Variation of the lattice parameter and thermal expansion coefficient of (U,Dy)O2 as a function of DyO1.5 content

Thermal expansions of (U,Dy)O₂ solid solutions were investigated between room temperature and 1673 K by using a thermo-mechanical analyzer. The lattice parameter of (U,Dy)O₂ pellets is lower than that of UO₂ and it decreases as Dy content increases. The linear thermal expansion and average thermal expansion coefficients of (U,Dy)O₂ are higher than that of UO₂. For the temperature range from room temperature to 1673 K, the average thermal expansion coefficient values for UO₂ and (U_0.8Dy_0.2)O₂ are 10.97 × 10⁻⁶ and 11.37 × 10⁻⁶ K⁻¹, respectively.     

Thermal expansion of the W5Si3 and T2 phases of the W–Si–B system investigated by high-temperature X-ray diffraction

The coefficients of thermal expansion (CTE) of the W₅Si₃ and T₂ phases of the W–Si–B system were determined using high-temperature X-ray diffraction in the 298–1273 K temperature interval. Alloys with nominal compositions 62.5W37.5Si (at%) and 58W21Si21B (at%) were prepared from high-purity materials through arc melting followed by heat treatment at 2073 K for 12 h under argon atmosphere. The highly different thermal expansion coefficients of W₅Si₃ along the a (5.0 × 10⁻⁶ K⁻¹) and c (16.3 × 10⁻⁶ K⁻¹) axes lead to a high thermal expansion anisotropy (α_c/α_a ≅ 3.3). On the other hand, the T₂-phase exhibits similar thermal expansion coefficients along the a (6.9 × 10⁻⁶ K⁻¹) and c (7.6 × 10⁻⁶ K⁻¹) axes, indicating a behavior close to isotropic (α_c/α_a ≅ 1.1).     

Thermal and mechanical properties of perovskite-type barium hafnate

Polycrystalline samples of the barium perovskite-type oxide, BaHfO₃ were prepared by solid-state reactions from HfO₂ and BaCO₃ powders. The thermal expansion coefficient, heat capacity, thermal diffusivity, thermal conductivity, elastic modulus, Debye temperature, and micro-Vickers hardness were measured. The crystal structure of BaHfO₃ is of the cubic perovskite type with the lattice parameter 0.4171 nm at room temperature. The sample bulk density is 91% of the theoretical density. The average linear thermal expansion coefficient is 6.93 × 10⁻⁶ K⁻¹ in the temperature range between 300 and 1500 K. The Young's modulus equals 194 GPa. The thermal conductivity at room temperature is 10.4 Wm⁻¹K⁻¹.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

Viscous flow behavior and thermal properties of bulk amorphous Mg58Cu31Y11 alloy

The viscous flow behavior of the Mg₅₈Cu₃₁Y₁₁ bulk amorphous rods in the supercooled viscous region is investigated using differential scanning calorimetry (DSC) and thermomechanical analyzer (TMA). Below the glass transition temperature, T_g, a linear thermal expansion coefficient of 3 ± 1 × 10⁻⁶ m/m K was obtained. In contrast, significant viscous deformation occurred as a result of a compressive load above T_g. The onset, steady state, and finish temperatures for viscous flow, determined by TMA, are slightly different from the glass transition and crystallization temperatures measured by DSC. The appropriate working temperature for microforming as determined by the steady state viscous flow temperature is about 460–474 K. The effective viscosity within this temperature range is estimated to be about 10⁷–10⁹ Pa s, and it increases with increasing applied stress. The onset, steady state, and finish temperatures all decrease with increasing applied stress, suggesting accelerated crystallization in the present Mg₅₈Cu₃₁Y₁₁ under stress.     

