Concentration Of Cation Vacancies Research Articles

Effects of MgF<sub>2</sub> Addition on Formation of Sintered MgFe<sub>2</sub>O<sub>4</sub> Ferrite Compacts and Its Magnetic Properties

The effects of MgF2 addition on the formation of sintered ferrite compacts were studied. They were prepared by mixing and sintering at 1300°C for 2h in air the following compositions. MgO; 50-x, MgF2; x, Fe2O3; 50mol% (x=0, 1, 4, 7, 12 and 20). During sintering almost of fluorine sublimed, its amount remained in the ferrite compacts was small, and seems to exist in ferrite lattice. A large amount of MgF2 addition (x≥12) resulted to precipitate the foreign phase on the surface of the ferrite compacts in the form of such as MgO. Sinterability decreased practically with increasing amount of MgF2. This decrease was attributed to a presumable decrease of cation vacancy concentration by addition of MgF2.The effects of MgF2 addition on magnetic properties and crystal lattice constant of magnesium ferrite were studied. Saturation magnetization and crystal lattice constant decreased by addition of MgF2, on the contrary, Curie temperature increased. B-H (magnetic hysteresis) characteristics also changed. Namely, magnetic flux density decreased, while coercive force increased.

Trivalent cobalt in nickel-zinc ferrites—I. chemical problems

By using special preparation techniques the amounts of trivalent cobalt present in irondeficient Ni-Zn-Co ferrites have been varied. Co 3+ concentrations have been measured very accurately by careful chemical analyses. It has been shown that under normal sintering conditions, e.g. 1200°C in oxygen, the amount of Co 3+ just compensates the iron deficiency. The cation vacancy concentration is low. Cation vacancies are formed at lower temperatures near pores and grain boundaries.

ChemInform Abstract: REACTIVITY AND CATALYTIC ACTIVITY OF COPPER CHLORIDES PART 4, ELECTRICAL PROPERTIES AND ACCEPTOR LEVELS PRODUCED IN CUCL BY CL2 GAS

AbstractMessungen der elektrischen Leitfähigkeit, der Thermo‐EMK und der Transportzahlen von reinem oder mit Cl2‐Gas behandeltem CuCl im Bereich von 24‐240°C zeigen, daß die beobachteten Erscheinungen weniger auf elektronischer als auf ionischer Leitung beruhen, abgesehen von den Messungen an reinem CuCl oberhalb 140°C. Zwei Akzeptorniveaus bei 0,51 und 0,88 eV oberhalb des Valenzbandes werden nachgewiesen.

Rate of Transition from the Disordered Lattice Structure to the Ordered One in the Lithium Ferrite, Li<sub>0.5</sub>Fe<sub>2.5</sub>O<sub>4</sub>

The rate of transition from the disordered lattice to the ordered lattice in the lithium ferrite has been found to depend strongly on the preparation conditions of the ferrite sample. The present study was done to investigate the governing factors of the transition rate. Various samples of ferrite were made by changing the preparation conditions variously and their long-range order parameter was measured by X-ray diffraction. The results obtained are as follows.1) The order-disorder transition temperature of the lithium ferrite was found to be 725°C in a flowing atmosphere of nitrogen and to be 750°C in a flowing atmosphere of oxygen.2) The disorder-to-order transition proceeds at an observable rate in a temperature range from 400°-450°C to the transition temperature. In a temperature below 400°-450°C the transition scarcely proceeds.3) The experimental results show that the disorder-to-order transition rate is governed by the oxygen content in the ferrite sample. The transition rate becomes larger in proportion to the oxygen content.4) It is considered that the concentration of cation vacancies on B-sites (octahedral sites) of the ferrite increases with the oxygen content, and through those vacancies the exchange of lattice position between Li+ and Fe3+ ions on B-sites will easily proceed to complete the transition.

