Zn-Mg合金的长程Finnis-Sinclair势

通过拟合Mg的晶格能、晶格常数、弹性常数,并将其与前人的结果相比较后获得了描述Mg的最优长程Finnis-Sinclair(F-S)势函数参数,使用同样方法并引入修正因子后得到了Zn的长程F-S势参数.基于单质Zn,Mg的F-S势参数,进一步拟合合金Mg₂₁Zn₂₅,MgZn₂,Mg₂Zn₁₁的晶格常数、晶格能获得Zn-Mg原子对的F-S势参数,构建了整套描述Zn-Mg合金的长程F-S势参数.在此 关键词： 长程Finnis-Sinclair势 Zn-Mg合金 分子动力学模拟     

63Cu NMR analysis of microstructure evolution in Al–Cu–Mg alloys

⁶³Cu NMR spectroscopy has been used to detect metastable Guinier–Preston–Bagaryatsky (GPB) zones and nanoscale precipitates of equilibrium S-phase (Al₂CuMg) in dilute alloys of aluminium containing copper and magnesium with compositions which lie in the α?+?S phase field. The GPB zones are observed to form rapidly at room temperature with a time development closely related to the Vickers hardness. The final development of S-phase in the alloy has been confirmed by the observation of a line shape in the alloy identical to that observed in a specimen prepared from stoichiometric Al₂CuMg. Analysis of the hyperfine structure of the ⁶³Cu line shape observed for S-phase shows clearly that two Cu sites are present with approximately equal population. This result suggests that possibly two crystallographically distinct Al₂CuMg phases are present. The addition of small amounts of silver to Al–Cu–Mg alloys in the α?+?θ phase field is known to induce the formation of Ω-phase: a slight distortion of tetragonal θ-phase Al₂Cu. A hyperfine-structured ⁶³Cu line shape assigned to Ω-phase, indicating one distinct Cu site, has been observed in two separate Al–1.7?at.%?Cu–0.33?at.%?Mg alloys containing 0.1 and 0.18?at.%?Ag, but not in the same Al–Cu–Mg alloy without Ag.     

In‐situ X‐ray diffraction of mechanically milled β‐Al3Mg2 powders

The structure evolution during heating of mechanically milled single‐phase β‐Al₃Mg₂ has been investigated by in‐situ X‐ray diffraction. The nanoscale supersaturated Al(Mg) solid solution formed during milling transforms back to the original β‐Al₃Mg₂ phase through a sequence of phase transformations. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At intermediate temperatures, the β′‐phase, a hexagonal phase with approximate composition Al₃Mg₂, forms. Finally, at higher temperatures the original β‐Al₃Mg₂ phase is restored, indicating that the formation of the supersaturated solid solution during milling can be reversed by appropriate heat treatment. The phase transformations during heating are gradual and the temperature ranges of stability of the different structure configurations are quite large, all exceeding 50 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Local composition and carrier concentration in Pb0.7Ge0.3Te and Pb0.5Ge0.5Te alloys from Te NMR and microscopy

Pb_0.7Ge_0.3Te and Pb_0.5Ge_0.5Te alloys, (i) quenched from 923 K or (ii) quenched and annealed at 573 K for 2 h, have been studied by ¹²⁵Te NMR, X-ray diffraction, electron and optical microscopy, as well as energy dispersive spectroscopy. Depending on the composition and thermal treatment history, ¹²⁵Te NMR spectra exhibit different resonance frequencies and spin-lattice relaxation times, which can be assigned to different phases in the alloy. Quenched and annealed Pb_0.7Ge_0.3Te alloys can be considered as solid solutions but are shown by NMR to have components with various carrier concentrations. Quenched and annealed Pb_0.5Ge_0.5Te alloys contain GeTe- and PbTe-based phases with different compositions and charge carrier concentrations. Based on the analysis of non-exponential ¹²⁵Te NMR spin-lattice relaxation, the fractions and carrier concentrations of the various phases have been estimated. Our data show that alloying of PbTe with Ge results in the formation of chemically and electronically inhomogeneous systems. ¹²⁵Te NMR can be used as an efficient probe to detect the local composition in equilibrium as well as non-equilibrium states, and to determine the local carrier concentrations in complex multiphase tellurides.     

