Showing papers on "Composite number published in 2009"

Research on Advanced Materials for Li‐ion Batteries

Abstract: In order to address power and energy demands of mobile electronics and electric cars, Li-ion technology is urgently being optimized by using alternative materials. This article presents a review of our recent progress dedicated to the anode and cathode materials that have the potential to fulfil the crucial factors of cost, safety, lifetime, durability, power density, and energy density. Nanostructured inorganic compounds have been extensively investigated. Size effects revealed in the storage of lithium through micropores (hard carbon spheres), alloys (Si, SnSb), and conversion reactions (Cr(2)O(3), MnO) are studied. The formation of nano/micro core-shell, dispersed composite, and surface pinning structures can improve their cycling performance. Surface coating on LiCoO(2) and LiMn(2)O(4) was found to be an effective way to enhance their thermal and chemical stability and the mechanisms are discussed. Theoretical simulations and experiments on LiFePO(4) reveal that alkali metal ions and nitrogen doping into the LiFePO(4) lattice are possible approaches to increase its electronic conductivity and does not block transport of lithium ion along the 1D channel.

Hybrid solar cells from P3HT and silicon nanocrystals.

Abstract: We are reporting new hybrid solar cells based on blends of silicon nanocrystals (Si NCs) and poly-3(hexylthiophene) (P3HT) polymer in which a percolating network of the nanocrystals acts as the electron-conducting phase. The properties of composite Si NCs/P3HT devices made by spin-coating Si NCs and P3HT from a common solvent were studied as a function of Si NC size and Si NC/P3HT ratio. The open-circuit voltage and short-circuit current are observed to depend on the Si NC size due to changes in the bandgap and surface-area-to-volume ratio. Under simulated one-sun A.M. 1.5 direct illumination (100 mW/cm2), devices made with 35 wt % Si NCs 3−5 nm in size showed 1.15% power conversion efficiency.

A self-sensing carbon nanotube/cement composite for traffic monitoring.

Abstract: In this paper, a self-sensing carbon nanotube (CNT)/cement composite is investigated for traffic monitoring. The cement composite is filled with multi-walled carbon nanotubes whose piezoresistive properties enable the detection of mechanical stresses induced by traffic flow. The sensing capability of the self-sensing CNT/cement composite is explored in laboratory tests and road tests. Experimental results show that the fabricated self-sensing CNT/cement composite presents sensitive and stable responses to repeated compressive loadings and impulsive loadings, and has remarkable responses to vehicular loadings. These findings indicate that the self-sensing CNT/cement composite has great potential for traffic monitoring use, such as in traffic flow detection, weigh-in-motion measurement and vehicle speed detection.

High Mechanical Performance Composite Conductor: Multi-Walled Carbon Nanotube Sheet/Bismaleimide Nanocomposites

Abstract: Multi-walled carbon nanotube (MWNT)-sheet-reinforced bismaleimide (BMI) resin nanocomposites with high concentrations (∼60 wt%) of aligned MWNTs are successfully fabricated. Applying simple mechanical stretching and prepregging (pre-resin impregnation) processes on initially randomly dispersed, commercially available sheets of millimeter-long MWNTs leads to substantial alignment enhancement, good dispersion, and high packing density of nanotubes in the resultant nanocomposites. The tensile strength and Young's modulus of the nanocomposites reaches 2 088 MPa and 169 GPa, respectively, which are very high experimental results and comparable to the state-of-the-art unidirectional IM7 carbon-fiber-reinforced composites for high-performance structural applications. The nanocomposites demonstrate unprecedentedly high electrical conductivity of 5 500 S cm−1 along the alignment direction. Such unique integration of high mechanical properties and electrical conductance opens the door for developing polymeric composite conductors and eventually structural composites with multifunctionalities. New fracture morphology and failure modes due to self-assembly and spreading of MWNT bundles are also observed.

