Self-Diffusion of Er and Hf in Polycrystalline Er2O3-Stabilized HfO2

Tracer self-diffusion coefficients for Er and Hf were measured in polycrystalline Er₂O₃-stabilized HfO₂ (fluorite phase) at compositions between 10 and 40 mol% Er₂O₃ and temperatures between 1700° and 2033°C. In some instances, grain-boundary coefficients were also determined. The Hf volume coefficients were generally slightly smaller than Er coefficients, but neither showed a strong composition dependence, a finding most likely attributable to extensive clustering of charged defects which apparently rendered a large proportion of the "notional" defects essentially immobile. The activation energies for volume coefficients were very large, ranging from 630 to 760 kJ-mol⁻¹, and generally decreased with increasing Er₂O₃ content.     

Cation Self-Diffusion in Polycrystalline Y2O3 and Er2O3

A tracer sectioning technique was used to measure cation self-diffusion coefficients in fully dense polycrystalline Y_aO₃ and Er₂O_s under oxidizing conditions. The results are described by the relations for Y₂O₃ (1400° to 1670°C), and for Er₂O₃ (1400° to 1700°C). The greater activation energy for erbium diffusion in erbia may be partly attributable to a mass effect.     

High-Temperature Tensile Strength of Er2O3-Doped ZrO2 Single Crystals

The deformation and fracture mechanisms in tension were studied in single-crystal Er₂O₃-doped ZrO₂ monofilaments processed by the laser-heated floating zone method. Tensile tests were carried out between 25° and 1400°C at different loading rates and the dominant deformation and fracture mechanisms were determined from the shape of the stress–strain curves, the morphology of the fracture surfaces, and the evidence provided by monofilaments deformed at high temperature and broken at ambient temperature. The tensile strength presented a minimum at 600°–800°C and it was controlled by the slow growth of a crack from the surface. This mechanism was also dominant in some monofilaments tested at 1000°C and above, while others showed extensive plastic deformation before fracture at these temperatures. The strength of plastically deformed monofilaments was significantly higher than those which failed by slow crack growth due to the marked strain hardening capacity of this material.     

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 .     

Self-Diffusion of 45Ca and 88Y in Pure and YF3-Doped CaF2 Single Crystals

Tracer self-diffusion coefficients for Ca and Y were measured in pure and YF₃-doped CaF₂ crystals for dopant levels ranging from 2 to 10 mol% and temperatures between 960° and 1212°C. Coefficients for both cations increased approximately linearly with the square of the dopant concentration at constant temperature, with D *_Ca> D *_Y in doped crystals but D *_Ca> D *_Y in pure crystals. The activation energies for diffusion of both cations decreased with increasing dopant concentration. Atomic jump pathways are considered and the decrease in activation energy is explained in terms of reduction in the enthalpy of motion accompanying the formation of Willis anion defect clusters in the more heavily doped crystals.     

Preparation, Sintering, and Properties of Translucent Er2O3-Doped Tetragonal Zirconia

Tetragonal zirconia doped with 3 mol% Er₂O₃ was prepared by the gel-precipitation wet-chemical method. The compaction response of ultrafine (∼8.5 nm), calcined, deagglomerated powders was studied. Initial sintering was studied using both isothermal and nonisothermal techniques, and an activation energy of 270 ± 40 kJ/mol was obtained; therefore, grain-boundary diffusion was probably the predominant mechanism in the sintering stage. The microstructural development in the high-temperature-aged, sintered samples and its effect on density and mechanical properties were also studied. A theoretically dense body of tetragonal zirconia solid solution of 3 mol% Er₂O₃–ZrO₂, obtained from sintering below 1400°C, was translucent.     

Oxygen Self-Diffusion in Single-Crystal Y2O3

Self-diffusion coefficients of the oxygen ion in single-crystal Y₂O₃ were determined by the gas-solid isotope exchange technique. The results in the range 1100° to 1500°C are described by D=7.3 X 10^-6 exp [-19l(kJ/mol)/RT] cm²/s. Comparison of the results with those for oxides with the fluorite-type structure indicates that the regularly arranged vacant anion sites in the C-type structure do not contribute eflectively to oxygen ion diffusion .     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Structural and Electrical Properties of Er2O3-Doped Na1/2Bi1/2TiO3 Lead-Free Piezoceramics

Sodium bismuth titanates Na_1/2Bi_1/2TiO₃ (NBT) doped with 0–3 wt% Er₂O₃ were prepared by the conventional solid-state reaction method. The X-ray diffraction results revealed that the sintered Er-doped NBT ceramics exhibited a pure perovskite structure with Er³⁺ concentrations ranging from 0 to 1 wt%. At a low Er₂O₃ concentration, the Er-doped NBT ceramics showed enhanced electrical properties with dielectric constant ɛ₃₃^T/ɛ₀=636, a low dielectric dissipation factor (tan δ=3.3%), a low coercive field ( E _c=4.56 kV/mm), and a high piezoelectric constant ( d ₃₃=75 pC/N). The relationship between the composition and properties of Er-doped NBT ceramics has been discussed.     

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.     

Creep of Doped Polycrystalline Al2O3

Creep behavior of polycrystalline A1₂O₃ doped with MgO + NiO, both as-hot-pressed in graphite dies and additionally annealed, was determined for 1300° to 1470°C and for 1000 to 15,000 psi in compression. The deformed specimens contained intergranular separations. Creep rates were proportional to stress to the 1.1 and 1.3 powers and were independent of grain size changes occurring during creep. The suggested creep mechanism is localized plastic deformation at stress concentration points accommodated by grain-boundary separations initiated by grain-boundary sliding.     

High-Temperature Deformation of Polycrystalline Fe2O3

The effects of stress, temperature, grain size, porosity, and O₂ partial pressure on the creep of polycrystalline Fe₂O₃ were studied in the range 770° to 1105°C by tests in 4-point bending and compression. Deformation rates are controlled by the stress-directed diffusion of either oxygen or iron. Diffusion coefficients computed from the Nabarro-Herring formula modified by including an empirical porosity-correction term are also consistent with the values reported for oxygen and iron.     

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.     

Stress Corrosion Cracking of Single-Crystal Tetragonal ZrO2(Er2O3)

The flexure strength of partially-stabilized tetragonal ZrO₂(Er₂O₃) single-crystal monofilaments manufactured by the laser-heated floating zone method was measured as a function of the environment (air versus water) and temperature (from 25° to 800°C) at loading rates spanning three orders of magnitude to ascertain their susceptibility to the environmental conditions. These mechanical tests were completed with parallel tests on fully annealed monofilaments (to relieve the thermal residual stresses induced during growth) and by detailed analysis of the fracture surfaces using scanning electron microscopy and micro-Raman spectroscopy. While environmental susceptibility of ZrO₂(Y₂O₃) in previous investigations was always associated with the destabilization of the tetragonal phase, monoclinic phase was not detected on the fracture surfaces of the ZrO₂(Er₂O₃) monofilaments and it was concluded that slow crack growth in this material at high temperature or immersed in water was due to stress corrosion cracking.