Low temperature synthesis of ultrafine LiTaO 3 powders
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Controlled chemical polymerization of tantalum ethoxide in the presence of glacial acetic acid (HOAc/Alk. = 16) and solubilized lithium acetate (Li/Ta = 1.00, H 2 O/Alk. = 55.55) was used for the preparation of an amorphous gel precursor to LiTaO3. Although additional investigations are required, the results suggest that successful formation of amorphous gel network, as opposed to that of colloidal tantalum (hydrous) oxide, may be due to the generation of a new organotantalum precursor via a structural modification reaction between the tantalum ethoxide and glacial acetic acid. The evolution of LiTaO3 ceramics from pre-ceramic gels was investigated using thermal analysis, electron microscopy, and x-ray diffraction. The results indicate that after the completion of gel pyrolysis (200-400 °C) and crystallization (Tc = 590 °C), ultrafine (average particle size 100-300 nm), single phase, crystalline (a = 5.243, c = 13.812 A) LiTaO3 powders can be prepared at low processing temperatures.
I. INTRODUCTION A. Chemical synthesis of LiTaO3 ceramics Lithium tantalate (LiTaO3), one of the technologically important high temperature ferroelectrics (Tc — 665 °C), is characterized by its useful piezoelectric, pyroelectric, and electro-optic properties.1 Although large single crystals of LiTaO3 are grown by the Czochralski method, the processing of polycrystalline LiTaO3 ceramics continues to be a difficult proposition. This is primarily due to the need for high densification temperatures (>1300 °C) at which volatilization of lithium oxide (Li2O) occurs rapidly. This makes it very difficult to process LiTaO3 in the form of single phase, polycrystalline ceramics. The problem may be alleviated—for instance, using sintering additives (e.g., SiO 2 , MgF 2 , etc.).2-3 Although such additives may promote densification at lower temperatures, formation of the secondary phases as distinct grains or at the grain boundaries can adversely affect the electrical and other properties of the sintered LiTaO3 ceramics. Therefore, there is considerable interest in development of low temperature, wet chemical routes such as the sol-gel processing for the synthesis of LiTaO3 powders or thin films. Such techniques, characterized by their unparalleled ability to manipulate surfaces and interfaces at a nanoscale and during the early stages of ceramic processing, can lead to ultrahomogeneous, stoichiometric, and ultrafine powders at temperatures lower than those required for conventional processing. Castaings and co-workers4 reported a gelation process for the synthesis of LiTaO3 powders using a
*Address correspondence to this author. J. Mater. Res., Vol. 6, No. 7, Jul 1991
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lithium ethoxide (LiOC2H5) and tantalum ethoxide [Ta(OC2H5)5]. The resultant gel, which was apparently colloidal in nature, was separated by filtration. They speculated that the formation of a bimetallic oxyalkoxide precursor [LiOTa(OC2H5)4] occurred as a result of the reaction between partially hydrolyzed tantalum ethoxide and
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