Migration Enthalpy Research Articles

Pressure and compositional dependence of electric conductivity in the (Mg 1 − xFe x) 1 − δO ( x = 0.01–0.40) solid-solution

(Mg 1 − xFe x) 1 − δO ( x = 0.01–0.43) single crystals (~ 8 mm in diameter) were made by a melt-growth method. Electrical conductivity measurements were carried out as functions of temperature and frequency by a complex impedance method under pressure (~ 43 GPa and ~ 673 K and at 0.1 MPa and ~ 1400 K). Our experimental results show a change in charge transport mechanism in the (Mg 1 − xFe x) 1 − δO solid solution at high temperature. The temperature of inflection point of the slope in Arrhenius plots depend greatly on both composition and extrinsic factors of crystals. The low-temperature conduction mechanism in (Mg 1 − xFe x) 1 − δO solid solution is small polaron. Pressure effect of the electric conductivity was observed and the conductivity increased to 0.5 at log scale of S/m with increasing pressure up to 43.4 GPa. The activation energy was decreased linearly with increasing pressure. Chemical composition and homogeneity of specimen rather than pressure greatly influence the electric conductivity. The activation energy of 2.37(4) eV for the (Mg 0.99Fe 0.01) 1 − δO solid solution might correspond to a migration enthalpy of O ions through thermally formed defects. It is proposed that a possible dominant electrical conduction mechanism in ferropericlase under the lower mantle conditions, at least in the higher temperature region, is super ionic conduction.

Niobium diffusion in niobium-doped titanium dioxide

The present work studied the self-diffusion coefficient of 93Nb in Nb-doped TiO2 single crystal (4.3 at.% Nb) at high oxygen activity [p(O2) = 21 kPa] over the temperature range 1,073 to 1,573 K. The diffusion-induced 93Nb concentration profile was determined by using secondary ion mass spectrometry (SIMS). The subsequently determined self-diffusion coefficient of 93Nb exhibits the following temperature dependence:\(D_{{}^{93}{\text{Nb}}} = 1.77 \times 10^{ - 9} \,{\text{m}}^2 \,{\text{s}}^{ - 1} \,\exp \left( {\frac{{ - 197 \pm 9\,{\text{kJ}}\,{\text{mol}}^{ - 1} }}{{{\text{RT}}}}} \right)\). This study builds upon a similar study performed previously for 93Nb tracer diffusion in undoped TiO2, and identifies the effect of compositional change on self-diffusion behaviour. The obtained activation energy has been considered in terms of migration and formation enthalpies of titanium vacancies.

Concentration of intrinsic defects and self-diffusion in GaSb

Early experiments have determined that the gallium and antimony diffusivities in gallium antimonide are similar, whereas recent more precise studies demonstrate that gallium diffuses up to three orders of magnitude faster than antimony. In the present study using electronic structure calculations we predict the concentrations and migration enthalpy barriers of important defects in gallium antimonide. It is predicted that the asymmetric self-diffusion in gallium antimonide is due to the insufficient concentration of the point defects that can facilitate the antimony transport. The results are in excellent agreement with the recent experimental evidence and theoretical studies in gallium antimonide and related materials.

Electronic Conductivity of 10 – 40 mol % YO1.5-Doped CeO2

The electronic conductivity of yttria-doped ceria, (, 0.1 step), was obtained as a function of oxygen partial pressure at , 1073, 1173, and by a dc polarization method using a Hebb–Wagner’s polarization (ion blocking) cell over an oxygen partial pressure range of . All obtained data were fitted as a sum of the electron conductivity having a dependence in the lower region and the hole conductivity having a dependence in the higher region. The activation energy for the electron and the hole conductivity exhibits the yttrium concentration dependence in an opposite way, and the sum of two activation energies has been found to be independent of the yttrium concentration. The thermodynamic considerations were made to estimate the concentrations of electron and hole, leading to the tendency that the migration enthalpy for electrons and holes is independent of the yttrium concentration, although their concentrations exhibit a strong dependence.

