Effect of Electric Field on Gas Hydrate Nucleation Kinetics: Evidence for the Enhanced Kinetics of Hydrate Nucleation by Negatively Charged Clay Surfaces

Natural gas hydrates are found widely in oceanic clay-rich sediments, where clay–water interactions have a profound effect on the formation behavior of gas hydrates. However, it remains unclear why and how natural gas hydrates are formed in clay-rich sediments in spite of factors that limit gas hydrate formation, such as small pore size and high salinity. Herein, we show that polarized water molecules on clay surfaces clearly promote gas hydrate nucleation kinetics. When water molecules were polarized with an electric field of 10⁴ V/m, gas hydrate nucleation occurred significantly faster with an induction time reduced by 5.8 times. Further, the presence of strongly polarized water layers at the water–gas interface hindered gas uptake and thus hydrate formation, when the electric field was applied prior to gas dissolution. Our findings expand our understanding of the formation habits of naturally occurring gas hydrates in clay-rich sedimentary deposits and provide insights into gas production from natural hydrate deposits

Effect of Organic Matter on CO₂ Hydrate Phase Equilibrium in Phyllosilicate Suspensions

In this study, we examined various CO₂ hydrate phase equilibria under diverse, heterogeneous conditions, to provide basic knowledge for successful ocean CO₂ sequestration in offshore marine sediments. We investigated the effect of geochemical factors on CO₂ hydrate phase equilibrium. The three-phase (liquid–hydrate–vapor) equilibrium of CO₂ hydrate in the presence of (i) organic matter (glycine, glucose, and urea), (ii) phyllosilicates [illite, kaolinite, and Na-montmorillonite (Na-MMT)], and (iii) mixtures of them was measured in the ranges of 274.5–277.0 K and 14–22 bar. Organic matter inhibited the phase equilibrium of CO₂ hydrate by association with water molecules. The inhibition effect decreased in the order: urea < glycine < glucose. Illite and kaolinite (unexpandable clays) barely affected the CO₂ hydrate phase equilibrium, while Na-MMT (expandable clay) affected the phase equilibrium because of its interlayer cations. The CO₂ hydrate equilibrium conditions, in the illite and kaolinite suspensions with organic matter, were very similar to those in the aqueous organic matter solutions. However, the equilibrium condition in the Na-MMT suspension with organic matter changed because of reduction of its inhibition effect by intercalated organic matter associated with cations in the Na-MMT interlayer

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(N^<i>t</i>Bu)(NEt₂)₃, Ta(N^<i>t</i>Bu)(NEt₂)₂Cp, and H₂O

The growth characteristics of Ta₂O₅ thin films by atomic layer deposition (ALD) were examined using Ta­(N^<i>t</i>Bu)­(NEt₂)₃ (TBTDET) and Ta­(N^<i>t</i>Bu)­(NEt₂)₂Cp (TBDETCp) as Ta-precursors, where ^<i>t</i>Bu, Et, and Cp represent <i>tert</i>-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta₂O₅ films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta₂O₅ films were 0.77 Å cycle^–1 at 250 °C and 0.67 Å cycle^–1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt₂) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta–Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta₂O₅ film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta₂O₅ film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta₂O₅ film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta₂O₅ films was 2.1–2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases