Activation Enthalpy Of Diffusion Research Articles

The Influence of Hofmeister Series Ions on Hyaluronan Swelling and Viscosity

The dissolution of hyaluronan in water leads to its degradation, and as a result its molecular weight decreases. The degradation of hyaluronan is mainly influenced by temperature, solution composition, and also its pH. This study describes the influence of Hofmeister series ions on hyaluronan behaviour and hyaluronan film swelling by solutions of these ions. It was found that Hofmeister ions show lyotropic effects influencing the entanglement of hyaluronan coils and their expansion from solid polymer films into swollen gel state. The hydrophobic and hydrophilic interactions in the structure of hyaluronan macromolecules are represented by the mutual diffusion coefficient D(c), the mean mutual diffusion coefficient Ds, the expansion work of coil swelling RAδ,s, the activation enthalpy of diffusion connected with swelling HD,s and kinematic viscosity of hyaluronan-ions solutions ν.

Vacancy-mediated dopant diffusion activation enthalpies for germanium

Electronic structure calculations are used to predict the activation enthalpies of diffusion for a range of impurity atoms (aluminium, gallium, indium, silicon, tin, phosphorus, arsenic, and antimony) in germanium. Consistent with experimental studies, all the impurity atoms considered diffuse via their interaction with vacancies. Overall, the calculated diffusion activation enthalpies are in good agreement with the experimental results, with the exception of indium, where the most recent experimental study suggests a significantly higher activation enthalpy. Here, we predict that indium diffuses with an activation enthalpy of 2.79eV, essentially the same as the value determined by early radiotracer studies.

Electrochemical behavior of lithium intercalated in a molybdenum disulfide-crown ether nanocomposite

A new nanocomposite, obtained from the intercalation of the cyclic ether 12-Crown-4 into MoS 2, Li 0.32MoS 2(12-Crown-4) 0.19, is described. The laminar product has an interlaminar distance of 14.4 Å. The electrical conductivity of the nanocomposite varies from 2.5 × 10 −2 to 4.3 × 10 −2 S cm −1 in the range 25–77 °C, being about four times higher than the analogous poly(ethylene oxide) (PEO) derivative at room temperature. The electrochemical step-wise galvanostatic intercalation or de-intercalation of lithium, leading to Li x MoS 2(12-Crown-4) 0.19 with x in the range 0.07–1.0, indicates a Li/Li + pair average potential of 2.8 V. The electrochemical lithium diffusion coefficients in the crown ether intercalates, determined by galvanostatic pulse relaxation between 15 and 37 °C at different lithium intercalation degrees, are higher than those of the PEO derivatives under similar conditions, being however the diffusion mechanism rather more complex. The variation of both, the lithium diffusion activation enthalpy and the quasi-equilibrium potentials, with the lithium content shows there are two different limit behaviors, at low and high lithium intercalation degree, respectively. These features are discussed by considering the high stability of the Li-crown ether complex and the different chemical environments found by lithium along the intercalation process.

The diffusion process of sodium hyaluronate (Na‐Ha) and Na‐Ha‐n‐alkyl derivatives films swelling

The process of diffusion of sodium hyaluronate and its hydrophobically modified alkyl derivatives in water was studied by measuring the kinetics of solid polymer film swelling. The following fundamental thermodynamic parameters of the swelling process were calculated: the apparent diffusion coefficient of swelling by water D(s) determined at three temperatures (25, 37, and 45 degrees C), the activation enthalpy of diffusion connected with the swelling DeltaH(D,s) and the activation enthalpy of the swelling process DeltaH(s). The thermodynamic activity of the solvent for the given sodium hyaluronate and its four alkyl derivatives has been expressed through the internal quantity RA(delta,s), that is expansion work of polymer coil accomplished by the action of the internal pressure.

Barium diffusion in mixed cation glasses

Diffusion of barium (Ba) in mixed cation glasses of the composition xNa2O x (3 - x)BaO.4SiO2 with x = 0.0, 0.1, 0.3 and 1.0 and 0.4K2O.2.6BaO.4SiO2 was investigated by means of the radiotracer diffusion technique below the respective glass transition temperatures. In accord with our previous results of calcium (Ca) diffusion in soda-lime silicate glasses (F. Natrup et al., Phys. Chem. Chem. Phys., 2005, 7, 2279), the mobility of alkaline-earth ions increases with the alkali content in all analyzed glass systems with no decrease in the diffusion activation enthalpy, but a raise in the pre-exponential factor. A distinct dependency of the activation enthalpy of alkaline-earth ions on the type and content of the alkali ions in the glass is observed. The results provide evidence for elastic and electrostatic contributions to cation diffusion in glasses and support the formation of dissimilar cation pairs, that were derived from nuclear magnetic resonance (NMR) investigations of soda-lime silicate glasses and glasses containing sodium and barium. Finally, a striking correlation between the pre-exponential factor of alkaline-earth ion diffusion in soda-lime and potassium barium glasses is found, the origin of which remains unsolved.