The resistance to sliding along Coulombic shear faults in ice

Sliding experiments were performed along Coulombic shear faults in laboratory-grown freshwater ice over a range of sliding velocities (4 × 10⁻³–8 × 10⁻⁷ m s⁻¹) and temperatures (−3, −10 and −40 °C). The Coulombic failure criterion was used to describe the observed linear relationship between the shear stress along and the normal stress across the fault. From this relationship the coefficient of friction was determined. At each temperature the coefficient of friction peaks at a transitional velocity (∼8 × 10⁻⁶ m s⁻¹). For a given velocity the coefficient of friction increases with decreasing temperature. We propose that the peaked shape of the coefficient of friction vs. sliding velocity graphs are a consequence of a change in sliding behavior from “ductile-like” at low velocities to “brittle-like” at higher velocities. The velocity-strengthening and velocity-weakening friction regimes are attributed to creep and to frictional melting, respectively.     

Thermal expansion of the V5Si3 and T2 phases of the V–Si–B system investigated by high-temperature X-ray diffraction

The thermal expansion anisotropy of the V₅Si₃ and T₂-phase of the V–Si–B system were determined by high-temperature X-ray diffraction from 298 to 1273 K. Alloys with nominal compositions V_62.5Si_37.5 (V₅Si₃ phase) and V₆₃Si₁₂B₂₅ (T₂-phase) were prepared from high-purity materials through arc-melting followed by heat-treatment at 1873 K by 24 h, under argon atmosphere. The V₅Si₃ phase exhibits thermal expansion anisotropy equals to 1.3, with thermal expansion coefficients along the a and c-axis equal to 9.3 × 10^?6 K^?1 and 11.7 × 10^?6 K^?1, respectively. Similarly, the thermal expansion anisotropy value of the T₂-phase is 0.9 with thermal expansion coefficients equal to 8.8 × 10^?6 K^?1 and 8.3 × 10^?6 K^?1, along the a and c-axis respectively. Compared to other isostructural silicides of the 5:3 type and the Ti₅Si₃ phase, the V₅Si₃ phase presents lower thermal expansion anisotropy. The T₂-phase present in the V–Si–B system exhibits low thermal expansion anisotropy, as the T₂-phase of the Mo–Si–B, Nb–Si–B and W–Si–B systems.     

3,6-Di(diphenylamino)-9-alkylcarbazole-based hole-transporting hydrazones

The synthesis of a series of new glass-forming hydrazones containing carbazole and diphenylamine moieties is reported. The thermal, optical and photoelectrical properties of mono-, di- and tetra-substituted compounds are compared. The glass transition temperatures of the newly synthesized hydrazones range from 75 to 145 °C. The ionization potentials of the films of hydrazones, measured by the electron photoemission technique, range from 4.74 to 5.15 eV. Hole-drift mobilities of the solid solutions of hydrazones in bisphenol Z polycarbonate (50 wt.%) are in the range of 6.3 × 10⁻⁶ to 1.1 × 10⁻⁵ cm²/Vs at electric field of 6.4 × 10⁵ V/cm as characterized by the time-of-flight method.     

The brittle–ductile transition in single-crystal iron

The fracture behaviour of single-crystal pure iron was studied by four-point bending of pre-cracked specimens at temperatures between 77 and 180 K and strain rates between 4.46 × 10⁻⁵ and 4.46 × 10⁻³ s⁻¹. Fracture behaviour changes from brittle to ductile with increasing temperature. The brittle–ductile transition (BDT) temperature increases with increasing strain rate. The relation between BDT temperature and strain rate follows an Arrhenius relation, giving an activation energy for the BDT of 0.33 eV. Dislocation-dynamics simulations of the crack-tip plasticity and resultant shielding of the crack tip were performed using two different variants of the dislocation velocity/stress/temperature relation. The models predict an explicit BDT, and give a good quantitative fit to the experimental transition temperatures.     

