Self-propagating high-temperature synthesis and formation of quasicrystals in Al/Mn bilayer thin films

Self-propagating high-temperature synthesis in Al/Mn bilayer thin films was investigated. It was found that a quasicrystalline phase forms after the passage of self-propagating high-temperature synthesis through the sample. It is shown that after multiple initiation (n>5) of self-propagating high-temperature synthesis in a sample the quasicrystalline phase transforms into a stable Al₆Mn phase. It is conjectured that self-propagating high-temperature synthesis can play the main role in the formation of quasicrystals in other film systems as well. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 121–124 (25 July 1998)     

Structural evolutions of the mechanically alloyed Al<Subscript>70</Subscript>Cu<Subscript>20</Subscript>Fe<Subscript>10</Subscript> powders

Elemental mixtures of Al, Cu, Fe powders with the nominal composition of Al₇₀Cu₂₀Fe₁₀ were mechanically alloyed in a planetary ball mill for 80 h. Subsequent annealing of the as-milled powders were performed at 600–800°C temperature range for 4 h. Structural characteristics of the mechanically alloyed Al₇₀Cu₂₀Fe₁₀ powders with the milling time and the heat treatment were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and differential thermal analysis (DTA). Mechanical alloying of the Al₇₀Cu₂₀Fe₁₀ did not result in the formation of icosahedral quasicrystalline phase (i-phase) and a long time milling resulted in the formation of β-Al(Cu,Fe) solid solution phase (β-phase). The i-phase was observed only for short-time milled powders after heat treatment above 600°C. The β-phase was one of the major phases in the Al₇₀Cu₂₀Fe₁₀ alloy. The w-Al₇Cu₂Fe₁ phase (w-phase) was obtained only after heat treatment of the short-time milled and unmilled samples. The present investigation indicated that a suitable technique to obtain a large amount of quasicrystalline powders is to use a combination of short-time milling and subsequent annealing.     

Formation and stability of Al-Mn quasicrystalline thin films prepared by high temperature successive deposition

This article reviews the results on the formation of i-QC thin films prepared by high-temperature successive deposition in the Al-Mn system and the metastable quasicrystalline Al₄Mn and crystalline Al₆Mn phase transformation in detail. Experimental evidence is given for the location of the solid-phase reaction. New data concerning the effects of preparation parameters (temperature, Mn deposition rate, composition) on the formation processes are reported. The phase formation and transformation processes are described by the combination of thermodynamic and kinetic factors.     

用EXAFS测定准晶Al-Mn合金中Mn原子的近邻结构

测定了晶态Al₆Mn,准晶Al₄Mn和准晶Al₆Mn的Mn原子K吸收EXAFS谱(包括Mn原子K的近限吸收谱)。研究了准晶Al₄Mn和准晶Al₆Mn中Mn原子周围的第一近邻结构,讨论了Mn原子周围第一近邻Al原子的分布特征。在准晶Al₆Mn和准晶Al₄Mn中,Al-Mn间原子平均距离约为2.52?,小于晶态中Al-Mn间平均距离,围绕Mn原子的配位数约为 关键词：     

Synthesis and mechanical properties of mechanically alloyed Al-Cu-Fe quasicrystalline composites

Al-Cu-Fe alloys were prepared by mechanical alloying starting from elemental powders in a high-energy planetary ball mill. Three different alloy compositions with the same c _Cu : c _Fe ratio of 2:1 but different aluminium contents, that is Al₅₅Cu₃₀Fe₁₅, Al₆₃Cu₂₅Fe₁₂ and Al₇₀Cu₂₀Fe₁₀, were investigated. A sequence of solid-state reactions resulting in quasicrystalline phase formation in Al₆₃Cu₂₅Fe₁₂ proceeds during milling and during annealing of the as-milled powder. These reactions were studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. In order to form an aluminium-matrix composite, Al₆₃Cu₂₅Fe₁₂ single-phase quasicrystalline powders were blended with different amounts of aluminium. In an intermediate milling step the powder blend was homogenized. The powders were consolidated by hot extrusion. The bulk samples revealed a homogeneous dispersion of the particles in the matrix but a rather heterogeneous size distribution. The mechanical properties at room temperature were tested by constant-rate compression tests. A rule-of-mixtures dependence of the ultimate strength and the yield strength on the volume fraction of the quasicrystalline particles was found.     