Reactivity and catalytic activity of copper chlorides. Part 4.—Electrical properties and acceptor levels produced in CuCl by Cl2 gas

Studies of electrical conductivity, thermoelectric power and transport numbers in CuCl, pure or after exposure to Cl2 gas, in the range 24–240°C, have shown that all phenomena observed are caused by electronic rather than ionic conduction, except in pure CuCl above about 140°C. Two acceptor levels are identified, at 0.51 and 0.88 eV above the valence band. The former is attributed to cation vacancies; the latter, tentatively, to anion interstitials. For high concentrations of cation vacancies, the conduction below 100°C is predominantly by electrons in a narrow band at the acceptor level, the activation energy of this “hopping” process being 0.36 eV. The difference between the two acceptor levels is briefly discussed in terms of ligand field splitting of the Cu 3d orbitals.

Phase relations and transformations in the system PbTe-GeTe

Results of a study of the pseudobinary system PbTe-GeTe are reported and discussed. A new phase diagram, the dependence of the lattice constants on alloy composition, and measurements of a phase transformation in Pb 1− x Ge x Te are presented. Complete solid solubility is found above 570°C. An exsolution dome extends from a maximum at 570°C (near 60 mole % GeTe) to about 5 and 96 mole % GeTe at 300°C. For alloys with compositions near GeTe the unit cell parameters depend markedly on the concentration of cation vacancies. The temperature for the cubic to trigonal phase transformation depends on alloy composition, decreasing from about 670°K for x = 1 to 0°K for x ≈ 0·01. The variation of lattice parameters at the transition temperature is continuous within experimental precision.

Disaccommodation in manganese-zinc ferrite

The temperature spectrum of disaccommodation for manganese-zinc ferrous-ferrite toroidal specimens is studied using two methods of magnetic shock. One of these methods, namely, disturbance using a capacitor discharge (ac demagnetization), produces a broad maximum in the disaccommodation against temperature characteristic. The other method of disturbance, using the field provided by a permanent magnet, is shown to give a discrete disaccommodation spectrum, which is used to analyse three room-temperature diffusion effects in terms of the partial pressure of oxygen during firing. One of these effects, which depends only on the oxygen partial pressure (and hence cation vacancy concentration), has not previously been reported at room temperature. The permanent-magnet disturbance is also used to investigate the magnetization states in the toroidal specimens.

The effect of cation vacancies on the distributions of cations in oxide solid solutions

A thermodynamic model taking account of cation vacancies has been formulated for oxide solutions of type ( A, B) O and A(A, B) 2 O 4. The chemical potentials of the cation species are found to be functions of the vacancy concentration and of the free energy of formation of vacancies. A form has been chosen for the excess free energy of the solid solution which includes cation-cation and cation-cation vacancy interactions. The predicted type of dependence of the cation vacancy concentration on the oxygen activity has been compared with the observed dependence for (Fe, Mn) O. According to the theory a composition gradient will be set up when the oxide solid solution is placed in an oxygen activity gradient. Assuming the same diffusion rate for all cation species, composition curves have been calculated for (Fe, Mn) O and (Fe, Ni) O growing on alloys and for an solated crystal of Fe (Fe, Co) 2 O 4 in an oxygen activity gradient.

Diffusion in polycrystalline magnesiowüstite determined by reduction kinetics

Polycrystalline magnesiowüstite samples (5 wt% Fe 2O 3 doped) were reduced (or oxidized) at various temperatures in CO 2CO gas mixtures and N 2O 2 gas mixtures. The reaction was followed continuously with a Cahn microbalance. The kinetics in the high oxygen partial pressure region (0.18−9.6 × 10 −6 atm) were controlled by cation vacancy diffusion. The chemical diffusion coefficient of iron in magnesiowüstite was found to increase linearly with the concentration of cation vacancies. The activation energy for diffusion was found to be 45 kcal/mol. The pseudochemical diffusion coefficient of iron in magnesiowüstite evaluated from the kinetics of reduction and oxidation in low oxygen partial pressure atmospheres (10 −7–10 −10 atm) was found to decrease as the concentration of cation vacancies increased. The reduction kinetics could be interpreted best by assuming an initial desorption of oxygen on the surface of the solid solution, and the migration of oxygen away from the grain boundaries of the sample followed by an associated solid state diffusion process.

