Giorgio Spinolo

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.

Combustion synthesis of Zr–Si intermetallic compounds

Abstract SHS experiments have been performed starting from the elemental compositions corresponding to all the Zr–Si intermetallic compounds. Analysis of the products shows that the Si rich compositions give, as the major product components, ZrSi2 and ZrSi, and the Zr rich composition the high temperature phase Zr5Si3. Melting of silicon has been identified as the triggering step for the combustion process. Once initiated the reaction involves solid (Zr)–liquid (Si) or liquid–liquid interactions. The phase formation is then controlled by kinetic parameters rather than thermodynamic ones. Solid–solid interaction can play an important role in the pre- and post-front regions; these have been studied through diffusion couples experiments.

Combustion synthesis of ZrAl intermetallic compounds

Abstract The combustion syntheses of all the intermetallic compounds in the ZrAl binary system have been investigated. Analyses of the reaction products show that for compositions of the reacting mixture richer than 60 at.% in Al the products always contain a mixture of ZrAl4, ZrAl2 and Zr2Al4, in different proportions. Zr-rich samples always produce a mixture of Zr5Al3, Zr3Al2, ZrAl2 and Zr2Al3. DTA investigations have shown that the exothermic process responsible for the front propagation is represented, for all starting compositions, by the reaction of molten aluminum with solid zirconium to form ZrAl3. When fine powdered reactants are used, solid-state interaction also contributes to the reaction ignition.

SHS (Self-sustained high-temperature synthesis) of intermetallic compounds: effect of process parameters by computer simulation

Abstract The very wide temperature ranges and the many different chemical and phase transformation steps experienced by an SHS process make the search for a unifying theory impractical and a numerical approach highly desirable. The work describes an investigation of the Al+Ni→AlNi SHS by a computer simulation method developed by the Authors and based on a finite difference numerical engine coupled to a detailed description of several assumed reaction steps. The model includes Al melting, Ni diffusion-controlled dissolution (and possibly Ni melting), nucleation and precipitation of AlNi, and eutectic deposition. The results are discussed from the point of view of feasibility of the process, transition from self-propagating to thermal explosion behavior, and influence of process variables such as reactant grain sizes, diffusion coefficient, initial composition, and thermal conductivity.

CHEMICAL MECHANISM OF THE ZR + O2 ZRO2 COMBUSTION SYNTHESIS REACTION

The oxidation of Zr particles under a combustion regime is investigated by discussing separately the macrokinetic aspects (Fouriers law of heat transfer) and the microkinetic aspects (the kinetic laws describing chemical and phase transformations of a solid particle). A very simple model with a single reaction step and two different mechanisms (and different laws) is assumed to describe the microkinetics, while filtration of the gaseous reagent through the porous sample is neglected. The microkinetic variables (conversion degree and temperature) are then identified with the corresponding global variables, and the set of macrokinetic equations is finally solved by numerical methods. The approach is used to investigate how the chemical mechanism is affected by process parameters such as oxygen partial pressure, size of the reacting particles, and dilution degree. It is shown that experimental determinations (such as wave speed, thermal and composition profiles, and their dependence on process parameters) c...

Nature and amount of carriers in Ce doped Nd2CuO4 I. High-temperature characterization

Abstract DC conductivity, ionic transport number, and thermopower have been measured of Nd2−xCexCuO4±δ materials (x = 0, 0.05, 0.10, 0.15, 0.20) at different temperatures [300 ⩽ T ⩽ 900°C] and under different oxygen partial pressures [1 atm ⩽ P(O2) ⩽ 3 × 10−6 atm]. For the most heavily doped samples, the current carriers are electrons essentially produced by the substitutional defect CeNd., while the crystal-gas equilibria involving Oi″ account for a smaller amount of carriers. The behavior of Nd1.95Ce0.05CuO4±δ is intermediate between more doped and undoped materials. When coupled with oxygen non-stoichiometry data, the results indicate that the mobility of the electronic carriers exhibits a transition from small-polaron to large-polaron behavior with increasing doping level.

Extreme undercooling (down to 90K) of liquid metal nanoparticles

Low-melting-point metal nanoparticles show remarkable undercoolings: exceptional values ΔT∕Tm≅0.5 were previously reported for confined Ga droplets in the 100–1000nm range. This was considered a characteristic limiting temperature for Ga. We here give evidence that Ga nanoparticles in the 3–15nm range can be undercooled at least down to 90K (ΔT∕Tm⩾0.7) without crystallization. Since computations and experiments on clusters with <50 atoms show on the contrary the signatures of melting at temperatures well above the bulk value, the result is particularly relevant in stressing the importance of interfacial and finite-size effects on the thermodynamics of the nanoscale.

A two‐color spatial‐scanning pyrometer for the determination of temperature profiles in combustion synthesis reactions

This article describes a new two‐color pyrometer able to obtain multiple temperature reading using charge couple device type sensors. The device was tailored to fit the characteristics of self‐propagating high‐temperature synthesis processes in order to be able to detect temperature profiles and propagation rates of the combustion front. The light intensities at the two working wavelengths and the relative temperature profile are detected in 512 or 1024 points on the sample surface. The time between the acquisition of two profiles can be varied between 100 ms and 10 s. The pyrometer is interfaced with the BUS of a PC and is fully software programmable. The constructive details, procedure for sensors alignment, and temperature calibration are described. Few examples of applications are discussed.

The atomic and electronic structure of cerium substitutional defects in Nd2−xCexCuO4+δ An XAS study

Abstract The local atomic and electronic structure of cerium substitutional defects in Nd 1.80 Ce 0.20 CuO 4+δ was investigated by means of X-ray absorption spectroscopy (EXAFS and XANES of Ce L III edge), as a function of the oxygen partial pressure. It has been found that in all of the experimental conditions investigated (1, 10 −3 and 10 −6 atm), the Ce formal oxidation state should be regarded as IV. This is explained in terms of the bond configuration in this material. The consequences for the electron counting in the CuO 2 planes are discussed. As is shown by EXAFS, the increased charge on the 4e site in the T′ structure mandates a local lattice distortion around cerium: the co-ordination cuboid collapses along the (001) direction. Consequently, the CeO bond length is shorter than the NdO bond length.

Electrical properties in the Bi-rich part of the Bi, Mo/O system

Abstract Mixed [(1− x )BiO 1.5 − x MoO 3 ] oxides in the composition range x = 0 to 0.25 have been prepared by solid state reaction at different temperatures and their electric transport properties have been characterised as a function of temperature and oxygen partial pressure P O 2 . Broad-range solid solutions belonging to the fluorite-related structure of δ-Bi 2 O 3 are formed at high temperatures, and their high conductivity can be partially retained during fast cooling, but it is not reproducible during successive heating runs. Reproducible electrical properties have been obtained as a result of long thermal annealing at 800°C. Materials so prepared show a conductivity maximum of ∼ 10 −3 or 10 −2 δ −1 cm −1 (respectively at 500 or 700°C) for compositions near x = 0.125. For x = 0.125 (Bi/Mo = 7) the total bulk conductivity is independent of oxgen partial pressure in the range 1 ⩽ P O 2 ⩽ 10 −13 atm. Below the latter pressure, an irreversible conductivity jump occurs, due to formation of a reaction layer on the electrode surface.