Inhomogenitätsuntersuchungen an keramischen Reaktorwerkstoffenmit Hilfe der Laser-Lokal-Emissionsspektralanalyse

Using Laser-localized emission spectroscopy, a method has been developed for the determination of elemental inhomogeneity within the grains of three coarse-grained ceramic materials - the mullite-based Duritals M70 and M90 and the silica-based Masrock -. The method had to be optimized to allow an acceptable detection of the elements to be analyzed in spite of the small quantities of evaporated material. A Laser-produced crater of 100 $\mu$m diameter and depth was found to be necessary to produce the required amount of evaporated material, 0,5 to 0,7 $\mu$g depending on the material composition. The results for Durital E90 showed that the large grains consisted of very pure mullite (2 Al$_{2}$O$_{3}$ $\cdot$ SiO$_{2}$) and the fine grained regions contained small particles of mullite, corundum and binder, the impurities in the binder being concentrated at the grain boundaries. In Durital M70, the coarse and fine grained regions consisted principally of mullite, the impurities introduced during manufacture being evenly distributed in both regions. Analyses of the Masrock material showed a concentration of impurities in the fine grained regions with a very inhomogeneous distribution. The large grains consisted of nearly pure SiO$_{2}$. The distribution of impurity elements on the fracture surfaces of ceramics tested showed that the fracture behaviour is mainly influenced by the physical properties of the materials, for example, the binding of the large grains to the matrix. Analyses ot the test pieces exposed for 1330 hours in test helium at 950°C showed that low melting point eutectics (for Durital E90 CaO-Al$_{2}$O$_{3}$-SiO$_{2}$ and MgO-Al$_{2}$O$_{3}$-SiO$_{2}$, for Durital M70 Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$) can form. In the exposed specimens a depletioin of MgO and CaO was found for Durital E90 and of Fe$_{2}$O$_{3}$ and TiO$_{2}$ for Durital M70, which may be attributed to the diffusion of these oxides to the surface, possibly along the grain boundaries in the fine grained regions. Exposed specimens of Masrock showed only for B$_{2}$O$_{3}$ a significant concentration change. The depletion of this oxide in the bulk material as well as on the surface could not be completely explained