**3. Approaches for lifetime improvement**

## **3.1 Polyimide nanocomposites**

PD characteristics and lifetime repetitive impulse voltages would be improved by using polyimide nanocomposites. **Figure 7(a)** shows the PDIV of PI and PI/ Al2O3 nanocomposites. PDIV increases with increasing nano-alumina content. The PDIV of PI/Al2O3 (8 wt.%) increases from 693 to 782 V (12.8% increase) compared with that of PI. This means the discharge threshold is much higher for PI/Al2O3 nanocomposites. **Figure 7(b)** gives the PD amplitude and number recorded in 1 min, which both decrease with increasing nano-alumina content, revealing that the decrease in PD intensity is attributed to the effect of nano-alumina.

As shown in **Figure 8**, the PD resistance is enhanced significantly as the content of nano-alumina increases. The lifetime values of PI/Al2O3 with nano-alumina content of 0, 2, 5, and 8 wt.% are 37.8, 52.1, and 61.9 min, increasing by 92.8, 165.8, and 215.8% respectively, compared with that of PI. This demonstrates that the nano-alumina helps to prolong the lifetime of PI.

In polyimide nanocomposites, the melting point of nano-alumina particles is higher than that of polyimide. Under the effect of surface discharge, nanoparticles float on the film surface, forming a protective layer to prevent the polyimide matrix from further erosion. This layer, therefore, is able to mitigate the PD effects on films and block the erosion path, which can help avoid the same local point being eroded continuously. In contrast, without nanoparticles, the erosion will follow a specific path and lead to breakdown much faster. This is why there are a number of microcavity groups in PI/Al2O3 nanocomposites but only a few big cavities in the PI films, as shown in **Figure 6 c** and **d**. Thus, the incorporation of nanoparticles improves the physical characteristics of PI/Al2O3 nanocomposites, because nanoparticles can act as obstacles in the erosion path, and then the degradation of polymer can be reduced, and prolonged lifetime of PI/Al2O3 nanocomposites can be obtained subsequently.

In addition, some acid compounds (e.g., the amic acid and the nitric acid) are generated during PD aging. Meanwhile, new bonds in PI/Al2O3 nanocomposites are the weakest, subsequently the ether linkage, imide ring, and then aromatic ring. Those weakest bonds in interfacial regions would be destroyed at the first stage of aging, so the dissociations of polyimide molecules would be reduced. Because of the chemical bonding between nanoparticles and PI molecules, the degradation of polymer can be reduced, and prolonged lifetime of PI/Al2O3 nanocomposites can be obtained consequently.

Moreover, higher surface conductivity facilitates surface charge dissipation and leads to low PD intensity. Lower trap density is responsible for the low possibility of charge recombination and the corresponding released energy. All these factors improve the electrical characteristics of PI/Al2O3 nanocomposites resulting in reduced degradation of polymer and longer lifetime.

### **3.2 Surface modification**

Lifetime of PI under repetitive impulse voltages would be further improved by surface modification. **Figure 9** shows the lifetime of PI and PI/Al2O3

**Figure 7.** *PD characteristics of PI and PI/Al2O3 nanocomposites. (a) PDIV and (b) PD amplitude [9].*

nanocomposites under repetitive impulse voltages, after being treated by nonthermal plasma in atmospheric air with different treating times. For both PI and PI/ Al2O3 nanocomposites, lifetime can be prolonged by plasma treatment. Apparent improvement of lifetime can be seen in the first 20 s of treating time. However, the lifetime decreases with increasing treating time, when the treating time is longer than 20 s. When the treating time exceeds 30 s, the lifetime decreases sharply, by reaching the values even shorter than that of the untreated samples. For plasmatreated samples, with shallower trap energy level, less charge accumulation facilitates charge dissipation and local electric field mitigation, leading to suppressed PD intensity and longer lifetime. Active groups containing oxygen and nitrogen, which are introduced during plasma treatment, are responsible for trap energy

**Figure 8.** *Lifetime of PI and PI/Al2O3 nanocomposites under PD aging [9].*

### **Figure 9.**

*Lifetime of PI and PI/Al2O3 nanocomposites with different plasma treating times under repetitive impulse voltage.*

distribution modification of PI films. An appropriate treating time is the key factor to introduce reactive groups on film surface and prolong the lifetime.
