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.     

Volume and Grain Boundary Diffusion of Hf in Polycrystalline Er2 O3

Hafnium diffusion was measured in polycrystalline Er₂O₃ with two impurity levels. A tracer sectioning technique was used and the diffusion coefficients were measured at 1601° to 1970°C. Volume diffusion of Hf showed both extrinsic and intrinsic behavior, with the transition temperature increasing with the impurity level present in Er₂O₃. The grain boundary diffusion was apparently extrinsic over the entire temperature interval.     

Grain-Boundary Grooving of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma-sprayed 7 wt% Y₂O₃–ZrO₂ thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma-sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain-boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat-treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.     

Cationic Self-Diffusion in Calcia-Stabilized Zirconia

Self-diffusion of Zr in 12 and 16 mol.% CaO and of Ca in 16 mol.% CaO cubic zirconia was measured by the sectioning technique from 1700° to 2150°C. Lattice self-diffusion coefficients are:
Autoradiographs show rapid grain boundary diffusion. D_B is estimated as <10⁵ D_L between 1800° and 2150°C. The low cation self-diffusion coefficients are compared with the high anion rate, and their effect on the diffusion-controlled properties of zirconia is discussed.     

Initial Sintering of BaTiO3 Compacts

Experimental measurements were made of the rate of initial shrinkage of high-purity BaTiO₃ compacts in air. The time dependence of the shrinkage rate was consistent with a model based on grain boundary vacancy diffusion. The apparent activation energy for the shrinkage rate in the range 700° to 1000°C is 112 ± 9 kcal per mole. Comparison with other data indicates that oxygen ion vacancy diffusion controls the initial sintering rate.     

Sodium Diffusion in SiO2 Glass

Diffusion of the radioactive tracer ²²Na in a commercial SiO₂ glass was investigated from 170° to 1000°C. The temperature dependence curve had discontinuities at about 573° and 250°C. The resulting Arrhenius equations are D = 3.44 × 10² exp(-21.1 kcal/RT) cm²/sec between 1000° and 573°C, D = 0.398 exp(-25.8 kcal/RT) cm²/sec between 573° and 250°C, and D = 2.13 exp(-28.3 kcal/RT) cm²/sec between 250° and 170°C. The two anomalies are discussed in terms of "quartz-like" and "cristobalite-like" precrystalline elements in the structure of the glass. Comparison of the Na diffusion in SiO, glass with that in soda-silica and soda-lime-silica glasses shows that SiO₂ glass occupies a boundary position with respect to these systems. A possible diffusion mechanism is discussed.     

Sodium Diffusion in the Nuclear Waste Glass GP 98/12

The diffusion of²² Na was measured in the nuclear waste borosilicate glass GP 98/12 in the temperature range of 350° to 450°C. The boundary condition of a thin initial tracer layer and serial sectioning were employed to measure the diffusion profiles. The activation enthalpy was found to be 1.08 eV. A parallel experiment with the base glass VG 98/12 not containing added (chemically simulated, inactive) waste showed that Na diffusion does not vary significantly when adding the waste.     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.     

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 by    
The interpretation developed is that this equation represents the diffusivity for a defect which contributes to aluminum self-diffusion.     

Oxygen Transport in Aluminum Nitride Substrates

The diffusion of oxygen in commercially available aluminum nitride substrates has been studied by performing interdiffusion-type experiments. The diffusion of oxygen both within AlN grains and along grain boundaries was investigated. By using as-received AlN substrates and an electron microprobe as an analytical tool, it was found that the rate of oxygen diffusion along grain boundaries was strongly influenced by the presence of impurities and/or other phases at these boundaries. The diffusion of oxygen within AlN grains was studied between 1600° and 1900°C in a flowing nitrogen atmosphere by using secondary ion mass spectrometry (SIMS) for determining oxygen concentration profiles. The chemical diffusion coefficient of oxygen in the AlN lattice as a function of temperature is described by the equation The oxygen concentration profiles determined by SIMS also show a contribution from diffusion along grain boundaries. Therefore, it was possible to determine values for the product δD'_o (δ= width of grain boundaries, D'_o= grain boundary diffusion coefficient of oxygen).     

Grain Boundary Grooving Studies of Yttrium Aluminum Garnet (YAG) Bicrystals

Grain boundary grooving experiments were conducted with Σ5 (210) twist boundaries in Y₃Al₅O₁₂ (YAG) with the goal of extracting information on diffusion in YAG. Planar boundaries oriented 90° to the surface were annealed in air at various times and temperatures. Atomic force microscopy was used to characterize the subsequent grooves. The Mullins approach leads to the following expression for the diffusion coefficient: D (m²/s) = 3.9 × 10⁻¹⁰ exp[−330 ± 75 (kJ/mol)/ RT ]. The relatively low activation energy agrees well with earlier oxygen tracer diffusion measurements on YAG, suggesting that oxygen is the limiting diffusing species in boundary grooving of YAG.     

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.     

Final-Stage Densification During Pressure-Sintering of CoO

The final-stage densification behavior of high-purity pressure-sintered CoO was studied as a function of temperature (950° to 1100°C) and applied pressure (6,000 to 11,750 psi). The stress-temperature regime during densification indicated that Nabarro-Herring diffusional creep should be the predominant deformation mechanism, whereas the experimental creep rate response as a function of stress level indicated the presence of a threshold stress. Interpretation of the data through use of a diffusion model for hot-pressing yields effective diffusion coefficients which agree well with those obtained by tracer techniques for Co self-diffusion. Activation energies of 33,460 and 39,600 cal/mol were obtained at relative densities of 0.90 and 0.95, respectively. On the basis of the observed densification kinetics and microscopic examination, it was concluded that densification is controlled by cation diffusion through the lattice and that oxygen diffusion is enhanced near the grain boundary.     

