Monique Gervais

Synthesis of YBa2Cu3O7−x by sol-gel route formation of YBaCuO oxycarbonate intermediate

Abstract High- T c superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders. However, sol-gel synthesis of this oxide involves the formation of intermediate phases which impedes the obtaining of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800 and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure — SG P4/ mmm — similar to the parent oxide with a stoichiometry close to YBa 2 Cu 2.95 (CO 3 ) 0.35 O 6.6 . The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the sol-gel precursor is annealed at 925°C in O 2 .

Structural study of polybutadiene‐poly(ethylene oxide) block copolymers. Influence of the nature of the amorphous block on the refolding of the poly(ethylene oxide) chains

Our earlier studies on block copolymers polystyrene-poly(ethylene oxide) has allowed to state the influence of the molecular weight of the copolymer and of its composition on the refolding of the crystallized poly(ethylene oxide) chains. To precise the effect of the nature of the amorphous block, we have studied by X-ray diffraction and differential scanning calorimetry, the lamellar crystalline structures exhibited by block copolymers polybutadiene-poly(ethylene oxide) (BEO) in the presence of a preferential solvent of one block: xylene for polybutadiene (PB), acetic acid or acrylic acid for poly(ethylene oxide) (PEO). In systems BEO copolymer/preferential solvent of PEO, the lamellar structure with crystallized PEO chains (LCC) exists at temperatures below about 45°C and for solvent concentrations ranging from 0 to a value characteristic of the copolymer. In the LCC structure, the chains of PEO crystallize in two layers; the solvent forms a third layer located between the PEO layers. We have studied the influence of the nature of the amorphous block on the variation of the structural parameters with the solvent concentration. In systems BEO copolymer/preferential solvent of PB, the lamellar structure with crystallized PEO chains (LC) appears at temperatures below about 50°C and for all solvent concentrations where the mesophase exists. In the LC structure, the chains of PEO crystallize folding in two superposed layers. We have established the influence of the solvent concentration and the nature of the amorphous block on the number of folds and the crystallinity of the PEO chains.

Use of freeze-fracture electron microscopy to study the refolding of crystallized chains in block copolymers

Abstract Block copolymers with an amorphous block or polystyrene (PS) of polybutadiene (PB) and a crystallizable block of poly(ethylene oxide) (PEO) or poly(ϵ-caprolactone) (PCL) exhibit lamellar crystalline structures. In these structures the elementary sheet results from the superposition of two layers: the first contains the amorphous blocks and the second contains the crystallized and refolded PEO or PCL chains. Study of the lamellar structure by freeze-fracture and electron microscopy provides micrographs that show crystallization of the PEO and PCL chains in two superposed layers and confirm the model proposed to explain the results of earlier X-ray diffraction and differential scanning calorimetry studies.

Liposomes injected intravenously into mice associate with liver mitochondria.

Liposomes encapsulating uranyl acetate or ferritin were injected intravenously into mice. At periods of 20 min, 1 h and 4 h post-injection, animals were killed, and livers were excised. Transmission electron micrographs of liver tissue showed association of oligolamellar liposomes with mitochondria for each time period. At 1 h post-injection, an average of one out of ten mitochondria was associated with liposomes. In most cases, the liposomes were clearly enclosed in a cytoplasmic vacuole. Phagocytosis by Kupffer cells as well as fusion with primary lysosomes and inclusion in secondary lysosomes was observed. No difference in intracellular fate was observed when lactosylceramide was incorporated in the liposome bilayers, suggesting that the differences observed in biochemical studies are at the level of liposome-plasma membrane interaction. When liposomes containing uranyl acetate were intravenously injected and hepatocytes were isolated by collagenase perfusion one hour later, transmission EM revealed the presence of liposomes in these cells, in cytoplasmic vacuoles in the cytoplasm and in association with mitochondria. A freeze-fracture-etching analysis of liver tissue excised 20 min after injection of liposomes encapsulating ferritin, further supported the observation that liposomes associate with mitochondria in the liver.

