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 (498%) ZnO compact produced by the TSS was smaller than 1 lm, while the grain size of those formed by the conventional sintering method was B4 lm. 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 8001C (step 1) and 7501C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 lm). Heating at 8501C (step 1) and then at 7501C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 8001C (step 1) and 7001C (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.

Two-Step Sintering of Ceramics

“…Furthermore Zhang et al proposed kinetic window for successful TSS profile. Mazaheri et al [22] confirmed the triple-point drag mechanism for controlled grain growth at the second-step sintering proposed by Chen and Wang [15] by using TEM image of two-step sintered ZnO compacts. ZnO sintered via TSS exhibited excellent I-V characteristics [16,74,76].…”

“…TEM images of the ZnO samples sintered (a) using TSS profile with the first-step sintering temperature of 800°C for 60 s and a second-step sintering temperature of 750°C for 72,000 s, (b) conventionally at 1100°C and (a1)-(a10) magnified images of grain boundaries marked in circles at (a) [22].…”

“…Electrical and optical properties of ZnO are mainly influenced by grain size. Grain growth of ZnO was successfully controlled using TSS [16,20,22,[74][75][76][77][78]. Zhang et al [20] and others [16] successfully sintered fully dense ZnO without grain growth at the final stage of sintering.…”

Densification and grain growth behavior of highly dense MgO ceramics in pressureless sintering

“…4a, we can see that at 1400 1C the main migration mechanism is grain boundary diffusion because q 4 3, and that volume diffusion has taken effect at 1410 1C but grain boundary diffusion still plays a main role for q ¼ 2.82 7 0.03 close to 3. Since grain boundary diffusion has little effect on grain growth [6,7,14] and volume diffusion is connected with grain growth [9], the grain growth rate plot (Fig. 3) below 1410 1C is very placid.…”

Dependence of dielectric properties on microstructural characteristics of (Ba0.7Sr0.25Ca0.05)(Ti0.9Zr0.1)O3 ceramics

“…High densities and homogeneous microstructures with small grain size have been obtained for BaTiO 3 [12], Y 2 O 3 , [13,14] cZrO 2 , [15,16], and ZnO [17] ceramics sintered by the two-step method that consists of the individual stages grain growth and densification.…”