Pierre Bracconi

On the difficulty of assessing the specific surface area of magnesium stearate

The water content of as-received commercial magnesium stearate batches from animal and vegetable sources have been modified by ageing in humid air at room temperature or by vacuum treatment. The complete adsorption-desorption isotherms of nitrogen and krypton vapours by samples of these as received and modified materials have been measured at liquid nitrogen temperature after standardised vacuum degassing. They are greatly affected by the initial water content of the material. In particular: (a) the BET surface area values computed from the adsorption branch vary widely and is increasing with increasing water content; (b) anomalous hysteresis of varying amplitude is observed in all cases except adsorption of krypton on the material with the lowest water content; (c) the hysteresis loops extend down to very low desorption pressure values and cannot be accounted for by capillary condensation. Lastly, the surface area value of a given material computed from nitrogen and krypton adsorption may differ by a factor as high as six. Accordingly, the very significance of BET surface area values obtained from routine adsorption experiments should be regarded as questionable, at least until the mechanisms of adsorption are fully clarified.

Structural properties of magnesium stearate pseudopolymorphs: effect of temperature.

A thorough review of the relevant literature reveals that the interaction between water vapour and magnesium stearate, in contrast to many other metal soaps, is not properly understood. The structural modifications associated with the up-take or loss of water of vegetable-derived commercial magnesium stearate powders exposed to humid air or vacuum at room temperature are investigated using standard powder X-ray diffractometry. It is found that in such conditions magnesium stearate reacts reversibly with the vapour phase with structural consequences very similar to the high temperature transition between the crystalline and rotator phases of other anhydrous metal soaps. When temperature is increased under dry nitrogen the diffraction band characteristic of the rotator phase shifts towards higher angle values and the corresponding lattice spacing increases at the rate of 6.9x10(-4)C(-1). Melting takes place gradually above 100 degrees C as revealed by the collapse of the diffraction band and the growth of the broader diffusion band characteristic of the liquid state. Full clarification of the structure of the hydrated and dried phases proves impossible based on powder diffraction spectra obtained with conventional high resolution X-ray diffraction equipment.

Surface characterization and reactivity of a nitrogen atomized 304L stainless steel powder

The oxide layer on the surface of the particles of a nitrogen atomized 304L stainless steel powder was quantitatively characterized using X-ray photoelectron spectroscopy (XPS), secondary-ion microprobe (IMP) analysis, selective reduction of surface iron oxide by hydrogen, and chemical analysis for total oxygen, all as a function of particle size. The composition and thickness of the oxide layer do not depend significantly on particle size. A 5.8-nm-thick outer layer made of MnO and Fe2O3 islands covers a 2.4-nm-thick inner layer of Cr2O3. The reaction of the powder with O2 impurity in either H2 or N2 exhibits different kinetics and mechanism in both cases. In H2, a selective oxidation of the alloying elements Mn, Cr, and Si takes place above 400 °C with a parabolic isothermal rate law. In N2, iron also is oxidized, isolated Fe2O3-rich microcrystals form over a Cr2O3-rich uniform underlayer, and the isothermal kinetics are accelerated.

Assessing the particle size of a broadly dispersed powder by complementary techniques

The experimental determination of reliable particle size distribution curves and statistical parameters of broad distributions is known to be a difficult task. This problem is addressed here in an attempt to characterize the granularity of three distinct batches of a pharmaceutical powder (fenofibrate from Fournier Laboratories). The methodology consists in comparing the results, expressed in terms of surface based mean diameter, as obtained by three complementary techniques, namely optical microscopy image analysis, laser light low angle diffraction and surface area measurement by krypton physisorption. These techniques are applied in parallel to the material of interest and to a certified reference material, a nearly spherical and narrowly distributed glass powder.

Electronic microdiffraction study of structural modifications resulting from the dehydration of gypsum. Prediction of the microstructure of resulting pseudomorphs

Abstract Endothermic decomposition reactions (solid 1→solid 2+gas) generally affect only part of the inter-atomic bonds of the solid 1 structure. In consequence, the morphology and external dimensions of particles remain unchanged (the particles of solid 2 are referred to as pseudomorphs of those of solid 1). The gas release normally leads to a decrease of the molar volume of the precursor solid, resulting in cracking and formation of intra-particle porosity. This work deals with such a reaction: the dehydration of gypsum into sub-hydrated and anhydrous phases. Ultrathin (010) gypsum cleaved plates have been dehydrated under controlled conditions and studied by electron microdiffraction. All reactions have been shown to preserve four different sets of three-dimensional orientation relations between all structures. These topotactic orientation relations allow us to clarify the associated atomic rearrangements. The reactions are described in a simplified net common to all structures, which allows to predict the microstructure of the resulting pseudomorphs.

