David Lambertin

Effect of aging and alkali activator on the porous structure of a geopolymer

Nitrogen sorption and small- and wide-angle X-ray and neutron scattering techniques were used to study the porous structure of geopolymers, inorganic polymers synthesized by reaction of a strongly alkaline solution and an aluminosilicate source (metakaolin). The effects of aging and the use of alkali activators (Na+, K+) of different sizes were investigated at room temperature. The influence of aging time on the microstructure of both geopolymer matrixes was verified in terms of pore volume and specific surface area. The results suggested a refinement of the porosity and therefore a reduction in the pore volume over time. Regardless of the age considered, some characteristics of the porous network such as pore size, shape and distribution depend on the alkali activator used. Whatever the technique considered, the potassium geopolymer has a greater specific surface area than the sodium geopolymer. According to the scattering results, the refinement of the porosity can be associated with, first, a densification of the solid network and, secondly, a partial closure of the porosity at the nanometre scale. The kinetics are much slower for the sodium geopolymer than for the potassium geopolymer in the six months of observation.

Temperature dependence and effect of oxide anion on the americium chemistry in the molten LiCl-KCl eutectic

Summary The chemical properties of americium chlorides have been studied in the fused eutectic LiCl-KCl as a function of temperature ranging from 743K to 943K. Titration of Am(III) chlorocomplex by O2- demonstrated the existence of soluble AmO+ species as well as the precipitation of AmOCl and Am2O3 solid compounds. The corresponding solubility products of AmOCl and Am2O3 have been determined. Calculated Am(II) disproportionation equilibrium constant indicates that the stability of Am(II) chlorocomplex increases with the increase of temperature.

Molten salt electrosynthesis of ZnSe

Abstract High temperature molten salt electrosynthesis is a promising method for preparing thin layers of metallic or composite compounds. In this case no post heat treatment (modifying the interface composition) is needed. In this work, we report the electrowinning of ZnSe which is performant for realizing diode lasers and solar cells. Thin films containing up to 90% well crystallized ZnSe with Se/Zn around 1.1 and a grain size equal to 1 μm was obtained in the molten mixture NaClCaCl2 fused at 550°C. Optical properties are currently studied.

Modelling and quantification of intergrowth in γ-MnO2 by laboratory pair distribution function analysis

γ-MnO2 is a material formed by random intergrowth of two phases, β-MnO2 and R-MnO2. It is demonstrated here on seven γ-MnO2 samples that pair distribution function analysis using a conventional X-ray diffraction setup (Bragg–Brentano geometry with a molybdenum anode) allows the quantification of this intergrowth simply via a simulation of the actual material by a mixture of β-MnO2 and R-MnO2 phases. Although this method does not take into account specifically the relaxed distances in the vicinity of the intergrowth zone, it is found to be very robust, accurate and in full agreement with the widely used quantification based on the empirical approach of Chabre & Pannetier [Prog. Solid State Chem. (1995), 23, 1–130].

Galvanic Corrosion of Mg-Zr Alloy and Steel or Graphite in Mineral Binders

The dismantling of UNGG nuclear reactor generates numerous nuclear wastes such as fuel decanning commonly composed of Mg-Zr alloy. A conditioning strategy consists in encapsulating these wastes into a hydraulic binder in a suitable state for storage. The eventual presence of steel and graphite accompanying the magnesium wastes could imply corrosion by galvanic coupling. This work is an experimental investigation of the galvanic coupling between Mg-Zr alloy and steel or graphite using ZRA electrochemical method in Portland cement or geopolymer pastes. The lowest corrosion activity of magnesium alloy while coupled to graphite or steel cathode has been observed in geopolymer pastes. Indeed, in this binder, an efficient corrosion protection of the magnesium alloy maintains the galvanic current very low during all the hardening process. In geopolymer paste, current densities of anodised Mg-Zr alloy is not dependent of the cathode/anode surface ratio in the range of 0.1 to 5 due to the dominance of the anode resistance.