Two‐Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Abstract

Two‐step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high‐density (>98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple‐point drags that result in the grain‐boundary curvature.