Creep behavior and its correlation with defect chemistry of La0.58Sr0.4Co0.2Fe0.8O3−δ

The creep behavior of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) perovskite was studied in the temperature range 750–950 °C in air and vacuum (P_O2 ≈ 4 mbar). A transition in the apparent activation energy was found between 800 and 850 °C for both oxygen partial pressures. The apparent activation energy is ～250 kJ mol^?1 for the temperature range 700–800 °C under vacuum (P_O2 ≈ 4 mbar) and ～480 kJ mol^?1 for 850–950 °C in both atmospheres. Above 850 °C, the creep rate of LSCF is higher in vacuum than in air although the same cubic structure exists. The stress exponent of the creep law is in the range 1.9–2.5 for all temperatures, which excludes a transition of creep mechanism. It is suggested that, below 800 °C, cation vacancies originate from the necessary balance with the substituted cations in LSCF, and the determined activation energy reflects the energy barrier for cation migration via these vacancies. Above 850 °C, additional vacancies appear to be formed intrinsically, and the activation energy represents the sum of the thermally activated formation energy of cation vacancies and migration energy of cations.     

Recrystallization kinetics of ultrafine-grained Ni studied by dilatometry

The release of excess volume upon recrystallization of ultrafine-grained Ni deformed by high-pressure torsion was measured with a high-precision difference-dilatometer employing constant heating rates in the range from 0.3 to 10 K min^?1. The kinetics of the recrystallization process was analyzed according to the Johnson–Mehl–Avrami–Kolmogorov theory adapted to the case of constant heating rates. An effective Avrami exponent of 2 and a value of 1.20 eV for the activation energy of recrystallization was determined. Analysis by the Kissinger method yielded the same result for the activation energy.     

Effect of growth rate on microstructure and mechanical properties of directionally solidified multiphase intermetallic Ni–Al–Cr–Ta–Mo–Zr alloy

The effect of growth rate on microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic alloy with the chemical composition Ni–21.9Al–8.1Cr–4.2Ta–0.9Mo–0.3Zr (at.%) was studied. The DS ingots were prepared at constant growth rates V ranging from 5.56 × 10⁻⁶ to 1.18 × 10⁻⁴ ms⁻¹ and at a constant temperature gradient at the solid–liquid interface of G_L = 12 × 10³ K m⁻¹. Increasing growth rate increases volume fraction of dendrites and decreases primary dendritic arm spacing, mean diameter of α-Cr (Cr-based solid solution) and γ′(Ni₃Al) precipitates within the dendrites. Room-temperature compressive yield strength, ultimate compressive strength, hardness and microhardness of dendrites increase with increasing growth rate. All room-temperature tensile specimens show brittle fracture without yielding. The brittle-to-ductile transition temperature for tensile specimens is determined to be about 1148 K. Minimum creep rate is found to depend strongly on the applied stress and temperature according to the power law with a stress exponent of n = 7 and apparent activation energy for creep of Q_a = 401 kJ/mol.     

Study of LiMgVO4 electrical conductivity mechanism

This paper deals with impedance spectroscopy on single-phase polycrystalline LiMgVO₄ in the temperature range of 25–500 °C. Thermogravimetric measurements show a weight loss of 2.7% in the temperature range between 25 °C and 175 °C due to humidity desorption. A conductivity mechanism along the grain boundaries (agb) is identified in the specific temperature range and is attributed to a reversible humidity absorption–desorption mechanism. Equivalent circuits are drawn using the results of the impedance measurements at each temperature. A unique conduction process within the material is assigned to each element of the equivalent circuit and Arrhenius plots are plotted. The calculation of activation energy of each conduction mechanism is based on the Arrhenius plots. The activation energy E_b of the bulk conductivity mechanism was found to be 0.62 eV. The activation energy E_gb of the grain boundaries conductivity mechanism was found to be 1.03 eV up to 275 °C and 0.50 eV in the temperature range of 300–500 °C. The absence of the conductivity mechanism along the grain boundaries above 175 °C can only be due to the complete removal of water from the material's grains.     

A molecular dynamics simulation study of the velocities,mobility and activation energy of an austenite–ferrite interface in pure Fe

Molecular dynamics simulations have been used to obtain the mobility, in pure Fe, of a face-centered cubic (fcc)–body-centered cubic (bcc) interphase boundary with an orientation given by (1 1 0)_bcc//(7 7 6)_fcc and [0 0 1]_bcc//[?1 1 0]_fcc. The interface is best described by a 4.04° rotation, about an axis lying in the boundary plane, from the Nishiyama–Wasserman orientation and the boundary consists of a parallel array of steps (disconnections). An embedded atom method interatomic potential was employed to model Fe, and the free energy difference as a function of temperature between the fcc and bcc phases, which provided the driving force for boundary motion, was determined by a thermodynamic integration procedure. Although the boundary was found to be very mobile, the transformation did not proceed by a martensite mechanism. The boundary mobility was obtained for several temperatures in the range 600–1400 K and Arrhenius behavior was found with an activation energy of 16.5 ± 2.7 kJ mol^?1 and a pre-exponential factor equal to 7.8( ± 0.9) × 10^?3 mmol J^?1 s^?1. The activation energy is much lower than that extracted from experiments on the massive transformation in Fe alloys and possible reasons for the discrepancy are discussed.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni₃Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol⁻¹, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.     

