Salutation Angelique

https://digitalcommons.library.umaine.edu/mmb-me/1629/thumbnail.jp

Rhombohedral calcite precipitation from CO2-H2O-Ca(OH)2 slurry under supercritical and gas CO2 media

The formation of solid calcium carbonate (CaCO3) from aqueous solutions or slurries containing calcium and carbon dioxide (CO2) is a complex process of considerable importance in the ecological, geochemical and biological areas. Moreover, the demand for powdered CaCO3 has increased considerably recently in various fields of industry. The aim of this study was therefore to synthesize fine particles of calcite with controlled morphology by hydrothermal carbonation of calcium hydroxide at high CO2 pressure (initial PCO2=55 bar) and at moderate and high temperature (30 and 90 degrees C). The morphology of precipitated particles was identified by transmission electron microscopy (TEM/EDS) and scanning electron microscopy (SEM/EDS). In addition, an X-ray diffraction analysis was performed to investigate the carbonation efficiency and purity of the solid product. Carbonation of dispersed calcium hydroxide in the presence of supercritical (PT=90 bar, T=90 degrees C) or gaseous (PT=55 bar, T=30 degrees C) CO2 led to the precipitation of sub-micrometric isolated particles (<1$\mu$m) and micrometric agglomerates (<5$\mu$m) of calcite. For this study, the carbonation efficiency (Ca(OH)2-CaCO3 conversion) was not significantly affected by PT conditions after 24 h of reaction. In contrast, the initial rate of calcium carbonate precipitation increased from 4.3 mol/h in the "90bar-90 degrees C" system to 15.9 mol/h in the "55bar-30 degrees C" system. The use of high CO2 pressure may therefore be desirable for increasing the production rate of CaCO3, carbonation efficiency and purity, to approximately 48 kg/m3h, 95% and 96.3%, respectively in this study. The dissipated heat for this exothermic reaction was estimated by calorimetry to be -32 kJ/mol in the "90bar-90 degrees C" system and -42 kJ/mol in the "55bar-30 degrees C" system

Hardness and friction behavior of bulk CoAl2O4 and Co–Al2O3 composite layers formed during Spark Plasma Sintering of CoAl2O4 powders

Materials made up of a Co–Al2O3 composite coating over a CoAl2O4 core are prepared during Spark Plasma Sintering of CoAl2O4 powders. The Co particles are precipitated because of a combination of high temperature and low O2 partial pressure. The precipitation and densification processes hamper each other and thus the way the uniaxial pressure is applied during the sintering cycle is an important parameter to control the microstructure of composite layer and its thickness (about 100 mm) and obtain a dense sample (about 4 g/cm3). The friction coefficient of the Co-Al2O3 composites against an Al2O3 ball is lower than that found for an Al2O3 specimen, which could reveal the lubricating role of submicrometer Co particles. However, increasing the load from 5 to 10 N load causes major changes in the friction contact, which are detrimental. Bulk CoAl2O4 was found to have a Vickers microhardness about 15.5 GPa and an average friction coefficient lower than that of an Al2O3 sample

Spark plasma sintering as a reactive sintering tool for the preparation of surface-tailored Fe–FeAl2O4–Al2O3 nanocomposites

Al1.86Fe0.14O3 powders were partially or totally reduced in H2. The fully reduced Fe–Al2O3 nanocomposite powder was sintered by spark plasma sintering (SPS) without any reaction taking place. For the other powders, the SPS induced the formation of FeAl2O4 and sometimes Fe. The most severe reducing conditions were found at the surface of the materials, producing nanocomposites with a surface layer composition and microstructure different to those of the core. This in situ formed composite layer confers a higher hardness and fracture strength

Mechanical and tribological properties of Fe/Cr-FeAl2O4-Al2O3 nano/micro hybrid composites prepared by Spark Plasma Sintering

Fe/Cr–Al2O3 nanocomposite powders are prepared by H2 selective reduction of oxide solid solutions. These powders, an alumina powder and a starting oxide powder, are sintered by spark plasma sintering. The microstructure of the resulting materials is studied. The composites show a lower microhardness and higher fracture strength than unreinforced alumina. The friction coefficient against an alumina ball is lower, revealing the role of the intergranular metal particles, whereas FeAl2O4 grains formed during SPS are beneficial for higher cycle numbers

Condensation of helium in aerogels and athermal dynamics of the Random Field Ising Model

High resolution measurements reveal that condensation isotherms of $^4$He in a silica aerogel become discontinuous below a critical temperature. We show that this behaviour does not correspond to an equilibrium phase transition modified by the disorder induced by the aerogel structure, but to the disorder-driven critical point predicted for the athermal out-of-equilibrium dynamics of the Random Field Ising Model. Our results evidence the key role of non-equilibrium effects in the phase transitions of disordered systems.Comment: 5 p + suppl. materia

Influence of pulse current during Spark Plasma Sintering evidenced on reactive alumina–hematite powders

Spark Plasma Sintering (SPS) is increasingly used. The temperature and current are not independent parameters, making it difficult to separate the current intrinsic role from Joule heating. There is a debate on whether there are any specific SPS mechanisms. The influence of a key parameter, the (on:off) pulse pattern, is studied on the SPS of reactive α-Al2−2xFe2xO3 (x = 0.02; 0.05; 0.07; 0.10) powders. Changing it modifies the current crest intensity and has a great influence on the materials microstructure. Comparisons with runs where the current is blocked and hot-pressing reveal three competing phenomena: formation of FeAl2O4, dominant in the core and not peculiar to SPS, formation of Fe, producing Fe-Al2O3 composite surface layers, and most notably electrical-field induced diffusion of Fe3+ ions towards the cathode, which could have far-ranging implications for the consolidation of ionic materials and the in situ reactive shaping of composites and multimaterials