Gaetano Chiodelli

Short range order in the network of the borophosphate glasses: Raman results

This paper presents a Raman investigation of glasses of the system MX:M 2 O:B 2 O 3 :P 2 O 5 (M = Li, Ag; X = Cl, 1). For the phosphate glasses, we discuss in detail the relationship among glass composition, density of the different phosphate (PO 4 ) units, and Raman spectra. For the borophosphate glasses, we analyze how the oxygens introduced with the modifier oxide M 2 O are shared among borate and phosphate units. When P 2 O 5 is partially substituted by boron oxide, and the M 2 O content remains constant, the negative charge of the phosphate units increases, on average. Comparison with the results of a companion 31 P NMR-MAS study allows identification of bands associated with structures containing the P-O-B motives. A discussion is also given of the changes induced into the borophosphate glass network by addition of a “doping” MX salt.

Electrical properties of Ni / YSZ cermets obtained through combustion synthesis

Abstract The synthesis of Ni/YSZ cermet with controlled microstructural characteristics presents a lot of interest for many solid-state electrochemical applications. These materials are generally obtained by reducing poorly sintered mixtures of YSZ with nickel oxide by hydrogen. We recently proposed an alternative route based on a thermite reaction performed in a combustion regime. This method makes it possible, in one step, to synthesize highly porous cermet, to sinter the ceramic fraction, and to dope it with yttria. Such an approach represents an application of combustion synthesis or the self-propagating high-temperature synthesis (SHS) technique. We present here electrical characteristics of Ni/YSZ cermets produced by this method. The total electronic conductivity shows an unusually smooth percolation threshold. This fact makes it possible to obtain a fairly high electronic conductivity even at relatively low metal volume fractions. The ceramic fraction shows an excellent degree of sintering and an ionic conductivity not too different from the conductivity of YSZ single crystals with the same composition.

Silver borophosphate glasses : Ion transport, thermal stability and electrochemical behaviour

Abstract An experimental study of the undoped and AgI-doped borophosphate glasses is presented which includes discussion of the glass forming properties, thermal and redox stability, ionic conductivities and transport numbers for the “quaternary” system AgI : Ag 2 O : B 2 O 3 : P 2 O 5 . For glasses without AgI and with the same Ag 2 O molar fraction, the conductivity at room temperature (rt) may be a highly non-linear function of the B/P ratio. By appropriately mixing boron and phosphorus oxides, undoped glasses can be obtained with better rt conductivities and thermal stability than corresponding glasses made with a single glass former (mixed anion effect). The electrochemical behaviour of several undoped and AgI doped glasses has been characterized by triangular voltammetry techniques.

Short range order and glass transition in AgIAg2OB2O3 vitreous electrolytes

Abstract Several experimental techniques are used to study the short range order, the dynamics and the glass transition in AgIAg2B2O3 compounds. Addition of Ag2O to B2O3, up to [Ag2O]/[B2O3] ⩽0.5 modifies the borate network by creating a BO4 unit for each silver added. Addition of AgI decreases the glass transition temperature (Tg) but has only minor effects on the short range structure of the borate network. Silver iodide is partially accomodated in the interstices of the glass network. The relationship among a tentative structural picture, the ion transport phenomena and the low temperature dynamics are discussed. An investigation of the dynamics in the AgI·Ag2O·2B2O3 glass near and above Tg is presented. With NMR techniques, we monitor the onset of tumbling of the borate units and the dynamical effects of crystallization and/or aging of the glass. Hysteresis effects in the ionic conductivity (σ) temperature dependence and the non-Arrhenian behavior of σT near Tg are interpreted in terms of structural modifications occurring at elevated temperatures in the glass.

Electrical properties of the ZrO2-CeO2 system

Abstract The electrical properties of ZrO 2 -CeO 2 solid solutions are investigated in detail as functions of the temperature between 200 and 1000 °C and of the ceria molar fraction between 16 and 100% (pure CeO 2 ). The samples are studied by combining impedance spectroscopy, ionic transport number and thermoelectric power measurements so as to define the role and the contribution of hole and electron carriers to the conductivity mechanism. In particular, the transition from p- to n-type conductivity of undoped CeO 2 at 750 °C is clearly demonstrated. The conductivity dependence on the oxygen partial pressure is also studied in the ZrO 2 -80 mol% CeO 2 composition that exhibits the highest electronic conductivity.

