Coating a polymeric membrane for gas separation is a feasible approach to fabricate gas sensors with selectivity. In this study, poly(methyl methacrylate)-(PMMA-)membrane-coated palladium (Pd) nanoparticle (NP) films were fabricated for high-performance hydrogen (H2) gas sensing by carrying out gas-phase cluster deposition and PMMA spin coating. No changes were induced by the PMMA spin coating in the electrical transport and H2-sensing mechanisms of the Pd NP films. Measurements of H2 sensing demonstrated that the devices were capable of detecting H2 gas within the concentration range 0–10% at room temperature and showed high selectivity to H2 due to the filtration effect of the PMMA membrane layer. Despite the presence of the PMMA matrix, the lower detection limit of the sensor is less than 50 ppm. A series of PMMA membrane layers with different thicknesses were spin coated onto the surface of Pd NP films for the selective filtration of H2. It was found that the device sensing kinetics were strongly affected by the thickness of the PMMA layer, with the devices with thicker PMMA membrane layers showing a slower response to H2 gas. Three mechanisms slowing down the sensing kinetics of the devices were demonstrated to be present: diffusion of H2 gas in the PMMA matrix, nucleation and growth of the β phase in the α phase matrix of Pd hydride, and stress relaxation at the interface between Pd NPs and the PMMA matrix. The retardation effect caused by these three mechanisms on the sensing kinetics relied on the phase region of Pd hydride during the sensing reaction. Two simple strategies, minimizing the thickness of the PMMA membrane layer and reducing the size of the Pd NPs, were proposed to compensate for retardation of the sensing response.
Polypyrrole/expanded graphite (PPy/EG) nanohybrids, with a hierarchical structure of a three dimensional EG framework with a thick PPy coating layer, have been synthesized via a vacuum-assisted intercalation in situ oxidation polymerization method. In the synthesis, pyrrole monomers were intercalated into the irregular pores of EG with the assistance of a vacuum pump. Subsequently, the intercalated pyrrole monomers assembled on both sides of the EG nanosheets and formed PPy by an in situ polymerization method. As electrode materials, the typical PPy/EG10 sample with an EG content of 10% had a high specific capacitance of 454.3 F g-1 and 442.7 F g-1 (1.0 A g-1), and specific capacitance retention rate of 75.9% and 73.3% (15.0 A g-1) in 1 M H2SO4 and 1 M KCl electrolytes, respectively. The two-electrode symmetric supercapacitor showed a high energy density of 47.5 W h kg-1 at a power density of 1 kW kg-1, and could retain superb stability after 2000 cycles. The unique self-supporting structure feature and homogeneous PPy nanosphere coating combined the contributions of electrochemical double layer capacitance and pseudo-capacitance, which made the nanohybrids an excellent electrode material for high performance energy storage devices.
s. Yttium-substituted bismuth titanate (Bi 4 - x Y x Ti 3 O 12 , BYT) thin films were deposited on the (111)Pt/Ti/SiO 2 (100) substrates by a modified Sol-Gel process and studied in this work in terms of Y 3+ -modified microstructure and phase development as well as ferroelectric properties. With the aid of the fist-principle, the position of Y 3+ substitution for Bi 3+ on the microstructure of BYT was studied.The phase change in the formation of BYT crystalline and the effect of Y 3+ substitution for Bi 3+ on the microstructure of BYT was studiedbyXRD. The results showed that the optimal properties of the obtained BYT ferroelectric thin films were x :0.6. The ferroelectric properties of the films were also investigated. When the Y-substituted content x was equal to 0.6, the remnant polarization was the largest. The remnant polarization 2 Pr value was equal to 16.02 μ C/cm 2 and the coercive field Ec value was 88 kV/cm.
During CO2 displacement, the diffusion characteristics of gas in the coal matrix are among the key factors that affect the gas extraction rate. In this study, we used a self-developed CO2–CH4 displacement experimental platform to study the diffusion characteristics of CH4 under different CO2 injection pressures and temperature. The increase in CO2 injection pressure from 0.6 MPa to 1.0 MPa was accompanied by a 9.44% increase in CH4 production, the CH4 limit diffusivity increased by 0.05, the effective diffusion coefficient extreme point increased by 111.45%, and the time to reach CH4 diffusion equilibrium was reduced by 121.15 min (approximately 2 h). As the gas injection temperature was increased from 40°C to 60°C, the CH4 production increased by 4.02%, the CH4 limit diffusivity increased by 0.03, the extreme point of the effective diffusion coefficient increased by 103.31%, and the time to reach CH4 diffusion equilibrium was shortened by 134.57 min (approximately 2.24 h). To better explain the gas diffusion characteristics during CO2 displacement, the influence mechanism of the increase in the CO2 injection pressure and temperature on the CH4 diffusion kinetics was discussed. The dynamic CO2 injection models were considered to be more effective in ensuring efficient CH4 recovery.
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