Effect of Yttrium and Lanthanum on the Final-Stage Sintering Behavior of Ultrahigh-Purity Alumina

Final-stage sintering has been investigated in ultrahigh-purity Al₂O₃ and Al₂O₃that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain-boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare-earth cations, which segregate strongly to the grain boundaries in Al₂O₃, block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al₂O₃, surface-diffusion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al₂O₃grain boundaries.     

Yttrium, an Isoelectric Donor in α - Al2O3

Measurements of dc electrical conductivity and emf of single crystal and polycrystalline α-Al₂O₃ doped, respectively, with 100 and 300 ppm yttrium show that yttrium, although isoelectronic with aluminum, is a donor. The electronic energy level of the donor is attributed to O^2- next to Y³⁺, its level being forced up as a result of the large size of the yttrium ion. The donor activity of Y supports the view that the favorable effect of Y on the oxidation of super alloys is due to reduced diffusion of Al in the bulk through a reduced concentration of Al_i^… without a marked increase in the concentration of V^…_Al.     

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.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.     

A Magnetic Resonance Investigation of the Process of Corundum Formation Starting from Sol–Gel Precursors

²⁷Al–MAS (magic angle spinning)–NMR and Fe³⁺ EPR measurements have been performed to follow the local process of corundum formation starting from xerogel on the Al³⁺ and Fe³⁺ sites yielding complementary information. Different heat treatments have been applied to the samples: an isochronous procedure and thermoanalytical measurements stopped by quenching. Despite the different mechanisms of the phase transitions deduced, both seeding and Fe³⁺ doping under the conditions of isochronous procedure favor the formation of α-Al₂O₃ at remarkably low temperatures. The first observed temperature of corundum formation following the isochronous procedure with iron-doped samples is as low as 750°C. The transition alumina, which could be clearly evidenced, is the γ-Al₂O₃ phase (four- and six-fold Al coordination). In undoped or unseeded samples, intermediate Fe species could be detected by ESR and evidence for θ-Al₂O₃ was obtained from ²⁷Al–NMR spectroscopy.     

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed poly crystalline AI₂O₃ made from AI-isopropoxide (AI₂O₃(II)). The undoped material was found to contain 1.5×10¹⁸ cm⁻³ fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor- or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of AI, involving V _AI"in donor-, AI," in acceptor-dominated material. In creep, oxygen is transported along grain boundaries in a neutral form (O_i^p). The p_O2 dependence of σ _t and σ _h are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed at low temperature, is attributed to the presence of AI-silicate second phase.     

Effect of Europium Concentration on Densification of Transparent Eu:Y2O3 Scintillator Ceramics Using Hot Pressing

Europium (Eu) was found to act as a solid-state sintering aid in Y₂O₃ optical ceramics by controlling ionic diffusivity, which in turn leads to enhanced optical transparency. Transparent ceramic samples of Eu-doped Y₂O₃, with no additional additives, were sintered by uniaxial vacuum hot pressing under 40 MPa and maximum temperature of 1580°C. Optical attenuation was found to decrease with increasing Eu concentrations between 0 and 5 at% for ceramics processed under the same sintering conditions. In order to study the effect of Eu concentration on ceramic densification, the strain rate and grain size during sintering at constant temperature and varied pressure were measured. A diffusional flow densification model was used to derive instantaneous effective diffusion constants for the densification process. Diffusion constants were found to increase with increasing Eu concentration according to a log–linear relationship. Eu²⁺ was detected in samples after hot pressing through fluorescence spectroscopy, and the extrinsic defect chemistry was found to be dominated by the reduced Eu in solid solution with Y₂O₃. A sintering model with diffusion rate limited by yttrium interstitial transport and controlled by the incorporation of Eu²⁺ onto the cation sublattice was found to be in good agreement with experimental diffusivity data.     

Effect of Firing Atmosphere on Sintered and Mechanical Properties of Vanadium-Doped Alumina

Vanadium-doped alumina was sintered at 1650°C in both an oxidizing (O) and a reducing (R) atmosphere and the sintered bodies examined. In the O-sintered body, vanadium was present preferentially between the alumina grains, forming a phase (AlVO₄). In the R-sintered body, on the other hand, most of the vanadium was dissolved in alumina as V³⁺, and a small proportion of the vanadium was present as V⁴⁺ in the grain-boundary region. During O sintering, V₂O₅ doping depressed both densification and grain growth, whereas R sintering had no effect on densification but did depress grain growth. The O-sintered, V-doped body exhibited low flexural strength and hardness, whereas the R-sintered body showed comparatively high hardness.     

