Strategies and practices for suppressing abnormal grain growth during liquid phase sintering

Abnormal grain growth (AGG), where a small number of grains grow to sizes much larger than the neighboring matrix grains, is a frequent occurrence in liquid phase sintering of ceramics and cermets. As AGG can be detrimental to the material properties, a considerable amount of research on the nature, causes and suppression of AGG has been carried out. In this review, we outline the mixed control theory of grain growth and the principle of microstructural evolution that have been developed by Kang and coworkers over the last two decades. The theory and the principle, which are based on theories of crystal growth from a liquid, state that grain growth behavior is controlled by the nature of the solid-liquid interfaces, either atomically rough (macroscopically rounded) or smooth (macroscopically faceted). For grains with atomically rough solid-liquid interfaces, growth is controlled by diffusion of solute through the liquid phase and normal grain growth always occurs. For grains with faceted solid-liquid interfaces (or a mixture of rough and faceted interfaces), growth is interface reaction-controlled and diffusion-controlled below and above a critical driving force for growth, respectively. Depending on the relative values of the critical driving force for growth Δg_c and the maximum driving force for the largest grain in the system Δg_max, pseudo-normal, abnormal, and stagnant grain growth can take place. Based on this theory and principle, we present strategies for suppressing AGG by adjusting Δg_c and Δg_max to avoid AGG and examples of the successful use of these strategies.     

Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain–liquid interfaces have either flat or hill-and-valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two-dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain–liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition.     

Effect of Interface Structure on the Microstructural Evolution of Ceramics

The interface atomic structure was proposed to have a critical effect on microstructure evolution during sintering of ceramic materials. In liquid-phase sintering, spherical grains show normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion-controlled, because there is little energy barrier for atomic attachments. On the other hand, kink-generating sources such as screw dislocations or two-dimensional (2-D) nuclei are required for angular grains having an ordered or singular interface structure. Coarsening of angular grains based on a 2-D nucleation mechanism could explain the abnormal grain growth behavior. It was also proposed that a densification process is closely related to the interface atomic structure. Enhanced densification by carefully chosen additives during solid state sintering was explained in terms of the grain-boundary structural transition from an ordered to a disordered open structure.     

Microstructural changes in (K0.5Na0.5)NbO3 ceramics sintered in various atmospheres

(K_0.5Na_0.5)NbO₃ is a potential lead-free piezoelectric ceramic, but often suffers from abnormal grain growth. Previous work on BaTiO₃ and SrTiO₃ has shown that abnormal grain growth can be suppressed by controlling the sintering atmosphere. In the present work, (K_0.5Na_0.5)NbO₃ was sintered in atmospheres ranging from O₂ to H₂ and the effect on grain growth behaviour studied. Sintering in reducing atmospheres causes a delay in the onset and a reduction in the amount of abnormal grain growth. The effect of sintering atmosphere on grain growth behaviour can be explained using the 2D nucleation-controlled theory of grain growth. Changes in the grain shape during sintering in reducing atmospheres indicate a reduction in the edge free energy of (K_0.5Na_0.5)NbO₃ caused by an increase in the concentration of oxygen vacancies. This decreases the critical driving force necessary for rapid grain growth and causes a transition from abnormal to pseudo-normal followed by abnormal grain growth.     

Texture Development by Templated Grain Growth in Liquid-Phase-Sintered α-Alumina

The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al₂O₃ grains after sintering at 1400°C. A CaO + SiO₂ liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.     

氟化物对低温烧结氧化镁陶瓷的影响

研究了添加不同种类氟化物的氧化镁陶瓷的各种烧结条件和显微结构.实验发现:添加MgF2可以在相对低的温度下显著提升制品的致密度并促进晶粒的生长,说明采用MgF2可以实现低温烧结MgO.这说明MgO-MgF2体系中烧结驱动力即总界面自由能随着粒子总表面积的不断降低而降低.     

Phase-field simulation of platelike grain growth during sintering of alumina

The toughness of alumina can be improved by utilizing the in situ formation of platelike anisotropic grains during sintering, that is, abnormal grain growth (AGG). Computer simulations of AGG may be effective to realize the conditions for obtaining the desired self-composite microstructure. In the first part of this study, sintering experiments of high-purity alumina powders were conducted to confirm the effects of powder size distribution as well as the amounts of additives. In the second part, a phase-field method for simulating the platelike grain growth was proposed. The large platelike grains were reproduced when the critical driving force of coarsening was set up. The incubation time of AGG was also observed in the case of the narrow size distribution. Although the morphology of the platelike grains did not exactly agree with the experimental observations, a possibility of the present method as a computational tool for simulating platelike AGG was verified.     

