Effect of nanosized carbon fillers on the hydrolytic stability of films of a heat-resistant aromatic polyimide

The effect of carbon nanoparticles (nanofibers, nanocones/nanodiscs) introduced into a polypyromellitimide matrix on the stability of the mechanical and thermal characteristics of the resulting nanocomposite film materials in the course of hydrolysis in aqueous alkali was studied. The nanocomposite films are less resistant to hydrolysis than the unfilled films of the matrix polyimide. The decreased packing density in the nanocomposite material is considered to be the major factor responsible for the observed effects. A certain excess void volume in the nanocomposite appears owing to insufficient compatibility of the matrix polyimide with the nanoparticles, manifested in the course of the material preparation.     

Effect of carbon nanoparticles of different shapes on mechanical properties of aromatic polyimide-based composite films

Nanocomposite films based on thermally stable polypyromellitimide are prepared via the introduction of cylindrical and laminar carbon nanoparticles—nanofibers and nanocones/disks—into the prepolymer solution. An increase in the filler concentration increases the elastic modulus and yield stress of the material. Moreover, in composites filled with nanofibers, both characteristics increase much less intensely than those of films containing nanocones/disks. Rigidity increases in composites filled with nanofibers when their concentration is below ～7 vol %. A further increase in concentration is accompanied by the aggregation of nanofibers. The introduction of the nanoparticles of both types into the polymer matrix stabilizes the behavior of the material under prolonged mechanical loading: both the creep strain and the relaxation intensity decline with an increase in the filler concentration. The introduction of nanocones/disks into the polyimide matrix results in a more pronounced stabilization of creep characteristics of films than that in the case of nanofibers.     

Hydrolytic Stability of Films of Aromatic Polyimides and Composites on Their Basis,Filled with Carbon Nanocones

Variation of the mechanical and thermal characteristics of poly(4,4′-oxydiphenylenepyromellitimide) and poly{1,3-bis(3′,4-dicarboxyphenoxy)benzene [4,4′-bis)4′-N-phenoxydiphenylsulfone]imide} films and nanocomposites based on these polyimides and filled by carbon nanocones/disks in the course of hydrolysis in an alkaline solutions was studied. The goal of the study was to obtain systematic information about the influence exerted by carbon nanoparticles introduced into polyimide films with varied chemical structure on the stability of the resulting film materials against a prolonged action of active hydrolyzing media. It was shown that the hydrolytic stability of the materials under study is largely determined by the molecular packing density. Introduction of nanoparticles into the polymers under study may result in that the concentration in the material of the excess free volume localized at polymer–filler interfaces increases. This, in turn, causes a decrease in the hydrolytic stability of the nanocomposite film as compared with unfilled films of the matrix polyimide. The opportunity was considered of raising the hydrolytic stability of the polyimide and nanocomposite material by making higher the average packing density.     

Carbon nanocones/discs – a new type of filler to improve the thermal and mechanical properties of polymer films

The mechanical characteristics and thermal properties of composite films based on the thermally stable aromatic polyimide (PI) (PMDA‐ODA) and carbon nanocones/discs (CNC) were studied. The introduction of CNC to PMDA‐ODA leads to the substantial increase of film stiffness. The Young's modulus values of the composite films are somewhat higher than those of the previously characterized composite films of this PI filled with nanoclay, carbon nanofibers, and asbestos‐like hydrosilicate nanotubes. The introduction of CNC into PMDA‐ODA (concentrations of CNC were up to 15 vol%) does not cause any marked aggregation of nanoparticles. The presence of CNC in the PI matrix does not affect the glass transition temperature of the polymer but hinders chain mobility at temperatures above T_g. This behavior makes it possible to increase the working temperature range of the composite films containing more than 5 vol% of CNCs, up to the temperature of thermal decomposition. The introduction of CNC into PMDA‐ODA leads to dramatic (～12 orders of magnitude) increase of active electrical conductivity of the material. Copyright © 2011 John Wiley & Sons, Ltd.     

