F. Beclin

Enhanced fluorescence from Eu3+ in low-loss silica glass-ceramic waveguides with high SnO2 content

We report on the sol-gel fabrication and characterization of (100−x)SiO2–xSnO2 (x=8, 16, and 25mol%) glass-ceramic waveguides doped with 1mol% Eu3+. A suitable top-down thermal process led to the formation of SnO2 nanocrystals ∼4nm embedding Eu3+ ions. The excitation spectra evidence the role of interband electronic transition of SnO2 nanocrystals on the luminescence of Eu3+. Monitoring the D05→F27 Eu3+ emission, we observe about 15 times increase in the intensity of SnO2 absorption band, moving from x=8to25mol%. These waveguides also exhibit low losses, making them quite promising for development of high-gain integrated optical amplifiers.

Elaboration of low shrinkage mullite by active filler controlled pyrolysis of siloxanes

Abstract The active filler controlled pyrolysis of polymers has been used to synthesize mullite from Al 2 O 3 -filled, Al-filled and Al/Al 2 O 3 -filled siloxanes. Due to the presence of transition alumina and to a finer scale of mixing of the mullite precursors provided from active Al-filler, the beginning of mullitization temperature was lowered in the two last systems (1200–1250°C) in comparison to the first one (1400°C). The mullite equiaxed-grains obtained from the polymer/Al 2 O 3 (PAO) system contained a large amount of dislocations whereas in polymer/Al (PA) and polymer/Al/Al 2 O 3 (PAAO) derived samples they were primarily dislocation free. The presence of small amounts of residual glassy phase was observed as glassy pockets at triple-junctions in the whole of the materials. The improved microstructure obtained with the Al/Al 2 O 3 -filled mixture indicates that this system seems to be a good choice for a low shrinkage and low defect mullite, in comparison to mullite obtained from Al-filled and Al 2 O 3 -filled siloxanes.

Microstructural superplastic deformation in MgO · Al2 O3 spinel

Abstract Polycrystals of hot-pressed and then hot-isostatically-pressed MgO · Al2O3 spinel, having large grains of about 10–20 μm embedded in a fine-grained matrix of 610 nm grain size, have been deformed in uniaxial compression. The results have been analysed by taking into account grain growth. The variation in stress exponent n suggests a transition from interface-reaction (n = 2) at low stresses to diffusional mechanism (n = 1) at high stresses, but cavitational creep was responsible for the non-linear behaviour at stresses above 80 MPa. The grain size exponent found at 60 MPa (p = 2.7), the absence of dislocations within fine grains and the similarity of grain shapes before and after large flow (55%), resemble the characteristics of “microgram superplastic flow”. The values of the grain size exponent and of the activation energy suggest that grain-boundary diffusion (p = 3) makes a significant contribution to the mixed deformation mechanism.

Experimental Study of SiO2 Soot Deposition using the Outside Vapor Deposition Method

The deposition of pure silica layers using the Outside Vapor Deposition method has been studied. The experimental study focuses on some fabrication parameters that have to be adjusted in order to work with a target temperature optimized for deposition of pure silica compatible with large volume deposition and post-treatments. It is experimentally shown that the optimized soot porous preforms are constituted by homogeneous silica spherical particles of 100 to 110 nm in diameter and have, in this case, densities approximately equal to 0.35 g.cm –3 .

Superplasticity of HIP MgO · Al2O3 spinel: Prospects for superplastic forming

Abstract Polycrystals of hot-isostatically pressed MgO · Al2O3 spinel, having a fine-grained size of 610 nm, have been deformed in uniaxial compression. The results have been analysed by taking into account grain growth. The variation in stress exponent n suggests a transition from interface reaction (n = 2) at low stresses to diffusional mechanism (n = 1) at high stresses, but cavitational creep was responsible for the non-linear behaviour at stresses above 60 M Pa. The grain size exponent found at 60 M Pa (p = 3), the absence of dislocations within the fine grains and the similarity of equiaxed grain shapes before and after large flow (≈ 40%) resemble the characteristics of micrograin superplastic flow. The value of the grain size exponent suggests that grain-boundary diffusion makes a significant contribution at medium stresses to the diffusion-accommodated grain-boundary sliding. Post-HIP treatment of superplastically deformed samples results in a general density increase and improved transparency for the less stressed sample.

Development of microstructure in hot isostatically pressed MgO·Al2O3 spinel

Abstract Fine-grained MgO·Al2O3 spinel was formed by hot isostatic pressing (HIP) using a titanium container. The cold isostatically pressed green bodies were subjected to heat treatment in air prior to HIP at different temperatures. The resulting microstructures were characterized by scanning and transmission electron microscopy and related to process parameters. It was possible by this route to elaborate almost dense spinel with the fine-grained microstructure required for superplastic behaviour. Compressive creep test has given a clear indication of the potential of this material to be superplastically deformed.

Self induced gratings in ternary SiO2:SnO2:Na2O bulk glasses by UV light seeding.

The diffraction efficiency of gratings written in ternary SnO2:SiO2:Na2O bulk glasses rises dramatically with time after the occultation of the cw 244nm light used to write the thick hologram. This self-induced behavior lasts for several hours and ultimately leads to refractive index changes as high as 3 10-3.

SiO2-SnO2 glass-ceramic planar waveguides activated by rare earth ions

Eu3+ doped (100-x) SiO2 - x SnO2 (x = 8, 16 and 25 mol %) glass-ceramic waveguides were fabricated by sol-gel technique with dip-coating processing at 800 °C. Higher thermal treatments led to the formation and growth of SnO2 nanocrystals in the matrix, enhancing the 5D0 → 7F2 emission of Eu3+. Spectroscopic investigations reveal the evolution of a centro-symmetric local environment around the Eu3+ ion. This property of glass-ceramic waveguides can be exploited for obtaining a higher rare-earth concentration in a smaller volume. The glass-ceramic waveguides exhibited low-losses (0.8 ± 0.2 dB/cm at 632.8 nm) and thus are promising for the development of high-gain integrated optical amplifiers.