Low temperature specific heat of diethyl-morpholinium ditetracyanoquino-dimethane [DEM (TCNQ)2]

Specific heat measurements are reported for 0.5 K<T<50 K. Anomalous effects are observed near T = 30 K, which possibly can be attributed to a spin-Peierls transition. At low temperatures the specific heat contains a term linear in T indicating that one half of the TCNQ stacks is still in its unpertubed state.     

Salt transport and crystallization in porous building materials

Salt weathering is a major cause of deterioration of porous building materials. To obtain information about the mechanisms underlying these damage processes we have studied the moisture and ion transport. We measured the time evolution of NaCl saturated samples of fired-clay brick during one-sided drying using Nuclear Magnetic Resonance. The moisture content and amount of dissolved Na ions could be measured quantitatively as a function of position. The NaCl concentration profiles obtained from these data reflect the competition between advection to the surface and redistribution by diffusion. By representing the measured moisture and NaCl profiles in an efflorescence pathway diagram (EPD) also the crystallization can be taken into account.     

Sodium NMR relaxation in porous materials

The NMR relaxation of hydrogen nuclei of a fluid in a porous material is generally interpreted in terms of the Brownstein-Tarr model, in which the relaxation rate of the signal is inversely proportional to the pore size. We have investigated whether this model can be applied to the relaxation of Na nuclei in a NaCl solution in a porous material. The results indicate that the ion distribution over the pores can be obtained from an analysis of the Na NMR signal decay, if the pore sizes are roughly below 1 microm. This information is very useful for studies of combined moisture and ion transport in porous building materials.     

Diffusion in porous building materials with high internal magnetic field gradients

An algorithm to calculate NMR signals of a multi-echo pulse sequence with arbitrary position dependent B0 and B1 fields taking into account relaxation and spin-diffusion is presented. The multi-echo pulse sequence consists of an initial RF pulse ("90 degrees " RF pulse) and a series of L refocusing RF pulses with arbitrary phases and flip-angles. The calculation is exact and takes into account all the magnetization pathways that contribute to the signal on a predefined spatial grid. The theoretical prediction is verified experimentally using a high field NMR microscopy system. The algorithm was implemented in a simulation program in order to optimize the design of an inside-out MR intra-vascular catheter that is used for characterization of vessel wall tissue. Measured data obtained with the catheter are in good agreement with the theoretical prediction of the simulation.     

One-dimensional scanning of moisture in heated porous building materials with NMR

In this paper we present a new dedicated NMR setup which is capable of measuring one-dimensional moisture profiles in heated porous materials. The setup, which is placed in the bore of a 1.5 T whole-body scanner, is capable of reaching temperatures up to 500 °C. Moisture and temperature profiles can be measured quasi simultaneously with a typical time resolution of 2-5 min. A methodology is introduced for correcting temperature effects on NMR measurements at these elevated temperatures. The corrections are based on the Curie law for paramagnetism and the observed temperature dependence of the relaxation mechanisms occurring in porous materials. Both these corrections are used to obtain a moisture content profile from the raw NMR signal profile. To illustrate the methodology, a one-sided heating experiment of concrete with a moisture content in equilibrium with 97% RH is presented. This kind of heating experiment is of particular interest in the research on fire spalling of concrete, since it directly reveals the moisture and heat transport occurring inside the concrete. The obtained moisture profiles reveal a moisture peak building up behind the boiling front, resulting in a saturated layer. To our knowledge the direct proof of the formation of a moisture peak and subsequent moisture clogging has not been reported before.     

Ion transport in porous media studied by NMR.

Moisture and salt transport in masonry can give rise to damages. Therefore a detailed knowledge of the moisture and salt transport is essential for understanding the durability of masonry. A special NMR apparatus has been made allowing quasi-simultaneous measurements of both moisture and Na profiles in porous building materials. Using this apparatus both the absorption of a 4 M NaCl solution in a calcium silicate brick and the drying of a 3 M NaCl capillary saturated fired-clay brick have been studied. It was found that during the absorption process the Na ions clearly stay behind, which this is caused by adsorption of these ions to the pore surface. For the drying it was found that at the beginning of the drying process the ions accumulate near the surface. As the drying rate decreases, diffusion becomes dominant and the ion profile levels off again.     

Microscopic properties of Co/Pd multilayers studied by ferromagnetic and nuclear magnetic resonance

The microscopic properties of Co/Pd multilayers were studied by ferromagnetic and nuclear magnetic resonance. The anisotropy of the layer is found to decrease for decreasing Co sublayer thickness and a switching of the magnetic preferential direction occurs below 8Å. The anisotropy is also a function of the Pd layer thickness: below 20Å it increases with decreasing thickness. This is attributed to a decrease in the Pd/Co interface contribution to the anisotropy. Hyperfine field spectra show the layers with [111] texture to be mainly polycrystalline fcc. The spectra shift to lower hyperfine fields upon decreasing Co sublayer thickness. This is attributed to a decrease in the magnetic moment caused by stretching of the Co lattice due to neighbouring Pd atoms. The results were checked by experiments on almost single phased [100] fcc Co/Pd multilayers. The similarity with the results on the [111] layers suggests that the expansion of the Co lattice is nearly isotropic.