Porous thermal insulation materials (PTIMs) are a class of materials characterized by low thermal conductivity, low bulk density and high porosity. The low thermal conductivity of the gas enclosed in their pores allows them to achieve efficient thermal insulation, and are they among the most widely used and effective materials in thermal insulation material systems. Among the PTIMs, polyurethane foam (PUF) stands out as particularly promising. Its appeal comes from its multiple beneficial features, such as low density, low thermal conductivity and superior mechanical properties. Such attributes have propelled its broad application across domains encompassing construction, heterogeneous chemical equipment, water conservation and hydropower, and the aviation and aerospace fields. First, this article outlines the structure and properties of porous thermal insulation PUF materials. Next, it explores the methods of preparing porous thermal insulation PUF materials, evaluating the associated advantages and disadvantages of each technique. Following this, the mechanical properties, thermal conductivity, thermal stability, and flame-retardant characteristics of porous thermal insulation PUF materials are characterized. Lastly, the article provides insight into the prospective development trends pertaining to porous thermal insulation PUF materials.

In this study, mullite single-crystal whiskers were prepared by sintering mullite gel powders using HF as a catalyst via the sol–gel process. The effects of the Al₂O₃:SiO₂ molar ratio on the morphology of mullite whiskers in the Al–Si–F system were comprehensively explored during the catalytic reaction. Furthermore, the synergistic effects of the Si:Al ratio and the catalyst content on the growth mechanism of mullite whiskers were evaluated. The morphological characteristics of the whiskers were determined using transmission electron microscopy and scanning electron microscopy. Moreover, morphological parameters, including the diameter and length of whiskers, were statistically analyzed using the Image J software. Additionally, the compositional variation and phase evolution during the whisker growth process were examined via energy-dispersive spectroscopy and X-ray diffraction, respectively, and the corresponding growth mechanism was elucidated. When HF-mediated catalysis reaches a sufficient level (Al₂O₃:SiO₂:HF = 1:1.5:4.3), the low SiO₂ content in the system leads to Al enrichment and the formation of flake-shaped Al₂O₃ structures, indicating an effect analogous to that of increasing catalyst content. Conversely, the simultaneous reduction in the contents of HF and SiO₂ induces different catalytic reactions because of their synergy. Specifically, at relatively low SiO₂ and HF contents, F ions enter the Al–Si–O system via SiF₄, leading to the generation of fluorine-containing topaz, which subsequently transforms into mullite. At relatively high SiO₂ and HF contents, mullite can be directly synthesized via the reaction of AlF3 and SiF₄. With a gradual reduction in the SiO₂ and HF contents, the mullite whiskers exhibit a varying morphology, predominantly transitioning from rod-shaped to flake-shaped and subsequently to rod-shaped structures. This is due to the synergistic effects of the phase transformation and catalytic reactions within the Al–Si–O system.