Nanoquasicrystalline Al–Fe–Cr-based alloys. Part II. Mechanical properties

Nanoquasicrystalline Al-based alloys show considerable promise for elevated temperature applications compared with commercial Al-based alloys. In particular, a group of Al–Fe–Cr-based alloys-containing Ti, V, Nb or Ta have outstanding thermal stability. In the present work, the elevated temperature mechanical properties of these nanoquasicrystalline alloys were studied by tensile tests at a constant strain rate. Tests were designed in order to compare the mechanical behaviour at different test temperatures. Fractographic analysis was also carried out. The apparent activation energy for plastic deformation was found to be close to that for lattice self-diffusion for pure Al in the Al–Fe–Cr ternary alloy and in the Ti-containing alloy, and for grain boundaries diffusion for pure Al in the V-containing alloy, whereas the activation energy of the alloy with Ta additions was three times higher. All of the alloys showed similar sensitivity of plastic deformation to the strain rate in the range of 10^?3–5 × 10^?6 s^?1 at 350 °C. The apparent true stress exponent was n_app ～ 7, which can be associated with a deformation process controlled by dislocation mechanisms.     

Kinetics study on non-isothermal crystallization of the metallic Co43Fe20Ta5.5B31.5 glass

The crystallization kinetics of metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass has been studied by continuous heating differential scanning calorimetry. The DSC traces have been analyzed in terms of activation energy and kinetic model. It is found that all the DSC traces have a single exothermic peak which is asymmetrical, with a steeper leading edge and a long high temperature tail. The heating rate has a significant influence on the shape of the DSC curve, activation energy and transformation mechanism. The existence of a critical heating rate, β_crit = 20 K min⁻¹, is evident. The activation energy for crystallization are determined as 594.8 and 581.4 KJ mol⁻¹ for the heating rates β = 5–20 K min⁻¹, and 437.7 and 432 KJ mol⁻¹ for the heating rates β = 25–65 K min⁻¹, when using the Kissinger equation and the Ozawa equation, respectively. For the volume fraction crystallized, α, E_c dependence was obtained by the general Ozawa's isoconversional method. Using the Suriñach curve fitting procedure, the kinetics was specified. Namely, the crystallization begins with the Johnson–Mehl–Avrami nucleation-and-growth mode and the mode which has been well described by the normal grain growth kinetic law. These two modes are mutually independent. The proportion between the JMA-like and the NGG-like modes is related to the heating rate. The JMA kinetics is manifested as a rule in the early stages of the crystallization. The JMA exponent, n, initially being larger than 4 and continuously decreases to 1.5 along with the development of crystallization. The NGG-like mode dominates in the advanced stages of the transformation with the NGG exponent, m = 0.5 and is the major and principal kinetic characteristics for heating rate, β > 25 K min⁻¹.     

Hydrogen diffusion and trapping in a precipitation-hardened nickel–copper–aluminum alloy Monel K-500 (UNS N05500)

Hydrogen uptake, diffusivity and trap binding energy were determined for the nickel–copper–aluminum alloy Monel K-500 (UNS N05500) in several conditions. The total atomic hydrogen (H) concentration increased from 0 to 132 wppm as the hydrogen overpotential decreased to ?0.5 V in alkaline 3.5% NaCl electrolyte at 23 °C. The room-temperature H diffusion coefficient ranged from 0.9 to 3.9 × 10^?14 m² s^?1 for single-phase solid solution, aged, and cold worked then aged microstructures. Diffusivity was independent of lattice H concentration but depended weakly on metallurgical condition, with slower H diffusion after aging. The apparent activation energy for H diffusion was in the range of 29–41 ± 1.5 kJ mol^?1 at the 95% confidence level. The lower value approached nearly perfect lattice transport, while the high value was strongly influenced by traps of low-to-intermediate strength. Atomic hydrogen trapping at metallurgical sites, strongly suggested to be spherical-coherent γ′ (Ni₃Al) precipitates, was evident in the aged compared to the solution heat treated + water-quenched condition. Both thermal desorption and classical Oriani trap state analyses confirmed that the apparent hydrogen trap binding energy interpreted as Ni₃Al (10.2 ± 4.6 kJ mol^?1) interfaces was significantly less than the activation energy for perfect lattice diffusion (25.6 ± 0.5 kJ mol^?1) in this nickel-based alloy system.     