Pb-Mg-Al合金腐蚀机理的电子理论研究

为了了解Pb-Mg-Al合金腐蚀的物理本质, 本文采用基于第一性原理的赝势平面波方法系统地计算了Pb-Mg-Al合金中各物相的结合能、费米能级和局域态密度等电子结构参数, 分析了合金的电化学腐蚀机理. 计算结果表明：Pb-Mg-Al合金中各主要组成物相稳定性大小关系为 Mg₁₇Al₁₂>Mg₂Pb>Mg;Mg,Mg₂Pb和Mg₁₇Al₁₂的费米能级存在E_f(Mg)>E_f(Mg₂Pb)>E_f(Mg₁₇Al₁₂)的关系, 说明Mg最容易失去电子, Mg₂Pb次之, Mg₁₇Al₁₂最难;局域态密度表明, 在同样的外界条件下, 体系中Mg相和Mg₂Pb相对于Mg₁₇Al₁₂均处于不稳定的状态, 容易失去电子, 即容易发生腐蚀. Pb-Mg-Al合金体系中不同物相的费米能级差构成了电化学腐蚀的电动势, 导致电子从费米能级高的Mg相和Mg₂Pb相流向费米能级低的Mg₁₇Al₁₂相, 使Pb-Mg-Al合金发生腐蚀.     

Hyperfine interaction parameters of the ScFe2 laves alloy with MgZn2 hexagonal structure

This paper reports on NMR measurement of hyperfine fields, enhancement factors, and quadrupole splitting for ⁴⁵Sc nuclei in the ScFe₂, Sc_0.95Fe_2.05, and ScFe_1.97Al_0.03 alloys at room temperature and 77 K. The NMR spectra are studied, and the hyperfine-field shifts at ⁴⁵Sc nucleus sites in the above alloys, caused by Fe substituting for Sc and Al, for Fe in the nearest Sc coordination shells, are determined.     

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

The substitution of the divalent cations Mg²⁺ and Zn²⁺ into the aluminophosphate (AlPO) framework of STA-2 has been studied using an “NMR crystallographic” approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and first-principles calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH⁻ or F⁻, and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined.For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg²⁺ for Al³⁺ and, where required, by hydroxide anions coordinated to the framework [27] Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations. The calculations also show that ³¹P chemical shifts depend not only on the topologically-distinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)₂OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the ³¹P isotropic chemical shift and the average 〈P–O–M〉 bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.     

Predictions of mechanical and thermodynamic properties of Mg17Al12 and Mg2Sn from first-principles calculations

Using first-principles calculations, we predict mechanical and thermodynamic properties of both Mg₁₇Al₁₂ and Mg₂Sn precipitates in Mg–Al–Sn alloys. The elastic properties including the polycrystalline bulk modulus, shear modulus, Young’s modulus, Lame’s coefficients and Poisson’s ratio of both Mg₁₇Al₁₂ and Mg₂Sn phases are determined with the Voigt–Reuss–Hill approximation. Our results of equilibrium lattice constants agree closely with previous experimental and other theoretical results. The ductility and brittleness of the two phases are characterized with the estimation from Cauchy pressure and the value of B/G. Mechanical anisotropy is characterized by the anisotropic factors and direction-dependent Young’s modulus. The higher Debye temperature of Mg₁₇Al₁₂ phase means that it has a higher thermal conductivity and strength of chemical bonding relative to Mg₂Sn. The anisotropic sound velocities also indicate the elastic anisotropies of both phase structures. Additionally, density of states and Mulliken population analysis are performed to reveal the bonding nature of both phases. The calculations associated with phonon properties indicate the dynamical stability of both phase structures. The temperature dependences of thermodynamic properties of the two phases are predicted via the quasi-harmonic approximation.     

Combined Rietveld refinement of BaMgAl10O17:Eu using X-ray and neutron powder diffraction data

The phosphor, BaMgAl₁₀O₁₇:Eu²⁺, showing a blue emission band at about 450 nm was prepared by a normal solid-state reaction using BaCO₃, Al₂O₃, MgO and Eu₂O₃ as starting materials with AlF₃ as a flux. The study of combined Rietveld refinement and photoluminescence spectra was carried out to determine the structural parameters, such as lattice constants, the valence state of Eu, the site preference of Mg and site fractions of Mg and Eu. The occupancies of Eu and Mg were 0.022 and 0.526, respectively. The valence state of Eu was the divalent state because there was only one broad line at about 450 nm in the photoluminescence spectrum. The site preference of Mg atoms was the tetrahedral site of Al atoms surrounded by oxygen atoms in the spinel block. Lattice parameters decreased due to the difference of two ionic radii, Eu²⁺(1.09 Å) and Ba²⁺(1.34 Å), compared with those of BaMgAl₁₀O₁₇.     

