Grain Boundary Diffusion in Polycrystalline Ferrites with Different Intergranular Potential Barrier

Using the method of layer-by-layer measurements of the electrical conductivity activation energy, an investigation is performed of grain-boundary diffusion of oxygen in polycrystalline Li–Ti-ferrites in the range of temperatures 873–1073 K. A correlation relation is established between the value of the intergranular potential barrier and the parameters of grain-boundary diffusion of oxygen in polycrystalline ferrites. An increase in this value gives rise to a growth in the diffusion activation energy. The grain-boundary diffusion coefficient increases with the value of the intergranular potential barrier.     

Self-diffusion in polycrystalline silver

An experimental study of self-diffusion in polycrystalline silver has been carried out using the radioisotope Ag-110. The electrolytic sectioning technique was used for measuring. This method made it possible to determine the temperature range in which the self-diffusion of silver is not influenced by grain-boundary diffusion. The temperature was found where the grain-diffusion predominates over the volume diffusion so that the grain-boundary diffusion coefficient can be estimated by Fischer's treatment. Finally, it was found that the temperature range of 619°C< <718°C is the transition region where the electrolytic sectioning technique does not permit the volume or grain-boundary diffusion coefficients to be determined.     

Grain-boundary diffusion of oxygen in polycrystalline ferrites

It is shown that layer-by-layer measurements of the electron-transfer activation energy in polycrystalline ferrites allow the profiles of depth distribution of oxygen introduced from the atmosphere into a ferrite at the stage of sintering or thermal treatment to be studied using a simple experimental technique. The analytical equations proposed here make it possible to determine the volume and grain-boundary diffusion coefficients of oxygen using a minimum number of adjustable parameters. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 64–69, May, 1999.     

Plasma-stimulated penetrability of hydrogen as a tool for studying phase transitions at grain boundaries

It is well known that the diffusion of hydrogen atoms through the intrinsic defects of a crystal lattice has characteristics different from those of bulk diffusion and, at certain parameters for some polycrystalline metals, ensures the determining contribution to the transfer of hydrogen atoms through the material. Grain boundaries (and dislocations) are the most important and shortest paths, the diffusion through which is much faster than bulk diffusion through a crystal lattice. It is particularly important to take into account this diffusion in materials with grains having sizes of about several nanometers. The possibility of using the method of the plasma-stimulated penetrability of hydrogen to analyze phase transitions at the grain boundaries is demonstrated on the example of polycrystalline niobium foils. In contrast to the existing methods, this method proposed for studying grain-boundary diffusion and phase transitions is simple and ensures control over the surface. The temperature characteristics of the diffusion of hydrogen atoms through niobium grain boundaries have been measured.     

Ionic conductivity of ThO2- and ZrO2-based electrolytes between 300 and 2000 K

A conductivity vs temperature relationship is derived which results from simplified geometrical considerations of a polycrystalline solid electrolyte, assuming that oxygen ion diffusion only takes place through the grains or along the grain boundaries. The relationship proves to be capable of describing conductivity data of YDT, CDT, YSZ and CSZ between 300 and 2000 K. The data analysis reveals that the activation enthalpy as well as the activation entropy of grain conductivity are significantly higher in comparison with grain-boundary conductivity. It may be concluded that BAUERLE's electrical equivalent circuit which underlies the interpretation of dispersive type measurements aiming at separate determination of grain and grain-boundary conductivity ought to be revised.     

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

An analysis of manifestation of the effect of acceleration of creep in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium (coating) is performed. The influence of the softened (due to impurity diffusion fluxes) layer thickness on the creep character in molybdenum is discussed for the case of its diffusion contact with nickel. An analysis of the effects of the regime of impurity grainboundary diffusion on the value of strain-rate sensitivity and on the behavior of grain-boundary migration as a function of external stress is made. Using submicrocrystalline nickel as an example, the influence of grainboundary state on properties of metallic materials in submicrocrystalline state is investigated. Direct measurements, theoretical calculations and indirect estimations of the differences in grain-boundary diffusion coefficients in coarse- and ultrafine-grained states are performed. The role of dispersion hardening in stabilizing the submicrocrystalline structure and improving the creep resistance is addressed, including the case where the grain-boundary fluxes of impurity atoms are forced from an external medium.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–64, August, 2004.     

Oxygen diffusion in SrZrO3

Oxygen diffusion coefficients in SrZrO₃ polycrystals were determined using the isotopic exchange method with ¹⁸O as oxygen tracer. Diffusion treatments were performed at different temperatures between 1173 K and 1473 K. Oxygen diffusion profiles were established by secondary ion mass spectroscopy (SIMS). Classical diffusion equations were used to fit experimental results and to determine bulk diffusion (D_vol) and surface exchange (k) coefficients of oxygen in SrZrO₃ polycrystalline materials. From these values, bulk diffusion and grain boundary diffusion coefficients as well as oxygen surface exchange coefficients were determined. The activation energy of oxygen diffusion in the bulk is 2.1 eV, while for the diffusion in the grain boundary, 1.8 eV was found. The surface exchange reaction has an activation enthalpy of 1.2 eV.     

