L.N. Dinh

The effects of moisture on LiD single crystals studied by temperature-programmed decomposition

Abstract Temperature-programmed decomposition (TPD) technique was performed on LiOH powders and LiD single crystals previously exposed to different moisture levels. Our results show that the LiOH decomposition process is rate-limited by an inward moving reaction front mechanism with an activation energy barrier of ∼122–149 kJ/mol. The LiOH structure is stable even if kept at 320 K. However, LiOH structures formed on the surface of LiD single crystals during moisture exposure at low dosages may have multiple activation energy barriers, some of which may be much lower than 122 kJ/mol. The rate-limiting mechanism for the decomposition of LiOH structures with reduced activation energy barriers is consistent with a unimolecular nucleation model. We attribute the lowering of the activation energy barrier for the LiOH decomposition to the existence of sub-stoichiometric Li(OH)x with x LiOH · H 2 O formation is observed. The release of H2O molecules from LiOH · H 2 O structure has small activation energy barriers in the range of 48–69 kJ/mol and follows a unimolecular nucleation process. The loosely bonded H2O molecules in the LiOH · H 2 O structure can be easily pumped away at room temperature in a reasonable amount of time. Our experiments also suggest that handling LiD single crystals at an elevated temperature of 340 K or more reduces the growth rate of LiOH and LiOH · H 2 O significantly.

The nature and effects of the thermal stability of lithium hydroxide

Temperature programmed decomposition and complimentary microscopy/spectroscopy techniques were performed on lithium hydroxide with micron-sized grains. The lithium hydroxide grains thermally decomposed into Li2O, releasing H2O, following a three dimensional phase boundary moving from the surface inward. The energy barriers measured for the decomposition of surface and near-surface lithium hydroxide are noticeably smaller than those of bulk counterpart. The conversion of Li2O grains back to lithium hydroxide during moisture exposure was also found to proceed from the surface inward such that surface states are filled before bulk states. In a different set of experiments, nanometer-scale composite grains composed of LiD inner cores and LiOH outer layers were observed to form on top of pressed polycrystalline LiD upon moisture exposure. A diffusion coefficient on the order of 10−23 m2/s was measured for the diffusion controlled reaction of LiOH with LiD in the nanopowder at room temperature in a dry environment. The measured kinetics were used to model the evolution of the LiD/LiOH composite system in a dry environment.

Molecular weight distributions of irradiated siloxane-based elastomers: A complementary study by statistical modeling and multiple quantum nuclear magnetic resonance

The statistical methodology of population balance (PB) has been applied in order to predict the effects of cross-linking and chain-scissioning induced by ionizing radiation on the distribution of molecular weight between cross-links (MWBC) of a siloxane-based elastomer. Effective molecular weight distributions were extracted from the quantification of residual dipolar couplings via multiple quantum nuclear magnetic resonance (MQ-NMR) measurements and are taken to reflect actual MWBC distributions. The PB methodology is then applied to the unirradiated MWBC distribution and considers both chain-scissioning and the possibility of the formation of three types of cross-links: random recombination of scissioned-chain ends (end-linking), random covalent bonds of free radicals on scissioned-chain ends (Y-cross-linking), and the formation of random cross-links from free radicals on side groups (H-cross-linking). The qualitative agreement between the statistical modeling approach and the NMR data confirms that it is possible to predict trends for the evolution of the distribution of MWBC of polymers under irradiation. The approach described herein can also discern heterogeneities in radiation effects in different structural motifs in the polymer network.

The moisture outgassing kinetics of a silica reinforced polydimethylsiloxane

A silica-filled polydimethylsiloxane (PDMS) composite M9787 was investigated for potential outgassing in a vacuum/dry environment with the temperature programmed desorption/reaction method. The outgassing kinetics of 463 K vacuum heat-treated samples, vacuum heat-treated samples which were subsequently re-exposed to moisture, and untreated samples were extracted using the isoconversional and constrained iterative regression methods in a complementary fashion. Density functional theory (DFT) calculations of water interactions with a silica surface were also performed to provide insight into the structural motifs leading to the obtained kinetic parameters. Kinetic analysis/model revealed that no outgassing occurs from the vacuum heat-treated samples in subsequent vacuum/dry environment applications at room temperature (∼300 K). The main effect of re-exposure of the vacuum heat-treated samples to a glove box condition (∼30 ppm by volume of H2O) for even a couple of days was the formation, on the silica surfa...