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Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a1Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X3Σ- ground state is performed by driving the spin-forbidden A3Π ← a1Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the groundstate NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.
Nanocrystalline hydroxyapatite (HAp) prepared by a precipitation route was investigated. The X-ray diffraction (XRD) powder patterns of the elongated nanocrystals with a typical diameter of about 10 nm and length of 30-50 nm (by transmission electron microscopy (TEM)) revealed the presence of HAp with significantly broadened XRD reflections. However, Ca deficiency was found, as the Ca/P ratio was 1.5 only (so-called calcium-deficient hydroxyapatite (CDHA)), and not 1.67. This Ca deficiency of nanocrystalline HAp is explained using NMR. It is shown unambiguously that (i) the nanocrystals consist of a crystalline core and a (disordered) surface region with a relative phosphate content of about 1:1, (ii) the crystalline core is HAp, and (iii) the surface region is dominated by hydrogen phosphate anions (with no hydroxyapatite-like structural motif) and structural water (hydrate). From the relative phosphate content and taking into account the crystal shape, the thickness of the surface layer along the main crystal axis could be estimated to be about 1 nm, and the average chemical composition of the surface layer has been determined. Finally, a Ca/P ratio of 1.52 was estimated from the NMR data that compares well with the value of 1.51 from chemical analysis. The important consequences are that the surface of nanocrystalline HAp has nothing in common with the bulk composition and that the chemistry of such materials (e.g. the binding of protein molecules to phosphate surfaces) must be reconsidered.
The hydrothermal stability of SAPO-11 was studied in water of autogenous pressure as function of the temperature. SAPO-11 remained crystalline up to 190 °C. Thereby, it was observed, parent as well as hydrothermally treated samples showed similarities to the isotypic structure of zeolite AlPO-11 called here AELdry. After fully hydration of the samples, AELdry changed completely into a hydrated AELwet structure of another framework symmetry. The transition from calcined to hydrated state was reversible for both the parent and the hydrothermally treated samples. 27Al and 31P NMR MAS spectra showed a single resonance typical for AlPO4 type tetrahedra for the calcined parent and hydrothermally treated samples. Upon hydration, the single AlPO4 resonance signal split into three different signals as is the case for the corresponding 31P signals. By means of NH3-TPD measurement a loss of acidity was detected after the hydrothermal treatment as follows from a framework desilication. Nitrogen and water adsorption decreased systematically. At 210 °C a non-zeolitic, non-porous phase was irreversibly formed. The XRD pattern of this phase shows resemblance to a synthetic aluminium phosphate of the tridymite structure type.
Separation of CO2. Microporous polyurethane networks were prepared based on a renewable resource. Betulin, extracted from birch bark, is used as a structure-directing diol monomer in A3-B2 monomers. The resulting microporous networks show very promising CO2/N2 selectivities. The state of adsorbed CO2 is analyzed by in situ NMR spectroscopy, and pure physisorption is proven. The preparation of monolithic materials is demonstrated as well.