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.     

Modeling of Grain-Boundary Segregation Behavior in Aluminum Oxide

It is believed that the segregation of oversized dopant ions to grain boundaries in Al₂O₃ hinders grain-boundary diffusion, thereby reducing the tensile creep rate in this system by ∼2–3 orders of magnitude. In order to explain this improvement in creep behavior, it is helpful to characterize both the effective cation and interstitial volumes at grain boundaries, because the relative openness of some boundary structures suggests a great accommodation of oversized ions. In this study, the boundary volume is determined by a spatially local Voronoi construction, which highlights cation (Al³⁺) substitutional sites as well as large interstitial voids. In particular, we examine the spatial distribution of free volume near grain boundaries and, in addition, the dependence of the driving force for segregation on misfit strain in doped Al₂O₃. We interpret our results in light of recent evidence that selective codoping can provide a more efficient means of filling available space near boundaries, thereby further enhancing creep resistance.     

Effects of Yttrium Doping α-Alumina: I, Microstructure and Microchemistry

The influence of yttrium doping on the microstructure and microchemistry of hot-pressed α-alumina was investigated using a combination of electron microscopy techniques. The implications of microstructure and microchemistry on the improved creep behavior of the doped material are discussed. The samples doped only with yttrium had a bimodal grain-size distribution that was strongly correlated to the frequency and distribution of Y₃Al₅O₁₂ (YAG) precipitates in the microstructure. Yttrium segregated to most of the grain boundaries, with a normal excess concentration of Gamma= 3.3 ± 0.9 atoms/nm² at random boundaries. Two types of twin boundaries (Sigma3 and Sigma7) accommodated no yttrium. None of the boundaries or triple-point junctions contained a glassy grain-boundary phase. Strong interaction of the grain boundaries and dislocations with YAG precipitates indicated a pinning mechanism by the precipitates. Yttrium doping did not appear to favor formation of special boundaries in α-alumina.     

Electrical Conductivities of (CeO2)1−x(Y2O3)x Thin Films

Electrical properties of CeO₂ thin films of different Y₂O₃ dopant concentration as prepared earlier were studied using impedance spectroscopy. The ionic conductivities of the films were found to be dominated by grain boundaries of high conductivity as compared with that of the bulk ceramic of the same dopant concentration sintered at 1500°C. The film grain-boundary conductivities were investigated with regard to grain size, grain-boundary impurity segregation, space charge at grain boundaries, and grain-boundary microstructures. Because of the large grain boundary and surface area in thin films, the impurity concentration is insufficient to form a continuous highly resistive Si-rich glassy phase at grain boundaries, such that the resistivity associated with space-charge layers becomes important. The grain-boundary resistance may originate from oxygen-vacancy-trapping near grain boundaries from space-charge layers. High-resolution transmission electron microscopy coupled with a trans-boundary profile of electron energy loss spectroscopy gives strong credence to the space-charged layers. Since the conductivities of the films were observed to be independent of crystallographic texture, the interface misorientation contribution to the grain-boundary resistance is considered to be negligible with respect to those of the impurity layer and space-charge layers.     

Structure of Alumina Grain Boundaries Prepared with and without a Thin Amorphous Intergranular Film

The presence of a thin amorphous intergranular film along grain boundaries in alumina is expected to affect the properties of the interface and hence those of the material. In the present study, two types of grain boundaries have been formed in hot-pressed alumina bicrystals. In one case, the surfaces of the sintered crystals were kept as clean as possible, while in the other a thin layer of SiO₂ was intentionally deposited onto the surface of one crystal. The distribution of SiO₂ along the resulting grain boundary was then monitored by transmission electron microscopy and compared with the morphological features of the interface. In the special cases chosen here, the glass receded into large pores which grew into the alumina itself. However, the presence of the glassy phase during the early stages of sintering clearly did influence the characteristics of the resulting grain boundaries.     

Orientation and Grain Boundary Microstructure of Alumina in Al/Al2O3 Composites Produced by Reactive Metal Penetration

The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al₂O₃ composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al₂O₃. The interconnected Al₂O₃ in the composite material is separated by Σ3 boundaries (twins) with a 60° rotation around the [0001] direction. A high frequency (∼100%) of Σ3 coincidence boundaries in composite alumina is remarkable since only ∼12% of boundaries in a sintered polycrystalline Al₂O₃ are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner.     

