Salt crystallization as damage mechanism in porous building materials--a nuclear magnetic resonance study

Salts can damage building materials by chemical reactions or crystallization, which is a serious threat to cultural heritage. In order to develop better conservation techniques, more knowledge of the crystallization processes is needed. In a porous material, the size of a salt crystal is limited by the sizes of the pores. It has been predicted that as a consequence, the solubility of a salt increases with decreasing pore size. This increase seems to be related to an increase of the stress generated by a crystal on the pore wall. It has been suggested that the resulting stress could become high enough to induce failure. We have studied the crystallization of salts in porous materials with well-defined pore sizes. Samples were saturated at 40 degrees C with saturated Na2SO4 and Na2CO3 solutions. Next we have cooled the samples to 0 degrees C and waited for nucleation. After nucleation occurred, the solubility in the porous material was measured with nuclear magnetic resonance (NMR) as a function of the temperature. The measurements on Na2CO3 indeed show an increase in solubility with a decrease in pore size. For Na2SO4, we did not observe a pore size-dependent solubility. However, we have to remark that these results show a metastable crystal phase. The results can be used to calculate the actual pressure exerted by the crystals onto the pore wall.     

Experimental evidence of crystallization pressure inside porous media

Crystallization pressure of salt in porous materials is one of the mechanisms that may induce serious damage, for example, weathering of buildings and monuments of cultural heritage. Since this pressure also causes the solubility of the salt inside a porous material to differ from the bulk solubility, it can be assessed experimentally by measuring the solubility inside the pores. We show that this is possible by NMR, and study Na(2)CO(3) and Na(2)SO(4) in a series of model porous materials. Using the solubility data the crystal-liquid surface energies are estimated as gamma = 0.09 N/m for Na(2)CO(3) . 10H(2)O and gamma = 0.06 N/m for Na(2)SO(4) . 10H(2)O. For pore sizes below about 30 nm, the resulting pressure exceeds the tensile strength of typical building materials (3 MPa). No pressure is induced by the metastable Na(2)SO(4) . 7H(2)O, which suggests for this crystal a value of gamma close to zero.     

Specific heat of (C6H11NH3) CuCl3 (CHAC), a system of ferromagnetic chains

The heat capacity of (C₆H₁₁NH₃) CuCl₃ (CHAC) has been measured for 0.45 < T < 60 K. Three-dimensional ordering is observed at T = 2.214 K. The data in the paramagnetic region can be described by a ferromagnetic $\text{S =}\text{1}\text{2}$ Heisenberg linear chain model system with J/k = +45 ± 5K.     

Curing processes in solvent-borne alkyd coatings with different drier combinations

The concern regarding the effect of chemicals on the environment has increased considerably in recent years. Nowadays, technological developments in the coating industry are largely influenced by environmental issues and subsequent legislation. One of these issues is the tendency to replace cobalt as a catalyst with more environmentally friendly alternatives, because studies have indicated possible carcinogenicity. Not much knowledge is available on the effects of catalysts (driers) on the in-depth drying of alkyd coatings. Therefore we have studied the effect of cobalt as a primary drier combined with Ca and Zr as secondary driers on the in-depth curing of high solid solvent-borne alkyds. The profiling of the curing of alkyd coatings is performed with a new high-spatial-resolution NMR setup. In this study, two effects observed in the solvent-borne alkyd coatings are investigated. One is that when Ca and Zr are added as secondary driers the speed of the observed cross-linking front increases. Second, in the deeper un-cross-linked region below the front, the signal of the NMR profiles was found to decrease proportional to . This could be explained by the presence of slowly reacting species that diffuse into the deeper uncured region of the coating, after which they cross-link. The model describing the effect of these reactive species also indicates that the signal decrease is inversely proportional to coating thickness L, which was confirmed by additional measurements.     

A comparison study of multishot vs. single-shot DWI-EPI in the neonatal brain: reduced effects of ghosting compared to adults

In the neonatal brain, it is important to use a fast imaging technique to acquire all diffusion weighted images (DWI) for apparent diffusion coefficient (ADC) calculation. Taking into account the occurrence of typical echo planar imaging (EPI) artifacts, we have investigated whether single-shot (SSh) or multishot (MSh) DWI-EPI should be preferred. In 14 neonates, 17 adult patients and 5 adult volunteers, DWIs are obtained both with SSh and MSh EPI. The occurrence of artifacts and their influence on the ADC are explored and further quantified using simulations and phantom studies. Two radiologists scored overall image quality and diagnosability of all images. Single-shot and MSh DWI-EPI scored equally well in neonates with respect to overall image quality and diagnosability. In newborns, more motion artifacts in MSh can be noticed while N/2-ghost artifacts in SSh occur less frequently than in adults. Both N/2-ghost and motion artifacts result in significant ADC abnormalities. There is a serious risk that these artifacts will be mistaken for genuine diffusion abnormalities. N/2-ghost artifacts are hardly noticed in the neonatal brain, which might be due to smaller cerebrospinal fluid (CSF) velocity than in adults. Apparent diffusion coefficient values in MSh are unreliable if motion occurs. We conclude that for ADC calculations in neonates SSh DWI-EPI is more reliable than MSh.     

High spatial resolution NMR imaging of polymer layers on metallic substrates

High spatial resolution NMR imaging techniques have been developed recently to measure the spatial inhomogeneity of a polymer coating film. However, the substrates of the polymer coatings for such experiments are generally required to be non-metallic, because metals can interact with static magnetic fields B(0) and RF fields B(1) giving rise to artifacts in NMR images. In this paper we present a systematic study on the effects of metallic substrates on 1D profiles obtained by high resolution NMR imaging. The off-resonance effect is discussed in detail in terms of the excitation profile of the RF pulses. We quantitatively show how the NMR signal intensities change with frequency offset at different RF pulse lengths. The complete NMR profiles were simulated using a Finite Element Analysis method by fully considering the inhomogeneities in both B(1) and B(0). The excellent agreement between the calculated and measured NMR profiles on both metallic and non-metallic substrates indicates that the experimental NMR profiles can be reproduced very well by numerical simulations. The metallic substrates can disturb the RF field of the coil by eddy current effect and therefore change the NMR profiles. To quantitatively interpret the NMR profile of a polymer layer on a metallic substrate, the profile has to be divided by the profile of a reference on the same metallic substrate located at the same distance from the coil.     

Dealing with the subvoxel vessel position relative to the reconstruction voxel grid in 2D MR quantitative flow measurements

A method is introduced that quantifies the error in 2D MR Quantitative Flow measurements induced by the position of the vessel relative to the reconstruction voxel grid, called the subvoxel vessel position. In this method, the vessel area and the volume flow rate are determined for all possible subvoxel vessel positions resulting in a mean value with standard deviation. Since the subvoxel vessel position in standard MR image reconstruction is completely arbitrary, the standard deviation can be considered as a measure of its random error contribution. Simulation studies and in vivo measurements show that our method can be used to quantify and subsequently eliminate this random error. It is further quantitatively shown that, for low noise levels, Fourier interpolation to a higher reconstruction matrix also decreases the random error. We conclude that the precision of a 2D MR Quantitative Flow measurement is improved either by using our method or by reconstruction to a higher matrix.