Graphene structure in carbon nanocones and nanodiscs

Carbon nanoparticles, like nanocones and nanodiscs, can be obtained by mechanical treatment of carbon nanofilaments. Microstructural studies suggest that in nanocones the conical graphene stacking with progressively increasing apex (cone) angles does not fully agree with current theoretical geometry models, such as a closed cones model and a cone-helix model. The unusual stacking form of nanocones was taken into account in a modified cone-helix model. The formation mechanism of the distinctive microstructure is attributed to the inclined anchoring effect, and the relaxation of internal stresses, which were induced by the confined pyrolysis process, resulting in easier disintegration by sonication the nanofilaments. This is disclosed for the first time in literature regarding the attainment of uniform carbon nanoparticles.     

Effect of apical defects and doped atoms on field emission of boron nitride nanocones

We present a systematic study of field-emission performance of prototype boron nitride (BN) nanocones using all-electron density-functional theory method. The effects of apical defects and doping/adsorption on the field emission have been evaluated on the basis of magnitude of ionization potential (IP) and electron affinity (EA). Among BN nanocones examined, two 120 degrees -BN nanocones, namely 120 degrees -4-B-N and 120 degrees -55-mol-B, have been identified as promising candidates for the field-emission electron source. Effects of the applied electric field on the electronic structures of BN nanocones have been investigated. In general, the electronic structures of BN nanocones can be significantly modified by a strong electric field, such as the reduction of the HOMO-LUMO gap and the change in density of states. The interaction between BN nanocones and applied electric field can be described by the second-order Stark effect. In addition, calculations show that the doping/adsorption of an impurity atom results in higher IP or EA values, which is unfavorable to the field emission. Our study suggests that BN nanocones can be considered as alternative cold-emission electron sources.     

Individual strings of conducting carbon cones and discs in a polymer matrix: Electric field‐induced alignment and their use as a strain sensor

We demonstrate micromechanical strain sensors with integrated readout based on carbon nanocones and discs (CNCs) which are aligned into a string‐like formation using an alternating electric field and studied by AC impedance spectroscopy and electromechanical methods. The CNC particles are first dispersed into a polymer matrix with a particle fraction of 0.1 vol %. This value is well below the percolation threshold (～ 2 vol %), which suppresses particle aggregation and facilitates transparency allowing the use of an UV‐curable polymer. Alignment was carried out with a 1 kHz, 4 kV/cm electric field and is a consequence of dielectrophoretic effect. It develops in minutes and makes the initially insulating, nonaligned material conductive. This is followed by UV curing of the polymer matrix, which renders a solid state device. The stretching of the aligned strings in the cured polymer leads to a reversible piezoresistive effect, and a gauge factor of about 50 is observed. This is in a sharp contrast to CNC films with particle fraction above percolation threshold (13 vol %), which are conductive but not sensitive to stretching. The strings are Ohmic in nature and moreover show higher DC conductivity (22–500 S/m) compared to identically prepared carbon black strings (1–22 S/m). © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Evaluation of carbon nanocones/disks as sorbent material for solid-phase extraction

The potential of carbon nanocones/disks as sorbent material in solid-phase extraction (SPE) procedures has been evaluated. For this aim, a model analytical problem, the determination of chlorophenols in water samples, was selected. An accurately weighed amount of 20 mg of purified carbon nanocones/disks was packed in 3 mL commercial SPE cartridges. Once conditioned, up to 8 mL of water samples can be preconcentrated without analyte losses. The chlorophenols were eluted by using 200 μL of hexane. Aliquots of 2 μL of the organic extract were injected in the gas chromatograph–mass spectrometer for separation and quantification. The purification of the commercial nanocones/disks to reduce the presence of amorphous carbon has been successfully achieved by heating the carbon nanocones/disks at 450 °C for 20 min. Detection limits of chlorophenols were in the range 0.3–8 ng mL⁻¹ by using 2 mL of sample. Moreover, excellent average recovery values (98.8–100.9%) have been obtained after the analysis of water samples from different nature. Finally, the performance of the carbon nanocones/disks as sorbent material has been compared with that of multiwalled carbon nanotubes, providing the former better results under the experimental conditions assayed.     

