Daniele Gozzi

Oxidation kinetics of refractory carbides at low oxygen partial pressures

Abstract The interaction kinetics of sintered refractory carbides such as NbC, TiC, VC and ZrC with atmospheres at low oxygen partial pressure is reported from ambient to 1000°C and from 0.8 to 20 Pa at the total pressure of 1.6 bar (rel) in Ar. Depending on the experimental conditions, the isothermal reaction rate was found decreasing or increasing on time. This behaviour has been linked to the parameter Γ , defined as the ratio between the weight change of the sample and the number of moles of oxygen consumed. A decreasing trend of the reaction rate corresponds to a null value of Γ . In this case, no growth of oxide occurs but an increase of atomic disorder on the surface of the carbide is expected. A positive trend of the reaction rate implies Γ ≠0 as well as the growth of an oxide layer driven by a solid state mechanism. The oxidation kinetics was never found parabolic. It is linear at the highest temperatures regardless of the oxygen partial pressure value and it approaches to be parabolic by decreasing the temperature and increasing the oxygen partial pressure.

High temperature reactivity of different forms of Carbon at low oxygen fugacity

The reactivity of non-oriented polycrystalline graphite, highly oriented pyrolitic graphite, glassy carbon, diamond and fullerene C60 has been studied at the oxygen partial pressure < 2 Pa and in the temperature range from room temperature to 1223 K. The common characteristic in their reactivity behaviour was essentially found in the low temperature region (≈ 550K) where a consumption of oxygen is observed which cannot be fully associated to chemisorption but to surface CO intermediates.

A new synthesis route to light lanthanide borides: borothermic reduction of oxides enhanced by electron beam bombardment

Abstract Praseodymium hexaboride, uncontaminated by other boride phases, has been quantitatively synthesized in a short time, at the very high temperatures obtainable by electron beam bombardment. The synthesis is carried out in vacuum (∼5×10 −5 mbar) and, after a short annealing, the product is obtained as pure phase which does not require further purification.

Ru K-edge absorption study on the La1-xCexRu2 system

We have measured high-resolution Ru K-edge x-ray absorption spectra for the intermetallic La1-xCexRu2 system. Multiple-scattering calculations are made to explore the origins of different features observed in the experimental x-ray absorption near-edge structure (XANES) and their variation with Ce doping. The experiments and theoretical analysis demonstrate that the Ce doping in La1-xCexRu2 has a direct influence on the states at the Fermi level and hence the hybridization between the f and d states induced by doping.

Multi-walled carbon nanotubes decorated with titanium nanoparticles: synthesis and characterization

A chemical route to decorate MWCNTs by titanium nanoparticles has been developed. MWCNTs, grown by catalytic decomposition of propane, have been used to prepare intercalated K-MWCNTs, which are then reacted with Ti(IV) isopropoxide. The amount of Ti present in the nanostructure is 8.7 wt%. Several techniques have been used for a detailed characterization of MWCNTs decorated by Ti nanoparticles, such as transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high resolution transmission electron microscopy (HREM) and electron energy loss spectroscopy (EELS) as well as Raman spectroscopy and x-ray diffraction (XRD). Titanium is found to be present as α-Ti(hcp) dispersed in very small particles, the size of the largest being ∼2 nm. The Ti-containing phase appears to be fairly homogeneously distributed in the space between the two sides of a MWCNT. No intercalation of Ti between the CNT walls has been detected.

X-ray, heat excess and 4He in the D/Pd system

Abstract The energy balance between heat excess and 4 He in the gas phase has been found to be reasonably satisfied even if the low levels of 4 He do not give the necessary confidence to state definitely that we are dealing with the fusion of deuterons to give 4 He. In the melted cathode, the data of which are reported, no 4 He was found at the achieved sensitivity. X-ray film, positioned at 50 mm from the cell, roughly gave the image of the cathode through spots. Extended considerations have been made to explain this evidence on the basis of the bundle nature of the cathode. From these considerations, the energy of the radiation and the total energy associated to it have been estimated, as 89 keV and 12 kJ, respectively. This value is ∼ 0.5% of the energy measured by calorimetry in the same interval of time. The highest values of energy and excess power are 8.3 MJ and 10 W, respectively.

