Ultra-high temperature ceramics (UHTCs) are considered as a family of nonmetallic and inorganic materials that have melting point over 3000 °C. Chemically, nearly all UHTCs are borides, carbides, and nitrides of early transition metals (e.g., Zr, Hf, Nb, Ta). Within the last two decades, except for the great achievements in the densification, microstructure tailoring, and mechanical property improvements of UHTCs, many methods have been established for the preparation of porous UHTCs, aiming to develop high-temperature resistant, sintering resistant, and lightweight materials that will withstand temperatures as high as 2000 °C for long periods of time. Amongst the synthesis methods for porous UHTCs, sol-gel methods enable the preparation of porous UHTCs with pore sizes from 1 to 500 urn and porosity within the range of 60%-95% at relatively low temperature. In this article, we review the currently available sol-gel methods for the preparation of porous UHTCs. Templating, foaming, and solvent evaporation methods are described and compared in terms of processing-microstructure relations. The properties and high temperature resistance of sol-gel derived porous UHTCs are discussed. Finally, directions to future investigations on the processing and applications of porous UHTCs are proposed.

中文翻译：

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溶胶凝胶衍生的多孔超高温陶瓷

超高温陶瓷（UHTC）被认为是一类非金属和无机材料，其熔点超过3000°C。化学上，几乎所有UHTC都是早期过渡金属（例如Zr，Hf，Nb，Ta）的硼化物，碳化物和氮化物。在过去的二十年中，除了在致密化，微结构剪裁和机械性能改进方面取得了巨大成就外，已经建立了许多方法来制备多孔性超高温TCS，旨在开发耐高温，耐烧结和轻量化的产品。可以长时间承受高达2000°C温度的材料。在多孔UHTC的合成方法中，溶胶-凝胶法能够在相对较低的温度下制备孔径为1至500 um，孔隙率在60％-95％范围内的多孔UHTC。在本文中，我们回顾了用于制备多孔UHTC的溶胶凝胶方法。描述并比较了模板，发泡和溶剂蒸发方法，方法涉及加工-微观结构。讨论了溶胶-凝胶衍生的多孔UHTC的性能和耐高温性。最后，提出了对多孔UHTC的加工和应用进行进一步研究的方向。讨论了溶胶-凝胶衍生的多孔UHTC的性能和耐高温性。最后，提出了对多孔UHTC的加工和应用进行进一步研究的方向。讨论了溶胶-凝胶衍生的多孔UHTC的性能和耐高温性。最后，提出了对多孔UHTC的加工和应用进行进一步研究的方向。