Effects of Thermal Pretreatment on Coarsening of Nominally Monodispersed Titania

The coarsening of nominally nwnodisperscd, ∼0.35-μm-diameter titania powders produced by controlled hydrolysis of titanium tetraethoxide was studied. Thermal treatments prior to coarsening experiments had a minimal effect on the apparent particle size distribution, but significantly modified the coarsening behavior and microstructural characteristics. Explanations attributing these differences to the effects of preannealing on the microstructure (grain size and phase composition) within particles are proposed.     

Microstructural Coarsening of Zirconia-Toughened Alumina Composites

In this work, the microstructural coarsening induced by heat treatment at temperatures between 1450° and 1750°C, for different zirconia-toughened alumina (ZTA) materials has been studied. In doing so, grain growth kinetics and fracture toughness have been documented and analyzed. Obtained results are discussed and rationalized in terms of microstructural effects on zirconia clustering and transformability (content and size) and operative toughening mechanisms. Based on the relatively broad microstructural and toughness ranges exhibited by the ZTA composites developed and characterized, it is concluded that fracture toughness tailoring through microstructure control may be effectively implemented for these materials.     

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the ‘use temperature’ of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments were studied. Microstructural stability experiments were conducted in air from 1200 to 1500°C and creep tests at temperatures of 1400 to 1700°C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400°C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior are discussed.     

Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites

Grain growth kinetics in alumina-rich, zirconia–alumina composites have been examined as a function of volume fraction of zirconia. Both he microstructural scale and character alter during grain growth. Grain growth exponents between n = 3 and 4 were observed and have been rationalized in terms of current models for grain growth and particle coarsening. Grain growth constants wore measured and compared with available data. Coupled grain growth was observed in that the ratio of the alumina-to-zirconia grain size was constant, for a given zirconia content. This behavior is discussed in terms of the topological constraint present during growth and the observed microstructural evolution. The measured activation energies were consistent with boundary-diffusion-controlled grain growth.     

Combined-Stage Sintering Model

By focusing on the similarities between the three stages of sintering, a single equation is derived that quantifies sintering as a continuous process from beginning to end. The microstructure is characterized by two separate parameters representing geometry and scale. The dimensionless geometry parameter, denoted T, comprises five scaling factors that relate specific microstructural featuers (e.g., surface curvature) to the scale (grain diameter). Calculations of T from experimental data show (a) agreement with computer simulations of initial-stage sintering, (b) the effect of surface diffusion on T, and (c) changes in T with microstructural evolution during sintering. Application of the model to the design of firing schedules and the study of microstructural geometry effects on sintering is discussed.     

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.     

Deterioration of a Classical Final-Stage Microstructure: A Study in Alumina

Classical final-stage microstructures have been obtained in an ultra-high-purity alumina by a latex sphere impregnation and burnout technique. The isolated equilibrium-shaped pores produced by the latex spheres deteriorated during long-term, high-temperature annealing under reducing atmospheres to produce a cracklike interconnected pore network. Several mechanisms of degradation are considered, including (i) the influence of entrapped gases, (ii) the effect of stresses produced during differential sintering, and (iii) the kinetics of competing mass transport mechanisms. Results suggest that the degradation is an inherent microstructural instability promoted by enhancing the ratio of coarsening rate/densification rate. Furthermore, it is shown that magnesia additions inhibit the process of microstructural deterioration.     

Effect of shear memory on the coarsening behaviour of polystyrene and poly(vinyl methyl ether) blend

The effect of shear memory on the coarsening behaviour of polystyrene/poly(vinyl methyl ether) (PS/PVME) blend which shows a typical lower critical solution temperature (LCST)-type phase diagram has been thoroughly investigated for the near critical composition (PS/PVME=30/70) using a time-resolved light scattering technique. The measurements were carried out at 135 °C (20 °C above the quiescent cloud point) at two different directions, parallel and normal to the direction of flow. Different shear memories were generated in the melt using a simple shear apparatus of parallel plate geometry. The coarsening process was influenced to a great extent by the shear history of the blend over the time scale of the measurement. The average domain size of the dispersed particles obtained from the analysis of the light scattering data on the basis of Deby Bueche theory was found to be shear memory dependent. The coarsening process was elevated and suppressed at low and high shear memory, respectively. This behaviour was attributed to the shift of the cloud point observed under same values of shear rates. In addition, the coarsening behaviour of this blend was found to be flow direction independent due to the very high viscosity ratio of the blend, which led to in turn rather circular domains of PS in PVME matrix without any elongation or orientation in the direction of flow. Furthermore, the coarsening process for all the measured samples was followed the general power low, regardless the shear history and the flow direction of the blends. This result indicated that; the shear could only retard or elevate the rate of domain growth without any effect on the coarsening mechanism.     

