Factors Influencing the Stability of Low Temperature Tetragonal ZrO2

Various factors that influence the appearance of a tetragonal (t-) ZrO2 polymorph at room temperature have been extensively investigated. Several proposed models emphasize the role of anionic impurities (SO42-, OH-), crystallite size (surface energy), structural similarities between the starting material and t-ZrO2, lattice strains, water vapor, lattice defects (oxygen vacancies), etc. Our investigations, focused on the stability of low temperature t-ZrO2, showed that, regardless of the structural differences in the starting zirconium materials, their thermal decomposition products crystallized into a metastable t-ZrO2. The t-ZrO2 -> m-ZrO2 transforma-tion occurred during the cooling or further calcination in the pres-ence of air at atmospheric pressure. On the other hand, if these processes are performed in vacuum, the metastable phase is pre-served. These observations indicate that a metastable t-ZrO2 appears at room temperature as a result of stabilization caused by introduction of oxygen vacancies, similarly as in the solid Solutions with aliovalent cations. A decrease in the specific surface area of ZrO2 grains or the presence of the substances that enter into strong surface interactions with ZrO2 (SO42-, Cr2O3) prevents the diffusion of oxygen from the atmosphere into the ZrO2 lattice and due to this fact the metastable t-ZrO2 is stabilized. On the other hand, lattice strain and grain size of metastable t-ZrO2 could not be clearly related to its stability

Hydrothermal Synthesis of Platinum Group Metal Nanoparticles

A novel route for the synthesis of platinum group metal nanoparticles has been reported. The synthesis is based on the addition of tetramethylammonium hydroxide (TMAH) to the aqueous PtCl(4), IrCl(3) or Rh(NO(3))(3) solution followed by the hydrothermal treatment of these precipitation systems at 160 degrees C. The mean size of nanoparticles was 9.2 nm for platinum, 21 nm for iridium, and 28 nm for rhodium. The average crystallite size was estimated at 7.4 nm for platinum, 3.1 nm for iridium and 3.5 nm for rhodium. The possible mechanism of platinum group metal nanoparticles formation is briefly discussed

Formation of Rust During the Corrosion of Steel in Water and (NH4)2SO4 Solutions

Formation of rust during the corrosion of steel in water and (NH4>2S04 solutions was monitored at 20 °C up to 3 months and at 90 °C up to 3 days. All corrosion products were analyzed by X-ray diffraction and Fourier transform IR spectroscopy. Four oxide phases, lepidocrocite (y-FeOOH), goethite (a-FeOOH), magnetite (Fe3C>4) and hematite (a-Fe203), were found in the corrosion products. Their distribution or absence of some oxide phases in corrosion products were strongly dependent on the experimental conditions. The strong influence of (NH4>2S04 electrolyte on the phase composition of the rust was explained by the cumulative effect of two aggressive ions, NH4 and SO4. Possible pathways for the formation of iron oxide phases in the rust were discussed

Formation of Rust During the Corrosion of Steel in Water and (NH4)2S04 Solutions

Formation of rust during the corrosion of steel in water and (NH4)2S04 solutions was monitored at 20 °C up to 3 months and at 90 °C up to 3 days. All corrosion products were ana1yzed by X-ray diffraction and Fourier transform IR spectrosropy. Four oxide phases, lepidocrocite (γ-FeOOH}, goethite (α-FeOOH), magnetite (Fe204) and hematite (α-Fe203), were found in the corrosion products. Their distribution or absence of some oxide phases in corrosion products were strongly dependent on the experimental conditions

Formation of Oxide Phases in the System Pr-Fe-O

The formation of oxide phases at 900 °C in the system Fe2O3-“Pr2O3” was investigated. With a decrease in the molar fraction of Fe2O3 a corresponding increase in PrFeO3 was observed. For equal molar fractions of Fe2O3 and “Pr2O3” the formation of PrFeO3 and very small fractions of α-Fe2O3 plus an addi¬tional oxide phase, which could not be identified with certainty, were observed. With further increase in “Pr2O3” fraction the praseodymium oxides Pr6O11 and PrO2 started to become dominant in the phase com¬position. The small fraction (< 0.02) of the same unidentified oxide phase was also obtained when Pr(OH)3 was calcined in air at 900 °C; this was probably a mixture of other praseodymium oxides with different average oxidation numbers of Pr. The results of XRD, 57Fe Mössbauer and FT-IR spectroscopies are discussed. (doi: 10.5562/cca2247