Effect of TiO2 addition on grain shape and grain coarsening behavior in 95Na1/2Bi1/2TiO3-5BaTiO3

Grain coarsening behavior in the 95Na_1/2Bi_1/2TiO₃-5BaTiO₃ system has been studied as a function of the addition of TiO₂. As the amount of added TiO₂ was increased, the grain shape changed to a more faceted cube, indicating an increase in the step free energy of the facets, and hence a rise in the critical driving force for appreciable growth of grains. Grain coarsening behavior also changed from pseudo-normal to abnormal with an increasing TiO₂ concentration and thus increased faceting. The pseudo-normal behavior observed in the system without TiO₂ addition also changed to quite abnormal behavior during extended sintering. These observations support our theoretical prediction based on the coupling effects between the maximum driving force for growth and the critical driving force for appreciable growth.     

Sintering and Nanostability: The Thermodynamic Perspective

Sintering and nanostability (defined as the stability against sintering) are critical phenomena present in the processing and application of nanoparticles. With important implications in obtaining high‐quality dense ceramics with fine grains or in enabling high surface areas in nanoparticles for catalytic applications, the control of these interrelated phenomena has been the focuses of several studies. From a thermodynamic perspective, it is recognized that surface energy is a fundamental parameter in both cases, since it is the main driving force for sintering and also the reason that nanoparticles are thermodynamically unstable and have the tendency to coarsen at elevated temperatures. The role of grain‐boundary energies is less recognized as relevant, but is also connected to densification, grain growth, and nanoparticle stability. In this paper, we review the critical aspects of the role of interfacial energies in the microstructure evolution, in particular addressing them as parameters to allow better control in addition to more conventional kinetic parameters. The concept is based on the nonsingularity of interfacial energies in a given system, which varies with temperature, atmosphere, and most importantly, chemical composition—this last offering a method to induce particular microstructural evolutions. While the model assumes isotropic grain boundaries but consequences to anisotropy are also discussed. The paper presents examples of the role of dopants on interfacial energies, how this is quantitatively related to their segregation at the interfaces, and the impact in sintering and nanostability. Given the importance of interface energetics to these phenomena, we also present a short review on the current methods used to obtain reliable interface thermodynamic data.     

Grain-Growth Behavior during Stepwise Sintering of Barium Titanate in Hydrogen Gas and Air

To study the effect of oxygen partial pressure on grain growth in BaTiO₃, TiO₂-excess samples have been sintered in air with and without a prior H₂ heat treatment. Without prior H₂ treatment, abnormal grain growth occurs below and above the eutectic temperature ( T _e). An introduction of H₂ treatment before air sintering, however, increases the average grain size and suppresses the formation of abnormal grains during subsequent air sintering below and above T _e. This H₂ treatment effect has been explained in terms of a decrease of the driving force for the growth of faceted grains below a critical value for formation of abnormal grains. The observed grain-growth behavior under various atmospheres demonstrates the possibility of having various microstructures via control of oxygen partial pressure and initial grain size.     

Shape Dependence of the Coarsening Behavior of Niobium Carbide Grains Dispersed in a Liquid Iron Matrix

In niobium carbide–iron (NbC-Fe) specimens where the grains were faceted, abnormally large grains appeared during coarsening. Normal and uniform grain growth occurred when the grain shape was changed to a spherical morphology by the addition of a small amount of boron. The results have been discussed, in terms of a coarsening mechanism, depending on the atomic structure of the interface. For faceted grains with an atomically smooth interface structure, the coarsening was suggested to occur via two-dimensional nucleation and a lateral-growth mechanism. For spherical grains with an atomically rough interfacial structure, diffusion was suggested to control the coarsening process.     

A Screening Design Approach for the Understanding of Spark Plasma Sintering Parameters: A Case of Translucent Polycrystalline Undoped Alumina

An experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process.     

Chiral angle-dependent defect evolution in CVD-grown single-walled carbon nanotubes

Defects are ubiquitous in nanomaterials and it is critical to understand and control defect densities in these materials for electronic, chemical, and mechanical applications. Until now the relationship between nanomaterial structure and defect density during synthesis was limited to theoretical studies with no experimental confirmation of the predictions. Here we study defect evolution during the synthesis of individual single-walled carbon nanotubes (SWCNTs) using in situ Raman spectroscopy. SWCNTs are an important class of nanomaterials, and offer the unique ability to study the effect of their chiral angle on defect evolution during growth – a widely explored theoretical area that still lacks experimental confirmation. Our data reveals the first experimental evidence of chiral angle dependence on the defect density in SWCNTs, with lower defect density for higher chiral angle SWCNTs despite their faster growth rate. Modeling of the kinetics of defect generation reveals formation energy as the critical factor driving steady-state defect densities, with higher formation energies for topological defects in higher chiral angle SWCNTs and lower energies for low chiral angle SWCNTs.     