A new empirical model for quiescent annealing of binary co-continuous polymer blends

The high sensitivity of the morphology and final properties of co-continuous polymer blends to thermal annealing has motivated many researchers to study the evolution of their morphology during thermal annealing process. In this work, phase coarsening of a low interfacial tension polylactic acid/polycaprolactone blend and a medium interfacial tension polylactic acid/polyethylene blend during quiescent annealing was studied in detail. To this aim, characteristic length scale of the microstructure of the polymer blends was determined at different annealing times. It was found that the phase size in both blends increased linearly by time at the early stage of the annealing and then the phase coarsening rate gradually decreased at longer times. Finally, the phase size of the blends approached a finite size. The mechanisms involved in the observed phase coarsening behavior were discussed in detail. Linear and exponential phase coarsening models in the literature could not explain the observed phase coarsening behavior in the studied blends. A new empirical model was presented which showed a very good agreement with both the obtained results in this work and the previous experimental data in the literature. The obtained results indicate the significant potential of the new model in analyzing phase coarsening behavior of co-continuous polymer blends.

     

Experimental and Theoretical Investigation of Degradation Mechanisms by Particle Coarsening in SOFC Electrodes

Performance degradation data obtained from single solid oxide fuel cells, tested at 850 °C with air and humidified H₂ and using Ni‐YSZ anode supported cells, are presented here. Microscopic investigation is carried out on both anode and cathode to quantify variations in the morphology at different operation times. The comparison between the measurements on the cells and the SEM image analysis allows to conclude that there is no relationship between the initial cell activation and microstructural modifications of the electrodes. On the other hand, it was found that cell degradation is strictly related to the coarsening of Ni particles occurring in the anode. A theoretical analysis based on an electrode micromodel has been performed in order to compare the variation in performance, expected from particle size change, with the observed data. The model confirmed the conclusions of the experimental results.     

Coarsening of Faceted Crystals

The influence of the nucleation energy barrier on the capillary-driven coarsening of faceted crystals that exchange material by diffusion is quantified. Our calculations are based on the assumption that the transport of material between particles must happen in series with the nucleation of partial layers on flat facets. Using a numerical model based on this idea, we simulate the time evolution of distributions of crystals that are made up of perfect faceted crystals (without step-producing defects), crystals containing step-producing defects, and mixtures of the two types. We find that the coarsening of a distribution containing only perfect faceted crystals is arrested at a size where the nucleation energy barrier becomes prohibitive. This critical size ranges from a few nanometers to several hundred nanometers, depending on material parameters and experimental conditions. When a small fraction of the crystals have step-producing defects (for these crystals the nucleation energy barrier vanishes), they can grow to large sizes at the expense of the perfect crystals and a bimodal grain size distribution is created. Based on these results, we hypothesize that when abnormal coarsening is observed in nature, it results from the presence of a small number of crystals with step-producing defects.     

Microstructural stability at elevated temperatures

The factors affecting the diffusion-controlled coarsening of microstructures at elevated temperatures are reviewed in this paper. Coarsening can occur in a variety of different ways. Perhaps the most familiar of these is Ostwald ripening of a dispersed phase, e.g. precipitates in a 2-phase alloy. However, there are other related Ostwald-ripening-types of processes, including coarsening of dispersed phases in 2-dimensions (e.g. precipitates in thin films, ‘fibers’ in directionally solidified eutectics) and coarsening of particles that grow in three dimensions in a 2-dimensional diffusion field (e.g. grain-boundary precipitates, particles on a substrate). Coarsening by fault migration, which is an important issue in the microstructural stability of directionally solidified rod and lamellar eutectics, and discontinuous coarsening, which affects the stability of cellular microstructures produced by discontinuous precipitation, eutectoid decomposition and eutectic solidification, are also discussed. The principal predictions of theories of these various types of coarsening mechanisms are described, and exemplified where possible by reference to published data. Emphasis is placed on ability of current theory to explain existing data obtained from real as well as computer simulation experiments. Possible reasons for the shortcomings of theory are discussed.     

