Sol-gel chemistry of transition metal oxides

In the previous decade, a considerable amount of work has been done on the alumina-zirconia ceramic composite system with a particular emphasis on improving the mechanical properties utilizing the recognized toughening mechanisms. Zirconia-toughened aluminas (ZTA) can be regarded as a new generation of toughened ceramics; for example, a toughness of >12 MPa m0.5 has been obtained, compared with 3 MPa m0.5 for commercial alumina ceramics. The fracture strength of ZTA is also greatly in excess of that for alumina. The mechanical properties of ZTA are critically dependent on their microstructures, which can be designed in terms of specific applications and controlled by means of powder preparation and densification processes. This review also includes details of the possible future development of ZTA; these are expected to involve the development and measurement of the mechanical properties for high-temperature engineering applications.

Zirconia-toughened alumina (ZTA) ceramics

In situ polymerization of tetraethoxysilane in poly(methyl methacrylate): morphology and dynamic mechanical properties

Precursors used for the synthesis of oxide systems by the sol-gel method are reviewed and their role in the various stages of the process is discussed. Emphasis is given to alkoxide precursors and to their physical and chemical properties. In particular, the following topics are discussed: degree of oligomerization, volatility, viscosity, reactions with alcohol, molecular association between alkoxides, hydrolysis, stabilization against hydrolysis. Some information about preparation methods, commercial products and availability is also given. Among non-alkoxide precursors, nitrates, carboxylates, acetylacetonates, chlorides and other inorganics are described from the point of view of their use in sol-gel preparations. They are compared, when possible, with the corresponding alkoxides.

Precursors for sol-gel preparations

Chelating agent, such as acetylacetone (AcAcH) and ethylacetoacetate (EAA) are used to avoid the undersirable precipitation of gelatinuous aluminum hydroxides during hydrolysis of aluminum-sec-butoxides, Al(OBus). This procedure leads to alumina sols with particle size distributions in the range of 1 to 15 nm which depend on the type of chelating agent as well as on the molar Al(OBus)3/chelating agent ratio. Generally, EAA leads to particles smaller than AcAcH. Because of the small particle size, gel powders are X-ray amorphous and do not crystallize at temperature < 750°C. Crystallization of α-alumina starts between 950 and 1000°C. Complete transformation to α-alumina is achieved between 1050 and 1100°C. Coating experiments with different sols on silica glass lead to nearly crack-free dried gel films with AcAcH system. An ultrafine, dense microstructure is obtained.

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Synthesis of an alumina coating from chelated aluminium alkoxides

Transparent non-crystalline stoichiometric magnesium aluminate gel-monolith was prepared by chemical polymerization reaction between aluminium alkoxide and magnesium salt. The reaction mechanisms of gel formation are discussed. The ovendried gel was freed from residual organics by chemical treatment with hydrogen peroxide solution. X-ray analysis of organic-free gel powder after heat treatment for 4h at 2000‡ C did not show any indication of phase separation. It is suggested that the use of sol-gel-derived powders could largely eliminate most of the problems associated with the conventional method of fabrication of optically transparent stoichiometric magnesium aluminate spinel.

Preparation of transparent non-crystalline stoichiometric magnesium aluminate gel-monolith by the sol-gel process

80Al2O3 · 20ZrO2 (wt%) powders were synthesized by three different chemical processes: (1) chemical polymerization, (2) destabilization of mixed sols, and (3) coprecipitation. The elementary particles produced by each of these processes were of the order of 1.5 to 3.0 nm and were amorphous to electron diffraction. The powders woe evaluated in terms of (a) weight loss (thermal gravimetric analysis, TGA) and thermal (differential thermal analysis, DTA) characteristics, (b) surface area (BET), pore size distribution, and density at different temperatures; and (c) the crystallization behaviour (XRD method) which showed that the physiochemical characteristics and crystallization behaviour of these chemically derived powders are significantly influenced by the chemistry of powder synthesis. Moreover, the powders produced by the chemical polymerization and the colloidal processes retained the tetragonal phase of zirconia completely during cooling from 1600° C, but significant transformation of tetragonal to monoclinic zirconia occurred in the coprecipitated powder under identical conditions. Furthermore, the 1550° C co!cined powder derived by the chemical polymerization process retained a substantial amount of tetragonal zirconia after annealing at 1000° C for 72h. The average alumina and zirconia crystal sizes of the calcined powders, calculated by the Scherrer formula, were in the 20 to 80 nm range.

Influence of synthesis chemistry on alumina-zirconia powder characteristics

Transparent barium hexa-aluminate gel was synthesized by chemical polymerization reactions involving commercial aluminium di(isopropoxide) acetoacetic ester and barium acetate. The gel was evaluated in terms of weight loss and thermal and crystallization characteristics by thermogravimetric analysis, differential thermal analysis, transmission electron microscope and X-ray diffraction. The as-prepared gel was amorphous to electron diffraction. X-ray diffraction results showed that fully crystalline barium hexa-aluminate can be produced at ≤1200°C which is 150°C less than the minimum synthesis temperature reported in the literature. The crystallization kinetics of the gel are discussed on the basis of the X-ray diffraction data. Furthermore, preliminary studies were conducted to examine the sintering behaviour of isostatically pressed pellets fabricated from 800°C heat-treated and milled gel powder. The sintered pellets were characterized in terms of density, microstructure, and linear thermal expansion. Sinterability of the material at 1550°C was unsatisfactory. The sintered density of the pellets was less than 60%. Microstructural examinations showed that sintered pellets were highly porous and consisted of a mixture of round, hexagonal plate-like and needle-like particles, the relative proportion of each type being dependent on the heat-treatment schedule. The linear expansion coefficient measured on a 1550°C sintered specimen was 7.3×10−6 in the 15 to 1000°C range.

J. C. Debsikdar

Papers

Preparation of transparent non-crystalline stoichiometric magnesium aluminate gel-monolith by the sol-gel process

Influence of synthesis chemistry on alumina-zirconia powder characteristics

Synthesis and characterization of gel-derived barium hexa-aluminate