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

超高温陶瓷(UHTCs)将用于高超声速飞行器前缘的发汗冷却系统。以前用于大气再入等应用的热防护系统(TPS)试图通过在前缘具有较大的曲率半径来最小化气动加热的影响[1]。这在减少车辆发热方面是有效的，但不利于机动性。这些防护罩还将依赖于烧蚀冷却，这是一种将防护材料移除的过程，有效地从受保护车辆中带走热量，但仅适用于一次性使用[1]。要设计一种也可重复使用的锋利前沿，TPS必须能够承受高达2000摄氏度而不会造成损害。只有UHTC才能维持这样的温度[2]。待测试的TPS是蒸腾冷却，这是一个冷却气体通过暴露的外层流动的过程。发汗冷却需要一种在保持结构稳定的同时允许流体流动的材料，而这种材料的制造将是当前工作的主要重点。将评估在二硼化锆中创建内部通道的不同方法。目前，部分烧结法生产的多孔二硼化锆已被证明允许足够的液体流动以进行发汗冷却[1]。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℃的应用温度下不会致密的多孔材料。材料的特性将包括使用膨胀计来评估高温下的致密性，而材料的性能，如导热系数、强度和渗透性将被用来评估材料的适宜性。此外，工作将涉及将多孔皮肤与致密的、有沟槽的子结构相结合，以高效和可控地输送冷却液，以及提供结构加固。同样由二硼化锆制成，将研究通道大小和间距的优化。无压烧结、机器人铸造和凝胶铸造等制造方法将被评估它们是否适合设计和开发有效的发汗冷却系统。[1]Rocher，M.E等人在IRS的等离子体隧道中测试发汗冷却的二硼化锆样品。2019年AIAA科技论坛。[2]Fahrenholtz，W.G，&Hilmann，G.E.“超高温陶瓷：用于极端环境的材料。”Scripta Materialia，129，94-99 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.