Lithium borophosphate vitreous electrolytes

Abstract This paper investigates the ionic (Li+) conductivity and thermal stability properties of glasses of the Li2O/B2O3/P2 O5 system. For the same molar ratio between lithium oxide and glass formers (B2O3 + P2O5), the ionic conductivities at room temperature can be substantially higher in borophosphate glasses than in pure borates or phosphates. An attempt is made to relate this technologically interesting conductivity enhancement to the coordination chemistry of borophosphate glasses.

Formation of high conductivity glasses in the system AgIAg2OB2O3

Glass phases showing high ionic conductivity at room temperature were prepared through a rapid quenching of the molten mixtures of the system AgIAg2OB2O3 (a fixed Ag2O/B2O3 = 1 molar ratio was always considered): the obtained specimens were homogeneous and transparent cylindrical blocks. Disk shaped cells prepared with such specimens did not show any grain-grain effect as supported by the comparison between four electrodes dc and ac conductivity determinations. A less than 10−9(ohm cm)−1 electronic conductivity was found. According to X-rays diffraction and DTA investigations, XAgI > 0.8 samples contained crystallized AgI, whereas 0.1 < XAgI < 0.8 samples could be considered actual vitreous homogeneous phases. AgI contents lower than 10 mole% were not considered due to the observed presence of segregated metallic silver. Room temperature density and conductivity data showed a regular behaviour vs XAgI in the vitreous phases range, whereas an evident discontinuity was observed about XAgI = 0.8. Linear fits in the Arrhenius plots of the bulk conductivity were obtained in the 120 K−Tg (glass transition temperature) range: the corresponding activation energies, as well as the high room temperature conductivities, allowed to closely compare these vetrous phases with the so called “superionic” conductors.

Role of oxygen content on the transport and magnetic properties of La1−xCaxMnO3+δ manganites

Abstract In this paper we report about the synthesis and X-ray powder diffraction characterization of La 1− x Ca x MnO 3+ δ samples with x =0.1, 0.3 and −0.025≤ δ ≤0.054. Transport and magnetization measurements on these samples were performed as a function of the oxygen content both in over and under-stoichiometric regimes. We point out the role of the cation and anion vacancies and the obtained results suggest taking in relevant account the role of oxygen content when dealing with this kind of materials.

Effect of alkaline-doping on the properties of La2Mo2O9 fast oxygen ion conductor

Synthesis by means of a modified Pechini method of pure and alkaline-doped (Na, K and Rb) La2Mo2O9 (LAMOX) is presented. The electrical conductivity of LAMOX prepared by this method is completely analogous to that of the solid state prepared material. Doping with K and Rb hinders the α → β phase transition found in the pure and Na-doped material around 580 °C. Electrical measurements as a function of pO2 shows that LAMOX has a mainly ionic conductivity down to 10−18 atm. The electrical conductivity of the doped samples is close to that of the pure molybdate for temperatures higher than about 600 °C, while for K- and Rb-doped samples the conductivity is higher for lower temperatures, as a consequence of the absence of the phase transition. Finally, a non-Arrhenius behaviour is observed for all the investigated samples and correlated, at a first approximation, to the structural peculiarities of the LAMOX family.

Electrical properties of plasma-sprayed yttria-stabilized zirconia films

Yttria-stabilized zirconia films, 100 to 200 μm thick, were prepared by plasma spraying. The electrical properties were investigated by complex impedance spectroscopy. The results are compared with those obtained on sintered pellets prepared with the same powders used for spraying and on commercial single crystals. The ionic conductivity and the activation energies of sprayed films and single crystals are found to be very similar, and no grain-boundary effect is observed in the film complex impedance plots. These results are explained by the high density and purity of the sprayed films.