A carbon nanotube/cement composite with piezoresistive properties

Abstract: This paper studies the piezoresistive property of the CNT/cement composite to explore its feasibility as an embedded stress sensor for civil structures such as roadways, levees and bridges. The experimental results show that the electrical resistance of the CNT/cement composite changes with the compressive stress level, indicating the potential of using the CNT/cement composite as a stress sensor for civil structures. The piezoresistive responses of the composite with different fabrication methods and CNT doping levels were also studied. It is found that dispersion-assistant surfactants could block the contacts among carbon nanotubes, thus impairing the piezoresistive response of the composite, while a higher CNT doping level could improve the sensitivity of the composite stress response.

Formation Mechanism of β-Phase in PVDF/CNT Composite Prepared by the Sonication Method

Abstract: Two poly(vinylidene fluoride)(PVDF)/carbon nanotube (CNT) composites are prepared by solution sonication and mechanical mixture approaches. It is found that α-phase coexists with β-phase in the composite prepared by sonicating the PVDF/CNT mixture solution, while no β-phase can be observed in the composite prepared from the mechanical mixture route. With the help of the density functional theory calculations, it is explained that a large amount of energy is required for transforming trans−gauche−trans−gauche′ (TGTG′) into trans−trans (TT) conformations and the TT molecular chain can be bound on the CNT surface tightly. The emergence of β-phases is independent of zigzag carbon atoms on the CNT surface. The formation mechanism of β-phase is proposed based on the theoretical calculations and experimental results.

Polymer Layered Silicate Nanocomposites: A Review

Abstract: This review aims to present recent advances in the synthesis and structure characterization as well as the properties of polymer layered silicate nanocomposites. The advent of polymer layered silicate nanocomposites has revolutionized research into polymer composite materials. Nanocomposites are organic-inorganic hybrid materials in which at least one dimension of the filler is less than 100 nm. A number of synthesis routes have been developed in the recent years to prepare these materials, which include intercalation of polymers or pre-polymers from solution, in-situ polymerization, melt intercalation etc. The nanocomposites where the filler platelets can be dispersed in the polymer at the nanometer scale owing to the specific filler surface modifications, exhibit significant improvement in the composite properties, which include enhanced mechanical strength, gas barrier, thermal stability, flame retardancy etc. Only a small amount of filler is generally required for the enhancement in the properties, which helps the composite materials retain transparency and low density.

Synthesis and Electrochemical Performance of Sulfur/Highly Porous Carbon Composites

Abstract: Sulfur/highly porous carbon (HPC) composites were synthesized by thermally treating a mixture of sublimed sulfur and HPC. The microstructure of the HPC and the composite was characterized by transm...

A novel fiber-reinforced polyethylene composite with added silicon nitride particles for enhanced thermal conductivity

Abstract: A novel thermally conductive plastic composite was prepared from a mixture of silicon nitride (Si 3 N 4 ) filler particles and an ultrahigh molecular weight polyethylene–linear low density polyethylene blend. The effects of Si 3 N 4 particle sizes, concentration, and dispersion on the thermal conductivity and relevant dielectric properties were investigated. With proper fabrication the Si 3 N 4 particles could form a continuously connected dispersion that acted as the dominant thermally conductive pathway through the plastic matrix. By adding 0–20% Si 3 N 4 filler particles, the composite thermal conductivity was increased from 0.2 to ∼1.0 W m −1 K −1 . Also, the composite thermal conductivity was further enhanced to 1.8 W m −1 K −1 by decreasing the Si 3 N 4 particle sizes from 35, 3 and 0.2 μm, and using coupling agent, for the composites with higher filler content. Alumina short fibers were then added to improve the overall composite toughness and strength. Optimum thermal, dielectric and mechanical properties were obtained for a fiber-reinforced polyethylene composite with 20% total alumina–Si 3 N 4 (0.2 μm size) filler particles.

High-Strength Composite Fibers: Realizing True Potential of Carbon Nanotubes in Polymer Matrix through Continuous Reticulate Architecture and Molecular Level Couplings

Abstract: Carbon nanotubes have unprecedented mechanical properties as defect-free nanoscale building blocks, but their potential has not been fully realized in composite materials due to weakness at the interfaces. Here we demonstrate that through load-transfer-favored three-dimensional architecture and molecular level couplings with polymer chains, true potential of CNTs can be realized in composites as initially envisioned. Composite fibers with reticulate nanotube architectures show order of magnitude improvement in strength compared to randomly dispersed short CNT reinforced composites reported before. The molecular level couplings between nanotubes and polymer chains results in drastic differences in the properties of thermoset and thermoplastic composite fibers, which indicate that conventional macroscopic composite theory fails to explain the overall hybrid behavior at nanoscale.