Thermodynamics of the YAl–YAl2 system

Abstract The emf method using solid-state galvanic cells based on a CaF 2 single crystal as electrolyte has been used to investigate the thermodynamics of the intermetallic couple YAl–YAl 2 . These represent two out of the five intermetallic compounds shown in the Y–Al phase diagram. Two kinds of cells were studied: − Pt/Y,YF 3 /CaF 2 s.c./YAl,YAl 2 ,YF 3 /Pt + and − Pt/YAl,YAl 2 ,CaAlF 5 ,CaF 2 /CaF 2 s.c./Fe Al, a + ,CaAlF 5 ;CaF 2 /Pt + The cell reactions expected are, respectively, 2YAl (s)=YAl 2 (s)+Y (s) and YAl (s)+Al (l, s)=YAl 2 (s). The standard thermodynamic quantities Δ G °, Δ H ° and Δ S ° have been measured as a function of temperature for both the reactions. The Δ f H ° 298 of YAl has also been derived by selecting from the literature the best value of Δ f H ° 298 for YAl 2 . The value to assign to the formation of YAl have been found equal to: Δ f H ° 298 =−107±6 kJ/mol. Moreover, Δ f S ° 298 =2±5 J/K.mol and Δ f G° T =(−100±5)+(1.8±0.5)10 −2 T kJ/mol have been estimated in the temperature range from 792 to 1007 K.

Thermodynamic study of intermetallic phases in the HfAl system

Abstract The vaporization thermodynamics of the aluminum-rich portion of the HfAl system have been investigated by Knudsen cell-mass spectrometry in the temperature range 1280–1680 K. The aluminum vapor pressures were measured in the two-phase regions in the composition range 50%–75% at. Al, and the enthalpy changes of the decomposition reactions were determined by second-law and third-law methods. Hence, the enthalpies of formation of the intermetallic compounds were derived: HfAl3, −44.7 kJ g−1 atom−1; Hf2Al3, −40.8 kJ g−1 atom−1. The results of the vaporization experiments which yielded a residue not assignable to a definite phase were tentatively submitted to thermodynamic analysis, by introducing the formation of HfAl or Hf5Al4, and their enthalpies of formation were derived.

High-temperature electrical conductivity of FeTiO3 and ilmenite

Resistivity measurements have been carried out on FeTiO3 and the mineral ilmenite to characterize these materials in terms of oxygen exchange in the gas phase. These measurements were obtained in the temperature range 400–1273 K and in the oxygen partial pressure range 0.5–101.3 Pa. The activation energies of conductivity were found in the range 0.10 ± 0.01–0.40 ± 0.01 eV for FeTiO3, and 0.13 ± 0.01–0.21 ± 0.01 eV for ilmenite, independent of the oxygen fugacity. FeTiO3, which behaves as p-type semiconductor, shows a σ∝(Po2)1/n experimental dependence with n=+4, supporting an oxygen-exchange process based mainly on the formation of singly negative metal vacancies. Under the same conditions, the resistivity measurements carried out on ilmenite indicated that the sample conductivity was independent of the oxygen partial pressure. This is to be expected if the concentrations of electrons and electron defects are much larger than those of the ionic defects. In this case, the disorder of electrons is not influenced by small deviations from ideal stoichiometry.

Solid Solutions of Rare Earth Cations in Mesoporous Anatase Beads and Their Performances in Dye-Sensitized Solar Cells.

Solid solutions of the rare earth (RE) cations Pr3+, Nd3+, Sm3+, Gd3+, Er3+ and Yb3+ in anatase TiO2 have been synthesized as mesoporous beads in the concentration range 0.1–0.3% of metal atoms. The solid solutions were have been characterized by XRD, SEM, diffuse reflectance UV-Vis spectroscopy, BET and BJH surface analysis. All the solid solutions possess high specific surface areas, up to more than 100 m2/g. The amount of adsorbed dye in each photoanode has been determined spectrophotometrically. All the samples were tested as photoanodes in dye-sensitized solar cells (DSSCs) using N719 as dye and a nonvolatile, benzonitrile based electrolyte. All the cells were have been tested by conversion efficiency (J–V), quantum efficiency (IPCE), electrochemical impedance spectroscopy (EIS) and dark current measurements. While lighter RE cations (Pr3+, Nd3+) limit the performance of DSSCs compared to pure anatase mesoporous beads, cations from Sm3+ onwards enhance the performance of the devices. A maximum conversion efficiency of 8.7% for Er3+ at a concentration of 0.2% has been achieved. This is a remarkable efficiency value for a DSSC employing N719 dye without co-adsorbents and a nonvolatile electrolyte. For each RE cation the maximum performances are obtained for a concentration of 0.2% metal atoms.