Numerical Simulation of the Fast Firing of Alumina in a Box Furnace

Abstract

Fast firing has been used to produce relatively dense ceramics in periods of time shorter than 5 min. High heat inputs during fast firing originates changes in the internal structure of the ceramic compact. The amount of energy available for sintering increases by the formation of a dense outer layer which controls the heat flux to the interior of the compact. In this paper, a numerical model based on the method of finite volumes was developed to simulate the fast firing process in an alumina compact. The heat transfer inside the ceramic sample was modeled through the whole process in order to predict the density changes induced by the fast furnace's heat transfer and compared to conventional sintering. SubscriptsBound boundary interfaces Conv convection Diff diffusion Fur furnace I index Rad radiation Sur surface Greek letters a thermal diffusivity Dt time increment Dx space increment in coordinate axis x Dy space increment in coordinate axis y Dz space increment in coordinate axis z ɛ emissivity w densification factor q density r Stefan-Boltzmann constant [W m À2 K À4 ] Symbols A area [m 2 ] c p specific heat [J kg À1 K À1 ] h heat transfer coefficient [W m À2 K À1 ] k thermal conductivity [W m À1 K À1 ] Nu Nusselt [-] Pr Prandtl [-] q″ heat flux [W m À2 ] Ra Rayleigh [-] S radiation heat source [W] T temperature [K] V volume [m 3 ]

Obtaining highly dense YSZ nanoceramics by pressureless, unassisted sintering

“…7. 104 Considering a ceramic green bar (20 × 20 × 100 mm) placed into a furnace at 1500°C, the compact's centre takes about 120 s to reach 98% of the furnace temperature. Owing to the elevated temperature difference between the centre of the body and the furnace, the sample heating occurs under non-isothermal conditions as shown by the isotherms.…”