Microstructure And Thermal Conductivity Of Tape Calendaring Formed Silicon Nitride Ceramics

With the increasing demand for high power electronic devices,heat dissipation has become a key issue of hindering development of high power electronic devices.Silicon nitride ceramics are considered to be a promising candidate heat dissipation substrate material for high speed circuit and high power electronic device,owing to good mechanical properties,oxidation resistance,superior electrical insulation and high theoretical thermal conductivity.However,the actual thermal conductivity is far from satisfying engineering applications,improving the actual thermal conductivity of silicon nitride materials remains the research focus.In this paper,α-Si₃N₄ powder andβ-Si₃N₄ whisker were used as raw materials,PVA,glycerin and liquid paraffin were selected as organic additives.The silicon nitride green body was prepared by roll forming,and finally sintered by gas pressure sintering to fabricate silicon nitride ceramic with oriented grains.The phase composite,orientation factor,microstructure,mechanical properties and thermal conductivity of the as-prepared silicon nitride ceramic are correlated with content of whiskers and sintering aid,based on X-ray diffractometer,scanning electron microscopy,universal testing machine and laser thermal conductivity analyzer.Studies have shown thatβ-Si₃N₄ act as template crystals,which can induce newβ-Si₃N₄ grains to grow adhering seed crystals during sintering process,resulting in preferred orientation similar to seed crystals.The degree of orientation of the silicon nitride ceramic increases with the increase ofβ-Si₃N₄ content,inducing that thermal conductivity increases in a specific direction.When the content ofβ-Si₃N₄ whisker is 15wt.%,the IOP,F orientation factors and thermal conductivity are 5.1,0.39 and 51W/（m·K）,respectively.With the increase ofβ-Si₃N₄ whisker content,the flexural strength of silicon nitride ceramics increases first and then decreases,the fracture toughness increases and reaches to the maximum value of 10.6 MPa·m¹/2/2 when theβ-Si₃N₄ whisker content is 20 wt.%.Sintering aids are indispensable in the preparation process of silicon nitride ceramics.The increased liquid phase content can promote the dissolution-precipitation and mass transfer rate during sintering process,which can accelerate the phase transformation and grain growth,as well as facilitate newβ-Si₃N₄ grains to grow adhering seed crystals.When the content of sintering aid is 10 wt.%,the IOP and F orientation factors are 5.27 and 0.36,respectively.The effect of sintering aids on thermal conductivity and mechanical properties can be mainly included to be the following aspect:the existence of the glass phase restrict the improvement of mechanical properties,but it can adjust the fracture toughness of silicon nitride by changing the grain size and aspect ratio;In terms of thermal conductivity,the glass phase formed by the sintering aid reduces the thermal conductivity of the silicon nitride ceramic,however,it can promote the densification of the ceramic,the growth of crystal grains and the phase transformation.Furthermore,the thermal conductivity in a specific direction will increase owing to the enhanced template effect of theβ-Si₃N₄ seed crystals.