Electron back-scattering diffraction (EBSD) can be successfully performed on SiC coatings for HTR fuel particles. EBSD grain maps obtained from thick and thin unirradiated samples are presented, ...along with pole figures showing textures and a chart showing the distribution of grain aspect ratios. This information is of great interest, and contributes to improving the process parameters and ensuring the reproducibility of coatings.
Origin of growth defects in pearl Bourrat, Xavier; Qiao, Li; Feng, QingLing ...
Materials characterization,
10/2012, Letnik:
72
Journal Article
Recenzirano
Odprti dostop
Pearls are produced from a natural biomineralisation process. The structural unit is a biocrystal whose formation is controlled by organic molecules. In the present paper we studied a major growth ...defect to highlight the role of the organics in the growth mechanism. Some freshwater pearls exhibit a lack of lustre, also known as ‘milky pearl’ defect. This defect is related to the formation of vaterite instead of aragonite during the biomineralisation growth process. We used Rock Eval pyrolysis, a new technique in this field, to quantify a noticeable increase of the organic matrix in milky pearls. At least 20% more organics were found when vaterite forms than aragonite. To further study its role during the growth process, the organic matrix was extracted using three different protocols and characterised by infrared spectroscopy (FTIR) and liquid chromatography (HPLC). The fraction of the organic matrix which is soluble in water (WSM fraction) was significantly different when analysed by FTIR and by HPLC. This fraction is very likely occluded within the mineral (intracrystalline matrix). To examine its role, WSM extracted from milky pearls was used as additive in controlled calcium carbonate growth experiments. In this case, vaterite crystals were obtained for the greater part. When WSM extracted from healthy pearls was used, aragonite and calcite crystals were grown. The polymorph (vaterite vs aragonite) appeared to be clearly connected to the WSM organic fraction. Several hypotheses may explain this result: e.g., a minor disorder of the organic composition of the extrapallial fluid, an excess of Mn or Mg or a blocking of the growth at a transient stage. Finally, we propose to use cathodoluminescence as a non‐destructive technique to sort these defective pearls.
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► Fresh water pearls sometimes exhibit a defect related to the formation of vaterite. ► It is revealed by a strong cathodoluminescence signal (non-destructive). ► An excess of proteins appears when it occurs. ► When extracted and added to a controlled growth, these proteins stimulate vaterite. ► Intracrystalline proteins of regular pearls stimulate aragonite.
We report an interesting finding of calcium carbonate (CaCO3) crystal growth in the silk fibroin (SF) hydrogel with different concentrations by a simple ion diffusion method. The experimental results ...indicate that the CaCO3 crystals obtained from silk fibroin gels with low and high concentrations are all calcites with unusual morphologies. Time-dependent growth study was carried out to investigate the crystallization process. It is believed that silk fibroin hydrogel plays an important role in the process of crystallization. The possible formation mechanism of CaCO3 crystals is proposed. This study provides a better explanation of the influence of silk fibroin concentration and its structure on CaCO3 crystals growth.
► Calcium carbonate crystals are grown into silk fibroin hydrogels. ► Silk fibroin (SF) gels with different concentrations have no effect on CaCO3 polymorph. ► Dissolution and recrystallization are involved in CaCO3 crystallization within SF gel. ► A possible growth mechanism of CaCO3 crystals in SF gel is proposed.
The oxidation of N ex-PAN carbon fibre preform as well as the oxidation of the related carbon/carbon (C/C) composites are compared with the oxidation of their counterparts based on M ex-PAN carbon ...fibres. The oxidation test is conducted on polished specimens by thermogravimetry. In order to localise and quantify the oxidation, SEM observation was performed on the specimens before and after the test. Whatever the matrix, oxidation of the composite is controlled by the oxidation of the preform itself. The N preform oxidizes more slowly than the M reference. The oxidation features of M-based materials are related to catalytic effects because of the presence of pitting at the interface. Oxidation of N-based composites is localised on the external surface and at the fibre/matrix interface. In the matrix, oxidation is controlled by the anisotropy of the pyrocarbon: smooth laminar pyrocarbon (C/C1) oxidising more slowly at the surface than rough laminar pyrocarbon (C/C2), but deeper in the bulk at the fibre/matrix interface.
