UC San Diego

Solid-state activated sintering of TiO2-CuO system was systematically studied to reveal the effects of grain boundary structures on low temperature densification. Specifically, the eutectic temperature and composition of the TiO2-CuO system were carefully measured to be 1010 ± 10 °C and 83CuO:17TiO2, respectively. Subsequently, a TiO2-CuO phase diagram was computed. Activated (enhanced) sintering of TiO2 with the addition of CuO occurring at >300 °C below the eutectic temperature was observed. High resolution transmission electron microscopy (HRTEM) characterization of water-quenched specimens revealed the formation of nanometer-thick, liquid-like, intergranular films (IGFs), a type of grain boundary (GB) complexion, concurrently with accelerated densification and well below the bulk eutectic temperature. Consequently, activated sintering is explained from the enhanced mass transport in this premelting-like complexion. An interfacial thermodynamic model was used to quantitatively explain and justify the stabilization of liquid-like IGFs below the eutectic temperature and the temperature-dependent IGF thicknesses. A GB λ diagram was computed, for the first time for a ceramic system, to represent the thermodynamic tendency for general grain boundaries to disorder.

Activated flash sintering of ZnO through a bulk phase transformation or a GB complexion transition was investigated. Specifically, in undoped and Al2O3-doped ZnO, the flash sintering is activated by natural thermal runways that can be quantitively predicted. In contrast, a bulk eutectic reaction and the associated formation of premelting-like IGFs in Bi2O3-doped ZnO can lead to a nonlinear rise in the specimen conductivity (above the Arrhenius extrapolation) to trigger flash sintering prior to the occurrence of the predicted natural thermal runaway. This work uncovers the roles of the bulk phase and interfacial (phase-like) complexion transformations in initiating flash sintering. Beyond high temperature interfacial liquids, water was used as an example of low temperature, transient, interfacial liquid to activate flash sintering at room temperature. ZnO powder pellets was water-assisted flash sintered (WAFS) to achieve ~ 98% of the theoretical density in 30 s without any external furnace heating. The specimen conductivity can be increased by > 10,000 times via absorbing water vapor to enable the room-temperature flash. The initial electric field must be higher than a critical threshold to lead to densification, suggesting bifurcation in kinetic pathways.

Another method called two-step flash sintering (TSFS) was proposed as a new ceramic fabrication method to achieve fast densification with suppressed grain growth. Using ZnO as an examplar, ~ 96.5% of theoretical density was achieved using TSFS with a grain size of ~ 370 nm, representing a > 3 times reduction of the grain size in comparison with conventional flash sintering. TSFS achieved this result in a few minutes, > 200 times faster than that needed for conventional two-step sintering to obtain comparable results.

Lastly, the effect of electric field on grain growth and grain boundary structure was studied in the presence of ionic grain boundary liquids at high temperatures. Using Bi2O3-doped ZnO as a model system, this study reveals the electrochemical polarization of ionic defects driven by an external electric field and current. A mixed conducting model was developed to explain the asymmetric grain growth of sandwiched single crystal, abnormal grain growth near the cathode, as well as the pore formations. The ionic blocking, electric conducting single crystal decouples the effect of electric potential and electrochemical potential on grain growth. It is assumed that electric field and current polarized ionic defects and thereafter modified local space charges. Grain boundary complexion transitions were also observed after annealing under electric field and current.