Fluorographene/polyimide composite films: Mechanical,electrical, hydrophobic,thermal and low dielectric properties

Nowadays, dielectric materials with excellent mechanical and hydrophobic properties are desired for use in the integrated circuits (ICs). For this reason, low dielectric constant fluorographene/polyimide (FG/PI) composite films were prepared by a facile solution blending method, suggesting that the mechanical, electrical, hydrophobic and thermal properties were significantly enhanced in the presence of FG. With addition of 1 wt% FG, the tensile strength, Young’s modulus and elongation at break were dramatically increased by 139%, 33% and 18% respectively when compared with pure PI film. Furthermore, composite films exhibit superior hydrophobic and thermal stability performance. Especially, the FG/PI film with 0.5 wt% of FG possessing a low dielectric constant of 2.48 and a good electrical insulativity that is lower than 10⁻¹⁴ S m⁻¹. Therefore, by their excellent performance, FG/PI hybrid films represent suitable candidate solutions with applications in the microelectronics and aerospace industries.     

Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Out-of-plane effective shear elastic properties of a novel cellular core for sandwich structures

When designing sandwich structures, different types of periodic cellular cores have been considered trying to achieve higher overall bending stiffness and strength to weight ratios as a main objective. Although most of them have proved to offer advantages in certain specific applications, there is a common drawback for all: complexity of the manufacturing process reflected in higher manufacturing costs. This article aims at investigating the out-of-plane shear elastic properties G_xz and G_yz of a novel cellular core named ExpaAsym, produced by a potentially simple manufacturing method: sheet material expansion. Numerical analyses are carried out in order to study the way in which the out-of-plane shear elastic properties modify in terms of several geometric parameters that define the topology of the structure. The numerical results are validated through experimental tests.     

Mechanical characterisation of a glass/polyester sandwich structure for marine applications

Composite materials have recently found application in various field, particularly for high performance equipment. Some examples might be found in the sport and sea transportation field (i.e. yacht hulls, windsurf boards).The aim of the present work is to deepen the knowledge of mechanical properties of sandwich structures used for marine applications, suggesting at the same time some solutions to increase their performances. In particular, a new lamination sequence – used to realise the cover exhaust of luxury yachts – has been investigated.The behaviour of the sandwiches was studied under static conditions, by performing the following tests: three point flexural, torsion, edgewise compression and flatwise compression test. The tests execution has allowed both to analyse the mechanical performances and to understand several fracture mechanisms that take place in these structures.     

Improving the Cyclic-Oxidation Resistance of Ti3AlC2 at 550°C and 650°C by Preoxidation at 1100°C

Preoxidation of Ti₃AlC₂ at 1100°C for 2 h was conducted to improve its cyclic-oxidation resistance at the testing temperature of 550°C and 650°C in air. The cyclic oxidation of the preoxidized Ti₃AlC₂ was found to follow a parabolic rate law rather than the linear oxidation rate for that without preoxidation. Through the X-ray diffraction and SEM analysis, the remarkable improvement of the cyclic-oxidation resistance of preoxidation Ti₃AlC₂ is suggested due to the existence of protective α-Al₂O₃ layers formed during the preoxidation treatment, which inhibits the formation of amorphous Al₂O₃, which can result in larger thermal stress and stress-induced microcracks.     

Biomimetic high toughness Si3N4 ceramics with inverse-bouligand structure

Inspired by the Bouligand structure in nature, a kind of Inverse-Bouligand structure was designed. Different from the bionic Bouligand structure of a one-dimensional material unit, the inverse-Bouligand structure is obtained by stacking and twisting the ceramic layer with a parallel groove structure. The grooves arranged in parallel are first etched on the ceramic green body and then sintered after stacking and twisting. After sintering, the groove structure can still be maintained, so the resulting laminated twisted groove structure in the bulk forms an inverse-Bouligand structure, which can promote crack deflection and improve the toughness of the material during the fracture process. After testing different twist angles, the results show that when the twist angle is 15°, the toughness of the material can reach 11.22 ± 1.77 MPa m^1/2, and high strength can be guaranteed. The synergistic effect of interlayer crack deflection and regional crack twist is the main mechanism for the improvement of material toughness.     

