# Accelerated Charge Dissipation by Gas-Phase Fluorination on Nomex Paper
## 3 Results and Discussion
### Surface Charge Accumulation and Dissipation  
Isothermal surface potential decay (ISPD) model was utilized to characterize the trap energy and distribution of the samples. Based on the ISPD model, the trap energy level \(E_{t}\) and trap density \(N(E_{t})\) can be calculated as [28],  
\[E_{t}=kT\ ln(vt) \tag{1}\]  
\[N(E_{t})=\frac{\varepsilon_{0}\varepsilon_{r}t}{kT\Omega t}\frac{dV_{S}}{dt} \tag{2}\]  
where \(k\) is the Boltzmann constant, \(T\) is the absolute temperature, \(v\) equaling to \(4.17\times 10^{13}\), \(s\) is the attempt to escape frequency of the trapped electron, \(\varepsilon_{0}\) and \(\varepsilon_{r}\) are the vacuum permittivity and relative permittivity of dielectric, \(q\) is the Coulomb's quantity of electrons, \(\delta\) = 2 \(\upmu\)m is the thickness of the top charge layer, and \(L\) is the sample thickness. Figure 4 presents the positive and negative surface potential decay curve of sample F0, F15, F30 and F45 after implementation of \(\pm\)5 kV DC (grid electrode) for 10 min. The initial potential and \(t_{50}\), the time when the surface potential decays to 50% of the initial potential, are also utilized to characterize the surface charge characteristic. It can be seen that both the positive and negative surface potential decay rate decreased with the extension of time; moreover, the decay rate increased when the fluorination duration was prolonged from 0 to 30 min and decreased when the duration was further prolonged from 30 to 45 min. During the corona charging, the carriers including electron, positive and negative ions would deposit on sample surface and would be captured by surface trap, followed by the thermal de-trapping just after the charging voltage was off. Since, the de-trapped time was shorter in shallow  
Figure 3: ATR-FTIR spectrums of Nomex paper before and after fluorination. Figure 2: Structure of PMIA (Nomex) molecule chain. It can be observed in Figures 5 and 6 that all of the trap moved toward to a lower energy band after fluorination. The shallow trap density and energy level showed a similar change in both electron and hole trap with fluorination duration extended: The shallow trap energy level decreased and trap density increased when the fluorination duration was prolonged from 0 to 30 min, and then developed in the opposite tendency when the fluorination duration was prolonged to 45 min. Meanwhile, both the energy level and the deep trap density decreased first and then increased, and reached the minimum at F30. As we know, the de-trapping probability of charge carriers can be calculated as follows [29]:  
\[P_{de}=ve^{-\frac{E_{t}}{RT}} \tag{3}\]  
where \(P_{de}\) is the de-trapping probability of charge carriers, which means that the decrease of energy level \(E_{t}\) will result in the promotion of de-trapping probability. Both the deep and shallow trap developed toward a lower energy level compared with original samples, thereby increasing the de-trapping probability of charge carriers, meaning the accumulated surface charges could easily dissipate and lead to a higher surface potential decay rate. It can be seen from the ATR-IR results that an obvious molecular structure change occurred during fluorination and a large amount of molecular chain scission and cleavage would significantly influence the trap characteristic of the samples. The mechanism of physical and chemical defects can be used to explain the trap density and energy level variety of the samples before and after fluorination. With the extending of fluorination from 0 to 30 min, the formation of physical defects were introduced by the structure change could be the main factor that leads to decrease in the deep trap, because the physical defects had much weaker bonding force than the chemical defects that were formed by introducing the highest electronegativity fluorine element [30].