Nanocomposites of MgO with Y2O3 have been produced from the respective nitrates by an esterification reaction with ethylene glycol and citric acid. The evolution of nitrous oxides during the reaction causes the product to foam, and the calcination of this foam gives nanocomposite powders with extremely fine, uniform grains, and phase domains. These microstructures are remarkably stable both under postcalcination heat treatment and during consolidation by hot pressing. These stable microstructures arise as a result of the decomposition sequence: this involves the formation of a metastable amorphous/vitreous intermediate followed by concurrent crystallization and phase separation on the nanoscale.
Nanocomposite powders with equal volume fractions of Y 2 O 3 and MgO have been produced by the thermal decomposition of precursor mixtures of yttrium nitrate and magnesium nitrate. Solutions of the precursor salts were mixed with ammonium acetate fuel, dried to form a gel-like substance, and then calcined to give nanocrystalline ceramic powders. The amount of ammonium acetate added to the metal nitrate precursors was varied systematically, and the morphology and distribution of the component phases in consolidated compacts of the resultant ceramic powders were examined by a combination of focused ion beam sectioning, scanning, and transmission electron microscopy. The dispersion of the Y 2 O 3 and MgO phases within the synthesized powders improved, and the sizes of the phase domains reduced, with increasing ammonium acetate content up to the quantity required for a stoichiometric redox reaction with the metal nitrates. The addition of excess ammonium acetate gave no further improvement in phase domain dispersion or reduction in phase domain sizes. These phenomena are related to the thermal characteristics for the decomposition of the precursors and their effect on phase separation during oxide crystallization.M. Cinibulk-contributing editor Manuscript No. 29465.
A combined sol–gel/thermal decomposition process has been used to produce powders of Y2O3‐50 vol% MgO nanocomposites from four different precursor mixtures of acetate and/or nitrate salts. The influence of precursor chemistry on phase homogeneity in the ceramic materials produced from these powders has been investigated. Solutions of the precursor salts were mixed, dried to form a gel, and then calcined to give ceramic powders. The effects of precursor chemistry were investigated using a combination of thermal analysis, X‐ray diffraction, scanning and transmission electron microscopy techniques. All of the calcined powders, and most of the consolidated materials produced from them, consist of nanosized grains of the cubic Y2O3 and MgO phases clustered into phase domains. For the materials synthesized from precursor mixtures containing only acetates or only nitrates, these grains and domains are rather coarse and are distributed rather inhomogeneously. Precursor mixtures containing an acetate and a nitrate, however, give materials with finer and more homogeneous phase domains. These phenomena are related to the thermal characteristics for the decomposition of the precursors and their effect on phase separation during oxide crystallization.
A combined sol–gel/thermal decomposition route has been used to produce powders of MgO–50 vol% ZrO2 nanocomposites and the influence of synthesis variables on the phase homogeneity in ceramic materials produced from these powders has been investigated. The starting precursors were solutions of magnesium nitrate and zirconium acetate, which were mixed, dried to form a gel, and then calcined to give ceramic powders. The effects of urea additions to the precursor mixture and changes in the calcination temperature were investigated using a combination of thermal analysis, X‐ray diffraction, and scanning electron microscopy techniques. Although all of the ceramic materials produced from the powders consist of fine grains of the MgO and ZrO2 phases, in most cases these are clustered into coarse‐phase domains that are distributed rather inhomogeneously. For ceramics synthesized from powders calcined at lower temperatures, and those produced from precursor mixtures containing 2 wt% or more of urea, the phase domains are finer and are distributed much more homogeneously. These observations are explained on the basis of the way in which the phase separation occurs during thermal decomposition of the precursors and crystallization of the oxides.
In solution precursor plasma spray chemical precursor solutions are injected into a standard plasma torch and the final material is formed and deposited in a single step. This process has several attractive features, including the ability to rapidly explore new compositions and to form amorphous and metastable phases from molecularly mixed precursors. Challenges include: (a) moderate deposition rates due to the need to evaporate the precursor solvent, (b) dealing on a case by case basis with precursor characteristics that influence the spray process (viscosity, endothermic and exothermic reactions, the sequence of physical states through which the precursor passes before attaining the final state, etc.). Desirable precursor properties were identified by comparing an effective precursor for yttria-stabilized zirconia with four less effective candidate precursors for MgO:Y 2 O 3 . The critical parameters identified were a lack of major endothermic events during precursor decomposition and highly dense resultant particles.
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