Enthalpy of formation of ye’elimite and ternesite

Calcium sulfoaluminate clinkers containing ye’elimite (Ca4Al6O12(SO4)) and ternesite (Ca5(SiO4)2SO4) are being widely investigated as components of calcium sulfoaluminate cement clinkers. These may become low energy replacements for Portland cement. Conditional thermodynamic data for ye’elimite and ternesite (enthalpy of formation) have been determined experimentally using a combination of techniques: isothermal conduction calorimetry, X-ray powder diffraction and thermogravimetric analysis. The enthalpies of formation of ye’elimite and ternesite at 25 °C were determined to be − 8523 and − 5993 kJ mol−1, respectively

Characterization of surface roughness of MCM-41 using methods of fractal analysis

We report here the characterization of surface roughness of the model mesoporous molecular sieve MCM-41, at various scales of resolution. This material comprises several levels of structure-that of the mesopores, the crystallites, the grains, and the particles-spanning four decades of resolution, each having its independent surface properties at its characteristic length scale. The apparent fractal dimension of this material, of various pore diameters, synthesized in our laboratory has been determined at various scales with the help of various characterization techniques such as adsorption, mercury porosimetry, small-angle X-ray and neutron scattering (SAXS and SANS). A new method for the estimation of fractal dimension from a gas adsorption isotherm is proposed which considers the effect of solid-fluid interactions and the meniscus curvature, neglected in earlier methods of fractal analysis. On the basis of the results, the MCM-41 structure is found to have a fractal dimension of 2 at molecular resolutions of 3-7 Angstrom and is therefore highly smooth at this scale. At lower resolutions corresponding to the mesopore diameter (20-50 Angstrom), it possesses an apparent fractal dimension of about 3, suggesting a higher roughness. While not suggestive of a fractal structure for the narrow pore size distribution of MCM-41, the roughness indicates the presence of constrictions in the mesopore channels. At still lower resolutions (80-250 Angstrom and 0.1-0.4 mu m), the structure is also found to be rough. It is also found that MCM-41 has a higher fractal dimension than HMS, and that vanadium incorporation into the structure of MCM-41 increases its roughness

The mechanisms of combustion and continuous reactions during mechanical alloying

Some materials exhibit a combustion event during mechanical alloying, which results in the rapid transformation of reactants into products, while others show a slow transformation of reactants into products, In this paper, the continuous W + C --> WC reaction is compared to the Ti + C --> TiC combustion reaction. Rietveld refinement of X-ray diffraction patterns is used to show that these particular reactions proceed through different pathways, determined by crystallographic factors of the reactants. When a crystallographic relationship exists between the reactants and the products, such as that between W and WC, the product forms slowly over a period of time. In contrast, insertion of C into the Ti structure is associated with atomic rearrangements within the crowded lattice planes and the subsequent catastrophic failure of the reactant lattices results in combustion to form TiC. (C) 2001 Academic Press

Experimental and theoretical investigations of adsorption hysteris and criticality in MCM-41: studies with O₂, Ar, and CO₂

MCM-41 materials of six different pore diameters were prepared and characterized using X-ray diffraction, transmission electron microscopy, helium pycnometry, small-angle neutron scattering, and gas adsorption (argon at 77.4 and 87.4 K, nitrogen and oxygen at 77.4 K, and carbon dioxide at 194.6 K). A recent molecular continuum model of the authors, previously used for adsorption of nitrogen at 77.4 K, was applied here for adsorption of argon, oxygen, and carbon dioxide. While model predictions of single-pore adsorption isotherms for argon and oxygen are in satisfactory agreement with experimental data, significant deviation was found for carbon dioxide, most likely due to its high quadrupole moment. Predictions of critical pore diameter, below which reversible condensation occurs: were possible by the model and found to be consistent with experimental estimates, for the adsorption of the various gases. On the other hand, existing models such as the Barrett-Joyner-Halenda (BJH), Saito-Foley, and Dubinin-Astakhov models were found to be inadequate, either predicting an incorrect pore diameter or not correlating the isotherms adequately. The wall structure of MCM-41 appears to be close to that of amorphous silica, as inferred from our skeletal density measurements

STRUCTURE OF CALCIUM ALUMINATE SULFATE CA4AL6O16S

The commensurately modulated structure of polycrystalline calcium aluminate sulfate was determined using spectroscopic and electron microscopic information and calculations with the bond-valence method. The neutron powder diffraction data were refined by the Rietveld profile technique. Calcium aluminate sulfate, Ca4Al6O16S, MW = 2440 Da, orthorhombic, Pcc2, a = 13.028(3) Angstrom, b = 13.037(3) Angstrom, c = 9.161(2) Angstrom, U = 1556(1) Angstrom(3), Z = 4, Dc = 2.60 g cm(3), lambda (neutron) = 1.893 Angstrom (Ge monochromated), mu R(neutron) = 0.42, R(Bragg) = 0.046, R(p) = 0.06, R(wp) = 0.08 is a twofold superstructure of sodalite which is generated by z modulation of the framework along [1 1 0] of the cubic subcell. (C) 1995 Academic Press, Inc

Mode of boron solubility in ferrous alloys

X-ray and neutron diffractometry and inelastic neutron scattering studies on a set of cast ferritic iron-chromium alloys containing boron indicate that the dissolved boron is present interstitially rather than substitutionally within the α-Fe,Cr matrix. Inelastic neutron scattering spectra of the alloys may be interpreted in terms of the phonon dispersion curves for α-Fe. These spectra show small shifts in the frequencies of particular phonons, indicative of lattice disruption by the presence of interstitial atoms. These results were subsequently confirmed by internal friction measurements