Research On Preparation And Performance Of Quasi One-dimensional Silicon Nitride Nanomaterials

Owingtotheiruniquestructuresandexcellentproperties,quasi-one-dimensional nanostructures are potentially applied in reinforcing composite materials,nano optoelectronics devices,nanocomposites,and so on.According to the development trend of quasi one-dimensional nanostructures with controlled microstructure and high yield,the main research object is quasi one-dimensional Si₃N₄ nanostructures with high temperature resistance and oxidation resistance.Large-scale quasi one-dimensional Si₃N₄ nanostructures were effectively prepared by the optimization of raw materials and methods,and the regulation of process parameters.The properties of quasi one-dimensional Si₃N₄nanostructures were characterized,and the essential relation between the properties and microstructures of quasi one-dimensional Si₃N₄ nanostructures was revealed.The growth mechanism of quasi one-dimensional Si₃N₄ nanostructures was also disclosed.Methods for adjusting the properties of quasi one-dimensional Si₃N₄nanostructures were preliminarily established and it could provide dates and theoretical support for expanding the application field of quasi one-dimensional Si₃N₄ nanostructures.Based on literature research and analysis,ultra-long quasi one-dimensional Si₃N₄ nanostructures were effectively fabricated using independent source materials and mixture sources through optimization of methods and the regulation of process parameters.Compared to other materials,in-situ and ex-situ ultra-long quasi one-dimensional Si₃N₄ nanostructures with high-yield could be obtained under a certain condition using graphite,Si and Si O₂ as independent source materials or polycarbosilane?PCS?as the mixture sources.The yield and size of quasi-one-dimensional Si₃N₄ nanostructures could be controlled by adjusting the preparation temperature,holding time,flowing rate,and other parameters.In the temperature range of 1300-1550?,the yield of quasi one-dimensional Si₃N₄nanostructures showed a trend of increasing first and then decreasing with the increase of preparation temperature,while the size of quasi one-dimensional Si₃N₄nanostructures showed an increasing trend with the increase of preparation temperature.With the extension of holding time within 4h,the yield and length of quasi one-dimensional Si₃N₄ nanostructures showed an increasing trend,while,the yield and length of quasi one-dimensional Si₃N₄ nanostructures showed a trend of increasing first and then decreasing with the increase of flowing rate ranging from50 to 400 m L/min.According to thermodynamic calculation analysis and literature research,large-scale ultra-long quasi one-dimensional Si₃N₄ nanostructures were successfully acquired using the mixture containing mixture source PCS as raw materials through the experiment optimization,in which the raw materials were prefabricated to a 3D connected porous body by a freeze-casting method.According to the results of various characterization methods,it can be seen that the main component of the ultra-long quasi one-dimensional Si₃N₄ nanostructures was?-Si₃N₄ with the length up to several millimeters or even centimeters,which purity was very high and just contained Si and N elements,and its growth direction was[101]direction when the independent source materials,including graphite,Si and Si O₂,or the mixture sources such as PCS used as raw materials.It was worth noting that the quasi one-dimensional Si₃N₄ nanostructures obtained by using graphite,Si and Si O₂ as independent source materials at 1500?or PCS as the mixture sources at 1400?exhibited a belt or straight line shape.In the experiments using the 3D connected porous body,containing PCS,prefabricated by a freeze-casting method as the raw material at 1400?to prepare large-scale quasi one-dimensional Si₃N₄ nanostructures,large amounts of quasi one-dimensional Si₃N₄ nanostructures not only grown on the surfaces of the porous body and the ceramic crucible,but also grown on the graphite paper as a lining of tube corundum furnace.Meanwhile,lots of Si₃N₄ nanofibers were growth in the pore channels of porous ceramics.According to the microstructure characterization and analysis,the morphologies of Si₃N₄ nanobelts?NBs?grown on the surfaces of mixture powder and the inner walls of ceramic crucible at 1500?were significantly different using graphite,Si and SiO₂ as raw materials.There were no droplets at the tips of Si₃N₄NBs grown on the surfaces of mixture powder,while there were droplets located at the tips of Si₃N₄ NBs grown on the surfaces of inner walls of ceramic crucible,which also contained Fe element.In this analysis,Fe element should be from the inner walls of ceramic crucible,and Al element was also detected in the Si₃N₄ NBs grown on the surfaces of inner walls of ceramic crucible.Combined with the results reported by relevant literature,alumina may be a novel and highly effective mediator playing an important role in controlling the concentration of Si during the growth of ultra-long Si₃N₄ NBs,and a vapor–solid?VS?mechanism was proposed for the growth mode of the Si₃N₄ NBs grown on the surfaces of mixture powder,while an alumina assisted vapor–liquid–solid?VLS?mechanism was used to disclose the growth of the Si₃N₄ NBs obtained on the inner walls of the crucible.The VS mechanism was proposed to illustrate the growth process of large-scale Si₃N₄ nanowires?NWs?formed in the pore channels of porous ceramics when a 3D connected porous body containing PCS was used as the raw material at 1400?,while the large-scale Si₃N₄ NWs grown on the surfaces of inner walls of ceramic crucible was controlled by an alumina assisted VLS mechanism.Similar to the growth process of large-scale Si₃N₄ NWs,the growth process of Si₃N₄ nanofibers grown on the graphite paper accorded with a carbon assisted VS model since no droplet and Al element were observed,and the carbon in graphite paper could promote the growth of the NWs.The size effect of the mechanical propertiy on the Si₃N₄ NBs with thickness ranging from 80 to 120 nm and width ranging from 260 to 350 nm,and Si₃N₄ NWs with diameters ranging from 360 to 960 nm were clearly observed when in situ nanoindentation was used to measure the mechanical properties of quasi one-dimensional Si₃N₄ nanostructures,and the maximum modulus values were548.6GPa for Si₃N₄ NBs and 526.0GPa for Si₃N₄ NWs,respectively.In the photoluminescence?PL?testing,the room-temperature PL spectra of Si₃N₄ NBs and Si₃N₄ NWs both showed red shifts,which could be attributed to the size effect controlled by the flowing rate ranging from 50 to 200 m L/min or Al-doped.Compared with the size effect,Al-doped played a more important role in affecting the PL property.Based on the results of characterization and measurement of microwave absorption properties,it can be obvious seen that both of compressive strength and microwave absorption property of porous composite ceramics containing Si₃N₄ nanofibers increased with the increase of the content of PCS.It was worth noting that the pyrolysis atmosphere played an important role in affecting microwave absorption properties of porous composite ceramics.