Lanthanide transport in stabilized zirconias: interrelation between ionic radius and diffusion coefficient.

The diffusion of all stable lanthanides was measured both in calcia stabilized zirconia (CSZ) and in yttria stabilized zirconia (YSZ) in the temperature range between 1,286 and 1,600 degrees C. The lanthanide diffusion coefficients obtained increase with increasing ionic radius. The experimental activation enthalpy of diffusion is near 6 eV for CSZ and between 4 and 5 eV for YSZ and is not strongly affected by the type of lanthanide. The results were correlated with defect energy calculations of the lanthanide diffusion enthalpy using the Mott-Littleton approach. An association enthalpy of cation vacancies with oxygen vacancies of about 1 eV (96 kJ/mol) was deduced in the case of CSZ, while there is no association in the case of YSZ. Furthermore, the change in diffusion coefficients can be correlated to the interaction parameter for the interaction between the lanthanide oxide with zirconia: The higher the interaction parameter, the higher the lanthanide diffusion coefficient.

On the mechanism of annealing effect in electrical resistivity of sub-100 nm Ag (1% W) films

Our 50 nm Ag(1% W) film deposited by electroless onto Pd activated SiO2 shows rather high resistivity ρ≈35 μΩcm, which is caused most likely by open porosity and weak contacts between the grains [E. Glickman, A. Inberg, V. Bogush, Y. Shacham-Diamand, N. Croitoru, Microelectron. Eng. 70 (2003) 45]. The resistivity does not decrease after long aging at 20 °C or vacuum annealing at 80 °C, but shows a 3-fold drop after 1 h of isothermal vacuum annealing at 100 °C. Assuming the diffusion nature of the process responsible for the resistivity decay, we derived from the annealing kinetics ρ(T,t) at 100–300 °C the diffusion activation enthalpy E=0.19 eV and pre-factor D0∼(10−8–10−12) cm2/s. It is shown that these extraordinary small values are not compatible with any mechanism of bulk- or grain boundary diffusion in Ag, but can be explained in terms of surface diffusion (SD) controlled, surface tension driven sintering. It is assumed that surface of the film is contaminated/oxidized that prevents production of thermal equilibrium adatoms. Formation of non-thermal Ag adatoms, which are considered to mediate SD mass transport, is explicable in terms of decomposition of non-stable silver oxides on the film surface. The AFM, HRSEM, DSC and SIMS observations are in general agreement with the proposed sintering mechanism.

Modelling diffusion in crystals under high internal stress gradients

Diffusion of vacancies and impurities in metals is important in many processes occurring in structural materials. This diffusion often takes place in the presence of spatially rapidly varying stresses. Diffusion under stress is frequently modelled by local approximations to the vacancy formation and diffusion activation enthalpies which are linear in the stress, in order to account for its dependence on the local stress state and its gradient. Here, more accurate local approximations to the vacancy formation and diffusion activation enthalpies, and the simulation methods needed to implement them, are introduced. The accuracy of both these approximations and the linear approximations are assessed via comparison to full atomistic studies for the problem of vacancies around a Lomer dislocation in Aluminium. Results show that the local and linear approximations for the vacancy formation enthalpy and diffusion activation enthalpy are accurate to within 0.05 eV outside a radius of about 13 Å (local) and 17 Å (linear) from the centre of the dislocation core or, more generally, for a strain gradient of roughly up to 6 × 106 m−1 and 3 × 106 m−1, respectively. These results provide a basis for the development of multiscale models of diffusion under highly non-uniform stress.

Oxygen diffusion in yttria stabilised zirconia—experimental results and molecular dynamics calculations

Bulk oxygen self-diffusion in yttria-stabilised zirconia (YSZ) was investigated using tracer diffusion experiments and molecular dynamics (MD) simulation as a function of the yttria content. Experimentally, 18O tracer diffusion was measured as a function of temperature (650–1200 K) and yttria content (8–24 mol% Y2O3) using gas-phase exchange of the stable isotope 18O and SIMS analysis. For a given temperature, the diffusivity was highest for YSZ containing 10 mol% yttria. The activation enthalpy of diffusion was 0.8 to 1.0 eV, independent of the yttria content. The diffusion process was simulated with molecular dynamics using the program DL_POLY and comparing different potential sets. The oxygen diffusion coefficient was found to be of similar magnitude to the experimental value, and also showed similar concentration dependence with a maximum for YSZ containing 10 mol% yttria. The calculated activation enthalpies of oxygen transport are close to the values observed experimentally.

