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The origin of negative thermal expansion (NTE) in the bond frustrated ZnCr2Se4 has been explored. ESR and FTIR document an ideal paramagnetic state above 100 K, below which ferromagnetic clusters coexist with the paramagnetic state down to TN. By fitting the inverse susceptibility above 100K using a modified paramagnetic Curie-Weiss law, an exponentially changeable exchange integral J is deduced. In the case of the variable J, magnetic exchange and lattice elastic energy couple with each other effectively via magnetoelastic interaction in the ferromagnetic clusters, where NTE occurs at a loss of exchange energy while a gain of lattice elastic one. [ABSTRACT FROM AUTHOR]

The electronic properties of epitaxial graphene (EG) on SiC (0001) depend sensitively on the surface morphology of SiC substrate. Here, 2-3 layers of graphene were grown on on-axis 6H-SiC with different step densities realized through controlling growth temperature and ambient pressure. We show that epitaxial graphene on SiC (0001) with low step density and straight step edge possesses fewer point defects laying mostly on step edges and higher carrier mobility. A relationship between step density and EG mobility is established. The linear scan of Raman spectra combined with the atomic force microscopy morphology images revealed that the Raman fingerprint peaks are nearly the same on terraces, but shift significantly while cross step edges, suggesting the graphene is not homogeneous in strain and carrier concentration over terraces and step edges of substrates. Thus, control morphology of epitaxial graphene on SiC (0001) is a simple and effective method to pursue optimal route for high quality graphene and will be helpful to prepare wafer sized graphene for device applications. [ABSTRACT FROM AUTHOR]

Fly ash is a coal-fired power plant waste. Its main chemical ingredient is aluminum silicate and its exterior appearance is mainly agraphitic glass phase sphere with loose structure and different size, characteristic of a wide source and low cost. In hydrothermal conditions, Holler and Wirsching took the lead to use fly ash and alkaline (NaOH or KOH) in the synthesis of zeolite. Aimed at actual demand of improving the absorption water profile of flooding well, in regards to the blockage of reservoir pore, the article explores the ingredient constitution and technological conditions for part transformation of fly ash into the zeolite under the conditions of hydrothermal condition. The chemical composition and the structural feature of the exterior appearance of the modified fly ash are studied, as are the blocking effect and profile control mechanism, all of which have important application value to improve the blocking effect and increasing oil recovery. [ABSTRACT FROM AUTHOR]

Metal Mn was introduced into Li 3N flux to grow GaN single crystals from Ga melt. Colorless, transparent GaN single crystals with an average size of 2−3 mm were obtained at 800 °C under N 2pressure of about 2 atm. The effects of Mn on the quality, size distribution, morphologies, and formation mechanism of GaN crystals were investigated. It was found that the proper addition of Mn favored GaN growth, impeded the homogeneous nucleation to some extent, and resulted in a more uniform size distribution and high quality of GaN crystals as compared with that of GaN grown using Li 3N as flux only. The formation mechanism was enhanced by Mn addition. These results suggested a promising new route for the growth of large size and high quality GaN single crystals from a Li−Ga−N system in the future by optimizing the molar ratio of Mn to Li 3N. [ABSTRACT FROM AUTHOR]

We have developed a modified solid-state metathesis method by selecting metal oxides and Li3N as precursors to prepare nitrides in sealed ampoules or tube furnace, which provide a general, facile, and high-efficient route to most binary metal nitrides including III-nitrides, transition metal nitrides, rare-earth nitrides, and some ternary metal nitrides. A two-step process is involved in this route and thermodynamical favored byproduct Li2O serves as a potent driving force, enhancing the nitrides formation process. Our results show that this route offers an attractive preparative route to nitrides due to the wide availability of metal oxides, high yield, and relatively simple operation. [ABSTRACT FROM AUTHOR]