Einfluß des Kationenleerstellengehaltes auf die Lage des Curie‐Punktes der MnZn‐Ferrite

AbstractA Curie point shift in MnZnFe‐ferrites after sintering or annealing at small partial oxygen pressures between 10−1 and 10−3 Torr at 1200°C was measured. The effect is explained with the increasing number of Zn‐ions on B‐sites and Fe3−‐ions on A‐sites of the spinel lattice at high temperatures. Because of the small cation vacancy concentration at these partial oxygen pressures, the Zn‐ions on B‐sites cannot return on the A‐sites during cooling and the Curie point is shifted to higher temperatures.The Curie point shift can be used to determine the stoichiometric state of MnZnFe‐ferrites, if the desaccommodation method and the determination of the active oxygen cannot be used.

Temperature dependence of passive film growth on iron at constant current

The temperature dependece of anodic oxide growth on passive iron under steady-state galvonostatic conditions has been studied in the temperature range 14-60°C. The activation energy of the ionic conduction process in γ-Fe 2O 3 is 1·39 eV. The temperature dependence of the field coefficient is linear but shows an intercept at zero 1/T. The pre-exponential term is used to compute the mobile ion concentration, which is compared with values estimated from the crystal lattice data. The experimental value is practically equal to the cation vacancy concentration, but firm conclusions on the kind of activation control (internal or interfacial) are premature in view of the logarithmic accuracy.

Transition kinetics during linear to parabolic oxidation of chromium

The transition kinetics during linear to parabolic oxidation of chromium in H 2O-H 2 gas mixtures have been measured thermogravimetrically. Based on a cation-deficit defect structure for Cr 2O 3, the transition follows a model first proposed by C. Wagner. Under conditions not far removed from the Cr, Cr 2O 3, H 2O-H 2 equilibrium the linear region of oxidation is controlled by the adsorption and dissociation of H 2O on Cr 2O 3; the phase boundary reaction is independent of the concentration of cation vacancies at the Cr 2O 3/gas phase boundary.

Diffusion of nickel into lead telluride between 200° and 400°C

The diffusion of nickel into lead telluride has been studied between 200° and 400° as a function of crystal composition and doping. Even at these temperatures, the diffusion of nickel is very dependent on the composition of the crystal. For the most part the nickel distribution profiles, as determined using radiotracer techniques, cannot be described by a simple solution of the diffusion equation. Except in crystals containing a large concentration of cation vacancies, the nickel distribution profiles are best fit by two overlapping curves, each of which is a solution of the diffusion equation with different values of the diffusion coefficient. Diffusion into crystals containing a large cation vacancy concentration can be described by a simple solution of the diffusion equation. The range of compositions which can be studied at these temperatures is restricted by the retrograde solubility of lead and tellurium in PbTe.

Low temperature suppression by chlorine of diffusion of gold in silver chloride

The tracer-sectioning technique was used to study the diffusion of aurous ion in silver chloride. Above 260°C, the diffusion proceeds interstitially with an activation energy of 0.47 eV. When the diffusion anneal was carried out in an external chlorine atmosphere, the diffusion coefficient was depressed. This behavior is explained in terms of the effect of chlorine on the cation vacancy concentration, and agrees quantitatively with results obtained from optical absorption studies by Ulrici, Koswig and Stasiw. Analysis of the data yields 0.62 eV for the heat of the reaction: ½Cl 2 = Cl − +cation vacancy + hole .