Synthesis of single crystal (Mg 1−xFe x) 1−δO ( x=0.001–1.00) solid-solution and electrical conduction mechanism at high temperature and pressure

The (Mg,Fe) 1− δ O rock salt-type solid solution is believed to be one of the major constituents in the Earth's lower mantle. The rock salt-type (Mg 1− x Fe x ) 1− δ O ( x=0.001–0.43) single crystals (∼8 mm in diameter) were made by a melt-growth method using high temperature graphite heater (up to 3300 K) in the Ar atmosphere and by a FZ method in the CO 2–H 2 atmosphere ( pH 2/ pCO 2=0.1). Electrical conductivity measurements were carried out as functions of temperature and frequency by a the complex impedance method under pressure (5 GPa and ∼1800 K, and, 0.1 MPa and ∼1400 K). The ratio of Fe 3+/(Fe 2++Fe 3+) in the solid solution decreases with MgO contents under the same oxygen fugacity. Our experimental results show a change in charge transport mechanism in the (Mg 1− x Fe x ) 1− δ O solid solution at high temperature. The temperature of inflection point of the slope in Arrhenius plots depend greatly on both composition and extrinsic factors of crystals. Chemical composition and homogeneity of specimen rather than pressure greatly influence the electric conductivity. The activation energy for the (Mg 0.99Fe 0.01) 1− δ O solid solution is 2.4 eV corresponding with a migration enthalpy of O ions vacancies. It is proposed that possible dominant electrical conduction mechanism in ferropericlase under the lower mantle conditions, at least in the higher temperature region, is a super ionic conduction.

DFT study of migration enthalpies in MgSiO3 perovskite

The effect of pressure on ionic diffusion in orthorhombic MgSiO3 perovskite has been investigated using density functional theory. An intensive investigation of possible silicon pathways revealed new positions of the saddle-points and an enthalpy of migration at 26.2 GPa of 4.7 eV that is in fair agreement with the experimental values of about 3.5 eV at 25 GPa. This is much lower than found in previous studies (~9 eV) and removes the need to explain silicon diffusion by a complicated process involving coupled oxygen vacancies, as has been previously proposed. Our migration enthalpies for oxygen and magnesium are in excellent agreement with experiments. We find that oxygen diffusion occurs via a chain of several inequivalent jumps along the octahedron edges, and that magnesium occurs via two inequivalent [110] jumps and one [001] jump. We also present activation volumes for all three species at 25 and 135 GPa.

Interconnection of the parameters obtained from the dielectric spectrum of CaF2 doped with ErF3 and YbF3

AbstractA model has been suggested long ago that interconnects point defect parameters with bulk properties. On this basis, the ra‐tio of the migration entropy sm and the migration enthalpy hm should be equal to a bulk quantity. The validity of this prediction is studied here for the migration sm deduced from the pre‐exponential factor τ0 over the activation energy E involved in the Arrhenius relation τ = τ0 exp (E /kT) for the relaxation time τ. The study is focused on the so called RI‐relaxation in CaF2 doped with ErF3 and YbF3. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Activation Enthalpy of Dislocation Migration in Aircraft (Aerospace) (2024) Alloy by Positron Spectroscopy

Positron annihilation lifetime is one of the most important nuclear techniques, used to study the isochronal and isothermal annealing in one of the most important engineering aluminum alloys which is 2024 alloy. Samples of 25 % deformation have been used for these studies. Two recovery stages during the isochronal annealing [1] were observed which were ascribed to the recovery of point defects and dislocations introduced by the deformation. The isothermal annealing measurements were performed at 583, 603, 623 and 643 K from which the activation energy obtained was 1.24 ± 0.08 eV.

Motion of the faceted 57° $$ [11\overline{2} 0] $$ tilt grain boundary in zinc

The impact of faceting on the high-angle grain boundary motion was studied. The non-steady-state motion of the 57° \( [11\overline{2} 0] \) tilt grain boundary (GB) half-loop in a Zn bicrystal has been studied in situ. Above 678 K the slowly migrating GB half-loop was continuously curved. Below this temperature moving GB was fully or partially faceted. The transformation of curved GB into a GB facet with increasing temperature was observed for the first time. Overlapping faceting/roughening of three crystallograpically different GB facets lead to the complicated non-steady-state motion. As a result, the GB mobility values and migration enthalpy were not unique, but lay in a certain interval.

26Al tracer diffusion in titanium doped single crystalline α-Al 2O 3

Simultaneous 18O and 26Al tracer diffusion experiments were performed on titanium doped single crystalline α-Al 2O 3. The simultaneous tracer diffusion of 18O and 26Al clearly demonstrates that the aluminium diffusivity in Ti doped alumina is orders of magnitude higher than the oxygen diffusivity. The enhanced aluminium diffusivity is caused by an increase of the concentration of mobile Al vacancies. The reasonably good agreement between earlier conductivity measurements and our transport measurements indicates in accordance with theoretical work that the migration enthalpy of aluminium vacancies is (3.8 ± 0.1) eV.

