Research On In-situ Driven Mechanism Of External Field For Liquid Phase Sintering And 4D Manufacturing Applications Of Si₃N₄-based Multiphase Ceramics

Additive manufacturing has entered an era of rapid development.Based on 3D manufacturing,the concept of 4D manufacturing was born by increasing the external environmental factors and time dimension.However,the research and application of 4D manufacturing of ceramic materials is still in its initial stage.The"shape control"of 3D printing and"capability control"of high-temperature sintering make 4D manufacturing of ceramics relatively complicated.How to apply the concept of 4D manufacturing to advanced ceramic materials and integrate the design and preparation of products with special macro-micro structures and properties will be a key issue to be considered and solved for the development of a new generation of additive manufacturing technology for ceramic materials.In this context,the basic and applied research of advanced ceramic 4D manufacturing method is carried out with high performance silicon nitride（Si₃N₄）-based ceramics as the model material,and the in situ driving phenomena and mechanisms of mass transfer,phase transformation and microstructure evolution during high temperature liquid phase sintering of Si₃N₄-based ceramics induced by electromagnetic,electric and temperature fields are systematically analyzed.It is found that,firstly,under the in situ driving effect of electromagnetic field,the coupled second phase Ti N has the ability to preferentially absorb electromagnetic wave energy,and the 50 nm particle size Ti N promotes the liquid phase formation process of Si₃N₄ matrix,which in turn enhances the densification of Si₃N₄-based ceramics,but the thermal gradient effect brought by its role as the"heating phase"may be very short in time and small in temperature range.However,its thermal gradient effect as a"heated phase"may be very short in time and small in temperature range,which cannot provide additional activation energy for the Si₃N₄phase transformation process.Although the thermal gradient and thermal diffusion effects of Ti N in the electromagnetic field-assisted sintering process are limited,its microscopic control mechanism in the liquid-phase sintering of Si₃N₄-based complex ceramics under the action of electromagnetic field can still provide the necessary methodological ideas and theoretical basis for the 4D fabrication of Si₃N₄-based ceramics by applying electromagnetic field.Secondly,under the in situ driving effect of electric field,the different physical and chemical state characteristics of the 50 nm particle size coated Ti N effectively promote the liquid phase formation process of the Si₃N₄ matrix,resulting in a lower apparent liquid phase temperature point.At the same time,the agglomeration phenomenon of the clad Ti N makes the heat in the complex ceramic relatively independent and concentrated,thus refining the grains of the Si₃N₄ matrix phase and changing the grain size distribution of Si₃N₄,but also affecting the mechanical properties of the Si₃N₄-based complex ceramic.The unique microscopic control mechanism of the conductively coupled phase Ti N in the liquid phase sintering process of Si₃N₄-based complex-phase ceramics under the action of DC electric field will accelerate the 4D fabrication process of Si₃N₄-based ceramics by applying DC electric field.Finally,the thermal and liquid phase gradients during the rapid temperature sintering process,driven by the temperature field,resulted in high-density Si₃N₄-based ceramics（with a relative density of about 95%）with"less outside and more inside"pore distribution and"smaller outside and larger inside"grain size distribution.The microscopic regulation of the temperature field in the liquid phase sintering process of Si₃N₄-based ceramics,combined with the pressureless rapid sintering method of spark plasma sintering（SPS）for 3D printed samples with complex macroscopic configurations,will eventually provide a new methodological idea and theoretical basis for the 4D fabrication of Si₃N₄-based ceramics with controlled shape and properties by applying the temperature field.This project innovatively uses the green and efficient external field assisted heating technology to combine the high temperature sintering of ceramics with"shape control-capability control"manufacturing,which is based on the technological frontier of 4D manufacturing of advanced ceramics,and focuses on the strategic vision of green manufacturing.The development of this project will provide new design ideas and implementation methods for the structural and functional integration design and manufacturing of high-performance ceramic materials,and enhance the independent innovation capability of ceramic materials and ceramic manufacturing industry.