Jean-Pierre Coutures

A thermochemical study of glasses and crystals along the joins silica-calcium aluminate and silica-sodium aluminate

Abstract Enthalpies of solution in molten 2PbO · B 2 O 3 at 985 K are reported for series of glasses xCa 0.5 AlO 2 -(1− x ) SiO 2 ( O ≤ x ≤ 0.99) and xNaAlO 2 -(1− x ) SiO 2 (0 ≤ x ≤ 0.56). The data are compared to values for the corresponding crystalline aluminosilicates and to preliminary data for systems containing KAlO 2 and Mg 0.5 AlO 2 . The enthalpies of mixing of glasses become more exothermic with increasing basicity of the mono- or divalent oxide. The tendency toward immiscibility on the silica-rich side, indicated by the shape of the heat of mixing curve between x = 0 and x = 0.4, is pronounced in the calcium aluminate system, but not in the sodium aluminate system. The shape of the heat of mixing curve, which is roughly symmetrical about x = 0.5, can be rationalized in terms of glass structure by considering essentially random substitution of Si and Al on a continuous three dimensional tetrahedral framework, with stabilization arising from electrostatic interactions between aluminum and the nonframework cation balancing the destabilizing effects arising from perturbation of the aluminosilicate framework by the nonframework cation. These trends are consistent with the variation of physical properties of aluminosilicate melts.

Solar gasification of carbonaceous materials

Abstract Charcoal, wood and paper have been gasified in a packed-bed reactor using steam and solar energy. The steam was generated by spraying water directly on to the surface of the fuel and, at the same time, heating the fuel at the focus of a solar furnace. Half of the steam reacted with carbon and 30 per cent of the incident solar energy was stored as chemical enthalpy. The performance of a fluidized-bed reactor was compared to that of a packed-bed reactor using charcoal and CO2. The fraction of the incident solar energy utilized to produce CO (stored) was 10 per cent in the case of the fluidized-bed reactor and 40 per cent for the packed-bed reactor. The fuel value of the gas produced from the steam-gasification of wood and paper was 65 kcal/mole (320 Btu/lb). On an ash free basis the volume yield of the gas was 1 ± 0.1 m3/kg.

Magnesium and Calcium Aluminate Liquids: In Situ High-Temperature 27Al NMR Spectroscopy

The use of high-temperature nuclear magnetic resonance (NMR) spectroscopy provides a means of investigating the structure of refractory aluminate liquids at temperatures up to 2500 K. Time-averaged structural information indicates that the average aluminum coordination for magnesium aluminate (MgAl2O4) liquid is slightly greater than for calcium aluminate (CaAl2O4) liquid and that in both liquids it is close to four. Ion dynamics simulations for these liquids suggest the presence of four-, five-, and six-coordinated aluminate species, in agreement with NMR experiments on fast-quenched glasses. These species undergo rapid chemical exchange in the high-temperature liquids, which is evidenced by a single Lorentzian NMR line.

X-ray photoelectron spectroscopy characterization of barium titanate ceramics prepared by the citric route. Residual carbon study

Powder and ceramics of barium titanate prepared by the citric process were studied by x-ray photoelectron spectroscopy (XPS). Spectra of C1 s , O1 s , Ti 2 p , Ba 3 d , and Ba 4 d levels are analyzed in powder and ceramics immediately after the sintering step and after several months of exposure in the air. Ar-ion etching allowed one to characterize the material intrinsic carbon. The results are discussed in comparison with works previously published on oxide single crystals.

Density of superheated and undercooled liquid alumina by a contactless method

The density of liquid alumina drops maintained in levitation with an aerodynamic device and heated with CO2 lasers is determined by analysis of high-speed video digital images between 2000 and 3100 K in various gases. It is shown that consistent results can be achieved for the lighter drops (m<100 mg) which do not depend on the nature of the gas. Experiments performed with lasers impinging the drop surface or during free cooling of the preheated drop gave similar results. The density is represented by the following expression: d=(2.79±0.01)(l−α(T−2500)) g·cm−3, where α=(4.22 ±0.14) × 10−5K−1.

