Alain C. Pierre

Sol-gel processing of ZnS

Abstract A sol-gel process has been developed for the chemical synthesis of ZnS at room temperature using zinc tert-butoxide and H2S as precursors in a toluene solution. The obtained reaction product was a yellow semi-transparent gel which dried to a reddish-orange solid. Characterization of the dried gel by XRD, IR and EDS confirmed that the product is ZnS with a Zn 5 atomic ratio ≈ 1:1.

Clay sediment-structure formation in aqueous kaolinite suspensions

Plate-like kaolinite particles have different crystal structures on edges and faces (van Olphen, 1977), thus the electric property of the edges is different from that of the faces. After treatment by the Schofield and Samson method (1954), kaolinite particles are converted to the Na form and carry negative electric charges on their faces and positive charges on their edges. In aqueous kaolinite suspensions, clay particles can interact in the edge to edge (EE), edge to face (EF), and face to face (FF) modes (van Ophen, 1977). They constitute different types of flocs or aggregates in suspensions (Michaels and Bolger, 1962) and can form sediments with different structures such as cardhouse, bookhouse, dispersed, stair-step structures, etc. (van Olphen, 1977; Bennett and Hulbert, 1986). The interaction of clay particles and structure formation of flocs or aggregates in aqueous media were studied systematically by sedimentation, rheology, and electrophoresis by Rand and Melton (1977), Melton and Rand (1977a, 1977b), and Michaels and Bolger (1962). In the present study, clay-sediment samples, prepared by supercritical drying, were observed under a scanning electron microscope (SEM). Since supercritical drying eliminates the capillary force on a sediment sample, this technique can relate sediment structures to their sedimentation behaviors. The process of microstructure formation in aqueous kaolinite suspensions is discussed by comparing the results obtained here with previous work (Rand and Melton, 1977; Melton and Rand 1977a, 1977b; Michaels and Bolger, 1962).

COLLOIDAL BEHAVIOUR OF MONTMORILLONITE IN THE PRESENCE OF FE3+ IONS

Abstract Three types of Fe additives were used to study the influence of the clay particle and Fe additive interaction on the dispersion–coagulation behavior of aqueous montmorillonite suspensions. They were unaged FeCl 3 , aged FeCl 3 and β-FeOOH particles. The present result could be explained in the light of the theories of Derjaguin, Landau, Verwey, and Overbeek (DLVO) and diffusion-limited aggregation (DLA). Fe 3+ ions not only acted as counterions to compress the electric double layer around clay particles but also hydrolyzed to produce Fe 3+ hydrolysis products. The hydrolysis products could modify the clay surface electric properties and act as a bonding agent to promote coagulation in the suspensions. Furthermore, the sediment behavior can be related to the microstructure of the corresponding sediments. A DLA fractal structure was observed in the montmorillonite sediments using a scanning electron microscope (SEM), after application of a supercritical-drying technique.

Structure of kaolinite flocs formed in an aqueous medium

The structure evolution of kaolinite flocs during their formation in aqueous medium was studied under a scanning electron microscope by hypercritical drying with liquid CO2, as a function of Fe electrolyte content. Without any Fe electrolyte, a sediment with uniform porosity formed by accumulation of particles under gravity. Fresh Fe electrolytes made it possible to aggregate the kaolinite particles to flocs with an architecture consistent with diffusion limited aggregation (DLA). With 0.67 mM Fe, a single shrinking DLA flocs layer formed. With 3.33 mM Fe, extensive face to face aggregation of the clay particles occurred and a sediment with preferred horizontal orientation accumulated. With 0.17 mM Fe a shrinking DLA floc layer and a uniformly porous accumulated sediment formed concurrently, while with 1.67 mM Fe a shrinking DLA floes layer and an accumulated sediment with preferred orientation formed concurrently.

Preparation of tungsten sulfides by sol—gel processing

Abstract Tungsten sulfides were synthesized by reaction of different tungsten alkoxides and hydrogen sulfide in toluene. The reaction was performed in toluene solution at room temperature. Tungsten ethoxide yielded a brown black gel, while tungsten dichloride ethoxide produced a brown—black colloidal powder. The W/S at% ratios were 1:2.7 and 1:1.4 in the gel and powder, respectively. Both sol—gel products were amorphous materials. The gel appeared to be highly viscous liquid formed from spherical colloidal particles connected into a three-dimensional network. However, the powder consisted of loose colloidal particles whose radii were in the range 0.5 to 1 μm.

