Processing and Properties of Entropy-Stabilized Boride Ceramics.

为了在高超音速飞行、核聚变、弹道冲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温度的材料。该项目研究了复合过渡金属硼化物陶瓷的组成和微观结构对其性能的影响，这种陶瓷的熔点在3000℃以上，是已知材料中熔点最高的。这类材料被称为超高温陶瓷。熵稳定是一种概念，即至少四种其他元素以大约相等的比例加入到二元化合物中，已被用于产生稳定的高熵硼化物。使用这种方法的最初研究集中在合成和致密化上，而没有分析性能。因此，本研究解决了硼化物陶瓷基本成分-微观结构-性能关系知识的空白。该项目的科学成果将包括确定加工条件、微观结构和性能之间的关系，从而能够设计选择组合物和微观结构，从而产生要求苛刻的应用所需的性能。更广泛的影响将包括增加具有较高研究活动的博士大学和提供终端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强的研究联系，以加强学生招生和增加对研究基础设施的利用。该研究将采用综合实验和计算方法，为含有多种过渡金属的硼化物陶瓷的原子结构、微观结构发展以及成分-微观结构-性能关系提供前所未有的知识水平。该项目的主要目标是：1)利用熵稳定效应生产含有通常不会形成硼化物的金属的新硼化物组合物；2)通过改变成分来控制致密化动力学，从而控制微观结构的发展；3)在这类新兴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形成行为、热力学性质和缺陷形成能量。补充的实验研究将集中在熵稳定硼化物陶瓷的加工和性能。反应热压将用于生产具有可控成分的陶瓷，从含有一种过渡金属的高纯度硼化物到含有五种过渡金属的熵稳定成分，其比例大致相等。此外，反应热压提供了控制微观结构发展的能力，使微观结构-性能关系的研究成为可能。该项目将利用先进的表征工具来量化具有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内在力学、热学和电学性质。这个项目将导致前所未有的热化学稳定性和熵稳定陶瓷的固有性质的知识。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.