Phase Relations and Behavior of Carbon-Containing Impurities in Ceramics Prepared from Mechanically Activated Ln2O3+2HfO2 (Ln = Nd, Dy) Mixtures

Ln(2)O(3) + HfO2 (Ln = Nd, Dy) powders and ceramics have been studied in an oxidizing (O-2) and a mild reducing (He) atmosphere using differential scanning calorimetry (DSC), thermogravimetry, mass spectrometric analysis of released gases, X-ray diffraction, IR spectroscopy, and Raman spectroscopy. The results demonstrate that both a mechanically activated oxide mixture of appropriate composition and the powders and ceramics prepared by heat-treating the mixture contain carbon-containing compounds (basic rare-earth carbonates and hydroxycarbonates) and/or at least 0.2-0.5 wt % carbon (X-ray amorphous or crystalline). As a result, during heating in an oxidizing atmosphere all of the samples release CO2 in the same temperature ranges (250-600 and 750-1200 degrees C), which is accompanied by exothermic peaks in their DSC curves. The CO2 release in the range 250-600 degrees C is due to the onset of decomposition of the basic rare-earth carbonates and hydroxycarbonates, which are present in small amounts in the starting mixture, powders, and ceramics. The CO2 release in the range 750-1200 degrees C is due to the burnout of strongly bonded carbon and thermally stable carbon-containing compounds (rare-earth dioxymonocarbonates, Ln(2)O(2)CO(3)). The exothermic peaks in the DSC curve are due to fluorite LnHfO(4 - delta) (Ln = Nd, Dy) crystallization processes. We believe that synthesis in air, involving the formation of X-ray amorphous (fine-particle and nanocrystalline) precursors containing rare-earth oxides, which tend to form basic rare-earth carbonates and hydroxycarbonates in air, will always yield high-temperature ceramics containing carbon compounds and at least 0.5 wt % X-ray amorphous carbon and/or graphite. The amount of carbon and carbon-containing compounds in the dysprosium-containing ceramics is markedly smaller (similar to 0.2%) than that in the neodymium-containing ceramics. The crystallization of the rare-earth hafnates is a rather slow process that can begin at temperatures as low as 550 degrees C. The formation of Nd2Hf2O7 with the pyrochlore structure involves fluorite NdHfO4 - delta formation as an intermediate step, and a single-phase product can only be obtained by high-temperature firing at similar to 1600 degrees C. Phase-pure DyHfO4 - delta with the fluorite structure can be obtained by firing at 1200 degrees C.