Calculated Activation Volumes Research Articles

The effect of constituent phases on microstructure and high temperature deformation mechanisms of IrRhRuWMo complex concentrated alloys

The alloys with different primary phases (BCC, BCC + HCP, HCP, FCC) were fabricated using the same alloying elements: Ir, Rh, Ru, W, and Mo. Comprehensive microstructure characterization and phase identification were performed to reveal all existing phases present within the selected alloys. In situ strain jump compression test was carried out to assess mechanical properties and explore underlying deformation mechanisms, both at room temperature (RT) and 1500 °C. While a noticeable discrepancy in 0.2 % proof stress was observed among the selected alloys at RT, the difference was significantly minimized at 1500 °C. Furthermore, while the dual-phase structure effectively increased the 0.2 % proof stress at RT, it had a detrimental effect at 1500 °C. All alloys obeyed the power-law relationship both at RT and 1500 °C, exhibiting relatively high stress exponents. Together with calculated activation volume, these findings suggested that different rate-limited deformation mechanisms were likely activated at RT and 1500 °C.

Displacive-Diffusive plasticity in nanoporous gold nanowires under tensile creep

While creep deformation of nanoporous gold (NPG) was argued to be mainly dislocation mediated, we show using molecular dynamics simulations of tensile creep of NPG nanowires that diffusion plays a significant role. Lower creep stress regimes are influenced by dominant atomic diffusion with negligible dislocation accumulation, while the interplay between surface diffusion and dislocation slips governs the deformation at higher creep stresses. The deformation mechanisms manifest themselves in the calculated activation volumes and creep stress exponents, which are of the same orders of those calculated from indentation creep experiments despite the different length and time scales. The contribution from the dislocation events decreases with increase in the ligament diameter, which is expressed as change in the creep exponents at the higher creep stresses. We suggest that this displacive-diffusion plasticity mechanism is essential to capture creep deformation of NPG structures.

Mg diffusion in Si on a thermodynamic basis

In the present work, magnesium diffusion in silicon studied recently in the temperature range 600–1200 °C (Astrov et al. in Phys Status Solidi A 214:1700192, 2017; Shuman et al. in Semiconductors 51:5, 2017) is investigated on the basis of the cBΩ thermodynamic model, which connects point defect parameters with the macroscopic elastic and expansion properties. The calculated activation Gibbs free energy, activation enthalpy, activation entropy, activation volume and activation specific heat of Mg diffusion exhibit non-linear temperature dependence, due to the anharmonic behavior of the isothermal bulk modulus of Si. The calculated activation enthalpy of diffusion (1.67–2.12 eV) is in agreement with the reported experimental value (1.83 ± 0.02 eV) of Mg diffusion in Si, whereas the calculated activation volume (60% of the mean atomic volume) is compatible with the reported interstitial diffusion of Mg impurities.

Thermodynamic modelling of fast dopant diffusion in Si

In the present study, nickel and copper fast diffusion in silicon is investigated in the framework of the cBΩ thermodynamic model, which connects point defect parameters with the bulk elastic and expansion properties. All the calculated point defect thermodynamic properties (activation Gibbs free energy, activation enthalpy, activation entropy, and activation volume) exhibit temperature dependence due to the non-linear anharmonic behavior of the isothermal bulk modulus of Si. Calculated activation enthalpies (0.15–0.16 eV for Ni and 0.17–0.19 eV for Cu) are in agreement with the reported experimental results. Small values of calculated activation volumes for both dopants (∼4% of the mean atomic volume) are consistent with the interstitial diffusion of Ni and Cu in Si.

Determining the activation volumes in ZnO

The study of the properties of the defects provides an effective way to control the physical properties of solids, such as conductivity. Using a thermodynamic model which correlates the activation Gibbs energy with the bulk elastic and expansivity data, we determine the activation volumes for the conduction processes for ZnO. The calculated activation volumes are in agreement with their experimental values.

Correlation between Activation Volume and Pillar Diameter for Mo and Nb BCC Pillars

AbstractCompression tests with varying loading rates were performed on [001] and [235] oriented small-scale bcc Mo and Nb pillars to determine the contribution of thermally activated screw dislocation motion during deformation. Calculated activation volumes were shown to be in the range of 2 - 9 b3 and by further examination were found to decrease with pillar diameter. This suggests that the kink-pair nucleation of screw dislocations is enhanced by surface effects in the micron and submicron range.

