Silicon Nitride Ceramics With High Thermal Conductivity Based On The Liquid Phase Composition And Microstructure Tailoring

In order to deal with the energy and environmental challenges,the energy resource structure in the world is shifting from fossil fuel to electric power.Driven by the need for efficient and effective control and conversion of electricity,power electronic devices are moving toward miniaturization,higher voltage,larger current,and greater power density.The resulting thermal stresses pose a significant challenge to the ceramic substrate in devices.Silicon nitride(Si₃N₄)ceramics with high thermal conductivity and high strength have aroused much attention as potential thermal substrate materials.In this work,we aim to fabricate Si₃N₄ ceramics with high strength and high thermal conductivity by gas pressure sintering(GPS).By selecting and optimizing raw Si₃N₄powders,designing the variety,amount,and proportion of sintering additives,the composition and properties of the liquid phase were adjusted to regulate the densification,phase transformation,and grain growth behavior.The microstructure of the sintered body was well tailored,and finally,the properties of Si₃N₄ ceramics were optimized.The following four innovative works were carried out.The three stages of liquid phase sintering were determined by liquid phase generation simulation experiments and sintering shrinkage curves.The particle rearrangement occurred at the early stage about 1400°C due to the liquid formation.The dissolution-precipitation process was performed during 1400?1750°C.The solid-state sintering controlled step entered at 1750?1900°C due to the formation of the solid skeleton.The evolutions of densification,phase transformation,and grain growth under different sintering regimes,different raw powders,different additives content and composition were studied by in situ dilatometry.The content,composition,and properties of the liquid phase were adjusted by selecting and optimizing the raw Si₃N₄powders,changing the content of sintering additives,the ratio of rare earth and alkaline earth oxides,and using non-oxide additives.The regulation of densification,phase transformation,and grain growth behavior by combining the adjustment of the liquid phase and the design of sintering regimes was achieved.Therefore,the possibility of optimizing the properties of the Si₃N₄ ceramics by tailoring the microstructure was demonstrated.Based on the above mechanisms,trace Si was creatively introduced to remove native oxygen in the form of Si O(g)by silicothermic reduction treatment(Si(s)+Si O₂(s)=2Si O(g))with Si O₂ in the raw Si₃N₄ powder,which effectively reduced the oxygen content in the liquid phase.The phase transformation rate was faster than the densification rate during sintering,resulting in an exaggerated bimodal microstructure.The fracture toughness was improved from 8.56±0.15 MPa·m^1/2 to 9.91±0.13MPa·m^1/2 and the thermal conductivity was increased from 90.03 W·m^-1·K^-1 to 104.50W·m^-1·K^-1 after sintered at 1900°C for 4 h.In addition to the above optimization of the raw Si₃N₄ powder,two kinds of non-oxide sintering additives:silicides and hydrides,have been developed.High thermal conductivity Si₃N₄ ceramics were prepared using Zr Si₂ as a sintering additive.Zr Si₂reacted with native Si O₂ in the raw Si₃N₄ powder(Si O₂(s)+Zr Si₂(s)+2N₂(g)=Zr O₂(s)+Si₃N₄(s)),then,Zr O₂ and rod-like?-Si₃N₄ particles were in situ generated.The rod-like?-Si₃N₄ particles acted as crystal seeds to promote the growth of large-sized?-Si₃N₄ grains.The in situ generated Zr O₂ participated in forming the liquid phase,which promoted densification and the growth of?-Si₃N₄ grains.TEM showed that Zr precipitated as Zr N phase in the sintered body.There was no obvious amorphous grain boundary film between?-Si₃N₄ grains,thus the scattering of phonons at the grain boundaries was alleviated.Consequently,the replacement of Zr O₂-Mg O by Zr Si₂-Mg O led to an increase of 28%in thermal conductivity from 84.58 to 113.91 W·m^-1·K^-1 after sintered at 1900°C for 12 h.What is more,a higher value of 117.32 W·m^-1·K^-1 was achieved by using binary non-oxide additives Zr Si₂-Mg Si N₂,which increased by 33%.Besides,rare earth metal hydride(REH₂)was first applied as a sintering additive for Si₃N₄.It was demonstrated that unstable La H₂ and Sm H₂ were oxidized to La(OH)₃and Sm₂O₃ during the preparation process.Gd H₂,YH₂,and Yb H₂ were stable and can be used as sintering additives for Si₃N₄.During pre-sintering,the highly reactive metal produced by the decomposition of REH₂ reacted with Si O₂(4RE(s)+3Si O₂(s)=3Si(s)+2RE₂O₃(s))to remove Si O₂,RE₂O₃ was produced simultaneously.The resulting“oxygen-lack and nitrogen-rich”liquid phase not only facilitated the precipitation and growth of?-Si₃N₄ but also benefited the reduction of lattice oxygen content and thus improved the thermal conductivity.The fracture toughness,flexural strength,and thermal conductivity of the samples doped with YH₂-Mg O were 9.30±0.37 MPa·m^1/2,693±19 MPa,and 123.00 W·m^-1·K^-1,respectively,after sintered at 1900°C for 12 h.After sintered at 1900°C for 24 h,the thermal conductivity of the Si₃N₄ ceramics doped with Yb H₂,YH₂,and Gd H₂ was 131.15 W·m^-1·K^-1,131.60 W·m^-1·K^-1,and 134.90W·m^-1·K^-1,respectively.The regulation effect of the rare earth ion radius on the thermal conductivity was shielded due to the substantial change in the composition and properties of the liquid phase when RE₂O₃ was substituted by REH₂.In addition,the transition metal hydride Zr H₂ can also be used as a sintering additive for high thermal conductivity Si₃N₄.Native oxygen in the Zr H₂-doped sample was removed to a greater extent through the Si O₂?Zr O₂?Si O(g)route.The substitution of Zr O₂ by Zr H₂enabled Si₃N₄ ceramics to obtain an increase in thermal conductivity from 90.20 W·m^-1·K^-1 to 116.40 W·m^-1·K^-1 after sintered at 1900°C for 12 h.