Silicon-Based Porous Ceramics via Freeze-Casting Preceramic Polymers

非技术描述：从柴油发动机排气中捕获颗粒，为燃料电池中的气体运输提供通道，并通过绝缘保持热量，所有这些都依赖于多孔材料。 这些应用中的每一个都需要具有独特尺寸、形状、粗糙度和连通性的多孔网络，以控制和缓解流动或过滤或绝缘能力。许多与能源相关的应用还要求材料（陶瓷）在超过1500 C（2732 F）的温度下保持强度和鲁棒性。 由陶瓷先驱体聚合物生产的高级陶瓷在高温应用中优于传统材料。该项目的重点是使用陶瓷先驱体聚合物，通过冷冻铸造来创建新的高温多孔陶瓷。聚合物溶解在有机溶剂中，当以受控方式冷冻并转化为陶瓷时，产生针对流动特性和机械完整性优化的定向多孔网络。通过发展对这些冷冻铸造系统的化学性质的理解，可以创建通用的孔结构，从而扩大多孔陶瓷在环境和能源需求中的应用。参与这项研究的学生接受最先进的陶瓷加工和表征方法的培训，为技术人员做好准备。 作为一个额外的好处，通过这些方法生产的多孔材料类似于自然界中发现的材料，并提供了惊人的图像，为通过艺术进行科学教育提供了机会。 技术规格：使用预陶瓷聚合物来制造硅基和硼基陶瓷提供了一种获得耐高温蠕变和耐化学腐蚀材料的途径。传统上，大的收缩伴随着通过聚合物热解转化为陶瓷，通常导致开裂。为了解决这个问题，部件仅限于薄纤维、板和泡沫多孔固体。本研究建立在陶瓷聚合物预制体陶瓷加工的基础上，并使用定向冷冻铸造扩展其可成形性。与其他多孔材料成型方法相比，定向冷冻铸造提供了各向异性的连续孔，其尺寸，形状和曲折度可以调节，因此，为能源相关的应用开辟了新的途径。 为了使预陶瓷聚合物的冷冻铸造可行，有必要使用光谱测量和相分离和固化的实时显微镜观察来发展对聚合物-溶剂对和交联催化剂的基本理解。 预陶瓷聚合物的另一个好处是它们能够使用反应性添加剂和短效相进行官能化，从而产生更复杂的化学性质和层次结构，以实现更大的功能。这些材料提供了一类新的冷冻铸造固体，其孔隙网络特性是特别感兴趣的，他们探索使用同步加速器技术和渗透性实验。此外，它们的力学性能，包括各向异性的孔和复杂的孔壁结构，相对于传统的多孔材料的力学模型进行评估。该研究与Paolo科隆博教授（意大利帕多瓦大学）进行了国际合作，他是国际公认的陶瓷前体聚合物和多孔陶瓷专家，并为学生提供了重要的国际培训经验。

NON-TECHNICAL DESCRIPTION: Capturing particulates from diesel engine exhausts, providing channels for gaseous transport in fuel cells, and retaining heat via insulation all rely on porous materials. Each of these applications requires porous networks with distinctive size, shape, roughness, and connectivity to control and ease the flow or filtering or insulating capabilities. Many energy-related applications also require materials (ceramics) that maintain strength and robustness to temperatures in excess of 1500 C (2732 F). Advanced ceramics produced from preceramic polymers surpass conventional materials for high-temperature applications. This project focuses on the use of preceramic polymers to create new high-temperature porous ceramics by freeze casting. The polymers are dissolved in organic solvents, that when frozen in a controlled manner and converted to a ceramic, produce directionally porous networks that are optimized for flow characteristics and mechanical integrity. By developing an understanding of the chemistry of these freeze-casting systems, creating versatile pore architectures is possible, thereby expanding the use of porous ceramics for environmental and energy needs. Students involved in this research are trained in state-of-the-art ceramic processing and characterization methods, as preparation for the technical workforce. As an added benefit, porous materials produced by these methods resemble those found in nature and offer striking images that afford opportunities for science education through art. TECHNICAL DETAILS: The use of preceramic polymers to create silicon- and boron-based ceramics provides a route to high-temperature creep- and chemically-resistant materials. Traditionally, large shrinkages accompany the conversion to ceramic by polymer pyrolysis often resulting in cracking. To deal with this issue, components are limited to thin fibers, plates, and foamed porous solids. This research builds on the foundation of ceramic processing from preceramic polymers, and extends their formability using directional freeze casting. Compared to other porous material-forming methods, directional freeze casting affords anisotropic continuous pores, whose size, shape and tortuosity can be tuned, and hence, opens new avenues for energy-related applications. To make freeze casting viable for preceramic polymers, it is necessary to develop a fundamental understanding of polymer-solvent pairs and crosslinking catalysts using spectroscopic measurements and real-time microscopic observation of phase separation and solidification. An added benefit of preceramic polymers is their ability to be functionalized using reactive additives and fugitive phases, resulting in more complex chemistries and hierarchical microstructures for greater functionality. These materials provide a new class of freeze-cast solids where their pore network characteristics are of particular interest; they are explored using synchrotron techniques and permeability experiments. Moreover, their mechanical properties, consisting of anisotropic pores and complex pore wall structures, are assessed with respect to conventional mechanics models for porous materials. The research seeds an international collaboration with Prof. Paolo Colombo (Universita di Padova, Italy), an internationally recognized expert in preceramic polymers and porous ceramics, and provides significant international training experiences for students.