考虑接头作用的全复合材料桁架结构多尺度分析

为提高全复合材料桁架分析的精度和效率,引入结构多尺度有限元思想,对接头进行精细化建模,通过建立两点位移约束实现不同尺度模型连接,从而将接头模型嵌入宏观桁架模型,并针对具体制备工艺赋予桁架材料属性。为验证多尺度模型的优势,同时进行全复合材料桁架实验,以及分别基于全部梁单元模型和全部实体单元模型的有限元分析。对比相关模型的计算精度与效率,结果表明多尺度模型能够较好地兼顾计算精度与效率。该文针对全复合材料桁架的结构多尺度有限元建模方法,可精确分析全复合材料桁架承载性能,并且能够提供有效的局部信息,可用于分析其他包含复杂细节构造的大尺度复合材料结构。     

Diamond as an active sensor material

Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Previously (J.L. Davidson, W.P. Kang, Examples of diamond sensing applications, Proceedings 3rd International Symposium on Diamond Film (ISDF-3), Polytechnical Institute of Russian Academy of Science, St. Petersburg, Russia, 16–19 June 1996) we reported the discovery and development of useful ‘secondary’ effects in diamond and applying them to interesting sensor applications. For example, boron-doped diamond piezoresistors for strain micro-gauges on rugged MEMS (microelectromechanical structures) pressure and acceleration sensors. This paper will present some recent developments with chemically vapor-deposited diamond for microelectromechanical sensing applications such as a new design all diamond pressure microsensor that measures pressure at high temperatures and an accelerometer with over 45 kHz resonant frequency. Also, presented are recent results on layered diamond films that behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pt/SnO_x/i-diamond/p⁺-diamond metal–insulator–semiconductor diode structure for oxygen sensing is described. In addition, the latest emission properties of fabricated diamond microtips for field emitters are reviewed.     

Effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites

The effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites was evaluated. Alkali treatment of the fibers and reaction with organosilanes as coupling agents were applied to improve fiber–matrix adhesion. Fiber loadings of 1, 3, 5, and 7 wt% were incorporated to the phenolic matrix and tensile, flexural, morphological and thermal properties of the resulting composites were studied. In general, mechanical properties of the composites showed a maximum at 3% of fiber loading and a uniform distribution of the fibers in such composites was observed. Silane treatment of the fibers provided derived composites with the best thermal and mechanical properties. Meanwhile, NaOH treatment improved thermal and flexural properties, but reduced tensile properties of the materials. Therefore, the phenolic composite containing 3% of silane treated cellulose fiber was selected as the material with optimal properties.     

The influence of temperature on the adhesion of mixed cultures of Staphylococcus aureus and Escherichia coli to polypropylene

The adhesion of Staphylococcus aureus and Escherichia coli, in mixed cultures, to polypropylene surfaces was evaluated at 12°C and 30°C. The micro-organisms were isolated from a chicken carcass and cultured in an aqueous extract, prepared from the same carcass, for the production of biofilms on polypropylene coupons. Adhered cells were counted by epifluorescence microscopy with acridine orange staining. Escherichia coli adhered in greater numbers to the coupons than S. aureus at both temperatures. Staphylococcus aureus adhered better at 12°C than at 30°C, while the reverse was true for E. coli. At 30°C, there was no increase in the number of adherent cells ofS. aureus over 8 h, while E. coli increased from a median of 5·0–19·0 per microscope field. At 12°C, the major increase in adherent cell numbers for both species occurred between 2 and 4 h, so that leaving cleaning until 8 h, as is common, would not result in greatly increased biofilms, 2-hourly cleansing is clearly unrealistic. However, total adherent cell numbers were the same at 12° and 30°C between 4 and 6 h incubation. Hence it seems that reduced temperature has little to offer for restricting biofilm formation on polypropylene work surfaces in a well-run food processing plant.