Processing and Properties of Entropy-Stabilized Boride Ceramics.

为了在高超声速飞行、核聚变、弹道撞击防护和聚光太阳能方面取得进展，需要能够承受远高于典型高性能材料熔点的材料。本项目研究成分和微观结构对复合过渡金属硼化物陶瓷性能的影响，这种陶瓷的熔化温度在3000℃以上，是已知材料中熔点最高的材料之一。这种材料被称为超高温陶瓷。熵稳定化的概念是将至少四个其他元素以大致相等的比例添加到二元化合物中，已被用于生产稳定的高熵硼化物。使用这种方法的最初研究集中在合成和致密化上，而不分析性能。因此，这项研究解决了关于硼化物陶瓷的基本成分-显微结构-性能关系的知识空白。该项目的科学成果将包括确定工艺条件、微结构和性能之间的关系，以便能够设计选择将产生要求苛刻的应用所需的性能的组合物和微结构。更广泛的影响将包括增加一所拥有更高研究活动的博士生大学与一所提供终极硕士学位的公立大学之间的校际合作。这项合作将包括扩大远程方法，以扩大两所学校的课程设置，并在两所学校之间建立更紧密的研究联系，以增加招生和获得研究基础设施的机会。研究将使用综合的实验和计算方法，提供关于含有多种过渡金属的硼化物陶瓷的原子结构、微结构发展和成分-微结构-性能关系的前所未有的知识水平。该项目的主要目标是：1)利用熵稳定效应来生产含有通常不形成硼化物的金属的新的硼化物成分；2)通过改变成分来控制致密化动力学来控制微观结构的发展；以及3)在这一新兴类别的材料中建立超高温结构-性能关系。计算方法将用于研究溶液的形成行为、热力学性质和缺陷形成能。补充的实验研究将集中在稳定熵的硼化物陶瓷的工艺和性能上。反应热压将被用来生产受控成分的陶瓷，范围从含有一种过渡金属的高纯硼化物到含有大致相等比例的五种过渡金属的熵稳定化合物。此外，反应热压提供了控制组织发展的能力，从而能够研究组织与性能之间的关系。该项目将利用先进的表征工具来量化复杂成分的硼化物陶瓷中的金属分布，这将有助于阐明致密化动力学，并测量高熵硼化物成分的固有机械、热和电性能。该项目将使人们对熵稳定陶瓷的热化学稳定性和固有特性有前所未有的了解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In order to make advances in hypersonic flight, nuclear fusion, ballistic impact protection, and concentrated solar power, materials are needed that can withstand temperatures far above the melting point of typical high performance materials. This project investigates the effects of composition and microstructure on the properties of complex transition metal boride ceramics which have melting temperatures above 3000 C and are among the highest melting points for any known materials. This class of materials is known as ultra-high temperature ceramics. Entropy stabilization, a concept whereby at least four other elements in approximately equal ratios are added to a binary compound, has been used to produce stable, high entropy borides. Initial studies using this approach focused on synthesis and densification without analyzing properties. Hence, this research addresses a gap in knowledge of fundamental composition-microstructure-property relationships for boride ceramics. The scientific outcomes of the project will include identification of relationships among processing conditions, microstructures, and properties to enable design selection of compositions and microstructures that will yield properties desired for demanding applications. The broader impacts will include increased inter-campus collaboration between a doctoral university with higher research activity and a public university that offers terminal M.S. degrees. This collaboration will include expanding distance methods to extend curricular offerings at both schools, and establishing stronger research links between both to enhance student recruitment and increase access to research infrastructure.The research will use an integrated experimental and computational approach to provide an unprecedented level of knowledge about the atomic structure, microstructure development, and composition-microstructure-property relationships for boride ceramics containing multiple transition metals. The main goals of the project are to: 1) utilize the entropy stabilization effect to produce new boride compositions containing metals that do not typically form borides; 2) control microstructure development by manipulating densification kinetics through changes in composition; and 3) establish ultra-high temperature structure-property relationships in this emerging class of materials. Computational methods will be used to investigate solution formation behavior, thermodynamic properties, and defect formation energies. Complementary experimental studies will focus on processing and properties of entropy stabilized boride ceramics. Reactive hot pressing will be used to produce ceramics with controlled compositions ranging from highly pure borides with containing one transition metal to entropy-stabilized compositions containing five transition metals in roughly equal proportions. In addition, reactive hot pressing offers the ability to control microstructure development to enable studies of microstructure-property relationships. The project will utilize advanced characterization tools to quantify metal distributions in boride ceramics with complex compositions, which will help elucidate densification kinetics, and measure intrinsic mechanical, thermal, and electrical properties of high entropy boride compositions. This project will lead to unprecedented knowledge of the thermochemical stability and inherent properties of entropy stabilized ceramics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.