Crystallization of Y3Al5O12 garnet from deep undercooled melt effect of the Al-Ga substitution

Abstract Melts of garnet Y 3 Al 5 O 12 and compositions lying on both sides of this compound were rapidly cooled in contactless conditions by using an aerodynamic levitation device coupled with a CO 2 laser. The crystallized phases were analyzed at room temperature by XRD and 27 Al MAS NMR. As the temperature reached by the supercooled melt decreases, the solidification was shown to lead successively to a mixture of YAlO 3 and Al 2 O 3 , a monophasic garnet type structure and a glass. The formation of Y 3 Al 5 O 12 starts at about 920 °C under the melting temperature and only 50 °C above the glass transition temperature. The solidification behaviour of liquid Y 3 Al 5− x Ga x O 12 ( x = 0.5, 1) was similar. Results clearly show the difficulties of garnet to nucleate. They are consistent with a rapid increase of the melt viscosity in the temperature range just above the formation of glass.

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.

Chemical inhomogeneities in YBaCuO produced by conventionnal sintering

This paper addresses chemical inhomogeneities that are frequently developed during the sintering process of the high T/sub c/ yttrium-barium copper oxide: YBa/sub 2/Cu/sub 3/O/sub 7-chi/ (123). To evaluate the potential benefit of chemical routes with respect solid state reaction synthesis, a new process has been developed using a spray pyrolysis technique. It provides excellent chemical homogeneity in the powder and a low synthesis temperature (850{sup 0}C) of the (123) phase. Several samples with different stoichiometries were prepared and sintered. The resulting chemical homogeneity has been studied by electron microprobe (EDS). The study (by DTA) of the phase diagram characteristics shows that in most cases it is a liquid phase that promotes the sintering but also chemical inhomogeneities.

Implications of the Y2O3BaOCuO liquidus for processing pure YBa2Cu3O7−x material

Abstract Results relating to the liquids relationships in the CuO-rich corner of the 1 2 Y 2 O 3  BaO  CuO phase diagram were used to identify the partial meltings occurring in YBaCuO powders at temperatures below the peritectic decomposition of the YBa2Cu3O7−x compound. The production of this compound in three atmospheres with different oxygen potentials was followed by X-ray diffraction and differential scanning calorimetry. The origin of the partial meltings in these powders was found in the failure of the synthesis to be complete (i.e. to reach the equilibrium state) at temperatures below the solid-liquid phase transitions, implying the presence of the compounds YBa2Cu3O7−x, BaCuO2 and CuO. This difficulty is overcome if the powders are annealed in an O2 atmosphere which allows treatment temperatures as high as 940°C. Sintering of such powders up to 1000 °C is not promoted by a liquid phase. Differential scanning calorimetry proved to be an easy and valuable method for checking the purity of the YBa2Cu3O7−x compound.

Surface studies of polypeptidic block copolymers by electron spectroscopy for chemical analysis: Poly(Nε-trifluoroacetyl-l-lysine)-polysarcosine diblock copolymers

Abstract The surface of poly (N e - trifluoroacetyl- l -lysine)-polysarcosine (Kt.Sa) block copolymers was studied by electron spectroscopy for chemical analysis (e.s.c.a.) on films cast from methanol. The e.s.c.a. results indicate that the copolymer composition of the surfaces (e.g. outermost ∼ 35 A) may be significantly different from the overall bulk composition. A surface excess of Kt is observed for copolymers containing less than about 45 mol% Kt; the Kt excess increases as the bulk content of Kt decreases. Angular-dependent studies show that the concentration of Kt increases as the air-copolymer interface is approached. X-ray diffraction studies of the Kt.Sa copolymers in concentrated solution in methanol and in the dry state show that they exhibit a lamellar structure. Knowledge of this structure makes it easier to interpret the e.s.c.a. results. A model of the surface topography is proposed, namely that the lamellar structure is perpendicular to the air-polymer interface and the Kt domains are elevated above the Sa domains.

Metastable solidification process of Ln3Al5O12 (Ln≡Dy, Y, Lu) garnets

Abstract The solidification process of liquid Ln 3 Al 5 O 12 (Ln ≡ Dy, Y, Lu) garnets was investigated by differential thermal analysis (DTA) and a critical temperature T cl was found. Cooling from temperatures below T cl leads to the crystallization of the single garnet phase. Cooling from temperatures above T cl leads to the solidification of a mixture of LnAlO 3 perovskite and α-Al 2 O 3 . For Ln ≡ Y, the results are in agreement with previously proposed stable and metastable phase diagrams. The variation of T cl with composition was investigated. Rapid cooling of liquid yttrium aluminium garnet in contactless conditions was performed, using an aerodynamic levitation device coupled with a CO 2 laser. The results are consistent with those obtained by DTA. In addition, cooling from temperatures above around 2400 °C leads to the crystallization of a metastable form of garnet.