Study of the electronic structure of the quasicrystalline Ti—Zr—Ni system

Photoelectron spectra and optical absorption spectra for clean surfaces of quasicrystalline samples of the Ti—Zr—Ni system are measured. The resonant photoelectron spectra are also measured in the range of photon energies corresponding to Ni 2p and Zr 3d absorption thresholds. The change in the intensities of the spectrum of the valence band near the Zr 3d threshold is insignificant. The emission of Auger electrons increases near the Ni 2p threshold in a resonance way. Our studies make it possible to reveal certain specific features of the electronic structure of quasicrystals, which can be used to construct models of actual electronic structures of quasi-crystalline materials.     

Structure of zinc-borate low-melting glasses derived from IR spectroscopy data

The IR spectra of glasses of the ZnO—SrO—B₂O₃ system with constant additions of PbO, Al₂O₃, and Li₂O (20 mol. % in sum) were studied. It is established that on replacement of B₂O₃ by ZnO, the structure of the glasses is characterized by the presence of groupings with the bridge bonds B^III— O—B^III, B^III—O—B^IV, B^IV—O—B^IV and end groups B^III— O⁻; ZnO practically exerts no influence on the coordination transition [BO₃] → [BO₄]. At a high content of ZnO, zinc ions are present in both a six-and a four-coordinated state. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 778–781, November–December, 2005.     

Decagonal quasicrystal in the Al–Pd–Mn system

Abstract

A stable decagonal quasicrystal in Al₇₀Pd_30?xMn_x alloys (x = 10–20) was examined by electron diffraction and high-resolution electron microscopy. The decagonal quasicrystalline grains are formed with definite crystallographic relationships to adjacent icosahedral and Al₃Mn crystalline grains. The structure of the decagonal phase, which is formed as the main phase at near Al₇₀Pd₁₀Mn₂₀ composition, is a mixture of decagonal quasicrystalline regions with some linear phason strain and microcrystalline regions. The structures of both regions may be interpreted in terms of quasiperiodic and periodic tilings, constructed with two types of bond lengths, S (about 2 nm) and L (= τ · S, where τ is the Golden ratio), of the same atom cluster with decagonal symmetry.     

Crystallization and structural relaxation in quasicrystalline Al6CuMg4

We report here electron microscopic, differential scanning calorimetric (DSC) and X-ray diffraction studies on the phase transition in Al₆CuMg₄ alloy. Structural analysis of the transformed crystalline phase suggests that some of the lattice sites, which define the vertices of triacontahedron - the basic volume element of three dimensional Penrose tiling for this system - are fractionally occupied. We also observed that the specific heat of the as grown quasicrystalline alloy reduces gradually from 1.08J/gm. K at 330K to the Dulong Petit value 0.84J/gm. K around 420K, due to structural relaxation of the quasicrystalline phase.     

X-Ray studies of Al6CuLi3 quasicrystal

A stable, single grain of quasicrystalline Al₆CuLi₃ alloy, was subjected to X-ray diffraction experiments. The quasicrystalline nature of the grain is established from the X-ray diffraction photographs taken along the 5-, 3- and 2-fold axes. The attempts to index the reflections in the conventional crystallographic way are not very successful.     