Creep in copper dispersion strengthened with alumina particles (ODS copper)

The creep behaviour of ODS copper is investigated in two distinctly different temperature intervals. In both intervals creep exhibits threshold behaviour; the true threshold stress decreases with increasing temperature. In the lower temperature interval (673–773 K) the creep strain rate is dislocation core diffusion controlled and the true stress exponent is close to 7. In the higher temperature interval (923–1023 K) creep is controlled by the lattice self-diffusion and the true stress exponent is close to 5. In both temperature intervals the apparent activation energies are much higher than the respective activation enthalpies of diffusion, i.e. the true activation energies of creep, and the apparent stress exponents are much higher than the respective true stress exponents. The differences of apparent and true activation energies are fully explained by the observed temperature dependences of the true threshold stresses. The differences between the apparent and the true stress exponents are shown to be inherent to the true threshold creep behaviour.

Diffusion of calcium and barium in alkali alkaline-earth silicate glasses

Diffusion of the radioactive isotopes 45Ca and 133Ba in alkali alkaline-earth silicate glasses of compositions 2O·2CaO·4SiO2 with = Li, K, Cs and K2O·2BaO·4SiO2 was investigated at temperatures below the respective glass-transition temperatures. The diffusion profiles of the radiotracers were recorded by means of an ion-beam sputtering technique in conjunction with activity measurements. Analysis of the Ca and Ba profiles is based on the thin-film solution of the diffusion equation and a contribution which takes into account the instrumental broadening of the profiles caused by sputtering effects. The diffusion coefficients of the alkaline-earth ions were determined down to values of as low as 10−21 m2 s−1. Our diffusion data enables us to determine accurate values for the activation enthalpy of diffusion and the pre-exponential factor. The activation enthalpies reveal a distinct correlation with the radius ratio of the alkaline-earth ions to the alkali ions and show that the alkaline-earth ions are most mobile when the radii of the alkaline-earth and alkali ions are similar. The diffusion study strongly supports recent results from mechanical loss spectroscopy. A simple model is proposed which qualitatively explains the correlation of the alkaline-earth mobility with the radius ratio of the cations.

59Fe Grain Boundary Diffusion in Nanostructured γ-Fe-Ni

For the first time, the grain boundary diffusion was investigated in a nanocrystalline material in the whole spectrum of possible kinetic regimes: types-C, B, AB, and A. The microstructure of the nanocrystalline γ-Fe-40 wt.% Ni alloy revealed two different levels of organization: the individual nano-scaled grains build up micrometer-large clusters or agglomerates of the grains. The existence of the two types of internal interfaces in the material - the boundaries between the nano-grains and between the agglomerates of these nanocrystallites (the inter- and intraagglomerate boundaries) - affected notably the C-regime of 59 Fe radiotracer diffusion and resulted in two-stage diffusion penetration profiles. The careful profile processing according to formal criteria of the given kinetic regimes allowed, for the first time, to elaborate a consistent picture of the diffusion behavior in this nanocrystalline material with a complex bimodal distribution of equilibrium and nonequilibrium internal interfaces in the same material. The absolute values of the measured diffusion coefficients and the Arrhenius parameters of Fe diffusion along the nanocrystalline boundaries in nano-γ-Fe-40 wt.% Ni (Do = 4.2 × 10 -3 m 2 s -1 , Q = 187 kJ mol -1 ) are very similar to the Fe grain boundary diffusivity in coarse-grained γ-Fe. The activation enthalpy of diffusion along the inter-agglomerate boundaries (D 0 = 3.4 × 10 -3 m 2 s -1 , Q = 148 kJ mol -1 ) is notably smaller and the absolute diffusivity is larger by two to three orders of magnitude than the corresponding values of Fe diffusion via the nanoboundaries. The observed features are explained by a quasi-equilibrium structure of the nanocrystalline GBs, related to the grain growth during sample preparation, whereas the residual interagglomerate boundaries remained in nonequilibrium state. The potential contribution of the numerous triple junctions in the nano-material to short-circuit diffusion was investigated in detail and was found to be negligible under the present experimental conditions.

