Fabrication of Porous Ultra-high Temperature Ceramics for Transpiration Cooling of Hypersonic Vehicles.

超高温陶瓷（UHTCs）将被用于高超声速飞行器前沿的蒸腾冷却系统。以前的热保护系统（TPS）用于诸如大气再入等应用，试图通过前缘[1]的大曲率半径来最小化空气动力学加热的影响。这在减少车辆发热方面是有效的，但不利于机动性。这些防护罩也将依赖于烧蚀冷却，这是一种去除防护材料的过程，有效地从被保护的车辆中除去热量，但只适用于一次性使用。为了设计一个锋利的前缘，也可以重复使用，TPS必须能够承受高达2000摄氏度的高温而不会持续损坏。只有超高温电池能够维持这样的温度。要测试的TPS是蒸腾冷却，这是一种冷却气体流过暴露在外的外层的过程。这既在内部冷却了材料，又在表面提供了一层保护流体。蒸腾冷却需要一种允许流体流动同时保持结构稳定性的材料，其制造将是当前工作的主要重点。在二硼化锆中创建内部通道的不同方法将被评估。目前，通过部分烧结制备多孔二硼化锆的方法已被证明可以为蒸腾冷却提供足够的流体流动。接下来的工作将是探索控制二硼化锆粗化和致密化的参数，目的是生产一种在2000摄氏度的应用温度下不会致密化的多孔材料。材料的表征将包括使用膨胀法来评估高温下的致密性，而材料的性能，如导热性、强度和渗透性将用于评估材料的适用性。此外，工作还包括将多孔外壳与密集的、通道状的下层结构结合起来，以有效和受控地输送冷却流体，并提供结构加固。同样由二硼化锆制成，通道尺寸和间距的优化将被研究。制造方法，如无压烧结、机器人铸造和凝胶铸造，将评估其是否适合设计和开发有效的蒸腾冷却系统Rocher, m.e.等，“在IRS等离子体隧道中测试蒸腾冷却的二硼化锆样品”。2019 AIAA科技论坛，2019. b[2]华伦霍茨，w.g.和希尔曼斯，G.E.超高温陶瓷：极端环境用材料。材料科学，29,94-99 2018。

Ultra-high temperature ceramics (UHTCs) are to be examined for their use in transpiration cooling systems for the leading edges of hypersonic vehicles. Previous thermal protection systems (TPS) for applications such as atmospheric re-entry have tried to minimise the effect of aerodynamic heating by having a large radius of curvature for the leading edge [1]. This is effective at reducing the heating of the vehicle but is detrimental to manoeuvrability. These shields would also rely on ablative cooling, a process by which the protective material is removed, effectively removing heat from the protected vehicle but is only suitable for single use [1].To design a sharp leading edge, which is also reusable, the TPS must be able to withstand up to 2000C without sustaining damage. Only UHTCs are capable of sustaining such temperatures [2]. The TPS to be tested is transpiration cooling, a process by which a cooling gas flows through the exposed outer layer. This both cools the material internally and provides a protective fluid layer at the surface [1].Transpiration cooling requires a material that allows for fluid flow while maintaining structural stability, the fabrication of which will be the primary focus of the current work. Different methods for creating internal channels in zirconium diboride will be assessed. Currently the creation of porous zirconium diboride by partial sintering has been shown to allow for sufficient fluid flow for transpiration cooling [1]. Following work will now be to probe parameters controlling coarsening and densification of zirconium diboride, with the aim to produce a porous material which will not then densify at application temperatures of 2000C. Characterisation of materials will involve use of dilatometry to assess densification at high temperature, while material properties such as thermal conductivity, strength, and permeability will be used to assess suitability of materials.Additionally, work will involve combining the porous skin with a dense, channelled, substructure for efficient and controlled delivery of cooling fluid, as well as providing structural reinforcement. Also made from zirconium diboride, optimisation of channel size and spacing will be investigated. Manufacturing methods such as pressureless sintering, robocasting, and gel casting are to be assessed for their suitability for the design and development of an effective transpiration cooling system.[1] Rocher, M.E, et al., "Testing a Transpiration Cooled Zirconium-Di-Boride sample in the Plasma Tunnel at IRS." AIAA Scitech 2019 Forum, 2019.[2] Fahrenholtz, W.G, & Hilmans, G.E. "Ultra-high Temperature Ceramics: Materials for extreme environments." Scripta Materialia, 129, 94-99 2018.