Showing papers on "Grain growth published in 1994"

A coupled finite element-cellular automaton model for the prediction of dendritic grain structures in solidification processes

Abstract: A new algorithm based upon a 2-dimensional Cellular Automaton (CA) technique is proposed for the simulation of dendritic grain formation during solidification. The CA model takes into account the heterogeneous nucleation, the growth kinetics and the preferential growth directions of the dendrites. This new CA algorithm, which applies to non-uniform temperature situations, is fully coupled to an enthalpybased Finite Element (FE) heat flow calculation. At each time-step, the temperature at the cell locations is interpolated from those at the FE nodal points in order to calculate the nucleation-growth of grains. The latent heat released by the cells and calculated using a Scheil-type approximation is fed back into the FE nodal points. The coupled CA-FE model is applied to two solidification experiments, the Bridgman growth of an organic alloy and the one-dimensional solidification of an Al-7wt% Si alloy. In the first case, the predicted boundaries between grains are in good agreement with experiment, providing the CA cell size is of the order of the dendrite spacing. For the second experiment, the quality of the coupled CA-FE model is assessed based upon grain structures and cooling curves. The columnar-to-equiaxed transition and the occurrence of a recalescence are shown to be in good agreement with the model.

Grain destruction in shocks in the interstellar medium

Abstract: Destruction of interstellar dust occurs predominantly in supernova shock waves in the warm neutral/ionized medium (density approximately = 0.25/cu cm, temperature approximately = 10(exp 4) K). Recent theoretical developments and laboratory data for sputtering processes and grain-grain collisional vaporization allows us to better evaluate the grain destruction rate in interstellar shocks in the warm medium. We find that, independent of composition, grain denstruction in supernova blast waves is dominated by nonthermal sputtering for shock velocities greater than 50 km/s and less than or equal to 150 km/s and thermal sputtering at higher shock velocities. We use a detailed scheme for the vaporization of grains colliding at high velocities (v(sub s) greater than or equal to 20 km/s) and show that the grain-grain collision destruction process is only dominant for shock velocities of less than or equal to 50-80 km/s and is less important than previously assumed. Nevertheless, the grain-grain destruction rates are of order 30%-90% of the sputtering rates at v(sub s) greater than 100 km/s and less than 200 km/s and are important in vaporizing the cores of grains. Detailed results for grain destruction as a function of grain size and composition are presented. We also present results for silicon carbide, iron, ice, and porous test particles. For carbonaceous grains we find that the fractional destruction is less than or equal to 0.29, and for silicate it is less than or equal to 0.45, for v(sub s) less than or equal to 200 km/s. We have calculated grain lifetimes, using the three-phase model of the interstellar medium, and find lifetimes of 4 x 10(exp 8) yr for carbonaceous grains and 2.2 x 10(exp 8) yr for silicate grains. Given that the typical stardust injection timescale of 2.5 x 10(exp 9) yr, we conclude that efficient mechanisms for grain growth in the interstellar medium must exist in order that a significant fraction of the refractory elements be incorporated in dust, as observed. Therefore, although our improved model has less vaporization of dust due to grain-grain collisions, sputtering still destroys dust efficiently and grain mantle growth in the interstellar medium is required, a conclusion reached in previous models of grain destruction in the interstellar medium. Carbonaceous mantles on silicate grains can protect the silicate cores from sputtering destruction in interstellar shock waves, provided that the protective mantles can efficiently reform in the interstellar medium. Also, if the grains are porous the postshock grain velocities are lower than for solid particles, and grain destruction is reduced. Porosity and mantling may increase the grain lifetime by factors of approximately 3 and approximately 3-4, respectively. The fraction of interstellar silicon in silicate stardust is therefore less than or equal to 0.25, but more silicon might be depleted in the form of a grain mantle.

Computer simulation of the domain dynamics of a quenched system with a large number of nonconserved order parameters: The grain-growth kinetics

Abstract: The domain dynamics of a quenched system with many nonconserved order parameters was investigated by using the time-dependent Ginzburg-Landau kinetic equations. Our computer simulation of a model two-dimensional system produced microstructures remarkably similar to experimental observations of normal grain growth. After a short transient, the average domain or grain radius was found to increase with time as ${\mathit{t}}^{1/2}$, in agreement with most of previous mean-field predictions and more recent Q-state Potts model Monte Carlo simulations.

