Low temperature liquid-phase sintering in the magnesium aluminosilicate glass - alumina syste

This study was carried out to obtain a fundamental understanding of the process conditions and strategies for lowering liquid phase sintering temperatures. Alumina coated with 9 vol% of a ternary eutectic glass in the MgO-Al$sb2rm Osb3$-SiO$sb2$ (MAS) was used as the model system. Homogeneous coating of the glass was ensured by using a sol-gel coating technique.;Sintering behavior of the glass coated alumina was investigated over a temperature range of 1400$spcirc$C-1460$spcirc$C. Mullite and spinel or sapphirine crystallized from the MAS liquid during sintering. The formation of spinel was observed at the interface between the liquid and alumina, whereas mullite was found to homogeneously crystallize in the liquid. The rate of glass crystallization was enhanced by finer powder. The time at which crystallization rate became significant corresponded to the onset of the decrease of densification rate at the temperature. Attaining a balance of these two competing phenomena at the sintering temperature is proposed as an additional criteria for successful liquid phase sintering at low temperatures. Additional experiments revealed that this balance is not easy to control in the case of bimodal powder mixture, and that homogeneous distribution of liquid at the initial stage is essential for the control of this balance.;Densification kinetics and crystallization behavior of 0.2 $mu$m alumina coated with boron or sodium doped glass in the MAS system were also investigated at 1400$spcirc$C and 1460$spcirc$C. Doping the Al$rmsb2Osb3$-MAS glass system is effective for liquid phase sintering of alumina at 1400$spcirc$C. The balance between densification and crystallization for the alumina coated with MAS glass was altered by the addition of boron and sodium. However, the mechanisms by which boron and sodium improve the sintering kinetics are different.;From the experimentally obtained result, the ratio of the densification rate to crystallization rate was calculated. Using theoretically determined equations and the analysis of above mentioned ratio, strategies to lower the temperature of liquid phase sintering was proposed.