Diffusion of 51Cr in Surface Layers of Magnesia, Alumina, and Spinel

Diffusion of ⁵¹Cr isotope on MgO (100), Al₂O₃ (0001, and MgAl₂O₄ (111) surfaces was investigated by using the edgesource method. The surface diffusion parameter, αD,δ, where α is the segregation factor, D , the surface diffusion coefficient, and δ the thickness of the high-diffusivity layer, was determined for the temperature region 650° to 1250°C. For calculation of experimental results Whipple's solution was used. Arrhenius plots show a break at ∼1050°C for MgO and at ∼900°C for MgAl₂O₄. Above these temperatures vapor transport seems to be the overriding diffusion mechanism. Below these temperatures ionic transport predominates. Ionic transport is the predominant mechanism for surface diffusion of ⁵¹Cr on Al₂0₃ over the entire investigated temperature region. This can be explained by the weak bond between Cr-vapor species and Al₂0₃ surface. The apparent activation energies for ionic transport of 51Cr are 110 ± 12, 121 ± 12, and 119 ± 12 kJ/mol on MgO, Al₂O₃, and MgAl₂O₄ surfaces, respectively. They include enthalpy of motion and binding enthalpy and are 2.5 to 2.8 times smaller than the activation energies for volume diffusion. Investigations of surfaces by LEED, Auger, and SIMS indicated structural nonperiodicity and surface segregation of impurities.     

Diffusion of 51Cr in MgO Crystals

An arc fusion technique was used to grow single crystals of MgO with an aliovalent impurity level of ∼ 250 ppm. Diffusion coefficients for Cr³⁺ in single-crystal MgO were measured from 1353° to 1553°C using a high-specific-activity isotope,⁵¹ Cr. The diffusion coefficient of ⁵¹Cr for both arc-grown and vapor-deposited MgO crystals can be characterized by an activation energy of 70.0±2.1 kcal/mol. These results are comparable with the interdiffusion coefficients in Cr₂O₃-MgO when the interdif-fusion D is extrapolated to low Cr concentrations.     

Influence of Ionic Charge on Diffusion in the Surface Layer of Magnesia

The diffusion of ⁵⁷Co isotope on the MgO (100) surface was investigated by the edge-source method. The surface diffusion parameter, αD_sδ, where α is the segregation factor, D_s the surface diffusion coefficient, and δ the thickness of the high-diffusivity layer, was determined over the temperature region 750° to 1250°C. An Arrhenius plot shows a break at ∼1100°C. Below this temperature ionic or localized transport predominates and above it nonlocalized transport seems to predominate. The divalent Co ion diffuses faster than the trivalent Cr ion in the surface layer. The apparent activation energies for the localized surface diffusion of ⁵⁷Co and ⁵⁹Cr are 59±12 and 110±12 kj/mol, respectively.     

Oxygen Grain-Boundary Diffusion in MgO

Proton activation and autoradiography were combined to measure ¹⁸O grain-boundary diffusion in undoped and Fe-doped MgO bicrystals. Additions of 7000 ppm Fe enhance O grain-boundary diffusion at 1700°C by at most a factor of 5 over that in samples that contain 1000 atomic ppm total Al and Ca. From these results, the relative importance of grain-boundary and volume diffusion to O transport in dense polycrystalline MgO at 1700°C was estimated.     

Grain-Boundary Diffusion of 60Co and 51Cr in NiO

Grain-boundary diffusion of ⁶⁰Co and ⁵¹Cr in bicrystals and polycrystals of NiO was measured at temperatures <0.6 T_m at various equilibrium oxygen partial pressures, using a tracer-sectioning technique. Values of D 'δ were obtained using the Whipple and Suzuoka analyses. The results show that, for both ⁶⁰Co and ⁵¹Cr, grain-boundary-enhanced diffusion is observed and the temperature dependence of D 'δ is about the same as that of the respective bulk diffusivity,/). The effects of equilibrium oxygen partial pressure and the type of gram boundary on D 'δ are not detectable within experimental error.     

