Differential scanning calorimetry and reaction kinetics studies of γ + α2 Ti aluminide

Reaction synthesis method for titanium aluminide processing consists of an exothermic reaction among alloying elements present and primarily between titanium and aluminium particles at specific temperature range. Study of this reaction helps in understanding the process of aluminide formation. Differential scanning calorimetry (DSC) study is the suitable method to study such reactions. In the present work, five different alloy mixtures based on Ti48Al2Cr2Nb0.1B are prepared and DSC study is carried out. Onset temperature, peak temperature and completion temperature of the major exothermic reaction is analyzed at different heating rates. Further, kinetics of the reaction is studied using Johnson–Mehl–Avrami equation. Activation energy and Avrami parameter are calculated and compared with the reported works on binary alloy. It has been observed that exothermic reaction is triggered by melting of aluminium. Boron assists in increasing the enthalpy of reaction by boride formation. Primary reaction product is found to be TiAl₃. Activation energy as well as Avrami parameter is found to have marginal variation due to small change in alloying elements in different alloys and due to heating rates in the same alloy.     

Effect of (α + β) heat treatment on microstructure and mechanical properties of (TiB + TiC)/Ti–B20 matrix composite

For the first stage, a metastable β titanium alloy, Ti–3.5Al–5Mo–4V–2Cr–2Sn–2Zr–1Fe reinforced with trace amounts of TiB whiskers and TiC particles was fabricated by vacuum arc melting process and hot forging followed by heat treatment at 780 °C/740 °C, then by aging at 500 °C, 550 °C, 570 °C and 600 °C. For the second stage, the unreinforced titanium alloy was also fabricated by the same process. The microstructural characteristics were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Traces of TiB whiskers and TiC particles (2.2 vol.%) with a volume ratio of 2:3 synthesized in situ exerted a hybrid reinforcing effect on the β titanium alloy. The reinforcements were uniformly distributed in the matrix and the elastic modulus was improved about 25 GPa. Ultimate tensile strength and yield strength achieves about 1625 MPa and 1500MPa respectively, with ductility at 7% when the aging temperature is 500 °C. The ductility of (TiB + TiC)/(Ti–3.5Al–5Mo–4 V–2Cr–2Sn–2Zr–1Fe) matrix composite could be enhanced by increasing the aging temperatures. After 780 °C followed by aging at 570 °C, excellent strength and plasticity properties were obtained (ultimate tensile strength of matrix alloy is 1350 MPa with elongation of 18% and ultimate tensile strength of composite is 1500 MPa with elongation of 13%).     

3D characterization of trans- and inter-lamellar fatigue crack in (α + β) Ti alloy

This paper presents a three dimensional image processing strategy that has been developed to quantitatively analyze and correlate the path of a fatigue crack with the lamellar microstructure found in Ti-6246. The analysis is carried out on X-ray microtomography images acquired in situ during uniaxial fatigue testing. The crack, the primary β-grain boundaries and the α lamellae have been segmented separately and merged for the first time to allow a better characterization and understanding of their mutual interaction. This has particularly emphasized the role of translamellar crack growth at a very high propagation angle with regard to the lamellar orientation, supporting the central role of colonies favorably oriented for basal 〈a〉 slip to guide the crack in the fully lamellar microstructure of Ti alloy.     

An XPS study of interaction of neptunyl(V) in aqueous solution with goethite (α-FeOOH)

Sorption and physicochemical state of Np on the goethite (-FeOOH) surface were studied. The oxidation state of Np on the -FeOOH surface was studied by the liquid extraction. The neptunium complexes formed on the surface were studied by X-ray photoelectron spectroscopy. The ionic and elemental composition of the goethite surface and NpO ₂ ⁺ complexes with -FeOOH were determined from the XPS data. No Np(IV) and Np(VI) compounds were detected. Neptunyl(V) Np(V)O ₂ ⁺ forms complexes with the surface groups of -FeOOH. The equatorial plane of these complexes is occupied by the oxygen atoms of -FeOOH and water and/or carbonate group CO ₃ ^2– .Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 503–506.Original Russian Text Copyright © 2004 by Yu. Teterin, Kalmykov, Novikov, Sapozhnikov, Vukcevic, A. Teterin, Maslakov, Utkin, Khasanova, Shcherbina.     

Features of nanostructuring in LaMnO3 + δ (polydisperse and monodisperse structures)

The formation of polydisperse and monodisperse structures in LaMnO_3 + δ has been investigated by X-ray diffraction (XRD) analysis and transmitting electron microscopy (TEM). The sizes of LaMnO_3 + δ particles have been evaluated for different types and conditions of synthesis (solid-phase synthesis and sol–gel method) and subsequent heat treatment in the controlled atmosphere. The effect of the size distribution on the correctness in the determination of particle sizes has been analyzed.     