Ordered Network Mesostructures in Block Polymer Materials

Abstract: Block polymers are formed by the covalent union of two or more chemically distinct homopolymers These composite macromolecules self-assemble into a variety of ordered morphologies with features on the nanometer length scale, a phenomenon that has interested researchers for roughly four decades The known ordered morphologies include numerous multiply continuous network mesostructures, the focus of this review Multiply continuous network morphologies contain two or more chemically distinct domains that continuously percolate through the specimen in all three dimensions They have captivated researchers because of their superior mechanical properties and could potentially find utility in technologies such as catalysis, photonic materials, solar cells, and separations This review summarizes experimental and theoretical investigations of the structures and properties of network morphologies in AB block copolymer and ABC block terpolymer systems and includes a discussion of some proposed technological appli

Increased Interface Strength in Carbon Fiber Composites through a ZnO Nanowire Interphase

Abstract: One of the most important factors in the design of a fiber reinforced composite is the quality of the fiber/matrix interface. Recently carbon nanotubes and silicon carbide whiskers have been used to enhance the interfacial properties of composites; however, the high growth temperature degrade the fiber strength and significantly reduce the composite's in-plane properties. Here, a novel method for enhancing the fiber/matrix interfacial strength that does not degrade the mechanical properties of the fiber is demonstrated. The composite is fabricated using low-temperature solution-based growth of ZnO nanowires on the surface of the reinforcing fiber. Experimental testing shows the growth does not adversely affect fiber strength, interfacial shear strength can be significantly increased by 113%, and the lamina shear strength and modulus can be increased by 37.8% and 38.8%, respectively. This novel interface could also provide embedded functionality through the piezoelectric and semiconductive properties of ZnO.

Influence of moisture absorption on the interfacial strength of bamboo/vinyl ester composites

Abstract: Moisture absorption is a major concern for natural fibers used as reinforcement in structural composites. This paper reports a detailed study on the moisture sorption characteristics of bamboo strips and their influence on the interfacial shear strength (IFSS) of bamboo/vinyl ester composite. The IFSS determined by pull-out test decreased dramatically as the fabrication humidity increased. The bamboo strips provide a reservoir of moisture which diffuses into the interfacial region and inhibits the hardening of vinyl ester matrix. The interface of the bamboo/vinyl ester composite can also be damaged due to moisture exposure after fabrication. Post-fabrication exposure of composites to moisture was found to be less damaging than the moisture exposure during the composite fabrication. The IFSS of the composite decreased by nearly 40% in the first 9 d of water immersion. Further immersion up to 100 d did not cause any further reduction in interfacial shear strength.

Sensing of Damage Mechanisms in Fiber-Reinforced Composites under Cyclic Loading using Carbon Nanotubes

Abstract: The expanded use of advanced fiber-reinforced composites in structural applications has brought attention to the need to monitor the health of these structures. It has been established that adding carbon nanotubes to fiber-reinforced composites is a promising way to detect the formation of microscale damage. Because carbon nanotubes are three orders of magnitude smaller than traditional advanced fibers, it is possible for nanotubes to form an electrically conductive network in the polymer matrix surrounding the fibers. In this work, multi-walled carbon nanotubes are dispersed into epoxy and infused into a glass-fiber preform to form a network of in situ sensors. The resistance of the cross-ply composite is measured in real-time during incremental cyclic tensile loading tests to evaluate the damage evolution and failure mechanisms in the composite. Edge replication is conducted to evaluate the crack density after each cycle, and optical microscopy is utilized to study the crack mode and growth. The evolution of damage can be clearly identified through the damaged resistance parameter. Through analyzing the damaged resistance response curves with measurements of transverse crack density and strain, the transition between different failure modes can be identified. It is demonstrated that the integration of an electrically conducting network of carbon nanotubes in a glass fiber composite adds unique damage-sensing functionality that can be utilized to track the nature and extent of microstructural damage in fiber composites.