sintering

Sintering additives
The additives help to control the microstructure of the sintered materials; the effects of additives can be explain by :
* Additives that react with the basic compound to give a liquid phase at low temperature for example sintering additives are selected to react with the silica layer (SiO2) is magnesia MgO reacts with SiO2 to form MgSiO3, which we have liquid phase at about 1550°C(SiO2 form liquid at 1700 °C).
* Additives that do not lead to the formation of a liquid phase and which
consequently enable the sintering to take place in solid phase. This is the case of the doping of Al2O3 with a few of MgO, because at lowest temperature a liquid can appear in the Al2O3-MgO system
*Sintering additives are usually added to powders to enhance the sinter ability and to control the microstructure and properties.
Sintering with liquid phase
Parameters of the liquid phase
Various technical ceramics, most metals and cermets are all sintered in the presence of a liquid phase. It is rare that sintering with liquid phase does not imply any chemical reactions, but in the simple case where these reactions do not have a marked influence, surface effects are predominant. The main parameters of the liquid phase:
1) quantity of liquid phase: as the compact stacking of spheres leaves a porosity of approximately 26%; this value is the order of magnitude of the volume of liquid phase necessary to fill all the interstices and allow the rearrangement of the grains observed at the beginning of the vitrification. However, the presence of a small quantity of liquid (a few volumes percent) does not make it possible to fill the interstices.
P= Dambd3?????? (tf-t)
Where P porosity , Damb is the ambipolar diffusivity , d is the average diameter of the sintering particles, K constant , T temperature , tf final time and t the initial time for sintering , L volume fraction of liquid phase.
2) viscosity of the liquid: this decreases rapidly when the temperature increases. Pure silica melts only at a very high temperature to produce a very viscous liquid. The presence of lkaline earths quickly decreases the softening temperature and the viscosity of the liquid.