Electrical behavior and positive temperature coefficient effect of graphene/polyvinylidene fluoride composites containing silver nanowires

Polyvinylidene fluoride (PVDF) composites filled with in situ thermally reduced graphene oxide (TRG) and silver nanowire (AgNW) were prepared using solution mixing followed by coagulation and thermal hot pressing. Binary TRG/PVDF nanocomposites exhibited small percolation threshold of 0.12 vol % and low electrical conductivity of approximately 10^-7 S/cm. Hybridization of TRGs with AgNWs led to a significant improvement in electrical conductivity due to their synergistic effect in conductivity. The bulk conductivity of hybrids was higher than a combined total conductivity of TRG/PVDF and AgNW/PVDF composites at the same filler loading. Furthermore, the resistivity of hybrid composites increased with increasing temperature, giving rise to a positive temperature coefficient (PTC) effect at the melting temperature of PVDF. The 0.04 vol % TRG/1 vol % AgNW/PVDF hybrid exhibited pronounced PTC behavior, rendering this composite an attractive material for making current limiting devices and temperature sensors.     

基于透红外视觉传感的GMA-AM熔池图像质量评价

针对熔化极气体保护电弧(gas metal arc,GMA)增材制造(additive manufacturing,AM)图像感知,提出一种透红外熔池视觉传感方法. 为客观评价熔池图像质量,综合图像灰度、纹理、形状和频谱等四类特征定义了熔池图像质量评价参数φ. 结果表明,φ值越大,图像质量越好. 透红外熔池图像质量评价参数φ远大于近红外窄带熔池图像,熔池更清晰,对比度更高. 相比800,850和930 nm等透红外滤光片,990 nm透红外滤光片能过滤大部分电弧连续谱和特征谱,获得的熔池信息量最大,对比度更明显,边缘更清晰,细节信息更丰富,熔池图像质量最佳,是最佳取像窗口.     

The formation of β‐polypropylene crystals in a compatibilized blend of isotactic polypropylene and polyamide‐6

Compatibilized polypropylene (PP)/polyamide (PA6) blends with and without β nucleating agent (β‐NA) are prepared, and are designated as Blend‐0.3 and Blend‐0, respectively. The melting and crystallization characteristic of the blends crystallized under different cooling rates and different crystallization temperatures are studied. It is observed that high β‐PP content can be developed in Blend‐0.3 only at slow cooling rates (<5°C/min), whereas high α‐PP content is formed at fast cooling rates. Isothermal crystallization analysis of Blend‐0 indicates that PA6 is an effective NA for α‐PP in the lower temperature range, whereas the α‐nucleating effect disappears in the higher temperature range. Blend‐0.3 can, therefore, be viewed as a system containing both α‐ and β‐NAs, simultaneously. PA6 is competing with β‐NA in inducing PP crystallization. Under the normal injection of Blend‐0.3, the melt will be cooled through the higher temperature that favors the effectiveness of β‐NA rapidly because of the faster cooling rate. However, the α‐nucleation effect from PA6 predominate at the lower temperature. This explains the difficulty in obtaining high β‐PP content in Blend‐0.3 from injection molding. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Spark Plasma Sintered Hydroxyapatite/Multiwalled Carbon Nanotube Composites With Preferred Crystal Orientation

Hydroxyapatite (HA) and its nanocomposites reinforced with low loading levels of multiwalled carbon nanotubes (MWNTs) are prepared by spark plasma sintering process. The structure, morphology, and hardness of sintered HA and MWNT/HA nanocomposites are characterized by means of X‐ray diffraction (XRD), scanning electron microscopy and nanoindentation techniques. XRD results show that the orientation of crystallographic planes of sintered HA are highly related to the applied pressure direction. The perpendicular section of sintered MWNT/HA nanocomposites shows predominantly oriented HA a‐and b‐planes while the parallel section displays a dominant c‐plane orientation. The hardness of MWNT/HA nanocomposites improves considerably with increasing MWNT content.     

