Key Engineering Materials Vols. 575-576

Abstract: Polyacrylonitrile (PAN) copolymers containg photocrosslinkable comonomer were obtained by the free-radical suspension copolymerization of Ultraviolet photocrosslinkable comonomer (UPC), methyl acrylate (MA) and acrylonitrile (AN) that was carried out in DMSO/H2O using redox as initiator in which have a typical composition of AN/MA/UPC in the mole ratio of 84/15/1. It was shown that MA was found to be suitable for enabling the melt processing of PAN. The ultraviolet sensitivity of UPC moiety improves the ultraviolet crosslinkability of the precursor fibers, as a result, the melt spun precursor fibers after ultraviolet irradiated can be oxidatively stabilized at 310 oC without melting and subsequently carbonized. The resultant carbon fibers with tensile strength of 602.2±151.3 MPa and modulus of 7.32±0.78 GPa were obtained.

Abstract: Cu-3wt. %MoS2-7wt. %Mo and Cu-3wt. %MoS2 composites were prepared by repressing, re-sintering and vacuum hot pressing, respectively. Microstructures were characterized by optical metallographic microscope, EDS, SEM and X-ray diffraction analysis, respectively. The micro hardness, electric conductivity and density of samples were separately measured as well. Results show that the micro hardness of Cu-3wt. % MoS2-7wt. % Mo composites is about 33.3% higher than that of Cu-3wt. %MoS2 composites. The increase in micro hardness is attributed to the presence of Mo. The electric conductivity of Cu-3wt. %MoS2 and Cu-3wt. %MoS2-7wt. % Mo prepared by vacuum hot pressing were 80.6 % and 63.8% IACS, respectively, which is an increase compared with values of 80.2 % and 57.3% IACS of samples obtained by repressing and re-sintering.

Abstract: There have been tremendous researches on carbon nanotubes (CNTs) and CNTs reinforced composites due to their excellent mechanical, electromagnetical and thermal properties. Radially aligned multi-walled CNTs grown in situ on the surface of carbon fiber or its woven fabric by the chemical vapor deposition method not only provide the significant three-dimensional reinforcement, but also overcome the poor dispersion of nanostructures. In order to investigate how the aligned multi-walled CNTs affect the mechanical properties of the multi-scale composites, a representative volume element is used to simulate the multi-walled CNT pullout process by means of the finite element method. Related influencing factors are taken into consideration: CNT length, CNT Youngs modulus, and the shear strength of the interface between multi-walled CNTs and epoxy matrix, which is characterized by a cohesive zone model. The results reveal that the aligned multi-walled CNTs make a significant contribution to the fracture toughness of the multi-scale composites, and the CNT/epoxy composites with high toughness require relatively high interfacial shear strength, reasonable CNT length and Youngs modulus. The study is helpful for the optimal design of multi-scale composites.

Abstract: AlN-SiC ceramics were prepared at 1950°C by hot pressing (HP) of AlN/SiC powders in stoichiometric proportion. The sintered product was characterized using X-ray diffraction (XRD). Scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) were utilized to investigate the morphology characteristics. The results show that AlN-SiC phase is well-developed with a close and lamellar structure. The grains are plate-like with the size of 1-3μm, the thickness of 3-5μm and elongated dimension. The results showed that the products had the density of 99 percent of the theoretical, bending strength of 550-800 MPa and fracture toughness of 5-6 MPa·m^1/2. The distribution of AlN-SiC grains is relatively uniform.

Abstract: sothermal oxidation behavior of the AlN-TiB₂ conductive ceramics prepared by self-propagating high-temperature synthesis and hot isostatic pressing (SHS-HIP) was evaluated in a temperature range from 900 to 1400 °C for exposure times from 1 to 16 h in air. The oxidation experimental results show that the conductive ceramics have a good oxidation resistance below 1200 °C. The oxidation products on the sample surfaces are mainly composed of Al₂TiO₅, TiO₂ and aluminum borate phases.

Abstract: Carbon fiber reinforced plastics (CFRP) were prepared by manual molding technology and the effect of loading speed on the piezoresistive property of CFPR was discussed. The piezoresistive sensitivity of CFRP with the different content of carbon fibers was contrasted and the interface morphology of CFRP was observed by SEM. The results show that CFRP has the obvious piezoresistive property and it can provide early warning as a kind of strain sensor. The piezoresistive sensitivity of CFRP decreases as the increasing of the content of carbon fibers in CFRP. Moreover the piezoresistive sensitivity of CFRP reduced as the increasing of loading speed. The SEM showed that the interface was good between carbon fibers and epoxy resin.

Abstract: The ZrB₂ particulate reinforced aluminum matrix composites were fabricated via melt direct reaction method using Al-K₂ZrF₆-KBF₄ components. 850°C and 30 min were the optimized synthesizing temperature and reaction time separately. The metallurgical thermodynamic and kinetic processes were then analyzed in detail. It reveals that the interphases include Al₃Zr, AlB₂, [Z and [ atoms. The ZrB₂ particulates can be acquired through the molecular combination between Al₃Zr and AlB₂ or atomic combination between Zr and B atomics. The in situ reaction between reactive salts and molten aluminum takes place spontaneously, which exhibits the character of liquid-liquid reaction. Scanning electronic microscope observation results demonstrate that the sizes of ZrB₂ particulates are almost 100-200 nm. The intervals between particles are almost 200-400 nm, demonstrating a unirom status of distribution.

Abstract: During the cure process of fiber reinforced resin matrix composites, the residual stress and deformation can be generated. The cure-dependent elastic model, which is the earliest constructed and the simplified form of the viscoelastic model, is most universally applied to the finite element analysis on the cure-induced deformation. However, stress relaxation and creep are the unique properties of resin matrix composites, which can affect the residual stress and deformation in some degrees. In this study, both the elastic model and the viscoelastic model were used to predict the residual stress and deformation of fiber composite laminates by means of the finite element method, and then a comparison between them was made. The results show that the difference between them decreases with the increase of fiber volume fraction or the decrease of resin curing shrinkage.

Abstract: On the smallest structural scale in the multi-scale structure composites, namely fiber scale, a numerical model was proposed for the analysis on the mechanical properties of unidirectional composites through the representative unit cell (RUC). The progressive method was used to simulate the failure behavior of fiber and matrix, and the debonding between fiber and matrix was characterized by the cohesive zone model (CZM). The failure strength of the unidirectional composite was predicted, and the influence of the interfacial strength on the mechanical behavior of unidirectional composite was discussed. It is shown that fiber dominates the failure strength of the material under the longitudinal load, whereas under the transverse load interfacial properties play an important role in the mechanical behavior of the material. The increase of the interfacial strength can significantly improve the capability of transverse compression and shear resistance.

Abstract: This paper is aim to investigate the interfacial effect on the bending property of Al/Cu/Al laminated composite produced by the asymmetrical roll bonding and annealing. The interfacial microstructure was observed by scanning electron microscope, and the three-point bending tests were conducted at room temperature. It is found that the interfacial layer near the faster roll is about 1 μm thickness and continuous, and the bending strength is increased by 4.4% in comparison with the interface near the slower roll. The results demonstrate that the shear deformation during asymmetrical roll bonding causes a severe interfacial fracture and makes a good interfacial bonding. The increase of bending load is ascribed to the interfacial improvement.