Zirconia, $ZrO_2$, is regarded as such an outstanding and versatile ceramic that it has attracted world-wide attention through many scientific and technological investigations. The high ionic conductivity of stabilized $ZrO_2$ has yielded many important applications at high temperatures such as oxygen sensors for metal processing or for combution control, fuel/oxygen cells for electricity generation, and oxygen pumps for partial pressure regulation. The stress-induced phase transformation in the partially stabilized $ZrO_2$ has led to an important application in the enhancement of the strength and fracture toughness of ceramics. Fast-firing (or rapid sintering) is a technique to sinter ceramic powder compacts in which powder compacts are rapidly heated and then fired at high temperature for a relatively short period of time. Rapid heating can be conducted by passing powder compacts quickly through the hot zone in a furnace or heating the compacts rapidly using nonconventional heating methods like as microwave or plasma. Fast-firing is known to often allow the preparation of a product with high density and small grain size. Fast-firing has been successfully applied to the sintering of $\alpha-Al_2O_3$, $\beta-Al_2O_3$, and $Al_2O_3$-TiC composite. These successful applications and the benefits of fast-firing, the high density and fine-grained microstructure and the possible reduction of process time and cost, have encouraged attempts at extending this technique to new systems. But, unfortunately, the attempts to apply this technique to $ZrO_2$ are found to be not as successful as $Al_2O_3$. In this study, the possibility of fast-firing zirconia ceramics was investigated and theories of fast-firing were re-illucidated. As a result, one main cause prohibiting fast-firing of zirconia was attributed to the residual chlorines in the commercial zirconia powders manufactured by chloride process. By calcining at 1,100$^\circ$C for 1 hr or laundering with distilled water to remove residual chlorines from the powders, high densities (up to 99\% TD) could be obtained both by fast-firing and microwave sintering which used rapid heating rate of about $500^\circ\!C/\min$. The 'point density' phenomenon which means that the density obtained in the heating stage has little dependence on the heating rate, appeared to be affected by the pore size distribution in the powder compacts. It was argued that the thermodynamic stability of pores should be one of the important premise for the appearence of the 'point density' phenomenon. Pore size distribution analysis showed that the differential sintering was restricted in the fast-riring. So, the high density and fine-grained microstructure usually obtained in the fast-firing, was attributed to the restriction of the differential sintering in which grain-growth without densification readily occur. This arguement was modelized without conflicts with general sintering theories.
