Structural model for the reinforcement of polymethyl methacrylate/carbon nanotube nanocomposites at an ultralow nanofiller content

The structural basis of the anomalously high reinforcement of polymer/carbon nanotube nanocomposites at an ultralow nanofiller content is studied. This effect is shown to be caused by the absence of interaction between carbon nanotubes and the related sharp increase in the interphase adhesion. From the standpoint of a nanofiller structure, the effect disappears when three critical points related to the structure of carbon nanotubes in a polymer matrix are reached. These points are a percolation threshold, an aggregative nanofiller stability threshold, and the beginning of formation of closed circular carbon nanotube structures.     

Enhancement of single-walled nanotubes luminescence intensity upon dithiothreitol doping

In the present work the influence of reducing agent dithiothreitol doping on photoluminescence spectra of nanotubes with adsorbed biopolymers (single-stranded DNA and polyC) in aqueous suspensions and films was studied. It is revealed that greater intensity enhancement at 10^?3 mol/L dithiothreitol concentration is observed for (7,5) and (6,5) nanotubes in suspension with single-stranded DNA (by more than 150% of initial intensity) comparing to polyC suspension (less than 60%) while for (6,4) and (9,1) nanotubes enhancement is less than 50% for both suspensions. Photoluminescence intensity increasing for nanotube film with DNA is less than 50% without noticeable dependence on nanotube chirality. It is assumed, that different influence of biopolymers on nanotube luminescence intensity enhancement is due to their different coverage of nanotube surface.     

Preparation and Characterization of Linear Low Density Polyethylene/Carbon Nanotube Nanocomposites

Linear low‐density polyethylene (LLDPE)/multiwalled carbon nanotube (MWNT) nanocomposites were prepared via melt blending. The morphology and degree of dispersion of nanotubes in the polyethylene matrix were investigated using scanning electron microscopy (SEM). Both individual and agglomerates of MWNTs were evident. The rheological behavior and mechanical and electrical properties of the nanocomposites were studied using a capillary rheometer, tensile tester, and Tera ohm‐meter, respectively. Both polyethylene and its nanocomposites showed non‐Newtonian behavior in almost the whole range of shear rate. Addition of carbon nanotubes increased shear stress and shear viscosity. It was also found that the materials experience a fluid‐solid transition below 1 wt% MWNT. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. With increasing nanotube content, the activation energy of flow increases. A decrease of about 7 orders of magnitude was obtained in surface and volume resistivity upon addition of 5 wt% MWNT. In addition, a difference between electrical and rheological percolation thresholds was observed. The results confirm the expected nucleant effect of nanotubes on the crystallization process of polyethylene. A slight increase in Young's modulus was also observed with increasing MWNT content.     

Aromatic Polyimide/MWCNT Hybrid Nanocomposites: Structure,Dynamics, and Properties

Two series of hybrid polyimide (PI)/multiwalled carbon nanotube (MWCNT) nanocomposites were prepared including COOH-functionalized or pristine nanotubes, and their structure, morphology and dynamics/mechanical properties at 20°C–500°C were studied using WAXD (Wide-angle X-ray diffraction), AFM (Atomic force microscopy), TEM (transmission electron microscopy), DSC (Differential scanning calorimetry), DMA (Dynamic mechanical analysis), CRS (creep rate spectroscopy) techniques, and stress–strain testing. The impact of nanofiller loadings of 0.125, 0.25, 0.5, or 1 wt% relative to PI was evaluated. Specific changes in the matrix morphology and different quality of nanotube dispersion in the nanocomposites with amorphous and semicrystalline matrices were determined. The best nanotube dispersion was observed in the composites with 0.5 wt% MWCNT-COOH. A peculiar high temperature dynamics, different for amorphous, and semicrystalline matrices, was revealed in these nanocomposites. The most dramatic changes in high temperature dynamics and a pronounced dynamic heterogeneity as well as substantially enhanced mechanical properties at room temperature were revealed in the case of a semicrystalline PI matrix. The results were treated in terms of the synergistic impact of nanotubes and matrix crystallites on dynamics in the intercrystalline regions of PI (“combined constrained dynamics effect”) and the peculiar interfacial dynamics.     

