High temperature x‐ray diffraction studies of zirconia thin films prepared by reactive pulsed laser deposition

Zirconium oxide thin films have been deposited on Si (100) substrates at room temperature at an optimized oxygen partial pressure of 3x10^‐2 mbar by reactive pulsed laser deposition. High temperature x‐ray diffraction (HTXRD) studies of the film in the temperature range room temperature‐1473 K revealed that the film contained only monoclinic phase at temperatures ≤ 673 K and both monoclinic and tetragonal phases were present at temperatures ≥ 773 K. The tetragonal phase content was significantly dominating over monoclinic phase with the increase of temperature. The phase evolution was accompanied with the increase in the crystallite size from 20 to 40 nm for the tetragonal phase. The mean thermal expansion coefficients for the tetragonal phase have been found to be 10.58x10^‐6 K^‐1 and 20.92x10^‐6K^‐1 along a and c‐axes, respectively. The mean volume thermal expansion coefficient is 42.34x10^‐6 K^‐1 in the temperature range 773‐1473 K. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of substitution on the thermo-physical properties of LaMnxV1−xO4−δ

LaMn_xV_1−xO_4−δ(0≤x≤1) samples were characterized using thermogravimetry, thermo-dilatometry, high-temperature X-ray diffraction (HTXRD) and temperature-programmed reduction techniques, with an objective to explore the role of substitution on their thermo-physical properties, which may have a direct bearing on their catalytic behavior. Even though the substituted compositions (x<0.8) were of a single phase, their reduction occurred in two steps, a lower temperature step corresponding to Mn⁴⁺→Mn³⁺/Mn²⁺ and another higher temperature one related to V⁵⁺→V³⁺. The dilatometric measurements gave similar values of linear thermal expansion coefficient (α₁) at temperatures up to 600 °C, both for LaVO₄ and substituted samples. A different behavior was, however, observed at higher temperatures, whereas thermal contraction was observed in case of LaVO₄ for measurements at temperatures above 700 °C, the value of α₁ remained almost constant in case of the substituted samples. Furthermore, the HTXRD data revealed expansion in cell volume for all temperatures up to 950 °C, irrespective of the substitution. These results therefore point to a higher degree of sintering in LaVO₄ as compared to Mn-doped samples on heating at temperatures above 700 °C. It is inferred that the resistance to sintering and the lowering of the reduction temperature are both responsible to the higher catalytic activity of the substituted samples and their compositional stability during the repeated cycles of reduction-reoxidation, as reported earlier [Appl. Catal. A 205 (2001) 295].     

Temperature-induced transformation of metavariscite to berlinite

The phase transition of metavariscite into berlinite has been studied by means of different techniques, namely electron probe micro analysis, environmental scanning electron microscopy, high-temperature X-ray diffraction, differential thermal analysis, thermogravimetry, thermoluminescence, and thermo Raman. The application of these techniques indicates that: (i) from room temperature to 150°C metavariscite (monoclinic dimorph AlPO₄ ? 2H₂O) appears as the main phase, (ii) in the range of 100–300°C metavariscite starts to lose the water molecules giving rise to form α-berlinite (trigonal AlPO₄) that is the stable phase up to ca 540°C, and (iii) from 550°C onwards the structure adopts the more stable configuration, tetragonal β-berlinite.     

Thermal activation of typical oxidative dehydrogenation catalyst precursors belonging to the Ni−Mo−O system

NiMoO₄ obtained by calcination of precursors has been shown to be a very effective catalyst for oxidative dehydrogenation of propane into propene. Preparation conditions and thermal decomposition of two precursors have been studied by TG-DTA, HTXRD, FFT-IR, and thermo-desorption coupled to mass spectroscopy in order to determine their composition and to define the best treatment to favour the oxidative dehydrogenation process. The selectivity and activity for propane transformation into propene are very different depending on the nature of the precursor and of the active phases obtained after thermal activation. The more selective high-temperature β phase of NiMoO₄ has been obtained at a lower temperature (500°C) than previously reported (700°C).     

Thermal behaviour of dickite-dimethylsulfoxide intercalation complex

The thermal behaviour of the intercalation complex of a dickite from Tarifa, Spain, with dimethylsulfoxide was studied by high-temperature X-ray diffraction, differential thermal analysis and thermogravimetry, and attenuated total reflectance infrared spectroscopy. The ATR-FTIR study indicated that the heating between room temperature and 75°C produced the elimination of adsorbed molecules. Above this temperature the elimination of intercalated molecules occurs through several stages. Loss of 6.5% of the intercalated DMSO first causes a slight contraction of the basal spacing at 90şC due to a rearrangement of the DMSO molecules in the interlayers positions. This contraction is followed by the formation of a single layer complex and the restoring of the dickite structure, at 300°C, when the loss of intercalated species have been completed. This revised version was published online in July 2006 with corrections to the Cover Date.