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.     

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.     

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 57Co in Surface Layers of Nickel Oxide and Alumina

Diffusion of ⁵⁷Co isotope on NiO (110) and AI₂O₃ (0001) surfaces was investigated by using the edge source method. The surface diffusion parameter α D ,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 range 750°–1200°C. For calculation of experimental results, the Whipple solution was used. The Arrhenius plot shows a break at ∽900°C for the surface diffusion of ⁵⁷Co isotope on AI₂O₃ (0001) plane. Above this temperature, vapor transport seems to be the overriding diffusion mechanism. Below this temperature, ionic transport predominates. The apparent activation energy for the ionic transport was calculated to be 120 ± 12 kj/mol.
Ionic transport predominated in the surface diffusion of ⁵⁷Co on NiO over the entire investigated temperature range. This can be explained by the weak bond between Co-vapor species and the NiO surface. The results obtained suggest that the surface diffusion of Co²⁺ ion on NiO at 750°C is ∼7 orders and at 1200°C ∼5 orders of magnitude faster than volume diffusion. Activation energies are 139 and 227 kJ/mol, for surface and volume diffusion, respectively.     

Microwave Sintering of Alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.     

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.     

Oxidation Kinetics of Participate Silicon Monoxide

The air oxidation of particulate silicon monoxide to cristobalite was studied in the range 820° to 1040°C. This SiO neither disproportionates nor volatilizes in this range. An activation energy of 23.8 ± 2.0 kcal/mole for the diffusion of oxygen was obtained from a measure of the differential rate of reaction at various temperatures for various fixed percents of conversion up to 24%. The reciprocal time needed to reach a fixed percent of conversion up to 30%, at various temperatures, gave an activation energy of 24.2 ± 1.5 kcal/mole. It is concluded that the diffusional activation energy is 24.0 ± 2.0 kcal/mole.     

Thermodynamic Properties of RuO2

The free energy change for the reaction RuO₂( s )+4Cu( s ) = 2Cu₂O( s )+Ru( s ) was determined from 600° to 1000°C from emf measurements on a solid oxide galvanic cell using a stabilized ZrO₂ electrolyte. The cell was designed to minimize the reduction of RuO₂ by the gas phase. The results were used to develop an equation for the standard molar free energy of formation of RuO₂:
The standard molar enthalpy and entropy of formation of RuO₂ at 298°K were calculated to be −72,430 ±200 cal/mol and –40.44±0.2 eu, respectively, using the available heat capacity data. The absolute entropy of RuO₂ at 298°K was calculated to be 15.46±0.2 eu.     

Grain-Boundary Diffusion of Cr in Pure and Y-Doped Alumina

The diffusive transport of chromium in both pure and Y-doped fine-grained alumina has been investigated over the temperature range 1250°–1650°C. From a quantitative assessment of the chromium diffusion profile in alumina, as obtained from electron microprobe analysis, it was found that yttrium doping retards cation diffusion in the grain-boundary regime by over an order of magnitude. The Arrhenius equations for the undoped and Y-doped samples were determined to be: δ D _b=(4.77±0.24) × 10⁻⁷ exp (−264.78±47.68 (kJ/mol)/RT)(cm³/s) and δD_b=(6.87±0.18) × 10⁻⁸ exp (−284.91±42.57 (kJ/mol)/RT)(cm³/s), respectively. Finally, to elucidate the mechanism for this retardation, the impact of yttrium doping on diffusion activation energies and prefactors was examined.     

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.     

Initial Sintering of Rutile

The initial shrinkage of powder compacts of rutile was measured in air at 700° to 1130°C. The shrinkage behavior agrees well with a model based on grain-boundary diffusion. The apparent activation energy for the shrinkage rate is 76.9 ± 2.5 kcal/mole. Changes in ambient atmosphere (O₂, N₂, vacuum) had no effect on the initial sintering kinetics.     

Metal Reference Line Technique for Obtaining Dihedral Angles from Surface Thermal Grooves

A metal reference line (MRL) technique is described for the measurement of surface–grain-boundary dihedral angles, Ψ_s, from thermal grooves at a sample surface using scanning electron microscopy (SEM). Metal lines deposited onto a thermally grooved surface using photolithography conform to the contours of the grain-boundary groove and provide a high-contrast reference line for measuring Ψ_s by SEM. Measurements of Ψ_s from optical interferometry and calculated from groove dimensions using surface diffusion models of thermal grooving are compared with the metal reference line measurements from the same thermally grooved surface of MgO-doped Al₂O₃. Distributions of Ψ_s are found to shift to lower angles and approach the true Ψ_s value as the resolution of the technique increases, with the MRL technique having the highest resolution, a median angle of 113°± 1° and a distribution of angles from 90°± 5° to 139°± 3°.     

