Low temperature sintering of nanosized ceramic powder: YSZ-bismuth oxide system

In this study, the mechanism of sintering and kinetics of nanosized YSZ powder in the presence of Bi2O3 were investigated. The sintering mechanism of YSZ-Bi2O3 system has not been understood well. It has been supposed that liquid phase sintering is main mechanism at temperatures 900°C or higher because Bi2O 3 melts at 825°C. However, the results of the present study indicate that this is not true and that the observed sintering enhancement by Bi 2O3 is entirely due to some solid state mechanism (at least for systems with less than about 5 mole% Bi2O3). A new mechanism termed "stress assisted rearrangement sintering" (STARS) is proposed.; STARS occurs when several conditions are met simultaneously. The first condition is the dissolution of Bi2O3 in YSZ. The dissolution of Bi2O3 is associated with a large increase in the concentration of oxygen ion vacancies. The concentration of vacancies is especially large on the surface of the crystallites. This highly defective surface has a high surface mobility. The second condition is the precipitation of monoclinic ZrO2. An additional driving force besides high surface energy of starting nanosized YSZ powder for densification comes from the large stresses caused by the inter-granular precipitation of ZrO2. The third condition is the inhibition of YSZ crystallite growth. The inter-crystallite ZrO 2 precipitates also act as Zener barriers preventing further growth of crystallites. Thus the driving force is not dissipated by coarsening and is used exclusively for densification process.; STARS does not occur at higher amounts (>about 5 mole%) of Bi2O 3 because the system composition moves out of the two phase region and into a 3-phase region containing a Bi-rich liquid phase at the sintering temperature. In this situation, liquid phase sintering dominates. Bi2O 3-rich liquid phase forms during heating at about 850°C. Significant growths of crystallites and of composite grains are observed in this region.; The overall porosity of YSZ-Bi2O3 system after STARS is controlled by the residual macropores formed by the large size of the initial Bi2O3 particles. Bi2O3 dissolves in YSZ leaving behind micronsize pores (macropores). These pores are removed very slowly only in the final stages of sintering.; The ionic conductivity of YSZ-4.5 mole% Bi2O3 sample sintered at 900°C for 1 hour is measured to be about two times higher than that of pure YSZ sintered at 1500°C for 1 hour. The reason for the higher conductivity in YSZ-4.5 mole% Bi2O3 is because of increased concentration of oxygen ion vacancies. This indicates that STARS may be an efficient way to sinter the YSZ electrolyte used in solid oxide fuel cells at low temperatures without loss of ionic conductivity.