Stem Reserve Mobilisation Supports Wheat-grain Filling Under Heat Stress

Abstract: The grain filling of wheat (Triticum aestivum L.) is seriously impaired by heat stress due to reductions in current leaf and ear photosynthesis at high temperatures. An alternative source of carbon for grain filling is stored stem reserves. Two spring wheat cultivars (V5 and V2183) of very similar phenology and plant stature, which had previously been found to differ in grain shrivelling under drought and heat stress conditions in the field, were used to evaluate the hypothesis that the mobilisation of stored stem reserves into the growing grain is an important source of carbon for supporting grain filling under heat stress. In two experiments in Israel (1990 and 1991), the rates of stem dry matter (DM) and stem total non-structural carbohydrates (TNC) loss, grain growth and leaf senescence were monitored under optimal (control) and high (stressed) temperatures in the glasshouse (1990) and the growth chamber (1991). Cultivar V5 always sustained a smaller reduction in grain dry weight under heat stress, than V2183. Irrespective of temperature, V5 had a higher stem DM and TNC content at the onset of grain filling, greater depletion of stem dry matter (or TNC) during grain filling, and longer duration of grain filling, than V2183. During grain filling V5 generally exported about two to three times more DM from the stems than V2183, under both non-stressed and stressed conditions. On the other hand, V5 was more heat-susceptible than V2183 in terms of leaf longevity, in vivo chlorophyll stability and grain abortion under heat stress. In a third experiment (1992) five cultivars (including V5 and V2183) were subjected to chemical desiccation (0.3% potassium iodide) of the canopy in the field in order to destroy the photosynthetic source ofthe plant after anthesis. The same cultivars were subjected to heat stress (35/25oC) or non-stressed (25/15oC) conditions after anthesis in the growth chamber. It was found that grain dry weight reduction by chemical desiccation was highly correlated with grain dry weight reduction by heat stress (r2 = 0.89). Therefore, the superior capacity of V5 for grain filling from mobilised stem reserves is a consti- tutive trait which supports grain filling under heat stress which can be tested for by chemical desiccation of plants under non-stressed conditions.

Microstructure Control of Silicon Nitride by Seeding with Rodlike β‐Silicon Nitride Particles

Abstract: The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si3N4 singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation Dn–D0n=kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2, retaining high strength levels of about 1 GPa.

Structure and properties of nanocrystalline materials

Abstract: The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed. A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed. New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.

InAs island‐induced‐strain driven adatom migration during GaAs overlayer growth

Abstract: The impact of the strain fields associated with partially strain relaxed InAs islands on GaAs (100) on the evolution of the growth front profile during subsequent GaAs capping layer growth as a function of the growth temperature is examined via placement of very thin AlGaAs marker layers. Transmission electron microscope studies reveal the presence of strain dominated atom migration away from the islands over dynamically evolving length scales of ∼100–400 A at higher growth temperature whereas at lower growth temperature such an effect is minimal. Anisotropy in the length scale of impact between the [011] and [011] directions is observed. Estimates based upon a suitably adapted formulation of the classical theory of grain growth shows the mass transport to be dominantly strain rather than surface curvature driven.

The study of grain size dependence of yield stress of copper for a wide grain size range

Abstract: Room temperature yield strength of copper has been measured by means of miniaturized disk-bend test as a function of grain size ranging from about 30 nm to 180 μm. It has been established that grain size dependence of strength does not obey the Hall-Petch relation in the entire range of grain sizes studied. The results obtained suggest that a gradual change of deformation mechanism takes place with decreasing grain size. Nanostructured samples appear to be rather ductile.