Origin of Grain-Boundary Diffusion in MgO

Preferential diffusion of Ni²⁺ and Co²⁺ along grain boundaries was observed in certain bicrystals of MgO. This enhancement is attributed to impurity segregation at the boundary. The identified impurities responsible for the effect are the principal impurities in the single-crystal MgO: Ca, Si, and Fe. No enhancement was observed in any bicrystal prepared above 1300°C, a temperature similar to that at which studies of the mechanical properties of MgO have implied a reabsorption of impurity precipitates into solid solution. It is concluded that enhanced grain-boundary diffusion of cations in MgO is an extrinsic, rather than an intrinsic, property of the boundary.     

Sintering and Characterization of Nanophase Zinc Oxide

Nanocrystalline, single-phase undoped ZnO was sintered to 95%–98% of theoretical density at 650°–700°C, using pressureless isothermal sintering. The density increased very rapidly at 500°–600°C, remained constant with sintering temperature until ∼900°C, and then decreased slightly. The estimated activation energy for densification at 600°–700°C (275 kJ/mol) was comparable to grain-growth activation energies previously reported for microcrystalline ZnO but much greater than the grain-growth activation energy measured in the present work. A bimodal microstructure, consisting of nanocrystalline grains within larger ensembles ("supergrains"), was observed, and both modes grew as the sintering temperature increased. The grain-growth activation energy for the nanocrystalline grains was extremely low, ∼20 kJ/mol. The activation energy for the growth of the supergrains depended strongly on temperature but was ∼54 kJ/mol at >500°C. The important mechanisms probably are rearrangement of the nanoparticle grains, with simultaneous surface and boundary diffusion, and vapor transport above 900°C.     

Relative Energies of Tilt Boundaries in Aluminum Oxide

The relative grain boundary energy and surface diffusion coefficient of aluminum oxide were determined by observing the thermal grooving behavior of a series of bi-crystals containing symmetric tilt boundaries. The relative energy for high tilt angles (30° to 150°) was roughly constant and equal to 0.54. The variation of relative energy with tilt angle was consistent with a dislocation core model for grain boundaries. The surface diffusion coefficient was:    
The agreement of this value for a vacuum etch with previous data for etches in air ruled out the oxygen ion as a possible rate-controlling diffusion species.     

Interdiffusion and Moving Boundaries in NiO-CaO and NiO-MgO Single-Crystal Couples

For single-crystal NiO-CaO diffusion anneals in air at 1346° to 1527°C, the chemical interdiffusion coefficient increased exponentially with increasing Ni concentration; for NiO-MgO at 1342° to 1346°C, a similar relation was found for [Ni] <0.41. The higher diffusivity and correspondingly greater flux in the NiO-rich phase resulted in reaction at the interface and movement of the crystallographic boundary with the growth of the NiO at the expense of the other crystal. The crystallographic orientation of the volume swept out transformed to that of the growing crystal.     

Superplastic Deformation in Fine-Grained YBa2Cu3O7−x

The superplastic behavior of YBa₂Cu₃O_{7− x} ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10⁻⁵ to 1 × 10⁻³/s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development.     

Mechanical Behavior of Polycrystalline Magnesium Oxide at High Temperatures

High-temperature tensile deformation of paolycrystalline magnesia prepared by ( a ) single-crystal recrystallization and ( b ) hot-pressing, is described. Recrystallized polycrystalline magnesia goes through a brittle-ductile transition at 1700°C (strain rate 10⁻⁴ sec⁻¹). The brittleness below 1700°C is due to a lack of slip systems and grain boundary sliding. At 1700°C grain boundary migration produces corrugations in the interface which interfere with sliding. Above 1700°C the matrix becomes sufficiently plastic through multiple slip and polygonization to accommodate any distortion. Polycrystalline specimens then neck down for completely ductile fracture. Hot-pressed magnesia starts through a transition at 2200°C, i.e. 500°C higher. The increase is attributed to pores and impurity. Porosity is considered to promote grain boundary sliding by ( a ) providing the source for intergranular sliding, ( b ) decreasing the interfacial contact area, and ( c ) preventing grain boundary migration and corrugation. These observations confirm that high-temperature deformation occurs by dislocation glide and climb and by grain boundary sliding and migration.     

Design of Electroceramics for Solid Oxides Fuel Cell Applications: Playing with Ceria

Nanostructured samaria- and gadolinia-doped ceria (SDC and GDC) powders were synthesized at low temperature (400°C) using diamine-assisted direct coprecipitation method. Fast-firing (f.f.) processes, where sintering temperatures are reached in a short time to promote lattice diffusion, were compared with conventional sintering, for the formation of dense microstructures from the nanostructured powders. Highly dense SDC and GDC samples (96%) with reduced grain size (150 nm) were obtained by f.f. even at 1300°–1400°C and, unexpectedly, high electrical conductivity and low blocking effect at grain boundary was obtained. Conventionally sintered samples showed that the grain boundary resistivity decreased with increasing the grain size, in agreement with the increase in geometrical bulk volume/grain boundary area ratio. Conversely, f.f. samples showed grain boundary resistivity smaller for small grain size. The above effect was observed only for high dopant (>10% molar) contents. The combined effect of powder grain size, dopant content, and sintering temperature–time profile, can be exploited to tune ceria microstructures for specific ionic device applications.