Conductivity of single-crystal and polycrystalline bilayer metal films under the conditions of interdiffusion

The electrical conductivity of a bilayer film with a single-crystal or polycrystalline structure is theoretically investigated under the conditions of metal interdiffusion at an arbitrary ratio between the thickness of layers and the mean free path of electrons in the layers. It is shown that analysis of the changes in the electrical conductivity of the bilayer film due to diffusion annealing allows one to elucidate the nature of the processes of bulk and grain-boundary diffusion, to determine the effective depth of penetration of impurity atoms into the bulk and grain boundaries of the sample, and to obtain information on the bulk and grain-boundary diffusion coefficient.     

Molecular-Dynamics Simulation of Grain-Boundary Diffusion Creep

Molecular-dynamics (MD) simulations are used, for the first time, to study grain-boundary diffusion creep of a model polycrystalline silicon microstructure. Our fully dense model microstructures, with a grain size of up to 7.5 nm, were grown by MD simulations of a melt into which small, randomly oriented crystalline seeds were inserted. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to MD simulations (of typically 10^-9s), our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogenous (i.e., involving no grain sliding), with a strain rate that agrees quantitatively with that given by the Coble-creep formula.     

Self-diffusion of lithium,hydrogen, and oxygen ions in crystalline lithium hydroxide

The self-diffusion coefficients of ions of the three chemical elements forming lithium hydroxide have been determined by the crystal-crystal and crystal-gas isotope exchange method in the temperature range 500–720 K. Crystal samples with different isotope compositions have been grown by the Bridgman method from melts. The melting temperature is 743 ± 2 K. Original methods have been developed for high-precision measurements of the isotope ratios of all three elements, i.e., lithium (⁶Li/⁷Li), hydrogen (H/D), and oxygen (¹⁶O/¹⁸O), and their changes after diffusion annealings with the use of the same sample. The self-diffusion coefficients of lithium and hydrogen ions differ but by a factor of no more than 3–5; however, their values exceed those for oxygen by several orders of magnitude. In particular, at 670 K, they are equal to 6.0 × 10⁻⁹, 3.2 × 10⁻⁹, and 2.0 × 10⁻¹² cm² s⁻¹ for hydrogen, lithium, and oxygen, respectively. In the range 680–720 K, the self-diffusion coefficients of hydrogen and lithium increase sharply with increasing temperature to approximately 10⁻⁶ cm² s⁻¹. A probable mechanism of migration of protons and lithium ions in LiOH and the role played in this process by the oxygen ions with a lower mobility have been discussed.     

Self-diffusion at grain boundaries with a disordered atomic structure

A change in the free energy of a grain boundary is analyzed in the case when lattice vacancies come to the boundary and are then delocalized in its disordered atomic structure. It is shown that the free energy of the boundary is minimized at some excess atomic volume Δv _b =Δv _b ^* , whose value depends on the energy of vacancy formation in the crystal lattice and the boundary energy. The formation of a metastable localized grain-boundary vacancy as a result of thermal fluctuations of the density in a group of n ₀=\gM_v/Δv _b atoms (\gM_v is the vacancy volume), followed by the jump of an adjacent atom into this vacancy, is taken as an elementary event of grain-boundary diffusion. Expressions for the activation energy of diffusion and the diffusion coefficient are derived for equilibrium (Δv _b =Δv _b ^* ) and nonequilibrium (Δv _b >Δv _b ^* ) boundaries.     

Diffusion of 22Na in Cd-doped silver bromide and silver chloride

The diffusion coefficients of ²²Na tracer in zone-refined, CdBr₂-doped, polycrystalline AgBr were measured. An approach is developed to calculate the intrinsic concentration of Frenkel defects in AgBr and AgCl based on the ²²Na diffusion data. The theory is based on the usual assumptions of random walk and electroneutrality.     

Influence of the temperature and duration of the annealing on the lattice structure and growth of the Mg-Al spinel layer

In this paper, MgO film is successfully grown on polycrystalline and monocrystalline alumina substrates using sol-gel method, and polycrystalline and monocrystalline Mg-Al spinels are fabricated by solid state reaction, respectively. The influence of annealing temperature and time on the lattice structure and growth of the formed Mg-Al spinel layer has been investigated. It is indicated that the annealing temperature and time on the as-synthesized polycrystalline Mg-Al spinel has more significant influence than that of single crystal Mg-Al spinel. The thickness of the Mg-Al spinel layer increases with the annealing temperature, both for polycrystalline and for monocrystalline alumina substrates. And the significantly intercrystalline diffusion of Mg²⁺ ions and Al³⁺ ions results in a quicker growth velocity of the Mg-Al spinel layer than that of intracrystalline diffusion.     