Critical Concentration of MgO for the Prevention of Abnormal Grain Growth in Alumina

Abstract: The effects of MgO on sintering and grain growth of alumina in the absence of any other impurities as well as in the presence of various amounts of CaO were investigated using ultrapure (>99.999%) alumina and sintering at 1900°C for 1 h in a clean contamination-free condition. Critical concentrations of MgO required for the prevention of abnormal grain growth were linearly dependent on the CaO concentration. For a given concentration of CaO, at least the same amount of MgO has to be added to prevent abnormal grain growth. MgO addition alone to the ultrapure alumina enhanced both grain growth and densification kinetics during pressureless sintering. The beneficial effect of MgO doping could not be explained based on the solute drag (or pinning) model. It was more likely to be understood in terms of either a glass modification model or a solid–liquid interface modification model.

Effects of α-SiC versus β-SiC starting powders on microstructure and fracture toughness of SiC sintered with Al2O3-Y2O3 additives

Abstract: Dense SiC ceramics were obtained by pressureless sintering of [beta]-SiC and [alpha]-SiC powders as starting materials using Al[sub 2]O[sub 3]-Y[sub 2]O[sub 3] additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from [alpha]-SiC powder was composed of equiaxed grains, whereas SiC obtained from [beta]-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the [beta][r arrow][alpha] phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using [alpha]-SiC powder increased slightly from 4.4 to 5.7 MPa [center dot] m[sup 1/2] with holding time, that is, increased grain size. In the case of the sintered SiC using [beta]-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa [center dot] m[sup 1/2] with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.

Non‐ohmic characteristics of ZnO‐V2O5 ceramics

Abstract: The densification behavior, microstructure, and electrical properties of ZnO‐V2O5 ceramics were studied with V2O5 as the only additive ranging from 0.01 to 1.0 mol %. The addition of V2O5 to zinc oxide shows a tendency to enhance the densification rate and promote grain growth. However, a microstructure that consisted of anomalously grown grains was found for the specimens containing V2O5≥0.05 mol % when sintered at 1100 °C for 2 h. The x‐ray diffraction and SEM‐EDS microanalysis revealed that the sintered specimens had a two‐phase microstructure, i.e., a vanadium‐rich intergranular phase formed between ZnO grains. The formation of the grain boundary barrier layer was confirmed by the non‐ohmic I‐V behavior and the quick drop of apparent dielectric constant with increasing frequency of the ceramics. A nonlinearity coefficient of 2.4–2.8 was obtained at a current density of 10 mA/cm2 for a series ZnO‐V2O5 ceramics, and a Schottky barrier height of 0.44–0.47 eV (at 25 °C) was determined from the I‐V and C‐V experimental data, based on the thermionic emission theory, and the model of back‐to‐back double Schottky barriers.

Effects of barrier layer and annealing on abnormal grain growth in copper thin films

Abstract: Abnormal (100) grain growth has been characterized in predominantly (111)‐textured Cu thin films as a function of deposition temperature, annealing temperature and the presence of a Ta or W underlayer. For films deposited at room temperature, bimodal grain size distributions are observed at annealing temperatures at or above 150 °C for Cu on Ta and 100 °C for Cu on W. Suppression of (100) abnormal grain growth was achieved by depositing Cu on either barrier layer at 150 °C. A bimodal grain size distribution was still observed for the film deposited on W at 150 °C but the large grains forming this distribution were found to be (111) oriented. These results are explained as the result of competition between strain energy minimization and surface and interface energy minimization. The (100) growth is shown to be driven by a reduction of the orientation‐dependent strain energy that builds up due to the elastic anisotropy of Cu. Films deposited at higher temperatures have a lower yield stress which limits the ...

Role of Defect Interaction in Boundary Mobility and Cation Diffusivity of CeO2

Abstract: Grain boundary mobility of CeO[sub 2] containing 0.1% and 1.0% trivalent dopant cations (Sc, Yb, Y, Gd, and La, in order of increasing ionic radius) has been measured. At the lower dopant concentration (intrinsic regime), mobility is controlled by grain boundary diffusion of host cations, whereas at the higher dopant concentration (extrinsic regime), mobility is controlled by solute drag through the lattice. The effect of trivalent dopants is closely associated with their ability to provide and to interact with oxygen vacancies. Evidence consistent with an interstitial mechanism for cation diffusion has been found which is remarkably affected by the presence of oxygen vacancies. Ce diffusion is enhanced by free oxygen vacancies in the system, while dopant diffusion is suppressed if a dopant-associated oxygen vacancy is not present. A bare Sc cation however, appears to be a fast-diffusing species, due to its highly distorted local environment, while Y at 1.0% emerges as the most effective grain growth suppressant.

