Superconducting fibers from organometallic precursors. Part II: Chemistry and low temperature processing 1
- PDF / 1,416,138 Bytes
- 13 Pages / 576 x 792 pts Page_size
- 17 Downloads / 168 Views
I. INTRODUCTION
Organometallic and inorganic preceramic polymers have proved extremely useful in processing a wide variety of nonoxide ceramic materials.2"9 By comparison, studies demonstrating their utility for processing oxide ceramic materials are limited to some thin film studies10 and silica-alumina fibers.11 The relative dearth of efforts in the oxide preceramics area is primarily the result of the success of sol-gel processing.1213 Sol-gel processing is distinguished from preceramic polymer processing by virtue of the need to hydrolytically cleave metal-ligand (typically metal-alkoxide) bonds. The advent of high temperature oxide ceramic superconductors provides an arena in which ceramics processing using organometallic14 polymers offers some advantages over sol-gel processing. The potential advantages that organometallic precursors offer over sol-gel processing1213 of oxide ceramics are particularly evident when processing multicomponent ceramics. The sol-gel approach requires careful attention to the disparate rates of hydrolysis of the component metal alkoxides to achieve effective atomic mixing during sol formation. In contrast, organometallic precursors rely on preformed bonding arrangements in the polymer for both atomic mixing and the rheological properties necessary for low temperature manipulation (e.g., spinning). Furthermore, close control of drying does not play a role in organometallic processing. However, alternate problems do a)
Address correspondence to this author at the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136. b) Taken in part from the thesis of R. A. Kennish. J. Mater. Res., Vol. 6, No. 5, May 1991 http://journals.cambridge.org
Downloaded: 14 Mar 2015
exist, such as (1) effective removal of carbon containing ligands so that carbon impurities are minimized; and (2) densification to nonporous, defect-free ceramic shapes. The latter problem arises because preceramic polymers have much lower densities than the resulting ceramic products. However, the significant volume changes associated with processing via organometallic preceramics are less problematic if one is interested in producing thin films or fibers. In this paper, principles, synthetic chemistry, and screening methods are developed for use in selecting "useful" organometallic polymer precursors for processing precursor fibers to 123 ceramic oxide fibers. The resultant criteria point to metal carboxylates as superior candidates for fabricating ceramic superconducting fibers. Preliminary results on the rheological properties and microstructural features (by optical microscopy) of the extruded precursor fibers are presented in support of the selection process. More detailed studies will be published elsewhere.15
II. BACKGROUND As a prelude to discussing specific research efforts, it is necessary to define the term "useful" in the context of preceramic polymers. To be "useful" for fiber forming, a preceramic polymer system must provide (A) controllable rheology, (B) lat
Data Loading...