Unexpected selective enhancement of the thermal stability of aromatic polyimide materials by cerium dioxide nanoparticles

The unusual effect of selective enhancement of the thermal stability of aromatic polyimide materials was established through the introduction of cerium dioxide nanoparticles into these polymers as nanofiller. Depending on the chemical structure of the polymers, a marked increase or a substantial decrease in the thermal stability of the nanocomposite material was registered by thermal analysis, as compared with that of unfilled polymer material. The positive effect was registered only for the composite materials based on the matrix polyimides containing the sulfur atoms located in the sulfonic groups arranged in the elementary units. The results of the thermogravimetric examination are compared with the data obtained during the mechanical tests of the same samples. The possible reasons for the alteration of the thermal stability of polymers by ceria nanoparticles are discussed. The effect above can be of substantial practical interest providing new options for the design of polyimide nanocomposite materials with enhanced thermal stability.     

Production of complex cerium-aluminum oxides using an atmospheric pressure plasma torch

Ceria-alumina particles of a wide variety of structures, from micrometer-sized hollow spheres to nanoparticles, were produced from aerosols of different natures, but all derived from nitrate salts passed through a low power (<1000 W) atmospheric pressure plasma torch. The amount of water present with the nitrate salts was found to significantly affect the morphology of the resulting material. A model was proposed that explains the mechanism in which water acts as a blowing agent to create hollow metal oxide spheres that then shatter to form metal oxide nanoparticles. Further examination of the nanoparticles revealed that they display a core/shell morphology in which the core material is crystalline CeO2 and the shell material is amorphous Al2O3. These unique core/shell materials are interesting candidates for catalyst support materials with high thermal durability. In addition, experiments have shown that the nanoparticles can be readily converted into CeAlO3 perovskite.     

From the surface to volume: concepts for the next generation of optical-holographic data-storage materials

Optical data storage has had a major impact on daily life since its introduction to the market in 1982. Compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs (BDs) are universal data-storage formats with the advantage that the reading and writing of the digital data does not require contact and is therefore wear-free. These formats allow convenient and fast data access, high transfer rates, and electricity-free data storage with low overall archiving costs. The driving force for development in this area is the constant need for increased data-storage capacity and transfer rate. The use of holographic principles for optical data storage is an elegant way to increase the storage capacity and the transfer rate, because by this technique the data can be stored in the volume of the storage material and, moreover, it can be optically processed in parallel. This Review describes the fundamental requirements for holographic data-storage materials and compares the general concepts for the materials used. An overview of the performance of current read-write devices shows how far holographic data storage has already been developed.     

Changes in the surface morphology induced by 1.0‐MeV Au ion bombardment of Ti and Ti‐6Al‐4V

The effects of surface sputtering by 1.0‐MeV Au ion implantation in commercially pure Ti and its alloy Ti‐6Al‐4V have been studied. These materials are associated with applications in orthopaedic implants. There are few studies that try to explain the ion implantation process of Au in these materials when considering the effects generated on the surface by sputtering, especially at energies of the order of MeV. Discs of these materials were mirror polished and then implanted with 1.0‐MeV Au ions for 4.7 × 10¹⁷ ions/cm² at 45° incident angle with respect to the surface. Part of the eroded material was deposited simultaneously on glass slides to determine their spatial distribution. These discs and the slides were analysed by Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM), optical microscopy and atomic force microscopy. The implanted materials show the initial production of surface ripples that evolve into banded structures. Copyright © 2014 John Wiley & Sons, Ltd.     

