Mechanism for the Peritectic Reaction and Growth of Aligned Grains in YBa2Cu3O6+x

It is experimentally observed that the peritectic reaction, 211 + liquid → 123, can be driven essentially to completion in 1 h at an undercooling of only ∽30°C. The kinetic data, together with the observed microstructures, are inconsistent with the normal mechanism of the peritectic reaction. It is proposed that the mechanism of the reaction involves dissolution of 211 particles into the liquid and precipitation of solid 123. The aligned grain structure is explained through sympathetic nucleation of new 123 grains on existing grains.     

Direct Observation of Multilayer Adsorption on Alumina Grain Boundaries

Grain-boundary films 0.6 nm in size have been observed on the grain boundaries of neodymia (Nd₂O₃)-doped alumina (α-Al₂O₃) sintered at 1800°C. Direct observation by high-angle annular dark-field imaging in the aberration-corrected scanning transmission electron microscope shows that this type of grain-boundary structure is the result of multilayer adsorption. Neodymium cations adsorb onto the faces of each of the two grains that comprise the grain boundary by substituting for aluminum cations. The positions of these cations are slightly distorted relative to the perfect lattice, and a third atomic layer in the core of the grain-boundary resides between these two layers. The measurements also confirm that the thickness deduced from high-resolution transmission electron microscopy lattice images are accurate.     

Electron Microscopy of Annealed (Ni,Zn,Co)Fe2O4

( Ni,Zn)Fe₂O₄ samples containing small amounts of Co were characterized in a transmission electron microscope to ascertain the micro structural changes accompanying low-temperature oxidation of the samples. Although no new features resulting from oxidation were observed, prominent surjace reduction occurred in the thin foil specimens. Formation and growth of Ni particles on the ferrite surface are explained using the heats of formation of the oxides.²     

Compositional tailoring of the thermal expansion coefficient of tantalum (V) oxide

The effect of Al₂O₃ and Nb₂O₅ additions on the microstructure and thermal expansion behavior of tantalum (V) oxide has been studied. Both singly doped and co-doped compositions were examined. Compositions with 3 wt% Al₂O₃ (or greater), contained AlTaO₄ as a second phase. Complete solubility was observed for Nb₂O₅ contents in the range 1–5 wt%. It was found that doping with Al₂O₃ results in a decrease of the coefficient of thermal expansion (CTE), such that increasing amounts of Al₂O₃ (1–5 wt%) cause a successive decrease in the CTE value. In the case of Nb₂O₅ additions, the result is an increase in the CTE of the tantalum (V) oxide. However, in the range 1–5 wt% Nb₂O₅, the CTE value is relatively insensitive to the amount of Nb₂O₅ added. Due to the countervailing influences of these two additive oxides, it was demonstrated that co-doping with Al₂O₃ and Nb₂O₅ is an effective strategy for tailoring the CTE of tantalum (V) oxide.     

Changes in the Grain Boundary Character and Energy Distributions Resulting from a Complexion Transition in Ca-Doped Yttria

The grain boundary character distribution and the relative grain boundary energy of 100?ppm Ca-doped yttria were measured before and after a previously identified grain boundary complexion transition. The grain boundary character distribution of samples exhibiting normal grain growth (before the complexion transition) favored {111} planes, whereas those exhibiting abnormal grain growth (after the complexion transition) favored {001} planes. Additionally, the relative grain boundary-to-surface energy ratios in the sample exhibiting abnormal grain growth were 33?pct lower than in the sample exhibiting normal grain growth. The results also indicate that the complexion transition increased the anisotropy of the grain boundary energy, and this may be responsible for the increase in the anisotropy of the grain boundary character distribution.     

Effect of Liquid Volume Fraction on Grain Growth of Magnesium Oxide Grains in Molten Calcium Magnesium Silicate Matrix

The grain-growth kinetics of fully dense MgO compacts containing various amounts of CaMgSiO₄ from 1 to 20 wt% was investigated. The relationship between grain size, volume fraction of liquid phase, and annealing time was determined. The exponent of grain growth ( n ) was 3, independent of the volume fraction of the liquid phase, and the rate constant ( k ) was inversely proportional to the volume of liquid. The overall grain-growth kinetics was governed by mass transport through liquid pockets at grain corners, which provided the longest diffusion paths between the grains. This result was modeled after a solid-state system containing isolated pores in which the pores move by vapor-phase diffusion.     

Alumina Agglomerate Effects on Toughness-Curve Behavior of Alumina–Mullite Composites

Toughness-curve ( T -curve) behavior of composites of spherical, polycrystalline, coarse-grained, alumina agglomerates dispersed throughout a constant-toughness, fine-grained, 50–50 vol% alumina–mullite matrix has been evaluated as a function of agglomerate content for the range 15 to 45 vol%. T -curve behavior was evaluated using the indentation-strength method. Increasing alumina agglomerate content resulted in a progressive increase of large indentation load strengths with negligible change of plateau strength levels at small indentation loads. This behavior is consistent with underlying T -curves that rise to greater values and are shifted toward longer crack lengths with increasing agglomerate content, suggesting that both bridge spacing and bridge potency increase with increasing agglomerate content over the range tested. The proposed relationships between bridge spacing and agglomerate content, and bridge potency and agglomerate content, are rationalized in terms of residual stress considerations. The indentation-strength data also demonstrated that the composite containing the greatest alumina agglomerate content, 45 vol%, exhibited the greatest flaw tolerance.     

Effects of Milling Liquid on the Reaction-Bonded Aluminum Oxide Process

The reaction-bonded aluminum oxide process begins with aluminum, Al₂O₃, and usually ZrO₂ powders that have been attrition-milled in an organic liquid. The attrition-milled powder is then compacted and heat-treated in air to produce polycrystalline, Al₂O₃-based ceramics. Safety considerations have made it desirable for the milling liquid to be changed from acetone to a less-flammable solvent. In this paper, mineral spirits, ethanol, and mineral spirits that contains 2 wt% stearic acid are presented as viable alternatives to acetone. The effects of changing the milling liquid on the reaction process and the properties of the final fired ceramic are investigated.     

A study of grain-boundary structure in rare-earth doped aluminas using an EBSD technique

Oversized rare-earth dopant ions such as Y³⁺, Nd³⁺, and La³⁺ segregate to grain boundaries and reduce the tensile creep rate of -Al₂O₃ by 2-3 orders of magnitude. It has been speculated that these dopant ions can modify the grain boundary structure in alumina by promoting the formation of special grain boundaries. If this were indeed the case, it would provide a possible explanation for the aforementioned creep rate retardation. In order to test this hypothesis, electron backscatter diffraction (EBSD) has been used to assess both the proportion of coincidence-site lattice boundaries, and the grain boundary misorientation distribution, in aluminas doped with various ions (Zr, Y, Nd, La, Nd/Zr). The results show that the grain boundary structure in alumina is not significantly altered by the addition of the above dopants, implying that the change in grain boundary chemistry is primarily responsible for the observed creep behavior.