Preparation And Mechanical Properties Of La₂O₃-doped Al₂O₃ Nanocrystalline Ceramics

Alumina ceramics is a widely used ceramic material,which has attracted much attention due to its excellent properties,and is often used in building materials,heat-resistant materials,electronic materials,optical materials,etc.In order to prepare dense alumina ceramics under pressureless conditions,it is usually necessary to sinter at high temperature,but high temperature sintering will cause the grains to grow rapidly,resulting in uneven microstructure of the prepared sintered body,which is not conducive to Al₂O₃The comprehensive mechanical properties of ceramics,so we expect to add a sintering aid to inhibit the growth of Al₂O₃grains and improve its microstructure,so that it has good mechanical properties.Previous research results show that La₂O₃can effectively inhibit the growth of Al₂O₃grains and improve the microstructure of the sintered body.However,due to the extremely difficult preparation of high-quality?-Al₂O₃nanoparticles,most of the current work mainly studies the effect of La₂O₃on the sintering behavior of micron/submicron?-Al₂O₃particles,and the grain size of the final sintered body.Usually on the micrometer scale.So far,the effect of La₂O₃on the sintering behavior of nano-scale?-Al₂O₃particles has not been reported,and La₂O₃-doped dense Al₂O₃ceramics with grain size less than 150 nm have not been obtained.Therefore,it is of great significance to study the sintering behavior of La₂O₃-doped?-Al₂O₃nanoparticles and obtain La₂O₃-doped dense Al₂O₃ceramics with fine grain size.In this paper,?-Al₂O₃aggregates with nanometer size were prepared by adding appropriate?-Al₂O₃seed and precipitation method at low phase transition temperature.Then dispersed,fine and equiaxed?-Al₂O₃nanoparticles were obtained by mechanical ball milling,hydrochloric acid corrosion and fractional polymerization separation.The sintering behavior of La₂O₃doped?-Al₂O₃nanoparticles was investigated by using high quality?-Al₂O₃nanoparticles as raw materials,and La₂O₃doped Al₂O₃nanocrystalline ceramics with different average grain sizes were prepared by two-step sintering method,and their microhardness and fracture toughness were also studied.The main contents are as follows:(1)?-Al₂O₃with average particle size of 150 nm was used as seed,and?-Al₂O₃aggregates with primary particle size at nanometer scale were obtained by direct precipitation method at low phase transition temperature of 1050?.Then,the primary particles were refined by mechanical ball milling in the process of eliminating the hard aggregates.Finally,?-Al₂O₃nanoparticles with dispersed,equiaaxial and average particle size of 5.0 nm and 8.4 nm were obtained by hydrochloric acid corrosion and fractional precipitation separation.In addition,the effect of seed size on decreasing?-Al₂O₃phase transition temperature is also analyzed.It is found that smaller?-Al₂O₃seed size can effectively reduce?-Al₂O₃nucleation barrier,promote?-Al₂O₃nucleation,and thus reduce?-Al₂O₃phase transition temperature.The low phase transition temperature can avoid the overgrowth of?-Al₂O₃,which makes the high yield of high-quality?-Al₂O₃nanoparticles.(2)La₂O₃doped Al₂O₃nanocrystalline ceramics were prepared by using La₂O₃as sintering agent.?-Al₂O₃nanoparticles with average particle size of 5.0 nm obtained in(1)were used as raw materials and added to La(NO₃)₃salt solution.After stirring and drying at room temperature,lanthanum-doped?-Al₂O₃nanoparticles were obtained,and then green billet was formed by dry pressing.Since La(NO₃)₃adsorbed on the surface of?-Al₂O₃nanoparticles can be completely decomposed into La₂O₃at800?,La₂O₃doped Al₂O₃sintered body can be obtained after sintering.The sintering behavior of?-Al₂O₃nanoparticles doped with 0.1 wt%La₂O₃was studied by one-step uninsulated sintering experiment.It was found that La₂O₃had obvious inhibition effect on grain growth.Then,the La₂O₃doped Al₂O₃nanocrystalline ceramics were successfully prepared by the two-step sintering method without pressure:The average grain size of La₂O₃doped Al₂O₃nanocrystalline ceramics with average grain size of65 nm and relative density of 98.2%were prepared under the sintering parameters of T₁=1250? and T₂=1225? for 40 h.La₂O₃doped Al₂O₃ceramics with average grain size of 136 nm and relative density of 99.3%were prepared at T₁=1350? and T₂=1300? for 20 h.Finally,the Vickers microhardness test of La₂O₃doped Al₂O₃ceramics with different grain size was carried out.According to the indentation crack length of microhardness test,the fracture toughness of La₂O₃doped Al₂O₃ceramics with average grain size of 136 nm is as high as 5.04±0.32 MPa·m^1/2.Compared with2.96±0.21 MPa·m^1/2of undoped Al₂O₃ceramics with average grain size of 130 nm,the fracture toughness is increased by 70%.