Density-functional-theory calculations for silicon vacancy migration

The saddle-point configurations and associated formation energies of a migrating silicon vacancy in the +2, +1, 0, −1, and −2 charge states were computed using density-functional theory with a plane wave basis set, norm-conserving pseudopotentials, and the generalized-gradient approximation for exchange and correlation. Spurious electrostatic and strain contributions arising from use of periodic boundary conditions were removed by performing maximum likelihood fits on results from 215-, 511-, and 999-atom supercells, and thereby obtaining formation energies corresponding to isolated vacancies. Migration enthalpies were computed by subtracting similarly obtained formation energies for vacancies in local-energy minimum configurations. The results (0.27eV in the +2 charge state, 0.19eV in the +1 charge state, 0.36eV in the 0 charge state, 0.04eV in the −1 charge state, and 0.15eV in the −2 charge state) are in good overall agreement with experimental results obtained at low temperatures.

Temperature-dependent diffusion coefficients fromab initiocomputations: Hydrogen, deuterium, and tritium in nickel

The temperature-dependent diffusion coefficients of interstitial hydrogen, deuterium, and tritium in nickel are computed using transition state theory. The coefficient of thermal expansion, the enthalpy and entropy of activation, and the pre-exponential factor of the diffusion coefficient are obtained from ab initio total energy and phonon calculations including the vibrations of all atoms. Numerical results reveal that diffusion between octahedral interstitial sites occurs along an indirect path via the metastable tetrahedral site and that both the migration enthalpy and entropy are strongly temperature dependent. However, the migration enthalpy and entropy are coupled so that the diffusion coefficient is well described by a constant activation energy, i.e., $D={D}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{exp}[\ensuremath{-}Q∕(RT)]$, with $Q=45.72$, 44.09, and $43.04\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ and ${D}_{0}=3.84\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $2.40\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $1.77\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for H, D, and T, respectively. The diffusion of deuterium and tritium is computed to be slower than that of hydrogen only at temperatures above $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At lower temperatures, the order is reversed in excellent agreement with experiment. The present approach is applicable to atoms of any mass as it includes the full coupling between the vibrational modes of the diffusing atom with the host lattice.

Defect Chemistry of LiFePO4

Based on experimental results, in particular, obtained on single crystals, the nature of the decisive point defects and charge carriers in is discussed, and the dependencies of their concentrations on the control parameters Li activity, doping content, and temperature are worked out. In the native regime characterized by Li deficiency δ, lithium vacancies being decisive for ion conduction are compensated by holes as decisive electronic carriers. Very close to the concentration of order (where δ is strictly zero) frozen-in native defects or non-intentional impurities dominate. We typically found lithium vacancies compensated by iron atoms on Li sites . Intentionally introduced donors such as or , have the same donor effect. The Brouwer diagrams displaying the logarithm of the defect concentration vs log lithium activity or vs log donor/acceptor content are derived. As to the temperature dependence of conductivities, trapping of holes by lithium vacancies is crucial and dominates the effective activation barrier at frozen lithium content. In particular, for the Al-doped samples, ionic association also proves important. Defect reaction and migration enthalpies are derived for electronic and ionic transport. Trapping of holes by lithium vacancies and association of lithium vacancies with impurities also turn out to be key for understanding the temperature dependence of the chemical diffusion coefficient of lithium.

The presence and nature of vacancy type defects in nanometals detained by severe plastic deformation

Excessive concentrations of deformation-induced vacancy type defects have been found in nanometals processed by severe plastic deformation (SPD) but not in others which suggest them to be responsible for SPD-specific properties i.e. considerable ductility and particular phase existencies. Methods of annealing resistometry as well as of calorimetry proved well in determining the concentration of these defects, and their activation enthalpy being typical of both the defect type and the actual diffusion mechanism. For identification of complex vacancy defects, comparisons with results from X-ray line profile analysis for the dislocation density revealed to be indispensable. The vacancy concentrations measured in SPD-deformed metals markedly exceed that of conventionally deformed ones, because of the presence of enhanced hydrostatic pressure as being shown by systematic investigations with various pressures. While in SPD-processed Cu only vacancy agglomerates have been found, in SPD-processed Ni also single/double vacancies are present, and the total concentration of vacancy type defects is higher. These effects are suggested to arise from the higher stacking fault energy of Ni, and of its higher enthalpy for vacancy migration.