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 Composition on the Oxygen Tracer Diffusion in Transparent Yttrium Aluminium Garnet (YAG) Ceramics

The grain boundary structure and oxygen tracer diffusion in transparent yttrium aluminum garnet (YAG) ceramics varying from 2% excess of Y₂O₃ to 0.5% excess of Al₂O₃ were studied. The characterization of the specimens is as follows: (i) For the Y₂O₃-excess specimen, a second phase (yttrium aluminum perovskite: YAP) containing silicon in the grain boundary was found, (ii) For the Al₂O₃-excess specimen, both aluminum-rich particles (alumina) and a silicon-rich segregant layer were observed in the grain boundary. The volume diffusion of the oxygen tracer is little influenced by the excess composition. In contrast, the grain boundary diffusion of the oxygen tracer is suppressed in the Y₂O₃-excess specimens, compared to Al₂O₃-excess specimens. These differences are thought to result from the chemical reaction between the second phase and the intergranular liquid phase during the sintering.     

Atomic Resolution Transmission Electron Microscopy of the Intergranular Structure of a Y2O3-Containing Silicon Nitride Ceramic

High-resolution transmission electron microscopy (HRTEM) employing focus-variation phase-reconstruction methods is used to image the atomic structure of grain boundaries in a silicon nitride ceramic at subangstrom resolution. Complementary energy-dispersive X-ray emission spectroscopy experiments revealed the presence of yttrium ions segregated to the 0.5–0.7-nm thin amorphous boundary layers that separate individual grains. Our objective here is probing if yttrium ions attach to the prismatic planes of the Si₃N₄ at the interface toward the amorphous layer, using Scherzer and phase-reconstruction imaging, as well as image simulation. Crystal structure images of grain boundaries in thin sample (<100 Å) areas do not reveal the attachment of yttrium at these positions, although lattice images from thicker areas do suggest the presence of yttrium at these sites. It is concluded that most of the yttrium atoms are located in the amorphous phase and only a few atoms may attach to the terminating prism plane. In this case, the line concentrations of such yttrium in the latter location are estimated to be at most one yttrium atom every 17 Å.     

Grain Growth Control and Dopant Distribution in ZnO-Doped BaTiO3

ZnO additions to BaTiO₃ have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine-grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO₃ particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO₃-ZnO, it is proposed that dopant redistribution takes place by vapor-phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM-EDX analysis revealed that solid solubility of ZnO into the BaTiO₃ lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO₃ dielectrics.     

Silica and Magnesia Dopant Distributions in Alumina by High-Resolution Scanning Secondary Ion Mass Spectrometry

Trace SiO₂ and MgO additive distributions in sintered alumina have been studied using high-resolution scanning secondary ion mass spectrometry (SIMS). When doped with each additive individually, evidence is seen for both strong silicon segregation to grain boundaries ( C _gb/ C _grain similar/congruent 300) in SiO₂-doped alumina and strong magnesium segregation to grain boundaries ( C _gb/ C _grain similar/congruent 400) in MgO-doped alumina. When codoped with both SiO₂ and MgO, segregation of both ions to grain boundaries is reduced by a factor of 5 or more over single doping. The additive concentrations increase proportionally in the grains, and both dopants become more uniformly distributed throughout the bulk. It is concluded that codoping with these additives increases their mutual bulk solid solubility and decreases their interfacial segregation over single doping. The beneficial effect of MgO additions in controlling microstructure development in alumina and improving corrosion resistance to aqueous HF stems from its ability to redistribute silicon ions from grain boundaries into the bulk.     

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.     

A Review of the Influence of Grain Boundary Geometry on the Electromagnetic Properties of Polycrystalline YBa2Cu3O7−x Films

Shortly after the discovery of high-temperature superconducting (HTS) materials in the late 1980s, it was revealed that grain boundaries in these complex oxides are strong barriers to current flow. This fact has remained one of the most significant challenges to a viable HTS conductor, and necessitated the development of technologies capable of producing biaxially textured substrates in long lengths. Multiple studies have reported that the critical current density ( J _c) across grain boundaries in the perovskite-like superconductor YBa₂Cu₃O_{7− x} (YBCO) falls off exponentially below the intragrain J _c beyond a critical misorientation angle θ_c of only ≈2°–3°. Here we review our recent work demonstrating that certain grain boundary geometries permit significant enhancements of J _c well beyond the conventional J _c(θ) limit, and also that the grain boundary structure in YBCO films is tied closely to the films' deposition technique. Pulsed laser deposition, a physical vapor deposition technique, results in a columnar grain structure and planar grain boundaries that exhibit the typical J _c(θ) dependence. Ex situ growth processes, where the YBCO film is converted from a previously deposited precursor, can result in laminar grain growth with highly meandered grain boundaries. These latter grain boundary structures are directly correlated to greatly improved J _c values over a wide range of applied magnetic fields. Consequently, very high J _c values are possible in polycrystalline HTS wire even when significant misorientations between grains are present.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

