Formation of CO2 bubbles in epoxy resin coatings: A DFT study.

The epoxy resin coating is a fundamental species with epoxy resins used as main components to form the final film. Unexpectedly, bulky CO2 bubbles that occasionally appeared during the curing process of epoxy resin coatings might destroy the final film properties. With an attempt to thoroughly understand the formation mechanism of CO2 bubbles and further propose countermeasures to control them, Density Function Theory (DFT) in this paper was employed to calculate the absorption process, the curing reaction and the formation mechanism of CO2 bubbles. The gas phase basicity (GB) values and pKa values of common amine curing agents were calculated. The total Gibbs free energies difference of the curing reactions between polluted curing agents and epoxy resins were calculated according to a thermodynamic cycle. Whether in gas phase or resin phase, the energetically negative ΔGsolv indicated that the curing reactions might occur spontaneously and CO2 molecules would be separated and released from amine molecules. The total Gibbs free energy calculations also revealed that the re-absorption of CO2 by the curing system was energetically unfavorable. Thus, the formation mechanism of CO2 bubbles of epoxy resin coatings could be summarize in three steps: (1) Carbon dioxide pollutes accidentally the curing agents. (2) CO2 molecules are gradually released as the curing process occurs. (3) CO2 molecules are collected to form big bubbles which can lead to seriously surface and/or internal defects. Finally, based on practical experiences three tips were proposed to control CO2 bubbles. The present results not only evidenced the nature of the unexpected bubbles of epoxy resin coatings, but also additionally paved to the way to full utilization of the formation mechanism to improve the epoxy coatings' properties.

Influence of material parameters on acoustic wave propagation modes in ZnO/Si bi-layered structures.

The influences of material properties on acoustic wave propagation modes in ZnO/Si bi-layered structures are studied. The transfer matrix method is used to calculate dispersion relations, wave field distributions, and electromechanical coupling coefficients of acoustic wave propagation modes in ZnO/Si bi-layered systems, in which the thickness of the substrate is of the same order of magnitude as the wavelength of the propagating wave modes. The influences of the thin film parameters on the acoustic wave propagation modes and their electromechanical coupling coefficients of the wave modes also are obtained. In addition, some experimental results for characterizing the wave propagation modes and their frequencies have also been obtained, which agree well with the theoretical predictions.

Influence of surface conductivity on sensitivity of acoustic wave gas sensors based on multilayered structures.

The influences of the surface conductivity on the velocity of an acoustic wave (AW) in a multilayered material are studied theoretically with the transfer matrix method and the conductivity sensitivity of the AW sensor is presented. It is found that the velocity of the AW increases with decreasing surface conductivity and vice versa. The result is used to explain the abnormal response of AW sensors, in which the central frequencies of AW sensors increase after they sorb the detected gases. Meanwhile, the conductivity sensitivity is found to be related to the dielectric constants of the multilayered material and the electromechanical coupling coefficient of the sensor. Finally, the sensitivities of AW sensors based on multilayered structures are optimized by considering the influences of the surface conductivities of the sensors with different initial conductivities and thicknesses of the sensitive layers.

Calculation of electromechanical coupling coefficient of Lamb waves in multilayered plates.

Two methods have been always used to calculate the electromechanical coupling coefficient of a Lamb wave in a multilayered plate: one is an approximate method using the acoustic velocity difference under different electric boundary conditions and the other is the Green's function method. The Green's function method is more accurate but more complicated, because an 8N-order matrix is used for calculating the electromechanical coupling coefficient of the Lamb wave in an N-layered plate, which induces great computation loads and some calculation deviations. In this paper, a transfer matrix method is used for calculating the electromechanical coupling coefficient of Lamb waves in a multilayered plate, in which only an 8-order matrix is needed regardless of the number of layers of the plate. The results show that the transfer matrix method can obtain the same accuracy as those by the Green's function method, but the computation load and deviation are greatly decreased by avoiding the use of a high order matrix used in the Green's function method.