Effect of addition of N on L10 transformation and atomic diffusion in FePt films

The effect of nitrogen incorporation on the kinetics of L1₀ transformation in FePt alloy films was studied. Films with a maximum possible amount of nitrogen were prepared by sputtering a FePt compound target with pure nitrogen. As-prepared films consisting of nano-grains of Pt and Fe_0.7N_0.3 transformed to face-centered cubic FePt phase with a certain amount of nitrogen incorporated in the grains after annealing at 573 K. It was found that the presence of N results in faster kinetics of L1₀ transformation compared with pure FePt alloy films, as well as a substantial reduction in grain size. Detailed structural and diffusion measurements were carried out to elucidate the mechanism of enhanced transformation kinetics as well as grain size reduction. The activation energy for volume diffusion of Fe in FePtN films was found to be 0.27 ± 0.14 eV, while that in FePt was 0.50 ± 0.17 eV. Faster atomic diffusion in nitrogen-containing films was the cause of an enhanced rate of L1₀ transformation. Further studies reveal that, in partially transformed FePtN alloy films, N is incorporated mainly in the grain boundary region, which hinders grain growth and results in a smaller grain size.     

Mobility of nitrogen in ε-Fe3N below 150 °C: The activation energy for reordering

Previously quenched (from 573 K) ε-iron nitride powder of a composition close to Fe₃N, which shows partial disorder in the interstitial N superstructure, can be reordered by annealing between 373 and 408 K. The kinetics of the reordering can experimentally be traced by the axial ratio (c_hcp/a_hcp), as measured by X-ray powder diffraction, and can be described by a first-order rate law for the time-dependence of (c_hcp/a_hcp) with an Arrhenius-type temperature dependence of the rate constant. The determined activation energy of 144 ± 5 kJ mol⁻¹ can be associated with specific jumps of N atoms from disorder to order sites. The order-of-magnitude of the pre-exponential factor of (7 ± 10) × 10¹³ s⁻¹ can be related with the vibration frequency of the N atom in an octahedral site.     

Adsorption and inhibition effect of Ascorbyl palmitate on corrosion of carbon steel in ethanol blended gasoline containing water as a contaminant

The adsorption and inhibition effect of Ascorbyl palmitate (AP) on carbon steel in ethanol blended gasoline containing water as a contaminant (GE10 + 1%water) was studied by weight loss and electrochemical impedance spectroscopic (EIS) techniques. The results showed that the addition of ethanol and water to gasoline increase the corrosion rate of carbon steel. AP inhibits the corrosion of carbon steel in (GE10 + 1% water) solution to a remarkable extent. The adsorption of AP on the carbon steel surface was found to obey the Langmuir adsorption isotherm model. The values of activation energy (E_a) and various thermodynamic parameters were calculated and discussed.     

Effect of CuO additives on sintering and dielectric behaviors of CeO2 ceramics

The effects of CuO addition (1 and 2 wt%) on microstructures and microwave dielectric properties of CeO₂ ceramics with CuO additions have been investigated. It is found that CeO₂ ceramics can be sintered at 1490 °C due to the liquid phase effect of CuO addition. At 1580 °C, CeO₂ ceramics with 1 wt% CuO addition possesses a dielectric constant (?_r) of 21.7, a Q × f value of 50,000 (at 9GHz) and a temperature coefficient of resonant frequency (τ_f) of ?59 ppm/°C. A large sintering temperature reduction (about 200 °C) can be achieved by adding CuO to the CeO₂ ceramics. The CuO–added CeO₂ ceramics can find applications in microwave devices.     

Thermal stability of Al1−xInxN (0 0 0 1) throughout the compositional range as investigated during in situ thermal annealing in a scanning transmission electron microscope

The thermal stability of Al_1?xIn_xN (0 ? x ? 1) layers was investigated by scanning transmission electron microscopy (STEM) imaging, electron diffraction, and monochromated valence electron energy loss spectroscopy during in situ annealing from 750 to 950 °C. The results show two distinct decomposition paths for the layers richest in In (Al_0.28In_0.72N and Al_0.41In_0.59N) that independently lead to transformation of the layers into an In-deficient, nanocrystalline and a porous structure. The In-richest layer (Al_0.28In_0.72N) decomposes at 750 °C, where the decomposition process is initiated by In forming at grain boundaries and is characterized by an activation energy of 0.62 eV. The loss of In from the Al_0.41In_0.59N layer was initiated at 800 °C through continuous desorption. No In clusters were observed during this decomposition process, which is characterized by an activation energy of 1.95 eV. Finally, layers richest in Al (Al_0.82In_0.18N and Al_0.71In_0.29N) were found to resist thermal annealing, although the initial stages of decomposition were observed for the Al_0.71In_0.29N layer.     