Al-Cu-Fe二十面体准晶的深过冷研究

通过循环过热净化法研究了Ａｌ－Ｃｕ－Ｆｅ合金系中Al₆₅Cu₂₀Fe₁₅合金液的深过冷采用循环过热净化法净化Al₆₅Cu₂₀Fe₁₅合金液，获得了９８Ｋ的过冷度，并制备出直径为６ｍｍ的高纯度的Ａｌ－ＣＳ－ＦＳ二十面体准晶球．随着Al₆₅Cu₂₀Fe₁₅合金液过冷度的不同，Al₆₅Cu₂₀Fe₁₅合金液的第一形核相也发生变化．此外，提高对Al₆₅Cu₂₀Fe₁₅合金液的净化程度，可以提高合金液的形成Ａｌ－Ｃｕ－Ｆｅ二十面体准晶的能力．     

The TDPAC study of quasicrystalline and T-phases

Time differential perturbed angular correlation (TDPAC) spectra of the quasicrystalline phase Al₇₉Mn₂₀Si₁ (800 ppm Hf) and the decagonal phase (T-Phase) Al₈₀Mn₂₀ (800 ppm Hf) were measured using the¹⁸¹Hf probe. It is obtained that the structure around the center in Mackay icosahedra in the decagonal phase seems to be distorted in comparison with that in the quasicrystalline phase.     

Cluster structure of the surface in the amorphous,quasicrystalline, and liquid states

The electronic structure of the Zr₆₀Ni₂₀Ti₂₀, Al₆₅Cu₂₀Fe₁₅, and Al₈₆Mn₁₄ systems has been investigated by X-ray photoelectron spectroscopy and band calculations of the total and partial densities of electronic states have been carried out by the TB-LMTO-ASA method with the aim of studying the chemical bonds and the nearest environment of atoms in the liquid, amorphous, and quasicrystalline states. A direct proof of the inheritance of the chemical structure of the liquid state in the amorphous and quasicrystalline states has been obtained for the first time. The similarity of the electronic structure and the nearest environment of atoms indicates identity of clusters in the liquid, amorphous, and quasicrystalline states. The degree of hybridization of valence electrons of atoms in clusters increases in going from the quasicrystalline to the amorphous state due to the increase the number of transition metal atoms in clusters.     

Local structural order and single-particle dynamics in metallic glass

The atomic dynamics of the amorphous metallic alloy Al₅₀Cu₅₀ is investigated in this work. The critical glass-forming temperature is defined from the behavior change of the Wendt—Abraham parameter and the pair-correlation entropy, and is T _c ≈ 500 K. It is shown that the power-fractional memory function allows us to describe in the framework of the mode-coupling theory the time dependences of the incoherent scattering functions obtained from the atomic dynamic simulations for the wide region of temperatures (T = 100–2000 K).     

Ordered structures in a nonideal dusty glow-discharge plasma

The formation of ordered structures of charged macroparticles in a constant-current neon glow-discharge plasma is investigated. Experiments were performed with two types of particles: thin-walled glass spheres 50–63 μm in diameter and particles of Al₂O₃, 3–5 μm in diameter. Formation of quasicrystalline structures is observed in the standing strata and in an artificially created double electric layer. The formation of extended filamentary structures of macroparticles in the absence of visible stratification of the positive column has been observed for the first time. The influence of the discharge parameters on the formation of the ordered structures and their melting is examined. The form of the interaction potential between the charged macroparticles is considered, as well as changes in the conditions for maintaining the discharge in the presence of high concentrations of dust particles. Zh. éksp. Teor. Fiz. 112, 2030–2044 (December 1997)     

Formation and properties of Al composites reinforced by quasicrystalline AlCuFeB particles

Al-based composites reinforced by icosahedral (i-) Al₅₉Cu_25.5Fe_12.5B₃ quasicrystalline particles were prepared by solid-state sintering. It was found that Al diffusion from the matrix to the quasicrystalline particles induces phase transformation into the ω-Al₇Cu₂Fe tetragonal phase. In order to preserve the i phase, we used an oxidation pre-treatment of the particles and studied its influence on the kinetics of the phase transformation (Al + i → ω) as a function of temperature by high energy X-ray diffraction. The oxide layer acts as a barrier, reducing efficiently the diffusion of Al up to a sintering temperature of 823 K, allowing the control of the phases in the composites. The mechanical properties and the friction behaviour of the composites were investigated and show the negative influence of the oxide on the interface strength.     

