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121.
ABSTRACT The thermoplastic and low dielectric constants polyimides were introduced. The polyimides were prepared by pyromellitic dianhydride (PMDA) or 4,4?-(4,4?-Isopropylidenediphenoxy)diphthalic anhydride (BPADA) as anhydride monomer and 4,4?-oxydianiline (ODA) or 2,2-bis(4-(4-aminephenoxy)phenyl)propane (BAPP) as amine monomer. The polyimides were well characterized by FT-IR, thermogravimetric analysis, dynamic thermomechanical analysis, dielectric measurement, and tensile test. The dielectric constants were 2.32–2.95 compared with 3.10 of ODA-PMDA polyimide, while partly polyimides were thermoplastic. The results indicated anhydride monomers, containing lateral methyl groups, made polyimides become thermoplastic. The results of molecular simulations via Materials Studio also proved this conclusion. 相似文献
122.
《Ceramics International》2020,46(13):21014-21020
Structural features of the glass family xLi2O- yMgO (4.8 Bi2O3 47.6 P2O5) obtained by melt quenching technique were studied taking into account the density, FTIR and UV–vis spectra and also the electrical response observed by impedance spectroscopy. In this work it becomes clarified how the alkaline earth oxides stabilize the glassy matrix and also, the fundamental importance of determining the optimal proportion in order to obtain a flabby easily polarizable matrix to enhance the electrical behavior due to a boosted cation mobility. It is evidenced that when the glass composition becomes complex it is needed to take into account a larger number of structural parameters to understand, to predict or to design the resulting physical properties. 相似文献
123.
Jialin Sun Jun Zhao Xiuying Ni Feng Gong Zuoli Li 《Journal of the European Ceramic Society》2018,38(9):3096-3103
This work investigates the critical roles of two-step sintering (TSS) and laminated structure on the sintering behavior and mechanical properties of functionally graded WC-TiC-Al2O3 nanostructured composite materials doped with Cr3C2/VC. Results show that excellent mechanical properties are achieved for tailored TSS conditions with a hardness of 27.91?±?2.3?GPa and a flexural strength of 1423.3?±?23.5?MPa. The desirable mechanical properties are attributed to the suppressed grain growth without densification deterioration. TSS is more effective in facilitating the favorable dispersion of secondary phase toughening nano-particulates in a WC matrix than conventional sintering (CS). Cr3C2/VC dopant plays an important role in maximizing and shifting the temperature range of the kinetic window for WC-Al2O3 composites. Al2O3 crack deflection, transgranular Al2O3, microcracking, WC crack bridging and plate-like WC crack deflection are the major toughening mechanisms. Residual surface compressive stress induced by the graded structure is also an appreciated contribution to the improvement of mechanical properties. 相似文献
124.
《Journal of the European Ceramic Society》2020,40(8):3035-3041
BaCu2-xCoxSi2O7 solid solutions with orthorhombic structure (Pnma) were prepared by solid-state reaction method. The phase synthesis process, structural evolution and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics were investigated. Single BaCu2Si2O7 phase was obtained when calcined at 950 °C for 3 h and was decomposed into BaCuSi2O6 phase when calcined at 1075 °C for 3 h. The sintering process was effectively promoted when Cu2+ was replaced by Co2+ and the maximum solubility of BaCu2-xCoxSi2O7 was located between 0.15 and 0.20. P-V-L complex chemical bond theory and Raman spectra were used to explain the structure-property correlations of BaCu2-xCoxSi2O7 ceramics. The corrected dielectric constant (εr-corr) of BaCu2-xCoxSi2O7 ceramics decreased monotonously with the susceptibility (Σχμ) and ionic polarizability of primitive unit cell. The quality factor (Q × f) increased with bond strength and lattice energy (Ucal), especially the lattice energy of the Si-O bond. The temperature coefficient of resonant frequency (τf) was determined by the susceptibility and lattice energy of the Cu/Co-O bond. The following optimum microwave dielectric properties were obtained at x = 0.15 when sintered at 1000 °C for 3 h: εr = 8.45, Q×f =58958 GHz and τf = -34.4 ppm/°C. 相似文献
125.
