<p>With the development of third-generation semiconductors, the heat dissipation problems of electronic packaging systems and the harsh usage environment of high-power devices have put forward higher requirements on the thermal conductivity and mechanical properties of silicon nitride ceramic substrates. As an almost defect-free ceramic forming technology, aqueous gel tape casting technology is very suitable for the forming of ceramic substrate materials because of the high density, low porosity and large forming size of the ceramic flakes produced. However, the existing acrylamide-methylene bisacrylamide (AM-MBAM) gel system has a complicated process, is subject to oxygen depolymerisation and has problems such as neurotoxicity. For this reason, in this study, a non-toxic and environmentally friendly isobutylene-maleic anhydride copolymer (Isobam) gel system was used to prepare silicon nitride ceramic substrates in-situ by combining the methods of gel tape casting and reaction sintering. The rheological properties of the slurry were optimized by adjusting the dosage of additives, and the effects of the nitriding reaction process of silicon powder and different sintering aids on the properties of Si3N4 ceramic substrates after resintering were investigated in detail. The results show that the slurries with different additive contents show shear-thinning behaviour and relatively high solid content. Under 0.1&#xa0;MPa nitrogen atmosphere, Si₃N₄ has been basically completely generated at 1400&#xa0;°C, and the comprehensive performance of Si₃N₄ ceramics is optimal with the addition of Y₂O₃-MgO-ZrO₂ sintering additives. The relative density and thermal conductivity were 98.4% and 61.5 W·m⁻¹·K⁻¹, respectively, and Si₃N₄ showed columnar morphology, which effectively improved the hardness, fracture toughness and flexural strength of Si₃N₄ ceramics, up to 13.39 GPa, 6.9&#xa0;MPa·m^1/2 and 684.5&#xa0;MPa, respectively.</p>