Bulk and interface boundary diffusion in group IV hexagonal close-packed metals and alloys

Bulk and grain boundary (GB) self-diffusion and substitutional solute diffusion in group IV hexagonal close-packed (hcp) metals (α-Ti, α-Zr, and α-Hf) are reviewed. The recent results obtained on high-purity materials are shown to approach closely the “intrinsic” diffusion characteristics. The enhancement effect of fast-diffusing impurities (such as Fe, Ni, or Co) is discussed for both self- and substitutional bulk solute diffusion in terms of the interstitial solubility of the impurity atoms. In GB self-diffusion, the impurity effect is found to be less dramatic. The results obtained on high-purity hcp materials can be interpreted in terms of intrinsically ‘normal’ vacancy-mediated GB diffusion, with the ratio of GB to volume diffusion activation enthalpies of Qgb/Q ≈ 0.6. The GB self-diffusion in group IV hcp metals reveals distinct systematics. Bulk self-diffusion and fast interstitial solute diffusion (Fe and Ni) in the hcp phase α2-Ti3Al are reviewed. Interphase boundary diffusion of Ti in the unidirectional lamellar α2/γ structure of the two-phase Ti48Al52 alloy is analyzed with respect to the phase boundary structure and GB self-diffusion in α2-Ti3Al.

Activation Enthalpy and Entropy for the Formation of 9-Cyanophenanthrene Exciplexes

Exciplexes of 9-cyanophenanthrene with a series of weak electron donors with the Gibbs energy of electron transfer ΔGet* varying in the range –(0.02–0.09) eV were studied. The exciplexes exhibited fairly intense emission both in nonpolar and aprotic polar solvents. The kinetics of the exciplex formation was found to be controlled mainly by diffusion and reactant orientation. This is clearly manifested in the low-temperature region in which the activation enthalpy of exciplex formation is very close to the activation enthalpy of diffusion, and the activation entropy of exciplex formation does not exceed 18 J mol–1 K–1 in absolute value.

Ni tracer diffusion in the B2-compound NiAl: influence of temperature and composition

The effect of composition and temperature on Ni bulk self-diffusion is investigated for nine different single crystalline NiAl alloys with well-defined compositions between 46.8 and 56.6 at.% Ni in the temperature range from 1050 to 1630K. The diffusion penetration profiles of Ni in NiAl were determined by applying two different techniques of profile detection. Radiotracer experiments have been carried out using the 63Ni tracer, a serial sectioning technique, and sensitive liquid scintillation counting for the high temperature measurements, while at lower temperatures the diffusion profiles were analyzed by secondary ion mass spectrometry (SIMS) using the highly enriched stable isotope 64Ni. In contrast to the literature data on Ni self-diffusion in NiAl alloys by Hancock and McDonnell [Phys. stat. sol. A4, (1971) 143], the present measurements show an unexpected concentration dependence of the Ni diffusion coefficients D with nearly constant diffusivities for stoichiometric and Al-rich alloys and increasing D values with increasing Ni content on the Ni-rich side of the NiAl composition range. The effective diffusion activation enthalpy Q is equal to about 3.0±0.07 eV for the Al-rich, stoichiometric, and slightly Ni-rich NiAl alloys, while for the compositions with larger Ni content a decrease of Q was observed with increasing Ni content, for example, Q=2.39 eV for the Ni 56.6Al 43.4 alloy. The present experimental results imply that mainly the same diffusion mechanism operates on both sides of stoichiometry in NiAl. This mechanism is identified with the triple defect mechanism. Its contribution is compositionally independent. The activation energy of Q=3.18 eV was calculated for the triple defect mechanism using empirical EAM potentials in agreement with the experimental data. The decrease of Q at large Ni concentrations on the Ni-rich side is explained by an additional contribution of the anti-structure bridge mechanism with the activation energy of Q=1.73 eV.

Diffusion in intermetallic phases of the Fe-Al system

Abstract Self-diffusion of Fe and impurity diffusion of In and Zn in three alloys of the Fe-Al system with the approximate compositions Fe3al, Fe2al and FeAl have been investigated by the radiotracer technique using the isotopes 59Fe, 114mIn and 65Zn. Zn and In also were studied with the intention of ‘simulating' Al diffusion for which no affordable radioactive isotope is obtainable. Diffusion of In and Zn is slightly faster than Fe diffusion in all cases. Most of the measurements took place in the B2 structure regions of the phase diagram, some in the A2 and few in the DO3 structure region. In Fe3Al there are two ordering transitions, i.e., between A2 and B2 and between B2 and DO3, both leading to an increase in the activation enthalpy of diffusion in the phase with higher degree of order. For increasing Al content the activation enthalpy of diffusion increases although the melting temperature decreases significantly. The results are discussed in the light of information on the defect structure in Fe-Al ...