Oxygen-18 surface exchange and diffusion in Li 2O-deficient single crystalline lithium niobate

18O/ 16O isotope exchange in combination with SIMS depth profiling was used to investigate oxygen transport in Li 2O-deficient single crystalline LiNbO 3 in the temperature range 983 ≤ T/K ≤ 1188 at 200 mbar oxygen. Within the limit of experimental error and for the investigated range of temperatures no significant differences between transport parallel and transport perpendicular to the c-axis were found. The following temperature dependencies were determined: for oxygen tracer diffusion D = 6.4 × 10 −3exp[−333 kJ/mol/( RT)] m 2/s; and for oxygen surface exchange k = 7.8 × 10 2exp[−288 kJ mol −1/( RT)] m/s. The activation enthalpy obtained for tracer diffusion can be interpreted as the enthalpy of migration of extrinsic oxygen vacancies induced by impurities with lower valency on niobium sites.

Reversible atomic processes as basic mechanisms of the glass transition

Reversible formation and disappearance of vacant spaces (vacancy-type defects) in bulk Zr57Cu15.4Ni12.6Nb5Al10 glass are directly evidenced by high-resolution, time-differential dilatometry studies. The vacancy kinetics are strongly temperature-dependent, with an effective migration enthalpy of HVM=3.34 eV. This may explain the strong temperature dependence of glass properties such as viscosity. The results presented here are of general importance for understanding amorphous condensed matter and biomaterials and for the technical development of amorphous steels.

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN) x , respectively) and silicon rich phases (amorphous Si 3N 4 and amorphous Si 3+(1/4) x C x N 4− x , respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10 −7 m 2/s can be explained with an appropriate expression for the entropy factor.

Grain Boundary Migration and Compensation Effect

Anisotropy of grain boundary motion in a Fe–6at.%Si alloy is represented by a spectrum of values of the activation enthalpy of migration and the pre-exponential factor, depending on the orientation of individual grain boundaries. The general plot of these values exhibits a pronounced linear interdependence called the compensation effect. It is shown that changes of these values, caused by changes of intensive variables, are thermodynamically consistent.

A computational study of ionic vacancies and diffusion in MgSiO 3 perovskite and post-perovskite

We have performed first-principles simulations within density functional theory to investigate the effects of pressure on the formation of defects (ionic vacancies) and ionic diffusion in the perovskite (pv) and post-perovskite (ppv) phases of MgSiO 3. Our results show that the predicted formation enthalpies of three Schottky (MgO, SiO 2 and MgSiO 3) defects are similar between the two phases at high pressures (100 to 150 GPa) with MgO Schottky defect being the most favorable. However, the calculated activation enthalpies and activation volumes of diffusion are shown to differ substantially between them. In particular, the activation enthalpies for Mg and Si diffusion in ppv are smaller than the corresponding values for pv, for example, by factors of 2.2 and 3.4, respectively, at 120 GPa, whereas the O migration enthalpy of ppv is only slightly larger than that of pv. The easy migration paths of the cations in ppv are shown to take place along the 〈100〉 direction in which Si–O octahedra share the edges. Visualization of the simulation data reveals that the vacancy defects and migrating ions induce substantial distortions in the atomic and electronic structures around them. It is suggested that diffusion is equally easy for all three species in ppv and is likely to occur through extrinsic processes near the bottom of the lower mantle.

Annealing effects on the reordering of severe plastic deformed boron doped Ni3Al

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to investigate the reordering process of a partially disordered Ni76Al24 (0.2 wt.%B) powder with annealing times and temperature. The long-range order parameter varied linearly with temperature and a significant ordered state was observed at 773 K. The DSC measurements showed one main exothermal peak in the temperature range of 493–913 K. The maximum rate of energy release occurred at 763 ± 2 K with a total thermal effect of ΔH = 52 ± 9 j/g. The DSC peak showed a shoulder around its maximum temperature of 763 ± 2 K which was attributed to order stabilization. The presence of boron in the alloy enhanced the transformation temperature by increasing the enthalpy of vacancies migration without affecting the reaction enthalpy.