Effect of Nd2O3 Doping on the Densification and Abnormal Grain Growth Behavior of High-Purity Alumina

The densification behavior and microstructural development of high-purity Al₂O₃ doped with different levels of Nd₂O₃ were investigated. Dopant levels ranged from 100–1000 ppm (Nd/Al atomic ratio). The densification behavior of the doped powders was studied using constant heating rate dilatometry. It was found that neodymium additions inhibited densification, with a corresponding increase in the apparent activation energy. The level of grain-boundary segregation was studied using high-resolution analytical electron microscopy. At dilute concentrations, the degree of neodymium grain-boundary excess was found to be consistent with a simple geometrical model relating this quantity to the overall dopant concentration and average grain size. For certain combinations of dopant level and heat treatment, supersaturation of the grain boundaries was observed, which was found to correlate with the onset of abnormal grain growth. Possible explanations for this behavior are discussed.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

The Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

Influence of Nd2O3 Doping on the Reaction Process and Sintering Behavior of BaCeO3 Ceramics

The influence of Nd₂O₃ doping on the reaction process and sintering behavior of BaCeO₃ is investigated. Formation of BaCeO₃ is initiated at 800°C and completed at 1000°C. When Nd₂O₃ is added to the starting materials, the formation of BaCe_1–xNd_xO_3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe_1-xNd_xO_3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd₂O₃ content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO₃ is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO₃ with Nd₂O₃ markedly enhances its sinterability. The enhancement of the sinterability of Nd₂O₃-doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering.     

Nd2O3-Doped Sialons with ZrO2/ZrN Additions Formed by Sintering and Hot Isostatic Pressing

β-sialon and Nd₂O₃-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO₂ (3 mol% Nd₂O₃) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO₂(Nd₂O₃) or ZrN were seen in both the HIPed β-sialon/ZrO₂(Nd₂O₃) composites and the HIPed Nd₂O₃-stabiIized α-sialons with ZrN additions.     

Phase Relations in the Pyrochlore-Rich Part of the Bi2O3−TiO2−Nd2O3 System

In this study we used solid-state synthesis to determine the phase relations in the pyrochlore-rich part of the Bi₂O₃−TiO₂−Nd₂O₃ system at 1100°C. The samples were analyzed using X-ray powder diffraction and scanning electron microscopy with energy- and wavelength-dispersive spectroscopy. A single-phase pyrochlore ceramic was obtained with the addition of 4.5 mol% of Nd₂O₃. We determined the solubility limits for the three solid solutions: (i) the pyrochlore solid solution Bi_{(1.6–1.08 x )}Nd _x Ti₂O_{(6.4+0.3 x )}, where 0.25< x <0.96; (ii) the solid solution Bi_{4− x} Nd _x Ti₃O₁₂, where 0< x <2.6; and (iii) the Nd_{2− x} Bi _x Ti₂O₇ solid solution, where 0< x <0.35. The determined phase relations in the pyrochlore-rich part are presented in a partial phase diagram of the Bi₂O₃−TiO₂−Nd₂O₃ system in air at 1100°C.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Solubility of NiO in Al2O3

Solubility of NiO in Al₂O₃ was determined by electron probe microanalysisy A diffusion couple method was used by coupling an NiO-doped Al₂O₃ polycrystal to a pure single crystal of Al₂O₃. The solubility of NiO in Al₂O₃ in air was 230 wt ppm (157 at. ppm of cations) and 170 wt ppm (116 at. ppm) at 2073 and 1973 K, respectively. The solubility of NiO in Al₂O₃ obtained in this work was compared with our previous work of the solubility of MgO in Al₂o₃.     

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.     