Modeling and characterization of irreversible switching and viscosity phenomena in perpendicular exchange-spring Fe-FePt bilayers

A numerical model has been developed for simulating magnetic domain configurations, remanence, and viscosity curves in systems with strong perpendicular anisotropy and strong disorder, starting from internal switching field distributions for pinning and nucleation processes in the slow dynamics regime. In the considered systems, the domains expand in a percolationlike manner and domain configuration displays fractal properties. The simulations show that even in the case of pinning not governed by nucleation an abrupt avalanche propagation of reversed domains occurs. It is moreover evidenced that a decrease of viscosity coefficients can coexist with lowered energy barrier heights. The model has been applied to perpendicular FePt thin films with granular morphology and corresponding exchange-coupled Fe/FePt bilayers, by exploiting magnetic force microscopy, dc demagnetization (DCD), isothermal remanence, and viscosity data. The comparison between magnetic viscosity phenomena in thin films and exchange-coupled bilayers has been achieved by the definition of an adimensional viscosity coefficient. The addition of the Fe layer causes a decrease of the maximum viscosity coefficient, thus demonstrating that viscosity measures can be utilized to verify the coupling of hard/soft layers. From experiments it is inferred that our samples are mainly influenced by the pinning energy barrier law, the linearity of which allows reconstructing universal viscosity curves starting from DCD data. The calculated activation volumes are comparable to the average grain volume of the FePt layer. The obtained results also demonstrate that the addition of the Fe layer leads to a widening and a shift to lower fields of the pinning field distribution, determining a decrease of both the maximum viscosity and the pinning energy barrier heights and an increase of the demagnetizing effective field.

Effects of pressure on structure and dynamics of model elastomers: A molecular dynamics study

On the basis of an idealized model of an elastomer, we use molecular dynamics simulations to explore the effects of pressure on the glass transition, structure, and dynamics of the model elastomer. The simulated results indicate that with the pressure increasing, the glass transition temperature T(g) increases while the glass transition strength decreases, which is in accordance with the experimental result from Colucci et al. [J. Polym. Sci., B: Polym. Phys. 35, 1561 (1997)] For the structure of the elastomer, it is found that the intramolecular packing remains nearly unchanged over the pressure range studied, also validated by the independence of the chain size and shape on the pressure, while the intermolecular distribution exhibits a more efficient packing effect at high pressures. By analyzing the end-to-end vector correlation and incoherent intermediate dynamic structure factor, which are well fitted by a stretched exponential Kohlrauch-William-Watts (KWW) function, we observe that the time-pressure superposition principle (TPSP) takes effect at the chain length scale, while at the segmental length scale the TPSP does not completely hold, attributed to the enhanced dynamic heterogeneity with the pressure increasing, which is evidenced by the beta values in stretched exponential fitting over the pressure range studied. Extracting the characteristic relaxation time from the KWW function, and then plotting the logarithm of the characteristic relaxation time versus the pressure, we observe a good linear relationship and find that the pressure exerts nearly the same effect on the relaxation behavior at both the segmental and chain length scales. This point is further validated by almost the same dependence of the alpha-relaxation time for three representative q wave vectors, indicating that the segmental and chain relaxations of the elastomer are influenced similarly by the pressure variation and the same physical processes are responsible for relaxation at the probed length scales. The calculated activation volume is independent of pressure at fixed temperature but increases with the temperature decreasing at fixed pressure. Finally, the pressure effect on the stress autocorrelation function is also examined, and a more difficult trend for stress relaxation and dissipation of the elastomer at high pressure is found. It is expected that all these simulated results would shed some light on the relevant experimental and theoretical studies.

Stress Dependence of Oxidation Reaction at SiO2/Si Interfaces during Silicon Thermal Oxidation

The microscopic processes of interfacial reaction of O2 molecules involving compressive stress normal to SiO2/Si(001) interface are investigated by first-principles total-energy calculations. It is found that the energy barrier height for the O2 reaction increases with residual stress at the interface. This is because the energy of the transition state structure for O2 diffusion before O2 insertion into the Si substrate is higher than that for the stress-free interface. The calculated activation volume (7.9 Å3) is comparable to those obtained by numerical simulations using experimental data, implying that the effects of interfacial stress on the reaction of O2 molecules play important roles during thermal oxidation of silicon nanostructures.

Coercivity and magnetic viscosity in mechanical milled nanocrystalline YCo5

High coercivity and magnetic viscosity in nanocrystalline YCo5 alloy were observed. The alloy was obtained by arc-melting the raw materials, mechanical milling for 4h with subsequent heat treatment for short times at 1103K and a final quenching in water. The x-ray diffraction pattern showed a single hexagonal 1:5 phase. The average crystallite size determined by transmission electron microscopy was 13nm. Magnetic measurements were carried out at 85K in a pulsed field magnetometer varying the field in the range 50–250kOe and keeping the pulse duration equal to 0.3s. A maximum coercivity of 50kOe was measured when applying a maximum magnetic field of 250kOe. The viscosity parameter was measured varying the dH/dt rate in the range 511–1113kOe∕s. The calculated activation volumes from viscosity data were smaller than the crystallite volume from transmission electron microscopy, consistent with their different physical meanings.

