Molecularly Modified Alkoxide Precursors for Synthesis of Dielectric Ceramics
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MOLECULARLY MODIFIED ALKOXIDE PRECURSORS FOR SYNTHESIS OF DIELECTRIC CERAMICS. PRADEEP P. PHULE AND FARIDA KHAIRULLA Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA
ABSTRACT: Chemical synthesis of titanate and zirconate based multicomponent electronic ceramics using molecularly modified alkoxide precursors is discussed. Controlled chemical polymerization of the modified alkoxides was conducted to prepare multicomponent gels which on calcination at moderate temperatures (700 -1000*C) formed high purity, ultrafine powders (e.g. BaTiO3 ,ZrTiO4,LiTaO 3 ). In the sol-precipitation technique modified precursors were used to prepare ultrafine (< 100 nm) and crystalline powders of such ceramics as BaTiO3 and BaZrO3 at low temperatures (< 1001C), under highly alkaline (pH > 13) conditions. INTRODUCTION: Following the pioneering works of Mazdiyasni et al. [1-2] , Flaschen [3], Claubaugh et al. [4] and Pechini [5], several other chemical processes for the synthesis of titanates and zirconates have been reported in the literature [6]. Alkoxide based chemical processing routes for synthesis of capacitor and microwave ceramics suffer from the extreme reactivity of the transition and alkaline earth metal alkoxides [7]. Furthermore, metal alkoxides exhibit a cation dependent differential reactivity i.e. different metal alkoxides (e.g. Ba and Ti) hydrolyze and polycondense at different rates. This can lead to chemical heterogeneities in the final ceramic product. Handling and storage of larger quantities and cost can also pose additional difficulties while using alkoxides as starting materials. Problems associated with differential and extreme reactivity can be alleviated using such physical methods as lower temperatures, smaller concentrations or pre-hydrolysis of the slower reacting precursor. Central to the theme of this paper is the concept of chemical ways to gain intrinsic control of the transition metal alkoxide reactivity via purposeful modifications in their structure. As will be discussed shortly, such modifications lead to metallorganic precursors which are relatively resistant to hydrolysis. This genre of precursors, referred to as molecularly modified alkoxides, can be effectively used for fabrication of multicomponent ceramics in the form of powders, thin films or fibers [8]. The objective of this paper is to discuss the concept of molecular modification and applications of modified precursors for synthesis of ceramics. Molecularly modified alkoxide precursors: Reactions of certain metal alkoxides with such reagents as acetic acid, acetylacetone, esters and alcohols, leading to changes at a molecular level, have been known to chemists for well over twenty years [7,9]. Other modifying agents (e.g. diethanolamine) have also been reported [10]. Following are some examples of such modification reactions 17,9]: Reactions with alcohols: M(OR)4 + 4 HOR' -...
> M(OR')
4
+ 4 R-OH -----
(1)
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
528
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