Creep microstructures and creep behaviors of pure molybdenum sheet at 0.7 Tm

The creep behavior of high purity molybdenum (99.995 wt.%) is investigated at temperatures of 1600 to 2000 °C (0.61–0.77 T_m) by direct ohmic heating. The stress and temperature dependency of creep strain and rupture time are described through optical microstructure observations. Under low load and low temperature conditions, coarse secondary recrystallized grains caused by dynamic recrystallization are observed far from the crack tip. In contrast, under high load and high temperature conditions, coarse secondary recrystallized grains are only fully formed near the crack tip, while coarse secondary recrystallized grains and small primary recrystallized grains coexisted further away from the tip. The recrystallized grain size of the Mo-B sheet is smaller than that of the Mo-A sheet, and small primary and large secondary recrystallized grains are mixed throughout whole specimens of the Mo-B sheet. Mo-A sheet shows elongated ductile fracture, but Mo-B sheet shows irregular brittle fracture under the same conditions. The steady-state creep strain rate at 1800 °C is found to be 7.34 × 10^− 6, 2.83 × 10^− 5 and 1.53 × 10^− 4 s^− 1 under a constant stress of 5, 10 and 20 MPa, respectively. The stress exponent is estimated to be 3.85–3.98 and the strain activation energy for steady state creep is 362–413 kJ/mol.     

Electrical properties,sintering and thermal expansion behavior of composite ceramic interconnecting materials,La0.7Ca0.3CrO3−δ/Y0.2Ce0.8O1.9 for SOFCs

This paper presents a high-performance interconnect ceramic for solid oxide fuel cells (SOFCs), based on a modification of La_0.7Ca_0.3CrO_3−δ (LCC). It was found that addition of a small amount of YDC (Y_0.2Ce_0.8O_1.9) into LCC dramatically increased the electrical conductivity. For the best system, LCC + 3 wt.% YDC, the electrical conductivity reached 104.8 S cm⁻¹ at 800 °C in air. The electrical conductivity of the specimen with 2 wt.% YDC in H₂ at 800 °C was 5.9 S cm⁻¹. With the increase of YDC content, the relative density increased, reaching 97.6% when the YDC content was 10 wt.%. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with YDC content, ranging from 11.12 × 10⁻⁶ K⁻¹ to 15.34 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating that it is still an electronically conducting ceramic. Therefore, it is a very promising interconnecting ceramic for SOFCs.     

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.     

Z-scan measurements of third-order optical non-linearities in poly(phenylacetylenes)

Poly(phenylacetylene) (PPA) and poly(p-methoxyphenylacetylene) (PMOPPA) were synthesized by catalytic polymerization of monosubstituted alkynes and were fully characterized by conventional techniques. Linear and third-order non-linear optical properties of polymers solutions were investigated at λ=780 nm using a Z-scan set-up equipped with a femtosecond laser source at different operating regimes. A high repetition rate (76 MHz) regime was used to measure the linear and non-linear absorption coefficients by means of thermo-optical effects. Values of the linear and non-linear absorption coefficients were α=4.9×10⁻³ cm⁻¹ and β=1.1×10⁻¹¹ cm W⁻¹ for PPA and α=2×10⁻² cm⁻¹ and β=2.1×10⁻¹⁰ cm W⁻¹ for PMOPPA. Very low repetition rate (14 Hz) excitation was used to evidence the purely-optical non-linear refractive indexes, that were γ=6×10⁻¹⁸ cm² W⁻¹ and γ=11×10⁻¹⁸ cm² W⁻¹, respectively, for PPA and PMOPPA.     

Variation of atomic spacing and thermomechanical properties in Ni–Mn–Ga/alumina film composites

Ni_51.4Mn_28.3Ga_20.3 thin films deposited on alumina ceramics have been studied by X-ray powder diffraction and dynamic mechanical analysis. Substantial temperature vs. film thickness dependencies of interatomic spacing measured in the direction of the film normal are observed in the range of 25–200 °C and 0.1–5 μm, respectively. The coefficient of thermal expansion (CTE) of the film in the paramagnetic cubic phase has been determined to be equal to (15 ± 1) × 10⁻⁶ K⁻¹ for all the films, in agreement with the CTE of bulk material. The thickness dependent shrinkage of the pseudo-cubic lattice along the film normal direction is attributed to the thermally induced tensile stress in the film plane. The thickness dependence of the elastic modulus of submicron films is obtained. It is shown that the internal stresses result in both the thickness dependence of martensitic transformation temperature and the reversible, thermally induced change in shape of the Ni–Mn–Ga/alumina cantilever actuator.     

