First-Principles Investigation of Mechanical and Thermodynamic Properties of Nickel Silicides at Finite Temperature

First-principles calculations are performed to investigate lattice parameters, elastic constants and 3D directional Young’s modulus E of nickel silicides (i.e., β-Ni₃Si, δ-Ni₂Si, θ-Ni₂Si, ε-NiSi, and θ-Ni₂Si), and thermodynamic properties, such as the Debye temperature, heat capacity, volumetric thermal expansion coefficient, at finite temperature are also explored in combination with the quasi-harmonic Debye model. The calculated results are in a good agreement with available experimental and theoretical values. The five compounds demonstrate elastic anisotropy. The dependence on the direction of stiffness is the greatest for δ-Ni₂Si and θ-Ni₂Si, when the stress is applied, while that for β-Ni₃Si is minimal. The bulk modulus B reduces with increasing temperature, implying that the resistance to volume deformation will weaken with temperature, and the capacity gradually descend for the compound sequence of β-Ni₃Si > δ-Ni₂Si > θ-Ni₂Si > ε-NiSi > θ-Ni₂Si. The temperature dependence of the Debye temperature ΘD is related to the change of lattice parameters, and ΘD gradually decreases for the compound sequence of ε-NiSi > β-Ni₃Si > δ-Ni₂Si > θ-Ni₂Si > θ-Ni₂Si. The volumetric thermal expansion coefficient αV, isochoric heat capacity and isobaric heat capacity C _p of nickel silicides are proportional to T³ at low temperature, subsequently, αV and C _p show modest linear change at high temperature, whereas C_v obeys the Dulong-Petit limit. In addition, β-Ni₃Si has the largest capability to store or release heat at high temperature. From the perspective of solid state physics, the thermodynamic properties at finite temperature can be used to guide further experimental works and design of novel nickel–silicon alloys.

     

Atomistic simulation of the coupled adsorption and unfolding of protein GB1 on the polystyrenes nanoparticle surface

Understanding the processes of protein adsorption/desorption on nanoparticles’ surfaces is important for the development of new nanotechnology involving biomaterials; however, an atomistic resolution picture for these processes and for the simultaneous protein conformational change is missing. Here, we report the adsorption of protein GB1 on a polystyrene nanoparticle surface using atomistic molecular dynamic simulations. Enabled by metadynamics, we explored the relevant phase space and identified three protein states, each involving both the adsorbed and desorbed modes. We also studied the change of the secondary and tertiary structures of GB1 during adsorption and the dominant interactions between the protein and surface in different adsorption stages. The results we obtained from simulation were found to be more adequate and complete than the previous one. We believe the model presented in this paper, in comparison with the previous ones, is a better theoretical model to understand and explain the experimental results.     

Stable High-Energy Density Super-Atom Clusters of Aluminum Hydride

With the concept of super-atom, first principles calculations propose a new type of super stable cage clusters Al_nH_3n that are much more energetic stable than the well established clusters, Al_nH_n+2. In the new clusters, the aluminum core-frame acts as a super-atom with n vertexes and 2n Al-Al edges, which allow to adsorb n hydrogen atoms at the top-site and 2n at the bridge-site. Using Al₁₂H₃₆ as the basic unit, stable chain structures, (Al₁₂H₃₆)m, have been constructed following the same connection mechanism as for (AlH₃)_n linear polymeric structures. Apart from high hydrogen percentage per molecule, calculations have shown that these new clusters possess large heat of formation values and their combustion heat is about 4.8 times of the methane, making them a promising high energy density material.     

Molecularly Imprinted Multi‐Layer Core‐Shell Nanoparticles – A Surface Grafting Approach

Surface initiated living‐radical polymerization (SIP) based on dithiocarbamate iniferters has been used to create molecularly imprinted core‐shell (CS) nanoparticles. Using this approach, propranolol, morphine and naproxen have been successfully imprinted in particle shells (the latter could not be imprinted using conventional aqueous‐based CS methods). Rebinding properties of the imprinted particles appear to be similar to those made by alternative methods. The living radical initiation mechanism makes it possible to build complex multi‐layer particles sequentially. As a demonstration, multi‐layer propranolol‐imprinted particles were generated. Two additional functional shells were grown over the imprinted shell, while the propranolol binding was retained, albeit at a reduced level.

