Highly concentrated ceramic bonding suspensions: Dispersion composition and porosity of casting

Abstract

In the production of materials based on highly concentrated ceramic bonding suspensions (VKVS), as a rule, one of the main problems is to obtain castings with a high density (low porosity). The porosity of the castings is a complicated function of the technical and rheological properties of the VKVS, the characteristics of the dispersion, and the molding or shaping conditions. The porosity of the casting to a large degree is governed both by the original and the "secondary" structure of the material formed in the sintering process or the heating procedure (for example, during the service of unfired materials).One of the main factors in reducing the porosity of the greenware formed by different methods is the choice of the optimum grain-size composition.In the present article* on the basis of generalized results of previous work [i, 2] and also new experimental data we were faced with the problem of determing the influence of the dispersion composition of the various types of VKVS on the density of the castings made from them.Characteristics and Features of the Dispersion Composition. Considering that the principle of the optimum dispersion and grain-size distribution [3] in the technology of VKVS is considered to be decisive, we need an all-round analysis of the main factors of their dispersion composition. The solid phase of VKVS should be characterized by a complete integral curve for the grain-size distribution, taking into account the particles of both the overscreen (more than 50 ~m) and the colloidal (less than 0.1-0.2 ~m) sizes. These curves for VKVS based on fused quartz are shown in Fig. i. In this case the content of particles plus 50 ~m was determined by the screen method, particles from 50 to 0.01 pm by the sedimentation method (on an automatic x-ray sedimentograph), and the colloidal component (less than 0.03 ~m) bv the ultracentrifuge method [4]. The section of the curves in the region d = 0.I-0.03 J ~m were extrapolated. As follows from the curves, the composition of the solid phases of VKVS is characterized by the polydispersion nature -extreme valuesof the particle sizes are distinguished by five orders (from 0.i mm to several nanometers).On the basis of the complete integral curve for the grain-size distribution of the particles it becomes possible to calculate the average dispersion parameters: average surface diameter of the particles d~v , pm [5]; and the specific surface of the solid phase Sspec , m2/g, as a function of dPav: 1 O0 6000 dPv = ~r S " = ,, spee dP ,Qs' av av