Showing papers on "Grain growth published in 1988"

Grain‐Growth Transition During Sintering of Colloidally Prepared Alumina Powder Compacts

Abstract: When the grain size in partially sintered compacts of alumina was measured as a function of density, we found that the grain-growth behavior fell into two distinct regions. In the region where the porosity remained interconnected, grain growth was negligible; when the continuous pore network collapsed into isolated pores, grains grew rapidly. The transition in grain-growth behavior was observed at approximately 90% of theoretical density. A simple phenomenological method for obtaining the transition in grain growth is suggested. It is based on the idea that an abrupt increase in grain size should be accompanied by a significant decrease in the rate of sintering since the sintering rate changes as the third or fourth power of the grain size. The method consists of fitting the sintering data to an exponential function. The transition then appears as a change in the time constant for the exponential. The low rate of grain growth in the region where the pores are interconnected contradicts earlier work in the literature where significant grain growth during intermediate-stage sintering has been reported. This difference is explained in terms of the homogeneity of packing of our powder compacts, which were prepared by colloidal processing.

Ceramic Microstructures '86: Role of Interfaces

Abstract: Progress in the Understanding of Ceramic Microstructures and Interfaces since 1976.- Ceramic Microstructures: The Art of the Possible.- Designing Material Structures.- Purposive Design of Nanocomposites: Entire Class of New Materials.- Design of High Thermal Expansion Glass-Ceramics Through Micros truetura1 Control.- Microdesigning of Ceramic-Metal Composites.- Characterization of Microstructures.- Grain Boundaries and Interfaces: Some Applications of Electron Microscopy and Microanalysis.- Quantitative Characterization of Microstructural Geometry of Interfaces.- Gas Transport as a Tool for Structural Characterization of Inter facial Phases.- Unit Cell Distortion of Pb(Zr,Ti)O3 by Pb(Co,Nb)O3.- Electron Microscopy Studies of BaTiO3~NaNbO3 Ceramics.- Analytical Electron Microscopy of the Glassy Phase in Mullite/Zirconia Composites.- Microstructure Characterization of Basalt Glass-Ceramics.- Interfacial Reaction Between Bioactive Glass and Synthetic Physiological Solution.- Oxidation and Microstructure of Sintered Silicon Nitride.- Origin of Cracking Phenomena Observed in Decomposition Reactions.- Characterization of Interfaces.- Oxide Interfaces: Theory of Oxide-Oxide and Oxide-Metal Interfaces.- Green Function Method for Calculation of Atomistic Structure of Grain Boundary Interfaces in Ionic Crystals.- Comparison of the Energies of Symmetrical and Asymmetrical Grain Boundaries.- The Structure of a Natural Grain Boundary in a Magnetite Bicrystal Studied by XRD Technique.- Interface Morphology in Ceramics.- HREM Study of a Tetragonal ? Monoclinic Martensitic Interface in a Y2O3-Stabilized ZrO2 Alloy.- Control of the Tetragonal to Monoclinic Phase Transformation of Yttria-doped Tetragonal ZrO2 Polycrystals by Annealing in Water.- Grain Boundary Energy of Twisted MgO Bicrystals on (100).- T.E.M. Statistical Characterization of Grain Boundaries in Polycrystalline NiO Scales Obtained by High Temperature Oxidation of Nickel.- Structure of High-Angle Grain Boundaries in NiO.- High-Angle Grain Boundaries in Sheet Silicates (Biotite/Chlorite): A TEM Study.- HRTEM Analysis of Ordered Grain Boundaries in High Purity Alpha-SiC.- Atomic Structure of Interfaces in Sialon Ceramics.- Characterization of Impurity Segregations.- Atomistic Lattice Simulations of the Structure, Energetics and Impurity Segregation Behaviour of Ceramic Boundaries.- Distribution and Influence of Minor Constituents on Ceramic Formulations.- Characterization of Interphase Interfaces.- Diffusion Bonding of Metal/Ceramic Interfaces-A Model Study at the Nb/Al2O3Interfaces.- Ceramic-Metal Reactions and Their Effect on the Interface Microstructure.- The Influence of Surface Roughness on Interface Formation in Metal/Ceramic Diffusion Bonds.- A Reappraisal of Wetting in the System AI-AI2O3 from 750-1000 C.- Microstructural Characterization of Ni/Al203 Diffusion Bonds.- Bonding Ceramic-Metal Interfaces and Joints.