Olivier Dezellus

Chemical reaction-limited spreading: the triple line velocity versus contact angle relation

The mechanisms of reaction-limited spreading of molten copper and nickel silicides on carbon substrates are studied by the dispensed drop variant of the sessile drop technique under high vacuum. Both standard wetting experiments and experiments which separate the effect of time and instantaneous contact angle on spreading rate are carried out. Equations are derived for the time-dependent variation of contact angle and spreading rate.

Progress in modelling of chemical-reaction limited wetting

In reactive metal/ceramic systems the wetting rate of small, millimeter sized droplets on smooth ceramic surfaces is controlled by the slower of two successive phenomena that intervene in the reaction process: diffusive transport of reacting species to or from the triple line, and local reaction kinetics at the triple line. The first case, of diffusive wetting, was modelled by Mortensen et al. [Scripta mater. 36 (1997) 645] a few years ago. The purpose of this paper is to present progress accomplished during the last few years in modelling the second type of wetting controlled by the reaction process at or close to the triple line. The predictions of equations derived from a model describing the change in contact angle and spreading rate with time are compared with experimental results obtained for different silicon alloys on carbon substrates.

The role of van der Waals interactions on wetting and adhesion in metal/carbon systems

Sessile drop expts. using pure Cu and Au on different C substrates under high vacuum showed that these metals wet monocryst. graphite better than vitreous C with the corresponding differences in the work of adhesion being 50-60%. These results were interpretted satisfactorily by assuming that the predominant interactions at the surface are van der Waals interactions due to dispersion forces. For nonreactive metal/C systems, wetting and adhesion of a metal is expected to vary only slightly around its m.p. This is also expected to be true for any metal/ceramic system in which adhesion is ensured mainly by van der Waals interactions.

Spreading of Cu-Si alloys on oxidized SiC in vacuum: experimental results and modelling

Abstract The wetting behaviour of a Cu–40 at.% Si alloy, non-reactive with SiC, on oxidized (0001) faces of α-SiC single crystals is studied at temperatures close to the melting point of copper by the “dispensed drop” technique under high vacuum. The experiments focused on wetting kinetics to determine the mechanisms controlling the rate of spreading under conditions allowing for “in situ” deoxidation of silicon carbide. It is shown that spreading occurs in three stages: (i) an initial stage consisting of a rapid spreading of the alloy on the oxidized SiC up to a contact angle which is close to that on vitreous silica, (ii) a second stage with a zero spreading rate during which the alloy goes through the oxide layer by dissolution near the triple line allowing an intimate contact between the alloy and the SiC surface to be established, and (iii) a final stage during which the alloy spreads with a constant rate up to a contact angle equal to that of clean SiC. These results are interpreted on the basis of a dissolution–diffusion–evaporation process occurring in the vicinity of the solid–liquid–vapour triple line.

DIFFUSION-LIMITED REACTIVE WETTING: SPREADING OF Cu-Sn-Ti ALLOYS ON VITREOUS CARBON

When wetting is promoted by interfaces, the rate at which the liquid spreads over the solid substrate is controlled by the rate at which the well-wetted interfacial product can be formed along the liquid/solid/atmospheric triple line. If this reaction involves a reactive alloy addition to the liquid, the rate of spreading can be limited by two phenomena. A simplified analysis of diffusion- limited reactive spreading of Cu-Sn-Ti alloys on vitreous carbon was conducted based on the assumption that the interfacial reaction is strictly localized at the triple line. The addition of 3at.%Ti to Cu-15at.%Sn led to very good wetting on vitreous carbon. This was due to the formation, at the interface, of a continuous layer of TiC.

