Preparation And Structural Characterization Of Al₂O₃ And TiC_0.3N_0.7 Whiskers

Ceramic whiskers,as one of the important members of inorganic non-metallic whiskers,are widely used to toughen and strengthen various composite materials due to their high elastic modulus,high strength and high toughness,which greatly promote the application of composite materials.In this paper,the preparation of Al₂O₃ whiskers and TiC_0.3N_0.7 whiskers was completed by hydrothermal synthesis and carbothermal reduction.The effects of process parameters on the composition and morphology of the product were studied.The internal structure of the product was combined with the morphology of the whisker to determine whether there were defects in the crystal and the growth mechanism of the whiskers.In this paper,Al₂?SO₄?₃·18H₂O was used as the aluminum source,and CO?NH₂?₂ was used as the precipitant by hydrothermal method at 150?with special morphology of preparation of?-Al₂O₃.The effect of the concentration of urea in the reaction solution on the morphology and composition of the precursor was investigated.With the increase of urea concentration,XRD and SEM results show that the phase composition is NH₄Al₃?SO₄?₂?OH?₆,AlO?OH?,NH₄Al?OH?₂CO₃,and the morphology of the precursor is from spherical to flower-like.When the molar ratio of Al³⁺to urea is 1:6,the length of the tip portion of the flower-like structure can be up to about 4?m.It was also found that all of the precursors were converted to?-Al₂O₃ when the precursor was calcined to 1000?,and the precursor retained its original morphology while undergoing phase transformation shrinkage.High resolution and selected area electron diffraction analysis by transmission electron microscopy showed that the tip portion of the flower-like structure was monoclinic twin crystal?-Al₂O₃,and the crystal structure was consistent with the XRD analysis.And the BET test results show that the flower-like structure with this special morphology has the characteristics of multi-layer adsorption.The preparation of single crystal?-Al₂O₃ whiskers was carried out by hydrothermal method,the urea concentration was further increased on the basis of the preparation of flower-like?-Al₂O₃.It was found that when the molar ratio of Al³⁺to urea was 1:9,the flower-like structure was destroyed,the needle-like morphology of the flower-like structure is completely dispersed to form a single elongated precursor.The effects of the initial pH of the reaction solution,calcination time and calcination atmosphere on the morphology and phase composition of the product were investigated.The results show that the aspect ratio of the precursor increases with the increase of pH value,but the phase transition of???is inhibited during the calcination process,the higher the pH,the stronger the inhibition;As the calcination time is prolonged,the alumina of the porous rod-like structure begins to change from???,and the volume shrinkage is finally calcined for 6h to form?-Al₂O₃ with smooth surface;Calcination of the precursor under different atmospheres indicates that the growth of?-Al₂O₃ is more favorable under low oxygen partial pressure.On the contrary,it is difficult to completely convert the precursor into?-Al₂O₃ under high oxygen partial pressure.Calcination in Ar and N₂ to 1200?for 6 h to obtain?-Al₂O₃ whiskers with an aspect ratio of about 25.The preparation of TiC_0.3N_0.7 whiskers was carried out by a carbothermal reduction method.The morphology and phase composition of the samples were observed by a series of different C/Ti,catalyst,sintering temperature and cosolvent content,and the growth mechanism of whiskers was judged by observing the morphology of the samples.The results show that the influence of C/Ti on the phase is quite important.When C/Ti is 2.7,the X-ray diffraction peak shifts to the left relative to TiN,EDS shows TiC_0.3N_0.7.However,the products with C/Ti of 2.3 and 2.5 are TiN.Catalysts with different melting points directly affect the microstructure of the product,and the melting point of the catalyst is close to the reaction temperature to form a whisker-like morphology,which is closely related to the growth mechanism of TiC_0.3N_0.7 whiskers.As the content of the co-solvent increases,the morphology of the product gradually changes from granular to long rod,excessive cosolvents will also hinder the growth of whiskers,resulting in the appearance of granules and long rods.The morphology of the products at different reaction temperatures is also different.As the temperature increases,the diameter of the whiskers decreases and the aspect ratio decreases.And the stepped streaks on the whisker surface are at 1400?,at which time the growth mechanism changes from V-L-S to V-S mechanism.