Densification and Grain Growth of Fe-Doped and Fe/Y Codoped Alumina: Effect of Fe Valency

The effect of iron and iron/yttrium codoping on the densification and grain growth of ultra high-purity (99.995%) fine-grained alumina has been studied. The experiments were carried out under both oxidizing (flowing air) and reducing conditions (N₂/H₂ mixture, p O₂∼5.1 × 10⁻¹⁴). For studies carried out in air, relative to undoped alumina, the addition of 1000 ppm Fe was found to reduce the densification rate by a factor of 5 and also retard the grain growth rate. This result, which was consistent with tensile creep data obtained in a separate study, was attributed to the retardation of grain-boundary diffusive processes by segregating Fe(III) ions. In contrast, under reducing conditions the 1000 ppm Fe- doped samples exhibited an increase in the densification rate of 2.5 orders of magnitude over that of the undoped samples. In the case of the codoped compositions (1000 ppm Fe/1000 ppm Y), for heat treatment in air, the densification behavior did not differ significantly from that of samples singly doped with Y (1000 ppm). However, under reducing conditions, the presence of the Fe²⁺ in the samples appeared to compensate for the retarding effect of the yttrium, such that the densification rate of the codoped samples was comparable with that of the undoped material. A mechanism based on compensating point defects is invoked to rationalize the more rapid kinetics under reducing conditions.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

Effects of Grain Size and Humidity on Fretting Wear in Fine-Grained Alumina, Al2O3/TiC, and Zirconia

Friction and wear of sintered alumina with grain sizes between 0.4 and 3 μm were measured in comparison with Al₂O₃/TiC composites and with tetragonal ZrO₂(3 mol% Y₂O₃). The dependence on the grain boundary toughness and residual microstresses is investigated, and a hierarchical order of influencing parameters is observed. In air, reduced alumina grain sizes improve the micromechanical stability of the grain boundaries and the hardness, and reduced wear is governed by microplastic deformation, with few pullout events. Humidity and water slightly reduce the friction of all of the investigated ceramics. In water, this effect reduces the wear of coarser alumina microstructures. The wear of aluminas and of the Al₂O₃/TiC composite is similar; it is lower than observed in zirconia, where extended surface cracking occurs at grain sizes as small as 0.3 μm.     

Effect of Grain Boundary Structure on Diffusion-Induced Grain Boundary Migration in BaTiO3

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO₃. SrTiO₃ particles were scattered on the polished surfaces of two kinds of BaTiO₃ samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H₂ for those with rough boundaries. In the BaTiO₃ samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO₃ samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.     

Scanning Transmission Electron Microscopy Analysis of Grain Boundaries in Creep-Resistant Yttrium- and Lanthanum-Doped Alumina Microstructures

High-spatial-resolution analytical electron microscopy using energy-dispersive X-ray (EDX) and electron energy-loss spectrometry (EELS) of yttrium- and lanthanum-doped Al₂O₃ has been conducted to ascertain the level of segregation of these impurities to grain boundaries. Line profile analyses indicate that the segregation is confined to a layer thickness of <3 nm. Similar amounts of excess solute have been observed in both dopant systems: 4.4 ± 1.5 and 4.5 ± 0.9 at./nm² for yttrium and lanthanum, respectively. Assuming all the segregant is uniformly distributed within ±0.5 nm of the boundary, this excess corresponds to 9 ± 3 at.% for yttrium-doped Al₂O₃ and 10 ± 2 at.% for lanthanum-doped Al₂O₃. For both dopant systems, examination of the spatially resolved electron energy-loss near-edge structures (ELNES) on the Al- L _2,3 edge suggests a loss in octahedral symmetry and a slight Al-O bond-length expansion. No significant change is noted in the O- K edge.     

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.