André Douy

Polyacrylamide gel: an efficient tool for easy synthesis of multicomponent oxide precursors of ceramics and glasses

The combination of citrate complexation in aqueous chemistry with the steric entrapment effect of an auxiliary organic gel affords the possibility of easy and rapid synthesis of high quality multicomponent oxide powders. Stable solutions of strongly chelated cations are obtained by controlling the amount of ligand and the pH. These solutions are then easily gelled by in situ formation of polyacrylamide gel. This hydrous gel is directly decomposed by thermal treatment, resulting in fine and nanoscale powders. The polyacrylamide gel makes the citrate process easier and more rapid. The chemical homogeneity of the powders, usually high for the citrate process, may be improved by the steric effect of the organic gel. The process, routinely used for hundreds of preparations is illustrated by the synthesis of four compounds as examples. Nearly single-phase calcium hexaaluminate CaAl12O19 is obtained after heat treatment at 1100°C for 1 h. Pure lanthanum manganite La0.8Sr0.2MnO3 and barium zirconate BaZrO3 are formed at 650 and 1000°C, respectively. The synthesis of anorthite CaAl2Si2O8 is an example of the use of this process for silicates or aluminosilicates in aqueous chemistry without citrate chelation. The amorphous precursor crystallises at 1000°C into hexagonal anorthite before transforming into the stable triclinic form. The steric entrapment effect of the polyacrylamide gel is advantageous for aqueous chemical processes, and in the same way, the same concept may be used for other organic gels in organic media.

Aqueous Syntheses of Forsterite (Mg2SiO4) and Enstatite (MgSiO3)

Forsterite MgSiO4 and enstatite MgSiO3 were synthesized by two different aqueous processes. TEOS was directly hydrolyzed in aqueous solutions of magnesium nitrate, giving solutions of magnesium nitrate and silicic acid. For the first process these solutions were spray-dried and the powders heat treated to decompose the nitrate; and for the second one they were precipitated in a solution of ethylenediamine as a base, the resulting precipitate was filtered, washed and dried. Spray-dried or precipitated, no specific thermal event was detected by thermal analysis for the crystallization of forsterite (500–1000°C) while a strong and sharp exothermic peak traduced the crystallization of enstatite at 800°C. Very minor secondary phases could be detected by X-ray diffraction up to 1200°C for the spray-dried powders, while the precipitated powders presented a higher chemical homogeneity, but much care had to be taken for a quantitative precipitation. As some minor secondary phases like SiO2 or some polymorphs of MgSiO3 could be not detected by XRD up to 1300°C, higher thermal treatments were necessary to control the purity or the desired phase.

Block copolymers with a polyvinyl and a polypeptide block: factors governing the folding of the polypeptide chains

Abstract AB block copolymers polystyrene-poly(γ-benzyl- l -glutamate) (SG) of various molecular weights and compositions were synthesized and studied by X-ray diffraction and infra-red spectroscopy. They exhibit lamellar mesophases in the solid state and in dioxane concentrated solution. Each sheet of the lamellar structure results from the superposition of two layers: one formed by the polyvinyl chains in a disordered conformation, the other formed by the polypeptide chains in an α-helix conformation arranged in a hexagonal array and generally folded. The comparison of the lamellar structure of copolymers polystyrene-poly(γ-benzyl- l -glutamate) (SG), polybutadiene-poly(γ-benzyl- l -glutamate) (BG) and polystyrene-poly(e-carbobenzoxy- l -lysine) (SCK) showed that: (1) for the three types of copolymers (copolymers SG or BG or SCK) the number of folds of the polypeptide chains increases with the molecular weight of both the polyvinyl and the polypeptide blocks; (2) for copolymers of fixed molecular weight of the polyvinyl block and fixed degree of polymerization of the polypeptide block the number of folds of the polypeptide chains depends upon the nature of both the polyvinyl and the polypeptide blocks: a polypeptide chain is more folded when it is linked to a polystyrene chain than to a polybutadiene chain but a poly(e-carbobenzoxy- l -lysine) chain is more rigid than a poly(γ-benzyl- l -glutamate) chain.

Characterization of mono- and diphasic mullite precursor powders prepared by aqueous routes. 27Al and 29Si MAS-NMR spectroscopy investigations

The structural evolution from amorphous to crystalline mullite, for different 3Al2O3 · 2SiO2 mono- and diphasic precursors, has been investigated by 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The crystallization has also been studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the 29Si NMR spectra. A close agreement is found with the composition deduced from the lattice parameters measured by XRD. For monophasic precursors the amount of hexa-coordinated aluminium atoms decreases when the temperature increases while Al(IV) and Al(V) increase. Al(VI) practically completely disappears just before the crystallization at 980 °C. An alumina-rich mullite 2Al2O3 · SiO2 (2∶1 mullite) is then formed through a strong exotherm. An enthalpy of 75 kJ per mol is determined for the crystallization of the 2∶1 mullite. At higher temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the 29Si NMR spectroscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO6 octahedra before the crystallization at 980 °C is higher. Spinel crystallizes and continues to become richer in alumina until it reacts with silica to form the stoichiometric 3∶2 mullite at 1260–1275 °C. The nature of the crystallization is related to the local composition of the amorphous alumino-silicate network and to the amount of AlO6 octahedra on approaching 980 °C.