Quantitative phase analysis and thickness measurement of surface-oxide layers in metal and alloy powders by the chemical-granular method

The principles of the chemical-granular analysis of metal and alloy powders are reviewed and the results are compared with those provided by the spectroscopic analytical techniques XPS, AES and SIMS, including ion etching in their depth-profiling mode, when they are applied to the same materials. Several examples are analysed and it is shown that the chemical-granular method alone can provide the very same information as depth profiling. However, it is averaged over a macroscopic powder sample in contrast to one or a few single particles. Nevertheless, it is the combination of the chemical-granular and depth-profiling analyses that really provides an unparalleled description in quantitative terms of the phase composition and microstructure of either multiphase and/or irregular surface layers resulting from oxidation, precipitation or contamination.

Quantitative analysis of the thermal degassing of a beryllium powder

Abstract The thermal degassing of a beryllium powder is investigated using quantitative quadrupole mass spectrometry and the Karl-Fisher technique adapted to measure both water and hydrogen which are the main desorbed species. Methane and carbon dioxide are also measured, but in much lower amounts. Based on the proportionality between the desorbed amounts and the surface area of the various size fractions analysed, all species are shown to originate from the surface of the beryllium particles. The relative proportions of degassed water vapour and hydrogen depend on the thermal conditions of the analysis whereas the total number of moles of both gases remains constant. This shows that the hydrogen is generated by the reduction of surface water molecules or hydroxyls, though a small fraction may result from the normal decomposition process of amorphous surface hydroxide.

Physical chemistry of the powder metallurgy of beryllium: Chemical characterization of the powder in relation to its granularity

Combining the systematic quantitative chemical analysis of the light impurities H, C, N, and O, the quantitative thermal desorption of molecular H2O and H2, and X ray diffractometry of various size fractions of a commercial Be powder (SP-65 grade from Brush-Wellman) allowed the precise de-termination of the mean composition and equivalent mean thickness of the surface impurity phases in the passivation-contamination layer on the surface of the particles. The overall surface stoichi-ometry is as follows: 0.2 BeOcrystallized, 0.8 [BeO - 0.59 H2O]amorphous, 0.14 H2Oads The result of the elemental analysis by X-ray photoelectron spectroscopy of the unetched surface of a powder pellet is compared. Analysis of the metallic impurities reveals increased concentrations of Mg, Ca, Ti, and Cr in the finest fraction, presumably due to the liberation of fine particles of intermetallic phases by attrition.

Heterogeneous kinetics: from solid-gas reaction to solid-liquid dissolution

Abstract The extension of the formal kinetics of solid–gas reactions to solid–liquid dissolution is investigated theoretically. The work is based on modelling the dissolution of a solid in a closed system by a stoichiometrically simple chemical reaction A solid ⇄A liquid proceeding up to either an equilibrium state if the solid phase is initially in excess or exhaustion in the reverse situation. The initial amount of solid compared to the capacity of the liquid phase appears as a major factor of complexity. First, the rate equation is formulated in terms of the intensive variables without any assumption about the mechanism and the rate-limiting step. The resulting equations are compared to those derived from Nersnt boundary-layer theory of dissolution. Only when the solid phase is initially in deficit does the kinetic approach turn to be in the wrong if the rate of dissolution is assumed to depend on the maximum of the concentration gap. At this stage, the dissolution mechanism has to be taken into consideration and the rate equation adjusted accordingly. Finally, algebraic expressions for the fractional dissolution curves can be obtained. Except for particular solid liquid ratios they are not superimposable by an affinity relation.

Modeling of the kinetics of chemically controlled double solid-gas reactions

Abstract The experimental isothermal kinetics of reduction by H 2 of Co(II, III) oxides pure and doped with chromium are compared to the simulated kinetics of a double isothermal reaction under pure chemical control. The calculation of the simulated overall rate laws and fractional reactions is based on the convolution integral of the rate laws of the elementary reactions. The linear (shrinking unreacted core) and Erofeevs nucleation and growth laws are considered as elementary reaction kinetics. Otherwise unpredictable overall kinetics result from their combination. From their comparison with experiment it appears that varying the prereactional protocol (e.g. with a thermal pretreatment in vacuo) and increasing the chromium doping rate may affect the kinetics law and the kinetic constant of both elementary reactions.