- Copper-Glass-Ceramic Interfaces and Composites: Reactions, Microchemistry, and Electrical Properties.- The Study of Microstructures for Pt-Silicide Interfaces after Nd:YAG Laser Irradiation by XRD, SEM, XPS, and RBS.- High Resolution Electron Microscopy of Silicon Nitride-Metal Bonded Interfaces.- Interfacial Chemistry and Bonding in Fiber Reinforced Glass and Glass-Ceramic Matrix Composites.- Effects of Interfacial Diffusion Barriers on Thermal Stability of Ceramic Fibers.- Solid State Bonding of Alumina and Steel by HIPING.- Thermochemical Analyses of Interface Reactions in Carbon-Fiber Reinforced Glass Matrix Composites.- Interfaces in Heterogeneous Dissolution of Oxides in Molten Ca-Al-Silicates.- Microstructure Development.- Powders, Interfaces, and Processing: Alumina as a Case Study.- The Role of Powder Packing in Sintering.- Effect of Pores on Microstructure Development.- Microstructure Development of Hydrothermal Powders and Ceramics.- The Colloidal Chemistry of Growing Silica Spheres.- Effects of Vapor Transport on Microstructure Development.- Controlling Grain Growth.- The Effects of Grain Growth on the Intergranular Porosity Distributions in Hot-Pressed and Swelled UO2.- Effects of Solutes on the Grain Boundary Mobility of TiO2.- Microstructural Development in Hot Pressed CaO: Density Decrease and Pore Growth During Post Sintering.- Diffusion Induced Interface Migration in Ceramics.- Grain Boundary Diffusion Artefacts in Polycrystalline Nickel Oxide Grown by High Temperature Oxidation.- Variations in Grain Growth of Donor-Doped SrTiO3 with Cation Nonstoichiometry.- The Wetting and Dewetting of Grain Boundaries.- Anion Controlled Microstructures in the Al2O3-AIN System.- Sintering.- Sintering Theory for Crystalline Solids.- Selected Sintering Conditions for SiC and Si3N4 Ceramics.- Colloidal Consolidation and Sintering Behavior of CVD-Processed Mullite Powders.- Sintering of Acicular NiZn-Ferrite Powder.- Continuous SiC Fiber/Glass Composites.- Electrical Properties.- Electrical Conductivity in Ceramics: A Review.- The Crystallographical and Chemical Relationship between Intergranular and Bulk Resistivity in Semi-Conductor Oxides.- Interface Effects in Zinc Oxide Varistors.- The Role of Inter facial Microstructure in ZnO Varistor Materials.- Electrical Properties and Microstructure of ZnO-Nb2O5~MnO Ceramics Sintered in the Liquid Phase.- New Composite PTC Materials Based on PbTiO3-TiO2.- Effects of Microstructure Control on Ferroelectric Ceramics.- Effect of Deviations from Stoichiometry on Microstructure and Magnetic Properties of Ferrites.- Optical Effects of Grain Boundaries in PLZT Ceramics.- Microstructures of High Dielectric Constant Materials.- Interfaces Generated by Electronic Device Development in Beta SiC Thin Films.- Electrical Barriers at Grain Boundaries in Silicon Carbide Materials with BeO Addition.- Microstructural Analysis of Silicon Carbide Materials with BeO Addition.- Mechanical Properties and Behavior.- The New Generation of High Toughness Ceramics.- Influence of Microstructure on Creep Rupture.- Mechanisms of Dynamic Failure in Debased Alumina.- Some Interfacial Related Properties of Transformation Toughened Ceramics.- Precipitation in a (Mg) Partially Stabilized Zirconia during Aging at 1000 C.- High Temperature Mechanical Properties of ZrO2-Based Ceramics.- Microstructure and Mechanical Properties of a Al2TiO5-Mullite-ZrO2 Composite Obtained by Reaction Sintering.- Interfaces in Ceramic Fiber Composites.- Delamination Toughening from Interfacial Cracking in Ceramics and Ceramic Composites.- Development of Structure in Silicon Nitride Grain Boundaries.- The Impact of Compositional Variations and Processing Conditions on Secondary Phase Characteristics in Sintered Silicon Nitride Materials.- Structure and Chemistry of Interfaces in Silicon Carbide-Containing Materials.- Interfaces in Alumina-SiC Whisker Composites.- Creep Performance of Silicon Carbide Whisker-Reinforced Alumina.- Temperature-Dependent Toughening in Whisker-Reinforced Ceramics.- Effect of High Temperature Oxidation on the Microstructure and Mechanical Properties of Whisker-Reinforced Ceramics.- The Role of Interfacial Reactions on the Mechanical Properties of Ceramic Brazements.- Fracture Energies of Ceramic-Metal Interfaces.- On the Fracture Behavior and Microstructure of Metal-to-Ceramic Joints.

