With the rapid development of high-technology fields, advanced aerogel acts as the thermal insulation materials under high-temperature environment attracted extensive attention. Polymer derived ceramic (PDC) aerogels were considered to be a promising thermal protection materials due to its low density, high porosity, excellent thermal insulation performance and high-temperature stability. Silicon oxycarbide (SiOC) ceramic aerogels, one kind of typical representative PDC materials, were mainly formed by partial substitution of C atoms for O atoms in SiO₂ tetrahedra. Compared the traditional silica aerogels, silicon oxycarbide (SiOC) aerogels exhibited better thermal and chemical stability as well as higher mechanical strength. Previously research reported that SiOC aerogels were mainly prepared by sol-gel method and PDC route, exhibited a higher density, which limited its applications and development in lightweight thermal insulation filed. Moreover, the obtained SiOC aerogels exhibited a necklace-like structure, poor high-temperature stability and mechanical strength due to the weak contact between nanoparticles. And most researches on SiOC aerogels focused on its high-temperature pyrolysis behavior. In this paper, a lightweight and high-strength SiOC aerogel were prepared using polyhydromethylsiloxane (PHMS) as precursor and tetramethyltetravinylcycletetrasiloxane (D₄Vi) as crosslinker through solvothermal, freeze casting and pyrolysis. The influence of D₄Vi content on the microstructure and thermal stability of SiOC aerogels were discussed. The evolution of phase composition, mechanical strength and thermal insulation performance was further explored via high-temperature heat treatment and oxidation treatment.

In a typical process, 1.5 g D₄Vi and 1.0 g PHMS were added into 52.4 g cyclohexane solvent and mixed uniformly by magnetic stirring for 30 min. Then, 1% Pt catalyst was added into the mixture for another 15 min stirring to obtain the transparent polysiloxane (PSO) solution. The PSO solution was then poured into an 80 ml Toflon-lined autoclave and underwent hydrothermal treatment at 150 ℃ for 20 h. Subsequently, the sample was freeze-dried at -50 ℃ for 48 h to obtain PSO aerogels. Finally, the PSO aerogels were pyrolyzed at 1 000 ℃ for 2 h with a heating rate of 5 ℃/min in Ar atmosphere. The pyrolyzed SiOC aerogels were further treated at 1 400 ℃ in Ar and 800 ℃ in air to explore their high-temperature stability and anti-oxidation.

The bulk density of SiOC aerogels was calculated by the measurement of the mass and dimension of the samples. The pore size distribution of SiOC aerogels was characterized using mercury intrusion porosimetry. The chemical structure and phase composition of SiOC aerogels was explored by Fourier transform infrared spectrometer and X-ray diffractometer. The microstructure of SiOC aerogels was characterized by scanning electron microscopy. The thermal conductivity of SiOC aerogels was measured by the hot-disk method using a thermal constant analyzer with the sample's dimension of Φ15 mm ×20 mm. The compressive strength of SiOC aerogels was investigated by an electronic universal testing machine using the samples with a dimension of Φ15 mm×20 mm.

lightweight SiOC aerogels were prepared by solution-based freeze-drying method under the mass ratio of D₄Vi: PHMS of 1.5 and 2. Both kinds of obtained aerogels exhibited low density of 0.12-0.13 g·cm^-3, much lower than that of previously reported SiOC aerogels and porous SiOC ceramics (0.23-0.62 g·cm^-3). With the increase in DVB content, the ceramic yield of SiOC aerogels decreased from 75.40% to 71.68% as the decreased crosslinking degree of PSO precursor, further leading to the transition from relatively disordered spider-web structure to ordered honeycomb-like structure. The stable porous structure endowed SiOC aerogels with low thermal conductivity of 0.061 9-0.065 5 W·m^-1·K^-1 and high compressive strength of 0.60-0.96 MPa.

Moreover, the SiOC aerogels possessed excellent high-temperature stability, when heat treatment at 1 400 ℃ in Ar, density of it was still ranged between 0.12 and 0.13 g·cm^-3 with original honeycomb-like structure. Meanwhile, the SiOC aerogels remained low thermal conductivity of 0.066 7-0.070 3W·m^-1·K^-1 and high compressive strength of 0.56-0.95 MPa. When oxidation at 800 ℃ in air, the SiOC aerogels still maintained porous structure and a density of 0.12-0.13 g·cm^-3 with a low mass loss of about 1.2%. The related thermal conductivity showed a minor variation of 0.060 7-0.064 3 W·m^-1·K^-1 and the compressive strength lightly decreased to 0.58-0.91 MPa, indicating a good anti-oxidation performance.

In this work, SiOC aerogels with lightweight, high-strength and thermal performance were prepared using PHMS as precursor and D₄Vi as crosslinker through solvothermal, freeze casting and pyrolysis. Porous honeycomb-like structure endowed SiOC aerogels with low density (0.13 g·cm^-3) and thermal conductivity (0.065 5 W·m^-1·K^-1) as well as high compressive strength (0.96 MPa). Meanwhile, SiOC aerogels exhibited excellent high-temperature stability and oxidation resistance. The corresponding bulk density, phase composition, thermal conductivity and compressive strength still remained and without any structural collapse even heat treatment at 1 400 ℃ in Ar, which could be an ideal candidate for lightweight, high strength, and high-temperature stability materials.