Preparation Of Nanocrystalline Ceramics By The Method Of Combustion Reaction Heating And Quick Pressing And Investigation On The Processing Technique And Mechanisms

The developing fast sintering method of combustion reaction heating and quickpressing (termed as CR/QP) provided a convenient and economical way of preparingdense nanocrystalline ceramics. This approach utilized combustion reaction asthermal source to supply the compact of nano-particles with a heating effect, and itadopted a high mechanical pressure to assist the sintering process of nanocrystallineceramics. In this process, the pressure played an important role in enhancing thedensification process, thereby overcoming the drawback of low density in the productof conventional combustion reaction method. Furthermore, by taking advantages ofthe particular sintering conditions characterized with the ultra-high heating rate(>1600K/min), high pressure (170MPa) and short effective soaking duration (limitedto several minutes), the grain growth of nanocrystalline ceramics could be effectivelyrestrained while without impeding densification.In this thesis, the CR/QP method was applied to fabricate dense nanocrystallineceramics. Moreover, the investigation on the densification and grain growth behaviorsof the nanocrystalline ceramics, as well as the responsible mechanisms of masstransfer were also carried out.Firstly, polycrystalline magnesia (MgO) with an average crystallite size of50nmwas utilized as the raw materials, and the combustion reaction between nickel andaluminum was used as the thermal source. Under the optimized sintering conditionsof CR/QP, pure nanocrystalline MgO ceramics with a relative density of99.1%wasobtained at1620K and170MPa, and the concurrent grain growth was almostcompletely restrained. Besides, the effects of the various pressure-dependentdensification mechanisms including plastic flow, diffusion and power-law creep wereinvestigated, respectively. The results indicated that, the grain-boundary diffusioncreep accommodated plastic flow was the rate-controlling densification mechanism ofthe as-obtained nanocrystalline MgO ceramics, and an undefined recovery mechanismof internal stress was predicted to assist plastic flow, which further elevated thedensification rate of plastic flow while avoiding inducing the propagation ofdislocation in the interior of grains.Subsequently, the polycrystalline MgO powders with an average particle size of100nm were adopted as the raw materials, to research the grain growth behaviors andresponsible mechanisms. Under the optimized pressure conditions and the sintering temperature of2080K, the average grain size of MgO compact increased to500nm,and the characteristic morphology of mass transfer by the path of gas was novelyobserved. According to the results of model analysis and microstructural observation,the achievement of grain growth (or neck growth) was likely to be dominated by gasdiffusion (as evaporation-condensation mechanism) rather than solid diffusion (assurface and volume diffusion mechanism), which was owing to both the specialty ofMgO and the particular CR/QP conditions. Moreover, it was found that the pressureof CR/QP had an impact of weakening the effect of the grain growth based on soliddiffusion while strengthening that based on gas diffusion, thereby converting thedominate mechanism of grain growth from surface diffusion mechanism toevaporation-condensation mechanism.Finally, we explored the phenomenon of grain growth stagnation in the densenanocrystalline yttria (Y2O3) ceramics prepared by CR/QP method. The results ofmicrostructural observation and quantitative analysis inferred that, at early stages ofsintering, the combined effect of the ultra-high heating rate and the pressure applied atthe sintering temperature was capable of inhibiting the grain growth based ontime-dependent surface diffusion mechanism, which facilitated the subsequentdensification process by preserving the high specific surface area of nano-particles.While at final stage of sintering, the retained nanostructure contributed to enhancingthe densification process based on diffusion mechanism, thus reducing the effect ofgrain growth via decreasing the sintering temperature and soaking duration requiredfor further densification of the yttria. Moreover, under the decreased sinteringtemperature, the high-density nano-pores derived from the preserved nanostructurewere capable of resisting the final-stage grain growth dominated by grain-boundarymigration mechanism. By comparison, the higher temperatures tended to exaggerategrain growth and retard densification process. In this view, there existed anappropriated sintering temperature range for fast sintering of nanocrystalline ceramics,while the excessive sintering temperature would not only cause significant graingrowth or even abnormal grain growth, but also lead to the low final-density.