168 / 2018-08-25 21:17:30

The mechanical and thermal properties study of polyimide/nano-Al2O3 composite films with sandwich structures

Polyimide, nano-Al2O3, Interlayer interface, Sandwich structure, Mechanical properties, Thermal properties

Polyimide has been extensively valued and applied and become a key element in precision machining, electrical and electronics, aerospace and other fields due to its excellent mechanical, thermal and electrical properties. Driven by the rapid growth and promising future of power electronic technology and high speed railway, pulse width modulation (PWM) technology has been widely used in inverter-fed motor of electric-multiple-units (EMUS), in which area polyimide has a broad application as corona resistance insulation materials. During the working process of EMUS, a large amount of discharge and space charge created by high frequency pulse voltage often bring about the premature aging of insulation materials, and traditional pure polyimide films simply do not have enough ability to satisfy the insulation requirements of variable-frequency motor. Therefore, the modification and functionalization of polyimide materials by incorporation of inorganic nano-particles into polyimide have become a new hot area of innovation and research in recent years.

In this paper, both of the pure PI films and mono-layer PI/Al2O3 composite films are first prepared by casting film method. On this basis, a new type of tri-layer PI/Al2O3 composite films, which has sandwich structure with composite layer on both sides and pure layer in the middle, has been produced. It is observed by transmission electron microscopy (TEM) that nano-Al2O3 particles could be evenly dispersed in the polyimide matrix without cracks, pores and other defects, and tri-layer PI/Al2O3 composite films have a more complete tri-layer structure with the clearly visible interlayer interface. X-Ray Diffraction (XRD) patterns demonstrate that PI/Al2O3 composite films still have the typical chemical structure of polyimide, the doping of nano-Al2O3 particles and sandwich structure do not affect the imidization reaction. The XRD analysis also show that the nano-Al2O3 particles have weaken the order degree of polyimide molecular chain.

Mechanical and thermal properties as the most basic and significant performance of material have been measured in this paper. Results of mechanical properties tests show that the pure PI films have the best tensile strength and elongation at break, and the incorporation of nano-Al2O3 particles would reduce the tensile strength and elongation at break of PI/Al2O3 composite films. Under the same doping content of nano-Al2O3 particles, mono-layer PI/Al2O3 composite films have the lower tensile strength and elongation at break, when compared with tri-layer PI/Al2O3 composite films. With the high doping content, in particular, the mechanical advantage of tri-layer PI/Al2O3 composite films become more apparent, which should owed much to the existence of pure PI layer and interlayer interface in tri-layer PI/Al2O3 composite films. The existence of pure PI layer could compromise the negative effects of nano-particles on composite films, and increase overall mechanical performance,. Interlayer interface could also help to block the development of the micro-crack, and enhance the tensile strength and elongation at break. Various results of thermal properties of mono-layer and tri-layer PI/Al2O3 composite films, such as thermo-gravimetry analysis (TGA), differential scanning calorimeter (DSC) and dynamic mechanical analysis (DMA), indicate that the presence of nano-Al2O3 particles could effectively slow down the thermal decomposition of PI molecule, lead to the increase in thermal stability of the composite films.