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.     

Grain-Boundary Diffusion of U in Pure and Doped Uranium Carbides with Different C/U Ratios

"The grain-boundary diffusivity of U in UC was measured between 1200" and 2200°C in pure carbides with a wide range of C/U ratios (0.93 to 2.00) and in material doped with up to 2.4 wt% W, V, or Ta. Grain-boundary diffusion is ∼ 10³ to 10⁵ times faster than volume diffusion, and, for UC_1,0, has an activation enthalpy of 75.9 ± 9.1 kcal/mol, ∼ 55% of that for volume diffusion. In the monocarbide region, grain-boundary mobility increases as the C/U ratio decreases and is impeded by the addition of impurities.     

Cation Self-Diffusion in Cr2O3

Self-diffusion of ⁵¹Cr was measured both parallel to and perpendicular to the c axis in single crystals of Cr₂O₃ as a function of oxygen partial pressure at 1490° and 1570°C. The oxygen-partial-pressure dependence of the diffusivity indicates that cation self-diffusion occurs by a vacancy mechanism. The values of the self diffusion coefficients determined in this experiment are about 10⁴ times smaller than those previously reported in this temperature range .     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

Concurrent Measurement of Volume, Grain-Boundary, and Surface Diffusion Coefficients in the System NiO-Al2O3

Surface, grain-boundary, and volume inter diffusion coefficients for the NiO-Al₂O₃ system were measured concurrently by using a diffusion couple consisting of an A1₂O₃ bicrystal and an NiO single crystal. The A1₂O₃ bicrystals having various tilt angles were fabricated by firing 2 single crystals to be joined in an H₂ atmosphere at 1800°C for 30 h. Diffusion profiles over the surface, along the grain boundary, and in the bulk of the bicrystal were determined with an electron probe microanalyzer. Mathematical analysis of the diffusion profiles gives D _s = 7.41×10^-2 exp (-35,200/ RT ), D _gb = 2.14×10^-1 exp (-63,100/ RT ) (tilt angle =30°), and D _v = 1.26×10⁴ exp (-104,000/ RT ). The grain-boundary diffusion coefficient increases with the mismatch at the boundary.     

Surface Diffusion in MgO and Cr-Doped MgO

The surface diffusion of MgO and Cr-doped MgO was examined using grain-boundary grooving. Measurements were taken at T=1100° to 1400°C for a concentration range of 0 to 0.442 at.% Cr. The activation energy for surface diffusion of MgO was found to be 239± kj/mol. The surface diffusivity did not change markedly with chromium concentration. How-ever, an abrupt change in activation energy from ∼240 to ∼180 kj/mol occurred between 0.038 and 0.254 at % Cr, which may be associated with a change in surface structure.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Oxygen Tracer Diffusion in La2-xSrxCuO4-y Single Crystals

The tracer diffusion of ¹⁸O in La_2-xSr_xCuO_4-y single crystals (x = 0 to 0.12) has been measured from 400° to 700°C in 1 atm of oxygen using SIMS analysis. Evidence for diffusion by a vacancy mechanism was found at low strontium contents. Oxygen diffusivities for x 2≥ 0.07 were depressed by several orders of magnitude below the diffusivity for undoped La₂CuO_4±y. The observed effects of strontium doping on oxygen diffusivity are discussed in terms of defect chemical models. The decreasing oxygen diffusivity with increasing strontium was attributed to the ordering of oxygen vacancies at large defect concentrations. A diffusion anisotropy, D _ab/ D _c, of ∼600) was also found at 500°C.     

