Influence of original greenware porosity and solid-phase hydration on the sintering of ceramic materials

Abstract

Accelerating or activating the sintering of ceramics is an urgent problem since it provides savings of fuel and energy.The term activation of sintering is normally understood to mean speeding up the change in porosity (shrinkage), making interparticle contacts and affecting the physicomechanical properties of the material under the action of factors other than temperature and sintering time [i, 2]. In such conditions the methods of bringing about these processes are classified into two groups: chemical and physical.The activity of the powders during sintering is ensured either by structural defects or by defects generated during sintering. However, we should note that the preparation and use of active powders, as a rule, causes inadequate density to develop in the greenware, and localization of the shrinkage, leading to the formation of gaps between the various microsections of the material ("subnormal" sintering) [3].In contrast to the above methods the present article will examine two special cases of accelerated (activated) sintering in ceramic materials.The first of them is revealed during the sintering of ceramic greenware with a high density, and the second with a certain hydration of the particles of solid phase in the original green article.Influence of the Original Porosity of the Greenware.In most cases in ceramic technology it is important to attain the maximum volume filling of the system by solid phase C d [4] at the forming stage, and also the minimum changes in C d at the stage of forming the final structure, for example, during drying and firing.The latter occurs because considerable volume changes in systems located in the compact form (greenware) frequently cause cracking, buckling, or destruction of the shapes.With a marked reduction in the original porosity of the greenware it becomes possible not only to reduce the temperature or duration of the heat treatment of the material to obtain the prescribed density (strength), but in most cases to attain the maximum values [4,5]. Moreover, there is also a significant fall in the shrinkage, which is of great practical importance in the production ofhigh-precision articles (especially large ones).Starting from the Ivensen phenomenological theory of sintering it is usual to consider [3, 6-8] that during the sintering, in the same conditions, of articles with different original porosity, we observe the same coefficient of relative reduction in the volume of the pores K:where m m Vg--m m v~=~ a~; a-g ~' ,~2) d k is the true density of the solid phase; dg is the apparent density of the greenware; d c is the apparent density of the material after sintering; and m is the mass of the specimen.As follows from Eqs. (1) and (2),the magnitude Vc/Vg represents the ratio of the absolute values of the pore volumes after sintering V c and before sintering Vg. It was shown in [9, i0] that for sinterable bodies with different original porosities the ratio of the porosities before and after sintering, expressed in percentages of the volume of the body, also is a consta...

Aluminosilicate Refractories Based on High-Alumina HCBS.

“…To all appearances, such a marked effect may be explained by two different mechanisms. In [11] in studying sintering of ceramic castings with a different starting density, a significant activation effect was noted, caused by reduced original porosity of a semifinished product. For quantitative evaluation of this effect there was a proposal and detailed analysis of an index of the sintering efficiency for highly dense semifinished products.…”

“…For quantitative evaluation of this effect there was a proposal and detailed analysis of an index of the sintering efficiency for highly dense semifinished products. According to [11,12] this effect of sintering activation is due to the following. It is well known that forces of surface tension, operating in a porous body, are the main moving forces for compaction of a dispersed porous system During sintering and are determined by the radii of curvature of pore channels.…”