Effect of Substitution of Manganese for Iron on the Structure and Electrical Properties of Yttrium Ferrite

The solubility of manganese in YFe_{1− y} Mn _y O₃ reaches y = 0.4. Among these compositions, YFe_0.6Mn_0.4O₃ has the highest conductivity with ς= 2.16 Ω⁻¹·cm⁻¹ (1000°C, air), which is >1 order of magnitude higher than that of yttrium ferrite, YFeO₃ (ς= 0.10 Ω⁻¹·cm⁻¹, 1000°C, air). In the range 0.2 ≤ y ≤ 0.4, the conductivity is linearly proportional to the manganese content. The mechanism of direct transport of polarons among manganese sites ( y ≥ 0.2) is proposed to explain the electrical behavior of manganese-doped samples. X-ray diffractometry and infrared spectroscopy have been applied for the structural characterization of the samples.     

Influence of Yttrium Doping on Grain Misorientation in Aluminum Oxide

Oversized dopant ions such as yttrium and lanthanum segregate to grain boundaries and reduce the tensile creep rate of α-Al₂ O₃ by 2-3 orders of magnitude. One explanation for this behavior is that the oversized segregants give rise to a "site-blocking" effect for grain boundary diffusion. It has also been speculated that the dopant ions modify the grain boundary structure in alumina and reduce the creep rate by promoting the formation of special (e.g., coincidence site lattice (CSL)) grain boundaries. In order to test the latter hypothesis, we have used electron backscattered Kikuchi diffraction to characterize the misorientation and special grain boundary distribution for undoped and 1000-ppm-yttrium-doped alumina. The results show that the grain boundary structure in alumina (as characterized by the frequency of selected CSLs and misorientation distribution) was not significantly changed by the addition of yttrium, indicating that creep retardation results mainly from site-blocking.     

Study by Extended X-ray Absorption Fine-Structure Technique and Microscopy of the Chemical State of Yttrium in α-Polycrystalline Alumina

Yttrium-doped a-poly crystalline alumina was studied by microscopy observations and by extended X-ray absorption fine-structure experiments. The latter results were compared to those obtained for a standard Y₂O₃ powder and to theoretical spectra for Y₂O₃, and Y₃Al₅O₁₂ calculated on the basis of crystallographic data. Most of the yttrium in the doped alumina is precipitated as a Y₃Al₅O₁₂ phase. An observed threshold displacement involves a slight valence electron delocalization for the Y³⁺ ion.     

Influence of Dopant Concentration on Creep Properties of Nd2O3-Doped Alumina

The microstructural features and tensile creep behavior of Al₂O₃ doped with Nd₂O₃ at levels ranging from 100 to 1000 ppm (Nd:Al atomic ratio) were systematically investigated. Compositional mapping, using both high-resolution scanning transmission electron microscopy and secondary ion mass spectroscopy revealed that, for all of the compositions studied, the Nd³⁺ ions were strongly segregated to the Al₂O₃ grain boundaries. Microstructural observations revealed that the solubility of Nd₂O₃ was between 100 and 350 ppm. Tensile creep tests were conducted over a range of temperatures (1200°–1350°C) and stresses (20–75 MPa). Both the stress and grain-size exponents were analyzed. In selected experiments, controlled grain-growth anneals were used to enable creep testing of samples of the same average grain size but different neodymium concentrations. Independent of dopant level, the neodymium additions decreased the creep rate by 2–3 orders of magnitude, compared with that of undoped Al₂O₃. The value of the apparent creep activation energy increased with increased dopant concentration and then saturated at dopant levels exceeding the solubility limit. Overall, the results of the present study were consistent with a creep-inhibition mechanism whereby oversized segregant ions reduce grain-boundary diffusivity by a site-blocking mechanism.     

Control of Dielectric Constant and Loss in Alumina Ceramics

The dissipation factors and dielectric constants of alumina ceramics containing less than 100 ppm of impurities and of specimens doped with Si, Ti, Ca, Mg, Fe, and Cr ions were measured in the region 10² to 8.5 × 10⁹ cps and 25° to 875° C. Multiple regression analysis of the data at 500° C and 10⁶ cps showed a linear relation between impurity concentration and tan δ, with a correlation coefficient of 0.93. Si ions caused the greatest rise of tan δ, Mg and Ti were second, Ca third, and Cr and Fe had no significant influence. These effects diminished with rising frequency and became negligible in the microwave region. Activation energies of conduction for pure and doped alumina were estimated from measurements of δ at 10⁵ cps and 500°C. Values between 1.2 and 1.6 ev were calculated for all compositions except the one containing Mg²⁺, for which 2.0 ev was obtained. At low frequencies, the dielectric constant (k') rose exponentially with temperature, reflecting a similar rise in the number of free charge carriers contributing to interfacial polarization. At higher frequencies the temperature variation of k' fell to a shallow positive slope of about 120 ppm per °C. This coefficient was not influenced by low concentrations of impurities but could be effectively compensated without excessive loss by additions of 10 to 20% SrTi0₃.     

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.     