Calculation of the energy of binding of titanium and scandium complexes to the surface of carbon nanotubes

Complexes of zigzag-type carbon nanotubes (CNTs) with transition metal atoms, scandium and titanium, were studied. It was demonstrated that the energy of binding of both atoms with a carbon surface decreases whereas the rate of diffusion along the surface increases with increasing nanotube diameter. The rate constant of migration of scandium atoms over a CNT surface are several orders of magnitude higher than that for titanium atoms, because the CNT surface—Sc atom binding energy is substantially lower.     

Study of titanium adsorption on perfect and defected BC3 nanotubes using density functional theory

Density functional theory calculations were used to study the titanium (Ti) adsorption on perfect and defected (4, 0) BC₃ nanotubes, considering Stone–Wales and vacancy defects. The binding energy values for these nanotubes were larger than the corresponding values for carbon nanotubes. The charge transfer from the Ti atom to nanotube was observed for all systems studied. The most exothermic binding process occurred for the Ti adsorption on a native V_B defect, which showed minimum structural deformation with respect to a perfect BC₃ tube. In the case of a nanotube with a reconstructed carbon vacancy, the adsorption of Ti generated a half-metallic anti-ferromagnet. The results obtained in this paper are relevant for spintronics and hydrogen adsorption applications.     

Raman investigations of TiO2 nanotube substrates covered with thin Ag or Cu deposits

A tubular array of TiO₂ nanotubes on Ti matrix was used as a support for Ag or Cu sputter‐deposited layers intended for surface‐enhanced Raman scattering (SERS) investigations. The composite samples of Ag/TiO₂–nanotube/Ti and Cu/TiO₂–nanotube/Ti were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) [and scanning Auger microscopy (SAM)] to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured after it had been adsorbed on the TiO₂–nanotube/Ti substrates covered with thin Ag or Cu deposit as well as on the bulk electrochemically roughened Ag or Cu reference substrates. It was found that the SERS spectra measured for pyridine adsorbed on the bulk silver substrate were significantly different than the spectra measured on the TiO₂–nanotube/Ti substrates covered the Ag layer. The spectra measured for pyridine adsorbed on the Ag/TiO₂–nanotube/Ti suggest that on the surface of such a composite substrate there are many Lewis acidic sites. Spectra typical for pyridine adsorbed on acidic sites were observed even after deposition of a relatively thick silver layer (e.g. an Ag layer with an average thickness of 80 nm) on the TiO₂–nanotube/Ti support. Our findings suggest that TiO₂–nanotube/Ti support is a promising substrate for the preparation of metallic nano‐clusters on a support containing acidic active sites. Copyright © 2009 John Wiley & Sons, Ltd.     

Growth of multiwalled carbon nanotube arrays by chemical vapour deposition over iron catalyst and the effect of growth parameters

Multiwalled carbon nanotube (CNT) arrays were grown by catalytic thermal decomposition of acetylene, over Fe-catalyst deposited on Si-wafer in the temperature range 700-750 °C. The growth parameters were optimized to obtain dense arrays of multiwalled CNTs of uniform diameter. The vertical cross-section of the grown nanotube arrays reveals a quasi-vertical alignment of the nanotubes. The effect of varying the thickness of the catalyst layer and the effect of increasing the growth duration on the morphology and distribution of the grown nanotubes were studied. A scotch-tape test to check the strength of adhesion of the grown CNTs to the Si-substrate surface reveals a strong adhesion between the grown nanotubes and the substrate surface. Transmission electron microscopy analysis of the grown CNTs shows that the grown CNTs are multiwalled nanotubes with a bamboo structure, and follow the base-growth mechanism.     

