Vapor phase synthesis of Al-doped titania powders
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Sebastian V. R. Mastrangelo DuPont Chemicals, Edge Moor, Delaware 19809 (Received 9 September 1993; accepted 6 January 1994)
The role of aluminum as dopant in gas phase synthesis of titania powders was experimentally investigated in an aerosol flow reactor between 1300 and 1700 K. Titania was produced by vapor phase oxidation of titanium tetrachloride in the presence of dopant aluminum trichloride vapor. The presence of aluminum altered the particle morphology from polyhedral to irregular crystals. Energy dispersive analysis and transmission electron microscopy indicated that the powders were mixtures of crystalline titania and amorphous alumina. Analysis by XPS indicated significant enrichment of aluminum on the particle surface. Some aluminum titanate (up to 17% by volume) was formed at 1700 K when a high concentration of A1C13 was used (Aia 3 /TiCl 4 3= 0.07). Measurements of lattice parameters by x-ray diffraction indicated that aluminum formed a solid solution in titania. While titania synthesized in the absence of aluminum was about 90% anatase, the introduction of aluminum resulted in pure rutile at AlQ 3 /TiCl4 = 0.07. The effects of aluminum on titania phase composition and morphology are explained by the creation of oxygen vacancies in the titania crystallites and by the enhancement of the sintering rate of titania grains.
I. INTRODUCTION Titania (TiO2) is a ceramic that is extensively used in pigments and to a lesser extent in microelectronics^ waste water purification, inorganic membranes, and catalysis. Titania, especially in its rutile form, is one of the most widely investigated oxides regarding defect structure and physical and chemical properties.1 Commercial titania is primarily manufactured by the "chloride process" where titanium tetrachloride (TiCl4) vapor is oxidized in a flame. Dopants are routinely added during the "chloride" process to control the phase composition and size of the product powders, but the fundamentals of dopant addition have not been understood. Titania is coated with aluminum oxide to improve gloss and resistance to degradation of pigments.2 Doped titania is used in making inorganic membranes for high temperature separations where it is necessary to improve the thermal stability of the membrane.3 Mezey4 reported that the presence of 0.01 to 10% of an aluminum-containing compound in the flame oxidation of TiCl4 led to the formation of rutile. Chen and Wu5 doped titanium dioxide with niobium, lanthanum, and calcium by wet chemical processes and studied the increase in crystallite size as a function of sintering
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to whom correspondence should be addressed. J. Mater. Res., Vol. 9, No. 5, May 1994
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temperature. Mackenzie6 investigated titania crystallite growth and the kinetics of the anatase to rutile transformation in the presence of copper, manganese, iron, and zinc oxides in the solid phase. He concluded that oxygen vacancies promoted crystallite growth and the anatase to rutile transformation. While some o
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