Magdalena Wencka

PdGa intermetallic hydrogenation catalyst: an NMR and physical property study

The PdGa intermetallic compound is a highly selective and stable heterogeneous hydrogenation catalyst for the semi-hydrogenation of acetylene. We have studied single crystals of PdGa grown by the Czochralski technique. The (69)Ga electric-field-gradient (EFG) tensor was determined by means of NMR spectroscopy, giving experimental confirmation of both the recently refined structural model of PdGa and the theoretically predicted Pd-Ga covalent bonding scheme. The hydrogenation experiment has detected no hydrogen uptake in the PdGa, thus preventing in situ hydride formation that leads to a reduction of the catalytic selectivity. We have also determined bulk physical properties (the magnetic susceptibility, the electrical resistivity, the thermoelectric power, the Hall coefficient, the thermal conductivity and the specific heat) of single-crystalline PdGa. The results show that PdGa is a diamagnet with metallic electrical resistivity and moderately high thermal conductivity. The thermoelectric power is negative with complicated temperature dependence, whereas the Hall coefficient is positive and temperature-dependent, indicating complexity of the Fermi surface. Partial fulfillment of the NMR Korringa relation reveals that the charge carriers are weakly correlated. Specific heat measurements show that the density of electronic states (DOS) at the Fermi energy of PdGa is reduced to 15% of the DOS of the elemental Pd metal.

Dynamics of CO2? radiation defects in natural calcite studied by ESR, electron spin echo and electron spin relaxation

ESR spectra were recorded in the X-band (9.6 GHz) and in the W-band (94 GHz) and electron spin relaxation was measured by electron spin echo (ESE) in the temperature range 4.2–300 K for radicals in natural calcite samples obtained from a cave stalactite and a dripstone layer. Four types of carbonate radical spectra and two sulfate radical spectra were identified and high accuracy g-factors were derived. Time and temperature behaviour of the spectra show that the dominating CO2− radicals are rigidly bonded or undergo free reorientations, whereas CO3−, SO2− and SO3− only undergo free reorientations. Below 200 K the free reorientations of CO2− are suppressed and a hindered rotation around single local axis appears. The ESE detected spectrum proves that the lines of free rotating radicals are homogeneously broadened, thus they cannot participate in electron spin echo formation. Spin–lattice relaxation data show that CO2− radicals are decoupled from lattice phonons and relax via local mode tunnelling motion between inequivalent oxygen positions of CO2− molecules. The tunnelling appears in two excited vibrational states of energy 71 and 138 cm−1. Librational motions of CO2− molecules were detected by electron spin echo decay (phase relaxation) with energy 153 cm−1. Two kinds of impurity hydrogen atoms were distinguished from ESEEM: in-water inclusions and water coordinated to the calcium ions.

Coronary stent as a tubular flow heater in magnetic resonance imaging

BackgroundA coronary stent is an artificial metallic tube, inserted into a blocked coronary artery to keep it open. In magnetic resonance imaging (MRI), a stented person is irradiated by the radio-frequency electromagnetic pulses, which induce eddy currents in the stent that produce Joule (resistive) heating. The stent in the vessel is acting like a tubular flow heater that increases the temperature of the vessel wall and the blood that flows through it, representing a potential hazard for the stented patient.MethodsHeating of a metallic coronary stent in MRI was studied theoretically and experimentally. An analytical theoretical model of the stent as a tubular flow heater, based on the thermodynamic law of heat conduction, was developed. The model enables to calculate the time-dependent stent’s temperature during the MRI examination, the increase of the blood temperature passing through the stent and the distribution of the temperature in the vessel wall surrounding the stent. The model was tested experimentally by performing laboratory magnetic resonance heating experiments on a non-inserted stainless-steel coronary stent in the absence of blood flow through it. The model was then used to predict the temperature increase of the stainless-steel coronary stent embedded in a coronary artery in the presence of blood flow under realistic MRI conditions.ResultsThe increase of the stent’s temperature and the blood temperature were found minute, of the order of several tenths of a degree, because the blood flow efficiently cools the stent due to a much larger heat capacity of the blood as compared to the heat capacity of the stent. However, should the stent in the vessel become partially re-occluded due to the restenosis problem, where the blood flow through the stent is reduced, the stent’s temperature may become dangerously high.ConclusionsIn the normal situation of a fully open (unoccluded) stent, the increase of the stent temperature and the blood temperature exiting the stent were found minute, of less than 1°C, so that the blood flow efficiently cools the stent. However, should the problem of restenosis occur, where the blood flow through the stent is reduced, there is a risk of hazardous heating.

