Corrosion behaviour of brazing material AA4343

This paper is part of a work devoted to corrosion of brazed AA4343/AA3003/AA4343 materials on water side of automotive heater cores. The microstructure of the superficial resolidified AA4343 brazing material has been previously characterised [1] and [2]. It is composed of large (Al) grains separated by valleys containing multiphase deposits of (Al), Si and α-Al(Mn,Fe)Si. The present study focuses on its electrochemical behaviours in neutral water–ethylene glycol mixtures at different temperatures with and without chloride ions. Three types of behaviour are revealed: (i) passivation, (ii) defective passivation and (iii) pitting corrosion at the corrosion potential. The defective passivation is investigated in greater depth. The results show that Si needles do not participate in the corrosion progress whereas the α-Al(Mn,Fe)Si particles are preferential sites for corrosion attacks. α-Al(Mn,Fe)Si particle/matrix interactions are responsible of the defective passivation at valleys level where the α-Al(Mn,Fe)Si phase particles are mainly concentrated. Increasing the temperature enhances this reactivity whereas addition of ethylene glycol decreases it and favours a transition from defective passivation to passivation for ethylene glycol content higher than 55%

Tuning the thermal conductivity of graphene nanoribbons by edge passivation and isotope engineering: a molecular dynamics study

Using classical molecular dynamics simulation, we have studied the effect of edge-passivation by hydrogen (H-passivation) and isotope mixture (with random or supperlattice distributions) on the thermal conductivity of rectangular graphene nanoribbons (GNRs) (of several nanometers in size). We found that the thermal conductivity is considerably reduced by the edge H-passivation. We also find that the isotope mixing can reduce the thermal conductivities, with the supperlattice distribution giving rise to more reduction than the random distribution. These results can be useful in nanoscale engineering of thermal transport and heat management using GNRs.Comment: 4 pages, 4 figure

Physics and chemistry of hydrogen in the vacancies of semiconductors

Hydrogen is well known to cause electrical passivation of lattice vacancies in semiconductors. This effect follows from the chemical passivation of the dangling bonds. Recently it was found that H in the carbon vacancy of SiC forms a three-center bond with two silicon neighbors in the vacancy, and gives rise to a new electrically active state. In this paper we examine hydrogen in the anion vacancies of BN, AlN, and GaN. We find that three-center bonding of H is quite common and follows clear trends in terms of the second-neighbor distance in the lattice, the typical (two-center) hydrogen-host-atom bond length, the electronegativity difference between host atoms and hydrogen, as well as the charge state of the vacancy. Three-center bonding limits the number of H atoms a nitrogen vacancy can capture to two, and prevents electric passivation in GaAs as well

An electrochemical investigation of the formation of CoSx and its effect on the anodic dissolution of iron in ammoniacal-carbonate solutions

It has been found that the co-presence of cobalt (II) and thiosulphate ions in ammoniacal-carbonate solutions promotes the passivation of iron, under conditions in which it would otherwise continue to dissolve anodically. Electrochemical experiments have shown a relationship between the immersion time required for passivation and the formation of a solid species on the iron surface, which is thought to be implicated in the mechanism of passivation, whilst not being itself the protective species. Based on a combination of electrochemical, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and grazing incidence X-ray diffraction (GIXRD) characterisation techniques, the said species has been identified as CoSx, resulting from the interaction of cobalt (II) and thiosulphate ions. It is thought to form as a product of the cathodic reactions taking place on the iron surface during its active dissolution. These findings are particularly relevant to the Caron process, in which the ammoniacal-carbonate solutions containing dissolved cobalt and thiosulphate ions are used to leach nickel and cobalt from pre-reduced laterite ores rich in metallic iron. Both the loss of cobalt into the CoSx layer and the passivation of iron and of its alloys with nickel and cobalt, are potential contributing factors to the low cobalt and nickel recoveries, which are typical of the Caron process. This study provides a better understanding of the conditions under which the CoSx layer forms and promotes the passivation of iron, and may therefore provide useful information to help minimise the effect this may have on the extraction efficiency of the process. In particular, at the cobalt and thiosulphate ion concentrations usually encountered at a Caron plant, the passivation of iron was found to be prevented by maintaining a high enough concentration of ammonia

Silicon Oxide Passivation of Single-Crystalline CVD Diamond Evaluated by the Time-of-Flight Technique

The excellent material properties of diamond make it highly desirable for many extreme electronic applications that are out of reach of conventional electronic materials. For commercial diamond devices to become a reality, it is necessary to have an effective surface passivation since the passivation determines the ability of the device to withstand high surface electric fields. In this paper we present data from lateral Time-of-Flight studies on SiO2-passivated intrinsic single-crystalline CVD diamond. The SiO2 films were deposited using three different techniques. The influence of the passivation on hole transport was studied, which resulted in the increase of hole mobilities. The results from the three different passivations are compared