The rapid advancement of new energy technologies such as photovoltaics, hydrogen energy, and electric vehicles has stringent demands on the heat dissipation capacity of high-power devices. Silicon nitride (Si₃N₄) ceramics with a high thermal conductivity are considered as an ideal substrate candidate for packaging applications, due to their excellent thermal and mechanical properties. However, the actual thermal conductivity remains far below its intrinsic value, primarily due to lattice oxygen impurities and various microstructural defects. The sintered reaction-bonded silicon nitride (SRBSN) process has attracted much attention due to its ability to effectively control lattice oxygen, tailor the microstructure, and its low cost. Sintering aids, particularly rare-earth oxide (RE₂O₃), play a key role in determining the final properties. RE₂O₃ contributes to liquid phase sintering and densification and acts as "oxygen scavengers," effectively purifying the Si₃N₄ lattice. Previous studies showed that the type of RE₂O₃ could significantly affect grain morphology, grain boundary characteristics, thereby affecting the thermal conductivity and mechanical properties. Nevertheless, the specific role and underlying mechanisms of RE₂O₃ in the SRBSN process have yet to be systematically elucidated. This work employed MgO-RE₂O₃ as sintering aids to fabricate Si₃N₄ ceramics via the SRBSN process, and systematically investigated the effect of RE₂O₃ type on the nitridation behavior, microstructural evolution, and final properties of the ceramics.

Commercial silicon powder was used as a raw material, with nine different commercial RE₂O₃ and MgO employed as sintering additives. The total amount of sintering aids was fixed at 2.4% RE₂O₃ and 6.8% MgO for all the samples. The raw materials mixed with alcohol as a solvent were ground in a planetary ball mill with Si₃N₄ balls as grinding media for 3 h. The resulting slurry was dried, uniaxially pressed, and then further cold isostatic pressed at 200 MPa to form green bodies. The green bodies were placed in a BN crucible filled with a 1∶1 (by mass) mixture of BN and Si₃N₄ powder. The nitridation and subsequent sintering processes both were conducted in a gas pressure sintering furnace (ZYS-3050, Hunan Jinteke Technology Co., Ltd., China) under a 2 MPa N₂. The thermal schedule consisted of a nitridation step held at 1350 ℃ for 3 h, followed by a sintering step at 1860 ℃ for 12 h. Some samples were cooled to 1600 ℃ for an additional heat treatment duration of 8 h.

The bulk density was measured by Archimedes' method. The theoretical density was calculated according to the rule of mixtures, and the relative density was determined as a ratio of the bulk density to the theoretical density. The phase composition was characterized by X-ray diffraction (XRD). The microstructure of plasma-etched surfaces was determined by field-emission scanning electron microscopy (FESEM). The thermal diffusivity (α) was measured by a laser flash method on the samples with dimensions of 2 mm × 10 mm × 10 mm. The thermal conductivity (κ) was calculated according to the formula κ = ρ · Cp · α, where a fixed specific heat capacity (C_p) value of 0.68 J·g^-1·K^-1 was used. The flexural strength was tested by a three-point bending method, and the fracture toughness of the bar specimens with dimensions of 3 mm × 4 mm × 36 mm was evaluated by a single-edge notched beam (SENB) method.

The XRD patterns reveal that the w(β)/w(α+β) in nitrided bodies displays a U-shaped profile as the RE³⁺ionic radius increases, primarily due to the synergistic regulatory effect of the interfacial reaction activation energy and the liquid phase viscosity on the phase transformation kinetics. After sintering at 1860 ℃, all the samples (except SSN-Eu) achieve densification. The grain size first decreases and then slightly increases with increasing RE³⁺ ionic radius, as a phenomenon primarily regulated by the differential binding energy (δ) and the content of β-Si₃N₄ in the nitrided bodies. RE³⁺ with δ > 0 (e.g., Lu³⁺, Yb³⁺) can enrich in the liquid phase, promoting diffusion-controlled grain growth, while those RE³⁺ with δ < 0 (e.g., La³⁺, Sm³⁺) can segregate on the surfaces of Si₃N₄ particles, suppressing the grain growth. Meanwhile, the β-Si₃N₄ content in the nitride bodies affects both dissolution-precipitation and Ostwald ripening process. These two factors synergistically lead to the variation in grain size. The flexural strength generally exhibits an initial increase and a subsequent decrease with increasing RE³⁺ ionic radius, reaching the maximum value of (908 ± 35) MPa in the SSN-Gd sample. The increase in strength is attributed to the grain refinement, while the subsequent decrease is due to the thermal expansion mismatch between the Si₃N₄ grains and the intergranular phase. After heat treatment at 1600 ℃, the samples SSN-Lu, SSN-Er, and SSN-Y show an improved flexural strength, whereas the properties of the samples SSN-Nd and SSN-La decrease. The fracture toughness decreases with increasing RE³⁺ ion radius, primarily due to the reduced grain size and grain boundary strength. Among the samples, the sample SSN-Yb exhibits the maximum toughness of (8.7 ±0.3) MPa·m^1/2. After heat treatment, the fracture toughness of most samples further enhances, while it decreases in the samples SSN-Sm, SSN-Nd, and SSN-La. The thermal conductivity generally declines with increasing RE³⁺ ion radius, and the thermal conductivity of SSN-Yb reaches 101.2 W·m^-1·K^-1. Heat treatment promotes grain coarsening in most samples, leading to an increase in thermal conductivity, such as 105.6 W·m^-1·K^-1 for the sample SSN-Yb. However, the thermal conductivity of the samples SSN-Nd and SSN-La decreases instead.

The effect of RE₂O₃ (i.e., RE = Lu, Yb, Er, Y, Gd, Eu, Sm, Nd, La) on the microstructure evolution and properties of Si₃N₄ ceramics prepared by the SRBSN process was systematically investigated. The results indicated that RE³⁺ with smaller ionic radius (such as Lu³⁺, Yb³⁺, and Er³⁺) could effectively promote the growth of β-Si₃N₄ grains, forming a microstructure composed of large-sized grains with high aspect ratios, thereby significantly enhancing thermal conductivity. In contrast, RE³⁺ with larger ionic radius (such as La³⁺ and Sm³⁺) suppressed grain growth, favoring the formation of a fine-grained structure. The flexural strength exhibited an initial increase and a subsequent decrease with increasing RE³⁺ ionic radius, reaching the maximum value of (908 ± 35) MPa in the sample SSN-Gd. The thermal conductivity and fracture toughness both could exhibit a general decreasing trend with increasing RE³⁺ ionic radius. The sample SSN-Yb demonstrated the most favorable combination, with a thermal conductivity of 101.2 W·m^-1·K^-1 and a fracture toughness of (8.7 ± 0.3) MPa·m^1/2. After heat treatment at 1600 ℃, a further grain coarsening led to an improvement in thermal conductivity for the most samples. Among them, the sample SSN-Yb could achieve the maximum thermal conductivity of 105.6 W·m^-1·K^-1, while the flexural strength of the samples SSN-Lu, SSN-Er, and SSN-Y also improve. In contrast, the thermal conductivity and flexural strength of the samples SSN-Nd and SSN-La both decreased.