Interdiffusion studies in the system MgO-Cr 2O 3

The rate of formation of single crystal MgCr 2O 4 from MgO single crystal and polycrystalline Cr 2O 3 in air can be described by the expression k = 16.3 exp(80,000±9,000/ RT) cm 2 sec −1. The single crystal of MgCr 2O 4 grew by the advancement of(110) planes at the MgCr 2O 4/Cr 2O 3 interface. Two different methods were used for the calculation of interdiffusion coefficients of Cr 3+ in MgO in the temperature range 1340–1640°C. Wagner's recent analysis was applied for the calculation of interdiffusion coefficients in the MgO-Cr 2O 3 system where an intermediate spinel phase exists. A different mathematical treatment was derived for the calculation of interdiffusion coefficients at the saturation concentration of Cr 2O 3, in MgO single crystal. The interdiffusion coefficients increased linearly with increasing mole fraction of Cr 3+ cations and with increasing mole fraction of cation vacancies. For a concentration of one mol.% Cr 2O 3 MgO, the interdiffusion coefficient can be expressed as D = 5·50 × 10 −3 exp(67,000 ±2,000/ RT) cm 2 sec −1. The enthalpy of activation was independent of cation vacancy concentration while the entropy of activation increased with increasing vacancy concentration. The measured interdiffusion coefficients were deduced to be the intrinsic diffusion coefficients for Cr 3+ cations in MgO. Interdiffusion coefficients were in the range of 10 −12−10 −9cm 2sec −1 for the temperatures and concentrations studied.

Effect of Cold Working and Annealing on Oxidation Rates of Copper in Dry Air at Room Temperature

Abstract The effect of plastic deformation and further annealing on the kinetics of growth of thin oxide films on copper at 30 C (86 F) has been investigated. Oxidation rate was found to decrease markedly with increasing deformation. Further annealing showed an increase in the rate, the most pronounced changes occurring during the recrystallization stage. Results are interpreted in the light of Cabrera-Mott theory of growth of very thin oxide films on metals. The change in rate has been related to the concentration of cation vacancies in the Cu2O semi-conductor, which is dependent on the lattice distortion and defect concentration of the substrate metal.

Structural vacancies and solid solution decay in ferrospinels

The formation of vacancies in nonstoichiometric ferrospinels and their effect, upon solid solution stability are considered. On the basis of a solution model including vacancies of both signs, the interdependency of cation and anion vacancy concentrations, and of degree of inversion and magnetite content in solutions of Mg 1− a Fe 2+ a O 4 at a fixed temperature are obtained. It is shown that vacancy concentration can be considered as a parameter determining the structure stability. Exceeding a critical value of this parameter in a given solution leads to instability. Within the temperature range considered, this critical vacancy concentration is not temperature-dependent. The estimated interdependencies are verified by X-ray, chemical and magnetic data.

The Photochromism of Ag2S-HgI2

Abstract The bleaching kinetics of the photochromic material formed by the solid state reaction between silver sulphide and mercuric iodide at a molar ratio of 1: 4 have been studied. The thermal bleaching followed the law: where n is the concentration of coloured systems at time i, e is an activation energy term for the bleaching, A 0 and b are constants. Preliminary measurements indicate that ∈≃0·56 ev. It is suggested that the bleaching depends upon the cation vacancy concentration in the material, with silver ions diffusing via these vacancies from regions of high concentration near the surface into the bulk of the material.

Effect of Water Vapor on Initial Sintering of Magnesia

The effects of water vapor on the initial sintering of magnesia powder compacts were studied from 800° to 1107°C. The magnesia was obtained by thermal decomposition of magnesium oxalate. Water vapor partial pressures from 8 × 10 –4 to 658 mm were used. Initial sintering occurred by a grain-boundary vacancy-diffusion mechanism with magnesium assumed to be the slow moving species. Increasing water vapor partial pressure increased sintering rates. A model based on the solubility of the hydroxide ion in MgO relates this increase to the increase in the cation vacancy concentration. The grain-boundary diffusion coefficient, DG, varied with PH2O. For partial pressures up to about 5 mm, n was approximately ½ and for partial pressures above 5 mm it was between 1.0 and 1.5. The activation energy for densification was 80 kcal/mole for partial pressures up to about 5 mm and 48 kcal/mole above this partial pressure.

Remarks on: “An explanation of the high cation vacancy concentration and p-type conductivity in semiconductors containing a multivalent metal in its lowest valence state”

Remarks on: “An explanation of the high cation vacancy concentration and p-type conductivity in semiconductors containing a multivalent metal in its lowest valence state”