Biocompatibility of pure titanium modified by human endothelial cell-derived extracellular matrix

Extracellular matrix (ECM) used to modify biomaterial surface is a promising method for improving cardiovascular material hemocompatibility. In the present work, human umbilical vein endothelial cells (HUVECs) are cultured and native ECM is obtained on pure titanium surface. Fourier infrared spectrum (FTIR) test proves the existence of amide I and amide II band on the modified titanium surface. X-ray photoelectron spectroscopy (XPS) further confirms the chemical composition and binding types of the ECM proteins on the titanium substrate. The results of light microscopy and atomic force microscopy (AFM) exhibit the morphology of HUVEC derived ECM. There are higher water contact angles on the ECM modified samples. Furthermore, some ECM components, including fibronectin (FN), laminin (LN) and type IV collagen (IV-COL) are presented on ECM-covered titanium surface by immunofluorescence staining. The biological behavior of cultured HUVECs and adherent platelets on different samples are investigated by in vitro HUVECs culture and platelet adhesion. Cells exhibit better morphology and their proliferation ability greatly improve on the ECM-covered titanium. At the same time, the platelet adhesion and spreading are inhibited on ECM-covered titanium surface. These investigations demonstrate that ECM produced by HUVECs cannot only improve adhesion and proliferation ability of endothelial cell but also inhibit adhesion and activation of platelets. Thus, the approach described here may provide a basis for preparation of modified surface in cardiovascular implants application.     

Fabrication and characterization of CdS-sensitized TiO<Subscript>2</Subscript> nanotube photoelectrode

CdS quantum dot (Qd)-sensitized TiO₂ nanotube array photoelectrode is synthesised via a two-step method on tin-doped In₂O₃-coated (ITO) glass substrate. TiO₂ nanotube arrays are prepared in the ethylene glycol electrolyte solution by anodizing titanium films which are deposited on ITO glass substrate by radio frequency sputtering. Then, the CdS Qds are deposited on the nanotubes by successive ionic layer adsorption and reaction technique. The resulting nanotube arrays are characterized by scanning electron microscopy, X-ray diffraction (XRD) and UV–visible absorption spectroscopy. The length of the obtained nanotubes reaches 1.60 μm and their inner diameter and wall thickness are around 90 and 20 nm, respectively. The XRD results show that the as-prepared TiO₂ nanotubes array is amorphous, which are converted to anatase TiO₂ after annealed at 450 °C for 2 h. The CdS Qds deposited on the TiO₂ nanotubes shift the absorption edge of TiO₂ from 388 to 494 nm. The results show that the CdS-sensitized TiO₂ nanotubes array film can be used as the photoelectrode for solar cells.     

Effects of enhanced hydrogen bonding on the mechanical properties of poly (vinyl alcohol)/carbon nanotubes nanocomposites

In this study, poly (vinyl alcohol) (PVA) composites reinforced by multiwall carbon nanotubes (MWCNTs) functionalized with either phenolic hydroxyl groups (MWCNTs-f-OH) or PVP molecule (PVP@MWCNTs) were fabricated. The objective was to elucidate the effect of different MWCNTs surface functionalization on the mechanical properties of the nanocomposites. It was found that both of PVP@MWCNTs and MWCNTs-f-OH had a good dispersion in PVA matrix. However, the MWCNTs-f-OH had stronger effective interfacial interaction with PVA matrix than PVP@MWCNTs, owe to the formation of hydrogen bonds between MWCNTs-f-OH and PVA. The stress-strain measurements showed that the Young’s modulus and tensile strength of MWCNTs-f-OH/PVA with only 1.0 wt.% contents increased by 200 and 100% compare with that of PVA, respectively. The findings of this experimental study emphasized the critical role of MWCNTs surface morphology in determining the mechanical properties of nanocomposites, and shed new light on understanding and advancing the properties of carbon nanotube based composites.     

