Matti Punkkinen

Bio: Matti Punkkinen is an academic researcher from University of Turku. The author has contributed to research in topics: Relaxation (NMR) & Spin–lattice relaxation. The author has an hindex of 11, co-authored 62 publications receiving 443 citations.

The ortho-to-para conversion in solid hydrogen, catalyzed by hydrogen atoms

Abstract: The combined techniques of ESR and NMR were used to investigate the process of ortho-para conversion in solid molecular hydrogen, containing small amounts (∼ 500 ppm) of hydrogen atoms as impurity. Although the impurity atoms catalyze the effective conversion of the neighboring ortho-H2, the total catalyzed conversion rate at temperatures from 2.2 K to 4.4 K is much less than expected from the rate of the H atoms recombination. A possible explanation is given in terms of the diffusion of H atoms, which is confined to some defects in the crystal.

Deuteron spin-lattice relaxation in (ND4)2SnCl6 below 60 K

Abstract: Spin-lattice relaxation of deuterons in an (ND4)2SnCl6 single crystal, with B0|[100], was studied at the resonance frequencies 5.7, 14.5 and 43.9 MHz as a function of temperature. A reorientation-related minimum appears in the relaxation time T1 near 60 K with activation energy 5.9 kJ/mol. A model is presented for the corresponding relaxation rate in the presence of librational tunneling. Below 30 K relaxation is clearly nonexponential and an additional minimum is observed near 15 K. This minimum is probably related to the fact that the equilibrium orientations of the ND bonds are not parallel to [111] and equivalent directions but they make a small angle with these directions. The ND4 ion moves then as an entity amongst such orientations keeping always, at low temperatures when tunneling is ignored, each ND bond near a fixed [111] or equivalent direction. An expression is derived for the relaxation rate through this low-temperature motion. Results are compared with neutron diffraction experiments.

NMR imaging of materials

Abstract: NMR imaging methods are provided for determining the spatial petrophysical properties of materials. These methods employ the generation of separate Mo, T1 and T2 images from which various petrophysical characteristics may be obtained, such as free fluid index, porosity, pore sizes and distributions, capillary pressure, permeability, formation factor and clay content.

In Situ X-ray Diffraction and Solid-State NMR Study of the Fluorination of γ-Al2O3 with HCF2Cl

Abstract: In situ X-ray diffraction (XRD) and NMR methods were used to follow the structural changes that occur during the dismutation reaction of hydrochlorofluorocarbon-22 (CHClF2) over γ-alumina. Use of a flow cell allowed diffraction patterns to be recorded, while the reaction products were simultaneously monitored downstream of the catalyst bed, by gas chromatography. No visible structural changes of γ-Al2O3 were observed at 300 °C, the temperature at which this material becomes active for catalysis. A new phase began to form at 360 °C, which by 500 °C completely dominated the XRD powder pattern. 19F/27Al cross-polarization (CP) experiments of γ-Al2O3 activated at 300 °C showed that AlF3 had already begun to form at this temperature. By 400 °C, resonances from a phase that resembles α-AlF3 dominate both the 19F and 27Al NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive α-AlF3 phase were performed to investigate the structural changes of this material, associated with the e...