Creep Deformation in MgO-Saturated Large-Grain-Size Al2O3

Samples of 65 μm-grain-size AL₂O₃ containing 0.05 wt% Mg and 40 μm-grain-size AL₂O₃ containing 0.13 wt% Mg, both MgO-saturated, undergo compressive deformation in the range 1580° to 1800°C with results interpreted as diffusional creep rate-limited by grain-boundary diffusion with the Coble boundary-diffusion model giving
A lower deformation rate for an unsaturated composition indicates that MgO additions enhance grain-boundary diffusion.     

Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High-purity Al₂O₃ compacts were doped with 0–350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al₂O₃ at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80–2.63 × 10⁴/ T . The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al₂O₃ were probably not done in single-phase Al₂O₃ solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A₂O₃ may be correct. The effects of SiO₂ impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al₂O₃ are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

A Diffusion-Compensation Relation for Normal Grain Growth and Grain-Boundary Diffusion of Oxygen in Oxides

Values of D₀/δ and Q for grain-boundary diffusion, derived from published studies of normal grain growth in Al₂O₃, BeO, CaO, MgO, SiO₂, and CaSiO₃, are fit by the linear compensation equation log D₀/δ=0.03170Q -7.6792 (r²=0.9384). Comparison of grain-boundary diffusion coefficients derived from grain growth in oxides with those obtained by direct experimental measurement suggests that the kinetics of normal grain growth are controlled by grain-boundary diffusion of oxygen.     

Grain-Boundary Diffusion in MgO

Grain-boundary diffusion of Ni²⁺ was investigated in polycrystalline MgO and also in isolated grain boundaries in natural and prepared bi-crystals of MgO. Concentration distributions were determined with the aid of both electron microbeam probe spectroscopy and X-ray absorption analysis. Results from diffusion couples in which a surface was maintained at constant concentration during the diffusion annealing indicate that grain-boundary diffusion is the predominant transport mechanism in polycrystalline MgO at temperatures below 1700°C. Lattice diffusion becomes increasingly important at higher temperatures and eventually becomes the dominant mechanism. Concentration distributions for diffusion couples in which a fixed amount of NiO was supplied to the surface of the couple as a thin initial plating resemble those for lattice diffusion but yield anomalously high values for apparent lattice diffusion coefficients. Grain-boundary diffusion in MgO is not confined to a layer of atomic dimensions but extends over a zone of the order of microns. The activation energy for grain-boundary diffusion is less than that for lattice diffusion and is between 1 and 2 ev. Grain-boundary diffusion was observed even in tilt boundaries with a mismatch as low as 6°.     

Electron Probe Micro Analysis of A-Site Inter-Diffusion Between LaFeO3 and NdFeO3

The A-site cation diffusion in LaFeO₃ has been examined by inter-diffusion experiments between LaFeO₃ and NdFeO₃. Dense, polycrystalline bodies were annealed in contact at temperatures between 1100° and 1300°C in ambient air. The bulk- and grain-boundary inter-diffusion coefficients were calculated from concentration profiles determined by electron probe micro analysis of cross sections. The bulk- and grain-boundary inter-diffusion coefficients showed Arrhenius-type behavior with activation energies 610±30 and 600±100 kJ/mol, respectively. Based on the assumption of 1 nm thick grain boundaries the grain-boundary inter-diffusion coefficient was ∼4 orders of magnitude higher than the bulk inter-diffusion coefficient.     

High-Temperature Deformation of Stoichiometric 239PuO2

The deformation behavior of Stoichiometric ²³⁹PuO₂ was examined at 800° to 1500°C, using direct and diametral compression. Maximum ductility was observed at 1000°C, but above this temperature both strength and ductility decreased and the fracture mode changed from transgranular to inter-granular. The deformation activation energy measured at 1000°C was 598 kJ/mol. Comparison to the deformation behavior of hypostoichiometric ²³⁹PuO_2-x suggests that high-temperature dislocation motion becomes more difficult with increasing O/Pu ratio due to effects of stoichiometry on diffusion rates. Deformation mechanisms in ²³⁹PuO₂ appear to be a combination of dislocation motion and grain-boundary sliding.