Structural and compositional gradients in alternating current sintered aluminum-doped zinc oxide

Flash sintering revitalizes the exploration of electric field and current effects on the microstructure with novel phenomena including rapid densification, low-temperature sintering, and unique microstructures. The aim of this study is to investigate the influence of electric current on the microstructural development of Al-doped ZnO (nAl=4at.%). We apply two sintering techniques, namely spark plasma sintering (SPS) and flash spark plasma sintering (FSPS), both conducted under identical conditions (AC, f=50Hz, |E→0|=25V/cm, p=180MPa) for three different temperatures 600∘C, 800∘C and 1000∘C. The distinguishing factor between the two techniques lies in the passage of electric current flow through the specimens in the case of FSPS. The resulting microstructures of the sintered pellets are examined by XRD, SEM, EDX, TEM and SAED to identify the process-related differences. While SPS specimens reveal a homogeneous microstructure, FSPS specimens exhibit a complex microstructure structure which consists of two distinct regions. The major volume of the FSPS pellets exhibits a structural gradient with coarse ZnO grains and ZnAl2O4 spinel precipitates in the center to fine ZnO grains at the periphery. This microstructure results from Joule heating and geometry-dependent heat dissipation, creating a symmetric temperature profile with a hot center. A minor volume of the FSPS specimens shows a gradient in microstructure and composition. Here, ZnAl2O4 spinel precipitates accumulate in the center and the periphery reveals Al-depleted, dense and very coarse grains. Electromigration of Al-cations and the thermodynamically favored formation of ZnAl2O4 in the hot center are proposed as underlying causes for the observed microstructure. The electric current in FSPS significantly alters the microstructure and composition of sintered specimen where dopant electromigration is most significant under high current densities above |J→|=29(5)A/cm2.