Sintering Behavior Study Of α-Αl₂O₃ Nanoparticles With Different Sizes

Αlumina ceramics show wide applications as mechanism,electronics power,aerospace,chemical and medical fields due to their unique physicochemical properties such as high hardness,high temperature resistance,chemical resistance,wear resistance,excellent insulation and small dielectric loss of high frequency.However,the brittleness of ordinary alumina ceramics seriously affects the utilization,reliability and safety of ceramic parts.For the nano-characteristics,nano-materials show different mechanical properites from ordinary materials.And fine-grained nanoceramics（CaF2 and TiO2）provide an effective way to solve the problem of brittleness of alumina ceramics for which exhibit excellent ductility at low temperatures.We expect to prepare alumina nanocrystalline ceramics with high density,small grain size and expect they have excellent mechanical properties.In the sintering of α-Al₂O₃ nanocrystalline ceramics,the excessive growth of grain has been restricted the preparation of nanocrystalline ceramics.In this work,the sintering behaviors of α-Al₂O₃ nanoparticles with different sizes were studied and the effects of sintering temperature on the densification and grain growth of ceramics were investigated for assuring the suitable sintering temperature range.Finally,fully dense fine grain α-Al₂O₃ nanocrystalline ceramics was prepared through two-step sintering method（first heated to a higher temperature T1 at rate of 10 °C/min,followed by a rate of 10 °C/min to a lower temperature T2 and holding for a period of time）.By utilizing the difference between the temperature required to active grain-boundary migration and grain-boundary diffusion,two-step sintering method can complete the densification in the case of inhibition of grain growth and effectively avoid the grain growth in the final stage of sintering.First of all,α-Al₂O₃ nanoparticles with mean size of 7.3 nm is chosen as the object of study to research the influence of temperature and time on the relative density and grain size.From the variation curve of the relative density and grain size with the temperature,we know that the rapid densification of ceramics and the rapid growth of grains occur at different stages（the relative density of green bodies rapid growth between 900 °C and 1125 °C,while the green grain size rapid growth above 1150 °C）.This result provides experimental basis for the two-step sintering method.It showed that when the ceramic get relative density more than 84% sintered after T1,all pores in ceramic become subcritical and unstable.In the subsequent sintering with the lower temperature insulation,α-Al₂O₃ nanocrystalline ceramics can be completely densified by grain-boundary diffusion while grain-boundary migration is suppressed,avoiding the grain growth.At last,α-Al₂O₃ nanocrystalline ceramics with average grain size of 43 nm and relative density of 99.5% were prepared by the appropriate two-step sintering method（T1: 1125 °C,T2: 1000 °C,holding time: 20 h）.By sintering α-Al₂O₃ nanoparticles with mean size of 13.2 nm and 16.3 nm,it was found that α-Al₂O₃ nanoparticles with different sizes had similar sintering characteristics and the rapid densification and rapid growth of grains of the ceramics were at different temperatures area.With optimum sintering temperature and hold time,α-Al₂O₃ nanocrystalline ceramics with average grain size of 66 nm（ceramics relative density is 99.3%）and 74 nm（ceramics relative density is 99.2%）were obtained by sintering 13.2 nm and 16.3 nm nanoparticles respectively through two-step sintering method.It can be seen that the smaller size of α-Al₂O₃ nanoparticles,the higher of the sintering activity,the lower of sintering temperature in the two-step sintering,and the ceramic grains size is larger than the growth factor of the initial particles.