Codoping of Alumina to Enhance Creep Resistance

The tensile creep behavior of both singly and multiply doped alumina samples has been investigated in order to understand better the impact of dopant segregation to grain boundaries on observed creep resistance. Previous studies have suggested that the segregation of the oversized dopant ions reduces the grain boundary diffusivity and thus the creep rate. The aims of the present work are to examine the possibly beneficial effects of selective codoping in enhancing creep resistance, and to elucidate the role (if any) of precipitates in creep inhibition. The specific singly and codoped systems considered in this work were as follows: hot-pressed alumina samples containing nominally (i) 100 ppm zirconium, (ii) 100 ppm neodymium, (iii) 100 ppm zirconium codoped with either 100, 350, or 1000 ppm neodymium, (iv) 100 ppm zirconium codoped with 1000 ppm scandium. Microchemical mapping using secondary ion mass spectrometry revealed direct evidence of cosegregation of the dopant ions to grain boundaries. Tensile creep tests were carried out in the temperature range of 1200-1350°C, utilizing stresses ranging from 20 to 100 MPa. In the case of the Nd/Zr codoped alumina, it was found that the creep rate decreased by 2 to 3 orders of magnitude relative to undoped alumina. This improvement was greater than that achieved by doping with either Nd or Zr alone, and demonstrates that the incorporation of ions of differing sizes may be beneficial. The observed enhancement in creep resistance was obtained for compositions both above and below the solubility limit of Nd in alumina; hence the phenomenon is primarily a solid solution effect.     

The Effect of Yttrium on Oxygen Grain-Boundary Transport in Polycrystalline Alumina Measured Using Ni Marker Particles

The grain-boundary transport of oxygen in polycrystalline α-Al₂O₃ (undoped and 500 ppm Y³⁺-doped) was studied in the temperature regime of 1100°–1500°C by monitoring the oxidation of a fine, uniform dispersion of Ni marker particles (0.5 vol%). The annealing treatments were carried out in a high-purity O₂ atmosphere (>99.5%). The Ni particles, which are visibly oxidized to nickel aluminate spinel, were used to determine the depth of oxygen penetration. The thickness of the reaction zone was measured as a function of heat-treatment time and temperature, and a comparison of the oxidation rate constants and activation energies for undoped and Y³⁺-doped alumina was made. The results indicate that the presence of Y³⁺ slows oxygen grain-boundary transport in alumina by a variable factor of from 15 to 3 in the temperature regime of 1100°–1500°C. The values of the activation energy for undoped and Y³⁺-doped alumina were determined to be 430±40 and 497±8 kJ/mol, respectively.     

Low-Temperature Sintering of Alumina with Liquid-Forming Additives

Simultaneous application of colloidal processing and liquid-forming additives to alumina resulted in a sintered density of >99% in 1 h at a temperature as low as 1070°C for a commercial high-purity alumina powder at a total dopant level of 2 mol%. The additives were 0.9% CuO + 0.9% TiO₂+ 0.1% B₂O₃+ 0.1% MgO. At higher temperatures or after prolonged sintering, the doped alumina ceramic developed a duplex microstructure containing large elongated grains and exhibited a relatively high fracture toughness of ∼ 3.8 MPa · m^1/2 as compared to a value of ∼ 2.6 MPa · m^1/2 for the undoped alumina.     

Measurement of Small Concentrations of Cr and Fe in α-Al2O3 Using Electron Spin Resonance

Electron spin resonance line widths for Cr³⁺ and Fe³⁺ were measured in α-Al₂O₃ samples doped with <1.2 mol% of Cr and Fe. Line width increased with increasing Cr and Fe concentration and decreased with increasing grain size. The experimental results show that this technique can be used to determine, rapidly and nondestructively, small concentrations of Cr and Fe in commercial Al₂O₃. Nomograms are provided to facilitate interpretation of the results.     

Initial Coarsening and Microstructural Evolution of Fast-Fired and MgO-Doped Al2O3

The effect of an initial coarsening step (50-200 h at 800°C) on the subsequent densification and microstructural evolution of high–quality compacts of undoped and MgO–doped Al₂O₃ has been investigated during fast–firing (5 min at 1750°C) and during constant–heating–rate sintering (4°C/min to 1450°C). In constant–heating–rate sintering of both the undoped and MgO–doped Al₂O₃, a refinement of the microstructure has been achieved for the compact subjected to the coarsening step. A combination of the coarsening step and MgO doping produces the most significant refinement of the microstructure. In fast–firing of the MgO–doped Al₂O₃, the coarsening step produces a measurable increase in the density and a small refinement of the grain size, when compared with similar compacts fast–fired conventionally (i.e., without the coarsening step). This result indicates that the accepted view of the deleterious role of coarsening in the sintering of real powder compacts must be reexamined. Although extensive coarsening after the onset of densification must be reduced for the achievement of high density, limited coarsening prior to densification is beneficial for subsequent sintering.     

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.