Defect Diffusion Coefficient Research Articles

ChemInform Abstract: CHEMICAL DIFFUSION IN POLYCRYSTALLINE AL2O3 AS DETERMINED FROM ELECTRICAL CONDUCTIVITY MEASUREMENTS

AbstractBei Untersuchungen der zeitlichen Änderung der elektrischen Leitfähigkeit von polykristallinem Al2O3 bei verschiedenen Og‐Drücken wurde ein Defektdiffusionskoeffizient ermittelt, der der jeweils schneller diffundierenden Spezies, dem Al‐ oder O‐Ion,zuzuordnen ist.

Chemical Diffusion in Polycrystalline Al2O3 as Determined from Electrical Conductivity Measurements

By analyzing the changes with time of the electrical conductivity of polycrystalline Al2O3 after the O2 pressure was changed, a defect diffusion coefficient was obtained which was assigned to the Al or O ion, whichever is the faster-diffusing species. Both decreased grain size and MgO addition increase the defect diffusion coefficient. The grain-boundary defect diffusion coefficient for the undoped material was estimated to be: and that for the MgO-doped material was over the range 1100° to 1350°C (δ is the effective thickness of the boundary and s the coefficient of segregation of defects to the boundary region). The mechanism of grain-boundary diffusion is discussed in terms of defect mobility.

Theory of diffusion creep in pure, impure and non-stoichiometric ionic materials

Abstract It is shown that in pure binary crystals the diffusion creep rate is essentially that which would be predicted for the slower moving species using established theory. By separating the tracer diffusion coefficient of the species in a crystal into a defect concentration and a defect diffusion coefficient it is possible to compare these factors with the creep rate in some detail and quantitative relationships are established. The creep rate of many non-stoichiometric crystals is likely to depend on the magnitude of the departure from stoichiometry. The presence of impurity should stiffen or soften the material significantly and simple use of tracer diffusion coefficients in the diffusion creep formula is inappropriate.

Diffusion of Oxygen Vacancies in Reduced Rutile (TiO2)

Diffusion coefficients of point defects in reduced rutile (TiO 2 ) due to the reduction in vacuum of 4×10 -9 Torr with the ion pump in the temperature range 800°C–1100°C were studied. The coefficients along c and a -axes are \begin{aligned} D_{c}{=}147\exp\left(-\frac{2.4{\pm}0.3}{kT}\right) \text{and} D_{a}{=}91\exp\left(-\frac{2.4{\pm}0.3}{kT}\right). \end{aligned} The activation energies are very close to tat of O 18 in rutile by Haul and Dumbgen. Diffusions are discussed in terms of oxygen vacancies from the crystal orientation dependence of the coefficients and the activation energies. It is concluded that the result in this experiment gives the clear evidence of the presence of oxygen vacancies in reduced rutile and their migration energies are 2.4 eV.

真空還元にもとづくルチル（TiO<SUB>2</SUB>）中の点欠陥の拡散に対する加熱中汚染の影響について

The single crystal rutile in the form of slices was reduced in vacuum. The reduction was performed in three kinds of vacuum atmospheres, (A) vacuum by a diffusion pump without traps, (B) vacuum by a diffusion pump with liquid nitrogen traps and (C) vacuum by an ion pump. The process of reduction is considered to represent the process of diffusion of point defects induced by reduction. The diffusion coefficients of point defects in the reduced rutile were determined by the theory of the diffusion in a plane sheet with an equal surface concentration and the uniform initial distribution of solute in the sheet. These coefficients obtained in vacuum by the diffusion pump varied with the reduction time and this tendency was more remarkable in the case of the diffusion pump without traps. But the diffusion coefficients obtained in vacuum by the use of the ion pump were nearly constant at any reduction temperature independent of the reduction time except the beginning. It seems that these differences are due to the contamination by the diffusion pump. It becomes evident that specimens reduced with the diffusion pump are influenced by contamination by the diffusion pump oil. For the investigation of the reduced rutile, therefore, the ion pump should be used instead of the diffusion pump. By using the ion pump, the diffusion equation of point defects in the reduced rutile can be expressed as(This article is not displayable. Please see full text pdf.) This activation energy is very close in value to that of the oxygen vacancy in rutile.

Defect Diffusion in Single Crystal Aluminum Oxide

A defect diffusion coefficient was determined in single crystal aluminum oxide from 1400° to 1850°C by measuring the movement of the color boundary in a Ti3+− doped single crystal heated in air. In this temperature range the experimental data can be represented byThe interpretation developed is that this equation represents the diffusivity for a defect which contributes to aluminum self‐diffusion.

Der Konzentrationsgradient der Eisenionen-Leerstellen in Wüstit-Zunderschichten und der mechanismus der oxydation des eisens

The concentration gradient of iron-ion-vacancies in Wüstite scaling films is measured electrochemically. The films were obtained by oxidation of pure iron in oxygen between 650° and 1000°C. The concentration gradient of vacancies is constant over the whole Wüstite film. According to Wagner's theory of scaling a constant concentration gradient of mobile defects is to be expected under two conditions : (1) the diffusion coefficient of the defects is independent of concentration, and (2) the thermodynamic activity of the defects is proportional to concentration (or the nth power of concentration). It follows from measurements of Himmel, Mehl, and Birchenall that both conditions are almost fulfilled in the case of the diffusion of iron-ion-vacancies in Wüstite between 800° and 1000°C. From the measured gradients and velocities of oxidation of the iron specimen the diffusion coefficient of the mobile defects are calculated. At higher temperatures these values are somewhat larger than the diffusion coefficients of iron-ion-vacancies obtained from measurements of self diffusion of iron in Wüstite. Marker experiments show that this difference may be due to the participation of oxygen in the transport of matter through the Wüstite. Deviations from the parabolic law of oxidation of iron are thought to be caused by the formation of cracks in the iron-Wüstite boundary.

Oxidation of Metals at High Temperatures

The available data on the oxidation rates of pure metals from 300° to 1000°C have been analyzed in terms of concentrations of defects in the oxide structures and defect diffusion coefficients. Quantitative agreement with the diffusion theory is obtained in the cases of p‐type oxides such as cuprous oxide and nickel oxide, in which cationic vacancies arise by solution of oxygen at the oxide‐oxygen interface. The parabolic oxidation rates in the cases of some n‐type oxides, such as zinc oxide and zirconium oxide, are characterized by low activation heats and large negative activation entropies. In some of these cases anionic diffusion seems likely. In the case of zinc oxide, the diffusion of radiozinc during oxidation supports the cationic diffusion mechanism. The data on linear oxidation rates fall into two classes; evidence is presented to show that one class represents reaction at the oxide‐oxygen interface, and the other, reaction at a metal‐oxygen interface.