Low - Temperature Sintering of Porous Ceramics via Sodium Borate Addition

In the current study, sodium borate-bonded highly open porous ceramics successfully produced by starch consolidation technique. Open porous ceramic production was carried out by using an economical grade a-Si3N4, corn starch, CC31 commercial-grade kaolin, and borax decahydrate (Na2B4O7.10H2O). Borax decahydrate was used as a sintering aid in the system and total ceramic (a-Si3N4 + CC31): borax decahydrate ratio was kept constant at 5:1. Sintering studies of the shaped samples carried out in an air atmosphere at a relatively low sintering temperature, 1100°C, for one hour. Scanning electron microscopy investigations of the porous ceramic samples revealed that due to the high amount of borax based sintering additive a significant amount of liquid phase formed during the sintering process of the designed ceramics. Highly open porous(~66-74%) and lightweight(~0.64-0.83 g/cm3) ceramics were produced via starch consolidation technique and low-temperature sintering at atmospheric conditions.

Porous Ceramics Obtained by Slip/Starch Casting Consolidation Method (SSCC)

Slip/starch casting consolidation (SSCC) is a technique for obtaining porous ceramics, which joins the forming process by starch consolidation with the slip casting method. In this work, a slip which contains ceramic powders, starch and dispersant, is poured into a porous mold and is taken to an oven so that the gelling process occurs. After sintering, it is noticed that the ceramics show different characteristics from the ones obtained exclusively by slip casting or by starch consolidation. Alumina ceramics were produced by using the three methods presented in this work. The ceramics were characterized by apparent porosity, mechanical resistance and scanning electron microscopy. The ceramics produced by SSCC presented the highest mechanical resistance value (289 MPa), while the ones produced by starch consolidation and slip casting presented values of 126 MPa and 191 MPa, respectively.

Use of Statistical Techniques in the Optimization of the Process "Starch Consolidation"

The Starch Consolidation method of molding technique is based on the ability of starch swell and gelatinize in water, so the green bodies are formed from suspensions, generating controlled porosity after firing; however, it is strongly influenced by gelatinization temperature, starch used, content and type of dispersant, pH and viscosity. This work involves the statistical analysis of possible relations between viscosity, content of dispersant and pH. Slurries containing alumina, water and starch were prepared, settling the solids in 40% and varying the starch content of 10, 20, 30, 40 and 50%, compared to the solids at the mixture. PH and viscosity measurements were made of the slurries as a function of added dispersant. Were obtained and analyzed measures of each parameter, using the Pearson correlation, verifying the existence of an almost perfect correlation between most of the parameters studied.