双光子荧光有机纳米粒子的细胞染色

本文采用具有较大双光子吸收截面的有机分子2,5,2′,5′-(4′-N,N-二苯胺苯乙烯基)联苯(DPA-TSB)(双光子吸收截面δ: 3288 GM, 1 GM=1×10^-50 cm⁴·s·photon^-1·molecule^-1), 通过再沉淀法制备水相分散的纳米粒子. 研究表明, 这种有机双光子纳米粒子可以有效地富集在细胞质中, 对细胞染色显示出良好的荧光成像能力.     

KARI酶及其抑制剂

酮醇酸还原异构酶(Keto-Acid Reducto－Isomerase, KARI或称作乙酰羟基酸异构还原酶, Aceto-Hydroxyacid Isomero-Reductase, AHIR)是植物、微生物中的支链氨基酸生物合成途径中第二步的关键性酶。由于哺乳动物和人体内并不存在这种酶,因此靶向KARI酶的抑制剂具有安全、高效等特点,成为农用除草剂以及医药抗真菌药物的一个引人注目的研究方向。本文综述了KARI的酶学,KARI抑制剂及其抑制机理和理论研究进展,并对其发展趋势进行了展望。     

On the Stability of Pt-Based Catalysts in HBr/Br2 Solution

Stability studies on supported metal nanoparticles are essential for gaining insight into the design and optimization of high-performance materials. In this work, the dissolutions of Pt-based catalysts in HBr/Br₂ mixture of various concentration regimes were studied and correlated with material structural properties. The dissolution of metal nanoparticles was enhanced by adding Br₂ to the HBr solution. Comparing with commercial Pt/C catalyst, the well-alloyed PtIr/C catalyst was observed to exhibit high resistance towards dissolution. In addition, regulating the accessibility of the metal sites to dissolution-inducing species contributed to the marked stability of the nanoparticles in HBr/Br₂ solutions, as shown for the surface-modified PtIr/C catalysts with organic diamine molecules.     

In situ Polymerization Technique for Obtaining Composite Materials Based on Polyethylene,Multi-walled Carbon Nanotubes and Cobalt Nanoparticles

Specific features of formation of composite materials based on polyethylene, multi-walled carbon nanotubes, and cobalt nanoparticles (MWCNT–PE and Co/MWCNT–PE) by in situ polymerization were revealed. The in situ polymerization technique is based on ethylene polymerization on a highly dispersed Ti-containing catalyst supported on the surface of MWCNT and Со/MWCNT. This method ensures uniform distribution of MWCNT and Co/MWCNT in the polyethylene matrix. The effect of MWCNT and Со/MWCNT on the final properties of the composite materials was determined. Introduction of MWCNT into the polyethylene matrix increases the specific electrical conductivity of the material. Such composite materials can be used for preparing concentrates with preset composition for the subsequent preparation of antistatic coatings. Introduction of the Со/MWCNT filler into the polyethylene matrix makes the material paramagnetic owing to Co nanoparticles and allows its use for preparing coatings that efficiently protect from electromagnetic radiation.     

Microstructure of metallic copper nanoparticles/metallic disc interface in metal–metal bonding using them

This paper describes a metal–metal bonding technique using metallic Cu nanoparticles prepared in aqueous solution. A colloid solution of metallic Cu particles with a size of 54 ± 15 nm was prepared by reducing Cu²⁺ (0.01 M (CH₃COO)₂Cu) with hydrazine (0.6 M) in the presence of stabilizers (5 × 10^?4 M citric acid and 5 × 10^?3 M cetyltrimethylammonium bromide) in water at room temperature in air. Discs made of metallic materials (Cu, Ni/Cu, or Ag/Ni/Cu) were successfully bonded under annealing at 400 °C and pressurizing at 1.2 MPa for 5 min in H₂ gas with help of the metallic Cu particle powder. Shear strength required for separating the bonded discs was 27.9 ± 3.9 for Cu discs, 28.1 ± 4.1 for Ni/Cu discs, and 13.8 ± 2.6 MPa for Ag/Ni/Cu discs. Epitaxial crystal growth promotes on the discs with a good matching for the lattice constants between metallic nanoparticles and metallic disc surfaces, which leads to strong bonding. Copyright © 2013 John Wiley & Sons, Ltd.     