Photoluminescence in MgxSr1−xAl2O4:Eu, Ndand electron-vibrational interaction in the Eu 5d states

Green phosphor compositions Mg_xSr_1−xAl₂O₄:Eu, Nd (with x=0.05-0.25) were prepared by solid state reaction method. The effect of Mg substitution on photoluminescence characteristics was investigated. The photoluminescence show intense green emission for MgSrAl₂O₄:Eu²⁺, Nd³⁺ with long persistence. This green emission corresponds to transitions from 4f⁶5d¹ to 4f⁷ of Eu²⁺ ion. Comparative analysis of the excitation and emission spectra were used to evaluate the crystal field splitting of the 5d states of Eu²⁺ and the parameters of electron-vibrational interaction, such as Huang-Rhys factor, effective phonon energy, and zero-phonon line position.     

High-resolution transmission electron microscopy study on reversion of Al2CuMg precipitates in Al–Cu–Mg alloys under irradiation

Image deconvolution analyses showed that reversion of S-Al₂CuMg precipitates occurred in an Al–Cu–Mg alloy during high-resolution transmission electron microscopy observations. A fraction of Mg and Cu atoms in the precipitates diffused into Al matrix due to electron beam irradiation at 300 kV, resulting in structural/chemical reversion of the precipitates. The structural reversion of the S-Al₂CuMg precipitates is closely related with irradiation-induced displacement of atoms. The strong attraction between Cu and Mg atoms might assist the sub-threshold displacement of Cu atoms. One transitional structure is determined to be S′′-Al₁₀Cu₃Mg₃, a precursor of S-Al₂CuMg. Two other transitional structures, Al₃CuMg and Al₁₈Cu₅Mg₅ which have the same lattice parameters of a = c = 0.405 nm as that of S′′-Al₁₀Cu₃Mg₃, but different b values, are suggested.     

NMR study of the intermetallic Laves alloys LuFe2 and LuFe1.96Al0.04

NMR spectra of ⁵⁷Fe and ²⁷Al in LuFe₂ and LuFe_0.96Al_0.04 alloys were measured by the spin echo method at 4.2 K. The main contributions to the hyperfine fields on ⁵⁷Fe nuclei are estimated, along with the magnetic moments on the Fe and Lu atoms in LuFe₂. Fiz. Tverd. Tela (St. Petersburg) 41, 1142–1144 (July 1999)     

Electrical and optical activation studies of high dose Si-implanted Al0.18Ga0.82N

Both electrical and optical activation studies of Si-implanted Al_0.18Ga_0.82N have been made as a function of anneal time and anneal temperature to obtain maximum possible electrical activation efficiency. Silicon ions were implanted at 200 keV with doses of 5×10¹⁴ and 1×10¹⁵ cm⁻², and the samples were annealed from 1100 to 1250 ^°C for 5-25 min with a 500 Å thick AlN cap in a nitrogen environment. The electrical activation efficiency and Hall mobility increase with anneal time and anneal temperature. Nearly 100 and 95% electrical activation efficiencies were obtained for Si-implanted Al_0.18Ga_0.82N with doses of 5×10¹⁴ and 1×10¹⁵ cm⁻² and annealing at 1250 and 1200 ^°C for 25 min, respectively. The photoluminescence measurements show an excellent implantation damage recovery after annealing at these optimum anneal conditions, showing a strong near band emission. These optical results correlate well with the electrical results.     

Effects of Ca/Al ratio and extrusion process on Mg–Al–Ca alloys to produce a high toughness in-situ composite

The microstructures and tensile properties of Mg–Al₂Ca–Mg₂Ca in situ composites (Mg–17Al–8Ca, Mg–14Al–11Ca and Mg–12.5Al–12.5Ca) with different Ca/Al ratios have been studied in both as-cast and extruded conditions. The results indicated that by increasing Ca/Al ratio, new Mg₂Ca intermetallic introduces to the Al₂Ca phase in eutectic structure. Computer-aided cooling curve analysis confirmed the formation of these phases during solidification. Extrusion process not only altered the size of large bulk Al₂Ca intermetallic, but also changed the size and morphology of intermetallics in eutectic structure considerably. The results showed that with increasing Ca/Al ratio, tensile properties of cast composites changes slightly, but significant enhancement is observed after extrusion process. The strength and elongation values of Mg–12.5Al–12.5Ca (Ca/Al = 1) alloy improved from 166 MPa and 2% in as-cast condition to 465 MPa and 12% in hot-extruded condition. The reason for the improved toughness may be attributed to the formation of finer and well-dispersed distribution of hard (Al₂Ca) and ductile (Mg₂Ca) phases. It was found that hot extrusion easily deforms ductile Mg₂Ca phase in comparison with Al₂Ca phase. In as-extruded condition, there are more very fine dimples than as-casted condition because extrusion process leads to formation of fragmented tiny particles and more uniformity distribution of Al₂Ca particles.     