Sedimentation behaviour of kaolinite and montmorillonite mixed with iron additives, as a function of their zeta potential

In order better to understand the packing structure of clay sediments mixed with iron additives, as observed in a scanning electron microscope and published in previous studies, an experimental investigation of the zeta potential of clay particles was undertaken. The results were used to build sedimentation diagrams comparable with the experimental sedimentation diagrams. The results were analysed in the light of the known behaviour of iron additives in aqueous solutions and their interaction with clay particles.

Effect of interaction between clay particles and Fe3+ ions on colloidal properties of kaolinite suspensions

Fine kaolinite suspensions were mixed with unaged or aged FeCl3 in this experiment. The interaction between clay particles and Fe3+ hydrolysis products was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The proportion of Fe adsorbed was measured and the electrical charge on the clay particles was determined by electrophoresis. The effect of this interaction on flocculation of clay suspensions was investigated in a series of sedimentation tests. The Fe3+ ions acted as counterions when their concentration was low and when unaged FeCl3 solution was used. Otherwise, their hydrolysis complexes acted as a bonding agent between kaolinite particles. The dispersion-flocculation behavior of kaolinite suspensions was found to be in agreement with the theory of Derjaguin, Landau, Verwey and Overbeek (DLVO), as the sedimentation behavior could be predicted from the data of zeta potentials (ζ).

SEDIMENTATION BEHAVIOR OF A FINE KAOLINITE IN THE PRESENCE OF FRESH Fe ELECTROLYTE

The sedimentation behavior of a fine kaolinite, comprising a substantial proportion of colloidal particles as well as non-colloidal ones, has been studied when fresh FeCl3 or F2(SO4)3 electrolytes are added. The sedimentation behavior depends on the pH and the nature of electrolytes and can be explained qualitatively, in our study, by the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO theory). Fe helps also to aggregate the kaolinite particles in flocs. Two extreme kinds of qualitative sedimentation have been observed: flocculation-sedimentation and accumulation-sedimentation. However, the transition between the two kinds of sedimentation is quite progressive. The present results are discussed in reference to the DLVO theory and the hydrolysis behavior of Fe electrolytes.The sedimentation behavior of a fine kaolinite, comprising a substantial proportion of colloidal particles as well as non-colloidal ones, has been studied when fresh FeCl3 or F2(SO4)3 electrolytes are added. The sedimentation behavior depends on the pH and the nature of electrolytes and can be explained qualitatively, in our study, by the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO theory). Fe helps also to aggregate the kaolinite particles in flocs. Two extreme kinds of qualitative sedimentation have been observed: flocculation-sedimentation and accumulation-sedimentation. However, the transition between the two kinds of sedimentation is quite progressive. The present results are discussed in reference to the DLVO theory and the hydrolysis behavior of Fe electrolytes.

Microstructure of kaolinite sediments made with unaged FeCl3

Abstract Fine kaolinite suspensions were prepared with unaged FeCl 3 to perform sedimentation tests in this study. The microstructure of kaolinite sediments was examined by scanning electron microscopy after CO 2 supercritical drying. The interaction between clay particles and FeCl 3 in an aqueous medium was investigated by means of electrophoresis and analytical transmission electron microscopy. Clay particles were connected in three basic modes — edge to edge (EE), edge to face (EF) and face to face (FF). Clay particles with different connection modes formed aggregates or flocs in sediments. The flocs not only exhibited a fractal structure themselves, but also formed sediments with fractal characteristics. This study determined the formation conditions of different clay particle connection modes, aggregates or fractal flocs in the presence of unaged FeCl 3 in an aqueous medium.

Hydrous copper oxide gels

Abstract Copper is an irreplaceable element in the synthesis of high temperature superconductors. In this paper, an investigation of the aqueous gelation of copper from two types of precursors, the cupric nitrate and the cupric acetate, is reported. The structure and properties of the gels obtained with various complexing agents and their transformations during heat treatment are compared. It is shown that a bloating property already reported in gels for superconductors, is maintained for gels with copper as the only metal, and is due to a layered structure.