DONG Fahai ^{1, 2}, ZHU Nannan ^{1, 2}, ZHANG Lijuan ^{1, 2, 3}, WEN Guangwu ^{1, 2}

(1. School of Materials Science and Engineering, Shandong University of Science and Technology, Zibo 255000, Shandong, China;

2. Engineering Ceramics Research Institute, Shandong University of Science and Technology, Zibo 255000, Shandong, China;

3. Additive Manufacturing Research Institute, Shandong University of Science and Technology, Zibo 255000, Shandong, China)

Extended Abstract:[Background and purpose] SiCN ceramics are interesting for extreme environmental applications, due to their excellent thermal stability, oxidation resistance and mechanical robustness. However, conventional synthesis routes face important trade-offs between achieving high ceramic yields (>80%) and engineering nanostructural features, such as nanowires or controlled crystalline phases. Polymer-derived ceramics (PDCs) using ceramic prepolymers offer molecular-level design opportunities to address this dilemma. Polysilazane (PSZ), as a high-performance ceramic precursor, has attracted much attention in preparing SiCN-based ceramics, due to its high ceramic yield, molecular designability and low-temperature molding properties. It has been confirmed that chemical modification of the molecular structure of the precursor can significantly affect the microstructure and properties of ceramics. In addition, current studies have not yet systematically elucidated the curing behavior of PSZ in the low-temperature region (<200 ℃) and its regulatory mechanism on the pyrolysis pathway. At the same time, there are still key scientific questions about the functional properties of the ceramics. However, the potential of PSZ to generate β-Si₃N₄ nanowires in-situ during high-temperature cracking and its formation mechanism still lack in-depth exploration. To address the above challenges, this work was aimed to study the low-temperature thermal curing behavior and high-temperature pyrolysis pathways of acrylate-modified polysilazanes (A-PSZ). (1) The evolution of the appearance and morphology and the dynamic changes of the infrared functional groups (e.g., Si-H, N-H, and C=C bonding) of A-PSZ under the vacuum heat treatment at 40–200 ℃ is systematically studied. Thermogravimetric analysis (TG) of 200 ℃ cured specimens was used to reveal the contribution of acrylate groups to the crosslinked network construction at the low-temperature curing stage. (2) Based on the XRD and TEM characterization of the pyrolysis products at 1450 ℃ and 1500 ℃, the growth mechanism of the nanowires was explored through the statistical analyses of the morphology, diameter distribution and elemental composition (EDS).[Methods] A-PSZ was obtained by the mixed reaction of isocyanoethyl acrylate and vinyl polysilazanes. The homemade A-PSZ was used as the starting precursor and aliquots of it were dispensed into specimen tubes, which were subsequently subjected to a programmed heat treatment in a vacuum drying oven. The temperature range was 40–200 ℃ (with a gradient at 20 ℃ intervals) and a constant temperature was maintained at each point for 3 h to complete the heat-curing process. The samples were placed in an alumina crucible after heat curing at 140 ℃, after which the samples were placed in a tube furnace protected with nitrogen, at pressure of 0.2 MPa and flow rate of 400 mL·min⁻¹.[Results] FT-IR analysis confirms that the acrylate group was successfully grafted to the main chain of the PSZ molecule through the addition reaction of the isocyanate group with the Si-H and N-H bonds, resulting in the A-PSZ. A staged cross-linking mechanism is revealed. Below 120 ℃, the characteristic peak of the C=C bond of the acrylate (1635 cm⁻¹) decreases by 62%, indicating that free radical polymerization dominates the cross-linking process. When the temperature rises to 120–200 ℃, the residual Si-H (2160 cm⁻¹) and N-H (3380 cm⁻¹) groups further strengthen the network structure through condensation reactions. Finally, 97.5% of the cross-linking density is achieved at 200 ℃. This layered network of acrylate carbon chains and Si-N-Si backbone synergistically provide stable structural support for subsequent high-temperature pyrolysis. When pyrolyzed in N₂, amorphous SiCN formed at 1000 ℃ exhibited a high yield of 81.7%, which was attributed to the effective suppression of volatile components released by the cross-linked network. When the pyrolysis temperature is 1450 ℃, the products are dominated by amorphous materials and show only a broadened diffraction envelope. As the pyrolysis temperature is increased to 1500 ℃, the crystallinity of the material increased significantly, while characteristic diffraction peaks could be observed. The diffraction peaks at 31.1°, 34.6° and 35.4° correspond to the (201), (102) and (210) crystallographic planes of β-Si₃N₄, respectively. The diffraction peaks at 20.6° and 26.6° are attributed to the (100) and (101) crystallographic planes of SiO₂, while those at 27.1° and 33.8° correspond to the (200) and (200) crystallographic planes of α-Si₃N₄, respectively. The characteristic diffraction peak of the β-SiC (102) crystal plane was detected at 35.8°. The multiphase coexistence phenomenon was further verified by high-resolution TEM analysis results. The Si reacts with the surface SiO₂ layer to generate gaseous SiO(g) during high-temperature sintering. Meanwhile, the introduction of acrylate groups leads to an increase in the content of C, which can also promote to generate gaseous SiO(g) with SiO₂, which interact with N₂(g) at the gas-solid interface, thereby generating silicon nitride nanowires.[Conclusions] The introduction of acrylate groups significantly reduces the curing temperature of A-PSZ (high crosslink density can be achieved at 200 ℃). During the pyrolysis process, the A-PSZ-derived SiO gas and N₂ facilitate the growth of β-Si₃N₄ nanowires via a gas-solid (VS) mechanism, revealing the modulation of nanowire morphology by oxygen-involved gas-phase transport. This mechanism can be extended to other nanostructure systems containing silica-oxygen precursors. A-PSZ achieves high cross-linking density at 200 ℃ with a high ceramic yield of 81.7%, as compared with the conventional PSZ that requires curing temperature of >300 ℃.

Key words: polysilazane; SiCN ceramics; ceramization; silicon nitride nanowires