Study of the thermal behaviour of hydroxide mixtures aimed at the preparation of Mg-Al spinels

The thermal behaviour of hydroxide mixtures aimed at Mg-Al spinel preparation is reported. The mixtures of hydroxides were prepared by precipitation reaction from Mg and Al nitrate solutions, stoichiometric to the spinel formation. Hydroxide mixtures with different phase distributions were investigated, prepared by varying the precipitation procedure. The results were related to the thermal behaviour of mechanical mixtures of separately precipitated hydroxides. The spinel formation was identified performing XRD analysis on powder samples heated at different temperatures. The coprecipitated mixtures are completely decomposed to spinel at 400C. The presence of the Mg-Al mixed hydroxide phase in the mixture is of primary concern to get spinel at low temperatures.

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Zusammenfassung Im Hinblick auf die PrÄparation von Mg-Al Spinell wird das thermische Verhalten von Hydroxid-Mischungen beschrieben. Letztere wurden durch FÄllung von Mg- und Al-Nitratlösungen im stöchiometrischen VerhÄltnis der Spinellbildung hergestellt. Durch Variation des FÄllungsvorgangs wurden Hydroxid-Mischungen unterschiedlicher Phasenzusammensetzung erzeugt. Die Resultate werden mit dem thermischen Verhalten von mechanischen Mischungen separat gefÄllter Hydroxide verglichen. Die Identifizierung der Spinellbildung erfolgte durch Röntgendiffraktion an pulverförmigen, auf verschiedene Temperaturen aufgeheizten Proben. Die durch simultane FÄllung erhaltenen Mischungen wurden bei 400C vollstÄndig zu Spinell zersetzt. Um bei tiefer Temperatur Spinell zu erhalten, kommt dem Vorhandensein der Mg-Al-Mischhydroxidphase in der Mischung vorrangige Bedeutung zu.

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This work was supported by the Italian National Research Council (C.N.R.), under the Targeted Project Special Materials for Advanced Technologies.     

The infrared spectrum of C2F5Cl in cryogenic matrices

We have measured the infrared spectrum of pentafluoroethyl chloride in cryogenic matrices of argon, nitrogen, krypton, and xenon from 880 to 1360 cm^?1. Numerous combination bands were observed; assignments and symmetries are reported for most. Appreciably more structure was observed in argon than in other matrices. The observed splitting of the fundamental bands in an argon matrix into two or more components may be due to multiple trapping sites.     

Infrared spectroscopic evidence of the Mg3O3 molecule formed by reaction of magnesium and oxygen atoms in inert gas matrices at 20.4°K

The reaction of magnesium and oxygen atoms in argon, krypton, xenon and oxygen matrices at 20.4° K was studied by infrared spectroscopy between 200 and 4000 cm^?1. The appearance of a band in the magnesium-oxygen stretching region, reproducible in all the matrices, indicates that reaction takes place. Oxygen-13 isotopic substitution suggests that the absorption is due to Mg₃O₃, whose formation is supported by the crystal structure of the solid phase. The force constants are calculated as f_R = 2.3 mdyne/Å and F_?/R² = 0.44 mdyne/Å assuming a planar six-membered ring of alternate magnesium and oxygen atoms with bond angles O-Mg-O 100° and Mg-O-Mg 140°.     

Consistent derivation of the constitutive algorithm for plane stress isotropic plasticity. Part II: Computational issues

The implementation of the projected algorithm and of the consistent tangent tensor for general isotropic three-invariant elastoplastic models under plane stress conditions discussed in Part I of this paper [Valoroso, N., Rosati, L., 2008. Consistent derivation of the constitutive algorithm for plane stress isotropic plasticity. Part I: Theoretical formulation. International Journal of Solids and Structures, doi: 10.1016/j.ijsolstr.2008.08.012.] is addressed. The connections between the general three-dimensional case and the plane stress problem are analyzed in detail and an algorithmic treatment taking full advantage of the isotropic properties of the model is presented. In particular, intrinsic (matrix-free) expressions are provided for all steps of the stress computation scheme that allow one to carry out the numerical implementation in a way that is completely independent from the matrix representations. The numerical performances of the present solution scheme are evaluated through representative numerical examples.     

Vibrational spectra and rotational barriers of methyl titanium halides CH3TiX3 and CD3TiX3 (X = Cl,Br, I)

The IR and Raman spectra of gaseous and solid CH₃TiX₃ and CD₃TiX₃ species (X = Cl, Br, I) are reported. The gas phase spectra have been recorded between 4000 and 20 cm^?1 at pressures of 1 atm and 4 atm at 350 K and the Raman spectra of the solid phase recorded at 4.2 K. Internal rotation barriers and thermodynamic functions have been calculated.     

Thermodynamic study of the vaporization of uracil

The vapour pressure of uracil was measured in the temperature range 452–587 K using different techniques and the pressure—temperature equation log P(kPa) = 12.13 ± 0.50 — (6823 ± 210)/T was derived. The thermodynamic functions of gaseous and solid uracil were also evaluated through spectroscopic and calorimetric measurements. The sublimation enthalpy of uracil, ΔH⁰₂₉₈ = 131 ± 5 kJ mole^?1, was derived from second and third law treatment of the vapour data.     

Study of the organic residue from a 2600-year old Etruscan plumpekanne

Excavation of the Etruscan Necropolis dell'Osteria near Vulci (VI Century b.c.) brought to light in Tomb 2 a bronze plumpekanne. According to preliminary X-ray observation, an unusually thick deposit of organic nature lay at its bottom. Numerous samples of the residue were investigated using microFTIR and diffuse reflectance FTIR spectroscopy. For comparative purposes similar analyses were performed on samples of resins mentioned in classical sources. Spectroscopic results identify the residue as mastic or incense, both triterpenic resins. The former seems to be preferred since, according to classical sources, it was used as must additive.     

Far IR and Raman spectra of some gaseous titanium,zirconium and hafnium cyclopentadienyl chlorides

The far IR and Raman spectra of gaseous (C₅H₅)TiCl₃ and (C₅H₅)₂MCl₂ species (M = Ti, Zr and Hf) are reported. The results are compared to the previous vibrational analysis of the corresponding species in the solid and matrix isolated phases. The assignment of the metal skeletal vibrations is reexamined in further detail on the basis of the new spectroscopic measurements. The torsional frequencies and the related potential barriers are investigated.