CHARACTERIZATION OF THE MICROSTRUCTURE OF YAG CERAMICS VIA STEREOLOGY-BASED IMAGE ANALYSIS

The microstructure of transparent YAG ceramics is investigated by stereology-based microscopic image analysis using SEM and FE-SEM micrographs. Interface densities, mean curvature integral densities and the related grain size measures (mean chord length and Jeffries size) have been determined with relative errors of 9-12 % for interface densities and mean curvature integral densities and 6-9 % for the corresponding grain size measures. A comparison of the two grain size measures confi rmed an excellent linear correlation between the Jeffries size and the mean chord length, with a mean-chordlength-to-Jeffries-size ratio of 0.928 +/- 0.086. The overall range of average grain sizes is approx. 11-34 μm. It has been found that the sintering time has a signifi cant influence on the grain size, especially for YAG ceramics without Yb doping. When the sintering time is increased by a factor 8 (from 2 h to 16 h) the grain size increases by more than 200 % in undoped YAG ceramics, whereas the grain growth is much weaker in Yb-doped YAG ceramics (grain growth only 50-70 % for YAG ceramics with 5-10 at.% Yb). Thus it can be concluded that the Yb dopant acts as a grain growth inhibitor in YAG ceramics, at least for suffi ciently long sintering times (8 h and more). The infl uence of the sintering additive (tetraethyl orthosilicate TEOS) content on the grain size is negligible in the concentration range tested (0.3-0.5 wt.%)

Transparent Yb:YAG ceramics

YAG ceramics doped with rare earth elements have been recently given a consistent attention as materials for various applications. The specific application depends on the doping element added. Addition of Yb is used for the production of active materials for solid state lasers, and YAG polycrystalline ceramics are promising materials for the replacement of single crystals, which are mostly used at present. The advantage of polycrystalline ceramics over single crystals are the lower fabrication costs, faster production or easier preparation of complicated and compositionally graded structures. The poster presents Yb:YAG polycrystalline ceramics prepared via solid-state reaction of commercial submicrometer and nanosized powders (Al2O3, Y2O3 and Yb2O3). Powders were homogenized by ball milling and two different powder drying methods were compared, rotary evaporation and spray drying. Samples were prepared by cold isostatic pressing of homogenized powders followed by calcination in flowing air and sintering in vacuum. High transparency was obtained: optical transmittance greater than 80% was achieved, while the theoretical maximum is 84% at 1064 nm. Furthermore, the effect of the dopant content on properties of sintered material was observe

Analysis of risk exposures encountered by maintenance technicians

Includes bibliographical references

Multilayered YAG-Yb:YAG ceramics: manufacture and laser performance

International audienceThermal effects in transparent laser crystals and ceramics are generally an unwanted consequence of the pumping process: temperature gradients give rise to an unevenly distributed refractive index variation and a distortion of the optical surfaces crossed by the laser beam (thermal lens); birefringence due to thermomechanical stress can cause depolarization losses; and absorption from the ground level usually increases with temperature in quasi-three-level systems. All these effects can seriously impair laser performance, especially in high-power devices. Layered structures with a tailored modulation of the doping level can be used to reduce the peak temperature, the temperature gradients and also the thermally induced deformation of the laser material, thus mitigating the overall thermal effects. In the present work, structures comprising two and three layers of different compositions (pure YAG/10 at% Yb:YAG and pure YAG/10 at% Yb:YAG/pure YAG) were designed with a view to control deformation and stresses, and to reduce the thermal lensing effect. The multilayered samples were assembled by linear and cold isostatic pressing, and co-sintered under a high vacuum in a clean-atmosphere furnace. The microstructure of the layered samples obtained was characterized by FEG SEM, ESEM and TEM. The Yb diffusion profile across the doped/undoped interface was identified and related to the laser's output power. An internal optical transmittance up to 96% was obtained. A laser output power up to 5 W, with a slope efficiency as high as 74.3%, was also achieved

Neutron/γ discrimination by an emission-based phoswich approach

Particle discrimination in a mixed radiation field using scintillators is a challenging topic in radiation detection research. We propose a novel approach relying on the possibility of identifying particles interacting in a phoswich detector from the emission spectrum of the produced scintillation signal. As a proof of concept, we focus on the discrimination between neutrons and gamma rays. Lu3Al5O12:Pr and Gd3Al2Ga3O12:Ce thin single crystal scintillators, coupled to two different silicon photomultipliers equipped with optical filters, are simultaneously used in the same phoswich detector. Their optical emissions peak at approximately 310 nm and 545 nm respectively, and thus they can easily be distinguished by optical filtering. While both crystals are sensitive to gamma rays, neutrons interact only with the Gd3Al2Ga3O12:Ce thanks to the presence of Gd acting as neutron converter. Optical filtration and an anti-coincidence algorithm are therefore used to perform particle discrimination, rejecting coincidence signals arising from gamma rays, which simultaneously deposit energy in both crystals, and counting anti-coincidence signals due to neutrons, which deposit energy only in the Gd3Al2Ga3O12:Ce. The simple neutron counter developed here is intended to be a proof of the principle of the feasibility of the color-based particle discrimination technique