Javier E. Garay

Enhanced growth of intermetallic phases in the Ni–Ti system by current effects

Abstract The effect of direct current upon interfacial reactions in the Ni–Ti system was investigated. Isothermal diffusion couple experiments were conducted under varying current densities to de-couple Joule heating from intrinsic effects of the current flux. Current densities of up to 2546 A cm−2 were used in the temperature range of 625–850 °C. All of the intermetallic compounds (NiTi, Ni3Ti and NiTi2) present in the equilibrium phase diagram were identified in the product layer. In addition, β-Ti solid solutions formed in samples annealed above the α→β temperature, 765 °C. The growth of all product layers was found to be parabolic and the applied current was found to significantly increase the growth rate of the intermetallic layers. Using Wagner’s analysis the present results were compared to published results on current-free diffusion couples. The intrinsic growth rate constant of the NiTi2 intermetallic was found to be 43 times higher under the influence of 2546 A cm−2 than that obtained without a current at 650 °C. The effective activation energy for the formation of all phases was found to decrease with increasing current density. The effect was strong for all phases but the decrease was most marked for Ni3Ti. In this case, the activation energy decreased from 292 kJ mol−1 under the influence of a current density of 1527 A cm−2 to 86 kJ mol−1 when the current density was 2036 A cm−2. The results are explained in terms of current induced changes in the growth mechanism arising from changes in the concentration of point defects or their mobility.

Electric current enhanced defect mobility in Ni3Ti intermetallics

The effect of the application of a dc current on the annealing of point defects in Ni3Ti was investigated by positron annihilation spectroscopy. An increased defect annealing rate was observed under the influence of the current and was attributed to a 24% decrease in the activation energy of mobility. The results are interpreted in terms of the electron wind effect and the complex nature of diffusion in ordered intermetallic phases. They provide direct evidence for an increase in defect mobility in ordered intermetallics under the influence of a current.The effect of the application of a dc current on the annealing of point defects in Ni3Ti was investigated by positron annihilation spectroscopy. An increased defect annealing rate was observed under the influence of the current and was attributed to a 24% decrease in the activation energy of mobility. The results are interpreted in terms of the electron wind effect and the complex nature of diffusion in ordered intermetallic phases. They provide direct evidence for an increase in defect mobility in ordered intermetallics under the influence of a current.

Electrical properties of nanoceramics reinforced with ropes of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) were used to convert insulating nanoceramics to metallically conductive composites. Dense SWCNT/Al2O3 nanocomposites with CNT contents ranging from 5.7 to 15 volu200a% and with nanocrystalline alumina matrices have been fabricated by spark-plasma-sintering that retains the integrity of SWCNT in the matrix. The conductivity of these composites increases with increasing content of CNTs. The conductivity has been increased to 3345 S/m in the 15 volu200a% SWCNT/Al2O3 nanocomposite at room temperature. This is an increase of 13 orders of magnitude over pure alumina and of more than 735% over previously reported results in CNT–ceramic composites.

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

Alumina-based nanocomposites consolidated by spark plasma sintering

Abstract Spark plasma sintering is a new process by which ceramics and composites can be consolidated very rapidly to full density. In the present study, piezoelectric Nd 2 Ti 2 O 7 second phase toughening nanocrystalline alumina composites with higher toughness were successfully developed at relatively low temperatures through this technique.

Spark-plasma-sintered BaTiO3/Al2O3 nanocomposites

Abstract Using spark plasma sintering (SPS), BaTiO3/Al2O3 nanocomposites were successfully consolidated to more than 99% of theoretical density at a sintering temperature as low as 1150xa0°C in only 3 min. The processing methods for these dense nanocomposites where the retained grain size of alumina matrix was in the nanometer level were developed. The maximum volume content of BaTiO3 in the nanocrystalline matrix for toughening was around 15 vol.%. A significant increase in fracture toughness up to 5.36 MPa1/2 has been achieved in the 7.5 vol.% BaTiO3/Al2O3 nanocomposite. The toughening mechanism might be related to ferroelastic domain switching of ferroelectric phase in these nanocomposites.

Effect of phase transformation during high energy milling on field activated synthesis of dense MoSi2

Abstract The effect of mechanical activation through high-energy ball milling of reactant powders on the subsequent synthesis of MoSi2 by field activation was investigated. Field activated synthesis of this silicide was made through the use of the spark plasma sintering (SPS) method using a pulsed DC current. Milling (0–6 h) produced significant changes in the dispersion and crystallite size of the reactants initially, and resulted in the partial formation of the product in both the low-temperature (α) and high-temperature (β) modifications when longer milling times were employed. The sequence of phase evolution during milling was determined from XRD, EDX and SEM analyses. Subsequent field-activated synthesis resulted in the formation of α-MoSi2 only. The initiation of the synthesis reaction required a threshold power level (equivalent to the level of the current), with the threshold decreasing with increased milling time. However, the initiation time increased when milling resulted in the formation of a significant amount the product phase, with the increase being markedly significant at low power levels.

Directional electromigration-enhanced interdiffususion in the Cu–Ni system

The effect of a dc on the interdiffusivity D in the Cu–Ni system was investigated over the temperature range of 650–850°C and at current densities in the range of 0–1000Acm−2. Interdiffusivities were calculated using the Sauer–Freise–den Broeder method and the values calculated in the absence of a current were in agreement with previously published results. The influence of the current on D depended on its direction relative to the two interfaces in the trilayered Cu–Ni–Cu samples. When the electronic flow was from Ni to Cu (cocurrent interface), the interdiffusivity showed a marked increase relative to copper content but was unchanged when the electronic flow was from Cu to Ni (countercurrent interface). The increase of D in the cocurrent interface depended on concentration and temperature. At lower temperatures, the increase becomes significant at higher copper concentrations but for the same value of current density, the increase is apparent at lower concentrations. The effective activation energy o...

High-flux current effects in interfacial reactions in Au–Al multilayers

Abstract The influence of high dc currents (up to 1019 A cm−2) on the interaction between thin Au-Al layers was investigated over the temperature range 400-500°C. In contrast with earlier diffusion couple results, only four of the five intermetalllic compounds in this system were observed, in both the absence and the presence of a current at all levels. The intermetallics Au5 Al2 and Au2 Al were the dominant products while AuAl and AuAl2 were relatively insignificant. The latter showed little change, growing only to a total thickness of a few micrometres, regardless of the magnitude of the current density and time of annealing. While the sequence of formation of the different intermetallics was not affected by the current, the incubation time and rate of growth of the layers were strongly influenced. The incubation time for the appearance of a measurable product layer was markedly reduced by the current, by nearly two orders of magnitude in some cases. The effect of the current on the nucleation of the product phases was investigated through the use of ‘pre-nucleated’ samples. The results are discussed in light of the effect of the current on atomic flux (electromigration) and on the formation of defects and compared with recent observations on field effects in multilayer systems.

Characterization of densified fully stabilized nanometric zirconia by positron annihilation spectroscopy

Fully stabilized nanometric zirconia samples with varying degrees of porosity and grain sizes were analyzed using the coincidence Doppler broadening mode of the positron annihilation spectroscopy (PAS). A decrease in the low-momentum fraction was observed and coincided with a decrease in porosity. In addition to pores, it is proposed that defects in the negatively charged grain-boundary space region act as positron trapping centers; their effectiveness decreases with an increase in grain size. It is shown that PAS is sensitive to small grain-size differences within the nanometric regime in these oxide materials.