Umberto Anselmi-Tamburini

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.

ELECTROMIGRATION EFFECTS IN Al-Au MULTILAYERS

Department of Physical Chemistry, Universityof Pavia, Pavia, Italy(Received August 14, 2000)(Accepted in revised form October 24, 2000)Keywords: Electromigration; Gold-aluminum; Multilayers; Intermetallic; Compound formation1. IntroductionThe use of electric and magnetic fields in the processing of materials has been investigated for decadesand has become the focus of renewed attention in recent years (1). In recent studies it was found thatthe imposition of an electric field plays a dominant role in self-propagating combustion reactions (2).The field was found to influence the dynamics of wave propagation, the kinetics of the reaction, andthe mechanism of phase formation. For intermetallic systems the field was found to influence the natureof the phases in the product of the reaction (3–6). However, under conditions of self-sustaining reactionwaves it is not possible to readily isolate any intrinsic effects of the field from the thermal effectsbrought about by Joule heating. While Joule heat is the main effect produced by the passage of a currentthrough metals, other effects may play a role in phase formation and transformation. Among these isthe well-documented effect of a current on mass transport, i.e., electromigration. In this paper we reportthe results of electromigration investigation in the system Al-Au.Solid–solid diffusion couple reactions were chosen for this investigation. The simple geometry of thesamples and the relatively long reaction time of the experiments were found to provide optimumconditions to study the effect of the electric current independent from the thermal effect. Althoughinvestigations on diffusion couples have been numerous, only a few have recently been made to assessthe role of the current in mass transport. An electric current passing through the sample can provide anadded driving force to the chemical potential for diffusion, such that the total flux is nowJ

Colloidal synthesis and thermoelectric properties of Cu2SnSe3 nanocrystals

Copper-based selenides are attracting increasing interest due to their outstanding optoelectronic and thermoelectric properties. Herein a novel colloidal synthetic route to prepare Cu2SnSe3 nanocrystals with controlled size, shape and composition is presented. The high yield of the developed procedure allowed its up-scaling to the production of grams of colloidal Cu2SnSe3 nanocrystals. These nanocrystals were used as building blocks for the production of Cu2SnSe3 bulk nanostructured materials by spark plasma sintering. The thermoelectric properties of the prepared nanocrystalline Cu2SnSe3 pellets were characterized in the temperature range from 300 to 720 K. The obtained results show the bottom-up production of nanocrystalline materials from solution-processed nanocrystals to be a potentially advantageous alternative to conventional methods of production of efficient thermoelectric materials.

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.

Reactive growth of niobium silicides in bulk diffusion couples

Abstract The diffusion-controlled growth of niobium silicides (NbSi 2 and Nb 5 Si 3 ) was studied in Nb/Si and Nb/NbSi 2 bulk diffusion couples annealed at 1200–1350 °C for 2–24 h. Both compounds were found to grow as parallel layers, according to the parabolic rate law. The concept of the integrated diffusion coefficient is used to describe the growth kinetics of the two silicides. The corresponding activation energy is 263 kJ/mol for Nb 5 Si 3 and 304 kJ/mol for NbSi 2 . The activation energy (in eV) scales as 0.98 T m (K)/1000 for Nb 5 Si 3 and as 1.4 T m (K)/1000 for NbSi 2 in agreement with the general behavior observed for many transition metal silicides. The position of the Kirkendall plane inside the Nb 5 Si 3 layer developed in Nb/NbSi 2 couples indicates that, in the present temperature range, the diffusion of Si in Nb 5 Si 3 is considerably faster than that of Nb.

Reactive diffusion in the system vanadium–silicon

Abstract The diffusion-controlled growth of vanadium silicides (V 3 Si, V 5 Si 3 , V 6 Si 5 , VSi 2 ) was studied on bulk V–Si diffusion couples annealed for 2–36 h at 1150–1390°C. The layer growth kinetics was parabolic for all of the silicides. Only at 1150 and 1200°C was an induction period observed before the formation of a continuous V 6 Si 5 layer. The parabolic growth constants of the II kind for the exclusive growth of each silicide from the adjacent phases were calculated from the parabolic constants of the I kind measured on the V–Si diffusion couples. The rate constants of the II kind were in turn related to the diffusion properties of the silicides. As a result, the interdiffusion coefficient, taking into account the diffusion of both elements, was obtained for each phase. The resulting activation energies were 240 kJ mol −1 for V 3 Si, 250 kJ mol −1 for V 5 Si 3 and 190 kJ mol −1 for VSi 2 . The activation energies scale well with the melting point of the compounds. For V 6 Si 5 , the activation energy is strongly dependent on the set of thermodynamic data used in the calculation owing to the uncertainty in the decomposition temperature of this phase.

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.

Combustion synthesis of mechanically activated powders in the Nb–Si system

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of niobium silicides was investigated. SHS experiments were performed on reactant powder blends of composition Nb:Si = 1:2 and Nb:Si = 5:3 pretreated for selected milling times. A self-sustaining reaction could be initiated when a sufficiently long milling time was employed. At short milling times, the reactions self-extinguished or propagated in an unsteady mode. Combustion peak temperature, wave velocity, and product composition were markedly influenced by the length of the milling treatment. Single-phase products could be obtained for sufficiently long milling times. Observation of microstructural evolution in quenched reactions together with isothermal experiments allowed clarification of the mechanism of the combustion process and the role played by the mechanical activation of the reactants.

On the diffusional growth of compounds with narrow homogeneity range in multiphase binary systems

Abstract A general analysis of the problem of diffusional growth of n compounds with narrow homogeneity range in a binary system is presented. Both constituents are assumed to be mobile. The end members of the diffusion couple can be any combination of pure elements (when terminal solubility is negligible), saturated terminal solid solutions and binary compounds. The kinetic equations follow from the coupling between chemical reactions at phase boundaries and partitioning of the diffusion flux between two adjacent layers. Different kinds of parabolic rate constant are used to describe the growth of compound layers in various conditions and the relationships between these quantities are established. In particular, the rate constant of the second kind for the exclusive growth of a given compound is a function of the n rate constants of the first kind measured on the complete couple where all the intermediate phases grow simultaneously. The rate constant of the second kind is related to the diffusion properties and the thermodynamic stability of the phase. The equivalence between the present approach and the purely diffusional model of Wagner is shown. Most of the models proposed in the literature can be obtained as special cases of the present analysis.