Fabrication of Calcite (CaCO3) Ceramics With High Density

Abstract

Calcium carbonate (CaCO3) exists in three polymorphic modi®cations: vaterite (hexagonal) [1], aragonite (orthorhombic) [2], and calcite (rhombohedral) [3]. The last form is stable at ordinary temperature and pressure. The former two are transformed into calcite at low temperatures on heating. It is dif®cult to densify CaCO3 by ordinary sintering methods due to the characteristic of thermal decomposition at 700 8C in air. Yamasaki and co-workers [4, 5] studied the low-temperature sintering of CaCO3 by the hydrothermal hot pressing technique; however, the sintered density was only 73%. According to this study, well-densi®ed CaCO3 ceramics can be fabricated by the combined use of hot pressing and hot isostatic pressing. The present letter deals with the fabrication of CaCO3 ceramics and their mechanical properties. As-received CaCO3 (calcite) powder (99.999% pure) with an average particle size of 1 im was used as a starting material. It is known that the decomposition temperature of CaCO3 into CaO increases with increasing pressure [6]. Ceramics were fabricated by hot pressing, followed by hot isostatic pressing using argon gas as the pressure-transmitting medium. Before hot pressing, the calcite powder was pressed into pellets without a binder at a pressure of 98 MPa and then isostatically cold pressed at 343 MPa. No calcination was performed because of a sintering temperature as low as 700 8C. The green compacts, covered with a-alumina powder ( 200 im), were hot-pressed for 2 h at 700 8C and 40 MPa to obtain the presintered compacts. The heating rate was 300 8C hy1, and the pressure was applied after the sample reached 600 8C. Presintered compacts were densi®ed by HIPing. HIPing conditions were as follows: (1) a heating rate of 300 8C hy1, (2) an increasing pressure rate of 90 MPa above 500 8C, and (3) sintering for 2 h at 700 8C and 196 MPa. X-ray diffraction (XRD) analysis showed that presintered and sintered compacts consisted of only calcite. Bulk densities after polishing with diamond paste were determined by the Archimedes method. Relative densities were calculated using 2.711 g cmy3 [3] for the theoretical density of calcite. Presintered CaCO3 with the relative density of 94% was produced after hot pressing. Scanning electron microscopy (SEM) was used for microstructural observations. As can be seen from an SEM micrograph for the fracture surface of presintered CaCO3 (Fig. 1), many pores are present at grain boundaries and interiors. The bulk and relative densities of sintered CaCO3 were 2.69 g cm y3 and 99.2%, respectively. Fig. 2 shows an SEM micrograph of sintered CaCO3, indicating a microstructure in which polyhedral grains ( 4 im) with sharp