A first-principles study of O 2 incorporation and its diffusion in compressively strained high-density silicon oxides

The microscopic mechanisms of O 2 diffusion in compressively strained high-density silicon oxides are investigated based on first-principles total-energy calculations. It is found that, both in high-density α-quartz and in α-cristobalite, the calculated incorporation energies and energy barriers increase with increase of oxide density. Independent of the structure of oxides, the calculated activation energies increase with increasing density. Furthermore, the calculated activation volumes suggest that the oxidation retardation by the oxidation-induced strain is due to the retardation of O 2 diffusion in the high-density region, qualitatively consistent with experimental results.

NMR investigation of water and methanol transport in sulfonated polyareylenethioethersulfones for fuel cell applications

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20–140 °C. The selectivity of the membrane, defined as ( D OH/ D CH3), decreased from 6 to 2.4 as temperature increased from 20 to 140 °C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 °C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 °C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm 3/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nafion-117.

Magnetic viscosity and activation volume in chromium substituted Pb–M hexaferrite

AbstractPbFe11CrO19 polycrystalline samples were prepared by the chemical coprecipitation method. X‐ray diffraction and Mössbauer spectroscopy confirm the formation of the M‐type hexagonal structure. Time dependence of the magnetization was recorded on the demagnetization curve of the hysteresis loop. It is well described by a simple logarithmic law but also non‐logarithmic behavior was detected. A two‐peak dependence of the viscosity coefficient S with the applied field was encountered. Considering reversible susceptibility measurements and a M(H) curve at constant dM/dt in the irreversible susceptibility determination, the calculated activation volume as a function of the applied field shows two well defined zones. The zones observed in the activation volumes are related to the two local maxima that the total susceptibility exhibits. It points to the fact that the viscosity study is sensible enough to characterize this system, encountering two apparent activation volumes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Pressure NMR Study of Water Self-Diffusion in NAFION-117 Membrane

Measurements of the self-diffusion coefficient of water in NAFION-117 as a function of pressure have been carried out. The natural field gradient of a 7.3 T superconducting magnet was used for the diffusion measurement. The measurements were carried out at 288 K and variable pressure up to 0.25 GPa. The high-pressure diffusion data were obtained for four different water contents between 6.6 and 22 wt %. The calculated activation volume decreased from 13.2 to 2.7 cm3/mol as the water content increased from 6.6 to 22 wt %. These results agree with previously published activation volumes extracted from electric conductivity and NMR T1 measurements. It is suggested that segmental motions within the polymer play a significant role in the transport mechanism for low water content membranes, and that the transport mechanism for high water content membranes is similar to that in liquid bulk water.

Activation parameters of the blue shift (Shibata shift) subsequent to protochlorophyllide phototransformation

The Shibata shift was analyzed in flash irradiated wheat ( Triticum aestivum, L., cult. MV17) leaf homogenates in the pressure range of 0.1 to 500 MPa, at temperatures of 20, 30 and 40 °C. The kinetics of the blue shift (called Shibata shift in case of intact leaves) was followed by repeated recording of fluorescence emission spectra after phototransformation. At 20 °C, above 100 MPa, the blue shift slowed down remarkably. Two components of the blue shift could be distinguished, one was pressure-dependent and the other was almost pressure-independent. The pressure-independent component can be associated with minor conformational changes of the NADPH:protochlorophyllide oxidoreductase (POR) enzyme, followed by molecular movements of the newly formed chlorophyllide molecules. The calculated activation volume of the pressure-dependent component was 43±11 cm 3 mol −1 at 20 °C. This value reflects major molecular reorganizations in the lipid system of the membrane and in the chlorophyllide–protein complexes, and corresponds to changes of the tertiary structure of proteins which can proceed directly or indirectly via structural changes of the membrane lipids. The process was inhibited by 300 and 400 MPa at 30 and 40 °C, respectively. The activation volume reduced to 35±1.5 cm 3 mol −1 at 40 °C. The decrease of the activation volume with increasing temperature indicates that the blue shift requires loosened lipid structures. The activation energy of the blue shift (measured between 10 and 40 °C at atmospheric pressure) was 100±20 kJ/mol, indicating that the structural change involves rearrangement of strong molecular interactions.

