**3.3 Structure design**

Structure design is another effective approach to improve lifetime of PI under repetitive impulse voltages. **Figure 10** shows the lifetime of PI, PI/Al2O3

### **Figure 10.**

*Lifetime of PI, PI/Al2O3 nanocomposites, and PI/Al2O3-PI-PI/Al2O3 films under repetitive impulse voltages as a function of applied voltage (room temperature) [10].*

nanocomposites, and PI/Al2O3-PI-PI/Al2O3 films under repetitive impulse voltages as a function of applied voltage. For all samples, lifetime decreases with increasing applied voltage. As the thickness of these three types of samples is all 60 mm, the lifetime of PI/Al2O3 nanocomposites are the longest. However, the results shown in **Figure 10** were tested at room temperature. Considering the effect of temperature, lifetime of PI/Al2O3-PI-PI/Al2O3 films is longest when the temperature is higher than 90°C, as shown in **Figure 11**. For applications such as inverter-fed motors, the

### **Figure 11.**

*Lifetime of PI, PI/Al2O3 nanocomposites, and PI/Al2O3-PI-PI/Al2O3 films under repetitive impulse voltages as a function of temperature (1 kV) [10].*

operating temperature of PI is usually higher than 90°C; thus layered structure is an important approach to improve lifetime for industry applications.
