Formation of Crystalline Ultramicrospheres of ZrO, of Narrow Size Distribution in Condensed Gel-Sol System

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T. Sugimoto, S. Waki, A. Muramatsu
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Particle Formation
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Zirconia (ZrO,) particles are one of the most important materials for refractory ceramics, However, commercially available ill-defined powders prepared by milling of calcined agglomerates or by gas-phase reaction of ZrCl, and 0, are normally difficult in preparation of crack-free compact for sintering or need high sintering temperatures above 1700 “C. On the other hand, the monosized amorphous powders of a high sinterability prepared by hydrolysis of the alkoxides are not free from the economical problem of their low productivity (< 0.2 mol dmm3 in final concentration).

The objectives of our project are to develop a new method for preparation of unagglomerated ultrafme crystalline spheres of ZrO, of a narrow size distribution with the mean diameter of the order of ten nanometers or less in large quantities on the basis of the gel-sol technique developed in our laboratory, to elucidate the formation mechanism, and to examine the sintering properties. The standard procedure is as follow. Zirconium (IV) n-propoxide (ZNP: Zr(OC,H,),) is mixed with triethanolamine (TEOA: N(CH,CH,OH),) at a molar ratio of ZNP:TEOA = 1:3 in a dry box filled with dry air to form a stable compound of Zr”’ against the exceedingly rapid hydrolysis of Zr4’. After aging the solution at room temperature for 24 h, doubly distilled water and NH, solution are added to the solution in order to make a solution of 0.5 mol drnm3 in Zr4’ and 1.0 mol dmw3 in ammonia. In this stage no reaction takes place. The resulting solution is then aged at 200 “C for 3 days without stirring in a preheated oil bath to nucleate and grow the zirconia particles.

From the UV spectra of the mixed compounds of ZNP and TEOA in n- propanol and FT-IR spectra in chloroform, it was found that the propoxy groups of ZNP were all replaced by ethoxy groups of TEOA. With the elevation of the internal temperature which took ca. 30 min to reach 200 + 1 “C, hydroxide gel was formed and the nucleation occurred on the gel network in 20 min. The nuclei were grown to fairly uniform particles of ca. 15 nm in mean diameter at the expense of the gel until, finally, the gel completely disappeared in 1 h. In this stage the particles were of a rough surface, but as they were further aged, they became spherical with a smooth surface due to the intra-particle recrystallization. When the aging was prolonged to 72 h, the particles were somewhat grown by Ostwald ripening. It is noteworthy that so-prepared particles were tetragonal ZrO, after 30 min as determined by X-ray diffractometry, even at the rather low temperature, 200 “C, while particles prepared at high pH (-13) in the absence of ammonia or those prepared in the presence of acetate ions at neutral pH (-7) i were both monoclinic. Also, high-resolution transmission electron microscopy revealed that the produced particles were mostly single crystals.

From the analysis of growth mechanism it is concluded that the fairly uniform unagglomerated ultrafme particles are nucleated on the gel network of the hydroxide, and grown by dissolution of the gel without significant coagulation owing to the gel network holding each particle. In addition, extensive renucleation is inhibited due to the lowered supersaturation by the formation of the gel precursor.

The so-prepared powders (-15nm and -2.5nm) showed an excellent sinterability, as compared to power prepared by gas-phase reaction (-6Onm) or monosized amorphous powder by hydrolysis of zirconium butoxide at 50 “C (-250nm).