We assume that the reader is already aware with the concept of ‘Sintering’, and also the types and effects of sintering on various properties of refractory bricks. In this article we will discuss on the effects of Powder characteristics (Grains size and their distribution) on sintering and densification behaviour of any refractory brick.
Densification is an important objective of sintering. Characteristics of starting powder e.g., particle size, size distribution, particle shape, particle aggregates, degree of agglomeration have a profound effect on the sintering kinetics as well as densification and Microstructural development of a refractory brick. The current understanding of ceramic powder processing has led to the following description of the desired powder and transport processes which led to the high density [R.L. Coble and R.M. Cannon in “Material Science Research Processing of Crystalline Ceramics, Vol.11” Plenum Press, New York, 1978, p.291]:
>> Small, non-agglomerated, monodisperse, spherical powders,
>> Uniform, dense packing of powder,
>> Mass transport during sintering by volume or grain boundary diffusion, no transport by surface diffusion or vaporization and condensation.
How can particle size help in achieving better sintering at a relatively lower temperature and thus, high sintered density in a refractory composition?
It has been found that a higher percentage of smaller particle size or fines in the starting powder lead to a faster densification rate. The theoretical basis of this argument is due to Herring’s scaling law which states that there are simple laws governing the times required top produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical except for a difference in particle dimensions. That is why; those who are interested in high sintered density or reduced sintering temperatures and times strive for fines starting powder (mixture). However, the success in sintering of the fine powder relies on the removal of agglomerates and aggregates. De-agglomeration treatments increase the sinterability of refractory mixtures. Homogeneous mixing of ingredients plays an important role in this regard. There are other limitations too as a very high percent of fines in the initial mixture may cause other problems like -
>> Formation of lamination cracks in green bricks during their pressing or ramming which get exposed after the bricks become dry or has been fired.
>> Less compressive strength, less load bearing capacity and low MOR.
A narrow grain size distribution is imperative for obtaining a high sintered density. H. Kent Bowen stated two postulates for improving the manufacturability of high value added refractory products [H.K. Bowen in “Proceedings of the First China-US Seminar on Inorganic Materials Research”, May 17-21, 1983, Shanghai, Eds. T.S. Yen and J.A. Pask, Science Press, Beijing, 1983, p.55] -
>> Postulate 1: Powders (individual homogeneous or heterogeneous units) with a narrow size distribution are easier to process into uniform microstructures (uniformity of size and distribution of the voids), which results in easier control of the microstructure during densification.
>> Postulate 2: Submicron particles require modification of the interparticle forces by controlling the surface chemistry (usually a liquid phase) such that a small repulsive interaction is achieved by electrostatic, salvation, or steric phenomena.
How can particle size distribution help in controlling the porosity of a refractory composition?
According to Kingery [W.D. Kingery in “Ceramic Fabrication Process, Part IV”, Technology Press, Cambridge and John Willey and Sons, New York], in practice it is found commonly that the porosity is about 40% for a single particle size refractory material, and a combination of cubic and hexagonal packing is observed. In a binary refractory mixture i.e. if two quite different particle sizes are mixed, the apparent volume varies as indicated in the adjacent Figure. with a minimum in apparent volume at about
Fig.- Particle Packing in a Binary Refractory Mixture
Fig.- Particle Packing in a Ternary Refractory Mixture
70% coarse material. At an infinite size ratio, the lower straight lines are reached, while for identical sizes the top horizontal line is followed. The heavy line is for a diameter ratio of about 10:1. Addition of a third size i.e. in a ternary refractory mixture decreases the pore volume even more as shown in the Figure.
Thus, during processing i.e. before firing of the refractory bricks, the powder characteristics (at first instance can be observed through the green mixture sieve analysis data) can be considered as a set of constant parameters. In conclusion, to achieve a dense fine-grained microstructure it is desirable to have a starting powder with small particle size and a narrow distribution, non-agglomerated particles with equiaxed shapes, and high purity (or controlled dopant / additive content). For all practical purposes and mass productions of refractory bricks and castables of different types like Fire-clay, High Alumina, Basic, Silica, Mag-chrome, Mag-carbon, Slide Gate refractories etc. at industry level there are established standard values for the range of coarse, medium, fine and ultrafine fractions which need to maintained and checked at regular intervals by observing mixture (powder) sieve analysis reports to ensure better control over fired properties of these refractory bricks and castables respectively.