Reassessing cold sintering in the framework of pressure solution theory

Cold sintering densification and coarsening mechanisms are considered from the perspective of the non-equilibrium chemo-mechanical process known in Earth Sciences as pressure solution creep (or dissolution-precipitation creep). This is an important mechanism of densification and deformation in many geological rock formations in the Earth’s upper crust, and although very slow in nature, it is of direct relevance to the cold sintering process. In cold sintering, we select particulate materials and identify experimental processing parameters to significantly accelerate the kinetics of dissolution-precipitation phenomena, with appropriate consideration of chemistry, applied stress, particle size and temperatures. In the theory of pressure solution, pressure-driven densification is considered to involve the consecutive stages of dissolution at grain contact points, then diffusive transport along the grain boundaries towards open pore surfaces, and then precipitation, all driven by chemical potential gradients. In this study, it is shown that cold sintering of BaTiO₃, ZnO and KH₂PO₄ (KDP) ceramic materials proceeds by the same type of serial process, with the pressure solution creep rate being controlled by the slowest kinetic step. This is demonstrated by the values of activation energy (E_a) for densification, which are in good agreement with the existing literature on dissolution, precipitation, or coarsening. The influence of pressure on the morphology of ZnO grains also supports the pressure solution mechanism. Other characteristics that can be understood qualitatively in terms of pressure solution are observed in the in systems such as BaTiO₃ and KDP. We further consider activation energies for grain growth with respect to the precipitation process, as well as evidence for coalescence and Ostwald ripening during cold sintering. For completeness we also consider materials that show significant plastic deformation under compression. Our findings point the way for new advances in densification, microstructural control, and reductions in cold sintering pressure, via the use of appropriate transient solvents in metals and hybrid organic-inorganic systems, such as the Methylammonium lead bromide (MAPBr) perovskite.     

Shape Evolution of a Coherent Tetragonal Precipitate in Partially Stabilized Cubic ZrO2: A Computer Simulation

The kinetics of shape evolution of a tetragonal precipitate coherently embedded in a cubic matrix are examined. Specifically, the morphology of tetragonal ZrO₂ particles in partially stabilized cubic ZrO₂ is discussed. A computer simulation, carried out without any a priori constraint on possible kinetic paths and particle morphologies, shows that a lenslike shape appears during growth of a tetragonal particle. Upon further coarsening , the shape relaxes into a rhombus bounded by facets. Depending on the balance between interfacial and strain energies controlled by the particle size, the facets can be smoothly curved or straight. The predicted particle morphologies are in good agreement with the experimental observations. The kinetic model proposed is quite general for simulating microstructural developments during decomposition involving a crystal lattice symmetry change where elastic strain accommodation plays an important role.     

Nanoindentation behaviour of nano BiFeO3

Bismuth ferrite (BiFeO₃) is a unique magnetoelectric multiferroic that exhibits the coexistence of ferroelectricity and antiferromagnetism at room temperature. This unique combination of properties has pumped a huge surge in current research on BiFeO₃ as a future material for very important technological applications such as magnetic detectors and as an active layer in magnetoelectric memories. For such applications involving miniaturized components and devices, it is essentially important to have an idea of the mechanical integrity of the system at the scale of the microstructure. In spite of the wealth of the literature, however, the attempt to evaluate the mechanical integrity of nano BiFeO₃ at a scale comparable with the local microstructural length scale was almost non-existent. Here we report, possibly for the first time the nanoindentation behaviour of a sol-gel process derived nano BiFeO₃ having particle size of 5-25 nm. The nanoindentation studies were conducted at 100-1000 μN loads on a green pellet annealed at a low temperature of only 300 °C to avoid particle coarsening. The results showed interesting dependence of nanohardness and Young's modulus on the nanoindentation load which could be explained in terms of elastic recovery and plastic deformation energy concepts.     

