Light-emitting Si:Er structures produced by molecular-beam epitaxy: High-resolution photoluminescence spectroscopy

Semiconductors - The photoluminescence spectra of light-emitting structures based on silicon layers doped with erbium in the course of growth by molecular-beam epitaxy at temperatures ranging from...     

Microstructure and mechanical properties of mechanically alloyed and spark plasma sintered amorphous–nanocrystalline Al65Cu20Ti15 intermetallic matrix composite reinforced with TiO2 nanoparticles

Multiphase Al₆₅Cu₂₀Ti₁₅ intermetallic alloy matrix composite, dispersed with 10 wt.% of TiO₂ nanoparticles, has been processed by mechanical alloying, followed by spark plasma sintering under pressure in the temperature range of 623–873 K. Differential scanning calorimetry and X-ray diffraction suggest that equilibrium crystalline phases evolve from the amorphous or intermediate crystalline phases. Transmission electron microscopy shows that the composite sintered at 873 K has partially amorphous microstructure, with dispersion of equilibrium, crystalline, intermetallic precipitates of Al₅CuTi₂, Al₃Ti, and Al₂Cu of 25–50 nm size, besides the TiO₂. The composite sintered at 873 K exhibits little porosity, hardness of 5.6 GPa, indentation fracture toughness in the range of 3.1–4.2 MPa√m, and compressive strength of 1.1 GPa. Indentation crack deflection by TiO₂ particle aggregates causes increase in fracture resistance with crack length, and suggests R-curve type behaviour. The study provides guidelines for processing high strength amorphous–nanocrystalline intermetallic composites based on the Al–Cu–Ti ternary system.     

Phase Equilibria,Crystal Chemistry and Physical Properties of Au-Ba-Ge Clathrates

In the Au-Ba-Ge system the clathrate type I solid solution, Ba₈Au _x Ge_46−x−y □ _y , extends at 800 °C from binary Ba₈Ge₄₃□₃ (□ is a vacancy) to Ba₈Au₆Ge₄₀. For the clathrate phase (1 ≤ x ≤ 6) cubic primitive symmetry (space group Pm[`3]n Pm{\bar{{3}}}n ) was confirmed by x-ray powder diffraction assisted by x-ray single crystal analyses of Ba₈Au_4.6Ge_40.3□_1.1. The lattice parameters of the solid solution show an almost linear increase with increasing gold content. Site preference from x-ray refinement shows that gold atoms preferably occupy the 6d site in random mixture with Ge and vacancies, which vanish at the solubility limit. Clathrate type ΙX (Ba₆Ge₂₅ type) has a maximum solubility of 2.7 at.% gold at 800 °C. Phase equlilibria at 800 °C are characterized by four ternary phases in the investigated region up to 33.3 at.% barium. The homogeneity range of Ba(Au_1−x Ge _x )₂ (AlB₂-type) and BaAu_1+x Ge_3−x has been established: Ba(Au_1−x Ge _x )₂ extends from BaAu_0.5Ge_1.5 to BaAu_0.9Ge_1.1 and BaAu_1+x Ge_3−x from BaAu_1.1Ge_2.9 (BaNiSn₃-type) to BaAu_2.7Ge_1.3 (Ce(Ni,Sb)₄-type). The crystal structures of two phases in the gold-rich part have been determined from single crystal x-ray data and were found to form new structure types: BaAu₃Ge with BaAu₃Ge-type (space group P4/nmm, a = 0.6459(2), c = 0.5487(2) nm) and BaAu_5+x Ge_2−x (x = 0, BaAu₅Si₂-type, space group Pnma, a = 0.8981(2), b = 0.7106(2) and c = 1.0363(2) nm), the latter revealing with increasing gold content a closely related derivative structure type (BaAu_5.3Ge_1.7, a = a_{\textBaAu5 \textSi2} ,  b = b_{\textBaAu5 \textSi2} ,  c = 2c_{\textBaAu5 \textSi2} a = a_{{{\text{BaAu}}_{5} {\text{Si}}_{2} }} ,\;b = b_{{{\text{BaAu}}_{5} {\text{Si}}_{2} }} ,\;c = 2c_{{{\text{BaAu}}_{5} {\text{Si}}{}_{2}}} ). Transport properties and particularly the thermoelectric behavior were studied for Ba₈Au₆Ge₄₀.     

Effect of cold deformation on age hardening of alloy VT35

The effect of cold deformation on the variation of hardness during aging of sheets from alloy VT35 with β-structure is studied. Diagrams of isothermal transformations due to aging of alloy VT35 with β-phase are plotted for sheets without work hardening and for sheets with an initial β-structure subjected to cold rolling.     

Ceramic composite materials

Conclusion The concept of developing new materials with prescribed properties based on ideas about "building" structures may be realized in creating ceramic composite materials. Creation of heterogeneous composite structures is the main path for achieving high crack resistance (a parameter which mainly governs the operating reliability of structural articles). These structures make it possible to realize energy-consuming processes of the dissipation of work for failure from external forces. The level of properties achieved (crack resistance above 300 N/mm^3/2, strength above 500 N/mm²), and the high permissible operating temperatures make ceramic composite materials irreplaceable for a number of fields of technology. However, the main problems connected with the development of ceramic composites remain unresolved. These are problems for achieving the optimum structure and composition of the interface for components, thermal stability of reinforcing elements, the production cost of both fibers and composite materials, and obtaining highly compact matrices in composite materials reinforced with continuous fibers.Scientific Production Association of the Institute of Aviation Materials. Interbranch Scientific Research Center of Technical Ceramics. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 36–40, February, 1992.     

Influence of quenching conditions on the mechanical properties and residual stresses of ÉP962 type heat-resistant nickel alloys

VILS. Stupinskii Metallurgical Works. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 10–13, January, 1992.     

Magnetic properties of a low-density Bi-based HTSC

The preliminary results of the investigation of magnetic properties of a novel low-density superconducting material Bi_1.8Pb_0.3Sr₂Ca₂Cu₃O_x with a low density and microfoam structure are presented. The temperature, magnetic field, and time dependences of magnetization M have been measured. The experimental hysteresis loops M(H) are well described in the framework of the Val’kov-Khrustalev theory developed for type II granular superconductors.