Study On Technology And Performance Of Self-propagating High-temperature Synthesis SiAION Powders

SiAlON materials are the novel solid solutions based on the Si₃N₄-SiO₂-Al₂O₃-AlN system,which has a similar structure with Si₃N₄ crystal.It has achieved wide application in metallurgy,aerospace,defense,advanced material manufacturing,wear-resistant materials and other industries or areas,because it has the characteristic of high strength,high wear resistance and excellent anti-oxidation resistance.At present,there exist diverse synthesis methods of SiAlON materials,and most of studies concentrated on the synthesis of SiAlON,while systematic researches on synthesis of pure phase SiAlON materials and SiAlON functional materials were quite limited.In this paper,the synthesis process parameters of SiAlON-based materials fabricated by SHS technology were studied and optimized,which provide the foundation for large-scale production in future.Based on SiAlON materials high temperature application characteristics,high-temperature oxidation resistance SiAlON materials was studied systematically,influence patterns of application conditions on material oxidation kinetics was discussed,oxidation kinetics model was established,influence patterns of different atmosphere on the oxidation kinetics of materials were investigated,and the functional scope of application of SiAlON materials was expanded.The main conclusions are summarized as follows:?1?The thermodynamics of Ca-?-SiAlON were investigated through FactSage software,and the variation of thermodynamic parameters under different reaction conditions including the pressures and temperatures were studied.The Gibbs free energy changes and the adiabatic temperature change were therefore obtained?The results show that:?a?NH₄F and NH₄Cl decomposition reaction occur at around700K and 900K,and the decomposition reaction changes radically;?b?Using FactSage database,thermodynamic data and adiabatic temperature(T_ad)formula were calculated theoretically;the adiabatic temperatures of identified three reaction paths were identified at 4139 K,3519 K and 1689 K;the maximum reaction temperature is significantly lower than theoretical adiabatic temperature,and?c?NH₄Cl and NH₄F significantly reduce the reaction temperature,compared with ammonium chloride,ammonium fluoride's dilution effect is more obvious.?2?The diverse SiAlON materials with different crystal structures were synthesized using SHS technology.The large scale synthesis parameters were obtained through investigating the synthesis process of Ca-?-SiAlON and?-SiAlON using SHS technology,and the synthetic process parameters were obtained.The effect of diluent and catalyst on the sysnthesis of Ca-?-SiAlON was investigated.The results show that:?a?The content of Si₃N₄ diluent has minor effects on Ca-?-SiAlON phase composition while has a significant impact on crystal morphology,catalysts have significant impacts on Ca-?-SiAlON phase and crystal morphology,NH₄F results in the existence of residual Si and AlN in combustion product,columnar crystals gradually become granular,NH₄Cl or a mixture of both catalysts,combustion products have no residual Si and AlN,and crystals becomes columnar and granular;?b?The increase of Si₃N₄ diluent content has no influence on?-SiAlON phase composition,and the combustion product is pure?-SiAlON,and the crystal morphology gradually changed from the coarse columnar crystals to small clusters columnar crystals.?3?The high temperature oxidation properties of Ca-?-SiAlON and?-SiAlON powders were investigated using Thermogravimetry?TG?,XRD and SEM techniques,and the high temperature oxidation kinetics were constructed.The oxidation mechanism under different conditions was therefore obtained.The results indicate:?a?Oxidation products of Ca-?-SiAlON include SiO₂ and CaAl₂Si₂O₈.With higher oxidation degree,its diffraction peak gradually increased,while the Ca-?-SiAlON diffraction peak decreased gradually with the increase of oxidation temperature.The columnar granular structure of Ca-?-SiAlON structure is damaged,the surface becomes rough,and the glass phase emerges.The Ca-?-SiAlON oxidation kinetics was simulated,the result shows that the oxidation of Ca-?-SiAlON is controlled by mixed chemical and diffusion steps.?b?Oxidation product of?-SiAlON is SiO₂,it is observed by SEM that the diluents result in the transformation from large hexagonal column-shaped crystals to fine isotropic grains with temperature increasing.Oxidation experiments were conducted by thermogravimetric analysis?TGA?in the temperature range from 1200?to1400?and the results indicate that oxidation is controlled by mixed chemical reaction and diffusion process.?4?The Eu²⁺doped Ca-?-SiAlON yellow phosphors were synthesized using the solid phase reaction method and SHS method.The crystal structure,morphology and luminescence properties of the composites were studied.The effects of different m and n values,Eu²⁺concentration,Si₃N₄ powder and the effect of acid pickling on the performance of the synthesized phosphor were studied.The influences of raw material,diluent,catalyst and acid pickling on Ca-?-SiAlON phosphor powder properties were systematically studied.The results show that:?a?When the Eu²⁺concentration does not reach the material concentration quenching,the luminous strength enhances with Eu²⁺content increases,and there exists an emission spectrum red shift,after reaching a certain concentration,a luminous intensity decrease significantly;?b?When m=2,n=1,Ca-?-SiAlONs synthesized by the two methods are relatively clean,and powders,with little impurity phase,have preferably fluorescent properties.When m,n value increases,the powder have better purity and fluorescent properties;?c?Compared with Ca-?-SiAlON synthesized by solid phase reaction,the Ca-?-SiAlON by SHS owes better phase purity,crystallinity and photoluminescence properties due to exothermic effect.