Ion‐bombardment‐enhanced grain growth in germanium, silicon, and gold thin films

Abstract: Grain growth has been studied in polycrystalline thin films of Ge, Si, and Au during ion bombardment. The phenomenon has been characterized by varying the ion dose, ion energy, ion flux, ion species, substrate temperature, and thin‐film deposition conditions. Films bombarded with Si+, Ar+, Ge+, Kr+, and Xe+ exhibited enhanced grain growth which was weakly temperature dependent and proportional to the energy deposited in elastic collisions at or very near grain boundaries. The effect of these parameters on grain size and microstructure was analyzed both qualitatively and quantitatively using transmission electron microscopy. A transition state model describing the motion of grain boundaries during ion bombardment has been applied to the present experimental data. The results suggest that bombardment‐enhanced grain growth may be due to thermal migration of bombardment‐generated defects across the boundary. The calculated defect yield per incident ion was found to be directly related to enhanced grain growth...

Grain growth in synthetic marbles with added mica and water

Abstract: Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 μm (D 0) to 65 μm in 24 hours. Growth rates fit the equation, D n -D 0 =Kt, where K is a constant and n≃2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, D max, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once D max is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.

Effect of Pore Distribution on Microstructure Development: II, First‐ and Second‐Generation Pores

Abstract: A sintering model has been developed to predict the consequences of independently varying the grain growth rate in alumina during final-stage sintering of a microstructure containing both small (first-generation) and large (inter-agglomerate second-generation) pores. The model shows that although it may be thermodynamically favorable to increase the grain growth rate, the kinetics of densification are such that it almost always pays to inhibit grain growth. This conclusion was verified by experiments on undoped, MgO-doped, and ZrO2-doped alumina impregnated with model spherical large pores produced by the burnt-out latex sphere method. A new type of ceramic processing map has also been developed to aid in the selection of the optimum processing conditions for the sintering of ceramics containing large pores.

Effect of Pore Distribution on Microstructure Development: I, Matrix Pores

Abstract: A model has been developed to describe the effect of the matrix (first-generation) pore distribution on microstructure development in the final stages of sintering. A model of simultaneous densification and grain growth was used to predict the effects of the number of pores per grain and the pore size distribution on microstructure evolution. Increasing the number of pores per grain was predicted to increase the densification rate, the grain growth rate, and the relative densification rate/grain growth rate ratio. Narrowing the pore size distribution was predicted to inhibit grain growth initially and to increase the densification rate indirectly. Overall, the pore distribution was predicted to have a strong influence on microstructure development and sintering kinetics.

Analytical and numerical modeling of columnar evolution in thin films

Abstract: A theory for the growth of columnar microstructure in thin films is presented. The zone II columnar microstructure is shown to result from a balance between shadowing (which results in zone I microstructures) and surface diffusion (which tends to smooth the surface). The zone I to zone II transition temperature is predicted together with the type of unstable modes. The surface morphology and the columnar grain structure are obtained both analytically and numerically. The scaling behavior of the grain size with film thickness is derived. Monte Carlo computer simulations follow the temporal evolution of three‐dimensional zone II microstructures, and account for growth competition between adjacent grains, and the formation of film texture. These results agree with experimental data.