Amphipathic block copolymers with two polypeptide blocks: synthesis and structural study of poly(Nε-trifluoroacetyl-L-lysine)-polysarcosine copolymers

Abstract AB block copolymers of poly (N e - trifluoroacetyl- l -lysine)-polysarcosine (KtSa) were synthesized and studied by X-ray diffraction. In order to obtain copolymers with a polydispersity small enough to allow the formation of mesophases, the first block of poly (N e - trifluoroacetyl- l -lysine ) was fractionated before using it as a macromolecular initiator of the polymerization of the second block of polysarcosine. Block copolymers of KtSa exhibit lamellar mesophases in the solid state and in concentrated aqueous solution. Each sheet of the lamellar structure results from the superposition of two layers: one formed by the hydrophilic polysarcosine chains in a disordered conformation, the other formed by the hydrophobic poly (N e - trifluoroacetyl- l -lysine ) chains in an α-helix type conformation arranged in a hexagonal array and tilted. The influence of the water content of the mesophase and of the composition of the copolymers on the structural parameters of the lamellar structure was analysed and it was shown that the angle of tilt of the poly (N e - trifluoroacetyl- l -lysine ) helices increases with both the water content of the mesophases and the length of the polysarcosine chains.

New aqueous mullite precursor synthesis. Structural study by 27Al and 29Si NMR spectroscopy

Abstract A mullite precursor gel is prepared by the slow and homogeneous generation of ammonia inside an aqueous solution of aluminium nitrate and silicic acid, urea being used as the base generator. At 80–100 °C a silica gel is rapidly formed, by a catalytic reaction, and then this gel is slowly digested by the partially hydrolysed aluminium atoms which link to silica by SiOAl bonds. The structural evolution has been studied by 27Al and 29Si NMR spectroscopy. The first aluminium atoms are incorporated in tetrahedral symmetry and the following ones remain hexacoordinated. The system evolves to soluble aluminosilicate colloids with a local structure close to that of imogolite or allophane: each silicon atom being linked to three AlO6 hexahedra and a hydroxyl group. By the completion of the hydrolysis the colloids are cross linked to a gel precursor of mullite. With thermal treatment the imogolite-like orthosilicate units arrangement is rapidly lost, but the xerogel remains chemically homogeneous with a random distribution of Al and Si atoms in the lattice, the Al atoms being four-, five- and six-fold coordinated. The xerogel crystallises at 980 °C into an alumina-rich mullite and amorphous silica, and the 3Al2O3· 2SiO2 stoichiometry is reached at 1300 °C by reincorporating the silica in the lattice.

Evolution of the Si environment in mullite solid solution by 29Si MAS-NMR spectroscopy

Abstract The evolution of the silicon environment has been investigated in a mullite solid solution, Al4 + 2xSi2 − 2xO10 − x, using 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and X-ray diffraction experiments. Various single phase precursors with compositions ranging from 50 to 71.4 mol% Al2O3 ( 2 ≤ Al Si ≤ 5 ) were synthesized, and the chemical compositions of the crystallized samples were determined from their lattice parameters. Four sites have been observed by 29Si NMR spectroscopy, whatever the composition in the solid solution, at −86 ppm, −90 ppm, −94 ppm and a shoulder at −80 ppm. Their occupancies have been obtained by simulating the spectra with Gaussian lines. They evolve linearly with the alumina content and with the number, x, of oxygen vacancies. The main contribution, corresponding to the shoulder at −80 ppm, is in fact a broad line centered at −88 ppm. It increases with a slope of 2x in the solid solution and is extrapolated to zero for sillimanite (x = 0). It has been attributed to a silicon site in triclusters or near oxygen vacancies. The occupancies of the sites at −86 ppm and −90 ppm decrease with a slope of 1x with the increase of Al2O3 content. They are assigned to sillimanite-type sites: the original one (−86 ppm) or one modified by replacement of an Al by a Si atom in the second sphere of coordination (−90 ppm). The relation between the structures of sillimanite and mullite is clearly shown. The occupancy of the minor fourth site (−94 ppm) slightly increases with x. This site may result from another replacement of Al by Si around the silicon atom.

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by 17O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and 17O, 27Al and 29Si MAS NMR spectroscopy.

Solid phase transformation and metlting of the compounds Ln4Al2O9 (Ln Gd, Dy, Y)

High-temperature studies of the compounds Ln4Al2O9 (Ln Gd, Dy, Y) were performed using both DTA and dilatometry. A solid phase transition is observed in the three compounds at a temperature which increases from 1090°C for Ln Gd to 1367°C for Ln Y. This transition is reversible with a hysteresis as large as 190°C. The enthalpy of transformation is about 2 kJ mol−1. During the heating, the transition is associated with a linear expansion of about 0.2%. Gd4Al2O9 is shown to display an incongruent melting at about 1900°C. The congruent melting point of the other compounds increases from 1917°C for Dy4Al2O9 to 1987°C for Y4Al2O9.

29Si MAS-NMR in lead silicates

Abstract High resolution 29 Si nuclear magnetic resonance spectra of four crystalline lead silicates have been studied: alamosite, PbSiO 3 ; lead barysilite, Pb 3 Si 2 O 7 ; H- and M-Pb 2 SiO 4 . The samples were synthesized by two different methods. Alamosite and barysilite were obtained by heating the glass of the corresponding composition. A spray-drying method was used for H- and M-Pb 2 SiO 4 synthesis in order to obtain pure phases, and not a mixture of these two polymorphs as is usually the case with classical methods. These phases give rise to six different silicon environments, well differentiated by their chemical shifts. All these sites are Q 2 , in classical silicate notation, except for the unique site Q 1 , in barysilite. The structural correlation was not limited to the SiO 4 tetrahedra, but extended to the second neighbours: starting from the local structure around each silicon, a correlation between the isotropic 29 Si chemical shift and the number of lead atoms in the second coordination sphere is proposed. Some consequences on glass formation by lead oxide chains are discussed.