Physical and optical properties of Co2+, Ni2+ doped 20ZnO + xLi2O + (30 − x)K2O + 50B2O3 (5 ≤ x ≤ 25) glasses: Observation of mixed alkali effect

Co²⁺ and Ni²⁺ ions doped 20ZnO + xLi₂O + (30 ? x) K₂O + 50B₂O₃ (5 ≤ x ≤ 25) mol% glasses are prepared using melt quenching technique. Structural changes of the prepared glasses by addition of transition metal oxides, CoO and NiO are investigated by UV–vis–NIR, FT-IR spectroscopy and XRD. The XRD pattern indicates the amorphous nature of prepared glasses. FT-IR measurements of the all glasses revealed that the network structure of the glasses are mainly based on BO₃ and BO₄ units placed in different structural groups in which the BO₃ units being dominant. The optical absorption spectra suggest the site symmetry of Co²⁺ and Ni²⁺ ions in the glasses are near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. The optical band gap and Urbach energies exhibited the mixed alkali effect. Various physical parameters such as density, refractive index, optical dielectric constant, polaron radius, electronic polarizability and inter-ionic distance are also determined.     

Phase transitions of bulk and nanocrystalline La1−xSrxMnO3 (x = 0.35 and 0.37) perovskite manganite materials using in situ ultrasonic studies

La_1−xSr_xMnO₃ perovskite manganite materials with two compositions, namely x = 0.35 and 0.37, both at bulk and nanoscale, were prepared by solid-state reaction and sonochemical reactor methods, respectively. The magnetic phase transition temperature of the prepared bulk and nanosamples was evaluated by in situ ultrasonic velocity and attenuation measurements. A home-made experimental setup was used for in situ measurement of ultrasonic velocities and attenuation over a wide range of temperatures (from 300 to 400 K). The observed anomalous lattice-softening behavior in the ultrasonic parameters was used to study the phase transition temperature (Curie temperature, T_C), i.e., from paramagnetic to ferromagnetic phase, both in bulk and nanostructured perovskite samples. Further, the ultrasonic measurements confirmed that sharp and broad transitions occur in bulk and nanostructured perovskite manganite materials, respectively. The Curie temperature for nanostructured perovskite samples was lower than that for the corresponding bulk perovskite sample, which was clearly identified by ultrasonic measurements.     

Multicolor-emitting Ca3 -x-ySry(PO4)2:xEu2 + (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors for light-emitting diodes

In this paper, a series of Ca_3 -x-ySr_y(PO₄)₂:xEu^2 +, (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors were prepared by flux assisted solid-state reaction method, and their photoluminescence properties were investigated. The β- to β′-phase transition of Ca_3 -ySr_y(PO₄)₂ for high Sr^2 + content was observed from the XRD patterns, and the corresponding optical bandgaps were obtained experimentally. Various Eu^2 + emission centers were found, which generate tunable emission depending on the Sr^2 + concentration. Broad and intense excitation bands exist in Eu^2 + activated Ca₃(PO₄)₂, and the introduction of Sr^2 + further extends and enhances the excitation bands beyond 350 nm, which is beneficial to the applications on near ultraviolet LEDs. The morphology measurement reveals that the average size of particles with smooth surface is about 11.2 μm, which is suitable for the practical applications. These results indicate that the Ca_3 -x-ySr_y(PO₄)₂:xEu^2 + phosphors could be promising candidates for LEDs.     

Vapor–liquid equilibrium measurements and assessments of fluoroethane + N,N-dimethylformamide and fluoroethane + dimethylether diethylene glycol systems for the hybrid refrigeration cycle

This study aimed to develop novel working pairs for the hybrid refrigeration cycle. To this end, the vapor–liquid equilibrium data of fluoroethane (R161) + N,N-dimethylformamide (DMF) and R161 + dimethylether diethylene glycol (DMEDEG) systems were obtained from 293.15 K to 353.15 K. Experimental data were correlated using the nonrandom two-liquid (NRTL) activity coefficient model. The average relative pressure deviations are 1.33% and 1.45%, and the maximum relative pressure deviations are 3.68% and 3.90%, respectively. Experimental results show that the two binary mixtures exhibit negative deviations from Raoult's law. Finally, the performance of R161 + DMF, R161 + DMEDEG, and R134a + DMF systems were simulated and evaluated based on the hybrid refrigeration cycle. Results show that both R161 + DMF and R161 + DMEDEG systems are potential working pairs for the hybrid refrigeration cycle, and the R161 + DMEDEG system is better.     

