Obtaining highly dense YSZ nanoceramics by pressureless, unassisted sintering

For technological applications, zirconia is commonly blended with other oxides to stabilise the tetragonal and/or cubic phases at low temperature, being yttria the most frequently added dopant. It is generally desirable to obtain highly dense ceramics while maintaining grain sizes in the nanoscale (<100 nm). Small grains contribute to stabilise the tetragonal phase and to improve the toughness and flexural strength. Moreover, a higher ionic conductivity for cubic zirconia electrolytes is achieved with smaller grain size and lower thickness of the intergranular regions. The sintering onset temperatures required for nanometric particles are significantly reduced when compared to conventional micrometric powders. However, densification is generally accompanied by an undesirable grain coarsening. A Ramp and Hold Sintering (RHS) is the simplest densification schedule, consisting of heating up to the peak temperature followed by a holding time at that temperature. Another approach, called Two-Step Sintering (TSS) is based on the principle that the activation energy for grain growth is lower than the activation energy of densification. The key elements in this method are heating up to a high temperature to achieve a density >75% Theoretical Density (TD) to render the pores unstable, and then cooling down rapidly to a lower temperature to finish sintering and hinder grain growth. Alternatively, considerable efforts have been made in increasing the heating rate and/or reducing the hold time at the peak temperature during sintering cycles in processes that are generally referred to as 'Fast Firing' (FF) or 'rapid sintering'. This review summarises the attempts in the literature for obtaining dense monolithic nanocrystalline Yttria-Stabilized Zirconia (YSZ) ceramics by pressureless sintering schedules carried out in conventional furnaces. RHS, FF and TSS schedules are reported only from YSZ as starting powders, i.e. without the aid of any additional dopant or grain growth inhibitor. For the sake of comparison, the discussion is focused mainly on YSZ nanoceramics with final densities >99% TD and average grain size <100 nm. Powder and shaping effects on microstructure and properties of bulk nanoceramics are discussed. A comparison among sintering approaches is then made taking into account the microstructural development of the nanostructures and some key properties of the products.