Understanding sintering characteristics of ZnO nanoparticles by FIB-SEM three-dimensional analysis

In this study, we utilize three-dimensional (3D) reconstruction by focused ion beam (FIB) cutting and SEM imaging to understand the evolution of pore volume, pore-solid interfacial area, pore shape, pore connectivity, and pore number during the two-step sintering of ZnO nanoparticles. After the first-step sintering, the density is 75% and all the pores are connected. During the second-step sintering, the decrease of pore volume leads to the segmentation of pores and formation of closed pores. The shape of closed pores is irregular. The pore number first increases with the formation of closed pores, and then decreases due to the disappearance of small pores. The pore-solid interfacial area keeps decreasing during sintering. FIB-SEM 3D reconstruction offers an opportunity to directly and quantitatively observe the pore evolution and understand the sintering process at nanoscale.     

Characterization of Heterogeneous Microstructure Evolution in ZrO2–3 mol%Y2O3 during Isothermal Sintering

Pore boundary tessellation and quantitative stereology were used to characterize microstructure evolution in ZrO₂–3 mol%Y₂O₃ (3YSZ) that had been pressed to a green density of 46% and isothermally sintered at 1275°C for 0.1–10 h. Scanning electron micrographs showed that, relative to classical sintering models, the sintered 3YSZ microstructure was spatially heterogeneous, and that this heterogeneity affected the way in which the microstructure evolved during sintering. Pore boundary tessellation cell maps revealed the presence of dense regions within the microstructure that grew by the preferential elimination of smaller pores and resulted in larger more widely spaced porosity at longer sintering times. In consequence, the average pore separation distance increased much faster than the average grain size. This would call into question the use of the grain size as a measure of microstructural scale for the prediction of densification kinetics for this material.     

Pore growth during the initial stages of sintering ceramics

Mercury porosimetry was used to measure changes in pore size distribution during initial stage sintering of compacts of submicron size particles of several oxides. Pore growth was observed in MgO and Fe₂O₃, and in Al₂O₃ under certain conditions. Pores can grow by these mechanisms: surface diffusion, particle size distribution effects, particle coalescence, phase transformation, and evaporation/condensation. Surface diffusion may be the mechanism in the case of an alpha alumina. Phase transformation was shown to be the cause when sintering gamma alumina. In the case of magnesia and ferric oxide, particle coalescence appears to be operating. Since pore growth competes with densification for the use of surface energy, it is an important sintering process.     

固相烧结—Ⅰ气孔显微结构模型及其热力学稳定性,致密化方程

讨论了二维及三维闭口气孔的稳定性，发现二维状态时气孔的稳定性问题可以用数学方法根据气孔的颗粒配位数和二面角的大小解析；而三维状态时气孔则可借助球形气孔模型近似地确定。在这一模型的基础上，建立了烧结中期和后期的气孔显微结构模型，并由此推导了因相烧结中，后期作用于气孔的烧结应力和固相烧结中斯和后期的致密化方程。     

Computer Simulation of Final-Stage Sintering: I, Model Kinetics, and Microstructure

A Monte Carlo model for simulating final-stage sintering has been developed. This model incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain-boundary diffusivity, and grain-boundary mobility. A preliminary study of a periodic array of pores has shown that the simulation procedure accurately reproduces theoretically predicted sintering kinetics under the restricted set of assumptions. Studies on more realistic final-stage sintering microstructure show that the evolution observed in the simulation closely resembles microstructures of real sintered materials over a wide range of diffusivity, initial porosity, and initial pore sizes. Pore shrinkage, grain growth, pore breakaway, and reattachment have all been observed. The porosity decreases monotonically with sintering time and scales with the initial porosity and diffusivity along the grain boundary. Deviations from equilibrium pore shapes under slow surface diffusion or fast grain-boundary diffusion conditions yield slower than expected sintering rates.     

Effect of Heating Rate on Pore Shrinkage in Yttria-doped Zirconia

The advantage of fast firing depends not only on the finer grain-size effect at the sintering temperature, but also on the smaller pore-size effect which results from the restricted pore growth. Pore growth due to the differential sintering during heating was observed to occur at relatively low temperature regions, and rapid passing through the region restricted the pore growth and enhanced the densification.     

Monoclinic Zirconia Microfiltration Membranes: Preparation and Characterization

Microfiltration zirconia membranes were prepared by slip casting from two pure zirconia powders derived from different processing techniques. Powders had almost the same mean particle size but were different in surface area, particle size distribution and morphology. Rheology of zirconia slips was studied in order to prepare a well-dispersed slip suitable for slip casting. The powders showed different dispersibility in the preparation of slips by colloidal processing. The effect of sintering temperature and holding time on porosity, pore size distribution, phase composition, microhardness and microstructure of unsupported membranes are studied and discussed in relation to the membrane processing and properties of powders resulting from different processing routes. Pore size distribution of membranes reflected the differences in morphology of particles and the state of agglomeration in the green samples.Isothermal sintering at 1100°C resulted in some tetragonal phase retained at room temperature in the monoclinic structure. Cracking occurred in membranes sintered above 1150°C due to the volume change in phase transformation. Densification behavior, removal of porosity and the hardness property showed differences that are attributed to the differences in powder processing and characteristics of powders. Crackfree membranes can be prepared by sintering at 1100°C from both powders.     