Computer simulation of microstructural evolution in thin films

Abstract: The nature of the microstructure of a thin film strongly affects its functionality in electronic applications. For example, the rate of electromigration-induced failure is a function not only of the grain size in an interconnect line, but also of the width and shape of the grain size distribution. We are developing techniques which allow prediction of the relationships between the conditions for thin film processing and the topology and geometry of resulting grain structures. We have simulated two-dimensional microstructural evolution by determining the location of grain boundaries after nucleation and growth of crystalline domains. We have allowed for nucleation under a variety of conditions including constant nucleation rates, decreasing nucleation rates and instantaneous saturation of nucleation sites. We have also allowed for increasing and decreasing growth rates which depend in various ways on the domain size. We have simulated grain growth in two-dimensional structures by allowing boundary and triple point motion in order to reduce the total grain boundary area. The rate and nature of the initial stages of grain growth are strongly affected by the conditions for nucleation and growth. Eventually, however, grain growth appears to proceed as expected from analytical treatments.

Grain growth and alignment in hot deformed Nd‐Fe‐B magnets

Abstract: Formation of grain oriented Nd‐Fe‐B magnets from melt‐spun ribbons by hot deformation has been studied using electron microscopy. It is shown that deformation and alignment of Nd2Fe14B magnets result from a combination of plastic deformation, grain boundary migration, and grain boundary sliding. Due to a limited number of available slip systems in this material, samples with large grains deform less easily. Small grain size materials, as encountered in the melt‐spun ribbons, are well suited for die‐upsetting to produce oriented magnets.

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

Abstract: The formation process of barium hexaaluminate (BaO 6Al2O3) from BaCO3/γ-Al2O3 powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO3 and Al2O3 via formation of BaO·Al2O3. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al2O3 proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m2/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al2O3.

Analysis of thermal stress‐induced grain boundary cavitation and notching in narrow Al‐Si metallizations

Abstract: Grain boundary voiding and notching have been found to produce failures in narrow metallizations during thermal aging. The nucleation and growth of grain boundary voids are considered to occur as a result of grain boundary sliding and the subsequent stress‐induced mass transport. A proposed model yields the linewidth and temperature dependence of the observed failure rate.

Transition from Amorphous to Crystalline Silicon: Effect of Hydrogen on Film Growth

Abstract: The transition from amorphous to crystalline silicon in films prepared in a hydrogen-diluted silane plasma has been studied. Emphasis was placed on the role played by hydrogen during film growth. Hydrogen is found to govern the film formation process by promoting the reverse process, ‘etching’. By preferentially eliminating energetically unfavorable configurations during the film growth, hydrogen controls the atomic structure, hydrogen incorporation, and grain growth of the film. It affects the total hydrogen content in the film, as well as the way hydrogen is bonded to the silicon. Excessive hydrogen dilution, however, reduces the grain size by changing the columnar grain growth to a more spherical-like grain growth, and eventually eliminates film growth. With appropriate hydrogen dilution, plasma deposition was found to yield ‘polycrystalline’ silicon films which has a large distribution of grain sizes, with the largest grains being about 2000 A.

Effect of Excess PbO on the Sintering Characteristics and Dielectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-Based Ceramics

Abstract: Additions of excess PbO to the perovskite Pb[(Mg1/3Nb2/3)0.92Ti0.08]O3 solid solution enhanced the formation of a liquid phase at 840°C, which served as a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30°C and dissipation factors less than 1%. Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase.

Rapid thermal annealing of high Tc superconducting thin films formed by metalorganic deposition

Abstract: Thin‐film superconductors of Y‐Ba‐Cu and Yb‐Ba‐Cu have been formed by the nonvacuum method of metalorganic deposition (MOD). The films produced in this manner were homogeneous and free of voids and cracks over large dimensions. A two‐step rapid thermal annealing of the MOD films, in oxygen, at 850 °C for 60 s followed by a second annealing at 920 °C for 30 s enhanced grain growth in the films and reduced the effects of substrate interaction. Preferred epitaxial grain growth, in the high Tc films, with the c axis both perpendicular and parallel to the substrate surface, occurred on 〈100〉 SrTiO3. Both the Y‐Ba‐Cu and Yb‐Ba‐Cu films showed superconducting onset temperatures above 90 K and zero resistance at 86 K.