     

ZnO/Ag球形异质结复合材料的制备及其吸光性能研究

采用水热法制备出球形ZnO颗粒,用微波辅助多元醇法对其表面进行修饰后得到ZnO/Ag异质结复合材料.利用X射线衍射仪、扫描电子显微镜、透射电子显微镜对样品的结构和形貌进行表征,用紫外-可见光谱分析了样品的吸光性能.结果显示:所制备的ZnO/Ag异质结是由面心立方的Ag纳米颗粒附着在纤锌矿结构的ZnO球表面形成的;与ZnO相比,ZnO/Ag异质结的紫外可见光吸收光谱发生明显红移,在紫外和可见光范围均有较强的吸收.     

Influence of citric acid on calcium sulfate dihydrate crystallization in aqueous media

The crystallization of Calcium sulfate dihydrate produced by the reaction between pure Ca(OH)₂ suspension and H₂SO₄ solution was investigated at different pH values, temperatures and citric acid concentrations. Crystal size distributions, filtration rates and zeta potentials of gypsum were determined as a function of citric acid concentrations at pH 3.5 and 65°C. The influence of citric acid on the morphology of gypsum was also investigated and discussed. The average particle size of gypsum was reached to maximum in the presence of approximately 2500 ppm citric acid concentration, where the minimum cake resistance and maximum filtration rate were obtained. In the presence of citric acid, various crystal morphologies such as tabular, plate‐like, double‐taper leaf‐like and flower‐like, etc., were obtained. The change of morphology is related to the preferential adsorption of citric acid on different crystallographic faces. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SiC crystal growth from transition metal silicide fluxes

SiC crystal growth in transition metal silicide melts was investigated by using spontaneous infiltration and solution methods. In the infiltration experiments, SiC powder preforms were infiltrated with Fe_xSi_y (Fe₃Si, Fe₅Si₃ and FeSi) and CoSi melts. The dissolution and precipitation of SiC led to SiC crystals growth in the infiltrated Fe₅Si₃ and CoSi melts, SiC particles coalescing in FeSi and free carbon precipitation in Fe₃Si. In the solution experiments, carbon from the graphite crucible dissolved in and reacted with FeSi₂ and Ti_2.3Si_7.7 to form SiC crystals. Scanning electron microscopy (SEM), X‐ray diffraction (XRD) and Raman scattering spectrometer were employed to investigate SiC crystals growth. Based on the investigation, the effect of solution content on the SiC crystal growth, the growth mechanisms in both methods and prototypes of the SiC crystals are also discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical study of the reaction mechanism and solvent effect on the regioselectivity of 1,3‐dipolar cycloaddition reaction between azide and acetylene derivatives

The reaction mechanism and solvent‐dependant regioselectivity of 1,3‐dipolar cycloaddition reactions between azide and acetylene derivatives have been studied using computational methods. The two possible reaction transition states were located. Geometry and NBO analysis found that the reactions take place along a synchronous and concerted mechanism for TS1 and an asynchronous and less concerted mechanism for TS2 . SCRF analysis found that TS2 is more sensitive to the polarity of solvent. In less polar solvent such as CCl₄, the difference of activation barriers of the two transition states is small. However, when the reactions were conducted in water, the activation barriers for TS2 increase which leads to the observed regioselectivity. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:203–207, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20236     

Effect of borane source on the enantioselectivity in the enantiopure oxazaborolidine‐catalyzed asymmetric borane reduction of ketones

The effect of borane source on enantioselectivity in the enantiopure oxazaborolidine‐catalyzed asymmetric borane reduction of ketones has been investigated by using (S)‐3,1,2‐oxazaborobicyclo[3.3.0]octane and (S)‐7,3,1,2‐thiaxazaborobicyclo[3.3.0]octane as catalysts. The results indicate that the enantioselective order of different borane sources is borane–dimethyl sulfide < borane–N,N‐diethylaniline < borane–THF for the asymmetric reduction of a ketone under the same conditions. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:740–746, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20370     

Intermediate products in solid‐state reactions of gadolinium oxide and ammonium dihydrogen phosphate

The solid‐state reactions of Gd₂O₃ with NH₄H₂PO₄ using various molar ratios (3:6, 2:8, 1:6 and 1:8) of the reactants were investigated at different temperatures (350‐1100°C) to observe the intermediate compounds. Analysis of the X‐ray powder diffraction patterns of the products obtained at 350°C; for the compositions of 2:8, 1:6 and 1:8 molar ratios revealed the formation of a new compound, NH₄GdHP₃O₁₀ which was reported previously for Nd, Ho and Er. At higher temperatures, gadolinium tripolyphosphate was obtained for all of the molar ratios. On the other hand, the IR data of the products also confirmed the proposed reaction pathways. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)