Genesis of Nanostructured, Magnetically Tunable Ceramics from the Pyrolysis of Cross-Linked Polyferrocenylsilane Networks and Formation of Shaped Macroscopic Objects and Micron Scale Patterns by Micromolding Inside Silicon Wafers

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Abstract
The ability to form molded or patterned metal-containing ceramics with tunable properties is desirable for many applications. In this paper we describe the evolution of a ceramic from a metal-containing polymer in which the variation of pyrolysis conditions facilitates control of ceramic structure and composition, influencing magnetic and mechanical properties. We have found that pyrolysis under nitrogen of a well-characterized cross-linked polyferrocenylsilane network derived from the ring-opening polymerization (ROP) of a spirocyclic [1]ferrocenophane precursor gives shaped macroscopic magnetic ceramics consisting of α-Fe nanoparticles embedded in a SiC/C/Si3N4 matrix in greater than 90% yield up to 1000 °C. Variation of the pyrolysis temperature and time permitted control over the nucleation and growth of α-Fe particles, which ranged in size from around 15 to 700 Å, and the crystallization of the surrounding matrix. The ceramics contained smaller α-Fe particles when prepared at temperatures lower than 900 °C and displayed superparamagnetic behavior, whereas the materials prepared at 1000 °C contained larger α-Fe particles and were ferromagnetic. This flexibility may be useful for particular materials applications. In addition, the composition of the ceramic was altered by changing the pyrolysis atmosphere to argon, which yielded ceramics that contain Fe3Si5. The ceramics have been characterized by a combination of physical techniques, including powder X-ray diffraction, TEM, reflectance UV−vis/near-IR spectroscopy, elemental analysis, XPS, SQUID magnetometry, Mössbauer spectroscopy, nanoindentation, and SEM. Micromolding of the spirocyclic [1]ferrocenophane precursor within soft lithographically patterned channels housed inside silicon wafers followed by thermal ROP and pyrolysis enabled the formation of predetermined micron scale designs of the magnetic ceramic.