The
mechanism for the dehydration of Mg(OH)2 and hydration
of MgO was investigated by near-infrared (NIR) spectroscopy. Mg(OH)2 showed three absorption bands in the NIR region at 7339,
7306, and 7157 cm–1, due to the different hydroxyl
groups (7339 cm–1: OH(e) and OH(c) located on the edge and corner of the Mg(OH)2 sheet, 7306 cm–1: OH(t) located
on the terrace of Mg(OH)2 sheet, 7157 cm–1: OH(i) located on the interlayer of Mg(OH)2 sheets). During the phase transition from Mg(OH)2 to MgO at approximately 400 °C, the interlayer OH(i) of Mg(OH)2 disappeared by a dehydration reaction and
the OH(t) on the (001) plane of Mg(OH)2 was converted to the hydrogen-bonded hydroxyl groups on the (111)
plane of MgO (7309 cm–1). Furthermore, when MgO
was in contact with water vapor at 85 and 50 °C (RH 85%), the
isolated hydroxyl groups of the MgO surface (7360 cm–1) were easily hydrated to form hydrogen-bonded ones (7309 cm–1). When a monolayer Mg(OH)2 sheet was formed
on the MgO surface, OH(e), OH(c),
and OH(t) were observed at 7345 and 7309 cm–1. As the hydration reaction proceeded, the intermediate state like
stacking of several Mg(OH)2 sheets, which are related to
the decrease in the OH(t) and the spontaneous increase
in the interlayer OH(i), was experimentally confirmed.
The adsorption states of NH 3 molecules and NH 4 + ions on various sites, such as hydroxyl, H + and Na + , of ZSM-5 zeolites with different SiO 2 /Al 2 O 3 ratios were investigated by near and middle infrared spectroscopy.For Na + -type and siliceous ZSM-5 zeolites, only hydrogen-bonded NH 3 clusters were observed at 5021and 4519 cm −1 , due to the lack of Brönsted acid sites. The NH 3 molecules interacting with the hydroxyl groups easily desorbed at room temperature (r.t.), while the molecules interacting with the Na + sites desorbed at 473 K. For the H + -type ZSM-5, when NH 3 adsorption amount was small, the NH 4 + formed on the H + sites as well as the NH 3 adsorbed on the H + (or possibly Al 3+ ) sites were independently observed at 4846 and 5044 cm −1 , respectively. As the amount of NH 3 adsorption increased, the NH 4 + stabilized by the 2nd layer of NH 3 molecules as well as the hydrogen-bonded NH 3 clusters were observed at 4782 and 5017 cm −1 , respectively. The hydrogen-bonded NH 3 molecules desorbed at lower than 423 K but the NH 4 + ions decomposed at 573-623 K to form Brönsted acid sites.Catal. Sci. Technol. This journal is
Mode II delamination properties of Vectran stitched composites were investigated, and tabbed end notch flexural specimen testing was used to prevent premature failure. The effects of stitch density and stitch thread thickness were explored, and fibre compaction due to the stitching process was also verified. The results show that, in moderately stitched laminates (low stitch density), the improvement in GIIC was negligible. Crack bridging by the stitch threads at the crack zone were mostly compensated for the effect of fibre compaction, which reduced the GIIC values. Conversely, in densely stitched laminates (high stitch density), GIIC values were improved significantly (2·4 times higher than those of unstitched laminates). The effects of stitch thread thickness appeared to be negligible in moderately stitched laminates. For densely stitched laminates, thicker stitch thread (500 denier) possessed GIIC values that were 45·7% higher than thinner stitch thread (200 denier).
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