Microstructure and related mechanical properties of hot pressed hydroxyapatite ceramics

Abstract: Polycrystalline hydroxyapatite (HAP) ceramics were densified by hot pressing. The effects of thermal treatments and of a sintering additive (Na3PO4) on the microstructure, flexural strength and fracture toughness were investigated. Hot pressing without additive resulted in dense HAP having a small average grain size (below 0.5 μm). Spontaneous microcracking of the material was also noted. This originated from the thermal expansion anisotropy of HAP crystals. The presence of the sintering aid promotes grain growth. Dense materials exhibited mechanical properties depending on the microstructure. The highest values obtained were 137 MPa and 1.2 MPa % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaci% GGTbaaleqaaaaa!36FA!\[\sqrt \operatorname{m} \] for the flexural strength and fracture toughness, respectively. A decrease of both strength and toughness was observed with increasing average grain size. This behaviour is attributed to the weakening of the grain boundaries by either the development of initial microcracking or the Na3PO4 addition. It is concluded that hot pressing is very useful to elaborate dense HAP having good mechanical characteristics.

Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites

Abstract: Grain growth kinetics in alumina-rich, zirconia-alumina composites have been examined as a function of volume fraction of zirconia. Both the 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 were 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.

Nb3Al Multifilamentary Wires Continuously Fabricated by Rapid-Quenching

Abstract: The Nb tube process has recently been developed at NRIM(Japan) for fabricating Nb3Al multifilamentary superconductors,1–3 which are characterized by the heat treatment at low temperature ( 1700°C), many attempts were made to form Nb3A1 by heat treating Nb/AI composite at high temperatures.4–6 However, rapid grain growth decreased the density of pinning center (grain boundary) in Nb3A1 and thereby degraded Jc in particular at low magnetic fields. To overcome this problem, Nb3A1 conductors fabricated through a rapid-quenching process have been studied. In this process the A15 Nb3A1 phase with fine grain structure can be precipitated from the supersaturated Nb-Al bcc phase by aging at ~800°C. However, no attempt was made to fabricate a practical-structure conductor such as a long multifilamentary wire through continuous rapid-quenching and subsequent annealing (post annealing) process.

Texture in multilayer metallization structures

Abstract: The effects of thin Ti, TiN, or Ti/TiN underlayers on the development of the crystallographic texture and the grain structure are explored. Metal layers ∼0.5 μm in thickness of Al‐0.5Cu or of Cu are deposited on these underlayers and on amorphous SiO2 as a reference. A strongly textured underlayer such as Ti〈0002〉 or Ti〈0002〉/TiN〈111〉 induces a similarly strong 〈111〉 texture in the AlCu. In copper with 〈111〉, 〈200〉, and random texture components, an underlayer induces a stronger 〈111〉 component compared to an analogous film deposited on SiO2. A nearly random texture in TiN significantly weakens the texture in subsequent metal films. Grain size distributions in all AlCu films are monomodal reflecting a process of normal grain growth. The grain size distribution for Cu sometimes deviates from lognormal. The bimodal distribution implies that grain growth is abnormal even though the median grain size does not exceed a low multiple of the film thickness.

Comprehensive Study of Lateral Grain Growth in Poly-Si Films by Excimer Laser Annealing and Its Application to Thin Film Transistors

Abstract: Lateral grain growth in nondoped poly-Si films was studied by using Si thin films (500 A) with different structures as a starting material for excimer laser crystallization. It was clarified that the lateral grain growth phenomenon (micron-size grains with strong (111) orientation) upon excimer laser annealing was strongly affected by both the microstructure and the orientation of the initial Si thin films. This result supports our previous speculation that the principal driving force of the lateral grain growth phenomenon is surface energy anisotropy. Poly-Si thin-film transistors using these films show a high field effect mobility of 440 cm2/Vs, achieved through a low-temperature process below 600° C. This excellent electrical characteristic is thought to be due to the large grain size of poly-Si thin film with controlled orientation, good crystallinity, and a smooth surface.