Sheet nacre is a natural biocomposite with a multiscale structure including a mineral phase of calcium carbonate (97 wt.%) and two organic matrices (3 wt.%). The mineral phase is constituted by an ...arrangement of CaCO₃ biocrystal nanograins (ca 40 nm in size) drowned in an “intracrystalline” organic matrix (4 nm thick) in order to form a microsized flat organomineral aragonite platelet. These platelets are themselves surrounded by an “intercrystalline” organic matrix (40 nm thick) building up a very tough materials. This microarchitecture referred to as “bricks and mortar” nacre structure, is mainly studied for the creation of new organic/inorganic hybrid materials. Currently, only little is known about the nacre mechanical behaviour under dynamical loading and more particularly under tribological conditions which involve shocks and thermal effects simultaneously. This paper brings out the thermal-induced damage mechanisms effect on the wear of sheet nacre by the assessment of the thermal component of the friction with a scanning thermal microscope. Results reveal that the mean contact pressure is the main driving force involved in the degradation of the organic constituents. For the lowest mean contact pressure (<0.4 MPa), wear is rather weak because the friction-induced thermal component is not sufficient for degrading the organic matrices. In contrast, beyond 0.4 MPa, the friction-induced contact temperature rises up over the melting point of the organic matrices, and may even reach the temperature of the aragonite-calcite phase transformation increasing dramatically the wear of sheet nacre.
The thermal behaviour of Pinctada margaritifera nacre was studied at different temperatures by means of thermal gravimetric, thermo-mechanical and Rock-Eval analyses. From the mechanical point of ...view nacre exhibited a complete reversible behaviour up to 230 °C. The bio-aragonite allotrope was seen to be as stable as the abiotic aragonite up to 470–500 °C. It was also evidenced that the organic phase was keeping cracking oxygen functions at temperatures as high as 650 °C. Nacre thermal behaviour could be described following four distinctive stages and discussed in comparison with previous data obtained in oxidative conditions.
The structural, morphological and textural characteristics of BN coatings processed by CVD from (BCl3, NH3, H2) gas mixtures, at low pressure (P=1.3 kPa) and low temperature (T=800NBDGC), with ...different QNH3/QBCl3 gas flow rate ratios, have been investigated. Whereas the as-processed coatings are amorphous, a high degree of crystallisation can be achieved after a post-deposition heat treatment. The sole post-elaboration heat treatment does not allow the improvement of the crystallisation degree of the boron nitride. The presence of a small amount of oxygen, resulting from a simple exposure of the coating to a controlled atmosphere (temperature, moisture rate), is also necessary. For given temperature and pressure, a wide range of microstructures of the heat-treated BN coatings, from isotropic to anisotropic, can be observed by varying the QNH3/QBCl3 ratio.
Micro (one single filament) and mini (one single fiber tow) non-oxide composites (C/C; C/SiC and SiC/SiC) with simple (PyC or BN) or complex interphases C (B) or (PyC-SiC)
n
multilayers are ...fabricated in a short time by CVD/CVI. The fiber/matrix interfacial zone is characterized by AES and TEM. Tensile tests are used to assess the mechanical properties and the Weibull statistical parameters of both the fiber and matrix, as well as the fiber–matrix interfacial parameters (τ
i;
l
d;
G
ic). The tensile stress–strain behavior has been modelled. The tensile curves exhibit the same features as those previously reported for real nD-composites. Lifetime at high temperatures in air is characterized through static/cyclic fatigue tests and modelled. It is improved by replacing conventional pyrocarbon by highly engineered interphases. The micro/mini composite approach is used in the optimization of processing conditions and to derive parameters necessary for the modelling of the thermomechanical and chemical behavior of composites with more complex fiber architectures.
Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BC1
3C
3H
8H
2 precursor mixtures. By increasing the
BCl
3
(C
3H
8 + BCl
3
) ratio up to 85%, the ...incorporation of boron can reach 33 at.%. A small amount of boron (e.g. 8 at.%) highly enhances the structural anisotropy of pyrocarbon, as evidenced by optical microscopy, X-ray diffraction and transmission electron microscopy (selected area diffraction and lattice fringes techniques). X-ray photoelectron spectroscopy has shown that a large fraction of the boron atoms are included by substitution in the carbon layers; the remaining boron atoms belong to a boron-rich amorphous part of the material. As the boron content increases beyond 8 at.%, the structural anisotropy of the boron-rich pyrocarbon decreases, due to the limited growth and stacking of the carbon layers. Also, amorphous boron-rich regions are more and more abundant as the total amount of boron increases. The oxidation resistance of the C(B) materials is better than that of pure pyrocarbon. This is mainly due to the improvement of the structural organization for the low boron content materials and to the coating of the whole material with a stable boron oxide for materials with a higher boron content.
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