Crystallisation and conformation-controlled breathing of the Al-MOF CAU-13

Within the last couple of years an enormous variety of metal-organic frameworks (MOFs) has been synthesized. Besides characteristics like large specific surface area or a defined interaction with gas molecules a "breathing" effect was described as well. During this breathing the atoms can undergo certain movements keeping the same structural topology and without losing crystallinity. Most known MOFs contain rigid aromatic linker molecules. A rare example of a MOF with flexible aliphatic linkers is [Al(OH)(trans CDC)] (CDC = cyclohexanedicarboxylate), known as CAU 13.[1] The structure was determined from PXRD-data using synchotron radiation. In analogy to Al-MIL 53,[2] CAU 13 is build up from chains of corner-sharing AlO6-octahedra interconnected by the linker molecules. Interestingly, the trans-CDC ion is incorporated in a,a- as well as e,e-conformation. Time-resolved in-situ EDXRD experiments at HASYLAB (DESY, Hamburg) show short induction times for the crystallization of CAU-13. Full crystallization occurs within two hours even at low reaction temperatures. CAU-13[3] shows porosity towards different adsoptives after thermal activation. In-situ temperature dependent PXRD experiments show a widening of the pores along the b-axis up to 3500C. The flexibility of the linker molecules allows structural changes of the compound during adsorption. While the adsorption of hydrophilic molecules only cause a small breathing effect, the adsorption of xylene leads to drastic changes in the crystal structure. The a,a-CDC2- ions change conformation to e,e-type to increase the cell volume per formula sum by 25 %. This combination of pore widening and conformational changes constitutes a new type of breathing in Metal-Organic Frameworks.

Development of Non-Destructive In-House Observation Techniques for Dislocations and Stacking Faults in SiC Epilayers

We have developed non-destructive in-house observation techniques for dislocations and stacking faults (SFs) in 4H-SiC epilayers. Low temperature photoluminescence (PL) mapping was carried out at 100K using He-Cd laser (325 nm) as an exciation source. PL mapping at ~420 nm was used to investigate basal plane dislocations (BPDs), Shockley stacking faults (SSFs) and boundary, while PL mapping at ~470 nm and 100K obtained in-grown SF images. In addition, using a high-resolution laboratory X-ray topography system with a four-crystal collimator, we succeeded in recording BPDs propagating along [11-20]. From the measurement results, new evaluation techniques for dislocations and SFs other than KOH etching and Synchotron radiation topography were demonstrated on Si- and C-face 4H-SiC epilayers.

Radio Emission from Young Supernova Remnants: Effects of an Inhomogeneous Circumstellar Medium

The evolution of young supernova remnants has been modeled using a 1-dimensional hydrodynamic code. Turbulent dynamo amplification of magnetic fields and both turbulent and shock acceleration of relativistic electrons have been included macroscopically to produce synchotron radiation. The observed radio morphology cannot be reproduced by expansion into a homogeneous medium; it appears that many small cloudlets must be present in the circumstellar material.