Using Master Curve on the Sintering of Nanocrystalline Alumina and Titania Ceramic Powders
Date Issued
2004
Date
2004
Author(s)
Chen, Mong-Hsia
DOI
zh-TW
Abstract
“Master Curve Model” was originally derived from the general equation of kinetic reactions by our group. It can be used in several fields, including crystallization kinetics and chemical kinetics, to adequately describe the variations of each reaction systems. It has also been proved that the model can describe and predict the densification behaviors of micron- and submicron-sized ceramic sintering in our preliminary study earlier. But there are no any experimental evidence yet whether it can be used in the sintering of nanocrystalline ceramic powders. Furthermore, the sintering of nanocrystalline ceramic powders involves more complicated mechanisms than the sintering of conventional ceramic powders, so the conventional sintering models cannot be applied to the sintering of nanocrystalline ceramic powders. To date no practical model has been developed to describe the sintering of nanoceramics. This study explores the feasibility of using the master curve model to quantitatively describe the densification behaviors of nanocrystalline ceramic powders in sintering. We believe the results will contribute not only to the sintering practice in industry but also to the understanding of the basic science of sintering.
Five powders were used in this study. They are ?Al2O3 (average diameter~50 nm)、γ-Al2O3 (~15 nm)、rutile-TiO2 (~35 nm)、anatase-TiO2 (~10 nm) and Degussa P25-TiO2 (~45 nm). The sintering is conducted in air at designed heating paths. The densities of the sintered specimens are measured by the Archimedes method. The phases and microstructure of the samples are determined by XRD and SEM, respectively.
Because the particle size effect, the nanocrystalline ceramic powders usually contain metastable phases. Therefore, phase transformation may occur during sintering. I summarize the experimental results into two parts： (1) Only a small fraction of the powders experiences phase transformation during the sintering. In this case, the densification behaviors of the powders show a clear master curve relationship, which is independent of heating path. (2) A large fraction of the powders experiences phase transformation during the sintering. That is when an unmistakable two-stage densification behavior is observed. The 1st stage is the results of both the volume shrinkage due to the phase transformation and the elimination of porosity during the early period of the sintering. The data of this stage cannot be merged into a single curve. The 2nd stage is the results of the sintering after the phase transformation has been completed, and again we observe a clear master curve relationship.
The values of the apparent sintering activation energy, which were derived from both the alumina and the titania, are greater than that of the conventional ceramic powders. In particular, we find that the starting powders with a smaller size will give a relatively greater value. The relationship indicates not only the complexity in nanosintering, but also that we cannon use the conventional sintering mechanisms to explain the densification behaviors of nanoceramics. The higher values of the apparent sintering activation energy might due to the predominant surface diffusion during the early stage sintering of nanoceramics. The surface diffusion quickly consumes the sintering driving forces by the shape-change of the grains (into the shape of rod or column), which in term makes the densification process harder in the later sintering stage and effectively makes the sintering apparent activation energies higher than their conventional counterparts.
Subjects
奈米二氧化鈦
奈米氧化鋁
主導曲線
燒結
nanocrystalline alumina
master curve
sintering
nanocrystalline titania
Type
thesis
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