Formation of Crystalline Sodium Hydride Nanoparticles Encapsulated Within an Amorphous Framework

A major research theme to emerge in the science and technology of materials is the incorporation of nanostructure into the functionality of properties. Such nanostructured materials can offer distinct advantages over bulk materials, partly because the physical properties of the material itself can vary in a tunable, size-dependent fashion. Of course, in addition, nanoparticles offer a greatly increased surface area for chemical reaction. Typical methods for nanoparticle synthesis include: reaction in the liquid phase using the sol–gel approach and mechanical ball-milling of the bulk material; both of these approaches are somewhat problematic for the preparation of reactive nanostructured materials which are sensitive to air and/or moisture. We report here the formation of crystalline nanoparticles of sodium hydride encapsulated in a host amorphous silica gel matrix. These nanoparticles are formed by in situ hydrogenation of a precursor material—Na loaded silica gel—under mild conditions. The resulting material is considerably less pyrophoric and less air-sensitive than the bulk hydride. We anticipate that this formation method of in situ modification of reactive precursor material may have wide applications.     

Small angle X-ray scattering analysis of the effect of cold compaction of Al/MoO3 thermite composites

Nanothermites composed of aluminum and molybdenum trioxide (MoO(3)) have a high energy density and are attractive energetic materials. To enhance the surface contact between the spherical Al nanoparticles and the sheet-like MoO(3) particles, the mixture can be cold-pressed into a pelleted composite. However, it was found that the burn rate of the pellets decreased as the density of the pellets increased, contrary to expectation. Ultra-small angle X-ray scattering (USAXS) data and scanning electron microscopy (SEM) were used to elucidate the internal structure of the Al nanoparticles, and nanoparticle aggregate in the composite. Results from both SEM imaging and USAXS analysis indicate that as the density of the pellet increased, a fraction of the Al nanoparticles are compressed into sintered aggregates. The sintered Al nanoparticles lost contrast after forming the larger aggregates and no longer scattered X-rays as individual particles. The sintered aggregates hinder the burn rate, since the Al nanoparticles that make them up can no longer diffuse freely as individual particles during combustion. Results suggest a qualitative relationship for the probability that nanoparticles will sinter, based on the particle sizes and the initial structure of their respective agglomerates, as characterized by the mass fractal dimension.     

Dispersing different nanoparticles in paraffin wax as enhanced phase change materials

Highly conductive nanoparticles were proposed to be dispersed into phase change materials (PCMs) such as paraffin wax for heat transfer enhancement. The mixture, often referred to as nanoparticle-enhanced phase change material (NePCM), has been studied extensively for latent heat energy storage but with conflicting results. This study attempts to understand this problem by investigating the stability of NePCMs under multiple thermal (melting–solidification) cycles, which has not been well explained in previous studies. We believe that stability of a NePCM is prerequisite for any experimental investigation of its thermal properties or application. In this study, paraffin wax was chosen as the base material. Three different types of nanoparticles were tested, i.e., multi-walled carbon nanotubes, graphene nanoplatelets, and aluminum oxide nanoparticles (Al₂O₃). The nanoparticles were dispersed into paraffin wax at varying mass fractions using mechanical dispersion methods (sonication, stirring) with and without different surfactants. Stability of different mixtures was investigated after consecutive thermal cycles performed in an environmental chamber. Significant coagulation and deposition of nanoparticles were found after a few thermal cycles regardless of the nanoparticle type, concentration, or dispersion method. Different boundary conditions in heating were also examined for their effects. None of these methods led to long-term stable NePCMs. The “negative” results from this study indicate that long-term stability of NePCM (at least for the paraffin wax and nanoparticles tested) remains a major challenge and requires further research with a multidisciplinary approach.