Crystal Chemistry and Phase Equilibrium Studies of the BaO(BaCO3)–½R2O3–CuOx Systems in Air: VI, R = Neodymium

Crystal chemistry and subsolidus phase equilibrium studies of the Ba-Nd-Cu-O system near the CuO and Nd₂O₃ corners have been carried cut at 950°C in air. Two solid-solution series have been identified in the Ba-Nd-Cu-O system. The first series involves the high- T _c superconductor phase, and has the formula Ba_2–xNd_1+xCu₃O_6+z, where × < ≅ 0.7. At the ideal compound stoichiometry of Ba₂NdCu₃O_6+z, the transformation from the high- T _c orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x ≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non-superconducting "brown phase" represented by Ba_2+2x-Nd_4–2xCu_2–xO_10–2z 0 ≤ x ≤ 0.1. Preliminary phase diagrams of the BaO–Nd₂O₃ and Nd₂O₃–CuO_x systems are also presented. Standard X-ray diffraction patterns of BaNd₂–CuO₅ and (Nd_1.9Ca_0.1)CuO_4–z are provided.     

Thermal, Dielectric, and Optical Properties of Neodymium Borosilicate Glasses for Thick Films

Because of their thermal, dielectric, and optical properties, new glass compositions and thick-filmed transparent dielectrics containing neodymium oxide (Nd₂O₃) were studied as a source of purer images in plasma display panels. In the present study, PbO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂–ZnO–Al₂O₃ were used as starting glass compositions, to which up to 25 wt% of Nd₂O₃ then was added. Increased amounts of Nd₂O₃ increased the glass transition temperature and dielectric constant of the bulk glasses and decreased the coefficient of thermal expansion. The fired thick films (around 30 μm) allowed selectively visible light to penetrate and showed deep absorption properties at 585 nm that were related to an extraneous gas from neon discharge.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Experimental Investigation and Thermodynamic Assessment of the Nd2O3–Y2O3 System

The phase relations in the Nd₂O₃–Y₂O₃ system were experimentally studied in the 1300°–1600°C range. X-ray diffraction, scanning electron microscopy, and electron probe microanalysis were applied to analyze the phase composition of annealed Nd₂O₃–Y₂O₃ mixtures with varying Y₂O₃ content. A thermodynamic assessment was conducted using the experimental data obtained. The excess Gibbs energies of the solution phases were described based on a simple substitutional solution model. A consistent set of optimized interaction parameters was derived for the Gibbs energy of the constituent phases, resulting in a good match between calculated and experimental data.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Phase Equilibria in the System Nd2O3-P2O5

The phase diagram for the system NdI₂O₃-P₂O₅ was constructed. Six intermediate compounds, having molar Nd₂O₃: P₂O₅ ratios of 3:1, 7:3, 1:1, 1:2, 1:3, and 1:5, were identified. The 3:1, 7:3, and 1:1 compounds are stable to at least 1500°C. The 1:2 compound decomposes to 1:1 and 1:3 at 730 ± 5°C. The 1:3 and 1:5 compounds melt congruently at 1280 ± 5° and 1055 ± 5°C, respectively. None of the neodymium phosphates show lower temperature limits of stability.     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion-conducting, composite membranes comprised of 14Li₂O·9Al₂O₃·38 TiO₂·39P₂O₅ glass–ceramic and polyethylene. The processing involved tape casting of 14Li₂O·9Al₂O₃·38TiO₂·39P₂O₅ glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid-like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO₄ at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Effect of Oxygen Partial Pressure on the Creep of Polycrystalline Al2O3 Doped with Cr, Fe, or Ti

Steady-state creep was studied in hot-forged polycrystalline Al₂O₃ (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm² at 1375° to 1525°C under O₂ partial pressures of 0.88 to 10⁻¹⁰ atm. Except in the 10-μm Fe-doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of P _o2 on the creep rate indicated that increased concentration of a divalent (Fe²⁺) or quadrivalent (Ti⁴⁺) impurity in solid solution enhances the creep rate of polycrystalline Al₂O₃. The activation energies for the creep of Fe- and Ti-doped Al₂O₃ samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr-doped material (114 kcal/mol). Taking into account the effects of P_o2, temperature, and grain size, it was concluded that the steady-state creep of transition-metal-doped Al₂O₃ is controlled by cation lattice diffusion.