Laws of the deformation and failure of molybdenum in creep with grain-boundary activation by diffusional nickel fluxes

The diffusion of nickel from the surface of polycrystalline molybdenum influences the crack formation and plastic deformation in creep. Investigation shows that diffusional fluxes of nickel from the surface reduce the critical deformation at which the first cracks appear in molybdenum. The contribution of grain-boundary slip to the deformation of molybdenum in creep is considerably increased in the presence of nickel diffusion from the surface as a result of easier crack formation and grain-boundary activation.Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 16–21, July, 1992.     

Grain-boundary sliding in elongated microstructures during diffusion creep

In diffusion creep, the contribution of grain-boundary sliding to the overall strain os can be evaluated in arbitrary polycrystals, if the angular distribution of grain boundaries is known. A os value of 0.5 is obtained for two-dimensional (2D) equiaxed microstructures consisting of regular hexagonal grains, equiaxed grains grown from a Voronoi structure or grains having a circular distribution of grain-boundary angles. The os value is also evaluated for uniaxially deformed 2D microstructures, both diffusionally and uniformly deformed. For the former, the deformed microstructure is obtained by the simulation of microstructural evolution in polycrystals with straight grain boundaries. The os value increases gradually with increasing or decreasing strain and is larger in the diffusionally deformed microstructures than in the uniformly deformed microstructures for a given grain aspect ratio. The os value for three-dimensional (3D) polycrystalline microstructures is also obtained from an ellipsoidal distribution of grain-boundary angles. The resultant os value is 0.60 for 3D equiaxed polycrystals and increases gradually with increasing strain.     

Kinetics of oxygen exchange in strontium titanate

The kinetics of oxygen exchange are of primary importance for the application of titanates as fast resistive oxygen sensors. The sensor’s conductivity is correlated with the oxygen partial pressure pO₂ of the surrounding atmosphere: Due to oxygen surface transfer and subsequent diffusion of oxygen vacancies V _O ^·· , a pO₂ change gives rise to a conductivity change of the sample. While bulk diffusion usually occurs very fast, the surface transfer reaction becomes the rate determining step for thin samples and for low temperatures. We have shown that in the case of acceptor doped SrTiO₃ the kinetics of the surface transfer reaction can be strongly influenced through stoichiometric changes brought about by thin coatings of alkaline earth metal oxides (e.g. SrO). In contrast to the commonly used jump method (conductivity response to a sudden pO₂ change in the time domain), a model is presented which is based on the frequency-domain analysis of amplitude and phase shift of the response signal obtained from a pO₂ modulation in a fast kinetic measurement set-up. This method allows not only for measuring response times in the sub-millisecond range but also for distinguishing between behaviour either controlled by volume diffusion or by surface transfer reaction. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Intergranular Melting of Ultrafine Grained Nd2Fe14B Studied by Means of Radiotracer Diffusion

Grain-boundary (Gb) diffusion was studied in ultrafine grained Nd₂Fe₁₄B-based permanent magnets below and above the melting transition of the Nd-enriched intergranular phase using the radiotracer technique with the isotope ⁵⁹Fe. The product D _Gb of interface diffusion coefficient and interface thickness shows a substantial increase above the intergranular melting transition. Assuming a volume self-diffusivity as in -Fe, an analysis in the framework of grain-boundary diffusion kinetic of type B yields an Arrhenius-type behaviour D _Gb = 1.53 × 10^–11 exp(–1.74 eV/kT) m³ s^–1 below the intergranular melting transition. Similar values D _Gb are observed for ultrafine grained Nd-Fe-B with reduced Nd excess in the grain boundaries. The diffusion characteristics are compared with the kinetics of the hot-deformation which is of technical relevance for the processing of high-performance permanent magnets.     

Grain Boundary Diffusion in C<Subscript>60</Subscript> Thin Films

This paper focuses on the effect of grain boundaries on the diffusion processes in polycrystalline C₆₀ thin films. Electrically induced diffusion of Au was investigated by in situ measurements of the film conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy was used to study diffusion of oxygen. Increase in grain sizes in polycrystalline C₆₀ thin films was found to result in the acceleration of gold and oxygen diffusion. The results are interpreted assuming that these impurities diffuse in C₆₀ films dominantly along grain boundaries.     

On physical nature of creep acceleration effect in submicrocrystalline materials

An analysis of special features of manifestation of the creep acceleration effect in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium is made. It is noted that in certain cases where the diffusion properties of sub-boundary regions differ greatly from those within the boundary and in the bulk of the grain (e.g., in materials with submicrocrystalline structure (grain size 0.1–0.5 µm) and/or in the case of diffusion along the grain boundary of a strongly segregating impurity), conventional theoretical methods of calculation of the temperature-temporal range of the occurrence of the above effect have to be corrected. Based on the analysis of the experimental data, feasible solutions to the problem are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 29–32, October 2004.