Preparation And Sintering Behaviors Of ?-Al₂O₃ Nanoparticles

Al₂O₃ ceramics are widely applied as structural materials,electronic materials,and optical materials due to their properties such as high hardness,high temperature resistance,corrosion resistance,wear resistance,and electrical insulation;but their brittleness limits their wider applications.Dense Al₂O₃ nanocrystalline ceramic with fine grains?even 10 nm?may have large plastic deformation or toughness,which may overcome their brittleness.In order to prepare dense Al₂O₃ nanocrystalline ceramic with fine grains,usually,disperse fine equiaxed ?-Al₂O₃ nanoparticles have to be prepared in advance.?-Al₂O₃ nanoparticles are usually obtained by calcining precursor powder.But the ?-Al₂O₃ nanoparticles obtained by this way usually have a vermicular microstructure,which is harmful to the preparation of Al₂O₃ nanocrystalline ceramic.So it has an important significance to avoid the vermicular microstructure and obtain disperse fine equiaxed ?-Al₂O₃ nanoparticles when the ?-Al₂O₃ nanoparticles are prepared by calcining precursor powder.The study of sintering behavior of disperse ?-Al₂O₃ nanoparticles is essential for preparing dense Al₂O₃ nanocrystalline ceramics with fine grains.But the sintering behavior of disperse ?-Al₂O₃ nanoparticles with a size distribution in the range of 0-100 nm has not been systematically explored so far.Meanwhile,there only have been a few reports about the preparation of almost fully dense?relative densities larger than 99%?Al₂O₃ nanocrystalline ceramics.In this dissertation,disperse fine equiaxed ?-Al₂O₃ nanoparticles were prepared by calcining precursor powder.Meanwhile,the sintering behaviors of different kinds of disperse ?-Al₂O₃ nanoparticles with different mean particle sizes obtained by different ways were studied and dense Al₂O₃ nanocrystalline ceramics with different mean grain sizes were sintered by two-step sintering.It mainly contains the following contents:?1?By optimizing the preparation cinditions,precursor powder was synthesized at an Fe3+/Al3+ mole ratio of 5 and a pH value of 7 by precipitation in the solution containing Al3+ and Fe3+ with NH3·H2O by a heterogeneous precipitation method.By calcining the precursor powder and then corroded by hydrochloric acid,disperse fine equiaxed ?-Al₂O₃ nanoparticles with a mean particle size of 12 nm and a size distribution from 2 to 40 nm without vermicular microstructure were obtained.By scanning transmission electron microscopy?STEM?and energy dispersive X-ray spectrometer?EDX?mapping analyses and the analysis of the effect of ?-Fe2O3 content,it has been found that a large amount of ?-Fe2O3 acts as seed which can reduce ?-Al₂O₃ nucleation barrier,facilitate the heterogeneous nucleation of ?-Al₂O₃,and lower the ?-Al₂O₃ formation temperature.Moreover,?-Fe2O3 acting as isolation phase can prevent the agglomeration and growth of ?-Al₂O₃ nanoparticles.For Fe3+/Al3+ mole ratios smaller than 1.5?the amount of ?-Fe2O3 is little?,?-Al₂O₃ particles obtained have a vermicular microstructure.In the Fe3+/Al3+ mole ratio range of 3 to 9?the amount of ?-Fe2O3 is large?,disperse ?-Al₂O₃ nanoparticles without any vermicular microstructure were obtained.?2?The sintering behavior of the disperse ?-Al₂O₃ nanoparticles with a mean particle size of 12 nm and a size distribution from 2 to 62 nm obtained by the heterogeneous precipitation method was studied.When no densification occurs,the mean grain size of the sample has increased from 12 to 31 nm.?-Al₂O₃ nanocrystalline ceramic with a mean grain size of 86 nm and a relative density of 99.3% was sintered by two-step sintering?heated to 1250°C without hold and then cooled down to 1100°C with a 10 h hold?.?3?The sintering behavior of the disperse ?-Al₂O₃ nanoparticles with a mean particle size of 4.5 nm and a size distribution of 2-8 nm obtained by ball milling,hydrochloric acid corrosion,and then separation was studied.The grain size of the sample increases rapidly with rapidly increasing relative density in the relative density range of 48%?the relative density of the green compact?to 75%.But the grain size increases obviously slowly with increasing relative density in the relative density range of 75-95%.When the relative density is larger than 95%,the grain size increases dramatically with increasing relative density.During two-step sintering,when the relative density is larger than 83% for the sample sintered at the first-step sintering temperature,the sample can achieve almost full density without grain growth in the second-step sintering with suitable sintering temperature and holding time.By calculation,it has been proved that the main mechanism of densification in the second-step sintering is grain boundary diffusion.In the second-step sintering,when the mean grain size of the sample is 36 nm,the activation energy of grain boundary diffusion is 199 kJ/mol.?-Al₂O₃ nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered by two-step sintering?heated to 1100°C without hold and then cooled down to 950°C with a 40 h hold?.?4?The sintering behavior of disperse ?-Al₂O₃ polycrystalline nanoparticles with a mean particle size of 62 nm?a size distribution of about 30-107 nm?and a mean grain size of 19 nm obtained by ball milling,hydrochloric acid corrosion,and then separation was studied.The mean grain size increases from 19 to 36 nm while the relative density of the sample increases from 58%?the relative density of the green compact?to 62%.During two-step sintering,when the relative density is larger than 84% for the sample sintered at the first-step sintering temperature,the sample can achieve almost full density without grain growth in the second-step sintering with suitable sintering temperature and holding time.?-Al₂O₃ nanocrystalline ceramic with a mean grain size of 75 nm and a relative density of 99.4% was sintered by two-step sintering?heated to 1250°C without hold and then cooled down to 1100°C with a 20 h hold?,with a 1.2-time size increase from the mean particle size of the starting ?-Al₂O₃ nanoparticles to the mean grain size of the sintered ?-Al₂O₃ nanocrystalline ceramic.