Infiltration and Top Seed Growth-Textured YBCO Bulks With Multiple Holes

The recently reported hole-patterned YBa₂Cu₃O _y (Y123) bulks with improved superconducting properties are highly interesting from material quality and application variety points of view. It is well known that the core of plain bulk superconductors needs to be fully oxygenated and some defects like cracks, pores, and voids must be suppressed in order that the material can trap a high magnetic field or carry a high current density. To minimize the above defects, we have used a combination of standard superconducting ceramic processing and an infiltration technique to prepare regularly perforated YBa₂Cu₃O _y (Y123) bulk superconductors. This process leads to negligible shrinkage upon annealing and a uniform distribution of Y211 inclusions. Texture was evidenced by neutron pole figure measurements. Flux mapping was used to verify the superconducting homogeneity of the samples and to investigate the field-trapping ability. In addition, the textured drilled samples were reinforced using resin or metal impregnation and the influence of the different processing steps on the hardness of the materials has been investigated.     

Granular and Lamellar Thermoelectric Oxides Consolidated by Spark Plasma Sintering

Using the spark plasma sintering (SPS) technique, dense nanostructured Ca_0.95Sm_0.05MnO₃ (n-type) and textured Ca₃Co₄O₉ (p-type) ceramics were prepared. Nanoceramic powders of doped n-type were synthesized using two routes: coprecipitation and solid-state reaction. The SPS method has been used to control the samples’ densification and grain growth. Microstructural investigations reveal that the SPS technique results in high bulk density and homogeneous morphology for Ca_0.95Sm_0.05MnO₃, and grain alignment for Ca₃Co₄O₉. The thermoelectric and mechanical properties were investigated, showing a dependence on the starting grain size and being governed by the SPS conditions.     

Properties of metakaolin based geopolymer incorporating calcium carbonate

An alkaline solution, thermally activated kaolinite clay and a mineral additive (calcium carbonate) were mixed with the aim to elaborate a geopolymer material with physical and mechanical properties comparable to those of classical construction materials.The starting reagents were characterized by quantitative chemical analyses (XRF), mineralogical analyses (XRD), thermal gravimetric analyses (TGA), and grain size distribution measurements. The setting of the mixture (polymerization) was implemented by measuring the evolution of the viscosity as a function of time at different temperatures.The geopolymers were synthesized at a temperature of 40 °C. The investigation of the mechanical behavior reveals that these materials display acceptable characteristics: the flexural and compression strength are around 4.6 and 26 MPa respectively, for an added calcium carbonate over dry matter ration up to 12% by weight.The promising results exposed in this paper show that the geopolymer formulations can be adapted for applications in construction and civil engineering structures as an alternative to conventional materials.     

An energy evaluation for C/SiC composite materials

An analysis of the load-load point displacement curve is presented in the general case where residual displacement appears. This method was used to determine R curves of fibre composite ceramic materials. The consequence of the residual displacements is a large increasing R curve, followed by a decrease towards a steady state value. The length required for the machined notch to be transformed in a crack and then to allow material parameters measurements gives information on the minimum size for test specimens.

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Résumé On présente une analyse de la courbe charge-déplacement, dans le cas général où se manifeste un déplacement résiduel. La méthode est utilisée pour déterminer les courbes R relatives à des matériaux composites à fibres céramiques. Les déplacements résiduels ont pour conséquence une croissance importante de la courbe R, suivie d'une décroissance vers une valeur stable. Des informations sur la taille minimum des éprouvettes d'essais sont fournies par la largeur nécessaire à une entaille usinée pour qu'elle se transforme en fissure et qu'elle permette des mesures sur les paramètres du matériau.

     

High temperature mechanical behaviour of an uncoated SiC-SiC composite material

Flexural tests have been performed on unnotched specimens of an uncoated SiC-SiC laminar composite material to check the influence of specimen thickness and temperature (22 to 1200C in air) on the strength and the elastic modulus. An important modification of the loading curves and crack paths occurred when the temperature was increased, corresponding to a strengthening of the fibre-matrix interface.     

Development of multilayer textured Ca3Co4O9 materials for thermoelectric generators: Influence of the anisotropy on the transport properties

Multilayer Ca₃Co₄O₉ (349) thick thermoelectric (TE) materials were fabricated by hot-pressing stacked dense and strongly textured single-layer samples. Microstructure and volume quantitative texture investigations were undertaken by using scanning electron microscopy and neutron diffraction techniques, respectively. The results show a bulk density similar to single-layer samples, but remarkable texture strength reinforcement. The electrical resistivity, ρ, and Seebeck coefficient, S, were reproducibly measured in directions parallel (ρ^c and S^c) and perpendicular (ρ^ab and S^ab) to the mean c-axis. ρ showed a high anisotropy ratio ${\rho }^c/{\rho }^{ab}$ of 13.5 and 8.8 at 300 and 900 K, respectively, and ρ^ab kept the same values whereas ρ^c decreased in the multilayer samples. S^ab and S^c unexpectedly revealed different values. The thermal conductivity also displayed a significant anisotropy, with ratio ${\kappa }^{ab}/{\kappa }^c=2.7$ at 900 K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio $Z{T}^{ab}/Z{T}^c=4.6$. ZT^ab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication.     

Structural modifications of a SiC-SiC material exposed to high temperatures in air

SiC-SiC specimens were exposed to high temperatures in air (from room temperature to 1400 ° C). A pronounced modification of the interphase between the fibres and the matrix appeared as a consequence of the oxidation of the carbon at the interface.     

Strength and creep behavior of geomaterials for building with tannin addition

A clay mined in Djénné (Mali) was used to elaborate geomaterials, with tannins addition from Parkia biglobosa pods (Néré). The compressed blocks of clay–sand mixtures show a composite microstructure. The clay contains significant quantities of kaolinite, illite, pyrophyllite, quartz and iron minerals as goethite and ferrihydrite. Quantitative mineralogical composition was assessed by X-ray diffraction and DTA/TG analyses. When tannin extract is added, the formation of a chemical complex with clay–iron hydroxides is evidenced by IR spectroscopy, which reveals specific bands. The compressive strength and creep behavior under 0.2 MPa during 20 days evidence different behaviors depending on humidity and tannin contents. Creep curves exhibit successive stages which can be described by the Granger model. The multi stage creep is explained by the visco-plastic behavior of clay constrained between large sand grains, where local and delayed deformation may occur with micro-cracking. Tannin addition has proved to increase the macroscopic strength, and reduce micro-cracking.