Numerical simulation of a coarsening two-dimensional network

Abstract: Topological correlations in a coarsening two-dimensional soap froth or polycrystalline network are studied by computer simulation. With use of a continuum model, grain growth in very large systems of over ${10}^{5}$ grains can be simulated. The correlations found between the size or the number of vertices of adjacent grains are in accordance with semiempirical rules of metallurgy. The average grain size grows in proportion to the square root of time, as predicted by mean-field theory. This result, that correlation effects do not modify the growth exponent, is consistent with dynamical scaling and agrees with simulations done on a lattice.

Effect of Grain Growth on Pore Coalescence During the Liquid‐Phase Sintering of MgO‐CaMgSiO4 Systems

Abstract: During the liquid-phase sintering of MgO-CaMgSiO4 systems in N2 atmosphere, the total porosity and the average pore size increase while the number of pores decreases. The negligible permeability of entrapped nitrogen through the liquid matrix and the observed linear relationship between the number of pores and that of the MgO grains suggest that the pores coalesce as a consequence of grain growth during sintering. An analysis of the balance between the pressure of the entrapped N2 gas and the capillary pressure shows that pore coalescence in turn causes the observed porosity increase. When Fe2O3 or Cr2O3 is added to MgO-CaMgSiO4, the pore size and the total porosity become larger or smaller, because the grain growth is accelerated or retarded, respectively.

Static recrystallisation and recrystallisation during hot deformation of Al–1Mg–1Mn alloy

Abstract: Plane strain compression tests at 5 s−1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 103 times faster in the material free from small particles.MST/751

A theoretical investigation of two-dimensional grain growth in the ‘gas’ approximation

Abstract: A master equation for grain growth is suggested for the one-particle distribution of grain areas and topological classes in two-dimensional polycrystals with uniform properties of grain boundaries. The ‘collision’ term for a self-similar mode (normal grain growth) is formulated within the ‘gas’ approximation, assuming equal probabilities of neighbour switchings for all the grain boundaries and ignoring mutual arrangement of grains.

Compressive deformation of CoZr and (Co,Ni)Zr intermetallic compounds with B2 structure

Abstract: B2 type (Co,Ni)Zr compounds which were prepared by arc-melting were deformed in compression at temperatures from liquid nitrogen temperature to 973 K. Their flow stress was anomalously dependent on the testing temperature, decreasing with increasing temperature up to room temperature and then increasing with temperature up to about 673 K, followed by a decrease. The peak of the flow stress was higher for PE specimens than for PA ones which were machined perpendicular and parallel to the direction of grain growth of the ingot, respectively. It is considered that this behaviour of the flow stress is caused, not by the phase transition but by the motion of superlattice dislocations. The ductility of CoZr was lowered by cracking at grain boundaries at which secondary phases were observed. The substitution of nickel for cobalt suppressed the grain boundary cracking and (Co,Ni)Zr had a higher ductility than CoZr.

Role of Zinc Volatilization on the Microstructure Development of Manganese Zinc Ferrites

Abstract: The role of Zn atmosphere on the microstructure development of Mn-Zn ferrite prepared by alcholic dehydration was investigated. Discontinuous grain growth and intragranular porosity were observed throughout the specimen when a low oxygen partial pressure was employed during sintering. The microstructure of Mn-Zn ferrite sintered in air resembled a mixture between Mn ferrite (uniform grain size) and Zn ferrite (exaggerated grain growth). Exaggerated grain growth was also observed on the surface of Mn ferrite sintered in the presence of a Zn-rich atmosphere. The presence of Zn vapor phase enhanced the grain growth kinetics. The overall results indicate that a strict control of atmosphere is required in order to achieve a uniform pore-free microstructure.