LU Tianlin，HAN Zhao，LIU Pengfei，LI Jie
School of Metallurgical Engineering， Anhui University of Technology， Ma’anshan 243000， China
Abstract
Objective Silicon nitride （Si3N4） ceramics， renowned for their exceptional mechanical properties， high-temperature resistance， and thermal shock resistance， hold broad application prospects in advanced technical fields. Among them， β-Si3N4， as the thermodynamically stable phase， has garnered significant attention in the field of semiconductor substrates， particularly due to its potential for high thermal conductivity. Currently， the industrial process predominantly utilizes α-Si3N4 as the raw material， inducing its phase transition to β-phase through high-temperature thermal treatment， with the goal of obtaining large-sized， high-aspect-ratio columnar grains to optimize material performance. However， conventional oxide additives suffer from issues such as high liquid-phase formation temperatures and high viscosity， leading to challenges in phase transition and imprecise control over grain size. This study aims to explore the regulatory effects of low-melting-point fluoride additives （LiF and CaF2） on the α-Si3N4-to-β-Si3N4 phase transition and grain morphology during thermal treatment. By systematically investigating additive types， mass fractions， and granulation pretreatment processes， a preparation method is established to quantitatively control the β-Si3N4 phase transition efficiency， grain size， and aspect ratio.
Methods High-purity α-Si3N4 was used as the raw material， with individual LiF additives introduced at varying mass fractions （5%–20%） and a composite LiF-CaF2 additive （total mass fraction fixed at 15%） prepared with mass ratios of 5：10， 7.5：7.5， and 10：5. The raw materials were uniformly mixed via ball milling， vacuum-dried at 50 ℃， and sieved. Thermal treatment was conducted in a vacuum carbon tube furnace under a nitrogen atmosphere， with a maximum temperature of 1 700 ℃， a holding time of 2 h， and a segmented heating program. For the composite additive system， an additional granulation process was applied to the mixed powder， involving preparing granulated powder through a cyclic process of pressing， crushing， and sieving， followed by thermal treatment under identical conditions. Phase analysis of the heat-treated products was performed using X-ray diffraction （XRD）， and the β-phase content was calculated. The microstructure was observed via scanning electron microscopy （SEM）， and the average grain length and aspect ratio were statistically analyzed using ImageJ software.
Results and Discussion The addition of LiF significantly promoted the α→β phase transition. As the LiF mass fraction increased from 0 to 15%， the β-phase mass fraction rose from nearly 0 to 72.15%， accompanied by an increase in the average grain length from 0.352 μm to 1.610 μm. This indicated that the liquid phase formed by LiF effectively reduced the system viscosity， enhanced mass transfer， and thereby facilitated the phase transition and grain growth. However， when the LiF mass fraction exceeded 15% （reaching 17.5% and 20%）， the β-phase mass fraction decreased to 67.65% and 53.24%， respectively， while the grain size also reduced to 1.325 μm and 1.116 μm. This was attributed to the excessive liquid phase， which lowered the supersaturation of Si and N atoms， thereby suppressing the nucleation and growth of the β phase. The composite additives exhibited remarkable synergistic effects. At 1 700 ℃， all three composite additive formulations achieved a complete α→β phase transition （100%）， significantly outperforming the single-LiF system. As the LiF mass fraction in the composite system increased from 5% to 10% （with the CaF2 mass fraction decreasing from 10% to 5%）， the morphology of the products changed in a regular pattern： the average grain length increased markedly from 3.39 μm to 7.64 μm， and the aspect ratio rose from 5.38 to 7.64. This indicated that increasing the LiF mass fraction more favorably promoted the one-dimensional preferential growth of crystals along the c-axis. The reaction mechanism lied in the formation of a deep eutectic liquid phase by LiF-CaF2， which further reduced the melting point and viscosity. Meanwhile， Li+， owing to its small ionic radius and high mobility， may preferentially adsorb on specific crystal planes to inhibit lateral growth， whereas Ca2+ contributed to regulating the liquid phase composition. Together， these factors enhanced the heterogeneous nucleation and anisotropic growth of the β phase. Granulation treatment had no effect on the phase composition of the products， which remained entirely β-phase， but it significantly optimized the grain morphology. Compared with the loose， ungranulated powder， the granulated powder， after thermal treatment， exhibited more distinct grain edges， enhanced anisotropy， and further increased grain size. Particularly in the composite system with 10% LiF and 5% CaF2， the average grain length after granulation reached 8.64 μm， and the aspect ratio increased to 9.31， surpassing those of the ungranulated system （7.64 μm， 7.64）. This was attributed to the mechanical pressure during granulation， which made particle contact tighter， reduced porosity， and formed a denser structure. Consequently， during thermal treatment， mass transport via grain boundary diffusion was facilitated， promoting anisotropic growth along low-energy crystal planes.
Conclusion The effective regulation of the Si3N4 phase transition and microstructure morphology was successfully achieved through the manipulation of the fluoride additive system and granulation process. The main conclusions are as follows： （1） LiF serves as an effective promoter of the α→β phase transition， with an optimal mass fraction of 15%. Excessive addition inhibits the phase transition and grain growth due to reduced supersaturation. （2） The LiF-CaF2 composite additive， through eutectic effects and ionic synergy， not only enables a complete α→β phase transition but also allows precise control over grain morphology. By adjusting the mass ratio， grains with varying shapes—from short and stout to elongated—can be obtained. （3） Granulation pretreatment is an effective approach for further optimizing grain morphology. By enhancing particle contact and facilitating grain boundary diffusion， it significantly improves the average length and aspect ratio of β-Si3N4 grains.
Keywords： Si3N4；LiF；CaF2；fluoride；α-β phase transition
Get Citation：LU Tianlin， HAN Zhao， LIU Pengfei， et al. Effect of LiF-CaF2 on phase transition and morphology of Si3N4［J］. China Powder Science and Technology， 2026， 32（5）： 1-11.
Received：2025-02-28，Revised： 2025-11-03，Online： 2026-07-29.
Funding：The research was supported by the National Natural Science Foundation of China (Grant No. 52074003).
DOI：10.13732/j.issn.1008-5548.2026.05.017
CLC No.：TB44；TQ170
Type Code：A
Serial No.：1008-5548（2026）05-0001-11
