Theoretical concepts of sintering were originally based upon ideas of the discrete nature of particulate media. However, the actual sintering kinetics of particulate bodies are determined not only by the properties of the particles themselves and the nature of their local interaction with each other, but also by macroscopic factors. Among them are externally applied forces, kinematic constraints (e.g. adhesion of the sample's end face and furnace surface), and inhomogeneity of properties in the volume under investigation (e.g. inhomogeneity of initial density distribution created during preliminary forming operations). Insufficient treatment of the questions enumerated above was one of the basic reasons hindering the use of sintering theory. A promising approach is connected with the use of continuum mechanics, which has been successfully applied to the analysis of compaction of porous bodies. This approach is based upon the theories of plastic and nonlinear-viscous deformation of porous bodies. Similar ideas have recently been embodied in a continuum theory of sintering. The main results of the application of this theory for the solution of certain technological problems of sintering are introduced including their thermo–mechanical aspects.

Theory of sintering: from discrete to continuum

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sli...

https://oatao.univ-toulouse.fr/2432/1/chaim_2432.pdf

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Experiments were performed with colloidally processed submicron alumina powders to investigate the effect of particle size distribution on their sintering characteristics. The results showed that in the absence of agglomerates and macroscopic size segregation, a broader particle size distribution leads to two opposing phenomena during sintering—enhanced overall sintering characteristics and a higher degree of local differential densification. The former is a result of both the higher initial green density and smaller isolated pores in the final stage of sintering brought about by enhanced grain growth during the intermediate stage. The latter is promoted by a higher degree of variation in local particle packing and may negate the enhanced sintering effect at sufficiently broad particle size distribution. There therefore exists an optimum range of particle size distribution for best sinterability. Since the optimum particle size distribution may vary considerably even for a given powder system, depending on the compaction technique and conditions used, narrow size distribution powder is preferred to monosized or broad size distribution powders for high sinterability and microstructure control of powder compacts, provided that agglomerates in the starting powder are removed by appropriate means. For the agglomerate-free, submicron alumina powder system studied, the optimum particle size distribution was found to have a geometric standard deviation value lying in between 1.6 and 1.9.

Effect of particle size distribution on sintering of agglomerate-free submicron alumina powder compacts

A model for the sintering of spherical particles of different sizes by solid state diffusion

Investigation on sintering mechanism of nanoscale tungsten powder based on atomistic simulation

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.

J. M. Ting

Papers

Effect of particle size distribution on sintering