Mechanism of Grain Growth and α-β'Transformation During Liquid-Phase Sintering of β'-Sialon

The rates of grain coarsening and α-β'transformation during the liquid-phase sintering of Si₃N₄-β'₆₀-YAG sialon have been measured at varying liquid fractions and z values in order to determine the rate-controlling mechanism. The average β'-grain size after sintering for 16 h at 1650°C shows no variation with the liquid-matrix fraction if the z value is fixed and a marked increase with the z value if the liquid fraction is fixed. Similarly, the amount of untransformed α-phase after sintering for 2.5 or 3.5 min at 1600°C shows no variation with the liquid-matrix fraction if the z value is fixed and a marked decrease with the z value if the liquid fraction is fixed. These results show that the grain coarsening and the α-β'transformation are controlled by the interface reaction. This conclusion is consistent with the observations in carbide-Co systems and with the theoretical predictions that the growth of faceted grains is controlled by interface reaction and that of spherical grains by diffusion. A general rule between the shape and the growth mechanism of grains in a liquid matrix is thus proposed.     

Grain Boundary Sliding Microdisplacement Measurements During the Creep of Alumina

Grain boundary sliding (GBS) is thought to be the principal driving force for the nucleation, growth, and coalescence of grain boundary cavities during compressive creep of polycrystalline ceramics. It has been shown theoretically that stochastic GBS gives rise to continuous cavity nucleation and transient cavity growth and coalescence, eventually leading to crack formation and failure. This paper will show through experimental measurements, using stereoimaging techniques, that GBS is in fact stochastic. Also, mode II GBS, in-plane grain rotation, and in-grain shear displacements, strains, and strain rate measurements during creep of Lucalox Al₂O₃ will be presented. These displacements, measured on a machine vision system, will be presented in terms of the surrounding microstructural constraint and their lack of angular relation to the compressive load axis.     

Electric Field‐Enhanced Solid‐State Conversion of Ceramic Sr5(PO4)3F to Crystals

The single crystal solid‐state conversion of fluorapatite‐type Sr₅(PO₄)₃F (Sr‐FAP) has been achieved by spark plasma sintering with the assistance of NaF additive. NaF was determined to act as both a sintering aid and impurity solute along the grain boundaries (GBs), controlling both the space charge and GB migration rate. Postsintering isothermal annealing was performed to examine the effect of DC electric field on grain growth. From the space charge potential determined from impedance spectra measurements, in combination with the theoretical contribution of space charge to grain‐boundary energy reduction, it was concluded that the magnitude of the space charge in Sr‐FAP is temperature dependent. As such, a moderate decrease in polycrystalline GB driving force is the main cause for the increased single crystal migration length that was observed in this study.     

Driving force evolution in solid-state sintering with coupling multiphysical fields

A phase field model based on coupled thermo–mechano–diffusional equations is presented to simulate the microstructure evolution of hot pressing sintering under nonisothermal conditions. Simulation results for different heating rates are basically consistent with the experimental densification curves. Further research shows that the temperature gradient driving force increases with the heating rates, whereas the driving force of concentration and strain gradients are the identical in the same shape (corresponding to a certain relative neck radius) with different heating rate. Moreover, Simulation results indicate the evolution trend of the concentration gradient driving force along with neck growth is consistent with that from the classical theory which derived using Fick's law in an ideal two-sphere equal-radius model. Finally, a dynamic equations of sintering driving force and neck growth rate are obtained under the assumption of constant temperature, and the dynamic equation of neck growth was proved to be valid in different phase field parameters and sintering temperatures.     

Effect of Li2O and PbO Additions on Abnormal Grain Growth in the Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 System

Abnormal grain growth in Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN-35PT) ceramics doped with Li₂O and PbO has been investigated. Replacing the PbO dopant with up to 2 mol% Li₂O caused an increase in the number of abnormal grains. For the composition containing 2 mol% Li₂O and 6 mol% PbO, the amount of abnormal grain growth decreased with increasing sintering temperature. Single crystals of ∼6 mm × 6 mm × 2 mm thickness were grown from the 2 mol% Li₂O, 6 mol% PbO-containing composition via the templated grain growth method. Grain growth behavior with temperature is explained in terms of the effect of Li₂O on interface-reaction-controlled grain growth and the critical driving force.     

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

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.     

Preparation of tabular corundum aggregate from different sources alumina raw powder: Sintering kinetics and establishment of kinetics model

In this paper, we report the use of four types of commercial alumina raw powders as raw materials for the preparation of tabular corundum aggregates under the same conditions. The influence of the transition phases of alumina raw powder on the sintering kinetics of tabular corundum is discussed, the sintering model of materials with pseudomorphic structure is established, and the mechanisms underlying the different performances of various commercial tabular corundum samples are evaluated. The following conclusions were drawn based on the results of the study. (1) A double tetrakaidecahedron model was established and was shown to satisfactorily describes the sintering mechanism of alumina raw powder with pseudomorphic structure, which accords with the porosity change trend of sintered body and provides a basis for perfecting the sintering theory. (2) Compared with the other transition phases, γ-Al₂O₃ shows the largest phase transformation volume contraction, which provides the driving force for the sintering process via an increase in surface energy and mainly acts in the densification and grain growth stages. Thus, high-quality refractory raw materials are prepared with optimized physical properties and Intracrystalline pores or pore clusters in the crystal structure. The preparation of these high-quality refractory products is of importance for prolonging the life of these materials and also meeting rising energy demands.