Uniform distribution in molecular level of ceramics could be achieved by polymer-derived method. Comparing to other methods, fewer crystal seeds existed in microstructure of polymer-derived ceramics, which made it more difficult to sinter. Up to now, few reports focused on the direct sintering of the polymer-derived ceramics. For silicon nitride sintering, the introduction of metal oxide additives caused defects in the lattice of silicon nitride because of oxygen atoms, which could affect the properties of the material. This work focused on the sintering behavior of polymer-derived ceramics and the issue of performance degradation caused by introducing oxygen elements in sintering additives. The metal elements were connected to the precursor molecule and formed the metal nitride sintering additives during pyrolysis process. The method could help to achieve the in-situ non-oxygen addition and uniformly mixing of sintering additives, and the followed low-temperature sintering could achieve the high-purity silicon nitride ceramics.

4 mL of propylamine was added into a solution of trimethylsilyl chlorosilane monomer (14 g), yttrium chloride (0.14 g) and magnesium chloride (0.085 g) in pyridine (20 mL) at room temperature in N₂ atmosphere. The mixture was warmed up to 120 ℃ and stirred for 2 d. The mixture was then heated to 400 ℃ for 4 h in N₂ atmosphere and further heated to 1 000 ℃ with the rate of 10 ℃/min, finally the mixture were holding at 1 000 ℃ for 4 h and then cooled to room temperature naturally. A white silicon nitride powder SiN-1 was obtained. Silicon nitride powders SiN-2 and SiN-3 were prepared by the same method, in which 0.055 g magnesium chloride was added in SiN-2 and no magnesium chloride was added in sample SiN-3. Silicon nitride ceramics was prepared through spark plasma sintering of silicon nitride powders in 1 450 ℃, 45 MPa for 30 min. Before characterization of the silicon nitride ceramics, surface of it was pre-polished.

Based on trimethylsilyl-substituted polysilazane precursors, the Y and Mg elements were successfully connected to N with the reaction of M-Cl to N-H and coordination. Through pyrolosis process, three kinds of ceramic powders SiN-1, SiN-2 and SiN-3 were obtained. The main component of these powders was silicon nitride, and content of C was below 0.1%, that of O was below 1.6%. As the content of metal decreased, there is a slight decrease in oxygen contents, being due to the high reactivity of metal nitrides. When treated at 1 000 ℃, a large amount of amorphous silicon nitrides with small amount of magnesium nitrides via the dope of magnesium. Small amount of YSi₃N₅ with Si₃N₄ via the dope of yttrium. When ceramics were heated to 1 500 ℃, amorphous silicon nitride began to crystallize, and sample SiN-3 only contained α-Si₃N₄. As the amount of magnesium increased, the content of β-Si₃N₄ crystal gradually increased. These results demonstrate that magnesium nitrides could promoted the transformation of amorphous Si₃N₄ to β-Si₃N₄ crystalline, while yttrium nitrides activated the lattice of Si₃N₄ crystal.

Silicon nitride ceramics were prepared by spark plasma sintering when three types of silicon nitride ceramic powders acted as main raw materials, but only SiN-1 powders with the highest magnesium content could obtain pure β-Si₃N₄. The frame of the obtained ceramics was mainly composed of stacked rod-shaped β-Si₃N₄ crystals. These results prove that the pores in porous ceramics were mainly through-pores, and few closed-pores. The porous ceramics showed a hardness of 29.1 GPa, an elastic modulus of 229.1 GPa, which attributed to the speical porous structure. At 25 ℃, the ceramics possessed a thermal conductivity of 20.451 (W/m·K), a specific heat capacity of 0.355 J/(g·K), a thermal diffusion coefficient of 18.169 mm²/s, and an overall thermal expansion coefficient less than 4×10^-6/K in the range of room temperature to 1 200 ℃. Due to the presence of pores, a significant decrease in thermal conductivity and thermal diffusion coefficient can be obtained, while the coefficient of thermal expansion remained relatively low.

Metal element Y and Mg were successfully connected to trimethylsilyl-substituted polysilazane precursors and contents could be adjusted as needed. Metal nitride sintering additives were in-situ generated during pyrolysis process and polymer-derived high-purity silicon nitride ceramics with sintering additives were prepared. This method could help to achieve both in-situ non- oxygen addition and molecular level uniform mixing of sintering additives in ceramic powders. Low-temperature sintering could help to achieve the silicon nitride ceramic powders which resulting in the obtained silicon nitride ceramics with good mechanical and thermal properties.