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.     

Mechanical properties of toughened ZrO2-Y2O3 Ceramics

Tetragonal zirconia polycrystals were produced from high-purity powders containing 1.5 to 5.0 mol% Y₂O₃ by cold isostatic pressing and sintering, hot-pressing, and hot isostatic pressing. The mechanical properties and microstructures of these resulting materials were examined, with emphasis on the relation between strength and fracture toughness.     

Transparent Nano-Composites Ceramics by Annealing of Amorphous Phase in the HfO2-Al2O3-GdAlO3 System

We have succeeded in fabricating transparent nano-structured ceramics by annealing of the amorphous phase obtained by the solidification of the eutectic melt in the ternary system HfO₂− Al₂O₃− GdAlO₃. The ceramics annealed at 1273 K for 6 hr contained 5-10-nm cubic hafnia grains, and those annealed at 1273 K for 72 hr contained both cubic hafnia and gadolinium aluminum garnet grains 5-10 nm in size. They showed high transparency. Annealing at 1473 K, however, resulted in grain growths that brought about non-transparency.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Sintering of Si3N4-Y2O3-Al2O3

Sintering kinetics of the system Si₃N4-Y2O₃-Al₂O3 were determined from measurements of the linear shrinkage of pressed disks sintered isothermally at 1500° to 1700°C. Amorphous and crystalline Si₃N₄ were studied with additions of 4 to 17 wt% Y₂O₃ and 4 wt% A1₂O₃. Sintering occurs by a liquid-phase mechanism in which the kinetics exhibit the three stages predicted by Kingery's model. However, the rates during the second stage of the process are higher for all compositions than predicted by the model. X-ray data show the presence of several transient phases which, with sufficient heating, disappear leaving mixtures of β ' -Si₃N₄ and glass or β '-Si₃N₄, α '-Si₃N₄, and glass. The compositions and amounts of the residual glassy phases are estimated.     

Effects of Sm2O3 and Gd2O3+ Nd2O3 on Electromechanical Properties of PbTiO3 Ceramics

The electromechanical properties of PbTiO₃ ceramics, modified by substitution of Sm or Gd + Nd (same average atomic radius as Sm) for Pb, were studied in the range of 6% to 14% substitution. The modified PbTiO₃ ceramics were stable, and the Curie temperature decreased linearly over this composition range. The 10% Sm composition had a large anisotropy in the coupling factor ratio, k _t / k _p , and a similar, but weaker, effect developed for 12% (1/2 Gd + 1/2 Nd). This indicates that other than average ion size may influence the electromechanical coupling factor ratio.     

Activated Sintering of ThO2 and ThO2-Y2O3 with NiO

The effect of additives on the sintering of ThO₂ and ThO₂-Y₂O₃ compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y₂O₃ as a separate phase increases the rate of sintering of ThO₂, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.     

Fabrication and Photoluminescence Characteristics of Eu3+-Doped Lu2O3 Transparent Ceramics

A novel co-precipitation process was adopted for the preparation of highly sinterable europium-doped lutetia powders using ammonium hydroxide (NH₃·H₂O) and ammonium hydrogen carbonate (NH₄HCO₃) as the mixed precipitant. The resultant powders calcined at 1000°C for 2 h showed good dispersity and excellent sinterability. Highly transparent polycrystalline lutetia ceramics with a relative density of ∼99.9% were fabricated by pressureless sintering in flowing H₂ atmosphere at 1850°C for 6 h without any additives. The average grain sizes of the transparent material were estimated to be 50–60 μm. Optical in-line transmittance in the visible wavelength region for Lu₂O₃ ceramics (1 mm in thickness) reached 80%. The luminescence and decay behavior of the obtained transparent plate and the corresponding nanophosphors were also investigated.     

Densification of Si3N4-Y2O3/Al2O3 by a Dual N2 Pressure Process

Sintering studies of Si₃N4-Y₂O3/Al₂O₃ compositions indicated that increased densification could be produced using a dual N₂ pressure process. RT modulus of rupture values and microstructures were found to be similar to hot-pressed material. The technique should be adaptable to complex shaped Si₃N₄ bodies where high density and strength are required .     