Influence of Sintering Temperature on Pore Structure and Apatite Formation of a Sol–Gel-Derived Bioactive Glass

The aim of this work was to investigate the influence of sintering temperature on the porous morphology, pore-size distribution, and apatite formation of sol–gel-derived porous bioactive glasses. For this purpose, three porous bioactive glasses were prepared by thermal phase separation cotemplate method in sol–gel process followed by sintering at 600°, 800°, and 1000°C. Pore structure of samples was characterized by various methods. The in vitro apatite formation test was carried out in simulated body fluid. The results showed that sintered bioactive glasses at 600°C exhibited a bimodal pore size distribution, mesopores, and nanopores (8–100 nm), macropores (100 nm–1 μm), and nanoscale pore walls (about 100 nm in width). The increase of sintering temperature induced the presence of a submicrometer pore-size distribution (300 nm–3 μm) with nanoscale pore walls (about 200 nm in width) and a morphological transformation from particle-like pore walls to dense pore walls. Depending on this porous structure, as-synthesized samples exhibited faster apatite formation capability.     

Densification and Shrinkage During Liquid-Phase Sintering

The process of densification and shrinkage during the final stage of liquid-phase sintering is described. The densification occurs by the liquid filling of pores during grain growth. The pore filling results in an instantaneous drop of liquid pressure in the compact and causes gradual accommodation of grain shape. The grain shape accommodation by the growth causes the specimen shrinkage. At the same time, the grains tend to restore their spherical shape, resulting in microstructure homogenization around filled pores. The process of densification and shrinkage appears to be determined by the growth of grains during sintering.     

影响多孔玻璃孔结构的因素

研究表明,利用填充法制备的多孔玻璃的孔参数(气孔率、孔径分布)可进行设计与控制,多孔玻璃的气孔率和孔径分布主要取决于成孔剂的体积比及其颗粒分布,前者与后者之间的偏差取决于生坯制备及烧结过程.     

Processing and microstructure characterization of porous corundum–spinel ceramics prepared by in situ decomposition pore-forming technique

Porous corundum–spinel ceramics were prepared from Al(OH)₃ and basic magnesium carbonate by an in situ decomposition pore-forming technique. Apparent porosity was detected by Archimedes’ Principle with water as medium. Pore size distribution and the volume percentage of micropores were measured by mercury intrusion porosimetry, and the microstructure was analyzed by SEM. The apparent porosity of the sintered sample decreased with increasing the Al(OH)₃ content in the raw mixture. With increasing temperature from 1200 °C to 1300 °C the porosity of the sample increased rapidly, from 1300 °C to 1500 °C the apparent porosity increased slightly, while it decreased rapidly when the temperature increased from 1500 °C to 1600 °C. The pores in the samples consist of two groups. One group is composed of micropores whose diameter is mostly in the range from 150 nm to 300 nm while the other is composed of bigger pores whose diameter is in the range from 0.5 μm to 1 μm. It was found that the composition of the starting powders and the sintering temperature are responsible for the apparent porosity and the pore size distribution of the samples. However the spinel formation and sintering play a more important role on porosity and pore size distribution.     

Behavior of Large Pores During Sintering and Hot Isostatic Pressing

A viscoelastic model that describes the shrinkage rate of large pores within a fine-grained body has been developed. The concept is based on a shrinkage potential derived from the surface and grain-boundary tensions and viscous response determined by the creep characteristics of the polycrystal. Pore removal rates are also derived and used to predict pore removal times during sintering and hot isostatic pressing.     

Sintering of Fine Oxide Powders: I, Microstructural Evolution

Microstructural evolution during sintering has been investigated using fine powders of CeO₂ and Y₂O₃ with excellent sinterability. A universal pore size distribution, normalized by particle size, has been determined and found to be a function of density only. Microstructure evolves toward the universal distribution, with or without densification, signifying homogenization at all stages. This may even involve the elimination of supercritical pores, at low densities, which are otherwise thermodynamically not sinterable. Theoretical justification for these observations is made by using a network model with a random, but spatially homogeneous, distribution of spherical particles. Final microstructure after full density is reached is also found to evolve toward a universal steady state of grain shape/grain size distribution regardless of initial state.     