Luminescence characterisation of alumina substrates using cathodoluminescence microscopy and spectroscopy

Polycrystalline alumina (Al₂O₃) substrates, found in many electronic devices and proposed as dosemeters in emergency situations, were invstigated using a scanning electron microscope (SEM) equipped with cathodoluminescence (CL) and elemental analysis probes. The characteristics of the CL spectra, surface morphology, and impurity content of the Al₂O₃ substrates were examined and compared with those of single crystal dosimetry-grade Al₂O₃:C. Whereas the CL spectrum, measured from 250 to 800 nm, for the Al₂O₃:C, contained resolved bands located at ∼340 nm and at ∼410 nm, the spectrum measured with the Al₂O₃ substrate was significantly broader, extending from ∼250 to ∼450 nm, and also included a narrow band at 695 nm. While it is likely that the accepted model of recombination at F⁺ (∼340 nm) and F (∼410 nm) in Al₂O₃:C also applies to the substrate, it is suggested that the presence of impurities within the alumina give rise to additional recombination centres. The 695 nm emission has been assigned to a Cr³⁺ ion impurity in previous work on alumina and a band indicated at ∼300 nm may be associated with Mg²⁺ or Ca²⁺_, the presence of which was confirmed by elemental mapping. Comparison of the spatial distribution of CL with the surface morphology and elemental composition of the samples indicates that the components of the emission spectrum can be qualitatively correlated with impurity content and morphological features of the samples.     

Solvothermal synthesis and luminescence properties of the novel aluminum garnet phosphors for WLED applications

The color rendering index (CRI) and structural stability of cerium doped yttrium aluminum garnet (YAG:Ce) based phosphors have been enhanced by replacing Y³⁺ ions by larger radius ions (Tb³⁺, Gd³⁺, Eu³⁺, and Sm³⁺) at the dodecahedral site and replacing Al³⁺ ions by larger ones (Ga³⁺, Y³⁺, Tb³⁺, Gd³⁺, and Sm³⁺) at the octahedral site. These aluminum garnet crystalline powders were prepared by solvothermal reaction method at 300 °C for 48 h. The lattice constant values of synthetic aluminum garnet crystalline powders are larger than that of YAG and the emission wavelength of Ce³⁺ ion of these samples is longer than that of YAG:Ce. FESEM and TEM studies revealed that the Ln₃Ga₂Al₃O₁₂ and Ln₃Al₂Al₃O₁₂ crystalline powders have 3-dimensional star-like morphology with submicron size and good crystallinity, while, Ln₃(LnAl)Al₃O₁₂ garnet crystalline powders were cubic crystalline phases and shaped as cubes with the round edge having an approximate diameter of about 200–400 nm. All the prepared powders were grown along (100) direction and crystallized into single crystal. Also, the effects of treatment time and reaction temperature on the structure of aluminum garnet crystalline powders have been investigated.     

Electronic structure of Mg–Zn-based compounds

We present a comparison between calculated densities of states of the Laves phase MgZn₂ and hexagonal Mg₂₈Zn₆₅Y₇ and experimental results obtained using both X-ray emission and photoabsorption spectroscopy techniques. We show that there is general agreement between both sets of data for both alloys. We also point out that the two samples retain a metallic character, yet the hexagonal approximant of the Mg–Zn–Y quasicrystal family is less metallic than the Laves phase.     

Broad-beam laser cladding of Al-Cu alloy coating on AZ91HP magnesium alloy

The resistance to wear and corrosion of AZ91HP Mg alloy was improved by laser cladding Al-Cu alloy. It was found that the clad layer was characterized by AlCu₄ and Mg₁₇Al₁₂ grains embedded in a AlMg matrix. The bonding zone exhibited a white-light planar crystal band with thickness of 10-13 μm. The heat-affected zone formed a eutectic structure due to the Mg diffusion. The microhardness and wear resistance of the coating were improved due to the formation of the hard phases AlCu₄ and Mg₁₇Al₁₂. Owing to the formation of dense Al₂O₃ oxide film, the coating exhibited better corrosion resistance in 3.5 wt.% NaCl solution.