Al65Cu20Fe15 in amorphous,crystalline and quasicrystalline states

Three structurally distinct states (amorphous, crystalline, and quasicrystalline) of Al₆₅Cu₂₀Fe₁₅ and neighboring compositions, have been observed. A new metastable state. Intermediate between the amorphous and the quasicrystalline states, has been found. Mössbauer-effect measurements on these samples Al₆₅Cu_20xFe_15+x (?10≤x≤ 10, at.%) In the three different states show that the differences in the effective isomer shifts and quadrupole splittings among the three states are small despile diametrically different structures.     

Crystal-quasicrystal local structural transition in Al-Cu-Fe

The rearrangement of the local environments of copper and iron in the ternary alloy Al₆₅Cu₂₂Fe₁₃ in transition from a crystalline phase to a quasicrystalline phase has been studied by combined extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis. It has been found that the nearest environment of copper retains symmetry characteristic of a crystal; however, a turn and small shifts of copper matrix atoms causes a considerable rearrangement of aluminum atoms around iron. As a result, icosahedral clusters with pentagonal symmetry are formed around iron atoms, and translational symmetry breaking is accompanied by the transition of Al₆₅Cu₂₂Fe₁₃ to a quasicrystalline state.     

The metal-semiconductor transition monitored by excited state lifetimes of Al4O m − clusters

The lifetimes of electronically excited states of Al₄O _m ⁻ clusters are measured for m=1,3,4,5, and 6 using time-resolved photoelectron spectroscopy. With increasing number of oxygen atoms the lifetimes increase. This can be explained qualitatively by a metal-semiconductor transition occurring between the metal-like Al₄ ⁻ cluster and the fully oxidized semiconductor-like Al₄O₆ ⁻ cluster. Long lifetimes of electron-hole excitations are characteristic for semiconductors, while in metals the strong interaction between the delocalized electrons causes short lifetimes.     

Eu2+ and Eu3+ based “concentrated phosphors” as converters for UV LED light: two approaches and two new examples

The absolute majority of phosphors are composed of a host lattice and some percentage of an activator. At higher activator concentrations the concentration quenching occurs. However, there are phosphors in which only minor quenching of the emission occurs with increasing of the activator content. Based on the existence of two different valence states of the Eu ion (2+ and 3+), two approaches for the development of “concentrated phosphors”, i.e. light emitting materials in which the activator ion is a main part of the crystal lattice, are discussed. In both approaches, reduced energy migration leading to the luminescence quenching is considered as a main condition to reach a high quantum efficiency of a concentrated phosphor. Two kinds of phosphors—Eu²⁺-doped alumosilicate and Eu³⁺-doped oxyfluoride—are used as an experimental basis for this discussion. Starting from the stoichiometric Ca_1-xEu_x²⁺Al₂Si₂O₈\mathrm{Ca}_{1-x}\mathrm{Eu}_{x}^{2+}\mathrm{Al}_{2}\mathrm{Si}_{2}\mathrm{O}_{8} anorthite and Eu³⁺OF oxyfluorides, the non-stoichiometric powders with Eu²⁺_0.92Al_1.76Si_2.24O₈\mathrm{Eu}^{2+}_{0.92}\mathrm{Al}_{1.76}\mathrm{Si}_{2.24}\mathrm{O}_{8}, Eu³⁺(O, F)_2,35 and Eu³⁺(O, F)_2,16 compositions were synthesized by a solid state reaction and investigated. It was shown that—in spite of the almost 100% Eu concentration—light converters with high quantum efficiency of more than 45% can be realized. A possible application of these materials as UV LED light converters for white light emitting diodes are discussed as well.