High-temperature solar pyrolysis of coal☆

Abstract Subbituminous coal from Western United States was pyrolyzed by directly exposing 50 mg powdered samples to concentrated solar radiation. It was found that exposure to flux levels > 200 W/cm 2 for 12.5 s devolatilized 51 per cent of the coal. At flux levels between 100 and 200 W/cm 2 devolatilization was slightly less. Gas yield was a maximum of 31 mmol/g coal at a flux of 100 W/cm 2 and decreased slightly with increasing flux. Gas yields were more than twice as great as those obtained by a laser technique developed to simulate solar pyrolysis. In experiments with spectral cut-off filters there was no effect due to changing the wavelength distribution of sunlight.

27Al NMR spectroscopy of aluminosilicate melts and glasses

The development of a new NMR device has been carried out to study high-temperature liquids: it includes a heating mode (CO2 laser) and a special radio-frequency (RF) coil that enables us to study aerodynamic levitating droplets which have no interaction with the nozzle; oxidizing, neutral or reducing atmospheres can be used as levitation gases. We present here 27 Al NMR results on calcium aluminosilicate melts; in all cases the narrow (200–300 Hz) resonance line shows a rapid exchange regime with a dependence of the chemical shift on the liquid composition.

Aerodynamic laser-heated contactless furnace for neutron scattering experiments at elevated temperatures

Conventional radiative furnaces require sample containment that encourages contamination at elevated temperatures and generally need windows which restrict the entrance and exit solid angles required for diffraction and scattering measurements. We describe a contactless windowless furnace based on aerodynamic levitation and laser heating which has been designed for high temperature neutron scattering experiments. Data from initial experiments are reported for crystalline and amorphous oxides at temperatures up to 1900 °C, using the spallation neutron source ISIS together with our laser-heated aerodynamic levitator. Accurate reproduction of thermal expansion coefficients and radial distribution functions have been obtained, demonstrating the utility of aerodynamic levitation methods for neutron scattering methods.

Fast x-ray scattering measurements on molten alumina using a 120° curved position sensitive detector

Fast x-ray scattering measurements on molten alumina were performed on the H10 beam line at the DCI Synchrotron of LURE (Orsay, France). A high-temperature chamber with a levitation device was coupled with the four-circle goniometer of the beam line. A 100 W CO2 laser was used to melt the sample and the temperature was measured using an optical pyrometer operating at 0.85 μm. Usually, measurements of the total structure factor S(Q) on molten materials are performed using a fixed detector scanned over an angular range. In this work, in order to reduce the total scan duration, x-ray scattered intensities were measured with a 120° position sensitive detector (INEL CPS120). We performed several measurements with different acquisition times varying from 10 s to 5 min. In 5 min it was possible to obtain a good determination of S(Q) with a usable signal up to the Q range limit (13 A−1). The intensity was comparable with a 1 h measurement with a NaI (Tl) scintillator scanned over the 120° 2θ range. On reducing th...

A time resolved 27Al NMR study of the cooling process of liquid alumina from 2450 °C to crystallisation

Abstract Nuclear magnetic resonance is able to give insight into the structure and dynamics of liquids at very high temperature (T > 2000 °C). 27Al NMR spectra have been recorded every 25 ms during the cooling of an aerodynamically levitated liquid alumina droplet from 2450 °C to crystallisation in less than 3 s. The temperature is measured jointly by pyrometry and NMR, and this time resolved experiment provides a unique way of exploring the temperature dependence of both the structure (shift) and the dynamics (relaxation time of 27Al of the liquid and the supercooled liquid alumina until the crystallisation of α-Al2O3. The apparent Arrhenian activation energy of 130 kJ mol−1, derived from T1 measurement, is understood as the signature of the macroscopic viscosity in this high temperature liquid.