Synthesis of nanoporous platinum thin films and application as hydrogen sensor

A nanoporous platinum (np-Pt) thin film based hydrogen sensor was fabricated and studied. The np-Pt thin films were fabricated through a method of chemical dealloying and coarsening starting from a CuPt alloy. The alloy thin films of Cu_xPt_1?x were deposited by sputtering copper and platinum at the same time. The dealloying process completely removed the copper from the film. We demonstrate a method to control the porosity of np-Pt by a method of coarsening. Scanning electron microscopy confirmed the presence of porosity with size ranging from a few nanometers to tens of nanometers. A sensor device with four electrodes was fabricated on the np-Pt thin films using a stainless steel mask and by sputtering copper. The electrical characteristics of the sensor exhibit marked sensitivity or current changes in the presence of hydrogen. The results demonstrate that np-Pt thin films configured as a gas sensor have high sensitivity to hydrogen.     

Factors Controlling the Sintering of Ceramic Particulate Composites: I, Conventional Processing

The effect of processing and material parameters on the sintering and coarsening of model composites consisting of a fine-grained ZnO matrix and coarse, rigid, inert particulate inclusions of ZrO₂ was investigated. The green composites were formed by two methods: (i) mixing the matrix and inclusion powders in a ball-mill followed by die-pressing and (ii) slip casting. For both forming methods, the inclusions caused a significant reduction in the density and the grain size of the composite matrix but had almost no effect on the grain size vs density relationship. The effects of inclusion volume fraction and sintering temperature were somewhat independent of the forming method. However, for the slip-cast composites, the effect of inclusion size was less severe and the packing of the matrix phase immediately surrounding the inclusion showed some improvement. The sintering kinetics and microstructural observations indicated that two main factors controlled the sintering of these composites: (i) interactions between the randomly distributed inclusion particles that created a constraint on the matrix and (ii) the packing of the matrix, especially in regions immediately surrounding the inclusion particles.     

Microstructure of ceramic foams

This paper describes the preparation of ceramic foams by expansion of a ceramic suspension based on a polyurethane system. The microstructure and degree of reticulation of the foamed ceramic were examined and analysed with the help of a simple geometrical model. Like the porous ceramics prepared by the replica processing method, these foamed ceramics possess open cells in a nearly equiaxed shape but the cell size is much finer. The ratio of the window size to the cell size is a useful parameter for characterising the geometry of the foam and is related to the qualitative concept of degree of reticulation. For a face centred cubic array of cells it is related geometrically to the volume fraction of porosity and this relationship is tested using microstructural measurements for a range of ceramic foams.     

Microstructure Development of Zinc Oxide in Hydrogen

Zinc oxide powder compacts with green densities of 60% of theoretical sintered in hydrogen do not densify but exhibit significant grain coarsening. The temperature dependence of the coarsening was greater than that expected from vapor-phase coarsening models and is close to that obtained for grain growth in dense ZnO. This suggests that the rate of particle coarsening is controlled by grain-boundary mobility. This possibility was reinforced by the reduction in coarsening with ZnAl₂O₄ present as a second phase. The results emphasize the importance of controlling grain growth throughout all stages of densification.     

On the flash sintering behavior of Li:ZnO—Evidence of electrode asymmetry driven by electrochemical reactions

The synthesis, subsequent flash sintering (FS) characteristics and microstructures of pure and Li-incorporated ZnO powders are reported. At low concentrations, Li is a substitutional occupant in ZnO but becomes an amphoteric dopant (substitutional and interstitial occupant) at higher concentrations inferred from a contraction reversal of the unit cell volume. Increasing Li reduces the average flash temperature of ZnO modestly by 15°C, and a doubling of the linear shrinkage. A discernible color smear (yellow–white–dark) stretching from the anode to cathode imputable to strong electromigration is also observed. Microanalyses of the electrode regions establish clear evidence of electrochemical (EC) lithiation into ZnO and the formation of Li–Zn compounds not observed in conventional sintering (CS). Interestingly, in contrast to CS, the addition of Li enhances coarsening during FS, suggestive of a dissolution–reprecipitation process concurrent with the EC lithiation process. Evidence for considerable (local) yet tangible temperature, chemical and microstructural asymmetry among electrodes driven by EC reactions is presented. Probable mechanisms, leading to these observations, are discussed.