Ceramic aerogels have promising applications in extreme environments, such as aerospace, but are severely limited by the sintering-prone nature of nanounits at high temperatures. However, they typically exhibit inadequate dimensional stability when exposed to high-temperature atmospheres, which can result in the deterioration of their macroscopic characteristics, eventually restricting their applications in extreme environments. Here, a (YYbErDyGd)₂SiO₅ ceramic aerogel (ESA) was prepared at 1050 °C by introducing high entropy into a ceramic aerogel, thereby enhancing its high-temperature thermal stability via the high-entropy effect. ESA was exposed to a temperature of 1600 °C for 2 h without undergoing sintering. In addition, different fiber-reinforced ceramic aerogel composites are also explored, with a thermal conductivity of only 0.032 W/(m·K) at room temperature and 0.108 W/(m·K) at high temperature of 1000 °C. These composites demonstrate no visible damage or deformation under extreme conditions across a substantial temperature range (−196 to 1300 °C), a property that is paramount for applications in extreme environments. At 1100–1300 °C, after 2 h of calcination, the composites exhibit a shrinkage rate of only 0.25%. After 600 s of butane torch ablation at 1300 °C, the back temperature is only 110 °C. Moreover, under 60% compression deformation, its maximum compression strength is 0.386 MPa. Even after 20 high-temperature thermal cycles (1300 °C for 2 h), the sample maintains a low thermal conductivity of 0.043 W/(m·K) and a compressive strength of 0.259 MPa. This work provides a new perspective for exploring the limits of the strength and thermal properties of ceramic composites in the field of high-temperature insulation, particularly under extreme conditions.

High-thermally conductive AlN-based microwave attenuating composite ceramics with spherical graphite (SG) as the attenuating agent were fabricated through hot-pressing sintering. The SG maintains its three-dimensional (3D) morphology within the sintered bodies, which considerably impedes the sintering of the composites to some extent but slightly influences on the growth of AlN grains. The addition of SG reduces the strength of the composites, but provides a moderate toughening effect at the optimal addition amount (3.8 MPa·m^1/2 at 4 wt% SG). Benefiting from the low anisotropy, high thermal conductivity, and the 3D morphology of SG, the composites exhibit a relatively higher thermal conductivity (76.82 W·m^-1·K^-1 at 10 wt% SG) compared with composites added with non-spherical attenuating agent. The dielectric constant and loss (8.2-12.4 GHz) increase remarkably as the amount of SG added increases up to 8 wt%, revealing that the incorporation of SG improves the dielectric property of the composite. The composite with 7 wt% SG exhibits the best absorption performance with a minimum reflection loss of -13.9 dB at 12.4 GHz and an effective absorbing bandwidth of 0.87 GHz. The excellent overall properties of the SG/AlN microwave attenuating composites render them as a promising material for various applications. Moreover, SG has a great potential as an attenuating agent for microwave attenuating composites due to its strong attenuation upon integration, high thermal conductivity, and low anisotropy.