Metal injection moulding (MIM) is an established process for high volume production of complex shaped metallic parts using commercially available feedstocks. The characteristics of parts after moulding, debinding, and sintering cannot be simply predictable from raw materials because the properties get altered with the process parameters and the corresponding levels of porosity during processing steps. In this study, physical properties, microstructure, and mechanical properties of the MIM parts have been characterised to understand the evolution of strength during various steps in MIM processing. Feedstocks with different binder loading show a considerable difference in physical as well as mechanical characteristics. During sintering of parts which have solid loading of grinding sludge, simultaneous in situ reduction and densification takes place, whereas only densification occurs in carbonyl iron parts. It is, therefore, possible to make complex shaped parts of different levels of porosity from downgraded shop floor metallic waste. 相似文献
126.
《Ceramics International》2020,46(10):15801-15811
In the present study, a heterostructure based on Zn0·1Cd0·9S, N-doped graphene quantum dots (N-GQDs), and graphene was successfully prepared by a simple method. Various analyses are conducted to determine the structure, morphology, and materials performance of the synthesized composite. The results exhibit that the Zn0·1Cd0·9S/N-GQDs/graphene heterostructure presents excellent photoelectric performance with a high photocurrent of 4.43 × 10−5 A/cm2 and 3.43 × 10−5 A/cm2 under light irradiation of 365 nm and 405 nm, respectively. It demonstrates a two-fold photocurrent enhancement in comparison to blank Zn0·1Cd0.9S. This remarkable improvement is ascribed to a mechanism in which the N-GQDs act as photosensitizers, enhancing the absorption ability. Concurrently, graphene serves as a carrier mobility substrate, facilitating the separation of the photogenerated electron–hole pairs. The synergetic effect between Zn0·1Cd0·9S, the N-GQDs, and graphene enhances the photoelectric performance. The Zn0·1Cd0·9S/N-GQDs/graphene heterostructure provides a new route for the enhancement of the photoelectric performance of a semiconductor under UV–visible light. 相似文献
127.
128.
Most researches on graphene/polymer composites are focusing on improving the mechanical and electrical properties of polymers at low graphene content instead of paying attention to constructing graphene’s macroscopic structures. In current study the homo-telechelic functionalized polyethylene glycols (FPEGs) were tailored with π-orbital-rich groups (namely phenyl, pyrene and di-pyrene) via esterification reactions, which enhanced the interaction between polyethylene glycol (PEG) molecules and chemical reduced graphene oxide (RGO) sheets. The π–π stacking interactions between graphene sheets and π-orbital-rich groups endowed the composite films with enhanced tensile strength and tunable electrical conductivity. The formation of graphene network structure mediated by the FPEGs fillers via π–π stacking non-covalent interactions should account for the experimental results. The experimental investigations were also complemented with theoretical calculation using a density functional theory. Atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), UV–vis and fluorescence spectroscopy were used to monitor the step-wise preparation of graphene composite films. 相似文献
130.
Zhiyuan Jiang Peng Liu Qihui Chen Hung-Jue Sue Tim Bremner Lorenzo P. DiSano 《应用聚合物科学杂志》2020,137(33):48966
Interest in developing high-performance blends for niche applications has grown significantly in efforts to meet ever-increasing harsh environment demands. In this work, four model poly(aryl-ether-ketone)/polybenzimidazole (PAEK/PBI) blends were chosen to study the influence of premixing methods, processing, and matrix polymers, on their mechanical properties. Among the model poly(ether ether ketone) (PEEK) and PBI blends, mechanical properties are greatly enhanced by melt premixing. The molding process mainly affects the matrix crystallinity, which in turn greatly influences fracture toughness of the blend. Poly(ether ketone ketone) (PEKK) and PBI blend exhibits a slightly lower tensile strength and fracture toughness than PEEK/PBI due to the differences in inherent properties of PEEK and PEKK matrices and their interfacial interaction with PBI. The processing−structure–property relationship of PAEK/PBI blends is established to help guide optimal design of high-performance polymer blends for structural applications. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48966. 相似文献