N-F共掺杂锐钛矿二氧化钛(101)面纳米管的第一性原理研究

共掺杂是提高二氧化钛纳米管可见光催化性能的一种有效方式. 采用基于密度泛函理论的第一性原理方法, 研究了N单掺杂、F单掺杂及N-F共掺杂二氧化钛纳米管的原子结构、电子性质和光学性质. 计算结果表明, 相比N单掺杂和F单掺杂, N-F共掺杂二氧化钛纳米管的形成能更低, 掺杂后的体系热力学稳定性更好. 此外, 相比未掺杂时的带隙, N-F共掺杂后体系的带隙变化最多, 减少了0.557 eV, 而这主要源于价带顶附近的杂质能级的贡献. 此外, 通过分析掺杂后的光催化活性发现, N-F共掺杂时纳米管的还原性和氧化性都有所降低, 但并没有丧失活性, 并且光吸收谱表明, 共掺杂体系的红移现象最为明显. 因此, N-F共掺杂可有效提高二氧化钛纳米管可见光的光催化性能.     

Fabrication of gas ionization sensor based on titanium oxide nanotube arrays

Gas sensors have been fabricated based on field ionization from titanium oxide nanotubes grown on titanium foil. Ordered nanaotube arrays of titanium oxides were grown by the anodization method. We measured breakdown voltages and discharge currents of the device for various gases. Our gas ionization sensors (GIS) presented good sensitivity, selectivity, and short response time. The GISs based on TiO₂ nanotube arrays showed lower breakdown voltage, higher discharge current, and good selectivity. An excellent response observed for Ar compared to other gases. Besides, by introducing 2 % CO and 4 % H₂ to N₂ flow gas, the amount of breakdown voltage shifts about 20 and 70 volts to the lower values, respectively. The GIS works at room temperature and has the ability of detect inert gases with high stability and good linearity. Besides, short response time of about 1 second for the GISs based on TiO₂ nanotube arrays makes them excellent for gas sensing applications. Sharp edges of the nanotubes, through enhancing the applied electric field, reduce operating voltage to the reasonable values and power consumption.     

Studies of chemical diffusion in La1-X SrXCoO3-δ

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15?K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T _c less than 1?K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T _c in a single-wall nanotube. Our theoretical results can explain the observed T _c in a single-wall nanotube. In particular, we find that T _c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T _c has some sort of oscillatory behaviour as a function of the surface number density of electrons.     

Comparison of the Mechanical Properties and the Ballistic Behavior in Heracron®/Phenol Composite after Plasma Treatment

In this study, aramid, Heracron^®, which was purchased from Kolon Inc., Korea, and phenol composites were prepared by plasma treatment and the influence of this plasma treatment on their mechanical properties and ballistic behavior were ascertained. By measurement of water contact angle and XPS analysis, we find that the plasma treatment may help the incorporation of functional oxygen groups on the surface of the Heracron^® fabrics. Additionally, the surface roughness of Heracron^® fiber may be proportional to the plasma treatment times. By SEM and mechanical tests, it was shown that plasma-treated Heracron^®/phenol composites exhibited higher mechanical properties compared with plasma-untreated Heracron^®/phenol composites. Based on this finding, we can conclude that plasma-treatment can promote the interfacial adhesion between Heracron^® and phenol, resulting in the superior mechanical properties of their composites. However, the ballistic behavior conflicted with the mechanical properties. That is to say, a helmet, manufactured from the plasma-treated Heracron^®/phenol composites, exhibited lower ballistic properties than that of the plasma-untreated Heracron^®/phenol composites. As a result, we demonstrated that for ballistic application, somewhat weak fiber–matrix adhesion leads to the improved ballistic behavior. However, it becomes apparent that the optimum strength condition for the composite interphase depends on the particular application, thus increased fiber–matrix adhesion may be more appropriate.     

Physico-chemical characterization of the fibre/matrix interaction in polyethylene fibre/epoxy matrix composites

The interphase in polyethylene fibre/epoxy matrix composites is studied with FT-IR microspectroscopy using a set-up to investigate the matrix as close to the fibre as a few μm or less. It is shown that moisture present on the fibre surface is able to influence the polymerization reaction of the epoxy/anhydride matrix in an irreversible manner. This effect is enhanced for composites from the more hydrophilic polyvinylalcohol fibre. The fibre/matrix interaction in these thermoplastic fibre composites is also studied with DSC through the characterization of the fibre melting. A decreased 'DSC interaction parameter' is found if the composition of the interphase is changed by moisture. For a composite with an epoxy/amine matrix, on the other hand, the DSC interaction parameter is unaffected by moisture from the fibre surface.     