Phase Equilibration in ZrO2-Y2O3 Alloys by Liquid-Film Migration

The tetragonal ( t ) and cubic ( c ) ZrO₂ solid solutions in two-phase ZrO₂-8 wt% Y₂O₃ ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y₂O₃ content. The enrichment in Y₂O₃ content of the c -ZrO₂ grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO₂ increases by the nucleation and growth of cap-shaped t -ZrO₂ lenses. The interfaces between the c -ZrO₂ matrix and the growing t -ZrO₂ lenses are semicoherent.     

Phase Equilibria and Ordering in the System ZrO2-Y2O3

The phase diagram for the system ZrO₂-Y₂O₃ was redetermined. The extent of the fluorite-type ZrO₂-Y_zO₃ solid solution field was determined with a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. Long range ordering occurred at 40 mol% Y₂O₃ and the corresponding ordered phase was Zr₃Y₄OL₁₂. The compound has rhombohedra1 symmetry (space group R 3), is isostructural with UY₆O_l2 and decomposes above 1250±50°C. The results indicate that the eutectoid may occur at a temperature <400°C at a composition between 20 and 30 mol% Y₂O₃ Determination of the liquidus line indicated a eutectic at 83± 1 mol% Y₂O₃ and a peritectic at 76 ± 1 mol% Y₂O₃.     

Electrical Properties of ZnO-Bi2O3 Ceramics

The nonlinear volt-ampere characteristics and small-signal ac capacitance and resistance of sintered ZnO containing 0.5 mol% Bi₂O₃ were measured. Many of the electrical properties are related directly to the microstructure, which consists of conductive ZnO grains separated by a continuous amorphous Bl₂O₃, phase. The origin of the nonlinear conduction in the intergranular phase was confirmed by experiments with evaporated thin films. The proposed conduction mechanism in varistors containing ZnO and Bi₂O₃ is a combination of hopping and tunneling in the amorphous phase.     

Room-Temperature Strength and Fracture of ZrO2-Y2O3 Single Crystals

Strength and fracture toughness results are presented for ZrO₂ single crystals stabilized with Y₂O₃. The crystals (2 cm in diameter by 5 cm long) were prepared by skull melting. The partially stabilized compositions with 4 to 6 wt% Y₂O₃ showed a dramatic improvement in mechanical properties over the fully stabilized samples containing 20 wt% Y₂O₃, i.e. a strength exceeding 1000 MPa and a fracture toughness of 8 Mpa,.m ^1/2 were achieved compared to 200 MPa and 2 Mpa.m^1/2, respectively, for fully stabilized ZrO₂ single crystals.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

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.     

Self-Diffusion of Er in Pure and HfO2-Doped Polycrystalline Er2O3

Using a tracer sectioning technique, the self-diffusion of Er in pure and HfO₂-doped polycrystalline Er₂O₃ was measured at 1614° to 1900°C. Up to ≊ 10 mol% HfO₂ dopant level, the Er self-diffusion coefficients followed a relation based on cation vacancies as the principal mobile defects present and available for cation diffusion. Above 10 mol% HfO₂, deviation from this relation occurred, apparently due to clustering of cation vacancies and oxygen interstitials around the dopant hafnium ions. The activation energy for the self-diffusion of Er in pure Er₂O₃ was 82.2 kcal/mol and increased with the HfO₂ dopant level present.     

The System Si3N4-SiO2-Y2O3

Subsolidus phase relations were established in the system Si₃N₄-SiO₂-Y₂O₃. Four ternary compounds were confirmed, with compositions of Y₄Si₂O₇N₂, Y₂Si₃O₃N₄, YSiO₂N, and Y₁₀(SiO₄)₆N₂. The eutectic in the triangle Si₃N₄-Y₂Si₂O₇-Y₁₀(SiO₄)₆N₂ melts at 1500°C and that in the triangle Si₂N₂O-SiO₂-Y₂Si₂O₇ at 1550°C. The eutectic temperature of the Si₃N₄-Y₂Si₂O₇ join was ∼ 1520°C.     

Defect Structure and Electrical Conductivity of ThO2-Y2O3 Solid Solutions

Compositions in the system ThO₂-YO_1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite-type structure were formed up to 20 to 25 mole % YO_1.5 at 1400°C and up to 45 to 50 mole % YO_1.5 at 2200°C. Density data showed that these solid solutions correspond to Th_{1— x} Y _x O_{2—0.5 x} , having a complete cation sub-lattice filled by Th⁴⁺ and Y³⁺ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO_1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO₂ is soluble in YO_1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y₂O₃ phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.