Control Over Al 2 O 3 Phase by Use Of Polymer Precursors
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CONTROL OVER A1203 PHASE BY USE OF POLYMER PRECURSORS
WILLIAM S. REES, JR." AND WERNER HESSE
Department of Chemistry and Materlals Research and Technology Center, The Florida State University, Tallahassee, Florida 32306-30S, U.S.A. ABSTRACT Reaction of Et2AIOEt with ethylene glycol or catechol produced polymers of the general form -[-AI(OEt)-O-R-O-]-n, for R = CH2CH2 or C6H4, respectively. Pyrolytic conversion of these polymers to ceramic materials produced A1203, at mild (-5000C) temperatures under a flowing atmosphere of dry air. The crystal phase obtained from the thermolysis is highly dependent upon the degree of cross-linking present inthe initial polymer. These results are discussed interms of the required solid-state atomic reorganization necessary to proceed from polymer to ceramic.
INTRODUCTION Alumina is widely employed for its combination of properties including highi hardness (Mohs 9), chemical inertness, and high melting point (>2,OOOOC).1 The refractory A1203 exists as the thermodynamically stable a form, obtained by exhaustive dehydration and densification of the minerals Gibbsite, Boehmite, Bayerite, or Diaspore (Figure 1).2 Inthe employment of ceramic fibers for support of both ceramic and metal-based composite structures A1203 containing candidates currently are used. 3 However, one drawback present in any ceramic fiber based system is the potential for crystallization and thereby strength loss due to grain boundary fracture. Therefore, inhibition of the onset of crystallinity is a desirable property to impart to A1203. Polymeric routes to SiC, Si3N4, BN, B4C, TiN, and mixed phases have been developed over the past decade. 4 One of the main applications for such preceramic polymers is inthe fabrication of ceramic fibers by pyrolysis of polymer fibers obtained by means of a processable material method such as melt spinning. 5 With the dual goal in mind of preparing precursors to A1203 that both inhibited the onset of crystallization and might serve as fiber-formers, we embarked on an exploration of polymeric aluminum alkoxides.
Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
52
EXPERIMENTAL The preparation of these polymers has been described previously (Eq. 1).6 The results of elemental analyses (C, H)are presented (Table I). TGA data (duPont systems instrument, to 1,000CC, air atmosphere) are summarized (Table II). Bulk pyrolyses were carried out ina Lindberg tube furnace equipped with a Eurotherm controller, inalumina boats held inquartz tubes, under an atmosphere of flowing air. The boats were prefired to 1,000 0C for 24 hours inpure 02 prior to use insample preparation. XRD measurements were performed on a Siemens diffractometer equipped with JCPDS peak search and match software. Et2AI(OEt) + HO-R-OH
----
(Eq. 1)
> 2 EtH , -[-AI(OEt)-O-R-O-]n-
TABLE I. ELEMENTAL ANALYSES FOR POLYMERS.
FORMULA
CHEMICAL ANALYSIS
GEt
I
(AI-OCH2CH20)n
Calc'd. Found
36.37
6.86
36.64
6.8
33.45
6.08 6.06
(OEt)0.57(OCH 2 -)o. 43 (AI-OCH2CH20)n
33.07
OCH2(AI-OCH2CH20)n
3
