INTEMA   05428
INSTITUTO DE INVESTIGACIONES EN CIENCIA Y TECNOLOGIA DE MATERIALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Forming and characterization of mullite green bodies obtained by novel starch consolidation routes
Autor/es:
M.H. TALOU; M.L. SANDOVAL; R. MORENO; M.A. CAMERUCCI
Lugar:
Mohs
Reunión:
Congreso; Fifth International Conference on Shaping of Advanced Ceramics; 2013
Resumen:
  The starch consolidation forming method can be used in the manufacture of porous ceramics. In this method, which is based on swelling and gelatinization properties of the starch in aqueous suspension at temperature (55-80 °C), the starch granules perform as a consolidator/binder of the green body as well as a pore former at high temperature. Commercially available powders of mullite, and potato and corn starches were employed as raw materials. Stable aqueous mullite/starch suspensions (0.25 starch volume fraction of 40 vol.% total solid loading) were prepared by intensive mechanical mixing and by homogenization in a ball mill for 6 h. The optimum stability conditions employed for preparing the aqueous suspensions were determined in a previous work. Taking into account the problems associated with the use of native starches in the forming of ceramic suspensions by direct consolidation method, and based on the premise to develop porous bodies with homogeneous microstructures and adequate mechanical properties, two shaping routes alternative to conventional processing by starch direct consolidation were evaluated. One of them was called Mixing Route (MR) and the other was denominated Soluble Route (SLR). Both routes were proposed with the aim to increase the viscosity of aqueous mullite/starch suspensions, and therefore, to prevent the segregation of particles. Thus, chemical or physical modifications of the native starches were performed. In MR, a mixture of ungelatinized native starch and a small portion (2.5 vol.% of the total solid loading) of gelatinized native starch (80 °C,10 min in a starch/water ratio of 1/10) was used by preparing the aqueous mullite/starch suspension. Regarding SLR, a fraction of starch (2.5 vol.% of the total solids in suspension) used in the aqueous mullite/starch suspension was a granular cold-water-soluble (GCWS) starch. GCWS was prepared by heating an aqueous suspension of the cassava native starch and a poliol (1,2-propanodiol) until 114 °C. Shear flow properties of these aqueous suspensions were analyzed to obtain information on their rheological behavior. With both routes, disks were formed by pouring the mullite/starch suspension at room temperature directly into metallic molds and heating at 80 °C, 2 h. Once the consolidation process was finished, samples were demoulded and dried (40 °C, 24 h). Green bodies shaped by two routes and obtained before (MRbb and SLRbb) and after (MRab and SLRab) burning out the starch, were characterized by bulk density and apparent porosity measurements, and microstructural analysis by SEM on the fracture surface. Both routes allowed minimize the segregation of mullite particles and starch granules and to obtain green bodies without defects or surface deformation, and homogeneous porous microstructure. The porosity was associated with tortuous cavities distributed throughout the thickness whose sizes corresponded to those original or swelled granules. An advanced gelatinization process occurred in all the samples. Only in disks prepared by MR with potato starch a few starch granules with integrity were observed in agreement with the high water retention capacity of the potato gel. Moreover, the forming of mullite bodies by routes proposed did not require the addition of others binders beside starch in order to increase the suspension viscosity. Thus, the complications that might arise due to the competition by superficial adsorption sites and rheological behaviour changes of suspensions that should be evaluated were avoided.