Growth and characterization of epitaxial Ba(Zn1/3Ta2/3)O3 (1 0 0) thin films

We have synthesized and characterized epitaxial and stoichiometric Ba(Zn_1/3Ta_2/3)O₃ (1 0 0) dielectric thin films grown on MgO (1 0 0) substrates by pulsed laser deposition. Advanced electronic structure calculations were used to guide the interpretation of the experimental data. Zn-enriched targets and high oxygen pressures were used to compensate for Zn loss during film growth. The Ba(Zn_1/3Ta_2/3)O₃ films had an indirect optical band gap of ～3.0 eV and a refractive index of 1.91 in the visible spectral range. Zn–Ta B-site ordering was not observed in the Ba(Zn_1/3Ta_2/3)O₃ thin film X-ray diffraction data. A dielectric constant of 25 and dissipation factor of 0.0025 at 100 kHz were measured using the interdigital capacitor method. The Ba(Zn_1/3Ta_2/3)O₃ films exhibited a small thermally activated ohmic leakage current at high fields (<250 kV cm^–1) and high temperatures (<200 °C) with an activation energy of 0.85 eV.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Enhancing magnetocrystalline anisotropy of the Fe70Pd30 magnetic shape memory alloy by adding Cu

Fe–Pd–Cu thin films are of great interest for applications in magnetic shape memory microsystems due to their increased martensitic transformation temperature. Here we analyse the consequences of Cu addition to Fe–Pd on the binding energy and magnetic properties by a combination of thin film experiments and first-principles calculations. Strained epitaxial growth of Fe₇₀Pd_30-xCu_x with x = 0, 3, 7 is used to freeze intermediate stages during the martensitic transformation. This makes a large range of tetragonal distortion susceptible for analysis, ranging from body-centred cubic to beyond face-centred cubic (1.07 < c/a_bct < 1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarization and Curie temperature are reduced only moderately, in agreement with our calculations. Beyond c/a_bct > 1.41 the samples undergo structural relaxations through adaptive nanotwinning. Cu enhances the magnetocrystalline anisotropy constant K₁ at room temperature, which reaches a maximum of ?2.4 × 10⁵ J m^?3 around c/a_bct = 1.33. This value exceeds those of binary Fe₇₀Pd₃₀ and the prototype Ni–Mn–Ga magnetic shape memory system. Since K₁ represents the maximum driving energy for variant reorientation in magnetic shape memory systems, we conclude that Fe–Pd–Cu alloys offer a promising route towards microactuator applications with significantly improved work output.     

Kinetics and microstructure of laser chemical vapor deposition of titanium nitride

Titanium nitride (TiN) films were deposited onto Ti–6Al–4V substrates by laser chemical vapor deposition using a cw CO₂ laser and TiCl₄, N₂ and H₂ reactant gases. Laser-induced fluorescence (LIF) and pyrometry determined relative titanium gas phase atomic number density and deposition temperature, respectively. Auger electron spectroscopy found substoichiometric films, caused by diffusion of nitrogen through TiN grain boundaries to the titanium alloy substrate. The morphology is a polyhedral structure with crystallite sizes ranging from 10 to 1000 nm. The activation energy was calculated to be 122 ± 9 kJ mol⁻¹ using growth rates measured by film height and 117 ± 23 kJ mol⁻¹ using growth rates measured by LIF signals. Above N₂ and H₂ levels of 1.25% and below TiCl₄ input of 4.5%, the growth rate has a half-order dependence on nitrogen and a linear dependence on hydrogen. The rate-determining steps of TiN growth are discussed.     

Efficient and low cost devices for solar energy conversion: Efficiency and stability of some natural-dye-sensitized solar cells

Dye-sensitized solar cells, named by us Dye-Cells, are one of the most promising devices for solar energy conversion due to their reduced production cost and low environmental impact, especially those sensitized by natural dyes. The efficiency and stability of devices based on natural sensitizers such as mulberry (Morus alba Lam), blueberry (Vaccinium myrtillus Lam), and jaboticaba's skin (Mirtus cauliflora Mart) were investigated. Dye-Cells prepared with aqueous mulberry extract presented the highest P_max value (1.6 mW cm^?2) with J_sc = 6.14 mA cm^?2 and V_oc = 0.49 V. Photoelectrochemical parameters of 16 cm² active area devices sensitized by mulberry dye were constant for 14 weeks of continuous evaluation. Moreover, the cell remained stable even after 36 weeks with a fairly good efficiency. Therefore, mulberry dye opens up a perspective of commercial feasibility for inexpensive and environmentally friendly Dye-Cells.     

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0]_β and line direction [1 1 1]_β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol⁻¹ was obtained, which is in reasonable agreement with experimental results.