Formation of Magneite in Highly Alkaline Media in the Presence of Small Amounts of Ruthenium

The effect of small amounts of ruthenium on the formation of magnetite in highly alkaline media was investigated using X-ray powder diffraction (XRD), Mossbauer and FT-IR spectroscopies, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Acicular alpha-FeOOH particles precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH) were used as a reference material. Initial addition of small amounts of Ru(NO)(NO3)(3) to that precipitation system had a drastic effect on the formation of iron oxide phases and their properties. The addition of Ru(NO)(NO3)3 favoured the formation of stoichiometric Fe3O4. With an increase of the initial Ru(NO)(NO3)3 concentration in the precipitation systems less time was needed for the formation of Fe3O4 as a single Fe-bearing phase in the precipitates. Ruthenium ions made solid solutions alpha-(Fe,Ru)OOH; however, there was no indication of the formation of solid solutions with alpha-Fe2O3 and Fe3O4, Mossbauer and FT-IR spectroscopies supported the conclusion on the formation of solid solutions a-(Fe,Ru)OOH. FE-SEM showed the formation of octahedral Fe3O4 Particles of a mu m range size. Ruthenium particles (approximate to 20 nm in size) were deposited onto the surfaces of Fe3O4 particles. They were also present in the form of clusters containing octahedral Fe3O4 particles in the nanosize range (approximate to 100 nm or less). The formation of Fe3O4 was interpreted as a combining effect of the thermal decomposition products of TMAH under autoclaving conditions and the catalytic action of ruthenium. In such a way strong reductive conditions in the investigated precipitation system were created

Microstructural Analysis of Boehmite Nanoparticles Prepared by Rapid Hydrolysis of Aluminum Sec-butoxide

Microstructural properties of six samples, prepared by rapid hydrolysis of aluminum secbutoxide, were investigated at RT using field emission scanning electron microscopy (FE SEM), X-ray powder diffraction (XRD) and Fourier transform infrared (FT-IR) spectrometry. The results of structural analysis show that, regardless of a significant difference in the processing parameters (pH, temperature, time of synthesis), all products contain boehmite as the only crystal phase present. The results of FE-SEM analysis indicate a significant difference in the morphology of obtained boehmites (plates, needles, granules). The results of line-broadening analysis of powder diffraction patterns (Le Bail method – program GSAS) indicate the presence of very small anisotropic crystal domains (around 1.5 to 7 nm in the direction 010; around 3 to 16 nm in the direction perpendicular to 010). In all cases the parameters that contribute to the strain broadening of diffraction lines decrease to nearly zero, which suggests that the obtained boehmites are almost strain-free. (doi: 10.5562/cca1884

In situ phase analysis of the thermal decomposition products of zirconium salts

X-ray powder diffraction at high temperature was used to determine the phase composition of the thermal decomposition products of two zirconium salts, Zr(SO4)(2). 4H(2)O and ZrO(NO3)(2). 2 H2O, and of a mixture of zirconium nitrates having Zr(OH)(2)(NO3)(2). 4.7 H2O and ZrO(NO3)(2). 2H(2)O as dominant components. Heating of the samples up to 1200 degrees C was performed inside a high-temperature chamber, attached to a diffractometer, at an air pressure of approximate to 2 x 10(-3) Pa. Regardless of the structural differences in the starting salts, thermal decomposition products crystallized to t-ZrO2 which remained stable up to 1200 degrees C. This result indicated that the structural nature of the starting materials was not the most important factor of metastable t-ZrO2 formation. The thermodynamically stable m-ZrO2 appeared after the cooling of the samples to room temperature. If the cooling was performed at low air pressure, the m-ZrO2 content was small. Introduction of air, even at RT, caused a considerable increase of m-ZrO2, which became the dominant phase in all cases. The important role of oxygen in the t-ZrO2 --> m-ZrO2 transition indicates that the lack of oxygen in the zirconia lattice favours the formation of metastable t-ZrO2