A Study Of Morphology Control Of Silicon Nitride Synthesized By Direct Nitridation Of Photovoltaic Silicon Sawdust

High-quality silicon nitride powder has strict requirements on phase,impurity content and morphology.The phase and impurity content have been studied for many years and have relatively mature theories and control methods.However,there is still a lack of understanding and effective means to control the morphology of silicon nitride particles.Compared with irregular-shaped particles,silicon nitride powder with spherical or spherical-like particles is more conducive to the sintering densification of silicon nitride ceramic and improves its performance.silicon nitride nanowhiskers can be used as toughening or thermally conductive phase for composite materials,and have application potential in the field of optoelectronic semiconductors due to their nanoscale effects.The former is so far can only be prepared by the Japanese company of UBE through the amination method based on complex chemical process,and our country relies on imports,the latter has so far lacked a method for macro-preparation.The author used the recycled and purified photovoltaic silicon sawdust as the raw material,and have a study of morphology control of the silicon nitride powder synthesized by the direct nitridation method.The main results are as follows:(1)The thickness of the flaky silicon sawdust is 100 nm,and have an amorphous layer in the surface.These characteristics are basis of the control of silicon nitride morphology.(2)In the conventional direct nitridation process of photovoltaic silicon sawdust,irregular-shaped silicon nitride particles and silicon nitride whiskers will appear.The former is mainly formed by the 3Si+2N₂—Si₃N₄pathway;the latter is formed by the 3Si O(g)+2N₂—3/2O₂+Si₃N₄pathway,in which Si O gas can be generated through the reaction of the surface oxide layer of photovoltaic silicon sawdust and silicon sawdust or H₂contained in the reaction gas.When the oxygen partial pressure is low,Si O gas can exist stably.(3)By using Ca F₂auxiliary agent and NH₃atmosphere,at a lower temperature of 1300?,?-rich spherical-like silicon nitride powder was prepared.The analysis shows that Ca F₂reacts with the oxide layer on the surface of silicon sawdust to form the Ca_xSi_yO_zliquid phase.The existence of the liquid phase makes the?-phase silicon nitride nucleate,and it grows rapidly and isotropically under the drive of the low-energy solid-liquid interface to form a spherical-like?phase silicon nitride particles.(4)By using a special compound halide assisted NH₃atmosphere nitriding method,at a lower temperature of 1200?/1300?,spherical-like?phase silicon nitride was successfully prepared.The possible mechanism is that the compound halide promotes the generation of Si O gas,so that the?phase silicon nitride nuclei are formed through the 3Si O(g)+2N₂—Si₃N₄+3/2O₂pathway.However,the situation is different from the conventional direct nitridation,due to the inhibition the compound halide,the silicon nitride crystal nuclei do not grow in a specific direction to form whiskers,but form spherical-like particles.(5)Based on the silicon nitride whisker formation mechanism proposed in this paper,a method for the marco-preparation of?phase silicon nitride whisker materials by the reaction of N₂/H₂(95:5)with silicon sawdust has been developed,and the yield of silicon nitride whiskers is 78.8%.