Ultra-lightweight and reinforced ZnO/Cellulose layered cryogel for thermal insulation

Cellulose cryogels (CA) with high porosity and environmental sustainability are expected to replace traditional petroleum-based insulation materials. However, the efficient thermal insulation and mechanical reinforcement of cellulose cryogel still face challenges. We develop zinc oxide/cellulose (ZnO/CA) cryogels through a straightforward strategy involving the dissolution of cellulose in a sodium hydroxide-urea solution followed by directional freezing. Specifically, the sodium hydroxide-urea solution disrupts the intermolecular and intramolecular hydrogen bonds within cellulose, converting its crystalline structure from cellulose I to cellulose II. The temperature gradient during directional freezing induces a uniformly ordered, three-dimensional layered porous structure with ultra-low density (0.071–0.102 g/cm³) and high porosity (95.75–93.47%). Importantly, the Young’s modulus of the ZnO/CA cryogel reaches 2.87 MPa, while its thermal conductivity is as low as 0.0138 W·m⁻¹·K⁻¹, indicating high compressive strength and excellent thermal stability. Within the ZnO/CA cryogel framework, the overall emissivity in the infrared band is below 0.5, effectively reducing thermal radiation intensity. On an 80 ℃ hot target, the surface radiation temperature of the cryogel drops to as low as 38 ℃. Furthermore, ZnO/CA cryogel-coated fabrics exhibit a 74% increase in insulation compared to untreated cotton fabric and demonstrate effective insulation for various heat sources. The lightweight, reinforced ZnO/CA cryogel offers new possibilities for developing wearable insulating devices and high-performance thermal insulation materials.