Mechanical properties of magnetite (Fe3O4), hematite (α-Fe2O3) and goethite (α-FeO·OH) by instrumented indentation and molecular dynamics analysis

Hardness and elastic properties of pure (crystal) and complex (product of corrosion) iron oxides, magnetite (Fe₃O₄), hematite (α-Fe₂O₃) and goethite (α-FeO·OH), were determined by means of molecular dynamics analysis (MDA) and instrumented indentation. To determine local mechanical properties by indentation, multicyclic loading is performed by using incremental mode. Moreover to study the influence of visco-elastoplastic behaviour of the material, various load-dwell-times were applied at each loading/unloading cycle. To support the indentation results, molecular dynamics analysis based on shell model potential is performed for pure oxides to determine Young's modulus, bulk modulus, Poisson's ratio and shear modulus. The comparison between experimental and theoretical values both with the literature data allows the evaluation of the mechanical properties of the pure oxides. Subsequently, this allows the validation of the mechanical properties of complex oxides which can only be deduced from indentation experiments.     

Synthesis and electrical characteristics of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 PTCR ceramics

Solid solutions of (1 − x)BaTiO₃–xK_0.5Bi_0.5TiO₃ were prepared by the solid state reaction technique. Samples were sintered in reducing atmosphere of N₂/H₂ in the temperature range 1100–1240 °C with subsequent oxidation at 700 °C. Phase composition and crystal structure were investigated by X-ray powder diffractometry (XRPD). It was shown that all samples of (1 − x)BaTiO₃–xK_0.5Bi_0.5TiO₃ (0 ≤ x < 0.4) exhibit a tetragonal structure at room temperature, the parameters a and c decrease with increasing x. With increasing x the Curie temperature of solid solutions (1 − x)BaTiO₃–xK_0.5Bi_0.5TiO₃ (0.1 ≤ x < 0.4) increases from 150 to 220 °C. The values of potential barriers at grain boundaries were calculated on the basis of Heywang model. With increasing x, the potential barrier at grain boundaries increases. It was shown that the grain size decreases with increasing the bismuth–potassium titanate content. The complex impedance results indicate that the grain boundary and the outer layer region, located between the boundary and the core of the grain, make a contribution to the positive temperature coefficient of resistance (PTCR) effect in (1 − x)BaTiO₃–xK_0.5Bi_0.5TiO₃ solid solutions. With increasing x ρ_max increases due to an increase in potential barrier at grain boundaries.     

Density functional study on the geometric features and growing pattern of AlnNm clusters (n = 1–4, m = 1–4, n + m ≤ 5)

The bonding features of Al_nN_m clusters (n = 1–4, m = 1–4, n + m ≤ 5) are studied within the framework of the density functional theory following a systematic growing mechanism. Various geometries are found to be more stable than structures reported elsewhere for small Al–N aggregates. In other cases, good agreement exists with results already reported by other authors. It is also found that Al–N clusters tend to grow mainly towards nonlinear bidimensional structures up to the pentamer. The ability to form triple N–N bonds is important for the relative stability of clusters as they grow. The growing patterns could be explained mainly in terms of electrophilic attacks of both Al and N atoms to the corresponding sites in the aggregates of lower size. It is found that atomic charges derived from molecular electrostatic potentials allow to rationalize the electrophilic attacks, being then useful to understand the growing paths followed by small Al–N clusters.     

Atomic diffusion in the Fe [0 0 1] ∑ = 5 (3 1 0) and (2 1 0) symmetric tilt grain boundary

Both the formation and diffusion activation energies of single vacancy migrating intra-layer and inter-layer near the Fe [0 0 1] Σ = 5 (3 1 0) and (2 1 0) symmetric tilt grain boundaries have been calculated by using the MAEAM and a MD method. From energy minimization, the vacancy concentration in the second layer is higher than the one in the other layers for both (3 1 0) and (2 1 0) STGBs. By the diffusion activation energies of the vacancies migrating intra-layer and inter-layer, the vacancies located from the first to the eighth layers of (3 1 0) STGB as well as the ones located from the first to the tenth layers of (2 1 0) STGB are favorably migrated to the second layer. Thus there is a vacancy aggregation tendency to the second layer near the grain boundary. For the vacancy migrating intra-layer and inter-layer, the influences of the grain boundary are respectively as far as to the fifth and eighth layers for (3 1 0) STGB as well as to the sixth and tenth layers for (2 1 0) STGB.     