The hydrogen dynamics of CsH5(PO4)2 studied by means of nuclear magnetic resonance.

We have investigated the hydrogen dynamics of cesium pentahydrogen diphosphate, CsH(5)(PO(4))(2), by means of nuclear magnetic resonance (NMR) spectroscopy, in order to address the question of why there is no superprotonic phase transition in this compound, in contrast to other structurally similar hydrogen-bonded ionic salts, where a superprotonic transition is frequently found to be present. The analysis of the NMR spectrum and the spin-lattice relaxation rate revealed that the temperature-dependent hydrogen dynamics of CsH(5)(PO(4))(2) involves motional processes (the intra-H-bond jumps and the inter-H-bond jumps at elevated temperatures, as a mechanism of the ionic conductivity) identical to those for the other H-bonded superprotonic salts. The considerably stronger H-bond network in CsH(5)(PO(4))(2) prompts the search for a higher superprotonic transition temperature. However, due to the relatively weak bonding between the {[H(2)PO(4)]}∞ planes in the [100] direction of the CsH(5)(PO(4))(2) structure by means of the ionic bonding via the cesium atoms and the small number of H bonds in that direction (where out of five H bonds in the unit cell, four are directed within the {[H(2)PO(4)]}∞ planes and only one is between the planes), the bonds between the planes become thermally broken and the crystal melts before the H-bond network rearranges via water release into an open structure typical of the superprotonic phase. Were the coupling between the {[H(2)PO(4)]}∞ planes in the CsH(5)(PO(4))(2) somewhat stronger, the superprotonic transition would occur in the same manner as it does in other structurally related hydrogen-bonded ionic salts.

A multicaloric material as a link between electrocaloric and magnetocaloric refrigeration

The existence and feasibility of the multicaloric, polycrystalline material 0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3, exhibiting magnetocaloric and electrocaloric properties, are demonstrated. Both the electrocaloric and magnetocaloric effects are observed over a broad temperature range below room temperature. The maximum magnetocaloric temperature change of ~0.26 K is obtained with a magnetic-field amplitude of 70 kOe at a temperature of 5 K, while the maximum electrocaloric temperature change of ~0.25 K is obtained with an electric-field amplitude of 60 kV/cm at a temperature of 180 K. The material allows a multicaloric cooling mode or a separate caloric-modes operation depending on the origin of the external field and the temperature at which the field is applied.

Quantitative determination of magnetic force on a coronary stent in MRI

To introduce an analytical method for a quantitative determination of magnetic force on a coronary stent in the magnetic resonance imaging (MRI) magnet that is generally applicable to metallic implants. Magnetic forces on metallic implants in the MRI magnets are traditionally determined empirically by measuring deflection from the vertical plane at the central axis of the magnet and at the point of the largest force along the longitudinal axis of the magnet.

Anisotropic electrical, thermal and magnetic properties of Al13Ru4 decagonal quasicrystalline approximant

Abstract We present measurements of the anisotropic electrical and thermal transport coefficients (the electrical resistivity, the thermoelectric power, the thermal conductivity), the magnetization and the specific heat of the Al13Ru4 monoclinic approximant to the decagonal quasicrystal, in comparison to the isostructural Al13Fe4. The electrical and thermal transport parameters of Al13Ru4 were found to exhibit significant anisotropy, qualitatively similar to that found previously in the Al13Fe4 (P. Popčević, et al., Phys. Rev. B 2010, 81, 184203). The crystallographic b direction, corresponding to the stacking direction of the (a,c) atomic planes, is the most conducting direction for the electricity and heat. The thermopower is strongly anisotropic with a complicated temperature dependence, exhibiting maxima, minima, crossovers and sign change. The electronic density of states (DOS) at the Fermi energy is reduced to 35% of the DOS of Al metal. The magnetic susceptibility is diamagnetic and the diamagnetism is by a factor of 2 stronger for the magnetic field along the stacking b direction.

Influence of the magnetic field on phase transitions in PrAlO3

Dielectric properties of PrAlO3 single crystals were studied under the influence of the magnetic field of 0.8 T. Below 300 K all phase transitions detected previously with different experimental methods are observed in e′[001](T) and e′[11¯0](T). The phase transition at 152 K is of the first order and under the influence of magnetic field the thermal hysteresis increases.