Why you can't use water to make cryoporometric measurements of the pore size distributions in meteorites - or in high iron content clays, rocks or concrete.

摘要

Many porous materials have high susceptibility magnetic gradients in the pores, due toudthe presence of iron or other magnetic materials. Thus if probe liquids are placed in theudpores they exhibit fast decaying signals with a short T2*. Usually the actual T2 of theudliquids is also reduced, due the presence of paramagnetic ions in the pore walls. Theudusual solution in NMR is to measure an echo (or echo train) at short times. However,udrecent work [J. Phys.: Condens. Matter 19, 415117, 2007.] has shown that water/iceudsystems near a pore wall form rotator phase plastic ice, with T2 relaxation times in theudregion of 100 to 200 ms. Thus if a NMR cryoporometric measurement is attemptedudwith a measurement time significantly less than 1 or 2 milli-seconds, the result is toudmake a measurement based on the phase properties of the brittle to plastic ice phaseudtransition, not that of the brittle ice to water phase transition. This gives rise toudartefacts of small pore sizes that may not actually be present. This work successfullyuduses a-polar liquids instead.