A Study on the Temperature of Ohmic Contact to p-Type SiC Based on Ti 3 SiC 2 Phase

In this paper, the electrical properties of Ti₃SiC₂-based ohmic contacts formed on p-type 4H-SiC were studied. The growth of Ti₃SiC₂ thin films were studied onto 4H-SiC substrates by thermal annealing of Ti-Al layers deposited by magnetron sputtering. In this study, we varied the concentrations of Ti and Al (Ti₂₀Al₈₀, Ti₃₀Al₇₀, Ti₅₀Al₅₀, and Ti), and the annealing temperature from 900 °C to 1200 °C for each concentration. X-ray diffraction and transmission electron microscopy analyzes were performed on the samples to determine the microstructure of the annealed layers and to further investigate the compounds formed after annealing. Using the transfer length method structures, the specific contact resistance (SCR) at room temperature of all contacts was measured. The temperature dependence up to 600 °C of the SCR of the best contacts was studied to understand the current mechanisms at the Ti₃SiC₂/SiC interface. The experimental results are in agreement with the thermionic field emission theory. With this model, the barrier height of the contact varies between 0.71 and 0.85 eV. Finally, ageing tests showed that Ti₃SiC₂-based contacts were stable and reliable up to 400 h at 600 °C under Ar.

Transformation Kinetics and Resulting Microstructure in MMC Reinforced with TiC Particles

The phase transformation kinetics on cooling and resulting microstructures of steel-based matrix composites (MMC) reinforced with TiC particles by powder metallurgy were studied. In addition, the phase transformation kinetics of the MMC were compared to those of the same steel without TiC and consolidated in the same conditions. The presence of TiC particles strongly favors the diffusive transformations in the steel matrix of the MMC. Different complementary techniques (XRD, SEM, TEM/EDX, atom probe tomography, in situ synchrotron XRD) were performed to analyze the chemical reactivity between TiC particles and the steel powders occurring during consolidation process and further heat treatments. Composition changes in the TiC as well as in the matrix were characterized. The chemical composition after treatment in the TiC particles tends toward the thermodynamic calculations with ThermoCalc. The effect of changes in chemical composition and the role of TiC particles acting as new favorable nucleation sites are discussed in regards to the obtained results.

Further Evidence of Nitrogen Induced Stabilization of 3C-SiC Polytype during Growth from a Si-Ge Liquid Phase

The influence of nitrogen impurity on the stabilization of 3C-SiC polytype has been studied during vapour-liquid-solid (VLS) growth on 6H-SiC(0001) seed with Si-Ge melt. By changing the partial pressure of N2 during growth, it was found that the proportion of 3C-SiC inside the grown material increases with N2 partial pressure. 6H inclusions are only found for high purity (low N2 content) conditions. The possible interactions proposed to explain this effect are divided in two effects: i) lattice parameter modification and ii) surface induced lateral enlargement variation. A combination of both effects is suspected.

Vapour-Liquid-Solid Induced Localised Growth of Heavily Al Doped 4H-SiC on Patterned Substrate

We are developing a new approach for SiC selective growth by vapour-liquid-solid (VLS) mechanism in which propane feeds an Al-Si melt. In a first step, a stacking consisting in 500 nm thick Si layer followed by 1 μm thick Al layer was deposited on top of a 4H-SiC, 8° off substrate. This stacking was patterned to obtain various geometrical motives. For the VLS growth, the sample was heated up to 1000°C so that the Al and Si layers reacted giving local melts which were fed by propane to grow SiC selectively. This process was successful to obtain selective SiC epitaxy with various dimensions and shapes. It was found that the width of the pattern lines have to be as small as possible (< 20 μm) in order to avoid circular droplet formation. Under this value, regular SiC lines were formed. Raman characterization shows that the areas where growth occurred are highly p-type doped.

Influence of evaporation–condensation in reactive spreading

Abstract In reactive wetting, considerable acceleration of spreading rate is usually observed in the high contact angle (short spreading time) range in comparison to the low contact angle range. This study examines the possibility to explain this acceleration by an evaporation–condensation process which may occur under high vacuum in front of the triple line. For this purpose, a method is developed to evaluate the thickness of the reaction product in front of the triple line during spreading. This method is based on calculations of ballistic exchange in vacuum between the drop and the substrate surfaces combined with Auger spectroscopic analysis of the substrate surface. The method is applied to Cu–40 at% Si and Cu–1.5 at% Cr droplets on vitreous carbon systems.