167 / 2018-08-25 20:09:28

Thermal Decomposition of Oil Impregnated Paper: A ReaxFF Molecular Dynamics Study

Oil impregnated paper,Thermal decomposition,Molecular dynamics simulation,Chemical reaction

Oil impregnated paper is a kind of insulating material which has been widely used in power transformers, cables, and electrical capacitors. It is inevitable that the oil impregnated paper is decomposed under high operating temperature, with complex chemical reactions occurring in the electrical power equipment. Experimental tests (eg. DGA, FTIR) provide an effective way to measure the contents of decomposition products. However, only few approaches are able to make the chemical mechanism of thermal decomposition clear. Reactive molecular dynamics (MD) simulations are a useful tool to examine the chemical reactions occurring in complex processes, providing that a realistic structural representation and an applicable reactive force field (ReaxFF) can be utilized. In order to analyze the processes of the main reactions and locate the origin of products in the oil-paper insulating system, the reactive MD method was applied to observe the process of atomic motion and collision in the interfacial structure of cellulose and oil molecules. The ReaxFF was applied to describe chemical bonds breaking and forming in this study. To build the molecular structure of oil impregnated paper, we went through the following 3 steps: (1) Concatenate the β-D-glucoses into 5 molecular chains, the degree of polymerization (DP) of each chain is 10; (2) Mix the 5 chains by the random walk method and obtain the structure of cellulose, and minimize its energy by using the conjugate gradient method; (3) Put the oil molecules (eg. dodecylbenzene (DB) and benzyltoluene (BT)) near the cellulose and do the equilibrium MD. Finally, we got two periodic cubic boxes of 6.0×6.0×6.0 Å box length, containing the cellulose structure composed of 1060 atoms along with 30 DB and BT molecules, respectively. To accelerate the chemical reaction rate, very high temperature (2500 K) was selected for ReaxFF simulation, but without any difference in chemical mechanisms with low temperatures according to the theoretical research. These systems were energy-optimized and equilibrated via low-temperature (300 K) ReaxFF simulation (time step of 0.25 fs) to prevent chemical reactions from occurring during equilibration. The temperature was controlled using the Berendsen thermostat with a 0.1 ps damping constant and stepped up to 2500 K. The thermal decomposition was started at 10 ps and ended at about 400 ps in our simulation. In this process, more than 800 kinds of reactions and 1000 products were recorded in the system. At the beginning of decomposition, the cellulose chains rupturing to small molecules and make the DP decrease. Then the unstable molecules (such as radicals) interact with each other, with electrons and proton transfer occurs. We found that the molecules of decomposition products such as water, hydrogen, hydrocarbon gases, carbon oxide, methanol, formaldehyde, and furan has appeared at the end of the simulation model, which has a good agreement with experimental results. The content of water is the highest than any other products and keeps increasing during the whole simulation. Other products were all increase before the first 100 ps and then level off. Our results not only obtained the microstructural weakness parts but also provided theoretical guidance for optimizing the electrical performance of oil impregnated paper.