Preparation Of Fine-grained ?-Al₂O₃ By High-pressure Phase Transformation Of Microcrystalline ?-Al₂O₃

Nano-ceramics are mostly prepared by using nano-powder as the starting material.The specific surface area and surface activity of the nano-powder are large,and the surface adsorption is easy to agglomerate.The nanoparticles tend to grow during the sintering process,and impurities are easily introduced,which affects the density and mechanical properties of the sintered product.These problems constrain the performance of the bulk material prepared by sintering the nano-powder as the starting material.High pressure can inhibit the long-range diffusion of atoms and inhibit the growth of crystal grains.The new phase intercepted under high pressure is not restricted by the grain size of the initial material.Thermodynamically controlled materials can be obtained with finer grain size.In this experiment,micro-crystalline?-Al₂O₃ was used as raw material,and the?-Al₂O₃to?-Al₂O₃ was realized by using the domestic hinged six-face pressing machine under the condition of 1-5.5 GPa pressure?force?with different temperature and holding time.The direct phase transition of Al₂O₃,when the pressure is 1 GPa,?-Al₂O₃ starts to convert to?-Al₂O₃ at 1000?,and complete conversion at 1100?,which gives a single pure?-Al₂O₃phase.When the pressure is 2 GPa,?-Al₂O₃ starts to convert to?-Al₂O₃ at around 950?,and complete conversion at 1050?,resulting in a single pure?-Al₂O₃ phase.When the pressure is 3 GPa,?-Al₂O₃ starts to convert to?-Al₂O₃ at around 800?,complete conversion is achieved at 1000?,and a single pure?-Al₂O₃ phase is obtained.When the pressure is 4 GPa,?-Al₂O₃ starts to transform to?-Al₂O₃ at about 700?,and complete conversion at 900?,resulting in a single pure?-Al₂O₃ phase.When the pressure is 4.5 GPa,?-Al₂O₃ starts to convert to?-Al₂O₃ at around 650?,and complete conversion at 850?,resulting in a single pure?-Al₂O₃ phase.When the pressure is 5 GPa,?-Al₂O₃ conversion to?-Al₂O₃ starts at about600?,complete conversion at 800?,yielding a single pure?-Al₂O₃ phase.When the pressure is 5.5 GPa,?-Al₂O₃ begin conversion to?-Al₂O₃ at about 550?,complete conversion is achieved at about 750?,can obtain a single phase pure?-Al₂O₃.Thus,a interval distribution diagram of the products from?-Al₂O₃ to?-Al₂O₃ at 1-5.5 GPa and different temperatures was obtained.X-ray diffraction analysis showed that the final powder and bulk materials contained only?-Al₂O₃ single phase.Scanning electron microscopy showed that the minimum grain size was 80 nm,and the average grain size was about 340 nm at the pressure of 5 GPa and the temperature of 800?.Submicron?-Al₂O₃ was prepared by high-pressure phase transformation of microcrystalline?-Al₂O₃.When the temperature is the same,the grain size of?-Al₂O₃ decreases gradually and the density increases with the increase of pressure.When the pressure is the same,the grain size of?-Al₂O₃ increases with the increase of temperature.When the pressure and the temperature is the same,the grain size of?-Al₂O₃ will increase with the increase of holding time.Compared with the common sintering method,the complete phase transformation of?-Al₂O₃ to?-Al₂O₃ can be realized at 5.5 GPa and 600?by using high temperature and high pressure sintering method,which significantly reduces the synthesis temperature of?-Al₂O₃material.The?-Al₂O₃ material is obtained at a lower temperature and in a shorter time.At the same time,the grain size of the obtained submicron?-Al₂O₃ is smaller than the initial raw material due to the suppression of the grain growth by the high pressure.The method of high-pressure phase transformation can reduce the sintering temperature,shorten the sintering time,and inhibit the grain growth.In this experiment,a method for preparing submicron?-Al₂O₃ by high-temperature phase transformation of microcrystalline?-Al₂O₃ was developed.Based on this,a fine-grained ceramic material technology was prepared by high pressure phase transformation with large particle powder as the starting material.