No abstract

No abstract

No abstract

Semi-alicyclic colorless and transparent polyimide (CPI) films usually suffer from the high linear coefficients of thermal expansion (CTEs) due to the intrinsic thermo-sensitive alicyclic segments in the polymers. A series of semi-alicyclic CPI films containing rigid-rod amide moieties were successfully prepared in the current work in order to reduce the CTEs of the CPI films while maintaining their original optical transparency and solution-processability. For this purpose, two alicyclic dianhydrides, hydrogenated pyromellitic anhydride (HPMDA, I), and hydrogenated 3,3',4,4'-biphenyltetracarboxylic dianhydride (HBPDA, II) were polymerized with two amide-bridged aromatic diamines, 2-methyl-4,4'-diaminobenzanilide (MeDABA, a) and 2-chloro-4,4'-diaminobenzanilide (ClDABA, b) respectively to afford four CPI resins. The derived CPI resins were all soluble in polar aprotic solvents, including N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). Flexible and tough CPI films were successfully prepared by casing the PI solutions onto glass substrates followed by thermally cured at elevated temperatures from 80 • C to 250 • C. The MeDABA derived PI-I a (HPMDA-MeDABA) and PI-II a (HBPDA-MeDABA) exhibited superior optical transparency compared to those derived from ClDABA (PI-I b and PI-II b ). PI-I a and PI-II a showed the optical transmittances of 82.3% and 85.8% at the wavelength of 400 nm with a thickness around 25 µm, respectively. Introduction of rigid-rod amide moiety endowed the HPMDA-PI films good thermal stability at elevated temperatures with the CTE values of 33.4 × 10 −6 /K for PI-I a and 27.7 × 10 −6 /K for PI-I b in the temperature range of 50-250 • C. Comparatively, the HBPDA-PI films exhibited much higher CTE values. In addition, the HPMDA-PI films exhibited good thermal stability with the 5% weight loss temperatures (T 5% ) higher than 430 • C and glass transition temperatures (T g ) in the range of 349-351 • C.The polymeric optical films with high thermal stability, high dimensional stability at elevated temperatures, high optical transparency, and high tensile properties are highly desired in modern optoelectronic areas, such as substrates or covering windows for flexible active matrix organic light-emitting diodes (AMOLED) devices, substrates for organic photovoltaic (OPV) solar cells, and so on [1][2][3]. Conventional polymeric optical films, such as polyolefin and polyester films usually suffer from the poor thermal and dimensional stability while the standard high-temperature resistant polymer films, such as the wholly aromatic polyimide (PI) films often exhibit highly colored appearance and poor optical transparency in the visible light region [4]. That is to say, the molecular structure factors that affect the optical and thermal properties of polymer films are usually contradictory. The structural factors that can improve the thermal stability of polymer films often reduce the optical properties of the films at the same time, and vice versa. Thus, it is a changeling research topic to dev...

Wide-bandgap power devices with voltage ratings exceeding 10 kV have the potential to revolutionize medium-and high-voltage systems due to their high-speed switching and lower on-state losses. However, present power module packages are limiting the performance of these unique switches. The objective of this work is to push the boundaries of high-density, high-speed, 10 kV power module packaging. The proposed package addresses the well-known electromagnetic and thermal challenges, as well as the more recent and prominent electrostatic and electromagnetic interference issues associated with high-speed, 10 kV devices. The module achieves low and balanced parasitic inductances, resulting in a record switching speed of 250 V/ns with negligible ringing and voltage overshoot. An integrated screen reduces the commonmode current that is generated by these fast voltage transients by ten times. This screen connection simultaneously increases the partial discharge inception voltage by more than 50 %. A compact, medium-voltage termination and system interface design is also proposed in this work. With the integrated jet-impingement cooler, the power module prototype achieves a power density of 4 W/mm 3 . This paper presents the design, prototyping, and testing of this optimized package for 10 kV SiC MOSFETs.

The relatively poor atomic-oxygen (AO) resistance of the standard polyimide (PI) films greatly limits the wide applications in low earth orbit (LEO) environments. The introduction of polyhedral oligomeric silsesquioxane (POSS) units into the molecular structures of the PI films has been proven to be an effective procedure for enhancing the AO resistance of the PI films. In the current work, a series of POSS-substituted poly (pyromellitic anhydride-4,4′-oxydianiline) (PMDA-ODA) films (POSS-PI) with different POSS contents were synthesized via a POSS-containing diamine, N-[(heptaisobutyl-POSS)propyl]-3,5-diaminobenzamide (DABA-POSS). Subsequently, the effects of the molecular structures on the thermal, tensile, optical, and especially the AO-erosion behaviors of the POSS-PI films were investigated. The incorporation of the latent POSS substituents decreased the thermal stability and the high-temperature dimensional stability of the pristine PI-0 (PMDA-ODA) film. For instance, the PI-30 film with the DABA-POSS content of 30 wt% in the film exhibited a 5% weight loss temperature (T5%) of 512 °C and a coefficient of linear thermal expansion (CTE) of 54.6 × 10−6/K in the temperature range of 50–250 °C, respectively, which were all inferior to those of the PI-0 film (T5% = 574 °C; CTE = 28.9 × 10−6/K). In addition, the tensile properties of the POSS-containing PI films were also deteriorated, to some extent, due to the incorporation of the DABA-POSS components. The tensile strength (TS) of the POSS-PI films decreased with the order of PI-0 > PI-10 > PI-15 > PI-20 > PI-25 > PI-30, and so did the tensile modulus (TM) and the elongations at break (Eb). PI-30 showed the TS, TM, and Eb values of 75.0 MPa, 1.55 GPa, and 16.1%, respectively, which were all lower than those of the PI-0 film (TS = 131.0 MPa, TM = 1.88 GPa, Eb = 73.2%). Nevertheless, the incorporation of POSS components obviously increased the AO resistance of the PI films. All of the POSS-PI films survived from the AO exposure with the total fluence of 2.16 × 1021 atoms/cm2, while PI-0 was totally eroded under the same circumstance. The PI-30 film showed an AO erosion yield (Es) of 1.1 × 10−25 cm3/atom, which was approximately 3.67% of the PI-0 film (Es = 3.0 × 10−24 cm3/atom). Inert silica or silicate passivation layers were detected on the surface of the POSS-PI films after AO exposure, which efficiently prevented the further erosion of the under-layer materials.

No abstract

No abstract