Preparation of interpenetrating ceramic–metal composites

Ceramic reinforced metals are attractive because of their enhanced elastic modulus, high strength, tribological properties and low thermal expansion. Most work in this sector has focused on particle- or fiber-reinforced composites where the ceramic phase is not continuous. This work presents aluminium–alumina composites where both phases are interpenetrating throughout the microstructure. Ceramic preforms were produced with sacrificial pore forming agents leading to porosities between 50% and 67%. Pore wall microstructure was varied by changing the sintering temperature. Permeability and strength was measured for the porous preforms and infiltration results were compared with theoretical predictions based on capillary law and Darcian flow. A direct squeeze-casting process was used to infiltrate the preforms with aluminium resulting in an interpenetrating microstructure on both macropore and micropore scale.     

Effect of Size Distribution of the Starting Powder on the Pore Size and its Distribution of Tape Cast Alumina Microporous Membranes

Thin disc type pure alumina membranes have been prepared by tape casting technique. Pore size distribution and pore volume have been determined by mercury porosimetry. Initial particle size of the alumina powder and the size distribution are found to have a strong influence on the ultimate median pore size and pore size distribution of the fired membranes. The spread of the particle size distribution of the powders is expressed by ‘quartile ratio’ which represents the size ratio corresponding to the cummulative finer percentages of 75 and 25 in the particle size distribution curve. With higher quartile ratio ( wider particle size distribution ) not only the median pore size increases but also the distribution tends to be bimodal. This is explained on the basis of certain basic sintering behaviors of the fine powders in general. ©     

Some observations on filter pressing and sintering of yttria-stabilized zirconia nanopowder

The aim of this work was investigation of the sintering behaviour of a material prepared by filter pressing of an yttria-stabilized zirconia powder with grain sizes of about 8 nm. The water suspension of the powder was filter-pressed under 5 MPa. The early shrinkage of the filter-pressed sample, during its non-isothermal sintering was attributed to removal of water layers adsorbed on the powder surface. The observation of the microstructure evolution in samples heat-treated at different temperatures was performed. Pore growth during sintering was related both to presence of agglomerates, and to pore coalescence. Particle arrangement in the material was very uniform, which led to uniform densification of the material. Heat treatment of the sample for 30 min at 1200°C resulted in the material of 99.96% relative density, and grains within nanometric range.     

Microstructure of Gas Diffusion Layers for PEM Fuel Cells

The gas diffusion layer (GDL) is a critical component of a proton exchange membrane fuel cell, and can play a key role in fuel cell performance. In order to design reliable and durable fuel cells, knowledge of the GDL microstructure is necessary. Currently, characterization of GDLs is generally based on porosity measurements to obtain a pore size distribution. However, the pore size distribution in GDLs may not be the only factor that affects the fuel cell performance. Additional microstructural characterization of GDLs manufactured by three different vendors (Toray, SGL, and Freudenberg) has been investigated. In addition to the pore size distribution, other statistical information of GDL microstructure including size, shape, orientation, and distribution of pores have been characterized and compared. Among these GDLs, the Freudenberg sample was found to have the smallest pore size and orientation analysis indicated that the pores were randomly distributed. Pore roundness was the lowest and pore clustering was highest in Toray sample. The effect of threshold setting on pore size data was also studied and found to have negligible influence on the calculated distributions. The microstructures of the GDLs were reconstructed in three‐dimension using computer simulations and good agreement with the two‐dimensional image analysis data was observed. The present work opens new opportunities for experimentalists and modelers in the area of fuel cell research to take into account the statistical characteristics of GDL microstructure.     

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

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.     

Effect of Phase Transformation on the Sintering of Lead Barium Metaniobate Solid Solutions

The densiflcation behavior and microstructure development were examined for Pb_1-xBa_xNb₂O₆ (x = 0.0, 0.15, and 0.30) with the low-temperature rhombohedral form. The rhombohedral form transformed to the high-temperature tetragonal form during sintering. The phase transformation increased the rate of grain growth, which influenced the microstructure development and densiflcation. The densiflcation proceeded partly before the phase transformation occurred, and the microstructure changed by the sequential formation of necks, chainlike grains, and equiaxed grains. The microstructure just after the phase transformation was determined by the microstructure just before the phase transformation. The chainlike and equiaxed grains changed to elongated and large equiaxed grains, respectively. Pore shape also changed.     

Domain coarsening in viscous sintering as a result of topological pore evolution

The microstructure evolution in 3D was studied by X-ray microtomography to reveal the relation between topology of pore networks and characteristic length in viscous sintering. The mean intercept length was defined from solid/pore interface for characterizing the length of solid phase and pore phase. The increase of the characteristic length with densification was termed as domain coarsening. The topological pore evolution was analyzed by using genus. The characteristic length increased with decreasing genus in the intermediate stage. The domain coarsening takes place as a natural consequence of pore evolution in viscous sintering, i.e., the decrease of total surface area concurrent with the topological transformations.