###### Abstract  
Si\({}_{3}\)N\({}_{4}\) ceramic materials have great potential in the field of insulation in SF6 gas ultra-high-voltage transmission and transformation equipment due to their excellent insulation performance and thermal stability. In this paper, Y\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\) was used as a sintering aid to prepare high-density (\(>\)99%) Si\({}_{3}\)N\({}_{4}\) ceramics through two-step pressureless liquid-phase sintering, and the mechanism of the influence of Y\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\) addition on the microstructure and electrical properties of Si\({}_{3}\)N\({}_{4}\) ceramics was studied. The results showed that increasing the sintering aid content could increase the grain size of Si\({}_{3}\)N\({}_{4}\) ceramics, while increasing the Y\({}_{2}\)O\({}_{3}\) ratio could refine the grain size. When Y\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\) addition was 8% and the ratio was 5.3, the room temperature volume resistivity of Si\({}_{3}\)N\({}_{4}\) ceramics was the highest, 7.33 \(\times\) 10\({}^{14}\) O\(\cdot\)m, and the volume resistivity was the most stable when the sintering aid content was 12%. The internal carrier migration type of Si\({}_{3}\)N\({}_{4}\) ceramics was mainly ion conduction, mainly along the grain boundaries. The temperature stability of the resistivity of Si\({}_{3}\)N\({}_{4}\) ceramics could be improved by doping with Y\({}^{3+}\) functional ions to reduce the potential barrier conductivity level and refine the grain size to improve the conduction path. The dielectric constant and dielectric loss of Si\({}_{3}\)N\({}_{4}\) ceramics were mainly affected by interface polarization. They gradually increased with the increase in sintering aid addition. Temperature had little effect on dielectric constant and dielectric loss in the range of  
SiO\({}_{2}\) activity in the liquid phase. There is less research on the electrical properties of Si\({}_{3}\)N\({}_{4}\) ceramics. Si\({}_{3}\)N\({}_{4}\) ceramics are electrical insulators owing to their large band gap. However, their electrical characteristics may change depending on their microstructure [17]. [PERSON] et al. [17] studied the effect of Yb\({}_{2}\)O\({}_{3}\) addition on the resistivity of Si\({}_{3}\)N\({}_{4}\) ceramics; the volume resistivity was related to the internal crystalline phase of ceramics, and the resistance of Yb\({}_{2}\)O\({}_{3}\) addition was decreased significantly compared with the Y\({}_{2}\)O\({}_{3}\) addition of Si\({}_{3}\)N\({}_{4}\) ceramics, the main reason of which is the formation of continuous Yb4 Si\({}_{2}\)O\({}_{7}\)N\({}_{2}\) crystalline phase. Ceramic dielectrics materials like Si\({}_{3}\)N\({}_{4}\) are reported to exhibit evidently better short- and long-term insulation performance under a DC electric field with satisfied charge suppression property [18,19]. However, a pillar insulator used in gas insulation equipment has not been reported. The SF6 gas-insulated high-voltage DC electrical equipment operating temperature is about 80 \({}^{\circ}\)C, and the insulation performance of the material under this condition has attracted much attention. Si\({}_{3}\)N\({}_{4}\) is a strong covalent bond compound with low self-diffusion coefficient and is difficult to be densified through solid-phase sintering. Si\({}_{3}\)N\({}_{4}\) ceramics are densified through liquid-phase sintering with the addition of sintering aids [13]. These sintering aids play an important role in changing the microstructure of Si\({}_{3}\)N\({}_{4}\) ceramics. Therefore, this article uses Y\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\) as a sintering aid to prepare high-density Si\({}_{3}\)N\({}_{4}\) ceramics through two-step liquid-phase sintering without pressure.