Crystallization Process of Polycrystalline Silicon by KrF Excimer Laser Annealing

Abstract: We have investigated the crystallization of a-Si films by means of pulsed KrF excimer laser annealing as a function of irradiation energy density (E L), using transmission electron microscopy (TEM), Raman scattering spectroscopy and secondary ion mass spectrometry (SIMS). The grain size increased gradually at 0.2–0.4 J/cm2, while a drastic enlargement of grains occurred with lateral growth at 0.6–0.8 J/cm2. The stress in the films decreased with a decrease in the thickness of the fine grain (FG) layer until the FG layer finally disappeared. We proposed a model in which a drastic enlargement of grains at high E L is controlled by the nucleation rate, the solidification velocity, and the nucleus density of initial growth. It was found that poly-Si films with large grains ( 0.5–0.9 µm), high purity of C ( ~3×1016 cm-3) and low stress were obtained in the high E L regime ( 0.6–0.8 J/cm2).

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Abstract: Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using Β-SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the Β-SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the Β→α phase transformation. With increasing holding time, the fraction of Β→α phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m1/2 to 8.3 MPa m1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.

Heteroepitaxial growth of chemically derived ex situ Ba2YCu3O7−x thin films

Abstract: Heteroepitaxial growth of Ba2YCu3O7−x (BYC) thin films prepared by postdeposition annealing on (001) LaAlO3 was characterized by TEM and x-ray diffraction studies of specimens rapidly cooled from various points in the growth heat treatment Heteroepitaxial nucleation of BYC occurred between 720 and 770 °C during heating at 25 °C/min to the annealing temperature of 830 °C The c-axis normal BYC rapidly coalesced into a continuous film with nearly complete coverage of the substrate surface after growth of a film of several unit cells thickness The experimental results were not consistent with purely solid phase heteroepitaxial nucleation and growth or epitaxial grain growth, mechanisms for microstructural evolution of other chemically derived epitaxial oxide thin films The nature of the transformation and the microstructure of the final superconducting films were consistent, instead, with growth of epitaxial BYC from a liquid phase that is present transiently during the anneal This hypothesis was supported by thermal analysis results obtained from the precursor material of which the films are composed prior to transformation to BYC

Preparation, characterization, and sinterability of well defined silica/yttria powders

Abstract: Dispersions of uniform submicron spherical particles consisting of silica cores and yttria coatings, or vice versa, were prepared by a precipitation technique. The overall size of the particles and the thickness of the shells could be varied over a wide range. Such powders were used to form green bodies by sedimentation, centrifugation, or pressure filtration, and the density and the pore size distribution of the resulting solids were evaluated. The green bodies were sintered and the changes in density, phases, and microstructure were followed with temperature. In general, the coated powders exhibited enhanced densification. On processing composite solids at temperatures <1000 °C, the formation of Y2Si2O7 took place, which caused a pronounced shrinkage of the samples. Powders of coated particles having the same silica/yttria ratios sintered at lower temperatures when the shell was composed of silica rather than of yttria. When either silica or yttria were in molar excess in the coated particles, the sintered products had a mixed composition of Y2Si2O7 and the component in excess. By terminating the sintering process before the grain growth started, the solids displayed a well-defined microstructure with a uniform distribution of areas of one phase in the matrix of the matter in excess. This property was mainly due to the uniformity of initial powders in terms of the particle size and the coating.

Analysis of Microstructural Development and Mechanical Properties of Si3N4 Ceramics

Abstract: The microstructural evolution in Si3N4 ceramics is analysed by studies of the growth of isolated Si3N4 grains dispersed in oxinitride glasses as well as model experiments of sintered materials. The experiments with the supersaturated oxynitride glasses offer the possibility to study the grain growth in the absence of steric hindrance and show the influence of the additive composition on the development of the grain morphology. The differences in grain growth behaviour is discussed with respect to the cation radius of the glass forming rare earth oxides. Investigations of polycrystalline Si3N4 ceramics show that the grain size and morphology are controlled by the properties of the Si3N4 starting powder, the sintering temperature and the additives. Different Si3N4 powders and various additive combinations were used in order to explore the dominant growth mechanisms.