Crack Healing in UO2

Isothermal annealing of thermally shocked UO₂ bars (O/M= 2.00 ±0.01) at 1600°, 1800°, and 2000°C caused crack healing, which was determined from recovery of room-temperature fracture strength. The activation energy for crack healing was ∼0.5 of that for volume diffusion, and healing occurred at the same rate as grain growth. This result has important implications with respect to crack healing in oxide fuels during in-reactor restructuring.     

Ion-Probe Measurement of Oxygen Self-Diffusion in Single-Crystal Al2O3

Anion self-diffusion coefficients normal to (1102) were obtained for single-crystal Al₂O₃ in a 1.3 × 10 ³ N/m² (10⁻⁵ torr) vacuum at 1585° to 1840°C. Tracer was supplied from an initial 650 to 1300 A Al₂¹⁸O₃ layer produced by the oxidation of vapor-deposited Al metal films in an ¹⁸O₂ atmosphere at 520°C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar⁺ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D₀ =6.4×10⁵cm²/s, with an activation energy of 188 ± 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.     

Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Raman Investigation of the Nitridation of Yttria-Stabilized Tetragonal Zirconia

Raman spectroscopy has been used to obtain Raman spectra of yttria-stabilized tetragonal zirconia subject to surface nitridation induced by contact with zirconium nitride. Raman spectra recorded from regions at increasing distance from the source of nitridation have been used to obtain diffusion profiles from samples treated at different times and temperatures. The coupling of X-ray diffraction data previously taken and of the Raman spectra shows that in the samples there is a two-phase region (tetragonal + cubic) near the nitrided surface and that, at larger distance inside the samples, there is only one phase (tetragonal). Fitting of the diffusive profiles in the single-phase tetragonal region with an appropriate diffusion function allows the determination of the diffusion coefficient of nitrogen in tetragonal zirconia which is expressed in terms of the preexponential factor, D ₀= (3.98 ± 0.5) × 10⁻³ cm²/s, and the activation energy, Q = 170 ± 10 kJ/mol.     

Sol-Gel Synthesis of Amorphous Calcium Phosphate and Sintering into Microporous Hydroxyapatite Bioceramics

A new route for preparing hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) bioceramic has been described. An amorphous, nanosized, and carbonate-containing calcium phosphate powder that had a Ca:P ratio of 1.67 was synthesized from calcium diethoxide and phosphoric acid in ethanol via a sol-gel method. The powder was pressed at 98 MPa into green specimens and then heated to a temperature range of 500°-1300°C. At 600°C, the powder crystallized to a carbonated hydroxyapatite and a trace of ß-tricalcium phosphate before converting to hydroxyapatite at 900°C. The thermal crystallization was associated with grain growth, shrinkage, and active surface diffusion. The activation energy of grain growth was 37 ± 2 kJ/mol. After sintering at 1100°C, the decomposition of carbonated hydroxyapatite generated a microporous ceramic with an average pore size of 0.2 µm and an open porosity of 15.5%. This microporous bioceramic can be used as a bone filler.     

Self-Diffusion of 60Co in Co1-xO Single Crystals of Controlled Stoichiometry

Cobalt tracer self-diffusion coefficients were determined in single crystals of Co_1-x O at 1037° to 1350°C and for x=0.002 to 0.00s. In crystals with x fixed at 0.005, the results were    
The activation energy in this constant-stoichiometry case was identified as the enthalpy of motion for Co ions moving via a vacancy jump mechanism. Based on known stoichiometry rela- tions, a published activation energy for Co self-diffusion mea- surements in air was corrected to yield an enthalpy of motion in excellent agreement with the number given above. At a constant temperature of 115O0°±2°C, the P_o2 dependence of Co self- diffusion was    
The P_o2 expynent suggests that the predominant mobile ion defects are singly ionized Co vacancies. Simple defect reactions with no modifications for defect clustering were adequate to explain all results obtained.     

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.     

Kinetic Studies of Mullite Synthesis from Alumina Nanoparticles and a Preceramic Polymer

The crystallization kinetics of mullite formation in a diphasic precursor consisting of a silicone resin filled with commercial γ-alumina nanoparticles (15 nm mean particle size, specific surface area of 100 m²/g), heated in air from 1250° to 1350°C, was studied by X-ray diffraction. Transitional γ-alumina and amorphous silica from the pyrolysis of the preceramic polymer exhibited a remarkable reactivity, as demonstrated by a very low incubation time (from 500 s at 1250°C to 20 s at 1350°C), a high mullite yield (about 80 vol%, after 100 s at 1350°C), and a low activation energy for nucleation (677±60 kJ/mol). The activation energy values found were lower than those reported previously for other diphasic systems, including sol–gel precursors. Besides the high specific surface of nanosized γ-alumina particles, the low energy barrier could be attributed to the highly reactive silica deriving from the oxidation of Si–CH₃ bonds in the silicone and to the homogeneous dispersion of the nanosized filler inside the preceramic polymer. Furthermore, the possibility of applying plastic shaping processing methods to the mixture of a preceramic polymer and nanosized filler makes this approach particularly valuable, in comparison, for instance, with sol–gel based alternatives.