Measuring spectral transmission and refractive index of AgCl1−xBrx (0 ⩽ x ⩽ 1) and Ag1−xTlxBr1−xIx (0 ⩽ x ⩽ 0.05) at the wavelength of 10.6 μm

We measured the complex refractive index at the wavelength of 10.6 μm with the help of Fourier transform infrared spectroscopy for polycrystalline plates of the following compositions AgCl_1−xBr_x (0 ⩽ x ⩽ 1) and Ag_1−xTl_xBr_1−xI_x, where x varied from 0 to 0.05. In order to do it we chose a segment of the spectrum, which was recorded with a high resolution (0.5 cm⁻¹) using the HgCdTe detector and which had a set of 10 identical peaks. It is shown that the real part of the refractive index rises along with increasing the substituting component fraction in the solid solution from 1.99 to 2.17 for AgCl_1−xBr_x and from 2.17 to 2.24 within the range of TlI mole fraction up to 0.05 for Ag_1−xTl_xBr_1−xI_x. We considered errors introduced by the spectrometer resolution and the accuracy rating of the micrometer, which was used to measure sample thickness. It is seen in the spectra, recorded for the second system with a lower resolution and using a deuterated and l-alanine doped triglycine sulfate detector, that increasing the thallium monoiodide fraction results in widening the transmission range towards bigger wavelengths. We also plan to use the obtained refractive index values for simulating mid-infrared optical fibers, the polycrystalline structure of which is close to the structure of the plates under investigation.     

Synthesis and characterization of xTiO2(1 − x)α-Fe2O3 magnetic ceramic nanostructure system

Rutile-doped hematite xTiO₂(1 ? x)α-Fe₂O₃ (x = 0.0–1.0) nanostructures were synthesized using mechanochemical activation by ball milling. Their complex structural, magnetic and thermal properties were characterized by X-ray diffraction, Mössbauer spectroscopy and simultaneous DSC–TGA. XRD patterns yielded the dependence of lattice parameters and grain size as a function of ball milling time. For the molar concentrations x = 0.1 and 0.3, the Mössbauer spectra were fitted with one, two, three or four sextets, corresponding to the degree of Ti ion substitution of Fe ions in hematite lattice. After 12 h of ball milling, the completion of Ti ion substitution of Fe ions in hematite lattice occurs for x = 0.1 and 0.3. For x = 0.5 and 0.7, Mössbauer spectra fitting required sextets and a quadrupole-split doublet, representing Fe ions substituting Ti ions in the rutile lattice. The completion of Fe ion substitution of Ti ions in rutile lattice was not observed, as indicated by XRD patterns and Mössbauer spectra for these two molar concentrations. Simultaneous DSC–TGA measurements revealed that the mechanochemical activation by ball milling has a strong effect on the thermal behavior of this nanostructure system. The enthalpy dropped dramatically after 2 h of milling time, indicating the strong solid–solid interactions between TiO₂ and α-Fe₂O₃ after ball milling. The change in weight loss of hematite was caused by the decrease of grain size and ion substitutions between Fe and Ti after mechanochemical activation.     

Microwave-assisted synthesis and characterization of Ti1 − xVxO2 (x = 0.0–0.10) nanopowders

Ti_1 ? xV_xO₂ (x = 0.0–0.10) nanopowders were successfully synthesized by a microwave-assisted sol–gel technique and their crystal structure and electronic structure were investigated. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and UV–Vis spectroscopy. The results revealed that TiO₂ powders maintained the anatase phase for calcination temperature below 600 °C, but gradually changed to the rutile phase above 800 °C. The formation of the rutile phase was completed at 1000 °C. For Ti_1 ? xV_xO₂ (x = 0.05) powders, the phase transformation appeared at 600 °C. The absorption edge of Ti_1 ? xV_xO₂ (x > 0) powders broadened to the visible region with increasing V concentration and a strong visible light absorption was obtained with 10% V doping. V doping and subsequent coexistence of both anatase and rutile phases in our Ti_1 ? xV_xO₂ nanoparticles are considered to be responsible for the enhanced absorption of visible light up to 800 nm.     

Electrical properties of nanostructures (CoFeZr)x + (Al2O3)1−x with use of alternating current

CoFeZr–Al₂O₃ nanocomposite films of 3–5 μm thickness, containing metallic alloy nanoparticles embedded into the dielectric alumina matrix, have been deposited on a glass ceramic substrate using magnetron sputtering of composite target in Ar gas ambient. Measurements of AC conductance and lagging have been performed within the frequency range of 50 Hz–1 MHz at the temperatures from 79 K to 373 K in the initial (as-deposited) samples as well as directly after their isochronous (15 min) annealings within the temperature range from 398 K to 648 K with 25 K step.The observed variations of real part AC electrical conductivity with temperature and frequency σ_real(T, f) in the as-deposited films display transition from dielectric to metallic behaviour when crossing the percolation threshold x_C in the studied nanocomposites. After annealing of the samples below the x_C the σ_real(T, f) progress follows the hopping law of electron conductivity with sigmoidal frequency dependence. The samples being far beyond the percolation threshold revealed transition from metallic to activational σ_real(T) law after high-temperature annealing attributed to the internal oxidation of metallic nanoparticle by excess of oxygen presented in the as-deposited samples.