Competition between strain and interface energy during epitaxial grain growth in Ag films on Ni(001)

Abstract: Epitaxial Grain Growth (EGG) is an orientation-selective process that can occur in polycrystalline thin films on single crystal substrates. EGG is driven by minimization of crystallographically anisotropic free energies. One common driving force for EGG is the reduction of the film/substrate interfacial energy. We have carried out experiments on polycrystalline Ag films on Ni(001) substrates. The orientation dependence of the Ag/Ni interfacial energy has been previously calculated using the embedded atom method. Under some conditions, EGG experiments lead to the (111) orientations calculated to be interface- and surface-energy-minimizing. However, when Ag films are deposited on Ni(001) at low temperature, EGG experiments consistently find that (111) oriented grains are consumed by grains with (001) orientations predicted to have much higher interface and surface energy. The large elastic anisotropy of Ag can account for this discrepancy. Strain energy minimization favors growth of (001) grains and can supersede minimization of interfacial energy if sufficient strain is present and if the film is initially unable to relieve the strain by plastic deformation.

Effect of particle size distribution on sintering

Abstract: A sintering model, taking into account the effect of particle size distribution and the effect of grain growth, was developed previously. Experimental data from the sintering of high-purity alumina were used to testify this model. The sintering was carried out at 1500 °C in air for various lengths of time. The results agreed with the prediction of the model. The powders with a narrower starting particle size distribution exhibited a lower sintering rate prior to the occurrence of grain growth, but a higher densification rate after the grain growth took place. The grain size/density trajectory was applied to reveal the effect of particle size distribution on sintering behaviour. It is suggested that powders with a narrower size distribution are preferable.

High‐Resolution Transmission Electron Microscopy Studies of a Near Σ11 Grain Boundary in α‐Alumina

Abstract: The atomistic structure of a near Σ11 (N211) grain boundary in ultrapure α-AI2O3 bicrystals was determined by quantitative high-resolution transmission electron microscopy (HRTEM). High-resolution imaging revealed an atomically sharp interface with a characteristic periodic pattern at (0111) ‖ (0111) facets. The pattern was analyzed by comparing the HRTEM micrographs with simulated images of different structures that had been relaxed by static-lattice calculations. The best agreement with experiment was reached for the grain boundary structure with the lowest energy (1.7 J·m−2). Although this structure reproduces the relative translation state of the adjacent crystals to within 0.02 nm, the calculated structure was not in complete agreement with the periodic pattern at the grain boundary. The origin of this difference is probably due to a slight misalignment of the two crystals adjacent to the boundary.

Yttria-ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation

Abstract: An analysis is presented of grain growth and densification of yttria-ceria stabilized tetragonal zirconia polycrystals (Y, Ce-TZPs) using both isothermal and non-isothermal techniques. The characteristics of Y, Ce-TZPs are compared to those of Y-TZP and Ce-TZP and the effect of increasing ceria concentration at constant yttria content is evaluated. During non-isothermal sintering two regimes are distinguished: below 900–1000°C the neck area increases strongly by surface diffusion accompanied by only very little densification and grain growth, in the temperature interval 900–1000°C to 1200°C the materials densify to 95% of the theoretical density via a grain boundary diffusion mechanism and grain growth accelerates. Dense materials with grain sizes of 0·15–0·20 μm can be prepared by isothermal sintering at 1100–1150°C. In Y, Ce-TZP it is yttrium that segregates to the grain boundaries at 1150–1400°C. The yttrium content of the grain boundaries in Y, Ce-TZP is independent of temperature and ceria-concentration under the investigated experimental conditions. Grain growth in dense TZP is controlled by a solute drag mechanism at elevated temperatures (>1200°C); this drag is highest for Y-TZP, absent for Ce-TZP and moderate for Y, Ce-TZP.