Solute diffusion of Ni in icosahedral Al-Pd-Mn quasicrystals

Abstract This paper presents the results of an experimental study of Ni solute diffusion in the single-grain icosahedral quasicrystalline compound Al70Pd21Mn9. Ni solute diffusion coefficients were measured over a temperature range from 699 to 1074 K. The radiotracer technique in combination with serial sectioning of the diffusion samples was used to determine concentration depth profiles. No deviation from Arrhenius behaviour was detected in the temperature range covered by the present experiments. The results are compared with the available diffusion data on quasicrystals and on diffusion in Al and various diffusion mechanisms proposed for quasicrystals are discussed. The most striking fact is that the diffusion of Ni and the diffusion of other transition-metal elements are found to be much slower than diffusion of non-transition-metal elements such as Ge and Zn in i-(Al-Pd-Mn) quasicrystalline alloys. Moreover, the activation enthalpies of diffusion of the transition-metal elements turned out to be con...

The effect of composition and temperature on grain boundary diffusion of 63Ni in Ni3Al alloys

The effect of stoichiometry on grain boundary self-diffusion of Ni in Ni3AI was investigated for compositions between 73 and 78 at.% Ni in pure Ni 3 Al alloys in the temperature range from 968 to 1190 K. Diffusion penetration profiles were obtained by a serial sectioning technique using the radiotracer 63 Ni and sensitive liquid scintillation counting. A remarkable V-shaped concentration dependence of the grain boundary diffusion parameter P with a minimum near the stoichiometric composition was observed. The grain boundary diffusion activation enthalpy Q gb Ni decreases with increasing deviation of the Ni concentration from approximately stoichiometric Ni 3 Al. This behaviour can be understood and will be discussed with respect to computer simulation studies of the change in composition and state of order of grain boundaries in non-stoichiometric Ni 3 Al.

Diffusion of hydrogen isotopes in the cubic Laves phase A12Gd

Abstract In the cubic (C15) Laves phase AI2Gd the diffusion of hydrogen and deuterium was investigated using the method of the magnetic after-effect. The samples were charged electrolytically; the hydrogen concentration obtained was determined by means of thermal desorption measurements and was shown to be at most only 0–01 hydrogen or deuterium atoms per formula unit. The after-effect spectra showed peaks at 58 and 69 K after hydrogen charging and peaks at 70 and 85 K after deuterium charging. Numerical analysis of the spectra yielded short-range diffusion activation enthalpies of 0·13 ± 0·01 and 0·16 ± 0·01 eV for hydrogen and 0·15 ± 0·01 and 0·18 ± 0·01 eV for deuterium, with pre-exponential factors of T0 = 0·5 × 10–11±0·5 S(hydrogen) and T0 = 3 × 10–11±0·5 S(deuterium). From these data, conclusions can be drawn concerning diffusion paths and occupation of interstitial sites by hydrogen and deuterium atoms in AI2Gd. By quantum-mechanical evaluation the lattice activation energy and the parameters of th...

Threshold creep behaviour of an Mg [sbnd]Zn [sbnd]Ca [sbnd]Ce [sbnd]La alloy processed by rapid solidification

The creep behaviour of an Mg 3Zn Ca 1.5Ce 0.5La alloy processed by rapid solidification was investigated at temperatures ranging from 473 K to 523 K and applied stresses up to 82.6 MPa. It is shown that the minimum or steady-state creep strain rate is not attained up to a true creep strain of about 0.35. At least up to this creep strain, the true creep strain rate decreases in time essentially linearly for strains higher than 0.03. Thus primarily the true creep strain rate ε 5 (the subscript 5 refers to a true creep strain of 0.05) is used to describe the creep behaviour of the alloy. In double logarithmic coordinates, the true creep strain rate ε 5 depends on the applied stress in a way that strongly suggests a true threshold stress. The threshold stress σ TH decreases with increasing temperature and the same holds for the true applied stress exponent n. The values of this stress exponent are low, decreasing from 3 at 473 K to 2 at 523 K. At applied stresses ranging from about 10 MPa to about 85 MPa, the activation enthalpy (true activation energy) of creep ΔH is significantly higher than the activation enthalpy of diffusion in the alloy matrix lattice ΔH L. Analysis of the creep data suggests that the process (or dislocation mechanism) controlling the creep strain rate in the alloy is either dislocation slip in the basal planes dependent on cross slip to non-basal planes or, more probably, dislocation glide in non-basal slip planes dependent on the nucleation of kink motion.