Electrical conductivity and dielectric response of poly(vinylidene fluoride)–graphite nanoplatelet composites

Poly(vinylidene fluoride)/graphite nanoplatelets (PVDF/GNP) composites were fabricated using solution mixing followed by compression molding. The electric conducting and dielectric behavior of such nanocomposites were determined over a wide frequency range from 10² to 10⁷. The results showed that the electrical behavior of PVDF/GNP nanocomposites can be well described by the percolation theory. Both conductivity and dielectric constant were found to be greatly enhanced at the percolation threshold. A large dielectric constant of 173 and low loss tangent of 0.65 were observed in the PVDF/2.5 wt% GNP nanocomposite at 1 kHz. Moreover, dynamic mechanical analysis was also used to characterize the relaxations of polymers in PVDF/GNP nanocomposites. Dielectric and mechanical relaxations of PVDF/GNP nanocomposites showed strong dependence with frequency and temperature. The activation energy for glass transition determined from mechanical relaxation is considerably higher than that evaluated from the dielectric analysis. This resulted from different operating mechanisms for dielectric and mechanical relaxation processes.     

Corrosion Protection of In Situ Al-Based Composite by Cerium Conversion Treatment

Cerium conversion films were deposited on the surface of in situ aluminum-based composite in solutions containing different cerium chloride (CeCl₃) and hydrogen peroxide (H₂O₂) concentrations at 30 °C. The morphology and composition of conversion films deposited in various solutions were investigated using scanning electron microscopy, energy dispersive x-ray analysis, and x-ray photoelectron spectroscopy (XPS). SEM observations revealed that only patches of film can be deposited on the composite surface when immersed in solutions with low CeCl₃ and H₂O₂ concentrations. However, entire composite surface was covered with a compact film when immersed in a solution containing 10 g/L CeCl₃ and 100 mL/L H₂O₂. XPS results indicated that cerium was incorporated as Ce⁴⁺ species in the hydrated oxide film. The formation of such hydrated conversion film on the composite resulted in low anodic current density and more noble pitting potential when exposed to 3.5% NaCl solution.     

Microstructural and mechanical characteristics of compatibilized polypropylene hybrid composites containing potassium titanate whisker and liquid crystalline copolyester

Maleic anhydride (MA) compatibilized polypropylene (MPP) hybrid composites reinforced with potassium titanate whiskers (K₂Ti₆O₁₃) and liquid crystalline polymer (LCP) were prepared in a twin-screw extruder followed by injection molding. The surface of whiskers were treated with tetrabutyl orthotitanate before blending. Scanning electron microscopic (SEM) examination showed that elongated LCP fibrils are formed in the skin section of hybrids reinforced with whiskers of various concentrations. Consequently, the hybrids reinforced with both LCP fibrils and whiskers exhibited anisotropic mechanical properties. Tensile test showed that longitudinal Young’s modulus and tensile strength of hybrids tend to increase with increasing whisker content. Moreover, the stiffness and tensile strength of hybrids were higher than those of MPP/K₂Ti₆O₁₃ composites. Such enhancement in mechanical properties resulted from the compatibilizing effect of MA-grafted-PP, and from the hybrid reinforcing effect of LCP fibrils and K₂Ti₆O₁₃ whiskers. Torque measurements revealed that LCP addition is beneficial in reducing the melt viscosity of MPP/K₂Ti₆O₁₃/LCP hybrids. The results of SEM observations generally correlate well with the mechanical measurements. The effects of MA compatibilization on the microstructure and mechanical properties of hybrids are discussed.     

TEM studies of strain-induced martensitic formation in fatigued Fe-18Mn alloy

Austenitic Fe-18Mn alloy was cyclically deformed at various total strain amplitudes. The structural changes induced by cyclic straining in Fe-18Mn alloy were investigated by transmission electron microscopy. At a low strain amplitude of 0.4%, the formation ofε-martensite associated with deformation twins and stacking faults was observed in this alloy. As the applied strain amplitude was increased to 1.0%,α′-martensite embryos were induced in the alloy investigated. These embryos coalesce into a lath structure upon subsequent cyclic deformation.