P.L. Cignini

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.

Reaction of rhodanese with dithiothreitol

The reaction between bovine rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) and reduced dithiothreitol has been studied. This reagent, in the absence of thiosulfate, reduces the amount of sulfur carried by rhodanese with formation of sulfide and oxidized dithiothreitol: E-S-SH + reduced dithiothreitol replaced by E-SH + HS- + oxidized dithiothreitol, (E = enzyme). An inactivation was observed at high dithiothreitol/enzyme ratios or at very low enzyme concentrations. The inactivation was not observed in the presence of thiosulfate and can be reversed by cyanide or thiosulfate. A thiosulfate reduction activity of rhodanese was also found using dithiothreitol as reductant.

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.

Kinetics of high temperature oxidation of refractory carbides

Abstract Data concerning the interaction with oxygen of refractory carbides such as TaC, TiC, NbC and VC is reported at the oxygen partial pressure of 0.8 Pa in the temperature range from ambient to 400 °C. All the carbides show consumption of oxygen around 350 °C. The amount of oxygen consumed is comparable with the amounts found previously on various forms of carbon in the same experimental conditions. Detailed results on TiC up to 1050 °C and at the oxygen partial pressure ranging from 0.8 to 19 Pa are reported. In particular, it is shown that at the lowest oxygen partial pressure and around 890 °C, oxygen interacts with TiC without producing any gaseous product. This occurs at ( O C ) = (2.7 ± 0.3) × 10 − 2 . At the highest temperatures, the oxidation kinetics was found quite linear with activation energy of 180 ± 10 kJ mol −1 .

Neutron and tritium evidence in the electrolytic reduction of deuterium on palladium electrodes

AbstractA Fleischmann and Pons type experiment was carried out for ∼3 months in a ten-cell electrochemical system. All the cells were connected in series, and electrolysis was performed in galvanostatic mode at a maximum current of 2.5 A, corresponding on the average to 500 mA/cm2. In this experiment, all cathodes were made of palladium, and the anodes were made of platinum. In nine cells out often, the cathodes were shaped into parallelepipeds (25 × 5 × 5 mm3) by high-vacuum sintering according to a previously reported procedure. The starting material for all these electrodes was palladium sponge powder. The tenth cathode was made of 32 short 0.5-mm-diam palladium wires, gold welded together at one end. A similar concentration of screw dislocations was produced in each wire. Three different groups of sintered cathodes were used in the experiment, corresponding to three different sintering procedures. Nine cells contained 0.2 M LiOD in D2O as electrolyte. The tenth cell, containing a sintered cathode, was...

Thermodynamics of intermetallics through emf measurements under effusion conditions

Abstract A general treatment is performed for a complete description of the thermodynamics of galvanic cells Pt Me″ , Me″ z F j CaF 2 s.c. Me h Me′ k , Me l Me′ m ,Me x F y Pt type working under effusion conditions where the vaporization of Me″ is considered. The treatment has been applied to three Me 2 Ni 17 Ni ss systems with Me≡ Eu, Tb, Ho and MehMe′k is the solid solution, Niss, produced by the eutectic reaction L ↔Me 2 Ni 17 s +Ni ss . From the emf vs T experimental data, ΔfH298θ and Δ f S T θ have been calculated. The ΔfH298θ values for Tb2Ni17(s) and Ho2Ni17(s) are, respectively, −16.76±0.06 and −18.8±0.1 kJ/mol atoms. In the same order, the Δ f S T θ values are −1.27±0.06 and −1.81±0.09 JK−1/ mol atoms, respectively, at T=1023 and 1085 K. For Eu2Ni17(s), Δ f H T θ and Δ f S T θ are, respectively, −48±4 kJ /mol atoms and 2.0±0.2 JK−1/mol atoms, both the values at T=910 K. The latter results should be considered unexpected and the reason for this is extensively discussed.

Heterogeneous equilibrium between YBa2Cu3O7-δ and oxygen gas studied by a kinetic approach

The kinetic behaviour of the YBa2CU3O7-δ system has been investigated via a ‘T-jump’ experiment. This is realized by a displacement of the thermodynamic system from equilibrium by a temperature jump. The subsequent relaxation process is followed via resistivity measurements. Kinetic models have been formulated by assuming the formation of singly and doubly charged oxygen vacancies and according to the kinetic law of mass action. A good agreement between the theoretical model and the experimental data has been achieved in the case of singly charged oxygen vacancies in the temperature range 400–800°C. At equilibrium the YBa2Cu3O7-δ conductivity is found to be a function of the oxygen partial pressure, P, according to σ∝Pn with ¼

Role of oxygen supply in high-temperature growth of compact oxide scale

High-temperature oxidation of metals at low oxygen impinging fluxes and low values of oxygen partial pressure were considered on the basis of the fundamental aspects of oxidation kinetics. To do this, the oxidation kinetics of copper to the monovalent oxide was studied under appropriate experimental conditions using apparatus consisting of two solid-state electrochemical cells, both with yttria stabilized zirconia as the solid electrolyte, coupled together. The cells operated as oxygen sensor and oxygen pump, respectively, in such a way that oxygen gas was generated and monitored very close to the surface of the oxidizing sample. The results obtained on copper foil at 1113 K over the oxygen pressure range 1 × 10−14 (highly purified argon) to 1500 Pa show a linear growth of the oxide for exposures up to 2000sec. This was tentatively explained by assuming the oxygen supply to the sample surface to be the rate limiting step, instead of the solid state diffusion into the growing oxide.

Transport processes at high temperatures related to oxygen activity at YSZ surface

Abstract In a coupled system of oxygen concentration cells with yttria stabilized zirconia as electrolyte, the behaviour versus time of their EMFs is given when the common atmosphere is initially constituted by argon with an oxygen partial pressure of 1 × 10 −14 Pa at 1 1 1 8 K. A breakpoint characterizes all the curves but it occurs at different times, depending on the experimental setup, but at the same EMF value. Adsorption of oxygen on porous platinum electrode is assumed to occur and the free energy change of desorption was calculated to be 234 kJ/mol.