Microstructural and magnetic characterization of Co/CN films fabricated by nanolamination

Nanolamination combined with appropriate annealing treatment has been used to produce high coercivity, heterogeneous Co–CN films with nanostructures ranging from classical granular to an interconnected network. Transmission electron microscopy and electron diffraction have been used to quantify the nanostructural evolution and resulting grain size distributions. As-deposited nanolaminates with initial layer thicknesses of 1.3 and 2.7 nm are essentially superparamagnetic. Annealing leads to coercivities of >1100 Oe. Viscosity and irreversible susceptibility measurements have been used to calculate activation volumes of ∼(18 nm) 3, in good agreement with grain size analysis. Measurements of the time dependence of coercivity have been used to calculate the thermal stability factor, KV/ kT∼480, which is independent of initial geometry. Effective first-order uniaxial anisotropy constants determined using calculated activation volumes are at maximum ∼75% of the value expected for bulk α-Co. This result is consistent with Co present in both α and β phases, as confirmed by electron diffraction.

Studies on the magnetic viscosity and the magnetic anisotropy of γ-Fe 2 O 3 powders

O3 powders, acicular γ-Fe2O3, and CoFe–γ-Fe2O3 powders are prepared by different methods. Particle shapes and mean particle sizes of samples are determined by transmission electron microscopy (TEM). Magnetic parameters are measured by a vibrating sample magnetometer (VSM) at different temperatures. Effective magnetic anisotropy constants KE of granular γ-Fe2O3 powders at different temperatures are obtained by using the law of approach to saturation (LATS). KE values of acicular γ-Fe2O3 and CoFe–γ-Fe2O3 powders are measured by a magnetotorquemeter. It is found for the first time that the variation tendency of KE with temperature for granular γ-Fe2O3 is about the same as that of shape magnetic anisotropy Ksh. Fluctuation field Hf and activation volume Vf of samples are measured. A theoretical expression of Vf is derived. For granular γ-Fe2O3 powders, calculated activation volumes are consistent with experimental ones at different temperatures. But as for acicular γ-Fe2O3 powders, calculated activation volumes are larger than experimental ones. Experimental results show that magnetization reversal of granular γ-Fe2O3 at different temperatures is close to homogeneous rotation.

Charge transport and water molecular motion in variable molecular weight nafion membranes: High pressure electrical conductivity and NMR

Measurements of the electrical conductivity and deuteron NMR spin-lattice relaxation times ( T 1) in three different molecular weights of acid form NAFION conditioned at various levels of relative humidity have been carried out. Complex impedance studies were made along the plane of the film at frequencies from 10-10 8 Hz at room temperature and pressures up to 0.3 GPa. The high pressure electrical data were only obtained for water contents less than 8 wt%. The NMR measurements were also made at room temperature and pressures up to 0.25 GPa. The NMR data are primarily for water contents greater than 6 wt%. The calculated activation volume exhibits a large decrease (from 16 to 3 cm 3/mol) as the water content is increased from 2.4–8 wt%. In addition, the activation volumes are larger and the electrical conductivity is smaller for the higher molecular weight material. These results represent further evidence that the transport mechanism in low water content materials is dominated by segmental motions of the polymer chain and that proton transport and water molecular rotation are correlated. The activation volumes extracted from the NMR data show only a small further decrease as the water content is increased from 6–22 wt%. Possible explanations for the high water content NMR pressure results are given.

Pressure effects on the rates of Hg 2+-assisted chloride release from some [CrCl(diamine)(triamine)] 2+ complexes

The rate of Hg 2+-assisted chloride release from several mer-[CrCl(diamine)(triamine)] 2+ complexes has been measured as a function of pressure, Hg 2+ concentration and temperature. The calculated activation volumes are independent of [Hg 2+] and temperature and kinetic parametes 10 4 k Hg (25 ° c) ( M −1 s −1), ΔH ‡ ( kJ mol −1), ΔS ‡ ( J K −1 mol −1), ΔV ‡ ( cc mol −1) are: (en)(dpt): 6.44. 75.5, −52, −5.0; (ibn)(dpt): 5.81, 89.5, −6, −0.03; (Me 2tn)(dpt): 22.2, 84.9, −11, −0.5; (tn)(dpt): 29.1, 87, −1, +0.3; (en)(2,3-tri): 1.94, 87.0, −24, −5.7; (en)(Medpt): 0.417, 94.6, −11, −0.8; (tn)(Medpt): 9.14, 98.3, +26, +1.8.

Self-Diffusion in Compressed Dimethylether: The Influence of Dipole-Dipole Interaction and Hydrogen Bonding Upon Translational Diffusivity in Simple Fluids

Abstract Self-diffusion coefficients of dimethylether have been measured as a function of temperature (185-458 K) and pressure (up to 200 MPa) by pulse-gradient field spin-echo NMR. Calculated activation volumes (ΔV*) fall from 15 (10-6 m3/mole) at the highest temperature to 7 (10-6 m3/mole) at the lowest. This trend is in keeping with values for other non-associating liquids. Comparison with self diffusion data for propane and ethanol reveals that the translational diffusivity in dimethylether is influenced by the substance's modest dipole moment (1.3 Debye). The effect is small but measur­able and becomes more pronounced at lower temperatures.