Theoretical insights into the effects of the diameter and helicity on the adsorption of formic acid on silicon carbide nanotube

The anchoring of small organic molecules onto the semiconductor surface has a great application for developing various molecular devices, such as novel solar cells, fuel cells, hybrid systems, sensors, and so on. In the present work, by carrying out detailed density-functional theory calculations, we have investigated the adsorption of the formic acid (HCOOH) molecule on planar and various curved silicon carbide (SiC) nanotubes. By considering both the molecular and dissociative adsorptions of HCOOH on these SiC nanomaterials, we found that the HCOOH molecule prefers to dissociate into HCOO and H group. Interestingly, different adsorption modes were found for HCOOH on SiC nanotubes, i.e. dissociative monodentate or bidentate adsorption, which depends on the tube diameter and helicity. For (n, 0) SiC nanotube, the monodentate adsorption mode is energetically favorable when n is less than 10. However, HCOOH prefers to be adsorbed on other (n, 0) SiC nanotubes in a bridged bidentate mode, which is similar to those of on (n, n) SiC nanotubes or planar SiC sheet. Moreover, upon HCOOH adsorption, these SiC nanomaterials remain to be of the semiconducting nature and their band gaps are decreased to different degrees. In addition, we also explored the effects of HCOOH coverage on its adsorption on SiC nanotube.     

Synthesis and characterization of CdS nanoparticle based multiwall carbon nanotube–maleic anhydride–1-octene nanocomposites

CdS nanoparticles were synthesized by sonication from cadmium chloride and thiourea using a multiwall carbon nanotube (MWCNT)–maleic anhydride (MA)–1-octene system as the matrix. The matrix was obtained by the “grafting from” approach from oxidized carbon nanotubes and maleic anhydride–1-octene. Multiwall carbon nanotubes used for reinforcing the matrix were synthesized by Catalytic Chemical Vapor Deposition using Fe–Co/Al₂O₃ as the catalyst. The obtained nanostructures were characterized by FTIR, XRD, Raman spectroscopy, TEM, SEM and UV–vis spectroscopy. The average CdS particle diameter was 7.9 nm as confirmed independently by TEM and XRD. UV–vis spectroscopy revealed that the obtained nanostructure is an appropriate base material for making optical devices. The novelty of this work is the use of the MWCNT–MA–1-octene matrix obtained via the “grafting from” approach for the synthesis of uniformly dispersed CdS nanocrystals by ultrasonic cavitation to obtain a polymer nanocomposite.     

Entropy-driven adsorption of carbon nanotubes on (0 0 1) and (1 1 1) surfaces of CeO2 islands grown on sapphire substrate

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO₂ surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO₂ islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO₂ surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO₂ and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO₂ islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO₂ surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.     

Interphase formation and fiber matrix adhesion in carbon fiber reinforced epoxy resin: influence of carbon fiber surface chemistry

The chemistry and morphology of the carbon fiber surface are important parameters which govern the properties of the interfacial region and the adhesion between carbon fibers and polymeric matrix in carbon fiber reinforced polymers. In the presented paper the surface chemistry of the fibers is varied while the surface morphology is left unchanged. We analyze chemical functionality and morphology of carbon fiber surfaces showing different degrees of activation, together with the adhesion of these fibers to an epoxy matrix and the width of the interfacial region between fiber and matrix. An increase of the oxygen and nitrogen concentration of the fiber surface, in particular in form of carboxyl functional groups, results in a significant increase of interfacial shear strength. Also the width of the interphase, as determined by scanning force microscopy in nanomechanical mode, depends on the activation degree of the carbon fibers. However, no direct correlation between interphase width, surface chemistry and fiber matrix adhesion is found, suggesting no direct influence of interphase width on adhesion properties.