Laser doping of chromium as a double acceptor in silicon carbide with reduced crystalline damage and nearly all dopants in activated state

Chromium, a p-type dopant, has been incorporated into silicon carbide by laser doping. Secondary ion mass spectrometric data revealed enhanced solid solubility (2.29 × 10¹⁹ cm⁻³ in 6H–SiC and 1.42 × 19¹⁹ cm⁻³ in 4H–SiC), exceeding the equilibrium limit (3 × 10¹⁷ cm⁻³ in 6H–SiC above 2500 °C). The roughness, surface chemistry and crystalline integrity of the doped sample were examined by optical interferometry, energy dispersive X-ray spectrometry and transmission electron microscopy, respectively, and showed no crystalline disorder due to laser heating. Deep-level transient spectroscopy confirmed Cr as a deep-level acceptor with activation energies E_v + 0.80 eV in 4H–SiC and E_v + 0.45 eV in 6H–SiC. The Hall effect measurements showed that the hole concentration (1.942 × 10¹⁹ cm⁻³) is almost twice the average Cr concentration (1 × 10¹⁹ cm⁻³), confirming that almost all of the Cr atoms were completely activated to the double acceptor state by the laser-doping process without requiring any additional annealing step.     

Theoretical and experimental study of photocurrent action spectrum of ITO/sexithiophene/Al structure

We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have measured the action spectrum obtained by illumination through the aluminum side which is compared to the measured absorption spectrum. Two models were used to interpret this experiment: the Ghosh model using the carriers diffusion property in the bulk, and the kinetic model describing the dynamic behavior of the charge carriers in the device. The confrontation of these models with the experimental spectra allows us to reach the diffusion length of electron–hole pair L=2×10⁻⁶ cm. We also give the mobility of carriers (μ_n≈1.5×10⁻⁴ cm²/V s, and μ_p≈1.5×10⁻⁵ cm²/V s) and their diffusion coefficients (D_n≈4×10⁻⁶ cm²/s, and D_p≈4×10⁻⁷ cm²/s).     

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹.     

Electrical and impedance spectral characterisation of ITO/DAG/In device

A Schottky device, with configuration ITO/DAG/In is fabricated using diazopheny diamino glyoxime (DAG) as an n-type organic material. Current–voltage characteristics and impedance spectroscopy measurements were carried out, which reveals that the injection and transport properties are dominated by negative charge carriers. Space charge limited current theory with an exponential distribution of traps is very well followed by observations resulted through current–voltage characteristics at high voltage region. This gives a traps density N_t of about 4.5×10²¹ m⁻³ and mobility of electron is about 3.9×10⁻¹⁰ m² V⁻¹ s⁻¹. It is found that DAG behaves as an n-type materials as it forms Schottky barrier with ITO (high work function electrode) and conduction is governed by majority carriers, i.e. electrons. Using temperature and bias dependence of impedance spectral characteristics in a broad frequency range, i.e. 40 Hz to 100 kHz, it is found that the ac behaviour of In/DAG/ITO device shows several features, described by the simple double RC circuit representing a depleted junction region and an undepleted bulk region. From the large frequency range of impedance spectroscopy two distinct processes were identified, corresponding to bulk DAG layer and junction region. The activation energy of the relaxation times coincides well with the results obtained from the temperature dependent dc conductivity. The temperature dependent capacitance–voltage measurements were analysed at a frequency of 40 Hz. The obtained inherent donor concentration from 1/C²–V plots varies from 1.8×10²³ m⁻³ at room temperature to 5.9×10²³ m⁻³ at 360 K.     

Superplasticity in cast A356 induced via friction stir processing

Superplasticity was investigated in friction stir processed A356 alloy at temperatures of 470–570 °C and initial strain rates of 3 × 10⁻⁴–1 × 10⁻¹ s⁻¹. Maximum superplastic elongation of 650% was obtained at 530 °C and an initial strain rate of 1 × 10⁻³ s⁻¹ where a maximum strain rate sensitivity of 0.45 was observed.