effects of ion bombardment on the growth of carbon nanomaterials on Fe substrates in microwave plasma chemical vapor deposition were investigated. The product morphologies are directly correlated with the kinetic energy of bombarding ions and range from nanosheets, through cones, to aligned nanotubes for bias voltages from 0to−250V, respectively. A change in growth mechanism from a base growth to tip growth induced by the competition between etching and growth is responsible for the observed changes. These findings bring insights into the growth mechanism and provide a strong tool to control the structure of carbon nanomaterials.